Abstract
Gate valves used in oil and gas production undergo stringent qualification before going into service. During qualification there is no external lubrication, leaving contact surfaces susceptible to friction evolution and wear. The work presented in this paper was carried out to better understand the changes that can occur during qualification and where and when the limit for mild wear and stable friction is reached. Ni-Cr alloy gate valve components were coated with WC-CoCr by HVOF spraying and dry sliding wear tests were conducted in nitrogen and in air. The coatings were then evaluated by means of SEM, EDX, and XRD analysis, nanoindentation and surface roughness measurements, and compression tests on micropillars milled out by FIB. Similar tests and analyses were performed on gate valves returned from the field. Examination of the valves that had been in service revealed the presence of oxygen rich layers on polished surfaces due to opening and closing of the gate. Such layers were also observed in coating samples following tribological testing. Initial surface roughness was found to play a role in the development of the oxygen rich layers as well as friction evolution.
