Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel?720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effects of environment and temperature on ceramic tensile strength–grain size relations

Overall strength ()–grain size (G), i.e. –G-1/2, relations retain the same basic two-branched character to at least 1200–1300°C. However, some polycrystalline as well as single crystal strength shifts or deviations are seen relative to each other, and especially relative to Young's moduli versus temperature for poly- and single crystals. The variety and complexity of these deviations are illustrated mainly by Al2O3, BeO, MgO and ZrO2 for which there is considerable data. At 22°C, Al2O3 polycrystals show substantial strength decrease due to H2O while MgO, ZrO2 and BeO polycrystals have limited, variable decreases. Al2O3 single crystals (sapphire) also show substantial strength decreases, but ZrO2 and MgO single crystals show little or none. Sapphire's strength markedly decreases from at least –196°C to a minimum in the 400–600°C range, then rises to a maximum at1000°C, followed by an accelerating decrease with further temperature increase. Polycrystalline Al2O3 shows similar (but less pronounced) strength minima and maxima, or alternatively an approximate strength plateau from 22 to 1000°C interrupting the normally expected strength decreases with increasing temperature at suitably large grain size and absence of defects (e.g. pores) dominating failure. BeO crystals show a linear strength decrease with increasing temperature (T) similar to that of Young's modulus. BeO polycrystals often show a significant strength (apparently grain size and impurity dependent) maximum (at 500–800°C) or plateau (from 22 to 1000°C) interrupting an otherwise continuous decrease. MgO shows similar temperature behaviour to BeO, but more pronounced crystal strength decrease and less pronounced polycrystalline strength maxima. Polycrystalline ZrO2 shows more rapid Young's modulus (E), and especially strength, decreases at 200–500°C than single crystals. More limited data for other materials also shows greater, variable –T versus E–T trends, e.g. MgAl2O4 has a similar, but less pronounced decrease than ZrO2. Collectively these deviations suggest variable impacts on primarily flaw controlled –G-1/2 behaviour due to factors such as microplasticity, machining stresses, and thermal expansion and elastic anisotropies requiring more comprehensive testing and evaluation to better sort out these effects.     

The role of oxidation in time-dependent response of ceramic–matrix composites

In this work, a model is constructed to account for the effect of oxidation of the fiber, fiber interface coating and surrounding matrix on the stress distribution and strain accumulation in ceramic–matrix composites. The model includes the role of the fabric architecture, the effect of porosity and the distribution of cracks in its formulation and utilizes oxidation rate constants and phenomenological models for the progress of oxidation as reported in literature.Dwell fatigue experiments were carried out for silicon carbide/silicon carbide nitride (SiC/SiNC) and Melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) composites to evaluate their time-dependent strain accumulation. Strain accumulation due to oxidation calculated by the model was compared to time-dependent strain obtained from experiment and showed that the rate of strain accumulation due to oxidation was low before the fibers were exposed to the environment but drastically increased after that. Such high rate of strain accumulation can be one of the main causes for failure of the composite.Model results showed that strain accumulation in both composites due to oxidation was dependent on the stress level with the SiC/SiNC accumulating more strain at similar stress levels. This can be explained by the higher modulus of the MI SiC/SiC that limits deformation, reducing crack density and accordingly decreasing the chance of oxygen to infiltrate the specimen and oxidize the fibers. Strain accumulation due to oxidation was also dependent on the fabric architecture and stress distribution within the unit cell. Additionally, comparing the effect of the value of the linear and parabolic oxidation rate constants reported by different researchers showed that not only is their absolute value important, but also their ratio to one another.     

Mechanism of synthesizing dense Si–SiC matrix C/C composites

The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 °C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 °C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si–SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 °C and 100 Pa. As a result, production of 1 × 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si–SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si–SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si–SiC matrix is synthesized so that a trunk-shaped Si–SiC matrix leads to the yarn inside.     

Tension–compression fatigue of a SiC/SiC ceramic matrix composite at 1200 °C in air and in steam

Tension–compression fatigue behavior of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200 °C in laboratory air and in steam. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. Tension–compression fatigue behavior was studied for fatigue stresses ranging from 80 to 200 MPa at a frequency of 1.0 Hz. Presence of steam significantly degraded the fatigue performance. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Aluminium nitride–molybdenum ceramic matrix composites: influence of molybdenum concentration on the mechanical properties

Aluminium nitride–molybdenum ceramic matrix composites are produced by hotpressing a mixture of two powders. Mechanical properties of a series of samples are measured in order to study the effect of molybdenum phase on the behaviour of composite. Three-point bend strength increases from a value of 270 MPa for pure aluminium nitride to 571 MPa for a composite containing 40% by volume of metallic phase. Fracture toughness measured by the single-edged precracked beam (SEPB) technique, is also increased as a function of molybdenum concentration. From 2.3 MPam^1/2 for pure AlN we obtain a value of 6.9 MPam^1/2 in the case of composite containing 40% by volume of metallic phase. This very important increase in the mechanical properties of AIN-Mo composites is attributed to higher mechanical properties of molybdenum and an adherent interface between AIN and Mo grains. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of interphase strength on the damage modes and mechanical behaviour of metal–matrix composites

A numerical version of the generalized self-consistent method previously developed by the authors is combined with the Gurson model to undertake a parametric investigation of the damage mechanisms and their relations with the macroscopic tensile properties of SiC reinforced aluminium, for three different interphase strengths. The results show that the interphase strength is a governing factor for damage propagation in the composite. Thus, transformation of the failure mechanism from reinforcement fracture to void nucleation and growth can be achieved by reducing the interphase bond strength, although the strengthening effects on the composite decrease unfavourably.     

Effects of fiber–matrix interaction on multiple cracking performance of polymeric fiber reinforced cementitious composites

The effects of polymeric fiber addition on the multiple cracking performance of composites have been investigated. For this purpose, cement-based matrices incorporating fly ash and a latex emulsion have been designed. Prismatic samples have been prepared and subjected to four-point bending load. The load-midpoint deflection curves and crack patterns have been determined. Meanwhile, flexural strength and relative toughness values have been calculated. Finally, the number of visible cracks formed throughout the testing period has been analyzed.Test results showed that the toughening improvement mechanisms of PP and PVA fibers in a cement-based matrix are extremely different and matrix modifications significantly change the multiple cracking performance. The addition of a latex emulsion in a weak matrix decreased the multiple cracking tendency of PP fiber reinforced composites. However, the same modification attempt improved the multiple cracking capacity of weak matrix in case of PVA fiber reinforcement. The possible causes of this performance improvement have been discussed with the aid of microstructure investigations.     

Application of time–temperature–stress superposition on creep of wood–plastic composites

Time–temperature–stress superposition principle (TTSSP) was widely applied in studies of viscoelastic properties of materials. It involves shifting curves at various conditions to construct master curves. To extend the application of this principle, a temperature–stress hybrid shift factor and a modified Williams–Landel–Ferry (WLF) equation that incorporated variables of stress and temperature for the shift factor fitting were studied. A wood–plastic composite (WPC) was selected as the test subject to conduct a series of short-term creep tests. The results indicate that the WPC were rheologically simple materials and merely a horizontal shift was needed for the time–temperature superposition, whereas vertical shifting would be needed for time–stress superposition. The shift factor was independent of the stress for horizontal shifts in time–temperature superposition. In addition, the temperature- and stress-shift factors used to construct master curves were well fitted with the WLF equation. Furthermore, the parameters of the modified WLF equation were also successfully calibrated. The application of this method and equation can be extended to curve shifting that involves the effects of both temperature and stress simultaneously.     

Effect of notches on creep–fatigue behavior of a P/M nickel-based superalloy

A study was performed to determine and model the effect of high temperature dwells on notch low cycle fatigue (NLCF) and notch stress rupture behavior of a fine grain LSHR powder metallurgy (P/M) nickel-based superalloy. It was shown that a 90 second (s) dwell applied at the minimum stress (“min dwell”) was considerably more detrimental to the NLCF lives than similar dwell applied at the maximum stress (“max dwell”). The short min dwell NLCF lives were shown to be caused by growth of small oxide blisters which caused preferential cracking when coupled with high concentrated notch root stresses. The cyclic max dwell notch tests failed mostly by creep accumulation, not by fatigue, with the crack origin shifting internally to a substantial distance away from the notch root. The classical von Mises plastic flow model was unable to match the experimental results while the hydrostatic stress profile generated using the Drucker–Prager plasticity flow model was consistent with the experimental findings. The max dwell NLCF and notch stress rupture tests exhibited substantial creep notch strengthening. The triaxial Bridgman effective stress parameter was able to account, with some limitations, for the notch strengthening by collapsing the notch and uniform gage geometry test data into a singular grouping.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrB2 composites

C/SiC–ZrB₂ composites prepared via precursor infiltration and pyrolysis (PIP) were treated at high temperatures ranging from 1200 °C to 1800 °C. The mass loss rate of the composites increased with increasing annealing temperature and the flexural properties of the composites increased initially and then decreased reversely. Out of the four samples, the flexural strength and the modulus of the specimen treated at 1400 °C are maximal at 216.9 MPa and 35.5 GPa, suggesting the optimal annealing temperature for mechanical properties is 1400 °C. The fiber microstructure evolution during high-temperature annealing would not cause the decrease of fiber strength, and moderate annealing temperature enhanced the thermal stress whereas weakened the interface bonding, thus boosting the mechanical properties. However, once the annealing temperature exceeded 1600 °C, element diffusion and carbothermal reduction between ZrO₂ impurity and carbon fibers led to fiber erosion and a strong interface, jeopardizing the mechanical properties of the composites. The mass loss rate and linear recession rate of composites treated at 1800 °C are merely 0.0141 g/s and 0.0161 mm/s, respectively.     

Oxidation behavior of a new Fe–Ni–Cr-based alloy in pure steam at 750 °C

The isothermal oxidation of a new Fe–Ni–Cr-based alloy has been investigated in pure steam at 750 °C for exposure time up to 500 h using secondary electron microscope (SEM)/ X-ray energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Results showed that the alloy was oxidized approximately following a parabolic law with a parabolic rate constant k_p of 2.36 × 10^?13 g²/m⁴/s. As revealed by SEM/EDS and XRD results, a duplex-layered external oxide scale was formed, consisting of a thin outer layer of Ni(Fe, Al)₂O₄ and a thicker inner layer of (Cr, Mn)₂O₃. Underneath the external oxide scale, the internal oxidation of Ti to be TiO₂ occurred particularly along the grain boundaries of the matrix alloy. Internal oxide of Al₂O₃ was also observed but at a deeper depth. Based on the detailed compositional and microstructural characterization of the oxidized zone, the mechanism of the external and internal oxidation in steam is presented.     

Effects of environmental aging on the mechanical properties of bamboo–glass fiber reinforced polymer matrix hybrid composites

Short bamboo fiber reinforced polypropylene composites (BFRP) and short bamboo–glass fiber reinforced polypropylene hybrid composites (BGRP) were fabricated using a compression molding method. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to improve the adhesion between the reinforcements and the matrix material. By incorporating up to 20% (by mass) glass fiber, the tensile and flexural modulus of BGRP were increased by 12.5 and 10%, respectively; and the tensile and flexural strength were increased by 7 and 25%, respectively, compared to those of BFRP. Sorption behavior and effects of environmental aging on tensile properties of both BFRP and BGRP systems were studied by immersing samples in water for up to 1200 h at 25°C. Compared to BFRP, a 4% drop in saturated moisture level is seen in BGRP. After aging in water for 1200 h, reduction in tensile strength and modulus for BGRP is nearly two times less than that of BFRP. Use of MAPP as coupling agent in the polypropylene matrix results in decreased saturated moisture absorption level and enhanced mechanical properties for both BFRP and BGRP systems. Thus it is shown that the durability of bamboo fiber reinforced polypropylene can be enhanced by hybridization with small amount of glass fibers.     

Effect of matrix strength on the mechanical properties of Al–Zn–Mg/SiCP composites

The mechanical properties of Al–Zn–Mg alloy reinforced with SiC_P composites prepared by solidification route were studied by altering the matrix strength with different heat treatments. With respect to the control alloy, the composites have shown similar ageing behaviour in terms of microhardness data at 135°C. It was shown that although composites exhibited enhanced modulus values, the strengthening was found to be dependent on the damage that is occurring during straining. Thus the initial matrix strength plays an important role in determining the strengthening. Consequently, compression data had shown a different trend compared to tension.     

Oxidation behavior of green coke-based carbon–ceramic composites incorporating micro- and nano-silicon carbide

The oxidation resistance of the carbon–ceramic composites developed using green coke-based carbon and carbon black as carbon source, boron carbide, and micro- and nano-silicon carbide was carried out in the temperature range of 800 to 1,200 °C. Silicon carbide particulate as such and silicon carbide obtained by the reaction of green coke and silicon provided micro silicon carbide while silicon and carbon black and sol–gel silica and carbon black used as silicon carbide precursors led to the formation of nano-silicon carbide. The oxidation resistance of these composites at 800 to 1,200 °C for 10 h showed that the size of the silicon carbide influenced the oxidation resistance. The weight gain due to protective coating formed on oxidation was higher in composites containing nano-silicon carbide as compared to the composites containing micro silicon carbide.     

Mechanical damping behavior of Al/C60-fullerene composites with supersaturated Al–C phases

Al-based materials with enhanced mechanical damping properties are of great interest in aerospace and automotive industries as engineering materials for critical components that suffer from severe dynamic environment. In this report, we developed Al/C₆₀-fullerene composites to increase damping capacity by the supersaturated Al–C phases. Carbon atoms, dissolved from individually dispersed C₆₀-fullerenes, are intercalated into the Al interstitial sites, producing Al–C phases with expanded lattice structures. These novel nanostructures exhibit a superior mechanical damping behavior compared to monolithic Al, throughout the temperature range of room temperature to 350 °C. The present approach to control the lattice structure thus represents a new engineering paradigm for atomic-level design of lightweight structural components.     

Foamed-metal-reinforced composites: Tribological behavior of foamed copper filled with epoxy–matrix polymer

To improve the thermal performance and wear resistance of epoxy–matrix polymers, foamed copper materials filled with a curable epoxy matrix have been developed for tribological studies. Graphite flakes were incorporated as friction additive in the epoxy matrix. The tribological properties of foamed-copper-reinforced composites were investigated using a UMT-2 friction and wear tester. The temperature of the frictional area was measured using an infra-red thermal camera, and the effect of metallic skeletons on temperature was calculated by the finite element analysis (FEA). The results show the foamed-copper-reinforced composites are effective in transmitting heat and sharing load along the interconnected metallic skeletons, and therefore possess better thermal conductivity and wear resistance.     

Effects of coupling agent on the preparation and dielectric properties of novel polymer–ceramic composites for embedded passive applications

The study was carried out to investigate the effects of silane coupling agent, γ-aminopropyl triethoxy silane (KH-550), on the preparation and dielectric properties of Barium titanate (BaTiO₃)/Bisphenol-A dicyanate (2,2′-bis (4-cyanatophenyl) isopropylidene)(BADCy) composites for embedded passive implications. It was found that KH-550 accelerated the polymerization of BADCy and was beneficial to improve the compatibility between BaTiO₃ particles and BADCy matrix. The dielectric constant (ε) and dielectric loss (tanδ) both increased at first and then decreased with the increase of the KH-550 content. With the increase of the frequency, the variation ranges of the dielectric constant and dielectric loss of these composites were not obvious since the dielectric properties of cyanate ester were stable at various frequencies.     

Effects of phosphorus on creep behaviour of nickel and Ni–20Cr and on creep and strength of Nimonic 80A

Abstract

The influence of P on the creep behaviour of Ni, Ni–20Cr (wt-%), and Nimonic 80A was investigated by carrying out creep tests under various loads and at different temperatures. After creep fracture the samples were investigated using optical, scanning electron, and transmission electron microscopy. The grain boundary segregation was examined using Auger electron spectroscopy (AES). It was found that P segregates to the grain boundaries in all the materials investigated. The creep rate of Ni–20Cr and Nimonic 80A is decreased by the addition of P. Grain boundary segregation of P and its influence on strength was also investigated using AES for specimens aged between 600 and 700°C after fracture by a tensile test inside an ultrahigh vacuum chamber. Maxima of tensile strength are observed to be time dependent as a result of carbide precipitation, which is affected by the P segregation.

MST/1679     

Effects of hydrothermal aging on glass–fiber/polyetherimide (PEI) composites

The moisture absorption behavior and the influence of moisture on thermal and mechanical properties of glass–fiber/polyetherimide (PEI) laminates have been investigated. The laminates were exposed to hydrothermal aging at two different temperatures and high moisture rates. The properties of as-received and hydrothermally aged samples were compared. The hydrothermally aged laminates contained a large amount of moisture which caused decrease in the glass transition temperature and deterioration in mechanical properties (interlaminar shear strength, flexural modulus, bearing strength, etc.). Fractographic analysis revealed interfacial debonding as the dominant failure mechanism, indicating a strong influence of water degradation on fracture toughness results. Alterations in visco-elastic properties of glass/PEI composite which was exposed to hydrothermal aging were analyzed with the dynamic mechanical thermal analysis (DMTA) method. DMTA tests give evidence of plasticization of the PEI matrix.