Investigating the effect of surface coatings on the wear behaviour of the friction contact in ultrasonic motors

Nowadays, ultrasonic motors (USMs) are used widely in a large variety of applications such as camera auto-focus mechanisms, devices in automobiles, industrial servo control in precision positioning, intelligent robotics, ultra-precision measuring, and medical equipment. USM is driven by traction force at high-frequency oscillations (>40 kHz) and the characteristics and life of the motor are strongly related to the friction material used in the tribological contact. In this study, two purposely built test rigs, including aUSMand a high-frequency (up to 4000 r/min) reciprocating machine (HFRM), were developed. The reciprocating machine ran with a small amplitude and a high-frequency motion was used to examine the wear behaviour of the friction pairs. Different materials such as advanced surface coating (chromium titanium aluminium nitride, and diamond-like coating) on stainless steel substrates were used to investigate their combined tribological performance with the aim to be used in the USM context. A matrix screening test for material selection purpose were also carried out using the HFRM and the tribological performance of the chosen material pairs were subsequently investigated using the USM. The ranking results of the material pairs from the USM tests agreed with those from the HFRM tests. HFRM results could correlate well with the USM results by considering energy input into the wear system. Thus, HFRM can be used as a screening tool for selecting materials using in USMs.