Estimation of Weibull parameters with linear regression method

Abstract

Based on minimum mean square error, a modified probability estimator is proposed by a Monte Carlo simulation for estimating the Weibull parameters with the linear regression method. It is shown that compared with the commonly used estimators, the modified probability estimator gives a more accurate estimation of the Weibull modulus and the same estimation precision of the scale parameter. Furthermore, it is more conservative than the commonly used estimator recommended by previous authors and hence results in a higher safety in reliability predictions.     

Macrostructural changes of polymer replicated open cell cordierite based foams upon sintering

Abstract

The goal of this work was to clarify the macrostructural changes that take place upon sintering of open cell cordierite based foams. A methodology, based on optical image analysis, was developed to assess the structure of open-cell foams, which allowed evaluating the macrostructure of both cordierite based foams obtained by the replication process and their polymeric templates. The parameters used to describe the structures were the size of the cell and the window, the window shape factor, the strut thickness and the volume fraction of the material. The experimental evidence gathered opened the way to understand the physical/chemical transformations involved in the polymer burnout and the ceramic sintering processes, as well as their influence on the ceramic final structure. The observed trends provide guidance for tailoring 'replicated' cordierite based foams, in view of the required application.     

Fast sinter crystallisation of waste glasses

Abstract

Two glasses, belonging to the CaO–Al₂O₃–SiO₂ system and corresponding to the melting of mixtures of industrial wastes (recycled glasses, mining residues, ashes, asbestos containing cements, etc.), have been successfully converted into dense glass ceramics by sintering with concurrent crystallisation. The usage of fine glass powders (<37 μm) allowed very short sintering treatments, due to the enhanced nucleating activity of glass surfaces. In particular, dense glass ceramics could be produced by direct insertion of pressed glass powders in the furnace at the sintering temperature, followed by rapid cooling at room temperature after a 30 min holding time. The proposed approach evidences the feasibility of sintered glass ceramics by the fast and economic processes employed for traditional ceramics, with the advantage of superior mechanical properties (bending strength exceeding 100 MPa, Vickers' microhardness exceeding 6 GPa). Like in traditional ceramics, clay and water could be used for the shaping of pressed tiles, thus posing the conditions for massive industrial production.     

Fracture toughness testing for advanced technical ceramics: internationally agreed good practice

Abstract

The development of robust methods for determining fracture toughness and for interlaboratory assessment of results is discussed with respect to procedural standardisation. Acceptable methods are based on meeting several criteria, including ease and cost-effectiveness of testpiece manufacture, the method of introducing a sharp precrack and the acceptability of appropriate fracture toughness analysis. The four simplest methods meeting these criteria are reviewed in detail, together with information on their reproducibility gathered during international round robin exercises. Not all these methods are effective on all materials, and users need to be aware of their limitations. Details of calibration equations and some experimental nuances are given.     

Effect of talc and bauxite on sintering,microstructure, and refractory properties of Egyptian dolomitic magnesite

Abstract

Refractories produced from mixes of Egyptian dolomitic magnesite with talc and low cost Chinese bauxite have been assessed as potential substitutes for the high temperature basic refractory bricks currently used in steelmaking and other industries. Fifteen batches of different compositions were prepared by firing for 2 h up to 1580° C. Six samples were selected for further investigation on the basis of their phase composition and densification parameters. The mineralogical composition and microstructure, pore size distribution, and mechanical and refractory properties of these samples were investigated. All six samples consisted mainly of MgO and MA spinel refractory phases, plus some calcium silicates and aluminoferrite phases. The latter phases contribute to densification during firing by promoting liquid phase formation. Most samples showed high refractoriness under load, good spalling resistance, better mechanical properties than current refractories, and compact microstructure. It is concluded that these new refractories are good candidate materials for the production of shaped linings for industrial kilns and furnaces.     

Properties and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Reaction between titanium dioxide and carbon in flowing helium stream

Abstract

The reaction between titanium dioxide and carbon in a flowing helium stream was studied by scanning electron microscopy and X-ray diffraction. Experimental results indicate that the order of phase transformations during the reaction was TiO₂(rutile) → Ti₄O₇ → Ti₃O₅ → Ti₂O₃ → TiO → TiC. A mechanism is proposed to explain the overall reaction. The reaction rate was found to increase with increasing sample thickness, reaction temperature, and initial bulk density and with decreasing helium flowrate, molar ratio of TiO₂/C, and grain size of titanium dioxide.     

Influence of nano–micrometre powder blends on microstructure and mechanical properties of silicon nitride

Abstract

A well known route to making tough silicon nitride compositions is to control the grain size and aspect ratio distributions. This is usually done by choosing the appropriate powder characteristics, sintering conditions, as well as sintering additives. The effect of hot pressing a blend of nano and micrometre scale silicon nitride powder is explored here. Microstructures and mechanical properties are determined for these hot pressed ceramics and are compared with a reference silicon nitride. Hardness and fracture toughness are determined at room temperature using hardness indents produced by a macro Vickers hardness indenter. Grain size and aspect ratio distributions and their impact on mechanical properties are presented. Blending of nano and micrometre scale powder is shown to result in a refined microstructure with an increase in the area/volume fraction of finer grains. Rising R curves are established for these ceramics demonstrating toughening behaviour. Crack bridging and crack path deviation are identified as possible toughening mechanisms.     

Crystallisation and characterisation of dielectric glass ceramics in Li2O–Al2O3–SiO2 system

Abstract

Glass ceramics in the Li₂O–Al₂O₃–SiO₂ system have been synthesised to produce bulk materials grown in a glass phase via quenching followed by controlled crystallisation. The crystallisation and microstructure of Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramic with nucleating agents (B₂O₃ and/or P₂O₅) are investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy and the effects of B₂O₃ and P₂O₅ on the crystallisation of LAS glass are also analysed. The introduction of both B₂O₃ and P₂O₅ promotes the crystallisation of LAS glass by decreasing the crystallisation temperature and adjusting the crystallisation kinetic parameters, allows a direct formation of β spodumene phase and as a result, increases the crystallinity of the LAS glass ceramic. Microstructural observations show that the randomly oriented, nanometre sized crystalline is found with residual glass concentrated at crystallite boundaries. Furthermore, it is interesting that codoping of B₂O₃ and P₂O₅ creates not much effect on the crystallisation temperature. The dielectric properties of the glass–ceramics formed through controlled crystallisation have a strong dependence on the phases that are developed during heat treatment. The dielectric constant is continuously increased and the dielectric loss is decreased with addition of additives where mobile alkali metal ions (e.g. Li⁺) are incorporated in a crystal phase and minimise the residual glass phase.     

Bioactivity investigation of glass and glass ceramics in Li2O–SiO2–CaO–P2O5–CaF2 system

Abstract

Glass samples in Li₂O–SiO₂–CaO–P₂O₅–CaF₂ system with different contents of P₂O₅, CaO and CaF₂ in relative ratios responded to fluoroapatite (FA) composition (referred to P₂O₅ addition) have been prepared and heat treated at 550 and 750°C to obtain glass ceramics. Bioactivity of all samples has been proved in vitro by the presence of new layer of apatite-like phases formed after soaking in simulated body fluid (SBF). The development and the apatitic character of created layers have been demonstrated by Fourier transform infrared analysis. Scanning electron microscopy and electron probe microanalysis have demonstrated that the density and the thickness of new layer depend on P₂O₅ content, crystallisation temperature and immersion time. The bioactivity has been enhanced by P₂O₅ addition as well in the case of the base glasses as in the case of glass ceramics. The additional heat treatment appeared to inhibit the bioactive behaviour, though the longer SBF acting leads to the additional formation of apatite-like layer. The mechanical properties, expressed as Vicker hardness, have been found higher and increasing with P₂O₅ in glass ceramics treated at 750°C comparatively with base glass samples and the highest value of 7˙37 GPa has been achieved by 14 wt-%P₂O₅ addition. The same content of P₂O₅ in glass ceramics heat treated at 550°C resulted in a decrease in hardness to a minimum value from all samples. The increase and decrease in hardness responded to development and suppression of crystallisation respectively. The inhibition of crystallisation has been affected by the presence of 'amorphous' FA according to X-ray diffraction and differential thermal analysis results.