Preparation and Characterization of Porous Molybdenum by Low Velocity Flame Spraying of Surface-Melted Particle Deposition

Porous molybdenum, combined with both porous and refractory properties, has promising applications in areas such as aerospace. However, its fabrications are limited by conventional powder metallurgy processing due to the high melting point of Mo. In this study, porous Mo deposits were prepared by flame spraying under different spray conditions to control porosity of the deposit. The deposit surface was kept at certain temperature to ensure the bonding formation of impacting spray particle with the underlying particles. To better understand the deposition process mechanism and the influence of spray particle state on microstructure and porosity, the velocity of spray particles was quantitatively measured and the morphology of deposited isolating particles was characterized by scanning electron microscopy to estimate the melting degree of spray particles before impact. The microstructure of the as-sprayed and annealed porous Mo in H₂ atmosphere was characterized by x-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The deposit porosity was measured by Archimedes method. The results showed that the porosity of the Mo deposit ranged from 40% to 70% with the change of spray particle melting degree, resulting from the change of spray parameters. The examination showed that 3D through-deposit pore-net structure was created by bonding necks between particles with melted particle fraction and the untouchable holes sheltered by main solid part of semi-melted particles. As melting degree decreased, pores in deposit became larger.       After reduction annealing, the sheet-shaped Mo oxides on the as-sprayed particle surface were changed to undulating Mo grains in several micrometers. The present results clearly demonstrated that a porous Mo material can be prepared by spray deposition by controlling the melting degree of spray Mo particles.