Studying tribological characteristics of alumina- and zirconia-based ceramics

Tribotests of ceramic specimens of various compositions (Al₂O₃-1% TiO₂, ZrO₂-5.3% Y₂O₃, Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃)) were carried out under dry friction conditions. It has been shown that all of the specimens have a high wear resistance, while the Al₂O₃-15% (ZrO₂ + 5.3% Y₂O₃) specimens have the lowest wear rate and the highest microhardness. This is due to the fine-grained structure of these specimens and their higher density compared to that of the other ceramic compositions.     

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 influence of the composition of nanostructural PSZ powders on ceramics wear

The results of tribological testing of ceramics with the composition ZrO₂ + Al₂O₃, ZrO₂ + Y₂O₃, and ZrO₂ + Y₂O₃ + Al₂O₃ made of nanostructural powders are presented. The nanopowders have been obtained by chemical precipitation from solutions of zirconium and yttrium chloride salts. The studies have been carried out as applied to machine parts: drawing dies and bearing plugs. The dependence of the wear of friction pairs on the composition of the ceramics is shown.     

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.     

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.     

Wear and fracture toughness of partially stabilized zirconia ceramics under dry friction against steel

The wear of two ceramic materials containing partially stabilized zirconia is studied under unlubricated friction against steel. The first material, with ZrO₂ and 3 mol % Y₂O₃, was obtained by cold isostatic pressing and sintering. The second material, comprising ZrO₂ and 4 mol % Y₂O₃, was fabricated by additional hot isostatic pressing. The samples of both materials were fabricated with high and low values of fracture toughness. The samples with high fracture toughness are found to wear more intensively. This fact can be explained by surface micro-cracking during braking as a result of phase transformations.     

Influence of the countermaterial on the dry sliding friction and wear behaviour of low temperature carburized AISI316L steel

Low temperature carburising (LTC) allows a significant hardness increase, with a consequent increase in wear resistance, without deteriorating corrosion behaviour. However, wear resistance strongly depends on contact conditions, therefore this work focuses on the dry sliding behaviour of LTC-treated AISI316L austenitic stainless steel against several countermaterials (AISI316L, LTC-treated AISI316L, hard chromium or plasma-sprayed Al₂O₃–TiO₂). LTC produced a hardened surface layer (C-supersaturated expanded austenite), which improved corrosion resistance in NaCl 3.5% and increased wear resistance, to an extent which depends on both normal load and countermaterial. The best results were obtained when at least one of the contacting bodies was LTC-treated, because this condition led to mild tribo-oxidative wear. However, LTC did not improve the behaviour in terms of friction.     

High-temperature tribological properties of strontium sulfate films formed on zirconia-alumina, alumina and silicon nitride substrates

In our present study, Al₂O₃, (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ and Si₃N₄ substrates coated with SrSO₄ and SrSO₄–10 mass% Ag films were prepared, and the friction and wear properties of these specimens were investigated using a reciprocating ball-on-disk tribometer in the temperature range from room temperature to 1073 K in air. It was clarified that (ZrO₂–3 mol% Y₂O₃)–39.6 mass% Al₂O₃ substrates coated with chemically precipitated SrSO₄ particles and the substrates coated with SrSO₄–10 mass% Ag films prepared by mechanically grinding and annealing at 1073 K for 3.6 ks exhibited low friction coefficients and low wear rates at all the testing temperatures. In addition, the average friction coefficients of Si₃N₄ substrates were reduced above 673 K by coating with chemically precipitated SrSO₄ particles.     

The Tribological Behaviors of Different Mass Ratio Al2O3-TiO2 Coatings in Water Lubrication Sliding against Si3N4

The tribopairs of water hydraulic plunger pumps are usually operated under severe conditions, due to the poor lubrication of water and silt suspended in natural water. It is essential to identify the desired engineering materials and material combinations for designing water pumps. As the candidate materials of tribopairs, the tribological characteristics of different Al₂O₃-TiO₂ coatings combined with Si₃N₄ ceramics under silt-laden water and tap water lubrication were investigated. The Al₂O₃-TiO₂ coatings with different weight percentages of TiO₂ in a wide range from 3 to 100% were tested. The tribological characteristics of the various couple pairs were researched using a ring-on-ring test rig. The microstructures of the contact surfaces were analyzed with a scanning electron microscope before and after the test to study the wear mechanism. The experimental results indicate that the friction coefficient of the Al₂O₃-TiO₂/Si₃N₄ tribopairs increases with an increase in the percentage of TiO₂ content in the Al₂O₃-TiO₂ coating. However, the TiO₂ content does not present a clear relationship with the wear rate. Considering the friction and wear properties, Al₂O₃-13%TiO₂ is the preferred coating to use in water hydraulic pumps when sliding against Si₃N₄.     

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.     

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.     

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.     

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.     

Effect of Mo and Ag on the friction and wear behavior of ZrO2 (Y2O3)–Ag–CaF2–Mo composites from 20 °C to 1000 °C

In this paper, a series of ZrO₂ matrix high-temperature self-lubricating composites were prepared by hot-press technique. The effect of Mo and Ag on the friction and wear behavior of the ZrO₂(Y₂O₃)–Ag–CaF₂–Mo composites in a wide temperature range was investigated. The XRD results showed that CaMoO₄ formed on the worn surface above 400 °C. The excellent lubrication performance of CaMoO₄ endowed the low coefficient of friction of the ZrO₂(Y₂O₃)–Ag–CaF₂–Mo composites at high temperatures. The ZrO₂(Y₂O₃)–10Ag–10CaF₂–10Mo composites showed favorable wear resistance at all the tested temperatures which was attributed to the combined action of hardness and phase transformation.     

Characterisation of ZrO2 layers deposited on Al2O3 coating

The paper describes a method of coating combining two different layer types. The first layer is Al₂O₃ produced by plasma spraying with a thickness of around 200 μm which was deposited on a stainless steel substrate. Subsequently, ZrO₂ layers were deposited on to the Al₂O₃ coating by a sol–gel process using a dip coating technique. The dip coating process was repeated in order to see the influence of the number of ZrO₂ layers. Moreover, the effect of annealing temperature was investigated. In order to study their tribological behaviour, the coatings were subjected to micro-scale abrasion, scratch testing and ball-on-disc tests. The result shows that sol–gel ZrO₂ top layers reduce friction and enhance the wear resistance of the coating system.     

Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)–Al2O3–Ba x Sr1?x SO4 (x?=?0.25, 0.5, 0.75) Composites at Elevated Temperature

Self-lubricating ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have been fabricated by spark plasma sintering (SPS) method. The tribological properties have been evaluated using a high-temperature friction and wear tester at room temperature and 760 °C in dry sliding against alumina ball. The composites exhibit distinct improvements in effectively reducing friction and wear, as compared to the unmodified ZrO₂(Y₂O₃)–Al₂O₃ ceramics. The ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have great low and stable friction coefficients of less than 0.15 and wear rates in the order of 10^− 6mm³/Nm at 760 °C. Delamination is considered as the dominating wear mechanism of the composites at room temperature. At elevated temperature, the formation and effective spreading of Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) lubricating films during sliding play an important role in the reduction of the friction and wear.     

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.     

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.     

Effect of Y2O3 content on Ni60 alloy cladding layer

Six kinds of Ni60 alloy coatings with different percentage of Y₂O₃ were prepared by laser cladding. A metallurgical microscope was used to analyze the morphology of the cladding layer. Scanning electron microscopy and EDS energy spectrum analysis were used to characterize the microstructure and element segregation of the cladding layer. A Vickers microhardness tester was used to measure the hardness of the cladding layer. Finally, a friction and wear tester established the friction and wear properties of the cladding layer. The study results show that Y₂O₃ can significantly reduce the height of the cladding layer and increase the width of the cladding layer; it can also improve the structure refinement and element segregation of the cladding layer. The microhardness of the cladding layer is significantly improved compared to the Ni60 alloy coating without Y₂O₃, thereby enhancing the wear resistance of the coating.

     

Sliding wear behavior of protective coatings for diesel engine components

The use of heat-insulating ceramic coatings on the cylinder walls of diesel engines is currently being considered for certain advanced engine designs. Since a major consideration in such an application is the wear resistance of the coatings, a series of tests has been carried out to determine the sliding wear behavior of several pairs of candidate materials systems, initially at room temperature. The tests were performed using a washer-on-disc specimen configuration and an oscillatory rotation movement to simulate the motion of a piston ring on a cylinder wall. It was determined that each material tested had a different pattern of sliding wear behavior. Impregnation of plasma-sprayed Y₂O₃-ZrO₂ with chromia markedly improved its wear resistance.