The microstructural and stress evolution of thick (25 ¼m) alumina films on dense alumina substrates sintered at temperatures from 1300 °C to 1600 °C has been investigated. In this study the constraint on sintering was monitored in the absence of significant differences in thermal expansion between the film and the substrate. For comparison purposes unconstrained alumina pellets sintered at 1300 °C-1600 °C were also examined. Overall, the constrained alumina densified less than the free alumina, as expected, although at intermediate temperatures densification rates were comparable. Sintering in the direction perpendicular to the substrate was enhanced with respect to that parallel to the substrate as a means of stress relaxation. Using fluorescence spectroscopy the residual stresses of the films parallel to the substrates were measured; residual tensile stresses as high as 450±40 MPa were exhibited by the films. The considerable stress development resulted in cracking and delamination of the film from the substrate, subsequently film constraint was reduced and densification was not impeded. © 2014 The Authors.
Wright GJ, Yeomans JA (2008) Constrained Sintering of Yttria-Stabilized Zirconia Electrolytes: The Influence of Two-Step Sintering Profiles on Microstructure and Gas Permeance, INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY 5 (6) pp. 589-596 WILEY-BLACKWELL
Kiani S, Pan J, Yeomans JA (2006) A new scheme of finding the master sintering curve, J AM CERAM SOC 89 (11) pp. 3393-3396 BLACKWELL PUBLISHING
The master sintering curve approach represents densification data in terms of a master variable that combines sintering time and temperature. Recently, a finite-element scheme to predict sintering deformation that requires only the master sintering curve instead of a full constitutive law as the input data has been developed. Here, a modification to the original master sintering curve approach, so that it is more suitable for finite-element analysis, is presented. Finite-element shape functions are used to represent the densification data as well as the master sintering curve. This approach confers extra flexibility to the master sintering curve approach, even when it is not used with finite-element analysis. For example, by using shape functions, a varying activation energy can be used to obtain a master sintering curve for a set of densification data that cannot be fitted using a constant activation energy.
Williams T, Yeomans J, Smith P, Heaton A, Hampson C (2015) Effect of interfacial area on densification and microstructural evolution in silicon carbide-boron carbide particulate composites, JOURNAL OF MATERIALS SCIENCE 51 (1) pp. 353-361 SPRINGER
Wright GJ, Yeomans JA (2009) Three-step sintering of constrained yttria stabilised zirconia layers and its effect on microstructure and gas permeance, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 29 (10) pp. 1933-1938 ELSEVIER SCI LTD
Surface treatments of alumina have been investigated with the aim of increasing the strength of the bond created between the alumina and a toughened epoxy adhesive. Four surface conditions have been assessed: as-fired; grit blasted; and krypton fluoride excimer laser treated under two sets of conditions. Compared with the as-fired surface, the grit blasted surface was rougher with poorer wettability, probably due to surface contamination. It was found that the laser treatments removed some of the sintering additives and caused rounding of the alumina grains, slightly increasing the surface roughness. Further, the laser treatment led to an increased surface energy and wettability, which has been linked tentatively to an observed increase in the hydroxyl groups on the surface. The adhesive bond strength was assessed by testing joints in tension and shear. It was found that the laser treated surfaces demonstrated slight improvements in bond strength, with a cohesive failure of the adhesive in tension for surfaces subjected to one of the two laser treatments, compared with failure at the interface for the as-fired, grit blasted and other laser treated samples in tension and for all samples in shear. Thus, it has been demonstrated that modifications to the surface of alumina can result in mechanical and chemical changes which affect roughness, wettability, bond strength and the locus of failure.
Millar L, Taherparvar H, Filkin N, Slater P, Yeomans J (2008) Interaction of (La1-xSrx)(1-y)MnO3-Zr1-zYzO2-d cathodes and LaNi0.6Fe0.4O3 current collecting layers for solid oxide fuel cell application, SOLID STATE IONICS 179 (19-20) pp. 732-739 ELSEVIER SCIENCE BV
Surface treatments of silicon carbide have been investigated with the aim of improving the strength of the bond between the ceramic and an epoxy adhesive. Three surface conditions have been characterised; as-fired, air re-fired and KrF laser processed. A number of characterisation techniques have been used to determine the morphological and chemical changes that have occurred to the surface. Scanning electron microscopy of the re-fired and laser processed samples showed surfaces that appeared glassy, with the laser processed surface showing a different morphology. X-ray photoelectron spectroscopy indicated both treatments had oxidised the surface and the laser processed surface also had a greater concentration of hydroxyl groups. The wettability of both surfaces had improved and the laser processed surface was found to be highly hydrophilic. Mechanical testing of joints prepared with this technique showed them to have the highest strength in tension, with the locus of failure being cohesive. © 2013 The Authors.
Wright GJ, Yeomans JA (2008) The influence of screen-printing parameters on the microstructure and gas permeance of a zirconia electrolyte, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 28 (4) pp. 779-785 ELSEVIER SCI LTD
Kastritseas C, Smith PA, Yeomans JA (2010) Thermal shock fracture in cross-ply fibre-reinforced ceramic-matrix composites, PHILOSOPHICAL MAGAZINE 90 (31-32) PII 923938348 pp. 4209-4226 TAYLOR & FRANCIS LTD
Mahdi D, Riches A, Gester M, Yeomans J, Smith P (2014) Rolling and sliding: Separation of adhesion and deformation friction and their relative contribution to total friction, Tribology International
© 2015 Elsevier Ltd. This study is concerned with determining the relative contribution of adhesion and deformation friction using rolling and sliding method. The challenges associated with in-vivo friction testing were overcome by utilising a novel substrate that mimics the viscoelastic behaviour and surface texture of human skin combined with a repeatable and reproducible test setup. The results show that in the dry state, deformation friction contributes 20% of the total friction while the remaining proportion is due to adhesion. These proportions are affected by probe material where for PTFE, deformation friction contributes 30% of the total friction. For the lubricated state, the contribution of deformation friction to total friction increases approaching 50-50% at the higher sliding speeds and normal loads investigated.
Kastritseas C, Smith P, Yeomans J (2006) Thermal shock of ceramic matrix composites, pp. 400-433
Hallam D, Heaton A, James B, Smith P, Yeomans J (2015) The correlation of indentation behaviour with ballistic performance for spark plasma sintered armour ceramics, Journal of the European Ceramic Society 35 (8) pp. 2243-2252
© 2015 Elsevier Ltd.The Knoop and Vickers indentation behaviour of spark plasma sintered SiC-5wt.% B4C, B4C and SiC-2.5wt.% AlN-3wt.% C armour ceramics have been investigated and observations correlated with ballistic performance. Surface and sub-surface indentation-induced damage has been characterised via cross-sectioning and serial ceramographic polishing techniques. The nature of the damage appears to be less influential than hardness in relation to ballistic performance, but variability in indentation behaviour appears to correlate with variability in ballistic performance. Examination of the indentation size effect curves shows that both Knoop hardness and predicted transition velocities correlate with V50 ballistic performance against an armour-piercing threat, further supporting the importance of hardness and the potential for indentation to be used as a screening method for armour materials.
Kiani S, Pan J, Yeomans JA, Barriere M, Blanchart P (2007) Finite element analysis of sintering deformation using densification data instead of a constitutive law, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 27 (6) pp. 2377-2383 ELSEVIER SCI LTD
The laser treatment of ceramics can lead to increased concentrations of hydroxyl ions on the surface, resulting in improved adhesive bond strength in quasi-static tests. Whether the improvement can be translated to armor applications is investigated here. The ballistic testing of composite-backed, surface treated and non-treated ?control? alumina and silicon carbide panels was undertaken. The failure locus of the ceramic to adhesive/composite joint and the qualitative degree of damage were assessed. Laser surface treated samples performed better than control samples, with silicon carbide moving from single shot to multi-shot capability, thus giving significant advantages for the deployment of these materials.
The only widely-accepted method of gauging the ballistic performance of a material is to carry out ballistic testing; due to the large volume of material required for a statistically robust test, this process is very expensive. Therefore a new test, or suite of tests, that employ widely-available and economically viable characterisation methods to screen candidate armour materials is highly desirable; in order to design such a test, more information on the armour/projectile interaction is required.
This work presents the design process and results of using an adapted specimen configuration to increase the amount of information obtained from a ballistic test. By using a block of ballistic gel attached to the ceramic, the fragmentation generated during the ballistic event was captured and analysed. In parallel, quasi-static tests were carried out using ring-on-ring biaxial disc testing to investigate relationships between quasi-static and ballistic fragment fracture surfaces. Three contemporary ceramic armour materials were used to design the test and to act as a baseline; Sintox FA alumina, Hexoloy SA silicon carbide and 3M boron carbide.
Attempts to analyse the post-test ballistic sample non-destructively using X-ray computed tomography (XCT) were unsuccessful due to the difference in the density of the materials and the compaction of fragments. However, the results of qualitative and quantitative fracture surface analysis using scanning electron microscopy showed similarities between the fracture surfaces of ballistic fragments at the edges of the tile and biaxial fragments; this suggests a relationship between quasi-static and ballistic fragments created away from the centre of impact, although additional research will be required to determine the reason for this.
Ballistic event-induced porosity was observed and quantified on the fracture surfaces of silicon carbide samples, which decreased as distance from centre of impact increased; upon further analysis this porosity was linked to the loss of a boron-rich second phase. Investigating why these inclusions are lost and the extent of the effect of this on ballistic behaviour may have important implications for the use of multi-phase ceramic materials as armour.
Pressed WC-Co hardmetal compacts of two different compositions, 6 and 10 wt.% Co, were heat treated under flowing atmospheres of nitrogen, hydrogen and methane at temperatures from 500 to 900 °C prior to sintering under argon. Microstructural examination showed excessive carburisation up to 2.5 mm into the compacts with regions most exposed to heat treatment atmospheres showing greatest carburisation. ·-phase was present in the 6 wt.% Co samples heat treated at low temperatures without methane but was not present with heat treatment temperatures of 700 °C or above with methane present. The hardness of both materials was significantly lower in highly carburised regions, highlighting the need for careful control of heat treatment parameters.
The purpose of the study is to accelerate the development of ceramic materials for armour applications, by substantially increasing the information obtained from a high-energy projectile impact event. This has been achieved by modifying an existing test configuration to incorporate a block of ballistic gel, attached to the strike face of a ceramic armour system, to capture fragments generated during the ballistic event such that their final positions are maintained. Three different materials, representative of the major classes of ceramics for armour applications, alumina, silicon carbide and boron carbide, have been tested using this system. Ring-on-ring biaxial disc testing has also been carried out on the same materials. Qualitative analysis of the fracture surfaces using scanning electron microscopy and surface roughness quantification, via stereoimaging, has shown that the fracture surfaces of biaxial fragments and ballistic fragments recovered from the edges of the tile are indistinguishable. Although the alumina and boron carbide fragments generated from areas closer to the point of impact were also similar, the silicon carbide fragments showed an increase in porosity with respect to the fragments from further away and from biaxial testing. This porosity was found to result from the loss of a boron-rich second phase, which was widespread elsewhere in the material, although the relevance of this to ballistic performance needs further investigation. The technique developed in this work will help facilitate such studies.
This work provides new insight and evidence that challenges and extends the accepted view of the oxidation of Kovar (ASTM-15). Specimens of 2 mm diameter Kovar wire were oxidised in air at 700 °C or 800 °C for 10 minutes. The resulting oxide layers were analysed by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, scanning transmission electron microscopy and Raman spectroscopy. Oxide layers of approximately 2 ¼m and 4 ¼m thick were formed at 700 °C and 800 °C, respectively. These were found to contain iron, cobalt and traces of nickel. The combination of analysis techniques revealed that the oxide contains Fe2O3 in addition to (Fe,Co,Ni)3O4, a spinel oxide, in contrast to the combinations of Fe3O4, Fe2O3 and FeO that are typically reported. The oxide layer was found to be complex, consisting of multiple layers with different compositions which is overlooked in the existing literature.
Shaving is an everyday act for many people and Gillette is at the forefront of this market. The complex process of designing a razor involves understanding the interaction between the cartridge and the face which are complicated systems in their own right. Wet shaving is a complex tribological process for which the mechanisms and parameters are not well understood. The time and high cost associated with designing razors are a major driving force for developing a technical model of shaving. Friction has been identified as an important parameter influencing consumer relevant attributes such glide and comfort. This thesis focused on breaking the problem down into two key areas, skin friction and hair cutting friction. By combining in-vivo and in-vitro testing capabilities, the key parameters affecting skin friction were determined and quantified. Due to the limited knowledge of the relative contribution of adhesion and deformation friction to total friction in the biotribology field, this thesis has confirmed past results and expanded on previous knowledge regarding the relative proportion of adhesion and deformation in three lubrication cases, namely, dry, water and oil contacts. Empirical models of skin friction for these three cases were developed to estimate the relative proportion of adhesion and deformation friction. The primary parameters affecting relative proportion of adhesion and deformation included the contact lubrication, probe material, sliding speed, and probe geometry. Further, the results indicated for the oil contact case, for high normal loads and sliding speeds, deformation friction contributed as much as 50% of the total friction. Hair cutting friction was also investigated focusing on two parameters, hair density and hair cutting profile. These two parameters significantly affected hair cutting friction, where increasing hair density and the area under the curve (hair cutting profile) increased hair cutting friction significantly. Two case studies were considered that combined data from skin friction and hair cutting to estimate the relative proportion of adhesion, deformation and hair cutting friction to shaving friction. The results showed, for contacts with water as a lubricant, hair cutting and adhesion friction contribute on average the same proportion (40-40%) and depends on the type of hair cutting profile considered. For contacts with oil as a lubricant, relative contribution of hair cutting friction significantly increases and can be as high as 80% of the shaving friction depending on the hair cutting profile considered.
WC-Co hardmetal, found in applications ranging from mining tools to valves in deep-sea gas pipelines, is valued for its hardness and toughness provided by the unique chemistry of the tungsten carbide ? cobalt pairing. Properties of WC-Co hardmetal are very sensitive to carbon content, variation of 0.01 wt.% can lead to alteration of hardness yet carbon content is very difficult to control in manufacture of hardmetal, a powder metallurgical process involving high temperatures and interactions with different atmospheres. Accurate measurement of carbon content is difficult in hardmetal as the total carbon content is high compared to the sensitivity.
Work has been reported on carbon measurement in hardmetals using x-ray diffraction (XRD) but attempts to replicate this work were unsuccessful due to limitations in accuracy of equipment. Examination of a commercial hardmetal production process sought to identify manufacturing variables that lead to changes in carbon content. The Vickers hardness, Palmqvist toughness, density and magnetic saturation of samples processed under different processed under different conditions found in commercial manufacture of hardmetal were compared. No clear correlation between any of the process variables examined and carbon content of sintered hardmetal could be found, motivating work to actively alter carbon content. An obvious solution to limited carbon control is to alter the balance of carbon and tungsten in powder blends to compensate for anticipated changes and work was undertaken to investigate this approach. In addition to mechanical and magnetic characterisation, the carbon content of samples was measured using the infra-red gas absorption method. Altering the carbon content of samples by adding carbon or tungsten did not appear to offer control of carbon content in sintered hardmetal as the amount of carbon added did not appear to correlate to the carbon content of sintered samples.
Heat treatments in carbonaceous atmospheres has also been explored and was demonstrated to have potential as a method of controlling carbon content in the manufacture of WC-Co hardmetal. Pre-sintering, a heat treatment often applied to compacts before sintering, was replicated at laboratory scale using an atmosphere of nitrogen and hydrogen with various additions of methane at different temperatures. Samples were examined using Vickers microhardness and confocal light scanning microscopy (CLSM) to capture spatial variation in sample properties and by electron backscatter diffraction (EBSD) to obtain information on grain size distributions. It was found that carbon content increases with the amount of methane in the heat treatment atmosphere and with heat treatment temperature though the reliability and accurate control required to make the technique commercially viable were not achieved. Results demonstrate that with further refinement heat treatment of pressed compacts in carbonaceous atmosphere could be used to accurately control the carbon content of sintered hardmetal components.
Ceramic armour must offer protection against armour piercing threats at low weight and affordable cost. As a possible means of improving armour, a range of SiC-B4C composites have been produced and characterised. The degree of contact between the two phases has been quantified and shown to have a strong effect on the densification and microstructure in these materials. This understanding has enabled independent variation of microstructural parameters which are normally interrelated. These were; porosity, SiC:B4C mass ratio, B4C distribution in a SiC matrix and SiC grain size distribution. To assess effects of each of these parameters on ballistic performance V50 testing was carried out, using 7.62 mm armour piercing rounds. The amount of porosity is shown to have a slight effect on V50 and a marked effect on scatter in V50. The pore size distribution is also shown to be important; across a range of pairs of materials with similar total pore volumes but differing pore size distributions, larger pores consistently give lower V50. SiC:B4C mass ratio does not seem to greatly affect V50, potentially allowing application specific cost/weight balances at constant protection level. B4C distribution has a strong effect. In general, for B4C features with diameters ranging from 1 mm to 100 mm, the coarser features performed better. Using coarse B4C particles in a SiC matrix, a V50 of approximately 980 ± 20 m s-1 at a density of 3.00 g cm-3 was achieved reproducibly. This material is preferred due to a combination of relatively lower cost, reduced density and repeatability. Knoop indentation has been used to derive possible merit indices which could potentially be used to rank ballistic materials. These includes analysis of failure probability of indents and the indentation size effect. A preliminary study indicates ballistic impacts may affect SiC polytype composition.
Glass-to-metal seals are used in a wide range of components. The nature of the interfaces between the constituents is often crucial to the performance of the seal and thus the aim of this study was to characterise the various interfaces in a novel seal made from a strontium boroaluminate glass-ceramic and the alloys Ti-6Al-4V and Kovar (Fe-29Ni-17Co). A titanium boride was found, by STEM, EELS and WDX, to have formed at the glass-ceramic to Ti-6Al-4V interface and to be bonded to both the metal on one side and the glass-ceramic on the other, in contrast to the classic view of glass to metal interfaces where bonding is thought to be promoted through metal dissolving into the glass / glass-ceramic. To establish bonding at the other interface, it was necessary to grow an oxide layer on the Kovar, by heating in air at 700 °C or 800 °C for 10 minutes. The oxide grown at both temperatures was shown (by XPS, XRD, SEM, EDX, STEM and Raman) to have the same composition, with the only significant difference being thickness (2.1 +/- 0.6) µm and (4.0 +/- 0.2) µm thick, for the oxides grown at 700 °C and 800 °C respectively. However, the oxide was found to be much more complex than was indicated by prior literature, comprising four layers. The top layer of the oxide was (Fe,Co)_3O_4, with an Fe_2O_3 layer beneath it. Below these layers were a further two layers of (Fe,Co,Ni)_3O_4. When heated to 800 °C, to simulate the sealing conditions, the oxide was changed to an Fe_3O_4 layer with metallic cobalt and nickel inclusions. Bonding was shown, by SEM and STEM, to occur between the oxidised Kovar and the glass-ceramic, as a result of dissolution of iron from the oxide into the glass. Although the interfaces were not definitively optimised, the seals produced were satisfactory and hermetic.