Ceramic and Inorganic Composites

He went on to say that the main problem is that ceramics wear too slowly to release sufficient solid lubricant. This is certainly a major problem, but another important difficulty is what might be called a mis-match between the temperature capabilities of the ceramics and of the more common solid lubricants. The greatest value of ceramics is for temperatures above 350°C, where steel loses its hardness, many non-ferrous metals become very soft, and only a few polymers can be used even for very short periods. Unfortunately this is also the temperature at which even the most stable of the common solid lubricants have limited lives except in vacuum or inert atmospheres. 

As a result, those workers who are trying to develop self-lubricating materials for these high temperatures are looking at much more exotic and much less efficient lubricants, such as the calcium fluoride/barium fluoride eutectic and silver used in the NASA PS200 and PM212 high-temperature composites  

One early report described the friction and wear performance of a high-temperature compact containing molybdenum disulphide, silica, lead oxide, silver and 

Gangopadhyay et al investigated the use of graphite intercalated with nickel chloride NiClj to lubricate silicon nitride or alumina sliding against a steel counterface. The lubricant was quite effective, reducing the coefficient of friction from 0.5 to 0.17 for silicon nitride, and from 0.55 to 0.18 for alumina. There is no obvious reason to expect that the behaviour of molybdenum disulphide would be significantly different. 

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. 

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