TEM studies of microstructures and interfaces of zirconia produced by plasma spraying.

摘要

Plasma spraying has been used extensively to produce a wide variety of, coatings or free-standing bodies. Although the plasma spray processing has been known for several decades, deep understanding of all physical and chemical, processes involved is rather limited often due to the lack of detailed enough experimental and diagnostic techniques supplying the necessary data. The applications produced by plasma spraying have in common their fine microstructure with features often of nanometer scale as a result of rapid solidification. This work reports on transmission electron microscopy studies of two types of zirconia ceramics. Nanocrystalline particles of pure zirconia from liquid feedstock and single splats of yttria stabilized zirconia (YSZ) produced from ordinary powder feedstock.; The modified wedge polishing technique of TEM cross-sectional sample preparation was engaged to prepare high quality samples of plasma sprayed ceramic-metal interfaces. That enabled us to describe the character of the splat-substrate interface and its dependence on substrate temperature and substrate surface chemistry. A rapid solidification model is proposed for the observed narrow columnar grain microstructure of the first and subsequent YSZ splats. The effect of liquid flow on the solidification of the subsequent splats was clearly demonstrated as well as the epitaxial growth of columnar grains.; The air plasma spraying of liquid zirconia precursors produces at relatively high rate nanocrystalline particles of pure zirconia. The particles free of defects were found to be composed of the high temperature tetragonal phase as long as their diameter is smaller than the critical diameter. An explanation of the size stabilization effect is proposed. During annealing of the tetragonal nanoparticles with lognormal size distribution, a phase transformation occurs which is associated with a change in distribution character. We conclude that lognormal distributions are not intrinsically preferred and particle coarsening before and after the transformation is self-similar.