Microstructure changes of ZrO₂/W/M_o coating layers on graphite after heat treatment at 2100 °C

A tungsten coating was deposited onto a graphite substrate using the atmospheric plasma spraying (APS) technique. In order to increase the adhesion strength between the metallic tungsten(W) and graphite, a molybdenum (Mo) interlayer was pre-deposited onto the graphite surface by utilizing the APS technique. Also, after deposition of a APS-W coating, a zirconia (ZrO₂) was deposited onto the W coating layer. For the APS process, argon and helium were used as the plasma-forming gases, and argon was used as the shield gas to protect the plasma from oxidation. After the APS coating process, heat exposure treatment was performed at 2100 °C for 360 h within a sapphire single crystal-growing furnace in order to evaluate the thermal stability of the coatings. After heat treatment, the ZrO₂/W/Mo coating layers were bound with the graphite without any peeling off. The microvickers hardness of the APS-W coating layer was increased after heat treatment due to the formation of carbides. Also, carbide phases such as Mo₂C, WC, ZrC and MO₃C₂ were identified by XRD diffraction and EDS analysis, by analyzing the depths below the coating surface. It was considered that the Mo interlayer served as a good buffer layer between the APS-W coating and the graphite after the heat exposure treatment because the lattice structure of the molybdenum carbide was similar to that of the graphite.