Dry Sliding Behavior of Qbe-2 Beryllium Bronze against 38CrMoAlA Steel in Pneumatic Downhole Motor under Different Loads

Abstract

In drilling engineering, the wear of tribo-pairs is the primary cause for the rapid failure of rotating seals in pneumatic downhole tools. In order to reduce the wear of tribo-pairs, a new type of rotating seals was designed in this work, which introduced copper alloys between the stator and rotor. To elucidate the wear and failure mechanism of the copper-steel tribo-pair rotating seals in pneumatic downhole motors, pin-on-disc dry sliding tests with Qbe-2 beryllium bronze pin against 38CrMoAlA steel disc under different loads were thus designed to simulate the friction and wear behavior of such tribo-pair. During the dry sliding process, the friction behavior of the copper pin would go through a running-in period and then become stable. As the load increases, the running-in period will be shortened, while the friction coefficient during the stable period decreases. Interestingly, a false stability occurs when the load is low. However, this phenomenon will disappear under heavy loads. The wear mechanism of the copper pins would change from adhesive wear to ploughing wear as the load increases, which is mainly related to the spalling of asperities and the filling of wear debris into the steel disc. The wear debris consists of copper and copper oxide. The surface roughness of the steel disc and copper pin decreased and the size of the wear debris increased with the increase of the load. The material removal mainly occurs on the copper pin, which will present a relatively small value under 45 N. On contrast, due to the filling of wear debris, the volume of the steel disc increased. Therefore, considering the value and stability of friction coefficients, as well as the wear amount of the sample, it would be better that such tribo-pair could work under 45 N. The present work will provide a fundamental understanding and solid support for systematically designing the tribo-pairs in pneumatic downhole tools under practical working conditions.