Adhesive strength and physical, mechanical, and triboengineering properties of nano- and microstructural Al2O3 coatings

The methods of atom-force microscopy and measurement of the nanohardness, elasticity modulus, cohesion forces, and friction coefficient with scratch testing (the scratching method) are applied to study the Al₂O₃ oxide coatings produced by magnetron deposition on Al targets. The morphologies of the coatings are compared when they are produced by the reactive synthesis and magnetron target sputtering and application of oxygen ions in high-frequency plasma. The accomplished study of the tribological and mechanical characteristics demonstrates that the resulting coatings possess stronger hardness and elasticity modulus. Application of the oxide coatings reduces the friction coefficient considerably compared with the substrate; the scratch test reveals higher strength and better adhesion to the substrate. Comparative analysis shows that the Al₂O₃ coatings applied to the nanostructural TiN film are 1.5 times harder than the coatings deposited on steel.     

Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films

With excellent lubricating property, zinc oxide (ZnO) films are promising candidates to act as protective coatings in Si-based microelectromechanical system devices for the purpose of decreasing friction forces of silicon (Si) material. In this paper, the nanotribological behavior of ZnO films prepared by atomic layer deposition on a Si (100) substrate is investigated by an atomic force microscope. The ZnO films have various thicknesses ranging from 10.0 to 182.1 nm. With the increase of film thickness, the root-mean-square roughness of the films increases, while the ratio of hardness to Young’s modulus (H/E) decreases. Due to their large surface roughness, the thick ZnO films are low in adhesion force. The friction force of the ZnO films is smaller than that of the Si (100) substrate and is greatly influenced by their adhesion force and mechanical property. In a low-load condition, the friction force is dominated by the adhesion force, and thus, the friction force of the ZnO films decreases as film thickness increases. While in a high-load condition, the friction force is dominated by plowing. Films with higher H/E possess smaller friction force, and thus, the friction force increases with the decreasing film thickness.     

Nano/Microtribological Properties of Ultrathin Functionalized Imidazolium Wear-Resistant Ionic Liquid Films on Single Crystal Silicon

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrile, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano- and microtribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nanoscale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity, and anti-wear durability at microscale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane microtribotester. Friction reduction, adhesion resistance, and durability of IL films were dependent on their cation chemical structures, wettability, and ambient environment.     

Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM

Molecularly thick perfluoropolyether (PFPE) films are considered to be good protective films for micro/nanoelectromechanical systems (MEMS/NEMS) to reduce stiction, friction, and improve their durability. Understanding the nanotribological performance and mechanisms of these films are quite important for efficient lubrication for MEMS/NEMS devices. These devices are used in various operating environments and their effect on friction, adhesion and durability needs to be clarified. For this purpose, mobile and chemically bonded PFPE films were deposited by dip coating technique. The friction and adhesion properties of these films were characterized by atomic force microscopy (AFM). The effect of rest time, velocity, relative humidity, and temperature on nanotribological properties of these films was studied. Durability of these films was also measured by repeated cycling tests. The adhesion, friction mechanisms of PFPE at molecular scale, and the mechanisms of the effect of operating environment and durability are subject of this paper. This study found that adsorption of water, formation of meniscus and its change during sliding, viscosity, and surface chemistry properties play a big role on the friction, adhesion, and durability of the lubricant films.     

Tribological and corrosion behaviors of Al2O3/polymer nanocomposite coatings

In this paper, the tribological and electrochemical corrosion properties of Al₂O₃/polymer nanocomposite coatings were studied by using micro-hardness test, single-pass scratch test, abrasive wear test, and finally electrochemical technique such as potentiodynamic polarization measurement. The coatings containing Al₂O₃ nanoparticles showed improvement in scratch and abrasive resistance compared with that of polymer coating. The improvement in scratch and abrasive resistance is attributed to the dispersion hardening of Al₂O₃ nanoparticles in polymer coatings. Corrosion test results showed that the embedded Al₂O₃ nanoparticles in polymer matrix do not sacrifice the corrosion resistance of the polymer itself.     

Steam and Water Lubrication of Quasicrystalline Films

The present work reports on the comparison of tribological properties between quasicrystalline and approximant films under steam and water lubrication. These films have nearly the same stoichiometry, which allows evaluation of the quasicrystalline structure as compared with the crystalline structure of the approximant. The goal of this study is to determine the effects crystal structure has on tribological properties and to study the general tribological behavior of quasicrystals in a steam and water environment. AlCuFe quasicrystals coatings were deposited by unbalanced magnetron sputtering. The test films were 10 m thick deposited on a 1inch diameter Al₂O₃ disk and a 1/4inch diameter Al₂O₃ ball. Data were collected under water and steam lubricated conditions. Friction coefficients and wear rates of quasicrystals and approximants were nearly indistinguishable. During water lubrication, quasicrystals act similar to ceramics where surface smoothing and hydrodynamic lift control the friction and wear properties. With steam-lubricated conditions, the friction and wear process is controlled by brittle fracture and third body abrasion. Generally, less than 5% difference in the friction and wear properties were observed between the icosahedral quasicrystal phase and the approximant phase.     

Investigation of antiwear coatings deposited by the pvd process

The tribological properties of various PVD‐deposited coatings (vacuum arc method) have been tested, both single‐layer coatings (TiN, CrN, Ti(C,N), and Cr(C,N)) and multilayer coatings (Cr(C,N)/CrN/Cr and CR(C,N)/(CrN+Cr₂N)/CrN/Cr). An unlubricated ball‐on‐disc tribosystem was used in which an Al₂O₃ ball is pressed against a coated steel disc rotating in the horizontal plane. A novelty of the method is the removal of wear debris from the contact zone using a draught of dry argon. This improves the repeatability of the test results and the stability of the tribological characteristics. It is shown that CrN coatings exhibit the best antiwear properties and Ti(C,N) the worst. Multilayer coatings have better antiwear properties than single‐layer ones. The friction coefficients for CrN and Cr(C,N) coatings are much smaller than for the commonly used TiN. A correlation has also been found between the physical properties of the coatings tested (adhesion of the coating to the substrate assessed in scratch tests, and coating hardness) and their antiwear properties. An improvement in coating‐substrate adhesion results in wear reduction, while greater hardness (causing a coating embrittlement increase and a change in the wear mechanism) brings about greater wear. There is no correlation between the physical properties and the friction coefficients of the coatings tested.     

Role of Al2O3 in Semi-Metallic Friction Materials and its Effects on Friction and Wear Performance

In this work the semi-metallic friction composites with moderately increased content of Al₂O₃ (0, 3.4, 5.6, 9.0, and 14.6 vol%) were successfully fabricated by casting technology and their tribological properties were evaluated. The character of friction surfaces was confirmed by SEM. The sample with 5.6 vol% of Al₂O₃ offers the best tribological property among the prepared formulations, and is characterized by high and steady friction coefficient (about 0.45) and low wear. With the temperature increasing, the stibnite patches population on friction surface increases and individual steel fibers spread due to their plastic deformation to form larger primary contact plateaus. The area covered by contact primary plateaus decreases with the alumina content increasing.     

Nanofretting Behavior of Monocrystalline Silicon (100) Against SiO2 Microsphere in Vacuum

With an atomic force microscopy, the tangential nanofretting between spherical SiO₂ tips and monocrystalline Si(100) surface was carried out at various displacement amplitudes (0.5–250 nm) under vacuum condition. Similar to fretting, the nanofretting of Si(100)/SiO₂ pair could also be divided into stick regime and slip regime upon the transition criterion. However, it was found that the energy ratio corresponding to the transition between two nanofretting regimes varied between 0.32 and 0.64, which was higher than the normal value of 0.2 for the transition criterion to determine the partial slip and gross slip regimes in fretting. One of the reasons may be attributed to the effect of adhesion force, since whose magnitude is at the same scale to the value of the applied normal load in nanofretting. During the nanofretting process of Si(100)/SiO₂ pair, the adhesion force may induce the increase in the maximum static friction force and prevent the contact pair from slipping. The higher the applied load, or the higher the adhesion force, the larger will be the transition displacement amplitude between two regimes in nanofretting. Different from fretting wear, the generation of hillocks was observed on Si(100) surface under the given conditions in nanofretting process. With the increase in the displacement amplitude in slip regime of nanofretting, the height of hillocks first increased and then attained a constant value. Compared to chemical reaction, the mechanical interaction may be the main reason responsible for the formation of silicon hillocks during the nanofretting in vacuum. The results in the research may be helpful to understand the nanofretting failures of components in MEMS/NEMS.     

Grey theory applied to evaluate the tribological performances of the a-C:H(N) coating films prepared by differing the nitrogen content and the film thickness

The purpose of this paper is to study the tribology performances of the aC:H(N) films by using a nanotester under different scratch loads and velocities. From the measurements of the friction coefficient and wear volume, the tribological performances including wear resistance and friction coefficients were evaluated for the hydrogenated amorphous carbon films prepared by differing film thickness and nitrogen volume friction in the gas mixture of (C₂H₂+N₂). Taguchi experimental design and the grey relational analysis were used to investigate the influence of specimen parameters (film’s thickness, nitrogen content in the film), and operating conditions in tribological tests (scratch load and scratch velocity) on the friction coefficients and the wear volume arising in the specimens with different coating films. It is found that the wear volume of thin film is increased by increasing either the nitrogen volume fraction or film thickness. Moreover, the optimal combination of the testing parameters was also determined in the use of the present model.     

Effect of CNFs content on the tribological behaviour of spark plasma sintering ceramic–CNFs composites

Alumina-carbon nanofibres (CNFs) and silicon carbide–CNFs nanocomposites with different volume fraction of CNFs (0–100 vol.%) were obtained by spark plasma sintering. The effect of CNFs content on the tribological behaviour in dry sliding conditions on the ceramic–carbon nanocomposites has been investigated using the ball-on-disk technique against alumina balls. The wear rate of ceramic–CNFs nanocomposites decreases with CNFs increasing content. The friction coefficient of the Al₂O₃/CNFs and SiC/CNFs nanocomposites with high CNFs content was found to be significantly lower compared to monolithic Al₂O₃ and SiC due to the effect of CNFs and unexpectedly slightly lower than CNFs material. The main wear mechanism in the nanocomposite was abrasion of the ceramic and carbon components which act in the interface as a sort of lubricating media. The experimental results demonstrate that the addition of CNFs to the ceramic composites significantly reduces friction coefficient and wear rate, resulting in suitable materials for unlubricated tribological applications.     

Tribological performance of Al2O3/NiCr coating

The tribological performance of Al₂O₃/NiCr coating deposited on steel (SM45C) was investigated under lubrication. The parameters of sliding wear consist of normal load and coating thickness. Test result showed that there was no evidence of an improved bonding strength in the coating. However, the wear resistance of the Al₂O₃/NiCr coating was significantly greater than that of the Al₂O₃ coating. It was evident that the residual stress for the AI2O3 coating was higher than that of the Al₂O₃/NiCr coating from the scratch test failure of coating. The bond coating played an important role in decreasing the residual stress. Also, it was found that the residual stress had a notable influence on the wear mechanism.     

Scratch Behavior of ZrO<Subscript>2</Subscript> Thin Film Prepared by Atomic Layer Deposition Method on Silicon Wafer

To evaluate friction behaviour and adhesion strength of 130 nm-thick zirconia (ZrO₂) film produceded by atomic layer deposition (ALD) on Si substrate, scratch tests were performed at two different scales; micro- and macro-scales. Surface morphology, roughness, crack propagations and interations between the surface and sliding indenter were also investigated. Scratch test was also conducted with Si substrate as a reference. The test results showed that friction coefficient (COF) was influenced by the scale of scratch test regardless of the tested materials. In microscale test, the 130 nm-thick zirconia film failed at the critical load of 96 mN and direct relationship between the generation of micro-cracks and friction coefficient was observed. Based on the Hertzian contact theory and experimental results, the macroscratch width was much greater than that in microscratch test although the maximum contact pressure were comparable in both cases. Further discussion was made with regard to the influence of the contact pressure on COF, crack generation and film removal. Various types of failure mode were identified through analyzing the mechanical response of scratch tracks both at micro- and macroscale tests. This study suggested that 130 nm-thick ALD-ZrO₂ film showed better tribological and adhesion properties at microscale contact than macroscale contact.     

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.     

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.     

Tribological Properties and Wear Mechanisms of NiCr–Al2O3–SrSO4–Ag Self-Lubricating Composites at Elevated Temperatures

NiCr–Al₂O₃–SrSO₄–Ag self-lubricating composites were prepared by powder metallurgy method and the tribological properties of composites were evaluated by a ball-on-disk tribometer against alumina ball at wide temperature range from the room temperature to 1,000 °C in air. The linear coefficient of thermal expansion was evaluated for investigation of thermal stability of composites. The tribo-chemical reaction films formed on the rubbing surfaces and their effects on the tribological properties of composites at different temperatures were addressed according to the surface characterization by SEM, XRD, and XPS. The results show that the NiCr–Al₂O₃ composite with addition of 10 wt% SrSO₄ and 10 wt% Ag exhibits satisfying friction and wear properties over the entire temperature range from room temperature to 1,000 °C. The composition of the tribo-layers on the worn surfaces of the composites is varied at different temperatures. The synergistic lubricating effect of SrAl₄O₇, Ag, and NiCr₂O₄ lubricating films formed on worn surfaces were identified to reduce the friction coefficient and wear rate from room temperature to 800 °C. Meanwhile, at 1,000 °C, the SrCrO₄ and NiAl₂O₄ was formed on the worn surfaces during sliding process, combining with the NiCr₂O₄, Al₂O₃, Cr₂O₃, Ag, and Ag₂O, which play an important role in the formation of a continuous lubricating film on the sliding surface.     

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 behavior and tribochemical reactions of alumina in HFC-134a environment

The tribological behavior of alumina (Al₂O₃) in CF₃CH₂F (HFC-134a) gas at pressures between 170 Pa and 10⁵ Pa were investigated using a ball-on-disk type tribometer. For comparison, the friction test was also carried out in a vacuum (10⁻⁵ Pa) under the same experimental conditions. X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ions Mass Spectrometry (TOF-SIMS) were used to identify the formation of tribochemical products.It was found that the friction and wear properties of Al₂O₃ were strongly dependent on the pressure of HFC-134a gas. A higher pressure correlated to a lower friction coefficient. When the pressure exceeded 10³ Pa, the wear strikingly decreased. The results of XPS and TOF-SIMS analyses identified that tribochemical films, mainly composed of fluorine-containing organic compounds and Al-oxyfluorides, were produced on the frictional surfaces. In addition, it was found that the amount of the fluorides correlated well to the friction and wear properties, that is, the more the fluorides were, the lower were the friction and wear. The results of research indicate that HFC-134a gas has a good lubricating effect for Al₂O₃ ceramic, which is attributed to the formation of the tribofilms at the frictional interface.     

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 niobium interlayer on high-temperature sliding friction and wear of silver films on alumina

In this study, we investigated the effect of a thin Nb bonding layer (15–20 nm thick) on the high-temperature sliding friction and wear performance of Ag films ( 1.5 m thick) produced on -alumina (Al₂O₃) substrates by ion-beam assisted deposition (IBAD). The friction coefficients of Al₂O₃ balls against the Ag-coated Al₂O₃ flats were 0.32 to 0.5 as opposed to 0.8 to 1.1 against the uncoated flats. Furthermore, these Ag films reduced the wear rates of Al₂O₃ balls by factors of 25 to 2000, depending on test temperature. Wear of Ag-coated Al₂O₃ flats was hard to measure after tests at temperatures up to 400°C. At much higher temperatures (e.g., 600°C), these Ag films (without a Nb layer) were removed from the sliding surfaces and lost their effectiveness; however, Ag films with the Nb bonding layer remained intact on the sliding surfaces of the Al₂O₃ substrates even at 600°C and continued to impart low friction and low wear.