Friction and wear behaviour of plasma sprayed WC–12Co coating sliding against Si3N4 under lubrication of ionic liquids

Abstract

The friction and wear behaviour of a WC–12Co coating prepared by plasma spraying sliding against a Si₃N₄ ceramic ball, under the lubrication of liquid paraffin and ionic liquids 1-methyl-3-butylimidazolium hexafluorophosphate and 1-methyl-3-hexylimidazolium hexafluorophosphate at room temperature, was investigated using an SRV tester. The morphology and elemental distribution of the worn coating surfaces were characterised by means of scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analyser (EDXA) attachment, and the chemical state of typical elements in the boundary lubricating film on the worn coating surface was analysed by means of X-ray photoelectron spectroscopy (XPS). The SEM/EDXA analysis shows that phosphorus is uniformly distributed on the worn coating surface lubricated by ionic liquids. The XPS results also indicate that the boundary lubricating film is mainly composed of CoF₂ and PF_x and the tribochemical reaction products contribute to reducing the friction and wear of the plasma sprayed WC–12Co coating.     

Wear study of Ni–WC composite coating modified with CeO2

In the present investigation, Ni–WC composite powder was modified with the addition of CeO₂ in order to form a new composition of Ni–WC–CeO₂. The Ni–WC and Ni–WC–CeO₂ compositions were used for coating deposition by high-velocity oxy-fuel (HVOF) spraying process so as to study the effect of CeO₂ addition on microstructure, distribution of various elements, hardness, formation of new phases, and abrasive wear behavior. Further, the effect of load, abrasive size, sliding distance, and temperature on abrasive wear behavior of these HVOF-sprayed coatings was investigated by response surface methodology. To investigate the abrasive wear behavior of HVOF-sprayed coatings four factors such as load, abrasive size (size in micrometers), sliding distance (meters), and temperature (°C) with three levels of each factor were investigated. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding distance were the main significant factors while load and abrasive size, load and sliding distance, abrasive size and sliding distance were the main significant interactions. Thus an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 4–9% error. The abrasive wear resistance of coatings increases with the addition of CeO₂. This is due to increase in hardness with the addition of CeO₂ in Ni–WC coatings.     

Wear behaviour of laser cladded and plasma sprayed WCCo coatings

The usefulness of WCCo cermets as wear resistant material for coatings is determined by the cladding technique employed. This paper compares the features of an 83% WCCo coating on an AISI 1043 steel substrate using two different application techniques: plasma spraying and laser cladding. Results show significantly less porosity, improved coating hardness and better layer-substrate adherence in laser cladded than in plasma sprayed coatings. This causes them to have different wear behaviour which was determined using a method developed on the basis of the PV factor theory using sliding linear contact of flat-cylinder type. The method proved that wear rate (V_d′) is directly proportional to the product of coefficient of friction (μ), load (C) and applied speed (V), V_d′ = KμCV, where proportionality constant, K, is different for every material and depends on conditions such as lubrication, temperature, etc. To study wear behaviour, laser cladded and plasma sprayed 83% WC-Co coatings, under extreme lubrication, were placed against a hardened and tempered AISI 1043 steel, at different load and sliding speed rates. As a result constant K was estimated for each coating. The tests also showed that wear rate in laser deposited coatings is approximately 34% lower than in plasma sprayed coatings.     

Comparative study of the tribological behavior of self-lubricating W–S–C and Mo–Se–C sputtered coatings

Transition metal dichalcogenides (TMD) have been one of the best alternatives as low friction coatings for tribological applications, particularly in dry and vacuum environments. However, besides their deficient behavior in humid containing atmospheres, their extensive application has also been restricted due to their low load-bearing capacity. In order to overcome these problems, recently the alloying with C has been tried with the expectation of simultaneously improving the coatings hardness and reaching sliding contacting phases more convenient for achieving low friction in humid environments.The practical application of this concept was extensively studied with the W–S–C system, with the C addition being achieved either by reactive or co-sputtering processes. The best tribological results were obtained by co-sputtering from a C target embedded with an increasing number of WS₂ pellets. Excellent results were reached from the more than one order of magnitude increase in the coatings hardness up to friction coefficients which are close to those of the references of self-lubricating coatings: TMD for dry or vacuum atmospheres or C-based coatings for terrestrial sliding conditions.Following the good results achieved with W–S–C system, other TMDs systems have been envisaged to be studied. The main focus was placed on the Mo–Se–C system.In this paper, the general comparison between W–S–C and Mo–Se–C coatings is presented. The main effort is pointed on the tribological behavior of both systems when tested by pin-on-disk against steel counterpart balls under different testing conditions: applied normal loads, temperatures and relative humidity of the atmospheres. Both coatings were deposited by co-sputtering from a C target with a varying number of TMD pellets which could lead to C contents in the films in the range from 30  up to 70 at.%. A Ti interlayer was interposed between the films and the substrates for improving the adhesion.Typically, W–S–C films are harder than Mo–Se–C films. From the tribological point of view, W–S–C films are more thermally stable than Mo–Se–C films although the friction coefficients of these last ones are lower when tested in humid containing atmospheres.     

Surface finishing: Impact on tribological characteristics of WC–Co hardmetals

Flat samples of WC–Co hardmetals with 6–12 wt%Co were surface finished by grinding, polishing and wire-electro-discharge machining (EDM). Comparative dry reciprocating sliding experiments against WC–6 wt%Co pins were performed using a Plint TE77 tribometer. Tribological characteristics were recorded online. Wear surfaces were characterized by surface scanning topography and scanning electron microscopy. Wire-EDM’ed samples exhibited higher friction and wear compared to ground and polished equivalents. This trend was correlated to X-ray diffraction measurements revealing tensile residual surface stresses in WC after wire-EDM contrary to compressive surface stresses after grinding and polishing. However, finer executed EDM reduces friction and wear significantly.     

Effect of active gas mixture composition on tribological behavior of coatings obtained by reactive magnetron sputtering of chromium in acetylene–nitrogen and acetylene–air gas mixtures

The tribological properties of chromium-based coatings deposited by reactive magnetron sputtering in argon–acetylene–nitrogen and argon–acetylene–air mixtures of different volume compositions have been studied. The coatings acquired in acetylene–nitrogen gas mixture have DLC structure and demonstrate high mechanical characteristics and low coefficient of friction values in dry friction. Although the coefficient of friction of these coatings has a tendency to decrease with the increasing nitrogen volume concentration in the mixture (until the values below 0.1 in dry friction in pair with silicon nitride), their performance diminishes at high contact pressures. This decrease was maximal for coatings deposited in acetylene–nitrogen gas mixture, with 80 vol % of nitrogen. The substitution of nitrogen with air is shown to considerably improve the performance, but a further increase in the air volume fraction above 85 vol % provokes a drastic decrease in the efficiency, especially at high contact pressures. The possible mechanisms of these effects and the abilities of their elimination are discussed, as well.     

The tribological performance of DLC-based coating under the solid–liquid lubrication system with sand-dust particles

Combination of solid and liquid lubricants to meet emission or environmental requirements of future tribological systems while providing the levels of desired friction and wear performance have received considerable research attention in the near term. The aim of the present work was to investigate the tribological behavior of oil-lubricated (PAO, PFPE, SO, IL and MAC) DLC coated surfaces under the conditions without and with sand-dust particles. The effects of applied load, frequency, and sand-dust particles on the tribological performance of DLC coating were systemically studied. The analysis results showed that solid–liquid lubricating coatings including SO and IL exhibited excellent anti-friction (～0.026) but relative poor wear-resistance performances under the conditions without and with sand-dust environments. But for PFPE and PAO, they demonstrated the worst tribological behaviors with high friction coefficient and wear rates. The added sand-dust particles lead to the wear rates to the one order of magnitude large than that without sand-dust conditions for all the selected liquid lubricants. The viscosity, contact angle and work of adhesion played an important part in affecting the tribological performances. The lubrication regimes in Stribeck curve for the five kinds of liquid lubricants were affected obviously by the sand-dust particles in different way. The formed transfer films on the coating surface and pin have much influence on the tribological behavior and the transition between lubrication regimes.     

Wear mechanisms of several cutting tool materials in hard turning of high carbon–chromium tool steel

The present study illustrates the performance of three different cutting tool materials, namely: PCBN, TiN coated PCBN, and mixed aluminum ceramic (Al₂O₃+TiC) in the turning of medium hardened D2 tool steel (52 HRC). Formation of Cr–O tribofilms on the ceramic tool surface as a result of interaction with the workpiece material and environment (identified by X-ray Photoelectron Spectroscopy) leads to improvement of lubricating properties at the tool/chip interface. Obtained results revealed that the mixed alumina ceramic tool can outperform both types of PCBN under different machinability criteria.     

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT

The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr₃C₂-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.     

Mechanical and tribological properties of Cr–Nb double-glow plasma coatings deposited on Ti–Al alloy

Double-glow plasma (DGP) coatings are recommended for metallic components to mitigate the damage induced by complex working conditions in previous studies. In this paper, Nb-rich (Cr–Nb₄) and Cr-rich (Cr₄–Nb) -alloyed layers were formed onto the Ti–Al substrate via a DGP process to enhance its wear resistance. Scratch and Nano-indentation tests were used to evaluate the mechanical properties of the coatings. The tribological behaviour of the coatings were investigated using a pin-on-disc tribometer by rubbing against the GCr15 ball. Results from surface analysis techniques showed that the coatings mainly comprised Cr, Nb and Cr₂–Nb phases, and were well bonded to the substrate. The hardness of the Cr–Nb₄ coating was 11.61GPa and the Cr₄–Nb coating was 9.66 GPa which all higher than that of the uncoated Ti–Al which was 5.65 GPa. However, the critical load of the Cr₄–Nb coating ~21.64 was higher than that of the Cr–Nb₄ coating ~17.6. And the specific wear rate of Cr–Nb₄ coating, Cr₄–Nb coating and uncoated Ti–Al were 3.54 × 10^?4, 0.01 × 10^?4 and 1.53 × 10^?4mm³ N^?1 m^?1_, respectively. The low-wear mechanism of the coatings is discussed in detail in this paper.     

A study on the tribological characteristics of duplex-treated Ti–6Al–4V alloy under oil-lubricated sliding conditions

The tribological characteristics of the polished, dimpled and over-coated dimpled specimens were investigated. Dimples were produced on a Ti–6Al–4V alloy specimen using a laser surface texturing (LST). A Cr-doped diamond-like carbon (DLC) film was deposited on a dimpled specimen using an unbalanced magnetron sputtering (UBMS). The effects of dimples and over-coated Cr-doped DLC film on the tribological characteristics were investigated by performing the friction tests against a Cr-plated steel pin. The test results showed that the over-coated dimpled specimen exhibited a lower friction coefficient and wear compared to those of the polished and dimpled specimens, which may be attributed to the storage of wear debris and high hardness. A model of the wear reduction mechanism of the specimens was discussed.     

Effect of 10 wt% VC on the Friction and Sliding Wear of Spark Plasma–Sintered WC–12 wt% Co Cemented Carbides

The effect of 10 wt% VC addition on the friction and sliding wear response of WC–12 wt% Co cemented carbides produced by spark plasma sintering (SPS) was studied. The SPS of WC–12 wt% Co alloys with and without 10 wt% VC, at 1100 and 1130°C, respectively, yielded dense materials with minimal porosity. No eta phase was found in any of the alloys. The WC–12 wt% Co–10 wt% VC alloy showed the formation of a hard WV₄C₅ phase, which improved the alloy's hardness. Friction and dry sliding wear tests were done using a ball-on-disk configuration under an applied load of 10 N and sliding speed of 0.26 m.s^?1, and a 100Cr-steel ball was used as the counterface. A significant improvement in the sliding wear response of the harder and more fracture tough WC–12 wt% Co–10 wt% VC alloy compared to the WC–12 wt% Co alloy was found. Analysis of the worn surfaces by scanning electron microscopy showed that the wear mechanisms included plastic deformation, preferential binder removal, adhesion, and carbide grain cracking and fragmentation.     

Some study on electrical discharge machining of ({WC+TiC+TaC/NbC}–Co) cemented carbide

Electrical discharge machining process is a potential method of shaping ({WC+TiC+TaC/NbC}–Co) cemented carbide known for its superior hardness and compressive strength at high temperature and resistance to diffusion wear. Yet, detailed study on electrical discharge machining of this material is lacking in the literature. In the present investigation, therefore, mathematical models are developed for material removal rate, wear ratio, and surface roughness in electrical discharge machining of this cemented carbide using the procedure of statistical design of experiments. Current setting, pulse on time, and pulse off time are chosen as input parameters. Based on available machine settings, a face-centered central composite design is selected for meaningful experimentations. The procedure may be extended to develop a data bank for such type of materials. Further, to reveal the attributes behind the removal of material from the work-piece surface, scanning electron micrographs are studied. It appears that sufficient superheating of work-piece material and subsurface boiling is essential for efficient material removal and that formation of pock marks due to burst of blisters and associated crack formation may be controlled by choosing a proper dielectric.