Research On Microstructure And Properties Of Boron-Aluminum Coating On Titanium Alloy Surface

Titanium alloys are widely used in aerospace,military and other fields.However,the poor high temperature performance and low wear resistance of titanium alloys severely limit its development.In this paper,the boron-aluminum co-infiltration coating was successfully prepared on the surface of titanium alloy by solid powder method,and its microstructure and properties were explored and analyzed.Defects resistant to high temperature oxidation and poor hot corrosion of single boronized coating were overcome.The research work carried out and the results achieved are as follows:Orthogonal method and single factor method were used to optimize process parameters and penetrating agent formulation while thickness and surface hardness of coating were taken as evaluation indexes.The optimum combination of penetrant and process parameters of boron-aluminizing were obtained as follows?each drug content refers to weight percentage?:the experimental temperature is 950?,the experimental time is 20h,the composition of boron-aluminizing agent is 20%Al,20%B₄C,5%NH₄Cl,55%SiC.Based on boron-aluminum co-infiltration,the infiltration agent formulation and process parameters of boronizing were determined as follows:the experimental temperature and time are 950?and 20h,and the infiltration agent composition is20%B₄C,5%NH₄Cl,75%SiC.The microstructure and mechanical properties of the coatings were observed and analyzed.The results show that the surface of both coatings were dense and non-porous.Boronized coating mainly composed of TiB₂,TiB and Ti phases and the thickness of the infiltrated coating was about 51.3?m.The bonding with the matrix was mainly mechanically combined,and the bonding force was 58N.The thickness of the boron-aluminum coating was 69.8?m that composed of Al₃Ti and Ti₃B₄.The diffusion of Al causes the boron needle to disappear,and the adhesion between the coating and the matrix was 67N because the coating and the matrix were metallurgically bonded.The surface hardness of the two coatings was 1996HV and 1221HV respectively.Al₃Ti reduces the hardness of boron-aluminum coating,but at the same time,its has lower hardness gradient and brittleness than the single boronized coating,and its friction coefficient was more stable in friction and wear experiments.The mechanical properties of the co-infiltrated coating were better than the single boronized coating under comprehensive comparison.The high temperature oxidation behavior of boronized coating and boron-aluminum co-infiltration coating at different temperatures?700°C,900°C?was studied.The results show that coatings both generated B₂O₃ which will volatilize at high temperature leading to weight loss in the oxidation experiment.The B₂O₃ formed in the early stage of the boronizing coating at 700°C has a certain protective effect on the matrix.However,as the oxidation temperature increases,the volatilization rate of the B element accelerated,and the protective effect of the boronized coating has been lost.At 900°C,boronized samples have poor antioxidant capacity and serious oxidation weight gain.The boron-aluminum co-infiltration coating has the strongest anti-oxidation ability.On the one hand,the co-infiltration coating provided rich aluminum element to absorb the diffused O element.On the other hand,in the oxidation process,a complete dense Al₂O₃ film was formed to insulate the substrate from the oxidize.The Al₂O₃ film can protect the boron and titanium compounds from oxidation and volatilization,but because of the volatilization of B element and the diffusion of Al element to the surface,holes were formed between the oxide layer and the infiltrated layer.The boron-aluminum co-infiltration coating and matrix were well combined in the oxidation process,and there was no delamination or abscission.The hot corrosion behaviors in a mixed salt of 80%NaSO₄+20%NaCl of boronized coating and boron-aluminum co-infiltration coating were investigated at different temperatures?700°C,900°C?.The results show that the boron-aluminum co-infiltration coating has the strongest resistance to hot corrosion,and the aluminum oxide film formed during the hot corrosion process effectively slows the diffusion of oxygen and sulfur.The B₂O₃ formed by the oxidation of the boronized coating left a large number of gaps and holes after volatile,and S passed through these defects to formed a sulfide coating.Based on the above,this paper creatively developed key technologies of boron-aluminum co-infiltration coating formula and co-infiltration process on titanium alloy surface,which greatly improved the hardness and wear resistance of titanium alloy and strengthened the ability to resist high temperature oxidation and hot corrosion.Provide theoretical basis for the development of titanium alloy surface modification technology.