The inﬂuence of grain size and surface treatment on the triboﬁlm formation on alumina components J. Olofsson n , S. Jacobson The Tribomaterials Group, The ˚ Angstr¨ om Laboratory, Uppsala University, Box 534, SE-75 121 Uppsala, Sweden article info Article history: Received 7 January 2012 Received in revised form 3 June 2012 Accepted 6 June 2012 Available online 15 June 2012 Keywords: Triboﬁlm Wear Alumina Friction Nano indentation abstract Ball-on-disc experiments have been performed in order to simulate the friction behaviour of, and triboﬁlm formation on the alumina friction drive system of an ultrasonic motor. The coefﬁcient of friction between the counter surfaces of the friction drive system is important since the driving force of the motor is limited to the available maximum friction force. The experiments have been performed with alumina balls against alumina discs with different surface treatments and grain sizes. The triboﬁlm formation depends on the amount and size of particles generated during wear. Surface treatment by grit blasting contributes to the formation of more extensive and thicker triboﬁlm. The coefﬁcient of friction showed to be substantially higher and more ﬂuctuating with grit blasted surfaces than with the other types. There was no triboﬁlm formation on top of the ground surfaces, however the level of the coefﬁcient of friction was about the same as on the as-sintered surfaces with triboﬁlm, irrespectively of grain size. & 2012 Elsevier B.V. All rights reserved. 1. Introduction There is a constantly growing interest in smaller and more portable products with an increased number of features utilising mechanical movement. Ultrasonic motors are well suited for such portable miniaturised systems due to their smallness and energy efﬁciency. Ultrasonic motors based on the converse piezoelectric effect have a friction drive system that transfers the movement form the piezoelectric elements to the drive rail. The friction drive system commonly consists of alumina (Al 2 O 3 ) due to its inertness, high wear resistance and relatively high coefﬁcient of friction. One particular such motor studied by the present authors has alumina drive pads that transfer the movement from the piezo- electric element by gripping and pushing an alumina drive rail [1]. The maximum available driving force of this kind of motors is limited by the friction force, thus the coefﬁcient of friction between the counter surfaces should be high. When rough alumina slides against alumina, the grains become polished or crushed, producing a lot of wear debris. This debris agglomerate, become pressed together and sinters to a triboﬁlm on the surfaces [2–10]. The wear debris is usually generated from both surfaces [11]. The triboﬁlm ﬁlls up cavities, covers grains and smoothens out the topography. Earlier studies have indicated a correlation between the build-up of the triboﬁlm and an increase in friction [1]. The wear rate of the alumina surfaces typically increases with increasing grain size [12–15]. When wear leads to triboﬁlm formation, the triboﬁlm tends to protect the surface from further wear [7, 11]. This study evaluates how the grain size and various surface treatments of the alumina surface affect the triboﬁlm formation and hence the coefﬁcient of friction. The purpose is to ﬁnd the best surface to achieve high driving force and long life for ultrasonic motors. 2. Experimental 2.1. Materials Alumina ceramics (96% a-Al 2 O 3 ) of three different grain size ranges (1–5 mm, 1–10 mm and 5–10 mm) were included in the present study. The micro hardness of the three types was measured to 16.5, 16.5, and 18 GPa, respectively. These values are mean values of 30 Vickers indentations with a load of 2000 g. The indents were approximately 6.7 mm deep and about 50 mm wide, which means that they can involve numerous grains and grain boundaries. All three types were tested tribologically as-sintered, without any surface treatment. In addition, four discs of grain size 5–10 mm were surface treated in four different ways prior to the tribological testing. The surface treatments were coarsely grit blasting with 200 mm SiC particles, ﬁne grit blasting with 40 mm sand, coarsely grinding with 320 grit SiC grinding paper and ﬁne grinding with 800 grit SiC grinding paper. The surface roughness values of the various surfaces are presented in Table 1. The as-sintered version of grain size 5–10 mm showed a roughness of Ra ¼ 0.83 mm. The coarsely grit Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear 0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.06.004 n Corresponding author. Tel.: þ46 70 259 85 17; fax: þ46 18 471 3572. E-mail address: johanna.olof@gmail.com (J. Olofsson). Wear 292–293 (2012) 17–24