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.     

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.     

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.     

Tribological properties of Ni3Al-Cr7C3 composite coating under water lubrication

The tribological properties of Ni₃Al-Cr₇C₃ composite coating under water lubrication were examined by using a ball-on-disc reciprocating tribotester. The effects of load and sliding speed on wear rate of the coating were investigated. The worn surface of the coating was analyzed using electron probe microscopy analysis (EPMA) and X-ray photoelectron spectroscopy (XPS). The results show the friction coefficient of the coating is decreased under water lubrication. The wear rate of the coating linearly increases with the load. At high sliding speed, the wear rate of the coating is dramatically increased and a large amount of the counterpart material is transferred to the coating worn surface. The low friction of the coating under water lubrication is due to the oxidizing of the worn surface in the wear. The wear mechanism of the coating is plastic deformation at low normal load and sliding speed. However, the wear mechanism transforms to microfracture and microploughing at high load with low sliding speed, and oxidation wear at high sliding speed. It is concluded that the contribution of the sliding speed to an increase in the coating wear is larger than that of the normal load.     

Relation Between Tribolayers and Wear Behavior During Dry Sliding of Fe-Al Hot-Dipped Coating

An Fe-Al coating consisting of FeAl and Fe₃Al was prepared on AISI 1045 steel by hot-dip aluminizing and subsequent high-temperature diffusion. Dry sliding wear tests were performed for Fe-Al coating against AISI 52100 steel under various sliding speeds and loads. During sliding, thin tribolayers formed on the worn surfaces of the Fe-Al coating. After wear, they were observed to be a nonoxidized mechanically mixed layer (MML) at 0.5 m/s, an oxide-containing MML at 0.75–2.68 m/s, and an in situ oxide layer at 4 m/s. The tribolayers presented a close relation with the wear behavior. Because of their different ingredients, structures, and types, the tribolayers resulted in significant changes in the wear behavior. At 0.75–2.68 m/s (except for 2.68 m/s, 40 N), the compact tribooxide layers exerted a protective function for Fe-Al coating to reduce the wear rate. However, for the tribolayers containing no or trace tribooxides at 0.5 m/s or the unstable ones formed at 2.68 m/s, 40 N and 4 m/s, no protection was presented. In these cases, the Fe-Al coating would be partly or totally ground off, thus presenting poor wear resistance at high wear rates.     

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.     

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 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.     

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 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.     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

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.     

Monitoring of Wear Characteristics of TiN and TiAlN Coatings at Long Sliding Distances

TiN and TiAlN thin hard coatings have been widely applied on machine components and cutting tools to increase their wear resistance. These coatings have different wear behaviors, and determination of their wear characteristics in high-temperature and high-speed applications has great importance in the selection of suitable coating material to application. In this article, the wear behavior of single-layer TiN and TiAlN coatings was investigated at higher sliding speed and higher sliding distances than those in the literature. The coatings were deposited on AISI D2 cold-worked tool steel substrates using a magnetron sputtering system. The wear tests were performed at a sliding speed of 45 cm/s using a ball-on-disc method, and the wear area was investigated at seven different sliding distances (36–1,416 m). An Al₂O₃ ball was used as the counterpart material. The wear evolution was monitored using a confocal optical microscope and surface profilometer after each sliding test. The coefficient of friction and coefficient of wear were recorded with increasing sliding distance. It was found that the wear rate of the TiAlN coating decreases with sliding distance and it is much lower than that of TiN coating at longer sliding distance. This is due to the Al₂O₃ film formation at high temperature in the contact zone. Both coatings give similar coefficient of friction data during sliding with a slight increase in that of the TiAlN coating at high sliding distances due to the increasing alumina formation. When considering all results, the TiAlN coating is more suitable for hard machining applications.     

Friction and wear behaviour of WC-Co-Cr3C2-VC cemented carbides obtained from nanocrystalline mixtures

The effect of adding Cr₃C_2, VC or a mixture of both as a grain growth inhibitors to cemented carbides obtained from WC-12 wt.%Co nanocrystalline mixtures on the behaviour of friction and dry sliding wear have been studied. All the wear tests were performed on a tribometer with ball on disc configuration, using a WC-6 wt.%Co ball as a counterpart with normal contact loads of 40 and 60 N, sliding distance of 2000 m and a sliding speed of 0.1 m/s. A significant reduction in the wear rates was observed by the effect of the aforementioned additives, in particular for the VC, which showed an increase in the wear resistance of the order of 90%. The analysis of wear and surface damage were correlated to the observed behaviour.     

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.     

Tribological Behavior of PTFE Composites Filled with PEEK and Nano-Al2O3

In this study, the tribological properties of polytetrafluoroethylene (PTFE) composites filled with polyetheretherketone (PEEK) and nano-Al₂O₃ particles were studied using a block-on-ring wear tester. The tribological performance of the composites was affected by the experimental parameters (sliding speed, normal load, and environmental temperature) and the composites achieved a high-speed sliding friction state. The results showed that the PEEK and nano-Al₂O₃ particles significantly improved the wear resistance of the PTFE composites. In addition, the nano-Al₂O₃ particles increased the hardness of the composites and enhanced the mechanical properties to enable applications in a wider range of industrial fields. The effects of the sliding speed and normal load on the tribological properties were more significant than that of the environmental temperature. In addition, the entire wear process was divided into three stages (the initial wear stage, severe wear transition stage, and ultralow stable wear stage), according to the evolution of the tribological characteristics (wear rate, morphology of the worn surface and transfer film, and wear debris morphology).     

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.     

Influence of Sliding Speed on the Tribological Characteristics of Si3N4-hBN Ceramic Materials

The influence of sliding speed on the unlubricated tribological behaviors of silicon nitride–boron nitride (Si₃N₄-hBN) composites was investigated with two modes in air by a pin-on-disc tribometer. Using the upper disc–on–bottom pin test mode, as the sliding speed increased, the friction coefficient of the sliding pairs showed an upward trend; for example, from 0.18 at the sliding speed of 0.40 m/s to 0.54 at the sliding speed of 1.31 m/s for the Si₃N₄/Si₃N₄–20% hBN pair. The surface analysis indicated that a tribochemical film consisting of SiO₂ and H₃BO₃ formed on the wear surfaces of the Si3N4/Si3N4–20% hBN sliding pair at sliding speeds of 0.40 and 0.66 m/s. Moreover, the formation of this film lubricated the wear surfaces. At the sliding speed of 1.31 m/s, no tribochemical film formed on the wear surfaces, most likely due to the increase in surface temperature. In the upper pin–on–bottom disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. The increase in sliding speed weakened the degree of abrasive wear, leading to a decrease in the friction coefficients.     

Dry sliding wear mechanism for P/M austenitic stainless steels and their composites containing Al2O3 and Y2O3 particles

Austenitic stainless steels are used in applications demanding general corrosion resistance at room or moderate operating temperatures. However, their use is often limited by the relative softness of these materials and their suceptibility to wear and galling. The present investigation deals with the dry sliding wear behaviour of two P/M austenitic stainless steels (AISI 304L and 316L) and their composites containing two different ceramic particles (Al₂O₃ and Y₂O₃) and two different sintering activators (BN and B₂Cr). Unlubricated pin-on-disc wear tests were carried out. Wear mechanisms were analysed by means of scanning electron microscopy and X-ray diffraction. A plastic deformation and particle detachment wear mechanism was revealed. Plasticity during sliding induced an austenite to martensite transformation. The presence of ceramic particles (Al₂O₃ and Y₂O₃) and sintering activators (B₂Cr, BN) improved significantly the wear resistance (especially the combination Al₂O₃ and B₂Cr). Ceramic particles limited plastic deformation while sintering activators decreased final porosity.