Tribological evaluation of soft metallic multilayer coatings for wear applications based on a multiple pass scratch test method

The multiple pass scratch test method was employed for characterising the tribological performance of soft metallic multilayer coatings under the linear sliding wear situation. For this purpose, three different structures of tin (Sn) based coating (<15Hv0.03) were produced by electroplating process on relatively hard metallic substrates (117Hv0.1). The coating structures studied include a benchmark monolayer coating with soft Sn matrix, and two multilayer coatings with 3-layers and 5-layers of soft Sn matrix and hard SnNi intermetallic sublayer (s). The test method involves sliding a 100Cr6 steel ball against the Sn coated surface under loaded condition for a certain length and then repeating the process 50 times in a unidirectional cyclic manner. Various loading conditions were applied to identify the correct load in order to eliminate the influence of surface roughness of coating and hardness of substrate on the tribological results. Finally, load of 10 N allowed the ball probe to remove a small amount of the coating material in each scratch cycle and differentiate between the characteristic friction and wear behaviour of different coating structures layer-by-layer. Post-test surface and cross-section of worn coating regions were analysed, and the scratch and Vickers hardness of fresh coating surfaces were measured, to correlate the tribological behaviour with the respective coating structure. Results clearly showed the beneficial effects of the lubricious intermetallic sublayer within 3-layers and 5-layers Sn coating structures. The improvement in performance was even higher with lesser thickness of sublayers in 5-layers structure. Apparently, the hard intermetallic sublayer strengths the multilayer structure and at the same time enhances its wear resistance due to increase in the overall hardness of coating. Therefore, the multiple pass scratch test method successfully compared the tribological performance of soft Sn based coatings under the carefully selected test conditions.