Oxidation resistance UHTC composites

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. 

Aside from the work performed at NASA Ames, recent work by Levine et al., at NASA Glenn, has been conducted to improve the oxidation resistance of monolithic materials and to develop fiber reinforced UHTC materials to improve composite fracture toughness and impart a level of graceful failure in materials. 

There is some evidence that composites based on ZrB including ZrB -SiC and ZrB -MoSi, which have a superior oxidation resistance up to 1600°C, also possess a pronounced degree of intrinsic spectral selectivity (Bogaerts Lampert, 1983 Kennedy, 2002) which could be useful for solar absorbers. However, studies of spectral emissivity characteristics of UHTCs refer to films and coatings and not to bulk materials (either porous or dense). 

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. 

UHTCs such as ZrB, HfB, and ZrC and their composites have been extensively investigated in recent years because of their excellent high temperature stiffness, high hardness, high thermal shock resistance, as well as the good oxidation and ablation resistance. These properties are essential for use as cladding materials in re-entry and ultra-high speed aircrafts to provide thermal protection under extreme environments. 

---