Julie Yeomans

Professor Julie Yeomans

Professor of Ceramic Materials, Associate Dean (Research and Innovation)
MA, PhD, CEng, CSci, FIMMM
+44 (0)1483 689613
28 AB 03
Personal Assistant: Charlotte Gray

Academic and research departments

Department of Mechanical Engineering Sciences.


Affiliations and memberships

Institute of Materials, Minerals and Mining.
American Ceramic Society.


Research interests

Research projects

My publications


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 m to 100 m, 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.
Edward H. Williamson (2020)Ceramic coatings for small modular reactors University of Surrey
In the nuclear sector, ceramic coatings can be applied to reactor components such as bearings and bolts to improve durability. Historically, electrodeposited hard chromium plate (HCP) has been used to provide wear and corrosion resistance in reactor environments. As a result of regulation changes and adverse health effects associated with Cr(VI), the use of HCP will soon be heavily restricted. Therefore, candidate ceramic materials have been identified which could match or improve upon the performance of HCP as the Rolls-Royce small modular reactor is developed. After a detailed materials selection process, ceramic coatings of Cr2O3, WC-(W,Cr)2C-Ni, Cr3C2-NiCr, CrN and CrAlN thermally sprayed or electron beam physical vapour deposited on to substrates of Inconel 625 were acquired. HCP, also deposited on Inconel 625, using electrodeposition was obtained for comparison. Tests to emulate nuclear reactor conditions including irradiation, corrosion and sliding wear were devised and undertaken to provide an initial screening programme for the candidate materials. A preliminary study into the irradiation performance used implanted ions, as surrogates for neutrons, with the HCP and Cr3C2-NiCr coatings showing no discernible damage in terms of peak shifts or broadening. Corrosion performance was also initially assessed, using a high temperature, high pressure autoclave, filled with simulated nuclear reactor coolant water. The Cr3C2-NiCr and CrN coatings were the least affected by this corrosive environment, as assessed by mass loss. The sliding wear performance of the coatings was examined in greater detail using an Inconel 625 ball undergoing reciprocating motion on a flat coating counterface under dry conditions, as well as in deionised water and simulated nuclear reactor coolant water environments, all at room temperature. Two wear rate analysis methods (mass loss and optical profilometry) were compared and were found to provide complementary information. Of the coatings, Cr2O3 was found to have the highest wear resistance, with no particle pull-out, likely due to its homogenous microstructure. Overall, the results from this initial screening programme indicated that the candidate coatings were unable to match the performance of HCP across all three testing regimes of irradiation, corrosion and wear. The Cr2O3 coating had the best overall performance in irradiation and wear testing but there were potential concerns over its resistance to pitting corrosion. The Cr3C2-NiCr coating was shown to offer comparable irradiation and corrosion performance to HCP and could be considered for use in applications where wear performance is not as critical. In addition to the outcomes for specific coatings, the study highlighted some key factors for further testing, such as the importance of using borated water in wear testing and the benefit of X-ray photon electron spectroscopy in providing additional information regarding wear mechanisms.
KL Powell, JA Yeomans, PA Smith (1993)Localised damage in continuous fibre reinforced ceramic matrix composites, In: Ceramic Engineering and Science Proceedings14(9 -10)pp. 890-895
As a precursor to a study of erosive near behaviour of ceramic matrix composites fracture by indentation and single particle impact has been studied in two glass-ceramic/silicon carbide fibre composite systems. The damage has been characterised and quantified using a combination of confocal scanning laser microscopy and scanning electron microscopy. Lateral cracks which form approximately parallel to the surface, have been found to be the predominant damage event. In the calcium alumino-silicate (CAS)/Nicalon system, lateral cracks tend, to form in regions of the matrix which have a high local fibre volume fraction, whilst in the barium magnesium alumino silicate (BMAS)/Tyranno system they tend to avoid fibre-rich regions. These results are consistent with an analysis of residual thermal stresses in the two systems. In CAS/Nicalon the coefficient of thermal expansion of the matrix is greater than that of the fibre. This puts the matrix into axial tension at room temperature with the stress increasing with local fibre volume fraction. In BMAS/Tyranno the reverse in the case. Thus in both systems, the observed damage is a consequence of the residual stress as well as the stress due to the contact event.
M Tillman, JA Yeomans, RA Dorey (2014)The effect of a constraint on the sintering and stress development in alumina thick films, In: Ceramics International4(7)pp. 9715-9721 Elsevier
The microstructural and stress evolution of thick (25 μm) 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.
C Kastritseas, PA Smith, JA Yeomans (2005)The onset of thermal shock damage in unidirectional-fibre-reinforced ceramic matrix composites, In: HIGH TEMPERATURE CERAMIC MATRIX COMPOSITES 5pp. 235-240
P Yates, Christopher Mallinson, P Mallinson, Mark Whiting, Julie Yeomans (2017)An Investigation into the Nature of the Oxide Layer Formed on Kovar (Fe-29Ni-17Co) Wires Following Oxidation in Air at 700 °C and 800 °C, In: Oxidation of Metals: an international journal of the science of gas-solid reactions88(5-6)pp. 733-747 Springer
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 μm and 4 μm 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.
C Kastritseas, PA Smith, JA Yeomans (2008)Thermal shock behaviour of angle-ply and woven dense ceramic-matrix composites, In: JOURNAL OF MATERIALS SCIENCE43(12)pp. 4112-4118 SPRINGER
AJ Harris, B Vaughan, Julie Yeomans, Paul Smith, ST Burnage (2017)Ballistic testing of surface treated alumina and silicon carbide with improved adhesive bond strength, In: International Journal of Applied Ceramic Technology: ceramic product development and commercialization14(3)pp. 323-330 Wiley
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.
Adam Healey, J Cotton, S Maclachlan, Paul Smith, Julie Yeomans (2016)Understanding the Ballistic Event: Methodology and Initial Observations, In: Journal of Materials Science52(6)pp. 3074-3085 Springer
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.
C Kastritseas, PA Smith, JA Yeomans (2010)Thermal shock fracture in cross-ply fibre-reinforced ceramic-matrix composites, In: PHILOSOPHICAL MAGAZINE90(31-32)PII 923938pp. 4209-4226 TAYLOR & FRANCIS LTD
Edward H. Williamson, Mark Gee, Daniel Robertson, John F. Watts, Mark J. Whiting, Julie A. Yeomans (2019)Wear performance and characterisation of coatings for nuclear applications: WC-(W,Cr)2C-Ni and hard chromium plate, In: Wear430pp. 169-182 Elsevier

The nuclear industry has used hard chromium plate for many years but is seeking alternatives, due to the adverse health effects of Cr(VI) employed in electroplating. In this study, testing and analysis regimes for the comparison of the sliding wear performance of candidate materials have been established and the performance of WC-(W,Cr)2C-Ni has been compared with that of hard chromium plate.

WC-(W,Cr)2C-Ni was applied to Inconel 625 substrates using a detonation gun thermal spray technique. Sliding wear testing was performed using a ball-on-flat configuration reciprocating tribometer at 20 °C in three environments: dry, deionised water and simulated nuclear reactor water chemistry. Wear rates have been evaluated, using both mass and volume loss, and the worn samples were analysed using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

The WC-(W,Cr)2C-Ni coating had a broadly comparable wear performance to hard chromium plate in all three environments. There were differences between deionised and borated water, such that the latter needs to be used in further evaluation. SEM and XPS analyses enabled the wear mechanisms for WC-(W,Cr)2C-Ni and HCP to be elucidated, including pull-out and tribolayer formation. XPS has shown that the tribolayer on WC-(W,Cr)2C-Ni is stratified and undergoes chemical changes as a result of wear.

D Hallam, A Heaton, B James, PA Smith, JA Yeomans (2015)The correlation of indentation behaviour with ballistic performance for spark plasma sintered armour ceramics, In: Journal of the European Ceramics Societypp. 2243-2252 Elsevier
The Knoop and Vickers indentation behaviour of spark plasma sintered SiC–5 wt.% B4 C, B4 C and SiC–2.5 wt.% AlN–3 wt.% 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.
RA Dorey, JA Yeomans, PA Smith (2002)Effect of pore clustering on the mechanical properties of ceramics, In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY22(4)pp. 403-409 ELSEVIER SCI LTD
JA Yeomans (2008)Ductile particle ceramic matrix composites - Scientific curiosities or engineering materials?, In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY28(7)pp. 1543-1550 ELSEVIER SCI LTD
This review is concerned with ductile particle ceramic matrix composites, which are a group of materials comprising micro- or nano-scale metallic particles in a ceramic matrix. The most studied materials have an alumina matrix; nickel, iron, molybdenum, copper, and silver are some of the more frequently used metals. In contrast to conventional cermets and composites containing an interconnected metallic phase, the particles are discrete. The larger particles provide a toughening increment by deforming plastically and bridging an advancing crack. For the nanoscale composites significant improvements in strength have been reported. Improvements in strength and toughness, coupled with changes to elastic properties and thermal conductivity, have led to improved thermal shock resistance and a consideration of these materials for wear applications.
RA Dorey, JA Yeomans, PA Smith, J Pan (2001)In situ optical dilatometric measurements of the initial stages of sintering of alumina, In: ACTA MATERIALIA49(3)pp. 519-527 PERGAMON-ELSEVIER SCIENCE LTD
MJ Blissett, PA Smith, JA Yeomans (1997)Thermal shock behaviour of unidirectional silicon carbide fibre reinforced calcium aluminosilicate, In: JOURNAL OF MATERIALS SCIENCE32(2)pp. 317-325 CHAPMAN HALL LTD
C Kastritseas, PA Smith, JA Yeomans (2005)Thermal shock fracture in unidirectional fibre-reinforced ceramic-matrix composites, In: COMPOSITES SCIENCE AND TECHNOLOGY65(11-12)pp. 1880-1890 ELSEVIER SCI LTD
MJ Whiting, MT Staff, JA Fernie, PM Mallinson, JA Yeomans (2016)Fabrication of a Glass-Ceramic-to-Metal Seal between Ti-6Al-4V and a Strontium Boroaluminate Glass, In: International Journal of Applied Ceramic Technology: ceramic product development and commercialization13(5)pp. 956-965 Wiley
Glass-ceramics are widely utilized in the electronics industry to provide electrical insulation and to form leak tight joints with a range of metals. The coefficient of thermal expansion (CTE) of the glass-ceramic can be controlled by the extent of crystallization to reduce detrimental tensile stresses in the joint. In recent years there has been interest in using titanium alloys, in place of stainless steels, due to their lower density and superior specific strength. In this study, the heat treatment of a strontium boroaluminate glass has been tailored to create glass-ceramics with mean CTEs ranging from 5.7 ± 0.1 × 10-6 K-1 to 9.7 ± 0.1 × 10-6 K-1 over the temperature range 303 K to 693 K. The resultant glass-ceramic consists of three crystalline phases and residual glass. A glass-ceramic with a mean CTE of 6.9 ± 0.1 × 10-6 K-1 was subsequently fabricated to form a compression seal with a Ti-6Al-4V housing and a pre-oxidized Kovar pin. Single pin assemblies were shown to be reproducible in terms of microstructure and all passed a standard helium leak test, indicating that a successful seal had been produced.
MJ Blissett, PA Smith, JA Yeomans (1998)Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites, In: JOURNAL OF MATERIALS SCIENCE33(16)pp. 4181-4190 KLUWER ACADEMIC PUBL
PM Delaforce, JA Yeomans, NC Filkin, GJ Wright, RC Thomson (2007)Effect of NiO on the phase stability and microstructure of yttria-stabilized zirconia, In: JOURNAL OF THE AMERICAN CERAMIC SOCIETY90(3)pp. 918-924 BLACKWELL PUBLISHING
C Kastritseas, P Smith, J Yeomans (2006)Thermal shock of ceramic matrix compositespp. 400-433
S Kiani, J Pan, JA Yeomans (2006)A new scheme of finding the master sintering curve, In: J AM CERAM SOC89(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.
D Mahdi, A Riches, M Gester, J Yeomans, P Smith (2015)Rolling and sliding: Separation of adhesion and deformation friction and their relative contribution to total friction, In: TRIBOLOGY INTERNATIONAL89pp. 128-134 ELSEVIER SCI LTD
S Kiani, J Pan, JA Yeomans, M Barriere, P Blanchart (2007)Finite element analysis of sintering deformation using densification data instead of a constitutive law, In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY27(6)pp. 2377-2383 ELSEVIER SCI LTD
GJ Wright, JA Yeomans (2009)Three-step sintering of constrained yttria stabilised zirconia layers and its effect on microstructure and gas permeance, In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY29(10)pp. 1933-1938 ELSEVIER SCI LTD
GJ Wright, JA Yeomans (2008)The influence of screen-printing parameters on the microstructure and gas permeance of a zirconia electrolyte, In: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY28(4)pp. 779-785 ELSEVIER SCI LTD
Simon Parker, Mark Whiting, Julie Yeomans (2017)Control of carbon content in WC-Co hardmetal by heat treatment in reducing atmospheres containing methane., In: International Journal of Refractory Metals and Hard Materials66pp. 204-210 Elsevier
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. η-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.
AJ Harris, B Vaughan, ST Burnage, JA Yeomans, PA Smith (2013)Surface preparation of silicon carbide for improved adhesive bond strength in armour applications, In: Journal of the European Ceramic Society33(15-16)pp. 2925-2934
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.
AJ Harris, B Vaughan, ST Burnage, JA Yeomans, PA Smith (2013)Surface preparation of alumina for improved adhesive bond strength in armor applications, In: Ceramic Engineering and Science Proceedings33(5)pp. 149-159 Wiley Blackwell
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.
C Kastritseas, PA Smith, JA Yeomans (2006)Damage characterisation of thermally shocked cross-ply ceramic composite laminates, In: JOURNAL OF MATERIALS SCIENCE41(3)pp. 951-962 SPRINGER
Currently, there are no reliable methods for screening potential armour materials and hence full-scale ballistic trials are needed. These are both costly and time-consuming in terms of the actual test and also in the materials development that needs to take place to produce sufficient material to give a meaningful result. Whilst it will not be possible to dispense with ballistic trials before material deployment in armour applications, the ability to shorten the development cycle would be advantageous. The thermal shock performance of ceramic armour materials has been highlighted as potential marker for ballistic performance. Hence the purpose of this study was to investigate this further. A new thermal shock technique that reproduced features relevant to ballistic testing was sought. As it would be beneficial to have a simple test that did not use much material, a water-drop method was adopted. This was combined with a variety of characterisation techniques, administered pre- and post-shock. The methods included measurement of the amplitude of ultrasonic wave transmission through the sample alongside residual strength testing using a biaxial ball-on-ball configuration and reflected light and confocal microscopy. Once the protocols had been refined the testing regime was applied to a group of ceramic materials. The materials selected were from two broad groups: alumina and carbide materials. Carbide ceramics show superior performance to alumina ceramics in ballistic applications so it was essential that any screening test would be easily able to differentiate the two groups. Within the alumina family, two commercially available materials, AD995 and Sintox FA, were selected. These were tested alongside three developmental silicon carbide-boron carbide composites, which had identical chemical compositions but different microstructures and thus presented more of a challenge in terms of differentiation. The results from the various tests were used to make predictions about the relative ballistic performances. The tests showed that all of the composites would outperform the alumina materials. Further, all of the tests led to the prediction that AD995 would be better ballistically than Sintox FA, possibly up to a factor of two better. The predictions were in very good agreement with literature values for depth-of-penetration testing. The situation was more complex for the carbide materials, with different tests leading to slightly different predictions. However, the predictions from the ultrasonic tests were consistent with the available ballistic data. Indeed, the ultrasonic data proved to be the most consistent predictor of ballistic performance, supporting the view that the total defect population is more relevant than a ‘critical flaw’ concept. Thus, it can be concluded that with further development, and subject to validation across a wider spread of materials and microstructures, thermal shock testing coupled with ultrasonic measurements could form the basis of a future screening test for ceramics for armour applications.
Ceramic hardness and plasticity have been highlighted as important characteristics in ballistic performance; both of which can be measured and semi-quantified from indentation experiments, respectively. However, relatively little work has investigated the accompanying type, on-set and evolution of indentation-induced damage that may also be contributing an influential role. Pressureless sintered SiC and spark plasma sintered B4C, SiC-AlN-C and range of SiC-B4C composite samples were investigated and their indentation damage characterised by cross- sectioning and serial ceramographic polishing techniques. Observations were compared with their surface and sub-surface sphere impact-induced damage and used to correlate with, and explain differences in, ballistic performance against an armour-piercing projectile. The results conclude that 19.62 N Knoop hardness and predicted transition velocities correlate with V50 ballistic performance, supporting the importance of high ceramic hardness and the propensity to sustain projectile dwell in ballistic performance. The type and evolution of indentation-induced damage appears less significant, although variability in indentation behaviour does appear to correlate to variability in ballistic performance. Despite the shallow indentation damage depth and high crack propagation resistance of SiC-AlN-C, the early on-set of concentrated damage beneath indentations (and impact craters) resulting in a decrease in hardness, appears to be the dominate factor governing the low ballistic performance of this material. In contrast, the high hardness, plasticity and apparent residual strength of the sub-surface indentation damage zone of SiC-B4C composites appears to offer the greatest ballistic performance potential. This work provides further evidence to support the use of indentation as a possible screening method to rank the ballistic performance potential of candidate armour ceramics.
Duale A. M. Mahdi (2015)Aspects of shaving friction
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.
Feed-through connectors are used in the electronics industry to route electrical current into isolated environments. Electrically insulating glasses are used to separate metals from each other to form leak tight glass-to-metal seals. The coefficients of thermal expansion (CTE) are controlled to reduce thermomechanical stresses in the seal. The majority of glass-to-metal seals consist of stainless steel housings and pins bonded to silicate-based glasses. Titanium and its alloys are alternative housing materials due to their low density and thus weight saving potential. Silicate-based glasses form weak and brittle interlayers when bonded to titanium, so borate-based glasses are used instead. Glasses are replaced with glass-ceramics due to improved toughness and refractory performance whilst allowing the CTE to be controlled. A knowledge gap is present as few heat treatments are published to manufacture robust glass-ceramic-to-metal seals with a Ti-6Al-4V housing. Hermetic glass-ceramic-to-metal seals were required to incorporate a Ti-6Al-4V housing and an electrically conducting pin, to survive for up to 30 years at standard laboratory conditions (20 ± 10 °C / 30 ± 20 %RH). The manufacture and testing of glass-ceramic-to-metal seals was divided into three parts: seal development, characterisation, and ageing. Hermetic seals were made between Ti-6Al-4V, a strontium boroaluminate glass-ceramic and an oxidised Kovar pin with a helium leak rate less than 1 × 10-9 mbar l s-1. Kovar was selected as the pin material predominantly due to the low CTE. Differential thermal analysis and push-rod dilatometry were used to tailor the CTE of the glass-ceramic (8.4 ± 0.1 × 10-6 °C-1) between the Ti-6Al-4V housing (10.1 ± 0.1 × 10-6 °C-1) and the oxidised Kovar pin (7.4 ± 0.1 × 10-6 °C-1). The CTEs were measured between 30 °C and 600 °C. The Kovar pin was oxidised in air at 700 °C for 10 minutes. The seal was heat treated in an argon atmosphere that peaked with an isothermal hold at 800 °C for 20 minutes, specifically to tailor the CTE of the glass-ceramic. Seals were reproducible in terms of hermeticity, microstructure and mechanical strength. Reproducible leak tight seals were manufactured in batches of 5 seals. A reproducible microstructure of the seal was confirmed with scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The seals had a load at failure of 1510 ± 10 N. Seals were aged between 20 – 85 °C, 35 – 93 %RH, for up to 32 weeks. There was no change in the hermeticity of the seals as a function of ageing time or temperature. The surface of the glass-ceramic aged at 85 °C / 93 %RH was degraded by contact with water vapour at a rate of 5 ± 1 μm yr-1. The resulting strontium enriched layer was mechanically weak. There was a 1 % change in the load at failure of the seals, aged as a function of time and temperature, which was accounted for by natural variation within the samples. Thus, the seals developed in this study are expected to remain hermetic following tens of years of ageing at standard laboratory conditions.
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.
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.