Mechanical properties and tribological behavior of ZrO<Subscript>2</Subscript> thin films deposited on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

A silane coupling reagent (3-mercaptopropyl)trimethoxysilane (abridged as MPTS) was self-assembled on a single-crystal Si substrate to form a two-dimensional organic monolayer (MPTS-SAM). The terminal –SH group in the MPTS-SAM film was in-situ oxidized to –SO₃H group to endow the film with good chemisorption ability. Then ZrO₂ thin films were deposited on the oxidized MPTS-SAM by way of the enhanced hydrolysis of aqueous zirconium sulfate (Zr(SO₄)₂·4H₂O) in the presence of aqueous HCl at 50 °C, making use of the chemisorption ability of the –SO₃H group. The thickness of the ZrO₂ films was determined with an ellipsometer, while their morphologies and corresponding friction forces were analyzed by means of atomic force microscopy. The hardness and elastic modulus of the ZrO₂ thin films were determined on a Nanoindentation II (MET) instrument. The macro-friction and wear behaviors of the ZrO₂ films sliding against an AISI-52100 steel ball were examined on a unidirectional friction and wear tester and the worn surface morphologies observed on a scanning electron microscope (SEM). As the results, the as-deposited ZrO₂ thin film at a deposition duration of 100 h is about 100 nm thick, it decreases to 48 nm after annealing at 500 °C and further decreases to 45 nm after heating at 800 °C. The as-deposited ZrO₂ film is relatively rougher, with the rms to be about 1.0 nm, while the ZrO₂ thin films heated at 500 and 800 °C have surface roughness rms of 0.76 nm and 0.68 nm, respectively. The ZrO₂ film annealed at 800 °C has a high hardness to elastic modulus (H/E) ratio (0.062) as compared to the as-deposited ZrO₂ film and the film annealed at 500 °C. Both the two annealed ZrO₂ films show excellent wear-resistance as they slide against AISI-52100 steel at a normal load below 2.0 N, while the one annealed at 800 °C has better wear-resistance. The differences in the friction and wear behaviors of the as-deposited ZrO₂ film, the ZrO₂ film annealed at 500 °C and that annealed at 800 °C are attributed to their different micro structures and compositions. Since the ZrO₂ films was well adhered to the underlying MPTS-SAM, it might find promising application in the surface-protection of single crystal Si and SiC subject to sliding at small normal load in microelectromechanical systems (MEMS).     

The Contribution of Thin PFPE Lubricants to Slider–Disk Spacing

We have investigated the effect of the molecular weight (MW) and film thickness of a perfluoropolyether lubricant, Zdol, on the slider–disk spacing loss, or clearance. The major conclusion of this work is that Zdol films as thin as 10 Å can reduce the slider–disk clearance by 2 nm or more in the molecular weight range of 1000–5000 amu. This is attributed to the attractive van der Waals interaction between the slider and the disk surface that causes the Zdol main chain to interact with the slider surface, giving rise to a friction force. When the film thickness of the lubricant exceeds the monolayer thickness, dewetting can take place. The droplets that form occupy the space between the slider and disk surface reducing the slider–disk clearance by as much as 4 nm. There is a step increase in the acoustic emission signal at the dewetting thickness transition, indicative of a slider–disk interference.     

An Accelerated Wear Test Method to Evaluate Lubricant Thin Films on Magnetic Hard Disks

A high speed ball-on-inclined-plane test method has been developed to evaluate the lubrication effectiveness of Z-Dol on magnetic hard disks. The test evaluates the combined durability of the lubricant film and the carbon overcoat under sliding conditions. A polished ruby (Al₂O₃) ball without suspension is used to simulate the head material. The ball slides over an inclined (at an angle of 0.055°±0.005°) section of the disk surface at 2.0 m/s linear velocity. The load is controlled by the geometric interference of the preloaded ball and the inclined plane. The contact forces are sampled periodically at 2 rpm and the frictional coefficients calculated. Repeated sliding between the ball and the disk sample leads to an increase in friction approaching that of the unlubricated case. Post test analysis using atomic force microscopy (AFM) suggests that the increase in friction is due to the loss of lubricant effectiveness of the lubricant and the wear of the carbon overcoat. X-ray photoemission microscopy (PEEM) results suggest progressive oxidation of Z-Dol as one of the degradation mechanisms leading to wear. The durability of the lubricating thin films is defined by the number of cycles to failure. Test repeatability is about 10%, depending on lubricant, film thickness, and surface roughness. The test can be used to evaluate different lubricant chemistries as well as different carbon overcoats. Compared to other pin-on-disk tests and step loading ball-on-disk methods, this test introduces two additional factors: high speed impact and wear acceleration by the inclined angle. The high speed impact simulates potential thermal stresses associated with head–disk contact. With an inclined angle, the load increases evenly for each contact cycle, hence simulates the ability of the lubricant layer to react to dynamic loads. The test is intended as a basic research tool to measure the fundamental resistance of the lubricant layer to resist repeated high speed contacts.     

Thin multilayer aluminum structures for superconducting devices

Various aluminum-based thin-film structures were manufactured and investigated at temperatures of 50 mK–3 K. Multilayer films of Al and Si, Al and Cr, and Al in the presence of oxygen were deposited by the thermal evaporation technique. As the thickness of pure-Al films decreases from 20 to 3 nm, the temperature of the superconducting transition increases from 1.30 to 2.45 K. An increase in the oxygen pressure to 5 × 10⁻⁶ mbar during deposition of Al films results in an increase in the critical temperature to 2.4 K. The presence of a chromium sublayer with a thickness of <0.5 nm may lead to complete suppression of superconductivity, whereas a thicker layer, 1–4 nm, deposited at a higher temperature with preliminary sputtering reduces the critical current of Al/Cr two-layer films to a lower degree. An atomic-force microscope was used to study the surface morphology and granularity and the roughness of manufactured film structures. The smallest linear roughness having a size of 0.29 nm for a 3-nm-thick film shows the advantage of using thinner films for creating a homogeneous tunneling barrier.     

Study of thin surfactant films under shear using the tribological surface force apparatus

Static and dynamic behaviour of thin surfactant films in aqueous solution of hexadecyltrimethylammonium salicylate (C16TASal) were investigated using the tribological surface force apparatus. Normal force measurements show that 0.15 mM C16TASal builds up an innermost film of approximately 8–11 Å thickness at each mica surface, indicating that the surfactant adsorbs in a flat conformation. Furthermore, the height of the force barrier at approximately 60Å is low (ca 2 mN/m) indicating that the second adsorbed layer is easily pushed out. Addition of salicylate salt to 0.15 mM C16TASal give rise to a more close packed structure, with a total thickness of 62–65 Å, indicative of a micellar or bilayer arrangement at the surfaces. Furthermore, the frequency dependence of the shear modulus was investigated both at close separation at the innermost force barrier and at larger separations (up to 300–400 Å). The visco-elastic measurements show that the elasticity modulus, G′, dominates over the loss modulus, G″, for all studied cases, indicative of a more solid-like than liquid-like film. Finally, it is shown that shear at high contact pressures induces new aggregate structures at the surface.     

Tribology of X-1P vapor lubricated hard disks

The tribological characteristics of vapor lubricated X-1P films on carbon coated disks were investigated as a function of lubricant thicknesses (0.2–2 nm) and compared with traditionally dip-coated X-1P and PFPE films. Glide and flyablity tests were performed and the lubricant redistribution in the ‘wear track’ was investigated using a surface reflectance analyzer (SRA). A critical lubricant thickness was found to exist for X-1P below which lubricant accumulation was observed, while lubricant loss was found to be present if the thickness of the lubricant film was greater than the critical thickness.     

Tribofilms generated from ZDDP and DDP on steel surfaces: Part 1, growth,wear and morphology

The growth and morphology of tribofilms, generated from zinc dialkyldithiophosphate (ZDDP) and an ashless dialkyldithiophosphate (DDP) over a wide range of rubbing times (10 s to 10 h) and concentrations (0.1–5 wt% ZDDP), have been examined using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy at the O, P and S K-edges and the P, S, and Fe L-edges. The physical aspects of the growth and morphology of the tribofilms will be presented in Part I and the chemistry of the films will be discussed in Part II. The major components of all films on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 Å thick) forms after 10 s, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600–800 Å after 30 min of rubbing, before leveling off or decreasing. The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations have also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed. The AFM results show that ZDDP forms larger and better developed “pads” than DDP at short rubbing times. At longer rubbing times, both films become more uniform. For the 1 h ZDDP films, the film thickness is surprisingly independent of the ZDDP concentration from 0.1 to 5 wt% ZDDP. The film thickness is also independent of the ratio of ZDDP/DDP concentrations.     

Structural, optical and mechanical properties of nanostructure diamond synthesized by chemical vapor deposition

Nanostructure diamond (NSD) films on Si substrate are prepared by microwave plasma enhanced chemical vapor deposition (MPECVD) using methane and hydrogen as the reactants with two-step negative substrate bias (SB). The dependencies of the NSD film morphology, grains, surface roughness, crystal and bonding structures and hardness on the negative SB at the bias-enhanced growth (BEG) step and substrate temperature during growth have been investigated by conducting atomic force microscopy (CAFM), X-ray diffraction (XRD), Raman spectroscopy and nanoindentation. The hardness of the NSD film is found to be as high as 80 GPa with CAFM average and root mean square roughness of 7 and 9 nm, respectively, under optimal negative SB at the BEG step. From the studies of substrate temperature effect, the hardness of the NSD film is as high as 70 GPa, with average and root mean square CAFM roughness of 9 and 11 nm, respectively, which were obtained at a substrate temperature of 500 °C. In both cases, the film hardness was found to be affected by the size of clusters, which are composed of many small NSD particles, the amount of NSD in an amorphous matrix as well as surface roughness. We also synthesized transparent NSD films by MPECVD under optimized single-step growth conditions on quartz substrates, which are scratched with several micrometers diamond powder. A hardness as high as 60 GPa and a maximum transmittance of 60% in the visible light region are achieved for an NSD coating of 1.0 μm thickness with small surface roughness.     

Study of the Chemistry of Films Generated from Phosphate Ester Additives on 52100 Steel Using X-ray Absorption Spectroscopy

X-ray absorption near-edge structure spectroscopy (XANES) has been used to investigate the chemistry and thickness of thermal and antiwear (AW) films generated on steel from oil solutions containing phosphate ester additives. DPP, a diaryl phosphate, reacted with steel to form a thermal phosphate film at lower temperatures than TPP, a triaryl phosphate and Irgalube 349, an amine phosphate. This phosphate film formation at lower temperatures resulted in better wear protection to the metal in tribochemical experiments, as indicated by a smaller wear-scar measurement for oil solutions containing the DPP additive. For TPP, a brief period of wear to the metal was necessary to initiate the tribochemical reaction between the additive and substrate. Once the tribochemical reaction begins, TPP is able to generate a tribochemical film of relatively the same thickness and chemistry as DPP. Irgalube 349 generated the thickest thermal films at temperatures greater than 150 °C, significantly thicker than any of the films generated from DPP and TPP. The substantial difference in thickness is believed to be due to the availability of alkyl/ammonium cations which enables continued growth of the phosphate film.     

Lubricant Spin-Off from Magnetic Recording Disks

As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 Å of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 Å thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.     

Friction and Wear of Various DLC Films in Water and Air Environments

Three types of diamond-like carbon (DLC) films, pure DLC, F-containing DLC, and a Si-containing DLC film, were deposited on a WC–Co substrate by a plasma-enhanced CVD technique. Friction and wear properties were determined using a ball-on-plate type reciprocating friction tester in water, comparing the water results to those in ambient air. The friction coefficient of DLC and F–DLC films in water was considerably lower than that in air. With Si–DLC, the friction was almost the same level in both water and air, and was less than 0.1. The specific wear rate of films in water was much smaller than that in air and varied around the low level of 10^–8 mm³/Nm in water, The mating ball wear was also less than 10^–8 mm³/Nm. With DLC and F–DLC films, the transferred amount of material on the friction surface of a mating ball was larger in a water environment than that in air. With a Si–DLC film, the difference in the transferred amount when exposed to either the water or air environment was negligible.     

Mechanisms of oiliness additives

Ultrathin film interferometry has been used to measure the boundary film-forming behaviour of long chain, carboxylic acid oiliness additives. It has been shown that in dry conditions, these acids form very thin films of around 2–3 nm thickness. However when water is present, some acids form significantly thicker films, around 10 nm in thickness. The behaviour of these films is very similar to that previously seen with metal carboxylate additive films, including thick film collapse at high rolling speeds followed by film reformation at slow speeds. It is suggested thick films formed by long chain carboxylic acid additives result from reaction of the acids at the rolling solid surfaces in the presence of water to form deposits of insoluble iron carboxylate.     

Thin lubricating films behaviour at very high contact pressure

Thin film colorimetric interferometry has been used to examine the behaviour of thin elastohydrodynamic (EHD) lubricant films under very high contact pressures of the order of 0.5–3 GPa. It has been shown that at moderate pressures, the variation of film thickness with speed follows the Hamrock and Dowson prediction down to one nanometer. As the load is increased, however, thin films behave differently from the prediction of the conventional EHD theory. For a certain lubricant and operational conditions, there is a critical rolling speed below which a reduction of film thickness is observed. This behaviour is very similar to that previously predicted computationally by Zhu.     

Cross-Section preparation for tem of film-substrate combinations with a large difference in sputtering yields

The preparation of cross-section samples of thin films for TEM, may be difficult and tedious, especially if the difference in chemical or physical etching rates is large between the film and the substrate. In this paper, a method is presented whereby cross-sections can be prepared even if the film and the substrate have a large difference in sputtering yield. By utilizing the strong angular dependence of the sputtering yield and sputter at a low ion incidence angle with respect to the substrate surface of 7–12° and, furthermore, by avoiding sputtering parallel to the interface, samples with homogeneous thickness can be obtained. The technique is demonstrated on reactively sputtered titanium nitride coatings on high speed steel substrates. The difference in sputtering yields is about three for this film-substrate combination with the substrate being sputtered fastest.     

Study of the Interaction of ZDDP and Dispersants Using X-ray Absorption Near Edge Structure Spectroscopy—Part 1: Thermal Chemical Reactions

The interactions of ZDDP and different dispersants have been investigated both in oil solutions and on steel substrates at 150–185°C. X-ray absorption near edge structure (XANES) spectroscopy at P and S L-edge and K-edge has been used to identify the chemical species both in solution and on the surface of the steel. It was found that noticeable ZDDP decomposition in solution starts at 175°C when no dispersant is present. In contrast, thermal oxidative films begin to form on the steel at 150 °C in the same solution. The products of decomposition in solution and in the film are phosphates and sulfides. N K-edge XANES spectroscopy has also been utilized to identify the reaction of the dispersant with the steel surface and ZDDP. The results show that dispersants enhance the decomposition of ZDDP in oil solutions as well on the steel surface. Dispersants, on their own, react/adsorb with the steel surface at 150–175 °C and also interact with ZDDP to form new products. Depending on the composition of the dispersants, the product is different.     

Soft-tribology: Lubrication in a compliant PDMS–PDMS contact

We investigate the influence of surface roughness and hydrophobicity on the lubrication of a soft contact, consisting of a poly(dimethylsiloxane) (PDMS) sphere and a flat PDMS disk. The full Stribeck curves, showing boundary, mixed and elasto-hydrodynamic (EHL) lubrication, are presented for varying surface roughness and hydrophobicity. It is found that neither surface roughness nor hydrophobicity influence the friction coefficient (μ) within the EHL regime. However, increasing surface roughness decreases μ in the boundary regime, while extending the limits of the boundary and mixed lubrication regimes to larger values of the product of velocity and lubricant viscosity (Uη). The transition from the mixed lubrication to EHL regime is found to take place at lower values of the film thickness parameter Λ for increasingly rough surfaces. We found Λ=0.7 in the case of a root mean square (r.m.s.) surface roughness of 3.6 μm, suggesting that the effective surface roughness in a compliant compressed tribological contact is lower than that at ambient pressures. Rendering the PDMS surface hydrophilic promotes full-film lubrication and dramatically lowers μ in the boundary regime by more than an order of magnitude. This influence of surface wetting is also displayed when examining a range of lubricants using hydrophobic tribopairs, where the boundary μ decreases with decreasing lubricant–substrate contact angle. Implications of these measurements are discussed in terms of the creation of model surfaces for biotribological applications.     

X-ray absorption spectroscopy of antiwear films on aluminum alloys generated from zinc dialkyldithiophosphate

The boundary lubrication of Al alloys 6061 (Al-6% Si) and A-390 (Al-18 wt% Si), by a sec-ZDDP (zinc dialkyldithiophosphate) oil blend was examined at 60 and 100°C. The wear performance in the ZDDP blend was an improvement on base stock. At 100°C the ZDDP prevented scuffing, though no antiwear films could be detected due to the severe wear. At 60°C all samples showed initial scuffing followed by a more controlled wear given by an embedded antiwear film. P L-edge X-ray near-edge structure (XANES) spectroscopy using synchroton radiation was used to characterize the antiwear and thermally generated deposition films on the Al alloys. P L-edge XANES results of the antiwear films at 100°C showed the absence of a polyphosphate film. A well-developed polyphosphate film was indicated at 60°C, very similar to that formed on steel at 100°C. P L-edge XANES results of the thermal deposition films showed the presence of only unchanged sec-ZDDP at 125°C with no polyphosphate present. A polyphosphate film was present at 200°C, similar to the antiwear film for Al alloy at 60°C and steel at 100°C.     

Effect of friction and adhesion on the load–displacement of a spherical probe in contact with a compliant incompressible elastic coating

The mechanical properties of thin films are extracted from the measured load displacement relation in a contact test conducted using micro or nano instruments. At this micro or nano force scale, the adhesion and friction operating between the test tip and thin film surface will contribute to the deformation. The well established Johnson–Kendall–Roberts (JKR) theory provided a relationship between the normal load and elastic central displacement for the adhesion contact. But because of its semi-infinite half-space hypothesis, the standard JKR theory is not applicable to thin film contact problem. Experimental verification demonstrates the numerical version of JKR theory is suitable for compliant thin film adhesion analysis, but it does not include the friction effect. In this paper, the load–displacement relation of totally bonded friction contact with adhesion is studied and compared with that of frictionless case. The practical thin film contact will lie in these two limits. The effect of friction to load and displacement seems very small except for the transition range from film to substrate response. Empirical expressions for the contact compliance are obtained from the detailed finite element study.     

Substrate surface energy effects on a liquid lubricant film at nanometre scalesubstrate surface energy effects on a liquid lubricant film at nanometre scale

In this paper is discussed the effect of the physical characteristics of substrate surfaces on the lubrication properties of thin films at nanometre scale. Different coatings with different surface energies have been formed on the surface of a steel ball by means of plasma assisted sedimentation (PAS). The ball was put in a pure rolling system in point contact, where the lubricant film is measured by relative optical interference intensity (ROII). Experimental results show that the film thickness is closely related to the substrate surface energy when the film is in the nanometre scale, and that the combined surface roughness in the contact region is closely related to the liquid lubricant film thickness and the contact pressure. The thinner the film and the higher the contact pressure, the smaller will be the combined surface roughness. Lastly, the relationship between critical film thickness and its influencing factors is discussed.     

A Position-Sensitive Optical Detector Based on Anomalous Photovoltage Films

A laboratory method for manufacturing position-sensitive optical detectors based on thin films of the CdTe-type semiconductors is described. These detectors are capable of generating anomalously high photoelectric voltages. To render a detector position-sensitive, the film thickness is increased stepwise, starting at ∼1 µm (for CdTe films).__________Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2005, pp. 125–126.Original Russian Text Copyright © 2005 by Rakhimov, Ser’eznov.