A Fundamental Study of Synthetic Sapphire as a Bearing Material

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Abstract
Some experiments are reported on the sliding behavior of synthetic sapphire against certain fairly pure metals as a function of temperature. The initial selection of the sapphire-metal couple was determined on the basis of thermodynamic and physical consideration so as to produce, theoretically, a weak interfacial contact, region. It was observed that this basis for selection of desirable sapphire-metal couples needed to be modified to include the effect of the surrounding atmosphere. For example, the couple sapphire-nickel should exhibit theoretically a low wear rate, weak adhesion and low friction. It was found, however, that the adhesion effects for this couple were highly sensitive to the presence of oxygen, and unless the oxygen was completely excluded, high friction and metal transfer occurred. The couple sapphire-gold, on the other hand, exhibited low friction and metal transfer regardless of atmosphere and elevated temperature. Hence, it would appear that the resistance to oxide formation by the metal is an important criterion for good sliding characteristics in metal-ceramics couples in sliding contact. Experiments on the sliding characteristics of these couples as a function of both high and low temperatures are included and discussed.