EFFECTS OF SURROUNDING ATMOSPHERE ON THE WEAR OF SINTERED ALUMINA

The dry-sliding wear of polycrystalline sintered alumina was found to increase by nearly one order of magnitude when the relative humidity (RH) of the surrounding air decreased from 50% to 10% RH and by a further order of magnitude as the humidity decreased still further down to 4% RH. Increasing the humidity above 50% RH had little effect on the amount of wear. While intergranular fracture is present at all humidity levels, below 50% RH transgranular fracture became increasingly common. At humidities of 10% RH and above wear was reduced by the formation of a film of material with reduced aluminium content, possibly hydrated alumina. Below 10% RH the film was not formed, resulting in the large wear rate observed at these low humidity levels. Another possible reason for the reduction in wear is the facilitation of plastic flow by the increase in humidity. This may have accommodated the contact shear stresses so that they were reduced below the level at which transgranular fracture was initiated. These effects have important consequences for the wear testing of alumina and for the design of components for antiwear applications.