Analysis Of Interfacial Failure In Aluminum Matrix Al₂O₃ Coating Under Thermal And Mechanical Loads

Aluminum alloy honeycomb structures have been widely used in aviation, aerospace and automotive fields due to their advantages such as high specific strength, high specific modulus and lightweight. But because of the disadvantage of poor heat resistance as well as wear resistance, the application has been limited in some fields. The Al₂O₃ coating was prepared on the surface of alu minum alloy honeycomb components by anodic oxidation technology to improve these problems. The damage and failure process of the coating and the interface of Al-substrate/coating materials under thermal and mechanical loads were studied by numerical simulation and experimental methods. The research work in this paper can provide guidance for the practical engineering application of Al-substrate/coating materials. The main contents are as follows:Firstly, 6061 alloy honeycombs and standard tensile specimens with 54 μm thick Al₂O₃ coatings were prepared by the hard anodic oxidation technique. Then a series of characterization and performance tests were carried out on the Al₂O₃ coatings. Micro Vickers hardness of the coating is 409.0HV. γ- Al₂O₃ was detected by X-ray diffraction（XRD）, indicating that the main phase of the coating is γ- Al₂O₃. And the ratio of atomic numbers of O and Al is greater than 2:1, which further indicates that the coating is composed of amorphous Al₂O₃ and crystal Al₂O₃. The material is composited by Al-substrate, a barrier layer and coating by a scanning electron micro-scope（SEM）. There is a barrier layer with thickness of about 3μm between the Al-substrate and the coating, which shows a good agreement with the Keller model.Secondly, the damage and failure of the coating and the interface between Al-substrate and coating under thermal loads were studied. The residual stress at the interfaces caused by heat treatment and quenching was determined by experimental and theoretical estimation methods. Then the influence of residual stress on the interface and coating was analyzed. Thermal stresses and interfacial failure of interface under thermal loads were simulated by introducing the cohesive zone model（CMZ）. The simulated results agree well with the experimental ones. The influences of the coating thickness and the thermal load as well as the coating crack density on the interfacial stress and failure were investigated. From the simulated and experimental results it can be found that there are no parallel interfacial cracks at a temperature below 300 ℃. But some parallel interfacial cracks were observed at the temperature of 400 ℃. The simulated results are consistent with the experimental ones. Under thermal loads, the interfacial damage will be aggravated with the increase of the prefabricated vertical crack density. While as the prefabricated vertical crack density reaches a critical value, the interface damage is decreased and then tends to be a stable value, indicating that there is an optimal prefabricated vertical crack density.Finally, the damage and failure of the interface and the coating under tension loads were studied. Based on the CMZ model, a two-dimensional finite element model was established to simulate the failure process of the interface under tensile loads, which was also verified by the experimental results. The influences of the coating thickness and the thermal load as well as the coating crack density on the interface stress and failure were investigated. Our simulated results showed that the critical tensile strain for the initiation of the parallel interface crack is about 0.8%, which is closed to the experimental tensile strain of 1%. For a low prefabricated vertical crack density, cracks initiate from the surface of the coating and propagate from the surface to interface in Al-substrate under the tensile load. For an intermediate prefabricated vertical crack density, cracks initiate from the surface and the central section of the coating, or the interface. While for a high prefabricated vertical crack density, cracks originate from the interface and extend to the surface of the coating. The simulation results are in good agreement with the experimental data.