Design and Manufacture of Ultra-High Temperature Ceramics with Oriented Strengthening and Toughening Phases

这项拟议研究的目的是为成熟一种名为CeraSGel的新工艺提供资金，通过该工艺可以制造出具有取向短纤维相增强的复杂几何形状的陶瓷。该工艺适用于任何可以无压烧结的粉末体系，但在本研究中，我们选择了一种很有前途的超高温材料，即具有短纤维碳化硅的二硼化锆或ZrB_2。CeraSGel将亚微米陶瓷粉末和短纤维悬浮在凝胶中，通过在水中溶解少量聚合物来提供陶瓷混合物的成形性。当旋转凸模插入陶瓷/聚合物混合物中时，局部剪应力会导致纤维相定向。纤维相增强了陶瓷基相的强度和韧性，无压烧结提供了一种低成本的制造方法。在这个项目中，我们将：(1)表征聚合物在最终部分的形成、粘结剂烧毁和密度方面的作用。(2)测定了聚合物在干、湿状态下与ZrB_2和SiC相的相互作用。(3)优化了坯体的烧结条件，包括聚合物加入量、纤维取向、烧结温度和时间对碳化硅增强ZrB_2的密度、组织和力学性能的影响。发动机整流罩进气口、火箭喷管插件和高速航空表面的鼻帽是需要复杂形状的高温部件的特殊应用。虽然现有的材料可以满足这些应用的苛刻温度要求，但加工和成型成本往往非常高，和/或部件的机械性能不足以满足预期的应用。在最广泛的意义上，拟议的工作将开发出一种普遍适用的成形工艺，从而可以生产出具有定向强化或增韧相的复杂零件。该工艺是廉价的，因为它服从现代复合材料制造技术，并最大限度地减少昂贵的材料损失和加工，因为可以制造近净形状的部件。

The objective of this proposed research is to provide funding to mature a new process called CeraSGel by which complex geometries of ceramic reinforced with oriented short-fiber phases can be manufactured. The process is applicable to any powder system that can be pressurelessly sintered, but for this study we have chosen a promising ultra-high temperature material, i.e. zirconium diboride or ZrB2 with short fibers of SiC. CeraSGel involves suspending submicron ceramic powders and chopped fibers in a gel created by dissolving small amounts polymer in water to provide formability of the ceramic mixture. Fiber phases are oriented by local shear-stresses that result when a spinning male mold is inserted into the ceramic/polymer mixture . The fiber phase enhances the strength and toughness of the ceramic matrix phase and pressureless sintering affords a low-cost manufacturing approach. In this project we will: (1) Characterize the role of the polymer on the formation, binder burnout, and density of the final part. (2) Determine the interaction of the polymer with the ZrB2 and SiC phases in the wet and dry states. (3) Optimize sintering conditions for green bodies including the effect of polymer loading, fiber orientation, and sintering temperature and time on density, microstructure, and mechanical properties of SiC reinforced ZrB2. Engine cowl inlets, rocket nozzle inserts, and nose-caps in high-speed aerospace surfaces are particular applications where complex-shaped, high-temperature components are required. While materials exist that meet the demanding temperature requirements for these applications, processing and forming costs are often very high and/or the mechanical properties of components are insufficient for the intended applications. In the broadest sense, the proposed work will develop a generally applicable forming process whereby complex parts can be produced with oriented strengthening or toughening phases. The process is inexpensive, as it is amenable to modern composite fabrication techniques, and minimizes expensive material losses and machining as near net-shape components can be fabricated.