Low COF Research Articles

Machining characteristics and mechanism of GO/SiO2 nanoslurries in fixed abrasive lapping

Abstract Water-based slurries with silica (SiO2) nanoparticles, graphene oxide (GO) nanosheets and GO/SiO2 hybrid nanostructures as abrasives were synthesised in order to achieve a high-efficiency and low-damage lapping. Tribological characteristics of SiO2, GO and GO/SiO2 nanoslurries were systematically investigated to optimise slurry formulae, and the lubricating mechanisms involved were revealed based on the analyses of worn surfaces and used slurries. Machining performance of the optimised slurries was then examined in the fixed abrasive lapping of glass substrates in terms of surface quality and material removal. The GO/SiO2 slurry of 0.16 wt.% at a mass ratio of 1:1 generated a lubricating layer consisting of C deposited and SiO2 dynamic films at the rubbing interface. This improved friction and wear conditions at the contact, producing a significantly lower COF and a better worn surface quality than those slurries containing only GO or SiO2. The lapping with the GO/SiO2 slurry thus reduced the surface damage on glass and achieved a crack-free subsurface. In comparison with a conventional lapping, the new lapping process resulted in a 35% reduction in surface roughness, but a 28% increase in material removal rate. Such improvements were attributed to the synergistic lubrication behaviour of the formed nanostructures of GO/SiO2 in water.

Effect of heat treatment on tribocorrosion behavior of 7B05 aluminum alloy in artificial seawater

Tribocorrosion behavior of 7B05 aluminum alloy under two heat treatment states has been studied in artificial seawater, using a pin-on-disk reciprocating sliding tester combined with a solartron electrochemical workstation. Both samples exhibit a face centered cubic crystalline structure with (200) preferred orientation. The T5 sample has a more uniform grain size and banded precipitate compared with the T4 sample. However, because of the uniform distribution of precipitated phase, the T4 sample has higher hardness, modulus, H/E and H3/E2. Experiments of both samples were performed under open circuit potential, potentiodynamic, and potentiostatic conditions at cathodic and anodic potentials. The results of OCP indicate the typical evolution of passive materials which present a significant cathodic shift at the beginning of sliding and return to initial after sliding. Under potentiodynamic polarization tests, the current under sliding of both samples are much larger than that without sliding at the same potential, especially the T5 sample. Compared with OCP and cathodic potential, both samples show a lower COF under anodic potential. The rate of volume loss of both samples increased with applied potential, and the rate of T5 sample is larger at the same potential. In general, under static condition, the T5 sample has better corrosion resistance. Reversely, it is found that the T4 sample possesses the better tribocorrosion resistance in artificial seawater, with the ratio of the synergistic effect S 54.6%, while the T5 sample is 74.0%.

Durability of glycaemic control with dapagliflozin, an SGLT2 inhibitor, compared with saxagliptin, a DPP4 inhibitor, in patients with inadequately controlled type 2 diabetes.

Dapagliflozin is associated with greater reductions in HbA1c and weight than saxagliptin in management of type 2 diabetes mellitus (T2DM). The present post hoc analyses compared the durability of these effects over short‐ and long‐term follow‐up in patients with T2DM who were inadequately controlled with metformin (≥1500 mg/day) and who were receiving either dapagliflozin (10 mg/day) or saxagliptin (5 mg/day). Failure of glycaemiccontrol was assessed using the slope of the change in HbA1c from baseline‐over‐time regression line (coefficient of failure [CoF]). CoF was compared directly (dapagliflozin vs saxagliptin) over the short term (NCT01606007, 24 weeks) and indirectly (placebo‐adjusted) over the long term (NCT00528879 and NCT00121667, 102 weeks). A low CoF value indicated greater durability. CoF was lower for dapagliflozin versus saxagliptin over 18–24 weeks (−1.38%/year; 95% CI, −2.41 to −0.35; P = .009) and 20–102 weeks (−0.37%/year; 95% CI, −0.73 to −0.02; P = .04). Fewer dapagliflozin‐treated patients versus saxagliptin‐treated patients required rescue medication or discontinued the study because of failure to achieve glycaemic control at 24 weeks (3.4% vs 9.4%; P = .0191). In patients with T2DM who were inadequately controlled with metformin, dapagliflozin was associated with greater durability of glycaemic control than saxagliptin over 18–24 and 20–102 weeks.

The Tribological Behavior of Hybrid Graphene/Tungsten Disulfide Nanoparticle Coatings

Environmental concerns regarding the disposal of fluid lubricants have led to the application of green technologies to metalworking processes in the form of dry lubricants. These dry lubricants are typically applied as spray coatings that employ the use of nanoparticles. The possible use of graphene in the form of graphene/tungsten disulfide (G/WS2) nanoparticle spray coatings has been examined in this research. The investigation was performed through the use of ball-on-disk tests at ambient temperatures to ascertain the possible application of the aerosol spray coatings to the aluminum forming processes. The coatings proved to possess good adhesion to the Al-Mg alloy substrate. The nanoparticle spray coatings were tested at various graphene to WS2 concentrations and under varying loads. A low steady-state COF was noted for all graphene concentrations during sliding contact under all the loads examined. The durability of the coatings was observed to improve with increasing loads. The low COF was attributed to the formation of wear-induced transfer layers on the steel balls and tribolayers on the coated Al-Mg surfaces. The coating durability was related to the concentration of graphene within the nanoparticle spray coatings and the stability of the tribolayers during sliding contact. The results highlight that the G/WS2 nanoparticle spray coatings could be considered for aluminum metalworking processes.

Investigation of mechanical and tribological properties of super-thick DLC films with different modulation ratios prepared by PECVD

Thick films can effectively improve the load-carrying capacity, but the thickness of films often fails to meet the requirements due to internal stress and other factors. The design of the multilayer film achieves an increase in film thickness by effectively reducing internal stress. The performance and structure of the super-thick DLC film can be structurally optimized by changing the modulation ratio. The super-thick DLC films consisting of Six-DLC layer (low Si content) and Siy-DLC layer (high Si content) with different modulation ratios (deposition time ratio of the Six-DLC to Siy-DLC) were successfully deposited on SUS304 stainless steel substrates by plasma enhanced chemical vapor deposition. The effect of modulation ratio on mechanical and tribological properties of the DLC-coating stainless steel was investigated by nano-indentation, scratcher and ball-on-disc reciprocating friction test. The results showed that hardening and toughening have been achieved through the modulation ratio design, and particularly remarkable when the modulation ratio ranges from is 1:5 to 1:1. The super-thick DLC film with a modulation ratio of 2:1 shows the best adhesion strength. Moreover, a film with a modulation ratio of 1:2 exhibits low COF about 0.03 at a load of 5 N due to the Si-rich transfer layer between the sliding interfaces; once the load reaches 20 N, the friction coefficient cannot establish such a low friction interface between the tribopairs, which is related to the peeling of the Si-rich transfer layer caused by high contact stress.

Microstructure and improved tribological performance of graphite/copper‑zinc composite coatings fabricated by low pressure cold spraying

It is a challenge to obtain metal-solid lubricant composite coatings with good tribological performance by cold spraying due to the poor interfacial bonding between solid lubricants and metal matrix along with uncompacted microstructure of the resulted composite coatings. Here, we report the preparation of graphite reinforced copper based composite coatings with improved self-lubricating and anti-wear performance by low pressure cold spraying via the rational feedstock design in combination with metallic pretreatment of graphite. Zinc particles are blended with the copper particles and used as the feedstock to produce a fully dense copper‑zinc matrix and ZnO during sliding process, which produces the durable binding of lubricating graphite ingredients to the contact surfaces. The graphite lubricants are pretreated with electroless copper coatings to improve their compatibility with copper‑zinc matrix and their content in the cold-sprayed composite coatings. It is found that graphite/copper‑zinc composite coatings with low COF, good wear resistance and high load capability can be obtained by tuning the content of copper-coated graphite in the feedstock. The mechanism on reduction of friction and wear for graphite/copper‑zinc composite coatings is also discussed.

Friction Behavior of TiAlN, AlTiN and AlCrN Multilayer Coatings at Nanoscale

In this study, 3.5 μm thick TiAlN, AlTiN, and AlCrN multilayer coatings were deposited on the H13 steel surface by physical vapor deposition (PVD) method. Friction behavior of these coatings was investigated by friction force microscopy for the first time at nanoscale in the literature. Friction force measurements were performed with the atomic force microscopy lateral force mode at various loads. Results showed that the AlTiN coating had three times lower COF value than the TiAlN and AlCrN coatings due to the lower surface energy. It was observed in this study that Amonton’s law was valid for the COF evolution at the nanoscale.

Tribological Behaviors of Cu/AlMgB14 Composite Under Deionized Water and Liquid Paraffin

In this work, a novel Cu/AlMgB14 composite was prepared by hot-press sintering, and its tribological responses to the AlMgB14 content and sintering temperature under deionized water and liquid paraffin media were investigated. As the results show, the wear resistance of pure copper is greatly enhanced by adding AlMgB14 in both media. Besides, the Cu/AlMgB14 composites sintered at lower sintering temperature show better lubricating irrespective of the AlMgB14 content due to the porosity of the specimens, while those sintered at higher temperature show gradually improved wear resistance with the AlMgB14 content increasing due to the growingly improved hardness. In addition, in liquid paraffin, a lower COF and a comparable wear rate to that in deionized medium are obtained due to the hydrophobic and lipophilicity of the composites.

The tribological evaluation of graphene oxide and tungsten disulfide spray coatings during elevated temperature sliding contact of aluminum-on-steel

The high tendency of aluminum to adhere to steel has led to extensive research into the application of lubricants to reduce adhesion and friction during forming processes. The behavior of lubricants like tungsten disulfide (WS2) and graphene oxide (GO) are sensitive to their deposition method. WS2 applied through aerosol spray has been observed to increase its working temperature range. This research aims to evaluate the tribological and durability properties of WS2 and GO (deposited as an aerosol spray coating) at temperatures between 25 °C and 450 °C, using ball-on-disc sliding tests. The spray coatings displayed low COF during sliding contact which has been attributed to the formation of transfer layers on the steel counter-faces and tribolayers on the coated Al-Mg surfaces, while their durability was observed to be related to the stability of the tribolayers. The WS2 spray coating was noted to possess more durable tribolayers, which could be a result of the superior adhesion properties of the carbon-bonded WS2 platelets to each other and the AlMg substrate.

Ru-induced microstructural change in ion-plated TiN coating and its tribological properties

TiN and Ti(Ru)N (3 wt%, Ru) coatings were deposited on Ti6Al4V substrate by arc ion plating method. Ti(Ru)N coating exhibits golden luster as the TiN coating does. From XRD, XPS and TEM investigations, Ru addition refines the TiN grains and Ti(Ru)N coating can be concluded as a kind of composite layer, which is mixed by Ru-containing amorphous part and crystalline TiN phases. The related hardening mechanism of Ru addition in TiN coating is fine grain strengthening. Wear resistance of Ti(Ru)N coating is significantly better than that of TiN coating, due to its higher hardness, higher H/E value and lower COF value. Improved wear performance of Ti(Ru)N coatings is mainly contributed to the mechanism of fine grain strengthening, which delays the occurrence of pitting of large-scale peel-off behavior like TiN coating.

Effect of load and sliding speed on the tribological properties of graphite-like carbon film under different aqueous environments

Closed field unbalanced magnetron sputtering system (CFUBM) was employed to prepare graphite-like carbon (GLC) film during deposition process. Structure and morphology of GLC film were measured by Raman spectrum and atomic force microscopy (AFM). Friction and wear properties of GLC film in different test conditions (applied load, sliding frequency, aqueous environment) were evaluated through multifunctional tribometer. The morphology of wear track was investigated by optical microscopy. Results indicated that the GLC film owned vast amounts of sp2-bonding clusters and displayed a flat surface. By friction and wear test, the GLC film presented relatively low COF and wear rate. Meanwhile, the applied load, sliding frequency and aqueous environment were crucial factors to influence the friction and wear behaviors of GLC film. Combined with the Hertzian contact theory, the micro-meshing regime might rationally explain the tribological mechanism of GLC film in this work.

Fluorine-carbon doping of WS-based coatings deposited by reactive magnetron sputtering for low friction purposes

WS2 self-lubricant coatings were fluorine-carbon alloyed to improve the load-bearing capacity, decrease the friction and, further, explore the water/oil wettability behaviour. The WS-C/F coatings were deposited by magnetron sputtering using an Ar/CF4 mixture. Up to a certain CF4 flow rate, F was effectively incorporated in the coatings (maximum of 9.5 at.%) which led to both compactness and hardness increase. Further CF4 flow rate increase led to fluorine content vanishing, while the oxygen and carbon contents increased, then hindering the mechanical properties. XPS results revealed that F and O are chemically bonded to W/S or W/C phases. The tribological performance evaluated at room temperature showed that, sporadically, the friction coefficient of the coating with 9.5 at.% F content reached lower values than WS2 (COF = 0.04 against 0.05). Meanwhile, when it was tested at 200 °C, the same coating showed lower COF values in the overall test (0.016 compared to 0.030), this attributed to the higher interplanar distance due to F insertion in the WS2 structure confirmed by XRD. F incorporation seemed to decrease the coating surface roughness and increased its hydrophilicity, with no effect on the oleophilicity. This study presents a further insight on the understanding of the fluorine doping of sputtered WS2 coatings for potential mechanical application.

Tribological Behaviour of Ti:Ta-DLC Films Under Different Tribo-Test Conditions

Diamond-like carbon (DLC) films are suitable applicants for cutting tools due to their high hardness, low friction coefficient and wear rate. Doping metals in DLC films have been improved its tribological properties. In this study, titanium and tantalum doped hydrogenated DLC films were deposited by closed-field unbalanced magnetron sputtering system onto M2 high speed steels in Ar/N2/C2H2 atmosphere. The friction and wear properties of Ti:Ta-DLC film were investigated under different tribo-test conditions including in atmospheric pressure, distilled water, commercial oil and Ar atmosphere. The coated specimens were characterized by SEM and X-ray diffraction techniques. The bonding state of C-C (sp3) and C=C (sp2) were obtained with XPS. The tribological properties of Ti:Ta-DLC were investigated with pin-on-disc wear test. Hardness measurements performed by micro-indentation. Our results suggest that Ti:Ta-doped DLC film shows very dense columnar microstructure, high hardness (38.2 GPa) with low CoF (µ≈0.02) and high wear resistance (0.5E-6 mm3/Nm).

Structure and properties of NbN/MoN nano-multilayer coatings deposited by magnetron sputtering

NbN/MoN nano-multilayer coatings with MoN layer thickness ranging from 1.0 to 4.3 nm were deposited using a closed field unbalanced magnetron sputtering system by varying Mo target current. The structure of the coatings was investigated by X-ray diffraction, scanning electron microscopy, atomic force microscopy and transmission electron microscopy. The mechanical and tribological properties of the coatings were evaluated by nano-indentation and ball-on-disc wear tests. It was found that all of the as-deposited NbN/MoN multilayer coatings exhibit fcc B1-NaCl structure and the preferred orientation of the coatings changed from (111) to (200) as MoN layer thickness exceeds 2.1 nm. The NbN/MoN multilayer coatings with MoN layer thickness 1.0 nm exhibit a superhardness with value of 40.0 GPa. Further increase of MoN layer thickness results in a decrease in coating hardness. The NbN/MoN multilayer coatings exhibit lower coefficient of friction and better wear resistance compared to NbN single layer coatings. A low COF of 0.53 and wear rate of 0.7 × 10−6 mm3/Nm can be achieved in the NbN/MoN multilayer coatings with a MoN layer thickness of 4.3 nm.

Stoichiometry and tribological behavior of thick Ta(N) coatings produced by direct current magnetron sputtering (DCMS)

Abstract Thick Ta(N) coating of 51 μm has been successfully obtained by DCMS technology. Ta(N) is a kind of distorted Ta matrix, which is inter-soluble with N-defect lattice structure, forming the disabled bcc structure. From the XRD and XPS investigations, the composition of Ta(N) coating is consisted of bcc-Ta and bcc-TaN0.06, while that of Ta coating mainly contains β-Ta phase. It can be concluded from wear test, nanoindentation test and SEM observations, wear resistance of Ta(N) coating is much better than that of Ta coating, due to its high hardness, H/E, H3/E2 value and low COF value. The wear mechanism of Ta coating is the compound fatigue and abrasive wear, while that of Ta(N) coating is transformed into adhesive wear mechanism. The secondary adhesion of the plastic deformation for the Ta(N) coating can reinforce the coated surface, to improve the load-bearing and anti-wear capacities, and thus improve the wear resistance.

Effect of WS2 Addition on Tribological Behavior of Aluminum at Room and Elevated Temperatures

The objective of this study is to evaluate the role of WS2-based two-dimensional material as solid lubricant additive to aluminum to reduce the friction. Al-2 vol% WS2 was consolidated by spark plasma sintering into a 99% dense composite. Tribological behavior of Al-2 vol% WS2 and pure Al was evaluated at room temperature and 200 °C using the ball-on-disk method in dry sliding wear conditions. Wear mechanism was studied using wear surface and sub-surface via electron microscopy and focused ion beam milling. Al-2 vol% WS2 showed the lowest coefficient of friction value of 0.55 at 200 °C as compared to 0.82 that of pure aluminum. The wear rate of Al-2 vol% WS2 showed 54 and 29% improvement at room temperature and 200 °C, respectively, as compared to pure Al. The improved tribological properties of Al-2 vol% WS2 are attributed to a tribofilm formation due to the breaking of the weak Van der Waals forces that hold the S–W–S structure. Due to shear forces, a portion of the tribofilm is transferred on the counter surface providing lubrication properties resulting in a stable and low COF.

Development of new duplex treatments on 100Cr6steel combining Thermochemical Treatments, Laser Shock Peening and Physical Vapour Deposition

100Cr6 steel (AISI 52100) is one of the most used steel grades in the manufacturing of through hardening bearings mainly due to its properties: controlled impurities during steel making process, high hardenability and well known mechanical properties such as wear and fatigue resistance on clean environments. These characteristics play an important role on the performance of a bearing together with the bearing design, loads and environment. However, there is an increasing set of demanding applications where the above mentioned steel does not fulfil the required needs and thus, bearing manufacturers continuously work on the development of technologies to improve the bearing performance.Nowadays thermochemical treatments (TCT), such as carbonitriding are being applied to this steel in order to enhance the performance of such pieces in contaminated environment, where particles can produce defects on the raceway, increasing the onset of defects that eventually lead to premature fail. These treatments induce the formation of carbides and nitrides which are directly related to the enhancement of the wear resistance and also to increasing the amount of Retained Austenite (RA) in the surface which may have a beneficial effect as it delays the crack propagation on subsurface regions, then increasing bearing fatigue life.In this work, different TCTs have been applied to 100Cr6 steel flat samples. Using a tribometer (ball-on-disc configuration) and a grinding machine, surface and in-depth wear resistance measurements have been carried out, obtaining wear resistance profiles that have been correlated with the microstructure, microhardness profiles and RA content.The most promising TCT has been combined either with Laser Shock Peening (LSP) treatments or carbonaceous Physical Vapour Deposition (PVD) coatings with the aim of improving not only the wear resistance but also the CoF of the duplex treated sample.The results obtained on flat samples are promising; the combination of treatments produces long-lasting low CoF and a reduction of 60% in the wear rate. However, the treatments should be applied on real pieces and tested in a test bench in order to obtain more appropriate data about the lifespan of duplex treated bearings.

Effect of nitrogen flow rate on structure and properties of MoNx coatings deposited by facing target sputtering

Molybdenum nitride (MoNx) coatings with different nitrogen content were deposited by reactively sputtering molybdenum metal target with various N2 flow rate. The structure of the coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectrometry (XPS). The mechanical and tribological properties of the coating were evaluated by nano-indentation and ball-on-disc wear tests, respectively. The structural analysis showed that these coatings changed from bcc α-Mo phase to fcc γ-Mo2N phase, and then to fcc B1-MoN with the increase in N2 flow rate. As the phase structure changed from α-Mo to γ-Mo2N, the MoNx coatings exhibit finer grain sizes and denser crystal structure, which have dramatic contribution on hardness enhancement. The γ-Mo2N coating exhibits the highest hardness with the value of 32.5 GPa. As phase structure further changed to B1-MoN coating, a slight decline of hardness can be observed. The γ-Mo2N structured coatings exhibited lower COF than that of mainly Mo structured MoNx coating or B1-MoN coating.

Simultaneously achieving superior mechanical and tribological properties in WC/a-C nanomultilayers via structural design and interfacial optimization

Nanomultilayered WC/a-C coatings with modulation period raning from 1.3 to 11.5 nm were successfully fabricated using unbalanced magnetron sputtering process and the evolution of their microstructure, mechanical and tribological properties with the modulation period were systemically investigated. It has been demonstrated that the columnar structure in WC/a-C nanomultilayers originates from the surface irregularities of intermediate Cr/WC/C layer and the column diameter is correlated with the initial Cr layer thickness in this intermediate stage. Hardening and toughening through the nanomultilayer structure design have been achieved and particularly pronounced when the modulation period ranges from 5.8 to 10 nm. The enhanced hardness and fracture toughness are mainly attributed to dislocation/crack pinning effect from the thin individual layer and the heterointerfaces between WC and a-C layers. It is also found that the more heterointerfaces, the lower internal stress. What's more, the WC/a-C nanomultilayers with small modulation period (≤5.8 nm) exhibit low COF about 0.05 and good wear resistance due to the interfacial sliding between a WO3-rich tribofilm and a carbon-rich wear surface; once the modulation period above 5.8 nm, it fails to build up such a low friction interface between the tribopairs, which combining with the high intrinsic friction caused by its own structure characteristics result in high COF about 0.18–0.2 and poor wear resistance.

Influence of Hot forging on Tribological behavior of Al6061-TiB2 In-situ composites

Al6061-TiB2 metal matrix composite was fabricated by stir casting technique via in-situ reaction, using mixture of Al6061 alloy, Potassium tetraflouroborate salt (KBF4) and tetraflourotitanate (K2TiF6). The cast composites were processed to hot forging, SEM studies; X- ray Diffraction studies (XRD), Microhardness and Dry friction and wear tests. Pin on disc type machine was used to perform tribological tests over a load range of 20-100N and sliding velocities of 0.314-1.57m/s. SEM and XRD studies confirms formation of fine in-situ TiB2 particles. Composites exhibit higher Microhardness, improved wear resistance and Lower COF with formation of TiB2 particles when compared with the unreinforced alloy. Compared to cast alloy and its Composites, forged alloy and its composites show superior Tribological behavior under similar test conditions.