Aluminium nitride-molybdenum ceramic matrix composites possessing high thermal shock resistance

Aluminium nitride-molybdenum ceramic matrix composites were produced by hotpressing a mixture of AlN and Mo powders. Thermal shock resistance of a composite (AM25) which contained 25 vol.% of metallic phase dispersed in the AlN matrix, was studied in order to verify the influence of metallic phase additional on the thermal properties. Results showed that AM25 possess a critical temperature difference (ΔT_c) for thermal shock of about 550°C compared to a mean value of 350°C measured in the case of pure hot pressed AlN. This improvement in thermal shock resistance was attributed to better mechanical properties and higher thermal conductivity of AM25 as compared to pure AlN.     

Thermal shock resistance of laminated ceramic matrix composites

The thermal shock resistance capability of laminated ceramic matrix composites is investigated through the study of three-dimensional transient thermal stresses and laminate failure mechanisms. A (–45°/45°)_s SiC/borosilicate glass laminate is utilized as a reference composite system to demonstrate the analytical results. The maximum allowable temperature change, T _max, has been taken as a measure of the thermal shock resistance capability of composites. The effects of fibre orientation, volume fraction, thermal expansion coefficient. Young's modulus, and thermal conductivity on the thermal shock resistance capability, expressed in terms of the maximum allowable temperature change, T _max, have been assessed. Numerical computations are also performed for six composite systems.     

Aspects of residual thermal stresses in continuous-fibre-reinforced ceramic matrix composites

An analytical model is presented that predicts the thermal stresses which arise from mismatch in coefficients of thermal expansion between a fibre and the surrounding matrix in a continuous fibre composite. The model consists of two coaxial isotropic cylinders. Stress transfer between the fibre and the matrix near an unstressed free surface has been modelled by means of a shear-lag analysis. Away from the free surface the theoretical approach satisfies exactly the conditions for equilibrium and continuity of stress at the fibre-matrix interface. Application of the model to a composite consisting of a glass-ceramic calcium alumino-silicate (CAS) matrix containing unidirectional Nicalon fibres points to a strong dependence of stress on fibre volume fraction. Surface effects are significant for depths of the order of one fibre diameter. Near-surface shear stresses resulting from cooling from the stress-free temperature are sufficiently high to suggest that a portion of fibre close to the surface is debonded at room temperature. Experimental results acquired with a scanning electron microscope (SEM) equipped with a heating stage are consistent with this prediction. Consequently, the model has been modified in a simple way to incorporate frictional slip at the interface, according to the Coulomb friction law. Although detailed measurements are limited by the resolution of the technique, experimental evidence suggests that the transfer length is within an order of magnitude of the model prediction.     

On the mechanical behaviour under cyclic loading of ceramic matrix composites

In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.     

Mechanical property degradation of a Nicalon fiber reinforced SiNC ceramic matrix composite under thermal shock loading

The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress–strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.     

Modeling behavior of cross-ply ceramic matrix composites under quasi-static loading

This paper presents a simplified analysis (model and failure criteria) for predicting the stress-strain responce of cross-ply fiber-reinforced ceramic composite laminates under quasi-static loading and unloading conditions. The model formulation is an extension of the modified shear-lag theory previously introduced by the authors for analyzing unidirectional laminates for the same loading conditions. The present formulation considers a general damage state consisting of matrix cracking in both the transverse and longitudinal plies, as well as fiber failure. These damage modes are modeled by a set of failure criteria with the minimum reliance on empirical data, and can be easily employed in a variety of numerical or analytical methods. The criteria used to estimate the extent of matrix cracking and interfacial debonding are closed-form and require the basic material properties. The failure criterion for fiber failure requires a priori knowledge of a single empirical constant. This parameter, however, may be determined without microscopic investigation of the laminate microstructure. The results from the present simplified analysis match well with the experimental data.The U.S. Government right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.     

Behaviour of unidirectional and crossply ceramic matrix composites under quasi-static tensile loading

Experimental results are presented for the quasi-static tensile behaviour of unidirectional, (0/90)_s, (0₂/90₄)_s and (0/90)_3s silicon carbide fibre (Nicalon) reinforced calcium aluminosilicate glass-ceramic matrix laminates. The stress-strain behaviour and associated damage development is described in detail for each laminate. The damage development is quantified by counts of crack density (in both the longitudinal and transverse plies) and stiffness reduction as functions of applied strain. The damage initiation and growth (and its effect on residual properties) are discussed with reference to the Aveston-Cooper-Kelly (ACK) theory for unidirectional ply cracking and crossply laminate shear-lag (originally developed for polymer matrix composites) to describe the transverse ply cracking behaviour.     

Fabrication of hybrid ceramic matrix composites

The desire to improve the transverse properties and microcracking stress of unidirectional continuous fiber reinforced ceramic matrix composites has led to development of the hybrid ceramic matrix composite (HCMC). This paper discusses the techniques we used in the fabrication of HCMC specimens used for mechanical characterization.     

Numerical method of thermal shock resistance estimation by quenching of samples in water

A temperature dependence of a transient heat transfer for cylindrical and ball samples (of different surface roughness) of 3–60 mm diameters heated up to the temperature range from 150 to 1200° C and quenched in a water bath of large volume was established. The measurement errors of the transient heat transfer defined by different methods with regard to hysteresis and statistical nature of boiling phenomena were evaluated. The study revealed, that the transition point from bubble to film boiling and vice versa differs essentially. The transient heat transfer in the field of bubble boiling did not depend on the size and the shape of the samples, their surface roughness and thermo-physical properties. But the magnitude of hysteresis in changing between the boiling regimes were substantially governed by the geometrical and thermo-physical characteristics of the samples. The examples of thermal stresses estimation which caused quenching damage to ZrC samples, heated up to a wide range of temperature from 150 to 1200 C, are given. The obtained data on the transient heat transfer and proposed recommendation on the temperature regimes of quenching for convenient sample sizes can form a basis of a standard for the numerical evaluation of the thermal shock resistance.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Thermal cycling response behavior of ceramic matrix composites under load and displacement constraints

Thermal cycling response of a two-dimensional carbon fiber reinforced SiC matrix composite (2D C/SiC) to load constraint (LC) and to displacement constraint (DC) in an oxidizing environment was investigated. During thermal cycling between 700 and 1200 °C, a constraint strain with a 0.208% range and a constraint stress with a 180 MPa range were, respectively, generated on the composites in LC and DC. It was found that with increasing cycles, the constraint strain increased in LC and the constraint stress decreased in DC. After 50 cycles, in contrast to the as-received composite materials, the as-cycled composites suffered greater loss in mechanical properties: the residual strength and failure strain are 204 MPa and 0.49% for the LC tested samples, and 223 MPa and 0.64% for the DC tested samples, respectively. Microstructural observations indicated that the LC could develop thermal microcracks and assist in oxidizing the internal fibers, whereas the DC reduced crack propagations and fiber oxidation because of decreasing tensile and increasing compressive stresses.     

Microstructure and thermal properties of binary ceramic particle-reinforced aluminum matrix composites with SiC/LAS

Multiphase particle-reinforced strategy shows promise for efficiently improving the comprehensive properties of aluminum matrix composites (AMCs) such as thermophysical and mechanical properties. In this work, AMC reinforced with β-eucryptite (LAS), and silicon carbide (SiC) particles were successfully prepared via a powder forging process. The microstructure morphology, interface compatibility, and coefficient of thermal expansion (CTE) of these composites were evaluated. Microstructural characterization illustrated that the co-effect of SiC and LAS resulted in a discontinuous phase with a microporous and deformation-free structure. The microporous structure of these composites was conducive for inward expansion and the elimination of internal stress, effectively limiting the outward thermal expansion behavior of the Al alloys. Moreover, SiC and LAS exhibited tight interfacial bonds with the Al grains, enhancing interfacial bonding strength. These composites provided practical and robust tensile stress that limited the thermal expansion of the Al matrix under heating. A fine Al grain size (53.5 nm) and low micro-strain (0.4?×?10^–4) were obtained with increasing LAS content. Consequently, the composites achieved a low CTE of 17.27?×?10^–6 K^?1 at 500 °C. The experimental CTE values were also compared with theoretical values calculated by a rule of mixture model to confirm that the excellent interfacial bonding between the LAS and SiC reinforcements and the Al matrix imposed an effective constraint on matrix expansion.

Graphical abstract

     

Toughness characterization of damageable ceramic matrix composites

As for concrete and many other heterogeneous materials, the damageable behavior of numerous ceramic matrix composites (CMCs) renders their toughness characterization particularly difficult. However, the need to compare CMCs' resistance to crack propagation has given rise to a new type of toughness test based on the use of a mixed CT–DCB specimen associated with steel frames and named Steel Framed Assisted Tension (SFAT). This type of specimen, whose shape and dimensions were adjusted by numerical simulation, allows the development and the steady state propagation of the process zone, while preventing the occurrence of damage outside the vicinity of notch or macrocrack tips. A study of the use of steel frames glued on each side of the specimens allowed the choice of a sufficiently rigid and resistant glue offering a good repeatability of the tests at a wide range of loading speeds. A compliance calibration procedure has been defined for SFAT specimens in view of the need to apply the method to anisotropic composites. Testing glass/epoxy composites with this procedure has shown its validity and pointed out the influence of the notch length on the R curves which can be derived in terms of crack growth release rate G from the related load–displacement curves. In addition, examination of the resulting G_R curves shows the possible use of various parameters to represent the tested material toughness. Finally, the whole testing procedure has been evaluated on the 2D–SiC/SiC and 2D–C/SiC composites.     

Effect of NiCrAlY platelets inclusion on the mechanical and thermal shock properties of glass matrix composites

NiCrAlY platelets modified glass matrix composites were prepared. Their microstructures were characterized, their Young's modulus, fracture strength in bending, Vickers hardness, and indentation toughness were measured, and their thermal shock resistance was studied using quenching-strength and indentation-quench methods. With increasing NiCrAlY content, evident enhancements of the Young's modulus and indentation toughness were obtained. The NiCrAlY alloy inclusion could exert significant influences on the retained bending strength of the samples after quench tests, from 9.6 MPa for NiCrAlY-free glass to 32.0 MPa for 30 wt.% NiCrAlY-containing composites. The indentation-quench tests showed that NiCrAlY alloy inclusion elevated the critical quenching temperatures for propagation of pre-crack, from 150 °C for NiCrAlY-free glass to 225 °C for 30 wt.% NiCrAlY-containing composites. Inclusion debonding and intersection, crack deflection and bridging were observed, and are likely the micromechanisms accounted for the improvement of fracture resistance.     

Characterization of the metal particles fraction in ceramic matrix composites fabricated under high pressure

This paper presents preliminary results concerning Al₂O₃–Ni composites fabricated by sintering under a high pressure of 7.7 GPa, at a temperature below the melting temperature of nickel. The microstructure of composites was characterized by scanning and transmission electron microscopy. Quantitative measurements of size, shape and distribution of metal particles were based on image analysis.

A correlation between the size of the Ni particles and their location has been found. Small Ni particles, with a grain size in the range of 50–500 nm, are mostly located inside the ceramic grains. Some Ni particles are also situated at the grain boundaries, and large particles are surrounded by ceramic grains. The shape of the ceramic grains suggests that the ceramic powder particles underwent deformation during the process of consolidation under high pressure.     

Ceramic thin film thermocouples for SiC-based ceramic matrix composites

Conductive ceramic thin film thermocouples were investigated for application to silicon carbide fiber reinforced silicon carbide ceramic matrix composite (SiC/SiC CMC) components. High temperature conductive oxides based on indium and zinc oxides were selected for testing to high temperatures in air. Sample oxide films were first sputtered-deposited on alumina substrates then on SiC/SiC CMC sample disks. Operational issues such as cold junction compensation to a 0 °C reference, resistivity and thermopower variations are discussed. Results show that zinc oxides have an extremely high resistance and thus increased complexity for use as a thermocouple, but thermocouples using indium oxides can achieve a strong, nearly linear response to high temperatures.