Indentation fatigue in silicon nitride, alumina and silicon carbide ceramics

Repeated indentation fatigue (RIF) experiments conducted on the same spot of different structural ceramics viz. a hot pressed silicon nitride (HPSN), sintered alumina of two different grain sizes viz. 1 μm and 25 μm, and a sintered silicon carbide (SSiC) are reported. The RIF experiments were conducted using a Vicker’s microhardness tester at various loads in the range 1–20 N. Subsequently, the gradual evolution of the damage was characterized using an optical microscope in conjunction with the image analysing technique. The materials were classified in the order of the decreasing resistance against repeated indentation fatigue at the highest applied load of 20 N. It was further shown that there was a strong influence of grain size on the development of resistance against repeated indentation fatigue on the same spot. Finally, the poor performance of the sintered silicon carbide was found out to be linked to its previous thermal history.     

Sintering of nano crystalline α silicon carbide doping with aluminium nitride

Sinterable silicon carbide powders were prepared by attrition milling and chemical processing of an acheson type α-SiC. Pressureless sintering of these powders was achieved by addition of aluminium nitride together with carbon. Nearly 99% sintered density was obtained. The mechanism of sintering was studied by scanning electron microscopy and transmission electron microscopy. This study shows that the mechanism is a solid state sintering process.     

Microstructure and mechanical properties of silicon carbide fibre-reinforced aluminium nitride composite

SiC continuous fibre (15 vol%)/AlN composite was fabricated using a sintering additive of 4Ca(OH)₂ · Al₂O₃ by hot-pressing at 1650 °C and 17.6 MPa in vacuum. Analytical transmission electron microscopy and scanning electron microscopy were used to investigate the microstructure of as-fabricated and crept SiC fibre/AlN composites. The room-temperature mechanical and high-temperature creep properties of the composite were investigated by four-point bending. The incorporation of SiC fibre into AlN matrix improved significantly the room-temperature mechanical properties. This improvement could result from the crack deflections around the SiC fibres. However, the incorporation degraded severely the high-temperature creep properties under oxidizing atmosphere. This could be attributed to the development of the pores and various oxides at the matrix grain boundary and matrix/fibre interface during creep test.     

Corrosion protection of aluminium-matrix aluminium nitride and silicon carbide composites by anodization

Anodization is an effective surface treatment for improving the corrosion resistance of aluminium-matrix composites. For SiC particle-filled aluminium, anodization was performed successfully in an acid electrolyte, as usual. However, for AlN particle-filled aluminium, anodization needed to be performed in an akaline (0.7 N NaOH) electrolyte instead of an acid electrolyte, because NaOH reduced the reaction between AlN and water, whereas an acid enhanced this reaction. The concentration of NaOH in the electrolyte was critical; too high a concentration of NaOH caused the dissolution of the anodizing product (Al2O3) by the NaOH, whereas too low a concentration of NaOH did not provide sufficient ions for the electrochemical process. The corrosion properties and anodization characteristic of pure aluminium, Al/AlN and Al/SiC were compared. Without anodization, pure aluminium had better corrosion resistance than the composites and Al/SiC had better corrosion resistance than Al/AlN. After anodization, the corrosion resistance of Al/AlN was better than Al/SiC and both composites were better than pure aluminium without anodization, but still not as good as the anodized pure aluminium.     

Thermal and grinding induced residual stresses in a silicon carbide particle-reinforced aluminium metal matrix composite

The residual stresses in a silicon carbide particle-reinforced aluminium (SiC_p/Al) metal matrix composite (MMC) were measured using the X-ray diffraction method. The thermal residual stresses induced by annealing were found to be hydrostatic tension for the Al matrix and hydrostatic compression for the SiC reinforcement. After grinding treatment, the force equilibrium between these hydrostatic stresses was disturbed and compressive stresses were measured in both constituents. The effect of grinding extended into the bulk, and depth profiles of the residual stresses in both constituents were obtained by layer removal. The behaviour exhibited in these depth profiles is explained and their usefulness is indicated.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Natural ageing behaviour of cast alumina particle-reinforced 2618 aluminium alloy

The effect of the presence of 10 and 15 vol% alumina particles on the natural ageing behaviour of cast 2618 aluminium alloy was investigated using microhardness measurements and differential scanning calorimetry. It was found that the addition of the alumina particles does not alter the ageing sequence of 2618 AI although certain aspects of the precipitation reactions are changed. In particular, the relative quantities of the various phases were changed by reinforcement addition. Increasing the alumina content decreased the volume fractions of the Guinier-Preston-Bagaryatskii (GPB) I phases. Also, the peak reaction temperature, (T _p), for the GPBII and S phases decreased with increasing volume fraction of alumina.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5?5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.     

Phase distribution and associated mechanical property distribution in silicon carbide particle-reinforced aluminium fabricated by liquid metal infiltration

The reaction between silicon carbide and aluminium to form silicon and Al₄C₃ in SiC particle-reinforced aluminium fabricated by liquid aluminium infiltration was most severe near the original interface between liquid aluminium and the SiC preform. This resulted in the highest concentration of Al₄C₃ and the lowest concentrations of silicon and SiC in the part of the composite near this interface. In particular, the silicon concentration was highest in the bottom centre of the composite when infiltration occurred from the top, because silicon diffused toward the surrounding aluminium melt before solidification. These non-uniform phase distributions, as measured by X-ray diffraction and differential scanning calorimetry, did not cause any non-uniform shear strength distribution. However, excessive reaction between SiC and aluminium, as observed for an infiltration (=mould=liquid metal) temperature of 780° C, caused the tensile strength to decrease. In the case where a steel mould was used during infiltration at 780° C, iron-containing precipitates, such as ternary Al-Fe-Si, were observed in the part of the composite within 5 mm from the above-mentioned interface; their formation was related to the silicon out-diffusion in the form of liquid Al-Si; they caused the shear strength to be lower in this part of the composite; larger such precipitates (up to 100 m) were observed in the excess aluminium adjacent to the cast composite. For pure aluminium as the infiltrating metal, the optimum infiltration temperature for the highest tensile strength was 700° C. An infiltration temperature of 670° C resulted in incomplete infiltration, which was more severe when a steel mould rather than a graphite mould was used because of the higher thermal conductivity of the former.     

Silicon carbide platelet-reinforced silicon nitride composites

Different grades of high aspect ratio SiC platelets were used to reinforce Si₃N₄. Dispersion of additives (4 wt % Y₂0₃ and 3 wt % Al₂0₃) was achieved by ball milling in ethanol using alumina balls, while dispersion of platelets was done by ball milling using plastic balls. Consolidation of the composites was carried out by uniaxial hot pressing. A slight decrease in flexural strength was measured, while significant increases in elastic properties, fracture toughness and Weibull modulus were noted. Microstructure and crack-propagation studies as well as reinforcement mechanisms are presented.     

Titanium diboride particle-reinforced aluminium with high wear resistance

A TiB₂ particle (61 vol%, 4 m mean size) reinforced aluminium fabricated by liquid-aluminium infiltration was subjected to unlubricated rolling wear and was found from the weight loss to be 1.5 times more wear resistant than 17-4 ph stainless steel, twice as wear resistant as 1020 steel, 7.5 times more wear resistant than 2024 aluminium, and 12.8 times more wear resistant than the aluminium matrix. This wear resistance is attributed to the lack of particle pull-out and the ability of the TiB₂ particles to protect the softer underlying matrix from abrasion. This composite was approximately three times more wear resistant than AlN particle (50 vol%)-reinforced aluminium. The greater wear resistance of Al/TiB₂ compared to Al/AlN is due to the slow wear of the TiB₂ particles and the AlN particle pull-out. A slight decline in tensile strength and no effect on the modulus was observed in Al/TiB₂ after heating at 300 or 600°C for 240 h. This high-temperature stability is attributed to the lack of reactivity between TiB₂ and the aluminium matrix.     

Effect of silicon carbide particulates on wear resistance of graphitic aluminium matrix composites

Use of graphite (Gr) reinforcement in aluminium matrix composites has been reported to be beneficial in reducing wear due to its solid lubricant property, but it results in reduction of mechanical strength. Addition of silicon carbide (SiC), on the other hand, improves both strength and wear resistance of composites, but high amount of SiC makes machining difficult and composites become brittle. Thus, SiC can be advantageously used as a second reinforcement to overcome the problem of strength reduction of Gr reinforced composites, resulting in what is known as hybrid composites. Aluminium matrix composites reinforced with equal weight fraction of SiC and Gr particulates up to 10% are studied with regard to hardness improvement and modified dry sliding wear behaviour. Studies based on design of experiments techniques indicate that there is an increasing trend of wear in Al–SiC–Gr hybrid composites beyond % reinforcement of 7.5%. Hybrid composites exhibit better wear characteristics compared to Gr reinforced composites. Interaction between load and sliding distance is noticed in both the composites and this may be attributed to the presence of Gr particulates. Decrease of wear with increase of speed and increase of wear with increase of either load or sliding distance or both were noticed.     

Hydrothermal corrosion of nitride and carbide of silicon

Silicon nitride (Si₃N₄) and carbide (SiC) are getting applied to engine parts in Japan. We have to estimate the life of these parts whenever we use them for engine applications. After burning of fuel in the air we get water vapour. Therefore, reactions between Si₃N₄ and/or SiC and water vapour are important for application for engine parts. The author presents a review on this topic.     

The microstructures of silicon nitride/alumina ceramics

The microstructures of materials formed by sintering or hot-pressing mixtures of silicon nitride and alumina have been studied by transmission electron microscopy. The probable mechanism of transformation of the reactants to form β′-silicon aluminium oxynitride (β′-sialon) via a liquid phase sintering process, which is analogous to a similar transformation in hot-pressed silicon nitride containing a magnesia additive, is proposed. The origin and crystal symmetry of an unknown second phase is discussed. The residual quantity of this phase, known as the X-phase, is controlled mainly by the silica impurity content of the initial silicon nitride powder.     

Oxidation behaviour of alumina–silicon carbide nanocomposites

Oxidation studies were conducted on Al₂O₂–SiC nanocomposites at 1400 °C. The composites were prepared by hot-pressing mixtures of commercial alumina and ultrafine SiC powders, in amounts of 5, 15 and 30 vol %. Linear kinetics were detected for the oxidation of composites containing 5 vol % SiC. Two stages were observed in composites containing 15 vol % SiC: the first linear and the second presumably parabolic. A parabolic behaviour was observed in the sample containing 30 vol % SiC. The oxidation rates were several orders of magnitude higher than those of monolithic SiC and the observed data were not consistent with the expected increase in weight associated with the oxidation reaction of SiC to SiO₂; in fact the most surprising feature is that the sample containing 30 vol % SiC showed a better oxidation resistance than samples containing 5 and 15 vol % SiC. The reaction products were alumina and mullite in samples with 5 and 15 vol % SiC, while mullite and silica were found on the oxidized surface of samples containing 30 vol % SiC. Explanations are given of the influence of the oxidizable phase amount, the presence of impurities, reaction product structure and composition.