Microtribological properties of thin films and bulk materials

Scratching behavior of thin films and bulk materials and anisotropic mechanical property of PET film have been investigated in this thesis.; For the uncrosslinked poly(n-butyl acrylate) (PnBA) film, the horizontal force during scratching decreases with reducing normal load and has a residual value as the normal load approaches zero. The relation can be understood by assuming that the horizontal force is proportional to the area of contact which can be estimated by using finite element analysis. For the crosslinked PnBA film, the horizontal force approaches zero at zero normal load. There are at least two damage mechanisms in the crosslinked films and the critical normal load for failure increases with the film thickness.; The friction coefficient (horizontal/normal force ratio) during scratching of poly(dimethylsiloxane) (PDMS) coatings decreases with increasing normal load. The contact area between the indenter and the coating is essentially elastic and the horizontal force appears to be proportional to the contact area. With increasing driving speed, the friction coefficient increases but the rate of increase decreases. The coating is damaged due to delamination and through-thickness cracking. The critical normal load for damage increases with the coating thickness and the driving speed.; The slip process of the stick-slip phenomena during scratching of a styrene-acrylonitrile (SAN) copolymer was studied. The scratch velocity during slip starts from zero, increases to a maximum and then decreases to zero again. The scratch groove made during slip shows a nonuniform depth which increases with the decrease of scratch velocity. The scratch force during slip remains almost constant for most of the scratch distance and is mainly determined by the normal load.; Expressions for the directional dependence of Young's modulus and Poisson ratio are derived for a general material under a plane stress condition. The experimental results of Young's modulus and Poisson ratio of PET films are interpreted in terms of the derived expressions. The aging effect of PET films is illustrated by dynamical mechanical property measurement. The PET film shows strong anisotropic shrinkage after annealed at 150°C for 30 minutes.