High‐Temperature Oxidation of ZrB2–SiC–AlN Composites at 1600°C

The effect of AlN substitution on oxidation of ZrB₂–SiC was evaluated at 1600°C up to 5 h. Replacement of ZrB₂ by AlN, with 30 vol% SiC resulted in improved oxidation resistance with a thinner scale and reduced oxygen affected area. On the other hand, substitution of AlN for SiC resulted in a deterioration of the oxidation resistance with an abnormal scale and significant recession. The effect of SiC content was also studied, and was found to be consistent with the literature for the composites without AlN additions. A similar effect was observed when AlN was added, with the higher SiC content materials showing improved oxidation resistance. X‐ray photoelectron spectroscopy showed the presence of Al₂O₃ and SiO₂ on the surface, which could possibly lead to a modification in the viscosity of the glassy oxide scale. Possibly, the oxidation behavior of ZrB₂–SiC composites can be improved with controlled AlN additions by adjusting the Al:Si ratios.     

Oxidation Behavior of ZrB2–SiC Composites at Low Pressures

ZrB₂‐60 mol%SiC composite with a eutectic microstructure was oxidized at 1573 to 1873 K with reduced total pressures (P_tot) and low oxygen partial pressures (). The mass change was continuously measured by a thermobalance, and then fit with a multiple paralinear model. Oxidation scale of SiO₂/ZrO₂+SiO₂/ZrO₂/ZrB₂ was formed at  > 0.13 kPa, whereas only porous ZrO₂ remained at  < 0.13 kPa, P_tot < 1.33 kPa and higher than 1773 K. With increasing , the parabolic oxidation constant decreased, whereas the linear oxidation constant increased.     

Initial Stages of ZrB2–30 vol% SiC Oxidation at 1500°C

The initial oxidation behavior of ZrB₂–30 vol% SiC was analyzed with the goal of understanding any relationship to the variable oxidation performance observed at longer times. A box furnace was used to oxidize samples for times as short as 10 s and up to 100 min at 1500°C in air. The samples were characterized using mass change, scanning electron microscopy, energy dispersive spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy to explore the oxidation behavior. The presence of borosilicate glass and ZrO₂ was observed on the surface at times as early as 10 s. Bubble formation in the borosilicate glass was observed after 30 s of oxidation and is attributed to uneven distribution of the glass. The impact of surface roughness on oxidation was also explored and found to be negligible for times greater than 30 s.     

Effect of AlN Substitutions on the Oxidation Behavior of ZrB2–SiC Composites at 1600°C

The oxidation behavior of ZrB₂–SiC composites, with varying amounts of AlN substituting for ZrB₂, was studied isothermally under static ambient air at 1600°C for up to 5 h. Small amounts of AlN substitutions (≤10 vol%) were found to result in marginal improvement in the oxidation resistance, whereas larger amounts resulted in a significant deterioration. The size of ZrO₂ clusters formed on the oxidized surface was found to be a function of the AlN content. This effect was more pronounced after longer oxidation times (~1 h) as opposed to shorter durations (~5 min). It was postulated that presence of AlN results in the formation of Al₂O₃ during the oxidation process, subsequently resulting in a lowering of viscosity of the glassy silica scale, which facilitates the coarsening of ZrO₂ clusters. This also increases oxygen permeation through the scale which adversely affects the oxidation resistance of the high AlN content composites.     

Volatility diagram of ZrB2‐SiC‐ZrC system and experimental validation

A volatility diagram of zirconium carbide (ZrC) at 1600, 1930, and 2200°C was calculated in this work. Combining it with the existing volatility diagrams of ZrB₂ and SiC, the volatility diagram of a ternary ZrB₂‐SiC‐ZrC (ZSZ) system was constructed in order to interpret the oxidation behavior of ZSZ ceramics. Applying this diagram, the formation of ZrC‐corroded and SiC‐depleted layers and the oxidation sequence of each component in ZSZ during oxidation and ablation could be well understood. Most of the predictions from the diagrams are consistent with the experimental observations on the oxidation scale of dense ZrB₂‐SiC‐ZrC ceramics/coatings after oxidation at 1600°C or ablation at 1930 and 2200°C. The reasons for the discrepancy are also briefly discussed.     

ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO₂ was used to produce an oxygen partial pressure of ∼10⁻¹⁰ Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO₂ produced a non-protective oxide surface scale.     

Oxidation Behavior of SiC Whiskers at 600–1400°C in Air

The oxidation behavior of SiC whiskers (SiC_W) with a diameter size of 50–200 nm has been investigated at 600°C–1400°C in air. Experimental results reveal that SiC_W exhibit a low oxidation rate below 1100°C while a significant larger oxidation rate after that. This can be attributed to the small diameter size of SiC_W, which determines that it is hard to form a protective SiO₂ layer thick enough to hamper the diffusion of oxygen effectively. Both nonisothermal and isothermal oxidation kinetics were studied and the apparent oxidation energy was calculated to further understand the oxidation behavior of the SiC_W.     

Cyclic Oxidation of Ternary Layered Ti2AlC at 600–1000°C in Air

The cyclic oxidation of bulk Ti₂AlC at intermediate temperatures of 600–1000°C in air was studied by thermogravimetric analysis. It was demonstrated that Ti₂AlC exhibited good cyclic‐oxidation resistance at temperatures above 700°C. The cyclic‐oxidation kinetics approximately follows a parabolic rate law at 700–1000°C range. The surface scales are dense, resistant to spalling and adhesive to Ti₂AlC substrate. An abnormal oxidation whose cyclic‐oxidation kinetics obeys a linear law is observed at 600°C. As revealed by scanning electron microscope (SEM), oxidation‐induced cracks present at 600°C results in poor protectivity and accounts for the abnormal oxidation. The cracks are caused by the stress associated with the volume expansion due the formation of anatase TiO₂ in the scale.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Thermo‐structural design of ZrB2–SiC‐based thermal protection system for hypersonic space vehicles

The leading edges of hypersonic space vehicle experience high temperature and stress due to prevailing aerothermodynamic conditions of extreme heat flux and pressure. The design of thermal protection system (TPS) to protect the metallic airframe structure can ensure longer life and reliability under flight conditions. The effective design of TPS system requires the precise quantitative understanding of thermo‐mechanical stresses and deformation, which demands careful computational study under flight simulated conditions. In the above perspective, TPS design for a leading edge exposed to Mach 7 hypersonic flow for 250 seconds has been carried out by performing finite element‐based thermo‐structural analysis with pressure and heat flux estimated from computational fluid dynamics analysis (CFD). The fidelity and robustness of CFD scheme is established using grid independence and convergence analysis. CFD analysis effectively captures the formation of bow shock around the leading edge and stagnation region near its nose. For finite element analysis, high‐quality structural elements have been generated using HyperMesh to precisely model the thermo‐structural behavior of TPS. In our computational analysis, TPS is modeled as a three‐layered system with outermost layer of ZrB₂‐SiC, middle layer of phenolic cork and innermost layer of Ti‐alloy. The analytical values of spatial variation of temperature, stress components, and displacement across the TPS have been critically analysed to rationalise specific structural configuration for better thermo‐structural stability. Together with temporal variation of temperature, the implication of such computational results has led us to propose a new design for TPS. The proposed TPS is capable of containing the stress and displacement within 32 MPa and 0.58 mm, respectively, when the leading edge is exposed to shock induced aero‐thermal heating as high as 2.11 MW/m² and pressure of 72.8 kPa for a hypersonic cruise flight of 500 km range.     

Experimental and computational analysis of thermo‐oxidative‐structural stability of ZrB2–SiC–Ti during arc‐jet testing

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.     

Oxidation behavior of C–SiC composites with a Si–W coating from room temperature to 1500°C

Oxidation tests of carbon fiber reinforced silicon carbide composites with a Si–W coating were conducted in dry air from room temperature to 1500°C for 5 h. A continuous series of empirical functions relating weight change to temperature after 5 h oxidation was found to fit the test results quite well over the whole temperature range. This approach was used to interpret the different oxidation mechanisms. There were two cracking temperatures of the matrix and the coating for the C–SiC composite. Oxidation behavior of the C–SiC composite was nearly the same as that of the coated C–C composite above the coating cracking temperature, but weight loss of the C–SiC composite was half an order lower than that of the coated C–C composite below the cracking temperature. As an inhibitor, the SiC matrix increased the oxidation resistance of C–SiC composites by decreasing active sites available for oxidation. As an interfacial layer, pyrolytic carbon decreased the activation energy below 700°C. From 800°C to 1030°C, uniform oxidation took place for the C–SiC composite, but non-uniform oxidation took place for the coated C–C composite in the same temperature range. The Knudsen diffusion coefficient could be used to explain the relationship between weight loss and temperature below the coating cracking temperature and the matrix cracking temperature.     

High‐temperature tensile strength and fracture behavior of ZrB2‐SiC‐graphite composite

The tensile behavior of ZrB₂‐SiC‐graphite composite was investigated from room temperature to 1800°C. Results showed that tensile strength was 134.18 MPa at room temperature, decreasing to 50.34 MPa at 1800°C. A brittle‐ductile transition temperature (1300°C) of ZrB₂‐SiC‐graphite composite was deduced from experimental results. Furthermore, the effect of temperature on the fracture behavior of ZrB₂‐SiC‐graphite composite was further discussed by microstructure observations, which showed that tensile strength was controlled by the relaxation of thermal residual stress below 1300°C, and was affected by the plastic flow during 1300°C and 1400°C. At higher temperature, the tensile strength was dominated by the changes of microstructures.     

Reactive Hot‐Pressed Hybrid Ceramic Composites Comprising SiC(SCS‐6)/Ti Composite and ZrB2–ZrC Ceramic

In this study, hybrid composites comprising SiC(SCS‐6)/Ti and ZrB₂–ZrC ceramics were prepared by sandwiching Ti/SiC(SCS‐6)/Ti sheets and Zr + B₄C powder layers, followed by reactive hot pressing at 1300°C. The microstructure of the obtained hybrid composites was characterized by field‐emission scanning electron microscopy, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. The results show that after reactive hot pressing, a highly dense matrix was achieved in the hybrid composites. A Ti‐rich zone was observed only in the hybrid composite prepared using a 10‐μm‐thick Ti foil. Interface reaction occurred during sintering and interface reaction layers were formed between the fibers and the matrix, and the phases were identified. In addition, the mechanical behavior of the hybrid composites was evaluated using by testing under four‐point bend testing. The results indicate that the hybrid composites exhibited greater flexural strengths and noncatastrophic fracture behavior. The flexural strength ranged from 440 to 620 MPa, depending on the thickness of the Ti foils and the fiber volume amount.     

Oxidation Behavior of Si-C-O Fibers (Nicalon) under Oxygen Partial Pressures from 102 to 105 Pa at 1773 k

Polycarbosilane-derived SiC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10² to 10⁵ Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 ×10² Pa and actively oxidized under an oxygen partial pressure of 10² Pa. Under oxygen partial pressures from 2.5 × 10² to 10³ Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer. Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was lost under the active-oxidation condition.     

Separating Test Artifacts from Material Behavior in the Oxidation Studies of HfB2–SiC at 2000°C and Above

Oxidation characteristics of HfB₂‐15 vol% SiC prepared by field‐assisted sintering was examined at 2000°C by heating it in a zirconia‐resistance furnace and by direct electrical resistance heating of the sample. Limitations of the material and the direct electrical resistance heating apparatus were explored by heating samples multiple times and to temperatures in excess of 2300°C. Oxide scales that developed at 2000°C from both methods were similar in that they consisted of a SiO₂/HfO₂ outer layer, a porous HfO₂ layer, and a HfB₂ layer depleted of SiC. But they differed in scale thicknesses, impurities present, scale morphology/complexity. Possible test artifacts are discussed.     

C–(N)–S–H and N–A–S–H gels: Compositions and solubility data at 25°C and 50°C

Calcium silicate hydrates containing sodium [C–(N)–S–H], and sodium aluminosilicate hydrates [N–A–S–H] are the dominant reaction products that are formed following reaction between a solid aluminosilicate precursor (eg, slags, fly ash, metakaolin) and an alkaline activation agent (eg NaOH) in the presence of water. To gain insights into the thermochemical properties of such compounds, C–(N)–S–H and N–A–S–H gels were synthesized with compositions: 0.8≤Ca/Si≤1.2 for the former, and 0.25≤Al/Si≤0.50 (atomic units) for the latter. The gels were characterized using thermogravimetric analysis (TGA), scanning electron microscopy with energy‐dispersive X‐ray microanalysis (SEM‐EDS), and X‐ray diffraction (XRD). The solubility products (K_S0) of the gels were established at 25°C and 50°C. Self‐consistent solubility data of this nature are key inputs required for calculation of mass and volume balances in alkali‐activated binders (AABs), and to determine the impacts of the precursor chemistry on the hydrated phase distributions; in which, C–(N)–S–H and N–A–S–H compounds dominate the hydrated phase assemblages.