Preparation Of Si-NbSi₂ Composite Coating On Niobium By Hot-dipping And Diffusion Annealing

Niobium and its alloys are considered to be promising high-temperature structural materials due to their high melting point,low density and excellent high-temperature mechanical properties.However,their oxidation resistance at high temperatures is extremely poor,and "pesting" oxidation will occur when it is higher than 600℃,which seriously limits their application in high temperature and oxygen-enriched environments.In order to prevent the oxidation failure of the niobium-based alloy under high temperature and oxygen-enriched environment,Si-Nb Si2 composite anti-oxidation coating was successfully prepared on the surface of the niobium-based alloy by hot-dipping silicon process.The effects of hot-dipping silicon and diffusion annealing on the surface and cross-section morphology,phase composition,surface roughness and surface hardness of silicide coating were studied.The conclusions of the study are as follows:(1)The Nb Si2-Si-based composite coating prepared by HDS process has a smooth surface and clear internal layers(Nb5Si3-Nb Si2-Si from the inside to the outside).The hardness of the sample is moderate(150~250 HV)and the roughness is extremely low(0.19~0.45 μm).The growth of HDS coating satisfies the diffusion kinetic equation:ΔX2= 2Kpt;the reaction activation Q and enthalpy change ΔHare calculated to be 90.6 k J · mol-1and-145.1 k J · mol-1respectively.(2)The results of hot-dipping test show that the Si-Nb Si2 composite coating prepared at 1500℃ for 20 min has a smooth and compact surface structure;the Nb Si2 crystal size on the coating surface is small(5~8μm)and uniformly distributed,with low surface roughness(0.24 μm)and high surface hardness(165.9 HV).(3)Diffusion annealing experiments showed that the surface roughness of the coating increase slightly(0.70~1.60 μm)and the surface hardness increased significantly(600~1200 HV)with the increase of annealing time and temperature,and the main phase Nb Si2 in the coating changed from inside to outside gradually.(4)Annealing temperature has a remarkable effect on the diffusion growth of Nb5Si3 interfacial layer.When the annealing temperature is lower than 1000℃,the diffusion growth of interfacial layer is slower,and the diffusion rate constant Kp is only 0.6;when the temperature is higher than 1400℃,the diffusion growth of interfacial layer is particularly intense,with the diffusion rate constant K_p>11.4.