High temperature wear resistance of (TiAl)N PVD coating on untreated and gas nitrided AISI H13 steel with different heat treatments

The wear resistance of a PVD (Ti_0.7Al_0.3)N coating deposited on an as-received and gas nitrided AISI H13 has been examined by using ball-on-disc tests at room temperature and at 600 °C. In order to determine the influence of a previous heat treatment on this type of steel on the wear resistance of the (Ti_0.7Al_0.3)N coating, two commercial heat treatments were employed which gave rise to the same substrate hardness. Surface microhardness measurements have been carried out to determine the load-carrying capacity of the coated systems. In general, the wear behavior was found to be independent of the nature of the heat treatment applied to the substrate prior to the nitriding process but strongly dependent on the testing temperature. At room temperature, there were small variations between the different systems tested, whereas at high temperatures, clear differences were found between them. At 600 °C, a typical temperature that could be achieved during the aluminum extrusion processes, the nitrided H13 steel/(Ti_0.7Al_0.3)N PVD duplex coating shows a satisfactory wear resistance compared to both the nitrided steel and the steel substrate only coated with (Ti_0.6Al_0.4)N, which exhibited the worst performance. The satisfactory wear resistance observed for the duplex coating system at high temperature is mainly a consequence of two different aspects. Firstly, its higher load-carrying capacity due to the existence of a hard nitrided layer, as well as its high H/E ratio. Both parameters allow the presence of higher elastic strains without the failure of the ceramic layer, which would normally occur in the case of TiAlN PVD coatings deposited directly on the AISI H13 steel. Secondly, the intrinsic characteristics of the coating, i.e. its chemical constitution, which allows the formation of a dense oxide mixture inside the wear track that impedes both its further oxidation and the deterioration of the mechanical properties as consequence of nitrogen diffusion.     

The friction and wear of electroless Ni–P matrix with PTFE and/or SiC particles composite

In this paper, the friction behaviour and wear mechanism of electroless Ni–P matrix with PTFE and/or SiC particles composite coating are investigated by virtue of ring-on-disk wear machine at a high load of 150 N. The worn surface, wear debris and the composition changes after wear were characterized using scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX). By comparison with Ni–P and Ni–P–SiC coatings, the results indicated that the combination of a PTFE-rich mechanical mixed layer (PRMML) formed on the worn surface and hard SiC were responsible for the good tribological properties of the hybrid Ni–P–PTFE–SiC composites at high load. After heat treatment at 400 °C for 1 h, the wear rate of Ni–P matrix composites decreased with corresponding increase in microhardness. During sliding, an obvious decrease in the temperature rise with PTFE addition was attributed to the good anti-friction of PTFE.     

Wear mechanism of coated tools in the turning of ductile cast iron having wide range of tensile strength

In this study different specimens of ductile cast iron with tensile strength ranking from 400 MPa to 675 MPa were turned with K15 carbide, TiN coated and TiAlN coated tool in order to investigate wear mechanism and performance. Cutting forces and cutting temperature were similar for both coated tools, however flank wear and BUE were the lowest on the TiAlN coated tool, for this reason the TiAlN coated tool is suitable in the machining of ductile cast iron. The proposed tool wear mechanism is based on like-intermittent cutting caused by the pass from hard matrix to the soft graphite occasioning wear by adhesion. The analysis of the flank wear on coated tools is proposed by means of the wear curves in logarithmic scale instead of the usual linear scale. In this way, the change in wear rate is easily observed. This phenomenon was related with the wear out of the coating layer. The partial loss of the coating layer on cutting edge was confirmed by the EDS mapping images and SEM photographs.     

Evaluation of braze bonded hard complex boride based coatings for sliding,erosion and abrasion wear

Materials requiring improved resistance to wear have been researched in coatings as well as in bulk form. A new process that aims to produce a wear resistant surface through powder metallurgy exists. The process brazes a green tape containing a reactive mixture of Mo, Fe, Cr, MoB and FeB and which produces a microstructure of hard complex borides dispersed in a soft metallic matrix, onto a compatible metallic substrate. Hence the process is called as “Braze-bonding”. In the process the phenomena of coating densification, microstructure development and interface development occur simultaneously. The resultant hard layer is evaluated for performance under sliding wear, erosive wear and abrasive wear conditions. It has been found that the coating is competitive to other hard materials. The process has inherent advantages like applying the coating in situ and easily allows for varying the thickness of the coated layer. A diffusion driven interface between coating and substrate improves bond strength. Complex borides of more than 50 vol.% dispersed in a Fe based ductile matrix help in abating wear through different mechanisms, which have been discussed here. The braze coatings can be used for applications involving wear resistance like pump impeller parts, machining tools, and injection molding screws.     

Enhancement of wear resistance of ductile cast iron by Ni–SiC composite coating

Wear resistance of unalloyed ductile iron (Dl) can be enhanced either by heat treatment or by deposition of hard coating. The electrodeposition of Ni–SiC composite on unalloyed Dl (GGG 40) has been applied. The effect of operating conditions including current density and SiC content in the plating solution on the SiC incorporation in the deposited layer were studied. It was found that the volume percent of SiC particles in the composite layer increases with increasing current density and SiC content in the bath. The maximum SiC incorporation could be attained at optimum conditions; 60 g/1 of SiC particles in suspension, 5 A/dm², pH 5 and 50 °C. Also the results reveal that the particle inclusion in the coating layer depends mainly on the treatment process (activation with PdCI₂). The mechanical properties of the composite such as hardness and wear resistance were examined comparing with the uncoated substrate. The reinforced particles incorporated with Ni-matrix improve the hardness and wear resistance of coated Dl comparing with uncoated substrate.     

Effect of three nitriding treatments on tribological performance of 42CrAlMo7 steel in boundary lubrication

Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investigated during boundary-lubricated rolling–sliding tests. The wear behaviour of nitrided steel with a thin compound layer (produced by plasma nitriding and by gas nitriding followed by oxidation) was characterised by the early removal of the compound layer, and the wear resistance was thus, given by the underlying diffusion layer. In the case of the material with a thick compound layer (produced by gas nitriding) wear was restricted to the compound layer. In this material, at low applied load (300 N, i.e. 485 MPa of Hertzian pressure, in this work), after the removal of the external porous layer wear tended to be negligible. At high applied load (1000 N, 890 MPa), on the other hand, the wear rate became higher than that of the diffusion layer. The friction behaviour was followed by determining the λ-factor evolution during each test. For a given λ-factor, the friction coefficients at 300 N were lower than at 1000 N.     

Comparative tribological study of air plasma sprayed WC–12%Co coating versus conventional hard chromium electrodeposit

In this work, the properties of air plasma sprayed WC–12%Co coating before and after heat treatment were compared with the properties of the hard chromium electrodeposit. WC–12%Co coatings were heat treated at 650, 900 and 1150 °C for 1 h in an argon atmosphere. XRD patterns confirmed the formation of an amorphous phase in the as-sprayed coating. This amorphous phase gradually transformed to η-carbides in the course of heat treatment of the coating. This transformation was confirmed by the XRD analysis of the coatings heat treated above 900 °C. Pin-on-disc wear test results showed that WC–12%Co coatings had a significantly better tribological performance as compared with that of the hard chromium electrodeposits. The results also indicated that heat treatment of the WC–12%Co coatings at 900 °C gave the highest wear resistance among the coatings, which was due to the formation of hard η-carbides at this temperature.     

Friction and wear scar analysis of carbon nanofiber-reinforced polymeric composite coatings on alumina/aluminum composite

Alumina/aluminum based composites with excellent physical and mechanical properties offer great potential for lightweight, wear resistant, and high temperature applications. The objective of the present research was to investigate a suitable coating material to provide a low coefficient of friction (COF) during sliding contact. The friction behavior of carbon nanofiber-reinforced aerospace polymer coatings prepared by the spin coating technique were investigated. Polymethylmethacrylate (PMMA), bis A polycarbonate, and two biphenyl endcapped poly(arylene ether phosphine oxide) compositions, namely BPETPP-E and 6FETPP-E, were used as the matrices. Pin-on-disc experiments were performed between 440C stainless steel balls and disc samples of coated alumina/aluminum interpenetrating phase composites at 0.2 m/s sliding velocity, in air, at room temperature under 0.25 and 0.74 N normal load. In all cases, formation of a lubricious carbon layer and its transfer to the steel counterface was observed to result in lower COF (∼0.2–0.3). Higher levels of fiber content (40 and 60 wt.% fibers) contributed to a faster formation of this layer. Wear scar analysis showed the dual roles of the carbon nanofibers, serving as solid lubricants and as reinforcement in the coatings. The amount of debris generated and the coverage of the lubricious carbon-rich film on the scar surface was dependent on the matrix material used. Adherent and uniform coverage of a lubricious carbon-rich film at the wear contact with the least amount of debris fragments was obtained only for composite coatings using BPETPP-E and 6FETPP-E matrices.     

Tribological and physical-mechanical properties of protective coatings from Ni-Cr-B-Si-Fe/WC-Co-Cr before and after fission with a plasma jet

A new type of coating is developed, which is a mechanical mixture of two different powders, namely, Ni-Cr-B-Si-Fe (PG-19N-01) and WC-Co (hard alloy). After the coatings from this mixture are deposited, their surface layer is fused with a plasma jet using an eroding electrode made of W. The additional treatment of the coatings with the plasma jet yields new phases and causes the redistribution of elements in the layer 45–60 μm deep; the percentage ratio of the phases WC, α-CoCr, Co, and Ni, as well as Cr₃Ni₂ + γ-(Fe, Ni) appearing during coating deposition also changes. The redistribution of elements occurs in the upper coating layer owing to fusion with the plasma jet. These processes yield variations in the physical-mechanical properties of the coatings, such as the hardness and elastic modulus; the coating wear rate decreases severalfold. It is found that with increasing load applied to the Berkovich pyramid the elastic modulus of the coating drops from 240 (at an indentation depth of 50 nm) to 175 GPa (at 150 nm). The elastic modulus of the substrate rises from 25 to 42 GPa. The coating hardness calculated from the loading-unloading curves is 15.3 to 10.6 GPa under increased load applied to the indentor. Specimens covered with the coating fused with the plasma jet in three passes demonstrate the lowest material wear.     

Deposition of duplex Al2O3/TiN coatings on aluminum alloys for tribological applications using a combined microplasma oxidation (MPO) and arc ion plating (AIP)

Microplasma oxidation (MPO) has recently been studied as a cost-effective plasma electrolytic process to provide thick and hard ceramic coatings with excellent surface load-bearing capacity on aluminum alloys. However, for sliding wear applications, such ceramic coatings often exhibit relatively high friction coefficients against many counterface materials. Although coatings deposited by physical vapour deposition (PVD) techniques such as TiN coatings are well known for providing surfaces with a high 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 this paper, these challenges were overcome by a duplex process of microplasma oxidation and arc ion plating (AIP), in which an alumina layer Al₂O₃ was deposited on an Al alloy substrate (using MPO as a pre-treatment process) for load support, and a TiN hard coatings were deposited (using AIP) on top of the Al₂O₃ layer for low friction coefficient. Microhardness measurements, pin-on-disc sliding wear tests, and antiwear tests using a Timken tester were performed to evaluate the mechanical and tribological properties. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc test. The research demonstrates that a hard and uniform TiN coating, with good adhesion and a low coefficient of friction, can successfully be deposited on top of an alumina intermediate layer to provide excellent load support. The investigations indicate that a duplex combination of MPO coating and TiN PVD coating represents a promising technique for surface modification of Al alloys for heavy surface load bearing application.     

Surface stresses in coated steel surfaces—influence of a bond layer on surface fracture

Thin hard coatings in the thickness range of only a few micrometers deposited by physical vapour deposition (PVD) on components or tools can improve the friction and wear properties by several orders of magnitude. A 2 μm thick TiN (E=300 GPa) coating on a high-speed steel substrate with a bond layer at the interface between the coating and the substrate was modelled by micro-level three-dimensional finite-element method (3D FEM) in order to optimise a coated surface with regard to coating fracture. Both compliant low modulus (E=100 GPa) and stiff high modulus (E=500 GPa) bond layers at the coating/substrate interface of 200 and 500 nm thickness were investigated. First principal stresses were simulated for scratch test geometry in the load range of 7.5-15 N. Very high stress concentrations of above 5700 MPa tensile stresses were observed in the bond layer just behind the contact zone for the stiffer bond layer. The stiff bond layer generated 5 times higher tensile stress maxima compared to the compliant bond layer. There was approximately 3.5 times larger strain in the compliant bond layer compared to the stiff bond layer. The general coating design advice based on this exercise is that when a bond layer is used e.g. for coating/substrate adhesion improvement should the bond layer be less stiff than the coating not to generate high and critical tensile stresses. The thickness of the bond layer may vary and is not critical with respect to generated stresses in the surface.     

Wear phenomenon in the hard steel machining using ceramic tools

The principle aim of this investigation is to recognize the wear phenomenon of the mixed ceramic tips against 60 HRC steel specimens in dry and hard turning operations. For this purpose both microscopic and microstructural aspects of ceramic tool wear were taken into consideration. Investigations were performed under varying feed rate, constant cutting speed of 100 m/min and small depth of cut of 0.2 mm to perform finishing cuts. Light optical microscopy (LOM), scanning electron microscopy (SEM), BSE technique and X-ray diffraction analysis (XRM) were applied for observations of worn tool surfaces, wear products and the distinction of wear mechanisms occurring. In general, wear mechanisms observed in the machining tests involve abrasion, fracture, plastic flow, material transfer and tribochemical effects which appear depending on the mechanical and thermal conditions generated on the wear zones. In particular, two types of transfer layer formation with different morphologies occurring at the rake-chip interface are distinguished.     

Effect of Interlayer Thickness of Ion-Plated Pb-Coating on Its Life (SUS440C Substrates)

High-precision ball bearings (SUS440C) used in spacecraft mechanisms that are exposed to deep space vacuum are solid lubricated by thin, adherent Pb ion-plated coatings. The high reliability requirements with long endurance life of these mechanisms solely depend on the tribological aspects of this solid-lubrication coating. Since ion-plated, soft metallic coatings have a graded coating–substrate interface structure, a gradual change in mechanical, chemical and thermal properties is expected across the depth of the coating. This aspect of the interlayer plays a major role in determining the extent of optimum tribological properties of the coating. By improving the interlayer, both in terms of increased thickness and concentration of Pb, a corresponding improvement in the lubricity and endurance life of this Pb ion-plated coating on AISI 440C stainless steel (bearing material) can be assured. For this, a detailed examination of the interlayer of the ion-plated Pb film on the nano-micro level is mandatory. This report is focused on the cross-sectional examination of the interlayer by scanning electron microscopy. SUS440C steel disk specimens having similar properties as the bearing material and with a thin, ion-plated Pb coating processed under two different substrate bias conditions of ion-plating are used. The thickness of the Pb/steel interlayer and the elemental composition are analyzed using SEM and EDS respectively. The endurance life of the coatings is measured by repeated sliding wear tests with a pin-on-disk tester in vacuum. It is confirmed that with an increased thickness of the graded interfacial layer of the Pb ion-plated coating, the coating endurance life is extended remarkably.     

Mechanical properties of thin carbon overcoats

Components used in magnetic storage systems (hard discs, tape heads and drums) are often very small and lightweight, and operate under very low loads (of the order of a few micrograms to a few milligrams). As a result, friction and wear processes occur on a nanometre scale and conventional tribological test methods and assessment tools are usually not appropriate. Furthermore, the assessment of the mechanical properties of the coatings or surface treatments used to protect these components from wear is complicated by the low thickness of the layers generally used. This paper details the problems associated with the assessment of the mechanical properties of thin diamond-like carbon coatings used to protect hard discs, tape heads and air bearings. Whereas thick coatings (>1 μm) are relatively easy to assess, even if the substrate has a low hardness and offers little support to the coating, there are many more problems when it comes to measuring the properties of the 5–10 nm layer on a hard disc. In many cases there is no plastic deformation of the coating which merely flexes and bends into the hole produced by plastic deformation of the substrate. Deformation of the coating is then limited to localised plasticity at the indenter edges, and/or fracture along the same edges and at the edge of the contact. The limits for use of Nanoindentation to assess the plasticity of the coating are discussed for such cases.     

Dry wear and friction behaviour of plasma nitrided Ti–6AL–4 V alloy after explosive shock treatment

The unlubricated wear behaviour of explosive shock treated and, subsequently plasma nitrided Ti–6Al–4 V alloy was studied using a ball-on-disc wear tester. Plasma nitriding was carried out at three different temperatures (700, 800 and 900 °C) for 3, 6, 9 and 12 h. Plasma nitriding after explosive shock treatment enabled a reduction in the wear rate of two orders of magnitude. Detailed investigations of this improved wear performance dependent on the nitriding temperature and time were carried out. The friction and wear data showed a clear breakthrough transition from the nitrided layer to the core of the Ti–6Al–4 V alloy matrix. The lowest wear volume was obtained for the sample, nitrided at 900 °C for 12 h, especially at loads of 2.5, 5 and 7.5 N. Obviously, the hard nitride layers were intimately associated with low wear rate, providing a smooth low friction surface. The coefficient of friction reduced from 0.46 to 0.2 due to a thick and hard compound layer resulting from a high nitrogen diffusion rate caused by explosive shock treatment that expected to increase point defects in the alloy. Detailed examination of the wear tracks showed that plasma nitriding changes the mechanism of wear from one of adhesion for untreated Ti–6Al–4 V to both delamination and mild abrasive.     

Ultrathin PFPE Film Systems Fabricated by Covalent Assembly: An Application to Tribology

In this study, we have employed covalent molecular assembly to fabricate robust thin film structures comprising molecular layers and have demonstrated its application in tribology. An anhydride-functionalized polymer (gantrez) was deposited over an amine-functionalized silicon surface through covalent binding and employed as an intermediate layer between derivatized silicon and perfluoropolyether (PFPE). X-ray photoelectron spectroscopy, atomic force microscopy, and ellipsometry were employed to study the interfacial chemistry, morphology, and thickness of the assembled films. The films show excellent stability and strength against sonication, which can be attributed to the covalent interlayer linkage. Such films showed wear life of >100,000 cycles in ball-on-disk sliding tests at a normal load of 0.5 N and a sliding rotation of 200 rpm at a track radius of 3.2 mm. The performance was superior compared to that of PFPE-coated self assembled monolayers used as the lubricating layer. The film systems and assembly technique can be employed as nano-lubrication in several technological applications, such as information storage devices and micro-electro-mechanical systems.     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

Self-Repairing Characteristics of Serpentine Mineral Powder as an Additive on Steel–Chromium Plating Pair under High Temperature

Friction and wear experiments on steel–chromium plating pairs were carried out with nanoscale serpentine (a magnesium silicate mineral) as a lubricating oil additive at 400°C. The tribological test results showed that self-repairing protective layers formed on the contact surfaces of both the steel matrix and hard chromium coating. Field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the morphology and elements of self-repairing layers were in accordance with that of serpentine. A generation mechanism of the layer was proposed that suggests that isomorphic replacement between Fe/Cr and serpentine mineral silicate occurs, which is the wear mechanism of the tribochemical reaction.     

Studies of the tribological and mechanical properties of laminated CrC–SiC coatings produced by r.f. and d.c. sputtering

Multilayer CrC–SiC coatings were produced by both direct current and radio frequency sputtering of Cr and Si targets in an Ar–C₂H₂ atmosphere. Coatings of constant thickness of about 4 μm, but with the number of layers varying between two and 200, were prepared and studied. Investigations of coating morphology were performed by scanning elecron microscopy (SEM), and coating composition was investigated using Glow Discharge Optical Spectroscopy (GDOS) and Auger Electron Spectroscopy (AES). Microhardness measurements, scratch adhesion, pin on disc, ASTM rubber wheel and impact wear tests were performed and the results were related to the individual layer thicknesses. It is shown that the improvement observed in hardness tests does not necessarily result in the improvement of other mechanical properties, e.g. adhesion and toughness. When two comparatively hard materials are combined in a multilayer coating, the result can be an increase in brittleness due to an absence of plastic release mechanisms for dislocation accumulation at layer boundaries. Thus, it is necessary to seek a compromise in the hardness of multilayer films in order to achieve optimal behaviour across a range of different surface contact conditions.     

Interlayer thickness influence on the tribological response of bi-layer coatings

The present work deals with the influence of coating thickness on the tribological response of bi-layer model coatings consisting of CrN with Cr interlayer with varying Cr/CrN thickness ratios on high-speed steel. Ball-on-disc experiments were carried out in ambient air at room temperature and alumina balls as counterbodies. The mechanical stresses in both layers generated during the tests were calculated with the software package Elastica. Wear tracks on the samples were characterised using both scanning electron microscopy and optical profilometry. The results show that the interlayer thickness plays a determinant role in the tribological response of the coatings provided that the CrN layer thickness exceeds a critical value to withstand mechanical wear.