Wear Track Surface Research Articles

Role of Testing Conditions in Formation of Tribological Layers at Line Contacts of Antifriction CF-Reinforced PI- and PEI-Based Composites.

High-strength PI and PEI polymers differ by chemical structure and flexibility of the polymer chains that ensure lower cost and higher manufacturability of the latter. The choice of a particular polymer matrix is of actuality at design of antifriction composites on their basis. In this study, a comparative analysis of tribological behavior of PI and PEI- based composites was carried out with linear contact rubbing. The neat materials, as well as the two- and three-component composites reinforced with chopped carbon fibers, were investigated. The third components were typically used, but were different in nature (polymeric and crystalline) being solid lubricant fillers (PTFE, graphite and MoS2) with characteristic dimensions of several microns. The variable parameters were both load and sliding speed, as well as the counterface material. It was shown that an improvement of the tribological properties could be achieved by the tribological layer formation, which protected their wear track surfaces from the cutting and plowing effects of asperities on the surfaces of the metal and ceramic counterparts. The tribological layers were not formed in both neat polymers, while disperse hardening by fractured CF was responsible for the tribological layer formation in both two- and three component PI- and PEI-based composites. The effect of polymer matrix in tribological behavior was mostly evident in two-component composites (PI/CF, PEI/CF) over the entire P⋅V product range, while extra loading with Gr and MoS2 leveled the regularities of tribological layer formation, as well as the time variation in friction coefficients.

Exfoliated nano-hBN additives for enhancing tribological performance of ATSP coatings deposited on AISI 316L steel: Role of SMAT pre-treatment

The current study investigates the role of hexagonal boron nitride (hBN) content (1, 5, and 10 wt%), the extent of exfoliation (magnetic stirring for 1, 5 and 12 h duration) and substrate conditions (with/without surface mechanical attrition treatment (SMAT)) on tribological performance (under dry sliding condition) of aromatic thermoset polymer (ATSP) coatings deposited on AISI 316L stainless steel using Mayers bar technique. The thickness of hBN particles is decreased from ~115 to ~14 nm after 12 h of stirring. The coating thickness is slightly higher for the SMATed (S) substrate than for the non-SMATed (NS) substrate. The coefficient of friction (COF) reduction of ~57 and ~86 % is observed for ATSP@hBN composite coating (containing 10 wt.% hBN) deposited on NS and S specimens (after 1 h stirring), respectively. The ATSP coating’s volume loss is reduced by ~70 % when deposited on SMATed substrate. Further, COF and volume loss are decreased with increased hBN content and stirring time. Composite coatings deposited on non-SMATed specimens show a considerable reduction in COF to ~0.036 after 12 h of stirring and 10 wt.% of hBN content. The minimum COF of ~0.018 and volume loss of ~0.0095 mm3 are achieved when this composite coating is deposited on the SMATed substrate. The ATSP-coated NS specimen suffers more severe damage during wear by adhesion, scratching, and oxide formation, which is considerably reduced after adding hBN. The wear track width and surface damage are negligible in the case of composite coated SMATed substrate. The factors like better load-bearing capacity, traces of coating on the wear track and thick transfer film on the counter ball are responsible for lowering the COF of ATSP and ATSP@hBN composite coatings deposited on SMATed substrate. The reduced COF generates less frictional heat, causing lower oxidation and enhanced coating durability.

Tribological properties of TiSiVN coatings: The effect of the counterpart on the wear performance

Self-lubricant coatings are a subject of continual research and improvements. In this research, the tribological performance of TiSiVN coatings produced by increasing V concentration is evaluated. The coatings were deposited by high-power impulse magnetron sputtering in deep oscillation mode, and were tested against three different counterparts (Al2O3, stainless steel and Titanium). The wear mechanisms were identified and the tribolayer was characterized to correlate with different specific wear rates and friction coefficient values. The coatings tested against Al2O3 balls displays a polishing type wear mechanism with some material adhered to the film wear track surface. When V is added to the reference film, V-O formed on the wear track has a beneficial influence by decreasing the friction and the wear of the coatings. For stainless steel balls, the wear track displays a mixture of severe abrasion and adhesion wear. Enclosure of hard particles at the softer stainless steel ball is the cause for the severe abrasion of the coating surface. This harsh abrasion removes the V-O from the contact and consequently decreases the efficiency of reducing friction coefficient and wear when compared to the tests against Al2O3 balls. For tests conducted against Ti ball, a strong adhesion of the material to the counterpart is observed. The adhered material protects the coatings from wear by giving rise to similar friction coefficient values irrespective of the coatings’ chemical composition.

Effect of Transfer Film on Tribological Properties of Anti-Friction PEI- and PI-Based Composites at Elevated Temperatures.

The structure, mechanical and tribological properties of the PEI- and PI-based composites reinforced with Chopped Carbon Fibers (CCF) and loaded with commercially available micron-sized solid lubricant fillers of various nature (polymeric-PTFE, and crystalline-Gr and MoS2) were studied in the temperature range of 23–180 (240) °C. It was shown that tribological properties of these ternary composites were determined by the regularities of the transfer film (TF) adherence on their wear track surfaces. The patterns of TFs formation depended on the chemical structure of the polymer matrix (stiffness/flexibility) as well as the tribological test temperatures. Loading with PTFE solid lubricant particles, along with the strengthening effect of CCF, facilitated the formation and fixation of the TF on the sliding surfaces of the more compliant PEI-based composite at room temperature. In this case, a very low coefficient of friction (CoF) value of about 0.05 was observed. For the more rigid identically filled PI-based composite, the CoF value was twice as high under the same conditions. At elevated temperatures, rising both CoF levels and oscillation of their values made it difficult to retain the non-polar PTFE transfer film on the sliding surfaces of the PI-based composite. As a result, friction of the ceramic counterpart proceeded over the composite surface without any protecting TF at T ≥ 180 °C. For the sample with the more flexible PEI matrix, the PTFE-containing TF was retained on its sliding surface, providing a low WR level even under CoF rising and oscillating conditions. A similar analysis was carried out for the less efficient crystalline solid lubricant filler MoS2.

Ionic liquids as boundary additives in water-based and PAO lubricants

Ionic liquids have been widely discussed as potential lubricants, however, their properties make them also very good potential candidates as lubricant additives (e.g., friction modifiers and anti-wear). In this work, the tribological study of two ionic liquids (tributylmethylphosphonium dimethylphosphate (PP), and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP)) as lubricant additives has been performed on stainless steel (AISI 316L) exposed to polar (water-glycol) and non-polar (polyalphaolefin) based lubricants under boundary lubricating conditions. The performance of these ionic liquids as lubricant additives has been compared to a classical organic friction modifier (dodecanoic acid (C12)). The water-glycol lubricant formulated with the two ionic liquids showed friction values higher than the same base lubricant formulated with dodecanoic acid, however, opposite results were observed for polyalphaolefin (PAO). A detailed surface chemical analysis using X-ray photoelectron spectroscopy (XPS) revealed differences in the passive/tribofilm thickness and chemical composition of the stainless steel surface tested in all lubricants. In the case of the polar lubricant additivated with ionic liquids, the tribochemical reaction accompanied by a tribocorrosion process led to the formation of an unstable passive/tribofilm resulting in high friction and wear. However, in the absence of tribocorrosion process (polyalphaolefin base lubricant), the tribochemical reaction led to the formation of a stable passive/tribofilm resulting in low friction and wear. A detailed surface and subsurface investigation of the microstructure using scanning electron microscopy equipped with a focused ion beam (SEM-FIB) showed that high wear rates resulted in thicker recrystallization region under the wear track surface. Among all lubricant additives tested in this work, BMP in non-polar lubricant media showed the best tribological performance.

Effect of NiTi metallic layer thickness on scratch resistance and wear behavior of high entropy alloy (CrAlNbSiV) nitride coating

In recent years, high entropy alloy (HEA) nitrides have shown outstanding properties in the field of protective hard coatings. However, for monolayer HEA nitride thin film, low adhesion strength and high residual stress lead to poor anti-wearing capabilities. This study aims at investigating the effect of metallic layer thickness in superelastic Ni51.3Ti48.7 on adhesion strength and wear resistance of HEA (Cr0.35Al0.25Nb0.12Si0.08V0.20) nitride coating. The results showed that the high compressive residual stress of top nitride layer was relieved by introducing NiTi metallic layer. The adhesion strength, wear track morphology and surface fracture were associated with the metallic layer thickness. Adding a proper thickness of NiTi metallic layer can improve the wear resistance of HEA nitride coating. Nevertheless, it was also observed that if the metallic layer thickness was not well controlled, the wear resistance enhancement was insignificant. The findings of this study are suggestive to provide a feasible approach for fabricating anti-wearing HEA nitride coatings.

Effect of Potential and Microstructure on the Tribocorrosion Behaviour of Beta and Near Beta Ti Alloys II

Titanium alloys, especially Ti6Al4V, are commonly applied in orthopaedic implants as a result of their relatively low density, good corrosion properties, satisfactory biocompatibility and bone ingrowth promoting properties. However, Ti implants are susceptible to mechanical failure. Although corrosion and wear related problems have been recognized as a major issue impeding their long-term application, there is still a lack of knowledge about the basic mechanisms. Previously, the tribocorrosion properties of 4 distinct titanium alloys (Ti13Nb13Zr, Ti12Mo6Zr2Fe, Ti29Nb13Ta4.6Zr aged at 300 °C and at 400 °C) was analysed in the published Part I of this study in regard to wear rates, electrochemical behaviour, and the tribocorrosion synergism estimations. This work, Part II, contributes to the previous study and investigates the tested surfaces of these 4 Titanium alloys from the same tribosystem aiming to characterize the wear track surfaces and identify the main wear mechanism, to characterize the tribofilm and to investigate the subsurface alterations occurring under varying contact pressures and electrochemical potentials. The results indicated a dominant abrasion wear mechanism regardless of microstructure, electrochemical potential and normal load (contact pressure). Additionally, grain refinement observed on the subsurface varied with alloy and electrochemical potential, with the variation being mostly independent of alloy microstructure. Finally, a graphitic tribofilm was detected in most conditions, which while inconsequential in regard to wear, may explain the previously observed reduction of friction.Graphic

HOT COMPACTION PROCESS OPTIMIZATION FOR IMPROVEMENT TRIBOLOGY BEHAVIOR OF ZIRCONIUM SILICATE STRENGTHENED BMCS

The paper describes the optimization of the hot compaction process to simultaneously increase hardness and decrease the wear coefficient of zirconium silicate reinforced BMCs. L9 orthogonal array is chosen for setup the experiment. Examining the influencing parameters is carried out on factors such as pressure, temperature, particle size, and particle content. Grey relation analysis is used to investigate to produce an optimal combination of parameter levels. The transmission electron scanning is used to study the morphology of zirconium silicate. The wear coefficient of the specimen was investigated by using the weight loss method. A scanning electron microscope was carried out to evaluate the wear track surface of the composite. The test results show that the particle size is the most influential hot compaction parameter. The optimal conditions for the hot compacting process are the temperature level at 350 °C, the pressure level at the 400 MPa level, the particle content level at 12 % weight, and the particle size level at 80 µm. In this optimal condition, the prediction GR-Grade value is 0.695. The validation test results showed that the GR-Grade value increased by 0.15, the hardness increased by 25%, and the wear coefficient decreased by 53%. This optimization method with Gray Relational Analysis has proven to be effective in the hot compaction process for improving the tribology behavior of the composites.

Acoustic emission and machine learning based classification of wear generated using a pin-on-disc tribometer equipped with a digital holographic microscope

The efficiency of processes involving frictional contacts between surfaces is often characterized by wear rates or friction coefficients. However, the classification and forecasting of wear rates in friction related processes is a real industrial challenge that is unsolved today. Hence, an on-line monitoring system able to classify wear rate can be crucial for many industries as it could help in preventing catastrophic failures. Applications include lifetime assessment of various industrial components where a range of wear failures occur such as scuffing (a typical sudden failure mechanism). These tribological processes can now be sensorized, and the corresponding sensor signatures can be modelled and monitored using state-of-the-art Machine learning (ML) algorithms. In this study, we use an Acoustic Emission (AE) sensor and ML frameworks to classify different wear categories simulated with a customized pin-on-disc tribometer. A real-time investigation of the wear track is necessary to find out the origins of the wear scar visible at the surface. To achieve this objective, the experiments were conducted on a pin-on-disc tribometer equipped with a Digital Holographic Microscope (DHM). Experiments were carried out using alumina and steel balls against steel discs at room temperature. Real-time DHM images of the wear track surface were recorded for each lap at the same position. An acoustic emission sensor recorded the AE signals during the complete duration of experiments. The AE signatures, in combination with the real-time DHM images, were correlated as input and ground truth labels for the ML algorithm. Several ML frameworks were compared; they are support vector machine, logistic regression, XGBoost, random forest, neural networks, k-Nearest Neighbor, quadratic discriminant analysis and Naive Bayes. The classifier was trained to differentiate the acoustic emission features of the different wear rates. Most ML algorithms had an average classification accuracy above 80%, and the highest was obtained with support vector machine (84.7%). The classification accuracy can be improved by combining two neighboring categories with limited differences in terms of wear rate. Hence, the proposed method has a significant industrial potential for in-situ and real-time quality monitoring of wear processes since it requires minimum modifications of commercially available industrial machines.

Influence of the ringer's solution on wear of vacuum mixed poly(methyl methacrylate) bone cement in reciprocating sliding contact with AISI 316l stainless steel

This paper presents microstructural properties and damage behaviour of a vacuum mixed poly(methyl metacrylate) (PMMA) bone cement, during the sliding contact with AISI 316L stainless steel, under micro-loads. Influence of the Ringer's solution on the wear was analysed in comparison to dry contact. The variation of load did not produce any significant change of the wear factor while the increase in the sliding speed induced significant increases in the wear factor, more pronounced in the case of dry sliding. The obtained wear factors were in average higher for the sliding in Ringer's solution than those obtained under dry conditions. Significant fragmentation of the worn tracks, of irregular shapes with broken edges, was observed, slightly more pronounced for the dry contact. Many cavities and voids were formed on the wear track surface, but they did not extend into the bulk material. Higher loads produced more uniform and less fragmented wear tracks. Abrasive, adhesive wear and plastic deformation grooves were observed, as well as fatigue and erosive wear. Fatigue cracks developed in the direction normal to sliding. Network of fine craze cracks was exhibited on the surface of wear tracks, especially pronounced in the case of dry sliding. These results are important since they contribute to understanding the sites of crack initiation, and development mechanisms on the surface of PMMA bone cements, also including synergistic effects of physiological environments pertaining to the non-steady crack and craze behaviour and crack pattern development in PMMA.

Effect of sliding wear and electrochemical potential on tribocorrosion behaviour of AISI 316 stainless steel in seawater

Sliding tribocorrosion behaviours of AISI 316 stainless steel were investigated against alumina ball in natural seawater environment (SWE) under 5 N loads. OCP (open circuit potential), potentiostatic (−1 V and 0.3 V) and potentiodynamic measurements were enforced to understand the influence of the sliding wear and electrochemical potential on tribocorrosion behaviours of AISI 316 stainless steel in SWE. Tribocorrosion experiments were conducted with a ball-on-disk type reciprocating tribometer integrated with the three-electrode potentiostat. The wear track surfaces were examined by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and optical profilometry. Mechanical effects reduced corrosion potential under both OCP and potentiodynamic scans in tribocorrosion conditions. Ploughing effects on the wear track were evident at the OCP and −1 V potential because of the predominance of mechanical effects. Pits were determined to develop due to the formation of the galvanic couple between worn and unworn surfaces after the tribocorrosion test, which was conducted under 0.3 V potential. The reduction of the contact area because of the pits and the lubricating effect of the oxides on the surface caused the lowest coefficient of friction (COF) in the wear zone at 0.3 V potential. Besides, COF obtained under −1 V cathodic protection potential (0.35) was less than in OCP condition (0.37) since it only occurred by the wear effects. The study also revealed that material loss from the wear track increased from cathodic potential towards anodic potential.

Optimization Study on Sliding Wear Characteristics and Heat-Treatment Conditions of Different Grades of Ferritic Ductile Cast Iron

The present work reports an experimental investigation on influence of microstructures on dry sliding wear performance of different grades of ferritic ductile iron. Ductile cast iron samples of ferritic grade have been subjected to different heat-treatment processes at different temperatures and times. Taguchi optimization technique (L16) has been applied to evaluate the influence of different process variables (load, time, heat treatment, and grade) during ball-on-plate wear test. Meanwhile, analysis of variance (ANOVA) method was adopted to know the significance of aforesaid process variables. ANOVA results confirmed that the heat-treatment process has highest significance (54.76%) within all process variables. Among heat-treated specimens, austempered samples have outstanding wear resistance while the DMS samples have lower wear resistance. In addition, overall utility values have been evaluated by using individual utility values of weight loss and hardness. An obtained overall utility value gives the optimum combination for achieving higher wear resistance and hardness. Additionally, morphology of wear surfaces was examined in scanning electron microscope and the micrographs confirm the existence of inferior surface in terms of abrasive wear, adhesive wear, particle pullout and delaminated sheets on wear track. Enrichment of oxygen element has been observed on the worn path through energy-dispersive X-ray spectroscopy. X-ray diffraction analysis confirms the existence of different compounds like iron and silicon oxides on the wear track surface which may improve its hardness.

In-situ cyclic voltammetry of an ionic liquid as a lubricant additive in a steel-ceramic contact

The tribochemical interaction of the ionic liquid triethylmethylammonium methyl sulphate as an additive in poly(ethylene glycol) base fluid with a steel surface sliding against alumina was investigated by in-situ electrochemical methods based on Open Circuit Potential (OCP) measurement and Cyclic Voltammetry (CV). In the current-potential relationships of the lubricant with and without sliding interaction, significant differences were observed. Additionally, contributions of the neat base fluid, selected added contaminants, and atmosphere composition to the entire reactivity were studied. In the presence of the ionic liquid, subtle changes of the current influence friction substantially. Protective atmosphere was shown to sensitize the chemical response to potential excursions, giving evidence of a redox couple possibly existing temporarily on the wear track surface.

Adsorption of Organic Friction Modifier Additives.

Organic friction modifier additives (OFMs) are surfactant molecules added to engine oils to reduce friction in the boundary lubrication regime. They are thought to work by forming an absorbed layer, which provides low friction. This paper studied the relationship between the adsorption of OFMs and their friction and wear-reducing properties in a rubbing contact formed by a stationary glass ball and a rotating silicon disk under the boundary lubrication regime. The effect of molecular structure was investigated by using OFMs of various tail saturation and head group chemistry. OFMs tested were oleic acid, octadecylamine, oleylamine, and glycerol monooleate. The thickness of an OFM-adsorbed layer in hexadecane, examined in situ by spectroscopic ellipsometry and quartz crystal microbalance (QCM), depends on the molecular structure and the concentration of the OFM. As expected, the saturated, linear chain gives the thickest film. A critical OFM layer thickness of about 0.6 nm is necessary to achieve low initial and maximum friction. The thicker OFM layers are accompanied by narrower wear tracks, which are rougher than the wider, smoother wear tracks formed with thinner OFM layers. The interplay between the thickness of the OFM layer and wear track surface roughness results in all OFM layers having similar steady state friction. This shows that the apparent effect of OFM depends on the stage of the rubbing test: initially on friction, and then subsequently on surface damage. Despite OFMs and the base oil having similar refractive indices, ellipsometry was found to be a suitable technique for examining the adsorption of OFM additives from an oil-based solution and showed reasonable correlation with QCM results.

Tribological behavior under high temperature of BCN films deposited by sputtering-PBII hybrid system

In this article, the results obtained from a study on the tribological behavior of boron carbon nitride (BCN) films under high temperature are reported. BCN films with different carbon concentrations (from 0 to 66.7 at%) were deposited by a sputtering-PBII hybrid system. The hardness and Young's modulus of the films were evaluated using a nanoindenter. The friction and wear behaviors of the films measured under room and high temperatures were characterized using ball-on-disk friction testing and an optical microscope. The element composition of the transfer layer formed on the ball and wear track surfaces was analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The results indicated that the nanohardness and Young's modulus of BCN films increased with increasing carbon concentration. A higher carbon concentration led to a lower friction coefficient when measured at room temperature. However, only the BC2.4N1.3 film showed a very low friction coefficient with a low specific wear rate when measured under high temperature. This behavior can be confirmed by the agglomerated debris and wear scars formed on the ball and wear track surfaces. Therefore, the BC2.4N1.3 film is a promising material as a protective coating to be applied in high temperature lubricating applications due to its good wear resistance and thermal stability.

Effect of Silicon Coupling Grafted Ferric Oxide and E-Glass Fibre in Thermal Stability, Wear and Tensile Fatigue Behaviour of Epoxy Hybrid Composite

In this present study the effect of adding silicon coupling grafted ferric oxide and E-glass fibre in thermal stability, wear and fatigue behaviour of epoxy resin hybrid composite was investigated. The principal aim of this research was explicating the importance of silicon coupling grafted E-glass fibre and ferric oxide particle in thermal stability, wear and fatigue properties of epoxy hybrid composite. Ferric oxide particles of 800, 200 and < 100 nm and E-glass fibre of 600 GSM was used as second phase additions in epoxy resin with surface grafted condition. The surface grafting was done using 3-Aminopropyletrimethoxylane via aqueous solution method with acetic acid as pH adjuster. The improvement of 80% was observed in initial thermal stability of surface grafted E-glass fibre epoxy composite on comparing with un-modified glass-epoxy composite. Similar improvements were noted in rapid and final decomposition stages also. The lower specific wear rate of 0.002 was observed for surface grafted E-glass and ferric oxide added composite designation EGFI11. The worn surface frcatograph explicated flat and smooth wear track surface for surface grafted composite designations. A highest fatigue life cycle of 18,724 is observed for surface modified composite designation EGFI21. These thermally stable and high wear resistance and fatigue strengthened composites could be used in automobile, aircrafts and domestic applications.

A new M50 matrix composite sintered with a hybrid Sns/Zno nanoscale solid lubricants: an experimental investigation

To improve the tribological behavior of AISI M50 steel (M), ZnO and SnS nanoscale solid lubricants were added to M50 matrix and sintered using spark plasma sintering technique (SPS) to fabricate two self-lubricating composites: M50 + SnS (MS) and M50 + ZnO + SnS (MZS). Friction tests were performed on a pin-on-disk tribometer using a silicon nitride ceramic under different loads (3, 6, 9, and 12 N) and different speeds (0.2, 0.4, 0.6, and 0.8 m s−1) to figure out the effects of theses parameters on the tribological performance of the developed composites. FESEM, EPMA, EDS, and XRD tests were performed to analyze the worn surface morphology and to understand the friction-reduction and anti-wear mechanisms of the developed composites. Wear and dry friction properties of MS and MZS were found to be superior at high loads and sliding speeds. Applied loads and sliding speeds change widely the wear mechanisms of M50 steel. Excellent tribological characteristics of the developed composites were attributed to the synergistic effects of ZnO-SnS and the created lubricious oxides, carbonates, and molybdenites on wear track surfaces.

Microstructure, phase and tribocorrosion behavior of 60NiTi alloy

Deep understanding the tribocorrosion performance of metal materials used in marine engineering offers an important guarantee to keep marine facilities run in safety, reliable and long-term. 60NiTi alloy possesses a unique set of desirable properties such as high hardness, excellent corrosion resistance and high specific strength which makes it quite suitably in marine environment. However, there is lack of investigation on the tribocorrosion behavior and mechanism of 60NiTi alloy. Therefore, systematic tribocorrosion tests including potentiodynamic polarization and open circuit potential under sliding contact were carried out to achieve a comprehensive understanding for the tribocorrosion performance of 60NiTi alloy. The wear track surfaces were analyzed by a field emission scanning electron microscope (SEM), energy dispersion spectrum (EDS) and X-ray photoelectron spectroscopy (XPS). The results clearly reveal that the negative synergy of tribology and electrochemistry/chemistry accelerates the material loss. The corresponded mechanism was also discussed in details.

Tribological behavior of WS2/oil-impregnated porous polyimide solid/liquid composite system

PurposeThis paper aims to study the tribological behavior of the WS2/oil-impregnated porous polyimide (PPI) solid/liquid composite system, in which both PFPE (perfluoropolyether) and SiCH (silahydrocarbons) oils with different hydrocarbon chains were used, respectively. Lubricating mechanism of the composite system was also explored.Design/methodology/approachThe tribological behaviors of the WS2 films against the PPI cylindrical pins before and after immersing oil were evaluated under different loads by a reciprocating-type ball-on-disc tribometer.FindingsThe composite system exhibited the low and stable friction coefficient after the running-in stage, and the lubricant oil played a positive effect. It was found that the WS2/PFPE composite system exhibited more excellent lubricating property, although sole SiCH far exceeds PFPE in lubrication. The abnormal phenomenon mainly resulted from the influence of the oil amount. XRD results on the wear track surfaces indicated that PFPE and SiCH oils with different hydrocarbon chains were likely to preferentially adsorb to the edge plane and basal plane of the WS2 crystals, respectively.Originality/valueIn previous studies, liquid lubricants were directly dripped or spin-coated on the solid lubricant surface. Based on its potential advantage in application, the tribological behavior and mechanism of the solid lubricating film/oil-impregnated PPI composite system were investigated in this study.

Sliding Wear Behavior of an AISI 440B Martensitic Stainless Steel Lubricated with Biodiesel and Diesel–Biodiesel Blends

The aim of this work was to analyze the effect of biodiesel and diesel–biodiesel blends on the sliding wear behavior of AISI 440B martensitic stainless steel. Lubricated tests were performed on AISI 440B martensitic stainless steel samples using Brazilian commercial diesel (containing 7 vol.% biodiesel), pure biodiesel, and diesel–biodiesel blends with biodiesel additions of 20% v/v, 30% v/v, and 50% v/v. Non-lubricated tests were also performed. The stainless steel was analyzed in the as-received condition (annealed) and after heat treatments (quenched and tempered at different temperatures), using a pin-on-disk device with an alumina pin, at a 1.8 m/s sliding speed, 14.7 N load, and 4400 m sliding distance, following the ASTM G99-04 Standard. Wear track widths, wear coefficients, and wear track surfaces were analyzed by optical and scanning electron microscopy. The results showed that AISI 440B presented the worst wear behavior in the dry condition with a microstructure characterized by a ferritic matrix and dispersed carbides (annealed condition). The wear resistance increased with the increase in biodiesel content due to the matrix strengthening by the martensitic transformation (heat-treated condition). When the biodiesel content was superior to 50% v/v, a reverse result was found. The observed wear mechanism was abrasive in all conditions. A − 1.35 power law coefficient characterized the wear coefficient as a function of biodiesel content for the annealed condition and a − 0.95 power law coefficient for the heat-treated conditions.