A computational analysis of a ZrO2–SiO2 scale for a ZrB2–ZrC–Zr ultrahigh temperature ceramic composite system

The success of a ceramic composite for ultrahigh temperatures (i.e., >1873 K) in an oxidizing atmosphere resides in the protective characteristics of a scale to limit oxygen ingress or to control the oxygen reaction into the substrate. With temperature changes from room temperature to ultrahigh temperatures, the mechanics of the scale and its reactivity becomes critical for ceramic composites to operate under extreme environments. A study was pursued to design computationally a SiO2–ZrO2 scale for a ZrB2/ZrC/Zr–Si composite by using conventional finite element analysis, which was used as a baseline microstructure for the extended finite element method. The model of the Zr boride/carbide composite with a SiO2/ZrO2/ZrSix scale simulates the development of local strain energetics under a thermal load from 300 to 1700 K. The computational analysis determined that the size of the SiO2 and ZrSix precipitates does not appreciably influence the durability of the microstructure. A simulated annealing optimization algorithm was also developed for an extended finite element program (called XMicro) with the purpose of optimizing the auto re-meshing of XMicro and thus minimizing its combinatorial selection of a composite's reinforcement architecture. After correcting for the overlapping of ZrO2 precipitates within a matrix, XMicro determined that 1.96 μm as the optimal spacing of precipitates within a cluster and 20 μm between clusters within a silica matrix of the scale interphase. The strategic experimentation determined that porosity developed during oxidation should be incorporated into the simulation of a ceramic composite. To probe into the efficacy of the silica layer for the scale, oxidizing experiments were performed at 1973 K, as well as microstructural analysis of the scale interphase. The computational mechanics coupled with consideration of the thermodynamic stability of phases for the Zr–Si–O system to set the oxygen potentials between layers can design a scale interphase for an ultrahigh-temperature, ceramic composite system. The processing challenge would be to attain the optimal configuration of the microstructure, for example, silicide precipitates developed with the appropriate spacing along a scale/matrix interface or ZrO2 clusters within a silicate phase.