Study Of HfO₂ Distribution And Growth Mechanism For The Oxide Scale Formed On CoNiCrAlHf Alloy In Water Vapour

Thermal barrier coatings are widely used in turbine engine hot sections,such as combustion chambers and turbine blades of gas turbine engines,to improve its working temperature and service life.It is one of the key technologies in the research and development of advanced gas turbines.With the continuous development of commercial verification of hydrogen-fired gas turbines,the service environment of hot sections is even harsher.Oxidation behavior is a key factor leading to the failure of thermal barrier coatings.The relevant researches on the oxidation behavior of the cobalt-based alloy bond coat at high temperature and high water vapor content,the distribution characteristics and formation mechanism of active element oxides,and the phase transition mechanism of Al2O3 under water vapor are still not clear enough.Therefore,this thesis takes CoNiCrAlHf alloy as the research object,and studies the internal defects of oxide layer formed on CoNiCrAlHf alloy and the interaction between HfO2 particle distribution in water vapor;the high-temperature oxidation resistance and HfO2 distribution characteristics of the co-doped alloys were characterized and studied under several content of water vapor atmospheres;the effects of different preparation methods on the oxide scale of 7YSZ+CoNiCrAlHf thermal barrier coatings were studied;The cyclic lifetime of the thermal barrier coating was analyzed.The distribution of HfO2 particles,the internal defects of the oxide scale and the phase transition process of alumina under different failure modes were analyzed,and the effect of multi-factor synergy on the cyclic lifetime of the coating was understood.The specific research contents are as follows:The distribution of HfO2 particles affects the formation of defects in the oxide scale.The uniform distribution of HfO2 particles in the oxide scale can effectively inhibit the growth of voids in the oxide scale.The difference in the internal defects of the oxide layer of the CoNiCrAlHf alloy prepared by different preparation methods is caused by the difference in the distribution of HfO2 particles.The generation of internal cracks and voids in spinel oxide on the surface of the alloy is caused by the rapid growth of spinel oxide in water vapor environment.The increase of water vapor content from 25%to 35%promoted the inward diffusion of O along the grain boundaries,resulting in the rapid growth of columnar alumina inside the oxide scale.Water vapor promotes the formation of linearly distributed HfO2 particles.With the increase of oxidation time from 24 h to 48 h in water vapor atmosphere,the band of linearly distributed HfO2 particles in the oxide layer of CoNiCrAlHf alloy gradually widened due to the growth and upward movement of HfO2 particles at the interface.The water vapor content can control the distribution of HfO2 particles.Water vapor increases the flux of O diffusion along grain boundaries,and different water vapor contents make the nucleation and growth of HfO2 particles different.When the water vapor content increases from 25%to 35%,the linearly distributed HfO2 particles in the oxide layer of CoNiCrAlYHf alloy move to the middle of the oxide scale.The water vapor content can control the size of HfO2 particles.As the water vapor affects the oxygen potential gradient inside the oxide layer,the outward diffused Hf reacts with O rapidly,which accelerates the massive nucleation of HfO2 particles.However,the outward diffused Hf cannot maintain the continuous growth of HfO2 particles,so only fine HfO2 particles are formed inside the oxide scale.The water vapor atmosphere delays the phase transition process of alumina,which is due to the fact that 25%water vapor promotes the formation of γ-AlOOH transient products,while the phase transition process of γ-AlOOH→γ-Al2O3→δ-Al2O3 prolongs the phase transition time of metastable alumina to steady state alumina.The phase transition process of vacuum cast CoNiCrAlHf samples is slower than that of arc smelted samples.The water vapor content does not affect the phase transformation of the oxide layer of the Y-modified CoNiCrAlHf alloy.The transient products formed at the interface of CoNiCrAlYHf alloy oxide layer under the condition of different water vapor content are all γ-Al(OH)3.The transient product of CoNiCrAlYHfSi alloy at the oxide scale/substrate interface is γ-AlOOH in air+25%water vapor atmosphere,while the transient product is γ-Al(OH)3 in 35%water vapor atmosphere.The water vapor content increases the hydroxyl groups of the transient product,while the dehydroxyl ati on reaction during oxidation retards the phase transition of alumina.Water vapor accelerates the generation and propagation of spinel cracks on the surface of the oxide scale.The failure of 7YSZ-CoNiCrAlHf coating is mainly due to the formation of spinel at the top coat/oxide scale interface.The propagation of internal cracks in the spinel lead to the spalling of the coating.High-frequency cyclic oxidation results in the formation of a large number of defects at the top coat/oxide scale interface of the 7YSZ-CoNiCrAlHf coating.The distribution of HfO2 particles can affect the voids inside the oxide layer.The refined HfO2 particles are evenly distributed in the oxide layer,which can effectively inhibit the generation of hole defects in the oxide layer.The 7YSZCoNiCrAlY coating loses the effect of active elements due to the complete consumption of Y in the alloy,resulting in the failure of the oxide layer after reaching the critical thickness.The failure of 7YSZ-CoNiCrAlYHfSi coating under high temperature water vapor mainly occurs at the oxide scale/substrate interface,and the formation and propagation of cracks at the interface are caused by metastable alumina.By adjusting the distribution of HfO2 particles in the oxide layer,the cyclic lifetime of the coating can be significantly improved.