Improving the Tribological Behavior of Graphite/Cu Matrix Self-Lubricating Composite Contact Strip by Electroplating Zn on Graphite

Studies to explore the nature of friction, and in particular thermally activated friction in macroscopic tribology, have lead to a series of experiments on thin coatings of molybdenum disulfide. Coatings of predominately molybdenum disulfide were selected for these experiments; five different coatings were used: MoS₂/Ni, MoS₂/Ti, MoS₂/Sb₂O₃, MoS₂/C/Sb₂O₃, and MoS₂/Au/Sb₂O₃. The temperatures were varied over a range from −80 °C to 180 °C. The friction coefficients tended to increase with decreasing temperature. Activation energies were estimated to be between 2 and 10 kJ/mol from data fitting with an Arrhenius function. Subsequent room temperature wear rate measurements of these films under dry nitrogen conditions at ambient temperature demonstrated that the steady-state wear behavior of these coatings varied dramatically over a range of K = 7 × 10⁻⁶ to 2 × 10⁻⁸ mm³/(Nm). It was further shown that an inverse relationship between wear rate and the sensitivity of friction coefficient with temperature exists. The highest wear-rate coatings showed nearly athermal friction behavior, while the most wear resistant coatings showed thermally activated behavior. Finally, it is hypothesized that thermally activated behavior in macroscopic tribology is reserved for systems with stable interfaces and ultra-low wear, and athermal behavior is characteristic to systems experiencing gross wear.     

Low Temperature Nano-Tribological Study on a Functionally Graded Tribological Coating Using Nanoscratch Tests

Previous studies on low temperature tribological investigations were limited to macro-scale studies because of the lack of suitable instrumentation. This limitation has been overcome using a newly developed low temperature nanoscratch tester capable of characterizing the scratch resistance of coatings down to −30 °C. The scratch resistance and mechanical properties of a functionally graded a-C:H(Ti)/TiCN/TiN/Ti coating have been investigated for temperatures ranging from 25 to −30 °C. It has been found that the a-C:H(Ti)/TiCN/TiN/Ti coating failed at high loads by cracking and spallation during the room-temperature scratch tests. Fractography suggests that these failures originate from or close to the interface between the top a-C:H(Ti) and the TiCN layers. Decreasing the test temperature from 25 to 0 °C resulted in increased values in H, H/E _r and H ³ /E _r², consistent with improved crack- and wear resistances, with further smaller improvements being achieved on further decreasing the temperature to −30 °C.     

Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)–Al2O3–Ba x Sr1?x SO4 (x?=?0.25, 0.5, 0.75) Composites at Elevated Temperature

Self-lubricating ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have been fabricated by spark plasma sintering (SPS) method. The tribological properties have been evaluated using a high-temperature friction and wear tester at room temperature and 760 °C in dry sliding against alumina ball. The composites exhibit distinct improvements in effectively reducing friction and wear, as compared to the unmodified ZrO₂(Y₂O₃)–Al₂O₃ ceramics. The ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have great low and stable friction coefficients of less than 0.15 and wear rates in the order of 10^− 6mm³/Nm at 760 °C. Delamination is considered as the dominating wear mechanism of the composites at room temperature. At elevated temperature, the formation and effective spreading of Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) lubricating films during sliding play an important role in the reduction of the friction and wear.     

Tribological Property of Ni<Subscript>3</Subscript>Al Matrix Composites with Addition of BaMoO<Subscript>4</Subscript>

The Ni₃Al matrix composites with addition of 10, 15, and 20 wt% BaMoO₄ were fabricated by powder metallurgy technique, and the tribological behaviors were studied from room temperature to 800 °C. It was found that BaAl₂O₄ formed during the fabrication process. The Ni₃Al composites showed poor tribological property below 400 °C, with high friction coefficients (above 0.6) and wear rates (above 10⁻⁴ mm³/Nm). However, the composites exhibited excellent self-lubricating and anti-wear properties at higher temperatures, and the composite with addition of 15 wt% BaMoO₄ had the lowest wear rate (1.10 × 10⁻⁵ mm³/Nm) and friction coefficient (0.26). In addition, the results also indicated that BaAl₂O₄ for the Ni₃Al composites did not exhibit lubricating property from room temperature to 800 °C.     

Wear behavior of Ni–P and Ni–P–Al2O3 electroless coatings

In this research, hardness and wear resistance of two types of electroless coating have been investigated including Ni–P and Ni–P–Al₂O₃ coatings. These coatings were applied on AISI 1045 steel discs by electroless deposition process and then they were heat treated at 200, 400 and 600 °C for 1 h. Wear resistance of deposits was measured by the pin on disc method and wear surfaces and debris were studied by scanning electron microscopy (SEM). Also, microstructural changes were evaluated by X-ray diffraction (XRD) analysis.The results showed that the existence of alumina particles in Ni–P coating matrix led to an increase in the hardness and wear resistance of the deposits. It was also found that heat treated coatings at about 400 °C have the maximum hardness and wear resistance.     

Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in situ composite

Resistance to wear is an important factor in design and selection of structural components in relative motion against a mating surface. The present work deals with studies on fretting wear behavior of in situ nano-Al₃Ti reinforced Al–Ti–Si amorphous/nanocrystalline matrix composite, processed by high pressure (8 GPa) sintering at room temperature, 350, 400 or 450 °C. The wear experiments were carried out in gross slip fretting regime to investigate the performance of this composite against Al₂O₃ at ambient temperature (22–25 °C) and humidity (50–55%). The highest resistance to fretting wear has been observed in the composites sintered at 400 °C. The fretting wear involves oxidation of Al₃Ti particles in the composite. A continuous, smooth and protective tribolayer is formed on the worn surface of the composite sintered at 400 °C, while fragmentation and spallation leads to a rougher surface and greater wear in the composite sintered at 450 °C.     

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

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

Sliding Wear Characteristics of AZ91D Alloy at Ambient Temperatures of 25–200 °C

The dry sliding wear tests were performed for AZ91D alloy under the loads of 12.5–300 N and the ambient temperatures of 25–200 °C. We studied the wear characteristics of AZ91D alloy as a function of the normal load and the ambient temperature. The mild-to-severe wear transition occurred with increasing the load and the critical load reduced with the ambient temperature rising. However, no matter how high the ambient temperature was in the range of 25–200 °C, the mild wear prevailed under the lower loads. Especially, the AZ91D alloy presented a lower wear rate at 200 °C than at 25 and 100 °C under the low loads of 12.5–25 N, but vice versa under the loads of more than 25 N. These phenomena seem to be contradictory to the popular view that the mild-to-severe wear transition is controlled by the critical surface temperature. These may be attributed to a thick and hard mechanical mixing layer (MML) containing the mixture of MgAl₂O₄ and Mg on the worn surface. The MML thickened with increasing the ambient temperature (under the low loads), effectively reduced wear and markedly elevated the critical surface temperature. The oxidative wear and delamination wear successively predominated in the mild wear regime; the gross plastic-induced wear would prevail in the severe wear regime.     

NiAl Matrix High-Temperature Self-Lubricating Composite

A high-temperature self-lubricating composite NiAl–Cr–Mo–CaF₂ was fabricated using the powder metallurgy technique, and the tribological behavior of the composite at a wide range of temperatures (room temperature to 1000 °C) was investigated. The results showed that the composite had a favorable friction coefficient of about 0.2 and an excellent wear resistance of about 1 × 10⁻⁵ mm³N⁻¹m⁻¹ at the high temperatures tested (800 and 1000 °C). The excellent self-lubricating performance was attributed to the formation of the glaze film on the worn surface consisting mainly of CaCrO₄ and CaMoO₄ as high-temperature solid lubricants.     

Friction and Wear Behaviour of Brake Pads Dry Sliding Against Semi-Interpenetrating Network Ceramics/Al-alloy Composites

Semi-interpenetrating network composites containing 40 vol.% ceramics (5Al₂O₃·8SiO₂) and 60 vol.% Al-alloy were fabricated in place of cast iron available for automotive brake rotors. The friction and wear performances of brake pads dry sliding against the composites were measured using a SRV testing machine. The test procedures include friction fade and recovery, load sensitivity at 100 and 250°C, and wear. The friction was found to increase first and then decrease with increasing temperature, followed by the inverse recovery upon cooling. Wear showed an incremental tendency over a wide temperature range. For loads from 40 to 160 N, the friction decreased at 100 and 250°C. At load below 128 N, the former friction was inferior to the latter while at load above 128 N the friction exhibited an inverse tendency. Wear mildly increased with load at 100 °C and decreased dramatically at 250 °C. SEM and EDS investigations revealed that the worn pad surfaces at 250 °C were covered by more tribofilms, including more coke and graphite with friction-reducing action as well as fewer compounds (corresponding to Si and Al) with friction-increasing action in comparison with those at 100 °C. The compression of the tribofilms contributed to a large decrease in the friction and wear with increasing load. However, at 100 °C E-glass fibers exposed at the worn surfaces inhibited the excessive wear of the pad despite lack of more tribofilms. Their glossy surfaces decreased the friction. The proposed friction models explain some friction and wear behaviour better.     

Tribological properties of nanostructured DLC coatings deposited by C60 ion beam

The frictional and wear characteristics of nanostructured DLC films were investigated. The coatings were deposited on silicon substrates by irradiation of a mass-separated C₆₀ ion beam with 5 keV of energy and a deposition temperature ranging from 100 to 450 °C. The effects of deposition temperature on the surface morphology, nano-structure, mechanical properties and tribological characteristics of the coatings were assessed. Results showed that deposition temperature strongly affects the nanostructure and surface morphology of the coatings. Coatings deposited at temperatures exceeding 350–400 °C exhibited an increase in surface roughness as well as compressive stress due to the formation of graphite, which led to a significant increase in the friction coefficient and wear rate. Coatings deposited at 300 °C showed the best tribological properties.     

Wear behavior of electroless Ni–P–B4C composite coatings

In this research, the wear of electroless Ni–P and Ni–P–B₄C composite coatings was reviewed. Auto catalytic reduction of Ni in nickel sulfate and sodium hypophosphate bath including suspended B₄C particles with different concentration was used to create composite coatings with 12, 18, 25 and 33 vol.% of B₄C particles. Coatings 35 μm thick were heat treated at 400 °C for one hour in an argon atmosphere and the wear resistance and friction coefficient of heat-treated samples were determined by block-on-ring tests. All wear tests were carried out at 24 °C, 35% moisture, 0.164 m/s sliding speed and about 1000 m sliding distance. Graphs show that an electroless Ni–P–B₄C composite coating with 25 vol.% of B₄C had the best wear resistance against a CK45 steel counterface.     

Effect of particle size on tribological behavior of Ni3Al matrix high temperature self-lubricating composites

The Ni₃Al matrix high temperature self-lubricating composites with different particle size were fabricated by the powder metallurgy technique. The effect of particle size on the mechanical and tribological properties of the composites was investigated in this paper. The results showed that the coarse particle composite exhibited the lowest friction coefficient and wear rate compared to the fine particle ones at a wide temperature range from room temperature to 1000 °C. The reason for the low wear rate was that the coarse bulk phase could provide better deformation resistance and higher load bearing capacity than the fine microstructure.     

Tribo-layer and its role in dry sliding wear of Ti–6Al–4V alloy

Dry sliding wear tests were performed for Ti–6Al–4V alloy under a load of 50–250 N at 25–500 °C on a pin-on-disk elevated temperature tester. Worn surfaces and subsurfaces were thoroughly investigated for the morphology, composition and structure of tribo-layers. Ti–6Al–4V alloy could not be considered to possess poor wear resistance at all times, and presented a substantially higher wear resistance at 400–500 °C than at 25–200 °C. The tribo-layer, a mechanical mixing layer, was noticed to exist on worn surfaces under various conditions. High wear rate at 25–200 °C was ascribed to no protective tribo-layer containing no or trace tribo-oxides. As more oxides appeared in the tribo-layers, they presented an obviously protective role due to their high hardness, thus giving a reasonable explanation for high wear resistance of Ti–6Al–4V alloy at 400–500 °C.     

Abrasive and Adhesive Wear Behavior of Arc-Evaporated Al<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N Hard Coatings

During the last decade, the usage of difficult-to-machine materials such as austenitic stainless steels has increased continuously in various industrial applications. Tools such as blind hole taps, punches, or deep drawing molds are often exposed to severe wear while machining/forming these materials, mainly due to excessive adhesion and material transfer. On combination with abrasive wear due to work-hardened wear debris, tool lifetime in these applications is often limited. In this study, ball-on-disc experiments were carried out with arc-evaporated AlCrN coatings with different Al/(Al + Cr) ratios against Al₂O₃ and austenitic stainless steel balls in ambient atmosphere. Test temperatures of 25, 500, and 700°C were chosen for the hard Al₂O₃ balls simulating severe abrasive loads, whereas 25, 150, and 250°C were used for the softer stainless steel material to evaluate the adhesive wear behavior. Characterization of the wear tracks was done by scanning electron microscopy in combination with energy-dispersive X-ray analysis and optical profilometry. The best abrasive wear resistance during testing against Al₂O₃ was observed for the coating with the highest Al content. In the case of the austenitic stainless steel balls, sticking of the ball material to the coating surface was the dominating wear mechanism. The influence of test temperature, chemical composition, and surface roughness was studied in detail.     

Effect of Cu Content on the Structure,and Performance of Substoichiometric Cr–N Coatings

Cu–Cr–N coatings with Cu contents between 3 and 65 at.%, Cu/Cr ratios in the 0.04–4.5 range and 21–27 at.% N, synthesized by twin electron-beam Physical Vapor Deposition at 450 °C, were investigated and compared against substoichiometric Cr–N reference samples. The main objective of this study is to study the influence of Cu on the structure, and the subsequent effects on the mechanical properties, room (22 °C) and high temperature (500 and 840 °C) tribological performance of Cu–Cr–N coatings. Using X-ray photoelectron spectroscopy, glancing angle X-ray diffraction and scanning electron microscopy, in combination with nanoindentation mechanical property measurements and laboratory-controlled ball-on-disc sliding experiments, it is shown that Cu–Cr–N coatings with low Cu content (3 at.%) possess sufficient wear resistance for high-temperature demanding tribological applications. The lubricious effect of oxide formation at high temperatures is also evaluated.     

Comparative tribological study of air plasma sprayed WC–12%Co coating versus conventional hard chromium electrodeposit

In this work, the properties of air plasma sprayed WC–12%Co coating before and after heat treatment were compared with the properties of the hard chromium electrodeposit. WC–12%Co coatings were heat treated at 650, 900 and 1150 °C for 1 h in an argon atmosphere. XRD patterns confirmed the formation of an amorphous phase in the as-sprayed coating. This amorphous phase gradually transformed to η-carbides in the course of heat treatment of the coating. This transformation was confirmed by the XRD analysis of the coatings heat treated above 900 °C. Pin-on-disc wear test results showed that WC–12%Co coatings had a significantly better tribological performance as compared with that of the hard chromium electrodeposits. The results also indicated that heat treatment of the WC–12%Co coatings at 900 °C gave the highest wear resistance among the coatings, which was due to the formation of hard η-carbides at this temperature.     

The tribological characteristics of TiCN coating at elevated temperatures

The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer (pin-on-disc, CSM Instruments) allowing measuring the dependency of friction coefficient on cycles (sliding distance) up to 500 °C. The evolution of the friction coefficient with the cycles was measured under different conditions, such as temperature or sliding speed and the wear rate of the ball and coating were evaluated. The 100Cr6 balls and the Si₃N₄ ceramic balls were used as counter-parts. The former were used at temperatures up to 200 °C, the latter up to 500 °C. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured.The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 °C; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 °C.     

Wear Behaviour of In Situ Al/TiB<Subscript>2</Subscript> Composite: Influence of the Microstructural Instability

In this present work, the in situ Al (A380)/5 wt%TiB₂ composites were fabricated through salt–melt reaction using halide salts such as potassium hexafluorotitanate (K₂TiF₆) and potassium tetra fluoroborate (KBF₄) salts as precursors. The composites were produced at four different melt temperatures (700, 750, 800, 850 °C). The formation of particle was confirmed from XRD results. The wear behaviour of Al/5 wt% TiB₂ composite was investigated by varying the wear test parameters such as sliding temperature (25, 100, 150, 200 °C), applied load (10, 20, 30, 40 N), sliding velocity (0.4, 0.7, 1, 1.3 m/s). The microstructure of Al/5 wt% TiB₂ composite was correlated with the wear characteristics of the composites. The wear resistance of Al/5 wt% TiB₂ composite was significantly improved due to the presence of TiB₂ particle in Al matrix material. The composite produced at melt temperature 800 °C showed a higher wear resistance at applied load: 10 N, sliding temperature: 25 °C and sliding velocity: 0.7 m/s. The wear mechanism for each of the tested condition was identified from the worn surfaces using scanning electron microscopy (SEM). ANOVA test was carried out to find out significant factor for the wear resistance of composite. The checking of adequacy of experimental value for the wear behaviour of composite for different testing condition was analysed by residual plots using statistical software.     

Ni3Al Matrix Composite with Lubricious Tungstate at High Temperatures

Ni₃Al–Ag–BaF₂/CaF₂–W composites were fabricated by the powder metallurgy route, and their tribological properties over a wide temperature range, starting from room temperature up to 800 °C, were investigated. The Ni₃Al matrix composite with 15 wt% BaF₂/CaF₂ exhibited a favorable friction coefficient (range 0.3–0.4) and wear rate (0.2–6.2 × 10⁻⁴mm³ N⁻¹ m⁻¹). The formation of BaWO₄ and CaWO₄ with lubricity on the worn surface due to a tribo-chemical reaction at high temperatures provided excellent lubricating properties. The low friction coefficient over a broad temperature range could be attributed to the synergistic effect of Ag, BaF₂/CaF₂, BaWO₄, and CaWO₄.