RECENT VISTAS IN ENGINEERING SURFACE

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					       INTERNATIONAL Engineering and OF MECHANICAL
 International Journal of Mechanical JOURNAL Technology (IJMET), ISSN 0976 –
 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 2, May-August (2012), © IAEME
         ENGINEERING AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 3, Issue 2, May-August (2012), pp. 92-107
                                                                     IJMET
© IAEME: www.iaeme.com/ijmet.html
Journal Impact Factor (2011): 1.2083 (Calculated by GISI)       ©IAEME
www.jifactor.com




            RECENT VISTAS IN ENGINEERING SURFACE
                 MODIFICATION TECHNIQUES
           Mohammed Yunus1, Dr. J. Fazlur Rahman2 and Dr.A. Ramesh3

            1. Research scholar, Anna University of Technology Coimbatore,
         Professor, Department of Mechanical Engineering H.K.B.K.C.E.,Bangalore,
                          India, yunus.mohammed@rediffmail.com

               2. Supervisor, Anna University of Technology Coimbatore,
                 Professor Emeritus, Department of Mechanical Engineering
                              H.K.B.K.C.E., Bangalore, India.
                              jfazlu2003@yahoo.co.in

               3. Supervisor, Anna University of Technology Coimbatore,
                 Professor and Head, Department of Mechanical Engineering,
        Srikrishna College of Engineering and Technology, Coimbatore, India.
                                ramesha@skcet.ac.in


 ABSTRACT

 The Thermal sprayed coatings are commonly used on many advanced industrial
 applications for their functional requirements like high strength at elevated
 temperatures, resistance to chemical degradation, wear resistance and environmental
 corrosion protection in Engineering components. The product design (design of a
 surface) is concerned with design of enveloping surface which is achieved with some
 suitable surface modifications. This paper highlights the several surface modification
 techniques used for producing high quality coatings which involve the requirements
 of one or more of mechanical and tribological properties.
 Keywords: Engineering Surface; Surface Modification; Mechanical and Tribological
 Properties; Thermal treatments; Thermo-chemical treatment; Plating and coating;
 Implantation.
 .




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1. INTRODUCTION
   The selection of technology to engineer the surface is an integral part of an
engineering component design. The first step in surface modification technique to
determine the surface and substrate engineering requirements which involves one or
more of the properties like wear resistance, corrosion and erosion resistance and
thermal resistance, fatigue, creep strength, pitting resistance etc. [1, 2 & 3].
   The various surface treatments generally used in engineering practice and
presented as under.

   2. SURFACE MODIFICATION METHODS/ TECHNIQUES
   A simplified classification of various groupings of non-mechanical surface
treatments could be reduced as [9, 10, 12, 13 & 14]
1. Thermal treatments 2.Thermo-chemical treatment 3.Plating and coating 4.
Implantation.
The figure illustrates different types of surface treatments and typical thickness of
engineered surface materials produced by them. The effectiveness depends on
particular surface and modification technique.




                Fig.1. Typical thickness of engineered surface layers
1.PVD process 2.CVD process 3.Electoless Nickel 4.Composite 5.Thermal spraying
6.Surface welding 7.Ion Implantation 8.Anodising 9.Boronizing 10.Nitriding
11.Carbonitriding 12.Carborising 13.Nitrocarburising 14.Surface alloying 15.
Thermal hardening.


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    There are two categories of vapor deposition processes: physical vapor deposition
(PVD) and chemical vapor deposition (CVD). In PVD processes, [4 ] the work piece
is subjected to plasma bombardment. In CVD processes, thermal [8] energy heats the
gases in the coating chamber and drives the deposition reaction.


2. 1. Physical Vapour Deposition (PVD)
        In this process, the work piece or substrate is subjected to high [ ]films by the
condensation of a vaporized form of the material onto substrate surfaces. This process
contains the three major techniques; evaporation, sputtering and ion plating. It
produces a dense, hard coating. The primary PVD methods are.ion plating, ion
implantation, sputtering and laser surface alloying.




Fig. 2. PVD process using Plasma           Fig. 3. PVD process using arc sputtering
evaporation




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    PVD is used in the manufacture of semiconductor wafers, aluminized PET film
for snack bags and balloons, cutting tools for metalworking and generally used for
extreme thin films like atomic layers and mostly for small substrates [8].

2. 2. Chemical Vapour Deposition (CVD)
     In these processes, thermal energy heats the gases in the coating chamber and
drives the deposition reaction and then this reactant gas mixture (mixture of gas
precursors and coating material also known as a reactive vapour) impinges on the
substrate [8]. CVD processes can be used to deposit coating materials, form foils,
powders, composite materials in the shape of spherical particles, filaments, and
whiskers and also in structural applications, optical, chemical, photovoltaic and
electronics.. Start-up costs are typically very expensive. CVD includes sputtering, ion
plating, plasma-enhanced CVD, low-pressure CVD, laser-enhanced CVD, active-
reactive evaporation, ion beam, laser beam evaporation, and many other variations.
These variants are distinguished by the manner in which precursor gases are
converted into the reactive gas mixtures.
    It is usually in the form of a metal halide, metal carbonyl, a hydride, or an organ
metallic compound [10]. The precursor may be in gas, liquid, or solid form. Gases are
delivered to the chamber under normal temperatures and pressures, whereas solids
and liquids require high temperatures and/or low pressures in conjunction with a
carrier gas. Once in the chamber, energy is applied to the substrate to facilitate the
reaction of the precursor material upon impact. The ligand species is liberated from
the metal species to be deposited upon the substrate to form the coating. Because most
CVD reactions are endothermic, the reaction may be controlled by regulating the
amount of energy input.
   Disadvantages of CVD [9 ], the precursor chemicals should not be toxic, and
exhaust system should be designed to handle any reacted and unreacted vapors that
remain after the coating process is complete. Other waste effluents from the process
must be managed appropriately. Retrieval, recycle, and disposal methods are dictated
by the nature of the chemical. For example, auxiliary chemical reactions must be
performed to render toxic or corrosive materials harmless, condensates must be
collected.




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                      Fig. 4. schematic diagram of CVD process.

2. 3. Electroless Nickel Plating
    Electroless nickel (EN) plating is a chemical reduction process that depends upon
the catalytic reduction process of nickel ions in solution containing a chemical
reducing agent and water and the subsequent deposition of nickel metal without the
use of electrical energy[15,16,18 & 20 ]. Thus in the EN plating process, the driving
force for the reduction of nickel metal ions and their deposition is supplied by a
chemical reducing agent in solution. This driving potential is essentially constant at
all points of the surface of the component, provided the agitation is sufficient to
ensure a uniform concentration of metal ions and reducing agents[15]. The electroless
deposits are therefore very uniform in thickness all over the part’s shape and size. The
process is advantageous when plating complex shape devices, holes, recesses, internal
surfaces, valves, threaded parts etc. Electroless (autocatalytic) nickel coating provides
a hard, uniform, corrosion, abrasion, and wear-resistant surface to protect machine
components in many industrial environments. EN is chemically deposited, making the
coating exceptionally uniform in thickness. If carefully process is controlled good
surface finish can be produced which eliminates costly machining after plating. In a
true electroless plating process, reduction of metal ions occurs only on the surface of a
catalytic substrate in contact with the plating solution. Once the catalytic substrate is
covered by the deposited metal, the plating continues because the deposited metal is
also catalytic [18 ].
    Deposits have unique magnetic properties. EN deposits containing more than 8%
P are generally considered to be essentially nonmagnetic in the as-plated condition. A
second generation of EN plating has been developed by code positing micrometer-
sized particles of silicon carbide with the nickel, thereby creating an extremely wear-
and corrosion-resistant coating. The nickel alloy matrix provides corrosion resistance,
and the silicon carbide particles, which are actually the contacting surface, add wear
resistance.




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                       Fig. 5. Electroless nickel plating process
2. 4. Composite
    A composite material is a macroscopic, physical combination of two or more
materials in which one material usually provides reinforcement [27]. Composites have
been developed where no single, quasi-continuous material will provide the required
properties. In most composites one phase (material) is continuous and is termed the
matrix, while the second, usually discontinuous phase, is termed the reinforcement, in
some cases filler is applied when the reinforcement is not a quasi-continuous fibre.
Matrix-filler nomenclature is one method of categorization. This yields the categories
metal matrix (MMC), polymer (plastic) matrix (PMC), and ceramic matrix (CMC)
composites — the major subdivisions of this section. Other categories are given the
shape and configuration of the reinforcing phase. The reinforcement is usually a
ceramic and/or glass. If it is similar in all dimensions, it is a particulate reinforced
composite; if needle-shaped single crystals, it is whisker-reinforced; if cut continuous
filament, chopped fibre-reinforced; and if continuous fibre, fibre composite. For fibre
composites configuration gives a further category. If fibres are aligned in one
direction, it is a uni-axial fibre composite; if arranged in layers, it is a laminar
composite; if a three-dimensional arrangement, it is a 3D weave composite.
Laminates and 3D weaves can be further divided by the weave used for the fibre.
2. 5. Thermal spraying
    Energy surface treatment involves adding energy into the surface of the work
piece for adhesion to take place [17]. Conventional surface finishing methods involve
heating an entire part. The methods described in this section usually add energy and
material into the surface, keeping the bulk of the object relatively cool and
unchanged. This allows surface properties to be modified with minimal effect on the
structure and properties of the underlying material. The different thermal spray
technologies, based on the process of heat source, type of coating material and spray
conditions etc., can be classified as follows:-




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   1. Detonation gun spaying
   2. Flame spraying
   3. Electric arc spraying
   4. Wire explosion spraying
   5. Liquid metal spraying and
   6. Plasma spraying
    Plasmas are used to reduce process temperatures by adding energy to the surface
in the form of kinetic energy of ions rather than thermal energy. Advanced surface
treatments often require the use of vacuum chambers to ensure proper cleanliness and
control [1, 2 & 3]. Vacuum processes are generally more expensive and difficult to
use than liquid or air processes. Facilities can expect to see less-complicated vacuum
systems appearing on the market in the future. In general, use of the advanced surface
treatments is more appropriate for treating small components (e.g., ion beam
implantation, thermal spray) because the treatment time for these processes is
proportional to the surface areas being covered. Facilities will also have to address the
following issues when considering the new techniques[ ]. The following methods are
widely used in engineering applications.
2. 5.1. High-velocity oxy-fuel spraying (HVOF) process




Fig.6. HVOF process                    Fig.7. HVOF process setup
    In general, the high velocity oxy fuel spraying (HVOF) process can be used for
the deposition of the bond coat materials, over which oxide coatings are sprayed for
good adherence. This process is based on the combustion of the fuel gas with oxygen
at high pressures within the combustion chamber. The exit jet velocity is generally
more than 1000 m/s and at this speed, the oxide powder which is axially injected is
moderately heated but highly accelerated through the expansion nozzle to large
particle velocities beyond 800 m/s. The stream of hot gas and powder is directed
towards the surface to be coated. The powder partially melts in the stream, and
deposits upon the substrate as shown in figure1. The process has been most successful
for depositing corrosion-resistant alloys (stainless steels, nickel-based alloys,
aluminium, etc). HVOF coatings are effectively used in the fields of general
manufacturing industry, gas turbine industry, petroleum industry, chemical process


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industry, pulp industry and automotive industry[ 1, 2, 3].
2.5.2. Atmospheric Plasma spraying process
    When a strong electric arc is struck between tungsten electrode (cathode) and a
nozzle (anode) in the presence of Argon and nitrogen / hydrogen mixture in the
chamber, the gas gets ionized producing high temperature plasma. Injected particles
of coating materials are heated inside the plasma jet and molten droplets sprayed on
the substrate with high velocities to form the coating [1&3]. APS ceramic coatings are
widely employed in the engineering applications which demand wear resistance,
corrosion resistance and high strength at elevated temperatures.




Figh.8. Atmospheric Plasma spraying             Fig.9.Atmospheric Plasma Spray set up.
process.
2. 6. Ion Implantation
    In the Ion plating (IP) process, the target material is initially melted while the
substrate is bombarded with ions before deposition to raise it to the required
temperature. The coating flux ion is attracted to the substrate by biasing the substrate
with a negative voltage. Thus sufficient ion energy [ 28 ] is available for good inter
mixing of coating and substrate at the interface. Ion implantation is the introduction of
ionized dopant atoms into a substrate with enough energy to penetrate beyond the
surface. The most common application is substrate doping. The use of 3 to 500 keV
energy for boron, phosphorus, or arsenic dopant ions is sufficient to implant the ions
from 100 to 10,000A below the silicon surface. The depth of implantation, which is
proportional to the ion energy, can be selected to meet a particular application.
    Implantation offers a clear advantage over chemical deposition techniques. The
major advantage of ion implantation technology is the capability of precisely
controlling the number of implanted dopant atoms. Furthermore, the dopants depth
distribution profile can be well-controlled.
    Disadvantages of Ion Implantation are very deep and very shallow profiles are
difficult, not all the damage can be corrected by annealing [9 & 10], typically has
higher impurity content than does diffusion. Often uses extremely toxic gas sources
such as arsine (AsH3), and phosphine (PH3) and expensive


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  They are generally used in Doping, SIMOX, H and He isolation in GaAs, and
Smart cut technologies.




Fig.10. Ion Implantation setup and doping process
2.7. Anodizing
        Anodizing involves the electrolytic oxidation of a surface to produce a tightly
adherent oxide scale that is thicker than the naturally occurring film. Anodizing is an
electrochemical process during which aluminium is the anode. The electric current
passing through an electrolyte converts the metal surface to a durable aluminium
oxide. The difference between plating and anodizing is that the oxide coating is
integral with the metal substrate as opposed to being a metallic coating deposition.
The oxidized surface is hard and abrasion resistant, and it provides some degree of
corrosion resistance [28].
   Anodic coatings can be formed in chromic, sulphuric, phosphoric, or oxalic acid
solutions. Chromic acid anodizing is widely used with 7000 series alloys to improve
corrosion resistance and paint adhesion, and unsealed coatings provide a good base
for structural adhesives. However these coatings are often discoloured and where
cosmetic appearance is important, sulphuric acid anodizing may be preferred.




                                Fig.11. Anodizing Process



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2. 8 Boronising
   Boronising is also called as boriding. It is a thermo-chemical treatment involving
diffusion of boron into the surface of a component from the surrounding environment
which results in the formation of a distinct compound layer of a metal boride. The
reaction takes place between boron and component, therefore it can be generally
limited to steels, titanium-based alloys and cobalt-based hard metals. In steels,
boronising is carried out in the austenite regime (between 810–1020 °C) for several
hours, resulting in the formation of layers commonly between 60 and 165µm thick.
The surface reaction layer thus formed consists of two separate phases, namely a layer
of Fe2B adjacent to the substrate and an outer layer of FeB. The proportions of the
two phases are dependent upon the composition of the boronising environment and
the alloy content of the steel (higher alloy content favours FeB formation). Care is
taken to reduce the proportion of FeB in the boride layer since this always exists in
tension; as such, high-alloy and stainless steels are unsuitable for boronising. The
hardness of the boronised layer is dependent upon the exact composition of the steel
but is commonly in the range 1600–2350 kgf/mm2 (as measured on the Vickers
scale). This is significantly higher than many commonly occurring abrasives and, as
such, boronising has been employed in situations requiring abrasive wear resistance.
Materials that can be processed for Ferrous materials such as irons, plain carbon,
alloy, stainless, and tool steels are all possible. This is because the boride compound
formed is an iron boride so we only need iron to be present in the material to do this
are Nickel-based alloys, Cobalt-based alloys, Molybdenum, Sintered carbides [5 & 6
].
    A variety of methods are employed to produce the boron-rich environment for the
boronising process such as pack boronising, paste boronising, salt bath boronising and
gas boronising[ ]. In pack boronising (the most commonly employed method), the
source of boron is B4C which is mixed with an activator and an inert diluent to make
up the pack powder.




                               Fig. l2. Boronising Process
2. 9. Nitriding
   Steels containing nitride-forming elements such as chromium, molybdenum,
aluminum, and vanadium can be treated to produce hard surface layers, providing


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improved wear resistance. Many of the processes employed are proprietary, but
typically they involve exposure of cleaned surfaces to anhydrous ammonia at elevated
temperatures. The nitrides formed are not only hard but also more voluminous than
the original steel[ ], and therefore they produce compressive residual surface stresses.
Therefore, nitrided steels usually exhibit improved fatigue and corrosion fatigue
resistance. Similar benefits can be achieved by shot-peening [28].




   Fig. 13. Nitriding process setup.             Fig. 14. Nitriding Process


2.10. Carburizing
     Carburizing is a heat treatment process in which iron or steel is heated in the
presence of carbon material (in the range of 900 to 950 °C). Depending on the amount
of time and temperature, the affected area can vary in carbon content. Longer
carburizing times and higher temperatures lead to greater carbon diffusion into the
part as well as increased depth of carbon diffusion. When the iron or steel is cooled
rapidly by quenching, the higher carbon content on the outer surface becomes hard via
the transformation from austenite to martensite, while the core remains soft and tough
as a ferritic and/or pearlite microstructure
        Generally it is used for low-carbon work-piece to increase their toughness and
ductility; and it produces case hardness depths of up to 6.4 mm.


   .




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                              Fig.15. Carburising Process
       Plasma carburization is increasingly used in major industrial regimes to
improve the surface characteristics (such as wear and corrosion resistance, hardness
and load-bearing capacity, in addition to quality-based variables) of various metals,
notably stainless steels [28]. The process is used as it is environmentally friendly (in
comparison to gaseous or solid carburizing). It also provides an even treatment of
components with complex geometry (the plasma can penetrate into holes and tight
gaps), making it very flexible in terms of component treatment.
   The process of carburization works via the implantation of carbon atoms in to the
surface layers of a metal.
   A main goal when producing carbonized work pieces is to insure maximum
contact between the work piece surface and the carbon-rich elements. In gas and
liquid carburizing, the work pieces are often supported in mesh baskets or suspended
by wire. In pack carburizing, the work piece and carbon are enclosed in a container to
ensure that contact is maintained over as much surface area as possible. It is possible
to carburize only a portion of a part, either by protecting the rest by a process such as
copper plating, or by applying a carburizing medium to only a section of the part.
2. 11.Carbo-nitriding
   Carbo-nitriding [28] is similar to cyaniding except a gaseous atmosphere of
ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be
quenched then the part is heated to 775–885 °C if not then the part is heated to 649–
788 °C.




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                          Fig. 16. Nitro- carburizing process
2.12. Ferritic nitro-carburizing
     Ferritic nitro-carburizing diffuses mostly nitrogen and some carbon into the case
of a work-piece below the critical temperature, approximately 650 °C. Under the
critical temperature the work piece’s microstructure does not convert to an austenitic
phase, but stays in the ferritic phase, which is why it is called ferritic nitro
carburization[28].
   It is used in Parts that are subject to high pressures and sharp impacts are
commonly case hardened, e.g. firing pins and rifle bolt faces, or engine camshafts.
2.13 Short Peening, Water-Jet Peening and Laser Peening
        In short peening the surface of the work piece[28] is hit repeatedly with large
number of cast-steel, glass or ceramic shot (size of 0.125mm to 5mm diameter),
making overlapping indentation on the surface; this action causes plastic deformation
of the surfaces[ ]. Thus improving the fatigue life of the component. Extensively used
on shafts, gears, springs, oil-well drilling equipment, and jet engine parts.
    In water-jet peening, a water jet at pressure as high as 400 MPa impinges on the
surface of the work piece, inducing compressive residual stresses. This have been
successfully used on steels and aluminum alloys[28].
In laser peening, the surface is subjected to laser shocks from high powered laser up
to 1KW and at energy levels of 100 J/pulse. This method has been used on jet engine
fan blades with compressive residual stresses deeper than 1mm [28 ].
2. 14.Thermal hardening
   In case of steels, to achieve a full conversion of austenite into hard martensite,
cooling needs to be fast enough to avoid partial conversion into perlite or bainite
[26]. If the piece is thick, the interior may cool too slowly so that full martensitic
conversion is not achieved. Thus, the martensitic content, and the hardness, will drop
from a high value at the surface to a lower value in the interior of the piece.
Hardenability is the ability of the material to be hardened by forming martensite.
   The shape and size of the piece, together with the heat capacity and heat
conductivity are important in determining the cooling rate for different parts of the
metal piece. Heat capacity is the energy content of a heated mass, which needs to be
removed for cooling. Heat conductivity measures how fast this energy is transported


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to the colder regions of the piece.
2.15. Precipitation Hardening
    Hardening can be enhanced by extremely small precipitates that hinder dislocation
motion. The precipitates form when the solubility limit is exceeded. Precipitation
hardening is also called age hardening because it involves the hardening of the
material over a prolonged time. Precipitation hardening is achieved by solution heat
treatment where all the solute atoms are dissolved to form a single-phase solution or
by rapid cooling across the solvus line to exceed the solubility limit[ ]. This leads to a
supersaturated solid solution that remains stable (metastable) due to the low
temperatures, which prevent diffusion or precipitation heat treatment where the
supersaturated solution is heated to an intermediate temperature to induce
precipitation and kept there for some time (aging).
    The requirements for precipitation hardening [28]are appreciable maximum
solubility, solubility curve that falls fast with temperature and composition of the
alloy that is less than the maximum solubility. Mechanism of Hardening involves the
formation of a large number of microscopic nuclei, called zones. It is accelerated at
high temperatures. Hardening occurs because the deformation of the lattice around
the precipitates hinder slip. Aging that occurs at room temperature is called natural
aging, to distinguish from the artificial aging caused by premeditated heating.
2. 16. Microwave Irradiation
    Microwave heating is fundamentally different from conventional heating process.
In conventional thermal processing energy is transferred to the material through
conduction, convection and radiation of heat from the surface of the material [3]. On
the other hand microwave energy is delivered directly to materials through molecular
interaction with the electromagnetic field. As microwave radiation penetrates and
interacts with molecules, transfer of electromagnetic energy takes place throughout
the volume of material, leading to volumetric heating. Microwave material interaction
depends on dielectric property of materials. While conducting, metals reflect,
insulators are transparent to microwaves.
    Once the ceramics composite material starts coupling with microwave at an
elevated temperature, temperature rises rapidly and induces phase transformation
associated with increase in volume. This results in micro structural changes like
partial filling of pores and voids, healing of micro cracks and consequent
densification. Typical enhancement in observed porosities across the microwave
glazed coating [3], typical improvement in hardness of glazed composite and a
significant improvement in Vickers hardness are observed, which is associated with
the densification of the coatings structures during post processing. Morphological
improvement during glazing is indicated by the observed improvement in surface
texture.
2.17. Laser Surface Alloying (LSA)
        The LSA is used to mix an additional material with the molten surface of the
substrate so that, upon solidification, an alloy surface with a different composition
from that of the substrate can be obtained[ 22, 23, 24, 24 25 & 26]. The properties of
the alloyed surface can be tailored to suit different requirements. Much work on LSA
has been done towards producing a hardfacing layer or an improved corrosion-
resistant layer [22]. The structure of the laser-alloyed layer often contains

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supersaturated solid solutions and sometimes intermetallic compounds. In addition to
the metallic components that can be alloyed to the substrate surface, ceramic
components can also be added so as to achieve a metal matrix composite surface with
significantly increased hardness on the metal substrate.
    Application of a laser can improve the properties of thermally sprayed coatings.
These improvements having been studied for the following applications such as
biomedical coatings, thermal-barrier coatings, wear-resistant composite coatings, wet
and hot corrosion-resistant alloys.

3.   CONCLUSION
     In an engineering component design, the selection of surface technology to
engineer the surface involves selection of suitable surface modification technique to
determine the surface and substrate requirements which involve one or more
properties. The very important proprties are mechanical, tribological, corrosion
reisistance, erosion resistance, creep strength, thermal resistance, pitting resistance
etc.
     The various surface treatments generally used in engineering practice are
highlighted in this technical paper. Details aspect of various surface modification
techniques have been dealt. Applications in engineering field has been mentioned.
Further, the most widely used ceramic material coating technique namely thermal
spraying technique ( e.g. HVOF and APS )is dealt in detail aspect of it. Besides, the
special post-processing methods of surface modification of ceramic coatings using
microwave glazing and laser treatment are also highlighted for the production of
better quality coating for any functional requirement of engineering surfaces.

REFERENCES
[1] Dr.J.Fazlur Rahman and Mohammed Yunus,“ Benefits of TBC Coatings on
     Engine applications”, International conference, INCAM 2009 at Kalsalingam
     University, Tamil Nadu, India, March 2009.
[2] Dr.J.Fazlur Rahman and Mohammed Yunus, “Mechanical and Tribological
     characteristics of Tungsten Carbide Cobalt HVOF coatings” International
     conference held at Anjuman college of Engineering, Bhatkal October 2008.
[3] Mohammed Yunus and Fazlur Rahman, J. “An investigation towards
Characterization of thermally sprayed industrial coatings”, International J.
Advanced Engineering Sciences and Technologies, Vol. 10, No.2, pp. 275 –         284,
2011.
[4] Moll, E. (1992), ‘Physical vapour deposition techniques II: ion plating, arc
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 [5] Wang, L., Snidle, R.W. and Gu, L. (2000), ‘Rolling contact silicon nitride
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[6] Sethuramiah, A. (2003), Lubricated Wear: Science and Technology, Elsevier
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[7] Stewart, S. and Ahmed, R. (2002), ‘Rolling contact fatigue of surface
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