Powders

Investigation of various Y-O^ stabilized Zrt^ coating systems has snown the extreme importance of powder quality. The effects of wide variations in coating performance are thought to be due to chemical purity and homogeneity in addition to the powder morphology which reflects the manufacturing process. Examples of different powder morphologies are shewn in Fig. 4.

Most of the powders used for thermal spraying have sizes between 150 urn and 5 um. The ideal powder is one where the grain size is minimal, and the grain size distribution variation from lot to lot is negligible. Unfortunately, this is not usually the case. The cost of sizing the powders to narrow limits can often be more than paid back by the improved yield from processing, and from achievement of more consistent properties of the deposit. The heating characteristics of the powder used in any of the thermal spraying processes are dependent on a variety of factors. The heat transfer 4) from the plasma flame to the material depends on the grain structure and the grain shape factor, in the following manner;

Porous ceramic

Dense ceramic Increased

Porous sintered metal heat

Dense sintered metal transfor-

Fused atomized metal mation

Powder shape: Spherical powder has the lowest surface to volume ratio and will not be heated in the plasma of the combustion torch as quickly as acicular or irregularly shaped particles of the same material.

Powder manufacturing process: For example, "spray dried" powders are manufactured by agglomerating fine powders Which have been mixed with a decomposable binder. The blend is then allowed to dry, and the mixture broken, or sprayed into a chamber to form small grains. These powders can vary substantially depending on binder material and content. In contrast, the "same" material might also be made by crushing or grinding from a large solid lump of the material system, forming irregularly shaped powders, or alternatively atomized from a large molten source of the material forming solid (and possibly hollow) spheres. Each of these products will have a different response in spray application.

For normal thermal spraying operations, the smaller powder sizes yield denser coatings. More highly stressed deposits by very finely divided powders tend to oxidize nore readily, and also tend to evaporate in the plasma flame. They also tend to flow with greater difficulty than larger powder sizes.

The way that powders are handled and prepared before spraying is too often neglected or underestimated. The thermal spray powder should be dried and kept clean prior to spraying. The carrier gas used to convey the powder from the feeder to the torch should also be free of oils, water vapor etc.

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