Tribological properties of spark-plasma-sintered ZrO2(Y2O3)-CaF2-Ag composites at elevated temperatures

Spark-plasma sintering is employed to synthesize self-lubricating ZrO₂(Y₂O₃) matrix composites with different additives of CaF₂ and Ag as solid lubricants by tailoring the composition and by adjusting the sintering temperature. The friction and wear behavior of ZrO₂(Y₂O₃) matrix composites have been investigated in dry sliding against an alumina ball from room temperature to 800 °C. The effective self-lubrication at different temperatures depends mainly on the content of various solid lubricants in the composites. The addition of 35 wt.% Ag and 30 wt.% CaF₂ in the ZrO₂(Y₂O₃) matrix can promote the formation of a well-covered lubricating film, and effectively reduce the friction and wear over the entire temperature range studied. The friction coefficients at low temperatures were at a minimum value for the composite containing 35 wt.% of silver. At this silver concentration, low and intermediate temperature lubricating properties are greatly improved without affecting high-temperature lubrication by the calcium fluoride in ZrO₂(Y₂O₃) matrix composites. The worn surfaces and transfer films formed during wear process have been characterized to identify the synergistic lubrication behavior of CaF₂ and Ag lubricants at different temperatures.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Tribological properties of ZrO2 (Y2O3)-Mo-BaF2/CaF2 composites at high temperatures

ZrO₂ (Y₂O₃) with different contents of BaF₂/CaF₂ and Mo were fabricated by hot pressed sintering, and the tribological behavior of the composites against SiC ceramic was investigated from room temperature to 1000 °C. It was found that the ZrO₂ (Y₂O₃)-5BaF₂/CaF₂-10Mo composite possessed excellent self-lubricating and anti-wear properties. The low friction and wear were attributed to enhanced matrix and BaMoO₄ formed on the worn surfaces.     

Wear properties of two-phase Al2O3/ZrO2 (Y2O3) ceramics at temperatures from 296 to 1073 K

Reciprocating sliding friction experiments were conducted with various two-phase, directionally solidified Al₂O₃/ZrO₂ (Y₂O₃) pins sliding on B₄C flats in air at temperatures of 296, 873, and 1073 K under dry sliding conditions. Results indicate that all the Al₂O₃/ZrO₂ (Y₂O₃) ceramics, from highly Al₂O₃-rich to ZrO₂-rich, exceed the main wear criterion requirement of 10⁻⁶ mm³ N⁻¹ m⁻¹ or lower for effective wear-resistant applications. Particularly, the eutectics and Al₂O₃-rich ceramics showed superior wear properties. The composition and microstructure of Al₂O₃/ZrO₂ (Y₂O₃) ceramics played a dominant role in controlling the wear and friction properties. The controlling mechanism of the ceramic wear, friction, and hardness was an intrinsic effect involving the resistance to shear fracture of heterophase bonding and cohesive bonding and the interlocking microstructures at different scales in the ceramics.     

The effect of Al2O3 on the friction performance of automotive brake friction materials

This study consists of two stages. In the first stage, bronze-based break linings were produced and friction-wear properties of them were investigated. In the second stage, 0.5%, 1%, 2% and 4% alumina (Al₂O₃) powders were added to the bronze-based powders and Al₂O₃ reinforced bronze-based break linings were produced. Friction–wear properties of the Al₂O₃ reinforced samples were aslo investigated and compared to those of plain bronze-based ones. For this purpose, friction coefficient and wear behaviour of the samples were tested on the grey cast iron disc. The hardness and density of the samples were also determined. Microstructures of the samples before and after the sintering and the worn surfaces of the wear specimens were examined using a scanning electron microscope (SEM). The sample compacted at 350 MPa and sintered at 820 °C exhibited the optimum friction–wear behaviour. With increase in friction surface temperature, a reduction in the friction coefficient of the samples was observed. The lowest reduction in the friction coefficient with increasing temperature was for the 2% and 4% Al₂O₃ reinforced samples. The SEM images of the sample indicated that increase in Al₂O₃ content resulted in adhesive wear. With increase in Al₂O₃ content, a reduction in mass loss of the samples was also observed. Overall, the samples reinforced with 2% and 4% Al₂O₃ exhibited the best results.     

Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

High-temperature tribological properties of spark-plasma-sintered Al2O3 composites containing barite-type structure sulfates

Al₂O₃–50BaSO₄–20Ag, Al₂O₃–50BaSO₄–10SiO₂, Al₂O₃–50(mass%)SrSO₄, Al₂O₃–50PbSO₄–5SiO₂, Al₂O₃–50BaSO₄ and Al₂O₃–50BaCrO₄ composites (mass%) were prepared by spark plasma sintering and their microstructure and high-temperature tribological properties were evaluated. Al₂O₃–50BaSO₄–20Ag composites (mass%) showed the lowest friction coefficients at the temperature ranging from 473 to 1073 K. Thin Ag film was observed on the wear tracks of the composites above 473 K. In addition, the friction coefficients of Al₂O₃ composites containing SrSO₄ and PbSO₄ were as low as those of Al₂O₃–BaSO₄ and Al₂O₃–BaCrO₄ composites at the temperatures up to 1073 K. The thin films formed on the wear tracks of the Al₂O₃–SrSO₄ composites were composed of Al₂O₃ and SrSO₄ phases, while the films formed on the wear tracks of the Al₂O₃–PbSO₄–SiO₂ composites consisted of Al₂O₃, PbSO₄ and SiO₂ phases.     

Friction and wear behaviour of Thordon XL and LgSn80 in sliding against plasma-sprayed Cr2O3 coatings

This paper studies the friction and wear behaviour of two important bearing materials, Thordon XL and LgSn80, in dry and lubricated sliding vs. plasma-sprayed Cr₂O₃ coatings. As a reference, AISI 1043 steel is also studied under the same conditions. SEM, EDS and surface topography were employed to study the wear mechanisms. The results indicate that the Thordon XL/Cr₂O₃ coating pair gives the lowest dry friction coefficient (0.16) under a normal load of 45.3 N (pressure 0.453 MPa) at a velocity of 1 m/s. The dry friction coefficient of Thordon XL/Cr₂O₃ coating increases to 0.38 under a normal load of 88.5 N (pressure 0.885 MPa). The dry friction coefficients of the LgSn80/Cr₂O₃ coating are in the range of 0.31–0.46. Secondly, both dry wear rate under low normal load (45.3 N) and lubricated wear rate under a load of 680 N for Thordon XL are lower than those of LgSn80 in sliding against plasma-sprayed Cr₂O₃ coatings at a speed of 1 m/s. However, under a normal load of 88.5 N the dry wear rate of Thordon XL is much higher than that of LgSn80. Thirdly, a high viscosity lubricant (SAE 140) leads to lower wear for Thordon XL and LgSn80 than a low viscosity lubricant (SAE 30). Finally, the dominating wear mechanism for Thordon XL is shear fracture when against the plasma-sprayed Cr₂O₃ ceramic coating. For LgSn80 against plasma-sprayed Cr₂O₃ ceramic coating, abrasive wear is the governing failure mechanism.     

Super-low friction of MoS2 coatings in various environments

The ultra-low friction coefficient (typically in the 10⁻² range) of MoS₂-based coatings is generally associated with the friction-induced orientation of ‘easy-shear’ planes of the lamellar structure parallel to the sliding direction, particularly in the absence of environmental reactive gases and with moderate normal loads. We used and AES/XPS ultra-high vacuum tribometer coupled to a preparation chamber, thus allowing the deposition of oxygen-free MoS₂ PVD coatings and the performance of friction tests in various controlled atmospheres. Friction of oxygen-free stoichiometric MoS₂ coatings deposited on AISI 52100 steel was studied in ultra-high vacuum (UHV: 5 × 10⁻⁸ Pa), high vacuum (HV: 10⁻³ Pa), dry nitrogen (10⁵ Pa) and ambient air (10⁵ Pa). ‘Super-low’ friction coefficients below 0.004 were recorded in UHV and dry nitrogen, corresponding to a calculated interfacial shear strength in the range of 1 MPa, about ten times lower than for standard coatings. Low friction coefficients of about 0.013–0.015 were recorded in HV, with interfacial shear strength in the range of 5 MPa. Friction in ambient air leads to higher friction coefficients in the range of 0.2. Surface analysis performed inside the wear scars by Auger electron spectroscopy shows no trace of contaminant, except after friction in ambient air where oxygen and carbon contaminants are observed. In the light of already published results, the ‘super-low’ friction behaviour (10⁻³ range) can be attributed to superlubricity, obtained for a particular combination of cystallographic orientation and the absence of contaminants, leading to a considerable decrease in the interfacial shear strength.     

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT

The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr₃C₂-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

Tribological Behaviour of Nanostructured Al2O3-3 wt% TiO2 Coating Against Steel in Dry Sliding

Nanostructured and conventional Al₂O₃-3 wt% TiO₂ coatings were deposited by atmospheric plasma spraying. The wear and friction properties of both coatings against a steel ball under dry friction conditions were examined. It was found that the wear resistance of the nanostructured Al₂O₃-3 wt% TiO₂ coating was superior to that of the corresponding conventional counterpart. The improvement in wear resistance of the nanostructured coating was attributed to its higher toughness and cohesion strength between splats. As for the nanostructured coating, the wear mechanism was mainly adhesion with micro-abrasion at low loads (20 N). At high loads (80 N), the wear of the nanostructured coating was controlled by plastic deformation and associated delamination along the splat boundaries, which was similar to that of the conventional coating at low loads. However, the failure of the conventional coating was predominantly brittle fracture within the splats and delamination between splats at high loads.     

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

Al₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al₂O₃ coating was superior to that of the Cr₂O₃ coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al₂O₃ coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al₂O₃ coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr₂O₃ coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation.     

Influence of Al2O3 reinforcement on the abrasive wear characteristic of Al2O3/PA1010 composite coatings

In the present study, the abrasive wear characteristics of Al₂O₃/PA1010 composite coatings were tested on the turnplate abrasive wear testing machine. Steel 45 (quenched and low-temperature tempered) was used as a reference material. The experimental results showed that when the Al₂O₃ particles have been treated with a silane coupling agent (γ-aminopropyl-triethoxysilane), the abrasive wear resistance of Al₂O₃/PA1010 composite coatings has a good linear relationship with the volume fraction of Al₂O₃ particles in Al₂O₃/PA1010 composite coatings and the linear correlation coefficient is 0.979. Under the experimental conditions, the size of Al₂O₃ particles (40.5-161.0 μm) has little influence on the abrasive wear resistance of Al₂O₃/PA1010 composite coatings. By treating the surface of Al₂O₃ particles with the silane coupling agent, the distribution of Al₂O₃ particles in PA1010 matrix is more homogeneous and the bonding state between Al₂O₃ particles and PA1010 matrix is better. Therefore, the Al₂O₃ particles make the Al₂O₃/PA1010 composite coatings have better abrasive wear resistance than PA1010 coating. The wear resistance of Al₂O₃/PA1010 composite coatings is about 45% compared with that of steel 45.     

Effects of anti-wear additives on friction and wear properties of Cr2O3 coating

The effects of some anti-wear additives on the friction and wear behaviour of plasma-sprayed Cr₂O₃ coating were investigated using a block-on-ring tester at ambient conditions. The results show that zinc dialkyldithiophosphate (ZDDP), tricesyl phosphate (TCP) and tributyl phosphate (TBP) significantly reduce the wear of Cr₂O₃ coating lubricated by paraffin oil. Additive concentrations as well as sliding time have great influence on the wear. The friction coefficient varies slightly with test conditions. The analysis by XPS of worn surfaces indicates that the wear resistance of these additives is due to the formation of tribochemical reaction films by reacting with Cr₂O₃ coatings.     

Friction and wear behaviour of plasma-sprayed Cr2O3 coatings against steel in a wide range of sliding velocities and normal loads

This study investigates the influence of sliding speed and normal load on the friction and wear of plasma-sprayed Cr₂O₃ coatings, in dry and lubricated sliding against AISI D2 steel. Friction and wear tests were performed in a wide speed range of 0.125–8 m/s under different normal loads using a block-on-ring tribometer. SEM, EDS and XPS were employed to identify the mechanical and chemical changes on the worn surfaces. A tangential impact wear model was proposed to explain the steep rising of wear from the minimum wear to the maximum wear. The results show that the wear of Cr₂O₃ coatings increases with increasing load. Secondly, there exist a minimum-wear sliding speed (0.5 m/s) and a maximum-wear sliding speed (3 m/s) for a Cr₂O₃ coating in dry sliding. With the increase of speed, the wear of a Cr₂O₃ coating decreases in the range 0.125–0.5 m/s, then rises steeply from 0.5 m/s to 3 m/s, followed by a decrease thereafter. The large variation of wear with respect to speed can be explained by stick-slip at low speeds, the tangential impact effect at median speeds and the softening effect of flash temperature at high speeds. Thirdly, the chemical compositions of the transfer film are a-Fe₂O₃ in the speed range 0.25–2 m/s, and FeO at 7 m/s. In addition, the wear mechanisms of a Cr₂O₃ coating in dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a mixture of abrasion and brittle fracture at high speeds. Finally the lubricated wear of Cr₂O₃ coating increases sharply from 1 to 2.8 m/s.     

Sliding friction and wear performance of Ti6Al4V in the presence of surface-capped copper nanoclusters lubricant

The friction and wear properties of Ti₆Al₄V sliding against AISI52100 steel ball under different lubricative media of surface-capped copper nanoclusters lubricant—Cu nanoparticles capped with O,O′-di-n-octyldithiophosphate (Cu-DTP), rapeseed oil and rapeseed oil containing 1 wt% Cu-DTP was evaluated using an Optimol SRV oscillating friction and wear tester. The wear mechanism was examined using scanning electron microscopy (SEM) and X-ray photoelectron spectrosmeter (XPS). Results indicate that Cu-DTP can act as the best lubricant for Ti₆Al₄V as compared with rapeseed oil and rapeseed oil containing 1 wt% Cu-DTP. The applied load and sliding frequency obviously affected the friction and wear behavior of Ti₆Al₄V under Cu-DTP lubricating. The frictional experiment of the Ti₆Al₄V sliding against AISI52100 cannot continue under the lubricating condition of rapeseed oil or rapeseed oil containing 1 wt% Cu-DTP when the applied load are over 100 N. Surprisingly, the frictional experiment of Ti₆Al₄V sliding against AISI52100 steel can continue at the applied load of 450 N under Cu-DTP lubricating. The tribochemical reaction film containing S and P is responsible for the good wear resistance and friction reduction of Ti₆Al₄V under Cu-DTP at the low applied load. However, a conjunct effect of Cu nanoparticle deposited film and tribochemical reaction film containing S and P contributes to the good tribological properties of Ti₆Al₄V under Cu-DTP at the high-applied load.     

Characterization and mechanical/tribological properties of nano Au-TiO2 composite thin films prepared by a sol-gel process

Nano Au-TiO₂ composite thin films on Si(1 0 0) and glass substrates were successfully prepared with a facile sol-gel process followed by sintering. The morphology and mircostructure of the films were investigated via X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The Au particles, of diameter 14-22 nm depending on the sintering temperatures used, were found to be well dispersed in the TiO₂ matrix, with a small amount of the particles escaped from the film. The surfaces of the films were uniform, compact and crack-free. Hardness and elastic modulus of the films were measured by using the nanoindentation technique. Friction and wear properties were investigated by using a one-way reciprocating tribometer. It was found that the highest hardness and elastic modulus values were obtained for the films prepared with 500 °C sintering temperature. The films displayed superior antiwear and friction reduction performances in sliding against an AISI 52100 steel ball. With 5.0 mol% Au, the friction coefficient was only 0.09-0.10 and the wear life was more than 2000 sliding cycles. The friction coefficient and wear life decreased with increasing sliding speed and load. The failure mechanism of the Au-TiO₂ films was identified to be light scuffing and abrasion. Those films can be potentially applied as ultra-thin lubricating coatings.     

Study on the tribological behaviors of the phenolic composite coating filled with modified nano-TiO2

In order to overcome the disadvantages generated by the loosened nanoparticle agglomerates dispersed in polymer composite coatings, nano-TiO₂ particles are modified using trifluoracetic acid. The friction and wear properties of the phenolic coatings filled with different surface treated nano-TiO₂, sliding against AISI-C-52100 steel ring under dry sliding, were investigated on a MHK-500 wear tester. Owing to the effective improvement of their dispersibility in the phenolic coating, compared with the cases of untreated nano-TiO₂, the employment of modified nano-TiO₂ provided the phenolic coating with much better tribological performance. Worn surfaces of the untreated nano-TiO₂ or modified nano-TiO₂ filled phenolic coating and transfer films formed on the surface of the counterpart ring sliding against the composite coating were respectively investigated by SEM and optical microscope (OM), from which it is assumed that the optimal content of TiO₂ or TF-TiO₂ is able to enhance the adhesion of the transfer films to the surface of counterpart ring. As a result, the wear resistance of the phenolic composite coating filled with modified nano-TiO₂ was significantly enhanced, especially at extreme wear conditions, i.e. high contact pressures.     

The friction behavior of Ni-, SiO2- and mica sodium silicate-based solid lubrication composites

The friction behavior of Ni⁻-, SiO⁻₂- and mica sodium silicate-based lubricant composites, which included BN, WS₂ and graphite as lubricants, were examined. A ring-on-disk apparatus, in which a solid lubricant composite disk was held against a rotating stainless ring, was used as the test configuration. The tests were run with a load from 62 to 250 N in temperatures from 20 to 800°C in the laboratory environment. The wear surface was characterized by scanning electron microscope and X-ray photo spectroscopy. The major findings were that both mica sodium silicate- and SiO⁻₂-based composites failed at above 500°C due to severe wear and surface damage; in contrast, Ni⁻-based composite showed a stable friction coefficient and low wear from 20 to 800°C.