有机前躯体裂解SiBCN非晶对非氧化物高温陶瓷的助烧结作用及其对材料性能的影响

Non-oxide ceramics are a class of widely used inorganic nonmetallic materials due to their high performance. Such materials have very strong covalent bonds which lead to not only excellent performance but also enormous difficulties in the ceramic sintering. A novel method to promote the sintering of the non-oxide materials is presented in this project. A polymer precursor polyborosilazane is used as sintering aids. During the sintering process，the precursor pyrolyzes into highly active SiBCN non-oxide amorphous phase which can keep stable even up to 2000℃. The amorphous phase is expectable to have excellent promoting effects on the mass transfer of the non-oxide materials and to achieve the lower temperature sintering of the ceramics. In this study, SiC without melting point and ZrB2 with ultra-high melting point are used as model materials. The interactions between the SiBCN amorphous phase and the main crystal phase at different stages and the influencing factors are investigated. The mass transfer mechanism of diffusion, dissolution - precipitation and viscous flow are explored through the pressure-free sintering and hot pressing process, respectively. The promoting sintering mechanism and the effects of SiBCN amorphous phase in the grain boundary on the properties of SiC and ZrB2 will be revealed. The results based on the two typical model materials have wider significance for the other non-oxide ceramics.

非氧化物陶瓷是一类用途广泛的高性能无机非金属材料，它具有很强的共价键，该特性给材料带来优异性能的同时，也给该材料的烧结制备带来了极大的困难。本项目提出一种全新的非氧化物助烧结方法。采用聚硼硅氮烷有机前躯体为烧结助剂，在非氧化物陶瓷烧结过程中有机物裂解形成较高活性的SiBCN非氧化物非晶相，（该非晶能够稳定存在至2000℃），通过该非晶相的传质促进作用使得非氧化物能在较低的温度下致密化。本研究以无熔点材料SiC和超高熔点材料ZrB2作为模型材料，揭示有机前躯体裂解SiBCN非晶在各温度阶段与非氧化物主晶相的相互作用及其影响因素；通过无压热处理和热压处理分别探讨扩散、溶解-沉淀和粘性流动传质机理，揭示SiBCN非晶的助烧结机理；揭示非晶SiBCN晶界相对SiC和ZrB2材料性能的影响。所选两个非氧化物极具代表性，因此该研究成果对整个非氧化物陶瓷的低温烧结以及性能提高具有普遍的指导意义。

本项目借鉴有机前驱体裂解陶瓷技术，提出并实现了利用聚硼硅氮烷（PBS）前驱体裂解非晶SiBCN作为难烧结非氧化物陶瓷烧结助剂的方法。选用SiC和ZrB2作为非氧化物陶瓷典型材料，系统研究有机物PBS裂解产生的非氧化物非晶相对这两种陶瓷材料烧结过程的影响，探讨其助烧结机理及影响因素，研究该技术所制备的非氧化物体系材料的性能。结果表明：PBS裂解非晶SiBCN作为一种新的烧结助剂，能够有效地促进SiC和ZrB2陶瓷的致密化程度。与传统氧化物烧结助剂表现不同，该助烧剂在1100-1450 ℃范围内（远低于体系烧结温度）由于大量活性基团的迁移、重组及游离碳与主晶相颗粒表面氧化物发生碳热还原反应等原因而实现了快速致密化。在高温阶段，则通过SiBCN非晶相发生形变或粘性流动、B、C原子促进晶界扩散等机制带来体系最终烧结。材料中SiC、ZrB2与SiBCN两相的界面及界面处缺陷成为决定材料性能的关键因素。随着非晶SiBCN添加量（0-20 wt.%）的增大，两种体系材料的室温力学性能、高温抗蠕变性及热导率、热膨胀系数等均呈现下降趋势。本研究揭示了PBS裂解产物SiBCN非晶相在SiC和ZrB2的烧结过程中的演变规律和传质机理及其影响因素；揭示了SiBCN非晶晶界相对SiC、ZrB2非氧化物陶瓷性能的影响规律。对其他高性能非氧化物陶瓷的制备提供了理论和技术支持。