Elevated Temperature Fretting Wear Behavior of Cobalt-Based Alloys

Structural parts in gas turbines undergo fretting wear, and as clearances open up, it turns into impact wear. Uncoated cobalt-based substrates that are used in such applications show poor resistance to fretting/impact damage and undergo extensive/unacceptable level of degradation. Commonly used substrate materials such as uncoated cobalt-based materials show poor resistance to fretting/impact wear. This study is focused on assessing the performance of high velocity oxyfuel (HVOF) tungsten carbide (WC) coatings and sintered tungsten carbide inserts as potential solutions for mitigating this issue. Sintered WC12Co with grain size range from 0.2 μm to 4.5 μm and HVOF coating with composition of WC–17%Co was tested. It was found that the HVOF coatings performed better than sintered material and the behavior was attributed to the hard WC particles surrounded by the higher volume fraction of cobalt binder. In the HVOF coating, the normal load was better accommodated by the decarburized WC but a fairly tough binder-surrounding matrix. An additional factor is that the sintered WC had significant volume fraction of undesirable W₂C phase, which apparently underwent fracture during the test, thus showing an inferior behavior compared to the HVOF WC coating.