FRICTION AND WEAR OF COATINGS FOR SUBSEA GATE VALVES OPERATING AT HIGH TEMPERATURE AND PRESSURE
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Gate valves for subsea oil and gas operation work under harsh conditions. Proper design ensures the valves functional integrity over the designed lifetime of the valve. The design of the valve includes material choice for protection against wear and corrosion, and functionality (i.e. friction). More specifically, sealing interfaces on the valves are usually coated with thermally sprayed tungsten carbide (WC) based materials to provide wear resistance. In order to assess the design of the valve, rigorous testing of the valves is required according to requirements from authorities and the customers (oil companies). Testing is done at the rated temperatures and pressures of the valve, in gas (nitrogen) and water. In order to understand the underlying mechanisms of friction and wear of coatings used in subsea gate valves a proper investigation of mechanisms acting on actual gate valve components is necessary. During this PhD work, several components from gate valves that have seen actual testing and subsea operation have been analyzed and wear mechanisms identified. Friction and wear of several candidate coatings were evaluated in small scale tribological testing and the acting mechanisms compared to the ones from full scale testing. Five different coatings of WC-CoCr, produced by high velocity oxy-fuel (HVOF), high velocity air-fuel (HVAF) and warm spray (WS) were evaluated. Tribological testing was conducted in controlled atmosphere (nitrogen gas) to best mimic the conditions in actual gate valve testing. The results indicate that testing in nitrogen reduces both friction and wear within the frame of testing. Surface analysis by x-ray photoelectron spectroscopy (XPS) and SEM revealed surfaces covered in tribofilms with large amount of oxide species of WC and Cr. These films were detrimental to the frictional performance, however they may act as a beneficial lubricant for reducing the incidence of galling during the initial sliding cycles (i.e. running-in). The results from this PhD work show that controlling the atmosphere during small scale tribological testing more closely correlates to testing of gate valves in large scale. This PhD work also contributes to the understanding of the role of oxide formation in friction and wear of thermally sprayed WC-CoCr coatings.