Mullite/Alumina Particulate Composites by Infiltration Processing: III, Mechanical Properties

The effect of incorporating mullite into alumina by an infiltration process on the mechanical properties was investigated. Data for Young's modulus, strength, and fracture toughness for various composite compositions were compared with those for the unreinforced matrix (alumina). Measurements of Young's modulus by a resonance technique showed that the addition of mullite decreased Young's modulus. Up to 14 vol%, these changes were close to those expected, but above this mullite content, the decrease was more dramatic and indicated specimen damage during processing. The addition of mullite led, in some cases, to increases of more than 60% in both the strength (biaxial flexure) and indentation fracture toughness. These increases have been attributed to the method of introducing mullite and the resulting residual compressive surface stresses. The strength of the indented composite bodies deviated from the ideal behavior, indicating the probability of R -curve behavior in these materials.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Monitoring progression of mode II delamination during fatigue loading through acoustic emission in laminated glass fiber composite

Acoustic emission (AE) was used to monitor damage development in a glass fiber/epoxy composite during monotonic and cyclic Mode II loadings of DCB specimens. AE parameters such as event count rates and accumulative event counts together with the distribution of events by time and location in the DCB specimen were used as proper indicators of the damage growth at various stages during testing. AE proved to be a powerful technique to predict in real time the interlaminar failure stress (delamination in Mode II) in both the static and the dynamic tests. An S-N curve constructed entirely on the AE signatures recorded during fatigue testing exhibits three distinct stages enabling delineation of period of damage initiation, the period of damage growth, and finally the occurrence of delamination. A generalized expression based on the nonlinear cumulative damage model was developed to correlate AE activity with the damage development in the composite during cyclic loading. The fatigue data to date agree well with this method of prediction of the fatigue life.     

Tailored Residual Stresses in High Reliability Alumina-Mullite Ceramic Laminates

A design and processing approach to fabricate ceramic laminates with high mechanical reliability, i.e., high failure resistance, limited strength scatter, and increased damage tolerance is presented in this paper. Different ceramic layers are stacked together to develop a specific residual stress profile after sintering. By changing the composition of the laminae and the composite architecture it is possible to produce a material with predefined failure stress which can be evaluated from the fracture toughness curve correlated to the residual stresses. In addition, by tailoring the fracture toughness curve, surface defects can be forced to grow in a stable way before reaching the critical condition, thus obtaining a unique-value strength ceramic material. Laminates composed of alumina/mullite composite layers are designed and created in this work by the implementation of the proposed approach. The material obtained shows a "constant" strength of 456 MPa (standard deviation <7%) even when large surface damage is produced by Vickers indentation.     

Temperature-induced fibre/matrix interactions in porous alumino silicate ceramic matrix composites

The thermal stability of alumino-silicate fibre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600°C. In the as-prepared state the fibres consist of mullite plus α-Al₂O₃, while the porous mullite matrix includes minor amounts of a SiO₂-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and α-Al₂O₃ at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 μm thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500°C under short time heat-treatment conditions (2 h) and at much lower temperature (1300°C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) fibre/mullite matrix composites ranges between 1500°C in short-term and 1300°C in long-term heat-treatment conditions.     

Fatigue damage identification of SiC coated needled C/SiC composite by acoustic emission

The fatigue damage process of SiC coated needled C/SiC composite specimen was monitored by acoustic emission (AE) under tension-tension cyclic loading. By analyzing the collected AE parameters of the composite, it is found that Kaiser effect enhances with the increase of stable cycles in the fatigue process. Moreover, multivariate K-means cluster analysis of AE parameters was carried out after the standardization of energy, amplitude, peak frequency and duration of AE signal. By comparing the objective function values of different number of clusters, and referring to the intra group variance and the variance between groups, the damage modes of the needled C/SiC composite are finally divided into four clusters, and the characteristics of AE parameters with different damage modes can be obtained. Furthermore, by referring to the microstructure characteristics of needled C/SiC composite, various damage modes at different fatigue stages were analyzed. In addition, the fracture morphology of the specimen was also observed by scanning electron microscope after fatigue fracture.     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.     

Modeling deformation of a melt-infiltrated SiC/SiC composite under fatigue loading

Results from fatigue experiments done on a SiC/SiC composite are presented. A micromechanics-based model is used to study the observed behavior under cyclic loading. The model includes consideration of progressive damage, creep and oxidation of the fiber and matrix. Comparison of model predictions with test data showed that the deformation during fatigue in this material is explained primarily by damage in the form of matrix microcracking and interface debonding, in combination with creep under the cyclic load. Stiffness of the material was observed to not change significantly during fatigue indicating that the contribution of fiber fracture to deformation is limited. Fiber fracture however was found to determine final failure of the composite. Failure under cyclic fatigue loading was found to be affected by load transfer from the matrix to the fiber due to damage and creep, and by progressive degradation of the load-carrying fibers due to the combined effect of oxidation and load cycling.     

Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography

To enhance the understanding of matrix cracking and damage progression on the macroscopic scale, within a 0/90° fibre reinforced SiC_f/SiC ceramic matrix composite (CMC), X-ray computed tomography (XCT) imaging and analysis have been performed in conjunction with a commercially available in-situ mechanical loading device. CMC test coupons were subjected to tensile cyclic loads and inspected using XCT without removal from the tensile loading device. Attempts to measure and quantify the resulting damage using volumetric image analysis techniques are presented, by characterising the crack network from XCT images acquired at both the maximum and minimum load condition during selected fatigue cycles. The XCT detection of significant crack development within the first loading half-cycle shows good agreement with cumulative acoustic emission energy data recorded under similar test conditions. The results are seen as an important step towards correlating the damage behaviour detected via different NDE and health monitoring techniques.     

Preparation of mullite-TiB2-CNTs hybrid composite through spark plasma sintering

A near fully dense mullite-TiB₂-CNTs hybrid composite was prepared successfully trough spark plasma sintering. 1 wt%CNT and 10 wt%TiB₂ were mixed with nano-sized mullite powders using a high energy mixer mill. Spark plasma sintering was carried out at 1350 °C under the primary and final pressure of 10 MPa and 30 MPa, respectively. XRD results showed mullite and TiB₂ as dominant crystalline phases accompanied by tiny peaks of alumina. The microstructure of prepared composites demonstrated uniform distribution of TiB₂ reinforcements in mullite matrix without any pores and porosities as a result of near fully densified spark plasma sintered composite. The fracture surface of composite revealed a proper bonding of TiB₂ with mullite matrix and also areas with CNTs tunneling and superficies as a result of pulling-out phenomenon. The flexural strength of 531 ± 28 MPa, Vickers harness of 18.31 ± 0.3 GPa, and fracture toughness of 5.46 ± 0.12 MPa m^−1/2 were achieved for prepared composites as the measured mechanical properties.     

A study of damage accumulation in unidirectional glass reinforced composites via acoustic emission monitoring

Damage accumulation in continuous unidirectional glass reinforced composites was studied by acoustic emission (AE) monitoring during three-point-bend loading. Results are presented for four composites monitored during quasi-static break loading, and one composite also monitored during cyclic fatigue and static creep loading. AE response was correlated with the mechanical (stress-strain) response and with visual observation of damage events to study the details of the damage accumulation process. Results show that the failure process is characterized by the sequential occurrence of three distinct damage mechanisms. Specifically, the failure process initiated with cohesive matrix damage, propagated with interfacial debonding, and ended with fiber breakage very near catastrophic failure. The same sequential damage process occurred in all four composites and all three test procedures examined. Results also demonstrate that AE analysis, in combination with mechanical testing and microscopic observation, is a valuable tool in understanding damage accumulation in composites.     

Experimental and numerical investigation into fatigue damage mechanisms in multifibre microcomposites

Abstract

Polarised light microscopy has been used to investigate the influence of stress level, interfibre spacing, and fibre–matrix adhesion on the fatigue micromechanisms in carbon–epoxy model composites consisting of a planar array of five intermediate modulus carbon fibres. It was found that an increase in fatigue stress results in an increase in the number of fibre breaks, a more coordinated fibre fracture pattern as a result of fibre–fibre interaction, and extensive interfacial damage. In addition, it was shown that a smaller interfibre spacing results in a higher level of fibre–fibre interaction. Finally, in the case of surface treated carbon fibres (good fibre–matrix adhesion), a more coordinated fibre failure pattern was observed owing to stronger fibre–fibre interaction, whereas in the case of untreated carbon fibres (poor fibre–matrix adhesion), extensive debonding was observed which resulted in a more random fibre failure pattern. Finally, the experimental results were validated by means of a three-dimensional finite element analysis.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

Theoretical and experimental analyses of Young's modulus and thermal expansion coefficient of the alumina-mullite system

Young's moduli (E) and thermal expansion coefficients (TECs) of the alumina–mullite–pore system (96.4–99.5% relative density) were measured for a wide mullite fraction range from 0 to 100 vol%. Both E and TEC values decreased at high mullite fractions. These properties were theoretically analyzed with four proposed model structures that were constructed by three-phase systems of mullite (or alumina) continuous phase 2–pore dispersed phase 1–alumina (or mullite) dispersed phase 3. The ratios of E(theoretical)/E(experimental) and TEC(theoretical)/TEC(experimental) were very close to unity, depending on the mullite fraction. That is, the measured E and TEC values are closely related to the change in the composite microstructure as a function of mullite fraction.     

Partial substitution of feldspar by B.F. slag in triaxial porcelain: Phase and microstructural evolution

Feldspar was gradually substituted by B.F. slag (a glassy by-product of Indian steel plant) to the extent of 5–20 mass% in a triaxial porcelain composition consisting of 45 mass% kaolinitic clay, 30 mass% feldspar, 25 mass% quartz. The green compacts were heated at 1200 °C for a period of 120 min. The microstructure and phase changes as they evolve on heating were studied using SEM and XRD techniques. The results reveal that quartz level was reduced from 26 to 9 mass% by the addition of 5% slag, while mullite level was reduced from 20 to 2 mass% by the addition of 10% slag. Beyond this, further addition of slag did not alter the quartz and mullite level much. Slag used in this study was enriched with alumina and contributed towards development of anorthite (CaO·Al₂O₃·2SiO₂) phase by crystallization of melted glassy phase and its quantity increased with increase in slag content. Sudden increase in flexural strength of 20% slag containing body is attributed to stronger pre-stress caused by the difference in thermal expansion coefficient between glassy matrix, quartz and anorthite during cooling process. This also caused circumferential cracking around quartz grains. The paper further discusses the variation in their physico-mechanical characteristics with respect to slag content.     

AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites

A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al₂O₃, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 °C. Pre-coating the fibers with AlPO₄ as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is 170 MPa at 25 °C ≤ T ≤ 1100 °C. At 1200 °C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO₄ coating acts as a weak layer even after thermal aging at 1300 °C for 100 h.     

Fatigue behaviour and residual strength evolution of 2D C/SiC Z-pinned joints prepared by chemical vapour infiltration

Z-pinned joints prepared by chemical vapour infiltration are widely used in ceramic matrix composite components. Excellent fatigue behaviour is important for structural safety. In this study, 2D C/SiC Z-pinned joints were loaded in axial direction of the pins under static and cyclic loading. Internal damage was monitored in situ by an acoustic emission system. The binding force between pin and hole is relatively strong. Meanwhile, the joints exhibite promising resistance to fatigue. The residual strength increased first with the fatigue cycles then decreased after 10⁵ cycles. Microstructural analysis indicated that full-developed cracks and local stress redistribution resultes in the increase in the strength of the joints. The acoustic emission analysis also provides a supplementary understanding of the damage mechanism. The results show that damage fully develops at the early stage of fatigue. When the specimen is reloaded, less AE events are collected before the fatigue maximum stress.     

Effect of mullite additions on the fracture mode of alumina

This work analyses the effect of mullite additions on the fracture mode of alumina. Mullite is proposed as an alternative to SiC for the second phase particles because the thermal expansion mismatch between alumina and mullite is of the same sign and order as that between alumina and SiC. Three alumina–5 vol.% mullite composites formed by alumina matrices with similar average grain sizes in the micrometric range (≈1 μm) and second phase sub-micrometric (50–350 nm) and nanometric mullite (<50 nm) particles located at grain boundaries and triple points were prepared. The fracture mode of the alumina matrix changed from predominantly intergranular to predominantly transgranular. This change became more significant as the size of the sub-micrometric fraction of mullite particles decreased.     

Mechanical characterisation of mullite-based ceramic matrix composites at test temperatures up to 1200°C

Nextel 610 fibre-reinforced mullite-based matrix fabricated by Dornier Forschung was characterised at DLR Institute of Materials Research. The material was produced by the polymer route after coating the fibres with a 0.1 μm thick carbon layer. The composite was manufactured by infiltrating the fibres with a slurry containing a diluted polymer and mullite powder, curing in an autoclave and subsequently heat treating and pyrolysis of the polymer. A final heat treatment in air is performed to remove the carbon coating and to reduce the residual stresses. A (0/90/0/90/0/90)_s-laminate was produced with an average fibre volume fraction of 45.6% and a porosity of 15.9%. Dog-bone-type tensile specimens with a width of 10 mm were cut from the plate by water jet and tested at temperatures up to 1200°C in air. The tensile strength at room temperature measured 177.4 MPa and linearly decreased to 145.2 MPa at a temperature of 800°C. A stronger decrease occurred at 1000 and 1200°C. In contradiction to ceramic matrix composites manufactured by the CVI-route the stress–strain behaviour is nearly linear up to failure. The modulus of the composite (at room temperature 108.8 GPa) is analysed on the basis of the expected moduli of the fibres and the mullite matrix. It can be concluded that the contribution of the matrix to the modulus of the composite is low, caused by porosity and components other than mullite. The intralaminar shear strength at room temperature measured 36 MPa. This value reflecting shear transfer capability of fibre to matrix limits the amount of fibre pull-out.     

Creep in Interlaminar Shear of a Nextel™720/aluminosilicate Composite at 1100°C in Air and in Steam

Creep in interlaminar shear of an oxide–oxide ceramic composite was evaluated at 1100°C in air and in steam. Composite consists of a porous aluminosilicate matrix reinforced with mullite/alumina (Nextel^?720) fibers, has no interface between fibers and matrix, and relies on the porous matrix for flaw tolerance. The interlaminar shear strength was 7.6 MPa. Creep behavior was examined for shear stresses of 2–6 MPa. Creep run‐out of 100 h was not achieved. Larger creep strains and higher creep strain rates were produced in steam. However, steam had a beneficial effect on creep lifetimes. Composite microstructure, damage, and failure mechanisms were investigated.