Hard, tough materials are becoming increasingly important for many applications. The use of ceramics in engines, machines, tools, and other wear-resistant components is steadily increasing due to the development of new ceramics with higher toughness and ductility. Most of these new types of ceramics belong to the SiAlON family. Another approach to meet new demands for hard materials is to apply hard coatings or to use various surface-hardening processes. The decision as to whether a hard material should be used in bulk form or as a coating is determined by the actual application. Hard bulk materials usually suffer from a low ductility and are thus susceptible to brittle fracture and cannot be used in many wear applications where the loading intensity is high. In such cases the alternatives are either to use a more ductile bulk material coated with a hard wear-resistant coating, or to use (develop) a more ductile hard solid material. Research today is expanding in both of these directions.
Hard coatings can be produced by a number of techniques which can be divided into two broad categories-film deposition or surface modification. Available techniques for film deposition include plasma and flame spraying, electroplating and fused salt electrolysis, chemical vapor deposition (CVD), and physical vapor deposition (PVD). Surface modification techniques include spark hardening, various laser-induced surface hardening procedures, ion implantation, and various nitriding processes. Each technique has specific applications for which it is best suited and each has specific processing problems.
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