Nano Surface Mechanical Properties of Semicrystalline Polymers

Abstract

This Thesis describes a systematic experimental study of the large strain surface mechanical properties of certain semicrystalline polymers, at the nanometre scale. A poly(etheretherketone) (PEEK) was selected as a model semicrystalline polymer for these investigations, as PEEK is currently finding extensive use in high performance composites. The central theme of the Thesis is to elucidate the nano normal indentation response and the scratch deformation mechanisms of semicrystalline polymers. Effective selection and design improvements of materials in surface engineering and tribological applications require knowledge of their near-to-surface mechanical properties. Therefore, this experimental study seeks to elucidate an understanding of the response of semicrystalline polymers in single point contacts. This Thesis approaches the nano surface mechanical property characterisation using indentation and scratching techniques. Indentation is a relatively simple and virtually nondestructive means of assessing mechanical properties of materials by an indenter, inducing a localized deformation into a solid surface. PEEK and other commercially available common polymers were indented and the data were analysed using a contact compliance method in conjunction with an MTS Nano IIs indenter system. The load-displacement curves, the hardness, the elastic modulus, the plasticity index and the creep response data and associated analysis for the PEEK surfaces are presented as a function of the contact displacement. A comparison of the data for the load-displacement curves, the hardness and the elastic modulus for common commercial polymers is described. A study of surface deformations of the PEEK surfaces when it undergoes scratching by means of conical indenters drawn along the surfaces under different contact conditions is also described. The scratch deformations produced considerable loss in optical appeal and surface mechanical properties of polymeric materials. The experimental investigation of the scratch response of the PEEK is focused upon the contact conditions. Scratch deformation maps have been constructed showing the effect of the normal load, the strain (the contact geometry), the strain rate (the scratching velocity), the contact temperature, the state of interfacial lubrication and the crystallinity of the polymer upon the scratching behaviour of the polymeric surface. The extent and the geometric characteristics of the surface damage produced are determined subjectively using scanning electron microscopy (SEM) and optical profilometry techniques. The nano hardness and the elastic modulus results as a function of contact displacement for PEEK composites are also presented. The fibre oriented PEEK composites were scratched using a pendulum sclerometer to analyze the orientation effects on scratch deformations. Finally, nano indentation results for the modified PEEK surfaces under thermal, solvent and mechanical disruptions are reported. The major conclusions of this Thesis are that the nanoindentations into the polymers show a surface hardening response and are dependent upon the contact conditions. The semicrystalline polymers have bimodal nanoindentation characteristics due to presence of the hard crystalline lamella and the soft amorphous phase. The semicrystalline polymers exhibit periodic fluctuations in surface mechanical properties with increasing penetration depth. The scratch deformations of semicrystalline polymers depend upon the contact conditions. A peculiar fibrillation of the polymeric surface was observed when scratched under severe contact conditions (high normal load and sharp conical indenters). The scratch deformations of fibre oriented polymers are highly fibre orientation dependent relative to the scratch direction. Surface plasticisation of amorphous PEEK has been observed in organic solvents mainly in chlorinated solvents. Semicrystalline PEEK was seen to exhibit considerable inert behaviour to common organic solvents but chlorinated organic solvents has caused decrease in surface mechanical properties. A surface hardening of amorphous PEEK has been observed after immersion in water. A qualitative methodology, based on nanoindentation data, to analyze subsurface deformations of polymers resulting from scratch deformations are also presented.