Sangkwon Park

Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales

The tribological behavior of self-assembled monolayers (SAMs) of ω-functional n-alkanethiol compounds were experimentally investigated at the nanoscale using friction force microscopy (FFM) and at the microscale using a microtribometer. Molecularly thin and well-ordered films of the surfactants with different terminal groups and chain length were prepared on gold substrates, gold-coated atomic force microscope (AFM) tips and steel balls by the spontaneous adsorption of the surfactant molecules. Nanoscale adhesion and friction forces between the films and gold-coated AFM tip were measured by FFM whereas microscale friction and wear properties between the films and gold-coated steel balls and uncoated steel balls were measured by the microtribometer. The nano- and microscale tribological behaviors between the films are compared and discussed in terms of the chemical structure such as terminal group and chain length of the film molecules. This investigation revealed that the molecular structure of the films significantly affects their tribological behaviors both in nano- and microscales.

A study on resistance of PECVD silicon nitride thin film to thermal stress-induced cracking

Abstract A potentiostatic method was used to investigate the ability of Plasma-Enhanced Chemical Vapor Deposited (PECVD) silicon nitride thin films to protect aluminum bonding areas of integrated circuit chips. Four different films in thicknesses of about 5 kA were deposited on 0.9-mm-thick stainless steel substrate and CERDIP test chips by varying deposition variables. Ultimate strain of the films was measured by controlled strain apparatus and was constant at about 0.2%. Thermal shock and cycling test was conducted to investigate the resistance of the films to thermal stress-induced cracking.. After the test, the number of cracked bondpads on the CERDIP test chips was counted and corrosion current through cracks of the films on the stainless steel substrate was measured. The results showed that more tensile films were more susceptible to crack-induced failure.

Adsorption and Thermal Desorption Behaviour of Asphalt-like Functionalities on Silica

The adsorption and thermal desorption behaviours of seven model compounds containing asphalt-like functionalities were studied on silica. Adsorption isotherms of the model compounds were examined at 25°C and analyzed via the Langmuir isotherm. A series of temperature-programmed desorption (TPD) experiments was also conducted on the silica samples, the activation energy for desorption (Ed) being determined by conventional TPD analysis. The adsorption and thermal desorption behaviours were correlated in terms of the activation energy (Ed tot) for complete desorption. Dimethyl sulphoxide showed the strongest adsorption and the largest value for Ed tot, implying that the high energy consumption necessary to desorb such functionalities plays a key role in the durability of asphalt–aggregate bonding.

Effect of pH on monolayer properties of colloidal silica particles at the air/water interface

Monodisperse colloidal silica particles were prepared by the Stöber method and hydrophobized by grafting a silane coupling agent, octadecyltrimethoxysilane. Two different types of silica particles, i.e., hydrophilic and hydrophobic silica particles were spread at the air/water interface to form the Langmuir monolayers. Monolayer properties of those particles were investigated by measuring surface pressure–area (π–A) isotherms at different subphase pH. At pH above the isoelectric point (IEP) of silica, as pH increased the π–A isotherms for the hydrophobic particles slightly shifted to larger surface area whereas those for the hydrophilic particles showed a reverse trend. At pH below the IEP, the π–A isotherms for both types of particles shifted to much larger surface area with different shapes. In order to analyze the π–A isotherm results further, the time dependence of π was examined. When pH is above the IEP, the π for the hydrophilic particles significantly decreased with increasing time and it did more at higher pH. On the other hand, the decrease in π for the hydrophobic particles was insignificant regardless of pH. For both types of silica particles, the decrease in π was minimal at pH below the IEP. These results were discussed in terms of particle desorption into the water subphase and interparticle electrostatic repulsion which is directly influenced by zeta potential.