Al₂O₃-Y₂O₃ Thin Films By Reactive Magnetron Sputtering: Fabrication And High-temperature Oxidation Resistance Properties

Al₂O₃-Y₂O₃,as a typical ceramic-ceramic composite material,has high melting point and low thermal conductivity.Besides,the fact that the thermal expansion coefficients of Al₂O₃ and Y₂O₃ are close makes it have potential in excellent high-temperature oxidation resistance and becoming a new alternative for coatings.Reactive magnetron sputtering,as a common and an attractive thin film deposition method,is frequently used in the industry scale as it is a flexible technique to deposit different kinds of films from metals to ceramics,and adjust properties of films which are uniformly forms over the substrates.However,this flexibility makes it difficult for reactive magnetron sputtering to control the deposition process.As a result,the texture and properties of thin films are very sensitive to the deposition conditions,which greatly limits the further practical applications.Therefore,it is of great value to systematically investigate the influence of these deposition conditions and the instability causes during depositions process on the texture and properties of thin films.Based on the results,improvement measures in deposition parameters and experimental configuration put forward to stabilize the Al₂O₃-Y₂O₃ thin film structures which are of important theoretical and practical significance.The objective of this work is to have a better understanding of the x Al O_1.5-（1-x）YO_1.5thin film properties deposited by reactive magnetron sputtering.In this thesis,the Al₂O₃-Y₂O₃ thin film with different Al content deposited by reactive magnetron sputtering are represented by x Al O_1.5-（1-x）YO_1.5thin film.We firstly focused on the influence of deposition parameters and the instability in the experimental configuration on the Y₂O₃ thin films texture and properties.Meanwhile,we suggested different measurements to improve the stability in reactive magnetron sputtering deposition of oxides.Then,we deposited the x Al O_1.5-（1-x）YO_1.5thin films based on the optimal experimental configuration using dual DC magnetron sputtering and investigated the texture and properties over the substrates.The oxidation behaviors of x Al O_1.5-（1-x）YO_1.5thin films are also studied.The main results are summarized as follows:（1）Y₂O₃ thin films were deposited on Si（100）substrates by reactive magnetron sputtering.A passive thermal probe and Monte Carlo simulations were used to calculate respectively the energy/momentum transfer per arriving atom（EPA/MPA）at the substrate which is strongly varied by the adjustment of the discharge voltage and the target-to-substrate distance.The relationship between EPA and the phase evolution,grain size,microstructure,packing density and residual stress was investigated in detail.A study of the thin film cross sections showed for all films deposited by EPA ranging from 52 to 132 e V/at straight columns throughout the thickness,typically for a zone II microstructure.The grains tend to grow in the fastest growth directions.At low EPA,the films consist of the monoclinic B phase.A minority cubic C phase appears at higher EPA.Under the given conditions,a linear relationship between MPA and EPA.The density and intrinsic stress were evaluated as function of MPA.The obtained results suggest that a densification effect occurs.It is commonly accepted that the energetic atoms transfer their momentum towards the growing film,and cause rearrangement of the surface atoms.As the momentum increases,the deposited atoms are more tightly packed,and the defects are annihilated.As a result,the impingement of sputtered atoms,and reflected neutrals,also result in a compressive stress at higher MPA values.（2）A strong variation in the film texture across the sample for experiments with a stationary sample stage was noticed.The instability in the texture and properties of Y₂O₃ thin films caused by number of pipes and the ratio of Ar/O₂ flows in the reactive magnetron sputtering deposition was investigated systematically.We found the instability can be traced back to the uneven local oxygen content.The（111）peak for the monoclinic phase is shifted to higher angle with decreasing oxygen content.The（111）texture coefficient didn’t change as a function of the the ratio of Ar/O₂ flows.Sample rotation resolves this problem.However,except for films produced at the lowest rotation speed,no change of the peak position and the texture coefficient as a function of the rotation speed is observed.This instability can be further improved by the adjustment of geometrical configuration.A support ring was installed to fix the oxygen pipes.It helps to improve the homogeneity of oxygen distribution over the substrate and the stability of Y₂O₃ thin films.The support ring somewhat works as an“energy filter”.It also collects the sputtered atoms,and makes it difficult for low energetic atoms reach the sample surface.Thus,MPA increased with the installation of support ring and facilitated the formation of monoclinic phase.（3）x Al O_1.5-（1-x）YO_1.5thin films with different Al concentration were deposited by dual reactive magnetron sputtering.Simulations based on Monte Carlo method were used to calculate the relative Al concentration（Al/（Al+Y））and momentum transfer during deposition process.Comparing to the measured relative Al concentration,it suggested that the angular distribution of sputtered Y and Al atoms corresponds to the cosine and under-cosine distribution,respectively.The simulation results also prove that sample rotation can compensate the inhomogeneous distribution of Al which results from the angle between the target center and the substrate holder.The MPA for each deposition condition was also calculated based on simulation results.It demonstrated that the momentum transfer was driven by almost Y（and Argon）as Al has a much lower mass compared to the average mass of the thin film and cannot contribute strongly to the total momentum transfer.At low Al content,the match with the theoretical density is good.The structure of x Al O_1.5-（1-x）YO_1.5thin films changes with Al concentration.x Al O_1.5-（1-x）YO_1.5thin films with Al concentration less than 4.0 at.%exhibited in a crystalline state.At higher concentration,the films became amorphous.（4）The oxidation resistance of x Al O_1.5-（1-x）YO_1.5thin films with different Al concentration prepared by reactive magnetron sputtering on the Si（100）and Ti Al alloys were investigated.When being exposure at 1000℃,there existed a phase transition from monoclinic to cubic Y₂O₃ among the x Al O_1.5-（1-x）YO_1.5 thin films with less than 7.2 at%Al content.For the samples with Al content from11.7 to 19.4 at.%,the oxidized thin films consisted of Y₂O₃,YAM andα-Al₂O₃ phases.For the samples with Al content from 30.2 to 37.0 at.%,the oxidized thin films consisted of YAM andα-Al₂O₃ phase.For For the samples with higher Al content to 70.0 at.%,the oxidized thin films consisted of YAG andα-Al₂O₃ phase.The x Al O_1.5-（1-x）YO_1.5 thin film deposited on Ti Al alloys exhibits double-layered structure after the high-temperature oxidation.It is composed of a x Al O_1.5-（1-x）YO_1.5outer layer,a Ti O₂+Al₂O₃ inner layer.The thermal stress at the outer layer/inner layer interface is tensile stress,while the stress at inner layer/Ti Al interface is compressive stress.With the increasing Al content in x Al O_1.5-（1-x）YO_1.5thin films,the stability of the outer layer increases and the net stress decreases.