Push-Out Tests on a New Silicon Carbide/Reaction-Bonded Silicon Carbide Ceramic Matrix Composite

Fiber push-out tests have been performed on a ceramic matrix composite consisting of Carborundum sintered SiC fibers, with a BN coating, embedded in a reaction-bonded SiC matrix. Analysis of the push-out data, utilizing the most complete theory presently available, shows that one of the fiber/coating/matrix interfaces has a low fracture energy (one-tenth that of the fiber) and a moderate sliding resistance τ∼ 8 MPa. The debonded sliding interface shows some continuous but minor abrasion, which appears to increase the sliding resistance, but overall the system exhibits very clean smooth sliding. The tensile response of a full-scale composite is then modeled, using data obtained here and known fiber strengths, to demonstrate the good composite behavior predicted for this material.     

Experimental Examination of the Push-Down Technique for Measuring the Sliding Resistance of Silicon Carbide Fibers in a Ceramic Matrix

The use of a push-down technique to measure the sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS) matrix has been examined experimentally. Tests have been conducted on more than 300 fibers in 2 different samples of the as-processed SiC/LAS III composite. Results show that the push-down measurements on an individual sample are reproducible, that the sliding resistance can vary significantly between samples, and that Poisson's expansion does not affect push-down measurements of fibers in this system.     

Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol–carbon nanotube fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers differing on their pre-stretching condition were embedded in glass fiber reinforced plastic (GFRP) composites and used as strain sensors for damage monitoring of the composite. Strain sensing of the composite was made by the in situ measurement of the embedded fiber’s electrical resistance change during the mechanical tests. Four glass fiber composite plates were manufactured; each one had embedded a different type of produced PVA–CNT fibers. The multi-functional materials were tested in monotonic tensile tests as well as in progressive damage accumulation tests. The electrical resistance readings of the PVA–CNT fibers were correlated with axial strain values, taking into account the induced damage of the composite. It has been demonstrated that increasing the fiber’s pre-stretching ratio, its electrical resistance response increases due to higher degree of the CNTs alignment in the PVA matrix. Higher fiber pre-stretching degree enables the better strain monitoring of the composite due to higher measured electrical resistance change values noticed for the same applied axial strain values. To this end, it enables for the better monitoring of the progressive damage accumulation inside the composite.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

Optimization of the Mechanical Properties of Silicon Carbide-Fiber-Reinforced Zirconia Titanate Matrix Composites through Controlled Processing

The effects of processing parameters on the microstructure and mechanical behavior of a SiC-fiber-reinforced ZrTiO₄ matrix composite were evaluated through a controlled study. The microstructure was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Mechanical behavior was characterized by strength and toughness measurements, which were correlated with the microstructure of the composite. The optimized processing schedule developed included incorporation of CO-heat-treated BN-coated fibers, composite calcination at 530°C, and consolidation via hot-pressing at 1270°C and 17.25 MPa applied pressure in an atmosphere of CO at an overpressure of 111.5 kPa (1.1 atm). Use of this processing schedule improved in situ fiber strength and modified the fiber/matrix interfacial microstructure to ameliorate its sliding and debonding resistance, leading to a composite with average strength over 1 GPa and a toughness of 26 MPa.m^1/2.     

Properties of Multilayered Interphases in SiC/SiC Chemical-Vapor-Infiltrated Composites with "Weak" and "Strong" Interfaces

The interfacial properties of SiC/SiC composites with interphases that consist of (C-SiC) sequences deposited on the fibers have been determined by single-fiber push-out tests. The matrix has been reinforced with either as-received or treated Nicalon fibers. The measured interfacial properties are correlated with the fiber-coatingbond strength and the number of interlayers. For the composites reinforced with as-received (weakly bonded) fibers, interfacial characteristics are extracted from the nonlinear portion of the stress-displacement curve by fitting Hsueh's push-out model. The interfacial characteristics are controlled by the carbon layer adjacent to the fiber. The resistance to interface crack growth and fiber sliding increases as the number of (C-SiC) sequences increases. For the composites reinforced with treated (strongly bonded) fibers, the push-out curves exhibit an uncommon upward curvature, which reflects different modes of interphase cracking and a contribution of fiber roughness.     

Effect-of Processing Varcables on Interfacial Properties of an SiG-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

Fiber/matrix interfacial debonding and frictional sliding stresses were evaluated by single-fiber pushout tests on unidirectional continuous silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. The debonding and maximum pushout loads required to overcome interfacial friction were obtained from load–displacement plots of pushout tests. Interfacial debonding and frictional sliding stresses were evaluated for composites with various fiber contents and fiber surface conditions (coated and uncoated), and after matrix densification by hot isostatic pressing (HIPing). For as-fabricated composites, both debonding and frictional sliding stresses decreased with increasing fiber content. The HIPed composites, however, exhibited higher interfacial debonding and frictional sliding stresses than those of the as-fabricated composites. These results were related to variations in axial and transverse residual stresses on fibers in the composites. A shear-lag model developed for a partially debonded composite, including full residual stress field, was employed to analyze the nonlinear dependence of maximum pushout load on embedded fiber length for as-fabricated and HIPed composites. Interfacial friction coefficients of 0.1–0.16 fitted the experimental data well. The extremely high debonding stress observed in uncoated fibers is believed to be due to strong chemical bonding between fiber and matrix.     

Influence of Interfacial Roughness on Fiber Sliding in Oxide Composites with La-Monazite Interphases

Room-temperature debonding and sliding of fibers coated with La-monazite is assessed using a composite with a polycrystalline alumina matrix and fibers of several different single crystal (mullite and sapphire) and directionally solidified eutectic (Al₂O₃/Y₃Al₅O₁₂ and Al₂O₃/Y-ZrO₂) compositions. These fibers provide a range of residual stresses and interfacial roughnesses. Sliding occurred over a debond crack at the fiber-coating interface when the sliding displacement and surface roughness were relatively small. At large sliding displacements with relatively rough interfaces, the monazite coatings were deformed extensively by fracture, dislocations, and occasional twinning, whereas the fibers were undamaged. Dense, fine-grained areas (10 nm grain size) resembling recrystallized microstructures were also observed in the most heavily deformed regions of the coatings. Frictional heating during sliding is assessed. Potential mechanisms for forming such microstructures at low temperature are discussed, and a parallel is drawn with the known resistance of monazite to radiation damage. The ability of La-monazite to undergo both debonding and plastic deformation relatively easily at low temperatures may enable its use as a composite interface.     

Investigation on mechanical and tribological behavior of naturally woven coconut sheath‐reinforced polymer composites

The article presents the results of experimental investigation on mechanical and dry sliding wear behavior of unsaturated polyester resin (USP), reinforced with naturally woven coconut sheath and glass fibers. The mechanical properties of coconut sheath (N) and glass fiber (G) reinforced polyester composites were studied, and the tribological behaviors were tested on pin‐on‐disc sliding wear tester. Mass loss was determined as a function of sliding distance for a sliding velocity of 3.5 m/s and an applied normal load of 40 N. The experimental result revealed that the mechanical properties and wear resistance of the composites depend on the wt% reinforcement of coconut sheath/glass fiber and sliding distance. The hybrid reinforcement (GGN) greatly increased the mechanical properties of USP. At lower sliding distance, the N‐reinforced USP had lower wear loss, whereas at higher sliding distance, the hybrid fiber‐reinforced (GGN) USP composite had lower wear loss. Furthermore, the work showed that the higher sliding distance bring about changes in the worn surface features such as interface separation, inclined fracture of fibers, loss of matrix, and the appearance of debris with the two different fibers. The worn surfaces were also examined by scanning electron microscopy. The study showed differing trends with load for the two types of reinforcements. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Acoustic emission and electrical resistance in SiC-based laminate ceramic composites tested under tensile loading

Acoustic emission and electrical resistance were monitored for SiC-based laminate composites while loaded in tension and correlated with damage sources. The ceramic matrix composites were composed of Hi-Nicalon Type S™ fibers, a boron-nitride interphase, and pre-impregnated (pre-preg) melt-infiltrated silicon/SiC matrix. Tensile load-unload-reload or tensile monotonic tests were performed to failure or to a predetermined strain condition. Some of the specimens were annealed which relieved some residual matrix compressive stress and enabled higher strains to failure. Differences in location, acoustic frequency and energy, and quantity of matrix cracking have been quantified for unidirectional and cross-ply type architectures. Consistent relationships were found for strain and matrix crack density with acoustic emission activity and the change in measured electrical resistance measured at either the peak stress or after unloading to a zero-stress state. Fiber breakage in the vicinity of composite failure was associated with high frequency, low energy acoustic events.     

Conductive eco-polymer composites: wear behaviour of recycled polycarbonate/crushed rubber microparticles

Abstract

Following an eco-design approach we have investigated the possible formulation of conductive polymer composite (CPC) from recycled poly(carbonate) (PC) and crushed rubber microparticles (CR) for tribological applications. Particularly, the abrasive wear behaviour of CPC has been studied as a function of smooth surface treatments applied to rubber fillers to improve their adhesion with the PC matrix. The effects of normal load, sliding velocity and treatments applied to CR on the wear rate and kinetics were investigated. Pin-on-disc tests carried out under water lubrication show that the wear rate increases with the increase in load and sliding velocity. Moreover, among all surface treatments, the most effective to improve the interface quality and thus wear resistance was a stripping of rubber microparticles with methanol whereas flaming was assumed to degrade filler surface and dichloromethane to swell the matrix. Additionally wear experiments proved to be effective in evaluating the quality of PC/CR interface.     

Degradation of a SiC-SiC composite in water vapor environments

The article examines degradation of a SiC-based fiber composite containing Tyranno ZMI fibers in water vapor at elevated temperatures (800°C and 1100°C). Degradation is characterized through mechanical tests under cyclic and quasi-static tensile loading in the near-threshold regime, at stresses at or slightly above the matrix cracking limit. These tests are augmented by examinations of fracture surfaces and polished cross-sections, measurements of fracture mirror radii, and measurements of interfacial debond toughness and sliding resistance. Degradation involves highly localized consumption of fibers through reactions of water vapor with the fibers and the BN coatings in regions adjacent to the few matrix cracks present at low stresses; the global hysteresis response and the average interfacial properties are minimally affected. Boria formed by oxidation of BN appears to play a fluxing role; it combines with silica on the fibers to form a non-protective molten glass. Inhomogeneous fiber consumption leads to stress concentrations in the fibers and hence reduced fiber strength. Spatial variations in the degradation process occur at two length scales: at the macroscopic scale, because of cracking of the external CVI SiC overcoat and subsequent water ingress through the cracks, and at the tow-scale, because of cracking of the CVI SiC around the tows. Parsing the kinetic processes over the two length scales remains a significant challenge.     

Effects of the combination of solid lubricants and short carbon fibers on the sliding performance of poly(ether imide) matrix composites

Solid lubricants, that is, graphite flakes and poly(tetrafluoroethylene) powders, were incorporated with short carbon fibers into a poly(ether imide) matrix to improve the tribological performance. Wear tests were performed with a polymer pin against a mild steel counterpart at a constant sliding speed of 1 m/s under various temperatures and contact pressures. Composites filled with equilibrium contents of solid lubricants and short carbon fibers, that is, 10 vol % of each filler, exhibited the lowest wear rate and friction coefficient. The relatively lower concentration of solid lubricants adversely affected the wear resistance, whereas the friction coefficient did not vary significantly in comparison with the friction coefficient of the composites filled with only short carbon fibers. The improved tribological behavior was attributed to more continuous and effective friction films formed on the material pairs during sliding. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1428–1434, 2004     

Use of carbon nanotubes for strain and damage sensing of epoxy-based composites

The interest in structural health monitoring of carbon fiber-reinforced polymers using electrical methods to detect damage in structures is growing because once the material is fabricated the evaluation of strain and damage is simple and feasible. In order to obtain the conductivity, the polymer matrix must be conductive and the use of nanoreinforcement seems to be the most feasible method. In this work, the behavior of nanoreinforced polymer with carbon nanotubes (CNTs) and composites with glass and carbon fibers with nanoreinforced matrices was investigated. These composites were evaluated in tensile tests by simultaneously measuring stress, strain and resistivity. During elastic deformation, a linear increase in resistance was observed and during fracture of the composite fibers, stronger and discontinuous changes in the resistivity were observed. Among other factors, the percentage of nanotubes incorporated in the matrix turned out to be an important factor in the sensitivity of the method.     

Interfacial Microstructure and Chemistry of SiC/BN Dual-Coated Nicalon-Fiber-Reinforced Glass-Ceramic Matrix Composites

Glass-ceramic composites with improved high-temperature mechanical properties have been produced by incorporating continuous SiC fibers into a barium magnesium aluminosilicate matrix. Control of the fiber/matrix interface was achieved by a dual-layer coating of SiC/BN(C) applied to the fibers by CVD. The weakly bonded interface resulted in composites with high fracture toughness and strength up to 1100°C, and the composite system was oxidatively stable during long-term exposure to air at high temperatures. Composites with different thermal and mechanical histories were studied, and interfaces were characterized using transmission electron microscopy (TEM), Auger electron spectroscopy, and fiber pushout tests. Observations of interfacial microstructure were correlated with the mechanical properties of the composite and with interface properties determined from fiber push-out tests.     

Effects of Combustor Rig Exposure on a Porous-Matrix Oxide Composite

The present study explores the effects of exposure in a laboratory combustor on microstructural stability and property retention of an all-oxide fiber-reinforced ceramic composite. The material consists of a porous mullite–alumina matrix and Nextel 720 fibers in an eight-harness satin weave. To assess the effects of matrix strength, two matrix conditions are used, distinguished from one another by the amount of alumina added through precursor impregnation and pyrolysis (1.8% and 4.8%). In both cases, the dominant damage mode upon exposure involves interply delamination along the panel midplane. However, significant reductions in the rate and extent of cracking are obtained in the material with higher alumina content: a result of the higher delamination resistance. Mechanical tests performed on exposed specimens reveal a slight (10–20%) reduction in tensile strength along the fiber direction and a comparable increase in shear strength. These trends suggest some sintering of the matrix upon exposure. Examinations of fracture surfaces provide additional supporting evidence. Implications for long-term performance and strategies for imparting improvement in microstructural stability and delamination resistance are discussed.     

Fiber Bundle Pushout: A Technique for the Measurement of Interfacial Sliding Properties

An experimental technique for the measurement of interfacial sliding properties in fiber-reinforced materials is presented. The technique involves pushing a bundle of fibers simultaneously through a thin section of the composite. Using this technique, sample averages are immediately available, eliminating the need for repeated single-fiber tests. The technique is particularly useful for composites that contain small-diameter fibers. Salient features of the technique are demonstrated through tests on a CAS/SiC composite.     

Effect of neutron irradiation on the mechanical properties of graphite fiber-based composites

Polymer and carbon composite materials reinforced with K-1100 ultra-high modulus fibers were subject to testing of their radiation resistance. Mechanical tests have been carried out, prior and after neutron irradiation at a dose of 7.1×10¹⁷ n/cm² (E>0.5 MeV) for organic matrix composite and 7.3×10¹⁷ n/cm² (E>0.5 MeV) for carbon matrix composite, to assess the radiation resistance. Flexural strength, deformation at break, Young’s modulus and dimensional changes were measured. Microstructure of the composites before and after irradiation was analyzed. The results showed that neutron irradiation causes significant changes in mechanical properties of composites with organic and carbon matrix and a slight variation in their dimensions. Stronger effects in mechanical properties changes for composites with carbon matrix were observed.     

Mode I Fracture Resistance of a Laminated Fiber-Reinforced Ceramic

The mode I fracture resistance of a ceramic matrix composite has been measured. Simultaneous observations have revealed that the resistance is dominated by frictional dissipation upon the pullout of fibers that fracture in the crack wake off the crack plane. Numerical and analytical crack growth simulations have been compared with the experimental results. One important feature in this comparison concerns the occurrence of large-scale bridging. With these effects taken into account, the simulations and the experiments are found to be in good correspondence for acceptable magnitudes of the interface sliding stress.     

Cyclic-Fatigue Behavior of SiC/SiC Composites at Room and High Temperatures

Tension-tension cyclic-fatigue tests of a two-dimensional-woven-SiC-fiber-SiC-matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000°C. The cyclic-fatigue limit (10⁷ cycles) at room temperature was ∼160 MPa, which was ∼80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000°C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic-fatigue life at room temperature and at 1000°C at stresses >180 MPa; however, cyclic-fatigue life decreased at 1000°C at stresses < 180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber-pullout length at 1000°C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000°C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interface between the matrix and the fibers.