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 . 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 . 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  and Hasselman published his theory on thermal crack propagation . Another significant publication was Ceramics in Machining Processes . 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 . 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 . 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 . + 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 . 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 . + 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 (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, 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, pp. 370-379 (1988). 17. M.D. Sacks, “Properties of Silicon Suspensions and Cast Bodies,” Am. Cer. Sm. Bull., 63, 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).