纳米ZrO2-Al2O3/碱土金属铬酸盐复合材料的制备与高温自润滑机理研究

Unlike traditional solid lubricants of graphite and MoS2, alkaline-earth metallic chromates do not have the layered crystal structure of a weak van der Waals bonding, however, they exhibit an excellent high-temperature tribological properties. Until to now, the reason why these chromates have a good self-lubricity at elevated temperatures remains still unclear. As self-lubricating composites or coatings incorporated with different contents of alkaline-earth metallic chromates are subjected to the changes in chemical composition and crystal structure during high-temperature friction and wear process, it is of great significance to understand the influences of these changes on lubrication mechanisms and developing friction-reducing and antiwear materials and their manufacturing techniques used in extreme environments. This project concentrates mainly on phase transformation mechanisms and high-temperature self-lubricating characteristics of alkaline-earth metallic chromates for developing novel solid lubricants by using both theoretical calculations and experimental methods. The first principles calculations are used to investigate the crystal structure, electronic structure, bonding energy and mechanical properties to establish a relationship between the microscale characteristics of chemical compositions and crystal structures and intrinsic mechanical properties. Based on the above-mentioned results, novel alkaline-earth metallic chromates and their self-lubricating composites will be designed by tailoring their chemical constituents and interfacial structures with powder metallurgy methods and appropriate post-heat treatments to unveil their mechanical properties, tribological properties, and self-lubricating mechanisms at different temperatures. A variety of novel high-temperature solid lubrication materials can be obtained by developing different fabrication methods.

与传统固体润滑材料(如石墨和MoS2等)不同，碱土金属铬酸盐不具有弱结合键的层状结构，但它具有非常优异的高温摩擦学性能，目前产生高温自润滑的原因仍不清楚。含有碱土金属铬酸盐的自润滑复合材料或涂层在高温摩擦磨损过程中出现了化学组成与晶体结构的变化，弄清结构变化对理解微观润滑机理和开发在极限环境下使用的减摩耐磨材料及其制备技术十分重要。本项目针对新型高温固体润滑材料的设计需求，以碱土金属铬酸盐为研究对象，拟从理论模拟计算和试验研究入手，采用第一性原理研究其晶体结构、化学键强度和力学性质等，建立微观尺度特性（成分组成和晶体结构）与本征宏观性质的关系。在此基础上优化设计与制备新型碱土金属铬酸盐与纳米陶瓷基自润滑复合材料，通过不同的后续热处理工艺调控材料中物相组成与界面结构，阐明其力学性能、不同温度下的摩擦磨损性能和自润滑机理，并通过制备技术的突破获得一系列新型的高温固体润滑材料。

虽然碱土金属铬酸盐不具有弱结合健的层状结构，但其高温摩擦学性能非常优异，目前产生高温自润滑能力的原因仍不清楚，弄清其结构演化对理解微观润滑机理和开发在极限环境下使用的减摩耐磨材料及其制备技术十分重要。本项目采用第一性原理对碱土金属铬酸盐的平衡晶体结构与本征力学性质进行了计算与设计，开发出不同碱土金属铬酸盐粉体材料、ZrO2-Al2O3陶瓷以及一系列高温自润滑复合材料与涂层的低成本制备工艺，高温磨损试验结果发现ZrO2-Al2O3基自润滑复合材料具有优异的高温摩擦学性能，通过设计不同的后续热处理工艺，阐明了碱土金属铬酸盐在不同温度和气氛下的化学组成与结构演化，揭示了ZrO2-Al2O3基复合材料与涂层在宽温域范围的自润滑机理。采用化学沉淀法和高温固相法制备出分散良好具有不同粒径和晶体形貌的BaCrO4和BaCr2O4粉体。两种铬酸盐粉体在不同热处理工艺条件下可以互相转变，且相转变过程与气氛中的氧分压有直接关系，发现了铬酸盐晶体在热处理过程中的再生长现象。采用热压烧结制备出具有微纳尺度晶粒的近全致密 ZrO2-Al2O3陶瓷。通过建立“致密度-晶粒尺度”曲线，确定了晶粒粗化的临界温度和最佳可控晶粒范围。ZrO2-Al2O3陶瓷在800℃时的摩擦系数和磨损速率为1.15和4.77×10-4mm3/Nm，高温磨损机制为沿晶脆性断裂和剥离磨损。调控氧化锆成分，采用固体激光对ZrO2-Al2O3 陶瓷表面进行重熔改性，获得外延生长极细纳米共晶层，阐明了共晶陶瓷集群结构的生长及取向演化。而烧结态ZrO2-Al2O3-BaCrO4复合材料具有突出摩擦学性能，磨损表面完全覆盖了一层BaCrO4细晶粒层。自润滑复合材料经过不同热处理工艺后，表面形成了针状、薄片状和类花妮状的细小析出物，并伴随复杂的氧/铬含量与结构的变化。采用不同热处理工艺可在表面生成形貌与结构不尽相同的新铬酸盐晶体，在摩擦磨损过程起到了固体润滑与修复作用，摩擦表面的明显塑性变形与流动降低了复合材料的摩擦与磨损。通过优化设计热处理工艺，可调控其摩擦表面结构与固体润滑性质，获得宽温域摩擦系数低于0.3、磨损速率在10-6mm3/Nm 数量级的ZrO2-Al2O3-BaCrO4自润滑材料与涂层。