Structure,mechanical Property,thermal Stability And Oxidation Resistance Of Cr-Al-O-N And （Cr,Al）₂O₃ Coatings

The physical vapor deposited Cr Al-based nitride coatings with great mechanical properties,excellent wear resistance and oxidation resistance are generally used as the surface protective coating of cutting tools to improve their performance and prolong their life.As the development of modern high-speed dry cutting technology and the growing demand for difficult-to-process materials,increasingly harsh machining environments,such as high temperature,high stress,high friction,and strong impact,etc.,have put forward higher requirements for the performance of coatings.The performance of traditional Cr Al-based nitride coatings needs to be optimized to meet those requirements.The addition of O gives rise to the structural transformation and property optimization of the nitride coating by introducing ionic bond into the material which is dominated by covalent and metal bond.The coating transforms from nitride to oxynitride and then to oxide with the increase of oxygen content,which results in the changes in mechanical properties,thermal stability,and oxidation resistance of the coating.For the quaternary Cr-Al-O-N coatings,most of the current studies focused on the composition,microstructure,and mechanical properties of the materials,and very few report on the oxidation behavior of coatings is available.For the ternary Cr-Al-O coating,the growth mechanism and the low temperature preparation technology ofα-（Cr,Al）₂O₃ coating with single-phase corundum structure need further exploration.Based on the above reasons,in this study,Cr-Al-O-N and Cr-Al-O coatings with different components were prepared.The structure and properties of the coatings were modified by adjusting the deposition parameters and constructing nano-multilayers.The main achievements were as follows:（1）The addition of O into(Cr_0.70Al_0.30)N leads to the formation of(Cr_0.70Al_0.30)(O_xN_1-x)_εsolid solution with face-centered cubic structure.A small amount of O-doping（x≤0.21）significantly improved the oxidation resistance by promoting the formation of a protective Al-rich oxide layer on the coating surface.Whereas a large amount of O-addition（x≥0.69）improved the thermal stability of the coating by inhibiting the decomposition of Cr–N bond.The(Cr_0.72Al_0.28)(O_0.86N_0.14)_1.40 coating showed red hardness ability whose hardness remained at 27.5 GPa after annealing at 1200°C.（2）With the increase of Al content,the oxidation resistance of the(Cr_1-xAl_x)(O_yN_1-y)_εcoating firstly increased and then decreased.The oxidation kinetics study showed that the oxidation activation energy of the coating reached a peak of 370.7 k J/mol（metallic atoms）when x=0.58.The addition of excessive Al increases the oxygen content of the coating,leading to the decreased molar volume after oxidation.Thus,the oxide scale formed on the surface of the coating is underdense and unable to protect the coating from oxidation.In addition,a self-organized multilayer structure with alternating N-rich and O-rich layers was observed in the(Cr_1-xAl_x)(O_yN_1-y)_εcoating.After annealing at 900°C,ordered diffusion of elements occurs to form Cr&N-rich and Al&O-rich layers.The Al&O-rich layer decomposed preferentially with the increase of annealing temperature.（3）The structure of the ternary(Cr_1-xAl_x)₂O₃ oxide coating is correlated to the Al content.when x≤0.3,the coating was in single-phase corundum structure,and when x≥0.52,the coating transformed to face-centered cubic/corundum/amorphous mixed structure.The hardness of coating was firstly increased and then decreased with the increase of Al content.Single-phase(Cr_0.70Al_0.30)₂O₃ coatings with corundum structure showed excellent thermal stability,whose hardness increased from26.2±1.7 GPa to 28.2±1.4 GPa after annealing at 1200°C.The(Cr_0.48Al_0.52)₂O₃ and(Cr_0.27Al_0.73)₂O₃ coatings with mixed structure transformed from fcc-（Cr,Al）₂O₃ toα-（Cr,Al）₂O₃ at～1100°C,resulting in a decrease in the hardness of the coatings.（4）The nucleation and growth ofα-(Cr_1-xAl_x)₂O₃ can be promoted by increasing oxygen pressure and substrate bias during deposition.The increase of deposition pressure from 0.8 Pa to 3.2 Pa reduces the energy of bombarding particles and the vacancy defects in the coating,resulting in the formation ofα-（Cr,Al）₂O₃.However,the decrease of energy of deposited particles leads to the decrease of compactness of coating and the deterioration of mechanical properties.The formation ofα-（Cr,Al）₂O₃ was also promoted by increasing the substrate bias.The single-phaseα-(Cr_0.46Al_0.54)₂O₃ coating was prepared at-120 V bias,and the hardness remained at～30 GPa after annealing at 1200°C.（5）Based on the template effect of nano-multilayer coatings,α-(Cr_0.70Al_0.30)₂O₃ was used as the template layer to modify the structure of(Cr_0.48Al_0.52)₂O₃ layer.The single-phase corundum structured(Cr_0.70Al_0.30)₂O₃/(Cr_0.48Al_0.52)₂O₃ multilayer coatings with outstanding thermal stability and red hardness were successfully prepared.