3D AMPLITUDE PARAMETERS FOR THE WEAR SCARS RESULTED AFTER TESTING ADDITIVATED RAPESEED OILS ON FOUR-BALL TESTER

Abstract

This paper presents an analysis of several 3D amplitude parameters of the texture, measured or calculated for the entire wear scar of the stationary balls used in four-ball tester: Sa, St, Sv, Sp, Ssk and Sku. The authors investigated balls tested in rapeseed oil, rapeseed oil + 1% TiO2 and rapeseed oil +1 % ZnO in order to establish the influence of the testing regime (sliding velocity and force on the main shaft of the tester) on the surface quality at the end of the test, by the help of several amplitude 3D parameters. Qualitatively, the wear process is uniform on the balls in one test, because the spread ranges of the discussed parameters are small enough. At low force (F=100 N), corroborating the values of the studied amplitude parameters, it could be concluded that wear processes take place on the peaks of the texture. The Sa parameter is not affected by additivation for v=0.38 m/s and forces from F=100 N to F=300 N. At high sliding velocity, a slope change of both parameters is observed between F=200 N and F=300 N. If the wear is approximately the same at the test end for F=300 N, the roughness of the wear trace increases greatly when the rapeseed oil is additivated, meaning that nano particles of TiO2 agglomerate and participate in the formation of a rougher texture. Lubricant with ZnO keeps the average roughness Sa close to that obtained with non-additivated rapeseed oil. In order to assess the quality of a surface as close as possible to reality, using 3D parameters, the profilometer pitch should be as fine as possible and the area of investigation as large as possible. However, a trade-off has to be done between the step size, the investigation area and the time to fulfill the study. The Sku parameter shows if there are large isolated peaks and Ssk indicates if the peak frequency is high. For the case of worn surfaces, the values of the parameters Ssk, Sku, St, Sp and Sv are of interest, because high peaks affect the tribological parameters. The surface quality when additivated with 1% TiO2 is much better up to F=300 N. Additivation with 1% TiO2 is recommended for low loads, F=100 N and F=200 N, for point contacts. The study should be completed with the same investigation in a severe regime where nano-additives should improve the seizure limit of load.