Understanding the influence of micro- and sub-micro structural features on the mechanical properties of HVO/AF sprayed WC-CoCr cermets

HVOF processes represent the state of art for the spray deposition of wear and corrosion resistant coatings since their supersonic gas velocities in combination with moderate flame temperatures allow the deposition of optimal coatings with very high bond strengths, fine surface finishes and low oxide levels. However, new generation coating materials (fine powders), stringent quality requirements and the high productivity demanded by the industry, push the HVOF technologies to their limits. Recently, a novel air-oxygen controlled high velocity combustion process has been development by Tecnalia. The system operates within the supersonic regime using a broad range of fuel/oxidant ratios thanks to the use of air-oxygen mixtures and a carefully optimized gun design. Extremely low flame temperatures can be achieved while keeping a supersonic flow of combustion products, thus allowing the solid state deposition of almost all industrially relevant metal alloys with superior deposit qualities. In this work, a systematic investigation of the influence of the powder particle size and gun configuration on resulting coating microstructural features has been performed. For comparison, two fine structured commercially available WC10Co4Cr powders with different particle size distributions have been investigated. The coating structure has been characterized with by high resolution SEM cross-section imaging and X-ray diffraction analysis. Resulting coatings are characterized by highly dense structures, a high reterion of the primary carbides, average microhardness of up 1885 HV0.3 and fracture toughness varying between 3 and 7 MPa.m^-1/2 depending on the powder particle size distribution and the process conditions used.