Ultra-high Temperature Ceramic Composites Research Articles

Densification behavior involving creep during spark plasma sintering of ZrB2-SiC based ultra-high temperature ceramic composites

A comparative study has been carried out on the densification behavior of the ZrB2–SiC composites with B4C and C as additives along with varying amounts of LaB6 (7, 10 and 14 vol%) during spark plasma sintering (SPS) at 1600 °C and 1800 °C under applied ram pressures of 50 MPa and 70 MPa, respectively. The densification kinetics for both the sintering conditions has been analyzed with emphasis on the role of additives and LaB6 for oxide reduction as well as deformation by creep. For SPS at both temperatures, the relative density is increased with increasing volume fraction of LaB6, with higher values being observed on sintering at 1600 °C. The creep during SPS at 1600 °C being analyzed using the Bernard-Granger model has exhibited stress exponent (n) ~5–6 suggesting dislocation-climb as the operating mechanism. However, for sintering at 1800 °C, observation of n~2 for the composites with 7 and 10 vol% LaB6 suggests grain boundary sliding aided by diffusion through grain boundary glassy phase, whereas n~3 for the 14 vol% LaB6 composite indicates dislocation glide-controlled creep. Lower densification on SPS at 1800 °C has been ascribed to formation and escape of gaseous products like B2O3, SiO and CO during sintering. For the composites sintered at 1600 °C with density ≥97.5%, hardness increases with decreasing grain size, whereas it is related to relative density in the composites with lower density being processed by SPS at 1800 °C.

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB2–SiC–ZrC–Cf ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB2:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB2–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (Cf: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:Cf components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% Cf had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.

An experimental study of ultra-high temperature ceramics under tension subject to an environment with elevated temperature, mechanical stress and oxygen

Ultra-high temperature ceramic (UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy (SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.

A passive oxidation, finite element kinetics model of an Ultra-High Temperature Ceramic composite

A finite element analysis of an Ultra-High Temperature Ceramic (UHTC) composite was conducted to observe the self-healing property of such a class of materials during damage propagation. In this work, a constitutive relation is presented to the composite that couples oxygen kinetics in from UHTCs additives with quasi-brittle damage propagation. The model is solved in a finite element framework, with polycrystalline multiscale material calculations obtained through a Representative Volume Element (RVE) calculation. The self-healing was obtained from self-healing time population and strength recovery during fracture for passive oxidation conditions, while accounting for loading beyond fracture limits and reloading past such limits. The oxidative model set by an Arrhenius relation is used to show the passive oxidative reaction role in changing the constitutive relation through change of a damage parameter for a quasi-brittle constitutive model. A single element study has been introduced to illustrate both the constitutive behavior and the self-healing population with time during passive oxidation at multiple temperatures. Also, a model was set to obtain the flexural strength of a Zirconium Diboride-30% Silicon Carbide ceramic composite to observe the strength recovery from oxidative additives for loading and reloading. The model captured the flexural strength recovery during reloading past the fracture limits and concurrent passive oxidation, where the strength has increased from 675 MPa to 746 MPa.

Development of a slurry injection technique for continuous fibre ultra-high temperature ceramic matrix composites

A simple and effective slurry injection method for producing dense and uniform ultra-high ceramic matrix composites from preforms of high fibre density was developed. As this method is based on slurry injection the homogeneity is not constrained to small preform sizes; dense components of high fibre volume can be produced in theoretically any size and shape. Samples produced by this method demonstrated high and consistent densities, with the injection method obtaining densities an average 27% higher and 87% lower in variability when compared to conventional vacuum impregnation. Tomography demonstrated no bias in the ceramic powder distribution for samples produced by injection, whereas samples produced by vacuum impregnation alone displayed poor powder penetration to the centre of large samples. The new approach yielded composites that were as strong and/or more consistent in strength compared to vacuum impregnation. Thermo-ablative testing demonstrated significant improvements in protective capability for materials produced by this route.

Carbon fiber reinforced ultra-high temperature ceramic matrix composites: A review

Abstract Recent studies on carbon fiber-reinforced ultra-high temperature ceramic matrix (C/UHTC) composites fabricated by hot-pressing, chemical vapor infiltration, polymer impregnation and pyrolysis, and melt infiltration (MI) are reviewed. Various efforts have been made to improve these preparation processes and to combine two or more of these because they have both the advantages and disadvantages in terms of the processing time, operating temperature, and the porosity of the resulting C/UHTC composites. In addition, the parameters governing the fracture toughness, thermal conductivity, and recession behavior (in oxidizing environments) of these composites have been discussed. This review demonstrates that C/UHTC composites with Zr- or Hf-based UHTC matrices fabricated via MI are potential candidates for advanced heat-resistant structural materials.

Ablation behaviour of ultra-high temperature ceramic matrix composites: Role of MeSi2 addition

A new class of ZrB2 composites reinforced with 40 vol% C short fibers and containing 5 vol% SiC in combination with 5 vol% MoSi2, HfSi2 or WSi2 successfully withstood extreme conditions in a oxyacetylene torch. Different responses to the torch testing were recorded depending on which secondary phase was present; this was primarily a result of the final density which ranged between 83 and 94% of the theoretical value. The temperatures achieved on the surfaces of the samples tested also varied as a function of the residual porosity and ranged from 2080 to 2240 °C. HfSi2 additions offered the best performance and exceeded that of the baseline material that contained only SiC. It is believed that this was due to its ability to promote the elimination of porosity during densification and to the refractory nature of its oxide, HfO2. In contrast, MoSi2 and WSi2 formed highly volatile oxides on the surface, which did not offer better protection than the ZrO2-SiO2 scale that developed in the baseline.

Development of Ultra-High Temperature Ceramics: From Monoliths to Composites

Regarding materials development, our studies have been mainly focused on ZrB2-SiC and HfB2-SiC compositions with TaSi2 or Y2O3 additions using hot pressing and spark plasma sintering. These additives have been used to decrease the sintering temperature and to improve the oxidation resistance. Interesting mechanical properties at room and high temperature have been measured. Moreover, excellent oxidation behaviors have been observed up to 2000-2200°C with Y2O3. Last developments are centered on the manufacturing of ultrahigh temperature ceramic matrix composites (UHTCMC) using slurry infiltration and pyrolysis for example. First results are encouraging.

Materials characterisation and mechanical properties of Cf-UHTC powder composites

Ultra-high temperature ceramic composites based on carbon fibre, Cf, preforms impregnated with hafnium diboride, HfB2, powder and then densified with carbon by chemical vapour infiltration, CVI, have been mechanically tested to measure the room temperature flexural, interlaminar shear, compressive and tensile strengths. The latter was also measured at 1000 °C. All the composites suffered a degree of delamination during the different mechanical tests but the strength values obtained were at least equal to, or better than, those previously reported in the literature for ultra-high temperature ceramic (UHTC)-based composites. Importantly, in spite of the oxidation of the tensile samples tested at 1000 °C, similar tensile strength values were obtained at both temperatures, suggesting that the materials can resist elevated temperatures. The samples tested at higher temperature did show greater evidence of fibre pull out, possibly due to a weaker fibre-matrix interface as a result of oxidative degradation. The results also suggested that the 0° orientation plies in the Cf preform structure offered greater resistance to mechanical stresses; this suggests that composites can now be designed to offer even greater strength values.

Enhanced thermo-mechanical damage tolerance of functionally graded ZrB2-20SiC ceramic reinforced with carbon nanotubes

The demand of a new generation spacecraft component has led to the development of a novel structure, which combines the traits of both functionally graded material (FGM) as well as an ultra-high temperature ceramics (UHTC). Herein, FGM based on the well-established ZrB2-20SiC (vol%) UHTC with the reinforcement of carbon nanotubes (CNT) is processed via powder metallurgy and spark plasma sintering (SPS). A densely graded microstructure with a crack-free seamless interface is witnessed along the cross-section of the FGM. The CNT in the optimized FGM is found to be efficaciously embedded in the matrix retaining its structure during sintering (at 1850 °C), shown microstructurally and supplemented by Raman spectroscopy. Enhanced mechanical and wear resistance of the FGM when compared to that of the conventional ZrB2 UHTC is attributed to the dense, crack-free microstructure and CNT pull-out mechanism. Oxidation test revealed an enhancement in the thermal stability as the onset of oxide formation starts above 886 °C (i.e. > 30% when compared to that of monolithic ZrB2). The thermo-structural performance of the processed FGM simulated using finite element method (FEM) indicated a reduction in the thermal residual tensile stresses (~ 8.1 times) which mitigates the crack propagation. The FEM analysis complements the high indentation fracture toughness values of the FGM even at the interfaces (similar to that found in each layer), establishing FGM approach superior to that of the conventional UHTC composites. Thus, addition of CNT in ZrB2-20SiC based FGM are conducive enough to be engineered as a potential material, poised to shape the future of re-entry vehicles.

Toughening effect of non-periodic fiber distribution on crack propagation energy of UHTC composites

Different configurations of continuous carbon fiber-reinforced ultrahigh temperature ceramics (UHTCs), by combining coatings and matrix, were produced via electrophoretic deposition (EPD) and slurry infiltration. The toughening of non-periodic fiber distribution induced by the EPD process was investigated through work of fracture analysis. The results show that a non-periodic fiber distribution results in toughness increase from 8 MPa√m to 11 MPa√m with respect to a periodic fiber distribution. This toughness improvement does not strongly affect the flexural strength, which is mainly related to the fiber volumetric amount. It is shown that the assembling of carbon fibers into bundles (i.e. by dispersing the fibers with a non-periodic distribution) increases the crack propagation energy dissipated on the crack-wake from 0.5 kJ/m2 to 1 kJ/m2, which can be mainly ascribed to the fiber/bundle pull-out. On the other hand, the energy dissipated on the crack-tip (as fiber/matrix debonding) is fiber distribution-independent and increases from 0.3 kJ/m2 to 0.4 kJ/m2 with increasing the fiber amount from 33 vol% to 40 vol%. Finally, WoF analysis is proposed as test to evaluate pull-out toughening instead of push-in and push-out tests.

Ultra-high temperature ceramic composite

ABSTRACTThe work carried out under the XMat research programme (Materials Systems for Extreme Environments, EPSRC Programme Grant number EP/K008749/1-2) in the field of ultra-high temperature ceramic matrix composites has been focused on the design, development and manufacture of complex shapes and large panels for use under extreme conditions. The composites are made from 2.5D woven carbon fibre preforms impregnated with HfB2 powders and with a pyrolytic carbon, PyC, matrix created using chemical vapour infiltration, CVI. More recently, the knowledge acquired during the development of these Cf-HfB2-C composites has been focused on shortening the densification time by moving from conventional CVI to Radio Frequency-heated CVI; the work has also switched to Cf-ZrB2-C composites. In addition, the use of 3D carbon fibre preforms has begun to be explored to improve the mechanical properties and also the replacement of PyC matrix with ZrB2 to reducing the oxidation of the composites at ultra-high temperature.

Influence of zirconium-based alloys on manufacturing and mechanical properties of ultra high temperature ceramic matrix composites

ABSTRACTFor the continuous development of ultra high temperature ceramic matrix composites manufactured using reactive melt infiltration (RMI), the influence of three different melt alloys and two preform routes on mechanical behaviour and melt infiltration is investigated. Contact angle and viscosity measurement are performed to describe the melt infiltration process. The purpose of these experiments is to further understand the RMI process and increase the mechanical performance of the material. The infiltration height of molten metals within capillary systems depends on the measured contact angles and viscosity as well as the different phases present in the preform, matrix and fibre coating. In order to successfully maintain mechanical performance, minimal reactivity between the melt and coating/fibres is desired. Resulting phase formations, for each manufacturing step, are investigated and analysed using SEM, EDX and XRD. Mechanical performance is determined using flexural strength by three-point-bending, achieving close to 400 MPa with fabrics.

Impact of residual stress on thermal damage accumulation, and Young's modulus of fiber-reinforced ultra-high temperature ceramics

Ultra-high temperature ceramic matrix composites (UHTCMCs) based on ZrB2-matrix reinforced with 45 vol% of unidirectional continuous carbon fibers are studied through the thermal mechanical hysteresis in order to investigate the thermal damage accumulation. The analysis carried out allowed to extrapolate the Young's modulus of the matrix from thermal expansion measures. It was found that the initial matrix Young's modulus of 195 GPa steadily decreases by thermal cycling the samples between RT and 1300 °C as a consequence of matrix cracking. On the other hand, the analysis suggested that carbon fibers keep their Young's modulus constant at 780 GPa. Finally, the residual stresses due to the different coefficient of thermal expansion between matrix and carbon fibers are discussed and let to justify the Young's modulus of 230 GPa, which cannot be explained with the so-called “rule of mixtures” generally valid and widely used in the composite science.

Theoretical prediction of temperature-dependent fracture strength for ultra-high temperature ceramic composites considering the evolution of damage and thermal residual stress

Based on the maximum storage energy density criterion of material fracture, a model of temperature-dependent fracture strength for ultra-high temperature ceramic composites is established. The combined impacts of the evolution of damage and thermal residual stress with temperature are considered. The model predictions are highly consistent with available experimental values. Besides, the critical crack sizes of ZrB2–30 vol%SiC in air from 1400 to 1600 °C are predicted using the proposed model, which agree well with the total oxidation thickness of the reported literature at 1400 and 1500 °C, and a more reasonable definition of critical crack size at 1600 °C are given. Moreover, the quantitative effect of crack size on the fracture strength is analyzed under different environment temperature, and a useful conclusion is obtained that decreasing crack size is more effective to improve the fracture strength of the composites at low temperatures. This study not only provides a feasible and convenient method to predict the fracture strengths at different temperatures, but also offers a theoretical support for the design of ultra-high temperature ceramic composites.

A novel and convenient temperature dependent fracture strength model for the laminated ultra-high temperature ceramic composites

At present, no suitable and convenient model has been developed to predict variation tendencies of fracture strength of laminated ceramic composites with temperature. In this paper, a novel and quite simple temperature dependent fracture strength model for the laminated ultra-high temperature ceramic composites was developed based on energy theories–the Griffith energy criterion and a concept of energy storage capacity. The complicated problems of characterization of combined effects of temperature, change in critical flaw size and laminated structure parameters on fracture strength are solved by using primary and simple method. The model prediction agreed well with the experimental value. The novel model can be used to determine both fracture strength and change in critical flaw size of laminated composite at elevated temperature. It should be noted that the model has no any fitting parameter, which is an easy-to-use method for the engineering.

Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic composites prepared via Zr-Si alloyed melt infiltration

Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic matrix composite (C/C-ZrC) from ultra high temperatures (particularly > 1500 °C) to the room temperature was evaluated using a novel self-developed equipment with high heating-cooling rates in controllable atmosphere. Residual strength and mass variation of the as-prepared composite under different thermal shock cycles and temperatures were tested to characterize the thermal shock resistance. Flexural strength of the composite slightly decreases initially without obvious weight loss below 1300 °C, while it decreases by relatively high values at 1300–1900 °C and then sharply decreases over 1900 °C with obvious increase of weight loss. Microstructure of the thermally shocked specimens was examined to reveal the thermal shock damage. Matrix cracking, interfacial debonding and matrix pores were clearly observed, which were the main reasons for the strength degradation of the composite thermally shocked for different cycles and temperatures.

On the thermal shock resistance and mechanical properties of novel unidirectional UHTCMCs for extreme environments

Aerospace provides a strong driving force for technological development. Recently a novel class of composites for harsh environments, based on ultra-high temperature ceramic composites reinforced with continuous fibers (UHTCMC), is being developed. The goal of this work is to overcome the current data patchwork about their microstructural optimization and structural behavior, by showing a consistent mechanical characterization of well-defined and developed UHTCMCs based on ZrB2-matrix. The obtained composites have a density of 3.7 g/cm3 and porosity of less than 10%. The flexural strength increased from 360 to 550 MPa from room temperature to 1500 °C, showing a non-brittle behaviour. The composites were able to sustain a thermal shock severity as high as 1500 °C. The maximum decrease of strength at 1400 °C was 16% of the initial value, indicating that the samples could be shocked at even higher temperature. Flexural strength, Young’s modulus and coefficient of thermal expansions (CTE) of the composites were measured both along transverse and longitudinal direction and correlated to the microstructural features. The presented microstructural and mechanical characterization well defines the potentiality of the UHTCMCs and can be used as reference for the design and development of novel thermal protection systems and other structural components for harsh environments.

ANFIS based model to forecast the Wire-EDM parameters for machining an Ultra High Temperature Ceramic composite

In this work, a ZrB2 based ceramic composites with 25 wt. % B4C sintered in spark plasma sintering furnace at 2000°C for 10 min under 50 MPa pressure has been machined with Wire electrical discharge machine (WEDM) and can readily be cut to intricate shapes to meet the requirements of industry. The adaptive neuro fuzzy inference systems (ANFIS) has been used to generate mapping relation between input parameters and output responses. ANFIS is a neuro fuzzy technique where the fusion is made between the neural network and the fuzzy inference system. Also the ANFIS models have been tested by a confirmative test and the results shows minimum root mean square error (RMSE) as 0.0416 for kerf width and 0.9605 for machining speed. This verification indicates that ANFIS modeling can be used to forecast the responses.

Reaction mechanism and microstructure development of ZrSi2 melt-infiltrated Cf/SiC-ZrC-ZrB2 composites: The influence of preform pore structures

Reactive melt infiltration (RMI) is an effective method for fabrication of highly dense carbon fiber reinforced ultra-high temperature ceramic matrix composites (Cf/UHTCs). In this work, Cf/SiC-ZrC-ZrB2 composites were fabricated by infiltrating ZrSi2 melt into porous Cf/B4C-C preforms, where the physical and chemical reactions involved during the RMI process were identified and analyzed. Inhomogeneous infiltration between the inter- and intra-bundle pores was revealed, and was found to be strongly related to the pore structures of the Cf/B4C-C preform. It is indicated that the inhomogeneous infiltration can be mitigated remarkably with increasing porosity and pore size of the preform. The effect of pore size on the RMI process was also investigated by a quantitative model, which agrees very well with the experiment results. It further indicates that the inhomogeneous infiltration can also be relieved at elevated RMI temperature. However, excessive infiltration at elevated temperature or more porous preform may cause serious erosion on interphase and fibers, leading to mechanical properties deterioration of the final composites.