Tribological evaluation of HVOF thermal-spray coatings as a hard chrome replacement

摘要

Hard chrome plating has been used in several different applications in industries that require abrasive sliding wear resistance, such as hydraulic pistons, shafts or bearings. However, the increasing environmental and worker safety pressures on electrolytic hard chrome are leading companies to adopt alternatives. The improvements of the high-velocity-oxy-fuel (HVOF) thermal spray process allow the chromium coating replacement with a comparable or superior surface treatment and are more environmentally friendly. This HVOF process, as a flexible dry-coating technology, avoids high-volume waste streams and enables a flexible choice of coating material for each application. The cobalt-chromium-cemented tungsten carbides are some of the easiest materials to spray and the WC-10Co-4Cr coatings have demonstrated superior performance over hard chrome with regard to mechanical and tribological properties. In this work, this coating has been deposited with a Sulzer Metco WokaJet-400 kerosene fuel spray gun, and the spray conditions have been optimized in order to ensure the best properties of the coatings. The mechanical and tribological properties have been evaluated in coatings sprayed with four deposition conditions that involve different gas flow rates. The most wear-resistant coating is obtained with those HVOF parameters that prevent decarburization of WC particles and, at the same time, allow an adequate agglomerate melting giving a good intersplat adhesion. The results indicate that HVOF-sprayed WC-CoCr coatings are a reliable alternative to electrolytic hard chrome (EHC) in the aeronautical industry to coat landing gear components. In particular, in the dry wear tests, the WC-CoCr coatings outperform hard chrome coatings in wear resistance.