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.     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Changes in chemical bonding of diamond-like carbon films by atomic-hydrogen exposure

We have deposited unhydrogenated diamond-like carbon (DLC) films on Si substrate by pulsed laser deposition using KrF excimer laser, and investigated the effects of atomic-hydrogen exposure on the structure and chemical bonding of the DLC films by photoelectron spectroscopy (PES) using synchrotron radiation and Raman spectroscopy. The fraction of sp³ bonds at the film surface, as evaluated from C1s spectra, increased at a substrate temperature of 400 °C by atomic-hydrogen exposure, whereas the sp³ fraction decreased at 700 °C with increasing exposure time. It was found that the sp³ fraction was higher at the surfaces than the subsurfaces of the films exposed to atomic hydrogen at both the temperatures. The Raman spectrum of the film exposed to atomic hydrogen at 400 °C showed that the clustering of sp² carbon atoms progressed inside the film near the surface even at such a low temperature as 400 °C.     

The effect of the surface nanostructure and composition on the antiwear properties of zirconia–titania coatings

This study describes the preparation, surface imaging and tribological properties of titania coatings modified by zirconia nanoparticles agglomerated in the form of island-like structures on the titania surface. Titania coatings and titania coatings with embedded zirconia nanoparticles were prepared by the sol–gel spin coating process on silicon wafers. After deposition the coatings were heat-treated at 500 °C or 1000 °C. The natural tendency of nanoparticles to form agglomerates was used to build separated island-like structures unevenly distributed over the titania surface having the size of 1.0–1.2 μm. Surface characterization of coatings before and after frictional tests was performed by atomic force microscopy (AFM) and optical microscopy. Zirconia nanoparticles were imaged with the use of transmission electron microscopy (TEM). The tribological properties were evaluated with the use of microtribometer operating in ambient air at technical dry friction conditions under normal load of 80 mN. It was found that nanocomposite coatings exhibit lower coefficient of friction (CoF) and considerably lower wear compared to titania coating without nanoparticles. The lowering of CoF is about 40% for coatings heated at 500 °C and 33% for the coatings heated at 1000 °C. For nanocomposites the wear stability was enhanced by a factor of 100 as compared to pure titania coatings. We claim that enhanced tribological properties are closely related to the reduction of the real contact area, lowering of the adhesive forces in frictional contacts and increasing of the composite hardness. The changes in materials composition in frictional contact has secondary effect.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

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.     

Microstructure and tribological properties of ternary BCN thin films with different carbon contents

Boron carbon nitrogen (BCN) thin films with different carbon contents are deposited on high-speed steel substrates by reactive magnetron sputtering (RMS) and their microstructure and tribological properties are studied. The BCN films with carbon contents from 26.9 wt.% to 61.3 wt.% have an amorphous structure with variable amounts of carbon bonds (sp²C–C, sp²C–N and sp³C–N bonds). A higher carbon content enhances the film hardness but reduces the friction coefficient against GCr15 steel balls in air. BCN films with higher hardness, lower friction coefficient, and better wear resistance can be obtained by increasing the carbon content.     

Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si₃N₄. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 °C, 300 °C, 500 °C, and 700 °C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 °C and 500 °C) coefficient of friction of monolithic Si₃N₄ and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 °C differences between the experimental materials disappeared and severe wear regime dominated in all cases.     

Effect of multi-walled carbon nanotubes as reinforced fibres on tribological behaviour of Ni–P electroless coatings

After multi-walled carbon nanotubes (MWNTs) were modified and dispersed uniformly in electrolyte, the MWNTs composite coatings were prepared by electroless deposition. Hardness tests were carried out using a Vickers Hardness indenter. The friction and wear behavior of the Ni–P–MWNTs composite coatings in carbon-steel rings were investigated by using a ring-on-plate wear tester at pure liquid paraffin. Moreover, the friction and wear behavior of nine kinds of wear combinations, which were composed of plates and rings of different composite coatings, were studied. The experimental results indicated that addition of MWNTs would result in an increase in microhardness and an improvement of tribological properties of the Ni–P composite coating significantly. The Ni–P–MWNTs composite coatings revealed lower wear rate and friction coefficient compared with Si–C composite coatings. Moreover, the wear combination, which composed of the Ni–P–MWNTs composite coatings, showed a more excellent ability of friction-reduction and wear resistance than other combinations, and their friction coefficient and wear rate were 0.1087 and 1.49 × 10^− 6 kg/m, respectively.     

Effect of structure of carbon films on their tribological properties

A comparative study of the tribological properties of a library of different carbon forms is presented. The library includes hydrogen free and hydrogenated carbon films with different bonding (CC, CH, different sp³ fractions) and structure configurations (amorphous, graphitic) leading to a wide range of densities and hardness. Reference samples (Si substrates, thermally evaporated amorphous carbon, graphitic foil) were studied as well. The tribological properties were measured using a reciprocal sliding tribometer under humid (50% RH) and dry (5% RH) air conditions. Friction coefficients were measured versus the number of sliding cycles and the wear was studied using optical profilometry and imaging as well as SEM.The friction and wear performance of the carbon films were found to depend on both the structure and the ambient conditions. Hydrogen free films have friction coefficients < 0.1 for 80% sp³ bonded films and > 0.1 for 100% sp² bonded films. The wear resistance of the hydrogen free films (much larger for sp³ bonded films) significantly decreases under dry conditions. In contrast, hydrogenated films show reduction in friction with decreasing humidity (from 0.2 under 50% RH to < 0.1 under 5% RH). The wear resistance of hydrogenated films is larger for dry and smaller for humid conditions.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Wear mechanisms of TiN–TiB2 ceramic in sliding against alumina from room temperature to 700 °C

TiN–TiB₂ ceramic was prepared by the reactive hot-pressing method using titanium and BN powders as raw materials. The friction and wear properties of TiN–TiB₂ ceramic were evaluated in sliding against alumina ball from room temperature to 700 °C in air. The TiN–TiB₂ ceramic has a relative density of 98.6%, a flexural strength of 731.9 MPa and a fracture toughness of 8.5 MPa m^1/2 at room temperature. The TiN–TiB₂ ceramic exhibits a distinct decrease in friction coefficient at 700 °C as contrasted with the friction data obtained at room temperature and 400 °C. Wear mechanisms of TiN–TiB₂ ceramic depend mainly upon testing temperature at identical applied loads. Lubricious oxidized products caused by thermal oxidation provide excellent lubrication effects and greatly reduce the friction coefficient of TiN–TiB₂ ceramic at 700 °C. However, abrasive wear and tribo-oxidation are the dominant wear mechanisms of TiN–TiB₂ ceramic at 400 °C. Mechanical polishing effect and removal of micro-fractured grains play important roles during room-temperature wear tests.     

A superhard diamond-like carbon film

A superhard hydrogen-free amorphous diamond-like carbon (DLC) film was deposited by pulsed arc discharge using a carbon source accelerator in a vacuum of 2×10⁻⁴ Pa. The growth rate was about 15 nm/min and the optimum ion-plasma energy was about 70 eV. The impact of doping elements (Cu, Zr, Ti, Al, F(Cl), N) on the characteristics of DLC films deposited on metal and silicon substrates was studied aiming at the choice of the optimum coating for low friction couples. The microhardness of thick (≥20 μm) DLC films was studied by Knoop and Vickers indentations, medium thick DLC films (1–3 μm) were investigated using a ‘Fischerscope’, and Young's module of thin films (20–70 nm) was studied by laser induced surface acoustic waves. The bonds in DLC films were investigated by electron energy loss spectroscopy (EELS), X-ray excited Auger electron spectroscopy (XAES), and X-ray photoelectron spectroscopy (XPS). The adhesion of DLC films was defined by the scratch test and Rockwell indentation. The coefficient of friction of the Patinor DLC film was measured by a rubbing cylinders test and by a pin-on-disk test in laboratory air at about 20% humidity and room temperature. The microhardness of the Patinor DLC film was up to 100 GPa and the density of the film was 3.43–3.65 g/cm³. The specific wear rate of the Patinor DLC film is comparable to that of other carbon films.     

Design of an inlaid interface structure to improve the oxidation protective ability of SiC–MoSi2–ZrB2 coating for C/C composites

To improve the oxidation protective ability of SiC–MoSi₂–ZrB₂ coating for carbon/carbon (C/C) composites, pre-oxidation treatment and pack cementation were applied to construct a buffer interface layer between C/C substrate and SiC–MoSi₂–ZrB₂ coating. The tensile strength increased from 2.29 to 3.35 MPa after pre-oxidation treatment, and the mass loss was only 1.91% after oxidation at 1500 °C for 30 h. Compared with the coated C/C composites without pre-oxidation treatment, after 18 thermal cycles from 1500 °C and room temperature, the mass loss was decreased by 30.6%. The improvements of oxidation resistance and mechanical property are primarily attributed to the formation of inlaid interface between the C/C substrate and SiC–MoSi₂–ZrB₂ coating.     

Friction and wear properties of scrap tire/potassium hexatitanate whisker composites

As a way of solving the environmental problem of waste tires, we developed a new type of friction material made of scrap tire composites with potassium hexatitanate in which rubber formed a continuous phase. The tribological behaviors of the scrap tire rubber composites were investigated by a friction and wear tester under dry conditions. According to the results, the optimum amounts of potassium hexatitanate were 5 phr in terms of the friction and wear properties up to 200 °C. The specimen containing other ingredients showed 0.72 of friction coefficient and 1.03 of wear rate which are highly compatible to those of the commercial ‘Sonata’ motor brake pad when it contains 5, 20, 10, 20, 10 phr of potassium hexatitanate, phenol, cashew, barium sulfate, and copper, respectively.     

Fabrication of apatite-type lanthanum silicate films and anode supported solid oxide fuel cells using nano-sized printable paste

Apatite-type lanthanum silicate based films have attracted significant interests to use as an electrolyte of solid oxide fuel cells (SOFCs) working at intermediate temperature. We have prepared Mg doped lanthanum silicate (MDLS) films on NiO–MDLS cermet substrates by spin coating and sintering of nano-sized printable paste made by beads milling. Changes in crystal structure and microstructure of the paste films with the sintering temperature have been investigated to show that porous network structure with a grain growth evolves up to 1300 °C, whereas densification occurred above 1400 °C. Anode supported SOFCs using the pasted MDLS films were successfully fabricated: an open circuit voltage of 0.91 V and a maximum power density of 150 mW cm⁻² measured at 800 °C were obtained with the electrolyte film sintered at 1500 °C.     

Tribological properties of NiCr–ZrO2(Y2O3)–SrSO4 composites at elevated temperatures

The effect of SrSO₄ content on the tribological properties of NiCr–30wt%ZrO₂(Y₂O₃) (NC30Z) cermet was evaluated over a wide temperature range from room temperature to 1000 °C. The results indicated that the inclusion of SrSO₄ effectively improved the friction coefficients and wear rates of NC30Z cermet above 400 °C. NC30Z–5SrSO₄ composite against alumina ball exhibited satisfactory tribological performance, which was attributed to synergistic lubrication of pseudocubic-SrZrO₃ and NiCr₂O₄ between 400 °C and 800 °C and cubic-SrZrO₃, NiCr₂O₄, NiO and Cr₂O₃ at 1000 °C.     

Pressureless sintering and tribological properties of WC–ZrO2 composites

In a recent work [Basu, B., Lee, J. H. and Kim, D. Y., Development of WC-ZrO₂ nanocomposites by spark plasma sintering. J. Am. Ceram. Soc. 2004 87(2), 317–319], the processing of ultrahard WC–ZrO₂ nanocomposites using spark plasma sintering is reported. In the present work, we investigate the processing and properties of WC–6 wt.% ZrO₂ composites, densified by pressureless sintering route. The densification of the WC–ZrO₂ composites was performed in the temperature range of 1500–1700 °C with varying time (1–3 h) in vacuum. The experimental results indicate that significantly high hardness of 22–23 GPa and moderate fracture toughness of ∼5 MPa m^1/2 can be obtained with 2 mol% Y–stabilized ZrO₂ sinter-additive, sintered at 1600 °C for 3 h. Furthermore, the friction and wear behavior of optimized WC–ZrO₂ composite is investigated on a fretting mode I wear tester. The tribological results reveal that a moderate coefficient of friction in the range from 0.15 to 0.5 can be achieved with the optimised composite. A transition in friction and wear with load is noted. The dominant mechanisms of material removal are tribochemical wear and spalling of tribolayer.     

Critical temperatures in the synthesis of graphene-like materials by thermal exfoliation–reduction of graphite oxide

We prepared a series of graphene-like materials by thermal exfoliation/reduction of a graphite oxide (GO) at temperatures between 127 °C and 2400 °C. The extent of the exfoliation and reduction of the GO at different temperatures, as well as the impact on the resultant graphene-like materials (TRGs), were studied through their chemical/structural characterization. The main oxygen loss was observed at 127 °C during the blasting of the GO, which produced its exfoliation into monolayer functionalized TRG with hydroxyl groups and minor amounts of epoxy and carboxyl groups. Above 600 °C, the reduction continued smoothly, with oxygen and hydrogen loss and the conversion of hybridised carbon atoms from sp³ into sp². 1000 °C appears to be a critical temperature for the efficiency of the reduction process, as the resulting TRG contained <2% oxygen and 81.5% sp²-carbon atoms. The materials obtained at 2000 °C and 2400 °C were almost oxygen-free and the layers exhibited a dramatic restoration of the pristine graphite structure, as confirmed by the increase in the average size of the sp²-domains. The typical disordered stacking of TRGs increases with temperature, although they can be dispersed yielding monolayers at 127 and 300 °C and stacks of up to 4–6 layers above 1000 °C, as determined by AFM.     

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.