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 characteristics of polycrystalline Magnéli-type titanium dioxides

The results presented in this paper have clarified experimentally, that titania-based Magnéli-phases (Ti₄O₇/Ti₅O₉ and Ti₆O₁₁) with (121)-shear planes exhibit more anti-wear properties than lubricious (low-frictional) properties. The results for dry sliding indicate that the coefficients of friction lie in the range of 0.1–0.6 depending on sliding speed and ambient temperature. The COF decreased with increasing temperature (T= 22–800°C) and increasing sliding speed (υ= 1−6 m/s). The dry sliding wear rate was lowest for the Al₂O₃ at 1 m/s at 800°C with values of 1.7 × 10−8 and 6.4 × 10−8 mm³/N m, comparable to boundary/mixed lubrication, associated with a high dry frictional power loss of 30 W/mm². The running-in wear length and, more important, the wear rate decreased under oscillating sliding tests with increasing relative humidity. The contact pressure for high-/low-wear transition increased under oscillating sliding tests with increasing relative humidity. At room temperature and a relative humidity of 100% the steady-state wear rate under dry oscillating sliding for the couple Al₂O₃/Ti₄O₇–Ti₅O₉ was lower than 2 × 10−7 mm³/N m and therefore inferior to the resolution of the continuous wear measurement sensor. TEM of wear tracks from oscillating sliding revealed at room temperature a work-hardening as mechanism to explain the running-in behavior and the high wear resistance. The hydroxylation of titania surfaces favours the high-/low-wear transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tribological behavior of RH ceramics made from rice husk sliding against stainless steel,alumina, silicon carbide,and silicon nitride

The tribological behavior of rice husk (RH) ceramics, a hard, porous carbon material made from rice husk, sliding against stainless steel, alumina, silicon carbide, and silicon nitride (Si₃N₄) under dry conditions was investigated. High hardness of RH ceramics was obtained from the polymorphic crystallinity of silica. The friction coefficients for RH ceramics disks sliding against Si₃N₄ balls were extremely low (<0.1), irrespective of contact pressure or sliding velocity. Transfer films from RH ceramics formed on Si₃N₄ balls. Wear-mode maps indicated that the wear modes were powder formation under all tested conditions, resulting in low specific wear rates (<5×10⁻⁹ mm²/N).     

Surface Energies of Magnetic Recording Head Components

The rate of material removal during fixed abrasive lapping is a function of friction coefficient, the surface tension of the lubricant and of the substrate, and the contact angles between the interfaces. In this study, the authors measured the surface energies of materials typically found in thin film magnetic recording heads using contact angle measurements and the Lifshitz–van der Waals acid/base approach. The different materials tested were Ni_xFe_y, Al₂O₃, and Al₂O₃-TiC. Sample preparation procedures were also considered. The chemical used to wash the surface was observed to affect the measured substrate surface energies. Surface energy values for samples washed with either acetone or hexane showed comparable results. The Ni_xFe_y gave the highest measured surface energy (46.3–48.8 mJ m⁻²) followed by Al₂O₃ (44.1–45.3 mJ m⁻²) and Al₂O₃-TiC (43.3–45.3 mJ m⁻²). In contrast, the oil-washed samples measured generally lower surface energy values. The study characterized the interaction of two lubricant types against the three materials. The oil-based lubricant spreads completely on oil-washed samples mainly because of the low surface tension of the oil (22.0 mJ m⁻²) and did not show measurable contact angles. In comparison, the water-soluble lubricant ethylene glycol, due to its higher surface tension (48.0 mJ m⁻²), formed higher contact angles ranging from 47.2 to 59.6° on the different substrates.     

Effect of counter materials on coefficients of friction of TiN coatings with preferred grain orientations

In order to investigate the effect of counter materials on the coefficients of friction of TiN coatings with preferred grain orientations, the coefficients of friction against six counter materials (ball) were measured. The ball materials were aluminum (A1050), stainless steel (SUS304, SUS440C), bearing steel (SUJ2), carburized steel (SWRM10), cemented tungsten carbide (WC-Co) and alumina (Al₂O₃). After tests, the worn flat surfaces of balls and the wear tracks of TiN coatings were analyzed by the electron probe micro analysis (EPMA) and the auger electron spectroscopy (AES) to observe the Ti oxide film on each ball material and the adhesion of ball materials to the TiN coatings. The Ti oxide film was observed on the worn flat surfaces of the ball materials and the ball materials did not adhere to the TiN coatings in case that the low coefficients of friction were obtained.     

Environmental Effects on the Tribology and Microstructure of MoS<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>–C Films

The tribology of molybdenum disulfide (MoS₂)–Sb₂O₃–C films was tested under a variety of environmental conditions (ambient 50% RH, 10⁻⁷ Torr vacuum, 150 Torr oxygen, and 8 Torr water) and correlated with the composition of the surface composition expressed while sliding. High friction and low friction modes of behavior were detected. The lowest coefficient of friction, 0.06, was achieved under vacuum, while sliding in 8 Torr water and ambient conditions both yielded the highest value of 0.15. Water vapor was determined to be the environmental species responsible for high friction performance. XPS evaluations revealed a preferential expression of MoS₂ at the surface of wear tracks produced under vacuum and an increase in Sb₂O₃ concentration in wear tracks produced in ambient air (50% RH). In addition, wear tracks produced by sliding in vacuum exhibited the lowest surface roughness as compared to those produced in other environments, consistent with the picture of low friction originating from well-ordered MoS₂ layers produced through sliding in vacuum.     

Effect of Tribofilm Formation on the Dry Sliding Friction and Wear Properties of Magnetron Sputtered TiAlCrYN Coatings

Nano-structured TiAlCrYN coatings, grown by unbalanced magnetron sputtering on various steel substrates, exhibited friction coefficients 0.6–0.8 and wear coefficients 10⁻¹⁶–10⁻¹⁵ m³ N⁻¹ m⁻¹ in dry sliding wear tests. This article reports comprehensive worn surface analyses using SEM, TEM, EDX, EELS and Raman spectroscopy. A ~80 nm thick tribofilm formed on the TiAlCrYN worn surface was found to have dense amorphous structure and homogeneous oxide composition of Cr_0.39Al_0.19Ti_0.20Y_0.01O_0.21. Viscous flow of the amorphous tribofilm was dominant in causing the high friction coefficient observed. The coatings showed combined wear mechanisms of tribo-oxidation and nano-scale delamination.     

Further investigation on the tribological behavior of Al2O3–20Ag20CaF2 composite at 650°C

The friction and wear behaviors of the self‐lubricating Al₂O₃–20Ag20CaF₂ disk against an Al₂O₃ pin pair have been investigated over a broad load range from 1 to 30 N and sliding velocities from 0.084 to 1 m/s at 650^°C. Four typical wear modes have been identified and the wear mode map was constructed to illustrate the influence of load and speed on the friction coefficient and wear rate. The results showed the effective self‐lubricating region (II) (continuous lubricating film) is almost independent of sliding speed, and mainly dependent on the load. It is suggested that the plastic deformation and plastic flow during sliding play an important role in the formation of the self‐lubricating film on the sliding surface. Furthermore, the worn surface in the region (II) (continuous lubricating film) was found to be much softer than the original surface and the distribution of Vickers hardness became more uniform due to the presence of the lubricating film on the worn surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wear properties of two-phase Al2O3/ZrO2 (Y2O3) ceramics at temperatures from 296 to 1073 K

Reciprocating sliding friction experiments were conducted with various two-phase, directionally solidified Al₂O₃/ZrO₂ (Y₂O₃) pins sliding on B₄C flats in air at temperatures of 296, 873, and 1073 K under dry sliding conditions. Results indicate that all the Al₂O₃/ZrO₂ (Y₂O₃) ceramics, from highly Al₂O₃-rich to ZrO₂-rich, exceed the main wear criterion requirement of 10⁻⁶ mm³ N⁻¹ m⁻¹ or lower for effective wear-resistant applications. Particularly, the eutectics and Al₂O₃-rich ceramics showed superior wear properties. The composition and microstructure of Al₂O₃/ZrO₂ (Y₂O₃) ceramics played a dominant role in controlling the wear and friction properties. The controlling mechanism of the ceramic wear, friction, and hardness was an intrinsic effect involving the resistance to shear fracture of heterophase bonding and cohesive bonding and the interlocking microstructures at different scales in the ceramics.     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

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.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

In situ formation of tribologically effective oxide interfaces in SiC‐based ceramics during dry oscillating sliding

The tribological behaviour of single‐phase SiC as well as both SiC–TiC and SiC–TiC–TiB₂ composite materials sliding against aluminium oxide has been investigated at room temperature with regard to the formation of wear‐reducing interfaces. The experiments were carried out in dry air for reasons of excluding the strong influence of water vapour. The introduction of the titanium phases into the SiC microstructure reduces the system wear by more than a factor of ten. The coefficient of friction is only slightly reduced but stabilised with time. The relatively abrasive oxide interface in the Al₂O₃/SiC pairing is altered to a soft and malleable oxide interface in the Al₂O₃/SiC–TiC–TiB₂ pairing. The wear reduction is mainly caused by a change of the wear mechanism so that the formed soft oxide wear debris is transferred to the counterbody, thus shifting the sliding plane further into the layer. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Dry Sliding Tribological Properties of Fe-Based Bulk Metallic Glass

The dry or unlubricated sliding friction and wear properties of as-cast and annealed BMG (bulk metallic glass) with nominal composition of Fe_66.7C_7.0Si_3.3B_5.5P_8.7Cr_2.3Al_2.0Mo_4.5 against Si₃N₄ ceramic ball was studied, along with a conventional material, using a ball-on-disk tribotester at room temperature. The overall average coefficient of friction value of the as-cast BMG was in the range of 0.26?C0.42, which was better than the conventional material SUJ2 (0.36?C0.46) and comparable with SUS304 (0.31?C0.40). The wear mechanism of the Fe-based BMG changed with wear condition. The wear rate increased with increasing load. The hardness of the BMG increased during annealing, however, the wear resistance did not increase proportionally.     

Improved Substrate Protection and Self-Healing of Boundary Lubrication Film Consisting of Polydimethylsiloxane with Cationic Side Groups

The substrate protection and self-healing capability of a cationic polymer lubricant (CPL) on a silicon oxide surface were tested with a pin-on-disc tribometer and atomic force microscopy (AFM). CPL was made of low molecular weight polydimethylsiloxane (PDMS) containing covalently attached quaternary ammonium cations and iodide counter-anions. CPL was spin-coated on the silicon oxide surface to form a 3–4 nm thick bound-and-mobile lubricant layer. The CPL film capable of binding to the SiO₂ surface through ionic interactions is superior in substrate protection than the neutral PDMS film which cannot form the bound layer. The mobile component in the CPL film readily flows into the lubricant-depleted sliding contact region from the surrounding film. The self-healing capability of CPL via lateral flow is slightly enhanced in humid environments due to water uptake in the film. The 3–4 nm thick CPL film on silicon oxide takes 30–40 s to flow into a ~50 μm wide track, which corresponds to an apparent spreading rate of 2–3 × 10⁻¹¹ m²/s.     

A Wideband spectrometer for NMR studies

The design of a wideband decimeter-wave (200–900 MHz) spectrometer with a magnetic induction of up to ∼10 T is described. This spectrometer is intended for studying electronic-nuclear oscillations in antiferromagnets at low temperatures (4.2−1.3 K). Critical field H _c = 2.5 ± 0.3 T of a reorientation transition in a noncollinear antiferromagnet Mn₃Al₂Ge₃O₁₂ at temperature T ≈ 1.3 K was determined from a ⁵⁵Mn²⁺ NMR spectrum.     

Reciprocating Sliding Friction and Wear Property of Fe<Subscript>3</Subscript>Si Based Alloys Containing Cu in Water Lubrication

Fe₃Si, Fe₃Si alloys containing Cu were fabricated by arc melting followed by hot-pressing. The friction and wear behaviors of Fe₃Si based alloys with and without Cu addition against Si₃N₄ ball in water-lubrication were investigated. The friction coefficient and the wear rates of Fe₃Si based alloys decreased as the load increased. The wear rate of Fe₃Si was higher than that of AISI 304. The addition of Cu can significantly improve the friction and wear properties of Fe₃Si based alloys and substantially reduce the wear rates of Si₃N₄ ball. The wear rate of Fe₃Si–10%Cu was 2.56 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 20 N and decreased to 1.64 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear rate of Si₃N₄ ball against Fe₃Si–10%Cu was 1.41 × 10⁻⁶ mm³ N⁻¹ m⁻¹, while the wear rate of Si₃N₄ ball against AISI 304 was 5.20 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear mechanism was dominated by micro-ploughing. The combination of mechanical action (i.e., shear, smear and transference of Cu) and tribochemical reaction of Si₃N₄ with water was responsible for the improved tribological behavior of Fe₃Si alloys containing Cu under high loads.     

Friction–wear behavior and tribochemical reactions of different ceramics in HFC-134a gas

The friction and wear properties of the ionic ceramics Al₂O₃ and ZrO₂, and the covalent ceramics Si₃N₄ and SiC rubbing against an Al₂O₃ ball in vacuum (10⁻⁵ Pa) and in CF₃CH₂F (HFC-134a) gas at 10⁴ Pa were investigated using a ball-on-disk type tribometer. Without exposure to air, the surface composition and chemical state of the wear tracks and debris on the disks were determined with X-ray photoelectron spectroscopy (XPS). It is found that HFC-134a gas significantly reduces the friction and wear of all the ceramic couples, and that the ionic ceramic pairs show lower friction and wear. On the other hand, metal fluorides and/or fluorine-containing organic compounds are detected on the sliding surfaces. The differences in the friction–wear behavior of the ceramics rubbing in HFC-134a gas may be due to the products of tribochemical reactions, which are dependent on the bond type of the ceramics.     

Microstructural and Tribological Properties of A356 Al–Si Alloy Reinforced with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles

In the present study, the effect of the Al₂O₃ particles (average size of 12 μm, 3 and 10 wt.%) reinforcement on the microstructure and tribological properties of Al–Si alloy (A356) was investigated. Composites were produced by applying compocasting process. Tribological properties of unreinforced alloy and composites were studied, using pin-on-disc tribometer, under dry sliding conditions at different specific loads and sliding speed of 1 m/s. Microhardness measurements, optical microscope and scanning electron microscope were used for microstructural characterization and investigation of worn surfaces and wear debris. During compocasting of A356 alloy, a transformation from a typical dendritic primary α phase to a non-dendritic rosette-like structure occurred. Composites exhibited better wear resistance compared with unreinforced alloy. Presence of 3 wt.% Al₂O₃ particles in the composite material affected the wear resistance only at specific loads up to 1 MPa. The wear rate of composite with 10 wt.% Al₂O₃ particles was nearly two order of the magnitude lower than the wear rate of the matrix alloy. Dominant wear mechanism for all materials was adhesion, with others mechanisms: oxidation, abrasion and delamination as minor ones.