UNLUBRICATED WEAR OF ALUMINA SLIDING AGAINST STEEL IN VACUUM AND IN AIR

Abstract

Unlubricated wear tests between pure alumina inserts and mild steel discs have been successfully carried out in a high-vacuum, high-speed test rig to characterize the wear behavior of alumina in different test environments. Furthermore, the series of tests in vacuum also demonstrate that the test configuration and conditions used presently can reasonably simulate the sliding conditions on the flank face of a tool during machining, as these could produce surface features that are similar to those seen on the flank face of tools used in actual machining. It has been found that the wear rate of alumina increases with increasing speed in vacuum, but the trend is reversed if the tests are carried out in air. Examinations of the worn alumina surfaces suggest that the dominant mechanism of wear changes with test environment and sliding speed. In a vacuum environment, plastic deformation dominates at low speed while the diffusion of some elements (Si and Mn) from the steel counterface into smeared regions on the alumina becomes more important at high speed. When tested in air, microfracture appears to be important when the speed is low, with plastic deformation of the alumina surface becoming dominant at high speed. However, no smeared region (with diffusion of Si or Mn from the steel counterface) is seen on the alumina surface during the high-speed tests. Transfer of steel materials onto alumina occurred in all three test environments, namely high vacuum, low vacuum and air. When the sliding distance is short, the extent of transfer and area of coverage are almost independent of the test environment; but the influence of the test environment becomes more significant with increasing sliding distance: at the slower speed of sliding, the transfer was more significant in vacuum than in air as a result of increased adhesion at the alumina/steel interface. The wear of steel discs appears to be independent of the test environment in that the same trend of decreasing wear rate with increasing speed is seen both in vacuum and in air. On the other hand, the primarily steel-based wear debris collected appears to be influenced, to a some extent, by the test environment: in air, most of the debris collected are flake-like; in vacuum, ribbon-like cutting chips become more numerous. This further supports the finding that the test configuration and conditions employed presently in the high-vacuum, high-speed test rig can reasonably simulate the condition on the flank face of a tool during machining.