Abstract
Thermal sprayed WC-Co coatings are widely used for various industrial applications due to their high hardness and corresponding wear resistance. Recently, it has been reported by many researchers, that the use of agglomerated and sintered micron powders with submicron or nanosized carbides can provide the deposition of WC-Co coatings with enhanced or even superior mechanical and tribological characteristics. This can only be achieved, as long as optimized coating conditions adapted to the specific thermo-kinetic behavior of such powders are considered. However, the porosity in the coating morphology represents an inherent problem when using powders with conventional agglomerate size (10-50 µm) and high internal porosity. Consequently, a minimum coating thickness is often necessary to provide suitable wear properties, which reduces the shape or dimensional accuracy when applying such coatings to complex surfaces. In addition, a reduction in surface roughness of the coating cannot be accomplished by fine carbides, since large agglomerates are employed. In this study we used two different fine WC-12Co powders in the HVOF process to manufacture nanostructured coatings with high hardness, moderate toughness, low surface roughness and low porosity. The first powder is a fine agglomerated and sintered powder with particle size of -10+2 µm and carbides in the ultrafine range (400 nm) The second one consists in two loose mixtures of fine Co (Fisher grain size FSS = 3.5 µm) with (a) WC (FSS = 3.0 µm) and (b) WC (FSS = 0.8 µm). Statistical design of experiments (DoE) were utilized to determine main effects of spray conditions on coating properties. Mechanical properties, microstructure and the phase development has been correlated to the in-flight particle behavior. Phase analyses were performed by XRD using synchrotron radiation.
