Evaluation of Novel Strontium Zirconate Systems for Investment Casting of High-Temperature Alloys

Atomic diffusion at the mold metal-interface in high-temperature investment casting systems is detrimental to processing costs for modern aerospace components, in the range of billions of dollars annually. In nickel superalloy systems, inward oxygen diffusion from the ceramic mold in combination with outward diffusion of various alloying elements such as Al, Cr, Ni, Hf, Ta, and Co results in the formation of a surface oxide scale, which is both expensive and time-consuming to remove. As such, mitigation of this diffusion provides a critical opportunity for making these castings more economical. Strontium zirconate (SZ), a novel perovskite ceramic, has demonstrated the potential for substantial reduction of surface scale on investment cast nickel superalloy components. Materials data regarding refractory performance of SZ and its processing-structure-property relations are, however, extremely limited.

In the work presented here, data with respect to processing of SZ are gathered to determine the effects of process parameters on thermal, mechanical, and physical properties. Included in these parameters are binder, composition, and firing temperature, which previously have each been determined to significantly influence sintered characteristics of ceramic components. By utilizing an orthogonal array of experiments, characterization of properties such as thermal conductivity, mechanical strength, and porosity have been conducted to determine a correlation factor relative to each parameter. Microstructural changes were characterized using optical and electron microscopy to correlate physical morphology of sintered SZ with processing criteria and the resultant characteristic ranges. Finally, preliminary solid-state diffusion experiments between Inconel 718 and SZ are used to relate these observations back to the formation of oxide products at the mold-metal interface as a result of the aforementioned atomic diffusion.