Effect of Microstructure on the Tribological and Mechanical Properties of CuO-Doped 3Y-TZP Ceramics

Dense 8 mol% CuO-doped 3Y-TZP ceramics were prepared by pressureless sintering for 8 h at 1500° and 1550°C, respectively. Transmission electron spectroscopy revealed that the ceramic sintered at 1500°C exhibits grain boundaries free of any amorphous phase, while crystalline copper-oxide grains were found in the zirconia matrix, whereas the sample sintered at 1550°C contains a Cu-rich amorphous grain boundary layer. The tribological behavior of these materials was tested under dry-sliding conditions using a pin-on-disk tribometer. The material sintered at 1500°C showed self-lubrication resulting in a low coefficient of friction ( f ) of 0.2–0.3 and a low specific wear rate ( k ) ≪ 10⁻⁶ mm³·(N·m)⁻¹. In contrast, the material sintered at 1550°C showed poor tribological behavior ( f =0.8–0.9; k ≫ 10⁻⁶ mm³·(N·m)⁻¹ under the same conditions. The difference in the tribological behavior of these two materials was interpreted on the basis of mechanical properties and microstructural characteristics.     

Tribological Behavior of Ti2SC at Ambient and Elevated Temperatures

The tribological properties of Ti₂SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al₂O₃) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10⁻⁴ mm³·(N·m)⁻¹ the specific wear rate (SWRs), was high. However, against Al₂O₃, at ∼5 × 10⁻⁵ mm³·(N·m)⁻¹ and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti₂SC/Inc718 and Al₂O₃ tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10⁻⁵ mm³·(N·m)⁻¹. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti₂SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al₂O₃ in the wide temperature range from ambient to 550°C.     

Viscosity and Density of Boron Trioxide

Extensive and accurate viscosity values for molten B₂O₃ are reported over the continuous range from 20 poises (at 1400°C) to 10¹⁰ poises (at 318°C). The measurements were made with a wide-range rotating-cylinder viscometer. The melts were pretreated by bubbling superdry nitrogen through them to ensure a minimum water content. Above 800°C the temperature dependence of the viscosity curve obeyed the Arrhenius equation; below 800°C the curvature was smooth showing no breaks. At the low temperatures, although its curvature decreased, the viscosity curve did not obey the Arrhenius equation. Densities were determined between 411° and 1400°C. Over this range the volume expansion coefficient changed by 1 order of magnitude from 3.35 × 10⁻⁴/°C to 3.34 × 10⁻⁵/°C. The liquid volume expansion coefficient above 1200°C was smaller than that of the glass below the glass transition temperature, indicating some type of structural rearrangement in B₂O₃.     

Tribological Behavior of Mo5Si3-Particle-Reinforced Silicon Nitride Composites

The tribological behavior of Mo₅Si₃-particle-reinforced silicon nitride (Si₃N₄) composites was investigated by pin-on-plate wear testing under dry conditions. The friction coefficient of the Mo₅Si₃–Si₃N₄ composites and Si₃N₄ essentially decreased slowly with the sliding distance, but showed sudden increase for several times during the wear testing. The average friction coefficient of the Si₃N₄ decreased with the incorporation of submicrometer-sized Mo₅Si₃ particles and also as the content of Mo₅Si₃ particles increased. When the Mo₅Si₃–Si₃N₄ composites were oxidized at 700°C in air, solid-lubricant MoO₃ particles were generated on the surface layer. Oxidized Mo₅Si₃–Si₃N₄ composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo₅Si₃–Si₃N₄ composite were 0.43 and 0.72 × 10⁻⁵ mm³ (N·m)⁻¹, respectively. Both values were ∼30% lower than those for the Si₃N₄ tested in an identical manner.     

Mechanical and Fretting Wear Behavior of Novel (W,Ti)C–Co cermets

In an effort to refine the composition and properties of existing hard metals, (W,Ti)C–20 wt% Co cermets have been developed. The present research reports the mechanical and tribological properties of these novel materials. Single-step as well as two-stage and three-stage sintering experiments were conducted on the cermets, processed from the (W,Ti)C solid solution powders. For property comparison, premixed WC/TiC powders were used to fabricate a reference (W,Ti)C–20 wt% Co cermet material. Higher sintered density (9.57 g/cm³) as well as better elastic modulus (467 GPa) and hardness (∼17 GPa) were obtained after three-stage sintering of solid solution powders. In order to evaluate the tribological properties, the fretting wear experiments (mode I, linear relative tangential displacement) were performed against bearing steel for varying normal load in the range of 2–10 N. The experimental results reveal that the steady-state coefficient of friction (COF) varies between 0.50 and 0.65, and a lower COF is recorded at 10 N load for cermets processed from solid solution powders. Under varying tribological conditions, the cermets sintered from solid solution powders exhibit low wear depth (∼1–4 μm) and lower wear rate (7 × 10⁻⁷–18 × 10⁻⁷ mm³·(N·m)⁻¹ when compared with cermet prepared from the premixed WC/TiC starting powders (wear rate ∼14 × 10⁻⁷–22 × 10⁻⁷ mm³·(N·m)⁻¹). The wear rate data are critically evaluated based on the phenomenological models. Broadly, abrasive wear is the dominant wear mechanism, and limited contribution from localized spalling of tribolayer and tribochemical wear was also observed.     

Sintering of Zinc Sulfide

The mechanisms of the sintering of ZnS were determined by measurement of the rate of growth of the necks between polycrystalline spheres. In vacuum (10⁻⁶ mm Hg) at temperatures higher than 600° C the mechanism of sintering is that of volume diffusion with coefficient D_v, = 1.06 × 10⁻² exp (-26,400/RT); below 600°C surface diffusion predominates, with coefficient D₃, = 9.14 × 10_-3 exp (-5700/RT). In Zn vapor (10⁻² mm Hg) between 550° and 650°C the predominating mechanism of sintering is surf ace diffusion, D₃, = 7.06 × 10⁻² exp (-8600/RT). It has been found that in an argon atmosphere the diffusion coefficient is much lower.     

Tribological Properties of a Hot-Pressed Ba-Doped S-Phase Sialon Ceramic

In this study, we report the tribological properties of hot-pressed S-sialon ceramics. In our experiments, the tribological properties have been studied against bearing grade steel, alumina, and sialon using a ball-on-flat fretting tribometer under dry unlubricated conditions. While the steady-state coefficient of friction (COF) remains the same (0.63) irrespective of the counterbody, the wear rate varies in the range of 10⁻⁵–10⁻⁶ mm³/N·m, depending on the counterbody. Tribochemical wear as well as adhesion and abrasion are found to be the underlying mechanisms for material removal. The possible influence of Hertzian contact stress and the contact temperature increase on the possible damage mechanisms/tribological interaction has been analyzed.     

Diffusion of Sputtered Vanadium, Nickel, and Silver in Lead Magnesium Niobate Ceramic

The sputtering of nickel-vanadium (Ni-V) and silver (Ag) to form thin films on lead magnesium niobate (PMN) ceramic substrates has been studied. An empirical equation was derived which correlated the average Ni-V film thickness with three sputtering variables: input power, argon pressure, and sputtering time. The films were dense, smooth, and feature less. Upon annealing at 800°C in argon, both Ni and V diffused into PMN; but reverse diffusion from the ceramic into the metal film was not observed. Vanadium diffused at a faster rate than Ni, leaving pores in the film. The diffusion coefficient of Ni in PMN was determined to be 4.1 × 10⁻¹² cm²/s, whereas the diffusion coefficient of V was determined to be between 3.5 × 10⁻⁹ and 2.9 × 10⁻¹¹ cm²/s at 800°C. Sputtered Ag was also dense, smooth, and featureless. However, the diffusion of Ag at 800°C in PMN was much slower than that of either Ni or V.     

Oxidation of Thin Sheet Reaction-Sintered Silicon Nitride

Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10⁻⁸ to 10⁻⁵ atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10⁻⁷ atm at 1050° to 1.5 to 2.5 × 10⁻⁵ atm at 1200°C.     

Microwave-Sintered MgO-Doped Zirconia with Improved Mechanical and Tribological Properties

This paper reports the results of microwave sintering (without post-sintering annealing) on the microstructure, phase assemblage, and properties of 8 mol% (PSZ—partially stabilized zirconia), and 16 mol% (FSZ—fully stabilized zirconia) MgO-alloyed zirconia. For the PSZ samples sintered at 1585°C, a maximum densification of ∼98%ρ_th, along with a hardness of ∼10.6 GPa and a fracture toughness of ∼6.8 MPa·m^1/2, was obtained. The results of tribological experiments on some selected samples revealed that a good combination of a lower coefficient of friction of 0.35 and a wear rate of 10⁻⁷ mm³/N m can be obtained with the optimally sintered Mg-PSZ.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.     

Effect of Grain Size on the Sliding Wear and Friction of Alumina at Elevated Temperatures

The sliding friction and wear of three different grain-size aluminas were studied from room temperature through 1000°C. The coefficient of friction revealed two distinct regions of decrease with increased temperature, with a transition at ∼700°C. Below 700°C, the coefficient of friction decreased rapidly with increased temperature (∼10^-3/°C). However, above 700°C, the decrease was more gradual (∼10^-5/°C). This was believed to be related to a brittle-to-ductile transition at the wear surface. The coefficient of friction was only weakly dependent on grain size, because the largest grain sizes exhibited slightly higher friction coefficients. However, the specific wear loss of the aluminas increased with increased grain size at room temperature and at 600°C, both below the 700°C transition. The primary mechanism of wear was ascertained to be brittle microfracture along grain boundaries. At 1000°C, above the 700°C transition, the specific wear loss was significantly decreased and appeared to be independent of the alumina grain size. At 1000°C, the wear surfaces developed a thin layer of fine grains formed by dynamic recrystallization. The grain size within the thin layer was in agreement with the previously reported grain-size/Zener-Hollomon parameter relationship.     

Thermal Expansion of Pb3O4

Thermal expansion of Pb₃O₄ was investigated by high-temperature X-ray diffraction. The coefficient in the a ₀ direction is 14.6×10⁻⁶/°C. Expansion in the c₀ direction is 32% greater, with a coefficient of 19.3×10⁻⁶/°C. Coefficients of expansion are linear from 25° to 490°C and are comparable with those of tetragonal and orthorhombic PbO.     

Alumina/Zirconia Micro/Nanocomposites: A New Material for Biomedical Applications With Superior Sliding Wear Resistance

In the present investigation, the sliding wear behavior is described for Al₂O₃/ZrO₂ micro/nanocomposites and monolithic alumina of similar grain size under defined conditions of a constant sliding speed and different loads (20–150 N). Nano ZrO₂ particles (1.7 vol%) were observed uniformly distributing throughout the composites, and most of them were located within the matrix alumina grains. The wear rate of the alumina and the micro/nanocomposites increased as the contact load increased and a clear transition in friction and wear behavior was observed in both materials. However, the nanocomposite wear resistance at low contact loads was one order of magnitude higher than that of the alumina. In the severe regime, no difference was observed among the materials. The low wear rate (10⁻⁷ mm³·(N·m)⁻¹) along with low pullout indicates higher wear resistance of micro/nanocomposites in the mild regime compared with monolithic alumina. Based on the morphological observation of worn surfaces by scanning electron microscope and on residual stress analysis performed by neutron diffraction, some wear mechanisms of Al₂O₃–ZrO₂ micro/nanocomposites are proposed. The high wear resistance of the nanocomposites is discussed in terms of fracture resistance properties and residual stress. Improvements in mechanical and tribological properties of these composites make them promising candidates for biomedical applications.     

Resistance of Tin Dioxide to Chemical Reaction with Vanadate—Sulfate Melts

A thermogravimetric analysis/SO₃ equilibrium technique has been used to show that tin dioxide (SnO₂) is inert to chemical reaction with NaVO₃ at 700° and 800°C under SO₃ partial pressures as high as 5 × 10⁻² bar (5 kPa). The results suggest that SnO₂ may be potentially useful as a material for protection against molten vanadate–sulfate hot corrosion at moderate temperatures (<800°C).     

Minimization of Parasitic Currents in High-Temperature Conductivity Measurements on High-Resistivity Insulators

An experimental setup and novel measurement technique are described which allow reliable conductivity measurements to be made at conductivities as low as 10⁻¹⁷Ω⁻¹.cm⁻¹ and temperatures up to at least 1300°C. This capability is of particular interest for conductivity measurements on high-resistivity insulators over a large range of temperatures. This technique has been used to measure the conductivity of single-crystal alumina from 400° to 1300°C in a 10⁻⁷ torr (∼1.3 × 10⁻⁵ Pa) vacuum, equivalent to an oxygen partial pressure of about 10⁻⁸ torr (∼1.3 × 10⁻¹¹ atm or ∼ 1.3 × 10⁻⁶ Pa). Surface and gas-phase conductance are determined as a function of temperature, and the requirements for their minimization are described. A key requirement is a very low voltage between the volume guard and the guarded electrode. The effect of leakage currents due to the sample fixture, electrical feedthroughs, and electronic instrumentation are also evaluated, and proper design features to make these effects negligible are outlined.     

Glass Formation and Properties in the System Calcia-Gallia-Silica

The glass-formation region of the calcia-gallia-silica system was determined. The glasses within this region were measured to have a density of 3 to 4 g/cm³, a refractive index of 1.6 to 1.73, an Abbe number between 35 and 58, a thermal expansion coefficient of 6.5 × 10⁻⁷/°C to 11.5 × 10⁻⁷/°C, softening temperatures between 730° and 790°C, and a Vickers microhardness of 5.2 to 7.3 GPa. Crystalline phases were identified along the glass-formation boundary. Infrared transmission spectra were used to explain glass structures and their effect on glass properties. The results suggest that the role of calcia in the glass structure is similar to that for calcia in calcium aluminosilicate glasses.     

Low-Temperature Creep of Nanocrystalline Titanium(IV) Oxide

Nanocrystalline TiO₂ with densities higher than 99% of rutile has been deformed in compression without fracture at temperatures between 600° and 800°C. The total strains exceed 0.6 at strain rates as high as 10⁻³ s⁻¹. The original average grain size of 40 nm increases during the creep deformation to final values in the range of 120 to 1000 nm depending on the temperature and total deformation. The stress exponent of the strain rate, n , is approximately 3 and the grain size dependence is d ^{− q} with q in the range of 1 to 1.5. It is concluded that the creep deformation occurs by an interface reaction controlled mechanism.     

Tribooxidational Effects on Friction and Wear Behavior of Silicon Nitride/Tool Steel and Silicon Nitride/Gray Cast Iron Contacts

Unlubricated pin-on-disk wear tests of Si₃N₄ against tool steel and gray cast iron were performed at 5 N of normal load, 0.5 m/s of sliding speed, and environmental temperature in the range 22°-600°C. The friction coefficient of Si₃N₄ sliding against tool steel and gray cast iron had maximum values of 0.88-0.98 for tests at 100°C. The friction coefficient of Si₃N₄ sliding against gray cast iron couples had minimum values of 0.48-0.57 at 400°C. Because of the increased third-body protection, the wear coefficient of the Si₃N₄ pins of the Si₃N₄/gray cast iron couples decreased by 1 order of magnitude from 1.6 10^-5 mm³/(Nm) at room temperature to 1.3 10^-6 mm³/(Nm) at 600°C. Fe₂O₃ and Fe₃O₄ resulting from tribooxidation of the metallic disks were the main constituents of the wear debris and adherent tribolayers. Activation energy values (6.3-13.7 kJ/mol) were comparable to those of oxidation wear of steel (7.3-11.8 kJ/mol) but were much lower than the activation energy for oxidation of iron alloys in static conditions. Calculations of the activation energy of the oxidation wear corroborate the morphological observations of a sacrificial action of the metallic surface protecting the ceramic material.     

Reactive Hot Pressing and Properties of Nb2AlC

Dense Nb₂AlC ceramic was synthesized from NbC, Nb, and Al powder mixture at 1650°C and a pressure of 30 MPa for 90 min using an in situ reaction/hot-pressing method. The reaction kinetics, microstructure, physical, and mechanical properties of the fabricated material were investigated. A thermal expansion coefficient of ∼8.1 × 10⁻⁶ K⁻¹ was measured in the temperature range of 30°–1050°C. At room temperature a thermal conductivity of ∼20 W·(m·K)⁻¹ and a Vickers hardness of ∼4.5 GPa were determined. The material attained Young's modulus, four-point bending strength and fracture toughness of ∼294 GPa, ∼443 MPa, and ∼5.9 MPa·m^1/2, respectively. The nanolayered grains with a mean grain size of 17 μm contributed to the damage tolerance of this ceramic. Quenching from 600°, 800°, and 1000°C into water at room temperature resulted in decrease in bending strength from 443 MPa for the as-synthesized Nb₂AlC to 391, 156, and 149 MPa, respectively.