Journal of the American Ceramic Society

In the present work thermal and chemical properties of a glass‐ceramic frit containing zirconium oxide are evaluated after milling in various polar and nonpolar solvents. Particle‐size distribution is one of the main variables investigated to evaluate the efficiency of several solvents. Milling in various solvents does not affect the thermal properties of the glass while the presence of polar groups in the solvent molecule increases the cation release from the glass.

The reaction‐bonded aluminum oxide process begins with aluminum, Al₂O₃, and usually ZrO₂ powders that have been attrition‐milled in an organic liquid. The attrition‐milled powder is then compacted and heat‐treated in air to produce polycrystalline, Al₂O₃‐based ceramics. Safety considerations have made it desirable for the milling liquid to be changed from acetone to a less‐flammable solvent. In this paper, mineral spirits, ethanol, and mineral spirits that contains 2 wt% stearic acid are presented as viable alternatives to acetone. The effects of changing the milling liquid on the reaction process and the properties of the final fired ceramic are investigated.

We use experimentally determined crack growth data for silica glass and a fracture mechanics model for delayed failure to predict the fatigue behavior for high‐strength silica‐glass fibers. The results of this model indicate that fracture mechanics methods can be used to adequately describe the fatigue behavior observed for high‐strength silica‐glass fibers at room temperature in humid conditions. The key feature to properly interpreting the fatigue of high‐strength fibers is the use of a fracture‐rate law in which the crack extension rate increases exponentially with applied stress. We show that a fracture mechanics approach to highstrength fiber fatigue can provide the basis for identifying additional fatigue mechanisms that may control failure in more aggressive fatigue environments.

We utilize a chemical‐kinetics‐based model to describe the rate of crack extension in vitreous silica as a function of the applied stress and the presence of reactive species. Our approach builds upon previous fracture models that treat the atomic bond rupture process at the crack tip as a stressenhanced hydrolysis reaction. We derive the stress dependence for siloxane hydrolysis from measurements of hydrolysis rates for strained silicate ring structures. The stress dependence determined for siloxane hydrolysis yields an activation volume of 2.0 cm³/mol, which is in good agreement with the stress dependence determined for silicate glass fracture. This result supports previous fracture models that are based on absolute reaction rate theory and predicts an exponential dependence of crack extension rate on applied stress intensity.

The emissivity and the catalytic efficiency related to atomic oxygen recombination were investigated experimentally in the range 1000–2000 K for ZrB₂ and ZrB₂–HfB₂‐based ceramics. In order to evaluate the effect of the machining method, two series of samples, one prepared by electrical discharge machining and the other machined by diamond‐loaded tools, were tested. High emissivity (about 0.7 at 1700 K) and low recombination coefficients (on average 0.08 at 1800 K) were found for all the materials. The experimental data showed an effect of the surface machining on the catalytic behavior only on the ZrB₂‐based composite; conversely, small variations were found in the recombination coefficients of ZrB₂–HfB₂‐based samples for the different machining processes. The surface finish affected the emissivity at lower temperatures in both compositions, with the effect becoming negligible at temperatures above 1500 K.

Experimental data on the oxidation kinetics of SiC‐containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter‐current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO₂ counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB₂ containing varying volume fractions of SiC, and for SiC–HfB₂ ultra‐high temperature ceramics (UHTCs) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory‐furnace‐based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc‐jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc‐jet environment.

The thermal stability of ZrB₂‐based composites containing SiC‐chopped fibers was tested in a bottom‐up furnace at 1200°C, 1500°C, and 1700°C for 30 min. The oxidation behavior was studied by X‐ray diffraction, scanning electron microscopy, and weight gain. The degradation induced by oxidation was also evaluated considering the strength decrease of oxidized bars. Baseline ZrB₂‐composites containing the same amount of SiC particles or SiC whiskers were tested in the same conditions for comparison.

The development of new ultra‐high temperature ceramics for thermal protection system (TPS) of hypersonic cruise and re‐entry vehicles requires performance‐qualification testing under simulated flight conditions. The present work, encompassing experiments and computational analysis, critically analyzes the thermo‐oxidative‐structural stability of flat surface disks of spark plasma sintered ZrB₂–18SiC–xTi composites (x=0, 10, 20; composition in wt%) under arc jet flow with heat flux of 2.5 MW/m² for 30 seconds. Such testing conditions effectively simulate the aero‐thermal environment in ground facility, as experienced by hypersonic vehicles. Based on the extensive XRD, SEM‐EDS and electron probe microanalyzer based analysis of the surface/sub‐surface of arc jet exposed ceramics, the oxidation mechanisms are qualitatively discussed. Importantly, thick oxide layers (~400‐950 μm) were found to be adherent, thereby providing good structural stability of such ceramics for reusable TPS. The careful finite element (FE) analysis with high quality structural elements, being generated using HyperMesh, was conducted to understand the underlying reasons for observed oxidation. Such analysis allows us to determine the temporal evolution of through‐thickness temperature distribution. FE‐based calculations were subsequently validated using experimentally measured backwall temperatures. The thermodynamic feasibility of competing oxidation reactions at the analytically computed front wall temperatures was thereafter realistically assessed to support the oxidation mechanisms. Taken together, the present work provides guidelines for better understanding of the thermo‐oxidative‐structural stability of ceramics under arc jet testing and also establishes the good stability of ZrB₂–18SiC–20Ti composites for potential application in TPS of hypersonic space vehicles.

Sharp leading edge (LE) samples of UHTC (20 vol%SiC–HfB₂) and SiC were exposed to simulated hypersonic flight conditions using a direct‐connect scramjet rig and their thermal and oxidation responses measured. The measured back‐wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics‐based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO₄) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates.

Effects of He irradiation on polycrystalline yttria‐stabilized zirconia (YSZ) are studied with the focus on irradiation‐induced damage buildup, He behavior, and volume swelling. The evolution of irradiation‐induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three‐step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single‐crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single‐crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.