Tribological Properties Of Self-assembled Monolayers Research Articles

Fabrication and tribological properties of self-assembled monolayers of alkanethiols on nickel substrates

Self-assembled monolayers (SAMs) are considered as the potential lubricants and can be used to control the adhesion and the wear in micro-electromechanical system. In this study, alkanethiols SAMs of single component and mixed alkanethiols SAMs with different alkyl chain lengths were fabricated on the surfaces of nickel (Ni) substrates by the solution deposition method. The surface morphology and the wetting property of bare Ni substrate and different SAMs were characterized by the atomic force microscopy and water contact angle measurement. Moreover, a ball-on-plate tribometer was selected to measure the tribological properties of bare Ni substrate and different SAMs. The surface free energy of Ni Substrate decreased significantly after the surface modified by different alkanethiols molecules, which effectively reduced the surface adhesion of Ni substrate. The friction coefficients of Ni substrates coated with different SAMs were much lower than that of bare Ni substrate. In addition, the mixed alkanethiols SAMs had the greatest lubricity and wear resistance compared to other alkanethiols SAMs of single component.

The Effect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological Features of Created Self-Assembled Monolayers.

The presented article shows the influence of concentration of perfluoroalkylsilanes in solutions on tribological properties of self-assembled monolayers (SAMs) deposited on three surfaces with different silicon content in the millinewton load range. The SAMs were created using the liquid phase deposition (LPD) method with 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) and (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) solutions, for which viscosity and surface tension were estimated. Deposited layers were analyzed in terms of thickness, coverage, wettability, structure and coefficient of friction. The obtained results demonstrated that SAMs created on the silicon-incorporated diamond-like carbon (Si-DLC) coatings possess the best microtribological properties. Systems composed of perfluoroalkylsilane SAM structures deposited on Si-DLC coatings are highly promising candidates as material for microelectromechanical applications.

Tribological properties of self-assembled monolayers prepared on titanium film

The self-assembled monolayers of octadecyltrichlorosilane (OTS) and 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (FOTS) were prepared on titanium surfaces, irradiated by ultraviolet irradiation device. Contact angle meter, atomic force microscope and Centre for Tribology (CETR) UMT-2 tribometer were used to explore the surface morphology and tribological properties. The influences of function groups, slide velocity, load and ultraviolet irradiation on tribological properties of samples were analysed. From experiments, it was found that OTS and FOTS SAMs (self-assembled monolayers) prepared on titanium films exhibit hydrophobic and tribological properties, especially under 5-minute ultraviolet irradiation. The frictional force of OTS SAMs is lower than that of FOTS SAMs. The wear time of FOTS SAMs is longer than that of OTS SAMs.

Effect of Humidity and Hydrophobicity on the Tribological Properties of Self-Assembled Monolayers

The tribological properties of two distinctive alkanethiol SAMs, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT), on gold substrates in various humidity conditions were examined by lateral force microscopy (LFM). The results suggest that hydrophobic ODT SAM is insensitive to humidity. The difference of lateral force signal is within ±10% regardless of humidity. The lateral force signal of hydrophilic MHA SAMs has a significant decrease in signal in humid environments. The influence of bulk water was also investigated by LFM. By imaging under water, the capillary force is eliminated on ODT SAMs, which leads to a lower lateral force. However, the lateral force image was reversed on MHA SAMs, which suggested that hydrophobic forces dominated in water.

Tribological Properties of Self-Assembled Monolayers of Catecholic Imidazolium and the Spin-Coated Films of Ionic Liquids

A novel compound of an imidazolium type of ionic liquid (IL) containing a biomimetic catecholic functional group normally seen in mussel adhesive proteins was synthesized. The IL can be immobilized on a silicon surface and a variety of other engineering material surfaces via the catecholic anchor, allowing the tribological protection of these substrates for engineering applications. The surface wetting and adhesive properties and the tribological property of the synthesized self-assembled monolayers (SAMs) are successfully modulated by altering the counteranions. The chemical composition and wettability of the IL SAMs were characterized by means of X-ray photoelectron spectroscopy (XPS) and contact angle (CA) measurements. The adhesive and friction forces were measured with an atomic force microscope (AFM) on the nanometer scale. IL composite films were prepared by spin coating thin IL films on top of the SAMs. The macrotribological properties of these IL composite films were investigated with a pin-on-disk tribometer. The results indicate that the presence of IL SAMs on a surface can improve the wettability of spin-coated ionic liquids and thus the film quality and the tribological properties. These films registered a reduced friction coefficient and a significantly enhanced durability and load-carrying capacity. The tribological properties of the composite films are better than those of pure IL films because the presence of the monolayers improves the adhesion and compatibility of spin-coated IL films with substrates.

Tribological properties of self-assembled monolayers covalently bonded to Si

The frictional properties of self-assembled monolayers (SAMs) covalently bonded to silicon (Si) substrates through Si–C or Si–O–C bonds were investigated using a pin-on-plate tribometer. Four kinds of molecules with different alkyl chain lengths and different head groups were used: 1-hexadecene, 1-hexadecanol, 1-dodecene and 1-dodecanol. The SAMs with longer alkyl chains exhibited lower friction coefficients than those with shorter alkyl chains. Of the SAMs with longer alkyl chains, the SAMs with the Si–C bond of the head group exhibited lower friction coefficients, higher wear resistance and higher durability than those with the Si–O–C bond. The head group and alkyl chain length of the molecules affected the frictional properties of the modified surfaces of the SAMs.

Tribological Properties of Self-Assembled Monolayers and Their Substrates Under Various Humid Environments

Using friction force microscopy (FFM) under controlled environments, we have systematically investigated the humidity effect on the frictional properties of two important classes of self-assembled monolayers (SAMs), i.e., N-octadecyltrimethoxysilane (OTE, CH3(CH2)17Si(OCH3)3) on SiO2(OTE/SiO2), and N-alkanethiols on Au(111), together with their respective substrates. Experimental results show that both OTE and alkylthiol SAMs can decrease the friction force between a Si3N4 atomic force microscope (AFM) tip and substrates. The nearly humidity-independent friction of the two kinds of SAMs indicates that these SAMs are ideal lubricants in applications of micro-electro-mechanical systems (MEMS) under different environments. The humidity dependence—as the humidity increases, the friction first increases and then decreases—of the two substrates, SiO2 and Au(111), can be explained by the adsorption of water. The decrease in the friction at high humidity is attributed to the low viscosity in the multilayers of water, while the increase in the friction at low humidity can be explained by the high viscosity between the water monolayer and the surfaces (AFM tip and sample), possibly due to the confinement effects. The effect of modification of the AFM tip with alkanethiol molecules on the humidity dependence of Au(111) friction has also been investigated.

Tribological Properties of Self-assembled Monolayers on Au, SiOx and Si Surfaces

Using interfacial force microscopy (IFM), the tribological properties of self-assembled monolayers (SAM) on Si surfaces produced by a new chemical strategy are investigated and compared to those of “classical” SAM systems, which include alkanethiols on Au and alkylsilanes on SiOx. The new SAM films are prepared by depositing n-alkyl chains with OH-terminations onto Cl-terminated Si substrates. The chemical nature of the actual lubricating molecules, n-dodecyl, is kept constant in all three thin film systems for direct comparison and similarities and differences in tribological properties are observed. The adhesion strength is virtually identical for all three systems; however, frictional properties differ due to differences in film packing. Differences in the chemical bonds that attach the lubricant molecules to the substrate are also discussed as they influence variations in film wear and durability. It is demonstrated that the new SAM films are capable of controlling the friction and adhesion of Si surfaces equally well as the classical SAMs and are potentially more reproducible and more durable.

Tribological properties of self-assembled monolayers of fluorocarbon and hydrocarbon thiols and disulfides on Au(111) studied by scanning force microscopy

The tribological properties of highly ordered self-assembled monolayers (SAMs) of fluorocarbon and hydrocarbon thiols and disulfides formed on triangular Au(111) terraces were studied by scanning force microscopy (SFM). Molecular resolution SFM images proved the high degree of lateral order in the films. The magnitude of friction between Si 3N 4 tips and the monolayers measured by SFM was related to the order of the alkane chains. The disorder of the hydrocarbon chains in SAMs of mixed hydrocarbon–fluorocarbon disulfides, or short chain hydrocarbon disulfides, was determined by Fourier transfrom infrared (FTIR) spectroscopy. Conformational and structural disorder was found to correlate with a significant increase in the magnitude of friction. The correlation between disorder (transition from `solid-like' to `amorphous-like' monolayers) and increase in observed friction force can be interpreted based on models by Yoshizawa et al. [H. Yoshizawa, Y.-L. Chen, J. Israelachvili, J. Phys. Chem. 97 (1993) 4128].