Tribochemical Changes Research Articles

Cellulose-armored “CNTs-soft metal” hybrid nanomaterials to improving the friction-reduction and anti-wear performance of bio-lubricants

Surface-inert carbon nanotubes (CNTs) exhibit poor interfacial compatibility with bio-oils and are prone to aggregation and precipitation which hinders their lubricity and anti-wear properties. Herein, polydopamine (PDA) was introduced to the surface of CNTs (CNTs-PDA) and constructed nanocellulose "armor" on the CNTs-PDA surface through hydrogen bonding effect (from amino and hydroxyl groups) to enhance the interfacial compatibility of carbon nanotubes with vegetable oils. The cellulose-armored “CNTs-soft metal” hybrid nanomaterial was used as a lubricant additive in sunflower oil, reducing the friction coefficient and wear rate of engineered steel surfaces by 61.9% and 79.5%, respectively. Furthermore, the tribo-chemical changes and lubrication mechanisms occurring at wear scars were revealed by the wear analysis, Raman spectrum, and lubrication theory calculations.

Ultralow Wear Poly(tetrafluoroethylene): A Virtuous Cycle of Wear Reduction and Tribochemical Accumulation

Certain nanofillers have demonstrated a remarkable wear-reducing effect on poly(tetrafluoroethylene) (PTFE), a common solid lubricant polymer. In these unusual cases, ultralow wear rates are accompanied by tribochemical changes that stabilize surfaces by anchoring polymer chains to dispersed nanoparticles and the countersurface. The leading hypothesis is that nanofillers catalyze these favorable tribochemical processes, which ultimately causes ultralow wear rates. However, the extent to which reduced wear rates contribute to the accumulation of favorable tribochemistry remains untested. In this study, we used indexed reciprocation to eliminate the protective effects of transfer films, control the wear rate, and independently determine how wear rate and filler morphology affect these tribochemical outcomes. Despite large and important differences between the fillers tested (from nanoparticles to microparticles), we observed surprisingly little effect from the filler on tribochemistry when controlling the wear rate. By contrast, tribochemistry was extremely sensitive to the changes in wear rate regardless of the filler. Standard reciprocation, which promoted the development of protective transfer films, caused low wear rates and significant tribochemical accumulation for all three filler conditions. By removing the protective effects of transfer films, indexed reciprocation increased wear rates by 100-fold while effectively eliminating tribochemical accumulation and tribofilm formation on the polymer pin. In contrast to prior studies, which demonstrate that tribochemical accumulation enables ultralow wear rates, this study demonstrates that a pre-existing condition of low wear is needed to enable tribochemical accumulation. The results from this study and prior studies of PTFE wear appear to be most consistent with the hypothesis that protective transfer films initiate low wear rates, that low wear rates enable tribochemical accumulation, and that tribochemical accumulation reduces wear rates in a virtuous cycle.

Tribological properties of YSZ and YSZ/Ni-YSZ nanocomposite coatings prepared by electron beam physical vapour deposition

Metal-ceramic nanocomposite coatings have been applied to many industrial applications owing to their remarkable properties such as wear, corrosion and high temperature oxidation resistance than that of metals and alloys in high temperature environments. In this study, YSZ and Ni-YSZ nanocomposite coatings deposited by electron beam physical vapour deposition (EBPVD) for high temperature environments have been investigated. Initially friction and wear behaviour of YSZ coatings deposited at various substrate temperature were studied. Then the effect on wear response of Ni-YSZ nanocomposites with different Ni content were investigated using a ball-on-disc micro tribometer. The structural and tribochemical changes that occurred in the wear tracks of YSZ and Ni-YSZ coatings were investigated using field emission scanning electron microscopy and Raman spectroscopy. The results obtained on sliding wear and friction behaviour of these nanocomposite coatings suggest that 50 wt.% of Ni in YSZ nanocomposite provides good wear resistance behaviour than that of other coatings. Such an improvement in tribomechanical and wear performance of the nanocomposite coating could be attributed to the optimum amount of Ni which promotes the formation of NiO from Ni due to the frictional heat between nanocomposite coating and the sliding counter body in wear track as confirmed by Raman analysis.

Leveraging trace nanofillers to engineer ultra-low wear polymer surfaces

Trace nanofillers have proven unable to stabilize polymer surfaces due to their inability to reinforce the subsurface. Thus far, their effects on surface wear have been obscured by potentially unrelated subsurface mechanics. In this paper, we addressed this problem by first eliminating matrix (PTFE) subsurface instability with a secondary filler (5 wt% PEEK) and then by stabilizing the sliding surface using variable loadings of trace alumina nanofillers. At the optimal loading of 0.1 wt%, the nanofillers reduced wear rates of an already low wear PEEK-PTFE blend by an unprecedented 50-fold. The minimum loading of 0.01 wt% reduced wear by 10-fold. Surface analysis revealed that trace nanofillers triggered ultra-low wear by catalyzing tribochemical changes that effectively crosslinked, anchored, and stabilized near-surface polymer chains. The degradation of wear performance at nanofiller loadings well above the 0.1 wt% optimum suggested that the mechanical detriments of greater loadings outweighed their tribochemical benefits. To our knowledge, this study is the first to isolate the effects of trace nanofillers on polymer surface stability and the first to demonstrate their radical wear-reducing potential. This hybrid approach – independently targeting surface and subsurface stability – serves as a roadmap for isolating the effects of other important nanofillers (e.g. CNT's) and engineering ultra-stable tribological surfaces.

Tribochemical changes of alloy 800HT under sliding contact at elevated temperature in impure helium environment

This study investigates the chemical changes of the surface of Alloy 800HT during sliding contact under helium atmosphere in the presence of impurities at 750 °C, which is of interest for the nuclear industry. The alloy samples are exposed to helium (He) atmosphere at high temperature for long duration of 500 h to identify the chemical and topographical changes of the alloy surface. The oxidation rate constant is reported based on the weight change data. For the tribo-experimental samples, the surfaces inside and outside the wear track are studied thoroughly using techniques such as secondary electron microscopy, energy dispersive spectroscopy, x-ray diffraction, Raman spectroscopy and secondary ion mass spectroscopy. The formation and evolution of surface oxides are discussed in He atmosphere, compared with air. The presence of Fe-rich thin layer of oxide on top of a thicker Cr-rich oxide and a mixed spinel-based inner layer on the bulk is found. The persistent presence of thicker Fe3O4in the wear track in high temperature air proved to be the reason of lower friction property. Pre-oxidation of the samples in air or He does not improve but worsen the tribological behavior in He atmosphere.

Reactive Pulsed Laser Deposition of Clustered-Type MoSx (x ~ 2, 3, and 4) Films and Their Solid Lubricant Properties at Low Temperature.

We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (−100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.

Mechanisms of change of superficial properties of polymeric materials in discharge plasma

In work analytical expressions for assessment of the tribochemical changes and definition of the most effective direction of regulation of contact interactions are received. These results can be used for the analysis of experimental data at a research of mechanisms of change of polymeric materials superficial properties.

Study of the interfacial mechanism of ZDDP tribofilm in humid environment and its effect on tribochemical wear; Part I: Experimental

Wear performance of any tribological system can be influenced in a complex way by water contamination. Water can be the cause of steel corrosion which, in turn, can accelerate wear. It can decompose the additives in the oil and create a more corrosive environment which leads to the higher wear in the system. A key novelty of this study is to investigate the effect of relative humidity and the tribochemical changes on the tribological performance and tribofilm characteristics of boundary lubricated systems by means of designing a humidity control system integrated to the Mini Traction Machine (MTM) and Spacer Layer Interferometry Method (SLIM) for the first time. The system is capable of simulating rolling-sliding conditions continuously where lubricant can be contaminated with water. This paper is the first part of a two-part study and the theoretical aspects of the work is the subject of the second part of this investigation. It was observed that humidity hinders the tribofilm formation, especially at higher values of relative humidity and lower temperatures and it can significantly affect the wear process. The correlation between tribofilm thickness, water concentration, temperature and wear of the system was studied. The experimental results suggest that the higher the humidity, the higher the wear of the system and it is more noticeable at lower temperatures where the tribofilm in thinner. The surface chemistry of zinc polyphosphates was investigated as a function of humidity.

Ultralow wear Perfluoroalkoxy (PFA) and alumina composites

Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1–5×10−4mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3MPa, v=50.8mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10−4mm3/Nm, and dropped to 4.0×10−8mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well.

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti3SiC2 and Ti3SiC2/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800°C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25°C (RT) to 600°C (friction coefficient ≈0.61–0.72, wear rate ≈10−3mm3 N m−1). An amount of Pb in the composites played a key role in lubricating with the temperature below 800°C. The friction coefficient (≈0.22) and wear rate (≈10−7mm3 N m−1) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti3SiC2/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O2=2PbO (below 600°C) and 6PbO+O2=2Pb3O4 (800°C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.

Tribofilm formation of lightly loaded self mated alumina contacts

A tribofilm is formed on alumina surfaces that have been slid against alumina surfaces. The tribofilm is formed by alumina wear particles that have been ground, agglomerated and tribosintered to a film. The tribofilm smoothens out the surface topography and fills up cavities. Tribofilms on alumina surfaces have been investigated with respect to surface appearance, hardness and chemical composition. Surface preparation and surrounding humidity have shown to affect the character and lateral distribution of the tribofilm. The tribofilm that was formed in humid air was softer than the tribofilm formed in dry air. XPS analysis revealed the chemical shift of the Al 2p peak did not differ between the tribofilms that was formed in different humidity, nor the unworn reference surface, finding that no hydroxide was found on the alumina surfaces. Also, no tribochemical changes could be detected by ToF-SIMS analysis.

Synthesis and tribological behavior of ionic liquid substituted fluoroalkoxycyclophosphazene derivatives in steel‐steel contacts

PurposeThe purpose of this paper is to synthesize a series of cyclophosphazene derivatives with better tribological properties.Design/methodology/approachTo improve the adsorptivity to substrate, a series of cyclophosphazene derivatives substituted with ionic liquids were synthesized. The lubricity characteristics of phosphazene derivatives were evaluated on an Optimol SRV IV oscillating friction and wear tester. Scanning electron microscope and X‐ray photoelectron spectroscope analyses were conducted to examine the morphology and chemical composition of the wear scars and the possible tribochemical changes involved in the friction process.FindingsAlkoxycyclophosphazene derivatives substituted with ionic liquids synthesized in the present work show better antiwear ability and load‐carrying capacity in steel‐steel contacts. The density of ion pairs plays an important role on tribological properties.Research limitations/implicationsThe paper deals with only a limited compounds and steel‐steel contacts. The paper works on the subject progressively to explore more combinations for better tribological properties.Practical implicationsAll the cyclophosphazene derivatives present better tribological performance and could be used as base oils, such as hard disk surface lubricants.Originality/valueIonic liquids are introduced to cyclophosphazene derivatives in this paper. This is a new method to improve the tribological properties of cyclophosphazene derivatives.

Tribo-physical and tribo-chemical aspects of WC-based cermet/Ti3SiC2 tribo-pair at elevated temperatures

The tribological behaviors of tungsten carbide (WC) based cermet/Ti3SiC2 tribo-pair at elevated temperatures were investigated. Lead oxide (PbO) was added as a solid lubricant. The tribo-physical and tribo-chemical changes on sliding surfaces were studied in detail. The results indicated that adhesive and abrasive wear due to removal of metallic binder and pullout of grains were the dominant wear mechanisms at room temperature. At high temperature, tribo-physical changes (i.e. mechanical mixture or sintering) and tribo-chemical reactions including complex reaction and oxidations were induced by frictional heat combined with high environmental temperature. As a result PbWO4 was formed as a reaction product and acted as a solid lubricant. PbWO4 and tribo-oxides along with the physically changed layer on the sliding surfaces were favorable to reduce wear of both materials. At high temperature, the wear mechanism varied from adhesive and abrasive wear at room temperature to lubrication by tribo-layer containing PbWO4, tribo-oxides, and sinters at high temperatures.

Characteristics of Acoustic Emission Signal Specific to Isolated Cracks Triggered by Scratch Tests in Stainless Steel Coatings

The tribological process during contact between two surfaces involves mechanical and tribochemical changes as well as material transfer. PVD coatings have a complex structure, and there is a need to characterize the failure of such coatings and this has been carried out by a non-destructive method, namely acoustic emission. In this paper simple scratch tests are described in order to study some characteristics of nitrogen-doped stainless steel coatings on 40CrMo4 construction steel. Scratch tests were performed in order to induce a mechanical failure in the coatings. The aim of this paper is to show that the nucleation of isolated cracks mechanism that leads to failure can be monitored by certain characteristics of the acoustic emission signal.

Tribochemistry and material transfer for the ultrananocrystalline diamond-silicon nitride interface revealed by x-ray photoelectron emission spectromicroscopy

The authors report tribochemical changes due to sliding of a silicon nitride (Si3N4) ball against an ultrananocrystalline diamond (UNCD) thin film. Unidirectional sliding wear measurements were conducted for 2000cycles using a ball-on-disk apparatus with a 3∕16in. diameter Si3N4 ball at a sliding speed of 3.3mm∕s and a normal load of 98.0mN (nominal Hertzian stress of 0.6GPa) in a nitrogen environment at 50% relative humidity at room temperature. The wear track produced on the UNCD film was analyzed by X-ray photoelectron emission spectromicroscopy (X-PEEM) combined with X-ray absorption near-edge structure (XANES) spectroscopy to identify and spatially resolve chemical changes inside the wear track, particularly rehybridization of carbon. XANES spectra show that SiOx complexes are deposited within the wear track. Very little rehybridization of the UNCD from its primarily sp3 bonding configuration to sp2 bonding is observed, and there is no observable oxidation of the UNCD, pointing to the impressive stability of the film under significant tribological loading conditions. Raman spectroscopy of the worn portion of the Si3N4 ball shows that disordered carbon is found on the worn surface. The authors attribute the formation of SiOx complexes within the wear track to wear of the ball and subsequent deposition into the track. The authors attribute the disordered carbon on the Si3N4 ball to removal of sp2-bonded carbon from the UNCD surface and subsequent accumulation onto the ball’s surface. This work demonstrates the potential of X-PEEM for the study of tribochemical modifications of carbon-based thin film surfaces.

Investigation of the tribological properties of polyfluo wax/polyurethane composite coating filled with nano-SiC or nano-ZrO 2

Effect of nano-SiC or nano-ZrO 2 on the friction and wear behaviors of the polyfluo wax (PFW)/polyurethane (PU) composite coating was studied using a ring-on-block wear tester under dry friction condition, and the worn surfaces of the PFW/PU composite coating and the transfer films formed on the surface of the counterpart ring were investigated by SEM and optical microscope, respectively. The structural changes of the PFW/PU coating filled with nano-ZrO 2 during the friction process were investigated with a Fourier transform infrared microscope. It was found that nano-SiC or nano-ZrO 2 as the fillers contributed to improve the friction-reducing and anti-wear abilities of the PFW/PU coating. Especially, the anti-wear ability of the filled coating is the most effective when content of nano-ZrO 2 is 5.0 wt.%. Furthermore, we studied the effect of curing temperature, sliding speed and applied load on the friction and wear behaviors of the PFW/PU coating filled with 5.0 wt.% nano-SiC or 5.0 wt.% nano-ZrO 2. The results showed that the friction and wear behaviors of all coatings were much better in lower curing temperature than those in higher curing temperature. The friction coefficient of the composite coatings reinforced with different fillers decreased with increasing sliding speed and applied load, and the anti-wear behavior of the coating filled with 5.0 wt.% ZrO 2 was the best under 420 N and at a speed of 2.56 m/s. The investigations of the frictional surfaces showed that the fillers of nano-SiC or nano-ZrO 2 were able to enhance the adhesion of the transfer films of the PFW/PU coating to the surface of counterpart ring, so they significantly reduced the wear of the PFW/PU coating. FTIR analyses indicated that the occurrence of the tribochemical changes during friction process mainly involved disordering and partial decomposition of the polyurethane binder.

Investigation of the tribological properties of polyfluo wax/polyurethane composite coatings filled with several micro-particulates

The tribological properties of the polyfluo wax (PFW)/polyurethane (PU) composite coatings filled with micro-MoS 2, graphite and SiO 2 were investigated using a ring-on-block wear tester under dry friction condition, and the worn surfaces of the composite coatings and transfer films formed on the surface of the counterpart steel ring were respectively investigated by SEM and optical microscope. The chemical changes of the composite coating filled with 10 wt.% MoS 2 during the wear process were analyzed by XPS and FTIR. It was found that the additions of MoS 2, graphite and SiO 2 are effective in enhancing the wear life of the PFW/PU coatings. Especially, the coating filled with 10 wt.%. MoS 2 can obtain the best anti-wear ability. Meanwhile, we studied the effect of curing temperature, sliding speed and applied load on the friction and wear behaviors of the PFW/PU composite coatings filled with several particulates. The results showed that the coatings filled with several particulates have excellent anti-wear behaviors as curing temperature is below 120 °C, and all coatings have the best anti-wear ability at 2.56 m/s and 320 N. Furthermore, the tribochemical studies by XPS revealed that MoS 2 in the PFW/PU composite coating is translated to MoO 3 during the wear tests. Accordingly, IR analyses indicated that the occurrence of the tribochemical changes during friction process mainly involved disordering and partial decomposition of the polyurethane binder.

Environmental effects on the friction of hydrogenated DLC films

We have investigated environmental effects on hydrogenated diamond-like carbon (H-DLC) films under various pressures of H2O, O2, and N2 by ultrahigh vacuum (UHV) tribometry. The H-DLC film exhibits an ultralow coefficient of friction (μ = 0.004 in UHV). The μ value increases with increasing pressure of H2O and O2. Specifically, μ increases up to 0.07 under 10 Torr of H2O, and up to 0.03 under 150 Torr of O2; these are typical H2O and O2 contents respectively in ambient air. Our results are consistent with similar environmental effects previously reported. But, we have also discovered that these friction changes are reversible, returning to the ultralow value when UHV is restored. The reversibility of the friction behavior in both environments, coupled with the lack of evidence of tribochemical changes by Auger electron spectroscopy, suggest that the observed friction changes are due to the weakly adsorbed gas molecules that influence the friction property by physically separating the H-DLC interface. Speed-dependent tribometry also supports this argument. In addition, two DLC films with different hydrogen contents and with widely different friction coefficients in UHV are shown to exhibit identical μ values under humid environments, further demonstrating that the frictional properties of these DLC films are essentially determined by the surface layer of adsorbed gas molecules.

Mechanism of boundary film formation from n‐hexadecane

AbstractIt is widely acknowledged that the process of surface film formation by tribological additives is very complex, because even saturated hydrocarbons — as major components of lubricating oils — undergo chemical changes under boundary lubrication conditions. To gain a better understanding of the tribochemical changes of paraffinic hydrocarbons, research was carried out using n‐hexadecane, which is widely used as a low‐viscosity model base oil. It has been hypothesised that the interactions between the products of n‐hexadecane triboreactions and steel surfaces are mostly initiated by the mechanical action of the system. Wear tests were performed, using a pin‐on‐disc machine. Fourier transform infrared microspectrophotometry and X‐ray photoelectron spectroscopy were used to analyse the wear tracks on the steel discs. Based on the analytical data obtained, a model of boundary film formation from aliphatic hydrocarbons is proposed.

The tribological behaviors of ordered system ultrathin films

The tribological behaviors of various long chain organic molecule LB films, organic molecule modified inorganic nanoparticle LB films, and the C60-LB films were systematically investigated. The correlation between the structure and tribological behavior of the LB films has been explored, while the structural changes of the LB films during friction process were analyzed with Fourier transform infrared (FTIR) microscope. Three kinds of C60-LB films were prepared; its micro-frictional behaviors were investigated by AFM. As the results, the tribological behavior of long chain organic molecule LB films is highly dependent on the molecular chain length and the characteristics of the polar end groups. The optimal tribological behavior is reached at a balanced stiffness and toughness of the molecular chain, which is also related to the bonding strength of the LB film to the substrate. The LB films of nanoparticles modified with organic molecules are superior to long chain organic molecule ones in terms of resisting wear. This is attributed to the enhanced load-carrying capacity of the inorganic nanocores in the LB films of nanoparticles modified with organic molecules. The tribochemical changes including ordering and partial decomposition of the organic modifier have been observed in the sliding of the LB films of inorganic nanoparticles modified with organic molecules against steel. C60-LB film shows excellent tribological behavior, which is highly dependent on the interaction between the C60 nanocluster and the organic long chain molecules.