A novel method to prepare self‐lubricity of Si3N4/Ag composite: Microstructure,mechanical and tribological properties

Si₃N₄/Ag composites were firstly prepared through SPS technology, using Si₃N₄ and AgNO₃ as raw materials. Utilizing the coordination bonding of Ag⁺ ions with nitrogen atoms of Si₃N₄, in situ generated Ag particles about 1 μm were tightly anchored on Si₃N₄ surface, thereby preventing the outflow of silver during sintering process. Meanwhile, smaller silver particles about 20 nm were located at the grain boundaries of Si₃N₄, which effectively improved the mechanical and tribological properties of Si₃N₄‐based composites. Finally, the Si₃N₄/Ag composites reinforced by Ag particles showed a friction coefficient of 0.48 ± 0.01, wear rate of 1.79 × 10^?6 mm³ N^?1 m^?1 and fracture toughness of 7.05 ± 0.2 MPa m^1/2, respectively.     

Tribological behavior of ceramic materials (Si3N4, SiC and Al2O3) in aqueous medium

The friction and wear behavior of self-mated Si₃N₄, SiC and Al₂O₃ in water were investigated by varying the test conditions of applied load and sliding speed. It was found that, for self-mated Si₃N₄ and SiC ceramics, the tribochemical reaction resulted in surface smoothening with low friction coefficient at high load and high speed condition. Al₂O₃ shows high friction coefficient, but better wear rate (10⁻¹¹ mm²/N) than other ceramic materials.     

Two-step strategy for improving the tribological performance of Si3N4 ceramics: Controlled addition of SiC nanoparticles and graphene-based nanostructures

The tribological behaviour of silicon nitride (Si₃N₄) ceramics is investigated using a two-step strategy. A set of ceramic composites containing silicon carbide nanoparticles (SiC_n) is developed and, subsequently, graphene-based fillers are added to the Si₃N₄/SiC composite with the best tribological performance. The friction coefficient and the wear rate of Si₃N₄ are reduced up to 22 % and 40 %, respectively, when a 10 vol.% of SiC_n is incorporated into the ceramic matrix due to its improved mechanical response. Si₃N₄/SiC composites containing 11 vol.% of graphene nanoplatelets (GNPs) or reduced graphene oxide sheets (rGOs) are analysed under isooctane lubrication and dry testing. rGOs composite leads to an important decrease of the friction coefficient (50 %) under lubricated conditions, and an enhancement of the wear resistance (44 %) under dry sliding tests, as compared to the reference Si₃N₄/SiC. The best performance of rGOs composite is due to the nature of the lubricating tribofilm and its excellent toughness.     

Friction and wear characteristics of ceramic particle filled polytetrafluoroethylene composites under oil-lubricated conditions

Five kinds of polytetrafluoroethylene (PTFE)-based composites were prepared: PTFE, PTFE + 30 vol % SiC, PTFE + 30 vol % Si₃N₄, PTFE + 30 vol % BN, and PTFE + 30 vol % B₂O₃. The friction and wear properties of these ceramic particle filled PTFE composites sliding against GCr15 bearing steel under both dry and liquid paraffin lubricated conditions were studied by using an MHK-500 ring-block wear tester. The worn surfaces and the transfer films formed on the surface of the GCr15 bearing steel of these PTFE composites were investigated by using a scanning electron microscope (SEM)and an optical microscope, respectively. The experimental results show that the ceramic particles of SiC, Si₃N₄, BN, and B₂O₃ can greatly reduce the wear of the PTFE composites; the wear-reducing action of Si₃N₄ is the most effective, that of SiC is the next most effective, then the BN, and that of B₂O₃ is the worst. We found that B₂O₃ reduces the friction coefficient of the PTFE composite but SiC, Si₃N₄, and BN increase the friction coefficients of the PTFE composites. However, the friction and wear properties of the ceramic particle filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by 1 order of magnitude. Under lubrication of liquid paraffin the friction coefficients of these ceramic particle filled PTFE composites decrease with an increase of load, but the wear of the PTFE composites increases with a load increase. The variations of the friction coefficients with load for these ceramic particle filled PTFE composites under lubrication of liquid paraffin can be properly described by the relationship between the friction coefficient (μ) and the simplified Sommerfeld variable N/P as given here. The investigations of the frictional surfaces show that the ceramic particles SiC, Si₃N₄, BN, and B₂O₃ enhance the adhesion of the transfer films of the PTFE composites to the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of the GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place. Meanwhile, the interactions between the liquid paraffin and the PTFE composites, especially the absorption of liquid paraffin into the surface layers of the PTFE composites, create some cracks on the worn surfaces of the ceramic particle filled PTFE composites; the creation and development of these cracks reduces the load-supporting capacity of the PTFE composites. This leads to the deterioration of the friction and wear properties of the PTFE composites under higher loads in liquid paraffin lubrication. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2611–2619, 1999     

Preparation,friction, wear,and fracture of the Si3N4-Ag-GNPs composites prepared by SPS

Silver and graphene nanoplatelets (Ag-GNPs) have been employed as reinforcements to prepare the self-lubricating silicon nitride matrix composites via 3D ball milling (Turbula) and spark plasma sintering. The prepared composites were characterized by scanning electron microscope with energy dispersive spectroscopy, Vickers hardness tester and reciprocating ball tribometer. Fracture surface morphology of the sintered composites indicated the potential reinforcement by the ductile silver phase. The mechanical property testing revealed that Si₃N₄ composites with Ag and GNPs incorporation exhibited lower hardness and slightly lower toughness compared with Si₃N₄ monolithic material. However, the coefficient of friction and wear in composites exhibited the lower values in 1 N friction force testing range.     

Effect of self-developed graphene lubricant on tribological behaviour of silicon carbide/silicon nitride interface

The research presented in this paper aims to investigate the effectiveness of different surface roughness and lubrication conditions on the interfacial tribological properties between silicon carbide (SiC) and silicon nitride (Si₃N₄) ceramics, particularly for providing insight into the mechanisms of how graphene reduces the friction and wear rate. The worn groove topography and surface composition were characterised in detail with 3D laser measuring microscopy and X-ray photoelectron spectroscopy. The tribological test results on the UMT-TriboLab show that a smooth initial surface is more likely to obtain a low friction coefficient and wear rate under water lubrication. The proper initial surface roughness for SiC and Si₃N₄ ceramics is approximately Ra 10?nm, and it will be lower in an alcohol or graphene aqueous solution. A large load does not worsen the tribological behaviour of a Si₃N₄ ball sliding against a SiC disk, and it reduces the friction coefficient and wear rate. Among the five lubrication states of dry friction, dry graphene lubrication, water lubrication, graphene solution lubrication, and self-developed graphene lubrication, the self-developed graphene lubricant can exhibit an ultra-low friction coefficient of 0.009 and ultra-low wear rate of 1.69?×?10^?7?mm³/N·m. The excellent tribological property of the graphene-coated ceramic surface helps the prepared lubricant to decrease the friction coefficient effectively. Furthermore, the graphene film can protect the SiC from being oxidised by water under the tribo-activated action, and therefore, lead to ultra-low wear rate under low friction condition. Alcohol improves the tribological property of the self-developed graphene lubricant, mainly because of the good wettability between graphene and ethanol. The self-developed graphene lubricant can be applied in water-lubricated ceramic bearings and motorised precision spindles.     

Effect of load on the friction and wear characteristics of Si3N4-hBN ceramic composites sliding against steels

The tribological behaviors of Si₃N₄-hBN ceramic composites sliding against steels (austenitic stainless steel (ASS) and 45 steel) under dry friction conditions at different loads were investigated by using an MMW-1 type vertical universal friction and wear tester. The experimental results showed that the friction coefficients and wear rates first showed a decrease and then an increase with an increase in the load under dry friction conditions. The better tribological performance was exhibited by the SN10/ASS sliding pair under a load of 20 N (the friction coefficient was as low as 0.27 and the wear rates of both pin and disc had a magnitude of 10⁻⁶ mm³ N⁻¹ m⁻¹). This may be attributed to the formation of a black surface film (consisting of B₂O₃, SiO₂, and Fe₂O₃). For the same sliding pair, when the load was 10 N, the dominating wear mechanism was abrasive wear. Hence, the friction coefficient was higher (0.7). When the load increased to 30 and 50 N, the wear mechanism of the SN10/ASS sliding pair was a combination of abrasive and adhesive wears, and higher friction coefficients (0.48 and 0.72 under loads of 30 and 50 N, respectively) were obtained. On the other hand, the contents of hBN also showed a significant impact on the tribological behaviors of the Si₃N₄-hBN/ASS sliding pairs. When the hBN content was less than 10%, the friction coefficients of the Si₃N₄-hBN/ASS sliding pairs decreased with an increase in the hBN content. On the other hand, at hBN contents of 10% or more, the friction coefficients of the sliding pairs increased with an increase in the hBN content. Under the same experimental conditions, the Si₃N₄-hBN/45 steel pairs showed poor tribological properties as compared with the Si₃N₄-hBN/ASS pairs.     

Mechanical and tribological properties of SiC whisker-reinforced SiC composites via oscillatory pressure sintering

SiC whisker (SiCw)-reinforced SiC composites were prepared by an oscillatory pressure sintering (OPS) process, and the effects of SiCw content on the microstructure and mechanical and tribological properties of such composites were investigated. The addition of SiCw could promote the formation of long columnar α-SiC, and the aspect ratio of α-SiC grains first increased and then decreased with the increase of SiCw content. When the SiCw content was 5.42 wt%, the relative density of the SiC–SiCw composite reached up to 99.45%. The SiC–5.42 wt% SiCw composite possessed the highest Vickers hardness, fracture toughness, and flexural strength of 30.68 GPa, 6.66 MPa·m^1/2, and 733 MPa, respectively. In addition, the SiC–5.42 wt% SiCw composite exhibited the excellent wear resistance when rubbed with GCr15 steel balls, with a friction coefficient of .76 and a wear rate of 4.12 × 10⁻⁷ mm³·N⁻¹·m⁻¹. This could be ascribed to the improved mechanical properties of SiC–SiCw composites, which enhanced the ability to resist peeling and micro-cutting, thereby enhancing the tribological properties of the composites.     

Effects of discrete and simultaneous addition of SiC and Si3N4 on microstructural development of TiB2 ceramics

The impact of Si₃N₄ and SiC additives incorporation in the microstructure and sintering behavior of TiB₂-based composites were studied. Three ceramic composites including TiB₂–Si₃N₄, TiB₂–SiC, and TiB₂–SiC–Si₃N₄ were manufactured by spark plasma sintering (SPS) at 1950 °C for 8 min under 35 MPa. The acquired ceramics were analyzed by X-ray diffractometry and scanning electron microscopy. In addition, the sintering thermodynamic was investigated using the HSC Chemistry package. X-ray diffraction patterns of the prepared ceramics revealed the in-situ formation of graphite and boron nitride in the final composites initiated from SiC and Si₃N₄, respectively. The thermodynamic assessments proved the role of liquid phase sintering on the sinterability enhancement of all composite samples. Field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy verified the in-situ formation of both BN and graphite components in the sample containing SiC and Si₃N₄ additives. Finally, the fractographical investigations clarified the transgranular breakage as the main fracture mode in the TiB₂-based ceramics.     

Nano/macro-hardness and fracture resistance of Si3N4/SiC composites with up to 13 wt.% of SiC nano-particles

Relations between composition and mechanical properties of the Si₃N₄/SiC micro/nano-composites were studied by combination of nano-indentation and Vickers indentation techniques. The Si₃N₄/SiC composites were prepared from crystalline Si₃N₄ powder doped with SiNC amorphous precursor and yttria as the sintering aid. During sintering the SiNC precursor crystallised to yield both SiC and Si₃N₄. The in situ formed SiC particles were located both inter- and intra-granularly. The presence of SiC nano-particles enhanced the nano- and macro-hardness, and the fracture toughness of the composites. The nano-hardness of Si₃N₄/SiC composites ranged between 20 and 24 GPa, and depends on the volume fraction of SiC. The nano-hardness of individual Si₃N₄ grains exhibited large scatter as the consequence of the presence of intra-SiC inclusions, which directly influence the measured values as the harder phase, or by generating large thermal stresses within Si₃N₄ grains. Consequently the scatter of nano-hardness was much larger than in case of macro-hardness where the measured values are averaged over large area. The nano-indentation of grain boundaries indicates that the boundaries are much softer than the surrounding matrix phase. Apart of indentation size effect (ISE) this is believed to be an additional reason why the measured values of macro-hardness are lower than the nano-hardness. The maximum fracture toughness (5.8 MPa m^1/2) was achieved for the composite with the total amount of 8 wt.% SiC, where a percolating network of intergranular SiC particles was formed, as indicated by the measurement of electrical resistivity.     

Friction and wear properties of Si3N4-hBN ceramic composites using different synthetic lubricants

This paper presents a tribological investigation of Si₃N₄-hBN composite ceramics using synthetic lubricants. The friction and wear properties of Si₃N₄-hBN ceramic composites sliding against TC4 titanium alloy (Ti6Al4V) were investigated via pin-on-disc tests. An axial compressive load of 10?N was applied with a sliding speed of 0.73?m/s. Three different lubrication conditions including simulated body fluid (SBF), physiological saline (PS) and bovine serum (BS) were used. For SBF lubrication, the friction coefficients and wear rates of Si₃N₄-hBN/Ti6Al4V pairs were varying with the increase of hBN contents. When using 20?vol% hBN, the average friction coefficient and wear rate of Si₃N₄ (0.28 and 3.5?× 10^?4 mm³ N^?1 m^?1) were as good as that of the pure Si₃N₄ (0.34 and 3.69?× 10^?4 mm³ N^?1 m^?1). Meanwhile, the processability of the Si₃N₄ material would be improved by adding hBN. It was worth to mention that when using 30?vol% hBN, the tribological performance of bearing combination deteriorated with extensive wear from the ceramic pin. This may due to the reduction of mechanical property caused by adding hBN and the occurring of tribochemical reaction. According to the worn surface examination and characterization, the main wear mechanism was abrasive and adhesion wear. Scratch grooves were observed on the metal disc, and metallic transform layers were seen on the ceramic pin. Moreover, surface lubrication film consisting of TiO₂, SiO₂·nH₂O, Mg(OH)₂, and H₃BO₃ were formed on the metal disc when using SBF lubrication and 20?vol% hBN content. Among the three lubrication conditions, SBF generally led to the best tribological performance. No surface lubrication film was found during BS and PS lubrications. This may be resulted from the absence of essential ions to promote the formation of surface lubrication film (PS lubrication) and the formation of a protein barrier on the surface of the metal disc (BS lubrication).     

Synthesis of Si3N4–TiN–SiC composites by combustion reaction under high nitrogen pressures

Si₃N₄–TiN–SiC composites were synthesized from TiSi₂ and SiC mixtures via the combustion reaction under high nitrogen pressure. The nitridation mechanism of TiSi₂ was analyzed. The results show that the nitridation of TiSi₂ produced TiN and Si firstly, and Si₃N₄ phase was formed by the further nitriding of Si. The molten eutectic phase and its agglomeration between Si and TiSi₂ formed one core-shell structure and affected the nitridation process. Under higher nitrogen pressure, the nitridation reaction was complete and the relatively dense Si₃N₄–TiN–SiC composites obtained. TEM observation revealed inhomogeneous Si₃N₄ grain size, amorphous phase, cavities, microcracks and dislocations, and graphite from the nitridation of SiC in the microstructure.     

Mechanical and Tribological Properties of a Nano-Si3N4/Nano-BN Composite

The mechanical and tribological properties of a nano-Si₃N₄/nano-BN composite were studied. The composite was prepared via high-energy mechanical milling and subsequent spark plasma sintering. Y₂O₃ and Al₂O₃ were used as sintering additives. After sintering, the average crystalline size of Si₃N₄ and BN was 50 nm. Hardness (Vicker and Knoop) was evaluated under a high load of 0.05–2.0 kg for the nano/nano- and the micro/micro-Si₃N₄/BN composite with the same composition. The indentation fracture toughness values of both composites were also evaluated. Tribological studies were conducted to study the friction and wear behavior of both composites. A friction coefficient of 0.4–0.7 was obtained for the nano-S₃N₄/nano-BN composite under a normal load of 20–22 N, whereas, a friction coefficient of 0.37 was obtained for the micro-Si₃N₄/micro-BN composite. Specific wear coefficients of 0.418 × 10⁻⁴ and 0.625 × 10⁻⁴ mm³/N/m were obtained for nano-sized and micro-sized Si₃N₄/BN composites, respectively. Higher hardness, higher fracture toughness, and lower wear were observed in the nano-sized composite, as compared with the micro-sized composite.     

Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si₃N₄/BN composites. In addition, the fracture toughness of Si₃N₄/micro BN composites was enhanced comparing to monolithic Si₃N₄ because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si₃N₄/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si₃N₄ matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.     

Microstructure and mechanical properties of superplastically deformed silicon nitride–silicon oxynitride in situ composites

Silicon nitride–silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine β-Si₃N₄ and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si₂N₂O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si₃N₄ matrix (∼ 0.30μm in diameter and ∼3.5 in aspect ratio), with the amount of Si₂N₂O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si₂N₂O grains, the texture formation, and the coarsening of β-Si₃N₄. Fracture toughness, however, was still low (∼5.2 MPa m^1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si₂N₂O-matrix interfacial bond and nearly equiaxed β-Si₃N₄ with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.     

Microstructure and properties of silicon nitride-matrix ceramic composites

A study is summed up on the microstructure and mechanical properties of ceramic-matrix composites using silicon nitride, Si₃N₄, as the matrix and SiC whiskers as the reinforcement. It has been found that an Si₃N₄ matrix synthesized by a plasmachemical process and long SiC whiskers (with a length-to-diameter ratio of more than 50) enhance crack resistance, high-temperature strength, and microhardness, whereas an Si₃N₄ matrix produced by self-propagating high-temperature synthesis and short SiC whiskers (with a length-to-diameter ratio of less than 50) enhance thermal endurance. The proposed composites hold promise as candidates for use in engineering applications where wear resistance, thermal endurance, and shock resistance are critical. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 1, pp. 23–26, January, 1998.     

Frictional behavior and wear resistance performance of gradient cermet composite tool materials sliding against hard materials

Gradient cermet composites possessing high surface hardness, flexural strength and interface bonding strength were fabricated using vacuum hot-pressing sintering. Ball-on-disk tests were performed to investigate the tribological properties of the gradient cermet composites against 440 C stainless steel, Al₂O₃ and Si₃N₄ balls at different sliding speed and load in comparison with traditional Ti(C,N) cermets. The tribological behavior was characterized in terms of friction coefficient and wear rate. The results showed that friction coefficient was significantly dependent on the sliding speed and load when sliding against Al₂O₃ and Si₃N₄. However, there was no obvious relation between them during sliding against 440 C stainless steel due to the formation of metal adhesive layer. Gradient cermet composites exhibited a higher friction coefficient but lower wear rate than traditional Ti(C,N) cermets. The main wear mechanism of gradient cermet composites was adhesion wear during sliding against 440 C stainless steel, while abrasion wear was the predominant mechanism during sliding against Al₂O₃ and Si₃N₄. It was expected that gradient cermet composites would be excellent candidates for cutting tool materials.     

High-temperature electromagnetic wave absorption properties of Cf/SiCNFs/Si3N4 composites

Carbon fibers reinforced Si₃N₄ composites with SiC nanofiber interphase (C_f/SiCNFs/Si₃N₄) were prepared by combining catalysis chemical vapor deposition and gel-casting process. Microstructures, mechanical properties, and electromagnetic wave absorption properties within X-band at 25°C-800°C of C_f/SiCNFs/Si₃N₄ composites were investigated. Results show that SiC nanofibers are combined well with Si₃N₄ matrix and carbon fibers, the fracture toughness is thus increased more than double from 3.51 MPa·m^1/2 of the Si₃N₄ ceramic to 7.23 MPa·m^1/2 of the as-prepared composites. As the temperature increases from 25°C to 800°C, C_f/SiCNFs/Si₃N₄ composites show a temperature-dependent complex permittivity, attenuation constant, and impedance. The relatively high attenuation capability of C_f/SiCNFs/Si₃N₄ composites at elevated temperature results in a great minimum reflection loss of −20.3 dB at 800°C with a thin thickness of 2.0 mm. The superior electromagnetic wave absorption performance mainly originates from conductive loss, multi-reflection, and strong polarization formed by the combined effects of carbon fibers and SiC nanofibers.     

Fabrication and wear behaviors of graded Si3N4 ceramics by the combination of two-step sintering and β-Si3N4 seeds

Graded Si₃N₄ ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si₃N₄ seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si₃N₄ phase were detected in the outer layers, and only β-Si₃N₄ phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (～21.2?GPa) was much higher than that of inner layer (～16.1?GPa), whereas fracture toughness of outer layer (～3.5?MPa?m^1/2) was much lower than that of inner layer (～5.9?MPa?m^1/2), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si₃N₄ ceramics exhibited superior wear resistance with low wear rate.     

Sn-containing Si3N4-based composites for adaptive excellent friction and wear in a wide temperature range

Ceramic design based on reducing friction and wear-related failures in moving mechanical systems has gained tremendous attention due to increased demands for durability, reliability and energy conservation. However, only few materials can meet these requirements at high temperatures. Here, we designed and prepared a Sn-containing Si₃N₄-based composite, which displayed excellent tribological properties at high temperatures. The results showed that the friction coefficient and wear rate of the composites were reduced to 0.27 and 4.88 × 10^?6 mm³ N^?1 m^?1 in air at 800 °C. The wear mechanism of the sliding pairs at different temperatures was revealed via detailed analyses of the worn surfaces. In addition, the tribo-driven graphitization was detected on the wear surfaces and in the wear debris, and the carbon phase was identified by SEM, TEM, and Raman spectrum.