Tribological Characteristics of α-Alumina in High-Temperature Water

The effects of temperature, sliding speed, and contact load on the tribological behavior of an α-alumina ceramic sliding on the same material in water were investigated in the range from room temperature to 300°C under the corresponding saturated vapor pressures. The specific wear rate increased remarkably at elevated temperatures. The primary wear mechanisms in high-temperature water are considered to be microfracture, promoted by the solution of grain boundary layers, and stress corrosion cracking. A film containing γ-alumina particulates appeared on the worn surface with increasing sliding speed and increasing contact load in 300°C water.     

Tribological Characteristics of Si3N4 Ceramic in High-Temperature and High-Pressure Water

The effects of sliding speed and dissolved oxygen on the tribological behavior of Si₃N₄ sliding on itself in water were investigated at room temperature and at 120°C saturated vapor pressure. The friction coefficients and specific wear rates at 120°C were much larger than those at room temperature and had a minimum at about 0.4 m/s, whereats -the specific wear rate of the disk increased with increasing the sliding speed. The wear rate at lower sliding speeds in water at 120°C is considered to be primarily controlled by the increase of the contact stress on the asperities which are formed by the dissolution of grain boundaries of the Si₃N₄ ceramic and the subsequent dissolution of the silica layer of the reaction product However, the wear rate at higher sliding speeds is governed by the direct oxidation and microfracture of the Si₃N₄ substrate. The tribochemical reaction to produce NH₃ mainly occurred at all sliding conditions in water at room temperature and 120°C, and the reaction to produce H₂ gas appeared slightly only at the sliding speeds above 0.4 m/s at 120°C. The tribological behavior was independent of dissolved oxygen concentration for all sliding conditions in water at room temperature and 120°C.     

滑动速率对La2O3–MoSi2与SiC摩擦副的高温摩擦磨损行为的影响

在XP-5高温摩擦磨损试验机上考察了La2O3–MoSi2与SiC摩擦副在1 000℃、30 N载荷以及不同滑动速率下的摩擦磨损行为。利用扫描电子显微镜和X射线衍射仪分析了La2O3–MoSi2复合材料和SiC的磨损表面形貌与相组成。结果表明:La2O3–MoSi2与SiC摩擦副的摩擦因数随滑动速率的增加而减小,在滑动速率为0.084 m/s时,La2O3–MoSi2复合材料磨损率最大;0.126 m/s时磨损率最小。其磨损机理除氧化磨损之外,还表现为黏着磨损、研磨和疲劳点蚀。SiC的磨损率随滑动速率的增加而减小,始终表现为磨损质量增加,这归因于氧化质量增加大于磨损质量损失。     

Temperature Dependence of the Electrical Properties and Seebeck Coefficient of AlN–SiC Ceramics

AlN–SiC ceramics composed of AlN–SiC solid solutions were fabricated by pressureless sintering without sintering additives. The microstructure and electrical properties of the AlN–SiC ceramics were investigated for compositions between 0 and 75 mol% AlN. The AlN–SiC ceramics had a porous structure, and a 2H polytype was found in all compositions. The electrical conductivities and Seebeck coefficients of the AlN–SiC ceramics increased with temperature. The electrical conductivity of 25 mol% AlN–75 mol% SiC ceramics was the highest in all compositions: 32.7 S/m at 300°C. In contrast, the electrical conductivity of 75 mol% AlN–25 mol% SiC ceramics was much lower than that of other samples: 10⁻² S/m at 300°C. The Seebeck coefficient of 50 mol% AlN–50 mol% SiC ceramics was the highest of all samples: 210 μV/K at 300°C. The electrical and thermoelectrical properties of SiC can be controlled by the formation of AlN–SiC solid solutions.     

Wear of Some Advanced Ceramics under a Sharp Indenter in Unidirectional Sliding

The wear of reaction-bonded SiC (RSSiC), dense silicon nitride (DSN), tetragonal zirconia ZrO₂ ( t ), and alumina-titanium nitride (Al₂O₃–TiN) composites have been studied under a sliding diamond indenter having an included angle of 90° and a tip radius of 6 μm at a sliding speed of 8 mm/s. The relative wear resistance as measured from the track diameter was in the order DSN > Al₂O₃–TiN > RSSiC > ZrO₂( t ). The track width showed a straight-line relationship with the square root of load, implying that lateral cracking was responsible for the wear. The wear rates decreased with load and varied from 1 × 10⁻⁵ to 2 × 10⁻⁵mm/(m.g) for the above ceramics. The wear rate increased sharply as the velocity increased over 1 m/min at a load of 300 g and above due to frictional heat generated because of the work of sliding.     

Creep Behavior of a SiC-Whisker-Reinforced Alumina

Creep tests in four-point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20-vol%-SiC-whisker-reinforced alumina and unreinforced single-phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain-boundary sliding by the SiC whiskers.     

Investigation of high temperature wear resistance of TiCrAlCN/TiAlN multilayer coatings over M2 steel

In this study, TiCrAlCN/TiAlN multilayer coatings were deposited on M2 high speed steel substrates by the Closed-Field Unbalanced Magnetron Sputtering system. The chemical composition, microstructure, morphology, mechanical and high temperature wear resistance properties of the coatings were characterized, analyzed and compared to the substrate. The high temperature wear tests were carried out under a load of 2 N at the lap (wear test distance) of 50 m and in dry sliding condition at Room Temperature (RT), 150, 300, 450, and 600 °C on atmospheric conditions. It has been found that the TiCrAlCN/TiAlN multilayer coatings have a higher wear resistance than the M2 substrate. The stable friction behavior and low friction tendency was determined at 600 °C. When the test temperature increased, the wear rates decreased. Narrow and smooth wear tracks and also the lowest wear rate were obtained at 600 °C.     

Effect of Grain Size on the Sliding Wear and Friction of Alumina at Elevated Temperatures

The sliding friction and wear of three different grain-size aluminas were studied from room temperature through 1000°C. The coefficient of friction revealed two distinct regions of decrease with increased temperature, with a transition at ∼700°C. Below 700°C, the coefficient of friction decreased rapidly with increased temperature (∼10^-3/°C). However, above 700°C, the decrease was more gradual (∼10^-5/°C). This was believed to be related to a brittle-to-ductile transition at the wear surface. The coefficient of friction was only weakly dependent on grain size, because the largest grain sizes exhibited slightly higher friction coefficients. However, the specific wear loss of the aluminas increased with increased grain size at room temperature and at 600°C, both below the 700°C transition. The primary mechanism of wear was ascertained to be brittle microfracture along grain boundaries. At 1000°C, above the 700°C transition, the specific wear loss was significantly decreased and appeared to be independent of the alumina grain size. At 1000°C, the wear surfaces developed a thin layer of fine grains formed by dynamic recrystallization. The grain size within the thin layer was in agreement with the previously reported grain-size/Zener-Hollomon parameter relationship.     

Tribooxidational Effects on Friction and Wear Behavior of Silicon Nitride/Tool Steel and Silicon Nitride/Gray Cast Iron Contacts

Unlubricated pin-on-disk wear tests of Si₃N₄ against tool steel and gray cast iron were performed at 5 N of normal load, 0.5 m/s of sliding speed, and environmental temperature in the range 22°-600°C. The friction coefficient of Si₃N₄ sliding against tool steel and gray cast iron had maximum values of 0.88-0.98 for tests at 100°C. The friction coefficient of Si₃N₄ sliding against gray cast iron couples had minimum values of 0.48-0.57 at 400°C. Because of the increased third-body protection, the wear coefficient of the Si₃N₄ pins of the Si₃N₄/gray cast iron couples decreased by 1 order of magnitude from 1.6 10^-5 mm³/(Nm) at room temperature to 1.3 10^-6 mm³/(Nm) at 600°C. Fe₂O₃ and Fe₃O₄ resulting from tribooxidation of the metallic disks were the main constituents of the wear debris and adherent tribolayers. Activation energy values (6.3-13.7 kJ/mol) were comparable to those of oxidation wear of steel (7.3-11.8 kJ/mol) but were much lower than the activation energy for oxidation of iron alloys in static conditions. Calculations of the activation energy of the oxidation wear corroborate the morphological observations of a sacrificial action of the metallic surface protecting the ceramic material.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Effect of oxide and carbide nanoparticles on tribological properties of phenolic-based nanocomposites

The phenolic-based composites and components are widely used because of their excellent thermal, tribological and mechanical behaviors. In the present study, phenolic resin composed of hexamine, novalac, furfural, and furfuryl alcohol has been used. The effects of two carbide nanoparticles (SiC and TiC) and two oxide nanoparticles (TiO₂ and ZrO₂) on the tribological properties of phenolic resin were experimentally investigated. This paper intends to identify the effects of different fillers, fraction of particles and normal load on wear rate and coefficient of friction in dry sliding wear of phenolic-based nanocomposites against hard metal. The proportions of fillers were 0.5, 1 and 2?vol% and experiments were carried out under 40, 50, 60 and 70?N loads and at 0.2?m/s speed. The fillers were mixed with phenolic resin and molded in the form of a cylinder (8.5?mm diameter?×?25?mm height). The samples were cured at 135?°C with a special heating cycle. The wear tests were performed on pin-on-disk testing apparatus at ambient temperature. The composite pins were tested in dry sliding against carbon steel disk. The worn surfaces of samples have been investigated by SEM and the effects of nanometer particles showed different wear mechanisms. Observations showed that carbide particles have better enhancing effect on tribological properties of phenolic resin as compared to the oxide particles. Nanocomposites with SiC particles showed the best tribological properties among the investigated samples. The optimal content of SiC nanoparticles were 1?vol%.     

Spontaneous Infiltration of Iron Silicides into Silicon Carbide Powder Preforms

The infiltration of liquid Fe₃Si (mp of ∼1300°C), Fe₅Si₃ (mp of ∼1210°C), and FeSi (mp of ∼1410°C) into SiC powder preforms was performed at various infiltration temperatures for 60 min under either argon flow or dynamic vacuum. The amount of infiltration under various infiltration conditions was studied as a function of infiltration temperature. For the preforms as-pressed from raw SiC powder, the amount of infiltration of the three silicides under argon flow was independent of their melting points, but suddenly increased within a common temperature range from 1450° to 1550°C. Thermodynamic analyses indicated that the common temperature range corresponded to the temperature at which the SiO₂ on the surface of the SiC particles was decreased under argon flow. Infrared spectroscopy showed SiO₂ on the surfaces of as-received SiC powder particles, but not on the surfaces of the SiC powder particles fired under argon at 1600°C. The amount of infiltration of the as-pressed SiC under vacuum and of fired SiC under argon and vacuum exhibited an obvious dependence on the silicide melting points. This was attributed to the SiO₂ reduction taking place at temperatures lower than the melting points of the silicides. The amount of infiltration was then controlled by the melt viscosity.     

Flexural Creep Deformation of ZrB2/SiC Ceramics in Oxidizing Atmosphere

Flexural creep of ZrB₂/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2-μm SiC, respectively. The stress exponent was about 2 for ZrB₂ containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB₂ containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain-boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.     

Cyclic-Fatigue Behavior of SiC/SiC Composites at Room and High Temperatures

Tension-tension cyclic-fatigue tests of a two-dimensional-woven-SiC-fiber-SiC-matrix composite (SiC/SiC) prepared by chemical vapor infiltration (CVI) were conducted in air at room temperature and in argon at 1000°C. The cyclic-fatigue limit (10⁷ cycles) at room temperature was ∼160 MPa, which was ∼80% of the monotonic tensile strength of the composite. However, the fatigue limit at 1000°C was only 75 MPa, which was 30% of the tensile strength of the composite. No difference was observed in cyclic-fatigue life at room temperature and at 1000°C at stresses >180 MPa; however, cyclic-fatigue life decreased at 1000°C at stresses < 180 MPa. The fracture mode changed from fracture in 0° and 90° bundles at high stresses to fracture mainly in 0° bundles at low stresses. Fiber-pullout length at 1000°C was longer than that at room temperature, and, in cyclic fatigue, it was longer than that in monotonic tension. The decrease in the fatigue limit at 1000°C was concluded to be possibly attributed to creep of fibers and the reduction of the sliding resistance of the interface between the matrix and the fibers.     

Reactive Synthesis and Phase Stability Investigations in the Aluminum Nitride–Silicon Carbide System

The effect of AlN on the structure formation of SiC was investigated. SiC was synthesized in the presence of AlN under vacuum at 1500°C, and the result was cubic SiC. The synthesis of AlN–SiC composites through the reaction Si₃N₄+ 4Al + 3C = 3SiC + 4AlN was also investigated and compared with synthesis via field-activated self-propagating combustion (FASHS). Reactants were heated in a vacuum furnace at temperatures ranging from 1130° to 1650°C. Below 1650°C, the reaction is not complete and at this temperature the product phases are AlN and cubic SiC. At 1650°C, the product contained an outer layer which contained β-SiC only and an inner region which contained AlN and cubic SiC. 2H-SiC and AlN composites synthesized via field-activated self-propagating combustion were annealed at 1700°C under vacuum. The AlN dissociated and evaporated and the 2H-SiC transformed to the cubic β phase. Reasons for the differences in products of furnace heating and FASHS are discussed.     

Lifetime-Applied Stress Response in Air of a SiC-Based Nicalon-Fiber-Reinforced Composite with a Carbon Interfacial Layer: Effects of Temperature (300° to 1150°C)

The lifetimes in air as a function of applied flexure stress and temperature (300–1150°C) are described for a Si–O–C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (∼7% closed porosity) with an ∼0.3 µm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent "threshold stress" (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pullout. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.     

Microstructural effects on the sliding-wear resistance of pressureless liquid-phase-sintered SiC under diesel fuel

The sliding wear of pressureless liquid-phase-sintered (PLPS) SiC ceramics under diesel fuel lubrication was investigated. It was found that the sliding-wear behaviour under diesel fuel is similar to what is observed under lubrication with the chemically similar paraffin oil: initial mild, plasticity-controlled wear followed by severe, fracture-controlled wear, with a well-defined wear transition. Compared to paraffin oil, however, diesel fuel lubrication decreases the extent of the mild-wear regime and results in damage that is more severe due to its lower viscosity. Also, it was found that the wear resistance under diesel fuel decreases with increasing contact load and sliding speed. Subsequent investigation of the effects of the intergranular phase content, grain size, and grain shape demonstrates nevertheless that the wear performance of PLPS SiC under diesel lubrication can be improved via microstructural control by (i) decreasing the intergranular phase content, and (ii) decreasing the SiC grain size or increasing its aspect ratio. The results of this work are likely to have relevant implications for the design of wear resistant PLPS SiC ceramic components for diesel engines.     

Crack-Healing Behavior of Si3N4/SiC Ceramics under Cyclic Stress and Resultant Fatigue Strength at the Healing Temperature

Si₃N₄/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm surface length was made on each specimen. The crack-healing behavior under cyclic stress of 5 Hz, and resultant cyclic fatigue strengths at healing temperatures of 1100° and 1200°C, were systematically investigated. The main conclusions are as follows: (1) Si₃N₄/SiC composite ceramics have an excellent ability to heal a crack at 1100° and 1200°C. (2) This sample could heal a crack even under cyclic stress at a frequency of 5 Hz. (3) The crack-healed sample exhibited quite high cyclic fatigue strength at each crack-healing temperature, 1100° and 1200°C.     

Influence of SiC ceramic reinforcement size in establishing wear mechanisms and wear maps of ultrafine grained AA6063 composites

This paper described the influence of varying sizes of SiC ceramic reinforcement (coarse (12 μm), fine (1 μm) and nano (45 nm)) in establishing the wear mechanisms and the wear mechanism maps of ultrafine grained (UFG) AA6063/4 wt%SiC composites. The UFG composites were developed by a hybrid manufacturing route of stir casting and cryorolling. The pin-on-disc machine under normal loads of 10–50 N and sliding speeds of 0.5–2 m/s was used to investigate wear behavior. The synergetic effect of cryorolling and varying ceramic reinforcement size has resulted in microstructural modification and has reducing effect on specific wear rate. Microstructural characterization revealed that, cryorolling has accumulated high dislocation density and arrested recovery and recrystallization at liquid nitrogen temperature. Frictional heat generated at wear surface with increase in sliding speed and load has activated the dynamic recovery, recrystallization and precipitation which further increased the hardness and reduced the specific wear rate. The wear mechanisms were established for UFG materials. The wear mechanism maps were constructed and correlated with the microstructures of worn surfaces of UFG materials to identify the dominant wear mechanism.     

Crack-Healing Behavior Under Stress of Mullite/Silicon Carbide Ceramics and the Resultant Fatigue Strength

Mullite/SiC composite ceramics were sintered and subjected to three-point bending of specimens made according to the appropriate JIS standard. A semicircular surface crack 100 to 250 μm in diameter was made on each specimen. We systematically studied crack-healing behavior and cyclic- and static-fatigue strengths at room temperature and 1000°C (crack-healing temperature) by using three types of specimens (smooth, cracked, and crack-healed). The main conclusions are as follows: (i) mullite/SiC composite ceramics have the ability to heal after cracking; (ii) crack-healed specimens exhibited higher static and fatigue strengths than those of smooth specimens, which was caused by crack-healing; (iii) a sample crack-healed at 1000°C had a high fatigue strength at 1000°C; and (iv) mullite/SiC ceramics can heal a crack under stress at 1000°C, and this behavior was considered using crack-driving force and crack-healing force, qualitatively.