Machinery's Handbook, 31st Edition
Cemented Carbides and Other Hard Materials
867
Table 3. Typical Properties of Cutting Tool Ceramics Group Alumina
Alumina/TiC Silicon Nitride PCD PCBN
Si 3 N 4 /Y 2 O 3 plus
Al 2 O 3 or Al 2 O 3 /ZrO 2
70 ⁄ 30 Al 2 O 3 /TiC
Typical composition types
Density (g/cm 3 )
4.0
4.25
3.27
3.4
3.1
Transverse rupture strength (N/mm 2 ) Compressive strength (kN/mm 2 )
700
750
800
800
4.0
4.5
4.0
4.7
3.8
1750
1800
1600
Hardness (HV)
Hardness HK (kN/mm 2 ) Young’s modulus (kN/mm 2 ) Modulus of rigidity (kN/mm 2 )
50
28
380 150
370 160
300 150
925 680 430 280
0.24
0.22
0.20
0.09 0.22
Poisson’s ratio
Thermal expansion coefficient (10 –6 /K) Thermal conductivity (W/m K) Fracture toughness (K 1c MN/m 3 ⁄ 2 )
8.5
7.8
3.2
3.8
4.9
23
17
22
120 100
2.3 10 Alumina-based ceramics were introduced as cutting inserts during World War II and were for many years considered too brittle for regular machine-shop use. Improved machine tools and finer-grain, tougher compositions incorporating zirconia or silicon carbide “whiskers” now permit their use in a wide range of applications. Silicon nitride, often combined with alumina (aluminum oxide), yttria (yttrium oxide), and other oxides and nitrides, is used for much of the high-speed machining of superalloys, and newer grades have been formulated specifically for cast iron—potentially a far larger market. In addition to improvements in toolholders, great advances have been made in machine tools, many of which now feature the higher powers and speeds required for the efficient use of ceramic tooling. Brittleness at the cutting edge is no longer a disadvantage, with the improvements made to the ceramics themselves, mainly in toughness, but also in other critical properties. 3.3 5.0 7.9 Although very large numbers of useful ceramic materials are now available, only a few combinations have been found to combine such properties as minimum porosity, hardness, wear resistance, chemical stability, and resistance to shock to the extent necessary for cutting-tool inserts. Most ceramics used for machining are still based on high-purity, fine-grained alumina (aluminum oxide) but embody property-enhancing additions of other ceramics such as zirconia (zirconium oxide), titania (titanium oxide), titanium carbide, tungsten carbide, and titanium nitride. For commercial purposes, those more commonly used are often termed “white” (alumina with or without zirconia) or “black” (roughly 70 ¤ 30 alumina/titanium carbide). More recent developments are the distinctively green alumina ceramics strengthened with silicon carbide whiskers and the brown-tinged silicon nitride types. Ceramics benefit from hot isostatic pressing to remove the last vestiges of porosity and raise substantially the material’s shock resistance, even more than carbide-based hard metals. Significant improvements are derived by even small parts such as tool inserts, although, in principle, they should not need such treatment if raw materials and manufac turing methods are properly controlled. Oxide Ceramics: Alumina cutting tips have extreme hardness—more than HV 2000 or HRA 94—and give excellent service in their limited but important range of uses such as the machining of chilled iron rolls and brake drums. A substantial family of alumina- based materials has been developed, and fine-grained alumina-based composites now have sufficient strength for milling cast iron at speeds up to 2500 ft/min (800 m/min). Resistance to cratering when machining steel is exceptional. Oxide/Carbide Ceramics: A second important class of alumina-based cutting ceramics combines aluminum oxide or alumina-zirconia with a refractory carbide or carbides,
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