PHASE TRANSITIONS IN THERMALLY SPRAYED CHROMIUM CARBIDE COATINGS

摘要

Thermal spray technologies are commonly used to apply surface coatings to enhance the properties of a substrate material, such as wear and corrosion resistance. Chromium carbide coatings are frequently used to mitigate wear and erosion at elevated temperatures, with typical spraying techniques aiming to melt the metallic binder but not the carbides themselves. However, dissociation of carbides can still occur during spraying, reducing the carbide concentration, the quality and the wear resistance of the final coating. Through heat treatment of the coatings, precipitation of carbides has been seen to occur in areas of carbide dissociation, increasing the hardness and wear resistance of the coating. Therefore, instead of trying to minimize the amount of carbide dissociation, this report investigates exploiting the precipitation hardening phenomenon that occurs in regions of high carbide dissociation when the coatings are heat treated. Two different spraying techniques were investigated: High Velocity Oxygen Fuel (HVOF) and Plasma spraying. HVOF uses high velocities but relatively low temperatures to achieve a dense coating with a limited amount of carbide dissociation. Plasma spraying uses lower velocities but much higher temperatures, significant enough to melt any known material, to produce a coating with high levels of dissociation and supersaturation. This investigation aims to determine the critical temperatures at which phase transitions occur in Cr3C2-NiCr coatings using these two different spraying techniques. This in turn determines the minimum heat treatment temperatures required to achieve equilibrium structures of the coatings and exploit precipitation hardening effects in industry. Differential Scanning Calorimetry was used for both coatings and the starting powder to determine the temperatures at which phase transitions occur. Following this, the coatings were heat treated to below and above each phase transition for one hour. Scanning Electron Microscopy, X-Ray Diffraction and Rietveld analysis were used to determine the phases present at each heat treatment temperature. The experimental results concluded that both the HVOF and plasma sprayed coatings experience at a phase and microstructure change around 550℃, while the plasma sprayed coating experiences an additional phase and microstructure change at 700℃. The first phase change was determined to be the crystallization of supersaturated Ni, causing Cr3C2 to precipitate out of solution. The second phase change was due to the precipitation of Cr3C2 out of the metastable (Cr,Ni)7C3 complex. This complex is only seen in the plasma sprayed coating due to the high levels of carbide dissociation achieved through spraying, therefore the HVOF coating does not experience this phase change. As a result, a lower heat treatment temperature is required for the HVOF coating to achieve an equilibrium structure. However, while the required heat treatment temperature is around 150℃ higher, more precipitation is achieved in the plasma coating. Therefore, the heat treated plasma coating may have better wear resistance properties compared with the heat treated HVOF coating.