From review of the literature, it was anticipated that TBCs with VPS bond coats would have significantly longer thermal cycle lifetimes than TBCs with APS bond coats, primarily due to the superior oxidation resistance of VPS bond coats. It was also expected that the durability of TBCs with VPS NiCoCrAlYHfSi bond coats would be equal to or superior to that of TBCs with NiCrAlY-type bond coats, especially since the NiCoCrAlY-type alloys do not undergo a significant volumetric increase due to high-temperature phase transformation (Fig.1). Contrary to these expectations, the average lifetimes of TBCs with VPS NiCrAl-1Y bond coats were only 20% greater than those of TBCs with APS NiCrAl-1Y bond coats, and were twice those of TBCs with VPS NiCoCrAlYHfSi bond coats. Even more surprising was the fact that APS NiCrAl-1Y bond coats yielded TBC lifetimes that were a factor of 1.6 greater than those of TBCs with VPS NiCoCrAlYHfSi bond coats.
The results described here suggest that the significant variations in TBC lifetimes may be associated with subtle differences in the oxidation behavior of the NiCrAlY and NiCoCrAlYHfSi bond coats. The oxidation rates of TBCs with VPS NiCoCrAlYHfSi bond coats were slightly higher than those with VPS NiCrAlY, which could be due to small differences in oxide scale growth rates or a larger NiCoCrAlYHfSi coating surface area. However, SEM analysis of metallographic cross sections suggested a slightly more irregular surface (and a potentially larger surface area) on the VPS NiCrAl-1Y bond coats.
Alternatively, the slight increase in TBC cyclic oxidation rate could be an indication that greater amounts of cracking and delamination were occurring in the interfacial Al2O3 scales on VPS NiCoCrAlYHfSi. Previous studies have reported significant localized damage in the interfacial Al2O3 scales after thermal cycling of APS TBCs. Since the YSZ top coat prevents scale spallation, an increase in the susceptibility to scale damage would accelerate the apparent TBC cyclic oxidation rate due to rapid oxide reformation beneath the cracked or delaminated interfacial Al2O3 scales. This would be the case even if the intrinsic isothermal oxidation rates of NiCrAlY and NiCoCrAlYHfSi were similar. Analysis of metallographic cross sections of cycled TBCs by SEM confirmed that varying amounts of localized cracking, delamination, and reformation of the interfacial Al2O3 scales had occurred on all bond coats after 100 cycles, as illustrated in Fig. 2(a) and (b). Further research will be required to effectively quantify the relative amounts and frequency of interfacial scale damage, as well as their potential influence on TBC durability.
Fig. 1 Comparison of the mean CTE (25 to 1200 °C) of single-crystal Rene N5 to cast versions of the bond coat alloys used in this study (Ni-20Cr-10Al-Y and Ni-22Co-18Cr-12.5Al-Y).
Fig. 2Secondary electron images of TBC cross sections after thermal cycling, showing the presence of localized damage in the Al2O3 interface scales after 100 cycles: (a) TBC with APS NiCrAl-1Y bond coat and (b) TBC with VPS NiCoCrAlYHfSi bond coat. Although there was significant scale damage (in the form of cracking and multilayering) in localized regions, scale cracking was not visible over the majority of the interface.
Comparison of the thermal cycle behavior of the bare MCrAlX coatings provides additional insight into the relative scale adhesion behavior of the various bond coats and supports the presumption that differences in APS TBC lifetimes might have been related to bond coat scale adhesion behavior. The TBC durability generally decreased as the time to initiation of scale spallation decreased or as the rate of scale spallation increased. Coincidentally, each type of TBC typically failed at or near the number of cycles that corresponded to the time at which their respective bond coat specimen spalled to its original weight, as illustrated in Fig. 3. This observation implies that the relative times to TBC failure may have been influenced by some critical amount of interfacial Al2O3 damage accumulation.
Fig.3Comparison of TBC and bond coat oxidation behavior at 1150 °C. Note that TBC failure occurs near the time at which the bond coat specimen weight decreases to the original specimen weight (due to scale spallation). This behavior was typical of all four bond coat compositions on René N5.
Although it is well known that TBC degradation is accelerated under oxidizing conditions, and that TBC lifetimes are dependent on both the number of thermal cycles and the timeat-temperature, there is no clear understanding of the influence of bond coat composition or oxidation behavior (i.e., scale growth rates and scale adhesion). Brindley and Miller reported that the intrinsic isothermal oxidation rates of various NiCrAlY bond coats were not necessarily an indicator of relative TBC performance. Other investigations have suggested that the relative durability of plasma-sprayed TBCs on various bond coats was strongly influenced by bond coat creep resistance or the interaction of bond coat stress relaxation behavior with bond coat CTE behavior. Bartlett and Dal Maschio suggested that failure of plasma-sprayed TBCs is facilitated by cracks that form in the brittle Al2O3 scales and then propagate into the adjacent YSZ layer. However, this conclusion does not agree with the observations of other studies, which reported that the critical YSZ cracks that caused TBC spallation did not initiate in the Al2O3 scale. Thus, the role of interfacial Al2O3 scale cracking within the TBC damage accumulation process is still unclear.
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