Axial suspension plasma spraying (ASPS) is a relatively new, innovative technique with which microstructures have been produced that are similar to the ones produced by electron beam physical vapor deposition. They have a columnar structure and consist of nm- and µm-sized pores. However, so far the formation of the microstructure is not fully understood because fragmentation and vaporisation of the liquid significantly affects the deposition process. Analysis of single splats can provide important information on the phenomena controlling the coating formation process and the final coating properties. Therefore, the present study aims at providing first results of 8 wt-% yttria-stabilised zirconia single splats sprayed onto a steel substrate by use of ASPS. Scanning electron microscopy and atomic force microscopy have been used to characterise the splats with respect to appearance, shape, and size distribution. © 2017 Institute of Materials, Minerals and Mining Published by Taylor & Francis on behalf of the Institute

The present work deals with the investigation of the corrosion and sliding wear behaviour of detonation gun sprayed (DG) conventional and nanostructured Al2O3-13TiO(2) coatings deposited on a biomedical grade Ti-13Nb-13Zr alloy. The microstructure and phase composition of the coatings were characterised by scanning electron microscopy (SEM) and X-ray diffraction ( XRD). Hardness measurements were carried out using a Vickers hardness testing machine. The experimental results suggested that the nano structured Al2O3- 13TiO(2) coating exhibited 43 and 33% increase respectively in corrosion and wear resistances compared to the conventional Al2O3-13TiO(2) coating. This improvement in the above properties of nanostructured Al2O3-13TiO(2) coating is due to the presence of larger volume fraction of nanosized particles and lower porosity attained by spraying using DG.

Plasma sprayed yttria stabilised zirconia (YSZ) coatings were investigated to assess the factors influencing their durability during thermal cycling. For any given powder, the best performance was found to be achieved at an optimum plasma arc current, all other spray parameters being held constant. The YSZ overlayer thickness was found to be an important lifetime determining factor. Use of a NiCoCrAlY bond coat instead of Ni–Cr led to a substantial improvement in coating lifetime, with the enhancement provided by NiCoCr AlY becoming more pronounced with increasing porosity level of the ceramic overlayer. A post-coating heat treatment was also found to be beneficial to coating longevity. The relative ranking of magnesium zirconate and YSZ coatings was found to depend upon the thermal cycle adopted during testing, which has important implications in designing accelerated tests to evaluate coating performance.

TiN and Ti–Al–Si–N nanocomposite coatings of the type nc-TiAlN/a-Si3N4 have been prepared by cathodic arc physical vapour deposition process using cylindrical cathodes on high speed steel substrates with different surface roughness values, where the roughness is induced by emery paper method and diamond hand polishing. Fracture toughness studies by indentation method have shown that TiN is tougher than Ti–Al–Si–N nanocomposite coatings. Scratch and pin on disc wear tests have been conducted on the specimens to study the adhesion and tribological behaviour of these coatings respectiely. The wear mode between two mating surfaces is complex, and the wear behaviour can be understood better by studying the progression of surface changes and wear debris. The adhesion pattern of harder nanocomposite coating on smooth substrate surfaces is different from that of tougher TiN coating. Wear volume of these coatings decreases with substrate roughness, but it is found more for nanocomposite than for TiN.

Cyclic nanoimpact tests were carried on nc-TiAlN/a-Si3N4 nanocomposite, TiN and multilayered TiN/nanocomposite (NC) coatings to evaluate their resistance to fracture under cyclic impact loads. Fracture behaviour of the coatings was ascertained from fracture probability obtained from time-depth curves and focus ion beam milling images of resulting indentation impressions. TiN coating mainly showed intercolumnar cracks while the other coatings showed other modes of cracking, that is, lateral, inclined, bending, edge cracks, during testing. The performance ranking of the coatings, TiN > TiN/NC > nc-TiAlN/a-Si3N4, is linked to their β0 value, representing relative indentation depth of the coating-substrate composite hardness system at which the fractional hardness improvement equal to 50% of the maximum is retained and also their corresponding microstructure. Apart from enabling prediction of fracture resistance of the coatings, these studies provide useful insights into design and selection of coating materials for targeted machining applications. © 2016 Institute of Materials, Minerals and Mining

Improving wear resistance of rails has a direct impact on the performance of rail-wheel system in railroad technology. Enhancement of sliding wear resistance at curved track, where factors such as adhesion, high slip ratios and contact fatigue act at contact patch of rail-wheel system, is particularly desirable. In the present investigation, influence of laser surface modification on sliding wear performance of a pearlitic rail steel (used in Indian railways) under two different conditions, namely, laser hardening (without any melting involved) and laser melting (with thin surface layer melting), has been studied under laboratory conditions. Before sliding wear testing, the effect of laser scanning speed on the treated layer depth has been optimised, utilising a 9 kW CO2 laser system. Sliding wear tests were carried out using a pin-on-disc device, with laser treated and untreated pearlitic rail steel discs and sliding pins made of wheel steel material, tungsten carbide (WC) and high speed steel (HSS). The tests were performed under normal prototypic loads and unlubricated conditions. Microhardness in the laser melted layer was in the range of 830-900 HV as against 890-1070 HV in the hardened layer, and was found to depend on the laser scanning speed. Sliding wear resistance of both hardened and melted layers was found to be significantly improved compared to untreated rail steel. The coefficient of friction was also marginally reduced in the laser surface melted layers.