Post spray treatment of plasma sprayed chromium carbide using Spark Plasma Sintering (SPS) technique

Thermal sprayed chromium carbide (CrC) coatings are used successfully to extend component lifetimes in a wide range of wear environments due to their excellent hardness and wear resistance, even at high temperatures or in corrosive environment. The high reproduction of parts concurrent with low cost has made plasma spray one of the best options in producing chromium carbide coatings. Unfortunately, spray parameters that produce good melting of the matrix metal normally will also result in oxidation of the chromium carbide particles, which deteriorates the surface hardness as well as the mechanical properties of the sprayed coating. Spark Plasma Sintering (SPS) process is introduced as a post-spray treatment to eliminated the less desirable carbon-depleted carbide phases. CrC coatings on SS316 substrate is prepared by Atmospheric Plasma Spray (APS) technique, and post-treated by the SPS process. The SPS treatment is carried out in a low vacuum for soak times of 5 minutes and at a temperature ranging from 700 to 1000°C. The microstructure of the samples is examined using the scanning electron microscope (SEM) and the crystalline phases present in the samples are identified by X-ray diffraction (XRD). The result shows that the SPS post-spray treatment has several beneficial effects on the properties of plasma sprayed chromium carbide based wear coatings. Microscopic inspection of polished metallographic sections shows a change in the coating/substrate bonding mechanism from mechanical to metallurgical bonding after SPS, and significant increases in coating hardness are observed. The microhardness and modulus are improved more than 58% at the SPS temperature of 900°C. The XRD analysis shows that, after SPS, the chromium carbide phase is recovered from the oxide phase Cr₂O ₃.