D Chakraborty

On the microhardness of silicon nitride and sialon ceramics

Influences of relative density, grain size, Youngs modulus, flexural strength and fracture toughness on microhardness characteristics of hot-pressed silicon nitride, sintered silicon nitride, reaction-sintered sialon and liquid-phase sintered sialon have been discussed. Three new semi-empirical equations have been proposed to correlate microhardness to relative density. Indentation size effects on microhardness measurement have also been discussed.

Fracture toughness of structural ceramics

Abstract A comparative study of fracture toughness evaluation at room temperature of three different structural ceramics viz. sintered alumina, silicon carbide and silicon nitride is reported. Four methods of fracture toughness evaluation such as the single edge notched beam (SENB) technique, chevron notched beam (CNB) technique, indentation fracture (IF) technique and fractographic methods (FM) were compared. In addition, for a given method, the influence of several experimental parameters, e.g. blade width, notch tip radius, normalised notch length and the loading rate on the measured value of fracture toughness was investigated in the cases of the aforesaid materials.

Hardness of silicon nitride and sialon

Abstract Hardness characteristics of hot pressed silicon nitride, sintered silicon nitride, reaction sintered sialon and liquid phase sintered sialon have been discussed in terms of porosity, grain size and mechanical properties, viz. Youngs modulus, flexural strength and fracture toughness. Hardness decreases with an increase in volume fraction open porosity and grain size, and in most of the materials hardness increases with an increase in the aforesaid mechanical properties. The statistical correlation coefficient is −0·91 between porosity and hardness, and almost equally high between mechanical properties and hardness of most of the materials examined. Among four proposed empirical hardness-porosity relationships, the equation H = H 0 (1 − f 1 P + f 2 P 2 ), where H is hardness, P is volume fraction open porosity, and H 0 , f 1 and f 2 are empirical parameters, represents the data of the present work best.

Scratch deformation behaviour of alumina under a sharp indenter

Abstract To understand the roles of grain size and a small amount of glassy phase in the abrasive wear mechanisms of brittle ceramics, controlled single-pass scratch experiments were conducted with a Vickers diamond indenter on a range of pure, polycrystalline aluminas of varying grain size (1–25 μm) as well as on a commercial, 10 vol% glass containing alumina (AD90). These tests were made at a constant speed of 0.36 m min −1 under a wide range of normal loads, P (≅ 0.05–20 N). The tangential frictional force, width and depth (and hence wear volume) of the groove all increased with the normal load. The wear volume of 1 μm alumina was about an order of magnitude smaller than those of 5–25 μm grain size pure, polycrystalline aluminas. However, at P > 10 N the wear volume of AD90 was the smallest of all aluminas examined. These observations of a strong grain size effect and considerable influence of glassy phase in the abrasive wear behaviour of alumina are discussed in terms of residual stress intensity factor and scanning electron microscopic evidence of a wear mechanism controlled predominantly by grain boundary microfracture.

High temperature Young's modulus of reaction-bonded Si3N4, liquid phase sintered Si3N4 and sialon

Temperature dependence (RT-1400 C) of Youngs modulus and strength of reaction-bonded silicon nitride (RBSN), liquid phase sintered silicon nitride (LPSSN) and Sialon has been investigated. Although strength and Youngs modulus are related through the equation σ = 7.39 × 10−6E1.41 the influence of temperature on strength and Youngs modulus is different. For RBSN strength increases with temperature, reaches a maximum at 1200 C and then decreases. But Youngs modulus steadily decreases with temperature. For LPSSN and Sialon both Youngs modulus and strength degrade with temperature. Details of Youngs modulus data have been presented and their implication in assessing temperature dependence of strength discussed.

Creep of sintered silicon nitride

The creep behaviour of silicon nitride with three different sintering aids has been studied. In addition to creep rates, the stress exponents and activation energies have been determined. The creep resistance has been related to the strength, fracture toughness and grain character (size and aspect ratio), as well as the liquid-phase content and its high-temperature refractoriness. The mechanisms of creep in the silicon nitrides are discussed.

High temperature strength and fractography of sintered silicon-nitride

Abstract Influences of the sintering liquid system, temperature, microstructure and post sintering heat treatment of high temperature (30–1250°C) strength, Youngs modulus and fracture toughness of sintered silicon nitride (SSN) have been studied. Based on quantitative fractography, typical fracture origin statistics has been presented for SSN. The measured strength of the SSN is in good agreement with the fractographically predicted strength.

Influence of some parameters on the strength and fracture toughness of reaction-bonded silicon nitride composites

Strength, Youngs modulus and fracture toughness data in the temperature range 30–1400°C are presented for reaction-bonded silicon nitride and its particulate composites with SiC (5–20 vol.%), BN (10 vol.%) and TiC (5 vol.%). The composites of Si3N4 with SiC showed the best mechanical properties of the three types of composites. The toughening mechanism and interdependence of various mechanical properties for these composites are discussed.

Influence of some Factors on Strength and Fracture Toughness of Reaction Bonded Silicon Nitride

Influence of density, Youngs modulus, temperature and other experimental variables on strength and fracture toughness of reaction bonded silicon nitride (RBSN) was studied. Fractographic study was made to identify the nature of flaws. The density dependence of strength could be expressed through proposed empirical equations. Strength of RBSN was proportional to Youngs modulus. In the range RT-1400°C some RBSN had constant strength, while others had a peak in strength at 1200°C followed by degradation at 1400°C. The density dependence of fracture toughness could be expressed through some equations. Fracture toughness of RBSN had direct dependence on strength and Youngs modulus. High temperature fracture toughness and strength were directly dependent on each other. Fracture toughness of RBSN at RT increased as notch tip radius was increased in the range 79–225 μm, but it did not show any systematic variation as the normalized notch length was varied.

Porosity dependence of strain of Si3N4 ceramics

Etude des relations deformation-porosite pour des ceramiques de Si 3 N 4 frittees, frittees avec reaction et pressees a chaud