Surface Engineering

Plasma spray coatings are produced by introducing powder particles of the material into a plasma plume, which melts and forwards them to the substrate. The flattening process of these individual molten droplets is one of the most critical factors as the coating quality strongly depends on the deposition of individual particles. Powders of aluminium bronze, a fine (−53 + 11 μ m) and a coarse one (−125 + 45 μm) were plasma sprayed onto stainless steel substrates (AISI 304L) under atmospheric condition with three different substrate temperatures (25, 165 and 270°C). Two different ranges of surface roughness R_a were used: mirror polished substrates with about 0·01–0·03 μm and grit blasted substrates with about 1·89–2·43 μm. A scanning electron microscopy equipped with energy dispersive spectrometry system and an electron probe microanalysis, were used in order to study the splat morphology and the chemical composition of the splats.

The present work intends to understand the failure mechanism of polymer coated metal sheets during deep drawing. Characterisation and degradation evaluations of the metal–polymer protective substrates were carried out by scanning electron microscopy, X-ray diffraction and electrochemical trials. Attenuated total reflection, Fourier transform infrared and confocal Raman spectroscopy were employed to analyse the structural changes of the polyethylene teraphtalate coatings on the electrolytic chromium coated steel plates resulting from the manufacturing processes by deep drawing and the electrochemical degradation caused by citric citrate solutions.

Mo ions extracted from a metal vapor vacuum arc ion source were implanted into H13 steel with a high implantation dose of 5 × 10¹⁷ cm⁻² and a pulsed ion beam flux of about 300 μA cm⁻². An optical interference microscopy and pin on disc apparatus investigated the wear and friction characteristics of the steel. The Mo concentration depth profile was measured by using Rutherford backscattering spectroscopy. It was observed by X-ray photoelectron spectroscopy and grazing angle X-ray diffraction that carbide of Mo appeared in the doped region. The concentration depth profile and microstructure analysis could serve to illuminate the wear resistance improvement mechanisms of the Mo implanted steel.

This paper discusses the scanning performance of a trimmed atomic force microscope (AFM) tip. A standard tip was trimmed by focused ion beam (FIB) sputtering to achieve a higher aspect ratio and sharpness. Microfeatures produced by FIB sputtering on a single crystal silicon substrate were scanned by AFM tapping mode using both untrimmed and trimmed probe tip. A comparison of the scanning results with analytical calculation showed that the trimmed tips were superior in imaging the profile for both the hole shaped and pin shaped microfeatures. But when using the trimmed high aspect ratio tip, the scanning speed was significantly low. Higher scanning speed usually resulted to tip breakage. The trimmed tips were expensive and justified for special applications.

Increasing concern over toxic wastes produced by nickel–chromium (Ni–Cr) plating industries has resulted in strong research efforts to replace conventional plating by physical vapour deposition techniques like evaporation and sputtering. Ni–Cr coatings on mild steel substrates produced by an electron beam evaporation process with layer thickness of ∼8–10 m m have been investigated with regard to the structural and corrosion properties. The structure of the coating was evaluated by X-ray diffractometer (XRD). The (110) Cr and (200) Ni predominant peaks with the crystallite size in the range of 25–30 nm was observed from XRD pattern. The electrochemical polarisation studies performed on samples showed the corrosion resistant nature of the evaporated Cr coating with Ni underlayer on mild steel substrates. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to investigate the surface characteristics of the samples before and after the electrochemical corrosion tests. The localised corrosion through a micropore in the evaporated Cr deposit and its penetration to the Ni layer was observed from SEM. The smaller change in roughness observed from AFM reflects the resistance of this coating to corrosion breakdown.

Models for simulation and monitoring of the evolution of surface layers during gas nitriding of titanium alloys were developed using own experimental results. They are based on analytical and numerical solutions of the diffusion equation and model the nitrogen distribution, the thickness of the nitrided layers and the incubation time for the formation of layers on the surface of titanium alloys.

The influence of the preflux bath composition upon the structure of zinc coatings deposited with hot dip galvanising has been examined. Various compositions were studied containing chloride salts of Na, K and Sr. The as formed coatings were examined by optical microscopy, scanning electron microscopy and X-ray diffraction. Depending on each specific compound and its concentration in the preflux bath, different crystallisation procedures are induced, which finally results in the formation of Fe–Zn intermetallic phases with different morphologies, thicknesses and qualities. Fluxing in an aqueous solution of NaCl or KCl introduced several cracks in and between the phases while fluxing in an aqueous solution of SrCl₂ caused total failure of the coating because it had no cohesion to the substrate. However, when the same salts were dissolved in an aqueous solution containing 50 wt-%ZnCl₂·2NH₄Cl instead of water, the coatings were improved.

Plasma sprayed and heat treated hydroxyapatite (HA) coatings, produced from fine HA powder (38–75 μm) under a spraying power of 45 kW (in short FCs) and coarse HA powder (75–106 μm) under a spraying power of 35 kW, (in short CCs) have been investigated, especially in terms of their surface characteristics. The X-ray diffraction results show that most of amorphous and decomposed phases transform into crystalline HA and the crystallinities of coatings are improved. The surface morphology of the coatings has been examined by electron probe microanalyser and field emission scanning electron microscope. At high magnifications the as sprayed FCs, with new nanosize particles and a netlike aggregated lamellar texture are observed on the surface. After heat treatment at 650°C, the nanograde particles of FCs have a tendency to grow and increase in quantity. While the netlike aggregation disperses into a uniform petal-like texture similar to a porous structure with a high surface roughness. Energy dispersive spectra show that these particles and netlike structure are primarily composed of calcium and phosphorous with adequate amount of oxygen and the Ca/P ratio is 1·75, i.e. exceeding that of pure HA.

Effects of such thermochemical processes as carburising, nitriding and boriding on properties of maraging steel grades have been investigated. Despite certain improvements in the properties of the obtained surface, at present, carburising and boriding cannot be recommended owing to problems in the layer quality and difficulties in dimension control processes. Nitriding has proved to be the most suitable process and subsequent surfacing with a TiN layer on the previously nitrided substratum of maraging steel provided additional improvement in properties.

The influence of two surface pretreatments, glass-bead blasting and grinding, on the development of aluminide diffusion coatings on 9%Cr steel substrates has been studied by optical, scanning electron and transmission electron microscopies. The results showed that pretreatment of the surface by glass-bead blasting yields an aluminised layer with a more uniform structure and less thickness variation than pretreatment by grinding. These beneficial consequences of glass-bead blasting are introduced by an increase in surface roughness which, by increasing the specific surface area that is accessible to gaseous metal halides to react with the substrate, to release aluminium, facilitates the surface reactions step in the coating development process, and by an increase in dislocation density. Furthermore, changes in dislocation arrangement take place as a result of glass-bead blasting, as the dislocations form into an arrangement of cells and tangles. As such defects contribute to coating nucleation and solid-state diffusion, it is apparent that glass-bead blasting of the substrate surface affects the surface reaction and the solid-state diffusion stages of the aluminising process. However, the dislocation arrangement developed during the glass-bead blasting pretreatment changes and the density of dislocation decreases during annealing at 650°C, the pack aluminising temperature. Coating development is, therefore, influenced by the treatment only in the early stages of the aluminising process.