Main Body - NiTi Project by xiuliliaofz

VIEWS: 21 PAGES: 37

									1. INTRODUCTION
NiTi shape memory alloys, SMA, also known as memory metal or smart alloy. It can recover
apparent permanent strains & return to some previously defined shape or size when subjected
to the appropriate thermal procedure.


There are other types of SMA called ferromagnetic shape memory alloys, FSMA, which
change shape under strong magnetic fields. These materials are of particular interest as the
magnetic response tends to be quicker and more efficient than temperature-induced responses.
However, metal alloys are not the only thermally responsive materials, as shape memory
polymers have also been developed and become commercially available in the late 1990‟s.
The two main types of SMA are the copper-base alloys such as Cu-Zn-Al and Cu-Al-Ni and
nickel-titanium, NiTi or TiNi, alloy. NiTi alloys possess superior mechanical properties when
compared to copper - based SMAs.


NiTi was named Nitinol (Nickel-Titanium Naval Ordnance Laboratory). The first efforts to
exploit the potential of NiTi as an implant material were made by Johnson and Alicandri in
1968 (Castleman et al. 1976). The use of NiTi for medical applications was first reported in
the 1970s (Cutright et al. 1973, Iwabuchi et al. 1975, Castleman et al. 1976, Simon et al.
1977). It was only in the mid-1990s, however, that the first widespread commercial
applications made their breakthrough in medicine. The use of NiTi as a biomaterial is
attractive because of its superelasticity and shape memory effect, which are completely new
properties compared to the conventional metal alloys.


Porous titanium-nickel, PTN, is found as new biomaterials for long-term implantation as well,
suitable for exchange of nutrition, growth of human tissue and it is light in weight. [1] The
porous have strong effects on corrosion characteristics of porous NiTi SMA; porous NiTi
SMA is found less corrosion resistant than solid one by potentiodynamic polarization tests.
[23]


The objective of this project is to, providing same condition, characterizes the wear
behaviours of alloy under difference heat-treatment temperature, porosity and applied normal
force.




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2. LITERATURE REVIEW
2.1 Wear, Friction & Hardness
2.1.1 Wear Behaviours & Mechanisms
Wear situations are common. It occurs when two solid bodies are in contact and move
relative to one another such as sliding, rolling, or impact; a liquid moving relative to a solid
surface such as erosion or erosive wear and occur when the wear caused by hard particles -
abrasive.


Wear is mechanically-induced surfaced damage & energy dissipation results in the
progressive removal of material. However the wear resistance of material is not a basic
material property, like elastic modulus or yield strength. The wear behaviours of material
depend on the conditions of its use. Hence, the right type of wear test, including the design of
test, control & selection of criteria & variables, results analyze, worn-surfaces examining,
testing parameters adjustment to better establish the usefulness and repeatability of the results,
can provide useful and meaningful engineering data. [3]


A good wear-resistant material should dissipate heat well, but not use the energy input to
create new surfaces (i.e. minimize the energy going into fracture, plastic deformation, and/or
micro chip cutting). [2]


There are four general types a material can wear in the aforementioned situations. They are
adhesive processes, abrasive or deformation processes, fatigue or fatigue-like processes
which associated with crack initiation and propagation or progressive deformation as a result
of repeated contact, and oxidative or corrosive processes which associated with the loss of
wear of in situ formed reaction product (e.g., oxide layers).[4]


In metal, wear is commonly classified into two categories: abrasive wear and adhesive wear.


In adhesive wear processes, wear occurs as a result of the bonding that takes place between
two surfaces in contact. With subsequent separation of the two surfaces, material from either
surface may be pulled out, separation and transfer of material from the surface, resulting in
wear. Adhesion is a major contributor to sliding resistance (friction) and can cause loss of



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material at the surface (wear) or surface damage without a loss of material at the surface (e.g.
galling). [5]


Galling is considered to be a severe form of adhesive wear that occurs when two surfaces
slide against each other at relatively low speed and high load. With high loads and poor
lubrication, surface damage can occur on sliding metal components. The damage is
characterized by localized macroscopic material transfer, that is, large fragments or surface
protrusions that are easily visible on either or both surfaces. For highly ductile materials,
asperities tend to plastically deform, thereby increasing the contact area of mated surfaces;
eventually, galling occurs.


Materials that have a hexagonal close-packed structure have a low dislocation cross slip rate
and are less prone to galling. Therefore titanium alloys tend to gall.


In abrasive, the wear processes are those fractures, cutting, and plastic deformation processes
that can occur when a harder surface engages a softer surface. These mechanisms tend to
produce machining-chip-like debris. Consequently, these mechanisms can be eliminated by
making the wearing surface harder than the particles causing the wear.


The contact pressure between abrasive and wearing surface is controlled by the size, shape,
and quantity of the abrasive. [6]


Wear mechanisms are not mutually exclusive. They can coexist and interact to form more
complex wear processes. When worn surfaces are examined, more than one mechanism are
usually found. However, in most tribosystems one type of mechanism tends to predominate
and ultimately be the controlling one. Except for adhesive wear processes, these processes
can result from sliding, rolling, and normal impact motions. Adhesive wear processes
normally occur only with sliding. However, adhesive wear processes can occur under
nominal impact and rolling conditions, because of the slip that is often present in those
situations.


There are system wear processes associated with the formation of tribofilms - composed of
wear debris that form on wear surfaces. If the layer is entirely composed of material from the



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counterface, it is called a transfer film. If it is composed of material from both surfaces, it is
called a third-body film.


With respect to wear, these films tend to act as lubricants by separating the two surfaces.
However, these films do not necessarily result in reduced friction.


Initial wear behavior of a tribosystem is often different from long-term behavior. Firstly,
wear and wear processes modify surfaces and it may take some time before stable surface or
contact conditions are established. Another is that the relative mix of mechanisms can change,
either as a result of the changes to the surfaces and the interface. While long-term wear rates
tend to be lower than initial wear rates, they are not necessarily constant. Depending on the
tribosystem, long-term wear rates may change as a result of the nature of the dominant wear
mode.




Fig. 1 Relation of friction force to metal substrate hardness. (a) Hard metal in contact with soft metal.
(b) Two hard metals of comparable hardness in contact with each other. (c) Two hard metals of
comparable hardness separated by a thin-film layer of soft metal deposited on one metal surface.
Deposition of a thin film of a soft metal on a hard metal substrate yields the lowest friction force of
the above-mentioned three cases. [26]




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2.1.2 Wear Measurement
Common techniques used to measure wear include measuring the length or thickness change
of the test specimen, profiling surfaces to determine the wear depth or cross-sectional area
worn away, using a precision balance to measure mass loss, measuring the relative
displacement of specimens on the testing machine (in situ ) with a mounted sensor of some
type, and making measurements of wear scar dimensions with microscopy. Other, less
common methods include making replicas of surfaces before and after testing, placing
hardness impressions in surfaces, and measuring the change in their sizes after wear and
measure surface layer activation by radionulcides.


Each method lends itself to certain techniques of wear measurement. However, wear data can
be related to the testing machine characteristics, the method of wear measurement, intrinsic
material variations, and sometimes the operator's judgment in making measurements. And
each measure of wear has limitations in both its precision and accuracy. [3]




2.1.3 Wear & friction
Surface is not completely flat at the microscopic level. At high magnification, even the best
polished surface will show ridges and valleys, asperities, and depressions; for this reason,
frictional force as the result, resists one surface move over another surface. In sliding contact,
the kinetic friction force, F, by definition, is the product of the kinetic friction coefficient, μk,
and the normal force, P. To overcome friction, the tangential force must be applied over the
entire sliding distance; the product of this μk & F consequence friction work - the resulting
energy, such as heat energy. [7]
.

However, not all the frictional energy turns into heat; part that goes into forming new
surfaces and deforming and fracturing the surface material.


General speaking, friction is independent of sliding velocity & contact area but proportional
to applied load, N, or F = μN. The proportionality constant is generally designated μ or f and
in termed the friction coefficient [7] . It is the ratio between the friction force, F, & the load,
N:
                                             μ=F/N


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Friction coefficient typical ranges from 0.03 for a very well lubricated bearing, 0.5 to 0.7 for
dry sliding, and even ≧ 5 for clean metal surfaces in a vacuum.


In fact, the factors of the friction coefficient between solids sliding are: composition of the
materials, surface finish, surface roughness, nature of the surrounding environment, velocity
& nature of relative motion, temperature of the interfacial region, prior sliding history of the
surfaces & characteristics of the machine and fixtures in which the materials are affixed. [8]
There are no single source has generated a comprehensive in spite of this complexity, the
values of μ obtained by different methods by different laboratories tend to fall into range that
is representative of the material pair in question under reasonable similar conditions.
Therefore the existing handbooks rely on compilations of data produced under a variety of
testing conditions. [9]


Such as wear, a constant, reproducible, and predictable friction values are necessary for the
design of components and machines that will function efficiently and reliably.


Moreover, in frictional test, when two surface are brought together, they touch intimately
only at the tips of a few asperities. At these points, the contact pressure may be close to
hardness of the softer material; plastic deformation takes place on very local scales, and cold
welding may form strongly bonded junctions between the two materials. When sliding begins,
these junctions have to be broken by the friction force, and this provides the adhesive
component of the friction. Some asperities may plow across the surface of the mating
material, and the resulting plastic deformation or elastic hysterics contribute to the friction;
additional contributions may be due to wear by debris particles that become trapped between
the sliding surfaces.


The deformation at asperities and junctions is extremely localized, and very high
temperature may therefore be generated over very short periods of time.


Friction oscillations may develop when the static coefficient of friction is greater than the
kinetic, as is the case for many unduplicated system. The resulting motion is often called
“stick-up”. The two surface stick together until the elastic energy of the system has built up




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to the point where a sudden forward slip takes place. The resulting oscillations may produce
equipment vibrations, surface damage, and noise. [10]


In many cases low friction is desired (bearings, gears, materials processing operations), and
sometimes high friction is the goal (brakes, clutches, screw threads, road surfaces). In all of
these cases, constant, reproducible, and predictable friction values are necessary for the
design of components and machines that will function efficiently and reliably.


The presentation of friction and wear data as a function of operational and structural
parameters of typical tribographs


The simplest type of tribotest involves subjecting a given tribosystem to a defined constant
set of structural and operational parameters and measuring friction or wear as functions of
time only.




Fig 2. Friction-Time Master Curves.


The typical friction-time curve during dry sliding consists of four stages of friction
coefficient. A typical example for a metal/metal system is:


The initial value of the friction coefficient value of stage I, is usually about f0   0.1, is
dependent on low loads, FN, and on the shear resistance of surface contaminants, but is
largely independent of material combinations. Surface layer removal and an increase in




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adhesion due to the increase in clean interfacial areas as well as increased asperity
interactions and possible wear particle entrapment lead to a gradual increase in the friction
coefficient.


Stage II, which produces the maximum value of the friction coefficient (fmax       0.3 to 1.0 for
most metal pairs), is reached when maximum interfacial adhesion, asperity deformation, and
wear particle entrapment occur.


In stage III, a decrease in the friction coefficient may occur due to the possible formation of
protective tribochemical surface layers and a decrease in plowing and asperity deformation
processes.


Stage IV is characterized by steady-state interfacial tribological conditions leading eventually
to almost constant friction coefficient values.

     Fig 3. Wear-Time Master Curve.




     The typical wear-time curve of dry sliding metal/metal systems consists of three wear
stages:

Stage I, which may have been preceded by a non-wear incubation period, is called the
running-in period. During stage I, the probability of the occurrence of elementary wear
events may decrease if, through changes in surface topography, the interaction rate of surface
asperity collisions decreases.




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In stage II, the tribosystem may exhibit relatively stable behavior under the action of the
tribological processes. In this case the probability of wear events remains constant. This
steady-state situation is characterized by a constancy of the wear-loss output per unit of time
without change in the tribological processes.


In stage III an acceleration of wear may occur through an accumulation of elementary wear
processes. During stage III, changes in the state of the system are of a directed nature and the
increments in the wear processes in this regime are mutually dependent. [11]




2.1.4 Hardness
Since wear is a resulting of the accumulation of plastic deformation and hardness is the
measure of a material‟s ability to combat plastic deformation. [12]


Hardness is the characteristic of a solid material expressing its resistance to permanent
deformation & commonly refers to a material‟s ability to penetrate softer materials. It can be
measured on the Mohs scale or various other scales. An object made of a hard material will
scratch an object made of a softer material. Indentation hardness seeks to characterize a
material‟s hardness, i.e. its resistance to permanent, and in particular plastic, deformation. [13]


Sometime the stress conditions experienced by a wear surface are more similar to those
imposed during a microindentation hardness test than in other cases.


However, in wear of metals and alloys, contact surfaces may change hardness greatly due to
work hardening during wear. Therefore, only using initial hardness numbers may be
misleading.


And in a traditional hardness test, the indenter moves vertically down and up. But in many
forms of wear, material is deformed tangentially to the plane of the surface giving rise to
shear stresses that do not occur in vertical penetration experiments.


In hardness testing, the rate of the indentation (strain rate) may be small compared to that
expertise, often increasing in stiffness and yield strength.


                                                9
Microindentaion hardness numbers obtained under normal room environments may not
accurately portray the mechanical behavior of a wear surface that is heated by friction.
Furthermore, tribochemcial effects such as oxidation or film formation may dominate wear
behavior in ways that cannot be described by hardness numbers.


Hard materials may be brittle and thus subject to fracture under the action of wear.
Microindentation hardness to wear resistance may not be established for very brittle materials
because the data simply cannot be obtained. [14]




2.2 SMA effect & pseudoelasticity
The behaviour of solids strongly influenced by structure transitions. The reversible phase
transition between the low temperature martensite phase and the high-temperature austenite
phase in NiTi alloys leads to well-know shape-memory and superelasticity effects. [12]


Shape memory effect describes the process of restoring the original shape of a plastically
deformed sample by heating it. This is result of a crystalline phase change known as
“thermoelastic martensitic transformation”. Below the transformation temperature, Nitinol is
martensitic. The soft martensitic microstructure is characterized by “self-accommodating
twins”, a zigzag like arrangement. Martensite is easily deformed by de-twinning. Heating the
material converts the material to its high strength, austenitic condition. The transformation
from austenite to martensite (cooling) and the reverse cycle from martensite to austenite
(heating) do not occur at the same temperature. There is a hysteresis curve for every Nitinol
alloy that defines the complete transformation cycle. The shape memory effect is repeatable
and can typically result in up to 8% strain recovery.


Martensite in Nitinol can be stress induced if stress is applied in the temperature range above
Af (austenite finish temperature). Less energy is needed to stress-induce and deform
martensite than to deform the austenite by conventional mechanisms. Up to 8%         strain can
be typically accommodated by this process. The material springs back to its original shape
when the stress is removed since austenite is the stable phase at this temperature under no-
load conditions, This extraordinary elasticity is also called “pseudoelasticity” or
transformational “superelasticity”.


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Fig 4. Temperature-induced phase transformation of an SMA without mechanical loading [15]




Figure 5. Shape Memory Effect of an SMA [15]




Figure 6. Pseudoelastic loading path.[15]         Figure 7. Pseudoelastic stress-strain diagram. [15]


It is feasible to vary the critical transition temperatures either by small variations of the Ti/Ni
composition or by substituting metallic cobalt for nickel. Lowering of Af is possible by
adding nickel. If nickel is added above 55.6 Wt%, a stable second phase (Ti-Ni3) forms and
the NiTi properties are lost. To avoid this problem, the cobalt substitution can be used. The




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properties of NiTi can also be greatly modified by mechanical working and through heat
treatment (time and temperature) (Buehler et al. 1967).


Excess nickel, in amounts up to approximately 1%, is the most common alloying addition.
Excess nickel strongly depresses the transformation temperature and increases the yield
strength of the austenite. Other frequently used elements are iron and chromium (to lower the
transformation temperature), and copper (to decrease the hysteresis and lower the
deformation stress of the martensite).




Figure 8 A) Martensitic transformation and hysteresis upon a change of temperature. As = austenite
start, Af = austenite finish, Ms = martensite start, Mf = martensite finish and Md = Highest
temperature to strain-induced martensite. Gray area = area of optimal superelasticity. B) Stress-strain
behaviour of different phases of NiTi at constant temperature.


While most metals deform by slip or dislocation, NiTi responds to stress by simply changing
the orientation of its crystal structure through the movement of twin boundaries.


A NiTi specimen will deform until it consists only of the correspondence variant which
produces maximum strain. However, deformation beyond this will result in classical plastic
deformation by slip, which is irrecoverable and therefore has no “memory effect”. If the
deformation is halted midway, the specimen will contain several different correspondence
variants. If such a specimen is heated above Af, a parent phase with an orientation identical
to that existing prior to the deformation is created from the correspondence variants in



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accordance with the lattice correspondences between the original parent phase and each
variant (Fig 8 C). The austenite crystal structure is a simple cubic structure, while martensite
has a more complex rhombic structure. This phenomenon causes the specimen to revert
completely to the shape it had before the deformation (Andreasen et al. 1987, Gil et al. 1998).




Figure 8. C). Transformation from the austenite to the martensite phase and shape memory effect. The
high-temperature austenitic structure undergoes twinning as the temperature is lowered. This twinned
structure is called martensite. The martensitic structure is easily deformed by outer stress into a
particular shape, and the crystal structure undergoes parallel registry. When heated, the deformed
martensite resumes its austenitic form, and the macroscopic shape memory phenomenon is seen.




Figure 8. D). Schematic presentation of lattice structure changes caused by outer stress in stainless
steel or superelastic NiTi alloy. In stainless steel, outer stress first causes reversible Hookian type
changes in the elastic area. In the plastic area, deformation takes place via a mechanism called slip.
This deformation is irreversible. In superelastic NiTi alloy, outer stress causes a twinning type of
accommodation which is recovered when outer stress is removed.




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2.3 Transformation in SMA
There are four major methods of characterizing the transformation in SMAs. The most direct
method is by differential scanning calorimeter, DSC. It measures the heat absorbed or given
off by a small sample as the material as it is heated and cooled through the transformation-
temperature range. The endotherm and exotherm peaks, as the samples absorb or given off
energy due to the transformation, are easily measured for the beginning, peak, and end of the
phases change in each direction. [24]


An alternative technique is differential thermal analysis, DTA. It is heat flow to the sample
and reference that remains the same rather than the temperature. When the sample and
reference are heated identically phase change and other thermal processes cause a difference
in temperature between the sample and reference. However, DSC is more widely used. [25]


Also method is to measure the resistivity of the samples as it is heated and cooled. The alloy
exhibit interesting changes and peaks in the resistivty over the transformation-temperature
range.


and direct method of characterizing an alloy mechanically is apply a constant stress to the
sample and cycle it through the transformation while measuring the strain that occurs during
the transformation in both directions, the value, then, obtained for the transformation points,
as Ms and Af. This test is directly indicative of the property one can expect in a mechanical
device used to perform some function using shape memory. However, this value is higher
than obtained form DSC tests because DSC test occurs at no applied stress, and the
transformation is not stress induced.


Finally, the stress-strain properties can be measured in standard tensile test at a number of
temperature across the transformation-temperature range, and from the change in properties
the approximate transformation-temperature values can be interpolated. [24]




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2.4 Titanium Nickel alloys
The nickel-titanium alloys were first developed in 1965 by the Naval Ordnance Laboratory
and commercialized under the trade name Nitinol. Nitinol is a family of intermetallic
material, which contain a nearly equal mixture of nickel (55 wt. %) and titanium. Other
elements can be added to adjust or “tune‟ the material properties. Nitinol exhibits unique
behavior, the material properties can be described as “Shape Memory” and “Superelasticity”.
The major physicals properties of basic NiTi system as: [16]
Properties                                    Property value
Melting temperature, ℃                        1300
Density, g/cm3                                6.45
Receptivity, µΩ •cm
    Austenite                                 ~100
    Martensite                                ~70
Thermal Conductivity, W/m • ℃
    Austenite                                 18
    Martensite                                8.5
Corrosion resistance                          Similar to 300 series stainless steel or Ti
                                              alloys
Young‟s modulus, GPa
    Austenite                                 ~83
    Martensite                                ~28 - 41
Yield strength, MPa
    Austenite                                 195 – 690
    Martensite                                70 – 140
Ultimate tensile strength MPa                 895
Transformation temperature ℃                  -200 to 110
Latent heat of transformation, kJ/kg • atom   167
Shape memory strain                           8.5% maximum




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2.4.1 Application
The range and applications for SMAs, particularly NiTi alloy, has been increasing in recent
years, with one major area of expansion being medicine. For orthopedic biomaterial
applications, strength (mechanical) and reactivity (chemical) are the two important properties.
NiTi alloy is extremely corrosion resistant, excellent in biocompatibility, can be fabricated
into very small sizes required, and has properties of elasticity and force delivery that allow
uses not possible any other material. However, these materials are not yet currently
appropriate for applications such as robotics or artificial muscles, due to energy inefficiency,
slow response times, and large hysteretic.


In engineering, several characteristics applications have an ordered structure, by minimizing
the number of crystallographic paths to the parent phase while making deformation by slip a
less favorable mechanism. Secondly, the martensite forms in a self-accommodating fashion
by the formation of twins. And finally, the martensite formed is thermoelastic,
crystallographically reversible, and consists of a highly mobile twin interface. Thermoelastic
martensite exhibits a small temperature hysteresis upon transformation [17]


Applications for NiTi SMA alloys can be grouped into four broad categories: actuation
devices, constrained recovery devices, superelastic devices, and martensitic devices. [18]




2.4.2 Porous TiNi shape memory alloy
Porous titanium-nickel (PTN) alloys represent new biomaterials for long-term implantation.


Their porosity properties might confer them capacity to trigger fluid capillarity, tissue
ingrowth, as well as good tissue-implant apposition and fixation. Porous titanium-nickel was
therefore extracted in a saline semi-physiological solution and materials were evaluated for
potential cytotoxicity and genotoxicity reactions. The cytocompatibility elution test was
performed in order to determine PTN toxic potential at the in vitro cellular level: no
reactivity was detected in cell layers exposed to PTN extracts or the negative controls.
Porous titanium-nickel can be considered completely cytocompatible and genocompatible,
and therefore represents a good candidate for long term implantation.




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2.4.3 Wear characteristics of TiNi shape Memory alloys
TiNi alloys have been found to possess excellent resistance to erosion, abrasive wear, and
especially to cavitations; and show superior performance compared with many standard
engineering materials and commercial wear resistance materials.


There was a strong correspondence between the wear resistance and the recoverable strain
resulting from the pseudoelasticity or pseudoplasticity. The large the recoverable strain, the
higher was the resistance to wear. [19]


Wear of a material are the factors of, contact and impact forces, mechanical properties of the
material, temperature, and surface-moving speed etc. The contact area between the bump and
the asperity may be significantly increased as the contact force increase. This happens
because TiNi alloy has a relatively low hardness due to the stress-induced martensitic
transformation or the rearrangement of martensitic variants. The increase in contact area
diminishes the contact stress and thus reduces the stress concentration at the rough surface.
The pseudo-elasticity can retard the propagation of micro-cracks. When an asperity passes
over a TiNi bump, little plastic deformation is because of the pseudo-elasticity. The high
flexibility due to the reversible martensitic transformation may allow the bump to withstand
impact from many moving asperities before it fails when the accumulated plastic deformation
reaches its critical value at fracture.
The pseudo-elasticity also makes TiNi alloy very resistant to wear under impact force, it is
because the impact energy can be readily absorbed by TiNi alloy due to its rubber-like
behavior and the resulting impact damage can thus be reduced significantly. Wear resistance
of TiNi alloy on its composition an optimal composition range from Ti-55% wt% Ni to Ti-
56.5wt% Ni. This optimal composition range is actually the range that allows TiNi alloys to
behave pseudo-elastically.


TiNi alloy has an order bcc structure (β phase) at ambient temperature. As the temperature
decreases, the β phase transforms to a martensitic phase (M), a monoclinic crystal structure,
vanishes as the temperature increase back to it initial level. When a tensile stress was applied,
the β phase transformed to a pre-martensitic phase (R phase – a rhombohedral structure) and
then to a martensitic phase (M), associated with a deformation.




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During a sliding wear process, the β phase transforms to the martensitic phase under the
stress caused by asperity contact and the transformation heat is released. After an asperity
passes over, the martensitic phase may absorb the friction heat and favors reverse phase
transformation M→R→β. The recovered β phase can take part again during the stress-
induced β→R→M phase transformation, and this enhances the pseudo-elasticity of the
material and thus its resistance to wear.


Alloying elements that improve the wear resistance of TiNi alloy require identification. The
wear rate of TiNi can be decreased effectively by alloying with 3% iron, which helps to form
oxide layer on surface during wear process.


Also, microstructure strongly influences the performance of TiNi alloy during wear process.
the coherent and fine Ti11Ni14 precipitates improve the wear resistance of TiNi alloys which
enhancement of pseudo-elasticity as well as the increase in strength of the material. [19]

2.4.4 Processing of TiNi SMA
Currently, three methods are commonly used for producing porous TiNi shape memory
alloys from elemental powders. These include conventional sintering. Self-propagating High
temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press
(HIP). Conventional sintering requires long heating times and sample are limited in shape
and pore size. SHS is initiated by a thermal explosion ignited at one end of the specimen,
which then propagated through the specimen in a self-sustaining manner. In HIP, elemental
Ni and Ti powders at elevated temperature and pressure using a HIP. Small and large pore
specimens containing average pore sizes ranging from 20μm up to 1mm have been produced
by slightly varying the HIPing sintering temperatures and times. (by Dimitris C Lagoudas).
[22]




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3. EXPERIMENTAL DETAIL
3.1 Sample Preparation – grinding                              Date: 22 May 06
Location: G2608, Lab for Mechanical & Micro Structural Characterization, Dept of Physics
           & Materials Science.
Background: Samples, difference in porosities, are used to test the wearing properties of
NiTi SMA. They are one dense sample and two samples with difference porosities. The
dense samples are about 12mm in diameter, samples A & B are about in 5.5mm in diameter.
They are dark grey in color with metallic outlook. All the samples are provided by Mr. S.L.
Wu, the project tutor.
                     The dense sample, 12 pieces of them, had been sliced and ready for test;
                     the two porous samples, samples A & B, in fact are
                     two short metal bars, were send to mechanical
                     workshop for slicing.
With the discussion with Mr. Wu, it is agreed that all three samples
should be tested at the same period of time & under same condition, which, not only, unified
the result but minimize the uncertainties & variables in lab as well.


Grinding: The purpose of surface grinding are to flat, remove of foreign material, impurities
and loosing matter of the samples; grinding can uniform the grade & quality of
                         surface for test among all samples. Therefore the grinding
                         of samples should go before treatment & tests. When the
                         entire samples are finished slicing, the grinding can be
                         carrying out.


Procedure: During grinding, the specimens are worked through series of sand paper, silicon
                   carbide grinding paper, starting with grade 240, 400, 600 and finishing on
                   grade 800, lubricated with water. The specimens were bonded on steel
                   disc because of efficiency, which can carry out grinding of ten specimens
at a time & better holding with more safe as well. When completed grinding,
                     the specimens are separated from the steel disc, washed
                     thoroughly and let dry.
                     3.2 study of samples surfaces by SEM                        Date: 25 May



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06
Location: G2609, SEM/Image Analyser Laboratory, Dept of Physics & Materials Science.
Background & Procedure:
After grinding the samples on 22 May 06, the samples are ready for SEM,
electronic microscope, to study their surface. One specimen from dense
sample & 2 specimens from both of Sample A & B are used for study.
The magnifications of 35, 50, 100, 200, 500 (& 1000) times were used on
this study.


From SEM, it can have a closer look on the sample surface; from these to have better
                        understanding & predication on their wearing behaviors. According
                        to the images, the porosity of the samples can be estimated by the
                        proportion of pours area to dense area and the pours‟ shape, diameter,
                        position, depth & distribution can then be studied as well. From
                        these „visual descriptions‟, their physical properties such as optically,
hardness / wearing resistance can then be expected
With the help of the technicians, samples were placed inside of SEM & the images were sent
to computer to store.




3.3 Heat Treatment of the samples                      Date: 30 May 06


                                              20
Location: Furnace Room I, Dept of Physics & Materials Science
Background: Metal consist of a microstructure of small crystals called grains or crystallites,
the nature of the grains size & composition. They are determining the overall mechanical
behaviours of the metal / alloy. Heat treatment is the temperature versus time history
necessary to generate a difference (desired) microstructures. Heat treatment provides an
efficient way to manipulate the properties of the metal by controlling the rate of diffusion and
the rate of cooling within the microstructure such as precipitation hardened. [20]


In this test, one third of number of samples, dense, porous A & B, are heat treated to
temperatures of 200℃, and one third for 400℃ and the rest for 500℃. The samples,
difference in porosity, will then be carried out DSC & wear test to evaluate the influence on,
firstly, the formation of microstructures which affecting their properties such as wearing
behaviours; and temperature of the phase transformation after heat-treated in difference
temperatures.


Procedure: When the furnace is heat up to 200℃; samples, 4 dense, 4 porous A & 4 porous
                      B, were placed inside to do the heat treatment for 1 hour with argon gas
                      passing through. The samples then air cool to room temperature.
                      Repeat the same procedure of other sample (3 dense, 4 porous A & B)
                      for the temperature of 400℃ & 500℃.




Out look of Furnace
                                                                 Close look & inside of furnace




3.4 DSC – Differential Scanning Calorimetry                 Date: 6 & 26 Jun 06



                                              21
Location: P2828 Thermal Analysis Lab II, Department of Physics & Materials Science
Background: DSC is a thermo-analytical technique of characterizing the transformation in
SMAs, the difference in the amount of heat required to increase the temperature of a sample
& reference are measured as a function of temperature. The basic principle is when the
sample undergoes a physical transformation such as phase transitions; more (or less) heat
will need to flow to it than the reference to maintain both at the same temperature. This
technique measures the heat absorbed or given off by a small sample of the material as it is
heated and cooled through the transformation-temperature range. The endotherm & exotherm
peaks, as the sample absorbs or gives off energy due to the transformation, are easily
measured for the beginning, peak, and end of the phase change in each direction.
DSC consists of two sealed pans – sample & reference (empty) pans. These two pans are
heated, or cooled, uniformly while heat flow difference between the two is monitored by
changing the temperature at a control (constant) rate – temperature scanning. During the
experiment, the instrument detects difference in the heat flow between the sample &
reference, and the information is sent to computer, the results then plot as differential in heat
flow between the reference and sample cell to temperature.


Procedure:
                     Cut the samples size of 10 to 20mg, to fit & to maximize surface contact
                     with the DSC sample pan. Clean the sample of all foreign materials such
                     as cutting fluid. Place sample in the pan &
                     empty pans on test & reference pedestal,
                     respectively. Flow the pans with pedestal                               gas,
                         nitrogen in this case, as 50ml/min.
                         Subsequently, run the heating &
                         cooling program as 100℃ to -50℃ then back to 100℃ with rate of
                         5℃/min. Record the data and plot the DSC curve as Temperature (x-
                         axis) against Heat flow (y-axis). Modified by adding baselines for
the cooling and heating portions of the curve; then draw the tangents to the cooling and
heating spikes through the inflection point. Obtain Ms, Mf, Mp, As, Ap & Af as the graphical
intersection of the baseline with the extension of the line of maximum inclination of the




                                               22
appropriate peak of the curves. Ap is the peak minimum of the endothermic cures, and Mp is
the peak maximum of the exothermic curves. [21]




3.5 Wear / Frictional Test & Surface Study of After test                     Date: 21 & 22 Jun
06
Location: Y1501 – Mechanics and Tribology Laboratory, MEEM, City University of HK.
Background: Surface is not completely flat at the microscopic level. At high magnification,
even the best polished surface will show ridges and valleys, asperities, and depressions; for
this reason, frictional force as the result, resists one surface move over another surface.
TEER-POD-2 Pin-On-Disc Wear Tester is a computer controlled device for creating wear
tracks to provide a quantitative measure of the wear properties. It uses a low speed-high
torque drive motor to rotate a flat sample under a loaded wear pin. The wear pin, 5mm dia.
Cr-steel ball, creates a circular wear track on the sample by off setting the pin relative to the
samples‟s centre of rotation. The sliding motion of the sample under the wear provides a
frictional force which is a proportional to the load applied and is detected by the load cell and
recorded by the computer. Afterward, the frictional coefficient, μ, can be interpreted. It is the
ratio between the friction force, F, & the load, N; i.e. μ = F / N


                     Procedure: Stick the sample on a steel plate, and then tighted the plate
                     on the sample table. Applied the selected load, 1N, and then lower the
                     beam until ball touches the sample. Power on the tester & activate the
                     programs, input rotational speed as 200 rmp, track diameter as 5mm,
maximum friction force (N) as 20N and operator name, temperature & humidity (for
reference only). Start the tester, recording the data as time vs. frictional force. Repeat loads
of 2N and 3N on new sample. From the data, the curve of Time
(x-axis) against Frictional Force Coefficient (y-axis) can then be
computed and plotted. SEM / Optical-microscopes, of x50, x100
& x200 (x500), study on the samples surface will be carried out
after wear test.




                                                23
4. Result & Discussion
4.1 Methods of manufacture & Porosity
The capsule free hot isostatic pressing, CF-HIP, technique was employed in this fabrication
of the porous equiatomic NiTi alloys. The equiatomic titanium and nickel powders of particle
size of 75µm & purity >99.5% were weighted and put into stainless steel can together with
stainless steel balls. Before mixing, the argon gas was intake for 2 hrs to prevent the
oxidation, the can then be sealed.


The powders were mixed for 12hrs by ball mill with a speed of 100rmp. The mixture then
was pressed into green compacts in a steel die using a hydrostatic machine under pressure.
The green compacts were put into capsule – free stainless canister. After that, the canisters
were placed into the HIP chamber, vacuum purged, and filled with 99.995% pure argon. It
was then pressurized via HIP to 100MPa, and heated for 3hrs for enough diffusion of nickel
and titanium. After „HIPing‟, the NiTi mixture was annealed in tube furnace following by
quenching with ice water. The produced NiTi alloy can be cut into specimen for tests.


The general porosity of porous samples was calculated by the equation:


Porosity ε = (1-m/ρ/V) x 100%, where m and V is the mass and volume of porous samples,
respectively. The ρ as 6.45g/cm3 is the theoretical density for bulk equiatomic NiTi.
It is given that the porosity of sample A is 21% and sample B is 36%.
(Methods of manufacture and samples porosity are provided by Mr. SL Wu, project supervisor).




                                                 24
4.2 Assumptions
In this study, due to the time concern, human resource, samples & equipments availability,
requirement & ability for the operator‟s skill, knowledge & experience; assumptions before
the result analysis should be made.


It is assumed that the production of the samples including the source of the raw material,
methods of manufacture, control in time & temperature, admixture and all factors &
condition which effecting the product, should be the same.


It is supposed that the physical & chemical composition of the dense sample, sample A & B
are the same including the Ni to Ti proportion, grain size & phase configuration. In addition,
it is think that all specimens in dense sample, all in samples A & all in sample B are exactly
the same in term of chemical and physical composition.


It is assumed that the dense samples have no any pores on the surface; the pores on the
sample A & B are to be the same in depth, shape and are uniform distributed. And the
determination of the porosity is using above calculation with round up in data, such as A =
21% & B = 36%.


It is believed that the sample sizes, sample A & B are in same size, about 5.5mm in diameter,
but small than dense sample about 12mm, have no influence to the results.


It is believed that the test parameter counting the temperature of the heat treatment,
temperature and setting in DSC, loads applied in wear test, etc, are suitable for this study.


It is accept as trued that the testing conditions are the same for all samples such as the degree
of grinding; heating and cooling rates in heat treatments, accuracy of the DSC equipment &
wear tester including the pin ball condition, etc.


And in DSC test, all samples are assumed to be unstressed or no residual stress.




                                               25
4.3 Test Result
     In this study, experiments including: samples grinding, samples surface study by SEM
and heat-treatments – sample preparation stages, DSC & wear teat – phase transformation &
wear properties and surface study by Optical microscopes, OM, after test were carried out.


Grinding of the sample




In visual inspection of samples after grinding, dense specimens are performing mirror likes
surface, specimen A results as reflecting surface as well but with few black spots, specimen
B are found many black spot/porous on the surface. By only the outlook of the specimens, it
can tell that the difference in porosity of the samples. The dense sample is the lowest in
porosity supposed to be pore free, sample A is in between in term of degree of porosity, and
sample B is the highest in porosity.




SEM Study
Dense sample images:
x35, x50, x100, x200, x500 & x1000




Sample A images:
x35, x50, x100, x200, x500 & x1000




                                             26
Sample B images:
x35, x50, x100, x200 & x500




From these images, it is found that the dense sample have nearly no opening, sample A have
some and sample B have many pours on surface. Therefore, it is not surprise that, as in visual
inspection, dense sample perform a minor like surface, sample A have good reflective
surface & sample B resulting poor reflective surface.




Heat treatment




In the heat treatment of the samples, argon gas is used which is, firstly, to uniform the
temperature inside the furnace and to reduce the oxidized of the samples. Nitrogen gas can be
used for some sample but not in this case because nitrogen may react with Ti at high
temperature. After the treatment, samples with 200℃ treated have no significant change,
visually; in the contrast, samples change in colour noticeable under treatment of 400℃ &
500℃. From the result, it is believed that the high-temperature-treated samples will
difference in properties comparing with lower-temp-treated sample; the difference in DSC
and wearing test result, therefore be expected.




                                              27
                          DSC Test
                          The result of DSC test:


                          DSC curve – Samples heat at 200 ℃ (figure in left), 400℃ (in middle) & 500℃ (in right)
                          (Blue line – Dense Sample, green – Sample A & red – Sample B)
                                                        DSC Curve - Samples heat-treated @ 200℃                                                                                      DSC Curve - Sample heat-treated @ 400℃                                                                                                        DSC Curve - Samples Heat-treated @ 500℃


                                                                                                                                                           1.8                                                                                                                                     2.0
                 1.8                                                                                                                                                                                                                                                                               1.8




                                                                                                                                                                                                                                                                                Heat Flow (mW)
                 1.6                                                                                                                                       1.6                                                                                                                                     1.6
                                                                                                                                                                                                                                                                                                   1.4
                                                                                                                                   Heat Flow (mW)
Heat Flow (mW)




                 1.4                                                                                                                                       1.4
                                                                                                                                                                                                                                                                                                   1.2
                 1.2                                                                                                                                       1.2                                                                                                                                     1.0
                                                                                                                                                                                                                                                                                                   0.8
                 1.0                                                                                                                                       1.0                                                                                                                                     0.6
                                                                                                                                                                                                                                                                                                   0.4
                 0.8                                                                                                                                       0.8                                                                                                                                     0.2
                 0.6                                                                                                                                       0.6                                                                                                                                     0.0
                                                                                                                                                                                                                                                                                                  -0.2
                 0.4                                                                                                                                       0.4                                                                                                                                    -0.4
                                                                                                                                                                                                                                                                                                  -0.6
                 0.2                                                                                                    Dense                              0.2                                                                                                                                    -0.8                                                                                 Dense

                 0.0                                                                                                                                                                                                                                                                              -1.0
                                                                                                                        Sample A                           0.0                                                                                                                                    -1.2
                                                                                                                                                                                                                                                                                                                                                                                       Sample A

                 -0.2                                                                                                   Sample B                        -0.2                                                                                                    Dense                             -1.4                                                                                 Sample B
                                                                                                                                                                                                                                                                                                  -1.6
                 -0.4                                                                                                                                   -0.4                                                                                                    Sample A                          -1.8
                 -0.6                                                                                                                                                                                                                                                                             -2.0
                                                                                                                                                        -0.6                                                                                                    Sample B

                        -50 -40 -30 -20 -10         0       10    20    30    40   50   60   70    80   90 100                                                                                                                                                                                           -50 -40 -30 -20 -10   0    10 20     30 40     50 60     70 80     90 100
                                                                                                                                                                     -50 -40 -30 -20 -10     0    10    20    30    40    50    60    70    80        90 100
                                                                                                                                                                                                                                                                                                                                                                  Temperature ℃
                                                                                         Temperature ℃                                                                                                                         Temperature ℃




                          DSC curve – Dense sample (figure in left), Sample A (in middle) & Sample B (in right)
                          (Blue line - 200℃, green - 400℃ & red - 500℃)
                                              DSC Curve - Dense Sample heat-treated 200℃, 400℃ &500℃                                                                                 DSC Curve - Sample A Heat-treated @ 200℃, 400℃& 500℃
                                                                                                                                                                                                                                                                                                               DSC Curve - Sample B Heat-treated @ 200℃, 400℃ & 500℃
Heat Flow (mW)




                                                                                                                                                                     1.8
                  1.8
                                                                                                                                                    Heat Flow (mW)




                  1.6                                                                                                                                                1.6
                  1.4
                                                                                                                                                                     1.4
                                                                                                                                                                                                                                                                            Heat Flow (mW)




                  1.2                                                                                                                                                                                                                                                                            1.4
                  1.0                                                                                                                                                1.2
                  0.8                                                                                                                                                                                                                                                                            1.2
                  0.6                                                                                                                                                1.0
                                                                                                                                                                                                                                                                                                 1.0
                  0.4                                                                                                                                                0.8
                  0.2                                                                                                                                                                                                                                                                            0.8
                  0.0                                                                                                                                                0.6
                 -0.2                                                                                                                                                                                                                                                                            0.6
                                                                                                                                                                     0.4
                 -0.4                                                                                                                                                                                                                                                                            0.4
                 -0.6                                                                                                                                                0.2
                 -0.8                                                                                                                                                                                                                                               200 ℃                        0.2
                 -1.0                                                                                                                                                0.0
                                                                                                                                                                                                                                                                    400 ℃                        0.0                                                                                   200 ℃
                 -1.2                                                                                                                                                -0.2
                 -1.4                                                                                                                                                                                                                                               500 ℃
                                                                                                                                                                                                                                                                                                 -0.2
                 -1.6                                                                                                     200 ℃                                      -0.4                                                                                                                                                                                                              400 ℃
                                                                                                                                                                                                                                                                                                 -0.4
                 -1.8                                                                                                                                                -0.6                                                                                                                                                                                                              500 ℃
                 -2.0                                                                                                     400 ℃                                                                                                                                                                  -0.6
                                                                                                                                                                     -0.8
                                                                                                                                                                                                                                                                                                 -0.8
                        -50   -40 -30   -20   -10       0        10    20    30    40   50   60    70   80   90   100     500 ℃
                                                                                                                                                                            -50 -40 -30 -20 -10   0    10    20    30    40    50    60    70    80    90 100                                           -50 -40 -30 -20 -10    0    10   20   30   40   50   60   70   80   90   100
                                                                                                                                                                                                                                           Temperature ℃                                                                                                          Temperature ℃
                                                                                                  Temperature ℃




                          From the DSC curve, it is found that under same treated temperature, Dense Sample, Sample
                          A & B have, give the impression that relatively, similar profile of Heat Flow to Temperature,
                          i.e. they have approximate points of Mf, Mp, Ms, As, Ap & As. That is, no mater the porosity
                          is, heat treatment with same temperature will result same transformation temperatures can be
                          assumed.


                          However; for the samples with same porosity, the transformation temperatures may various
                          with the treated temperature.


                          (Zoom-in Figure refer to APPENDIX)




                                                                                                                                                                                                                          28
     The summary of the Phase Transformation Temperature of the samples:


                                 Phase Transformation Temperature , ℃
Heat-Treated
Temperature          Dense Sample                  Sample A                    Sample B

                                             Mf=-18, Mp=-8 &
   200 ℃                  Not Clear          Ms=5;                             Not Clear
                                             As=6, Ap=26 & Af=43
                                             Mf=-3, Mp=8 & Ms=15;      Mf=-17, Mp=-15 &
   400 ℃                  Not Clear          As=22, Ap=27 & Af=44      Ms=-10;
                                                                       As=20, Ap=39 & Af=55
                Mf=-17, Mp=-12 &             Mf=-15, Mp=-4 &           Mf=-9, Mp=-5 & Ms=3;
                Ms=-7;                       Ms=3;                     As=29.0, Ap=37 & Af=41
   500 ℃
                As= 29, Ap=36 &              As=15, Ap=23 &
                Af=39                        Af=28



     However, the DSC test is not satisfied. Since some cures are so flat, the intersection of the
     base line & the inclination of the appropriate peaks are not clear. Nor the scales of curves are
     in huge difference. Further investigate should be needed.




     Wear / Friction Test
     In the frictional test, time of 600 seconds was used at the beginning; yet, after tested for few
     samples, time of 750 seconds was used. It is because the longer of period of time, the more
     steady result will be expected.


     In the test of sample A heat-treated at 400℃, time of 600 second then 750 second was carried
     out. It is because, in 600s test, no steady value results but increase frictional coefficient;
     hence the same area was tested for further 750s. Due to the worn surface being tested twice,
     in this case, the result is in doubt.


     SEM was used to study the wearing tracks of the samples at first, but, tracks were too light to
     realize under SEM, especially on porous sample, sample B, optical microscopes was used
     instead therefore.




                                                   29
The summary of Frictional Coefficients of difference samples with difference heat-treated as
follow: (refer to LIST OF FIGURE)

                                FRICTIONAL COEFFICIENTS, µ
     Heat Treated
                           Dense
                                               Sample A                    Sample B
                          Sample
        Porosity             0%                     21%                    36%

     Applied Force       1, 2 & 3 N      1N         2N     3N       1N     2N      3N

         200 ℃                          1.55    1.55       1.45     1.3    1.3     1.2

         400 ℃            1.3 ~ 1.6      0.4    0.65      1.0/1.3   0.7    1.15    0.6

         500 ℃                           0.9        0.7    0.65     0.65   0.6     0.5


Theoretically, the applied force, no matter of 1N, 2N & 3 N, have no effect on the frictional
coefficient performance as long as the load / force are not too large to break the sample. It is
because µ = F/N; fictional coefficient is the ratio of the frictional force & applied load / force
only. It is, more or less, true in dense samples.


In the frictional test result of dense sample, it is found that nearly constant value of frictional
coefficient of 1.3 ~ 1.6, no matter the applied load & heat treated temperature. Therefore, the
result for dense sample can conclude as the frictional coefficient is independent to applied
load & heat treatment temperature in dense sample.


It is also seen that the tendency of the result, firstly, the higher in porosity, dense sample to
sample A to sample B, the lesser in frictional coefficient would be. And secondly, samples,
especially porous samples, the higher temperature in treatment, lesser in frictional coefficient
will be as well. However, the wear test results in sample A & B are appear to be fluctuated &
unpredictable; the relation between loads applied and heat-treated temperature for the
samples need further investigation.


Before the test, it was expected that the dense sample would have the best wear resistance
among all, and sample A is the lesser in and sample B should be the worst. It was sense that
the dense samples have no pores on surface and should be the hardest which could provide



                                                30
the best wear resistance and then sample A and sample B. It was thought that the dense‟s are
fully pack with no space / void for surface deformation and therefore no material would be
worn away. It was believed as the case of hard surface to hard surface in contact mentioned
in LITERATURE REVIEW section. Under this “accept as true”, sample A, due to its pores /
voids on the surface, would be deformed under loading and then surface material was worn
away easily; and in sample B, as well.


From the OM study, it is found that, for all treated temperatures 200℃, 400℃ & 500℃ and
applied loads, heavy tracks on dense sample, but lighter tracks on sample A and the lightest
tracks on sample B (refer to APPENDIX 1). The case in dense samples as the result of hard
metal in contact with soft metal – deformation & pull-off of surface material; in contradict,
the case of sample A & B as the result of hard metal contact with hard metal or two hard
metal separated by thin-film layer – deformation of the surface maybe but without material
being pull-off. It is agreed with the above wear-result tendency that higher degree in porosity,
less in frictional force.


In this study, the frictional test instead of wear test was carried out. Wear test by the mass
lost method, may be the most direct; however, for the matter of time, availability & occupy
of equipments, frictional test & surface study after test are alternative approach. It is believed
that the lower in fictional coefficient will be lower in frictional force. And lower in frictional
force indicate that the less in wear, such as stick-up, deformation and pull off of surface
materials. Therefore, it can compare the wear behavior using frictional coefficient. However,
to test the samples by using direct wear test, as weight lost method, is strongly recommended
in further study.


In this study, the samples perform well in lower frictional coefficients, having µ value of
about 0.5 – 1.5 comparing ≥5 for clean metal surfaces in a vacuum. It is an indication of
lower in wear, that mean good in wear resistance. The good in wear-resistance can be
explained by the pseudo-elasticity & SMA effect as rubber-like behaviour under applied
loading and β phase transformation by friction heat during friction test, mention in
LITERATURE REVIEW section.




                                               31
Also, from the test results, the higher degree in porosity appear enhancing & reinforcing
these behaviour which grades lesser in friction coefficient and better in wear resistance. The
explanation for this including how & how much of the porosity to these performances should
be in further investigation.


Due to the time concern, human resource, samples & equipments availability, requirement &
ability for the operator‟s skill, knowledge & experience; assumptions such as the uniformity
of the samples production, composition, quality; test parameter and condition are made on
the results analysis. The minimization and study of these uncertainties should be in strongly
recommended when carry out further study.


Cause of the dissatisfaction of DSC curve with uncertain peak, repeat of the DSC for original
and new samples is admirable. It is not only good to the confirmation of the original samples
but with better understanding from additional information.


In this project, only one sample for one test condition is applied, accordingly the result may
not be well representable for the real case. Consequently, the amount of samples & tests
should be increased with difference in test parameter, such as temperature & time in heat
treatment; applied load, speed of rotation, testing temperature & the temperature of contact
should be evaluated in wear test to perfect the study.


In further study, it is suggested that to carry a parallel test of the NiTi with common /
standard material such as steel to standardized the result.


Hardness test, which is not carried out in this study, is strongly recommendation in additional
study. It is because wear is a resulting of the accumulation of plastic deformation and
hardness is the measure of a material‟s ability to combat plastic deformation. Hardness is the
characteristic of a solid material expressing its resistance to permanent deformation &
commonly refers to a material‟s ability to penetrate softer materials. Sometime the stress
conditions experienced by a wear surface are more similar to those imposed during a
hardness test than in other cases. Therefore, hardness study of the samples is an important
indicator for study of porosity of NiTi.




                                               32
It may not have “perfect-make” standard of procedure for the comparison of the porous NiTi
ASM study; it is still a good consideration to associate with international standard i.e. ASTM,
BS for (or partly) the tests.

.




                                             33
5. Conclusion & Recommendation
In this project, the study of comparison of the wearing of porous and dense NiTi Shape
Memory alloy was presented.


Experiments including samples grinding, surface study by SEM before test, heat-treatment,
DSC, wear test & surface study by optical microscopes after test were carried out.


In this study, the samples perform well in wear-resistance. It can be explained by the pseudo-
elasticity & SMA effect and β phase transformation of the NiTi SMA.


From the test results, the higher degree in porosity appear tendency lesser in friction
coefficient and better in wear resistance. The explanation for this should be in further
investigation.


Due to the time concern, human resource, samples & equipments availability, requirement &
ability for the operator‟s skill, knowledge & experience; assumptions such as the uniformity
of the samples production, composition, quality; test parameter and condition are made on
the results analysis. The minimization and study of these uncertainties should be in strongly
recommended when carry out further study.


Cause of the dissatisfaction of DSC curve with uncertain peak, repeat of the DSC for original
and new sample is admirable.


To perfect the study, the amount of samples should be increased, the hardness study & direct
wear test, as weight lost method, is strongly recommended in further study


In further study, it is suggested that to carry a parallel test of the NiTi with other common /
standard material such as steel to standardized the result.




                                               34
6. REFERENCE
[1] Microstructure and martensitic transformation behaviour of porous NiTi shape memory alloy
     prepared by not isostatic pressing processing, B. Yuan, , M. Zhu, College of Mechanical
     Engineering, South China University of Technology, Grangzhou; C Y Chung Department & of
     Physics Material Science, City University of Hong Kong
[2] Wear Mechanisms, Wear Testing by Peter J. Blau, Metals and Ceramics Division, Oak Ridge
     National Laboratory, ASM Handbooks Online
[3] Measuring Wear and Reporting Wear Test Results, Wear Test, Peter J. Blau, Metals and Ceramics
     Division, Oak Ridge National Laboratory
[4] Wear Mechanisms, Wear Behavior, Design for Wear Resistance, Raymond G. Bayer, Triblogy
     Consultant.
[5] General Classification of Wear, Abrasive Wear Failures, Jeffrey A. Hawk and Rick D. Wilson,
     Albany Research Center; Daniel R. Danks, Danks Tribological Services; Matthew T. Kiser,
     Caterpillar Inc.
[6] Jeffrey A. Hawk and Rick D. Wilson, Albany Research Center; Daniel R. Danks, Danks
     Tribological Services; Matthew T. Kiser, Caterpillar Inc.
[7] Basic Theory of Solid Friction, Jorn Larsen-Basse, National Science Foundation, ASM
     Handbooks Online
[8] Appendix: Static and Kinetic Frictioin Coefficient for Selected Material, Peter J. Blau, Metals and
     Ceramics Division, Oak Ridge National Laboratory, ASM Handbooks Online.
[9] Appendix: Static and Kinetic Friction Coefficients for Selected Materials by Peter J. Blau, Metals
     and Ceramics Division, Oak Ridge National Laboratory, ASM Handbooks Online.
[10] Introduction to Fiction, Jorn Larsen-Basses, National Science Foundation, ASM Handbooks
     Online.
[11] Presentation of Friction and Wear Data by Horst Czichos, BAM (Germany), ASM Online
     Handbook
[12] Anomalous relationship between hardness and wear properties of superelastic nickel-titanium
     alloy, Linmao Qian and Xudong Xiao, Department of Physical and Institute of Nano Science
     and Technology, The Hong Kong University of Science and Technology; Qingping Sun and
     Tongxi Yu, department of Mechanical Engineering, The Hong Kong University of Science and
     Technology
[13] Hardness from Wikipedia, the free encyclopedia
[14] Correlation of Microindentation Hardness Numbers with Wear, Microindentation Hardness
     Testing by Peter J. Blau, Oak Ridge National Laboratory, ASM on line Handbook.




                                                  35
[15]     Definition     of    Shape      Memory           Alloy   <Texas   A&M       Smart       Lab>,
       http//smart.tamu.edu/overview/smaintro/simple/defintion.html, with modification.
[16] Commercial SME Alloys, Shape Memory Alloys, Darel E. Hodgson, Shape Memory
       Applications, Inc; Ming H. Wu, Memry Corporation; and Robert J. Biermann, Harrrison Alloys,
       Inc. ASM Handbooks Online
[17] Rick Noecker, Lehigh University
[18] Appliction, Shape Memory Alloys, Darel E. Hodgson, Shape Memory Applications, Inc; Ming H.
       Wu, Memry Corporation; and Robert J. Biermann, Harrrison Alloys, Inc. ASM Handbooks
       Online
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       Material Engineering University of Alberta.
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Science Foundation, ASM Handbooks Online




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APPENDIX
1. Surface Study by Optical microscopes of samples after frictional test.
    (Remark: 200D1Nx50 - 200℃ dense sample with applied load of 1N magnifications of 50 times)
2. DSC Curve – Dense Sample Heat-treated at 200℃, 400℃ & 500℃.
3. DSC Curve – Sample A Heat-treated at 200℃, 400℃ & 500℃.
4. DSC Curve – Sample B Heat-treated at 200℃, 400℃ & 500℃.
5. DSC Curve – Dense sample, Sample A & B Heat-treated at 200℃
6. DSC Curve – Dense sample, Sample A & B Heat-treated at 400℃
7. DSC Curve – Dense sample, Sample A & B Heat-treated at 500℃
8. DSC Curve – Dense sample Heat-treated at 200℃
9. DSC Curve – Sample A Heat-treated at 200℃
10. DSC Curve – Sample B Heat-treated at 200℃
11. DSC Curve – Dense sample Heat-treated at 400℃
12. DSC Curve – Sample A Heat-treated at 400℃
13. DSC Curve – Sample B Heat-treated at 400℃
14. DSC Curve – Dense sample Heat-treated at 500℃
15. DSC Curve – Sample A Heat-treated at 500℃
16. DSC Curve – Sample B Heat-treated at 500℃




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