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.     

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.     

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.     

High temperature tribology of ceramics

The friction and wear behaviour of self-mated couples of MgO---ZrO₂, Al₂O₃ and two types of SiSiC were studied under dry sliding conditions in a special pin-on-disc high temperature tribometer. The temperature was varied between 25 and 1000°C, and the sliding speed from 0.03 m s⁻¹ to 3 m s⁻¹. The morphology of the worn surfaces was studied by means of SEM, and their phase distribution by X-ray diffraction and TEM analyses. The results show that the wear coefficients of all couples mostly increase with increasing temperature and sliding velocity. The wear of MgO---ZrO₂ is influenced by tribo-induced phase transformations while α-Al₂O₃ retains its original structure for all test conditions. For SiSiC delamination and fatigue of the interface Si/ß-SiC predominate. At higher temperatures and sliding velocities tribo-oxidation is effective. The friction coefficients lie between 0.5 and 1.0 under steady-state conditions but for short test durations lower values can occur. The couple SiSiC/SiSiC has low friction coefficients at low sliding velocities and temperatures, even if the steady-state region is reached.     

Dry and oil-lubricated sliding wear of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/BN fibrous monoliths

A high-temperature ball-on-flat tribometer was used to investigate dry and oil-lubricated friction and wear of sintered Si₃N₄ and Si₃N₄/hexagonal boron nitride (H-BN) fibrous monoliths. The friction coefficients of base Si₃N₄ flats sliding against Si₃N₄ balls were in the range of 0.6–0.8 for dry and 0.03–0.15 for lubricated sliding, and the average wear rates of Si₃N₄ were 10^–5 mm³ N^–1 m^–1 for dry sliding and 10^–10–10^–8 mm³ N ^–1m^–1 for lubricated sliding. The friction coefficients of Si₃N₄ balls against composite fibrous monoliths were 0.7 for dry sliding and 0.01–0.08 for lubricated sliding. The average specific wear rates of the pairs were of the same order as those measured for the conventional Si₃N₄ pairs. However, the fibrous monoliths, in combination with sprayed dry boron nitride, resulted in reduction in the lubricated friction coefficients of the test pairs and significant reduction in their wear rates. The most striking result of this study was that the coefficients of friction of the Si₃N₄/H-BN fibrous monolith test pair were 70–80 lower than those of either roughened or polished Si₃N₄ when tests were performed under oil-lubricated sliding conditions over long distances (up to 5000 m). The results indicated that Si₃N₄/H-BN fibrous monoliths have good wear resistance and can be used to reduce friction under lubricated sliding conditions.     

On the use of fractal geometry methods for the wear process characterization

Irregular fluctuations of friction coefficient around the mean value were analysed by means of fractal geometry methods. Particular data were recorded during the steady-state stage of dry sliding tests carried out in Si₃N₄ ball - on - Si₃N₄/SiC nanocomposite disc configuration. It was proven that the set of points {sliding distance (time), friction coefficient}, considered as a geometric object in a two-dimensional space, has the property of a fractal curve. The fractal dimension of this curve increased with increasing wear rate gained in a particular wear test. This could indicate a possible correlation between the wear rate and the fractal dimension of friction coefficient as a function of sliding distance (time).     

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 and wear of self-mated SiC and Si3N4 sliding in water

The friction and wear of self-mated SiC and Si₃N₄ with different initial roughness sliding in water were investigated with pin-on-disk apparatus at normal load of 5 N and sliding speed of 120 mm/s in ambient condition. It was found that, for self-mated Si₃N₄, the wear mechanism for surface smoothening to obtain low friction was tribochemical wear, but for self-mated SiC, it changed from mechanical wear into tribochemical wear with increasing sliding cycles. After running-in in water, self-mated Si₃N₄ exhibited lower steady-state friction coefficient than self-mated SiC did. For these two ceramics, initial and steady-state friction coefficients were hardly dependent on initial roughness. Initial roughness mainly affected the running-in period. The larger the initial roughness, the longer the running-in period, but the running-in period was much shorter for self-mated Si₃N₄ at each initial roughness than that for self-mated SiC.     

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si₃N₄) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000°C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000°C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750°C was caused by brittle fracture, whereas from 750 to 1000°C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.     

Tribological characterisation of a‐C:H coatings at room temperature: Effect of counterbody material

Metal‐free amorphous carbon (a‐C:H) coatings with 15% hydrogen were deposited on bearing steel surfaces. The friction and wear performance of these specimens was characterised in oscillating sliding tests with a ball‐on‐flat geometry. Balls of four ceramic and four metallic materials were investigated in tests at room temperature. Special attention was paid to the effect of moisture by testing in dry, normal, and moist air. The effect of water vapour on the friction and wear of the a‐C:H coatings was quite different for the different counterbody materials. The wear was in all cases very low, with a coefficient of wear below 10⁻⁷ mm³/N m for most cases. The coefficient of friction was also very low, between 0.04 and 0.12 for most of the tests. The smallest wear and friction coefficients were found for oxide ceramics, while during tests against SiC and Si₃N₄ the coating was worn through during the test. The effects of counterbody material and the humidity of the surrounding air are discussed in terms of friction and wear mechanisms.     

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.     

Effect of temperature on friction and oscillating sliding wear of dense and porous 3Y-TZP zirconia ceramics

Microstructures of 3 mol% Y₂O₃-ZrO₂ (3Y-TZP) with systematically varying porosity up to about 15% were produced by sintering. Hardness and fracture toughness of the ceramics as well as the amount of tetragonal, cubic and monoclinic phase were measured. Wear tests were carried out on the different self-mated microstructures under dry reciprocating sliding contact using ring-on-block geometries in air at five different contact temperatures up to 500°C. The microstructures and worn surfaces were extensively analysed using scanning electron microscopy (SEM) and X-ray diffraction techniques. The experimental results revealed a reduction of the amount of wear (independent of porosity) by more than one order of magnitude compared with room temperature if the test temperature was increased to 250°C. Between room temperature and 250°C, wear increased with increasing porosity while at 500°C the highest wear was measured on the dense structure. Microscopic observations showed that plastic deformation, surface layers consisting of compacted wear debris and also intercrystalline, transcrystalline or delamination type fracture influenced friction and wear.     

Wear transition diagram for silicon carbide

To obtain information on the tribological behaviour of silicon carbide at elevated temperatures, unlubricated ball-on-flat wear tests were conducted on sintered silicon carbide in self-mated sliding in air. The contact load was varied from 3.2 to 98.0 N, and a temperature range of 23°C to 1000°C was used. Scanning electron microscopy, Fourier transform infrared spectroscopy and energy-dispersive spectroscopy were used to elucidate the wear mechanisms. The results of the tests and observations were employed to construct a wear transition diagram, which provides a summary of tribological information including friction coefficient, wear coefficient and wear mechanisms as a function of temperature and load. The wear transition diagram for the sintered silicon carbide studied is divided into four regions plus one transition zone. At room temperature, under high loads and high environmental humidity, the tribological behaviour is controlled by tribochemical reactions between the silicon carbide surface and water vapour in the environment. Under low loads and at temperatures below 250°C, wear occurs by ploughing and polishing. At temperatures about 250°C and under low loads, tribooxidation and formation of cylindrical wear particles control the tribological behaviour. Wear occurs by microfracture when the load is increased above a critical value; and both the friction coefficient and the wear coefficient increase.     

Aqueous Lubrication of SiC and Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Ceramics Aided by a Brush-like Copolymer Additive,Poly(<Emphasis Type="SmallCaps">l</Emphasis>-lysine)-<Emphasis Type="Italic">graft</Emphasis>-poly(ethylene glycol)

We have examined the adsorption properties of poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)—a brush-like polymer—on Si₃N₄ and SiC surfaces and determined its impact on the aqueous lubrication of Si₃N₄ and SiC at various speeds and applied loads. The addition of PLL-g-PEG in aqueous solution reduces the interfacial friction forces significantly for self-mated sliding contacts of these two ceramics, as compared to lubrication with water or buffer solution alone. For SiC, the improved lubricating performance by addition of PLL-g-PEG was apparent for all tested speeds (from 1.4 to 185 mm/s under 2 N load). For Si₃N₄, the effect was more apparent in the slow-speed regime (≤20 mm/s under 2 N load) than in the high-speed regime (>100 mm/s), where extremely low coefficients of friction (μ ≤ 0.006) are readily achieved by aqueous buffer solution alone. It was further observed that the optimal lubricating effect with Si₃N₄ is achieved when the tribopairs are first run-in in polymer-free aqueous buffer to render the sliding surfaces smooth, after which the PLL-g-PEG copolymer is added to the buffer solution.     

Fretting at high temperatures

The fretting damage to an austenitic stainless steel, type 321, in CO₂ is much reduced at temperatures above 400°C by the formation of a glaze type oxide. Increasing the normal pressure from 2 to 6.9 MN m⁻² at 650°C greatly increased the extent and quality of the glaze. The nickel-based alloy, Inconel 718, developed glaze oxide when fretted at 540°C in air, as indicated by a low coefficient of friction and wear rate. At 280°C, the glaze was only found at greater amplitudes of slip. Although the titanium alloy Ti-6Al-4V in air at 200 to 400°C developed a surface oxide which had some of the superficial features of a glaze, it nevertheless did not reduce the coefficient of friction to values characteristic of glaze. The common feature of high-temperature alloys which develop protective glaze oxides is that they are capable under conditions of sliding and fretting of forming a spinel type oxide which, however, must be adequately supported by a creep-resistant substrate at the operating temperature     

Tribological properties of γ-Y2Si2O7 ceramic against AISI 52100 steel and Si3N4 ceramic counterparts

Reciprocating ball-on-flat dry sliding friction and wear experiments have been conducted on single-phase γ-Y₂Si₂O₇ ceramic flats in contact with AISI 52100 bearing steel and Si₃N₄ ceramic balls at 5–15 N normal loads in an ambient environment. The kinetic friction coefficients of γ-Y₂Si₂O₇ varied in the range over 0.53–0.63 against AISI 52100 steel and between 0.51–0.56 against Si₃N₄ ceramic. We found that wear occurred predominantly during the running-in period and it almost ceased at the steady friction stage. The wear rates of γ-Y₂Si₂O₇ were in the order of 10^?4 mm³/(N m). Besides, wear debris strongly influenced the friction and wear processes. The strong chemical affinity between γ-Y₂Si₂O₇ and AISI 52100 balls led to a thick transfer layer formed on both contact surfaces of the flat and counterpart ball, which changed the direct sliding between the ball and the flat into a shearing within the transfer layer. For the γ-Y₂Si₂O₇/Si₃N₄ pair, a thin silica hydrate lubricant tribofilm presented above the compressed debris entrapped in the worn track and contact ball surface. This transfer layer and the tribofilm separated the sliding couple from direct contact and contributed to the low friction coefficient and wear rate.     

Friction, wear and N2-lubrication of carbon nitride coatings: a review

Recent results of tribological properties of carbon nitride (CN_x) coatings are reviewed. CN_x coatings of 100 nm thickness were formed on Si-wafer and Si₃N₄ disks by the ion beam mixing method. Friction and wear tests were carried out against Si₃N₄ balls in the environments of vacuum, Ar, N₂, CO₂, O₂ or air by a ball-on-disk tribo-tester in the load range of 80-750 mN and in the velocity range of 4-400 mm/s.It was found that friction coefficient μ is high (μ=0.2-0.4) in air and O₂, and low (μ=0.01-0.1) in N₂, CO₂ and vacuum. The lowest friction coefficient (μ<0.01) was obtained in N₂. It was also found that N₂ gas blown to the sliding surfaces in air effectively reduced the friction coefficient down to μ≈0.017. Wear rate of CN_x coatings varied in the range 10⁻⁹-10⁻⁵ mm³/N m depending on the environment.The wear mechanisms of CN_x in the nanometer scale were studied by abrasive sliding of an AFM diamond pin in air. It was confirmed that the major wear mechanism of CN_x in abrasive friction was low-cycle fatigue which generated thin flaky wear particles of nanometre size.     

Influence of Sliding Speed on the Tribological Characteristics of Si3N4-hBN Ceramic Materials

The influence of sliding speed on the unlubricated tribological behaviors of silicon nitride–boron nitride (Si₃N₄-hBN) composites was investigated with two modes in air by a pin-on-disc tribometer. Using the upper disc–on–bottom pin test mode, as the sliding speed increased, the friction coefficient of the sliding pairs showed an upward trend; for example, from 0.18 at the sliding speed of 0.40 m/s to 0.54 at the sliding speed of 1.31 m/s for the Si₃N₄/Si₃N₄–20% hBN pair. The surface analysis indicated that a tribochemical film consisting of SiO₂ and H₃BO₃ formed on the wear surfaces of the Si3N4/Si3N4–20% hBN sliding pair at sliding speeds of 0.40 and 0.66 m/s. Moreover, the formation of this film lubricated the wear surfaces. At the sliding speed of 1.31 m/s, no tribochemical film formed on the wear surfaces, most likely due to the increase in surface temperature. In the upper pin–on–bottom disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. The increase in sliding speed weakened the degree of abrasive wear, leading to a decrease in the friction coefficients.     

The friction behavior of Ni-, SiO2- and mica sodium silicate-based solid lubrication composites

The friction behavior of Ni⁻-, SiO⁻₂- and mica sodium silicate-based lubricant composites, which included BN, WS₂ and graphite as lubricants, were examined. A ring-on-disk apparatus, in which a solid lubricant composite disk was held against a rotating stainless ring, was used as the test configuration. The tests were run with a load from 62 to 250 N in temperatures from 20 to 800°C in the laboratory environment. The wear surface was characterized by scanning electron microscope and X-ray photo spectroscopy. The major findings were that both mica sodium silicate- and SiO⁻₂-based composites failed at above 500°C due to severe wear and surface damage; in contrast, Ni⁻-based composite showed a stable friction coefficient and low wear from 20 to 800°C.     

Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6

The development of wear surfaces formed during limited debris retention sliding wear of Incoloy MA956 against Stellite 6 between room temperature and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ (7 N applied load, 4522 m sliding distance) were investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to oxidation of Stellite 6-sourced debris and transfer to the Incoloy MA956; this debris separated the Incoloy MA956 and Stellite 6 wear surfaces. Between room temperature and 450 °C, the debris mainly took the form of loose particles with limited compaction, whilst between 510 °C and 750 °C the debris were compacted and sintered together to form a Co–Cr-based, wear protective ‘glaze’ layer. The behaviour was identical to that previously observed on sliding Nimonic 80A versus Stellite 6 at 0.314 m s⁻¹.At 0.905 m s⁻¹, mild oxidational wear was only observed at room temperature and 270 °C and dominated by Incoloy MA956-sourced debris. At 390 and 450 °C, the absence of oxide debris allowed ‘metal-to-metal’ contact and resulted in intermediate temperature severe wear; losses in the form of ejected metallic debris were almost entirely Incoloy MA956-sourced. This severe wear regime was also observed from 510 up to 630 °C, but increasingly restricted to the early stages of wear by development of a wear protective Incoloy MA956-sourced ‘glaze’ layer. This ‘glaze’ layer formed so rapidly at 690 °C and 750 °C, that severe wear was all but eliminated and wear levels were kept low.The behaviour observed for Incoloy MA956 versus Stellite 6 at 0.905 m s⁻¹ contrasts sharply with that previously observed for Nimonic 80A versus Stellite 6, in that the Incoloy MA956-sourced high Fe–Cr debris formed a protective oxide ‘glaze’, whilst the Nimonic 80A-sourced Ni and Cr oxides formed an abrasive oxide that at high sliding speeds assisted wear. The data indicates that the tendency of oxide to form a ‘glaze’ is readily influenced by the chemistry of the oxides generated.