Composition, Microstructure and Mechanical Properties of Flame Sprayed Molybdenum Coatings

Abstract

SummaryFlame sprayed molybdenum coatings applied to ferrous engineering components can combat wear and fretting action effectively. The micro-hardness of coatings was shown to increase (up to 1700 HV) as the oxygen partial pressures of the oxy-gas flame increase. ‘Soft’ and ‘hard’ coatings have been examined in cross section by optical and scanning electron microscope techniques. Different etching characteristics were noted in the layered structure. At a magnification of ~ × 20 K, regions were revealed in the hard coatings where the etchant had apparently removed a fine dispersion of second phase particles. EDAX microanalysis indicated that the molybdenum content of the softest layers was 92·6% and 87·1% in the harder regions, whilst X-ray diffraction data revealed that the oxide dispersed throughout the coating was MoO2. Heat treatment of the coated samples at 1150°C coarsened the second phase oxide dispersion, a 50% reduction in microhardness resulted. The possibility of ultra fine grain sizes and dispersions of fine oxide particles are seen as the major strengthening agents in the coatings. The influence of internal oxide distribution on wear and fretting behaviour is considered to be important.