基于晶粒尺寸效应的金属基热障涂层隔热机制与热稳定性研究

Low Heat Rejection (LHR) engines are considered to be a trend of development, and an effective approach to reduce exhaust emissions and improve fuel economy of vehicles. However, the service lifetime and stability of thermal barrier coatings (TBCs), which is regarded as one of the key technologies, have significantly restricted the extended application of LHR engines. The concept of metal-based thermal barrier coatings (MBTBCs) initiates a novel idea to overcome the problems in which ceramic-based thermal barrier coatings (CBTBCs) can not solve, such as thermal fatigue. Nevertheless, it is still lack of sufficient understanding on the insulation mechanism and thermal stability of MBTBCs up to now. Therefore, in this work, related investigations will be conducted based on the fundamental of grain size effect on material thermal conductivity. The selection and optimization of metallic alloy components should be processed firstly to satisfy the demands of coatings microstructure and properties, in terms of high glass forming ability and crystallization temperature. To clarify the insulation mechanism of MBTBCs, uncoupled analysis method will be used to study the separate contribution to coating thermal conductivity of impact factors, respectively. Consequently, the insulation mechanism of MBTBCs could be characterized in scale of electrons, phonons and structure defects effect, after the insulation mechanism and grain-size-dependent established systemically for bulk metallic glasses and related nanocrystalline alloys. Furthermore, the thermal stability of MBTBCs will be detected under prolonged heat treatment periods, and the microstructure evolution and performance degradation of MBTBCs will also be illustrated. In a word, the results of this research could extend the industrial fields of amorphous/nanocrystalline alloys, and provide important fundamental and instruction for the practical design and engineering application of MBTBCs.

低散热发动机技术是降低汽车尾气污染物排放、提高能源利用率的有效途径和必然发展趋势，但是其关键技术之一，即热端面热障涂层材料的服役稳定性，是目前制约其大范围推广应用的技术瓶颈。针对陶瓷基热障涂层难以解决的热疲劳等问题，提出金属基热障涂层的设计理念是较为新颖的研究思路，但对于其隔热机制和热稳定性等基础科学问题，还缺乏足够的认识。基于此，本项目围绕晶粒尺寸对材料热导率的影响这一基本思路，在合金体系选择设计的基础上，通过涂层热导率影响因素解耦的分析方法，首先对块体非晶/纳米晶材料的隔热机制、及晶粒尺寸的影响规律进行系统研究，进而在电子、声子和结构缺陷等微观本质上，揭示出金属基热障涂层的隔热机制。进一步开展涂层长周期热稳定性研究，阐明涂层组织结构随温度和时间的演化规律及隔热性能的退化程度。相关成果为非晶/纳米晶材料应用领域的开拓、及金属基热障涂层的实际设计与工程应用，提供重要的理论基础和科学依据。

针对陶瓷基热障涂层难以解决的热疲劳等问题，本项目提出适合汽车工业领域的金属基热障涂层的设计理念，围绕晶粒尺寸对材料热导率的影响这一研究思路，优化设计Fe基块体非晶合金体系，并通过热喷涂方法制备Fe基非晶热障涂层，系统研究了喷涂方法与工艺条件对涂层微结构及热传输性的影响规律。在此基础上，通过开展不同温度与时长的热处理，进一步阐明涂层组织结构的演化规律及隔热性能的退化程度。主要研究成果如下：. 优化设计了Fe基块体非晶合金体系，得到晶化温度高于620℃，室温热导率低于8W/mK的Fe基非晶合金。对其传热机制进行了系统研究和定量表征，其中电子传热大于60%。结构弛豫与晶化转变的发生会导致合金热导率有一定程度的上升，主要来源于晶体结构有序化带来的电子传热的增强。晶粒尺度在特定热载下有饱和趋势，而合金热导率与其呈正相关变化。. 系统研究了不同喷涂工艺及参数条件变化对Fe基非晶涂层微观组织结构与热传输性等的影响规律，制备得到室温热导率低于2W/mK的Fe基非晶涂层，显著低于常规金属合金涂层，达到了国际同类研究的先进水平。涂层中的非晶相含量与结构缺陷是影响其热传输性的主要因素，而非晶相含量的影响更为显著。对于不同喷涂工艺，超音速火焰喷涂涂层兼具了较为优异的热障与力学性能；等离子喷涂涂层更易实现高非晶相含量和一定量的结构缺陷，因此具有更低的热导率；而电弧喷涂涂层由于大量结构缺陷的产生，使得其在升温过程中呈现出更为稳定的隔热能力。. 通过长周期热处理研究了Fe基非晶涂层的热稳定性及热传输性变化规律。在400℃条件下，涂层相结构能够长期保持稳定，随热处理时间延长，其热导率升高并不显著。而在接近晶化温度的600℃条件下，涂层在过冷液相区内发生明显的结构弛豫，原子排列有序化，隔热性能有一定程度的退化。