Comparative study for physical properties and corrosion mechanism of synthetic and in situ MgAl2O4 spinel formation zero cement refractory castables

Abstract

Magnesium aluminate spinel (MgAl₂O₄) is an excellent castable refractory product due to its high temperature thermal, chemical and mechanical properties. Alumina spinel castables are produced by addition of synthetic spinel or in situ spinel formation during the firing process. In the first part of the experimental studies, alumina rich MgAl₂O₄ spinel castable was produced using a solid state reaction technique. Tabular alumina and sea water magnesia (<100 μm) were used as starting raw materials. In the second part of the experimental studies, commercial synthetic spinel added castables were produced. In order to compare experimental results, both parts of the experimental study involved compositions with the same proportions of MgO. α-500 hydratable alumina was used as binder. Castables were sintered at 1500 and 1600°C. Water absorption, apparent porosity, bulk density and cold crushing strength values were considered and the optimum sintering temperature, proportions of synthetic spinel and sea water magnesia were determined. The XRD patterns confirm the phase formation of MgAl₂O₄. Moreover, the physical properties of the castables were supported by this XRD analysis. Scanning electron microscopy investigations of the fired samples were carried out to compare the effect of synthetic spinel addition and in situ phase formation on the physical properties of the castables. The mechanism of slag penetration to two types of zero cement castables for steel ladles was examined and the penetration layer chemically analysed by energy dispersive X-ray analysis studies.     

Production of MgAl2O4 – Ti(C,N) composite ceramics by aluminothermic reduction in reducing atmosphere

Abstract

In the presence of N₂ and CO gases, MgAl₂O₄ – Ti(C,N) composite has been synthesised by aluminothermic reduction. The phase characterisation and microstructure of this novel composite were investigated by powder X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that the nitridation reaction begins at 1100°C. With an increase of temperature, TiC starts to appear and forms Ti(C,N) solid solution with TiN; the grain size of Ti(C,N) grows with increasing temperature and the final product is MgAl₂O₄ – Ti(C,N).     

Comparative study of three different types of dental diamond burs

Abstract

Wear mechanisms of three different types of dental burs were studied by means of cutting experiments performed on machinable glass ceramic using a laboratory system designed for this purpose. The dental handpiece used for this research was subjected to a constant feed rate in order to better simulate the actual working conditions of a dental bur. The new and the worn-out burs were studied by optical and scanning electron microscopy. Diamond particle wear-out was found to be the dominant wear mechanism in all cases; a quantitative analysis was performed on the optical micrographs of the new and worn burs. In situ force measurements showed that the forces exerted by the bur increase with the blunting process in order to keep the required feed rates; each bur type seems to have a different characteristic curve of force versus the number of cuts.     

Examination of HA230 turbine transition duct

Abstract

The present paper reports the results of an examination of a service run transition duct from one of Siemens' industrial gas turbines and the correlation between the findings for this, and those from the studies undertaken on laboratory exposed Haynes alloy 230 material. The studies have shown that there are changes in the microstructure and hardness of the material that can be related to temperature and time of exposure, and the information gained can lead to estimations of the average component operating temperature. Metallographic studies revealed a high level of heterogeneity of the grains in this transition duct.     

Thermal characteristics of tubular single lap adhesive joints under axial loads

When an adhesive joint is exposed to high environmental temperature, the tensile load capability of the adhesive joint decreases because both the elastic modulus and failure strength of the adhesive decrease. The thermo-mechanical properties of a structural adhesive can be improved by addition of fillers to the adhesive. In this paper, the elastic modulus and failure strength of adhesives as well as the tensile load capability of tubular single lap adhesive joints were experimentally and theoretically investigated with respect to the volume fraction of filler (alumina) and the environmental temperature. Also the tensile modulus of the filler containing epoxy adhesive was predicted using a new equation which considers filler shape, filler content, and environmental temperature. The tensile load capability of the adhesive joint was predicted by using the effective strain obtained from the finite element analysis and a new failure model, from which the relation between the bond length and the crack length was developed with respect to the volume fraction of filler.     

The effect of silane hydrolysis on aluminum oxide dispersion stability in ceramics processing

The controlled hydrolysis of organosilanes has been shown to greatly influence both the surface adsorption of silanes on alumina powder, and the resultant dispersion stability of alumina in organic solvent slurries. Plateau adsorption concentrations from settling experiments show that complete surface coverage for hydrolyzed forms of both n-octyltrimethoxysilane (NOS) and N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) is typically achieved at concentrations of approximately 6 μmol/m². Moreover, the settling densities of dispersions prepared with hydrolyzed silanes are consistently higher than densities achieved with monomeric silanes alone (as seen in case studies involving NOS in toluene, and AAPS in isopropyl alcohol). Similarly, ceramic slips prepared with polystyrene and alumina, or with poly(isobutyl methacrylate) and alumina also lead to ceramic green bodies with increased densities when the slips are prepared with hydrolyzed silanes. In addition, solid state NMR and dynamic mechanical analyses of resultant green bodies reveal that the molecular motional behavior of these polymers is strongly influenced by the presence of hydrolyzed silanes. These results collectively add to a growing body of evidence which supports the idea that not only is hydrolysis required for silanes to produce controllable and predictable effects in many industrial processes, but hydrolysis must be made to occur at the 'right time' within any sequence of steps that define an industrial process. In the case of ceramic slurries, the 'right time' is the period just prior to the mixing of the ceramic slip ingredients.     

Effect of powder size on microstructure and mechanical properties of Ti(C,N) based cermets

Abstract

Effect of the particle size of TiC and TiN on the microstructure and mechanical properties of Ti(C,N) based cermets has been evaluated. Ti(C,N)–WC–Co cermets made from four groups of mixed raw powders of different sizes were manufactured by vacuum sintering. The microstructure and composition were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDX). The result shows that the four samples have the typical microstructures of 'black core/grey rim'. The mechanical properties of the cermet manufactured from submicron TiC and nano TiN are the best among the four samples.     

Stress corrosion cracking behaviour of 2091 Al—Li alloy in sulphate and chloride containing solutions

Abstract

The stress corrosion cracking resistance of 2091 Al—Li alloy in underaged, and in peak aged condition was investigated in chloride solutions with or without sulphate addition using both the static load technique and the slow strain rate technique. It is shown that the underaged material is more resistant than peak aged material. Sulphate additions to chloride solutions increase the stress corrosion cracking susceptibility. Metallographic cross-section observations show the simultaneous occurrence of other kinds of corrosion: generalised. corrosion, pitting corrosion, intergranular corrosion, and exfoliation corrosion. It appears that stress corrosion cracking susceptibility increases as the extent of intergranular corrosion decreases.     

Effect of laser fluence on surface microstructure of alumina ceramic

Abstract

The present study discusses the microstructure development during surface modifications of alumina ceramic using high power continuous wave Nd:YAG laser. Laser fluence influenced the microstructure in terms of changes in morphology and (1 1 0) crystallographic texture of the surface grains. The microstructural observations can be used to establish the guidelines for optimising the laser fluence to achieve the desired morphology of the surface grains and extent of texture in the surface modified alumina ceramic.     

Utilisation of South Libyan clay for preparation of mullite-alumina based refractory ceramics

Abstract

This work aims to benefit from the wide spreaded clay in south Libya with some alumina (Al₂O₃) additions to prepare and characterise mullite-alumina containing refractory ceramics. The starting materials used in this investigation are local raw clay and chemical aluminium oxide (Al₂O₃). Based on a previous geological survey carried out by the industrial research centre (Tripoli), four different clay samples were selected from different locations of the south of Libya. Representative samples of the four selected clays were investigated using X-ray diffraction (XRD) and differential thermal analysis. The four clay samples were used separately together with aluminium oxide to prepare six different batches containing different proportions of alumina from 0 up to 60% with an increament of 10%. The mullite containing bodies were investigated through their mineralogical compositions using XRD and scanning electron microscopy (SEM) attached with energy dispersive X-ray unit (EDX). The sintering, mechanical and refractory properties of the prepared mullite containing bodies were tested according to the international standard specifications. It is concluded that the densification, mechanical and refractory properties are improved as the alumina content increases on the expense of clay. Mixes containing 60% clay : 40% alumina and 50% clay : 50% alumina respectively, especially those prepared from Agar clay are characterised with good sintering, high mechanical and high refractory properties.