100

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** DETERMINED TO BE 2% USING Cu Ka RADIATION

** DETERMINED TO BE 2% USING Cu Ka RADIATION

is graphically demonstrated. In general, the coatings with the longest life were very porous and had some degree of cubic phase in the as-sprayed coating. The intermediate porosity levels and were fully tetragonal in the as-sprayed condition. Finally, the shortest life samples were dense and had a relatively high concentration of monoclinic phase in the coating (Table 5).

It can be seen that all of the coatings tended to become tetragonal upon thermal cycling and, that after 1000 cycles this transformation was fairly complete even for the least-tetragonal as-sprayed coatings .

This observation is very different from the observations made by Herman et al.1121 and Miller et al16-131. They observed that continuous exposure of the coatings at a temperature greater than 1100*C for periods of time up to 100 hours tended to produce varying amounts of monoclinic and cubic phases with decreasing tetragonal phase. This difference between the observations of the previous investigations and this work may be explained by differences in the types of thermal exposures involved in the investigations. For example, the coatings of Herman et al. and Miller et al. were heated to a constant temperature continuously for an extended period of time. From Figure 8 one can see that at their heat treatment temperatures, the equilibrium phases are the cubic and the transformable tetragonal phases. The time involved in their exposures is sufficient to complete the diffusion controlled processes involved in the formation of the equilibrium phase; therefore, upon cooling the stable monoclinic and cubic phases were formed. Opposed to this, in the present investigation, the coatings were thermally cycled between 1100"C and 200*C for periods of 40 minutes at the high temperature. This repeated heating and cooling of the coatings may be compared to the repeated heating and cooling of martensite in steel. Such cycling was found to significantly retain the high temperature austenitic phase at lower temperatures1141. It appears that the result of thermal cycling of Zr02-Y203 coatings is similar to this. Thus, after cycling testing a greater fraction of the high temperature phases are retained.

As indicated previously, the thermal cycle performance of tested coatings varied widely. Several powders were resized to shift the Mean Volume to a level which more closely approximated Vendor 'B's distribution (Figure 10), sprayed and thermal cycle tested. The results comparing the first test sequence (as received) with the second, after resizing, are shown in Figures 13 and 14. Significant improvements in the thermal cycle life of all coatings produced from the cast and sintered powders were observed. Conversely, the, spray dried powders each showed a significant decrease in thermal cycle life. A spe-

LIFE, CYCLES

FIGURE 14: THERMAL CYCLE PERFORMANCE OF RESIZED SPRAY DRIED POWDERS

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