Study On Preparation Of Aluminide Antioxidant Coating By Thermal Spraying + Vacuum Heating

Titanium and titanium alloys are widely used in modern industry and have many advantages,especially in military,aviation,biomedicine and other fields.However,titanium and titanium alloys have limited anti-oxidation properties,and have certain limitations in practical applications,especially in high temperature environments,so their service life are greatly reduced.Therefore,it is of great significance to improve the oxidation resistance of titanium and titanium alloys at high temperature.Spraying a composite coating with excellent high temperature oxidation resistance on the surface of titanium and titanium alloys can improve the oxidation resistance of titanium and titanium alloys and prolong their service life.Currently,the excellent performance of Ni-Al coatings is favored in the research of coatings for titanium and titanium alloys.However,most studies on Ni-Al coatings have been carried out in atmospheric environment,where oxygen is prone to react with the Al elements in the coating to form oxides and stunt the growth rate of Ni-Al intermetallic compounds.Therefore,this thesis conducts a vacuum heat treatment study on the basis of atmospheric heat treatment to determine the precipitation of intermetallic compounds and the growth rate of the diffusion layer of Ni-Al coatings and to study the oxidation resistance of the coatings.In this thesis,pure titanium is used as the substrate.Ni-Al coatings are prepared on the surface of pure titanium by arc spraying.Firstly,the atmospheric heat treatment of Ni-Al coatings was used as a basic experiment.Then,the Ni-Al coating was heat treated in a vacuum environment.Microstructure analysis of the Ni/Al coatings before and after heat treatment under different environments and temperatures was carried out using scanning electron microscopy（SEM）and X-ray diffraction（XRD）.The diffusion kinetics curve of Ni/Al coatings under different environments and temperatures were fitted to obtain the diffusion kinetic equations of Ni/Al coatings.Combined with the Ni-Al phase diagram,the intermetallic compounds in the coatings after vacuum heat treatment were analyzed.Finally,oxidation weight gain experiments were carried out on the coatings after heat treatment in different environments,and the oxidation kinetic curves were plotted to study the oxidation resistance of Ni/Al after vacuum heat treatment.The results show that the Ni/Al coating in the atmospheric environment formed an aluminium-rich diffusion layer dominated by Al₂O₃,Ni₂Al₃and Ni Al₃phases during the heat treatment at 700°C and 800°C,which prevented the infiltration of oxygen and formed an effective protection for the titanium substrate.In the vacuum environment,Ni/Al coatings formed mainly Ni₂Al₃,Ni Al₃,Ni Al phase dominated diffusion layers in the heat treatment at700°C and 800°C.The diffusion kinetic analysis reveals that the diffusion rate of Ni/Al coatings increased with increasing temperature in the two different environments,and the diffusion rate of Ni/Al coatings was higher after vacuum heat treatment than atmospheric heat treatment.The oxidation weight gain experiments demonstrate that the Ni/Al coating after vacuum heat treatment has better oxidation resistance than the Ni/Al coating after atmospheric heat treatment,and the coating is more uniform,which can improve the high temperature oxidation resistance of titanium and titanium alloys in practical applications.