UHTC composites properties

Basic property evaluation of UHTCs and UHTC composites has resumed at a number of government facilities within NASA and the military, as well as at some universities. " With this resurgence in basic research, investigations into UHTC carbides and nitrides are seeing added attention as new processing techniques make the fabrication of these materials easier. 

Table 5. Properties of bulk UHTC composites obtained by SPS...

In this chapter, the pyrolysis of synthesized polymers, microstructure of their derived UHTC composite matrices, as well the preparation and properties of C/C-ZrB2-ZrC-SiC composites are investigated. Preparation and microstructures of the composites were studied in part I of the paper. The ablation behaviors and mechanical properties of the composites in a ground arc-jet wind tunnel and plasma torch with temperatures above 2000 °C and heating rate around 30K/s were studied in Part II. The major aims of these studies were to evaluate the possibility of producing a nano-size dispersed ZrB -ZrC-SiC matrix using these complex polymeric precursors, which are expected to exhibit improved oxidation and ablation resistance, compared to those composites with layered or large particle incorporated UHTC matrices. 

Ceramic borides, carbides and nitrides are characterized by high melting points, chemical inertness and relatively good oxidation resistance in extreme environments, such as conditions experienced during reentry. This family of ceramic materials has come to be known as Ultra High Temperature Ceramics (UHTCs). Some of the earliest work on UHTCs was conducted by the Air Force in the 1960 s and 1970 s. Since then, work has continued sporadically and has primarily been funded by NASA, the Navy and the Air Force. This article summarizes some of the early works, with a focus on hafnium diboride and zirconium diboride-based compositions. These works focused on identifying additives, such as SiC, to improve mechanical or thermal properties, and/or to improve oxidation resistance in extreme environments at temperatures greater than 2000°C. 

To achieve fracture in UHTC materials, ManLabs researchers decided to notch all subsequent test specimens. Notches were 6 mm deep x 1.6 mm wide and parallel to the axis of the cylinder, extending inward from the outer surface along the entire length of the specimen. The main disadvantage of using notched specimens was the lack of adequate experimental and analytical data on the shape factor required to compare the experimental results with predicted properties as well as the inability to compare these results with those of previous evaluations. Their results showed that materials with SiC and carbon additions displayed somewhat higher steady state thermal stress resistance than the other compositions. Nonetheless, all the diboride compositions tested showed a level of thermal stress resistance considerably above any other ceramics they had tested. ... 

In this work. Zirconium based UHTCs were applied to modify the matrix of Cp/SiC composites and fabricate ultra-high-temperature coatings for C/SiC composites. The properties and microstructures were studied as well. 

Densities and open porosities of composites were measured by the Archimedes method. The mechanical properties of Cf/SiC-ZrC composites were tested through three-poinl-bending tests in an Instron-SS66 machine, operated at a crosshead speed of0.5mm/min and a span of 24mm, the dimensions of the testing bars were 4mmx2mmx40mm. The fracture surfaces and polished cross-sections of the composites as well as the surfaces and polished cross-sections of the UHTC coatings were observed by electron probe microanalyzer (EPMA, JXA-8100, Joel, Tokyo, Japan) to characterize the microstructures. 

The composition or the presence of second phases also affects the hot hardness properties of the UHTCs. For instance, the effect of sintering additive on the hot hardness properties of TiB is presentedin Figure 7 (Raju, 2009 Raju, 2009 Jungling, 1993). It can also be noticed that the hardness of TiB samples decreases with temperature. It is well known that hardness has an exponential dependency on temperature and can be given by the following relation... 

Having looked into and critically analyzed some of the processing issues, property requirements and more recently developed advanced composites based on the ultra-high temperature ceramics, in this section we summarize the various applications (current and potential) of these material classes. As has been emphasized in the preceding sections, UHTCs are nsed for applications that demand withstanding temperatures above 2000°C, along with harsh atmospheric conditions. Such conditions are typically enconntered dnring the re-entry of space shuttles into the atmosphere. Hence, a lot of research on UHTCs has been conducted with such applications in mind. 

These are just some basic features of Mo and Ta siiicides, however, in combination with UHTC matrices and in the sintering environment, the chemistry of these compounds can locally change, inclnding the incorporation of carbon, oxygen, boron in the crystal lattice, which in tnrn indnces the formation of metastable structures with different properties. As an example, creep and oxidation properties of MoSi can be improved with small addition of boron, which facilitate the development of mnltiphase composite material in the Mo-Si-B system (Meyer, 1996). 

In this section, the mechanical properties of carbides and borides of Zr-, Hf- and Ta-based composites are presented and correlated to the morphological features with particular attention to the effect of the secondary phases on the high temperature behaviour. The mechanical properties of the UHTCs presented in this chapter are summarized in Table 4. 

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