Characterization of Co-Ti-C Composite Coatings Formed by GTAW In the carrent study the in-situ fabrication process of a layered metal matrix composite coating was The coating consists of a matrix of cobalt-titanium intermetallics and the reinforcement phase of titanium carbide. The fabrication process is welding was Gas Tungsten Arc Welding (GTAW), conducted using a pre-placed powder mixture of elemental titanium, cobalt, and graphite. Several materials characterization methods including microstructure, Vickers microhardness, dilution, phase formation, wear, fracture toughness were employed to study the coating and coating–substrate interface. The intermetallic phases in the matrix vary from Co-rich phases at the coating surface to Ti-rich compounds near the substrate. The microstructure and microhardness of the samples were surveyed across the cross section. Various samples were used to study the microstructure. Metallographic samples were prepared using standard mechanical polishing procedures and etching in a solution containing hydrofluoric acid (HF), nitric acid (HNO 3 ) and water with volume ratio of 1:6:7. Then, microstructure, hardness and phase structure of samples were investigated using scanning electron microscopy, microhardness measurements and XRD analysis, respectively. The interface is revealed to have a smooth profile, free of any porosity or cracks, with good metallurgical bonding to the substrate. The microstructures of these coating were studied by optical microscopy, scanning electron microscopy (SEM) and X-Ray Diffraction. The specimens were investigated by energy dispersion spectroscopy (EDS) and micro hardness test was used. A relatively uniform hardness in the range of 760- 980 HVN is achieved through a depth of 500m into the coating. The surface hardness of specimen with 40% TiC was near 980 HVN and The hardness gradually decreases to 400 HVN at the substrate interface. The hardness evolution, which is predictable using the Rule of Mixtures, is explained by the fraction of the carbide particles and the type of intermetallic compounds in the matrix. The mechanism of wear was abrasive and adhesive wear. The weight loss of specimen with increase in weight fraction of reinforcement decrease. Mathematical models were developed to predict hardness and dilution percent of claddings. Response surface method (RSM) was used not only for development of the regression models but also for better understanding of the effect of variables. In addition, analysis of variance (ANOVA) was employed to check the adequacy of the developed models. Based on the results of the mathematical models, it was observed that reinforcement parameters can significantly affect the hardness while they have not a significant .
