MAX陶瓷抗热震和耐烧蚀性能及其机制研究

MAX ceramics have abnormal thermal shock resistance, crack healing ability and good ablation resistance, which make these materials potential candidates in key components for oxidizing and high temperature environment applications. However, the main mechanisms of their abnormal thermal shock resistance and ablation resistance have not been revealed yet. This project focuses on the investigation of the thermal shock and ablation resistances of Ti2AlC and Cr2AlC as two representative members of MAX ceramics, based on the previous research and enlightening results. The main contents include: firstly, confirming key processing parameters to prepare Ti2AlC and Cr2AlC materials with tunable microstructure-performance and to synthesize MAX-based high temperature composites as well as to compare their high temperature properties including oxidation resistance, thermal shock behavior and ablation resistance; secondly, researching on the influences of cooling media, reactive products formed at high temperature, and microstructures on thermal shock behavior of Ti2AlC and Cr2AlC; thirdly, investigating the contributions of ablation products, formation of liquid, oxidation layer, microstructure and second phases to the enhanced ablation resistance. The main purpose of this project is to elucidate the mechanism of the thermal shock resistance of MAX ceramics which has resisted interpretation for about 17 years, and to further reveal the mechanism of their ablation resistance. It is reasonable to believe that creative results can be achieved by performing this project. The present study is beneficial to promote the applications of MAX phases in oxidizing and high temperature environments.

MAX陶瓷具有不同于传统高温陶瓷的异常热震行为及最近发现的裂纹自愈合和耐烧蚀性能，是氧化和高温环境下服役的关键部件的优选材料，但其异常热震行为及耐烧蚀机制一直没有得到合理的解释。本项目以Ti2AlC和Cr2AlC两种代表性的 MAX陶瓷为研究对象，在前期有启发性的实验结果基础上，集中研究材料的抗热震和耐烧蚀性能。主要研究内容包括：确定关键工艺参数，制备具有组织-性能可调的Ti2AlC和Cr2AlC单一材料及新型高温复合材料，对比研究单一和复合材料的高温性能（氧化、热震及烧蚀）。研究关键影响因素如淬冷介质、高温反应产物、微观组织等对材料抗热震性能的影响；研究烧蚀产物、液相形成、氧化层结构、微观组织以及第二相颗粒等对烧蚀性能的影响。本项目旨在揭示MAX材料的耐烧蚀机制及困扰人们近17年的异常热震行为机制。本项目的开展，能取得创新性成果，对促进MAX材料在氧化和高温环境中的应用有推动作用。

本课题系统研究了Cr2AlC材料在不同淬冷介质中热震行为，揭示了异常热震机制；对比研究了Ti2AlC、Cr2AlC以及ZrC/Cr2AlC的超高温烧蚀性能及其机制。通过系统研究，获得了工艺-组织-性能之间的关系，为MAX相材料在高温环境及其它领域的应用提供了依据。本研究取得了如下创新性成果： .1）揭示了MAX材料在不同淬冷介质中的异常热震行为及机制.本研究发现Cr2AlC为代表的MAX材料不仅在水中，而且在淬火油和熔盐中皆表现出异常热震行为，即经过热震后材料性能逐渐降低，但在某一热震温度后力学性能开始恢复升高。其异常热震行为机制主要是热震裂纹被氧化物填充。另外，还首次发现Ti2SnC材料在导电方面存在异常热震现象，即Ti2SnC在ΔT=500-800 ºC范围内，热震后电导率在600 ºC存在异常升高现象。主要机制是部分Sn从Ti2SnC中脱离出来填充了细裂纹。.2）对比研究了Ti2AlC、Cr2AlC及ZrC/Cr2AlC复合材料的高温烧蚀行为，并揭示了相关机制。氧-乙炔焰实验表明，ZrC/Cr2AlC复合材料的耐烧蚀性能好于Cr2AlC，但两者的耐烧蚀性能皆低于Ti2AlC。主要因为Cr2AlC在2100 ºC高温分解成液相的Al8Cr5和Cr-C化合物。液相易被高速氧-乙炔焰吹走，造成Cr2AlC和ZrC/Cr2AlC材料的线和质量烧蚀率较高。但Ti2AlC在2100 ºC高温分解成固相TiCx（熔点3250 ºC）和Al或Ti-Al化合物。即使Al或Ti-Al化合物变为气相或液相，TiCx仍可作为固体骨架支撑烧蚀表面。这是Ti2AlC的高温烧蚀性能优于Cr2AlC的主要原因。.3）实现了Ti2SnC材料在相对低温（800 ºC）和短时间（1h）内愈合裂纹。Ti2SnC在800 ºC，1小时内可愈合长度为1-2mm裂纹。愈合后材料的电导率和强度接近于初始值。主要机制是表面粗裂纹被TiO2和SnO2所填充，有利于强度恢复；而材料内部的细裂纹由金属Sn填充，有利于电导率恢复。.4）在其它方面亦取得了成果，如利用等离子氮化处理，有效提高了Ti2AlC“软陶瓷”的表面硬度；批量合成了TiB2六方片晶并揭示形成机制；揭示了含有Al4C3的MAX块体材料在室温自粉化机制；利用MAX材料原位分解形成TiCx细化高铬铸铁组织，有效提高了高铬铸铁力学性能和耐磨性能。