Effect of heat treatment in air on the thermal properties of SiC fibre reinforced composite: Part 2 A magnesium aluminium silicate (MAS) matrix glass ceramic composite

The thermal properties of a magnesium aluminium silicate (MAS) glass ceramic matrix composite reinforced by SiC (Nicalon) fibres have been investigated before and after heat treatment in the temperature range 600–1,200 °C. Within this temperature range, during the heat treatment at lower temperatures such as 600 and 700 °C, the oxidation of the carbon layer occurred and mixture of silicon and carbon was formed in the interface. This results in a decrease in thermal diffusivity values. After heat treatment at the temperatures higher than 1,000 °C, the carbon layer was thickened and resulted in the higher thermal diffusivity values.     

Effect of heat treatment in air on the structure and properties of barium osumilite reinforced with Nicalon fibres

Microstructural studies have been carried out on glass-ceramic matrix composites, consisting of barium osumilite reinforced with Nicalon fibres, which have been subjected to heat treatment in air in the range 600–1100 °C. Parallel studies have involved the measurement of the friction stress between fibre and matrix and the flexural strength of the composite. The matrix was shown to consist of barium osumilite, hexacelsian, mullite and a silica-rich glass, the thermal mismatch of these different phases leading to the development of appreciable strains. Whilst high-temperature treatments caused the formation of voids due to flow of the glassy phase, the major factor controlling the mechanical properties of the composite was the fibre/matrix interface. A change in microstructure, from a weak carbon-rich interface to one where the fibre and matrix were strongly bonded together by a silica layer, was thus reflected in an increase in the interfacial friction stress and a change in fracture behaviour from one showing fibre pull-out and delamination to one with brittle characteristics.     

Development and properties of a SiC fibre-reinforced magnesium aluminosilicate glass-ceramic matrix composite

The preparation of a magnesium aluminosilicate glass-ceramic matrix composite having an approximately matched thermal expansion coefficient to the silicon carbide fibre reinforcement is described. Data are presented on the process conditions necessary to produce a composite with matched thermal expansion coefficients, the strength and work of fracture of the composite at ambient temperature, and also the effect of temperature on the mechanical properties.     

Comparative study of strain rate effects on mechanical properties of glass fibre-reinforced thermoset matrix composite

The tensile dynamic behaviour of glass fibre-reinforced phenolic and polyester resins has been determined in order to find the influence of strain rate on the mechanical properties of composite materials produced by the resin transfer moulding (RTM) and pultrusion processes. Data and experimental test systems from the literature are analysed. A new specimen design is created and validated using drop-weight dynamic tests. The dynamic elastic modulus and strength tend to increase in an important ratio for the majority of the materials studied. The shear modulus measured with off-axis and ±45° coupons produces different effects as a function of strain rate. The influence of the reinforcement structure is emphasized and shown to be effective.     

Effect of SiC content and orientation on the properties of Si/SiC ceramic composite

Silicon/silicon carbide ceramic composites are made by infiltrating carbonaceous material with liquid silicon to form SiC crystallites dispersed in a silicon matrix. The present study was conducted to determine the effects on the room temperature properties of density, elastic modulus, strength, and fracture toughness by varying the amount and distribution of the SiC crystallites. Most of the work involved uniaxially aligned SiC crystals of varying volume fraction tested both longitudinally and transversely to the converted fibre axis.     

Effect of viscous flow on the thermal residual stresses in a monofilament SiC/borosilicate glass composite

Residual strains of SiC (SCS-6) single fibres embedded between borosilicate slides were measured by comparing their post-fabricated in situ lengths with their original lengths as a function of processing temperature and load in an open die. If cooled without load the residual strains agreed with the values one would expect if the stress-free temperature was taken to be the strain point of the glass. However, if cooled under load, the residual strains were found to be significantly enhanced, and increased linearly with the applied load, but were insensitive to processing temperature. A hydrodynamic model, where glass flow normal to the vertically applied pressure entrains the embedded fibres and results in the enhanced residual strains, is proposed. The implications of these results in calculating thermal residual stresses in viscous media are discussed. At higher applied loads, the fibre edges broke into smaller fragments. The fragment length distribution was measured and analysed using Weibull statistics. Heat treatment of the fibres at 1600 °C for 4 h in vacuum, resulted in approximately a 50% decrease in their strength and critical length as compared to the as-received fibres. The Weibull moduli, however, were not affected by the heat treatment.     

Effect of heat treatment on microstructure and interface of SiC particle reinforced 2124 Al matrix composite

The microstructure and interface between metal matrix and ceramic reinforcement of a composite play an important role in improving its properties. In the present investigation, the interface and intermetallic compound present in the samples were characterized to understand structural stability at an elevated temperature. Aluminum based 2124 alloy with 10 wt.% silicon carbide (SiC) particle reinforced composite was prepared through vortex method and the solid ingot was deformed by hot rolling for better particle distribution. Heat treatment of the composite was carried out at 575 °C with varying holding time from 1 to 48 h followed by water quenching. In this study, the microstructure and interface of the SiC particle reinforced Al based composites have been studied using optical microscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), electron probe micro-analyzer (EPMA) associated with wavelength dispersive spectroscopy (WDS) and transmission electron microscopy (TEM) to identify the precipitate and intermetallic phases that are formed during heat treatment. The SiC particles are uniformly distributed in the aluminum matrix. The microstructure analyses of Al–SiC composite after heat treatment reveal that a wide range of dispersed phases are formed at grain boundary and surrounding the SiC particles. The energy dispersive X-ray spectroscopy and wavelength dispersive spectroscopy analyses confirm that finely dispersed phases are CuAl₂ and CuMgAl₂ intermetallic and large spherical phases are Fe₂SiAl₈ or Al₁₅(Fe,Mn)₃Si. It is also observed that a continuous layer enriched with Cu and Mg of thickness 50–80 nm is formed at the interface in between Al and SiC particles. EDS analysis also confirms that Cu and Mg are segregated at the interface of the composite while no carbide is identified at the interface.     

Transverse properties of a Ti-6-4 matrix/SiC fibre-reinforced composite under monotonic and cyclic loading

The transverse response of a Ti-6-4/SM1140+ fibre-reinforced composite to both monotonic and cyclic loading has been investigated. Five distinct regions were found in the monotonic stress versus strain curve: (I) elastic deformation of the composite, (II) failure of the fibre/matrix interfaces, (III) elastic deformation of the remaining matrix ligaments, (IV) yielding of the matrix ligaments, and (V) gross plastic deformation, which ultimately leads to specimen failure. The stresses at which interface debonding, matrix yield and final failure occurred rose with increased displacement rate. Stressing to levels above the interface failure stress caused significant damage and limited (0.025%) plastic deformation. A non-linear stress-strain response was observed on unloading/reloading, because the presence within the specimen of constrained holes (containing debonded fibres) resulted in non-homogeneous elastic straining of the matrix. The transverse low-cycle fatigue lives of Ti-6-4/SM1140+composite specimens were strongly dependent on maximum stress for values up to the interfacial failure stress, but less so for maximum stresses greater than 260–265 MPa, where full fibre/matrix debonding had occurred. Fatigue life was also dependent on the uniformity of fibre spacings within the composite.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

The effects of thermal fatigue on a SiC fibre/aluminosilicate glass composite

The thermal fatigue (TF) of ceramic matrix composites introduces stresses within the composite due to the thermal expansion mismatch of fibre and matrix; this will affect the lifetime and dimensional stability of the composite. A review of various laboratory TF methods is given, and the controlled, photon heating method used in this research is explained. A Nicalon fibre/glass matrix composite was subjected to rapid, controlled TF from 250 to 700 °C and 250 to 800 °C under no load and dead load conditions in order to illustrate a variety of elastic and inelastic cyclic strain conditions. To characterize simple environmental exposure at elevated temperature, ageing experiments were also run. After TF, the surfaces of the composites were characterized using SEM for evidence of thermal damage and microcracking. The composites were then tested for flexural strength. Results show that the tensile modulus after TF testing remains constant, and that dimensional changes are slight, except near any local hot spots. The 700 °C maximum TF specimens showed appreciably greater embrittlement and lower strength than the 800 °C maximum TF specimens. Observations in the SEM of the surfaces of the 700 °C specimens showed little matrix flow of the type which could decrease oxygen infiltration. Greater matrix flow was observed for the 800 °C specimens. Thermally aged specimens gave results similar to those for the TF experiments.     

Effect of dynamic loading on tensile strength and failure mechanisms in a SiC fibre reinforced ceramic matrix composite

The tensile strength and associated failure micromechanisms have been characterized for a SiC fibrereinforced ceramic matrix composite subject to strain rates approaching 1000 s^–1. It is found that behaviour under such conditions is not described by the current matrix fracture/fibre pull-out models. This is a consequence of the rapid and extreme frictional heating produced at the fibre-matrix interface by sliding velocities on the order of 100 ms^–1. At sufficiently rapid loading rates, the near-interface matrix appears to melt, and the frictional interface shear resistance is reduced to the point that the fibres debond throughout the specimen, and pull out without failing. This suggests that for sufficiently rapid loading, the stress to fail the composite will approach that merely to create the initial matrix crack, i.e., a stress level well below the ultimate strength normally attainable under quasi-static conditions.     

Effect of fiber coating on the mechanical properties of a Nextel-480-fiber-reinforced glass matrix composite

The effect of fiber coatings on the mechanical properties of glass matrix composites reinforced with Nextel 480 fiber was investigated. Two different fiber coatings, namely tin dioxide and boron nitride, were investigated. The presence of the coatings on the fiber led to an improvement in mechanical behavior. In particular, the boron nitride coating improved the total work of fracture of the composite without reducing the strength. A marked difference was observed in the fractographic features between the uncoated and tin-dioxide-coated samples as compared to the boron-nitride-coated samples. The uncoated and tin-dioxide-coated fiber composites failed in a brittle fashion, without any fiber pull-out, whereas the boron-nitride-coated fiber samples exhibited extensive fiber pull-out. This improvement in the toughness or energy absorbed during the fracture process was attributed to the relatively weak bonding between the fiber and matrix, induced as a result of the coating.     

Effect of heat treatment on the microstructure and properties of CVD SiC fiber

In this study, the effect of heat treatment on the room temperature strength of W-core Si C fiber produced by chemical vapor deposition(CVD) was investigated. Thermal exposure in the temperature range of 900–1000?C decreases the strength of the Si C fiber. Fracture morphology analysis indicates that failure initiations predominantly take place at the W-core/Si C interface. A reaction layer that formed at the W-core/Si C interface during thermal exposure degraded the fiber strength and an empirical linear relationship of strength vs thickness of the reaction layer can be obtained. The kinetics of the growth of the W-core/Si C reaction layer were determined.     

Effect of high thermal expansion glass infiltration on mechanical properties of alumina-zirconia composite

This work studies the effect on the mechanical properties of alumina-10 wt% zirconia (3 mol% yttria stabilized) composite by infiltrating glass of a higher thermal expansion (soda lime glass) on the surface at high temperature. The glass improved the strength of composite at room temperature as well as at high temperature. This could be attributed to the drastic drop in the coefficient of thermal expansion due to the compositional change in the soda lime glass during infiltration. There was a significant improvement in the Weibull modulus after glass infiltration. Glass infiltrated samples showed better thermal shock resistance. The magnitude of strength increment was found to be in the order of the surface residual stress generated by thermo-elastic properties mismatch between the composite and the penetrated glass.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effect of thermal treatment on the mechanical and toughness properties of extruded SiCw/aluminium 6061 metal matrix composite

Mechanical, instrumented Charpy V-notch (CVN) energy and plane strain fracture toughness properties of SiC whisker reinforced-6061 aluminium metal matrix composite material from an extruded tube have been determined. The effect of thermal treatment and orientation have been studied. The mechanical strength properties are higher than wrought Al 6061 in the T6 condition. CVN energy values, however, were reduced by an order of magnitude.K _lc fracture toughness of the as-received, T6 and degassed + T6 thermal treatments were 50% of the wrought Al 6061 alloy. The effect of orientation showed that the orientation with the least amount of SiC whisker in the crack plane (i.e. greatest mean free path between reinforcements) yields the highest toughness value.