Microstructure And Performance Of Cr-Al-Y Co-deposition Coating On The Surface Of DZ125 Alloy

DZ125 alloy is widely used in the aerospace field,but its application is seriously restricted due to its relatively poor resistance to high-temperature oxidation and high-temperature hot corrosion.In order to improve the comprehensive performance of DZ125 alloy in the high-temperature service environment and solve its failure behavior caused by hot corrosion and high-temperature oxidation,a Cr-Al-Y infiltrated layer was prepared on the surface of DZ125 alloy by the embedded infiltration technology,and scanning electron microscope（SEM）、X-ray diffraction（XRD）、energy dispersive spectroscopy（EDS）、atomic force microscopy（AFM）and other characterization methods analyzed the influence of different catalysts,different Al element content,and different rare earth element content on the microstructure of the infiltrated layer,revealed the principle of forming the infiltrated layer,and studied the thermal shock resistance,friction and wear performance,high temperature oxidation resistance,high temperature hot corrosion resistance of the Cr-Al-Y coating,and expounded its failure mechanism and protection mechanism.The research shows that there are great differences in the thickness and compactness of each layer of the carburizing layer prepared by using different types of catalysts,different rare earth element contents and different aluminizing agents,and there are differences in the thickness and compactness of each layer of the carburizing layer.Among them,AlCl₃·6H₂O is used as the catalyst to prepare the infiltration layer,which is divided into three layers.The infiltration layer is obviously layered,uniform and dense;However,the permeation layer prepared with NH₄Cl and NaF as catalysts has only two layers,with a high degree of heterogeneity and a large number of pores;The thickness of the infiltration layer increases first and then decreases with the increase of rare earth element content.When the rare earth element content is close to 2 wt.%,the prepared infiltration layer has a three-layer structure,with uniform tissue distribution,and each layer is more compact.The penetration layer prepared by adding 7 wt.%Al into the penetration agent has a three-layer structure,with uniform distribution of the structure,dense structure and appropriate thickness.To sum up,the optimal process proportion for the preparation of the infiltration layer is determined as:10Cr-7Al-8AlCl₃·6H₂O-2Y₂O₃-73Al₂O₃（wt.%）,the preparation temperature of the coating is 1050℃,and the holding time is 2 h.Thermal shock test shows that the surface of DZ125 alloy sample will be oxidized unevenly during thermal shock.Under the effect of thermal shock,the oxidation area protruding into the matrix produces impact stress concentration,which becomes a potential crack source.Some crack sources expand along the crack tip during the subsequent thermal shock process,but a large number of oxygen elements are detected at the crack tip,indicating that O elements enter along the crack channel.Under the synergistic effect of oxidation and thermal shock stress,the thermal shock cracks of DZ125 alloy continue to expand.For the Cr-Al-Y coating,during the thermal shock process,the surface will also be oxidized,and cracks will be generated under the effect of thermal shock stress.However,after the crack extends,the oxide mainly composed of Al and Cr inside the crack blocks the entry of O elements to some extent.At the same time,the gradient structure of the infiltrated layer has a certain buffer effect on the thermal stress impact.Therefore,the crack growth in the infiltrated layer is mainly caused by the thermal shock stress,Moreover,the progress of cracks is slower than that of DZ125 matrix.In addition,there is no cracking,peeling and other phenomena at the membrane substrate junction after multiple thermal shock tests,which proves that the bonding strength of the membrane substrate is good.The analysis of friction and wear experiment shows that ploughing wear and abrasive wear occur to DZ125 alloy in the friction process,while abrasive wear only occurs to the surface of the infiltrated layer.Because the oxide film on the surface of the infiltrated layer can effectively prevent the GGr15 ball from directly contacting with the matrix during the wear process,and the Al₂O₃and Cr₂O₃formed simultaneously play the role of lubricating phase,thereby reducing the friction coefficient and the wear rate,It shows that the Cr-Al-Y coating can effectively improve the friction and wear resistance of DZ125 alloy.The high temperature oxidation experiment shows that the Cr-Al-Y coating shows a stable oxidation rate at different temperatures,and its high temperature oxidation kinetics curve shows the law of weight gain first and weight loss later.The average weight loss of the coating is only 0.85 mg/cm²after oxidation for 100 hours at different temperatures.The excellent high temperature oxidation resistance of the Cr-Al-Y coating is mainly due to the selective oxidation of components in the coating in the high temperature environment,thus forming a protective double-layer Al₂O₃and Cr₂O₃oxide film structure,double oxide film can effectively prevent the oxidizing atmosphere from directly contacting with the matrix,thus reducing the oxidation rate.The hot corrosion experiment analysis of DZ125 alloy and Cr-Al-Y coating in mixed molten salt shows that the high-temperature hot corrosion resistance of DZ125 alloy is significantly improved due to the existence of Cr-Al-Y coating in the Na₂SO4-K₂SO4molten salt system.The main reason is that during the hot corrosion process,the surface of the infiltrated layer will form a double-layer Al₂O₃,Cr₂O₃oxide film structure,which is continuous and dense,and on the one hand,it blocks the external diffusion rate of Al,Cr,Ni elements,On the other hand,it slows down the corrosion of the corrosion medium on the interior of the substrate.With the continuous hot corrosion,sulfur and oxygen elements infiltrate into the coating along the microcracks of the carburizing layer and self sustained vulcanization and oxidation occur,which is the main reason for the corrosion degradation of the coating.