Vladimir D. Krstic

Pressureless sintering of β-SiC with Al2O3 additions

Abstractβ-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 α-SiC grains (maximum length ≈ 10 μm and width 2–3 μm) in a matrix of fine equi-axed grains (2–3 μm) and plate-like grains (2–5μm). 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 β-phase was transformed to the α-phase.

Silicon nitride: the engineering material of the future

The purpose of this review is to present the recent developments in silicon nitride (Si3N4) ceramics and to examine the achievements regarding our understanding of the relationship between processing conditions, chemical composition, microstructure and mechanical properties of Si3N4. Si3N4 is one of the most important structural ceramics because it possesses a combination of advanced properties such as good wear and corrosive resistance, high flexural strength, good fracture resistance, good creep resistance and relatively high hardness. These properties are obtained through the processing method involving liquid phase sintering in which a tailored microstructure, with high aspect ratio grains and chemistry of intergranular phase, triggers the toughening and strengthening mechanisms leading to the development of high fracture toughness and fracture strength. However, despite high fracture toughness and strength, Si3N4 ceramic materials still break catastrophically, and the fracture behaviour of this ceramic is considered to be the major obstacle for its wider use as a structural material. In addition to the macrostructure–mechanical properties relationship, this paper also reviews new designs involving laminates possessing no plane of weakness and some theoretical developments involving crack opening displacement. Proposals of how to improve the fracture resistance were also discussed.

Strength and machining of gelcast SIC ceramics

The aqueous gelcasting technique was employed to form silicon carbide green samples for mechanical strength measurements and machining tests. The monomers used in this work were acrylamide (AM) and methylenebisacrylamide (MBAM). Polymerisation reaction was promoted by the addition of catalyst (N, N, N′, N′-tetramethyl ethylenediamine) and initiator (ammonium persulfate). Characterisation of the green, dried gelcast samples included measurements of density and bend strength (flexural strength). It was found that the bend strength of dry, green, gelcast samples depends critically on the slurry solid loading, the amount of monomers and the ratio of monomers contents (AM/MBAM). The highest strength value of 29 MPa was found with samples produced from the slurry with 25 vol% solid loading, the monomers content of ∼10 wt% and the ratio AM/MBAM of ∼16. An increase in solid loading from 25 to 35 vol% caused a decrease in the green sample strength to 20 MPa. Successful machining was achieved with all samples having bend strengths of over ∼6 MPa using standard machining equipment. In one set of samples, machining was done with cemented carbide (W-Co) tools and in the other set with ceramic cutting tools (Al2O3-TiC) using the same cutting speeds, feed rate and depths of cut. Ceramic cutting tools exhibited approximately three times longer life than the cemented carbide tools. The effect of solid loading and the concentration of monomers on the sintered density were also investigated.

High toughness silicon carbide/graphite laminar composite by slip casting

A slip casting method has been developed to manufacture high toughness laminated SiC ceramics. The samples are produced by slip-casting alternate layers of SiC and graphite of various thickness ranging from 120 μm to 600 μm. After casting, the samples were dried and then pressureless sintered to high density. The graphite layer serves to deflect the crack and raise the apparent fracture toughness from 4 MPa · m12 to 14 MPa · m12. A strong effect of SiC and graphite layer thickness on apparent fracture toughness and flexural strength was observed.

Grain-size dependence of fracture stress in anisotropic brittle solids

The stress concentrations that occur at grain boundaries due to thermal expansion anisotropy and elastic stress concentration are discussed, and the stress intensity factor that results from these stresses is estimated. The procedure for the stress intensity factor calculation is based on the model in which a spherical crystal (grain) is forced into a cavity of equal size possessing annular or radial cracks emanating from the boundary. The stress intensity factor equation thus obtained is extended to include the effect of elastic stress concentration due to the presence of a cavity, and is subsequently used to predict the grain-size dependence of strength in anisotropic brittle ceramics. In assessing the degradation of strength with increasing grain size in non-cubic ceramics, it is shown that, in addition to grain size, the effect of pre-existing crack size must also be considered. Cubic ceramics, on the other hand, are known to exhibit no thermal expansion anisotropy and, based on the present model, their strength is predicted to be governed by the pre-existing flaw size, rather than the grain size.

Roles of porosity, residual stresses and grain size in the fracture of brittle solids

Abstract The effects of porosity, grain size and residual stresses on the thermal shock resistance and the strength of brittle solids are treated employing the stress intensity factor approach. Maximum resistance to thermal shock is found for materials possessing small pores and relatively large pre-existing cracks. The theory shows that it should be possible to synthesize a brittle material with high strength without the need to reduce the grain size significantly, provided that the grain size is considerably smaller than the flaw size. Excellent agreement was obtained between predicted and experimental values for strength.

Role of residual stress field interaction in strengthening of particulate-reinforced composites

A quantitative analysis was conducted on the effect of residual thermoelastic stress concentrations on the strength of particle-reinforced brittle matrix systems. The analysis is derived from the stress intensity factor for a periodic array of coplanar cracks emanating from the matrix-particle interface. It is shown that the major drop in strength occurs at smaller volume fractions of second phase where the residual stress field interaction effects are minimal. The effect of volume fraction on strength becomes important at larger volume fractions (normally above 10–15%). The theory is compared with experimental measurements of strength for glass and alumina matrix composites as a function of the particle volume fraction, its size, and thermal mismatch Δα.

Effect of porosity on the elastic response of brittle materials: An embedded-atom method approach

Abstract The values for Youngs modulus of porous single-crystal Ni in the [100] and [111] directions are computed using the embedded-atom method (EAM). Both pore volume fraction and pore size (defined by ratio S/R of the flaw size to pore radius) are varied. A reduction in Youngs modulus with increasing pore volume fraction and with increasing S/R ratio is observed in the EAM simulations, in good agreement with a recent theoretical model proposed by Krstic and Erickson. A porous Σ = 5, [100] grain boundary also demonstrates a marked reduction in Youngs modulus compared with the pore-free Σ = 5, [100] grain boundary. These results suggest that recent literature values demonstrating greatly reduced Youngs modulus for some nanocrystalline materials (compared with conventional polycrystalline materials) may be a consequence of residual porosity in the material. Poissons ratio is calculated for aligned pores with stress applied in the [100] direction. The crack-opening displacement is qualitatively and qu...

Reaction-inhibition during sintering of sic with Al2O3 additions

Abstract The mechanisms and products of the reactions which occur during the sintering of SiC with Al 2 O 3 as a sintering aid were analysed. The effects of two sintering atmospheres, namely pressurised CO and Ar gases, on the reaction thermodynamics and reaction products were investigated. Thermodynamically predicted reaction products were correlated with experimentally determined reaction products. The observed weight losses and the final sintered densities are explained on the basis of the proposed reaction mechanisms. The major finding of the present analysis is that a pressurised CO gas atmosphere greatly enhances the densification characteristics of SiC ceramics.

Porosity dependence of strength in brittle solids

Abstract The effect of pore size and pore volume fraction on strength in brittle solids is evaluated. The analysis considers that the strength degradaton of a solid containing a large number of spherical pores is due to a strong effect of porosity on Youngs modulus. Each pore is assumed to possess radial or annular flaws emanating from the pore surface whose lengths are considered to be independent of pore size. The effect of stress concentration induced by the presence of the pore is included in the equation for strength through the Youngs modulus dependence of porosity originally developed using the concept of crack opening displacement. It is shown that the strength of a solid containing spherical pores is controlled by the pore size, pore volume fracton and the radial (or annular) crack size to pore size ratio. Predicted variation of strength with pore volume fraction is tested against experimental data for glass and polycrystalline alumina.