To evaluate coating reliability at high temperature, thermal cycling tests were conducted at 1100 ◦C in a horizontal muffle tube furnace. The thermal cycling was continued until detachment failure over 10% of the area was observed. As illustrated in Fig. 1, one piece begins to peel off at the rim within 13 cycles at 1100 ◦C. Usually, spallation is much easier to achieve at the bevel edge because relatively large residual stress concentrates there in comparison with the center of coating. Upon 28 cycles at 1100 ◦C, the coating appears to have experienced thermal cyclic fatigue failure, exhibiting a much shorter lifetime than expected. As indicated by SEM micrographs in Fig. 2(a)-(b), a large horizontal crack exists near the bond coat/top coat interface, detaching the YSH ceramic coating from substrate and eventually causing the spallation failure. In fact, the coating shows a failure mode akin to other APS coating such as La2Ce2O7 and 8 YSZ. Phase transformation, thermal expansion mismatch, sintering and/or thermally-grown oxide (TGO) growth are possible contributing factors to the thermal cyclic failure of a coating. In failed 18YSH coating, the TGO is apparently too thin to drive delamination of the ceramic coating. Below we discuss the possible roles of phase stability and thermal expansion behavior in the failure mechanism of 18YSH coating. Fig. 3illustrates a TG-DSC curve which presents thermal response of plasma sprayed 18YSH coating from room temperature to 1250 ◦C. It reveals that no phase evolution occurs in 18YSH coating, leading to a good phase stability below 1250 ◦C. Sintering and the resultant thermal expansion behavior may also be responsible for thermal cyclic failure.
Fig. 1. Surface photographs and microstructures of 18YSH coating after thermal cycling.
Fig.2. Cross-sectional morphology of the 18YSH ceramic coating after thermal cycling.
Fig. 3. TG-DSC curve of the 18YSH coating from room temperature to 1250 ◦C.
They are both assessed in Fig. 4(a), where it can be noted that the 8 YSZ coating starts to sinter above 1200 ◦C. In contrast, the 18YSH coating shows no signs of sintering at temperatures as high as 1300 ◦C, well supported the fact that the 18YSH coating has an improved sintering resistance in comparison with 8 YSZ coating. The low degree of sintering excludes the possibility that sintering leads to 18YSH TBC spallation at 1100 ◦C. It is interesting to note that, there is an abnormal volume expansion (~1.16%) after non-isothermal treatment from 1300 ◦C to 1500 ◦C. XRD has been performed on the sample recycled from this thermal expansion determination. As illustrated in Fig. 4(b), almost half of cubic phase has converted into m-HfO2 within the 30 min heating from 1300 ◦C to 1500 ◦C. Thus as-sprayed 18YSH coating is not stable at temperatures above 1300 ◦C. This phase transformation, might be related to Y2O3 diffusion. It is generally believed that the APS method tends to develop a coating with a metastable phase during rapid quenching. An 18YSH coating that crystallizes into a metastable state may experience stronger Y3+diffusion if temperature activated above 1300 ◦C. This phase transformation and sharp volume change might be responsible to a microstructure development, special residual stress distribution and even thermal cycling failure. Thus the 18YSH coating itself is limited to applications below 1300 ◦C.
Fig. 4. Dilatometric measurement of the plasma-sprayed 18YSH coating (a) and corresponding XRD pattern after TEC measurement (b).
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