Surface wear silicon nitride

In fact Van Wyk ° used something similar to a composite structure for the lubrication of silicon nitride and alumina in plain spherical bearings. He incorporated a 90% molybdenum disulphide/8% molybdenum/2% tantalum compact in holes drilled in the surface of the alumina outer ring, and the details have been described in Chapter 8. The system was very successful, giving a forty times increase in wear life. 

Disks with an abrasive layer of resin-bonded or metal-bonded diamond grains are used to process ceramic surfaces because of their excellent cutting quality, their low rates of wear, and the ability of diamond to hold its edge. Disks with a synthetic resin bond are suitable for hard materials, such as silicon nitride ceramics and sialon ceramics. Metal-bonded grinding disks are used for oxide ceramics and silicate ceramics. The type of bond on the disk helps determine the surface quality of the polished section. 

Figure 27 Raman spectra of the carbon deposited on the wear scar of the silicon nitride plate in the experiment referred to in the text (a) 180 sec run time (b) 240 sec run time [60]. (Reproduced from Lubrication Engineering, 49, Vlcek, B. L., et aL, Lubrication of ceramic contacts by surface-deposited pyrolytic carbon, pp. 463-471. Copyright 1993, with permission from Society of Tribol-ogists and Lubrication Engineers.)...

Figure 28 Comparison of Raman spectra of surface carbon produced by ethylene on a metal (Ni3Al), lower spectrum) and a ceramic (Sialon, upper spectrum) [61]. (Reproduced from Applied Spectroscopy, 50, Lauer, J. L., Wear-induced changes in the Raman, infrared, and fluorescence spectra of silicon nitride surfaces, pp. 1378-1388. Copyright 1996, with permission from Society for Applied Spectroscopy.)...

The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

To increase the wear resistance of surfaces, silicon and metals are often coated with a hard nitride, carbide, boride, or oxide film. Nanoindentation and fracture simulations have been used extensively to elucidate failure mechanisms of these typically more brittle surfaces, which include crack propagation and film delamination. Considerable attention has also focused on nanocomposite materials, which possess nanocrystalline inclusions in an otherwise amorphous matrix. The nanocrystalline component is sufficiently small to preclude the formation of stable dislocations, and thus provide a higher hardness. 

Abrasive wet cutting, using metal or bakelite abrasive cut-off wheels impregnated with alumina, boron nitride, silicon carbide, or diamond, is most appropriate and gives rise to minimum deformation of the cut surface. Wheels may be distinguished as hard or soft depending on their ability to retain or discard the abrasive particles. In general, the harder (more difficult to cut) materials require softer wheels because these provide a continuous supply of new abrasive particles to the surface (but at the same time wear away more quickly). 

A number of other ceramics have been subjected to biomedical tests for implantation, without ciurently being developed industrially. Among these ceramics, we can cite silicon carbide, titanimn nitrides and carbides, and boron nitride. TiN has been suggested as the friction surface in hip prostheses. While cell culture tests show a good biocompatibility, the analysis of explants shows significant wear, related to a delaminating of the TiN layer [HAR 97]. Silicon carbide is another modem day ceramic which seems to provide good biocompatibihty and can be used as bone implant [SAN 98]. 

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