Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Sintering of 6H(α)-SiC and 3C(β)-SiC powders with B4C and C additives

The sintering behavior of three fine industrial SiC powders (two 6H()-type and one 3C()-type) has been comparatively investigated. The powders were pressureless sintered with B₄C and C additives between 1950°C and 2250°C in a high temperature dilatometer with flowing Ar atmosphere. The densification and shrinkage rate curves, polytype content, and grain growth were correlated with physical and chemical characteristics of starting powders. One of 6H()-type powders presented good sinterability only after extensive milling, even though it presented small average particle size, narrow particle size distribution and high specific surface area. The main difference in densification behavior among powders was the narrower shrinkage rate curve of -SiC powder, with its maximum shifted to higher temperature. Grain growth and phase transformation simultaneously occurred. In -SiC, 6H polytype partially transformed to 4H. This transformation was favored by aluminum impurity and resulted in a microstructure with more elongated grains. In -SiC, 3C transformed mainly to 6H, 15R and 4H, introducing many stacking faults which resulted in elongated SiC grains.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Sinterability,strength and oxidation of alpha silicon carbide powders

Pressureless sintering of commercially available -SiC powders was investigated at temperatures between 1900 and 2150° C for times of 10 to 240 min under one atmosphere of argon pressure. Alpha-SiC powder containing boron and carbon sintering aids was sinterable at 2150° C for a period of 30 min to a high final density greater than 96 per cent of theoretical. In contrast, a final density only about 80 per cent of theoretical was achieved in -SiC powder containing aluminium and carbon sintering aids. Room temperature and high temperature (1370° C) flexure strength and oxidation resistance were determined on sintered high density (>96% of theoretical) -SiC (boron, carbon) material. Both the strength and the oxidation resistance were found to be equivalent and comparable to those of Carborundum Company sintered -SiC, which is representative of the current state-of-the-art material.     

Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.     

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part II: Microstructural observations

Additions of AlN and Al₂O₃ to -SiC hot pressed at 2100°C strongly effect the - to -SiC phase transformation and the resultant -SiC polytypes which are formed. Scanning and transmission electron microscopy were utilized to investigate the microstructural changes occurring in SiC due to these additions and to correlate these observations to their mechanical properties. The results suggest that Al₂O₃ additions stabilize the formation of the 6H-polytype of -SiC which grows rapidly into an elongated plate-like morphology, while AlN additions stabilize the 2H-polytype of -SiC resulting in fine equiaxed 2H-SiC: AlN solid solution grains. It is speculated that the elongated growth of 6H-SiC with Al₂O₃ additions can be controlled through the simultaneous addition of AlN. The formation of 2H-SiC : AlN solid solution grains inhibits the growth of the 6H-SiC grains since AlN(2H) will not go into solid solution in the SiC(6H) structure, effectively pinning the growth of the 6H-SiC grains.     

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from to -SiC reveals a dependency on the initial -content and the sintering atmosphere. The transformation rate decreases with an increasing -content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure -SiC starting powders when the additive system consists of 10.34 wt% Y2O3 and 2.95 wt% AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial /-ratio (>1:9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.     

Effective Łojasiewicz inequalities in semialgebraic geometry

The main result of this paper can be stated as follows: letV ⁿ be a compact semialgebraic set given by a boolean combination of inequalities involving only polynomials whose number and degrees are bounded by someD > 1. LetF, G[X₁,, X_n] be polynomials with degF, degG D inducing onV continuous semialgebraic functionsf, g:VR. Assume that the zeros off are contained in the zeros ofg. Then the following effective ojasiewicz inequality is true: there exists an universal constantc ₁ and a positive constantc ₂ (depending onV, f,g) such that for allxV. This result is generalized to arbitrary given compact semialgebraic setsV and arbitrary continuous functionsf,g:V . An effective global ojasiewicz inequality on the minimal distance of solutions of polynomial inequalities systems and an effective Finiteness Theorem (with admissible complexity bounds) for open and closed semialgebraic sets are derived.     

Influence of Saffman's lift force on the motion of a particle in a Couette layer

The article is concerned with the study of the effect of E. S. Asmolov's corrections to Saffman's lift force for the wall vicinity and a nonzero ratio of Reynolds numbers. It is shown in what way these corrections change the particle paths in a Couette layer and the conditions of deposition.Notation x=X/D, y=Y/D dimensionless longitudinal and transverse coordinates - u=U _p /U , =V _p /U dimensionless projections of particle velocity on the longitudinal and transverse axes - =tU /D dimensionless time - U ²/(18D) Stokes number - = _g / _p , coefficient of the gas kinematic viscosity - particle diameter - /D - _g , _p densities of the gas and particle material - du/d - dv/d - P _s Saffman's force - C coefficient in the formula for Saffman's force - yRe _d ^1/2 - A v _r Re _d ^1/2 - 3.08 - Re V _r / - Re _k ²/)U _g /Y - A Re/Re _k ^1/2 - Re _d U D/ - V _r ((U _g –U _p )²+V _p ² )^1/2 Indices g refers to gas parameters - p refers to the parameters of particles - 0 at the time momentt=0 - S Saffman's force - k Reynolds number based on the velocity gradient - based on velocity - r relative velocity - x projection on thex axis     

Gas pressure sintering of β-silicon nitride

The sintering behaviour of -Si₃N₄ powder was investigated in 980 kPa (10 atm) nitrogen at 1800–2000 °C. It is shown that -Si₃N₄ has a higher sinterability than the finer -Si₃N₄. The solution of small grains and reprecipitation on large grains occurred during sintering at >1600 °C. The rate-determining step in the liquid-phase sintering is believed to be the diffusion of material through the liquid phase at grain boundaries. There was no abnormal grain growth during gas pressure sintering of -Si₃N₄. The microstructures of gas pressure sintered materials from -Si₃N₄ were more uniform than those from -Si₃N₄. The densification mechanism of -Si₃N₄ is discussed in relation to that of -Si₃N₄.     

Anomalous Depression of the Debye-Waller Factor and Observations of the Laue Spots Only at Forward X-ray Diffraction in the Light Atom Crystals as Quantum Solid

The ratio of 6·(1/2m)·(E ₀/c)² of 12.9 meV at E ₀=4 keV to k _{B D} of 2.27 meV for hcp-⁴He in the exponent 2M=[6·(1/2m)·(E ₀/c)²/k _{B D} ]·(t)sin² _B of the Debye-Waller factor D=exp(–2M) becomes 5.7, because of the small atomic mass m and the low Debye temperature _D . Conversely that of the heavy atom crystals becomes smaller than 1. An experiment on hcp-⁴He as an example of a light atom crystal at low temperature limit of (t)1 reveals that not only the maximum value of D is significantly reduced below 0.249 (=e ^–1.395) but also the values of D for all other Laue spots except the following observed six spots are crowded into a range of D<0.05. Therefore, only the six Laue spots of at 11.2 keV, (0002) at 20.3 keV, and at 14.4 keV and and at 23.9 keV in a low angular range of _B10.22° were observed over low values of e ^–1.395>De ^–3. At high angle of _B 45°, the values of E ₀ for the above six Laue spots are assigned to the region below 4.07 keV from the Bragg condition under constant values of D. Therefore, detection of them over the higher angular range of _B 45° is not impossible but quite difficult at low temperature X-ray diffraction, considering X-ray absorption by multi-thermal shields of the refrigerator, the thick sample holder as a pressure vessel and massive air in space. Generally this means that the large reduction of the Debye-Waller factor of hydrogen and helium drastically encloses the diffraction intensity and the number of observable Laue spots within lower bounds. This is the reason why definite Laue spots in the light atom crystals of hcp-⁴He could be observed only by forward X-ray diffraction. Conversely the recoil fraction of the diffuse scattering expressed by 1–D concomitantly increases as a major part of the total scattering. A proposal to measure thermal diffuse scattering as well as pressure experiments will provide insight into the wave-like character of constituent atom in solid helium.     

In situ enhancement of toughness of SiC—TiB2 composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated -SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from -SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated -SiC grains, due to the phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated -SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.     

Microstructural evolution in nanocrystalline alumina containing silicate glasses

Pure nanocrystalline -alumina powders were coated with different fractions (5, 10, and 15 vol%) of SiO₂-SrO glass using the sol-gel technique. The isostatically cold pressed powders were pressureless sintered in air for 5 h in the temperature range of 1250°C to 1550°C. The relative densities were ranged between 60 to 90% of the theoretical and were composition dependent. The density was increased with the sintering temperature. In pure alumina, the to phase transformation went to completion by sintering at 1250°C. However, in the glass-coated samples, transition -alumina was mostly retained after sintering at the same temperature. Pure nanocrystalline alumina sintered at 1350°C exhibited vermicular structure with isolated pores. The microstructure of the low glass-containing samples exhibited nanocrystalline to submicron size grains arranged in platelet-shaped clusters. Samples with higher glass contents exhibited also micron-size needle-shape grains of strontium aluminate.     

Microstructure and fracture toughness of liquid-phase-sintered β-SiC containing β-SiC whiskers as seeds

The effect of seeding on microstructural development and fracture toughness of -SiC with an oxynitride glass was investigated by the use of morphologically rodlike -SiC whiskers. A self reinforced microstructure consisting of rodlike -SiC grains and equiaxed -SiC matrix grains was obtained by seeding 1–10 wt% SiC whiskers, owing to the epitaxial growth of -SiC from the seed whiskers. Further addition of seeds (20 wt%) or further annealing at higher temperatures led to a unimodal microstructure, owing to the impingement of growing seed grains. By seeding -SiC whiskers, fracture toughness of fine-grained materials was improved from 2.8 to 3.9–6.7 MPa · m^1/2, depending on the seed content.     

Microstructure of SiCp-reinforced A356 cast Al metal-matrix composite

The microstructure of SiCp-reinforced A356 cast Al metal-matrix composite (MMC) has been investigated by means of transmission electron microscopy and energy-dispersive X-ray analysis. It is found that the MMC contains -SiC, ( + ₂) SiC composite, eutectic Si, GP zones of Si sequence, a new J phase and the amorphous phase Al _x Si₉Ca₂. The ₂-SiC is lathlike with a triclinic crystal structure having a=0.308 nm, b=0.305 nm, c=1.262 nm, = 93.8 °, = 90.0 °, and = 60.0 °. The J phase is bulk-like with a C-face-centred orthorhombic crystal structure having a = 0.680 nm, b = 1.170 nm and c = 0.826 nm.     

Synthesis of aluminum infiltrated boron suboxide drag cutters and drill bits

Synthesis of boron suboxide (B₆O) was made by reactive sintering of crystalline boron and zinc oxide powders at 1450 °C, in argon, for 12 h. After sintering, Vickers microhardness testing was performed on the material synthesized and an average hardness value of 34 GPa was obtained. Sintered suboxide (in crushed and ground powder form) was then analyzed through optical and scanning electron microscopies and X-ray diffraction. Following the completion of the analyses, consolidation of the powder was performed. Two different routes were carried out: (1) explosive consolidation which was performed in double tube (with a density value of 2.22 g/cm³) and single tube (with a density value of 2.12 g/cm³) canister design arrangements and (2) hot pressing which was performed in a graphite die assembly, at 1600 °C, in vacuum, for 2 and 4 h (with density values of 2.15 and 2.18 g/cm³ respectively). Consolidated samples of both routes showed different levels of mechanical attachment, agglomeration, porosity, fracture toughness and fracture strength values, whereas microhardness values and X-ray diffraction plots (as shown in Table I and Figs 6 and 8 respectively) were determined to be similar. Following characterizations, compacts of both routes were then given a high temperature sintering treatment (pressureless sintering) at 1800 °C, in vacuum, for full densification. Both in the as consolidated and densification sintered stages test results revealed the most desirable and well-established properties for the explosively consolidated double tube design compacts (with densification sintered density, microhardness and fracture toughness values of 2.46 g/cm³, 38 GPa and 7.05 MPa m^1/2 respectively). Consolidation and desification sintering steps were then followed by a pressureless infiltration step. Aluminum was infiltrated into densification sintered double tube design consolidated and 4 h of pressed samples (better-compacted and better-sintered compacts) in the temperature range 1100–1250 °C, in argon, for 10 h. During infiltrations, the optimum temperature of the infiltration process was determined to be 1200 °C. Characterization results revealed the most uniform and well established properties once more for the double tube design explosively consolidated compact (with aluminum infiltrated density, microhardness and fracture toughness values of 2.55 g/cm³, 41 GPa and 8.70 MPa m^1/2 respectively).     

Characterization of pyrolysed silacyclobutasilazane

Silacyclobutasilazane (SCBZ) is a candidate organosilicon polymer suited to many hightemperature applications. Pyrolysis of SCBZ occurs over two distinct temperature ranges 400–800 and 1400–1800 °C. X-ray diffraction analysis showed that amorphous SCBZ transforms to crystalline SiC above 1400 °C. Thin foils of the pyrolysed (1800 °C) SCBZ were prepared and examined using the analytical electron microscope. The material was found to contain 200 nm-SiC particles scattered through a matrix consisting of 50 nm radiating clusters of pyrolytic graphite crystals and a highly carbonaceous amorphous background. It is suggested that during 1400–1800 °C pyrolysis two major reactions occurred. Initially, SCBZ through nitrogen loss, decomposed to crystalline SiC and amorphous carbon. Subsequent graphitization then produced the radiating crystal clusters.     

Microstructure stability of fine-grained silicon carbide ceramics during annealing

Fine-grained silicon carbide ceramics with an average grain size of 140 nm or smaller were prepared by low-temperature hot-pressing of very fine -SiC powders using Al₂O₃-Y₂O₃-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure.     

Microstructural studies on silicon nitride

Thin specimens of reaction sintered and hot pressed silicon nitride have been prepared by ion beam thinning and examined in the Harwell million volt microscope. It has been found that reaction sintered material consists of large grains, which are mostly-Si₃N₄, in a fine grained matrix of-Si₃N₄. Fibres are frequently observed within the pores, the type of fibre depending on the size of the pore. The hot pressed material consists largely of two types of grain, small angular grains of-Si₃N₄ and larger irregular grains. There is also some non-crystalline material between the angular grains and there are numerous small unidentified inclusions.The grains of-Si₃N₄ generally contain dislocations and examination of these shows that most have a 0001 Burgers vector. The remaining dislocations appear to be more complex, frequently occurring as multiple images, and have not been unambiguously identified. An analysis of dislocations in-Si₃N₄ shows that 0001 dislocations are the most stable and are also likely to be most mobile with {10¯10} as the primary slip plane.     

Wettability and interfacial energies in SiC-liquid metal systems

The sessile drop technique is used to measure the contact angles of molten Si, Sn, Cu and Ni in contact with mono- and polycrystalline -SiC as well as CVD -SiC in purified argon atmosphere and at various temperatures. The contact angle of silicon, near its melting point, is about 38° on a mono- as well as polycrystalline -SiC substrate and about 41.5° on -SiC. Tin does not wet the SiC. Using data from the available literature, the work of adhesion and the interfacial energy between SiC and Si or Sn were calculated. In the -SiC-Sn system, both quantities are linearly dependent on temperature in the investigated temperature range 523–1073 K. The metals copper and nickel react with silicon carbide. The silicon content of the copper drop depends on the annealing temperature. The nickel drop after cooling forms the compound Ni₃Si₂. The interferometric measured groove angle of SiC (thermal etching) in vacuum at 2020 K gives a mean value of 157.6±5.8°.