A. Matthews

On the significance of the H/E ratio in wear control: a nanocomposite coating approach to optimised tribological behaviour

Although hardness has long been regarded as a primary material property which defines wear resistance, there is strong evidence to suggest that the elastic modulus can also have an important influence on wear behaviour. In particular, the elastic strain to failure, which is related to the ratio of hardness (H) and elastic modulus (E), has been shown by a number of authors to be a more suitable parameter for predicting wear resistance than is hardness alone. There is presently considerable interest in the development of nanostructured and nanolayered coatings, due to the fact that materials with extreme mechanical properties (which are difficult to synthesise by other methods) can be created, particularly when using plasma-assisted vacuum processing techniques. Until now, scientific research has been directed mainly towards the achievement of ultra-high hardness, with associated high elastic modulus, the latter of which, conventional fracture mechanics theory would suggest, is also desirable for wear improvement (by preventing crack propagation). In this study, we discuss the concept of nanocomposite coatings with high hardness and low elastic modulus, which can exhibit improved toughness, and are therefore better suited for optimising the wear resistance of ‘real’ industrial substrate materials (i.e. steels and light alloys, with similarly low moduli). Recent advances in the development of ceramic–ceramic, ceramic–amorphous and ceramic–metal nanocomposite coatings are summarised and discussed in terms of their relevance to practical applications. We also discuss the significance of elastic strain to failure (which is related to H/E) and fracture toughness in determining tribological behaviour and introduce the topic of metallic nanocomposite coatings which, although not necessarily exhibiting extreme hardness, may provide superior wear resistance when deposited on the types of substrate material which industry needs to use.

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti-6Al-4V alloy

Abstract The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al 2 TiO 5 and TiO 2 phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm 2 hardness and high adhesion and provide a low wear rate (3.4×10 −8 mm 3 /Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO 2 /TiO 2 -based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H 2 SO 4 solution, due to the greater chemical stability of the film phase components in this environment.

Deposition of layered bioceramic hydroxyapatite/TiO2 coatings on titanium alloys using a hybrid technique of micro-arc oxidation and electrophoresis

Abstract Titanium alloys have been used with some success in several bioimplant applications. However, they can suffer certain disadvantages, such as poor osteoinductive properties and low corrosive-wear resistance. Attempts to overcome the first of these drawbacks have involved coating the metal with the bioceramic material hydroxyapatite (HA), a primary component of bone and a very good osteoinductor. Since TiO 2 coatings are also known to be effective as chemical barriers against the in-vivo release of metal ions from the implants, a double layer HA–TiO 2 coating on titanium alloys with HA as the top layer and a dense TiO 2 film as the inner layer should possess a very good combination of bioactivity, chemical stability and mechanical integrity. This paper describes efforts to improve implant biocompatibility and durability by applying a hybrid treatment of micro-arc discharge oxidation (MDO) and electrophoretic deposition. The most common structural titanium alloy (Ti-6Al-4V) was used as the substrate material. A phosphate salt solution and an HA powder aqueous suspension were used as the electrolyte for micro-arc oxidation and the solution for HA electrophoretic deposition, respectively. It is shown that a relatively thick and hard TiO 2 coating can be produced by anodic micro-arc oxidation of titanium, and an HA coating incorporated on top of the TiO 2 layer can simultaneously be formed using a combination of plasma electrolysis and electrophoresis, with the suspension held at high values of pH. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the microstructure and morphology of the coatings. The adhesive strength between the coating and substrate has been assessed using scratch adhesion testing. The corrosion resistance of the specimens was examined using potentiodynamic tests in a buffered physiological solution. The results indicate that a hybrid combination of micro-arc oxidation and electrophoretic deposition can provide a phase-pure HA top layer and anticorrosive TiO 2 interlayer, which should show good mechanical and biochemical stability in the corrosive environment of the human body.

Abrasive wear/corrosion properties and TEM analysis of Al2O3 coatings fabricated using plasma electrolysis

Abstract Alumina coatings were deposited on Al alloy substrates using an electrolytic plasma technique, based on a dielectric barrier discharge created during anodic oxidation in an aqueous electrolyte. The substrate material (BS Al 6082) was biased anodically with an unbalanced AC high voltage. During processing, a plasma current density of 100 mA/cm2 was used, at which a coating deposition rate of 1.67 μm/min was achieved. Coating abrasive wear and corrosion properties were assessed by conducting dry and wet rubber wheel abrasive tests and potentiodynamic polarization experiments, respectively. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the coating microstructure, and the coating/substrate interface. The property test results show that the coatings possess excellent abrasive wear and corrosion resistance. XRD analyses indicate that the coatings consist of α- and γ-Al2O3. An amorphous+nanocrystalline inner layer (1.5-μm thick) and a nanocrystalline (50–60 nm) intermediate layer in the coating were observed by TEM. The higher resistance to wear and corrosion can in part be attributed to the presence of these interlayers.

Discharge characterization in plasma electrolytic oxidation of aluminium

Digital video imaging of the plasma electrolytic oxidation (PEO) of aluminium has been performed, which allowed evaluation of both dimensional characteristics of individual microdischarges appearing at the oxide–electrolyte interface and their collective behaviour throughout the oxidation process. It has been shown that the microdischarge cross-sectional dimensions vary within the range 0.01–1.35 mm2. In the course of PEO processing, small localized events (<0.03 mm3) always dominate in the microdischarge spatial distribution and the relative proportion of medium-sized to very large microdischarges is gradually redistributed in favour of the latter. Temporal dependences have been found for the fraction of surface area instantaneously experiencing the discharge, as well as for the spatial and current densities of the microdischarge. Discharge mechanisms occurring during PEO are discussed and a model of microdischarge formation is suggested, assuming the possibility of free-electron generation and glow discharge ignition in the gaseous media developed at the oxide–electrolyte interface. First approximation evaluations of thermal processes in the oxide layer under the discharge conditions have been considered. The estimated ranges of the microdischarge current density (50–18 kA m−2) and duration (0.25–3.5 ms) sufficient for initiating phase transitions (e.g. γ–α transformation and melting) in the surface oxide layer are shown to be in good agreement with experimental data.

Coatings tribology—contact mechanisms and surface design

Abstract The fundamentals of coating tribology are presented by using a generalised holistic approach to the friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, nanomechanical and tribochemical contact mechanisms, and material transfer. The important influence of thin tribo- and transfer layers formed during the sliding action is shown. Optimal surface design regarding both friction and wear can be achieved by new multi-layer techniques which can provide properties such as reduced stresses, improved adhesion to the substrate, more flexible coatings and harder and smoother surfaces. The differences between contact mechanisms in dry, water- and oil-lubricated contacts with coated surfaces is illustrated by experimental results from diamond-like coatings sliding against a steel and an alumina ball. The mechanisms of the formation of dry transfer layers, tribolayers and lubricated boundary and reaction films are discussed.

An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behaviour

Abstract In Part I, of this work the equivalent circuits for electrochemical impedance spectroscopy (EIS) modelling of PVD coated steels in 0.5 N NaCl solution were established. In this paper, Part II, the EIS spectra of such coated systems are modelled using the equivalent circuits. The circuit parameters obtained are correlated with the dielectric characteristics, and microstructure of steels and PVD hard coatings. Coating porosity and localised corrosion with exposure time have also been determined using the corrosion potential difference (Δ E corr ) between mild steel and PVD coatings and polarisation resistance R p , which was obtained through EIS modelling using equivalent circuits. In addition, diffusion rates of the reactants (e.g. oxygen) through ‘permeable’ defects (e.g. pores) are studied by introducing the diffusion impedances W and O in EIS modelling. It has been found that the usage of impedances W and O is closely related to the crystallite features of PVD coatings. Warburg impedance ( W ) is most suitable for columnar crystallites, while the co-tangent-hyperbolic diffusion impedance ( O ) is best for the equiaxed crystallite structure. Finally, visual inspection, SEM examination, and the scanning reference electrode technique were employed to observe the corrosion progress of PVD coated steels with immersion time, in order to validate the EIS interpretation.

An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N nacl aqueous solution: Part I. Establishment of equivalent circuits for EIS data modelling

Abstract Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can provide a wealth of information on the corrosion reactions, the mass transport and the electrical charge transfer characteristics of physical vapour deposition (PVD) ceramic coated steels in an aqueous solution. Although a huge amount of potentially useful data can be generated using the EIS technique, these data need to be carefully interpreted. This is usually done using an ‘equivalent circuit’ which comprises an assembly of electrical circuit elements that model the physicoelectric characteristics of the electrode/solution interface. A systematic study of PVD ceramic (TiN and CrN) coated mild steel and AISI 316L stainless steel was carried out using the EIS technique as the coated systems were immersed in 0.5 N NaCl solution. The relevant equivalent circuits (ECs) are developed by non-linear least square curve fitting to the exponential data to build up a description of the influence of different coatings deposited on steels on the temporal evolution of corrosion in such systems. Constant phase elements describing the non-ideal (e.g. capacitive) characteristics of the electrochemical interface, designated as Q , are introduced to achieve a more accurate simulation of electrochemical corrosion. The mass transport behaviour is also dealt with, through the introduction of diffusion-related elements such as Warburg (designated as W ) and cotangent–hyperbolic (designated as O ) impedance. The use of these elements significantly improves the quality of fit of the simulation to the EIS data. Finally, the physical validity of the proposed models is discussed in terms of the current–frequency response of the coated steel electrode, during extended corrosion degradation over a period of immersion of up to one week.

Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process

In this study, a plasma electrolytic oxidation (PEO) process was used to produce oxide coatings on commercially pure aluminium (1100 alloy) at a pulsed dc power mode. The effects of process parameters (i.e. current density and treatment time) on the plasma discharge behaviour during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285–800 nm). The elements present in the plasma were identified. Stark shifts of spectral lines and line intensity ratios were utilized to determine the plasma electron concentrations and temperatures, respectively. The plasma electron temperature profile, coating surface morphology and coating composition were used to interpret the plasma discharging behaviour. The different coating morphologies and compositions at different coating surface regions are explained in terms of three types of discharge, which originate either at the substrate/coating interface, within the upper layer, or at the coating top layer. The high spike peaks on the plasma intensity and temperature profiles corresponded to discharges originated from the substrate/coating interface, while the base line and small fluctuations were due to discharges at the coating/electrolyte interface.

Thickness effects on the mechanical properties of micro-arc discharge oxide coatings on aluminium alloys

Abstract Weight-saving materials are becoming increasingly important, especially in the automotive and aerospace industries. Design engineers would thus like to make more extensive use of light metals such as aluminium, titanium, magnesium and their alloys; however, these materials tend to have poor wear resistance. Previous treatments and coatings applied to aluminium alloys, for example by traditional processes such as hard anodising and thermal spraying, have suffered from the low load support from the underlying material and/or insufficient adhesion, which reduces their durability. Also, although TiN-, CrN- or DLC-coated aluminium alloys (using various PVD methods) can achieve a high surface hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In the work reported here, a plasma electrolysis technique known as micro-arc discharge oxidation (MDO) was investigated; thick and hard oxide ceramic layers were fabricated on BS Al-6082 aluminium alloy by this method. The phase composition and microstructure of the MDO coatings were investigated by XRD, SEM and EDX analyses. A number of adhesion and tribological sliding and impact wear tests were also performed. It was found that Al–Si–O coatings with a hardness of up to 2400 HV and with excellent wear resistance and load support could be formed. The thickness of the coatings significantly influenced the mechanical properties. In terms of tribological performance, the thicker coatings performed best in sliding, scratch and impact tests whilst thin coatings were also surprisingly effective in both impact and low-load sliding. Coatings of intermediate thickness provided relatively poor performance in all tribological tests.