Research On Thermal Stability And Mechanical Properties Of Yttrium Oxide Thin Films

Y₂O₃ has a great application potential as reaction barrier coating of hightemperature composites due to its high thermodynamic stability and high melting point,and the phase structure stability at high temperature and structure dependent mechanical property are key parameters for this application.In this work,Y₂O₃ thin films were deposited on silicon?100?wafers by DC magnetron sputtering with various oxygen partial pressure and substrate bias,and then vacuum annealing at 1000?C was performed to investigate the phase structure stability.The microstructure,stress and hardness of as-deposited and annealed Y₂O₃ thin films have been explored by X-ray diffraction,transmission electron microscope,and nanoindenter.The result shows that as-60 bias voltage is applied to substrate,cubic-c phase forms regardless of variation of oxygen partial pressure,and the cubic-c phase remains stability and crystal inity becomes better after annealing at 1000?C.In addition,the hardness and modulus also just have minor changes as a function of oxygen partial pressure.Characterization of elemental ratio shows that the oxidation state did not change with different oxygen partial pressure.The main causes is that oxygen chemical potential gives little contribution to various defects formation energy.As oxygen partial pressure is kept at 0.043 Pa,phase transition from cubic-c to monoclinic-b phase takes place with increasing substrate bias,accompanying by the increment of hardness and modulus,and 1000?Chigh-temperature annealing results in that as-deposited monoclinic-b phase transforms to cubic-c phase.Y₂O₃ thin films with different oxygen vacancy contents have been achieved through varying substrate temperature?Ts?.O/Y atomic ratio decreases continuously from 1.52 to 1.37 with increasing Ts from 25 ?C to 600°C,corresponding to an increase in oxygen vacancy concentrations.For as-deposited Y₂O₃ films,oxygen vacancies present at high Ts can promote the nucleation of monoclinic phase,whereas high Ts can also induce the instability of monoclinic phase.Consequently,the observed structural evolution is dominated by the competition between formation of monoclinic phase caused by oxygen vacancy concentrations and instability of monoclinic phase induced by high Ts,in which cubic phase present at 25 ?C transforms to monoclinic phase at 200 ?C,and then changes to a mixture of cubic and monoclinic phases at higher Ts?400 ?C,600 ?C?.During post annealing treatment,the cubic structure goes on keeping stable,whereas the monoclinic phase with oxygen deficiency present in as-deposited Y₂O₃ films is not thermodynamic stable at high temperature,which can transfer to more stable phases including cubic or oxygen defective phase depending on oxygen vacancy contents.There is a critical value of oxygen vacancy content,below which monoclinic phase transits to cubic phase for film deposited at Ts=400 ?C,and above which transforms to the oxygen defective phase for film grown at Ts=600 ?C.Annealing treatment also leads to grain growth and relaxation of compressive stress,further reducing the hardness by weakening stress hardening and grain size strengthening.Compressive stress releases mainly through film horizontal contraction caused by grains growth and phase transition.The high hardness just appears in Y₂O₃ films with mixed phase configuration including cubic + monoclinic for as-deposited films deposited at Ts= 400 and 600 ?C as well as cubic + oxygen defective phase for postannealed film deposited at Ts= 600 ?C,in which the interfaces between different phases may block the dislocation movement and give rise to the enhanced hardness.To maintain stable as well as favorable mechanical performance in a large temperature range,oxygen vacancies could be introduced to help maintaining multiphase structure in Y₂O₃ films.