Enhanced High Temperature Oxidation Resistance For TiAl Alloy With Composite Coating Comprised Of Electrodeposited SiO₂ And Air Sprayed Glass-ceramic

Due to the low density?3.7⁴.2 g/cm³?,excellent high temperature specific performance,high temperature creep resistance,and resistance to"titanium fire",TiAl-based intermetallic?TiAl alloy?has become a new type of high-temperature structural material and have drawn a great deal attention in the area of aerospace,automotive,weapons industry and so on.However,when the service temperature is higher than800°C,a non-protective mixed oxide scale?Al₂O₃+TiO₂?would generate on the surface of the TiAl alloy,which will deteriorate the specimen and seriously shorten the service life of the TiAl alloy.Till now,alloying design and surface treatment have been proposed to improve the high temperature oxidation resistance of TiAl alloy.Among them,glass-ceramic coating has become an ideal high temperature oxidation protective coating material for TiAl alloy due to its wide composition range,large thermal expansion coefficient range,good thermal stability and chemical stability.Recently,our group proposed to prepare SiO₂ protective coating with a rough outer layer and a dense inner layer on TiAl alloy via electrochemical deposition.Results shown that this electrodeposited SiO₂ coating can dramatically improve the high temperature oxidation resistance of TiAl alloy at 900°C.Based on our previous research,this thesis proposed to fabricate composite coating on TiAl alloy by the combination of porous electrodeposited SiO₂ with interlayer and air sprayed glass-ceramic coating.Then,ceramic particles were doped into the glass-ceramic coating to further improve the oxidation resistance of TiAl alloy.The high temperature oxidation behavior of the coating protected TiAl alloys was studied.The surface morphology,phase composition,and the evolution of the coating/substrate interface during the oxidation process were investigated by SEM,EDS and XRD.The high temperature oxidation mechanism of the coatings was investigated.In the third chapter,tetraethyl orthosilicate?TEOS?and potassium silicate water glass?K₂O:SiO₂=1:3?were used as raw materials to prepare SiO₂-glass-ceramic composite coating on TiAl alloy by the combination of electrodeposition and air spraying.The oxidation behavior of the composite coating under 1000?was investigated.Results shown that the composites coating protected TiAl alloy exhibited good high temperature oxidation resistance.After 100 h oxidation,the weight gain of the composite coating was only 1.16 mg/cm².The improved oxidation performance of this composite coating can be attributed to two aspect.Firstly,the internal diffusion of oxygen was efficiently hindered by the dense glass-ceramic coating.Secondly,the electrodeposited SiO₂ interlayer not only prevented the outward diffusion of Ti and Al from the alloy substrate,but also prevented the oxygen from contacting the substrate,and improved the adherence between the glass-ceramic coating and the substrate.In the fourth chapter,the 8YSZ particles were doped into the water silicate glass precursor in order to reduce the flowability of the glass-ceramic coating at high temperature.The oxidation behavior of this YSZ particle doped composite coating was studied.Results shown that the composite glass-ceramic coating with 3 wt.%8YSZ revealed good oxidation resistance.After 100 h oxidation,the weight gain was 1.12mg/cm².The doped 8YSZ particles not only acted as a support for glass-ceramic coating but also slowed down the high temperature flowability for the ceramic coating,therefore improving the high temperature oxidation resistance.Whereas,the doped8YSZ particles impeded the evolution for the generated gas,leading to the increase of outward diffusion of Ti,Al and generation of Ti₅Si₃ and Kirkendall's voids at the coating/substrate interface.Besides,interfacial structure consisted of Al₂O₃ layer/TiO₂+Ti₅Si₃mixed layer/Z phase layer was observedIn the fifth chapter,ZrB₂ particles was doped into the 3 wt.%8YSZ-doped silicate ceramic glass to improve the interfacial structure of the specimen.The oxidation behavior of the co-doped composite coating was investigated.Results revealed that the introduction of ZrB₂ particles further reduced the weight gain for the composite coating.After oxidation at 1000?for 200 h,the weight gain of the composite coating with 3wt.%ZrB₂ was only 0.66 mg/cm².This is because that the oxidation reaction between ZrB₂ particles and inward diffused oxygen not only consumed some oxygen,but also generated B₂O₃?l?which could improve the local flowability of the glass-ceramic coating,and heal the interface voids.Meanwhile,the high temperature gasification process of B₂O₃ could take away the heat of the coating,reducing the weight gain of the specimen,and improving the high temperature oxidation resistance However,the 1wt.%ZrB₂ doped composite coating exhibited a more stable interfacial structure?Al₂O₃layer/Z phase layer?due to ZrO₂ which formed by the process of oxidation of ZrB₂would react with the silicate glass phase at high temperature to form K₂ZrSi₃O₉,which consumes the silicate glass coating gradually and reduce the oxidation resistance of composite coating.But the electrodeposited SiO₂ interlayer dissolves into the glass phase at high temperatures delaying the coating failure.