Effect of high temperature annealing on the microstructure and thermal shock resistance of tungsten coatings grown by chemical vapor deposition

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Thick tungsten (W) coatings were prepared by chemical vapor deposition (CVD) at a rapid growth rate of 0.6 mm/h. Annealing experiments have been performed in a relatively wide temperature range from 1200 °C to 2300 °C for 3 h to examine the effect of high temperature exposure on the microstructure, micro-hardness and thermal shock resistance of the CVD-W coatings. The columnar grain structures with a preferred <001> orientation of the CVD-W coatings are well preserved up to 2300 °C. Bubbles of polyhedral forms formed at the grain boundaries of the sample annealed at 2300 °C. The dislocation structures were characterized by a high density of dislocation tangles and a low density of long and straight screw dislocations in the samples annealed below and above 1900 °C, respectively. The micro-hardness of the CVD-W coatings decreased with the increase of annealing temperature. The polished CVD-W coating surfaces exhibited high thermal shock resistance to ELM-like thermal loadings. Surface crack formation was suppressed at elevated temperature except for the sample annealed at 2300 °C.