Microstructural and tribological characterization of molybdenum-molybdenum carbide structures produced by spark plasma sintering

Pure molybdenum is one of the most important refractory metals owing to its high melting point, low thermal expansion coefficient and good thermal conductivity. Molybdenum carbide exhibits superior wear resistance to molybdenum owing to its low friction coefficient and high hardness; therefore, in this study, to endure abrasive conditions, the surface of molybdenum was modified as molybdenum carbide. In accordance with this purpose, pure molybdenum powder was sintered under constant pressure (40 MPa), various temperatures (1 650 °C, 1 700 °C and 1 725 °C) and holding periods (180 s, 360 s and 540 s) using the spark plasma sintering technique. The effects of sintering temperature and holding period on relative density, microhardness, microstructure and wear properties of the specimens were investigated. Moreover, carbide formation on the surface due to carbon diffusion was also investigated. Relative density values decreased with increasing sintering temperature and period, obtaining the highest value of 97.55 % at 1 650 °C and 180 s. A microhardness value of 15.20 GPa was reached on the surface while the maximum value measured at the cross-section was 2.17 GPa, showing the formation of molybdenum carbide structure on the surface of all specimens. Furthermore, microstructural analysis supported the microhardness results and showed a minimum 288-μm thick molybdenum carbide layer on the surface. In addition, a typical eutectic microstructure was observed between molybdenum and molybdenum carbide layers; however, the decrease in temperature decreased the thickness of the eutectic layer and it gradually disappeared from the centre to edge of disc-shaped specimens. Wear resistance of surfaces was improved by decreasing holding period where a non-porous carbide layer and highest microhardness values were achieved. The cross-sectional analysis corresponding to molybdenum base metal proved that the formation of molybdenum carbide layer because of carbon diffusion improved the wear properties of the sample with the low friction coefficient and low wear depth.