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.