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.     

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).     

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.     

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.     

Wear of aligned silicon nitride under dry sliding conditions

The wear behaviour of textured silicon nitride (Si₃N₄) ceramics with aligned microstructures was analyzed under abrasive wear conditions. Dry reciprocating self-mated ball-on-flat wear tests were performed to study the influence of different microstructural plane/orientation combinations on the Si₃N₄ tribological behaviour. Textured materials showed superior wear resistance than non-textured reference Si₃N₄ for the whole range of loads and contact pressures, 5–50 N and 1.7–3.6 GPa, respectively, with an increase of about 70% for the maximum applied load. Within textured materials, the plane perpendicular to the extruding direction exhibited a 50% higher wear resistance (4 × 10^?5 mm³ N^?1 m^?1) than the parallel plane where the elongated grains were aligned. The severe wear process involved debonding, fracture and debris formation mechanisms. The progress of this sequence depended on the particular microstructure of each plane/orientation combination. A relationship between abrasive wear resistance and selected microstructural parameters has been established.     

Polymer Jet Printing of SU-8 Micro-Dot Patterns on Si Surface: Optimization of Tribological Properties

Tribological properties of optimized SU-8 patterns (micro-dots with varying pitch) on Si (silicon) were evaluated using a ball-on-disk tribometer. Sliding tests on the patterns were conducted against a 2-mm diameter Si₃N₄ ball at varying normal loads and sliding velocities. It was observed that the pitch of the SU-8 pattern on Si substrate had a significant effect on the initial coefficient of friction and wear durability. Initial coefficient of friction studies have concluded that the SU-8 polymeric micro-dots improved the tribological properties by sharing the normal force and reducing the contact area. For the wear durability test, ultra-thin layer of perfluoropolyether was over-coated onto SU-8 micro-dot specimens, and the optimized pitch specimens have shown wear durability of more than 100,000 cycles at a normal load of 350 mN.     

Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces

The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si₃N₄) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation.     

Friction and wear behavior of Ni-P coated Si3N4 reinforced Al6061 composites

Al6061 matrix composite reinforced with nickel coated silicon nitride particles were manufactured by liquid metallurgy route. Microstructure and tribological properties of both matrix alloy and developed composites have been evaluated. Dry sliding friction and wear tests were carried out using pin on disk type machine over a load range of 20-100 N and sliding velocities of range 0.31-1.57 m/s. Results revealed that, nickel coated silicon nitride particles are uniformly distributed through out the matrix alloy. Al6061-Ni-P-Si₃N₄ composite exhibited lower coefficient of friction and wear rate compared to matrix alloy. The coefficient of friction of both matrix alloy and developed composite decreased with increase in load up to 80 N. Beyond this, with further increase in the load, the coefficient of friction increased slightly. However, with increase in sliding velocity coefficient of friction of both matrix alloy and developed composite increases continuously. Wear rates of both matrix alloy and developed composites increased with increase in both load and sliding velocity. Worn surfaces and wear debris was examined using scanning electron microscopy (SEM) for possible wear mechanisms. Energy dispersive spectroscope (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) techniques were used to identify the oxides formed on the worn surfaces and wear debris.     

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.     

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.     

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.     

Tribological Characterization of Several Silicon-Based Materials Under Ionic-Liquids Lubrication

The tribological behaviors of three silicon-based materials (low temperature silicon oxide (LTO), polysilicon (Poly Si) and silicon nitride (Si₃N₄) films) under ion liquids (ILs) lubrication have been investigated by varying the applied load and the sliding velocity. An atomic force microscope and a nanoindentor were used to characterize the deposited films, and the worn surfaces after frictional tests were analyzed by an optical microscope, a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) and an X-ray photoelectron spectrometer (XPS). The results show the best lubricating properties of the IL lubricants are obtained at the intermediate load of 150 g for the three tribo-pairs. For the Si₃N₄/Si₃N₄ tribo-pair, the coefficients of friction (COFs) and the wear rates under ILs lubrication are larger than those under dry friction. The COF values decrease with sliding velocity, and the COFs of the Si₃N₄/Si₃N₄ tribo-pair are greater than the ones of the other two tribo-pairs (LTO/Si₃N₄ and Poly Si/Si₃N₄) at low velocities. The COF of 1-ethyl-3-methylimidazolium tetrafluoroborate is nearly 3 times larger than the other two IL lubricants (1-butyl-2,3-dimethylimidazolium tetrafluoroborate and N-butylpyridinium tetrafluoroborate) for the Si₃N₄/Si₃N₄ tribo-pair. The differences in COFs between the latter two lubricants for the three tribo-pairs are negligible. Different mechanisms of these results have been analyzed in the paper.     

Tribological Behavior of Si3N4-hBN Ceramic Materials without Lubrication under Different Test Modes

Unlubricated tribological behaviors of silicon nitride–boron nitride (Si ₃ N ₄ -hBN) composites were investigated with two test modes in air by using a pin-on-disc tribometer. Under upper-disc-on-bottom-pin test mode, the addition of hBN to Si ₃ N ₄ resulted in a significant decrease of the friction coefficient, from 0.54 for Si ₃ N ₄ against Si ₃ N ₄ to 0.19 for Si ₃ N ₄ -20% hBN against Si ₃ N ₄ . The surface analysis indicated that a tribochemical film consisting of SiO₂ and H ₃ BO ₃ was formed on the wear surfaces. The formation of tribochemical film might be attributed to the embedment of wear debris into the spalling pits on the wear surfaces of Si ₃ N ₄ -hBN specimen. The wear debris reacted with moisture in air, and the resultant tribochemical film lubricated the wear surfaces. Under 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. A slight decrease of the friction coefficient, from 0.85 for Si ₃ N ₄ /Si ₃ N ₄ to 0.56 for Si ₃ N ₄ /Si ₃ N ₄ -30% hBN, was obtained, which might be attributed to the layered structure of hBN.     

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.     

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.     

Ceramic-ceramic composite materials with improved friction and wear properties

Dry friction and wear tests were performed with self-mated couples of SiC containing 50% TiC, Si₃N₄---BN, SiC---TiB₂ and Si₃N₄ with 32% TiN at room temperature and 400°C or 800°C.Under room temperature conditions, the friction coefficient of the couple SiC---TiC/SiC---TiC is only half of that of the couple SiC/SiC and the wear is one order of magnitude smaller. At 400°C, it exceeds the friction coefficient of SiC/SiC except at the highest sliding velocity of 3 m s⁻¹. At lower sliding velocities the wear coefficient of SiC---TiC/SiC---TiC is lower than that of SiC/SiC.The couple Si₃N₄---TiN/Si₃N₄---TiN exhibits high friction coefficients under all test conditions. At room temperature the wear volume of the self-mated couples of Si₃N₄ and Si₃N₄---TiN after a sliding distance of 1000 m is similar, but Si₃N₄---TiN shows a running-in behaviour. At 800°C the wear coefficient of Si₃N₄---TiN/Si₃N₄---TiN is approximately two orders of magnitude smaller than that of Si₃N₄/Si₃N₄, and equal to those at room temperature. At 22°C the addition of BN reduces the friction of Si₃N₄. The wear coefficient is independent of sliding velocity and the self-mated couples showing running-in. Friction and wear increase with increasing temperature. The wear coefficient of SiC---TiB₂ above 0.5 m s⁻¹ at 400°C is advantageously near 10⁻⁶ mm³ (Nm)⁻¹. With the other test conditions the wear behaviour is similar to SSiC.     

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).     

Preparation,microstructure and tribological behavior of laser cladding NiAl intermetallic compound coatings

Nickel aluminide (NiAl) intermetallic compound coatings were in situ synthesized from pre-placed mixed powders of Ni and Al by laser cladding. The phase composition and microstructure of the NiAl coatings were studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The effects of laser cladding parameters on the microstructure and friction and wear behavior of the NiAl coatings were investigated. It has been found that laser power density had a crucial influence on the microstructure and friction and wear behavior of NiAl coatings. Namely, the NiAl coatings synthesized under a lower power density have more dense and fine microstructure, and lower friction coefficient and wear rate. Besides, the friction and wear behavior of the laser cladding NiAl coatings is highly dependent on applied normal load and sliding speed; and the resulting coatings sliding against Si₃N₄ in a ball-on-disc contact mode is more suitable for tribological application at a moderate normal load of 3–7 N and sliding speed of 0.16–0.21 m/s.     

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.