Chemically vapour-deposited Mo/Re double-layer coating on graphite at elevated temperatures

Abstract

The diffusion process operating in a double-layer coating at elevated temperatures was investigated experimentally by performing isothermal annealing tests (at 900–1200 °C) for a chemically vapour-deposited molybdenum coating on graphite with an interlayer of rhenium. The Mo/Re double-layer coating with low impurity and good adhesion to the graphite substrate was fabricated by thermal decomposition of the metal carbonyls at fairly low temperatures (<700° C). Isothermal annealing tests showed a parabolic growth of Mo 2C within the molybdenum layer with annealing time and no rhenium carbide in the rhenium layer. The parabolic rate constant of the carbide layer growth was decreased by increasing the thickness of the rhenium layer, and this tendency was more significant at lower annealing temperatures. The carbide layer growth also depended on the microstructure of the rhenium layer; it was restricted as the columnar diameter of the rhenium layer increased.