Thermal insulation materials for sharp leading edges on hypersonic vehicles must be stable at very high temperatures (e.g., 2000 C, or 3630 F); must resist evaporation, erosion and oxidation; and they should have low thermal diffusivity to limit heat transfer to support structures. The most promising materials for the application are a family of ultrahigh temperature ceramics (UHTCs) comprising ZrB2 and HfB2, and composites of those ceramics with SiC. ZrB2 and HfB2 ceramics have high hardness and very high melting temperatures (3245 C for ZrB2 and 3380°C for HfB2), and they form protective, oxidation resistant coatings and have low vapor pressures at potential use temperatures. However, despite their attractive properties, these material have seen limited use due to difficulties in processing that precluded achieving optimal properties. Development work at Sandia National Laboratories is overcoming these problems.
Improving UHTC capabilities
The cornerstone of ultrahigh-temperature ceramics is a small group of diboride ceramic-matrix composites (CMCs) including ZrB2/SiC, HfB2/SiC and ZrB2/SiC/C. They have a unique set of material properties including unusually high thermal conductivity, good thermal shock resistance, and modest thermal expansion coefficients that make them particularly well suited for sharp-body applications in hypersonic flows. They can be fabricated in either billet or plate stock using a proprietary uniaxial hot pressing process. Complex parts, including 1-mm (0.04 in.) radius leading edges, are readily machined using electrical discharge machining techniques.
While these materials are particularly well suited for use in sharp-body applications in hypersonic flows, their use has been limited due to poor strength and thermal shock behavior, a deficiency that has been attributed to the inability to make them as fully dense ceramics with good microstructures. Initial evaluation by Sandia of UHTC specimens provided in 2002 by the NASA Thermal Protection Branch suggested that their poor properties were due to agglomerates, inhomogeneities and grain-boundary impurities, all of which were believed to be associated with errors in ceramic processing. The inititial research and development work at Sandia was aimed at significantly improving UHTC properties by applying fundamental understanding of ceramic processing to eliminate the defects that had so far limited UHTC properties.
The results of research during the first year of the project have exceeded expectations. Researchers fabricated UHTCs in both the ZrB2 and HfB2 systems that are 100% dense, or nearly so, and that have favorable microstructures as indicated by electron microscopic examination. In addition, they hot pressed UHTCs having a much wider range of SiC content than ever before. Availability of a range of compositions and microstructures will provide systems engineers the added flexibility to optimize designs.
Sandia proposed a multipart project to fabricate and characterize improved zirconium and hafnium diboride UHTCs. The tasks involved developing an improved process using either conventional or reactive hot pressing; characterizing UHTC specimens using Sandia's Automated X-Ray Spectrum Image Analysis (AXSIA) technique to determine compositions at the submicron scale and relating them to processing conditions; determining the relevant physical and mechanical properties of the UHTCs (particularly high temperature strength); and collaborating with the NASA Ames Research Center, exchanging information on UHTC development. The project team, led by the author, is part of the Sandia Thermal Protection Materials Program, and also represents the work of Sandia researchers Jill Glass, Paul Kotula, David Kuntz and University of New Mexico doctoral student Hans-Peter Dumm.
Because all subsequent project tasks were dependent on the availability of high-quality UHTC materials, the initial effort was focused on developing the CMC fabrication process. The plan was to initially try hot pressing to produce fully dense material, and if unsuccessful, then to use reactive hot pressing. However, hot pressing was extremely successful. The material compositions and densities made thus far are shown in Tables 1 and 2. The theoretical densities are calculated as linear combinations of the handbook values for SiC and the diboride, which may not be strictly correct, and, therefore, accounts for values that slightly exceed 100% of theoretical.
Fully dense HfB2 and ZrB2 UHTCs with SiC contents down to 2% have been fabricated. There is no previously reported success in producing nearly full density UHTCs having less than 20% SiC, and conventional wisdom was that the larger amount of SiC was necessary to achieve full density in hot pressed HfB2 and ZrB2. The Sandia results provide a much broader range of possible compositions for application as thermal insulation, and it is expected that properties such as strength, thermal conductivity and oxidation resistance will vary with SiC content.
The availability of a wide range of compositions with acceptable densities allows characterization using AXSIA analysis. The initial focus is on the role of SiC in the development of the ceramic microstructure. Kotula performs microstructural and microchemical analysis on the ceramic materials, applying the Automated eXpert Spectral Image Analysis (AXSIA) software (developed by Kotula and Michael Keenan, and recently patented and winner of a 2002 R&D 100 award). The materials are evaluated at the micron to subnanometer length scale for grain size and phase distribution, as well as for impurities or contaminants that can adversely affect their mechanical properties.
Boron and carbon are difficult to analyze because they give off low-energy or soft x rays when excited with an electron beam as in a scanning or transmission electron microscope typically used for such analyses. Instead of using x-ray analysis techniques, the research team has developed other analytical capabilities based on electron energy-loss spectrometry to determine amounts and nanometer-scale lateral distributions of the light elements in the UHTCs.
Oxygen, in particular, is an important impurity because in combination with the silicon present in the UHTCs and other impurities, it can form glasses or other phases, which typically cannot take the required high-operation temperatures and would melt or crack in service, causing the material to fail. If enough of the wrong contaminants find their way into the process, the material will have no high-temperature strength or stability.
Jill Glass conducts testing for high-temperature mechanical properties and fracture analysis. Specimens have been made in the required shapes to do the mechanical tests. Properties to be determined initially include microindentation hardness and fracture toughness, because they are good screening tests for material quality and the tests are easier to perform than some of the other planned measurements.
Vickers indentations over a range of loads were used to measure the hardness and fracture toughness of nine experimental compositions. Test results show that the ZrB2-20% SiC material has the highest toughness and second highest hardness value. The hardness and fracture toughness of the HfB2-20% SiC material was a close second to those of the ZrB2-20% SiC UHTC. It is believed that these material properties are sufficient for proposed leading edge applications.
Collaboration with the University of New Mexico was established to provide data on thermal conductivity/diffusivity and oxidation resistance of the different UHTC compositions. Thermal diffusivity values for different compositions and microstructures in both diboride families demonstrate that the material thermal diffusivities can be engineered to obtain desired values by varying composition and microstructures in rational way.
NASA Ames has tested one ZrB2-SiC specimen in its Arc Jet test facility and the material performed extremely well, showing just slight surface oxidation after testing at 1800 C (3270 F) in a simulated reentry environment.
Sandia plans to demonstrate successful performance at the lab scale in 2004 with scale-up in 2005.