In the absence of materials that could withstand extreme heat generated by high speeds upon reentry to Earth’s atmosphere, engineers have designed blunt-edged spacecraft to slow their speeds and to thermally protect their structures as they returned from space.
With part of a $1.7 million grant from the U.S. Air Force, Ken White, UH professor of mechanical engineering, has developed a new ceramic refractory material, a diboride-tungsten carbide solid solution alloy, to replace zirconium diboride in aircraft and spacecraft construction. The superior material will enable engineers to design aerodynamic new-generation reentry spacecraft and hypersonic aircraft that will withstand extremely high temperatures under load with improved maneuverability.
Before White and his students could begin their research, they had to invent the experimental methods necessary to study creep, or deformation of material under load at high temperatures.
“It was a unique experiment that no one had ever done before, so we had to come up with all the ways to do it,” White said.
With image analysis software, White observed and tracked zirconium diboride grains under load at 1800 degrees Celsius in a specially equipped furnace, which provided the vacuum environment necessary to avoid corrosion that would otherwise eat away the surface of the material. He observed that the shapes and sizes of the grains were relatively unchanged but that the grains slid across each other without creating cavities. White attributed the majority of the creep to this movement.
While the core of the grains remained rigid, he discovered a mechanism present in the narrow perimeter, or mantle, of the individual grains that was responsible for unlocking the grain-sliding deformation. He removed silicon carbide from the zirconium lattice and added tungsten, another heavy refractory element, to make deformation more difficult in the mantle. As a result, the mantles became more rigid, and the creep slowed 100 times the original rate of creep. White and his students are in the process of laying groundwork, such as additional creep modeling and development of the appropriate phase diagram that maps tungsten solid solution’s microstructure.
“Determining the mechanism that causes creep was our most important accomplishment,” White said. “No one understood what allowed the material to deform.”