Improved zirconium- and hafnium-based ceramic composites have been invented in an effort to obtain better resistance to ablation at high temperature. These ceramics are suitable for use as thermal-protection materials on the exterior surfaces of spacecraft reentering the terrestrial atmosphere, and in laboratory and industrial environments that include flows of hot oxidizing gases.
The predecessors of these ceramic composites are ZrB2/SiC and HfB2/SiC composites, which exhibit high resistance to oxidation and thermal shock, high configurational stability, and high resistance to ablation. The ablation resistance of ZrB2/SiC and HfB2/SiC composites is believed to arise from the formation of coherent passivating oxide scales on their surfaces. However, each such composite exhibits a transition point
within its operational envelope of mach number vs. stagnation heat flux; beyond this point, the cohesiveness of the oxide scale decreases in such a way that microspallation of the oxide scale and concomitant accelerated conversion occur. Thus, resistance to ablation decreases beyond the transition point. The present innovative ceramic composites offer superior resistance to ablation.
A ceramic of the present type is a multiphase composite of (1) zirconium diboride and zirconium carbide with silicon carbide, (2) hafnium diboride and hafnium carbide with silicon carbide, or (3) mixed diborides and/or carbides of zirconium and hafnium with silicon carbide. The composite material is made by sintering a mixture of the metal diboride, metal carbide, and silicon carbide powders at a temperature of about 1,900 °C or greater. Typical composition ranges in volume percentages of the starting ceramic powders are the following:
Constituent .... Proportion, Volume Percent
ZrB2 and/or HfB2 ....................... 20 to 64
ZrC and/or HfC ........................... 20 to 64
SiC ................... < 20 (Preferably 10 to 16)
A given composition is said to be diboride- or carbide-rich, depending on the ratio between the metal diboride and metal carbide contents.
Figure 1 is a pseudo-ternary phase diagram for the special case of composites made from either (1) ZrB2, ZrC, and SiC or (2) HfB2, HfC, and SiC but not (3) mixtures of the diborides and/or carbides of both Zr and Hf. "Pseudo-ternary phase diagram" as used here signifies that the three points of the triangle represent the starting ZrB2 or HfB2, ZrC or HfC, and SiC components, as distinguished from a true ternary diagram in which the individual elements Zr or Hf, Si, and C are represented. The composition ranges stated above are represented by the shaded trapezoidal area.
The table in Figure 2 presents some results of a test in which two composites of the present type and two of the previous type were exposed to an arc jet at a heat flux of 400 W/cm². The results show that with respect to conversion rates, the present ceramic composites resist ablation or conversion better than do the corresponding predecessor ZrB2/SiC and HfB2/SiC composites. The results are even more impressive when densities and surface temperatures are taken into account: About 50 seconds into the test, the ZrB2/SiC ceramic underwent a transition in which the surface temperature rapidly climbed to a maximum of 2,200 °C, with resultant micro and macro spallation, and stability was not reestablished.
This work was done by Jeffrey Bull of Ames Research Center, Michael White of White Materials Engineering, and Larry Kaufman of Cambridge Technology Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Materials category.
This invention has been patented by NASA (U.S. Patent No. 5,750,450). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center; (650) 604-5104.
Refer to ARC-12087.