Wear and Wear Transition Mechanism in Silicon Carbide during Sliding

Wear mechanisms in SiC during sliding have been investigated experimentally under paraffin oil lubrication. The wear and friction data indicate that a transition in wear mechanism occurs abruptly after a defined period of sliding. The transition point is reached earlier as load increases. Examination of wear samples reveals that surface material is predominantly removed by a plastic grooving process in the initial stage, and by a grain pull-out process after the transition. It is also observed that severe plastic deformation in the form of dislocations is produced during the sliding. The abrupt occurrence of the grain pull-out after a definite sliding time is discussed in relation to the internal stresses associated with this plastic deformation.     

Plasticity‐Controlled Friction and Wear in Nanocrystalline SiC

Wear resistance of ceramics can be improved by suppressing fracture, which can be accomplished either by decreasing the grain size or by reducing the size of the deformation zone. We have combined these two strategies with the goal of understanding the atomistic mechanisms underlying the plasticity‐controlled friction and wear in nanocrystalline (nc) silicon carbide (SiC). We have performed molecular dynamics simulations of nanoscale wear on nc‐SiC with 5 nm grain diameter with a nanoscale cutting tool. We find that grain‐boundary (GB) sliding is the primary deformation mechanism during wear and that it is accommodated by heterogeneous nucleation of partial dislocations, formation of voids at the triple junctions, and grain pull‐out. We estimate the stresses required for heterogeneous nucleation of partial dislocations at triple junctions and shear strength of GBs. Pile up in nc‐SiC consists of grains that were pulled out during deformation. We compare the wear response of nc‐SiC to single‐crystal (sc) SiC and show that scratch hardness of nc‐SiC is lower than that of sc‐SiC. Our results demonstrate that the higher scratch hardness in sc‐SiC originates from nucleation and motion of dislocations, whereas nc‐SiC is more pliable due to additional mechanism of deformation via GB sliding.     

Wear Properties of Alumina/Aluminum Composites with Interpenetrating Networks

The tribological behavior of an A1₂O₃/AI composite against a steel and an alumina was investigated in pin-on-disk wear tests using unlubricated conditions in air. Various composite compositions of aluminum contents ranging from 0 to 28 vol% were investigated over a variety of contact loads and sliding speeds. Wear rate and friction force were continuously monitored during testing. Following completion of the test, pin weight loss and the profile of the wear tracks were determined. Scanning electron microscopy accompanied by EDAX analysis was used to investigate the worn surfaces and the wear debris. The wear behavior of the composites with a low metal content (<15%) during sliding against steel and alumina was found to be comparable to the wear of pure alumina. Wear occurred either on the steel or apparently simultaneously on the pin and alumina disk. With higher Al contents, wear shifts to the composite, the wear rate increases abruptly and is accompanied by fracture of the A1₂O₃ matrix. Wear appears to occur in the composite when the mechanical strain in the composite, near the contact surface, as a result of frictional loads, exceeds the fracture strain of the alumina matrix.     

Effect of Microstructure on Sliding-Wear Properties of Liquid-Phase-Sintered α-SiC

We have studied, for the first time, the effect of the content of intergranular phase and grain size on the sliding-wear resistance of pressureless, liquid-phase-sintered (LPS) α-SiC (silicon carbide) ceramics. It was found that the sliding-wear behavior of these ceramics is similar to what is observed in other polycrystalline ceramics: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with well-defined wear transition. We have found that the increase in the content of intergranular phase and the grain coarsening leads to the degradation of the sliding-wear resistance in LPS α-SiC ceramics. A mechanistic model is used to rationalize these wear results, and to provide guidelines for the design and fabrication of low cost, highly wear-resistant SiC-based ceramics. This is likely to have important implications because SiC-based materials are being used increasingly in contact-mechanical and tribological applications.     

Grain-Size and R-Curve Effects in the Abrasive Wear of Alumina

Results of sliding wear tests on three alumina ceramics with different grain sizes are discussed in the light of crackresistance (R-curve, or T-curve) characteristics. The degree of wear increases abruptly after a critical sliding period, reflecting a transition from deformation-controlled to fracture-controlled sulface removal. This transition occurs at earlier sliding times for the aluminas with the coarser-grained microstructures, indicative of an inherent size effect in the wear process. A simplistic fracture mechanics model, incorporating the role of internal thermal expansion mismatch stresses in the crack-resistance characteristic, is developed. The results suggest an inverse relation between wear resistance and large-crack toughness for ceramics with pronounced Rcurve behavior.     

莫来石陶瓷的摩擦磨损特性

采用环-块式摩擦副,研究了自相配莫来石陶瓷在不同的介质和载荷下的摩擦磨损特性.试验结果显示,莫来石陶瓷以水为介质时在20N、以机油为介质时在1000N附近存在磨损突变.磨损突变前主要的磨损机理为塑性变形和犁耕;磨损突变后断裂磨损成为主要的磨损机理.机油为介质时,磨损率与载荷基本成线性关系;在以水为介质时,磨损突变前,这种关系更接近于0.40次幂.弹性模量和抗弯强度高、断裂韧性和硬度低的样品,其磨损率高.通过扫描电子显微镜形貌观察,发现在水为介质时,磨损表面被一层反应膜所覆盖.     

Wear and friction properties of spark plasma sintered SiC/Cu nanocomposites

In this study, in order to determine the effect of SiC nanoparticles on tribological properties of nanostructured copper, the dry sliding wear and friction behaviors of nanostructured copper and copper reinforced with silicon carbide nanoparticles, produced by high energy ball milling and spark plasma sintering, were investigated by using an oscillating friction and wear tester under different normal loads. To determine the dominant wear mechanism, the worn surfaces and obtained debris after wear tests were analyzed by scanning electron microscope (SEM). The results showed that the addition of 4 vol% silicon carbide to copper matrix reduced the wear track depth and the coefficient of friction. Investigation of the worn surfaces revealed that SiC nanoparticles on the top of worn surface decreases the plastic deformation in subsurface region and alleviate severe wear. Lower plastic deformation during dry sliding wear test was attributed to high hardness of the nanocomposite that has been resulted from grain growth inhibiting and reinforcing effects of the nanoparticles. Plastic deformation and delamination were determined as major wear mechanisms in both materials.     

Lubricated Rolling and Sliding Wear of a SiC-Whisker-Reinforced Si3N4 Composite against M2 Tool Steel

Mineral oil lubricated rolling and sliding wear of SiC whisker (SiC_w) reinforced Si₃N₄ composite and monolithic Si₃N₄ prepared identically against M2 tool steel were investigated using a cylinder-on-cylinder apparatus. Wear of this Si₃N₄ was higher than that of the composite. Wear of the steel against Si₃N₄ was also higher than that against the composite. Relatively larger scale microfracture occurred in the Si₃N₄ than in the composite; more pullout and microchipping of carbide particles were observed in the steel against Si₃N₄ than against the composite. Polishing of the worn surfaces of the steel occurred in both sliding and rolling tests. This was attributed to fine, hard wear debris circulating in the contact area. Spalling was observed in the steel sliding against Si₃N₄ but not in the steel sliding against the composite.     

Studies on Abrasive Wear of Monolithic Silicon Nitride and a Silicon Carbide Whisker-Reinforced Silicon Nitride Composite

Results presented in this paper demonstrate the roles of the most important parameters that govern abrasive wear of experimental ceramics. SEM studies of the abraded surfaces evidenced two main kinds of failures: microdropping of grain(s) and powder-type wear track(s) resulting from the brittle microfracturing of parts of grain(s). There were whiskers in the surface, and whisker pullouts occurring during wear processes are believed to be the reasons for the lower wear rates of SiCw/Si₃N₄ composite ceramics under experimental conditions.     

Modeling of Grain-Size-Dependent Microfracture-Controlled Sliding Wear in Polycrystalline Alumina

To quantify grain-size-dependent sliding wear of polycrystalline alumina induced by grain-boundary microfracture, an attempt is made to extend and combine Cho et al. 's fracture mechanics analysis and Fu and Evans' simple micro-cracking theory. An analytical equation is derived to relate wear with microstructural parameters. Wear by intergranular microfracture occurs, provided that the combined stresses are greater than a threshold value. The critical sliding time to the wear transition decreases with increasing grain size, and the wear rate after the threshold is proportional to the grain size. The theoretical predictions are correlated with the lubricated sliding wear data of aluminas with different grain sizes reported by Cho et al.     

Evaluation of tribological behavior of silicon carbide and silicon nitride ceramics under different conditions

A comparative analysis of the tribological behavior of commercially available sintered silicon carbide (SiC) and three different types of silicon nitride (Si₃N₄) ceramics have been carried out using the ball-on-disk method in dry and lubrication (deionized [DI] water and ethanol) environment. Scanning electron microscopy (SEM) was used to understand the morphology and chemical composition of the tribo-surfaces. Sintered SiC (Hexoloy-SA) had the highest friction coefficient during dry sliding with an average of ∼0.34. Deionized water showed a minor improvement in friction (∼0.27) while ethanol reduced the friction greatly to ∼0.18 compared to dry sliding. During dry sliding, the presence of an abrasive third body was responsible for the high wear rates (WRs) in these compositions. Hexoloy-SA showed a lower WR during ethanol and DI water lubrication due to the formation of stable tribofilms as well as higher hardness which resisted the formation of third bodies. In comparison, Si₃N₄ samples showed a lower WR in DI water and ethanol. The samples also showed composition-dependent behavior which indicates that grain structure and grain boundary chemistry are playing a vital role in the tribological process.     

Microstructure and wear properties of SiC woodceramics reinforced high-chromium cast iron

Wood ceramization is a promising preparation technology. Ceramics made from natural wood can retain the original structural characteristics and unique microstructure of the wood, and also offer acceptable mechanical properties and wear resistance. In this study, the ceramization process of natural poplar wood is optimized. Three-dimensional silicon carbide prepared by ceramization of wood is used to strengthen high-chromium cast iron, and three-dimensional silicon carbide reinforced high-chromium cast iron (3D-SiC/HCCI) composite materials are obtained. The results demonstrate that the treated wood retained acceptable network structure and uniform pore size after ceramization. Based on the size, the pores can be classified into smaller pores (approximately 8 μm), medium-sized pores (20–40 μm), and larger pores (100–300 μm). The 3D-SiC/HCCI composites, obtained by casting and infiltrating these pore channels, show continuous network-like interpenetrating structures in space. The wear resistance of the 3D-SiC/HCCI composite materials was investigated and compared with high-chromium cast iron. The results indicate that under the same friction condition, the wear resistance of the composites is significantly improved, and the abrasion loss is reduced from 3.8% to 0.32%. The three-dimensional silicon carbide in the composites produces a shadow effect during friction, which can provide acceptable support and protection for the high-chromium cast iron matrix.     

Wear Behavior of a Polymer-Matrix Composite Reinforced with Residues from a Hydrometallurgical Process

The residues from the final hydrometallurgical refining operation from a zinc plant were characterized in respect to their morphology, and were sieved in different granulometric fractions. This waste was incorporated into an epoxy resin, and the wear behavior of the composites was evaluated. The effect of the volume fraction of filler, and of the mean particle size was analyzed and the optimum values of these parameters were determined. Wear mechanisms were evaluated by scanning electron microscopy. With the experimental values obtained for the material with the best wear behavior a case study was performed, and the expendable coating of a conveyor belt used by the cement industry was designed, and the thickness of the composite wear resistant coating layer was evaluated. The results show that the composite developed can be feasible for practical use.     

Mechanical properties and sliding wear behaviour of Al2O3-SiC nanocomposites with 3–20 vol% SiC

Al₂O₃/SiC composites containing different volume fractions (3, 5, 10, 15, and 20 vol%) of SiC particles were produced by conventional mixing of alumina and silicon carbide powders, followed by hot pressing at 1740 °C for 1 h under the pressure of 30 MPa in the atmosphere of Ar. The influence of the volume fraction and size of SiC particles (two different powders with the mean size of SiC particles 40 and 200 nm were used), and final microstructure on mechanical properties and dry sliding wear behaviour in ball-on-disc arrangement were evaluated. The properties of the composites were related to a monolithic Al₂O₃ reference. Microstructure of the composites was significantly affected by the volume fraction of added SiC, with the mean size of alumina matrix grains decreasing with increasing content of SiC particles. The addition of SiC moderately improved the Vickers hardness. Fracture toughness was lower with respect to monolithic Al₂O₃, irrespective of the volume fraction and size of SiC particles. Al₂O₃/SiC nanocomposites conferred significant benefits in terms of wear behaviour under the conditions of mild dry sliding wear. Wear resistance of the alumina reference was poor, especially at the applied load of 50 N. The wear rates of composites markedly decreased with increasing volume fraction of SiC. Wear of the composites was also influenced by the material of counterparts, especially their hardness, with softer counterparts resulting in lower wear rates. All composites wore by a combination of grain pull-out with plastic deformation associated with grooving and small contribution of mechanical wear (micro-fracture). No influence of SiC particle size on wear rate or mechanism of wear was observed in the materials with identical volume fractions of SiC.     

Grinding behavior of biomimetic fractal-branched silicon carbide ceramic inspired from leaf-vein structure

Silicon carbide ceramics are widely used in many industrial fields owing to their outstanding physical and chemical characteristics. However, their inherent hardness and brittleness make the grinding process very difficult compared to that involving ductile materials. In the present study, the effects of the biomimetic fractal-branched structure, inspired from the leaf-vein, on the grinding behavior of silicon carbide were investigated. Two biomimetic fractal-branched structures with different densities of micro-channels were designed and compared with the non-structured silicon carbide surface. The surface of the silicon carbide ceramic was textured through pulsed-laser ablation. Thereafter, the grinding experiment was conducted on the biomimetic fractal-branched and non-structured workpieces. The surface topography, subsurface damage, grinding force, grinding force ratio, surface roughness and grinding wheel wear were examined throughout the experiment. The experimental results indicated that the normal and tangential grinding forces for the fractal-branched structure surface are 7.61–18.21% and 8.34–26.13% lower than those for the non-structured surface. The grinding force ratio also increased significantly with an increase in the micro-channel density. In addition, a larger volume of coolant was transported through the grinding zone of the fractal-branched structure. The research results confirmed that the biomimetic fractal-branched structure on the silicon carbide surface enhanced the grinding performance and improved the grinding quality.     

Grain-Size Dependence of Sliding Wear in Tetragonal Zirconia Polycrystals

Using a pin-on-plate tribometer with the reciprocating motion of SiC against yttria-doped tetragonal zirconia polycrystal (Y-TZP) plates, the friction and wear of Y-TZP ceramics were investigated as a function of grain size in dry N₂ at room temperature. The results showed that the overall wear resistance increased as the grain size of Y-TZP ceramics decreased. For grain sizes ≤0.7 μm, the wear results revealed a Hall-Petch type of relationship ( d ^−1/2) between wear resistance and grain size. In this case, the main wear mechanisms were plastic deformation and microcracking. For grain sizes ≥0.9 μm, the wear resistance was proportional to the reciprocal of the grain diameter. In this regime, delamination and accompanying grain pullout were the main mechanisms. In this case, the phase transformation to monoclinic zirconia had a negative effect on the wear resistance of TZP ceramics. The coefficient of friction tended to be higher for fine-grained TZP-SiC couples than for coarse-grained TZP-SiC couples, whereas, for a specific regime of grain size, the coefficient of friction was almost independent of the grain size.     

The Relationship Between Multiple Scratch Tests and Wear Behavior of Hot-Pressed Silicon Nitride Ceramics with Various Rare-Earth Additive Systems

The wear behavior of Si₃N₄ ceramics sintered with various rare earth additives was studied for nonlubricated sliding under different conditions, and scratch tests carried out in an attempt to correlate the wear behavior. When multiple scratch testing is used the results can be used to indicate the initial wear behavior under fracture-dominated wear of the materials. The additive system used in the sintering of the Si₃N₄ ceramics affected the specific wear rate under nonlubricated sliding conditions, and under high load conditions, where fracture is dominant, the specific wear rate was shown to increase in samples sintered with lutetium as a consequence of a strong bonding strength between the grains and grain boundary resulting in a higher degree of brittle fracture.     

Microstructure evolution process of porous silicon carbide ceramics prepared through coat-mix method

Porous silicon carbide ceramics were prepared through the coat-mix method and molding, carbonization and sintering process with silicon powders and phenolic resin as raw materials. The crystalline phase, microstructure, and porosity of samples treated at different stages were characterized. Results showed that the fabricated porous silicon carbide ceramics consist of pure β-SiC phase with a homogeneous structure and porosity of above 60%. Each of the processing stages, including coat-mix, molding, carbonization, and sintering, has certain contribution to the porosity of the final porous silicon carbide ceramics, in which the mole ratio of resin carbon to silicon and the molding temperature are the main factors to affect the porosity. A porosity evolution process of porous silicon carbide ceramics during fabrication process was also proposed.     

Highly Creep-Resistant Silicon Nitride/Silicon Carbide Nano–Nano Composites

A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/silicon carbide composite with a nano–nano structure (nanosized SiC and Si₃N₄ in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si–C–N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nano-sized SiC included in sub-micron Si₃N₄) to nano–nano type. Nanocrystalline silicon nitride/silicon carbide ceramic composite with 30–50 nm grain size was synthesized without using sintering additive.     

Oxygen penetration into silicon carbide ceramics during oxidation

The penetration of oxygen into polycrystalline silicon carbide ceramics, in advance of the oxide/substrate interface, during oxidation for 1–100 hrs at 1200–1400°C was studied using SIMS and TEM techniques. Fully dense hot pressed ceramics containing aluminum additives, with and without an oxide grain boundary phase and CVD silucon carbide exhibited sharp interfaces. Sintered silicon carbides with boron and carbon additives (～ 97% dense) and aluminum carbide additive (～ 90% dense) exhibited a region of oxygen penetration ～10–15 μm in depth beneath the oxides scale, the depth of which was insensitive to the time and temperature of oxidation. The amorphous oxide phase in this zone was located at three and four grain junctions but the two grain junctions were unaffected in this zone by oxidation. This oxygen affected region, which is responsible for the slow crack growth susceptibility of these ceramics after oxydation, results from gaseous oxygen penetration along interconnected or nearly interconnected pores and oxidation of impurity laden channels and SiC surfaces. The depth of penetration is presumably limited by closure of the channels by the oxidation products.