As shown in the previous section, the significant sintering of APS La2Zr2O7 coating occurred at the exposure temperatures of 1200℃and 1300℃. Such sintering effect leads to the densification of the coating,i.e, reduction of porosity. According to the previous studies, porosity of the coating has great effect on both the mechanical properties and thermo-physical properties of the ceramic coatings . Accordingly, the change of porosity during thermal exposure eventually results in the change of coating properties. Moreover, as has been well known, the limited bonding ratio between splats is a main factor that results in the low thermal conductivity and stiffness of APS TBCs. Therefore, the bonding ratio evolution during thermal exposure was also examined in this study.
Fig.1(a) shows the evolution of the volume shrinkage based on image analysis and the characterization of the size of the samples.For the results obtained from image analysis, after 5 h of thermal exposure at 1200℃, the volume decreased by about 0.6% while during the later 95 h of thermal exposure, the volume further decreased by 1.4%. When the exposure temperature was increased to 1300℃, the volume of the samples decreased more in certain duration. For the volume shrinkage based on the characterization of the sizes of the samples, it has the same tendency. However, compared with the result based on image analysis, the volume shrinkage increases faster during the first 5-20 h of thermal exposure and becomes lager after certain exposure duration. That may be resulted from some partially-melted particles, which have a loosen structure with some nanoscaled pores, which could be hardly observed at a magnification of 1000x. Accordingly, the apparent porosity, which was obtained from image analysis, of the coatings is actually lower than the real value. Obviously, there are more nanoscaled pores in the as-sprayed coatings compared with that after thermal exposure.
Fig.1(b) shows the evolution of the bonding ratio during thermal exposure, compared with the increase tendency of the r volume shrinkage during the first 5 h of thermal exposure, the bonding ratio increased more significantly from 0.48 to about 0.85.Then, after thermal exposure for another 95 h at 1200℃, the bonding ratio increased to 0.9. As the exposure temperature in-creased to 1300C, both the relative density and the bonding ratio increased to a higher level than 1200 C during certain duration.
Fig.1. Change of the volume shrinkage (a) and the bonding ratio(b) of the coating during thermal exposure.
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