Abstract: Recent literature on the electrodeposition of metallic coatings containing nanosized particles is surveyed. The nanosized particles, suspended in the electrolyte by agitation and/or use of surfactants, can be codeposited with the metal. The inclusion of nanosized particles can give (i) an increased microhardness and corrosion resistance, (ii) modified growth to form a nanocrystalline metal deposit and (iii) a shift in the reduction potential of a metal ion. Many operating parameters influence the quantity of incorporated particles, including current density, bath agitation (or movement of work piece) and electrolyte composition. High incorporation rates of the dispersed particles have been achieved using (i) a high nanoparticle concentration in the electrolyte solution, (ii) smaller sized nanoparticles; (iii) a low concentration of electroactive species, (iv) ultrasonication during deposition and (v) pulsed current techniques. Compositional gradient coatings are possible having a controlled distribution of particles in the metal deposit and the theoretical models used to describe the phenomenon of particle codeposition within a metal deposit are critically considered.

Abstract: This paper reviews commonly used methods of analysing and interpreting nanoindentation test data, with a particular emphasis on the testing of thin films. The popularity of nanoindentation testing is evidenced by the large number of papers that report such measurements in recent years. Unfortunately, there appear to be several issues that are emerging as common sources of error in using this technique. The present paper is aimed at highlighting these errors for the benefit of those practitioners who wish to use the technique but are not fully conversant with the field.

Abstract: Indium Tin Oxide (ITO) thin films with a variety of microstructures were deposited using a large area conventional DC magnetron sputtering system for flat panel displays manufacturing. Highly uniform ITO films with an average thickness of ∼100 ± 3 nm on the ∼0.6 m2 substrate area were obtained. Film structures with small amounts of crystalline sites were produced by room temperature deposition, and an entirely amorphous structure with excellent etching properties was achieved through optimized incorporation of hydrogen in the film, providing a significant increase in the crystallization temperature of ITO. Post-annealing of such a sample yielded a randomly orientated polycrystalline structure with superior conductivity and transparency. The polycrystalline ITO films, produced at the sputtering substrate temperature of 200 °C, provided structures with preferential grain orientation in both and directions, controlled by the amount of oxygen and increased process pressure. The impact of oxygen and pressure with related structures on the macroscopic properties of the layers was studied. Morphological features of the films such as phase/grain structure and surface roughness were investigated using SEM and AFM. Layers with an equiaxed grain structure of about 30 nm crystal size revealed an ultra smooth surface with RMS values of about 1 nm. Specific resistivities as low as 150 μΩ cm and transmittance values above 92% at 550 nm wavelength were obtained for polycrystalline layers with preferential grain orientation.

Abstract: The electrodeposition and surface morphology of aluminium on tungsten (W) and aluminium (Al) electrodes from 2 : 1 molar ratio AlCl 3 –[EMIm]Cl ionic liquids were investigated. Analyses of the chronoamperograms indicate that the deposition process of aluminium on W substrates was controlled by instantaneous nucleation with diffusion-controlled growth, while the deposition processes of aluminium on Al electrodes were found to be associated with kinetic limitations. Constant potential deposition experiments showed that the electrodeposits obtained on both W and Al electrodes between − 0.10 and − 0.40 V (vs. Al(III)/Al) are dense, continuous and well adherent. Dense aluminium deposits were also obtained on Al substrates using constant current deposition between 10 and 70 mA/cm 2 , and the current efficiency was found to be dependent of the current density varying from 85% to 100%.

Abstract: Multi-element nitride films of AlCrTaTiZr high-entropy alloy have been prepared in this study by reactive radio-frequency magnetron sputtering The influences of nitrogen flow ratio on the chemical composition, microstructure and mechanical properties of the deposited nitride films have been investigated The AlCrTaTiZr alloy film exhibited an amorphous structure, while a simple face-center-cubic solid-solution structure was observed in the nitride films prepared under different nitrogen flow ratios The multi-element AlCrTaTiZr nitride films exhibited much improved mechanical properties as compared with conventional nitride hard coatings of transition metals

Abstract: Titanium and its alloys are being used in many orthopedic and bioimplant applications. In order to render these materials bioactive and to enhance osteointegration, the surfaces are coated with hydroxyapatite (HAp). Adhesion of bone cell to the implant surface, bond strength and durability of the implants are highly dependent upon the characteristics of the Ti substrate and the methods utilized in the hydroxyapatite coating process. In this paper we have reported an innovative method of preparation of a nanotubular titania surface and subsequent electrodeposition of hydroxyapatite nanocrystalline coating. Growth of the hydroxyapatite onto the nanotubular titania surface was accomplished by a pulsed electrodeposition process. Prior to the electrodeposition, the nanotubular titania surface was subjected to an alkaline treatment, which provided a template for nucleation of the hydroxyapatite inside the nanotubes. This process resulted in a vertical growth of the hydroxyapatite crystals and increased the bond strength of the coating. Bond strength was further improved by annealing the hydroxyapatite coated nanoporous titania in an argon atmosphere.

Abstract: A new cold plasma jet has been developed for surface modification of materials at atmospheric pressure. This new cold plasma jet generator is composed of two concentric cylindrical all-metal tube electrodes. The argon is fed into the inner-grounded electrode, the outer electrode is connected to the high-voltage power supply and covered with a layer of dielectric, and then a stable cold plasma jet is formed and blown out into air. The plasma gas temperature is only 25–30 °C. Preliminary results are presented on the modification of polypropylene (PP) and polyethylene terephthalate (PET) fibres by this cold plasma jet. The water contact angle of these materials is found to decrease after plasma treatment and it will recover a little in two months. The chemical changes on the surface of polymers are studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) is used to study the changes in surface feature of polymers due to plasma treatment. The hydrophilicity and surface structure of these materials after plasma treatment are discussed. The results show that such a plasma jet is effective.

Abstract: For the example of copper, the present investigation directly compares the different microstructures and properties of coatings like hardness, bond strength or electrical conductivity obtained by cold spraying and thermal spraying. A deeper view into the type of defects obtained, their stability and their influence on recovery and recrystallization is supplied by subsequent annealing procedures and respective analyses. In particular, these investigations after different heat treatments prove to be a suitable tool for a deeper understanding of cold spraying and thermal spraying.

Abstract: The aim of this work is to study the efficiency of different intumescent formulations designed to protect steel in the case of hydrocarbon fire. The coating is based on a thermoset epoxy–amine resin system into which fire retardant agents, boric acid and ammonium polyphosphate derivative have been incorporated. The first part of the study evaluates, using large scale industrial furnace tests, the behavior of the thermoset resin containing alone or in combination with additives. It is revealed that in this epoxy resin, the combination between ammonium polyphosphate and boric acid leads to the best protective results. The second part of the study attempts to investigate more precisely the effect and the mode of action of the additives in terms of thermal stability, mechanical resistance and rheological properties using small scale lab tests, to explain why this combination works better than using the two fire retardants used separately. The experiments show that this combination leads to the smallest decrease of viscosity when the resin degrades, the highest mechanical resistance and the highest expansion.

Abstract: The active screen plasma and DC plasma nitriding of the low alloy steel 722M24 are investigated. Experimental results showed that the metallurgical characteristics and hardening effect on 722M24 steel nitrided by AS plasma nitriding at both floating potential and grounded potential were similar to those nitrided by DC plasma nitriding. Particles sputtered from the active screen and deposited on the specimen surface play the role of the nitrogen carrier in AS plasma nitriding. XRD and high-resolution SEM analysis indicated that the particles with sizes in sub-micron scale were Fe x N ( x > 2). Based on metallurgical analysis and Optical Emission Spectrometer (OES) experimental results, an AS plasma nitriding model has been proposed considering that AS plasma nitriding is a multi-stage process, involving sputtering, physical adsorption, desorption, diffusion and deposition.

Abstract: A comprehensive study of the mechanical properties of TiAlN and AlCrN coated cutting tools has been performed at room and elevated temperatures (up to 500 °C) using Micro Material's NanoTest Platform System. Micro-mechanical properties have been measured such as microhardness, elastic modulus, H/E ratio, microhardness dissipation parameter (MDP), critical load values ( L c1 —first crack event; L c2 —load of dramatic coating failure) during scratch testing; a scratch crack propagation resistance parameter, CPR s = L c1 ( L c2 − L c1 ) as well as nano-impact fracture resistance. Cutting tool life was studied under end milling conditions of the structural AISI 1040 steel. A correlation between CPR s was found with H/E ratio and MDP values. These parameters could be used to characterize the fracture toughness of the coatings. It was shown that mechanical characteristics such as H/E ratio, MDP and CPR s as well as nano-impact fracture resistance can be used to assess the resistance to adhesive-fatigue wear that is typical for end milling conditions. It was found that the microhardness of the coating and the H/E ratio reduces with rising temperature while the MDP value grows. The data obtained during quick laboratory nanohardness, nanoscratch as well as nano-impact fatigue testing can be used to rank the coatings studied and in some cases predict the relative life of a coated tool.

Abstract: In this work, the influence of the processing conditions on the microstructure and abrasive wear behaviour of a NiCrBSi hardfacing alloy is analysed. The hardfacing alloy was applied in the form of coatings onto a mild steel substrate (Fe–0.15%C) by different techniques: laser cladding (LC) and flame spraying (FS) combined with surface flame melting (SFM). In both cases, the appropriate selection of the process parameters enabled high-quality, defect-free NiCrBSi coatings to be obtained. The microstructure of the coatings was analysed by scanning electron microscopy (SEM), with attached energy dispersive spectroscopy (EDS) microprobe, and by X-ray diffraction (XRD). Their tribological properties were evaluated by micro-scale ball cratering abrasive wear tests using different abrasives: diamond, SiC and WC. Microstructural characterisation showed that both coatings exhibit similar phases in their microstructure, but the phases present differ in morphology, size distribution and relative proportions from one coating to another. Wear tests showed that in three-body abrasive conditions, despite these microstructural differences, the wear behaviour is comparable for both coatings. Conversely, in two-body wear conditions with diamond particles as the abrasive, it was observed that the specific wear rate of the material is sensitive to microstructural changes. This fact is particularly apparent in LC coatings, in which the zones of the layers with higher proportions of very small hard particles present a lower wear resistance. These results indicate that it is important to have good microstructural control of this material, in order to obtain coatings with an optimized and homogeneous tribological behaviour.

Abstract: The present investigation compares the mechanical properties of cold-sprayed and thermally sprayed copper coatings. The mechanical properties of the Cu-coatings are determined by in plane tensile test using micro-flat tensile specimen technique. A deeper view into the type of obtained defects, their stability and their influence on coating performance, is supplied by subsequent failure analyses and the comparison to annealed copper coatings. The results demonstrate that cold-sprayed coatings, processed with helium as propellant gas, show similar performance as highly deformed bulk copper sheets and respective changes in properties after annealing. In the as-sprayed condition, cold-sprayed coatings processed with nitrogen and thermally sprayed coatings show rather brittle behavior. Whereas subsequent annealing can improve the properties of the cold-sprayed coating, processed with nitrogen, such heat treatments have only minor influence on the tensile properties of thermally sprayed copper coatings. The investigation of failure modes for the as-sprayed states and after different heat treatments provided further information concerning particle–particle bonding and the effect of oxides on mechanical properties.

Abstract: A series of polymer–clay nanocomposite (PCN) materials that consist of siloxane-modified epoxy resin and inorganic nanolayers of montmorillonite (MMT) clay has been prepared through a thermal ring opening polymerization using 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane as a curing agent. These PCN materials at low clay concentration in the form of coating on cold-rolled steel (CRS) were found to be much superior in corrosion protection over those of pure epoxy resin when tested for performance in a series of electrochemical measurements of corrosion potential, polarization resistance, corrosion current, and impedance spectroscopy in 5 wt.% aqueous NaCl electrolyte. The as-prepared materials were characterized by infrared spectroscopy, wide-angle X-ray diffraction, and transmission electron microscopy. After measurements, we found advanced protection against corrosion on CRS coupon compared to bulk epoxy resin. Molecular (e.g., O2, N2, and H2O) permeability of epoxy resin–clay nanocomposite membranes was found to be lower than that of bulk epoxy resin along with the loading of nanoclay based on the studies of gas and vapor permeability analysis. Moreover, the epoxy resin–clay nanocomposite materials have significant advantages over standard epoxy resins such as lower water absorption, lower cure shrinkage, moderate glass transition temperature (Tg), and higher tensile strength.

Abstract: Ni–Co alloys with varying cobalt content were electrodeposited employing sulphamate electrolyte. The changes in microstructure and corrosion behavior of electrodeposited nickel with respect to cobalt addition were studied. Scanning electron microscope, optical microscope and energy dispersive X-ray analysis were used to characterize the alloy coatings. The alloy co-deposition was observed to be anomalous type. The cross-section microhardness measurement indicated that the hardness reached the maxima for a cobalt content of 50 wt.% and then dropped with the increase in cobalt content. A correlation between microhardness and microstructure has been attempted. The optical micrographs indicated a change in microstructure from mixed columnar-fibrous to lamellar and finally to fibrous with increase in cobalt content. The X-ray diffraction (XRD) studies indicated the crystal structure to be cubic for cobalt content in the range of 0–50 wt.%. A transition to hexagonal structure was observed for a cobalt content of 70 wt.% and beyond. A change in preferred orientation was also observed with respect to cobalt addition. Potentiodynamic polarization and electrochemical impedance studies were used to study the corrosion behavior of Ni–Co alloys. The physical behavior was quantified with equivalent circuit. These studies indicated that the Ni–20% Co alloy exhibited better corrosion resistance in comparison to other Ni–Co alloys, plain nickel and plain cobalt coatings irrespective of the substrate (mild steel, brass) employed for deposition.

Abstract: The nanocrystalline microstructure in the surface of 1Cr18Ni9Ti stainless steel induced by high-energy shot peening has been characterized by means of X-ray diffractometry and transmission electron microscopy. The effects of shot peening on corrosion resistance of the steel have been investigated by polarization curves and pit corrosion tests, and the surface morphologies of the corrosion samples have been characterized by scanning electron microscopy. The results show that shot peening can cause surface nanocrystallization; that is, a nanocrystalline microstructure with an average grain size of ∼ 18 nm forms in the surface layer of the sample, and furthermore, can induce α-martensite of about 15% in volume fraction. With increasing depth from the shot-peened surface, the grain size increases, but the volume fraction of the α-martensite decreases. The surface nanocrystallization can improve the potentiodynamic polarization behavior of 1Cr18Ni9Ti stainless steel in 3.5% NaCl solution. Comparing to the as-received coarse crystalline counterpart, the passive film on the surface of the shot-peened sample is easier to form and more stable. Shot-peening-induced surface nanocrystallization can markedly enhance the corrosion resistance of 1Cr18Ni9Ti stainless steel in the chlorine–ion-contained solution.

Abstract: In this work, several mixtures of self-fluxing NiCrBSi alloy powder and a nickel-clad WC powder (10 wt.% Ni and balance WC) were laser cladded on stainless steel substrates of austenitic type (AISI 304). The aim of the study was to determine the influence of the volume fraction of the reinforced WC particles on the formation and performance of the composite layer. The effect of other parameters of the treatment, such as the laser energy, beam profile, traverse speed and the mass rate of the feed powder was also investigated. Clad layers of 0.5–1.5 mm height were obtained, its microhardness measured and the microscopic morphology and distribution of tungsten carbide particles within the layer characterized by scanning electron microscopy (SEM). It was found that most clad layer properties such as its porosity, microhardness and homogeneity are determined by the percentage of WC particles in the mixture. Pores were observed for volume fractions roughly above 50%. Below this limit, homogeneous, dense and crack free clad layers were obtained, with measured hardness ranging between 600 and 1000 HV depending on the WC content.

Abstract: Nanocrystalline (nc) Ni coating was direct-current electrodeposited on the AZ91D magnesium alloy substrate aimed to improve its corrosion resistance using a direct electroless plating of nickel as the protective layer. As comparison, two electroless Ni coatings on the magnesium alloy with different thickness were also presented in the paper. The surface morphologies of the coatings were studied by SEM and FESEM. The nc Ni coating had an average grain size of about 40 nm and an evident {200} preferred texture revealed by XRD. The hardness of the nc Ni coating was about 580 VHN, which was far higher than that (about 100 VHN) of the AZ91D magnesium alloy substrate. The electrochemical measurements showed that the nc Ni coating on the magnesium alloy had the lowest corrosion current density and most positive corrosion potential among the studied coatings on the magnesium alloy. Furthermore, the nc Ni coating on the AZ91D magnesium alloy exhibited very high corrosion resistance in the rapid corrosion test illustrated in the paper. The reasons for an increase in the corrosion resistance of the nc Ni coating on the magnesium alloy should be attributable to its fine grain structure and the low porosity in the coating.

Abstract: Thermal spray coatings are formed by successive impingement and interbonding among the splats (solidified individual molten particles). Depending on the processing conditions employed during the spray process, deposits are produced with an assortment of microstructures and properties. This study brings out how the basic microstructural differences are influenced by mechanisms involved during the spraying processes. The Ni–5 wt.% Al metallic system is chosen for a systematic study of cross comparison across different spraying techniques because of the potential variety in coating microstructure and phases possible through application of the different spray processes as indicated in the literature. Spray techniques such as Air Plasma spraying (APS), Wire Arc Spraying and, High velocity oxy-fuel spraying (HVOF), each differ with respect to their feedstock injection and melting methods, spraying parameters and deposition efficiency as well as oxidation involved during spraying. The goal of this study is to address the mechanisms of oxidation involved and present schematic models to explain their role in the microstructural evolution of Ni–5 wt.% Al coatings in case of different processes. In-flight oxidation and post-impact oxidation occurring on top surface of splats are discussed in detail and the effects of these mechanisms on intersplat contact and coating buildup are addressed.

Abstract: Nickel matrix composite coatings reinforced with sub-microsized Al 2 O 3 particles were produced by pulse electrodeposition and the effect of pulse frequency on the microstructure, hardness and wear resistance of Ni–Al 2 O 3 composite coatings were investigated. The results showed that the pulse frequency significantly influenced the preferred orientation of Ni–Al 2 O 3 composite coatings; the texture of the coatings progressively changed from a strong (111) preferred orientation to a random orientation when pulse frequency increased. The hardness of composite coatings decreased slightly with the increase of volumetric content of alumina particles. The wear behaviors of composite coatings under dry sliding wear and oil-lubricated wear conditions were different significantly. The wear resistance of Ni–Al 2 O 3 coatings decreased with the increase of incorporated alumina particles under dry sliding wear condition, which was largely influenced by the microstructure of Ni matrix due to the presence of adhesive wear. However, the wear resistance of composite coatings increased with the increase of volumetric content of the reinforcements under oil-lubricated wear condition, which mainly depends on the volumetric content of incorporated alumina particles because the adhesive wear can be avoided under oil-lubricated wear condition.

Abstract: Morphologies of copper deposits obtained at overpotentials belonging to the plateau of the limiting diffusion current density and at higher overpotentials were examined by the scanning electron microscopy (SEM) technique. Copper dendrites are formed at overpotentials belonging to the plateau of the limiting diffusion current density. The shape of copper dendrites depends on the electrodeposition overpotential. At higher overpotentials (800 and 1000 mV) and larger values of current densities, porous and very disperse copper deposits were obtained. These morphologies were a consequence of a very vigorous hydrogen evolution at these electrodeposition overpotentials. Also, the obtained copper structures consisted of agglomerates of copper grains. The size of copper grains is a function of the overpotential of electrodeposition, thus approaching to nano-sized dimensions is achieved when the electrodeposition overpotential is increased.

Abstract: Electroless Ni-plating on Mg alloy AZ91 has been studied to understand the effect of the substrate microstructure and roughness on the deposition rate, nucleation, coating microstructure, and mechanical property of the coatings. Experimental results indicate that the growth of Ni deposit in the early stage was influenced by the substrate microstructure and roughness. The electroless Ni-plating on the abrasive blasted AZ91 (rough) substrate showed a higher deposition rate than that on the finely polished one, indicating that the mechanical roughening enhances the nucleation and coalescence of Ni crystallites. Scratching tests showed that higher coating adhesion is achieved on the roughened AZ91 substrate. Wear tests, however, showed that the Ni plating on the rough substrate has a higher friction coefficient than that on the polished surface. The hardness and adhesion property of Ni coatings before and after heat treatment were also characterised.

Abstract: Electroless Ni–P coatings provide high hardness and excellent resistance to wear and abrasion The present work aims to study the formation of electroless Ni–P graded coatings, with varying nickel and phosphorus contents of the individual layers and to evaluate their corrosion resistance by polarization and electrochemical impedance spectroscopic studies The possibility of preparing electroless Ni–P graded coatings by sequential immersion in three different plating baths is discussed The study reveals that electroless Ni–P graded coatings offer better corrosion resistance than non-graded Ni–P coatings

Abstract: Considerable progress has been made in improving the tribological properties of titanium alloys by application of surface engineering technologies such as physical vapour deposition (PVD) and plasma nitriding. PVD coatings produce very hard, low friction surfaces, but may be susceptible to coating fracture and delamination under high normal load sliding wear conditions. Plasma nitriding produces a nitrogen-strengthened diffusion layer beneath the coating, which should have beneficial effects on the coating's ability to support high normal load sliding wear. There do not appear to be any published works which clearly demonstrate and quantify this effect for TiAlV alloy under controlled experimental conditions. The current work investigates the comparative sliding wear behaviour of TiN and TiN/Ti2N thin films deposited by PVD and plasma nitriding processes, respectively, and seeks to quantify the relative improvements to sliding wear behaviour afforded by the two surface treatments. The results confirm the theory, showing that the increased hardness/strength of the substrate in the plasma nitrided material is responsible for a significant improvement in sliding wear behaviour under all normal loads considered.

Abstract: This paper reports on studies of the phase composition and electrochemical performances of microarc oxidation (MAO) films formed on AZ91D alloy using step-down current method in phosphate electrolytes (P-film) and silicate electrolytes (Si-film). The results showed that the P-film was mainly composed of Mg, MgAl 2 O 4 , MgO and the Si-film was composed of Mg 2 SiO 4 , MgO. There clearly existed a fluoride-enriched zone of about 1∼2 μm for P-film and 0.7∼1 μm for Si-film at the MAO coating/substrate interface. The electrochemical tests showed that both P-film and Si-film could enhance the corrosion resistance of AZ91D magnesium alloy significantly; the corrosion failure processes of the two films in 5 wt.% NaCl solution were quite different.

Abstract: In this paper we report on the electroplating of mild steel by aluminium in a first generation ionic liquid [EMIm]Cl/AlCl3 (40/60 mol.-%). The study was performed by means of cyclic voltammetry and galvanostatic polarization complemented by SEM/EDAX and optical microscopy. The results show that the pretreatment of the substrates plays a key role in the coating adhesion. The aluminium coating made on a conventionally pretreated mild steel substrate is of high quality but it does not exhibit good adherence to the substrate. However, we have found that an in situ electrochemical etching of the substrate by anodic polarization prior to electrodeposition leads to well adherent Al coatings of mild steel which resist even mechanical scratching. This is due to dissolution of the pre-formed iron oxide layer and the subsequent re-deposition of iron or Fe–Al alloy formation prior to Al bulk deposition resulting in excellent adhesion. It is also shown that with the electrochemical pretreatment in the ionic liquid not only steel sheets but also complex shapes like e.g. screws can be coated with well adherent aluminium.

Abstract: Conductive oxide coatings are used as protection layers on metallic interconnects in SOFCs to improve their surface stability and electrical performance, as well as to mitigate or prevent chromium poisoning to cells. This paper discusses materials requirements for this particular application and summarizes our systematic study on varied conductive oxides as potential candidate materials for protection layers on stainless steel substrates. Overall, it appeared that chromites such as (La,Sr)CrO 3 improved surface stability, but might not be good candidates for protection layer applications due to chromium vaporization, albeit at a lower rate than Cr 2 O 3 , from these oxides at high temperatures in air or moist air. The application of non-chromite perovskite (La,Sr)FeO 3 (LSF) protection layers resulted in improved oxidation resistance and electrical performance. It is doubtful, however, that LSF can be an effective barrier to prevent chromium release during long term SOFC stack operation due to chromium diffusion through the LSF coatings. With a high oxygen ion conductivity, the coatings of Sn-doped In 2 O 3 failed to provide protection to the metal substrate and are thus not suitable for the protection layer applications. The best performance was achieved using thermally-grown (Mn,Co) 3 O 4 spinel protection layers that substantially improved the surface stability of the metal substrates, and prevented chromium outward migration.

Abstract: A series of the electrochemical and long-term immersion tests was carried out in a strong sulfuric acid (5 wt.% H2SO4) solution on thermal sprayed WC cermet coatings having various kinds of metallic binder in order to examine the effect of composition of binder materials on the corrosion behavior. In the present study, the coatings were processed via a high velocity oxygen fuel (HVOF) spraying technique with WC–Co, WC–Co–Cr, WC–CrC–Ni and WC–Ni composite powders. The experimental results revealed that a considerable galvanic corrosion occurred between WC particles and metallic binders in the aerated 5 wt.% H2SO4 solution. The corrosion resistance of the coatings containing Cr was better than that of the coatings without Cr. For the coatings without Cr, general corrosion occurred in binder materials in addition to galvanic corrosion between WC particles and metallic binders. By contrast, the formation of passive film in the form of surface oxide in the coatings containing Cr suppressed the binder material dissolution into the solution. However, the overall corrosion resistance of the WC–CrC–Ni coating was inferior to that of the WC–Co–Cr coating due to the presence of microcracks which act as the infiltration paths of the solution. Conclusively, it is found that chemical composition of metallic binder materials and control of microcracks are the most important factors influencing the corrosion resistance of the HVOF sprayed WC cermet coatings in the strong acidic environment. Also, the wear–corrosion results such as surface morphologies and friction coefficients have been presented.

Abstract: The binary systems of Mo–N and W–N offer a high potential to enhance the tribological properties of common hard coatings, owing to their ability of forming lubricious oxides (often also referred to as Magneli phases) at elevated temperatures. The aim of this work is to characterize Mo–N and W–N hard coatings prepared by reactive unbalanced magnetron sputtering, and to verify the concept of solid/liquid oxide lubrication. Oxidation of the coatings and possible melting of the oxides are investigated by dynamical thermo-gravimetric analysis and differential scanning calorimetry (DSC) up to 900 °C in air. During heating the samples, the exothermal reactions, detected during the DSC measurements, indicate oxidation in the temperature range between 200 and 750 °C. These exothermic reactions are superimposed by endothermic melting reactions between 650 and 850 °C. The formed oxide phases are investigated by X-ray diffraction after DSC and tribometer testing. To evaluate the effect of formation and melting of the oxides on the coefficient of friction, dry sliding experiments in the temperature range between 25 and 700 °C against alumina and austenitic stainless steel balls are performed using a ball-on-disc tribometer. All coatings investigated show lubricious oxide formation, resulting in a decreasing coefficient of friction at temperatures above 500 °C. For Mo–N, these beneficial properties suffer from the formation of the volatile oxide MoO 3 at temperatures higher than 500 °C. Nevertheless, the lubricious oxide concept within both systems strongly contributes to the reduction of friction, which is important for several industrial applications. We therefore infer that the addition of Mo–N and/or W–N phases to hard coatings can effectively improve their tribological properties, especially in the temperature range between 200 and 700 °C.