Results And Discussion

Powder Characteristics:

The results of the various powder analyses are shown in Table 3. Included in the list are three powder groups from vendors A, H, and D added to the program after the program was well under way. These have an incomplete data list. While each material is unique, clearly the powders can be classified by their method of manufacture. In general, spray dried TBC powders have the lowest true density, the highest specific surface area, and the lowest mass flow rates of all powders. Sintered materials have intermediate values of density and flow, and surface areas which are an order of magnitude lower than those for spray dried. Finally, fused materials were dense, had high mass flow rates, and very low surface areas, another order of magnitude below the sintered powders. The size distribution provided by the various vendors fit into a very loose specification, and the different distributions are a function of the vendors' processes and/ or of the size distributions which the vendors chose to supply, as shown graphically in Figure 1. The distributions are represented by the Mean Volume (MV) of an equivalent sphere as calculated by the microprocessor and the standard deviation about that calculated mean. Except for the 'Y',and the two cast and crushed samples from 'J', the distributions of the powders are unimodal. It is important to note that vendor 'Y' and vendor 'B' are the same source; only the distribution (Figure 1) of the powders are different, with the 'Y' sample having many more fine particles and a bimodal distribution.

Metallographic evaluations of the powders (Figure 2) demonstrated the wide difference in powder shape, density and homogeneity. Scanning electron photomicrographs of the major groupings also demonstrate this (Figures 3 and 4). The spray dried materials were composites of discrete Zr02 and Y203 particles. These materials have a wide size distribution of Y203 particles, which caused compositional

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