ALUMINA-BASED CERAMICS
Ceramic tool materials, introduced in the early 1950s, consist primarily of fine-grained, high-purity aluminium oxide. They are cold-pressed into insert shapes under high pressure and sintered at high temperature; the end product is referred to as white, or cold-pressed, ceramics. Additions of titanium carbide and zirconium oxide help improve properties such as toughness and thermal-shock resistance.
Alumina-based ceramic tools have very high abrasion resistance and hot hardness. Chemically, they are more stable than high-speed steels and carbides, so they have less tendency to adhere to metals during cutting and a correspondingly lower tendency to form a built-up edge. Consequently, in cutting cast irons and steels, good surface finish is obtained with ceramic tools. However, ceramics lack toughness, and their use may result in premature tool failure by chipping or catastrophic failure.
Ceramic inserts are available in shapes similar to the shapes of carbide inserts. They are effective in high-speed, uninterrupted cutting operations, such as finishing or semifinishing by turning. To reduce thermal shock, cutting should be performed either dry or with a copious amount of cutting fluid, applied in a steady stream. Improper or intermittent applications of the fluid can cause thermal shock and fracture of the ceramic tool.
Ceramic tool shape and setup are important. Negative rake angles (large included angles) are generally preferred in order to avoid chipping due to the poor tensile strength of ceramics. Tool failure can be reduced by increasing the stiffness and damping capacity of machine tools, mountings, and workholding devices, thus reducing vibration and chatter.
Cermets. Black, or hot-pressed, ceramics were introduced in the 1960s. They typically contain 70% aluminium oxide and 30% titanium carbide, and are also called "cermets" (from ceramic and metal). Other cermets contain molybdenum carbide, niobium carbide, and tantalum carbide. Although they have chemical stability and resistance to built-up edge formation, their brittleness and high cost have been a problem.
Further refinements of these tools have resulted in improved strength, toughness, and reliability. Their performance is somewhere between that of ceramics and carbides and has been particularly suitable for light roughing cuts and high-speed finishing cuts. Chip-breaker features are important for cermet inserts. Although cermets can be coated, the benefits of coatings are somewhat controversial; the improvement in wear resistance appears to be marginal.
