Design and Synthesis of Novel High Temperature Materials

Ongoing efforts to create more efficient gas turbine engines able to operate at temperatures above those feasible today have stimulated an interest in new high temperature materials and methods for making them. This dissertation investigates the synthesis of two candidates. The first is a silicon carbide foam with submicron pores made from a preceramic polymer via a space holder method. The second is a class of materials called refractory complex concentrated metal alloys (RCCA) with potential maximum use temperatures significantly above those of superalloys, but whose optimal compositions remain to be discovered. The dissertation describes the exploration of a combinatorial approach intended to accelerate their discovery.
A method was developed for making silicon carbide foams with an average pore diameter of 650 nm and 150 nm wide interpore ligaments using spherical polymethylmethacrylate (PMMA) particle templating of a SiC nanoparticle loaded polycarbosilane (PCS) preceramic polymer. Differential scanning calorimetry and thermogravimetric analysis were used to investigate the effects of PMMA decomposition and the presence of SiC nanoparticles upon PCS cure, pyrolysis, and partial crystallization mechanisms. Incomplete thermal decomposition and evaporation of the PMMA in the presence of the PCS resulted in retention of excess carbon in the amorphous pyrolyzed foam and the appearance of small PMMA remnant particles in each foam pore. During the highest temperature crystallization treatments, these remnant particles began to decompose along with the solid foam ligaments. The foam’s Young’s modulus and compressive strength at first increased with maximum processing temperature, reaching a maximum after processing at 1300 ˚C. Further increases in temperature resulted in a rapid fall both in elastic modulus and compressive strength. Well established micromechanical foam models were combined with metallographically determined relative density to estimate the foam’s solid ligament Young’s modulus and modulus of rupture. These results indicated that changes to the solid (inter-pore ligament) material properties were primarily responsible for the observed trends in nanofoam mechanical properties rather than changes of foam density.
While much work is underway to develop SiC composites for use in load supporting applications in engines, an alternative superalloy replacement strategy is to develop a refractory alloy whose maximum use temperature meets or exceeds the needs of future engines (i.e. an alloy that functions at temperatures T >1300 °C). Complex Concentrated Alloys (CCAs) have emerged as a promising new class of advanced metallic material for high temperature applications. CCAs utilize four or more principal elements in non-dilute concentration to achieve high strength, toughness, and fatigue endurance, as well as reduced thermal and electrical conductivity. When made from combinations of three, four or more high melting temperature refractory metals (tungsten, tantalum, molybdenum, niobium together with hafnium, vanadium, chromium, zirconium and titanium) these properties could potentially be achieved in alloys with exceptionally high melting temperatures. However, gaps in the fundamental understanding of the mechanistic origins of many of the properties of CCAs has impeded alloy design and development. This is amplified by the lack of experimental tools for the rapid exploration of the extraordinarily large compositional design space; especially tools suitable for the difficult to process refractory metal alloys. While refractory CCAs (RCCAs) promise high strength, in combination with high melting temperature, the tendency for brittle fracture at low temperatures, the difficult processing paths, the extreme sensitivity to interstitial impurities (C, N, O), their susceptibility to oxidation in general, and particularly to volatile oxide formation, and the combinatorial complexity of the compositional space, make the discovery of new compositions via experimental approaches challenging. The goal of this aspect of the dissertation was to explore the application of an electron beam evaporation-based combinatorial synthesis tool to refractory, body centered cubic (BCC) CCAs based upon mixtures of W, Ta, Mo, Nb, Hf, V, Cr, and Ti. By using inert gas jet-controlled intermixing of the vapor plumes from two, three or four laterally separated metal sources, combinatorial libraries with concentration gradients up to 12 at% across 50 mm wide combinatorial libraries have been achieved using an electron beam directed vapor deposition (EB-DVD) approach. It is shown that the region of library concentration space could be adjusted by varying the electron beam power applied to each source. The high electron beam power (10 kW) available enabled high rates of evaporation from the sources and the growth of thick (greater than 10 m thickness) coatings on 50 mm x 50 mm molybdenum substrates. This enabled the synthesis of Ta-Cr-Nb-Ti and Ta-Hf-Nb-Ti libraries that permitted x-ray diffraction assessments of the composition - phase space relationships for libraries formed at 1200 °C. These films were sufficiently thick that they permitted application of nano-indentation methods for assessments of their ambient temperature elastic modulus and hardness.