390

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ity and phase structure have a major effect on the as-sprayed TBC structure. Coatings made from spray dried powders tend to be predominantly tetragonal with some cubic and retained some of the mono-clinic phase of the powder (Figure 5); coatings made from fully tetragonal fused materials remained tetragonal (Figure 6), and the coatings sprayed with sintered materials retained a substantial amount of the original cubic phase, although some shifts towards the tetragonal phase is evident (Figure 7). These observations are consistent with the phase diagram (Figure 8) in that quenching of the liquid powder particles during spraying would

Thermally Cycled Coating Characteristics :

All of the samples sprayed for this test were subjected to thermal cyclic testing until 20% of the TBC had spalled. The metallographic evaluations produced after cyclic exposure are shown in Figure 12. XRD analyses are shown in Figures 57, and the number of cycles to failure are given in Table 4. The quantitative phase analysis performed on each of the initial powder materials after thermal cycle testing is shown in Table 5. The failure mode observed is typical for thermal barrier coatings in all cases. Typical failure occurs by spallation of the ceramic coating just above the bond coat interface. Since the time to spall varied greatly, between 40 and >1000 cycles, the characteristic structure of the coating plays a major role in the capability of the particular coating to withstand strains developed by the oxidation of the bond coating. Thus, the strain tolerance of the 'W' coatings, which survived only 40 cycles, and the 'B' coatings, which withstood 1000 cycles

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