Correlation of wear behavior and indentation fracture resistance in silicon nitride ceramics hot-pressed with alumina and yttria

Nine kinds of silicon nitrides with different microstructures were fabricated by controlling both sintering conditions and amounts of sintering additives, Al₂O₃ and Y₂O₃. The wear behavior of the various Si₃N₄ ceramics was investigated in sliding contact test without lubricant. The specific wear rate varied notably from 6 × 10^?6 to 4 × 10^?4 mm³ N^?1 m^?1 depending on the microstructures, whose ranking was difficult to predict directly from the hardness or fracture resistance obtained by the indentation fracture (IF) technique as well as the single-edge-precracked beam (SEPB) method. A good correlation was obtained between the specific wear rate and both mechanical properties when a lateral-crack chipping model was applied as the material removal process. However, the correlation was lost when the fracture toughness obtained by the SEPB method was employed, indicating that the conventional long-crack toughness is inappropriate for analyzing the wear behavior of Si₃N₄ exhibiting a rising R-curve behavior.     

Tribological properties of polymer composites with diamond-like carbon flakes

Composites of epoxy resin with diamond-like carbon (DLC) flakes were fabricated. The DLC flakes were prepared from a DLC film deposited by chemical vapor deposition on an aluminum substrate. The tribological properties of composites were evaluated in air and water environments using a reciprocating friction tester and an AISI 440C mating ball. The friction coefficient of the epoxy composite decreased from 0.90 to 0.69 in air and from 0.71 to 0.29 in water with the addition of DLC flakes. The specific wear rate of the composite also decreased from 5 × 10^? 5 to 7 × 10^? 6 mm³/N m in air and from 4 × 10^? 5 to 4 × 10^? 6 mm³/N m in water. In contrast, the wear of the mating ball increased. Furthermore, the tribological properties of DLC flakes as an additive in water were evaluated. The suspension of powdered DLC in water reduced the friction coefficient of epoxy resin against the AISI 440C mating ball. Furthermore, the wear of the resin was negligibly small, although severe abrasive wear on the mating ball was observed.     

Tribological study of hydrogenated amorphous carbon films with tailored microstructure and composition produced by bias-enhanced plasma chemical vapour deposition

We investigated the mechanical and tribological properties of hydrogenated amorphous carbon (a-C:H) films on silicon substrates by nanoindentation, ball-on-disc tribotesting and scratch testing. The a-C:H films were deposited from an argon/methane gas mixture by bias-enhanced electron cyclotron resonance chemical vapour deposition (ECR-CVD). We found that substrate biasing directly influences the hardness, friction and wear resistance of the a-C:H films. An abrupt change in these properties is observed at a substrate bias of about ?100 V, which is attributed to the bias-controlled transition from polymer- to fullerenelike carbon coatings. Friction coefficients in the range of 0.28–0.39 and wear rates of about 7 × 10^?5 mm³/Nm are derived for the polymeric films when tested against WC–Co balls at atmospheric test conditions. On the other hand, the fullerenelike hydrogenated carbon films produced at ion energies > 100 eV display a nanohardness of about 17 GPa, a strong reduction in the friction coefficient (～ 0.10) and a severe increase in the wear resistance (～ 1 × 10^?7 mm³/Nm). For these films, relative humidity has a detrimental effect on friction but no correlation with the wear rate was found.     

Tribological study of boron nitride films

Boron nitride (BN) films with different cubic and hexagonal phase compositions were deposited on silicon substrates via diamond interlayers by magnetron sputtering and electron cyclotron resonance microwave plasma chemical vapor deposition. The tribological behaviors of the BN films were investigated systematically using a ball-on-disc tribometer with silicon nitride as the counterpart. Comparison studies were also performed on sintered cubic and hexagonal BN compacts. The influence of phase compositions and surface roughness of BN coatings on their tribological characteristics was studied. The cubic BN (cBN) films showed excellent wear resistance against silicon nitride. The wear rate of the cBN films was estimated to be about 1.0 × 10^?7 mm³/N m by measuring the cross-sectional area of the wear track after the sliding test over a distance of 12 km.     

Friction and wear property of a-CNx coatings sliding against ceramic and steel balls in water

The amorphous carbon nitride coatings (a-CN_x) were deposited on Si₃N₄ disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CN_x coatings' hardness was measured by nano-indentation and the friction and wear property of the a-CN_x coatings sliding against Si₃N₄, SiC, Al₂O_3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CN_x coatings contained 12 at.% nitrogen and the major chemical bonding was sp² C = N and sp³C–N. The nano-hardness of the a-CN_x coatings was 29 GPa, higher than those of balls. Among five kinds of tribo-systems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si₃N₄ balls, the largest wear rate of the a-CN_x coating of 1.77 × 10^− 7 mm³/Nm was obtained as sliding against Al₂O₃ ball, while the smallest wear rate of a-CN_x coating of 1.44 × 10^− 8 mm³/Nm was gotten as sliding against Si₃N₄ ball. However, SUJ2 ball showed the highest wear rate of 7.0 × 10^− 7 mm³/Nm, whereas Al₂O₃ ball exhibited the lowest wear rate of ball of 3.55 × 10^− 9 mm³/Nm. The XPS analysis on the worn surface for the a-CN_x coatings displayed that the nitrogen concentration decreased and the sp²-bonding-rich structure was formed after sliding tests in water.     

An efficient method of producing stable graphene suspensions with less toxicity using dimethyl ketoxime

A highly stable graphene suspension has been prepared using dimethyl ketoxime (DMKO) as reductant. Nitrogen was doped into the graphene plane at the same time as the graphene oxide (GO) sheets were reduced. X-ray photoelectron spectroscopy indicated that the C/O ratio of graphene was significantly increased after GO was treated with DMKO and the quantity of nitrogen incorporated into the graphene lattice was 3.67 at.%. The electrical conductivity of the graphene paper was found to be ～10² S m^?1, which was 5 orders of magnitude better than that of GO, and this demonstrated the effective chemical reduction of GO. The mechanism of the chemical reaction of GO with DMKO was also discussed. The as-produced graphene material showed good capacitive behavior and long cycle life with a specific capacitance of ～140 F g^?1.     

Biotribological performance of NCD coated Si3N4–bioglass composites

A nanocrystalline diamond (NCD) coated Si₃N₄–bioglass composite, with potential use for hip and knee joint implants, was tribologically tested in simulated physiological fluids. NCD was deposited using a hot-filament chemical vapour deposition (HFCVD) apparatus in an Ar–H₂–CH₄ gas mixture. Self-mated reciprocating experiments were performed using a pin-on-flat geometry in Hanks' balanced salt solution (HBSS) and dilute fetal bovine serum (FBS). A nominal contact pressure of 25 MPa was applied for up to 500,000 cycles. Very low friction coefficients of 0.01–0.02 were measured using HBSS, while for FBS lubricated tests the values are slightly higher (0.06–0.09), due to a protein attaching effect. AFM assessed wear rates by an approach using the bearing function for volume loss quantification, yielding wear rates of k ∼ 10^− 10 mm³ N^− 1 m^− 1 in HBSS and k ∼ 10^− 9–10^− 8 mm³ N^− 1 m^− 1 for FBS, characteristic of very mild wear regimes.     

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti₃SiC₂ and Ti₃SiC₂/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800 °C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25 °C (RT) to 600 °C (friction coefficient ≈0.61–0.72, wear rate ≈10⁻³ mm³ N m⁻¹). An amount of Pb in the composites played a key role in lubricating with the temperature below 800 °C. The friction coefficient (≈0.22) and wear rate (≈10⁻⁷ mm³ N m⁻¹) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti₃SiC₂/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O₂=2PbO (below 600 °C) and 6PbO+O₂=2Pb₃O₄ (800 °C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.     

Tribological properties of TiB2 and TiB2–MoSi2 ceramic composites

Recently, dense monolithic TiB₂ and TiB₂–20 wt.% MoSi₂ composites with high hardness (24–26 GPa) have been processed by hot pressing. To assess the tribological potential, the present study was performed in analyzing the influence of load on the fretting wear of TiB₂ and TiB₂–MoSi₂ composites against bearing steel. Under the investigated conditions, a higher coefficient of friction (COF) of 0.5–0.6 was recorded with all the materials with a lower COF at a higher load of 10 N. Detailed microstructural investigation of the worn surfaces was carried out using SEM–EDS and XRD in order to understand the fretting wear mechanisms. Severe wear (order of 10⁻⁵ mm³/N m) was measured for the investigated materials under the selected fretting conditions with lower wear rate for TiB₂–20 wt.% MoSi₂ composite at all loads (2–10 N). While abrasive wear dominates the material removal process in the case of monolithic TiB₂, the tribochemical wear is observed to be the predominant wear mechanism for the composite.     

Parametric study of intrinsic thermal transport in vertically aligned multi-walled carbon nanotubes using a laser flash technique

The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films synthesized by thermal chemical vapor deposition was measured by a laser flash technique, and shown to be ～30 mm² s^?1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT films and the individual CNTs were ～27 and ～540 W m^?1 K^?1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube–tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube–tube interaction in dense VACNT films decreased the effective thermal conductivity of the individual CNTs in the films. The tip-to-tip contact resistance was shown to be ～1 × 10^?7 m² K W^?1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.     

Wear behavior of graphene/alumina composite

In the present work, the dry sliding behavior of a graphene/alumina composite material was studied against alumina in air. The tests were carried out in a reciprocating wear tester with an applied load of 20 N, a sliding speed of 0.06 m s⁻¹ and a sliding distance of up to 10 km. Under the testing conditions, the graphene/ceramic composite showed approximately half the wear rate and a 10% lower friction coefficient than the monolithic alumina. It has been found that this behavior is related to the presence of graphene platelets adhered to the surface of friction that form a self-lubricating layer which provides enough lubrication in order to reduce both wear rate and friction coefficient, as compared to the alumina/alumina tribological system.     

Nanoindentation and tribology of VC,NbC and ZrC refractory carbides

Deformation, damage and wear characteristics of spark plasma sintered VC, NbC and ZrC refractory carbides have been investigated using nanoindentation, tribology and micro/macro − indentation tests. Fractography, using SEM, AFM and confocal microscopy, was used for the characterization of deformation and damage mechanisms. Considerable indentation load − size effect was found in all systems with hardness values from 30 to 36 GPa to 13–17 GPa corresponding to the applied loads of 1 mN and 100 N, respectively. During nanoindentation, characteristic stress-drops were observed on hardness-displacement profiles of NbC and ZrC at depth region of 15–30 nm while this was not typical in VC. The highest coefficient of friction was measured for NbC with a value of 0.45 and the lowest for ZrC with an average value of 0.3. The wear rate of the NbC, and VC was similar, approximately 3 × 10⁻⁶ mm³/Nm only the wear rate of ZrC was larger, approximately 2 × 10⁻⁵ mm³/Nm.     

C60/graphene/g-C3N4 composite photocatalyst and mutually- reinforcing synergy to improve hydrogen production in splitting water under visible light radiation

g-C₃N₄ as a new metal-free photocatalytic material for water splitting has attracted much attention in recent years, but its photocatalytic efficiency needs further improvement. Here we synthesized novel C₆₀/graphene/g-C₃N₄ composite photocatalytic materials with high hydrogen generation ability for water splitting under visible light radiation (λ>420 nm). These materials take full advantage of the electron conduction expressing of graphene and the superior-strong electron-attracting ability of C₆₀. The mutually-reinforcing synergy between graphene and C₆₀ improves the migration and utilization efficiency of photo-generated electrons and accelerates the separation of photo-generated charges, thus significantly enhancing the hydrogen generation capacity of g-C₃N₄. The hydrogen production amount and rate of C₆₀/graphene/g-C₃N₄ (10 mg/L C₆₀ and graphene) after 10 h are 5449.5 µmol/g and 545 µmol/g/h, which is 539.6 times of pure g-C₃N₄ under the same condition. The values are 50.8 and 4.24 times of graphene/g-C₃N₄ (10 mg/L graphene) and C₆₀/g-C₃N₄ (10 mg/L C₆₀), respectively. The apparent quantum yield of C₆₀/graphene/g-C₃N₄ (10 mg/L C₆₀ and graphene) in 97 h is about 7.2%. The improvement of hydrogen generation activity in 97 h suggests the high long-time stability of C₆₀/graphene/g-C₃N₄ in photocatalytic water spitting. The photocatalytic ability of C₆₀/graphene/g-C₃N₄ can be controlled by regulating the addition of graphene and C₆₀. The mutually-reinforcing synergy between graphene and C₆₀ was proved by X-ray photoelectron spectroscopy, photoluminescence spectrum and organic electron acceptors of MV²⁺. Thus, the joint action of C₆₀ and graphene promotes the migration, separation and utilization of photo-generated electrons, which is responsible for the significant enhancement of photocatalytic performance.     

Grand canonical Monte Carlo simulations of nitrogen adsorption on graphene materials with varying layer number

Fabrication of monolayer graphene is a challenge and many processes yield few-layer or multi-layer graphene materials instead. The layer number is an important property of those materials and a quality control variable in graphene manufacture. We demonstrated that N₂ adsorption on graphene materials was used to distinguish its layer number. We performed grand canonical Monte Carlo simulation of N₂ adsorption on graphene materials with 1–10 layers to indicate the possibility of distinction of layer number by evaluating the dependence of N₂ adsorption characteristics on the layer number of graphene materials as well as the adsorption mechanism. The threshold relative pressures of monolayer adsorption of N₂ on monolayer and two-layer graphene were 1 × 10⁻³ and 2 × 10⁻⁴, respectively, while those of the others were 1 × 10⁻⁴. In contrast, the threshold pressures of second layer adsorption of N₂ were similar to each other. The difference of threshold pressures is attributed to stabilized energies induced by interactions with graphene materials. Therefore, the layer number of graphene materials could be evaluated from the threshold pressures of adsorption, providing a guide to aid fabrication of graphene materials.     

Densification and grain growth during sintering of porous Ce0.9Gd0.1O1.95 tape cast layers: A comprehensive study on heuristic methods

The sintering behavior of porous Ce_0.9Gd_0.1O_1.95 (CGO10) tape cast layers was systematically investigated to establish fundamental kinetic parameters associated to densification and grain growth. Densification and grain growth were characterized by a set of different methods to determine the dominant sintering mechanisms and kinetics, both in isothermal and at constant heating rate (iso-rate) conditions. Densification of porous CGO10 tape is thermally activated with typical activation energy which was estimated around 440–470 kJ mol^?1. Grain growth showed similar thermal activation energy of ～427 ± 22 kJ mol^?1 in the temperature range of 1100–1250 °C. Grain-boundary diffusion was identified to be the dominant mechanism in porous CGO10 tapes. Grain growth and densification mechanism were found strictly related in the investigated temperature range. Porosity acts as a grain growth inhibitor and grain boundary mobility in the porous body was estimated around 10^?18–10^?16 m³ N^?1 s^?1 at the investigated temperature range.     

Effects of MgO addition on the microwave dielectric properties of high thermal-conductive silicon nitride ceramics sintered with ytterbia as sintering additives

1 mol% of MgO was added together with 7 mol% of Yb₂O₃ as sintering additives to silicon nitride powder to fabricate advanced silicon nitride ceramics with both high thermal conductivity and low dielectric loss at 2 GHz. The mixed powder was CIPed at a pressure of 120 MPa and was gas-pressure sintered at 1900 °C to >98% of theoretical density. The sintered Si₃N₄ sample exhibited a high thermal conductivity of ～100 W m^?1 K^?1 and a loss tangent (tan δ) of ～4 × 10^?4, concurrently. The tan δ was further reduced by half after the heat treatment at 1300 °C for 24 h. The improvement in tan δ due to the annealing was explained from the point of crystallization of the intergranular glassy phase.     

Synthesis of large-area graphene on molybdenum foils by chemical vapor deposition

We report the synthesis of large-area graphene films on Mo foils by chemical vapor deposition. X-ray diffraction indicates that the dissolution and segregation process governs the growth of graphene on Mo foils. Among all processing parameters investigated, the cooling rate is the key one to precisely control the thickness of graphene film. By optimizing the cooling rate between 1.5 and 10 °C/s, we managed to achieve graphene films ranging from mono- to tri-layer. Their uniformity and thickness were confirmed by Raman spectroscopy and optical measurements. The carrier mobility of films reaches as high as 193 cm² V^?1 s^?1. Our experiments show that the Mo substrate has the similar simplicity and large tolerance to processing conditions as Cu.     

One step synthesis of nanoparticles of cobalt in a graphitic shell anchored on graphene sheets

Cobalt core/graphite shell nanostructures anchored on graphene sheets have been prepared by a chemical vapor deposition process. Transmission electron microscope images show that the cobalt nanoparticles (10–30 nm) are encapsulated by a graphitic shell with a thickness of ～3 nm. Magnetic measurements reveal that the material has a typical ferromagnetic behavior with a saturation magnetization of 71.7 emu g^?1 at room temperature.     

Synthesis of a graphene–carbon nanotube composite and its electrochemical sensing of hydrogen peroxide

Graphene, whose structure consists of a single layer of sp²-hybridized carbon atoms, provides an excellent platform for designing composite nanomaterials. In this study, we have demonstrated a facile process to synthesize graphene–multiwalled carbon nanotube (MWCNT) composite. The graphene–MWCNT composite material is endowed with a large electrochemical surface area and fast electron transfer properties in Fe(CN)₆^3?/4? redox species. A graphene–MWCNT composite modified electrode exhibits good performance in terms of the electrocatalytic reduction of H₂O₂; a sensor constructed from such an electrode shows a good linear dependence on H₂O₂ concentration in the range of 2 × 10^?5 to 2.1 × 10^?3 mol L^?1. The detection limit is estimated to be 9.4 × 10^?6 mol L^?1. This study provides a new kind of composite modified electrode for electrochemical sensors.     

Friction and wear mechanisms of K2O–B2O3–Al2O3–SiO2–MgO–F glass-ceramics

Among various brittle materials, the mica-based glass-ceramics are of greater scientific interest, because of their machinability. Considering the potential of these materials as dental implants, an understanding of the wear behavior in oral environment is important. In the present investigation, K₂O–B₂O₃–Al₂O₃–SiO₂–MgO–F glass-ceramics containing about 70% crystals, heat treated at 1040 °C for 12 h was subjected to fretting against steel ball in artificial saliva (AS) environment. In order to elucidate the influence of environment on the friction and wear behavior, control experiments were also performed under dry ambient conditions. A systematic decrease in wear rate with test duration was recorded with a minimum wear rate of 10^?5 mm³/Nm after 100,000 fretting cycles in AS medium. Scanning electron microscope–energy dispersive spectroscopy (SEM–EDS) analysis indicated the formation and brittle fracture of tribochemical layer in AS medium, whereas mica crystal pull-out was a dominant mechanism in dry conditions.