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.     

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.     

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.     

界面滑移条件下点接触Stribeck曲线的实验研究

采用高黏度聚异丁烯润滑油,在光学弹流实验机上考察球-盘接触纯滑条件下的摩擦因数随卷吸速度和载荷的变化。结果表明,随着卷吸速度的增加,球-盘接触副进入弹流润滑并向流体动压润滑转变的过程中,摩擦因数并不像传统的Stribeck曲线一样,随着卷吸速度的增加而单调增加,而是呈现先上升、后下降、再上升的趋势,其中摩擦因数下降时的起始速度大致为凹陷出现的速度,摩擦因数再上升时的速度大致是润滑进入动压润滑的速度。初步论证界面滑移为产生上述波动的主要原因。     

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.     

The Failure and Damage Mechanisms Under Friction of Copper in the EHL and Mixed EHL Regions

Recently, friction and wear behaviour of Copper (Cu)–steel pairs rubbed under different lubrication conditions were studied. The Stribeck curve was used to identify the different regimes of friction of copper with different virgin grain sizes: the elasto-hydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication (BL) regimes. The aim of this work is the detailed analysis of the damage evolution under friction in the EHL and mixed EHL regions. The effects of load on the friction and the wear and damage mechanisms have been studied. The surfaces of Cu samples before and after friction have been analysed using SEM and AFM techniques as well as roughness and hardness measurements. It was shown that the mechanisms of damage and failure of Cu samples under friction in the EHL region are similar to the damage and failure mechanisms in Very High Cycle Fatigue. Friction in the EHL region is accompanied by initiation and coalescence of pores and microcracks. The effects of the coalescence of pores and microcracks observed in the EHL region are enlarged with a subsequent loading in the mixed EHL region. The effect of the loading rate on the transition from the EHL to BL region has been studied. It was shown that decreasing the loading rate increases significantly the load of the transition from the EHL to BL region. The pore and microcrack coalescence remain as the dominant damage mechanism under friction in the mixed EHL region with the low loading rate, while a lot of ploughing tracks and large delaminated regions appeared on the surface of Cu sample after friction with the higher loading rate.     

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.     

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 duality of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>

We report here on the friction behavior of fine- and coarse-grained Ti₃SiC₂ against steel and Si₃N₄ balls. Two successive friction regimes have been identified for both grain sizes and both counterparts. First, Type I regime is characterized by a relatively low (0.1–0.15) friction coefficient, and very little wear. Sliding occurs between a tribofilm on the ball and the Ti₃SiC₂ plane when against steel. Then, a Type II regime often follows, with increased friction coefficients (0.4–0.5) and significant wear. Compacted wear debris seems to act as a third body resulting in abrasion of the ball, even in the case of Si₃N₄. The transition between the two regimes occurs at different times, depending on various factors such as grain size, type of pin, and normal load applied. Some experiments under vacuum showed that the atmosphere plays also a major role. The reason for this evolution is not fully clear at that time, but its understanding is of major technological importance given the unusual good properties of this material.