SiC Ceramic Matrix Composites Research Articles

Microstructures and mechanical properties of spark plasma sintered Al–SiC composites containing high volume fraction of SiC

Al–SiC ceramic–matrix composites containing high volume fraction of SiC were synthesized by spark plasma sintering (SPS) technique with sintering temperatures ranging from 1500 °C to 1800 °C, a pressure of 50 MPa, and a heating rate of 150 °C/min, using Al and SiC powders. Microstructures and mechanical properties of the composites sintered at different temperatures were investigated. Results reveal that the Al–SiC composites synthesized by SPS process consist of Al, SiC, Si, and Al 4C 3 phases. The volume fraction of Al 4C 3 and Si phases in the composites were reduced remarkably with increasing the sintering temperature up to 1700 °C. As a result, the composite sintered at 1800 °C provides the optimal combination of dense microstructures and excellent properties, including the relative density of 99.6%, micro-hardness of 25.5 GPa, bending strength of 451 MPa, and fracture toughness of 6.05 MPa m 1/2. Compared with the monolithic SiC ceramics, both the bending strength and the fracture toughness of the composites are improved due to the dense microstructures, fine SiC grains, and infiltration of the molten Al into the interfaces of the SiC grains.

Measurement and calculation of thermal residual stress in fiber reinforced ceramic matrix composites

In this study, thermal residual stresses in two SiC-ceramic matrix composites reinforced with carbon fiber (C/SiC) and silicon carbide fiber (SiC/SiC) were completely investigated. Thermal residual stress in the C/SiC was quantified by three different methods of experimental measurement, analytical calculation, and theoretical prediction, and then compared with that in the SiC/SiC. Good agreement between these methods was observed and their applicability to the present composite systems was validated. Relationships between the thermal residual stress state and macroscopic mechanical properties of the composites (e.g., the first matrix cracking stress) are discussed.

Morphology and microstructure of 2.5 dimension C/SiC composites ablated by oxyacetylene torch

SiC ceramic matrix composites reinforced by 2.5 dimension carbon fibers were prepared by low-pressure chemical vapor infiltration. The ablation performance of the composites was characterized by an oxyacetylene torch. The morphology and microstructure of the as-ablated composite were examined by scanning electron microscopy. The composition of the new phase was confirmed by energy dispersive spectroscopy and X-ray diffraction. Three clear annular regions appeared on the surface of the as-ablated sample and each region had different ablation mechanism. Sublimation was the main ablation behavior in the centre region. Oxidation was the main ablation behavior in the middle region. Silica particles mainly resulted from the oxidation and deposition of Si and SiO gas from the centre to the outer region. The ablation mechanism of the C/SiC composites under oxyacetylene flame was a combined effect of thermo-physicals attack and thermo-chemicals ablation.

Mechanical properties of short carbon fiber reinforced ZrB 2–SiC ceramic matrix composites

A ZrB 2–20 vol.% SiC based ceramic matrix composite containing 20 vol.% short carbon fibers was top pressed at 2000 °C and 30 MPa for 1 h. The microstructure and mechanical properties of the composite were investigated. The fracture toughness was 6.56 MPa·m 1/2, which is 54% higher than that of ZrB 2–20 vol.% SiC composite. The toughening mechanisms were fiber debonding, fiber pull-out and crack deflection. Moreover, it was found that a graphitization transition layer between fiber and matrix led to the decreased in flexural strength. The results presented here point to a potential way for improving the fracture toughness of ultra-high temperature ceramics.

Inelastic behaviour of ceramic-matrix composites

A methodology for the formulation and identification of the constitutive laws of ceramic-matrix composites is summarised. It relies on an anisotropic damage evaluation that accurately separates the effects of the various damage mechanisms on the non-linear behaviour. A mixed approach takes into account the basic strain and damage mechanisms by using a homogenisation method that provides the relationship between the mechanical response and the intensity of damage in the individual modes. That leads to a non-arbitrary choice of internal variables in the macroscopic constitutive relationships. A successive process of prediction/experimental-data confrontation allows the optimal determination of the evolution laws of those internal variables. This methodology is illustrated on various behaviours of various CMCs; several crack arrays, tilted cracks, tensile test, cyclic loading, off-axis solicitation, in 1D SiC–SiC, 2D C–SiC, 2D C/C–SiC ceramic-matrix composites. Predictions of the three-dimensional changes in elasticity and of the inelastic strains are shown to compare favourably with experimental data measured with an ultrasonic method.

Microstructure and R-Curve Behavior of Al2O3–SiC Ceramic Matrix Composites

Microstructure and R-Curve Behavior of Al2O3–SiC Ceramic Matrix Composites

Current status of SiC/SiC composites R&D

Advantages of SiC-based ceramic matrix composites (CMCs) as structural materials in fusion applications rely on their high-temperature properties and stability, low density and reduced neutron activation. In recent years, experimental activities on industrial SiC CMCs have highlighted their main features under irradiation and provided important guidelines for further development of a radiation compliant material. Parallel efforts included design studies, development of advanced fibres and interfaces, alternative composite processing methods and joint development. In this paper, the current status of SiC/SiC R&D is reported and it is demonstrated that future activities require a strong collaboration with the industry as well as common efforts involving the different laboratories.

Pinning effect of SiC particles on mechanical properties of Al 2O 3–SiC ceramic matrix composites

The fracture mechanical properties of SiC particle reinforced Al 2O 3 matrix composites were investigated at room temperature and T=1200°C. The results showed that the fracture strength and toughness of composites are enhanced, compared with those of monolithic Al 2O 3, especially at elevated temperature, and the strength and toughness of composites increase with the increase in the content of SiC particles. It was found that the improvement in fracture mechanical properties of composites is due to the fracture mode transformation from intergranular in monolithic Al 2O 3 to transgranular in the composites, and the fracture mode change might result mainly from the pinning effect of SiC particles on the Al 2O 3 boundaries. The study on flexure creep behavior of monolithic Al 2O 3 and the composites with 20 vol% SiC particles at T=1260°C indicated that the composites also exhibited some improvement in their creep resistance, compared to the creep behavior of monolithic Al 2O 3, due to the pinning effect of SiC particles at Al 2O 3 grain boundaries on grain boundary sliding during creeping.

Investigation of incubation time for sub-critical crack propagation in SiCSiC composites

The effects of fiber thermal creep on the relaxation of crack bridging tractions in SiCSiC ceramic matrix composites (CMCs) is considered in the present work, with the objective of studying the time-to-propagation of sub-critical matrix cracks in this material at high temperatures. Under the condition of fiber stress relaxation in the bridging zone, it is found that the crack opening and the stress intensity factor increase with time for sub-critical matrix cracks. The time elapsed before the stress intensity reaches the critical value for crack propagation is calculated as a function of the initial crack length, applied stress and temperature. Stability domains for matrix cracks are defined, which provide guidelines for conducting high-temperature crack propagation experiments.