Influence of the condensation surface temperature on the microstructure and intrinsic stress in evaporated chromium films

Abstract

Chromium was evaporated onto die cast zinc at a pressure of better than 10 −3 Pa (10 −5 Torr). The deposition rate was varied between 0.1 μm min −1 and 0.6 μm min −1. The films were found to have a b.c.c. structure and to consist of needle-like crystallites, with fine needles for low deposition rates and coarse needles for high deposition rates. A rapid growth of the crystallites occured for a deposition rate of 0.2−0.3 μm min −1. During deposition the films were found to crack because of intrinsic stresses developed in the films. There was a strong dependence of the mean distance between cracks on the deposition rate with a maximum at about 0.25 μm min −1 for a substrate temperature of 323 K (50°C). The relation of the root mean square microstrain to the deposition rate is the inverse of the relation between the mean distance between cracks and deposition rate. A model is proposed that separates the experimentally obtained root mean square strain into two parts, one due to distortions at the grain boundaries and one due to defects in the crystallites. The results given by the model for the two strain components explain the obtained Vickers hardness dependence upon deposition rate.