It is usually difficult to deposit dense ceramic coatings with splats well bonded by plasma spraying at room temperature. Following the recent research progress on the splat interface bonding formation, it was found that there is a well defined relationship between the critical bonding temperature and materials melting point. Thus, it was proposed to control the lamellae bonding through the deposition temperature. In this study, to examine the feasibility of the bonding theory, a novel approach for the development of coating microstructure through materials design is proposed. Accordingly typical ceramic materials were selected of relative low melting point for plasma spraying of dense coating with well bonded splats. The experiment was conducted by using K 2 Ti 6 O 13 for splat deposition at ~110°C cooling down from a higher temperature to avoid substrate adsorbates and coating was deposited at room temperature in ambient temperature without substrate preheating. Results show that the splat is fully bonded with a ceramic substrate, while the coatings present a dense microstructure with a similar fracture morphology to sintered bulk ceramics. Moreover, the erosion test at 90° further confirmed the formation of a ceramic coating with lamellae fully bonded.

The corrosion resistance of thermal barrier coatings against CMAS deposit at high temperature is significantly affected by the microstructure of the coatings. Enhancing the bonding ratio between splats can reduce the inter-connected pores and then obstructs the penetration of the molten CMAS into the coatings. In this study, atmospheric plasma sprayed ZrO 2 contains 8 wt. % Y 2 O 3 (8YSZ) coating with improved lamellar bonding ratios was deposited with full-molten droplets at an enhanced deposition temperature. The microstructure of the dense 8YSZ coating and conventional 8YSZ coating before and after thermal exposure with CMAS were characterized. It was clearly revealed that by adjusting the microstructure and designing a ceramic layer with high bonding ratio, the corrosion resistance of the thermal barrier coating could be enhanced. Moreover, by designing double-ceramic-layer (DCL) TBCs composed of a porous ceramic layer and well-bonded ceramic layer, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

Thermally sprayed coatings exhibit a lamellar structure with a bonding ratio less than 32% when a coating is deposited at ambient temperature. The lamellar bonding is one of the most important factors controlling the mechanical, thermal and electrical properties of the coatings. However, it is not clear why only limited lamellar bonding exists in a thermal spray coating even though many studies have been focused on the formation of bonding. In our previous study, it was found that there exists a critical deposition temperature for depositing ceramic splats to form the bonding with the underlying identical substrate, i.e., critical bonding temperature. Moreover, the critical bonding temperature is related to the interface temperature prior splat solidification which is determined by the glass transition temperature of splat material. In the present study, the critical bonding temperature and its relationship with interface temperature are used to understand the limited lamellar bonding ratio in a coating. A numerical simulation model involving heat transfer among depositing splat was proposed to establish the sufficient condition for liquid splat to form the bonding with the underlaying splats. The non-uniform splat thickness model was established to calculate theoretically the interface bonding formation. The calculation based on the model yielded a bonding ratio of 38.5% which agrees reasonably with the observed maximum interface bonding ratio.