Journal Club November 2010: Ultra-high Temperature Resisting Materials and Structures
Advanced materials that can sustain extreme heat fluxes and heat loads are in increasing demand in recent years, driven by human desire to travel faster and facilitated by major nations’ recently reiterated initiatives on (manned) space exploration. A recent example is the establishment of a National Hypersonic Science Center for Materials and Structures, supported jointly by AFOSR and NASA and dedicated to the research and development of novel materials and structures that can withstand high temperature environment of > 1700 ºC for sustained period of time (for readers wish to know more about the fascinating history and current challenges specific to hypersonic vehicles, we recommend a recent article by Bertin and Cummings . But here we shall focus our discussions on UHT-materials and the related thermal mechanics).
In general, increasing material operating temperature can greatly improve the performance of a variety of important mechanical and material systems such as reentry space shuttles, hypersonic vehicles, rocket nozzles, turbine blades and combustor liners, and other thermal protection systems.
The challenges in materials development for UHT applications have been fully appreciated since the Apollo era . The very basic requirements are that the microscopic crystal structures should be stable and not subject to extensive oxidation. However, this has proved to be difficult to meet: most metals or metal alloys under UHT environment tend to suffer from microstructure evolution which leads to significant creep deformation and oxidation; UHT(monolithic) ceramic materials, on the other hand, can sustain UHT with much less creep deformation, but they are typically too brittle to meet structural requirements. Recent developments in diborides of zirconium (ZrB2)and of hafnium (HfB2) have demonstrated very promising oxidationresistance under UHT environments (up to 2200 ºC) . Another type of promising ceramics are the polymer-derived ceramics (PDCs) which are made directly by heating Si-based polymers to about1000 ºC, during which the scission of C-H bonds evolves hydrogen leaving behind a ceramic constituted from Si, C, N and O. These ceramics show zero creep up to 1500 ºC despite being "amorphous" [3,4]. However,such materials are still very expensive to make and their strength-to-weight ratios remain too small for large scale usage. They now are used only in those most critical UHT areas such as the leading edge of high-speed aero-vehicles.
With consideration of strength-to-weight ratio, material selection favors more toward the toughed ceramic matrix composites (CMCs). In such materials, high strength, high temperature resisting fibers (C, SiC or Oxides) are embedded in (brittle) ceramic matrices (C or SiC) . It has been well established that through secondary matrix cracking and interface debonding, such materials can achieve reasonable high strength at high temperature. However, due to the multiple cracking nature of such materials, careful consideration of progressive damage evolution is mandatory. A very comprehensive review on the physics and mechanics of classic CMC laminates can be found in  and the references therein. Note that in this paper the mechanisms of most of the failure modes are assumed to operate independently. In realty, multiple failure modes may operate in a strongly coupled manner and how to account for the coupled evolution of multiple damages (which may operate at different length and time scales) remains a challenging issue up to date. Another critical issue with CMC laminates is that they tend to delaminate under severe thermal gradients, which are inevitable under UHT environments.
Recently, an exciting new paradigm of CMCs based on the use of three-dimension fiber reinforcement that enables functionality in heat exchange, transpiration, detailed shape, and thermal strain management that significantly exceeds the prior art have been developed Two such examples are given in Figure 1 and Figure 2. Such structures can be tailored to the specific shape, stress, and thermal requirements of a structural application and therefore generally requires innovative textile methods for each realization. The current state-of-art design of such materials and structures has been given in , wherein the design considerations, textile processing, thermal management, thermal-mechanical stress analyses, and prototype demonstrations of several high temperature CMC components are presented as detailed case studies. It is worth pointing out that the unique design for targeted applications (i.e. non-standard design procedure) can pose potential challenges in assuring product quality and predict performance prior to investment in fabrication and testing . Regard to this, high fidelity numerical models that can faithfully simulate the material response under UHT environments become extremely valuable, which have prompted increasing investment into this research area recently. The major challenges in modeling the progressive failure of UHT materials and structures include but not limited to: 1) stochastic initial imperfections due to fiber tow displacement, void allocation, and processing induced cracking/damages; 2) coupled multiple cracking evolution and their instantaneous interaction with temperature field(Figure 2 & 3); 3) transient coupling among multiple arbitrary cracking, material softening (due to HT) and material damage due to oxidation; and 4) linking and cross-talking among vastly different temporal and spatial scales that the various mechanisms operate on. Possible methods to build such high fidelity capabilities are discussed in .
Figure 1 3D woven CMC tube structure for UHTapplication including its textile weave (a), 3D interlacing (b), and (c) finalprototype and its multiscale features. For this panel the inside of the tubes passes liquid H2 fuel while the bottom side are exposed tocombustion environment with temperature ~ 3300 0C
Figure2 A SiC-SiC combustor liner with transipiration holes.
Figure 3 Cross-section view of initial imperfections and complex damages in a CMC, which pose significant challenges forhigh-fidelity modeling
 Bertin, J. J. and Cummings,R. M. (2003). FiftyYears of Hypersonics: where we’ve been, where we’re going. Progressin Aerospace Science. Vol 39 (6-7): 511-536.
 William G. Fahrenholtz,w and Gregory E. Hilmas, et al. (2007). RefractoryDiborides of Zirconium and Hafnium. Journal of American Ceramic Society. 90(5): 1347-1364.
 PaoloColombo, Gabriela Mera, et al.(2010). Polymer-DerivedCeramics: 40 Years of Research and Innovation in Advanced Ceramics. Journal of the American Ceramic Society, 93(7),1805-1837.
 Brahmandam, S. and Raj, R. (2007). Novelcomposites constituted from hafnia and a polymer-derived ceramic as aninterface: Phase for severe ultrahigh temperature applications. Journal ofthe American Ceramic Society, Vol 90(10): 3171-3176.
 A. G. Evans and F. W. Zok (1994). The physics and mechanicsof fiber-reinfirced brittle matrix composites. Journal of Material Science. Vol. 29 (15): 3875-3896.
 D. B. Marshall and B. N. Cox (2008). IntegralTextile Ceramic Structures. Annual Review of Materials Research. Vol 38:425-443.
 B. N. Cox and Q. D. Yang (2006). InQuest of Tests for Structural Composites. Science. Vol 314 (5802):1102-1107.