Effect of faceting on the thermal grain-boundary grooving of tungsten

Abstract

The grain-boundary grooving of electropolished surfaces of polycrystalline tungsten annealed at 1350°C has been studied. Atomic force microscopy images of grooves were taken in the same locations after different annealing times. The profiles of the grooves developed between unfaceted grains were in qualitative agreement with the predictions of Mullins' theory of grooving by surface diffusion mass transport. In particular, the predicted secondary maxima next to the main groove maxima were often observed. Surface faceting strongly affected the grooving kinetics and groove shapes. Grooves developed between faceted and unfaceted grains were often asymmetric with unusual growth kinetics. Our observations suggest that certain faceted grains exhibited a negligible surface diffusion coefficient and that the surface fluxes at the associated groove root were non-zero. Numerical simulations assuming anisotropy of the surface diffusion coefficient (i.e. high diffusion coefficient on the unfaceted side, and negligible diffusion coefficient on the faceted side of the groove) were performed. The qualitative agreement between the simulated and observed groove shapes shows that the groove asymmetry can be explained by surface diffusion anisotropy.