Design of Novel Buffer Layers for Excellent Performance UHTCs; Investigation of Diffusion Mechanisms

ZrB 2是一种超高温陶瓷（UHTC），具有超过3000°C的熔点和有趣的工程和物理特性。因此，ZrB 2陶瓷被认为是空间和高超音速部件的潜在候选者。限制ZrB 2陶瓷用于更广泛应用的主要缺点主要与其低损伤容限、差的抗氧化性和相对高的密度有关。然而，不连续SiC纤维的引入可以使ZrB 2的韧性提高到6 MPa·m½以上，同时显著降低密度。此外，添加第二相如MoSi 2可以进一步提高高温强度，最重要的是，提高硼化物基体的氧化性能。该项目的最终范围是获得由ZrB 2-MoSi 2外层组成的功能梯度（FG）复合材料，以提供抗氧化和抗烧蚀性，并从上到下逐渐增加SiC纤维，以保证整个结构的更高故障容限和更低密度。要克服的主要障碍是在烧结过程中Mo化合物与SiC纤维之间发生有害的化学反应，这导致纤维的微观结构退化，从而失去其增韧功能。该项目的重点是开发一种新型的基于ZrB 2的缓冲层，能够防止SiC纤维和Mo之间的化学反应，化合物和研究的相互扩散过程中的界面，无论是在烧结和氧化。设想利用ZrB 2和Si 3 N4、SiCN和HfSiN的混合物作为扩散阻挡层。了解各层之间的化学反应将是设计用于极端环境下应用的新型UHTC的基础。工作计划预计将开发具有不同缓冲层成分的各种基线陶瓷接头，并通过透射电子显微镜对烧结和氧化试样进行彻底的微观结构表征。这些分析，再加上热力学预测，将是必不可少的，在揭示当地的扩散过程，并将导致一个功能的UHTC设计的优化缓冲层组合物的定义。将对优化的复合材料进行热机械表征和电弧喷射测试，以评估新材料的性能。

ZrB2 is an ultra-high temperature ceramic (UHTC) possessing a melting point exceeding 3000°C and interesting engineering and physical properties. As such, ZrB2 ceramics are considered potential candidates for space and hypersonic components. The main drawbacks that restrict the employment of ZrB2 ceramics for a wider spectrum of applications are mainly related to its low damage tolerance, poor oxidation resistance and relatively high density. However, the introduction of discontinuous SiC fibers can both increase the toughness of ZrB2 above 6 MPa·m½ and parallel a notably decrease in density. Moreover, the addition of secondary phases such as MoSi2 can further improve the high-temperature strength and, most importantly, the oxidation performance of the boride matrix. The ultimate scope of the project is to obtain a functionally graded (FG) composite made up of a ZrB2-MoSi2 outer scale to provide oxidation and ablation resistance and a progressively SiC fiber-enriched body from top to bottom to guarantee a higher failure tolerance and a lower density of the entire structure. The major obstacle to overcome is the detrimental chemical reaction between Mo-compounds and SiC fibers occurring during sintering, which leads to the microstructural degradation of the fibers and, hence, to the loss of their toughening function.To reach the above mentioned aim, the focus of the project is on the development of a novel ZrB2-based buffer scale able to prevent chemical reactions between SiC fibers and Mo-compounds and on the study of interdiffusion processes across the interface, both during sintering and upon oxidation. It is envisioned to utilize mixtures of ZrB2 and Si3N4, SiCN and HfSiN for the diffusion barrier. The understanding of the chemical reactions across the various layers will be fundamental for the design of a novel UHTC for applications under extreme environments.The work plan foresees the development of various baseline ceramic joints with different composition of the buffer layer and a thorough microstructural characterization of the as-sintered and oxidized specimens by transmission electron microscopy. These analyses, coupled to thermodynamic predictions, will be essential in revealing local diffusion processes and will lead to the definition of an optimized buffer layer composition for the design of a functional UHTC. Thermo-mechanical characterization and arc-jet tests of the optimized composite will be carried out to assess the performance of the novel material.