Scratching of alumina in various environments

The purpose of this work was to study the frictional and wear behavior of alumina under scratching in various environments. The scratch tests were performed at loads from 0.5 to 50 N and a sliding velocity of V = 0.4 mm s(-1) during 100 cycles in water, paraffin oil and in laboratory air (humidity similar to 50%). Diamond standard spherical indenter was used as the contrabody. The acoustic emission, friction coefficient, wear losses and failure development in the scratch tracks were evaluated. It was found that the friction coefficient increased during some cycles and then a steady friction state with approximately constant value of the friction coefficient was reached in all environments. The maximal friction coefficient; (f = 0.2), was in contact with paraffin oil, while the minimal value of f = 0.09 was obtained in the wear test with water. The opposite effect was obtained in the determination of the wear rate. The width of the wear track was maximal and minimal in contact with water and paraffin oil, respectively. The trapping and agglomeration of wear particles was observed in the wear track under contact with air and water. Intergranular and transgranular fractures were found to be the dominant damage mechanisms in contact with paraffin oil. Close relation between signals of acoustic emission and friction and wear properties was exhibited under different environmental conditions. The bond between the trapped wear particles and the substrate was analyzed. It is supposed that the trapping of the wear particles is caused by triboelectrification of alumina. This effect was maximal in dry friction. Almost no trapping was observed in contact with paraffin oil. (C) 1999 Elsevier Science Ltd. All rights reserved.