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Purpose This paper aims to study the effectiveness of using thermal spray (TS) coated bores in reducing friction under the mixed lubrication (ML) and elastohydrodynamic lubrication (EHL) regimes. Design/methodology/approach A reciprocating tribometer with a stroke length of 100 mm, was built to measure the coefficient of friction (COF) at the mid-stroke and ring reversal positions and to conduct sliding tests at a speed range of 0.31–3.14 m/s. Samples taken from fine-honed TS coated bores and also from cast iron (CI) liners that underwent a standard-honing process were tested against ring segments coated with chromium nitride (CrN) and diamond-like carbon. Findings Construction of Stribeck curves demonstrated that TS coatings showed a transition from ML to EHL at a lower speed (0.94 m/s) compared with CI (1.26 m/s) regardless of the counterfaces used. Lower COFs of 0.05–0.08 in ML was measured for TS coatings compared with those of 0.06–0.09 for CI in ML. Once EHL was reached, the COF of TS coatings decreased to 0.02–0.03 similar to those of CI. Examination of wear patterns suggested that the low roughness combined with high oil retention capability might be responsible for the reduced transition speed and the expanded EHL region for the TS coated surfaces. Originality/value With the EHL friction captured in a bidirectional sliding mode using a long-stroke tribometer, this work contributes to the understanding of the low-friction behaviour of TS coatings.

In order to improve drilling performance of Inconel 718 alloy, a cutting fluid (CF) consisting of 70% water and 30% vegetable oil blended with graphene nanoplatelets was used. Sliding of Inconel 718 workpiece using WC-Co drills with CF containing 54 × 10−5 wt% graphene (GCF) reduced the coefficient of friction (COF) between the tool and workpiece surfaces from 0.16 to 0.08 and resulted in the formation of tribolayers. The maximum drilling torque of 6 Nm for CF was reduced to 2 Nm using GCF. Additionally, arithmetic mean height (Sa) of drilled surfaces decreased from 8 to 4 μm and a reduction in the amount of material transferred from the Inconel 718 to the cutting edge of the drill was observed. The drilling temperature decreased from 150 to 100 °C using GCF during the process. Subsurface plastic deformation that occurred at the drilled surfaces of Inconel 718 was examined by cross-sectional optical microscopy which revealed that both the depth of deformation zone and the magnitude of subsurface strains were lower compared with flooded drilling. Therefore, graphene added in small quantities to the cutting fluid can provide an energy efficient method for drilling of Inconel 718. Graphical abstract

Tribological behaviour of Ti–6Al–4V alloy sliding against WC-Co was evaluated by employing WS2 nanoparticles blended in a cutting fluid used for machining of this alloy. Pin-on disk tests were carr...

Relationships between tribological properties of Ti-6Al-4V and the machining behaviour of the same alloy drilled by a WC-Co cutting tool using minimum quantity lubrication (MQL) were studied. The tribological behaviour of Ti-6Al-4V sliding against WC-Co was evaluated by determining the changes in the coefficient of friction (COF) using cutting fluids employed in MQL and flooded drilling tests. When pin-on-disk tests were carried out under the boundary lubricated condition, the MQL fluid (MQLF) provided a low COF of 0.16 at 25 ℃ and 0.24 at 250 ℃. The observed low COFs were attributed to formation of a tribolayer on the WC-Co counterfaces. A high COF of 0.45 observed at 300 ℃ indicated that the tribolayer was no longer stable. Application of conventional water-based cutting fluid (CF) also resulted in a low COF of 0.26 at 25 ℃ but the COF rose to 0.32 at 100 ℃. During drilling tests, MQLF was supplied at a rate of 80 ml/h using an external MQL system. The average drilling torque was reduced by 35% compared to CF as a result of the formation of tribolayer on the cutting edge of the tool as well as on the drilled hole surfaces, as the cutting temperature remained

The objective of this research was to investigate the effectiveness of application of liquid nitrogen (LN2) in turning of Inconel 718 compared to flooded cutting and select suitable LN2 cutting parameters using response surface methodology (RSM). The results of turning experiments conducted by spraying LN2 to the cutting area of Inconel 718 bar showed that using either low or high cutting parameters, cutting performance of Inconel 718 under the cryogenic condition was generally worse than the flooded cutting. However, using the medium cutting parameters the cutting performance was as good as flooded cutting showing the values of 90 N of cutting force, 60 µm of flank wear and 0.5-0.6 µm of surface roughness (Ra). These parameters were further optimized using desirability function of RSM to determine the set of parameters that provide the lowest cutting force, flank wear and Ra and the highest material removal rate (MRR) under cryogenic cutting. Analysis of variance (ANOVA) performed on the developed regression models showed that cutting speed was the significant factor on cutting force. Feed rate was the most influential parameter on flank wear. Feed rate and depth of cut was significant factors affecting Ra. Multi-objective optimization showed that a cutting speed of 87 m/min, a feed rate of 0.06 mm/rev and a depth of cut of 0.37 mm constituted the optimum cutting parameters for achieving the, cutting force of 78 N, flank wear of 58 µm, Ra of 0.49 µm and the MRR of 1.97 cm3/min under cryogenic cutting condition.

Titanium alloys are promising lightweight materials to replace the conventional use of stainless steels in engine valve applications, but their high temperature wear resistance should be improved. In this study, a thermal oxidation (TO) treatment (at 600 °C for 60 h) was utilized on Ti-6Al-4V to generate a surface rutile, TiO2, layer with a hardened oxygen diffusion zone underneath. Sliding wear tests were performed on untreated Ti-6Al-4V and TO-Ti-6Al-4V disks against cast iron pins at 350 °C and 550 °C, typical temperatures at which intake and exhaust engine valves operate. In addition, high temperature impact tests were conducted to study the deformation and fracture mechanisms of surface layers produced on TO-Ti-6Al-4V. The results showed that compared with untreated Ti-6Al-4V, TO-Ti-6Al-4V exhibited lower sliding wear losses at both test temperatures. The lowest wear loss was observed at 550 °C due to the preservation of both TiO2 layer and oxygen diffusion zone, and the propensity of impact crack growth was also reduced. The oxygen diffusion zone reduced the extent of subsurface plastic deformation and contributed to improved wear resistance.

Die wear during trimming of advanced high strength steel (AHSS) sheets deteriorates the edge quality of trimmed sheets. In this work, the mechanisms of AISI D2 steel trim die wear and their effects on plastic deformation and fracture behaviour of the sheared edge of DP980 steel sheet were examined. A mechanical press equipped with D2 inserts was used to trim DP980 sheets with a clearance of 0.14 mm (10%). Abrasion and microchipping were identified as the wear mechanisms operating at the upper die, with microchipping becoming more dominant after 60,000 trimming cycles. The burr height and the length of the burnish zone of sheared DP980 sheets increased linearly with the chipped die edge percentage. The shear strains in the shear effected zone (SAZ) were estimated using martensite displacements as metallographic markers in the ferrite matrix as a function of the depth beneath the sheared edge. The depth of SAZ, and the plastic strains at a given depth within the SAZ increased with the number of trimming cycles. Correlation of the local stress and strain values generated in the SAZ showed that a saturation flow stress was reached near the sheared edge. The damage in SAZ occurred in the form of crack formation at the martensite/ferrite interfaces and fracture of martensite. Die wear reduced the tensile ductility of the DP980 sheets, and the fracture mode in tension changed from ductile with localized neck to more brittle sheared edge fracture initiating from surface cracks after 60,000 cycles.

Plasma electrolytic oxidation (PEO) treatment of Mg alloys improves their wear resistance by increasing their surface hardness, but also leads to high coefficient of friction (COF) values. The sliding counterfaces and the conditions under which PEO-coated Mg alloys operate affect their COFs. PEO-coated AZ31 alloy sliding against hydrogenated DLC (a-C-H) coated steel yields a low COF of 0.13 under the ambient conditions. The current study investigates the effect of the test temperature on the tribological behavior of PEO-coated AZ31 Mg samples sliding against the a-C-H coated counterface at temperatures up to 300 °C. According to the COF vs. wear rate diagram constructed in the temperature range of 25–250 °C, lower COF values and wear rates were exhibited by PEO-coated AZ31 sliding against a-C-H compared to uncoated AZ31 sliding against a-C-H, and PEO coated AZ31 sliding against an uncoated ASTM 52100 steel. The PEO-coated AZ31 produced the lowest COF of 0.03 at 200 °C. The application of PEO to the Mg alloy automotive cylinder bores running against DLC-coated piston rings and/or PEO-coated Mg alloy pistons running against DLC-coated bores could provide a new approach for the prevention of seizure and hot scuffing in lightweight engines in the temperature range between 150–250 °C.

Mechanical properties of a low carbon steel thermal spray coating, deposited from AISI 1010 feedstock, on Al-Si cylinder bores were studied using nano- and micro-indentation tests before and after the sliding tests. The coating’s microstructure consisted of α-Fe splats surrounded by FeO stringers and occasional oxide aggregates. Iron oxide aggregates fractured at the lowest indentation load (10 gf). Vickers indentations performed on the ductile α-Fe matrix of the coating caused separation of the oxide stringers within the indentation plastic zone. The probability of chipping-type fracture by separation of oxide stringers depended on the indentation load applied. Reciprocating dry sliding tests, performed on the coating against a CrN counterface, generated a 770 ± 261-nm-thick tribolayer (hardness = 0.36 ± 0.1 GPa; elastic modulus = 38.4 ± 5.3 GPa). Raman spectroscopy revealed that the tribolayer consisted of Fe3O4, FeO and Fe2O3. The increase in the Weibull modulus, m, from 0.84 for the coating to 1.51 for the tribolayer-covered surface showed that the tribolayer decreased the probability of chipping fracture. A fracture mechanics argument was put forward to show that the tribolayer reduced the driving force that was available for chipping fracture and, in this way, protected the surface from damage during sliding contact.

The low coefficient of friction (COF) of sp2 rich non-hydrogenated diamond-like carbon (a-C) coatings make them suitable for tribological applications including machining of lightweight alloy castings and for certain components in internal combustion engines. Retaining the low COF of a-C at temperatures >100 °C is however a challenge that has limited further industrial applications of these coatings. This study examines the high temperature stability of a non-hydrogenated a-C coating (H

The formability of electro-hydraulically formed AA5182-O aluminium sheet was investigated by means of experimental testing and numerical modelling. The experimental results were compared with quasi-static, as-received forming limit curve (FLC) and were used to calibrate a finite element model of EHF. It is found that the formability improvement of AA5182-O sheets was insignificant when electro-hydraulically formed without a die. However, when specimens were electro-hydraulically formed with sufficient energy into 34 or 40° conical dies, the effective strain measured in safe grids increased by 40 and 70% respectively when compared with a conservative quasi-static FLC. Finite element simulation results suggested that a combination of different mechanical parameters contributed to the formability improvement. The increased strain rate in areas close to the apex of the speimen, the negative stress triaxiality just before specimens contact the die, and significant compressive through-thickness stress generated by high-velocity impact all contributed to the improvement of formability in electro-hydraulic die forming.

W containing diamond-like carbon (W-DLC) coating is of interest to manufacturing industry, as they exhibit low coefficient of friction (COF) values when sliding against aluminum, titanium and steel at elevated temperatures. The low COF of W-DLC observed at 400 °C and 500 °C could be attributed to the formation of tungsten oxide WO3 at the contact surfaces. However, at intermediate temperatures between 100 °C and 300 °C, high COF values of ~ 0.60 were recorded. In this work, the friction reduction mechanisms of W-DLC coatings were investigated in a dry oxygen atmosphere as a function of testing temperature up to 500 °C against an Al-6.5% Si alloy. The purpose of maintaining an oxygen rich environment was to increase the propensity of WO3 formation at the sliding surfaces at the intermediate temperatures. A steady state COF (μs) of 0.11 was observed at 25 °C and the low friction values were maintained up to 500 °C including the 100–300 °C range. Micro-Raman spectroscopy revealed that at 25 °C the transfer layers were rich in carbon, whereas between 100 °C and 500 °C the transfer layers primarily consisted of monoclinic γ-WO3, which could be responsible for the low μs of 0.09–0.15 observed in this temperature range.

Tetrahedral amorphous carbon based coatings generally exhibit a low coefficient of friction (COF) while sliding against steel and aluminum, but their tribological properties are sensitive to the test environment. In this study, tribological behaviour of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C-F) coatings were investigated under different atmospheres including ambient air (53% relative humidity, RH) and dry N2 (3% RH) against a steel counterface. The ta-C showed a period of low COF of 0.27 in ambient air and a C transfer layer was formed on the counterface, but the COF increased with sliding duration to ~ 0.4 due to transfer of Fe2O3 debris to ta-C surface. A high COF of ~ 0.60 with high fluctuations occurred in dry N2 as no carbon transfer layers were formed on the counterface. The ta-C-F coating exhibited a steady-state behaviour with low COF (μs) values under both ambient (μs = 0.25) and dry N2 (μs = 0.23) atmospheres. Formation of an F containing carbonaceous transfer layer on the counterface provided low COF for ta-C-F.

When sheets of aluminum alloys are pierced or trimmed, tool failure occurs by the transfer of material from the sheet to the surface of the tool, which results in the adhesion and damage of the workpiece surface. This phenomenon is referred to as galling, which adversely affects sheared edge quality and increases energy consumption. An instrumented pneumatic press was designed and built to conduct shear–punch tests on 2 mm-thick AA5754-O sheets and to investigate the progression of galling to AISI M2 steel punching tools during dry and lubricated punching. The punching tests were performed using a clearance of 2.0% of the lower die diameter. Cumulative galling volumes were measured using a non-contact optical surface profilometer, and the rate of material transfer (the galling rate) was estimated for both dry and lubricated punching. The galling initially occurred at a high rate, and for dry punching, the galling rate was reduced to 74.6 × 10 4 μ m 3 / s t r o k e between 20th and 125th strokes. Lubricating the aluminum sheet with an oil-based lubricant mitigated the material transfer and galling rate after the 20th stroke ( 3.1 × 10 4 μ m 3 / s t r o k e ) that was about 25 times lower compared to the dry punching. Punching force-displacement curves indicated that, in dry punching, higher amount of energy was expended to shear AA5754-O sheets compared to the lubricated punching.

Molybdenum disulfide (MoS2) displays low coefficient of friction (COF) in vacuum and under inert atmospheres, but a higher COF occurs under humid atmospheres. A significant aspect of this increase in COF that is still not well established is the decoupling the effects of dissociated and undissociated water molecules. The MoS2 thin films used in this study with an intrinsic defect structure incorporating misoriented and fragmented layers as observed by HR-TEM, exhibited an increase in COF from 0.007 in N2 with O Mo bonds, but this process would not change EB and had a minor effect on friction. An undissociated H2O molecule physisorbed between these layers formed H bond with S atoms increasing EB, a tribochemical mechanism that had a pronounced effect on increasing the COF of MoS2.

High temperature sliding wear tests were performed on samples taken from NCF 3015 austenitic steel (Fe-32% Ni–14% Cr based 0.8% C alloy with Al, Ti, Nb, Mo) and Inconel 751 superalloy (Ni–16%Cr alloy with Al, Ti, Nb) engine valves using the counterfaces made of PL12 chromium alloyed cast iron seat insert. The contact surfaces were characterized by SEM-EDS, Optical Profilometer, Raman, XRD. The wear mechanisms of the valve systems were explored using the results of these analyses that determined compositions of the tribolayers that formed on the contact surfaces. The Calphad based thermodynamic analyses that revealed stability of the precipitates in the alloys at the test temperatures. An increase of wear occurred between 25 °C and 350 °C, but for T > 350 °C, a continuous reduction in volumetric wear losses were observed with increasing temperature for both alloys. At all temperatures Inconel 751 showed higher wear performance than NCF 3015. It was found that the formation of (Fe1-x-yCrxNiy)3O4 mixed spinel oxides controlled the high temperature wear behavior of these alloys and these structures support the formation of tribolayer under local pressure and high temperature during sliding. Besides the direct effect of oxide compositions within tribolayer, the formation of high amount of (Ti,Nb)C carbides, Ni3(Ti,Nb,Al) and Ni3Nb precipitates play important role in transition from severe to mild wear regime above 350 °C.

During hot forming of Al sheets, sliding contact against the forming die causes material transfer and adhesion to die surfaces. This work uses in-situ microscopy methods during sliding experiments conducted inside an environmentally controlled high temperature pin-on-disk tribometer to investigate both formation of junctions during initial contact and growth of adhesively transferred material. For this purpose, AA5083 (Al-0.1 wt%Cu-4.8 wt%Mg) pins placed in contact with a flat quartz glass counterface mounted on to the motorized stage of an optical microscope were tested at 573 K and 673 K in argon and ambient air atmospheres. The instantaneous changes in the contact area size and morphology were recorded along with the tangential force. Evidence for plastic deformation during junction formation and growth was provided by the formation of slip lines and grain boundary sliding. Junction growth occurred continuously in argon without separation of the adhesive junctions moving into the steady state friction stage, whereas in air, initial junction growth was interrupted due to oxidation, giving rise to a stick-and-slip behaviour, and the tangential force at the onset of junction break was determined as 1.75 ± 0.15 N at 573 K and 1.00 ± 0.08 N at 673 K. The junction growth rate in the absence of oxide formation followed an Arrhenius-type relationship in the temperature range investigated with an activation energy in the range of 144 ± 2 kJ mol-1, implying that the plastic deformation (creep) during junction growth was controlled by diffusional flow.

Lithiation/de-lithiation cycles induced cracks in isostatically pressed graphite samples subjected to constant-load bending tests while simultaneously conducting cyclic voltammetry (CV) experiments in propylene carbonate-based Li-ion battery electrolyte solutions. A large cyclic strain of Δe = 0.95% was generated by lithiation/de-lithiation cycles as determined by micro-Raman measurements carried out concurrently with CV experiments. In-situ optical microscopy measurements of crack lengths, a, showed that the crack-growth rate, da/dt, depended on the stress intensity factor at the crack tip and could be expressed as da/dt = AΔK I . A two-stage crack-growth behaviour was determined with n = 51.3 in the first stage. However, lower crack propagation rates observed in the second stage (n = 9.9) were affected by crack closure due to (1) a rough fracture surface morphology of the graphite cracks, and (2) the deposition of solid electrolyte reduction products on facetted crack surfaces, where both the factors likely contributed to reducing ΔK to ΔK eff.

Generation of high localized cutting zone temperatures leading to dissolution wear hinders machinability of titanium alloys using uncoated carbide tools. In addition, the thermo-plastic instability exhibited by titanium alloys promotes serrated chip formation that causes fluctuations in the cutting forces leading to chatter and severe flank wear. This work considers two methods to mitigate these problems during cutting of Ti-6Al-4V, namely, cryogenic machining and the use of ethanol blended metal removal fluids (MRF). Cryogenic machining decreased the cutting force and surface roughness compared to dry machining under the ambient conditions. The decrease in the fracture energy of Ti-6Al-4V as measured by Charpy impact tests was the likely reason for the predominance of brittle type fracture leading to formation of shorter chip segments during cryogenic machining. The use of ethanol blended MRF reduced the cutting force by over 65% compared to dry machining. Alternatively the use of ethanol blended MRF further reduced the cutting force by over 65% compared to dry machining. The use of ethanol blended MRF resulted in adsorption of OH groups from ethanol to carbon surfaces of the cutting tool resulting in negligible adhesion to the cutting tool.

In this study, damage mechanisms in AA5182-O aluminium sheets are investigated using quasi-static (QS) Marciniak tests and high strain-rate electro-hydraulic forming (EHF) process. The results confirm that void nucleation, growth and coalescence are the main damage mechanisms of AA5182-O at both high and low strain rates. The EDS analysis suggests that cracking of Al 3 (Fe-Mn) intermetallic particles is the main source of void nucleation, whereas Mg 2 Si particles do not majorly influence void formation. Void growth analysis suggests that specimens deformed under QS conditions contained more voids in areas away from the sub-fracture surface but specimens deformed at high strain rate exhibit more significant rate of void growth close to sub-fracture areas. Void formation is suppressed by increasing the applied energy in EHF. And more significantly, the growth of voids is suppressed due to the high-velocity impact of the sheet against the die which plays an important role in increasing formability of AA5182-O aluminium sheet in EHF process. When the void percentage increase remains less than about 0.5% AA5182-O can be formed safely. However, when the void percentage increases beyond 0.6–0.8% fracture becomes inevitable.

In-situ Raman spectroscopy was performed on 1010 steel coatings thermally sprayed on an Al-Si alloy on A 380 (Al-9.0% Si) cylinder bore and subjected to reciprocating sliding against various coated rings to investigate the progression of iron oxide formation during sliding. The coatings evaluated for the top compression ring (TCR) included diamond-like carbon (DLC) and chromium nitride (CrN). The reciprocating tests were conducted under unlubricated and boundary lubricated conditions. During unlubricated sliding, a running-in period was observed with increasing coefficient of friction (COF) values which corresponded to the conversion of the initial FeO, present in the as-deposited coating, to Fe 3 O 4 and then to Fe 2 O 3 as identified by changes in Raman spectra obtained at short intervals during the test. Once a Fe 2 O 3 layer was formed, a steady state regime was obtained with a COF of 0.4 that was accompanied by the adhesion of the oxide debris generated during sliding to the counterfaces. The DLC coated TCR provided low steady state COF (0.18) and volumetric wear of 1010 steel, with negligible debris formation. Boundary lubricated sliding tests, with an initial period of unlubricated sliding, resulted in low steady state COF value of 0.10 for DLC and 0.16 for CrN. The low friction and wear of 1010 coatings observed during boundary lubricated sliding against DLC coated TCR was due to a composite tribolayer consisting of an amorphous carbon layer on top of an oil residue layer that was formed on the sliding surfaces.

Friction and adhesion mechanisms of a Ti incorporated multilayer MoS2 (Ti-MoS2) coating, intended for use in elevated temperature engineering applications like warm forming or machining of lightweight Al alloys, were investigated. Pin-on-disk type tests were performed at temperatures between 25 °C and 500 °C to measure variations of the coefficient of friction (COF) values and wear rates of Ti-MoS2 sliding against Al-6.5% Si (319 Al) counterfaces. The steady state COF, μs, of Ti-MoS2 decreased from 0.11 at 25 °C to 0.05 at 200 °C. A μs of 0.13 was recorded at 350 °C. Then an increase in COF to 0.34 at 500 °C was observed. Progression of surface compositional changes during sliding at different temperatures was monitored using micro-Raman and X-ray photoelectron (XPS) spectroscopy. It was found that formation of a MoS2 rich transfer layer was responsible for maintaining low COF values up to 200 °C at which point MoO3 formation started. At T > 350 °C, MoO3 covered most of the contact surfaces resulting in an increase in the COF and volumetric wear losses accompanied by Al transfer to the coating surfaces. Accordingly, it can be suggested that the safe operation window for use of Ti-MoS2 coating against Al-Si alloys ranges between 25 °C and 350 °C.

Parasitic friction losses during piston ring-cylinder liner interactions in an internal combustion (IC) engine consume large portions of the available fuel energy. This work investigates whether the tribological performance of grey cast iron (CI) cylinder liner would be improved using a piston ring coated with non-hydrogenated diamond-like carbon (NH-DLC) in comparison to uncoated CI and steel rings. Coefficient of friction (COF) values and volumetric wear losses of CI were determined using ball-on-disk type tests as a function of sliding distance. CI tested against itself during boundary lubricated sliding with synthetic engine oil showed a COF of 0.14 and a volumetric wear of 11.0×10 −4 mm 3 after 6×10 5 cycles, whereas the use of NH-DLC coated counterface resulted in a COF of 0.11 and lesser wear of 0.5×10 −4 mm 3 . Dilution of the engine oil by ethanol containing E85 biofuel was beneficial as COF was further reduced to 0.08 for CI tested against NH-DLC while maintaining low wear of 0.2×10 −4 mm 3 . According to TEM and XPS analyses, an oil residue layer (ORL) formed on the CI contact surfaces as a result of sliding-induced degradation of zinc-dialkyldithiophospahte (ZDDP) additive in the oil. The ORL, which consisted of nano-crystalline particles of sulphides and phosphates of zinc (anti-wear components) embedded in an amorphous carbon matrix, was responsible for maintaining low wear. Ethanol dilution of the synthetic oil facilitated the formation of ORL. TEM/EELS studies of the NH-DLC counterface provided evidence for OH adsorption and passivation of carbon bonds at the surface that reduced the friction. It is anticipated that piston rings coated with NH-DLC run against CI liners would show low friction and wear during boundary lubricated sliding, an effect that could be enhanced when ethanol diluted oil is used.

Boron carbide (B 4 C) is a potential tool coating for machining of titanium alloys due to its high hardness and high temperature stability. In addition, B 4 C coatings annealed at 600 °C, form a layer that acts as solid lubricant leading to a low coefficient of friction, COF, against titanium. This paper shows that a transfer layer consisting of graphite could be established on B 4 C surface when sliding against Ti–6Al–4V and passivation of graphitized carbon would reduce COF. Ball-on-disk (B 4 C) type sliding tests were performed in dry air, argon and nitrogen atmospheres ( 4 C samples were also tested while they were immersed in water, ethanol (C 2 H 5 OH) and iso-propyl alcohol (C₃H₇OH). The B 4 C coatings exhibited high COF values of 0.59–0.65 in dry atmospheres. Sliding under an air atmosphere containing 70% RH reduced the steady state COF to 0.25 and a further reduction of COF to 0.20 was recorded in air with 85% RH. A COF of 0.15 was measured when the tests were carried out in ethanol and 0.07 in iso-propyl alcohol. Low COF values of B 4 C were attributed to H and OH passivation of the graphitized transfer layers observed by the Raman, Fourier transform and X-ray photoelectron spectroscopy techniques.

By examining sliding-induced damage and interfacial material transfer processes that occur when multilayer graphene (MLG) is placed in contact with diamond-like carbon (DLC) and N- based coatings as well as uncoated steel the role of counterface materials on the friction of MLG has been elucidated. Sliding friction tests conducted on MLG in an ambient testing environment (45% RH) and under the same loading conditions showed that tribo-chemical mechanisms would differ depending on the counterface material. While the N-based coatings, e.g., TiCN counterface, inflicted the most extensive sliding wear damage to MLG resulting in a high coefficient of friction (COF), lesser damage occurred against 52100 steel counterface accompanied by the formation of graphene transfer layers on the counterface which was a necessary condition for low friction in tribosystems not involving carbonaceous counterfaces. A common feature of the worn MLG was the sliding induced defects which comprised of edge fracture, fragmented/bent graphene stacks compared to pristine graphene and disordered regions between them. When MLG was run against a C-based counterface, namely hydrogenated (40% H) diamond like carbon (H-DLC) coating a low COF of 0.08 resulted without formation of detectable transfer layers. Sliding induced defect activated H 2 O dissociation and adsorption of H and OH at the defect sites were likely to be responsible for the low friction of MLG sliding against H-DLC.

Tribological behavior of cast Al–Si alloys was studied by considering samples from hypo-eutectic (Al–6.5% Si), eutectic (Al–12% Si) and hyper-eutectic (Al–18.5% Si) compositions that were subjected to dry drilling experiments. Tool materials tested consisted of hydrogenated diamond-like carbon (H-DLC), CVD diamond coated and uncoated WC–Co drills. Tool failure mechanisms were identified and correlated with the drilling torques. The drilling torque vs. number of holes curves typically exhibited three stages, each identified with a characteristic slope (m). A failure criterion was established, such that when m ≥ 1.0 × 10− 2 N m the onset of tool failure in dry drilling of Al–Si alloys could be predicted. For hypo-eutectic and eutectic Al–Si alloys drilled using uncoated WC–Co the failure criterion was satisfied rapidly (

A WC–12%Co coating was thermally sprayed on a 52100 steel substrate and subsequently friction stir processing (FSP) was performed on this layer. The wear resistance and hardness was compared before and after FSP. Optical and SEM revealed that FSP intermixes the sprayed layer with the substrate, reduces porosity, and enhances both hardness and wear performances. 3D profilometry mapping was conducted to evaluate the wear track depth and its morphology. Refined grain structures and a martensitic structure with retained austenite are promoted by the FSP treatment. This leads to formation of new intermetallic and carbides which were detected by X-ray diffraction, thus accounting for the increased hardness and improved wear resistance. Keywords: Friction stir processing, Sliding wear, Thermal spray coatings, Wear testing, Wear mechanism

Friction and wear behavior of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C–F) coatings were studied in the temperature range between 25 °C and 500 °C. Pin-on-disk type tests conducted on ta-C against Al–6.5% Si (319 Al) showed that the steady-state coefficient of friction (μ s ) decreased from 0.36 at 25 °C to 0.11 at 400 °C. The reduction in friction was attributed to sliding-induced sp 3 to sp 2 transformation (graphitization) of the coating and formation of oxygen rich transfer layer as revealed by X-ray photoelectron and micro-Raman spectroscopy. The coefficient of friction of ta-C–F also decreased with an increase in the test temperature. A low μ s of 0.13 was observed at 400 °C compared to 0.39 at 25 °C. The maximum value of running-in coefficient of friction (μ R ) for ta-C–F tested at 400 °C was 0.38, lower than the μ R of ta-C (0.65). Both coatings maintained low wear at elevated temperatures but at 500 °C they showed high wear and high coefficient of friction and did not reach a steady state friction regime. The atomic % of F transferred to the Al surface increased with an increase in the test temperature up to 400 °C. It was suggested that a sp 2 carbonaceous transfer layer passivated by F atoms was responsible for the further decrease in μ s and μ R of ta-C–F at elevated temperatures (200 °C–400 °C) compared to ta-C.

Particulate reinforcing phases in metal–matrix composites (MMCs) can provide resistance to both sliding wear and abrasion by protecting the softer metallic matrix. In this review, the wear behavior of particulate MMCs in dry and lubricated sliding, and also under conditions of abrasive wear, is discussed. Stable tribological performance can be achieved under conditions of mild sliding wear. While wear rates normally reach an equilibrium (or steady state) in the mild wear regime, accelerated wear rates (with sliding distance and load) are observed in the severe wear regime due to large scale plastic deformation, and in some instances, local melting of the surface layers. MMCs can show significantly lower wear rates compared to unreinforced alloys over a substantially wider range of pressure and sliding speed. Wear mechanism maps are effective ways of delineating the regimes of dominant sliding wear mechanism (oxidation, delamination, seizure etc.) and show the effect of frictional temperature on wear transitions. The very small wear rates observed in lubricated sliding define a new wear regime different from mild and severe wear not only in terms of the infinitesimal rate of material removal but the mechanisms of material removal involved are also different. Tribolayers that form during the running in period are responsible for the very low wear rates observed in this ultra-mild wear regime. Reinforcement can also provide excellent improvements in resistance to low-stress abrasive wear. However, major enhancement of wear resistance in the severe sliding wear regime, or to high-stress abrasion, cannot be expected from these composites.

The mechanisms of electrochemical capacity retention and eventual degradation in composite anodes prepared by electrodepositing nanocrystalline Sn coating on carbon fibres (CF), Sn-CF, were studied using in situ optical microscopy, high-resolution scanning and transmission electron microscopy. Specific capacity changes of Sn-CF anodes (vs Li/Li+) were observed to take place in three stages: during the first two galvanostatic cycles, a rapid capacity decrease (from 1045 to 930 mAh g−1) occurred, which was followed by a steady-state stage where the capacity remained constant at 922 ± 22 mAh g−1. The fast capacity drop of Sn-CF in the first cycle was attributed to the partial decohesion of Sn from CFs although the carbon substrate remained unaffected due to formation of a layer from the solid electrolyte reduction products. The pure Sn electrode with a higher initial specific capacity than the Sn-CF displayed a rapid decrease in the same range, whereas the specific capacity of the uncoated CF was already much lower as the fibres were severely damaged in the first cycle.

Si added DLC coatings may be of value for engineering applications where a low COF and high wear resistance are required for the extended service life, during which the operating temperatures may increase above 200 °C. Hot/warm forming dies, machining tools, or critical sliding components such as the upper compression rings in combustion engines are among the components that operate under demanding conditions. This study examines friction, wear and adhesion behavior of multilayered DLC coatings consisting of a top layer of H-DLC and an inner layer rich in Si and O (a-C:H/a-Si:O) tested against Al-6.5% Si (319 Al) at temperatures above 200 °C where DLC coatings typically show high COF and wear. The a-C:H/a-Si:O coatings maintained the low room temperature (25 °C) steady state COF ( μ S of 0.17) up to 400 °C with a μ S of 0.04 at 200 °C and 0.11 at 400 °C. Low wear rates ( W ) were observed in the same temperature range with W =2.86×10 −5 mm 3 /Nm at 25 °C and 16.23×10 −5 mm 3 /Nm at 400 °C. Micro-Raman spectroscopy studies of the coating surface revealed a constant I D / I G ratio of 0.4–0.6 between 25 °C and 400 °C. It was suggested that incorporation of Si hindered temperature induced bulk graphitization (due to suppression of sp 2 bond formation) of the coating and thus accounted for the low coating wear. Scanning electron microscopy observations indicated formation of a transfer layer on 319 Al contact surface when tested against 319 Al. Cross-sectional transmission electron microscopy of the transfer layer formed on 319 Al at 400 °C revealed that they consisted of amorphous C with traces of SiC, transferred from the a-C:H/a-Si:O and mechanically mixed with Al. X-ray photoelectron spectroscopy of the transfer layer revealed the formation of C–OH, C–H and Si–O bonds. Accordingly, the low steady state friction obtained up to 400 °C could be attributed to H and OH passivation of the C and Si in the coating surface and in the transfer layers.

Elevated temperature sliding contact experiments were carried out on an Al–Mg alloy AA5083 to study the formation of oxide fibres on their surface. Deformation and friction tests were conducted at a constant strain rate of 4×10 −2 s −1 at temperatures 350 °C, 450 °C and 545 °C. The samples were tested in two surface conditions, namely in the as-received condition, with surfaces covered with a tribolayer generated during previous rolling processes and after being polished to remove the tribolayer. The coefficient of friction, COF, and the wear of the AA5083 surfaces as well as the amount of material transferred to the counterface increased with the temperature. The presence of tribolayers on the surface was beneficial, as the polished surfaces showed generally higher COF and wear, with a tendency to form localised adhesive joints with the counterface. Oxide fibres formed during plastic deformation (with diameters of about 100 nm) were generated within the cracks on the tribolayers and inside the cavities at the Al–Mg grain boundaries. High resolution scanning and transmission electron microscopy showed that the fibres were composed of an outer shell of MgO, Al 2 MgO 4 and Al 2 O 3 and an inner core of Al nanograins. The visco-plastic deformation behaviour of oxide fibres observed within the sliding track could have contributed to the reduction of COF.

Tribological tests were performed on hydrogenated diamond-like carbon (H-DLC) and F containing H-DLC (F-H-DLC) coatings sliding against a magnesium alloy (AZ91) in air with relative humidity (RH) ranging between 0% and 75%. Pin-on-disk type friction tests typically exhibited high running-in COF, followed by a steady state regime with a low and constant coefficient of friction (μ S ). At 32% RH both the F-H-DLC and the H-DLC showed similar friction behavior, but μ S of H-DLC increased with an increase in RH while μ S of F-H-DLC decreased. Carbonaceous transfer layers were formed on the AZ91 surfaces and incorporated F transferred from F-H-DLC. The transfer layers were passivated by OH molecules, as detected by the Fourier transform infrared and micro-Raman spectroscopy. A first principles interface model that examined the hydrophobic interactions between F and OH terminated diamond (111) surfaces showed that high repulsive electrostatic forces would contribute to low friction in high humidity atmospheres.

The role of an ethanol environment on the tribological behavior of non-hydrogenated DLC (NH-DLC) coatings sliding against Ti–6Al–4V was investigated. Pin-on-disk tests were performed under different test atmospheres consisting of dry argon (0% RH) and ambient air (40% RH) and also while the samples were submerged in water (H 2 O) and ethanol (C 2 H 5 OH). The running-in friction of the NH-DLC coatings under each condition was studied and the results were used to rationalize the drilling behavior of Ti–6Al–4V by NH-DLC coated tools. The highest running-in coefficient of friction (μ R = 0.55) was observed under a dry argon atmosphere. In ambient air, the μ R was 0.42, while a reduction in μ R to 0.21 was recorded when the tests were carried out in H 2 O. The observed reductions in μ R were attributed to OH passivation of the NH-DLC surfaces. Ethanol was effective in providing OH passivation, as evident from the lowest μ R of 0.15. Consistent with the friction measurements, a 45% reduction in drilling torque was obtained when 10% ethanol was blended with the metal removal fluid (MRF) consisting of water–oil (1:3) mixture compared to an ethanol free MRF.

MoS2 coatings exhibit low coefficient of friction (COF) when sliding against aluminum; however, the magnitudes of their COF show high sensitivity to environmental conditions. Ti could reduce the sensitivity of the frictional behavior of MoS2 coatings to moisture. This study examines the tribological properties of Ti containing MoS2 coating (Ti–MoS2) tested against an aluminum alloy (Al-6.5% Si) in ambient air (58% relative humidity, RH), dry oxygen, dry air and dry N2 (

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N2 atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (E b ) for dissociative adsorption of H2O was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (E B ) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.