Abstract
M.Ing. (Mechanical Engineering)
The document reports on research conducted on titanium bolt assemblies pertaining to the
surface friction behaviour between the sliding surfaces of a screw and nut within a fastener
assembly. The research was motivated by problems encountered in the dental implant
industry whereby the prescribed preload after application of the fixation torque was not
achieved. The insufficient preload in the abutment screw led to implant failure due to the
dynamic nature of its maxillofacial function and resulting in tissue growth into micro-spaces
because of joint separation between the abutment and implant. Commercially pure Grade
4 titanium screws were sulphurically anodised to various oxide layer thicknesses in an
attempt to reduce the coefficient of friction within the implant assembly.
Testing procedures involved the design and manufacturing of the experimental specimen
and testing fixture, anodising of screws, friction coefficient measurement, oxide layer
thickness measurement, surface hardness measurement, and surface typography
investigation.
The design process of the specimen and the testing fixture included various FEA (Finite
element analysis) in optimising both of the aforementioned systems of the research and
conclusively generate accurate results and findings to compare with each other. Oxide layer
thickness measurement was conducted by wavelength reflectance theory and visual
measurement of the oxide layer using a scanning electron microscope. The surface
hardness of the material was measured using a Vickers hardness tester while surface
typography investigation was performed using a scanning electron microscope.
Findings of the experiments revealed that the coefficient of friction reduces by 10% to 40%
as the anodising voltage is increased up to an oxide layer thickness of 0.4μm, where after
the surface friction coefficient increases to eventually surpass the coefficient of friction for
untreated fastener assemblies. Untightening examination exhibited similar behaviour to
fixation, where the removal friction coefficient reduces to attain a minimum of 0.69 at an
oxide thickness of 0.06μm. The untightening coefficient of friction was at a maximum at the
increased oxide layer thicknesses of 0.97μm to 3.4μm.
Oxide layer thickness investigation revealed that the thickness of the oxide layer increases
exponentially as the anodising voltage is increased. The increase in anodising voltage also...