Role of transfer layer on tribological properties of nanocrystalline diamond nanowire film sliding against alumina allotropes

The tribological properties of nanocrystalline diamond nanowire (DNW) film treated in CH₄ atmosphere at 400 °C were studied in ambient atmosphere at room temperature using various allotropes of alumina ball as sliding counterbodies. Super low value of friction coefficient (~ 0.003) and extremely high wear resistance (~ 2.8 × 10^− 21 mm³/Nm) were observed when the Al₂O₃ ball slides against the film. In contrast, high friction coefficients with the values ~ 0.06 and ~ 0.07 were observed while using sapphire and ruby balls, respectively. Wear loss was also high ~ 4 × 10^− 19 mm³/Nm and 2.8 × 10^− 15 mm³/Nm in DNW/sapphire and DNW/ruby sliding pairs, respectively. Such a behavior is fundamentally explained in terms of the chemical nature of the sliding interfaces and surface energy of ball counterbodies. As a consequence, the chemical affinity of Al₂O₃ ball towards the carbon atoms is less, which resulted in the absence of carbonaceous transfer layer formation on the Al₂O₃ ball scar. However, in the case of sapphire and ruby balls, the wear track was found to be highly deformed and significant development of carbonaceous transfer layer was observed on respective ball scars. This phenomenon involving transfer layer formation is related to high surface energy and strong chemical affinities of sapphire and ruby balls towards carbon atoms. In such a condition, sliding occurs between film and the carbonaceous transfer layer formed on the ball exhibiting high energy due to covalent carbon bonds that chemically interact and enhance sliding resistance.     

Frictional properties of diamond and fullerene nanoparticles sprayed by a high-velocity argon gas on stainless steel substrate

Lubricating abilities of diamond nanoparticles with size between 50 nm and 200 nm were studied in vacuum and in air to clarify the effective use of diamond fine powders for tribological purposes. Spraying of powders with a high-velocity argon gas jet was performed to form deposits on stainless steel (SUS304) substrates. For sliding in vacuum against SiC and Al₂O₃ balls under a 0.5 N applied load and 3.5 mm/s sliding speed, the deposits of microcrystalline diamond powders with a mean particle size of 50 nm and detonation nanodiamond with a mean aggregate size of around 75 nm demonstrated friction coefficients of less than 0.01 and 0.03, respectively, and ball wear rates of less than 2 · 10^− 6 mm³/(Nm). This means that diamond fine powders smaller than 100 nm can be considered as good solid lubricants in vacuum, because they demonstrate not only a low friction coefficient, but also wear rate of SiC ball lower than non-lubricated SUS304 does. A C₆₀ deposit, formed by the same method on the SUS304, was readily scratched from the substrate in vacuum; however, under open-air conditions, a friction coefficient of around 0.1 and a SiC ball wear rate of about 2 · 10^− 6 mm³/(Nm) were observed. This fact calls attention to the influence of the deposition method on C₆₀ frictional properties.     

Microstructure and tribological properties of cubic boron nitride films on Si3N4 inserts via boron-doped diamond buffer layers

Cubic boron nitride (cBN) coatings were deposited on silicon nitride (Si₃N₄) cutting inserts through conductive boron-doped diamond (BDD) buffer layers in an electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD) system. The adhesion and crystallinity of cBN coatings were systematically characterized, and the influence of doping level of BDD on the phase composition and microstructure of the cBN coatings were studied. The nano-indentation tests showed that the hardness and elastic modulus of the obtained cBN coatings were 78 GPa and 732 GPa, respectively. The tribological properties of the cBN coatings were evaluated by using a ball-on-disc tribometer with Si₃N₄ as the counterpart. The coefficient of the friction and the wear rate of the cBN coatings were estimated to be about 0.17 and 4.1 × 10^− 7 mm³/N m, respectively, which are remarkably lower than those of titanium aluminum nitride (TiAlN) coatings widely used in machining ferrous metal. The results suggest that cBN/BDD coated Si₃N₄ inserts may have great potentials for advanced materials machining.     

Long-term irradiation effects of visible light on amorphous carbon nitride films

The effect of long-term visible-light irradiation on the photo-induced deformation of amorphous carbon nitride (a-CN_x) films was investigated. a-CN_x films were deposited on SiO₂ substrates (30 × 2 × 0.05 mm³) using reactive radio frequency magnetron sputtering. Deformation of the a-CN_x films was measured using continuous wave (CW) or pulsed monochromatic light with a wavelength of 470 nm. Pulsed light irradiation was applied for a total of 60 min with an on/off pulse period of 60 s, while CW light irradiation was performed for 120, 190, and 759 min with different light intensities so that the total photon flux remained constant. In all cases, the extent of photo-induced deformation of the a-CN_x films before and after irradiation did not change. The chemical bonding states determined from X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses indicated no significant changes after illumination. In addition, electron spin resonance (ESR) spectroscopy measurements indicated that there was no increase in the defect density after illumination. The long-term stability of a-CN_x films is one of the main advantages for their use in light-driven microactuator systems.     

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.     

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 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 characterization of NCD in physiological fluids

NCD films deposited on silicon nitride (Si₃N₄) ceramic substrates by hot-filament chemical vapour deposition (HFCVD) technique were biotribologically assessed under lubrication of Hank's balanced salt solution (HBSS) and dilute fetal bovine serum (FBS), using a pin-on-flat test configuration. The reciprocating tests were conducted under an applied load of 45 N during 500,000 cycles using a NCD coated Si₃N₄ biocompatible ceramic substrates with two different surface preparations: i) polished (P) and ii) polished and plasma etched (PE). Friction coefficient values of 0.02 and 0.12 were measured for the P samples under HBSS and FBS lubrication, respectively. PE samples showed increased adhesion relatively to P ones and withstood 6 km of sliding distance without any evidence of film fracture but with friction coefficients of 0.06 for HBSS and 0.10 for FBS experiments. Evidences of protein attachment and salt deposition were found, being the responsible for the enhancement of friction under FBS relatively to HBSS. The wear rates measured for the NCD films are in the range of ~10^− 9–10^− 8 mm³·N^− 1m^− 1, values that are similar to the best values found for ceramic-on-ceramic combinations.     

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.     

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.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti⁴⁺ in the lattice of apatite-type La_9.83Si_6−xTi_xO_26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La_9.83Si_6−xTi_xO_26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O₂) range from 10⁻²⁰ to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H₂–90%N₂ gradients at 700–950 °C, suggesting the stabilization of Ti⁴⁺ in the apatite structure. Titanium addition in La_9.83Si_6−xTi_xO_26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O₂) = 10⁻²⁰ atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr⁴⁺ in the La_9.83Si_6−xZr_xO_26.75 system was found to be negligible, while the maximum concentration of Ce⁴⁺ in La_9.4−xCe_xSi₆O_27−δ is approximately 5% with respect to the number of lanthanum sites.     

Tribological behaviour of RF-magnetron sputter deposited hydroxyapatite coatings in physiological solution

The aim of the paper is to explore the tribological performance of hydroxyapatite (HA) coatings deposited by radio frequency (RF) magnetron sputtering on AZ31 magnesium alloy (96% Mg, 3% Al, 0.7% Zn, 0.3% Mn) for biomedical applications. In this study, the position of the samples on a substrate holder, relative to a target erosion zone was taken into consideration in order to elucidate its impact on the coating characteristics, such as composition, morphology, surface topography and tribology. Substrate rotation and arc-movement were foreseen in the experimental set-up to increase the uniformity of thin film properties. The deposited HA thin films were revealed to exhibit an increase of the Ca/P ratio from 1.83 to 1.97, a decrease of (002) texture and thickness, as the samples were shifted towards the target erosion zone. By coatings, the roughness of Mg alloy was decreased (R_{a Mg alloy}=31.3 nm; R_{a coating}=29 nm and 21 nm). The coating placed in the centre of the substrate holder showed high hardness and Young's modulus (H =8.3±0.9 GPa; E=89±10 GPa) than the coating prepared under the target erosion zone (H =6.9±1.1 GPa; E=75±6 GPa). The coating deposited under target erosion zone exhibits superior friction behaviour in simulated body fluid environment, with the friction coefficient (μ) of 0.184, while the sample located in the centre of the substrate holder possesses the friction coefficient (0.306) comparable to the AZ31 substrate (0.307). The low wear rate was determined in the case of coating deposited under target erosion zone (4.83×10⁻⁵ mm³ N⁻¹ m⁻¹) than uncoated AZ31 substrate (0.00518 mm³ N⁻¹ m⁻¹) or than coating placed in the centre of the substrate holder (0.00294 mm³ N⁻¹ m⁻¹).     

Synthesis and characterization of S2O82 −/ZnFexAl2 − xO4 solid acid catalysts for the esterification of acetic acid with n-butanol

New spinel-types of S₂O₈^2 −/ZnFe_xAl_2 − xO₄ solid acid catalysts were prepared by sol–gel method. Their catalytic performances for the synthesis of n-butyl acetate were investigated. The catalysts were characterized by means of XRD, IR, XPS, FT-IR of adsorbed pyridine and NH₃-TPD. The experimental results showed that S₂O₈^2 −/ZnFe_xAl_2 − xO₄ solid acid catalysts maintained the spinel structure as well as the support of ZnFe_xAl_2 − xO₄. Fe^3 + ions were well incorporated and highly dispersed into the spinel lattice. S₂O₈^2 −/ZnFe_0.15Al_1.85O₄ exhibited the maximum conversion of acetic acid with 98.2%. Moreover, S₂O₈^2 −/ZnFe_0.15Al_1.85O₄ showed better reusability, which remained above 72.7% conversion of acetic acid even after being used five times.     

Electrochemical properties of amorphous GeOx-C composite microspheres prepared by a one-pot spray pyrolysis process

Amorphous GeO₂-GeO-C (GeO_x-C) composite powders, containing a small amount of the GeC phase, are prepared by a one-pot spray pyrolysis process. The GeO_x-C composite powders have a completely spherical shape and are non-aggregated. The Ge 3d components in the XPS spectrum of the composite occupy 53.3%, 40.1%, and 6.6% of the total for GeO₂, GeO, and GeC, respectively. The amount of amorphous carbon in the GeO_x-C composite powder is estimated at 18.3%, based on the TG and XPS analysis. The initial discharge and charge capacities of the GeO_x-C composite powders at a current density of 1 A g⁻¹ are 1873 and 908 mA h g⁻¹, respectively. The discharge capacities of the GeO_x-C composite and commercial GeO₂ powders for the 1200th cycle are 723 and 169 mA h g⁻¹, respectively, and their corresponding capacity retentions from the 2nd cycle are 70.1% and 19.0%, respectively. The high structural stability of the composite during repeated lithium insertion and desertion processes results in excellent long-term cycling performance.     

Polymorphism in the Lu2 − xYxSi2O7 system at high temperatures

Samples in the system Lu_2 − xY_xSi₂O₇ (1.25 ≤ × ≤ 2) have been synthesised following a sol–gel method and calcined to high temperatures (≥1400 °C). X-ray diffraction (XRD) has shown that all compositions crystallize as β-Lu_2 − xY_xSi₂O₇ at the low temperatures, while increasing calcination temperature produces the formation of the γ- and δ-polymorphs, the temperatures of formation of each polymorph depending on the Y/Lu ratio. Unit cell parameters of the samples crystallizing as γ-Lu_2 − xY_xSi₂O₇ have been calculated and plotted as a function of composition. They show a linear change with increasing Y content, indicating a degree of solid solubility of Lu₂Si₂O₇ in γ-Y₂Si₂O₇. Based on these data and on those reported in our previous studies [Becerro, A.I. and Escudero, A., XRD and ²⁹Si MAS NMR spectroscopy across the β-Lu₂Si₂O₇–β-Y₂Si₂O₇ solid solution. J. Solid State Chem., 2005, 178; Becerro, A.I. and Escudero, A., Phase transitions in Lu-doped Y₂Si₂O₇ at high temperatures. Chem. Mater., 2005, 17, 112] a temperature–composition diagram of the Lu₂Si₂O₇–Y₂Si₂O₇ system is given. Finally, the influence of Lu on the reversibility of the γ-Y₂Si₂O₇ → β-Y₂Si₂O₇ transition is studied by means of XRD and ²⁹Si MAS NMR spectroscopy.     

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.     

Direct catalytic decomposition of nitrous oxide gas over rhodium supported on lanthanum silicate

The environmental impacts of nitrous oxide (N₂O) have received much attention, including contributions to the greenhouse effect and ozone depletion. Currently, the direct catalytic decomposition of N₂O is considered to be the simplest and most promising method for N₂O abatement. In this study, we focused on the high activity of rhodium and the oxide-ion conducting property of lanthanum silicate and prepared novel Rh/La₁₀Si_6 − xFe_xO_27 − δ catalysts. From the results of catalytic N₂O decomposition activities, Rh/La₁₀Si_6 − xFe_xO_27 − δ (x = 1.0) exhibited the highest catalytic activity and N₂O was completely decomposed at 600 °C.     

Substrate effect on wear resistant transition metal nitride hard coatings: Microstructure and tribo-mechanical properties

Four types of different hard transition metal nitrides (TMN:ZrN, CrN, WN and TiN) coatings were deposited on Si (100) and 316LN stainless steel substrates using DC magnetron sputtering. A comprehensive study of microstructure and substrate dependent tribo-mechanical properties of TMN coatings was carried out. Higher hardness (H) and elastic modulus (E) were obtained for WN (H=40 GPa and E=440 GPa) and TiN (H=30 GPa and E=399 GPa) coatings. This is related to the formation of (100) and (111) preferred orientations in WN and TiN coatings, respectively. However, the less hardness and elastic modulus were obtained for ZrN and CrN coatings where (200) orientation is preferred. Remarkably, low friction coefficient (0.06–0.57) and higher wear resistance in the coatings deposited on steel substrates are directly associated with the higher resistance to plastic deformation (H³/E²) and the presence of intrinsic compressive stress. Three body wear modes enhanced the friction coefficient (0.15–0.62) and the wear rate in the coatings deposited on Si substrates. This is primarily associated with low fracture toughness of brittle single crystalline Si (100) substrates. Steel-on-steel contact was dominated in ZrN/steel sliding system. This occurs due to the severe adhesive wear mode of steel ball, whereas, the abrasive wear modes were attained for the CrN, WN and TiN coatings sliding against steel balls.