Studies Of Preparation Of MoSi₂ Base Composite Coating On Niobium Surface And Its Oxidation Resistance Properties

Based on its superior physical and chemical properties, niobium and niobium alloy have good intensity and performance when the limited work temperature is higher than that of traditional alloy. Therefore, niobium and niobium alloy are known as a replacement of traditional high temperature alloy materials in the 21 st century. However, the oxidation resistance of niobium and niobium alloy is poor, and pure niobium begin to occur "pest" oxidation at 600℃, which restricts its application in industrial production severely. It requires us to improve the high temperature oxidation resistance of niobium and niobium alloy. A protective coating coated on the surface of niobium alloy is an effective way to improve the high temperature oxidation resistance.The design and preparation of Mo Si2 and Mo-Si-B composite coating on niobium based surface, the interface between the substrate and the coating and the oxidation resistance of coatings are discussed in this paper. Microstructure of the material was characterized by using OM, XRD, SEM and other testing technology. The main results were as follows:(1) With porosity and hardness of the coating as the optimization goal, the process parameters of preparation Mo coating were optimized. The best spraying parameters are: voltage 60 V, main gas flow 45L·min-1, spraying distance 120 mm and feeding rate 68g·min-1. The Mo pore porosity of coating is 2.5%, and the microhardness is 315.3HV0.5. The density of coating was decreased significantly after vacuum heat treatment, from 2.5% to 0.9%. After heat treatment, the mutual diffusion occur between Mo coating with Nb base, and the metallurgical combination improve the thermal match between coating and substrate.(2) The Mo Si2 coating and Mo-Si-B composite coating was prepared on the niobium base through siliconizing and Si-B co-deposition. The optimal process parameters:the heating temperature is 1050℃, and the holding time at maximum temperature is 5h. The cross section of coating after siliconizing treatment consists of Mo Si2 phase, Mo5Si3 phase, Mo phase, and Nb-Mo transition phase from surface to substrate. The cross section of coating after Si-B co-deposition consists of Mo Si2 phase, Mo5Si3-Mo B transition phase, Mo3 Si phase, Mo phase and Nb-Mo transition phase from surface to substrate.(3) The oxidation kinetics of Mo Si2 coating and Mo-Si-B composite coating both followed the parabolic rate law. But Mo-Si-B composite coating shows better oxidation resistance than that of Mo Si2 coating. B2O3-Si O2 mixed phases oxide layer with good fluidity is formed after adding boron. And it can repair the micro cracks and pores which results self-healing capacity of coating. The oxygen partial pressure between the coating and substrate was decreased after adding boron. B2O3-Si O2 composite oxide layer also could protect the substrate by preventing the oxygen diffusing to substrate.