A new type of ZrO_2 coating doped with SiO_2 was investigated and found to posses good adhesion with the substrate, and long lifetime with respect to thermal shock and even "self-closing-pore" effect. The thermal stresses in this coating are easy to release. The (ZrO_2-SiO_2) coating can effectively be upgraded by hot isostatic pressing (HIP) or laser remelting treatment and the coating so treated is rarely cracked. The interface between the ZrO_2 coating and the substrate (bond coat) formed by plasma spraying was found not to be a mechanically bonded one, but a zone with a certain thickness and composed of several phases or structural constituents. In some regions, interfacial phases formed by ceramic-metal reaction were observed, showing locally metallurgical bonding nature. The metallurgical bonding becomes more obvious, when the coating was subsequently subjected to HIP or laser remelting treatment. TEM revealed that liquid sintering occurred within the ZrO_2 powder particle during spraying. Within the particles, melting took place along the grain boundaries. Stress-induced martensitic transformation was also observed in the ZrO_2 coating. The bonded layer of plasma spray-welded Co-Cr alloy on 0.45% carbon steel was observed to consist of both multi-phase and single phase (decarburized) sublayers.
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