The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al2O3 composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al2O3. The interconnected Al2O3 in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al2O3 are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner.

In this study, the effects of relative humidity and doping elements are highlighted on the friction and wear properties of SiC in unlubricated condition. At higher humidity, the friction coefficient values (~0.25) are low and are independent of doping elements. The effect of doping (varying from 400 to 3000 ppm) is more pronounced at lower humidity and the friction coefficient values decrease (as compared to undoped SiC) with the addition of Al, Mg or P. The specific wear rate is lower at the higher humidity (60% RH) as compared to the lower humidity (i.e. at 30% RH) values. As the relative humidity increases from 30% to 60%, the wear mechanism gradually changes from mechanical to tribochemical. The tribo-layer consisting of carbon and hydrated silica progressively smears with relative humidity and becomes instrumental in controlling the friction and wear properties. Finally, the formation of large needle-like debris is elucidated through SEM observations.

The effect of doping elements on the friction and wear properties of silicon carbide is investigated in unlubricated sliding under varying humidity conditions. Doping elements (Al, Mg and P) appreciably affect the friction coefficient at lower humidity (30% RH) than at higher humidity (60% RH). The change in the kinetics of tribochemical reactions at lower humidity is attributed to chemisorption and microstructural changes. At higher humidity, coefficient of friction (COF) decreases but the effect of doping elements is rather insignificant. This is attributed to faster kinetics of tribochemical reactions. Continuing adhesion, twisting and rolling of small wear particles during the wear process results in the formation of long needle-like debris.

The objective of the present research is to analyze the role of SiC addition and load on the fretting wear of Mg–SiCp composites. The friction and wear properties of Mg–SiCp/steel contacts are studied with variation of SiC particulate volume fraction and load and compared to those of pure Mg/steel contacts. The wear mechanisms are investigated in detail using analytical techniques such as EDX, EPMA and Raman spectroscopy. In all the fretting contacts, the tribochemical reactions are observed to be dominant. In the early stage of fretting, abrasive wear dominates due to formation of MgO. In composites, the presence of harder SiC particulates causes severe abrasion. As the fretting continues, MgO undergoes tribochemical reaction and forms soft, viscous hydrated magnesia. In composites, hydrated silica formed from SiC, reacts with hydrated MgO and forms a dense hydrous magnesium silicate (DHMS), clinoenastatite. These soft, viscous triboproducts (hydrated MgO and DHMS) smear on the worn surfaces and decrease the friction coefficient. The experimental results reveal that the effect of SiC particulate loading (especially higher volume fractions) on friction coefficient is more significant compared to the normal load applied during fretting.

Doping as well as processing methodology exhibited pronounced effect on microstructure and grain boundary character of SiC. Higher frequency of special boundaries, formation of a phase and extensive grain growth were observed in the Spark plasma sintering (SPS) samples as compared to the hot-pressed (HP) samples. Al doping further enhanced the frequency of special boundaries. An exaggerated grain growth was noticed with Mg doping. Finally, the hardness values measured on different SiC materials were related to the grain size as well as the grain boundary character.

AC and DC resistivity of Cr-Al2O3 and ZrxAly-Al2O3 composites with varying metal content were measured. A strong percolation behavior was observed in the Cr-Al2O3 system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 degreesC indicate that the intermetallic phases ZrAl3 and ZrAl are PTC conductors. The electrical properties of ZrxAly-Al2O3 Samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction. (C) 2002 Elsevier Science Ltd. All rights reserved.

High strength 'engineering' ceramics have emerged as an important new class of structural materials within the latter half of the twentieth century. The silicon nitrides and their derivative 'Sialons', both of which are based largely on early research in the UK, have been particularly prominent in the field of engineering ceramics but have not fulfilled the early predictions for extensive ceramic substitution of metallic components. Reasons underlying the limited market penetration are based either on unacceptable component costs or on mechanical or thermal performance which has not achieved the levels based on optimistic extrapolation of early properties. Toughness is the most elusive goal which has promoted the evolution of ceramic matrix composites, based largely on spun-precursor fibres of SiC, with a recent drive to produce oxide/oxide systems with high temperature stability. The ability to relate 'model' structure and properties has enabled incremental advances in performance of real ceramics which are inevitably constrained by the requirements of convenient processing and fabrication. Examples are given of novel processing routes and microstructures within both monolithic and composite ceramics and their varied potential applications from ultra-hard tribological solids to high temperature components for engines of the twenty-first century.

The compressive flow behaviour of lithium aluminosilicate (LAS) glass, with and without SiC particulate reinforcements, was studied. The LAS glass crystallized toβ spodumene during high-temperature testing. The flow behaviour of LAS glass changed from Newtonian to non-Newtonian due to the presence of crystalline phase. Further, with the addition of 40 vol.% SiC additions, the strain rate sensitivity of flow stress decreased. While the activation energy for flow in LAS was 300 kJ/mole, it increased to 995 kJ/mole with the addition of 40 vol.% SiC reinforcements.

Silicate matrix composites are potential candidates for high-temperature applications. In the present investigation, the effect of metallic (Cu) and non-metallic (SiC particulates, platelets, short fibres and whiskers) additions on the rheological behaviour of borosilicate matrix composites has been evaluated. The hot-pressed composites were tested both in compression and tension in the temperature range of 625–725°C. SiC reinforced composites tested in compression exhibited varying degree of strengthening and strain rate sensitivity depending on the volume fraction and morphology of reinforcements. The degree of strengthening and strain rate sensitivity depends on the volume fraction and morphology of reinforcements. Strengthening effect increased with the volume fraction and aspect ratio of reinforcements. The flow behaviour of composites changed from Newtonian to non-Newtonian with strain rate sensitivity index value changing from unity to 0.48. A similar trend was seen in the rate sensitivity of copper composites. However, copper additions decreased the strength of the composites at lower temperatures because of the softer copper phase. Pre-oxidation of copper particles had certain strengthening effect on the composite. The apparent viscosity of SiC reinforced composites increased with volume fraction and aspect ratio of reinforcements. However, in particulate composites, the viscosity found to increase with particle size. The mechanical/hydrodynamic interactions among the particulates appeared to be responsible for such a behaviour. With increasing strain rate, the viscosity decreased progressively confirming the shear thinning of the composites. The tensile ductility of the composites with 40 vol% reinforcements was evaluated at 700°C. While 400% elongation was observed in SiC particulate, platelet and copper composites, in short fibre/whisker composites, the tensile elongation values were only 150%. Further, the elongation of SiC platelet and copper composites improved by decreasing temperature and volume fraction of reinforcements, and also elongation values >500% were recorded. The tensile ductility of borosilicate composites was limited by onset and growth of cavities nucleated at the reinforcement/matrix interfaces.

The high-temperature flow behavior of different borosilicate composites was investigated in compression and tension. The flow behavior of composites changed from Newtonian to non-Newtonian as the volume fraction and aspect ratio of reinforcements increased. Generally, platelet/particulate composites exhibited higher tensile elongations compared with short fiber/whisker composites. The tensile ductility increased with decreased volume fraction of reinforcements and temperature. Cavitation in these samples varied across the length, and maximum cavitation occurred near the fracture zone.

The rheological behavior of SiC particulate glass composites was investigated in the present study. The nature and extent of flow modifications are addressed with respect to solid content in the suspension, temperature and dispersoid size. A transition from Newtonian to non-Newtonian viscous flow and characteristic shear thinning behavior were observed. With progressive strengthening and deviation from Newtonian flow, a significant loss in rate sensitivity occurred. The apparent viscosity of the composites increased with the concentration and size of reinforcements. The increase in viscosity is explained in terms of hydrodynamic/mechanical interactions between particles in the composites.

An attempt was made for the first time to model the pressureless infiltration of a porous preform, from first principles, as flow through a porous medium. To this end, a transient, two-dimensional, laminar flow model in a cylindrical polar coordinate system has been developed to predict the flow phenomena in the molten alloy-porous ceramic filler material system. Two different modeling approaches were considered for the porous preform, and their relative accuracy/adequacy was assessed for predicting the infiltration rate ( i.e. , composite growth). The sensitivity of model predictions to various assumptions and numerical approximations, such as the initial preform wetting height, gridsize distributions, and time-step size, were tested computationally. For a first approximation, a structure-sensitive parameter ( β ) on the order of 10 −4 m, which is the same as that reported in the literature, gave reasonably accurate results. A hypothetical computation which assumed no nitride formation provided an activation energy value of 18 kJ/mol for a surface tension-driven infiltration process. In addition to these, the influence of process parameters such as particle size and operating temperature on the predicted results was investigated numerically and was shown to compare well with experimentally obtained data.

The effects of metallic (Cu) and non-metallic (SiC particulates, platelets, whiskers and short fibres) reinforcements on the elevated temperature flow behaviour of borosilicate matrix composites were explored. Hot-pressed composites were tested both in compression and tension in the temperature range of 625 degrees C-725 degrees C. SiC reinforccd composites tested in compression exhibited varying degree of strengthening and strain rate sensitivity depending on the volume fraction and morphology of reinforcements. The degree of strengthening increased with the volume fraction and the aspect ratio of the reinforcements. The now behaviour of all the composites changed from Newtonian to non-Newtonian as the volume fraction of reinforcements increased and strain rare sensitivity index value varied from 1 to 0.5. A similar trend was seen in the late sensitivity of copper reinforced composites. However, copper additions lowered the strength of the composites at lower temperatures due to softening of copper. Pre-oxidation of copper particles had cer tain strengthening effect on the composite. The tensile ductility of the composites with 40 vol.% reinforcements was evaluated at 700 degrees C. While 400% elongation was observed in SiC particulate, platelet and copper composites. in SIC short fibre/whisker composites, the tensile elongation values were only 150%. Further, the elongation of SIC platelet and copper composites improved with decreasing temperature and elongation values similar to 500% were recorded at 675 degrees C. The apparent viscosity of SiC reinforced composites increased with volume fraction and aspect ratio of reinforcements whereas the metallic dispersions had a marginal effect. The tensile ductility of borosilicate composites was limited by the onset and growth of cavities nucleated at the reinforcement/matrix interfaces.

The reactive infiltration of nickel aluminide powder preform at temperatures above 700°C yielded dense composites. The microstructural evolution of the constituent phases at different processing temperatures are interpreted via the vacancy diffusion mechanism and the peritectic reaction occurring at 854°C. Optimum mechanical properties were obtained for composites processed at lower temperatures.

The effects of reinforcement morphology, volume fraction and test temperature on the flow behavior of SiC-borosilicate glass composites have been investigated in the present work. The deviation from Newtonian behavior of the matrix glass with the addition of reinforcements and subsequent shear thinning are observed. With the increase in volume fraction and aspect ratio of reinforcements, the strength and apparent viscosity of composites increase, but their rate sensitivity decreases. The activation energy for flow gradually drops with the increase in concentration of reinforcements.

The abrasive wear behaviour of SiC whisker-reinforced borosilicate and magnesium aluminosilicate (MAS) composites is investigated under different conditions. Generally, MAS composites exhibited superior wear resistance compared to borosilicate composites. The superior wear resistance of MAS composites is attributed to its higher mechanical properties. The wear resistance increased by increasing volume fraction of whiskers. Sliding distance, sliding velocity and load exhibit significant influence on the material loss. During abrasive wear, silicate matrices undergo extensive contact damage due to repeated indentation of SiC particulates/whiskers. Under repeated contact events, cracks generated at various locations of the matrix simultaneously grow and join resulting in delamination of composite material. In the composites, material removal is delayed by various toughening mechanisms like crack deflection, bridging and whisker pull-out. (C) 1998 Published by Elsevier Science S.A. All rights reserved.