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

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 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.