New Quaternary MAX Phase Thin Films: Understanding the Thermally Induced Microstructural Evolutions and Reaction Mechanisms in Nanostructured Multilayers via Experimental Combinatorial Study

MAX相材料（Mn+1AXn, M =过渡金属，A = A族元素，X =碳或氮，n = 1,2,3）是一种原子层状化合物，具有金属和陶瓷的独特性质。单个原子层的多元素合金化是合成新型四元MAX相的有力工具，并提供了通过调整其局部化学和结构复杂性来广泛调整其性质的多种机会。近年来，除了随机固溶体结构外，还发现了M层上新的化学有序的n+1AXn结构（M M″），包括面外有序结构（o-MAX）和面内有序结构（i-MAX）。然而，在单相结构下合成这种大块的四元MAX相仍然是一个很大的挑战。四元MAX相薄膜的合成是材料科学中的一个新兴研究领域，化学有序的四元MAX相薄膜尚未见报道。在本提案中，我们的目标是在三个四元模型体系（Cr-M″-Al-C与M″：V， Ti, Zr）中合成单相和潜在的基面织构四元（M M″）n+1AXn薄膜材料。这将通过适当的处理工艺，利用薄膜前驱体设计和合成的实验组合方法，将具有预定义纳米结构结构（“薄膜前驱体”）的纳米级元素多层转化为纳米级，从而实现。研究工作将首先针对四元随机固溶体MAX相的合成，然后将研究两种复杂有序结构的合成。本课题的科学目标是：1)了解纳米多层膜在热加工过程中形成不同的四元MAX相薄膜的微观结构演变及其潜在的反应机制；2)探索四元MAX相薄膜的成分-微观结构-性能关系。薄膜前驱体的热处理将涵盖不同加热速率和动力学（即非常低和快速加热速率）的退火实验，包括使用新型薄膜量热计的选定实验。科学工作将基于原子尺度分辨率的密集薄膜表征方法。因此，所提出的研究将有助于开发合适的薄膜前驱体结构和热处理工艺，以合成相纯四元MAX相薄膜。科学成果将导致理解局部化学（组成和化学顺序）和微观结构对这种新的MAX相薄膜材料性能的作用。

MAX phase materials (Mn+1AXn, M = transition metal, A = A group element, X = carbon or nitrogen, n =1, 2, 3) are atomically layered compounds that possess unique properties combining attributes of both metals and ceramics. Multi-elemental alloying of individual atomic layers is a powerful tool to synthesize novel quaternary MAX phases and offers manifold opportunities to widely tune their properties through tailoring their local chemistry and structural complexity. Recently, apart from random solid solution structures, new chemically ordered quaternary (M′M″)n+1AXn structures on M layers have been discovered, including out-of-plane ordering (referred to as o-MAX) and in-plane ordering (referred to as i-MAX). However, the synthesis of such bulk quaternary MAX phases in single-phase structure remains a great challenge. Thin-film synthesis of quaternary MAX phase materials is an emerging research field in materials science, and, chemically ordered quaternary MAX phase thin films have not been reported yet. In this proposal, we aim to synthesize single-phase and, potentially, basal-plane textured quaternary (M′M″)n+1AXn thin film materials in three quaternary model systems (Cr-M″-Al-C with M″: V, Ti, Zr). This will be achieved by transformation of nanoscale elemental multilayers with pre-defined nanostructured architectures (“thin film precursors”) through appropriate treatment processes, utilizing an experimental combinatorial approach for the thin film precursor design and synthesis. The research work will target at first the synthesis of quaternary random solid solution MAX phases, and will then address the synthesis of the two complex ordered structures. The scientific objectives of this proposal are: 1) to understand the microstructural evolutions and their underlying reaction mechanisms during thermal processing of the nanostructured multilayers towards the formation of different quaternary MAX phase thin films, and, 2) to explore the composition–microstructure–properties relationship of the quaternary MAX phase thin films. The thermal processing of the thin film precursors will cover annealing experiments at various heating rates and kinetics (i.e. very low as well as fast heating rates), including selected experiments with a novel thin film calorimeter. The scientific work will be based on intensive thin film characterization methods with atomic-scale resolution. Thus, the proposed research will contribute to developing suitable thin film precursor architectures and thermal treatment processes for the synthesis of phase-pure quaternary MAX phase thin films. The scientific outcome will lead to an understanding of the role of local chemistry (composition and chemical ordering) and microstructure on the properties of such new MAX phase thin film materials.