Friction and Wear Transitions in Carbons (Temperature and Stress Analysis)

In this paper, the friction and wear behavior of a carbon composite is studied. Friction tests were performed under constant friction mode. The power inputs were increased gradually to study their effects on friction transition at ambient air environment. Friction surfaces were examined using various macro and nanoscopic techniques to understand the evolution of friction transition. Structural changes of friction film have been observed from graphitic allotropic form to amorphous type structure. This sudden change leads to an increase of the coefficient of friction. It seems that the inter-laminar shearing, disruption of the film and sample cracking might cause the friction transition. Analysis from stress field simulation shows that cracks were more likely to occur at the trailing edge of the sample at the end of transition. This study shows that any type of failure (mechanical or thermal fatigue) of the friction film or bulk material is sufficient to generate friction and wear instabilities.     

Friction and Wear Behavior of Irradiated Polyethylene Sliding Against a Rough Steel Surface

The aim of this study was to evaluate the tribological behavior of polyethylene crosslinked by gamma radiation sliding against a steel surface. Two high-density polyethylenes were irradiated to a total dose in the range of 2?20 Mrad under vacuum and at room temperature. After irradiation, the materials were annealed at 423 K and then cooled slowly to room temperature. The same thermal treatment was applied to the non-irradiated polymer. The wear behavior of the polymers was determined under controlled ambient temperature of 298 and 333 K using a homemade tribometer. Sheet-shaped specimens were loaded against the surface of a steel disc with different normal loads to generate nominal contact pressures in the range of 0.25–1.5 MPa. The tests were performed under dry conditions using a disc rotation to produce an average sliding speed of 0.6 m/s and during a period of time to provide an average sliding distance of 1,080 m. The wear rate was obtained as the mass loss by the sample divided by the sliding distance, and the friction coefficient was determined by measuring the friction force. The results indicate that the wear rate increases with load in the case of non-irradiated polyethylene and low-dose irradiated polymers, while the wear rate reaches a maximum value with the load in the case of the irradiated samples with high doses. The samples irradiated with a dose of 10 Mrad showed the lowest wear. The coefficient of friction (COF) increases slightly with the load in all the cases. Most irradiated polymers show higher COF than the non-irradiated material when compared at a given load. The results show that the irradiation dose applied to the polyethylenes produced no noticeable effect on the COF values when a comparison was made at a given applied load.     

Friction and Wear at High Sliding Speeds

Sliding tests have been carried out using a variety of soft metal and nonmetal pins on a rotating steel disk at speeds up to 150 m/s. A new high-speed friction apparatus in which the normal force, the friction force and the friction coefficient are recorded, was used. In general, the wear rate increased drastically, and the friction coefficient decreased moderately as the sliding speed was raised, these changes being especially pronounced when pin materials of low melting temperature were used. The friction data are in good agreement with those obtained by others using the pin-on-disk geometry. However, although in many cases the interface reached the melting temperature of the lower melting sliding material, the very low friction coefficient values of under 0.05 reported by some investigators were not reached.     

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.     

Friction and Wear Characteristics of BaCr2O4 Ceramics at Elevated Temperatures in Sliding Against Sintered Alumina Ball

In this article, friction and wear characteristics of BaCr₂O₄ ceramics have been investigated using a high-temperature friction and wear tester from room temperature to 800?°C in dry sliding against sintered alumina ball. At room temperature, the friction coefficient and wear rate of BaCr₂O₄ ceramics are quite high. BaCr₂O₄ ceramics exhibit low friction coefficients and small wear rates with temperature increasing up to 400?C600?°C. The oxidation reaction of BaCr₂O₄ during high-temperature wear tests is responsible for the tribological properties. The oxidized product of BaCr₂O₄ is BaCrO₄, which forms a smooth self-lubricating film on the worn surface to effectively reduce friction and wear. However, at 800?°C, severe oxidation reduces the relative density of sintered BaCr₂O₄ ceramics, and further expedites the materials removal process.     

Origin of Friction in Running-in Sliding Wear of Nitride Coatings

To investigate the origin of running-in friction in unlubricated sliding wear, a magnetron sputtered multilayer coating TiAlN/VN was tested on a ball-on-disc tribometer for a series of sliding durations from 10 to 1000 cycles, followed by careful observation of the obtained worn surfaces using an field-emission gun scanning electron microscope. Three steps of friction variation were found: (1) prior to wear particle generation, low initial friction coefficient was around 0.2–0.25 purely attributed to the asperity contact; (2) then it increased steeply to a range of 0.4–0.5 in the first 100 cycles following the generation, breaking and agglomeration of wear particles, and in particular the scaling-up of fish-scale-like tribofilm; (3) eventually it approached to a steady-state value around 0.5 when the friction was governed by the viscous shearing of the tribofilm. It is concluded that, under unlubricated sliding wear, the friction behaviour of transition metal nitride hard coating is dominated by the viscous shearing of tribofilm adhesively bonding to the parent nitride coating.     

Friction and Wear Behaviors of Several Polymers Sliding Against GCr15 and 316 Steel Under the Lubrication of Sea Water

Ocean tribology, a new research field of tribology, is currently being established and developed. The tribological behaviors of polyether ether ketone (PEEK), poly(phenyl p-hydroxybenzoate) (PHBA), polyimide (PI), and perfluoroethylene propylene copolymer (FEP) sliding against GCr15 and 316 steel rings under the lubrication of sea water were studied and compared with that under the lubrication of pure water. The results show that the friction and wear behaviors of a polymer under the lubrication of aqueous medium are not only related to the properties of polymer itself, but also to the corrosive effect and lubricating effect of the medium. When a polymer slid against GCr15 steel under sea water lubrication, the friction coefficient and wear rate of polymer were much larger than that under pure water lubrication because of indirect corrosive wear. However, when sliding against corrosion-resistant 316 steel, polymers PEEK, FEP, and PI exhibited lower coefficients of friction and wear rates under sea water lubrication, this was attributed to better lubricating effect of sea water as a result of the deposition of CaCO₃ and Mg(OH)₂ on the counterface. On the contrary, the friction coefficient and wear rate of PHBA sliding against 316 steel under sea water lubrication were larger than that under pure water lubrication, which may be related to the properties of PHBA itself.     

Wear and Friction of Mechanical Carbons in Liquid Oxygen as Influenced by Transfer Films

Experimental wear and friction studies were conducted with a series of mechanical carbons sliding against metal surfaces in liquid oxygen (-298 F) at sliding velocities to 6500 feet per minute and a load of 1000 gm. The data reported shows that dense highly graphitic carbons have potential use as seal and bearing materials for liquid oxygen applications.

High density graphitic carbons with a greater oxidation resistance and a greater capability of forming a transfer film gave the lowest wear and friction. Metals that form the most stable oxide films promote greater adherence of the graphite to the mating surface. Impregnated carbons must be selected with caution because frictional heating generated during sliding can initiate hazardous reactions between oxygen and certain unstable organic compounds.     

青铜-石墨复合粉末热喷涂层的干滑动摩擦磨损性能

研究了石墨含量和载荷等对青铜-石墨复合粉末热喷涂自润滑涂层干滑动摩擦磨损性能的影响。结果表明：青铜-石墨热喷涂层有很好的自润滑性能，石墨含量为6％的涂层抗磨损性能最优。     

Sliding Friction and Wear Characteristics of Novel Graphitic Foam Materials

While the use of ORNL developed high-thermal-conductivity graphitic foam materials has been focused on thermal management applications, recent experiments have revealed their potential as bearing surfaces as well. The three primary tribological advantages are: (1) they can efficiently remove frictional heat, (2) their natural porosity can trap wear debris, and (3) their porosity can serve as a lubricant reservoir for the contact surface. A series of pin-on-disk experiments were conducted at both room temperature and 400 °C to compare the sliding friction and wear characteristics of the densified graphitic foam (mated against M-50 tool steel or against alumina) to those of conventional bearing materials like graphite, bearing bronze, poly- tetrafluoroethylene, bearing steel, and a Co-based superalloy. At room temperature and under low contact pressure, the tribological behavior of the densified graphitic foam material was comparable to that of graphite and better than that of other bearing materials. At 400 °C, traditional graphite exhibited a dusting wear regime accompanied by a high friction coefficient. In contrast, the graphitic foam demonstrated an ability to maintain low friction and wear at that temperature.     

巴氏合金干滑动摩擦磨损机制分析

以巴氏合金(ZChSnSb11-6)和45*钢为摩擦副,在SST-ST销盘式摩擦试验机下进行干摩擦试验,采用超景深三维显微镜、SEM、XRD对巴氏合金试样的摩擦表面进行表征,探讨其磨损机制.结果表明:干摩擦条件下,巴氏合金的摩擦因数较大,且波动较大;干摩擦条件下巴氏合金试样表面的物相发生明显变化,且试样表面同时存在磨粒磨损、氧化磨损和应变疲劳剥落.     

High-Temperature Friction and Wear Studies of Nimonic 80A and Nimonic 90 Against Nimonic 75 Under Dry Sliding Conditions

The present research focuses on dry sliding friction and wear behaviour of Nimonic 80A and Nimonic 90 against Nimonic 75 at high temperature up to 1023 K. The influence of temperature, sliding distance and normal load on friction and wear behaviour of Nimonic 80A and Nimonic 90 against Nimonic 75 was studied using pin (Nimonic 75)-on-disc (Nimonic 80A and Nimonic 90). Lower wear and lower friction of superalloys was observed at high temperatures, as compared to room temperature. Surface morphological and surface analytical studies of fresh and worn surfaces were carried out using optical microscopy, 3D profilometer, scanning electron microscope, energy-dispersive X-ray spectroscopy and Raman spectroscopy to understand the friction and wear behaviour. The mechanism of the formation of microscale glaze layer is also discussed.     

Friction and Wear Behavior of WS2/Zr Self-lubricating Soft Coatings in Dry Sliding Against 40Cr-Hardened Steel Balls

WS₂ and WS₂/Zr self-lubricating soft coatings were produced by medium-frequency magnetron sputtering, multi-arc ion plating and ion-beam-assisted deposition technique on the cemented carbide YT15 (WC + 15 % TiC + 6 % Co) substrates. Microstructural and fundamental properties of these coatings were examined. Sliding wear tests against 40Cr-hardened steel using a ball-on-disk tribometer method were carried out with these coated materials. The friction coefficient and wear rates were measured with various applied loads and sliding speeds. The wear surface features of the coatings were examined using SEM. The results showed that the WS-1 specimen (with WS₂/Zr composite coating) has higher hardness and coating/substrate critical load compared with that of the WS-2 specimen (only with WS₂ coating). The friction coefficient of WS-1 specimen increases with the increase in applied load and is quite insensitive to the sliding speed. The wear rate of the WS-1 specimen is almost constant under different applied loads and sliding speeds. The WS-1 specimen shows the smallest friction coefficient and wear rate among all the specimens tested under the same conditions. The WS-1 specimen exhibits improved friction behavior to that of the WS-2 specimen, and the antiwear lifetime of the WS₂ coatings can be prolonged through adding Zr additives. The self-lubricating and wear mechanism of the WS₂/Zr coating was also found from the sliding wear tests.     

干滑动条件下45CrNi材料摩擦磨损规律研究

以某自行火炮底座传动箱摩擦副所用材料45CrNi为研究对象,考察了该材料在不同热处理状态下（530℃回火、380℃回火、830℃淬火及原始状态）组成摩擦副时的干滑动摩擦磨损特性。试验结果表明：530℃回火盘和摩始销组成摩擦配副时结合最好,具有较好的摩擦磨损性能,销试样磨损率最小;在较低速度下载荷是影响材料摩擦因数的主要因素,而速度对摩擦因数的影响较小;粘着磨损是材料配副磨损的主要形式。     

铜碲硒铁合金的干摩擦磨损性能

利用环-盘式摩擦磨损试验机研究了铜碲硒铁合金的干摩擦磨损行为,分析了载荷和摩擦速度等参数对该合金摩擦磨损性能的影响,并用扫描电子显微镜对磨损形貌进行了观察.结果表明:铜碲硒铁合金的摩擦因数随载荷的增加变化不大,但随摩擦速度的增加而明显增大;合金的磨损率随载荷和摩擦速度的增加均增大;在轻载低速条件下,合金的磨损机制以犁削磨损和粘着磨损为主;在重载高速条件下,磨粒磨损和粘着磨损加剧.     

Friction and Wear Behavior of Steels under Different Reciprocating Sliding Conditions

The friction and wear behavior of ISO 100Cr6 steel ball sliding against conventionally hardened carbon and low-alloy steels was studied. The effect of hardness, hardening capacity, normal load, and sliding speed on the coefficient of friction and friction energy was investigated. Friction tests were carried out, without lubrication and under ambient conditions, on a reciprocating friction tester in which a ball-on-flat contact configuration was adopted. The results showed that there is a relative tendency for the friction properties to decrease with increased hardening capacity and decreased hardness. The results showed that increasing normal load decreases the coefficient of friction for the two steel nuances. However, increasing sliding speed increases the coefficient of friction of low-alloy steel and decreases the coefficient of friction of carbon steel. The oxidation of wear debris influences the wear mechanisms and friction behavior.     

Wear,Friction, and Electrical Noise Phenomena in Severe Sliding Systems

The sliding mechanisms of unlubricated gold and palladium are described. Transfer, roughening, wear, friction, and contact resistance phenomena involve the same discrete events.

With rider-flat geometry, severely Worked transfer particles form a prow-shaped wedge which adheres to the smaller member. Even when rider and flat are different, prow material comes from the larger part. Loss of prow occurs by adhesive weld-back transfer and, to form loose debris, by shearing or fatigue fracture.

Friction rises in the early stages of sliding as prows grow. Soon, back transfer increases hardness of the larger member and friction falls to an equilibrium level. Friction also falls when prows are kneaded into rollers and become loose debris. During these stages, wear rate diminishes.

Contact resistance noise originates in stick-slip, roller formation, surface hardening, and changing composition at the sliding interface when dissimilar contact metals are involved.     

SiCp/AI有机摩擦材料摩擦磨损特性研究

对20%Vol SiCp/AI复合材料与有机摩擦材料组成的摩擦副的摩擦磨损特性进行了试验研究.试验表明:该摩擦副在不同的速度情况下都具有平稳的摩擦系数.摩擦系数在一定速度范围内随速度增加而减小,而在超过一定速度后随速度增加而增大.对摩擦系数随速度变化的机理进行了讨论.     

Dry Sliding Friction and Wear Characteristics of Cu-Sn Alloy Containing Molybdenum Disulfide

The wear and sliding friction response of a hybrid copper metal matrix composite reinforced with 10 wt% of tin (Sn) and soft solid lubricant (1, 5, and 7 wt% of MoS₂) fabricated by a powder metallurgy route was investigated. The influence of the percentages of reinforcement, load, sliding speed, and sliding distance on both the wear and friction coefficient were studied. The wear test with an experimental plan of six loads (5–30 N) and five sliding speeds (0.5–2.5 m/s) was conducted on a pin-on-disc machine to record loss in mass due to wear for two total sliding distances of 1,000 and 2,000 m. The results showed that the specific wear rate of the composites increased at room temperature with sliding distance and decreased with load. The wear resistance of the hybrid composite containing 7 wt% MoS₂ was superior to that of the other composites. It was also observed that the specific wear rates of the composites decreased with the addition of MoS₂. The 7 wt% MoS₂ composites exhibited a very low coefficient of friction of 0.35. The hardness of the composite increased as the weight percentage of MoS₂ increased. The wear and friction coefficient were mainly influenced by both the percentage of reinforcement and the load applied. Wear morphology was also studied using scanning electron microscopy and energy-dispersive X-ray analysis.     

Acoustic Emission and Its Relationship with Friction and Wear for Sliding Contact

Further investigation of the relationships between friction and wear properties and the characteristics of acoustic emission was conducted in the case of dry and grease-lubricated sliding contact using a ball-on-cylinder testing apparatus. The effect of contamination simulated by the inclusion of glass bead particles was also explored. Experiments were performed at sliding speeds ranging from 0.09 m/s to 1.47 m/s, while maintaining a fixed load and duration. As a first observation and contrary to what could be expected, the higher speed did not contribute to the decrease in friction interpreted by a worsening of the starved regime that had a consequence of increasing wear. However, the results revealed a good correlation between the friction coefficient and acoustic emission (AE) rms voltage for dry sliding. Such a relationship may allow the prediction of a reasonable friction coefficient μ from an AE signal. It was also determined that the friction work correlated well with the corresponding integrated AE voltage over time, intRMS. The detection of the sliding speed threshold beyond which accelerated wear would occur was possible from the intRMS variation. Proportionality between the theoretically determined grease film thickness and the intRMS was observed.