Metal shaping processes are clear examples of engineering applications where a hard material is worn by a softer one - i.e. the tool and workpiece respectively. The soft impressor technique, introduced by Brookes and Green (1973), has proved valuable in measuring the relevant mechanical properties of tool materials - e.g. the measurement of the flow stress of diamond single crystals at temperatures up to 1500°C (Brookes, 1992). In this work, the technique has been extended further in order to form a basis for the comparison and evaluation of ultra-hard materials. Three main aspects of the performance of these tool materials have been covered: the effect of temperature on flow stress; cumulative deformation under point loading conditions; wear due to repeated traversals (fatigue). In the first part, the technique has been extended to determine the flow stress of polycrystalline diamond and cubic boron nitride as a function of temperature and a mathematical model has been proposed to estimate the flow stress in isotropic polycrystalline materials. This model was first analysed by Love (1928) and was used as the basis on which to identify the threshold pressure above which dislocation movement is initiated in diamond single crystals (Brookes et al (1990)). The applicability of this model for polycrystals was verified by correlating the yield strength of polycrystalline copper, measured in tension, with the determination of minimum contact mean pressure to plastically deform the same material. According to the model, the first evidence of plastic deformation should be observed at the contact periphery and this has been verified in this work. Consequently, using this approach, the effect of temperature on the flow stress of polycrystalline diamond (Syndax) and polycrystalline cubic boron nitride (Amborite) has been established and it is shown that there are three distinct regimes. In regime I, the deformation is brittle and fracture occurs above a given mean pressure; in regime II dislocations are mobile and the flow stress decreases sharply as the temperature rises; and in regime III the flow stress is independent of the temperature. In the earlier work, the brittle-ductile transition temperature (BOT) has been identified as that temperature where regime I ends and II begins. Above the BDT, time dependent plastic flow has been observed, in all of these materials, leading to a measurable increase in the size of the impression. However, this particular type of cumulative deformation, described as impression creep, is shown to be different to conventional creep as measured under uniaxial stress conditions. Finally, the room temperature friction and deformation of various polycrystalline diamond based specimens, Le. aggregates with a binder phase of cobalt (Syndite) or silicon carbide (Syndax), a polycrystalline coating produced by a chemical vapour deposition processes (CVDite) and cubic boron nitride (Amborite) were studied when softer metallic and ceramic sliders were used. As a result of increasing the number of traversals, significant wear of the CVDite diamond coating by softer metallic sliders (aluminium and mild steel) was observed. This could be attributed to the high level of residual stresses in the diamond layer which is thought to be due to the difference in the thermal expansion coefficients of the coatings and their substrates. Burton et al (1995) reported a strain of 0.3% on the surface of the diamond coating and hence the tensile stress on the upper side of the coating will be equivalent to about 3.0 GPa. This value is comparable to the theoretical cleavage strength of diamond. It is suggested an additional tensile stress, due to the sliding friction, could have caused cleavage of individual diamond crystals. The resultant wear debris then becoming embedded in the metallic slider. These embedded diamond particles in the tip of the slider could be responsible for the increased friction and wear.

No description available.

An in-depth study of the biodeterioration of the Hulett Aluminium hot roll mill emulsion, Prosol, was conducted. Samples of the emulsion in use at the hot roll mill were taken from various areas of the emulsion reticulation system in order to identify regions of highest microbial contamination. Standard plate count techniques and diagnostic procedures were employed to quantify and identify the microorganisms in these samples. In some of the highly contaminated areas of the emulsion reticulation system, microorganisms exceeded lxlO'CFUml'1 emulsion. A range of bacteria was identified which included members of the genera: Bacillus; Pseudomonas; Escherichia; Enterobacter; Sporosarcina; Micrococcus; Aeromonas; Chromobacterium and Desulfovibrio. Various fungi, including several yeasts, were also isolated and some of the filamentous spore-forming types were identified zsAspergillus spp.; Penicillium spp. and a Cladosporium sp. A visual scale was developed to assess the growth rate of the isolated microorganisms on a range of specific media containing various emulsion components as carbon and energy source. Although the results obtained by using this scale were not conclusive, a few biodegradable components were nonetheless identified. It was found that mixed cultures of the above microorganisms had a greater biodeteriorative effect on the emulsion than did any of the pure cultures when applied separately. This suggested complex microbial interactions were involved in the breakdown of the emulsion. A laboratory-scale model system representative of the Hulett Aluminium hot roll mill was designed and constructed to carry out a series of tests on unprotected and biocide-treated emulsions. A range of biocide concentrations were tested from which the minimum biocide inhibitory concentration was calculated. It was shown that microorganisms exposed to sublethal doses of the biocide Busan (active component glutaraldehyde) over a prolonged period of time, exhibited greater levels of tolerance and resistance to the biocide than did those microorganisms not previously exposed. It was deduced that less frequent, shock doses of biocide are more effective in the control and eradication of emulsion degrading microorganisms than are frequent, low level doses of the same biocide. In addition to the biocide studies, three imported so-called 'biostable' emulsions were evaluated as possible replacements for the susceptible Prosol. Of these three imported emulsions, two viz. HRF3 and Houghton Biostable were shown to be more resistant than Prosol to biodeterioration. After assessing the current hot roll mill management practices, a number of recommendations were made, including: the improvement of plant hygiene; education of the mill workers; improvement of emulsion monitoring; improvement of down-time management and improvement of biocide dosing regimes. Recommendations are also made for minimizing potential microbial growth in the new hot roll mill currently under construction at the Hulett Aluminium processing plant at Pietermaritzburg, South Africa. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.

During firing, the deformation of ceramic articles under their own weight may be problematic particularly in the sanitary ware industry where articles are large. A model has been developed that predicts the viscoelastic deformation of a range of vitreous china testpieces during the firing process. The model constitutes a novel application of the transmission line modelling technique to viscoelastic deformation. The applicability of the model to the sanitary ware industry is addressed.

The aim of this work has been to use the soft impressor technique to investigate the plastic deformation of single crystal diamond and in particular to determine the effect that single substitutional nitrogen has on plasticity. Traditionally hardness tests in the form of Vickers or Knoop rigid indenters have been used to investigate the mechanical properties of materials which cannot be fabricated into tensile or three point bend test specimens. The high stress concentrations created by these types of test introduce a large degree of brittle failure in ultra-hard, covalently bonded materials. The soft impressor technique, on the other hand, allows large pressures to be applied without large stress concentrations. The result is that plastic deformation can be more readily induced into super hard materials such as diamond. This work has shown that not only can diamond be readily plastically deformed but that traces of nitrogen impurities within the lattice have a significant effect on the conditions necessary to produce dislocations. For this work, several different soft impressors were used to produce a range of pressures in the temperature range 800° to 1400°C. A selection of synthetic (HPHT) diamonds with various nitrogen concentrations were impressed and compared with impressions placed in natural type IIa specimens containing no nitrogen but heavily dislocated. Numerous analytical techniques were used to determine the level of deformation produced and gain a better understanding of the effect of nitrogen related defects. The first two chapters of this thesis review, first plasticity and then diamond, with reference to those properties/characteristics relevant to this topic. The third chapter discusses the principle of the soft impressor technique and the methodologies used. In the fourth chapter, models by which single crystal diamond plastically deforms are introduced, together with results that have extended the brittle-ductile transition schematic produced by Brookes, EJ. (1992). Results on the effect of dwell time and the phenomenon of impression creep are also presented. The fifth chapter identifies the predominant defects associated with substitutional nitrogen in HPHT diamond and presents profiles of impressions for diamonds with different 'grown-in' defect levels. The results are discussed and conclusions are made, in conjunction with suggestions for further work in chapter 6.

This thesis describes the novel test techniques that were developed to measure the parameters associated with the plastic shrinkage, and subsequent possible plastic shrinkage cracking, of high strength concrete. The parameters measured during the first 24 hours after placing were the stress- strain relationship, negative pore pressure and free shrinkage strain development. The plastic behaviour of eight high strength concrete mixes was quantified and these mixes were then tested to assess their propensity towards plastic shrinkage cracking, using restrained ring tests. A review of the parameters associated with plastic shrinkage cracking was carried out. The general view was that as the particle size in a cement matrix gets smaller, then the negative pore pressures developed are greater and hence shrinkage increases. This meant that the presence of secondary cementing materials, of very small diameter, such as microsilica, in high strength concretes would explain their apparent susceptibility to plastic shrinkage cracking. Eight high strength concrete mixes were tested in exposed and sealed conditions. It was found that when tested in sealed conditions none of the parameters measured presented itself as the sole driving force behind plastic shrinkage or plastic shrinkage cracking. Also, when cured in sealed conditions, none of the mixes tested in the restrained ring test apparatus cracked. When tested in exposed conditions, the presence of wind had little effect on the stress-strain relationship of the mixes tested. However the presence of wind seemed to cause negative pore pressures to develop earlier than in the sealed samples and increased free shrinkage by 3 to 40 times depending on the mix. The samples that exhibited the highest free shrinkage strains, in exposed conditions, were the mixes that cracked when tested in the restrained shrinkage rings. The mixes that cracked all contained microsilica and these mixes did not crack when the same mixes were tested without microsilica. Polypropylene fibres were found to reduce the cracked area of the samples that cracked. The supplementary cementing materials used in this study were ground granulated blast furnace slag, metakaolin, microsilica and pulverised fuel ash.

This investigation examines the structure-property relationships of high modulus fibres. Five fibre classes were chosen for examination. These are p-aromatic copolyamide (Armos and SVM) and poly-p-aramids (Terlon and Kevlar) obtained from rigid chain polymers; poly-m-aramids (Phenylon and Nomex) obtained from semi rigid chain polymers, and aliphatic polyamide (Capron and Nylon) and Polyethylene obtained from flexible chain polymers. The thermo-mechanical properties studied include tensile properties, thermal shrinkage, creep-recovery, stress-relaxation and residual deformation over a range of temperatures. Results show that mechanical properties are highly related to chain rigidity, orientation and crystallinity of the fibres. The presence of aromatic rings in polymer chains increase the polymer rigidity. The higher the intermolecular attractive force and chain rigidity, the greater the resistance to heat. Study of the creep-recovery properties of polyamide fibres shows that irrecoverable residual deformation for the rigid chain polymers is accumulated within a very short initial period of time (15 seconds) when the load is applied. However for semi-rigid or flexible chain polymer fibres, the residual deformation is accumulated during the whole creep process. The characteristics of tensile stress-strain properties and the accumulation of residual deformation are found to be temperature dependent, especially in the case of Armos and SVM. The mechanical properties of polyamide fibres are also influenced by moisture which is associated with intermolecular interaction. Supplementary studies using FTIR, SEM and DSC were also undertaken. FTIR was used for preliminary investigation into the intermolecular hydrogen bonding and associated moisture in fibres. The results support the explanation of the thermomechanical properties of polyamide fibres. SEM results show the fibre rupture mechanism related to the fibre structures.