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.     

Relations of counterface materials with stability of tribo-oxide layer and wear behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy

Dry sliding wear tests of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy (TC11 alloy) sliding against AISI 52100 and AISI M2 steels were performed under the load of 50–250 N at 25–600 °C. For two kinds of counterface materials, the titanium alloy presented totally different wear behaviours as the function of temperature. The appreciable variations of the titanium alloy sliding against different counterface materials were attributed the fact that a hard counterface caused unstable existence of tribo-layers by its microcutting action, thus resulting in the increase of wear rate. It is suggested that the hard counterface must be avoided as the counterface for the titanium alloy/steel sliding system.     

Cavitation erosion of Fe–Mn–Si–Cr shape memory alloys

Cavitation erosion tests of three Fe–Mn–Si–Cr shape memory alloys were carried out at speed 34 and 45 m/s using a rotating disc rig, and their cavitation damage has been investigated by comparison with a referring 13Cr–5Ni–Mo stainless steel used for hydraulic turbine vanes. The research results proved that the cavitation erosion of the Fe–Mn–Si–Cr shape memory alloys is a failure of low cycle fatigue and fracture propagates along grain boundaries. After 48 h cavitation erosion the cumulative mass losses of the studied alloys at speed 45 m/s are more than theirs at speed 34 m/s; however, the effect of velocity on cavitation damage of the Fe–Mn–Si–Cr alloys is much lower than that of 13Cr–5Ni–Mo stainless steel. The cumulative mass loss of the 13Cr–5Ni–Mo stainless steel are 26.3 mg at speed 45 m/s and 3.2 mg at speed 34 m/s, and the mass losses of the Fe–Mn–Si–Cr alloys are within the range of 3.6–7.3 mg at speed 45 m/s and 2.0–4.1 mg at speed 34 m/s. The surface elasticity of the Fe–Mn–Si–Cr shape memory alloys is better than that of the 13Cr–5Ni–Mo stainless steel, and the effect of surface elasticity on cavitation damage increases with velocity. The excellent surface elasticity of the cavitation-induced hexagonal closed-packed (h.c.p.) martensite plays a key role in contribution of phase transformation to the cavitation erosion resistance of the Fe–Mn–Si–Cr shape memory alloys. The cavitation damage of the studied alloys at speed 45 m/s mainly depends on their surface elasticity, and the variation of 48 h cumulative mass loss (Δm) as a function of the elastic depth (h_e) can be expressed as Δm=2.695+[1371.94/(4(h_e−46.83)²+12.751)] with a correlation factor of 0.99345.     

EDM of hybrid Al–SiCp–B4Cp and Al–SiCp–Glassp MMCs

Hybrid metal matrix composites are a class of material system, with two or more discrete particulate reinforcement. Notwithstanding their superior properties, their widespread application is constrained by the difficulty in machining them. Non-conventional processes such as electrical discharge machining can be applied to machine such composites. This work reports on the application of EDM to machine cast aluminum–silicon carbide–boron carbide and cast aluminum–silicon carbide–glass hybrid metal matrix composites and how the metal removal rate and surface finish vary in response to the various EDM parameters     

Analysis of Fretting Fatigue of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened titanium alloy, Ti–6Al–4V coupons was investigated using finite element method and a critical plane-based multi-axial fatigue parameter. Cavitation shotless peening (CSP)-induced compressive residual stress, which was larger at the contact surface than its counterpart from the shot peening (SP). However, compressive residual stress decreased more sharply with distance from the contact surface in CSP than in SP. Analysis using a critical plane-based multi-axial fatigue parameter demonstrated that the crack initiation would occur inside the cavitation shotless peened specimen which matched with the experimental observations. On the other hand, crack initiation would occur on the contact surface in the shot peened specimen which again was in agreement with experiments. The analysis also showed that the crack propagation part of the total fretting fatigue life was longer in the shot peened specimen than in the cavitation shotless peened specimen while the crack initiation part was almost equal from both peening methods. Therefore, CSP could not improve the fretting fatigue life/strength as much as the SP did but it improved relative to the un-peened specimen.     

Evaluation of performance of nanofluid using multiwalled carbon nanotubes for machining of Ti–6AL–4V

Machining of titanium alloys generate very high temperature in the cutting zone. This results in rapid tool wear and poor surface properties. Therefore, improvement in cutting performance in machining of titanium alloys is very much dependent on effectiveness of the cooling strategies applied. In the present work, performance of nanofluid using multiwalled carbon nanotubes (MWCNTs) dispersed in distilled water and sodium dodecyl sulfate (SDS) as surfactant is evaluated for turning operation on Ti–6Al–4V workpieces. Turning operations were carried out under three different conditions – dry, with conventional cutting fluid and with nanofluid. Nanofluid application was limited to 1 L/h and it was applied at the tool tip through gravity feed. Various machining responses like cutting force, surface finish and tool wear were analyzed while turning at optimum cutting parameters as 150 m/min, 0.1 mm/rev and 1 mm depth of cut. Later on, machining performance of nanofluid is confirmed at low cutting speed of 90 m/min. Nanofluid outperformed conventional cutting fluid with 34% reduction in tool wear, average 28% drop in cutting forces and 7% decrease in surface roughness at cutting speed of 150 m/min.     

Stress–Strain State of Welds

Russian Engineering Research - A method is proposed for analysis of the stress–strain state of welds in large rolling-stock components, using SolidWorks/Simulation software.     

Influence of Parameters of Reactive Magnetron Sputtering on Tribomechanical Properties of Protective Nanostructured Ti–Al–N Coatings

The influence of substrate temperature and bias voltage on the structure and tribomechanical properties of the Ti–Al–N coatings obtained by reactive magnetron sputtering technique has been investigated. The structure and elemental and phase compositions have been studied by scanning electron microscopy, Rutherford backscattering, and X-Ray diffraction. The results of friction and wear experiments indicated that the lowest coefficient of friction (three times lower than 12Cr18Ni10Ti) corresponded to a coating deposited at a bias voltage of–200 V and a substrate temperature of 340°С, while the most wear-resistant coating (under a load of 700 mN and the testing time of 1080 s) was Ti–Al–N sputtered at a bias voltage of–200 V and a substrate temperature of 440°С.     

Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V

Plasma nitriding was performed on Ti–6Al–4V samples at 520 °C in two environments (pure nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using alumina ball counterbody. Plasma nitriding reduced the tangential force coefficient of Ti–6Al–4V. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen–hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in pure nitrogen. The samples plasma nitrided in nitrogen–hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume and specific wear rate of the plasma nitrided samples were lower than those of the unnitrided samples. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume and specific wear rate at different loads.     

Determination of a Mobile-Ion Concentration in the Dielectric Films of Metal–Insulator–Semiconductor Structures

An experimental technique for determining the surface concentration N _Sof mobile ions in dielectric films of metal–insulator–semiconductor (MIS) structures is described. The technique is based on synchronous recording of the dynamic volt–ampere and low-frequency capacity–voltage characteristics of a sample under investigation. These experimental dependences are shown to ensure accurate extraction of the ion current peaks whose areas are proportional to N _S. These characteristics also allow the relaxation of the surface semiconductor potential to be found, which is needed for reconstructing the dependence of the convection ion current on the voltage drop across the insulation gap of the MIS capacitor. A comparative analysis with other known methods for determining N _Sis carried out. The proposed technique helps find a mobile-ion concentration from a 5 × 10⁹to 10¹³-cm^–2range, including the case when ion current peaks do not appear on the current–voltage characteristics.     

Investigation of Electrical Characteristics of Thin Silicone–Carbon Coatings

The complex resistance of silicone–carbon films of thickness more than 3 m is measured on the basis of an analysis of the electrotechnical substitution circuit containing a capacitor and a resistor. It was found that the parameters of the equivalent circuit determined experimentally make it possible to estimate whether the film thickness is greater than 3 m or not, i.e., to measure the film thickness more than 3 m. In this case, at frequencies higher that 100 kHz only the capacitive component of the impedance can be measured, whereas the active component associated most likely with the presence of silicon carbide in the film material was detected at lower frequencies. It is shown that for carrying out similar experiments with thinner coatings it is necessary to use a probe covered by a dielectric material with the higher dielectric constant.     

Tribological behaviour of carbon–carbon composite

Abstract

In the present study, the wear behaviour of cross ply (0/90°) C–C composite with 60 vol.-% fibres has been studied with sliding distance, applied load and sliding velocities. The measurement of specimen temperature has been carried out to study the effect of frictional heating. Furthermore, wear debris and wear track observations are correlated to understand the wear mechanism. The bulk wear increases linearly with distance after an initial running-in period. The temperature studies reveal that frictional heating is more with increase in load or sliding velocity under dry conditions, however, presence of lubrication reduces frictional heating, because exposure of surface for direct contact is reduced, and hence wear rate in all studies with lubrication is less than that under dry condition. The wear track studies show graphite powder, peeling of fibres and dislodging of the surface. At low loads, smearing of graphite powder keeps the wear rate low, but as the load increases; dislodging, delamination of surface and breaking of fibres dominate, and wear rate sharply increases, however, sliding velocity initially enhances the graphite formation reducing the wear, but as the velocity reached an optimum value, there is extensive breakage of fibres, dislodging and delamination of surface, and the wear rate increases sharply.     

Review of oil–water through pipes

In oilfield operations, co-current flow of oil–water mixture is very common and can cause significant flow assurance problems during production. Accurate knowledge of behaviour of oil–water in a pipeline flow is crucial to design/optimize production, transportation, and processing facilities. Many researchers have attempted to generalize the liquid–liquid or the two phase flow systems through mechanical models and develop means for predicting the flow patterns, pressure drop, and water holdups. However, the crude oil properties differ and the oil–water system can be operated over various range of operating conditions and subsequently encounter an altered hydrodynamics behaviour in two phase flow system. To worsen the situation, almost all the empirical correlations and mechanical models developed were for low viscosity oil and with majority focused on gas–liquid flow. Thus, comprehensive models still remain unsolved for some of the flow issues especially when involving high viscosity or waxy crude oils. Hence an attempt to generalize liquid–liquid flow in multiphase systems seems impractical. Since conventional light crude oil or easy oil reserves are gradually depleted, thus many research works on high viscosity oil–water flow in pipes have been initiated. Presently, there is limited number of papers on high viscous oil–water flow in open literature which has confined the understanding of the flow behaviour of two-phase flow. In this communication, we report a review on the current state of research on two phase flow oil–water phenomena, highlighting what is so far has been realized and how this knowledge can be applied as a basis to understand better of the more complex cases of gas-crude oil–water three phase flow.     

Mechanical and tribological properties of sputtered Mo–Se–C coatings

Transition metal dichalcogenides belong to the more developed class of materials for solid lubrication. However, the main limitation of these materials is the detrimental effect of air humidity causing an increase in the friction. In previous works, molybdenum diselenide has been shown to be a promising coating retaining low friction even in very humid environment. In this study, Mo–Se–C films were deposited by sputtering from a C target with pellets of MoSe₂. Besides the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the tribological characterization.The C content varied from 29 to 68 at.% which led to a progressive increase of the Se/Mo ratio. As a typical trend, the hardness increases with increasing C content. The coatings were tested at room temperature with different air humidity levels and at temperatures up to 500 °C on a pin-on-disc tribometer. The friction coefficient of Mo–Se–C coatings increased with air humidity from ～0.04 to ～0.12, while it was as low as 0.02 at temperature range 100–250 °C. The coatings were very sensitive to the elevated temperature being worn out at 300 °C due to adhesion problems at coating–titanium interface.     

Milling of gamma titanium–aluminum alloys

The present research is focused on the machinability of the gamma-TiAl alloys, also called in literature as titanium aluminides, under the milling process. In comparison with the alpha–beta alloys, the alloys solidified in the gamma phase present improved ratio between strength and density even at higher temperatures; however, hard and new machining problems appear. Currently, there is a lack of machining parameters for this kind of materials, and therefore, its inclusion in aeronautical and automotive applications is being delayed. This work summarizes the results of a testing program which offers an outlook on the cutting conditions when machining even more difficult-to-cut titanium alloys than the more spread and known structural alpha–beta alloys.     

Arc Evaporation of Ti–Al–Ta–N Coatings: The Effect of Bias Voltage and Ta on High-temperature Tribological Properties

Recently, titanium aluminium tantalum nitride (Ti–Al–Ta–N) coatings have been shown to exhibit beneficial properties for cutting applications. However, the reason for the improved behaviour of these coatings in comparison to unalloyed Ti–Al–N is not yet clear. Here, we report on the tribological mechanisms present in the temperature range between 25 and 900 °C for this coating system, and in particular on the effect of the bias voltage during deposition on the tribological response. Based on these results, we provide an explanation for the improved performance of Ta-alloyed coatings. An industrial-scale cathodic arc evaporation facility was used to deposit the coatings from powder metallurgically produced Ti₄₀Al₆₀ and Ti₃₈Al₅₇Ta₅ targets at bias voltages ranging from −40 to −160 V. X-ray diffraction experiments displayed a change with increasing bias voltage from a dual-phase structure containing cubic and hexagonal phases to a single-phase cubic structure. Investigations of the wear behaviour at various temperatures showed different controlling effects in the respective temperature ranges. The results of dry sliding tests at room temperature were independent of bias voltage and Ta-alloying, where the atmosphere, i.e. moisture and oxygen, were the most important parameters during the test. At 500 °C, bias and droplet-generated surface roughness were identified to determine the tribological behaviour. At 700 and 900 °C, wear depended on the coating’s resistance to oxidation, which was also influenced by the bias voltage. In conclusion, Ta-alloyed coatings show a significantly higher resistance to oxidation than unalloyed Ti–Al–N which could be an important reason for the improved performance in cutting operations.     

Effects of Strain-Induced Martensitic Transformation on the Solid Particle Erosion Behavior of Fe–Cr–C–Ni/Mn Austenitic Alloys

The effects of Ni and Mn concentrations and also the impact velocity on the solid particle erosion behavior of Fe?C12Cr?C0.4C?CxNi/Mn (x?=?5 and 10) alloys were investigated with respect to strain-induced martensitic transformation. The critical strain energy (CSE), which is defined as the energy required to initiate the martensitic transformation increased with increasing Ni and Mn concentrations. As the impact velocity decreased, the solid particle erosion resistance of the low CSE alloy improved compared to that of the high CSE alloy under the given ranges of impingement angles and impact velocities. This result was most likely due to an increase in the volume fraction of martensite that formed during the solid particle erosion test in the low CSE alloy when the impact velocity was decreased.     

An experimental study of process variables in turning operations of Ti–6Al–4V and Cr–Co spherical prostheses

Ti–6Al–4V and Cr–Co alloys are extensively used in manufacturing prostheses due to their biocompatibility, high strength-to-weight ratio and high resistance to corrosion and wear. However, machining operations involving Ti–6Al–4V and Cr–Co alloys face a series of difficulties related to their low machinability which complicate the process of controlling the quality levels required in these parts. The main objective of this paper is to study the influence of cutting parameters, machine tool control accuracy and metrology procedures on surface roughness parameters and form errors in contouring operations of Ti–6Al–4V and Cr–Co workpieces. The machining performance of the two biocompatible materials is compared, focusing the study on part quality at low feed per revolution and the stochastic nature of plastic deformations at this regime. The results showed a better surface roughness control for Ti–6Al–4V, whereas for Cr–Co alloys, the performance presents high variability. In the case of form errors (sphericity), contouring errors and metrology procedures are important factors to be considered for quality assurance. In addition, the study analyses the correlation of the machining performance with different sensor signals acquired from a low cost non-intrusive multi-sensor, showing a high correlation of signals from acoustic emission sensors and accelerometers in the machining of spherical features on Ti–6Al–4V parts. The findings of this research work can be taken into account when designing prostheses components and planning their manufacturing processes.     

Studies of microstructures made of Zn–Al alloys using microcasting

The microstructures of Zn–4 wt.% Al alloy with 0.1 mm diameter were prepared by microprecision casting based on gypsum-bonded investment casting. Aspect ratio up to 200 can be replicated in the case of the centrifugal speed of 1,500 rpm and the mold temperature of 270°C. The flow length was significantly influenced by the centrifugal speed and the preheating temperature of the mold. The flow length increases as the rotational speed and the mold temperature increase. The grain size and mechanical properties can be varied within a wide range by choosing different preheating temperatures.     

Effects of Silicon Content on the Mechanical Properties and Lubricated Wear Behaviour of Al–40Zn–3Cu–(0–5)Si Alloys

In this work, one ternary Al–40Zn–3Cu and seven quaternary Al–40Zn–3Cu–(0.25–5)Si alloys were synthesized by permanent mould casting. Their microstructure, mechanical and lubricated wear properties were investigated using appropriate test apparatus and techniques. As the silicon content increased the hardness of the alloys increased, but their elongation to fracture decreased. Tensile strength of the alloys decreased with increasing silicon content following a sharp decrease and a slight increase. Among the silicon-containing quaternary alloys the highest and the lowest tensile strength values (348 and 305 MPa) were obtained with the Al–40Zn–3Cu–2Si and Al–40Zn–3Cu–5Si alloys, respectively, while the base alloy (Al–40Zn–3Cu) exhibited a tensile strength of 390 MPa. However, the volume loss due to wear of the alloys increased with increasing silicon content after showing an initial increase and a sharp decrease. The lowest wear loss was obtained with the alloy containing approximately 2% Si which has the highest tensile strength among the quaternary alloys containing more than 0.25% Si. Wear surfaces of the alloys were characterized mainly by smearing indicating that adhesion is the dominant wear mechanism for the experimental alloys.