Hardness measurements of Ar⁺-beam treated polyimide by depth-sensing ultra low load indentation

Abstract

Polyimide Kapton and spin-cast polyamic acid (PAA) on sapphire have been implanted with 1 MeV Ar ions to a dose of 4.7 × 10¹⁵ cm⁻² at ambient temperature. The properties of both pristine and implanted surfaces were characterized by a depth-sensing low-load indentation technique. Experiments were carried out to investigate the effects of substrate, indentation rate, relaxation, and indentation technique. The results showed that (1) hardness was depth-dependent and decreased with increasing indentation depth, (2) measurements of the ion beam hardened surface with the untreated material as a substrate underestimated the hardness while measurements over the sapphire substrate overestimated it, (3) the effects of loading/unloading rates were apparent in the load displacement results, and (4) hardness values measured using the force modulation technique showed very little depth dependence. The hardness value at 100 nm depth is used for comparison purposes since the hardness value at this depth was almost independent of substrate, indentation rate, and indentation method. The hardness of Kapton, which was measured using the techniques described herein, was increased by over 30 times after Ar implantation, from 0.43 to 13 GPa at 100 nm indentation depth. A similar increase in hardness was also observed for polyamic acid. This result suggests that spin-cast PAA film may have potential technological applications for protective coatings where hardness and wear resistance are required.