A study on HVOF coatings of micron and nano WC–Co powders

Abstract

High velocity oxy-fuel (HVOF) spray coating of micron (m) and nano (n) WC–Co powders has been studied for the improvement of durability of sliding machine components (SMC). In this work, optimal coating process (OCP) is obtained from the best surface properties of coating prepared by the Taguchi program. Hardness of coating is strongly dependent on powder size and spray parameters (SP) because of their strong influence on in-flight parameters. Hardness of n WC–Co is lower than that of m WC–Co since the degree of hard WC decomposition to less hard W2C, W and graphite is larger due to the larger specific surface area. Coating is porous since the decomposed graphite forms carbon oxide gasses by reaction with excess oxygen, and the gas evolution from coating makes porous coating. Porosity of n WC–Co coating is larger than that of m WC–Co because of larger evolution of carbon oxide gasses through n WC–Co coating. Friction coefficient (FC) is strongly dependent on the coating process (CP) since hardness and porosity of coating are dependent on the CP. FC of n WC–Co is lower than that of m WC–Co both at 25 °C and 500 °C, because of the more decomposition of n WC–Co. FC increases with increasing coating temperature (CT) from 25 °C to 500 °C both at m and n WC–Co because of the increase of adhesion by increasing surface temperature. WC–Co coating is very protective for the machine component since hardness of the coating is 2–3 times higher than those of machine component materials. Stick friction on WC–Co coating surface occurs easily at higher temperature due to the higher FC at the higher temperature.