Susmit Datta

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.

Microstructure and corrosion behavior of laser processed NiTi alloy

Laser Engineered Net Shaping (LENS™), a commercially available additive manufacturing technology, has been used to fabricate dense equiatomic NiTi alloy components. The primary aim of this work is to study the effect of laser power and scan speed on microstructure, phase constituents, hardness and corrosion behavior of laser processed NiTi alloy. The results showed retention of large amount of high-temperature austenite phase at room temperature due to high cooling rates associated with laser processing. The high amount of austenite in these samples increased the hardness. The grain size and corrosion resistance were found to increase with laser power. The surface energy of NiTi alloy, calculated using contact angles, decreased from 61 mN/m to 56 mN/m with increase in laser energy density from 20 J/mm(2) to 80 J/mm(2). The decrease in surface energy shifted the corrosion potentials to nobler direction and decreased the corrosion current. Under present experimental conditions the laser power found to have strong influence on microstructure, phase constituents and corrosion resistance of NiTi alloy.

Nano- and micro-tribological behaviours of plasma nitrided Ti6Al4V alloys

Plasma nitriding of the Ti-6Al-4V alloy (TA) sample was carried out in a plasma reactor with a hot wall vacuum chamber. For ease of comparison these plasma nitrided samples were termed as TAPN. The TA and TAPN samples were characterized by XRD, Optical microscopy, FESEM, TEM, EDX, AFM, nanoindentation, micro scratch, nanotribology, sliding wear resistance evaluation and in vitro cytotoxicity evaluation techniques. The experimental results confirmed that the nanohardness, Youngs modulus, micro scratch wear resistance, nanowear resistance, sliding wear resistance of the TAPN samples were much better than those of the TA samples. Further, when the data are normalized with respect to those of the TA alloy, the TAPN sample showed cell viability about 11% higher than that of the TA alloy used in the present work. This happened due to the formation of a surface hardened embedded nitrided metallic alloy layer zone (ENMALZ) having a finer microstructure characterized by presence of hard ceramic Ti2N, TiN etc. phases in the TAPN samples, which could find enhanced application as a bioimplant material.

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.

High Temperature Failure Mechanisms of Sintered Silicon Nitride

A clear understanding of high temperature failure mechanisms of engineering ceramics is essential for their successful use as hot zone components in the turbine engine and for related structural applications. As an engineering ceramic the sintered silicon nitride (SSN) has shown tremendous potential for commercial exploitation in terms of thermomechanical properties, lower production cost, easy machinability, and fabricability to close tolerance of complex shapes. The various oxide additives used in sintering contribute to formation of a silicate rich liquid phase that aids densification, but remains on cooling from sintering temperature as an amorphous phase at the grain boundary of SSN 1–4. Softening of the amorphous phase at high temperature causes strength degradation1–5. In contrast the use of a nitrogen rich liquid phase has recently led to superior thermomechanical behaviour of SSN at high temperatures 6–8. In this paper we report the high temperature failure mechanisms of these SSN ceramics.In most of the previous works 1–5 emphasis have been focussed on the study of temperature dependence of flexural strength and the interdependence of flexural strength and fracture toughness have been only cursorily explored 7.8. The major aim of present work has therefore been directed to evaluate the high temperature strength, fracture toughness and creep simultaneously, for SSN ceramics of given compositions with nitrogen rich grain boundary phase.For the purpose of comparison only similar tests were conducted also on SSN synthesised with a conventional silicate-rich liquid phase. Following these tests,extensive use of both scanning and transmission electron microscopy were made to elucidate the failure mechanims at ambient as well as at high temperatures of SSN.