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 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 Properties of Reactive Magnetron Sputtered V2O5 and VN–V2O5 Coatings

Next generation of advanced hard coatings for tribological applications should combine the advantages of hard wear resistant coatings with low-friction films. In this study, the tribological behaviour of vanadium pentoxide (V₂O₅) single-layer as well as VN–V₂O₅ bi-layer coatings was investigated in the temperature ranging between 25 and 600 °C. For VN–V₂O₅ bi-layer coatings, the V₂O₅ top-layers were deposited by dc and bipolar-pulsed dc reactive magnetron sputtering, where the V₂O₅ phase shows preferred growth orientation in (200) and (110), respectively. The V₂O₅ single-layer coatings were prepared by dc reactive magnetron sputtering with a substrate bias of −80 V which leads to a preferred (200) growth orientation. Tribological properties were evaluated using a ball-on-disc configuration in ambient air with alumina balls as counterpart. The structure of the as-deposited films and eventual changes after tribometer testing were identified using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The friction coefficient of VN–V₂O₅ bi-layer coatings deposited in dc and pulsed dc mode decreases from room temperature to 600 °C, where the pulsed dc VN–V₂O₅ coatings have a significantly lower coefficient of friction over the whole testing temperatures reaching a value of 0.28 at 600 °C. Up to 400 °C, V₂O₅ single-layer coatings showed almost the same coefficient of friction as pulsed dc VN–V₂O₅ bi-layer coatings but reached a value of 0.15 at 600 °C. It seems that thermal activation of crystallographic slip systems is necessary for V₂O₅ films to show a low-friction effect.     

Tribological Characteristics of Combined Layered Phosphate and Silicate Additives in Mineral Oil

A mixture containing α-Zr(HPO₄)₂·H₂O, a small particle layered-phosphate, and α-Na₂Si₂O₅, a microparticle-layered silicate, is investigated for its potential as an additive in mineral oil. A four-ball tester is employed to measure the maximum non-seizure load, wear scar diameter, and friction coefficient of the combined additives. Friction and wear tests indicate that synergistic behavior exists between the two base materials; the load capacity, anti-wear, and friction reducing ability of the base oil are markedly increased when the combined materials are added. More important is the combined materials exhibit considerable advantage over typical lubricant additives, such as MoS₂ and graphite, presenting good application prospects as lubricant additives. The X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy results illustrate that the synergistic behavior of the combined material stems from the small α-ZrP particles confined in the valleys of the contacting surfaces that polish the wear surface, as well as the layered α-Na₂Si₂O₅ microparticles that effectively fill voids in the wear surface.     

A study of the characteristics of CeF3 scintillating crystals

The properties of cerium trifluoride scintillating crystals, grown in Russia using the Stockbarger's method, were investigated. The characteristics of CeF₃, PbWO₄, Bi₄Ge₃O₁₂, and BaF₂ crystals were compared for use in electromagnetic calorimeters to detect electrons and γ-rays of medium and high energies.     

Fabrication In Situ TiB<Subscript>2</Subscript>–TiC–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Multiple Ceramic Particles Reinforced Fe-Based Composite Coatings by Gas Tungsten Arc Welding

A Fe-based composite coating reinforced by multiple TiB₂–TiC–Al₂O₃ ceramic particles was developed by gas tungsten arc welding (GTAW) melting process. Mixture of aluminum (Al), boron carbide (B₄C), and titanium dioxide (TiO₂) powders was used as precursors, and as a consequence TiB₂–TiC–Al₂O₃ multiple ceramic particles were in situ synthesized during GTAW melting process. Microstructural investigations showed that TiB₂ particles exhibit a blocky morphology, TiC particles are of flower-like shape, and the Al₂O₃ particles exist as small black dots and located in the core of reinforced particles. The hardness and wear resistance of the coatings increased drastically in comparison with that of the substrate.     

Study of the heating rate effect on the glass transition properties of (60 − x)V2O5–xSb2O3–40TeO2 oxide glasses using differential scanning calorimetry (DSC)

The glass-transition temperature (T_g) and crystallization temperature (T_Cr) have been determined for the system (60 − x)V₂O₅–xSb₂O₃–40TeO₂ with 0 < x < 10 (in mol%) using differential scanning calorimetry (DSC) at heating rates φ = 3, 6, 9 and 13 K/min. The effect of the heating rate and the Sb₂O₃ content on T_g is discussed. It was observed that the transition region shifts to higher temperatures when the measuring time is reduced (or, conversely, when the applied temperature rate is increased). Using differential scanning calorimetry, the compositional dependence of T_g has been determined and so, an empirical equation has been deduced which relates the glass-transition temperature with the Sb₂O₃ content.     

Synthesis of CuInGaSe2 nanoparticles by low temperature colloidal route

CIGS nanoparticles were synthesized by a low temperature colloidal route for the absorber layer of photovoltaic devices. The CIGS nanoparticles were prepared by reacting CuI, InI₃, GaI₃ in pyridine with Na₂Se in methanol at 0°C under inert atmosphere. The reaction products of dark red and yellow colors were turned out to be NaI and CIGS nanoparticles, respectively, by ICP-AES and SEM-EDS analyses. Chalcopyrite structure of the CIGS nanoparticles was confirmed by XRD and TEM diffraction patterns. As compared to the particles from Cu_0.9In_0.8 Ga_0.3Se₂ ratio, more uniform and smaller nanoparticles were obtained from Cu_1.1In_0.68Ga_0.23Se_1.91 stoichiometric ratio. The CIGS nanoparticles were measured to be in the ranges of 5-20 nm. However, tube like CIGS particles with length of several γn and width in the range of 100-300 nm were obtained from Cu_0.9In_0.8Ga_0.3Se₂, and Cu_0.9In_0.7Ga_0.4Se₂. The morphological change of the CIGS particles seems to be closely related to the ratio of Cu/(In+Ga).     

Tribological properties of ZrO2 (Y2O3)-Mo-BaF2/CaF2 composites at high temperatures

ZrO₂ (Y₂O₃) with different contents of BaF₂/CaF₂ and Mo were fabricated by hot pressed sintering, and the tribological behavior of the composites against SiC ceramic was investigated from room temperature to 1000 °C. It was found that the ZrO₂ (Y₂O₃)-5BaF₂/CaF₂-10Mo composite possessed excellent self-lubricating and anti-wear properties. The low friction and wear were attributed to enhanced matrix and BaMoO₄ formed on the worn surfaces.     

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.     

Experimental study of frost growth on a horizontal cold surface under forced convection

Frost formation on a horizontal copper surface under low air temperature and forced convection conditions is investigated experimentally. Both the frost crystals pattern and the frost layer thickness formed on the cold plate are compared under different experimental conditions. The environmental variables considered in this study include the ambient temperature (T _∞ ), air relative humidity (φ), and velocity (v), as well as the cold surface temperature (Tw). The tested ranges are −5≤T _∞ ≤5 °C, 50%≤ φ≤80%, 2.2≤v≤8.0 m/s, −16.8≤T _w ≤−25.5 °C. The experimental results show the cold surface temperature and the air relative humidity have obvious effects on the frost growth: the frost layer thickness increases strongly with the decreasing cold surface temperature and increasing air relative humidity. The air temperature and air velocity or Reynolds number are also important factors affecting the frost crystals’ growth and thickness. With the increase of the air temperature and velocity, the frost crystals become denser, and the frost layer thickness become thicker, but this trend becomes weaker under higher air temperature and velocity.     

High-temperature tribological properties of strontium sulfate films formed on zirconia-alumina, alumina and silicon nitride substrates

In our present study, Al₂O₃, (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ and Si₃N₄ substrates coated with SrSO₄ and SrSO₄–10 mass% Ag films were prepared, and the friction and wear properties of these specimens were investigated using a reciprocating ball-on-disk tribometer in the temperature range from room temperature to 1073 K in air. It was clarified that (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ substrates coated with chemically precipitated SrSO₄ particles and the substrates coated with SrSO₄–10 mass% Ag films prepared by mechanically grinding and annealing at 1073 K for 3.6 ks exhibited low friction coefficients and low wear rates at all the testing temperatures. In addition, the average friction coefficients of Si₃N₄ substrates were reduced above 673 K by coating with chemically precipitated SrSO₄ particles.     

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.     

Preparation of ultrafine α -Al2O3 using precipitation-azeotropic distillation method

Ammonium aluminum carbonate hydroxide (AACH) was prepared by a precipitation-azeotropic distillation method, which uses aluminum sulfate as the Al source and ammonium carbonate as the precipitant. Then, AACH was calcined into ultrafine α-Al₂O₃ powder. The factors that influence the dispersion property of ultrafine α-Al₂O₃ powder are discussed in this paper, such as the methods of adding materials, surfactant, and drying methods. The changes of the structure and property of ultrafine alumina in the thermal treatment process are also studied. The morphological structure and properties of AACH are characterized by DTA/TGA, SEM, XRD, and ICP measurements. The results show that ultrafine α-Al₂O₃ powder with a uniform particle size and well-distributed property can be synthesized only after aluminum sulfate atomizes into ammonium carbonate, proper amount of PEG1000 is added as the dispersant, and the product is treated by azeotropic distillation. The phase transformation of alumina during the calcination process can be described as amorphous Al₂O₃→γ-Al₂O₃→ϑ-Al₂O₃→α-Al₂O₃. The crystal grain size and density of ultrafine alumina powder increase with the increase of the calcination temperature. After AACH has been calcined at 1200°C for 2 h, the ultrafine α-Al₂O₃ with uniform particle size, spherical shape, and more than 99.97% purity is obtained and its powder is well dispersed. __________ Translated from Journal of Hunan University of Science & Technology (Natural Science Edition), 2007, 22(2): 35–39 [译自: 湖南科技大学学报(自然科学版)]     

Bonding of Hard Disk Lubricants with OH-Bearing End Groups

Typical lubricants for magnetic hard disks comprise the central perfluoropolyether section and the short hydrocarbon end groups bearing hydroxyl unit(s). It had been shown earlier that chemical bonding of these lubricants to the carbon overcoat of disks involves (1) dangling bonds shielded inside the carbon, (2) transfer of the hydrogen atom of the hydroxyl unit to a dangling bond site, and (3) attachment of the remaining alkoxy system, R–CF₂–CH₂–O^·, to the carbon surface as a pendant ether unit. Dangling bonds at or near the surface react immediately with H₂O or O₂ in the atmosphere. It follows that, in order to bond, the hydrocarbon end group must move into crevices of the carbon film. It was postulated that the bonding rate would depend on the length of the hydrocarbon end-group, –(CH₂) _n –OH. The longer the hydrocarbon sector is, the faster and the more extensively the bonding would proceed. Bonding rates were examined for a set of samples differing only in the dimension of the hydrocarbon end-group. Results clearly in accordance with the postulate were obtained. The sample set included two novel lubricants, D-2TX2 and D-2TX4, with the following end-groups, –O–CF₂–CH₂–O–(CH₂) _n=2,4–OH. Excellent bonding rate, coverage, and potential anticorrosion property were revealed for these lubricants.     

A method for measuring the surface impedance of superconductors in the temperature range 0.4–120 K

A method and a setup for making precision measurements of the temperature dependences of components of surface impedance Z(T) = R(T) + iX(T) of small superconductor crystals in the temperature range 0.4–120 K have been developed. The setup combines a high-quality-factor resonance system for measuring the microwave response of a sample at a frequency of 28 GHz and a refrigerating unit with the use of evaporation of ³He vapors. Measurements of Z(T) of optimally doped YBa₂Cu₃O_6.93 single crystals in the superconducting and normal states were performed to illustrate the operation of the setup.     

Effect of pre-readout annealing treatments on TL mechanism in tellurite glasses at therapeutic radiation doses level

The higher sensitization for thermal annealing on TL mechanism in the region 550–600 °C for 80(TeO₂)–5(TiO₂)–(15 − x) (WO₃)–(x) A_nO_m where A_nO_m = Nb₂O₅, Nd₂O₃, Er₂O₃ and x = 5 mol% has been measured. The behavior of trap centers and luminescence centers has been investigated for tellurite glasses doped with rare earth oxides irradiated at 0.5 up to 2 Gy and annealed at different temperatures in the range 350–700 °C. The behavior of the three types of tellurite glasses is analyzed regarding to their kinetic parameters and luminescence emission which enhance the claim of tellurite glasses for use as TLD material at therapeutic radiation doses.     

A proximate and nondestructive quality control of epitaxial ferrogarnet films

A procedure of the proximate and nondestructive quality control of ferrogarnet epitaxial structures obtained by the liquid-phase epitaxy method is proposed. The procedure is based on the method that regards a magnetized ferrite film as an oscillating system in a ferromagnetic resonance (FMR) spectrometer. This spectrometer is intended to determine the FMR line width locally over the entire surface of Y₃Fe₅O₁₂ films with thicknesses of ≤100 μm in a frequency band of 1.2–4.0 GHz.     

Effect of Fluoride Content on Friction and Wear Performance of Ni<Subscript>3</Subscript>Al Matrix High-Temperature Self-Lubricating Composites

The self-lubricating composites Ni₃Al–BaF₂–CaF₂–Ag–Cr, which have varying fluoride contents, were fabricated by the powder metallurgy technique. The effect of fluoride content on the mechanical and tribological properties of the composites was investigated. The results showed that an optimal fluoride content and a balance between lubricity and mechanical strength were obtained. The Ni₃Al–6.2BaF₂–3.8CaF₂–12.5Ag–10Cr composite showed the best friction coefficients (0.29–0.38) and wear rates (4.2 × 10⁻⁵–2.19 × 10⁻⁴ mm³ N⁻¹ m⁻¹) at a wide temperature range (room temperature to 800°C). Fluorides exhibited a good reduced friction performance at 400 and 600°C. However, at 800°C, the formation of BaCrO₄ on the worn surface due to the tribo-chemical reaction at high temperatures provided an excellent lubricating property.     

Tribological properties of NiCr–Al2O3 cermet-based composites with addition of multiple-lubricants at elevated temperatures

The tribological properties of NiCr-40 wt% Al₂O₃ (NC40A) cermet-based composites containing SrSO₄ and other lubricant (graphite, MoS₂ and Ag) against alumina ball were evaluated to identify their self-lubrication mechanisms from room temperature to 800 °C. The composites demonstrated distinct improvements in effectively reducing friction and wear, as compared to NC40A cermet. The best results were observed for NC40A–10SrSO₄–10Ag composite, which exhibited satisfactory reproducibility of friction coefficient over a wide temperature range (200–800 °C) through high temperature cyclic friction tests due to the formation of synergistic lubricating films SrAl₄O₇, NiCr₂O₄ and Ag on the contact surface.