Browsing by Author "Isern, Luis"
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Item Open Access Elevated temperature micro-impact testing of TiAlSiN coatings produced by physical vapour deposition(Elsevier, 2019-06-08) Beake, Ben D.; Bird, Andrew; Isern, Luis; Endrino, José L.; Jiang, FengA high temperature micro-impact test has been developed to assess the fracture resistance of hard coatings under repetitive dynamic high strain rate loading at elevated temperatures. The test was used to study the temperature dependence of the resistance to micro-scale impact fatigue of TiAlSiN coatings on cemented carbide at 25–600 °C. Nanoindentation and micro-scratch tests were also performed over the same temperature range. The results of the micro-impact tests were dependent on the impact load, coating microstructure, coating and substrate mechanical properties, and their temperature dependence. At higher temperatures there was a change in failure mechanism from fracture-dominated to plasticity-dominated behaviour under the cyclic loading conditions. This was attributed to coating and substrate softening.Item Open Access Influence of probe geometry in micro-scale impact testing of nano-multilayered TiAlCrN/NbN coatings deposited on WC-Co(Elsevier, 2020-11-15) Beake, Ben D.; Bergdoll, L.; Isern, Luis; Endrino, José L.; Fox-Rabinovich, German S.; Veldhuis, Stephen C.Hard nano-multilayered TiAlCrN/NbN coatings on cemented carbide have shown promise in dry high speed machining applications involving repetitive contact, such as end milling of hardened H13 steel. In this study the fracture resistance of TiAlCrN/NbN coatings under repetitive dynamic high strain rate loading has been evaluated by the micro-scale impact test method. Although the fatigue mechanisms can vary with the ratio of coating thickness t to the indenter radius R, macro-scale tests of thin coatings using probe radii in the mm range are necessarily at low t/R. Micro-impact tests at higher t/R have been performed with a range of diamond indenter geometries (R = 8, 20, 100 μm) to investigate the role of varying t/R (0.03–0.375) on the deformation behaviour. With the largest radius probe there was no clear failure for the coatings or substrate under the test conditions. With the 8 and 20 μm radius probes the behaviour of the coatings was strongly load-dependent and they were more susceptible to impact-induced damage than the carbide substrate. As the load increased there was a change from coating to substrate dominated deformation behaviour as the stress field extended further into the substrate. At lower load the dominant fracture behaviour was coating fracture through ring cracking, radial cracking and chipping. At higher load chipping became less prevalent and break-up of the carbide substrate more extensive.Item Open Access Micro-scale impact resistance of coatings on hardened tool steel and cemented carbide(Elsevier, 2020-11-12) Beake, Ben D.; Isern, Luis; Endrino, José L.; Liskiewicz, T. W.; Shi, X.Micro-impact, a novel accelerated test method for assessing coating durability under repetitive contact, has been developed to concentrate impact-induced stresses close to the interfaces in coating systems. Test results are described for carbon coatings on hardened tool steel and nitride-based coatings on cemented carbide. At higher load it was possible to show the increasing contribution of the substrate properties (load support and ductility) to the coating system response whilst retaining high sensitivity to the coating properties. Hard and elastic carbon coatings on hardened tool steel displayed very low impact resistance under these conditions. Relatively soft carbon-based coatings with more metallic character and high plasticity (low H/E) deposited on hard but tough tool steel were resistant to radial cracking and lateral fracture at high load. Lateral fracture at high load and extensive substrate cracking was observed at higher load for hard nitrides on cemented carbide. The micro-impact test has the potential to significantly speed up the pace of coating system selection for durability under highly loaded repetitive contacts, as occur in coatings applications in engine components and in discontinuous cutting operationsItem Open Access Modelling evaporation in electron‑beam physical vapour deposition of thermal barrier coatings(Springer, 2021-08-18) Chevallier, Julie; Isern, Luis; Almandoz Forcen, Koldo; Chalk, Christine; Nicholls, John R.This work presents computational models of ingot evaporation for electron-beam physical vapour deposition (EB-PVD) that can be applied to the deposition and development of thermal barrier coatings (TBCs). TBCs are insulating coatings that protect aero-engine components from high temperatures, which can be above the component’s melting point. The development of advanced TBCs is fuelled by the need to improve engine efficiency by increasing the engine operating temperature. Rare-earth zirconates (REZ) have been proposed as the next-generation TBCs due to their low coefficient of thermal conductivity and resistance to molten calcium-magnesium alumina-silicates (CMAS). However, the evaporation of REZ has proven to be challenging, with some coatings displaying compositional segregation across their thickness. The computational models form part of a larger analytical model that spans the whole EB-PVD process. The computational models focus on ingot evaporation, have been implemented in MATLAB and include data from 6 oxides: ZrO2, Y2O3, Gd2O3, Er2O3, La2O3 and Yb2O3. Two models (2D and 3D) successfully evaluate the evaporation rates of constituent oxides from multiple-REZ ingots, which can be used to highlight incompatibilities and preferential evaporation of some of these oxides. A third model (local composition activated, LCA) successfully predicts the evaporation rate of the whole ingot and replicates the cyclic change in composition of the evaporated plume, which is manifested as changes in compositional segregation across the coating’s thickness. The models have been validated with experimental data from Cranfield University’s EB-PVD coaters, published vapour pressure calculations and evaporation rate formulas described in the literature.Item Open Access Nano- and micro-scale impact testing of zirconia, alumina and zirconia-alumina duplex optical coatings on glass(Elsevier, 2020-10-06) Beake, Ben D.; Isern, Luis; Bhattacharyya, Debabrata; Endrino, José L.; Lawson, Ken; Walker, TrevorOptimising the mechanical properties of optical coatings to improve their durability will be critical if they are to be used successfully in harsh environments where they may be subject to degradation by mechanical contact. In this study zirconia, zirconia-alumina duplex and alumina experimental coatings were deposited on soda lime and borosilicate glass and their resistance to repetitive impact under different experimental conditions evaluated in nano- and micro-scale impact tests. The influence of changing probe geometry (sharp and blunt contacts) and applied load on the deformation was studied. Spheroconical indenters were found to be more suitable to study the load sensitivity of the impact response than sharp cube corner indenters. Increased resistance to plastic deformation in the coating-substrate system (H3/E2) proved detrimental to the damage tolerance to the repetitive nano- and micro-impact tests. To compare the deformation behaviour in nano-impact and nano-scratch, tests were performed using the same spheroconical probe, revealing cracking and blistering of the glass substrate in both types of test. The change in probe depth after the first impact was found to be a very useful metric to effectively compare the evolution of surface damage on continued impact in nano- and micro-impact tests at different applied load and probe geometryItem Open Access Non-destructive thickness measurement of thermal barrier coatings using terahertz radiation(Springer, 2021-08-06) Isern, Luis; Waddie, Andrew J.; Chalk, Christine; Moore, Andrew J.; Nicholls, John R.A non-destructive thickness measurement technique based on terahertz (THz) reflectivity was successfully deployed to interrogate 7 wt.% yttria-stabilised zirconia thermal barrier coatings (TBCs) produced by electron-beam physical vapour deposition (EB-PVD). The THz technique was shown to produce accurate thickness maps for different samples with a resolution of 1 × 1 mm over a surface of 65 × 20 mm that were compared with direct examination of key cross-sections. All thickness measurements on different samples were calculated using a single value of refractive index. Small defects characteristic of EB-PVD, such as “carrot growths” and variations on column inclination, were evaluated and did not produce significant variations in the refractive index of the TBC. Moreover, the thickness maps correctly display thickness variations that are a consequence of the point-source nature of EB-PVD evaporation. In summary, this paper demonstrates the technique can be successfully deployed on large surfaces, and across different coatings of the same material produced under the same deposition conditions. It is shown that a single n value is required to map the thickness distribution for all samples. This combination of qualities indicates the potential of the technique for in-line control of TBC manufacture.Item Open Access Particle distribution, film formation and wear performance of brush plated Ni/WC(Electrochemical Society, 2018-06-16) Isern, Luis; Impey, Susan A.; Clouser, S. J.; Milosevic, D.; Endrino, José L.Nickel-matrix composite coatings with tungsten carbide particles were produced by brush electroplating using different current densities and materials of the brush. Non-abrasive materials and high current densities produce coatings with high particle content and non-uniform dispersion. Abrasive wear testing showed premature coating failure in areas with high particle concentrations (>21.3 at.% of W). Changes to the solution flow direction were undertaken to decrease ‘solution pooling’, as it was related to areas with excessive particle content. This, together with the use of abrasive brushes at lower current densities, gives Ni/WC coatings with a low and narrow composition range (from 13.2 ± 4.8 to 2.8 ± 0.8 at.% of W). Such optimized coatings minimized premature coating failure and improved the wear resistance to 1.8–4.4 times that of the original nickel matrix, achieving values similar to hard chrome coatings tested under the same conditions. Unlike other brush plated composite coatings, changes in coating morphology are not heavily influenced by processing parameters, but are sensitive to the presence of WC particles.Item Open Access Probe geometry and surface roughness effects in microscale impact testing of WC-Co(Taylor & Francis, 2020-05-25) Beake, Ben D.; Isern, Luis; Harris, A. J.; Endrino, José L.Depth-sensing repetitive microimpact tests have been performed on cemented carbide cutting tool inserts with spheroconical diamond probes with end radii of 8, 20 and 100 µm. Results were strongly dependent on the probe radius and applied load. At higher load, there was a transition to a faster damage rate marking the onset of more variability in rate and in the residual depth of the impact crater when using 8 and 20 µm probes. SEM images show the breakup of the WC skeleton at the periphery of the contact zone. Lower surface roughness slowed the initial damage rate at a higher load but did not significantly influence the final crater depth. The load-dependent fatigue mechanism displayed by the cemented carbide also has implications for the study and optimization of coatings when these are depositedItem Open Access Randomised nano-/micro- impact testing – A novel experimental test method to simulate erosive damage caused by solid particle impacts(Elsevier, 2024-04-09) Beake, Ben D.; Goodes, Stephen R.; Zhang, Hannah; Isern, Luis; Chalk, Christine; Nicholls, John R.; Gee, Mark G.A novel randomised nano-/micro-scale impact test method has been developed to experimentally simulate particulate erosion where statistically distributed impacts with defined energy occur sequentially within the test area. Tests have been performed on two brittle glasses (fused silica and BK7) to easily highlight the interaction between impacts, as well as on two ceramic thermal barrier coating systems (TBCs, yttria stabilised zirconia, 7YSZ, and gadolinium zirconate, GZO) that experience erosion in service. Differences in erosion resistance were reproduced in the randomised impact tests, with GZO less impact resistant than 7YSZ, and BK7 significantly worse than fused silica. The impact data show that erosion resistance is influenced by different factors for the glasses (crack morphology, longer-length interaction of radial-lateral cracks in BK7 vs cone-cracking in fused silica) and TBCs (fracture toughness).Item Open Access Structure zone diagram and particle incorporation of nickel brush plated composite coatings(Nature Publishing Group, 2017-03-16) Isern, Luis; Impey, Susan A.; Almond, Heather; Clouser, S. J.; Endrino Armenteros, JoseThis work studies the deposition of aluminium-incorporated nickel coatings by brush electroplating, focusing on the electroplating setup and processing parameters. The setup was optimised in order to increase the volume of particle incorporation. The optimised design focused on increasing the plating solution flow to avoid sedimentation, and as a result the particle transport experienced a three-fold increase when compared with the traditional setup. The influence of bath load, current density and the brush material used was investigated. Both current density and brush material have a significant impact on the morphology and composition of the coatings. Higher current densities and non-abrasive brushes produce rough, particle-rich samples. Different combinations of these two parameters influence the surface characteristics differently, as illustrated in a Structure Zone Diagram. Finally, surfaces featuring crevices and peaks incorporate between 3.5 and 20 times more particles than smoother coatings. The presence of such features has been quantified using average surface roughness Ra and Abbott-Firestone curves. The combination of optimised setup and rough surface increased the particle content of the composite to 28 at.%.Item Open Access Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location(Optical Society of America, 2020-10-06) Waddie, Andrew J.; Schemmel, Peter J.; Chalk, Christine; Isern, Luis; Nicholls, John R.; Moore, Andrew J.We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trialsItem Open Access Wear-resistant nickel-matrix composite coatings incorporating hard chromium carbide particles(Frontiers, 2024-01-08) Isern, Luis; Impey, Sue; Milosevic, Danijela; Clouser, Sidney J.; Endrino, José L.This work evaluates the influence of plating variables on the morphology, composition homogeneity, and abrasive wear resistance of metal matrix composite coatings. A set of Ni/Cr3C2 coatings were brush plated onto steel coupons modifying two key variables: particle size and brush material. Compositional maps of unprecedented detail have been produced and analysed statistically to enhance understanding of composition distribution. The use of Abbott-Firestone curves to analyse surface morphology enabled the evaluation of valley and peak features. The coating differences highlighted by previous analyses have been compared with their behaviour in abrasive environments, simulated using Taber testing. Moreover, coupling Taber testing with partial compositional maps at different wear stages enabled monitoring of coating wear evolution. This methodology has revealed the importance of particle sedimentation during plating, which increased particle incorporation in the composite coating but also increased composition heterogeneity. The smaller 1.7 μm carbides and abrasive brushes produced coatings with more homogeneous morphologies, higher particle content, and increased resistance against abrasive wear, with a 60% reduction in material loss in comparison to the standard nickel coatings.