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.     

Tribo-oxidation of Self-mated Ti3SiC2 at Elevated Temperatures and Low Speed

The tribological behavior of self-mated Ti₃SiC₂ is investigated from ambient temperature to 800?°C at a sliding speed of 0.01?m/s in air. The results show that at the temperatures lower than 300?°C, friction coefficient and wear rates are as high as 0.95 and 10^?3?mm³/N?m, respectively. With the temperature increasing to 600?°C, both the friction coefficient and wear rates show consecutive decrease. At 700 and 800?°C, friction coefficient and wear rates are 0.5 and 10^?6 mm³/N?m, respectively. According to the wear mechanism, the tribological behavior of Ti₃SiC₂ can be divided into three regimes: mechanical wear-dominated regime from ambient temperature to 300?°C characterized by pullout of grains; mixed wear regime (mechanical wear and oxidation wear) from 400 to 600?°C; and tribo-oxidation-dominated wear regime above 700?°C. The tribo-oxides on the worn surfaces involve oxides of Si and Ti. And, species transformation occurs to these two oxides with the increasing temperature. In the competition oxidation of elements Ti and Si, Si is preferably oxidized because of its high active position in the crystal structure. Additionally, plastic flow is another notable characteristic for the tribological behavior of self-mated Ti₃SiC₂.     

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.     

Stribeck Curve Under Friction of Copper Samples in the Steady Friction State

The friction and wear of a pure copper block (99.98 wt% Cu) against a hardened steel disc were studied. The effect of sliding velocity and load on the friction coefficient and wear rate of Cu samples during steady tests was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. The transitions from one lubrication region to another are discussed. The mixed EHL region is characterized by stable low values of the friction coefficient, wear rate and temperature. Straight asperity contact is the dominant mechanism under friction of Cu–steel pair in the BL region. High-friction coefficients and wear rates, thin lubricant films and large wear grooves indicate straight asperity contact between rubbed surfaces in the BL region. Although the dominant mechanisms in the mixed EHL and BL regions are different in principle, a steady friction state is preserved in both cases. It is expected that the steady friction state in the BL and mixed EHL regions is associated with deformation and fracture of surface layers but these process occur at different scale levels. It was shown that under friction of Cu–steel pair, two types of ML regions are observed. The first is the stable steady friction of mixed EHL with low values of the friction coefficient and wear rate. The second type of the ML region is the region of unstable friction and wear when a decrease of lubricant film leads to a change of external (roughness, temperature, friction and wear) and internal (strain and stress) parameters. It was found out that a transition to the unstable ML region occurs within a narrow range of Z parameter under definite values of the load and sliding velocity.     

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.     

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.     

The comparisons of sliding speed and normal load effect on friction coefficients of self-mated Si3N4 and SiC under water lubrication

The effects of sliding speed and normal load on friction coefficients of self-mated Si₃N₄ and SiC sliding in water after running-in in water were investigated with pin-on-disk apparatus at sliding speeds of 30 to 120 mm/s, normal loads of 1 to 14 N in ambient condition. The results showed that, after running-in in water, for two kinds of self-mated ceramics, friction coefficient increases with both decreasing sliding speed and increasing normal load when normal load is larger than a critical normal load. Friction coefficient was independent of normal load when normal load is smaller than the critical load. The lubrication film of Si₃N₄ under water lubrication exhibited larger load carrying capacity than that of SiC did. Stribeck curves indicated that, for self-mated Si₃N₄ ceramics, hydrodynamic lubrication will change into boundary lubrication abruptly when the sommerfeld number is less than a critical value; while for self-mated SiC ceramics, hydrodynamic lubrication will change into mixed lubrication and then into boundary lubrication gradually when the sommerfeld number is below critical value.     

The Difference in Running-In Period and Friction Coefficient Between Self-Mated Si3N4 and SiC Under Water Lubrication

Running-in periods and friction coefficients of SiC and Si₃N₄ sliding against themselves under water lubrication were investigated with a pin-on-disk apparatus at sliding speed of 120 mm/s and a normal load of 5 N under ambient conditions. It was found that the running-in period of self-mated Si₃N₄ is much shorter than that of self-mated SiC, and also that the steady-state friction coefficient of self-mated Si₃N₄ was lower (0.0035) than that of self-mated SiC (0.01). The difference in mechanism was analyzed from the point of view of electronic structure and surface chemistry.     

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.     

Friction and wear of copper samples in the steady friction state

The effect of sliding velocity and load on the friction and wear of Cu-steel pairs was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. Steady friction states were observed in the mixed EHL and BL regions, however two types of the ML region are revealed. The first type is the stable ML range. The second one is the range of unstable friction and wear when a decrease of the lubricant film leads to abrupt change of all controlled parameters. It was found that a transition to the unstable ML region occurs within a narrow range of Z parameter. Wear modes in the lubrication regions were studied. Deformation hardening in the lubricant regimes is discussed.     

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.     

Tribo-corrosion behaviors of Ti3SiC2/Si3N4 tribo-pair in hydrochloric acid and sodium hydroxide solutions

This paper is a continuation of our work to characterize the tribo-corrosion behaviors of Ti₃SiC₂ in common acids and alkalis [Ren et al. (2010) [1]]. Tribo-corrosion behaviors of Ti₃SiC₂/Si₃N₄ in hydrochloric acid (HCl) and sodium hydroxide (NaOH) with different concentrations were investigated in the present paper. The worn surfaces of Ti₃SiC₂ and Si₃N₄ were examined using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that oxide films form on the worn surfaces of both Ti₃SiC₂ and Si₃N₄ by corrosive oxidation and/or tribo-oxidation. The SiO₂ film, however, does not effectively prevent grains from detaching. Mechanical wear dominates the friction process rather than chemically corrosive wear for Ti₃SiC₂. Chemically corrosive reactions, in turn, decelerate mechanical wear to a certain extent. Corrosive wear resulting from oxidation and dissolution of material by corrosive solutions and mechanical wear from abrasion contribute to material loss for Si₃N₄. The high viscosity of NaOH solutions decreases friction and wear.     

Wear Behavior of (TiB2–TiC)–Ni/TiAl/Ti Gradient Materials Prepared by the FAPAS Process

(TiB₂–TiC)–Ni/TiAl/Ti functionally gradient materials were prepared by field-activated pressure-assisted synthesis processes. (TiB₂–TiC)–Ni composite ceramic, the top layer of the functional gradient materials, was prepared in situ by the combustion synthesis process using Ti and B₄C powders as raw materials. Scanning electron microscope (SEM) images of the ceramic layer revealed that the TiB₂ and TiC particles in the composite were fine and homogeneously dispersed in the Ni matrix. The friction and wear properties of the (TiB₂–TiC)–Ni ceramic were evaluated by sliding against a GCr15 disk at temperatures from ambient up to 400 °C. The experimental results showed that the friction coefficient of the (TiB₂–TiC)–Ni ceramic decreased with the increasing testing temperature, load, and sliding speed. However, the loss rate decreased at higher temperature and increased at higher load and higher sliding speed. The wear mechanisms of (TiB₂–TiC)–Ni ceramic mainly depend upon thermal oxidation at higher temperature, load, and sliding speed. The worn topography and phase component of the worn surfaces were analyzed using SEM, energy dispersive spectroscopy, and X-ray diffraction. The oxide films of Fe₂O₃, TiO₂, and B₂O₃ formed during the friction process play an important role in lubrication, which results in a smaller friction coefficient.     

The influence of TiC,CaF2 and MnS additives on friction and lubrication of sintered high speed steels at elevated temperature

Friction behaviours of sintered high speed steels containing TiC, CaF₂ and MnS additives and lubrication mechanisms of these additives have been investigated at sliding conditions at 600°C. Results shown that these additives strongly affected friction behaviours of the sintered high speed steels. Ceramic carbide TiC, as a bonding agent and enhancement phase, bonded the solid lubricant CaF₂ and MnS surrounding it and supported these solid lubricant particles so that friction process become stable. Fluoride calcium CaF₂ has a better high temperature lubrication properties than MnS, and that the addition of TiC + CaF₂ in the sintered high speed steels achieved excellent friction performances both a stable and a low friction coefficient value. Analysis results by X-ray indicated that the surface film appearing on worn path consists of some molten metal matters and very fine carbide particles. During sliding, the surface film separated contacts and resulted in a lower friction coefficient. Because of the sintered high speed steels usually to be used to manufacture high temperature components, these results are helpful for their engineering use.     

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.     

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 Behavior of Novel Ti3SiC2 (Natural Nanolaminates)-Reinforced Epoxy Composites during Dry Sliding

In this article, we report for the first time the synthesis and characterization of Ti₃SiC₂–epoxy (MAXPOL) composites. Three novel composites were designed by adding 20.7, 30.6, and 71.6 vol% Ti₃SiC₂ particulates to an epoxy matrix. The microstructure evaluation by scanning electron microscopy (SEM) showed that the Ti₃SiC₂ particles are well dispersed in the epoxy matrix. The addition of Ti₃SiC₂ enhanced the ultimate yield strength (UYS) and hardness of all of the composites compared to epoxy. Tribological studies were performed by a tab-on-disc method against Inconel 718 and alumina substrates. In both cases, the mean friction coefficient (µ_mean) decreased as the concentration of Ti₃SiC₂ in the epoxy matrix was increased. The concomitant wear rates (WRs) also decreased steadily and then increased slightly after reaching a concentration of ~32.6 vol% Ti₃SiC₂. The tribological studies proved conclusively that the addition of Ti₃SiC₂ in the epoxy matrix imparts self-lubricity to the composites. The tribofilms formed on different tribosurfaces were also characterized by detailed SEM investigations.     

Tribological properties of Ni3Al-Cr7C3 composite coating under water lubrication

The tribological properties of Ni₃Al-Cr₇C₃ composite coating under water lubrication were examined by using a ball-on-disc reciprocating tribotester. The effects of load and sliding speed on wear rate of the coating were investigated. The worn surface of the coating was analyzed using electron probe microscopy analysis (EPMA) and X-ray photoelectron spectroscopy (XPS). The results show the friction coefficient of the coating is decreased under water lubrication. The wear rate of the coating linearly increases with the load. At high sliding speed, the wear rate of the coating is dramatically increased and a large amount of the counterpart material is transferred to the coating worn surface. The low friction of the coating under water lubrication is due to the oxidizing of the worn surface in the wear. The wear mechanism of the coating is plastic deformation at low normal load and sliding speed. However, the wear mechanism transforms to microfracture and microploughing at high load with low sliding speed, and oxidation wear at high sliding speed. It is concluded that the contribution of the sliding speed to an increase in the coating wear is larger than that of the normal load.     

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.     

Surface chemistry of fluorine containing ionic liquids on steel substrates at elevated temperature using Mössbauer spectroscopy

Ionic liquids have properties that make them attractive as solvents for many chemical synthesis and catalysis reactions. Consequently, research has focused on their application as advanced solvents. Recently, ionic liquids were shown to have promise as a lubricant due to many of the same properties that make them useful as solvents. The focus of this paper is to study the surface chemistry of ionic liquid lubricated steel in sliding contact to temperatures from room to 300 °C. Tribological properties were evaluated using a pin on disk tribometer with high temperature capability (up to 800 °C). Chemistry was studied using Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Samples used for tribological evaluation were 1 inch diameter polished M50 disks. Samples used for studying the surface chemistry were enriched ⁵⁷Fe grown via thermal evaporation. Some ⁵⁷Fe samples were oxidized to Fe₂O₃ and Fe₃O₄ prior to treatment with ionic liquids. The metallic and oxidized ⁵⁷Fe samples were then reacted with ionic liquids at elevated temperatures. Three ionic liquids were used in this study; 1-n-ethyl-3-methylimidazolium tetrafluoroborate (BF₄), 1,2-di-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (TFMS), and 1,2-di-methyl-3-butylimidazolium hexafluorophosphate (PF₆). This study was focused on understanding the high temperature stability of the liquids in contact with metal and under tribological stress. Therefore, the friction data was collected in the boundary (or mixed boundary/EHL) lubrication region to enhance surface contact. BF₄ provided a friction coefficient of 0.04 for both the room and 100 °C tests and varied between 0.07 and 0.2 for the 300 °C test. The results from TFMS lubrication showed a friction coefficient of 0.025 at room temperature and 0.1 at 100 °C. The 300 °C test friction coefficient ranged between 0.1 and 0.3. Chemical analysis of the surface revealed corrosion of the surface due to reaction between the ionic liquids and steel/iron substrates.