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.     

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.     

Microstructure and tribological characteristics of ZrO2–Y2O3 ceramic coatings deposited by laser-assisted plasma hybrid spraying

ZrO₂–Y₂O₃ ceramic coatings were deposited on AISI 304 stainless steel by both a low-pressure plasma spraying (LPPS) and a laser-assisted plasma hybrid spraying (LPHS). Microstructure and tribological characteristics of ZrO₂–Y₂O₃ coatings were studied using an optical microscope, a scanning electron microscope, and an SRV high-temperature friction and wear tester. The LPHS coatings exhibit distinctly reduced porosity, uniform microstructure, high hardness and highly adhesive bonding, although more microcracks and even vertical macrocracks seem to be caused in the LPHS coatings. The ZrO₂ lamellae in the LPHS coatings before and after 800°C wear test consist mainly of the metastable tetragonal (t′) phase of ZrO₂ together with small amount of c phase. The t′ phase is very stable when it is exposed to the wear test at elevated temperatures up to 800°C for 1 h. The friction and wear of the LPHS coatings shows a strong dependence on temperature, changing from a low to a high wear regime with the increase of temperature. At low temperatures, friction and wear of the LPHS coatings is improved by laser irradiation because of the reduced connected pores and high hardness in contrary to the LPPS coating. However, at elevated temperatures, the friction and wear of the LPHS coatings is not reduced by laser irradiation. At room temperature, mild scratching and plastic deformation of the LPHS coatings are the main failure mechanism. However, surface fatigue, microcrack propagation, and localized spallation featured by intersplat fracture, crumbling and pulling-out of ZrO₂ splats become more dominated at elevated temperatures.     

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.     

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.     

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.     

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.     

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.     

Effect of particle size on tribological behavior of Ni3Al matrix high temperature self-lubricating composites

The Ni₃Al matrix high temperature self-lubricating composites with different particle size were fabricated by the powder metallurgy technique. The effect of particle size on the mechanical and tribological properties of the composites was investigated in this paper. The results showed that the coarse particle composite exhibited the lowest friction coefficient and wear rate compared to the fine particle ones at a wide temperature range from room temperature to 1000 °C. The reason for the low wear rate was that the coarse bulk phase could provide better deformation resistance and higher load bearing capacity than the fine microstructure.     

Comparison of wear behaviour of plasma-sprayed Al2O3 coatings with and without laser treatment

This paper studies experimentally the effects of CO₂ laser-treatment on the wear behaviour of plasma-sprayed Al₂O₃ coatings, in linear contact sliding (dry, abrasive and lubricated) against SAE 4620 steel. Tests were carried out using a block-on-ring friction and wear tester, under different loads at different speeds. The wear mechanism and the changes in adherence, porosity and microstructure by laser treatment were also investigated. Results show a better wear behaviour for both laser-treated ceramic coating and its paired steel under dry and abrasive conditions, compared with the case without laser treatment. The lubricated wear behaviour of the laser-treated ceramic coating, however, is not improved. The changes in microhardness, porosity and adherence caused by the laser treatment are responsible for the change in wear behaviour of the ceramic coating.     

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.     

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.     

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.     

Microtribological Properties of Two-Phase Al2O3 Ceramic Studied by AFM and FFM in Air of Different Relative Humidity

Commercially available, monolithic alumina ceramic was modified using CO₂-laser irradiation by surface remelting and adding HfO₂ powder. Scanning electron microscopy revealed that the microstructure of the modified ceramic consisted of a fine lamellar eutectic Al₂O₃–HfO₂ phase embedded in the Al₂O₃ matrix. Differences in the microtribological properties of the matrix and the eutectic phase could be measured by friction force microscopy (FFM) during unlubricated sliding contact with a silicon tip at room temperature as a function of relative humidity of the surrounding air and normal load. The dependence of the friction coefficient and the pull-off force on humidity was explained by the formation of lubricating tribochemical surface layers and described by theoretical models.     

Friction and Wear Characteristics of BaCr2O4 Ceramics at Elevated Temperatures in Sliding Against Sintered Alumina Ball

In this article, friction and wear characteristics of BaCr₂O₄ ceramics have been investigated using a high-temperature friction and wear tester from room temperature to 800?°C in dry sliding against sintered alumina ball. At room temperature, the friction coefficient and wear rate of BaCr₂O₄ ceramics are quite high. BaCr₂O₄ ceramics exhibit low friction coefficients and small wear rates with temperature increasing up to 400?C600?°C. The oxidation reaction of BaCr₂O₄ during high-temperature wear tests is responsible for the tribological properties. The oxidized product of BaCr₂O₄ is BaCrO₄, which forms a smooth self-lubricating film on the worn surface to effectively reduce friction and wear. However, at 800?°C, severe oxidation reduces the relative density of sintered BaCr₂O₄ ceramics, and further expedites the materials removal process.     

Tribological properties of self-lubricating CMC/Al2O3 pairs at high temperature in air

Three different self-lubricating ceramic matrix composites (CMCs) were fabricated by hot-pressed sintering. They are: Al₂O₃-50CaF₂, Al₂O₃-20Ag20CaF₂, and Al₂O₃-10Ag20CaF₂. Tribological tests were performed at temperatures ranging from 20°C to 800°C in air using a pin-on-disk tester. The experimental results show that the addition of the solid lubricants CaF₂ and Ag can evidently reduce the friction coefficients of alumina between 200°C and 650°C but not at room temperature and the wear rate of disks and pins at elevated temperature. The improvements in the friction and wear properties of CMC were due to the formation of a well-covered solid lubricating film. However, breakdown of the lubricating films at 800°C resulted in high friction and wear. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tribological performance of MoS2---B2O3 compacts

A recent investigation suggests that selected oxides perform well as additives in molybdenum disulphide (MoS₂) because of their ability to soften at asperity contacts with the result that the solid lubricant can attain and retain a preferred tribological orientation.This research determined the effectiveness of boric oxide (B₂O₃), when used as an additive in MoS₂, for substrate temperatures ranging from 21°C to 316°C. This range was used to allow the asperity contact temperature to vary below and above the softening point of B₂O₃. It was found that a moderate friction coefficient and high wear, which is attributed to the additive acting abrasively, occurred when the asperity contact temperature was well below the softening point of the oxide. When the asperity contact temperature neared the softening point of the oxide, the friction coefficient increased dramatically and wear volume was reduced. It is postulated that B₂O₃ acted adhesively at the interface resulting in a higher coefficient of friction, and wear decreased due to an attainment of a preferred orientation by the MoS₂. For asperity contact temperatures significantly above the softening point of B₂O₃, the friction coefficient returned to about the same value as for temperatures below the softening point. It is speculated that wear continued to increase moderately because of localized melting of the B₂O₃, permitting the MoS₂ to be removed from the interface. These observations support a hypothesis that an additive, such as boric oxide, can soften as the asperity contact temperature approaches the softening point temperature of the additive so that the overall tribological conditions may be improved resulting in reduced interfacial wear. Significant changes in temperature, load or sliding velocity would, of course, dramatically alter the wear characteristics observed at the interface.     

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.     

Comparative study on tribology and microhardness of laser synthesized Ti-Al2O3/Zn coatings on Ti-6Al-4V alloy

ABSTRACT

The study of laser cladding of 90Ti-10Al₂O₃, 90Ti-8Al₂O₃-2Zn and 90Ti-4Al₂O₃-6Zn coatings onto Ti-6Al-4V alloy, with intention to produce defect-less, high microhardness and wear resistant coating was carried out. The coatings were deposited onto Ti-6Al-4V alloy at 900 W laser power and 0.6 m/min laser scan speed. Microstructures and phase constituents of the developed coatings were investigated by using a scanning electron microscope (SEM) and X-ray diffractometer correspondingly. Vickers microhardness tester and pin-on-disk tribometer were employed to characterize microhardness and wear behaviour of the Ti-Al₂O₃/Zn coatings respectively. SEM was also used to examine the worn track. It was observed that 90Ti-10Al₂O₃ coating yielded optimal microhardness along with maximal wear resistance in comparison to the other coatings and Ti-6Al-4V alloy. It has been established that laser cladding of Ti-Al₂O₃ coating with Zn contents on Ti-6Al-4V alloy alleviates the formation of cracks, however, microhardness and wear properties are negatively affected.     

Ni3Al matrix high temperature self-lubricating composites

A Ni₃Al matrix high temperature self-lubricating composite Ni₃Al-BaF₂-CaF₂-Ag-Cr was fabricated by the powder metallurgy technique, and tribological behavior at a wide temperature range from room temperature to 800 °C was investigated. The results indicated that the composite exhibited low friction coefficients (0.30-0.36) and wear rates (0.65-2.45×10⁻⁴ mm³ N⁻¹ m⁻¹). It was found that the low friction coefficient was attributed to the synergistic effects of Ag, fluorides and chromates formed in the tribo-chemical reaction at high temperatures. The low wear rate of the composite was due to the high strength and the excellent lubricating properties.