Tribological Behavior and Tribochemistry of Self-mated Ti3SiC2 in Ethanol

The tribological behavior of self-mated Ti₃SiC₂ in low viscosity fluids, that is, water and ethanol, is investigated. The results show that both water and ethanol effectively reduce the friction and wear of self-mated Ti₃SiC₂. The friction coefficient and total wear rates of Ti₃SiC₂ tribo-pair are 0.82 and 2.17 × 10^?2 mm³/N m under dry condition, 0.41 and 6.50 × 10^?4 mm³/N m in water and 0.16 and 2.22 × 10^?4 mm³/N m in ethanol. As sliding distance is prolonged from 0.5 to 22.9 km, the total wear rates of Ti₃SiC₂ in ethanol can be further reduced. Mechanical wear of Ti₃SiC₂ is partly inhibited in water and greatly inhibited in ethanol. Surface polishing of Ti₃SiC₂ pin and tribochemistry are the two distinct characteristics of self-mated Ti₃SiC₂ in ethanol. The surface film in ethanol is composed of TiO₂ and SiO. Tribo-products in ethanol, that is, silica gel and titanium dioxide, are found. Polycondensation of ethanol by the catalysis of TiO₂ and SiO_2?x gives birth to paraffins and olefins.     

Tribological Behavior of Self-mated Ti3SiC2 in Short-Chain n-Alcohols,Glycol and Glycerol under Boundary Lubrication

The tribological behavior of self-mated Ti₃SiC₂ in bath of alcohols are investigated at sliding velocity ranging from 0.005 to 0.3 m/s. The results show that the friction coefficient and wear rates of self-mated Ti₃SiC₂ are reduced greatly under lubrication of alcohols compared to that under dry condition. In alcohols, smooth worn surfaces of Ti₃SiC₂ can be obtained. The mechanical wear is inhibited and the oxidized Ti and Si species on the worn surface are TiO₂ and silica gel. The friction coefficients decrease with carbon numbers and sliding velocity in the n-alcohols. The friction coefficient decrease with sliding velocity in glycol and glycerol as well. The decrease is determined by the increase of the viscosity of alcohols. In glycerol, at 0.1 m/s and 5 N, the friction coefficient of self-mated Ti₃SiC₂ is 0.08 which is the lowest in this paper. The lubrication regimes are calculated according to classical lubrication theory. At velocity from 0.005 to 0.3 m/s, in all of the alcohols except glycerol, the λ are below 1 which indicates that the lubrication regime is BL. While in glycerol, the lubrication regimes vary from BL to BL and EHL, then to EHL as the velocity increases.     

Effect of Ag and Ti3SiC2 on Tribological Properties of TiAl Matrix Self-lubricating Composites at Room and Increased Temperatures

Dry sliding tribological properties of TiAl matrix self-lubricating composites (TMSC) containing Ag, Ti₃SiC₂, Ag and Ti₃SiC₂ were investigated from 25 to 800 °C under ball-on-disk test conditions against Si₃N₄ counterface under the same conditions of 10 N-0.234 m/s. The results indicated that the tribological properties were strongly dependent on the lubricant additives. TMSC with the addition of Ag and Ti₃SiC₂ (TAT) exhibited the lower friction coefficients (0.32–0.43) and less wear rates (1.23–4.13 × 10^?4 mm³ N^?1m^?1) in the wide temperature range of 25–800 °C. The excellent tribological properties of TAT over the wide temperature range were attributed to the synergetic effect of Ag and Ti₃SiC₂ lubricants, silver diffusion forming a rich-silver smooth tribo-film on the frictional surface of TAT at low and moderate temperatures from 25 to 400 °C, while Ti₃SiC₂ oxidation reaction forming rich-oxide tribo-film on the worn surface of TAT at higher temperatures of 600 and 800 °C.     

Tribological Behavior of Ti3AlC2 Against SiC at Ambient and Elevated Temperatures

In this paper, we reported the tribological behavior of Ti₃AlC₂ disk sliding against SiC ball from room temperature (RT) to 1,000 °C. The tribological properties are highly dependent of testing temperature. At RT, the coefficient of friction (CoF) is as low as 0.34 in the steady state, but the wear rate is relative high (4.26 × 10^?4 mm³/Nm). At 200 and 400 °C, the CoF is as high as 1.21, and the wear rates are very high, about on the order of 10^?3 mm³/Nm. From 600 to 1,000 °C, however, Ti₃AlC₂ exhibits quite low wear rate on the order of 10^?6 mm³/Nm and relative moderate CoF, 0.60–0.80. The compacted continuous oxide layer at 600 °C and above might be responsible for the outstanding wear resistance.     

Friction and Wear Properties of TiAl-Ti3SiC2-MoS2 Composites Prepared by Spark Plasma Sintering

TiAl matrix self-lubricating composites (TMC) with various weight percentages of Ti₃SiC₂ and MoS₂ lubricants were prepared by spark plasma sintering (SPS). The dry sliding tribological behaviors of TMC against an Si₃N₄ ceramic ball at room temperature were investigated through the determination of friction coefficients and wear rates and the analysis of the morphologies and compositions of wear debris, worn surfaces of TMC, and the Si₃N₄ ceramic ball. The results indicated that TMC with 10 wt% (Ti₃SiC₂-MoS₂) lubricants had good tribological properties due to the unique stratification subsurface microstructure of the worn surface. The friction coefficient was about 0.57, and the wear rate was 4.22 × 10^?4 mm³ (Nm)^?1. The main wear mechanisms of TMC with 10 wt% (Ti₃SiC₂-MoS₂) lubricants were abrasive wear, oxidation wear, and delamination of the friction layer. However, the main wear mechanisms of TMC without Ti₃SiC₂ and MoS₂ lubricants were abrasive wear and oxidation wear. The continuous friction layer was not formed on the worn surfaces. The self-lubricating friction layer on the frictional surface, different phase compositions and hardness, as well as density of TMC contributed to the change in the friction coefficient and wear rate.     

Friction and Wear Behaviors of Ag/MoS2/G Composite in Different Atmospheres and at Different Temperatures

Silver-based composite with 15?vol% MoS₂ and with 5?vol% graphite was prepared by powder metallurgy method. The impacts of the counterface materials, atmosphere, and temperature on the tribological behavior of the composite were investigated. It was found that when sliding against brass less effective lubricating film formed, causing a higher friction and wear comparing with ASTM-1045 steel. With the increasing proportion of oxygen in the O₂/N₂ atmosphere, the wear rate and friction coefficient ascended slightly. At 200?°C, the combination lubrication of graphite, MoS₂, and Ag contributed to a low friction coefficient (0.07) and wear rate (6.56?×?10^?6?mm³/Nm). At 400?°C, graphite lost its lubricating role, while silver became excessively soft. Large amount of MoS₂ was oxidized into MoO_3, and the residual MoS₂ formed some island-like lubricating films. Severe adhesive wear occurred on the contact surface, which led to a high friction coefficient (0.25) and a great increase of the wear rate (23.2?×?10^?6?mm³/Nm). At 600?°C, a relatively low friction coefficient (0.1) was obtained because of the formation of high-temperature solid lubricants, (Ag₂Mo₄O₁₃ and Ag₂Mo₂O₇) and liquid Ag₂Mo₂O₇. However, the wear rate at 600?°C was the highest (32.6?×?10^?6?mm³/Nm) due to the thick transfer layer.     

Tribological Properties of Adaptive Ni-Based Composites with Addition of Lubricious Ag2MoO4 at Elevated Temperatures

The Ni-based self-lubricating composites with addition of 10 and 20?wt% Ag₂MoO₄ were fabricated by powder metallurgy technique, and the effect of Ag₂MoO₄ on tribological properties was investigated from room temperature to 700?°C. The tribo-chemical reaction films formed on rubbing surface and their effects on the tribological properties of composites at different temperatures were addressed according to the surface characterization by SEM and Micro-Raman. The results show that the Ag₂MoO₄ decomposed into Ag and Mo during the high-temperature fabrication process. The friction coefficient and wear rate of the composites decrease with the increasing of temperature and Ag₂MoO₄ contents and the composites with addition of 20?wt% Ag₂MoO₄ exhibits the lowest friction coefficient (0.26) and wear rate (1.02?×?10^?5?mm³?N^?1?m^?1) at 700?°C. The composition of the tribo-layers on the worn surfaces of the composites is varied at different temperatures. It is proposed that the improving of tribological properties of the composites at high temperatures are attributed to the synergistic lubricating effect of silver molybdate (reproduced in the rubbing process at high temperatures) and Fe oxide (transfer from disk material to the pin) formed on the worn surface.     

Tribological Properties of TiAl Matrix Self-Lubricating Composites Containing Multilayer Graphene and Ti3SiC2 at High Temperatures

An investigation is conducted on the unexplored synergistic effects of multilayer graphene (MLG) and Ti₃SiC₂ in self-lubricating composites for use in high-temperature friction and wear applications. The tribological properties of TiAl matrix self-lubricating composites with different solid lubricant additions (Ti₃SiC₂-MLG, MLG) are investigated from room temperature to 800°C using a rotating ball-on-disk configuration. Tribological results suggest the evolution of lubrication properties of MLG and the excellent synergistic lubricating effect of MLG and Ti₃SiC₂ as the testing temperature changes. It can be deduced that MLG has great potential applications as a promising high-temperature solid lubricant within 400°C, and a combination of MLG and Ti₃SiC₂ is an effective way to achieve and maintain desired tribological properties over a wide temperature range.     

Effect of the Synergetic Action on Tribological Characteristics of Ni-Based Composites Containing Multiple-Lubricants

Ni-based self-lubricating composites with multiple-lubricants addition were prepared by a powder metallurgy technique, and the effect of multiple-lubricants on tribological properties was investigated from room temperature to 700?°C. The synergetic effects of graphite, MoS₂, and metallic silver lubricants on the tribological characteristics of composites were analyzed. XRD analysis showed that new Cr _x S _y and Mo₂C phase were formed in the composites containing graphite, MoS₂ and metallic Ag lubricants during the sintering process. The average friction coefficients (0.69?C0.22) and wear rates (11.90?C0.09?×?10^?5?mm³?N^?1?m^?1) were obtained when rubbing against Inconel 718 alloy from room temperature to 700?°C due to synergetic lubricating action of multiple-lubricants. A smooth lubricating was gradually generated on the worn surface, and the improving of tribological properties was attributed to the formation of lubricious glaze film on the worn surface and their partially transferred to the counterface. The graphite played the main role of lubrication at room temperature, while molybdate phase and graphite were responsible for low friction coefficients and wear rates at mid/high temperatures. The synergetic lubricating effect of molybdate (produced in the rubbing process at high temperatures) iron oxide (transfer from disk material to the pin) and remaining graphite multiple-lubricants play an important lubricating role during friction tests at a wide temperature range.     

Wear and Friction of TiAl Matrix Self-Lubricating Composites against Si3N4 in Air at Room and Elevated Temperatures

More durable, low-friction bearing materials over a wide temperature range are needed for turbine components and other high-temperature bearing applications. The current study reported the tribological properties of TiAl matrix self-lubricating composites (TMC) containing MoS₂ (a low-temperature lubricant, below 500°C), hBN (a medium-temperature lubricant, below 600°C), and Ti₃SiC₂ (a high-temperature lubricant, above 600°C) designated as MhT against an Si₃N₄ counterface at temperatures ranging from 25 to 800°C in air. The load was 10 N and the sliding speed was 0.2 m/s for all tests. Tribological studies indicated that TMC containing MhT showed a lower friction coefficient and wear rate in comparison to TiAl-based alloy at all test temperatures, which was attributed to the excellent synergetic lubricating effect of MoS₂, hBN, and Ti₃SiC₂. TMC containing 5 wt% MhT exhibited the best tribological properties over a wide temperature range.     

Tribological properties of Ni-17.5Si-29.3Cr alloy at room and elevated temperatures

The tribological properties of Ni-17.5Si-29.3Cr alloy against Si₃N₄ were studied on a ball-on-disc tribotester between room temperature and 1000 °C. The effects of temperature on the tribological properties of the alloy were investigated. The worn surfaces of the alloy were examined using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results indicated that the tribological behavior of the alloy expressed some differences with increase in testing temperature. At low and moderate temperatures (below 800 °C), the alloy showed excellent wear and oxidation resistances, and the wear rate of the alloy remained in the magnitude of 10^?5 mm³/Nm; but at elevated temperature (800–1000 °C), the wear and oxidation resistances decreased, and the wear rate of the alloy increased up to 10^?4 mm³/Nm. The friction coefficient decreased from 0.58 to 0.46 with the rising of testing temperature from 20 to 600 °C, and then remained nearly constant. The wear mechanism of the alloy was mainly fracture and delamination at low and moderate temperatures, and transformed to adhesive and oxidation at elevated temperatures.     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

Friction and wear behaviors of C/C-SiC composites containing Ti3SiC2

In the present paper, friction and wear behaviors of a carbon fiber reinforced carbon–silicon carbide–titanium silicon carbide (C-SiC–Ti₃SiC₂) hybrid matrix composites fabricated by slurry infiltration and liquid silicon infiltration were studied for potential application as brake materials. The properties were compared with those of C/C-SiC composites. The composites containing Ti₃SiC₂ had not only higher friction stability coefficient but also much higher wear resistance than C/C-SiC composites. At an initial braking speed of 28 m/s under 0.8 MPa pressure, the weight wear rate of the composites containing 5 vol% Ti₃SiC₂ was 5.55 mg/cycle, which was only one-third of C/C-SiC composites. Self-lubricious film-like debris was formed on the composites containing Ti₃SiC₂, leading to the improvement of friction and wear properties. The effect of braking speed and braking pressure on the tribological properties of modified composites were investigated. The average friction coefficient was significantly affected by braking speed and braking pressure, but the wear rate was less affected by braking pressure.     

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.     

Effect of Sliding Speed and Applied Load on Dry Sliding Tribological Performance of TiAl Matrix Self-lubricating Composites

More durable, low-friction self-lubricating materials in modern industry are greatly needed for tribological systems. The current paper presents the tribological performance of TiAl matrix self-lubricating composites (TSC) containing MoS₂, hexagonal BN and Ti₃SiC₂ designated as MhT against GCr15 steel counterface under several sliding speeds from 0.2 to 0.8 m s^?1 and applied loads from 6 to 12 N. The results suggested that MhT played an important role in decreasing friction coefficients and wear rates. The covering percentage of transfer layers on worn surfaces varied with the changing of sliding speeds and applied loads, hence resulting in the distinct friction and wear characteristics of TSC. TSC containing 10 wt% MhT exhibited the best excellent tribological performance at 10 N–0.8 m s^?1, which could be due to the formation of the best compaction and largest coverage of transfer layer on the worn surfaces.     

Microstructural,Mechanical, and Tribological Properties of Rice Husk–Based Carbon: Effect of Carbonizing Temperature

In this study, we investigated the microstructural, mechanical, and tribological properties of rice husk (RH)-based carbon carbonized at various carbonizing temperatures under dry conditions. All samples exhibited amorphous carbon structures and the X-ray diffraction spectra of the samples carbonized at 1300 and 1400?°C indicated the presence of a polymorphic crystals of silica. The hardness increased with temperature due to the densification of the structure and the presence of the hard crystalline silica. At low normal loads, the mean friction coefficient of the material decreased as the carbonizing temperature was increased from 600 to 800?°C and slightly decreased as the carbonizing temperature was further increased from 800 to 1400?°C. At the highest load, all samples, except for that carbonized at 600?°C, exhibited extremely low friction coefficients (around 0.05). The wear rates of the all samples were smaller than 10^?5 mm³/N·m, indicating that RH carbon exhibits sufficient wear resistance. A Raman spectroscopic analysis of the worn surface of a steel ball revealed that the transfer layer at 600?°C had a less graphitic structure compared to the other carbonizing temperature. Based on these findings, we recommend an optimal carbonizing temperature for applications of sliding materials exposed to dry sliding contact.     

Friction and Wear of Thermal Oxidation-Treated Ti3SiC2

Ti₃SiC₂ was thermally oxidized (TO) at 1,000 °C for 10 h. An oxide scale of ca. 25 μm was composed of rutile TiO₂ and Al₂O₃ for the outer sub-layer and mixtures of TiO₂ and SiO₂ for the inner sub-layer. The tribological behavior of Ti₃SiC₂ and TO–Ti₃SiC₂ sliding against Si₃N₄ at 25 and 600 °C was investigated. Results indicated that at both 25 and 600 °C, the oxide scale significantly improved the tribological performance of Ti₃SiC₂. The wear mechanisms of Ti₃SiC₂ and TO–Ti₃SiC₂ sliding against Si₃N₄ at 25 and 600 °C are briefly discussed.     

Ceramic-ceramic composite materials with improved friction and wear properties

Dry friction and wear tests were performed with self-mated couples of SiC containing 50% TiC, Si₃N₄---BN, SiC---TiB₂ and Si₃N₄ with 32% TiN at room temperature and 400°C or 800°C.Under room temperature conditions, the friction coefficient of the couple SiC---TiC/SiC---TiC is only half of that of the couple SiC/SiC and the wear is one order of magnitude smaller. At 400°C, it exceeds the friction coefficient of SiC/SiC except at the highest sliding velocity of 3 m s⁻¹. At lower sliding velocities the wear coefficient of SiC---TiC/SiC---TiC is lower than that of SiC/SiC.The couple Si₃N₄---TiN/Si₃N₄---TiN exhibits high friction coefficients under all test conditions. At room temperature the wear volume of the self-mated couples of Si₃N₄ and Si₃N₄---TiN after a sliding distance of 1000 m is similar, but Si₃N₄---TiN shows a running-in behaviour. At 800°C the wear coefficient of Si₃N₄---TiN/Si₃N₄---TiN is approximately two orders of magnitude smaller than that of Si₃N₄/Si₃N₄, and equal to those at room temperature. At 22°C the addition of BN reduces the friction of Si₃N₄. The wear coefficient is independent of sliding velocity and the self-mated couples showing running-in. Friction and wear increase with increasing temperature. The wear coefficient of SiC---TiB₂ above 0.5 m s⁻¹ at 400°C is advantageously near 10⁻⁶ mm³ (Nm)⁻¹. With the other test conditions the wear behaviour is similar to SSiC.     

HVOF-Sprayed Adaptive Low Friction NiMoAl–Ag Coating for Tribological Application from 20 to 800 °C

An adaptive NiMoAl–Ag composite coating was deposited by high-velocity oxy fuel spraying, and its tribological properties from 20 to 800 °C under unlubricated conditions were evaluated using a CSM high-temperature tribometer. Scanning electron microscopy, X-ray diffraction and Raman spectroscopy were used to characterize the coating and corresponding wear tracks to determine the lubrication mechanisms. The results showed that the friction coefficient of the NiMoAl–Ag composite coating was around 0.3 from 20 to 600 °C and reached the lowest value of 0.09 at 800 °C. Meanwhile, wear rates of the coating were maintained on the order of 10^?5 mm³/N m at the test temperatures except for 400 and 600 °C. Characterization of the NiMoAl–Ag coating revealed that silver provided lubrication below 400 °C. Ag₂Mo₂O₇ and Ag₂MoO₄, which were formed through tribochemical reactions, acted as high-temperature lubricants above 400 °C. It was especially proposed that silver in a nearly molten state was effective in reducing the friction of the NiMoAl–Ag coating at 800 °C. Moreover, a comprehensive lubrication mechanism model of an NiMoAl–Ag composite coating at 800 °C was established to explain the extremely low friction coefficient and wear rate of the coating.     

Effects of sintering aids and solid lubricants on tribological behaviours of CMC/Al2O3 pair at 650°C

Ten kinds of self-lubricating composites with different amounts of sintering aids and solid lubricants in Al₂O₃ matrix were fabricated by hot-pressed sintering. Their friction and wear behaviours in unlubricated sliding against Al₂O₃ were tested by using a pin-on-disk wear tester at 650°C. It was shown that the amount of sintering aids strongly affected friction coefficient and wear rate of the Al₂O₃–20Ag20CaF₂ composite, the appropriate amount of sintering aids was 10 wt% for beneficial effect on the reduction of wear at 650°C. Also it was shown that the addition of equal quantities of Ag and CaF₂ in Al₂O₃ matrix can promote the formation of the well-covered lubricating film, and effectively reduce the friction and wear. The composite with 40 wt% of lubricants (20 wt% Ag, 20 wt% CaF₂) presented an optimum tribological behavior at 650°C (friction coefficient μ is about 0.3, wear rates are about 4 x 10^-6 mm³/N,m and 5 x 10^-7 mm³/N,m for the disk and pin, respectively). This revised version was published online in August 2006 with corrections to the Cover Date.