氧化铝薄膜与镍基底的高温界面强度跨尺度表征

Ceramic coatings are widely used as surface thermal protections for high temperature parts of airspace engines. Thermal growth Al2O3 layers, formed by oxidation of precipitated Al from bond coats between superalloy substrates and ceramic top coatings at high temperatures, will cause an interface crack easily and lead a failure of the related parts. In order to understand the related interface failure mechanism and design a new multi-layer structure, the evolution of interface strength between Al2O3 thin films and Ni substrates with temperature and the related failure behavior will be studied in this project. Firstly, a coupling effect among the temperature, composition, and the grain size of Al2O3 thin films on interface energy between the thin films and the substrates will be studied theoretically. Combining the interface energy with interface fracture energy and the interface constitutive (i.e. interface stress-displacement relation), a trans-scale model on interface strength with temperature will be developed. Further, by introducing the interface cohesive elements based on the interface constitutive, simulations of interface failure of Al2O3 thin films/Ni substrates under a series of high temperatures or four-point bending loading will be carried out by using the finite element method. The effects of interface strength, toughness and film (substrate) thickness, strength, and toughness on failure behaviors will be studied. Meanwhile, nanocrystalline and polycrystalline Al2O3 thin films, with a series of thicknesses, deposited or sprayed on Ni substrates will be prepared, and the related interface failure study of these samples at high temperatures will be performed to verify the theoretical model and simulation results.

航空发动机热端部件广泛使用陶瓷涂层作为高温合金部件的表面隔热层，表层与合金基底之间的合金粘结层在高温服役时铝元素析出，形成的热生长氧化铝层易引起界面开裂导致表层剥离，致使部件失效。为了厘清氧化铝层与界面失效的关系及设计新型多层涂层，本项目研究氧化铝薄膜与镍基底的界面强度随温度、成分及微结构特征的演化及其失效行为。首先从理论上研究成分、尺度及温度效应对界面能的耦合影响，结合界面能与界面断裂能的热力学关系及界面本构，发展界面断裂能及界面强度的跨温度尺度耦合模型。进一步结合界面粘聚模型开展氧化铝薄膜/镍基底在高温下界面失效的有限元模拟，分别研究界面强度、薄膜韧度和薄膜厚度等与微结构相关的材料性能和几何参数对薄膜/基底体系的界面应力演化及损伤失效影响。同时制备镍基底上系列厚度的纳米晶和微米晶氧化铝薄膜，对系列薄膜样品在高温下的损伤失效行为进行实验研究，验证理论和模拟，以提供新型多层涂层设计依据。

项目针对航空发动机高温材料中常涉及到的氧化铝薄膜与镍基底的高温界面强度表征这一关键科学问题，开展了氧化物薄膜（涂层）/金属基底界面力学性能的宏观、微观多尺度研究，发现了膜/基结构界面强度、断裂能、损伤与断裂行为的尺度效应，取得了系列创新成果，构建了膜/基结构界面断裂能与界面强度的跨尺度模型，揭示了界面损伤与断裂行为的跨尺度机制。考虑到理解氧化物/金属界面失效的微观机制对有效表征界面强度至关重要,项目首先研究并揭示了氧化物/金属界面剪切失效的微观机制是由界面失配位错主导。创新性地表征了氧化铝薄膜/镍基底界面损伤与断裂性能的尺度效应：当氧化铝薄膜晶粒尺度从微米尺度减小到百纳米以下时，其与基底间的界面拉伸断裂能可减小近一半；而宏观界面强度可增加一倍多；当薄膜厚度从几十微米增加到几百微米，界面损伤速率增加，即界面断裂更趋向灾变脆断。更重要地，发现面内残余应力降低了界面强度。最后通过开展氧化物涂层(薄膜)/基底样品的高温热震循环实验，揭示了涂层及界面的实际断裂失效特征及应力机制：淬火热震后，应力为压应力，涂层越厚，压应力越大，从而厚涂层界面更易开裂失效，热震循环失效次数更少。这些结果为设计界面结构宏微观尺度、提高相关部件界面可靠性提供了重要依据。