Sandia researchers develop ultra-high-temperature ceramics to withstand 2,000 degrees Celsius
The ultra-high-temperature ceramics (UHTCs) created in Sandia’s Advanced Materials Laboratory can withstand up to 2,000 degrees C (about 3,800 degrees F).
Ron Loehman, a senior scientist in Sandia’s Ceramic Materials Dept. 1843, says the results from the first seven months of the project have exceeded his expectations.
"We plan to have demonstrated successful performance at the lab scale in another year with scaleup the next year," says Ron, adding that results suggest these materials meet the thermal insulation requirements of Sandia’s Prompt Global Response project. The program also provides NASA Ames Research Center with analysis of UHTCs.
Ron says thermal insulation materials for sharp leading edges on hypersonic vehicles must be stable at very high temperatures (near
2,000° C). The materials must resist evaporation, erosion, and oxidation, and should exhibit low thermal diffusivity to limit heat transfer to support structures. Materials with those properties are required for development of hypersonics for prompt and precise delivery to difficult targets.
UHTCs are composed of zirconium diboride (ZrB2) and hafnium diboride (HfB2), and composites of those ceramics with silicon carbide (SiC). These ceramics are extremely hard and have high melting temperatures (3,245°C for ZrB2 and 3,380°C for HfB2). When combined, the material forms protective, oxidation-resistant coatings, and has low vapor pressures at potential use temperatures.
"However, in their present state of development UHTCs have exhibited poor strength and thermal shock behavior, a deficiency that has been attributed to inability to make them as fully dense ceramics with good microstructures," says Ron.
Ron says the initial evaluation of UHTC specimens provided by the NASA Thermal Protection Branch about a year ago suggests that the poor properties were due to agglomerates, inhomogeneities, and grain boundary impurities.
"All of which we believed could be traced to errors in ceramic processing," Ron says.
During the first seven months the researchers have made UHTCs in both the ZrB2 and HfB2 systems that are 100 percent dense or nearly so, and that have favorable microstructures, as indicated by preliminary electron microscopic examination. In addition, the researchers have hot pressed UHTCs with a much wider range of SiC contents than anyone previously has been able to make. Availability of a range of compositions and microstructures will give system engineers added flexibility in optimizing their designs.
The project is part of the Sandia Thermal Protection Materials Program and represents work from various Sandia researchers. The
primary research team includes Jill Glass, Brian Gauntt, and Dale Zschiesche (all 1843), Paul Kotula (1822), David Kuntz (9115), and University of New Mexico PhD student Hans-Peter Dumm.
David Kuntz, project investigator, says his primary responsibility is to compute aeroheating, design thermal protection systems (heat shields), compute material thermal response on high-speed flight vehicles, and develop tools to improve these capabilities.
"If a vehicle flies fast enough to get hot, we analyze it," David says. "Our tools consist of a set of computer codes that compute the flowfield around a high-speed flight vehicle, the resultant heating on the surface of the vehicle, and the subsequent temperatures and ablation of the materials which form the surface of the vehicle."
Jill works with high-temperature mechanical properties and fracture analysis.
Paul’s role in the project involves microstructural and microchemical analysis of this important class of ceramic materials. Paul applies the Automated eXpert Spectral Image Analysis (AXSIA) software (developed by Paul and Michael Keenan (1812), recently patented and winner of a 2002 R&D 100 award) to the characterization of hafnium and zirconium diboride/silicon carbide UHTCs. Paul looks at these materials at the micron to subnanometer length scale for grain size and phase distribution as well as 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 upon electron energy-loss spectrometry to determine amounts and nanometer-scale lateral distributions of the light elements in the UHTCs.
In particular, oxygen is an important impurity since in combination with the silicon present in the UHTCs and other impurities it can form glasses or other phases.
These other phases typically can’t 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," says Paul.