| Nb and Nb-based alloys as refractory metal have many advantages,such as high melting point,good ductility,moderate density,high strength at high temperature and so on.However,monocrystal Nb is characterized with poor high temperature oxidation resistance and the multiple alloying of Nb cannot meet the actual requirements as well.Thus,it is necessary to manufacture the anti-oxidation coating to promote the performance of high temperature oxidation resistance.It has been reported that series of Mo-Si-B coating have excellent high temperature oxidation resistance,which can provide effective protection for Nb alloy in high temperature service.In this paper,Mo Si2-B4C composite coating was prepared on the surface of Nb alloy by spark plasma sintering with Mo powder,Si powder and B4C powder as raw materials.The influence of sintering process parameters on the microstructure of the coating was investigated by XRD,SEM and EPMA measurements.Moreover,the rapid sintering behavior of the coating was analyzed to evaluate its high temperature oxidation resistance.The results are as follows:The coating is mainly composed of Mo Si2phase and B4C particles.The microstructure of the coating tends to be compact and homogeneous and the thickness of the interdiffusion region at the coating/substrate interface increases obviously with the increase of sintering temperature.When the sintering temperature rises to 1450?,partial B4C particles in the coating begins to react with Mo and Si elements and form the Mo B and Si C phases.Although sintering pressure has no obvious effect on the structure and composition of the coating,the density of the coating increases slightly with the increase of the sintering pressure.However,a remarkable deformation occurs at the coating/substrate interface.When the holding time of the coating is 0s during spark plasma sintering,Mo-Si-B4C mixed powder immediately sinter into coating with the generation of Mo Si2phase and dispersed B4C particles.When holding time increases to 30s,B4C particles begins to react with Mo and Si elements to form Mo B and Si C phase whose fraction continuously increases with the increase of holding time until it becomes stable after 180s.In addition,when the holding time increases from 60s to 180s,the thickness of the interdiffusion region at the coating/substrate interface increases obviously,and the growth rate is high at the initial stage and then declines subsequently.At the interface of the coating and substrate,the layers of the diffusion region are obvious with Nb B layer,?Nb,Mo?Si2layer,and?Nb,Mo?5Si3layer from top to bottom.The Nb B layer delays the diffusion of Si element into the matrix alloy,and improves the anti-degradation ability of the coating.After oxidation at 1450?for 100 hours,a continuous and compact Si O2-B2O3oxide film was formed on the surface of the coating.The weight gain per unit area is3.8mg/cm2,while that of the matrix alloy is as high as 108.9mg/cm2.The oxidation kinetics of the coating at 1450?fits to the parabolic law,and the oxidation parabolic rate constant is only 2.9%of that of the matrix alloy.Besides,the microstructure of the coating after oxidation at 1450?for 100 hours only slightly degraded compared with the coating before oxidation.The microstructure evolution of the coating after oxidation is mainly manifested in two aspects:the first is the transformation of B4C particles in the coating.The B4C particles are reacted into Si C,and Mo B phase around the B4C particles.The second is the change of microstructure in the interdiffusion region at the coating/substrate interface.The original Nb B layer disappears or disperses into discontinuous island shape with?Nb,Mo?Si2layer disappearing and?Nb,Mo?5Si3layer thickening. |
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