(405d) Investigation of Interactions between Magnesium Silicate Particles and Diamond-like Carbon Surface By Atomic Force Microscopy

Contamination of nanoparticles on surfaces becomes a growing concern in various industries, especially in microelectronic industries since the size of the components have drastically reduced. Although the atmospheric dust is generally controlled, particles from various sources such as processing components and operating environments are often detected. This work emphasizes on the adhesion of magnesium silicate particles onto diamond-like carbon (DLC) surface. Magnesium silicate particles are applied in many daily uses such as skin-care products, cosmetics, or even papers, while DLC thin film has been considered as a promising material for protection of electronic components because of its high hardness, low friction, high thermal conductivity and chemical inertness. Moreover, due to low hardness of magnesium silicate that leads to strong adhesion to the surface, removal of magnesium silicate particles becomes a challenging problem.

In this work, two types of magnesium silicate, i.e., commercial talcum powder and Mg₂SiO₄ particles synthesized by the hydrothermal method, were studied. The magnesium silicate-DLC interaction was investigated by atomic force microscopy (AFM) operated in force-volume mode, in which DLC coated silicon AFM tip was used. The particles were deposited on a silicon wafer, washed with deionized water, and dried at room temperature before subjected to the AFM analysis.

Preliminary experiments, which compare AFM analyses of talcum particles using a bare silicon tip to that using a DLC coated silicon tip, revealed that the adhesion forces measured are significantly different, although the spring constants of both tips are similar. The average adhesion forces measured using the bare silicon tip and the DLC coated silicon tip are 80 nN and 40 nN, respectively. Hence, tip-particle interaction can be used to represent particle-surface interaction.

In the experiments comparing talcum and Mg₂SiO₄ particles, the normal spring constants of the DLC coated tips were calibrated by thermal noise method (k = 0.02 N/m). The adhesion measurements were conducted several times to achieve statistical representation of the data. According to the measured force-displacement curves, both adhesion force and force acting on both types of particles are significantly different. The average adhesion force on talcum powder tends to be larger than that on Mg₂SiO₄ particles. Although the difference is suggested to be a result from different hardness of the particles (i.e., the hardness of talcum is 1-1.5 in Mohs’ scale, while that of Mg₂SiO₄ is 7), other factors such as chemical composition may also involve.

To verify the effect of chemical functional group on the surface of magnesium silicate particle on the tip-particle interaction, surface modification was conducted. Talcum naturally contains surface hydroxyl, but the content of the hydroxyl group can be varied by a treatment with various kinds of acid. For surface modification, 100 g of talcum powder was immersed in 1 L of concentrated acid at 80°C for 2 h. The acids employed were formic acid, acetic acid and hydrochloric acid. The treated particles were filtered, washed with deionized water and dried in an oven at 70°C for 48 h. According to Fourier transform infrared spectroscopic analyses, the talcum powder treated with hydrochloric acid showed significant increase in the signal corresponding to hydroxyl group, while that of the powders treated with organic acids is roughly unchanged. However, the adhesion force of hydrochloric acid-treated talcum particle is about the same as the untreated talcum. On the other hand, the talcum treated with organic acids showed much increased adhesion forces. It is therefore suggested that hydroxyl group does not contribute to magnesium silicate-DLC interaction. Instead, grafting of the surface of talcum by organic groups, which leads it to be hydrophobic, can enhance the adhesion because DLC is also hydrophobic. Hence, hydrophobicity of the particles plays an important role in adhesion onto the DLC surface in addition to the softness of the particles.