Introduction to Ceramic Cutting Tools

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					INDEX


                                                            INDEX

  1. Introduction to Ceramic Cutting Tools

  2. Tool Life

  3. Selection of Cutting Tool Materials

  4. Aluminum Oxide or Titanium Carbide Composite Cutting Tool

  5. Cermet Cutting Tools

  6. Alumina-Silicon Carbide Whisker Composite Tools

  7. Phase Transformation Toughened Materials for Cutting Tool

  8. Silicon Nitride Cutting Tools

  9. Aluminum Oxide Coatings for Cemented Carbide Cutting Tool

10. Polycrystalline Diamond and Cubic Boron Nitride

11. The New Diamond Technology and its Application in Cutting

12. Advanced Processing Concepts for Increased Ceramic Reliability




file:///D|/PG2004/TOOL/Anup/Ceramic%20tools/Index.htm [8/16/2004 3:34:15 PM]
1
Introduction to Ceramic Cutting Tools




                           Alan G. King
                         Twinsburg, Ohio


INTRODUCTION

         Ceramic science in the first third of the 20th century was
primitive. Ceramic engineering was largely by rote. Over time,
the slide rule and log tables gave way to the calculator or
computer.     However, the contributions of the early ceramic
engineer should not be discounted. Their skill, craftsmanship and
attention to detail laid the foundation for many of today’s technical
ceramics.
         Ceramic cutting tool development is paralleled by
developments in ceramic processing science, materials selection
and improved instrumentation.           This chapter discusses the
evolution of cutting tools in relation to the above criteria.


SIGNIFICANT      ADVANCES

        Ceramic cutting tools have been in use for approximately
90 years. As new materials were developed during a specific era,
the properties of the cutting tool improved. However, as new
cutting tools were developed, new materials that demanded even
more rigorous machining requirements were also developed. The
following sections discuss in chronological order some advances in
ceramic cutting tools.
2      Ceramic Cutting Tools

Alumina - Glass Bonded

         Ceramic tools have been in existence since the early
1900’s. These early tools consisted primarily of alumina and were
bonded to metallic cutting tools with a glassy phase derived from
additions of talc or clay. Because of the poor quality of the
alumina powders available at the time, fracture toughness values
of 3.0 MPa-m1’2 would have been typical (however, “fracture
toughness” had not been conceptualized at the time). Strengths of
even the best materials were low with values of = 340 MPa (50,
000 psi). The fundamental problem with glass-bonded alumina was
the glassy phase softened at metal cutting temperatures.
Therefore, these materials did not gain acceptance and their use
was abandoned.
         A bridge to the next era of ceramic cutting tools could be
attributed to Bridgman for his work in the field of high pressure
physics for which he was awarded the Nobel Prize. His research
required a material that could withstand both high stresses and
temperatures. As no suitable material was available, Bridgman
developed a device which now bears his name, the Bridgman
anvil. Another result of his research was the investigation of
diamond/graphite stability fields in the carbon phase diagram.

Sintered Alumina

         During the early 1930’s Ryschkewitsch experimented with
a relatively pure Al,O, cutting tool. The tool was marketed under
the company name of Degussit. The addition of MgO as a
sintering aid eliminated the glassy phase thereby improving the
strength of the material. The tool was = 98% dense with a grain
size of 3 pm. As is often the case today, application was found in
metal cutting of cast iron where stresses are lower than those for
machining steel.
         Another cutting tool, referred to as Microlite, was
developed during the same time period in what was then known as
the Soviet Union.       Microlite consisted of pure alumina and
magnesium oxide. This tool generated considerable interest among
tool engineers even though its physical properties were comparable
to the Degussit product. It is speculated that there may have been
                   Introduction to Ceramic Cutting Tools          3

a slight improvement in fracture toughness due to the 5 pm grain
size.
        In the 1960’s, several different types of sintered alumina
tools with a variety of additives were developed in the United
States, Europe and Japan.
        Goliber at General Electric’s Carboloy Division developed
a ceramic cutting tool based on alumina with a 10% addition of
TiO. Prior to Goliber’s work it was known that TiO, could be
used as a sintering aid, however, TiOz also caused discontinuous
grain growth. The Al,O,/TiO tool was referred to as the O-30
grade. It had an equiax grain structure of approximately 2 pm,
was sintered to nearly full density and had a transverse rupture
strength of 586 MPa (85,000 psi). This was a remarkable material
for the time, and received great acceptance.

Hot-Pressed Alumina

         There were two principle hot pressed tools with significant
market shares during the 1960’s. These were Carborundum’s
CCT-707 and Norton’s VR-97. Hot pressing as a densification
process is more forgiving than sintering in that full density is
virtually assured. Powder properties are still important but not as
critical as in sintering. For example, soft agglomerates can be
devastating with sintering but are of little or no consequence when
the ceramic is hot pressed. Given the full density and good
microstructure of the CCT-707 and VR-97 both had excellent
properties for ceramic tools at that time.
         CCT-707. The CCT-707 was developed under the trade
name Stupalox by VonMickwitz.           Carborundum acquired the
technology and for a time marketed this single point turning tools
along with its abrasive line. While Carborundum was a principle
supplier of abrasive tools, it was not generally thought of as a
cutting tool supplier.     This, along with internal management
difficulties, caused them to cease operations for both the abrasive
and cutting tool industries.
         VR-97. This material was a pure alumina with MgO
hotpressed to full density. Research on VR-97 was done by
Norton Company where investigators observed that there was a
generic connection between grinding wheels and single point
4       Ceramic Cutting Tools

machining. Unlike Carborundum, Norton management realized
the difficulty in building a distribution network. To solve this
problem, a Norton arranged a partnership with Vascoloy Ramet to
distribute the inserts.     Profits were split between the two
companies and neither one realized a profit. Eventually Norton
sold the VR line to Vascoloy who continued to market the tool for
several years.
         The application of ceramic tools was beginning to mature
to some degree. Machining costs leveled out at a low level as the
surface speed of the workpiece was increased. While this was
exciting, there were some provisos which limited the realization of
this advantage in practice.      One limit was, and still is, the
capability of the machine tool to function well at high speeds
without undo vibration. High speeds are acceptable if the cut is
long and straight, but can be difficult if the part is intricate and/or
delicate. As a result, ceramic tools found their only significant
application on cast iron, where abrasion resistance was the
overriding tool attribute.

Early Advances in Science and Technology

        In the early 1960’s, ceramic materials science was
beginning to flourish. Kuczyunski, et al. developed a sintering
theory bringing about a resurgence in materials research [ 11.
Also, a great deal was being learned about dislocations in metals
and this work was applied to the study of ceramics.
        Bridgman worked cooperatively with a consortium which
included General Electric, Carborundum, and Norton in an attempt
to synthesize diamond. They were not successful, but advanced
technology for achieving high pressures and temperatures. Later,
GE scientists developed the belt apparatus and the chemistry for
practical diamond synthesis.
        Most cemented carbide tools were ground with synthetic
diamond grinding wheels. This technology was undoubtedly a
factor in the search for a process to make very fine polycrystalline
diamond materials. GE developed a process to synthesize this type
of diamond by discharging a large capacitor bank into the “belt”
apparatus.    DuPont scientists also working in this area, used
explosives to obtain the phase change from hexagonal to cubic
                   Introduction to Ceramic Cutting Tools         5


carbon. In their process, graphite powder was floated onto a water
bath and the shock wave from the explosion provided the particles
with sufficient energy to cause a phase change.
         Coes working at Norton Co., developed a mechano-
chemical theory of grinding [2]. A portion of his research focused
on the chemical reactions occurring at the metal-abrasive interface
during metal cutting. Spine1 (Fe0*A1,03) was identified as a
reaction species suggesting that oxygen had to be available for the
ceramic to wear by this process. Wear research on alumina
cutting tools followed Coes’ lead and it was found that oxygen was
an important constituent in some wear processes.
         Several significant works were published during this time
frame.      Kingery, Bowen and Uhlmann authored the book
Introduction to Ceramics [3]. This work provided a basic text
for the scientific study of ceramic engineering and continues to be
used as a teaching and reference source. Kingery also published
his work on thermal shock crack initiation [4] and Hasselman
published his theory on thermal crack propagation [5]. Another
significant publication was Ceramics in Machining Processes [6].
This book combined science and experience into one source
making research and development accessible to all interested
parties.
         Instrumentation was advancing as transmission electron
microscopy on surface replicas was providing detailed information
on microstructure and wear phenomena. Surface area analyses
were becoming more accessible. Optical microscopy had been
available but its’ application was expanded - principally by German
instruments. Mechanical testing equipment had become routine.
Emission spectrograph was perhaps the central instrument for
analyzing the relatively pure materials available at that time.
         Also during this period, serious attention was given to
processing of high quality ceramic powders. Mazdiyasni and co-
worker conducted a sustained research effort on ceramic powders
using organic precursors [7]. While a one-to-one relationship
between this work and its direct application to tool materials was
difficult to ascertain, the research stimulated thought about very
pure ceramic powders with a controlled particle size distribution
in the near sub-micron range. This was a significant advance in
the technology we now call “advanced ceramics. ”
6      Ceramic Cutting Tools

         Coble, then at G.E., developed the translucent alumina
referred to as “Lucalox. ” Prior to this development, alumina
ceramics were opaque.           A translucent alumina was quite
astonishing.      The microstructure had to be fully dense. By
controlling the sintering aid (MgO), ceramic powder properties,
sintering atmosphere, and sintering cycle a translucent alumina was
realized. Lucalox became an important key material in sodium
vapor lamps. While this advance did not directly impact the tool
material research, it did serve to focus attention on critical
processing and sintering technology.
         Statistical experiment design was beginning to emerge as a
valuable mathematical tool. These techniques had been around for
about 30 years but were not extensively used until the 1960’s.
Factorial experiment design was the child of Ronald A. Fisher in
Great Britain in the 1930’s. Fisher was knighted for his valuable
application of mathematics to experimental methodology. At Bell
Laboratories, Shewhart adapted statistics to quality control
systems. Deming was a staunch advocate for “statistical process
control” (SPC). Although he was not successful in convincing
U.S. industry of the merits of this program, he was effective in
post-World War II Japan. While SPC was initially thought of as
a manufacturing quality control tool, it gradually evolved into a
process for continuous product improvement.
         Statistical methods apply to the ceramic cutting tool
research and control just as they apply to other fields. Major
customers, such as the automotive industry, require their tool
suppliers to use SPC methods.

Recent Developments    in Science and Technology     (198O-1990)

       There has been a profound change in technical (advanced)
ceramics since about 1980. A great deal of interdependent science
and technology became available resulting in improved ceramics.
Some of these ceramics are now being used as cutting tools.
                   Introduction to Ceramic Cutting Tools          7


        A detailed discussion of advanced ceramics is beyond the
scope of this chapter. However, a summary some of the salient
advances which made advanced ceramics possible follows.
        Advanced ceramic powders. These ceramic powders were
pure, finely divided and essentially free of contaminants. Powders
were generally derived by chemical methods, with a major thrust
coming from the Japanese. Superior powders were developed
where each particle was spherical, had a very narrow submicron
size distribution and were of high purity [8]. Morgan did some
remarkable work with non-aqueous powder synthesis which may,
in the future, see wider spread application. At Norton, extremely
pure alumina was being made by distillation of aluminum
isopropoxide which was hydrolyzed with water vapor and calcined.
The emission spectrograph plates were devoid of any spectra other
than Al. However, the Japanese were still the major source of
high quality ceramic powders including: Al,O,, yttria stabilized
zirconia, silicon nitride and silicon carbide.
        Advances in processing. A summary of some important
advances in ceramic processing follows:

       + Prochazka sintered dense polycrystalline      beta silicon
       carbide [9].

       + The toughening mechanism of partially stabilized
       zirconia was first observed by Garvie, et al. [lo], and then
       explained by Evans and Heuer [ 111.

       + Claussen fabricated transformation toughened alumina
       lxl.

       + A much better understanding of suspension chemistry
       was provided by several researchers including Askay, Sacks
       and Lange [ 13- 181. The work done by Lange focused
       attention on the importance of flaws in the ceramic
       structure which act as crack nuclei. By progressively
       removing crack nuclei populations by intelligent processing
       he was able to attain 2000 MPa (300,000 psi) transverse
       rupture strengths in yttria stabilized zirconia [19].
8      Ceramic Cutting Tools

      + Higher strength, hot pressed alumina (with zirconia
      additives) was developed in Japan.

      + The Soviet Union revealed that they had produced a
      polycrystalline diamond ceramic.

       +   Silicon nitride was developed principally for the
               ceramic heat engine. Jack and Wilson in England
explored the chemistry of SiAlONs [20].

      + Cutler made silicon carbide whiskers from calcining rice
      hulls in a reducing atmosphere.

      Advances    in processing   equipment    and   techniques.
During this period there were parallel advances in process
equipment and techniques some of which are discussed in the
following section.

      + It may appear inconsequential, but the ability to mill
      ceramic powders is crucial. Advanced ceramic milling
      media made in Japan are essentially free from producing
      mill chips.

      + Hot isostatic presses originally developed at Batelle are
      now widely used for densifying ceramics.

       + Much improved sintering furnaces are available that are
      cleaner and programmable. Graphite free furnaces and hot
      presses using refractory metals and vacuum purging
      provide the cleanest environment for sintering with the
      important option of neutral or reducing gas atmospheres.

       + Mensuration and instrumentation have been greatly
      improved. The scanning electron microscope (SEM-EDS)
      with energy or wave length dispersive capability is one of
      the most powerful problem solving tools available. Other
      instruments now available to the researcher include:
      particle size measuring equipment, the TEM, Fourier
      transform    infrared  spectroscopy   (FTIR),     electron
                  Introduction   to Ceramic   Cutting Tools      9


       microprobe       (EMP),     inductively-coupled plasma
       spectrometry    (ICP), gas chromatography (GC), raman
       spectroscopy,    nuclear magnetic resonance (NMR) and
       secondary ion   mass spectrometry (SIMS).

       + The development of the transistor at Bell Labs resulted
      in an explosion of instruments, sensors, and most
      importantly the computer.     Inexpensive and powerful,
      computers and the extensive array of software make many
      things possible which were prohibitively laborious not too
      long ago.


CURRENT     CERAMIC       CUTTING      TOOLS

       Ceramic materials in the cutting tool market are becoming
more diverse and differentiated. Major materials are:

      + Alumina-silicon carbide whisker composites.         The
      addition of Sic, increases the fracture toughness to
      approximately 6MPa-m1’2. This composite must be hot
      pressed as the whisker tangle prevents sintering to a high
      density.

      + Silicon nitride has a toughness of 4-5MPa-m1’2. It is
      widely used for machining cast iron where the material’s
      abrasion resistance is excellent. S&N4 is shock resistant,
      with a high thermal conductivity and a moderate thermal
      expansion.

      + Titanium carbide/titanium nitride materials are identified
      as cermets having good abrasion resistance.

       + SiAlONs are solid solutions principally between silicon
      nitride and alumina. The presence of alumina provides
      improved resistance to oxidation.
10     Ceramic Cutting Tools

        + 70%A1,03-30%TiC is used for machining carbon alloy,
       tool steels, and stainless steel.

       + Polycrystalline diamond has excellent abrasion resistance
       and is used for cutting metals, glass and ceramics. It is
       also used in drill bits for oil and gas exploration.

        + Cubic boron nitride is second only to diamond in
       hardness. Whereas carbon is soluble in iron, cubic boron
       nitride is not. This makes its application on abrasive
       ferrous metals a good choice.

       + Alumina continues to be used as a cutting tool insert.

        + Cemented carbide is actually a cermet where the WC
       part is the ceramic constituent. Hardness and fracture
       toughness values can be manipulated to produce a family of
       cutting tool materials. It is incredibly strong, resistant to
       thermal shock, has a toughness up to 15MPa-ml’*, and at
       lower cutting speeds is very wear resistant.


SUMMARY

        The advantages of ceramics over tool steel and cemented
carbide are inherent as they result from the composition and
crystal lattice. Ceramics are hard, inert and retain properties at
high temperatures.    When the tendency for brittle fracture is
substantially reduced, ceramics have the potential for general
application for machining steel and. displacing much of the
cemented carbide inserts.


REFERENCES

1.  G.C. Kuczyuski, N.A. Hooton and C.F. Gibson, eds.,
      Sintering and Related Phenomena, Gordon and Breach,
      NY (1967).
2. L. Goes, Jr., Abrasives, Springer-Verlag, NY (1971).
                   Introduction to Ceramic Cutting Tools         11

3. W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction
      to Ceramics, 2nd ed., John Wiley & Sons, NY (1976).
4. W.D. Kingery, “Factors Affecting Thermal Stress Resistance
         of Ceramic Materials, ” J. Am. Cer. Sot. 38:3 (1955).
5. D. P. H. Hasselman, “Unified Theory of Thermal Shock Fracture
         Initiation and Crack Propagation in Brittle Ceramics, ” J.
        Am. Cer. Sot., 52:600 (1969).
6. A.G. King and W.M. Wheildon, Ceramics in Machining
        Processes, Academic Press, NY (1966).
7. K.S. Mazdiyasni, C.T. Lynch and J.S. Smith, “Preparation of
         Ultra-High Purity Submicron Refractory Oxides, ” J. Am.
         Cer. Sot., a:372 (1965).
8.    Processing of Crystalline Ceramics, Materials Science
        Research, Vol. 11, (Palmour, Davis and Hare, eds.)
         Plenum Press, NY (1978).
9. S. Prochazka, “Sintering of Silicon Carbide,” in Ceramics for
        High Performance       Applications,   (Burke, Gorum and
        Katz, eds.) Brook Hill, MA (1974).
10. R.C. Garvie, R.H.J. Hammink and R.T. Pascoe, “Ceramic
         Steel,” Nature (London), 258:703 (1975).
11. A.G. Evans and A.H. Heuer, “Transformation Toughening in
         Ceramics: Martensitic Transformations in Crack Tip Stress
        Fields,” J. Am. Cer. Sot., a:241 (1981).
12. N. Claussen, “Fracture Toughness of Al,O, With an
        Unstabilized ZrO, Dispersed Phase, ” J. Am. Cer. Sot.,
        s:49 (1976).
13. I.A. Askay and C.H. Schilling, “Colloidal Filtration Route to
        Uniform Microstructures, ” in Ultrastructure Processing
         of Ceramics, Glasses and Composites, (L.L. Hench and
        D.R. Ulrich, eds.), John Wiley & Sons, New York, pp.
        439-447 (1984).
14. I.A. Askay, F.F. Lange and B.I. Davis, “Uniformity of A&O,-
         ZrO, Composites by Colloidal Filtration, ” Comm. Am. Cer.
        Sot., C-190 - C-192, 66[10] (1983).
15. J. Cesarano III, I.A. Askay and A. Bleier, “Stability of
        Aqueous       cr-A&O, Suspensions         Stabilized  with
        Polyelectrolytes, ” J. Am. Cer. Sot. ,7_l[4], pp. 250-255
        (1988).
12      Ceramic Cutting Tools

16.    M.D. Sacks, H-W Lee and O.E. Rojas, “Suspension
         Processing of Al,O,/SiC Whisker Composites, ” J. Am. Cer.
         Sm., 71[5], pp. 370-379 (1988).
17.    M.D. Sacks, “Properties of Silicon Suspensions and Cast
         Bodies,” Am. Cer. Sm. Bull., 63[12], pp. 1510-1515
          (1984).
18.   M.D. Sacks, C.S. Khadlikar, G. W. Scheiffele, A.V Shenoy,
         J.H. Dow and R.S. Sheu, “Dispersion and Rheology in
          Ceramic    Processing, ” in Ceramic Powder Science,
         Advances in Ceramics, Vol. 21, (G.L. Messing, K.S.
         Maxdiyasni,     J.W. McCauley and R.A. Haber, eds.)
         American Ceramic Society, Inc., Westerville, OH, pp. 495-
         515 (1987).
19.   F.F. Lange, “Processing Related Fracture Origins,” J. Am.
          Cer. Sot., f$:396 (1983).
20.   K.H. Jack and W.I. Wilson, “Ceramics Based on the Si-Al-O-
         N and Related Systems,” Nature Physical Science, 238128
          (1972).

				
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