Oxidation of zirconium diboride based and molybdenum based ultra-high temperature ceramics

Designing resilient high temperature alloys has been a long-standing engineering challenge. The driver for high temperature material research ranges from the need of developing robust thermal protection systems for hypersonic vehicles to increased operating temperatures (and hence Carnot efficiencies) of gas turbines. Ni based superalloys have been widely used in a variety of high temperature applications, but these alloys are limited by the melting temperature of Ni3Al and a variety of topologically close-packed phases that melt in the 1200--1300°C range. Transitioning to higher temperatures call for materials with high thermal stability. Hence, ceramics such as silicides, borides, nitrides and carbides have emerged as viable candidates for ultra-high temperature applications. Among these materials, Mo-Si-B alloys and ZrB 2-SiC composites have received a lot of attention due to the formation of a protective scale at elevated temperatures.;In ZrB2-SiC system, improving the oxidation resistance at elevated temperatures has long been a challenge. At temperature ranges where silica rich scale provides the oxidation resistance, the scale viscosity greatly affects the oxidation behavior. This research focuses in tuning the viscosity of the silica scale to tailor the oxidation behavior. The isothermal oxidation tests done at 1600°C demonstrated the overall thickness of the oxide scale has a strong function of the AlN content. Lower levels of AlN had thinner oxide scale and thus higher oxidation resistance, while high AlN substitution resulted in severe oxidation. Oxidation of AlN results in the formation of stable oxide Al2O 3. The presence of Al2O3 significantly reduces the viscosity of SiO2. The observation of coarsening of surface ZrO2 crystallites by Ostwald ripening as a function of increasing AlN content further corroborates a reduction in scale viscosity. Lower viscosity silica rich scale has better flow characteristic and hence better surface coverage. However, the lower viscosity scale also provides faster diffusion for oxygen, which promoted rapid oxidation for high AlN content materials. We also found a marked change in oxidation mechanism as a function of temperature, which was reflected in the structure of the oxide scale as well. At 2000°C in low PO2, the sample cross-sections were characterized by ZrO2 rich external layer with marginal presence of SiO 2 on the surface. The ZrO2 rich external layer served as thermal barrier for underlying materials but suffered from severe spallation during thermal cycling. An interfacial SiO2 rich layer under the oxide scale was observed in high Si and Al samples. The tenacity of the external ZrO2 scale is thought to depend on this interlayer. The presence of a low viscosity interlayer was found to be beneficial since the lower viscosity can accommodate the volumetric changes arising from thermal strains.;In Mo-Si-B system, the central challenge is posed by the phase diagram. The Si rich compositions show excellent oxidative stability, but suffer from poor fracture toughness. The metal rich compositions result in improved toughness, but poor oxidation resistance. In order to resolve this conundrum, the phase fields need to be modified to form a phase assemblage comprising of metal rich solid solution (for improved toughness), the T1 phase (for improved creep and oxidation resistance) and the T2 phase (for oxidation resistance). This requires the destabilization of the A15 phase that forms at compositions between the metal rich solid solution and the T1 phase. We demonstrate that the substitution of Mo by W results in the destabilization of the A15 phase. Furthermore, transient oxidation studies show that the addition of W does not have a deleterious effect on the oxidation resistance at elevated temperatures (1400°C), although the oxidation resistance is adversely affected at the lower temperature regime (≤ 1300°C). Unlike MoO3, which starts volatilizing around 700°C as (MoO3)3, (WO3)3 volatilizes at higher temperatures (∼1350°C). If WO3 persists on the surface, it prevents complete surface coverage by the silica scale. The transient oxidation studies also indicated a strong dependence of the oxidation behavior on the alloy microstructure. Finer grained materials, synthesized by drop-casting result in improved scale coverage in comparison to coarser grained materials synthesized by sintering. Finer length scales require the silica to flow over shorter distances, ensuring relatively rapid scale coverage.