Dry and water-lubricated slip-rolling of Si3N4- and SiC-based ceramics

Tests on self-mated Si₃N₄- and SiC-based ceramics as well as ceramic-ceramic composites were performed in an Amsler-type wear tester under dry and water-lubricated rolling conditions with 10% slip. Under dry friction, wear coefficients of the materials varied by four decades. Unlubricated wear coefficients below 10⁻⁷ mm³/(N.m), defined as a practical limit for applicability, can be achieved with Si₃N₄-TiN below 775 MPa and with HIP-SiC below 750 MPa. HIPped Si₃N₄ and hot-pressed SiC-TiC under dry friction exhibit a small dependency of wear coefficient on Hertzian pressure, with wear coefficients below 10⁻⁶ mm³/(N.m). The lowest wear coefficient below 10⁻⁶ mm³/(N.m) with water lubrication was found for Si₃N₄-TiN and S-RBSi₃N₄; water reduces the variability in wear coefficient for Si₃N₄- and SiC-based ceramics.     

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 Tribological Behaviors of Different Mass Ratio Al2O3-TiO2 Coatings in Water Lubrication Sliding against Si3N4

The tribopairs of water hydraulic plunger pumps are usually operated under severe conditions, due to the poor lubrication of water and silt suspended in natural water. It is essential to identify the desired engineering materials and material combinations for designing water pumps. As the candidate materials of tribopairs, the tribological characteristics of different Al₂O₃-TiO₂ coatings combined with Si₃N₄ ceramics under silt-laden water and tap water lubrication were investigated. The Al₂O₃-TiO₂ coatings with different weight percentages of TiO₂ in a wide range from 3 to 100% were tested. The tribological characteristics of the various couple pairs were researched using a ring-on-ring test rig. The microstructures of the contact surfaces were analyzed with a scanning electron microscope before and after the test to study the wear mechanism. The experimental results indicate that the friction coefficient of the Al₂O₃-TiO₂/Si₃N₄ tribopairs increases with an increase in the percentage of TiO₂ content in the Al₂O₃-TiO₂ coating. However, the TiO₂ content does not present a clear relationship with the wear rate. Considering the friction and wear properties, Al₂O₃-13%TiO₂ is the preferred coating to use in water hydraulic pumps when sliding against Si₃N₄.     

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.     

Influence of Al2O3 reinforcement on the abrasive wear characteristic of Al2O3/PA1010 composite coatings

In the present study, the abrasive wear characteristics of Al₂O₃/PA1010 composite coatings were tested on the turnplate abrasive wear testing machine. Steel 45 (quenched and low-temperature tempered) was used as a reference material. The experimental results showed that when the Al₂O₃ particles have been treated with a silane coupling agent (γ-aminopropyl-triethoxysilane), the abrasive wear resistance of Al₂O₃/PA1010 composite coatings has a good linear relationship with the volume fraction of Al₂O₃ particles in Al₂O₃/PA1010 composite coatings and the linear correlation coefficient is 0.979. Under the experimental conditions, the size of Al₂O₃ particles (40.5-161.0 μm) has little influence on the abrasive wear resistance of Al₂O₃/PA1010 composite coatings. By treating the surface of Al₂O₃ particles with the silane coupling agent, the distribution of Al₂O₃ particles in PA1010 matrix is more homogeneous and the bonding state between Al₂O₃ particles and PA1010 matrix is better. Therefore, the Al₂O₃ particles make the Al₂O₃/PA1010 composite coatings have better abrasive wear resistance than PA1010 coating. The wear resistance of Al₂O₃/PA1010 composite coatings is about 45% compared with that of steel 45.     

Effects of fullerene C60 on the friction and wear characteristics of ceramics in ethanol

For the combinations of a pin of Si₃N₄ and five kinds of disk, the friction and wear test was carried out in ethanol, and in ethanol containing C₆₀ particles (1 wt%). A topographical analysis was also performed on the micro-asperities of the wear surfaces to estimate the behavior of C₆₀ particles, and the degree of surface damage. As a results, the following facts were found. (1) The addition of C₆₀ particles in ethanol decreased the wear rates of such ceramic disks as Al₂O₃, SiC and TiC and of the mating Si₃N₄ pins. (2) The addition of C₆₀ particles decreased the mean coefficient of friction for SiC, Si₃N₄, and TiC disks. (3) The wear rates of pin and disk depended on the topographies of worn surfaces such as the mean depth of micro-grooves, the mean tip radius of micro-asperities and so on.     

Improved wear resistance of Si3N4 tool inserts by addition of Al2O3 platelets

In the present work, Si₃N₄ matrix composites reinforced with different amounts of Al₂O₃ platelets (0, 30 and 50vol%) were produced with the aim of increasing the tribochemical resistance in the machining of steels. Tool wear was related to the linear increase of the main cutting force (F_c) with time (dF_c/dt); a real-time parameter that can be used to assess the cutting edge damage and to stop machining before the tool fails. For all machined steels, tool wear resistance increased with increasing Al₂O₃ platelet content.     

Ultra-low friction coefficient in alumina–silicon nitride pair lubricated with water

In a ball-on-disc wear test, an alumina ceramic body sliding against a silicon nitride ceramic body in water achieved an ultra-low friction coefficient (ULFC) of 0.004. The profilometer and EDX measurements indicated that the ULFC regime in this unmated Al₂O₃–Si₃N₄ pair was achieved because of the formation of a flat and smooth interface of nanometric roughness, which favored the hydrodynamic lubrication. The triboreactions formed silicon and aluminum hydroxides which contributed to decrease roughness and shear stress at the contact interface. This behavior enables the development of low energy loss water-based tribological systems using oxide ceramics.     

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si₃N₄ sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO₂ results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si₃N₄, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si₃N₄ disks sliding against 1/4 in Si₃N₄ balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BF_x and PF_x films on the surface and creation of an electric double layer of ionic liquid.     

Tribochemical Behavior of Si3N4–hBN Ceramic Materials with Water Lubrication

The main objective of this article is to study the tribological behavior of Si₃N₄–hBN composites with different hexagonal boron nitride (hBN) volume fraction under distilled water lubrication. Water-lubricated sliding tests were carried out on a pin-on-disc tester, and Si₃N₄ was used as friction pair. The results showed that the addition of hBN to Si₃N₄ resulted in a severe decrease of the friction coefficient, from 0.35 for Si₃N₄ against Si₃N₄ to 0.01 for Si₃N₄-20% hBN against Si₃N₄ with drip-feed water lubrication; the friction coefficients of Si₃N₄–hBN/Si₃N₄ pairs sliding with full immersion water lubrication were as low as 0.01. The morphological and chemical characterization of the worn surfaces were conducted using scanning electron microscopy (SEM), laser scanning microscope, X-ray photoelectron spectroscopy (XPS). The analysis indicated that, with drip-feed water lubrication, hBN in Si₃N₄–hBN was spalled off during the wearing tests and spalling pits were formed on the wearing surface of Si₃N₄–hBN composites, then the wear debris were dropped into the pits and reacted with water, and thus a tribochemical film was formed on the wearing surface. The tribochemical film facilitated the wear surfaces of Si₃N₄–hBN and Si₃N₄ to smooth with drip-feed water lubrication, while the tribochemical remove facilitated the wear surfaces to smooth with full-immersion water lubrication.     

Frictional deformation and fracture in polycrystalline SiC and Si3N4

Electron optical microscopy was employed to study the friction and wear of commercial polycrystalline varieties of SiC and Si₃N₄ in air at ambient temperature. Friction and wear tests were conducted in a reciprocating configuration with conical riders (both diamond and ceramic) sliding on a flat ceramic substrate. Worn surfaces were examined by both scanning electron microscopy and transmission electron microscopy. In general, friction and wear in the diamond-ceramic couples were severe. Friction with ceramic-ceramic couples was low, with friction coefficients between 0.1 and 0.4, wear being absent in single-pass tests.With ceramic-ceramic couple multipass systems, wear of Si₃N₄ occurs by plastic deformation which increases in severity with sliding distance accompanied by a corresponding increase in friction coefficient. With SiC, wear occurs by a mixture of intergranular fracture due to grain boundary weakness and plastic deformation.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

The effect of Al2O3 on the friction performance of automotive brake friction materials

This study consists of two stages. In the first stage, bronze-based break linings were produced and friction-wear properties of them were investigated. In the second stage, 0.5%, 1%, 2% and 4% alumina (Al₂O₃) powders were added to the bronze-based powders and Al₂O₃ reinforced bronze-based break linings were produced. Friction–wear properties of the Al₂O₃ reinforced samples were aslo investigated and compared to those of plain bronze-based ones. For this purpose, friction coefficient and wear behaviour of the samples were tested on the grey cast iron disc. The hardness and density of the samples were also determined. Microstructures of the samples before and after the sintering and the worn surfaces of the wear specimens were examined using a scanning electron microscope (SEM). The sample compacted at 350 MPa and sintered at 820 °C exhibited the optimum friction–wear behaviour. With increase in friction surface temperature, a reduction in the friction coefficient of the samples was observed. The lowest reduction in the friction coefficient with increasing temperature was for the 2% and 4% Al₂O₃ reinforced samples. The SEM images of the sample indicated that increase in Al₂O₃ content resulted in adhesive wear. With increase in Al₂O₃ content, a reduction in mass loss of the samples was also observed. Overall, the samples reinforced with 2% and 4% Al₂O₃ exhibited the best results.     

Triboemission, tribochemical reaction, and friction and wear in ceramics under various n-butane gas pressures

The triboemission behaviour of negatively and positively charged particles and photons generated by scratching a Si₃N₄ disc with a conical diamond stylus was studied under various gas pressures of n-butane as a model compound of typical paraffinic lubricating oil. The triboemission behaviour of photon emission in a frictional system of an Al₂O₃ ball sliding on Al₂O₃ and Si₃N₄ discs was also measured under various n-butane gas pressures. Further, the friction and wear in the frictional system of an Al₂O₃ ball sliding on an Si₃N₄ disc were studied under various n-butane gas pressures. The triboemission intensities of three kinds of energetic particles, negatively and positively charged particles and photons, become a maximum at a particular n-butane gas pressure. When the n-butane gas pressure is such that triboemission is a maximum, the formation of friction polymer-like high molecular weight products is at a maximum, and friction and wear of the ball decreases to a minimum. It has been concluded that the friction polymer-like high molecular weight products are formed through tribochemical reactions triggered by triboemission phenomena and help reduce the friction and wear of the solids.     

Friction and wear properties of duplex MAO/CrN coatings sliding against Si3N4 ceramic balls in air, water and oil

It is evident that the micro-arc oxidation (MAO) ceramic coatings often exhibit relatively high friction coefficients as sliding against many mating materials. To reduce the friction coefficient for the MAO coatings, the duplex MAO/CrN coatings were deposited on 2024Al alloy using combined micro-arc oxidation and reactive radio frequency magnetron sputtering. The microstructure and phase of the duplex coatings were observed and determined using scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. The friction and wear behaviors of the duplex coatings sliding against Si₃N₄ balls in air, water and oil were investigated using a ball-on-disk tribometer. The wear rate of the duplex coating was determined by non-contact optical profilometer and the wear tracks on the duplex coatings were observed by SEM. The results showed the CrN coatings mainly consisted of Cr, CrN and Cr₂N phases. The duplex coatings/Si₃N₄ tribopair exhibited the highest friction coefficient in air, while displayed the lowest friction coefficient in oil. When the normal load and the sliding speed increased, the friction coefficient in air increased from 0.65 to 0.72, whereas decreased from 0.58 to 0.36 in water and 0.20 to 0.08 in oil. The specific wear rates for the duplex coatings in air were higher than those in oil. In comparison to the MAO coatings, the duplex MAO/CrN coatings displayed excellent tribological properties under the same conditions.     

High-temperature tribological properties of spark-plasma-sintered Al2O3 composites containing barite-type structure sulfates

Al₂O₃–50BaSO₄–20Ag, Al₂O₃–50BaSO₄–10SiO₂, Al₂O₃–50(mass%)SrSO₄, Al₂O₃–50PbSO₄–5SiO₂, Al₂O₃–50BaSO₄ and Al₂O₃–50BaCrO₄ composites (mass%) were prepared by spark plasma sintering and their microstructure and high-temperature tribological properties were evaluated. Al₂O₃–50BaSO₄–20Ag composites (mass%) showed the lowest friction coefficients at the temperature ranging from 473 to 1073 K. Thin Ag film was observed on the wear tracks of the composites above 473 K. In addition, the friction coefficients of Al₂O₃ composites containing SrSO₄ and PbSO₄ were as low as those of Al₂O₃–BaSO₄ and Al₂O₃–BaCrO₄ composites at the temperatures up to 1073 K. The thin films formed on the wear tracks of the Al₂O₃–SrSO₄ composites were composed of Al₂O₃ and SrSO₄ phases, while the films formed on the wear tracks of the Al₂O₃–PbSO₄–SiO₂ composites consisted of Al₂O₃, PbSO₄ and SiO₂ phases.     

The Sliding Friction of Hybrid Composite Journal Bearing Under Various Test Conditions

The friction and wear behaviour of SiC, Si₃N₄ and SiC/Si₃N₄ composite ceramics were investigated with oscillating sliding (gross slip fretting) at room temperature. The influence of counter body material and the humidity of the surrounding air was studied with a ball-on-disc configuration with different ball materials (1000Cr6, Al₂O₃ SiC and Si₃N₄). The effect of RH on friction is marginal with exception of SiC (low friction) as counter body material. The wear behaviour, however, is strongly affected by humidity, showing inverse trends for different counter body materials. Consequently, the wear behaviour of a tribo couple can be improved by selecting an adequate mating material. The results reveal the necessity to control RH in tribological tests. For estimation of the performance of tribo couples under varying environmental conditions, a variation of RH is required. In tribo couples with single phase SiC, either as ball or disc, the tribological behaviour of the system is dominated by SiC. The friction behaviour of the composite material is in between the behaviour of the two single phase materials, Si₃N₄ and SiC, whereas the wear behaviour is very similar to that of single phase Si₃N₄.     

Friction Reduction and Wear Resistance Enhancement of SiC and Si3N4 Ceramics under Dry Conditions

In this study, an effort was made to control the friction and wear behavior of silicon carbide (SiC) and silicon nitride (Si₃N₄) ceramics using an ultrasonic nanocrystalline surface modification (UNSM) technique. The friction and wear behavior of the ceramic specimens was investigated using a ball-on-disk tribotester under dry conditions against two different Si₃N₄ and bearing steel (SUJ2) balls. The experimental test results revealed the possibility of controlling the friction and wear behavior of ceramics, where the friction coefficient and wear resistance of the specimens were improved by the UNSM technique. The hardness of the specimens also increased after UNSM treatment, but it decreased abruptly with increasing depth from the very top surface. Microscratch tests showed that the critical load of the specimens was improved by the UNSM technique. In addition, Raman spectra results revealed that no additional phase was detected after UNSM treatment, but the intensity decreased after UNSM treatment. Hence, the UNSM technique ensures stronger ceramics and enables better friction and wear behavior than available conventional sintered ceramics.     

Observation of delamination wear of lubricious tribofilm formed on Si3N4 during sliding against WC-Co in humidity air

Hot pressed silicon nitride that was exposed to high (90%) and low (32%) relative humidity was examined in ball-on-disc geometry against cemented carbide ball at various normal loads. The study indicated that Si₃N₄ tested at high R.H. gave less specific wear rate compared with Si₃N₄ at low R.H. The friction coefficient of Si₃N₄−WC-6% Co tribopairs was found in the range of 0.32–0.39 and 0.05–0.17 at low humidity and high humidity respectively. It is suggested that adsorbed moisture markedly affected the wear and friction properties of silicon nitride.Following the tests, SEM was used to elucidate the wear mechanism and particularly to delineate the effects of relative humidity on the wear and friction. SEM micrographs showed that the main wear mechanism at low relative humidity (32%) was caused by mechanical wear including abrasive grooves, large holes and polishing, whereas at high relative humidity (90%) the main mechanism was highly influenced by a tribochemical reaction related to the moisture adsorption from the environment. It is concluded that the removal of lubricious tribolayer was occurred by delamination induced crack propagation.