Scratching of alumina in various environments L. Rapoport * , I. Lapsker, A. Rayhel Department of Science, Center for Technological Education P.O.B. 305, Holon 58102, Israel Received 9 March 1998; accepted 14 December 1998 Abstract 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, paran oil and in laboratory air (humidity  50%). Diamond standard spherical indenter was used as the contrabody. The acoustic emission, friction coecient, wear losses and failure development in the scratch tracks were evaluated. It was found that the friction coecient increased during some cycles and then a steady friction state with approximately constant value of the friction coecient was reached in all environments. The maximal friction coecient, (f  0.2), was in contact with paran oil, while the minimal value of f  0.09 was obtained in the wear test with water. The opposite eect was obtained in the determination of the wear rate. The width of the wear track was maximal and minimal in contact with water and paran 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 paran oil. Close relation between signals of acoustic emission and friction and wear properties was exhibited under dierent 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 triboelectri®cation of alumina. This eect was maximal in dry friction. Almost no trapping was observed in contact with paran oil. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Scratching; Alumina; Tribo®lm; Brittle fracture 1. Introduction Alumina is widely used in mechanical seals, roller bearings and cutting tools over a broad range of contact parameters [1,2]. The tribological behavior of alumina has been extensively studied [3±15]. Recently the anal- ysis of friction and wear of ceramics was based on in- dentation fracture mechanics concepts [3,4]. However these concepts are generally in poor agreement with the experimental results. The other concept based on the determination of transition from one dominant wear mechanism to another was discussed in [5±11]. Wear transition from deformation-controlled to fracture- controlled removal of alumina was revealed in dierent tests with a change in sliding speeds [5,6], loads [7±9], temperatures [10] and environments [5,6,11]. In view of controversial results in the tribological behavior of ce- ramics, essential attention has been given to the for- mation of tribo®lms [12±15]. For alumina, the reduction of the friction coecient and wear rate was attributed to the formation of hydroxide on the contact surface due to the tribochemical reaction [12]. The agglomeration process of wear particles of alumina was observed at various water vapor pressures [15]. Scratching is usually used to study the mechanisms of abrasive wear and grinding of ceramics. The review of the literature shows that the eect of environment on the tribo®lm formation under scratching of ceramics has not been obtained yet. The main goal was to study the frictional and wear behavior of alumina under scratch- ing in various environments. 2. Experimental procedure In order to determine the eect of original roughness on machining, single-point scratching of alumina in virgin state was used. The experiment was carried out on home-made scratch tester at loads from 0.5 to 50 N and a sliding velocity V  0.4 mm s 1 in water, paran oil International Journal of Refractory Metals & Hard Materials 17 (1999) 111±115 * Corresponding author. Tel.: +00972 3 5026601; fax: +00972 35026619; e-mail:rapoport@barley.cteh.ac.il. 0263-4368/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 9 8 ) 0 0 0 7 6 - 6