Sintering process changes the microstructure of the coating and eventually affects the properties of the coatings. The present results evidently suggest that the properties of the coating are not only affected by the porosity, but are affected more significantly by the morphology of the pores in the coating. In the first 5 h of thermal exposure at 1200℃, the shrinkage of the coating is about 0.6% and during the later exposure duration for another 95 h, the shrinkage reached to 1.4%, which is nearly 2.5 times of the shrinkage occurred in the first 5 h thermal exposure. However, it can be found that the mechanical properties and thermal conductivity change in a different fashion. For example, as shown in Fig.1(a), the microhardness increased by 244 HV from 355 HV to 599 HV during thermal exposure in the first 5 h, and in the later duration for another 95 h exposure, the microhardness increased by only 79 HV from 599 HV to 678 HV. The results indicate that the increment contributed by the later 95 h of thermal exposure is only one third of that achieved in the first 5 h. Therefore, the change of coating properties cannot be explained by the change of porosity or density.
Fig.1.Change of thermal conductivity of APS La2Zr2O7 coating measured at 1000℃with thermal exposure duration at 1200℃ and 1300℃.
Although the change of both the shrinkage and density of the coating is associated with the change of porosity during thermal exposure, no information on pore geometry is included in all these parameters. According to the results reported by Wang et al.the shape of the pores has significant effect on the thermal conductivity of the coatings. As shown by Figs.2 and 3, interface bridging changes the shape of the long inter-lamellar 2D pores.which leads to the increase of the inter-splat bonding ratio. It is evident that the evolution tendency of the properties of the La2Zr2O7 coatings during thermal exposure is consistent with that of the bonding ratio during thermal exposure.
Fig.2.Typical morphology of pores in as-sprayed La2Zr2O7 coating and La2Zr2O7 coating after thermal exposure at 1200℃ for 5h.
Fig.3.Topography of the La2Zr2O7 splats before and after different thermal exposure treatments.
The bonding ratio was defined as the ratio of the bonded interface to the total interface between the splats. According to previous studies, the bonding ratio of the as-sprayed ceramic coatings is usually less than 1/3. It is clear that after thermal exposure at 1200℃ for 5 h many grain bridges arise in the inter-lamellar 2D pores which increase inter-lamellar bonding. The bonding ratio of the coating after thermal exposure was estimated.
For APS La2Zr2O7 coating under thermal exposure at 1200℃, the bonding ratio increased by a factor of 77% from 0.48 to 0.85 in the first 5 h thermal exposure and then increased by further 5.9% to 0.90 in the later 95 h of exposure duration. On the other hand. the volume shrinkage of the coating only increased by a factor of ~0.65% at the first 5 h of exposure and further by 0.15% with the further 95 h exposure. It is clear that the density increment at the later 95 h of exposure is ～2.4 times of the first 5 h in comparison with ～0.136 times for the bonding ratio. Therefore, the effect of sintering causing rapid increase in the properties of APS La2Zr2O7 coatings at the early exposure at high temperature is mainly attributed to the healing of unbonded inter-splat interfaces, leading to rapid increase of the lamellar bonding ratio, by their rapid bridging rather than density change due to sintering.
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