TiO2纳米颗粒促进热生长Al2O3相转变的根本原因研究

Al2O3-forming coatings have been widely used to protect the mechanical components against oxidation at high temperatures. Under this condition, the Al2O3 phase often undergoes a transition from metastable (gamma and theta phase) phase to stable (alpha) phase, which might trigger the formation of cracks in the coating and accelerate the oxidation rate. Therefore, the key to improve the high temperature oxidation resistance of a coating is to form a protective layer of stable Al2O3, quickly, on the surface. Recently, we found that although the crystalline structure of the TiO2 differs from that of the alpha-Al2O3, it can also promote the transition of Al2O3 from metastable state to the stable state, and thus slow down the oxidation rate, improve the coating’s oxidation resistance. In this project, we will systematically study the oxidation behavior of the NiAl coating modified by TiO2 particles. By characterization at nanoscale even atomic scale using Transition Electron Microscopy (TEM) and High Resolution TEM (HRTEM), we will investigate the evolutions of structure and composition at the boundary between the TiO2 particle and the thermally-grown Al2O3 phase, during high temperature oxidation. The ultimate aim is to unravel the physical origin of the TiO2 promoted formation of stable Al2O3 during oxidation. This study will provide new insights into understanding of the phase transformation in thermally-grown -growth Al2O3 and its influencing factors. It will also provide a new approach to develop an oxide-modified coating with excellent oxidation resistance, which can avoid the negative effects induced by the phase transition of Al2O3, and accelerate the growth of stable Al2O3 during oxidation.

NiAl等Al2O3热生长型涂层广泛用于金属结构材料高温防护。但在高温下，Al2O3通常发生由亚稳态（gamma和theta相）向稳态（alpha相）的转变，导致裂纹产生并加速氧化。因此，快速生成稳态Al2O3保护层是提高涂层抗氧化性能的关键。我们近期发现，虽然和稳态Al2O3的晶型不同，TiO2纳米颗粒也能抑制亚稳相形成并催化稳态Al2O3生长，降低涂层氧化速率。基于该“反常”现象，本项目提出在NiAl涂层表面涂覆TiO2纳米颗粒，用透射电镜和高分辨电镜等分析手段，从微观乃至原子尺度研究涂层氧化过程中TiO2颗粒/热生长Al2O3界面结构和化学成分随时间的演化规律，揭示TiO2促进稳态Al2O3形成的基本原因。研究结果将有助于深入认识热生长Al2O3相转变及其影响因素，为高温涂层避免Al2O3相转变对氧化性能的负效应、促进稳态Al2O3生长提供一种表面涂覆特定氧化物改性的新途径。

本工作对空白Ni2Al3，表面喷洒TiO2以及弥散TiO2的Ni2Al3这三种涂层的高温氧化动力学及其氧化产物就行了研究。发现虽然TiO2的晶体结构为四方结构，但是它同样能像具有密排六方结构的Cr2O3一样，显著提升该涂层的高温氧化性能。TEM研究表明，对于弥散TiO2的Ni2Al3来说，当中分布的部分TiO2颗粒会转变为具有密排六方结构的Ti2O3，所以它能够像Cr2O3一样作为alpha-Al2O3外延生长的平台，促进Al2O3由gamma相向alpha相的转变，从而降低氧化速率。而对于表面喷洒TiO2的Ni2Al3涂层来说，alpha-Al2O3可以直接在表面的TiO2上形核（两者之间存在固定的位相关系），从而跳过了非稳态的gamma相的形核和生长，所以其抗氧化性能最优。