Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO₂) and doped TiO₂ ceramic thin films were prepared on a glass substrate by a sol–gel and dip-coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of TiO₂ and doped TiO₂ thin films sliding against Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO₂ films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO₂ films are inferior to the TiO₂ films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO₂ thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO₂ thin films show lower plasticity than the TiO₂ thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.     

Tribological performance of hybrid organo-silicate coatings

Hybrid organo-silicate coatings (hybrid coatings) present many advantages and find use in many applications. However, their tribological performance has not been widely investigated due to the non-uniformity of the synthesized hybrid materials. In this work, nanoscratch tests have been employed for the investigation of wear mechanisms and organic–inorganic coatings adhesion. The tribological performance was investigated under low applied normal forces, in order to estimate the coefficient of friction (μ) values, the coatings resistance and the humidity effect when the synthesized coatings were submerged on a cell culture medium. Also, the performed plastic deformation was correlated with the experimental data and the surface images taken with a scanning probe microscope after the scratch tests. Furthermore, the experimental data were fitted and the relative coefficient of friction and cohesive force of each synthesized coating were calculated, since organic and hybrid materials do not conform with the classic tribology laws and are greatly affected by the adhesion forces observed between tip and sample's surface.     

Effect of sidewall modification in the determination of friction coefficient of vertically aligned carbon nanotube films using friction force microscopy

The significance of the sidewall surface of vertically aligned carbon nanotubes (VACNTs) and the effect of humidity in the determination of VACNT film friction coefficient have been investigated. VACNT films of 2 μm thick were sidewall-modified by means of CF₄ and O₂ plasma treatments, and verified for the functionalization of the sidewalls. They were then characterized for wettability properties, as well as friction coefficient using friction force microscopy at different humidity levels. It was found that humidity had insignificant effect on the friction coefficient, and sidewall friction formed a major component of the friction force experienced by the tip. Sidewall modifications resulted in friction coefficient changes of up to 50%.     

Structure and friction properties of laser-patterned amorphous carbon films

In the paper we report on laser surface modification of super hard micrometer-thick tetrahedral amorphous carbon (ta-C) films in the regime of single-shot irradiation with KrF laser pulses (wavelength 248 nm, pulse duration 20 ns), aimed at investigations of the laser-induced changes of the structure and surface properties of the ta-C films during graphitization and developing ablation processes. Based on the analysis of surface relief changes in the laser-irradiated spots, characteristics of the single-shot graphitization and ablation of the 2-μm-thick ta-C film are determined. Using Raman spectroscopy, it is found that during the graphitization regime the structure transformation and growth of graphitic clusters occur according to the relationship I(D)/I(G) ∝ L_a², but after reaching the ablation threshold the Tuinstra-Koenig relationship I(D)/I(G) ∝ 1/L_a describes further growth of the graphitic cluster size (L_a) during developing ablation of the ta-C film with nanosecond pulses. The maximal size of graphitized clusters is estimated as L_a = 4–5 nm. The studies of nanomechanical properties of laser-patterned ta-C films using the lateral force microscopy and force modulation microscopy have evidenced lower friction forces (between diamond-coated tips and film surface) and lower stiffness in the laser-graphitized areas. The laser-produced graphitic layer acts as a solid lubricant during sliding of the diamond-coated tips on the ta-C film surface in ambient air (~ 50% RH); the lubricating role of adsorbed water layers is suggested to be significant at low loads on the tips. The results of this work demonstrate that the UV laser surface texturing in the regime of graphitization is a promising technique to control the friction and surface elasticity of super hard amorphous carbon films on the micro and nanoscale.     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

Wang-Landau Monte Carlo simulation of capillary forces at low relative humidity in atomic force microscopy

A lattice gas model is used with Wang-Landau Monte Carlo sampling to predict the capillary force between a model of an atomic force microscopy (AFM) probe and a smooth surface as a function of separation, relative humidity (RH), and tip hydrophilicity. Completely wetting AFM tips exhibit a maximum in the capillary force as the RH increases, while the magnitude of the capillary force in the presence of partially wetting and partially drying tips is relatively independent of the RH. Capillary forces can also be significant in low RH environments and should not be discounted in AFM studies involving hydrophilic surfaces.     

Tribological analysis of glass fibers using atomic force microscopy (AFM)/lateral force microscopy (LFM)

By using atomic force microscopy (AFM)/lateral force microscopy (LFM), a comparative study of the topography as well as the tribological properties (at a micrometer scale) of sized E‐glass fibers was done. Normal and lateral deflection signals are recorded when an AFM tip scans a fiber surface. Friction force data were obtained from the forward and backward scans of lateral force images whose contrasts reveal differences in friction coefficient values and, hence, surface chemical heterogeneity of certain‐sized glass fibers. Sizes having an epoxy film former lead to a higher friction coefficient value than those containing a starch film former. Moreover, the epoxy‐containing size is more readily plowed by the AFM tip. Annealing of this size lowers its friction coefficient. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1013–1025, 2000     

Low Friction in CuO-Doped Yttria-Stabilized Tetragonal Zirconia Ceramics: A Complementary Macro- and Nanotribology Study

The tribological behavior of CuO-doped yttria-stabilized tetragonal zirconia (3Y-TZP) ceramics in the absence of additional lubricants was characterized by macroscale pin-on-disk measurements and nanoscale atomic force microscopy (AFM) for a broad range of velocities. The previously observed low shear strength interfacial layers generated in pin-on-disk tracks by Al₂O₃ ball counter surfaces on CuO-doped 3Y-TZP, as well as virgin surfaces, were probed quantitatively by AFM with Si₃N₄ tips as the counter surface. The observed trends in nanoscale coefficient of friction determined by AFM were found to be in agreement with data acquired using a pin-on-disk tribometer. The combined data support the notion that a layer of surface contaminations is removed during the initial sliding, and wear of high asperities occurs. Subsequently, an interfacial layer with low shear strength is generated during sliding. While these results do not provide an exhaustive explanation for the process of layer formation, they represent the first report of bridged nano- and macrotribological analysis of a compositionally heterogeneous low-friction, low-wear ceramic material and further confirm some of the key assumptions for the deterministic model reported previously by Pasaribu and Schipper.     

Characterization and tribological investigation of sol-gel Al2O3 and doped Al2O3 films

Thin films of Al₂O₃ and doped Al₂O₃ were prepared on a glass substrate by dip coating process from specially formulated ethanol sols. The morphologies of the unworn and worn surfaces of the films were observed with atomic force microscope (AFM) and scanning electron microscope (SEM). The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of obtained thin films sliding against Si₃N₄ ball were evaluated and compared with glass slide on a one-way reciprocating friction tester. XPS results confirm that the target films were obtained successfully. The doped elements distribute in the film evenly and exist in different kinds of forms, such as oxide and silicate. AFM results show that the addition of the doped elements changes the structure of the Al₂O₃ films, i.e., a rougher and smoother surface is obtained. The wear mechanisms of the films are discussed based on SEM observation of the worn surface morphologies. As the results, the doped films exhibit better tribological properties due to the improved toughness. Sever brittle fracture is avoided in the doped films. The wear of glass is characteristic of brittle fracture and severe abrasion. The wear of Al₂O₃ is characteristic of brittle fracture and delamination. And the wear of doped Al₂O₃ is characteristic of micro-fracture, deformation and slight abrasive wear. The introduction of ZnO is recommended to improve the tribological property of Al₂O₃ film.     

Synthesis and Tribology of Carbide-Derived Carbon Films

Carbide-derived carbon (CDC) films are produced at atmospheric pressure on the surfaces of carbide-based ceramic materials and coatings by a high-temperature chlorination process. These nanoporous carbon films contain carbon nano-onions and amorphous carbon, and may contain nanocrystalline diamond and graphite as well, depending on the synthesis conditions. The combination of such diverse carbon phases in one material or coating provides unique and potentially useful properties for a wide range of engineering applications. In this paper, we will present the results of a comprehensive study on the tribological behavior of these films. The friction coefficient of CDC in open air is comparable with that of graphite and is typically in the range of 0.15–0.25. However, the friction coefficients of CDC tend to decrease with decreasing humidity. In dry nitrogen, its friction coefficient is ∼0.1 or less. Such behavior is in contrast to that of crystalline graphite, which normally exhibits low friction at high humidity, but high friction at low humidity or in vacuum. The friction coefficient of CDC becomes increasingly lower under heavier loads; however, increasing sliding velocity does not seem to affect its frictional behavior significantly. Using a hydrogenation process that removes residual chlorine from the CDC film, the friction coefficients of CDC can be further lowered to values as low as 0.03. In an attempt to understand some of the underlying mechanisms, we carried out comprehensive chemical and structural studies of the sliding surfaces as well as bulk films and correlated these findings with the friction and wear behavior of CDC films.     

The Effect of Humidity on the Adhesion Behavior of AC Anodised Aluminium

The effect of humidity on the surface forces of hot AC anodised AA6060 alloy was studied. The surface adhesion forces were measured in various humidity environments using an atomic force microscope (AFM). The environmental durability of the joints bonded at different humidity conditions was investigated using wedge test experiments. The results from AFM and durability tests indicate that there is a strong capillary effect at around 70–80% relative humidity. This effect was attributed to the change in the adsorption behavior of water on the anodised oxide surface forming bulk liquid and hence dissociating the bonds across the interface. At the lower humidity levels below 60% RH, no capillary effect was observed and the total adhesion forces followed the dry case values.     

Humidity dependence on the friction and wear behavior of diamond-like carbon film in air and nitrogen environments

Diamond-like carbon (DLC) films were deposited on Si (100) wafers by a plasma enhanced chemical vapor deposition (PECVD) technique using CH₄ plus Ar as the feedstock. The friction and wear behaviors of the resulting film sliding against Si₃N₄ balls were investigated on a ball-on-disk test rig in air and nitrogen environments at a relative humidity from 5% to 100%. The worn surface morphologies of the DLC film and the Si₃N₄ counterpart were observed on a scanning electron microscope (SEM), while the chemical states of some typical elements thereon were investigated by means of X-ray photoelectron spectroscopy (XPS). It was found that the DLC film recorded continuously increased friction coefficient and wear rate with increasing relative humidity in air. It showed linearly increased friction coefficient with increasing relative humidity in nitrogen, in this case the wear rate sharply decreased and reached the minimum at a relative humidity of 40%, which was followed by an increase with further increase of the relative humidity. The interruption of the transferred carbon-rich layers on the Si₃N₄ balls, and the friction-induced oxidation of the films in higher relative humidity were proposed to be the main reasons for the increases of the friction coefficient and wear rate. Moreover, the oxidation and hydrolysis of the Si₃N₄ ball in higher relative humidity, leading to the formation of a tribochemical film that mainly consists of silica gel on the wearing surface, were also thought to have effects on the friction and wear behaviors of the DLC films.     

Boron carbide films deposited by a magnetron sputter–ion plating process: film composition and tribological properties

Amorphous boron carbide films were deposited onto silicon substrates by a magnetron sputter–ion plating process in an argon plasma atmosphere (0.25 Pa) using a B₄C target. The substrates were polarized with a d.c. bias voltage in the range from 0 to −100 V. The film composition and the presence of contaminants were determined by ion beam analysis (IBA). The nanoscale tribological properties were investigated by atomic force microscopy (AFM). IBA revealed that the boron/carbon atomic ratio is around 4 and that oxygen contamination does not exceed 10 at.%. The hydrogen content is below 2 at.%. The film density is nearly the bulk value for all biases applied to the substrate. AFM measurements show that the surface roughness decreases with increase of bias from 0.85 to 0.15 nm. The friction coefficient obtained by lateral force measurements follows the same trend, decreasing with increasing bias from 0.25 to 0.1. Wear measurements were performed and the wear depth decreased for films with lower friction coefficients. A mechanism based on the removal of a modified B₄C surface layer is proposed to explain the wear results.     

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R_a and R_z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” (R_sk) and “Kurtosis” (R_ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R_a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R_a = 10 nm. This phenomenon is explained by analyzing the R_sk and R_ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.     

Nano-scale friction of polystyrene in air and in vacuum

Using atomic force microscopy (AFM), we measured friction between an AFM tip and a polystyrene surface at 25 °C, as a function of the sliding velocity and the applied normal load, both in air and under vacuum conditions. The objective was to analyze the influence of humidity on the frictional behavior of polystyrene. Our experimental results as a function of sliding velocity revealed a logarithmic increase of the friction force in air whereas a logarithmic decrease of this force is found in vacuum. Our comparative results unveil that two different dissipation mechanisms are dominating the frictional behavior of polystyrene in air and in vacuum. We propose a tentative explanation.     

Tribological Characteristics of α-Alumina at Elevated Temperatures

The tribological characteristics of a high-purity α-alumina sliding on a similar material under unlubricated conditions are divided into four distinct regimes. At low temperatures, T < 200°C, tribochemical reactions between the alumina surface and water vapor in the environment control the tribological performance. The coefficient of friction in this temperature range is approximately 0.40 and the wear coefficient is less than 10⁻⁶, independent of contact load. At intermediate temperatures, 200°C < T < 800°C, the wear behavior depends on the contact load. At low loads, wear occurs by plastic flow and plowing; the coefficient of friction is approximately 0.60 and the wear coefficient is less than 10⁻⁶. At loads larger than a threshold value, severe wear occurs by intergranular fracture. The coefficient of friction increases to 0.85 and the wear coefficient increases to a value greater than 10⁻⁴. At temperatures above 800°C, formation of a silicon-rich layer on the wear track by diffusion and viscous flow of the grain-boundary phase reduces the coefficient of friction to 0.40, and the wear coefficient is reduced to a value less than 10⁻⁶. The results of the wear tests and observations of the fundamental mechanisms controlling the tribological behavior of this material are consolidated in a simple wear transition diagram.     

原子力显微镜对材料表面微摩擦性能研究进展

原子力显微镜(AFM)被广泛应用于材料表面微摩擦学的研究,利用AFM针尖对样品表面进行扫描,来模拟在微摩擦中存在的点面接触的情况。综述国内外研究发现:微摩擦性能受多因素的影响,摩擦力与表面形貌呈现相同的周期性,随表面湿度增加先增大后减小,并与表面分子基团类型、相对滑动速度、表面势的改变有很大关系。     

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm³) and sp³ content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm², producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.     

A theoretical analysis of frictional and defect characteristics of graphene probed by a capped single-walled carbon nanotube

Using molecular dynamics simulations, we show that a probing tip using a short capped single-walled carbon nanotube is able to capture the frictional characteristics and faithfully resolve the graphene lattice through the measurements of oscillatory lateral force or normal force. By averaging the oscillatory lateral force and normal force along the tip moving path, we extract the friction coefficient. It is found that the friction coefficient decreases with increasing both the initial tip–surface distance and the number of graphene layer. The underlying energy dissipation arises from the periodical acceleration–deceleration of the tip, causing the conversion of kinetic energy into thermal energy. We also study the interaction of the tip with a single-layer graphene containing a vacancy or Stone–Thrower–Wales defect, and reveal that the change in lateral and normal forces can be used to differentiate these defects. The present study demonstrates that a short single-walled capped nanotube can serve as an ideal candidate for high-resolution surface probing.     

A comparative study on the effect of VUV radiation in plasma SiOx-coated polyimide and polypropylene films

Wettability of polyimide (PI) and polypropylene (PP) films have been improved using SiO_x-like thin layers deposited from a mixture of hexamethyldisiloxane (HMDSO) and oxygen in a microwave distributed electron cyclotron resonance plasma reactor. The films wettability evolution behaviors were evaluated through the results of contact angle measurements, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The plasma depositions of SiO_x thin layers in presence of VUV radiation induce a contact angle decrease to about 7° and 35° for PI and PP films, respectively. XPS data showed that such difference in wettability is attributed to the increase of hydrophilic group's proportion at the surface of coated PI films due to VUV irradiation. AFM images showed that the PI surface topography remains relatively smooth when coated in presence of VUV radiation. However, in the case of PP films, AFM images revealed the growth of irregular structure due to a substrate etching effect supported by VUV radiation. For polymers coated without VUV irradiation, the deconvolution of the C1s peaks showed a significant decrease of CO bonds for both PI and PP substrates.