Surface Metrology and the National Science Foundation

This project evaluates the use of Surface Metrology—the study of surface texture—in research publications funded by the National Science Foundation. It involves a critique on these research publications based on how they characterize surfaces towards fulfilling their research objectives. By conducting a comparative analysis of various measurement instruments and parameters, this project seeks to improve the use of Surface Metrology in research and industry

Improved Criteria for Acceptable Yield Point Elongation in Surface Critical Steels

Yield point elongation (YPE) is considered undesirable in surface critical applications where steel is formed since "strain lines" or Luders bands are created during forming. This project will examine in detail the formation of luders bands in industrially relevant strain states including the influence of substrate properties and coatings on Luders appearance. Mechanical testing and surface profilometry were the primary methods of investigation

DEVELOPMENT AND APPLICATION OF ON-MACHINE SURFACE MEASUREMENT FOR ULTRA-PRECISION TURNING PROCESS

Optical freeform components, featured with high functional performance, are of enormous demand in advanced imaging and illumination applications. However, the geometrical complexity and high accuracy demand impose considerable challenges on the existing ultra-precision freeform machining technologies. Surface measurement and characterisation become the key to further improving machining performance. In order to further increase the metrology availability and efficiency, a shift in the approach of surface metrology from offline lab-based solutions towards the use of metrology upon manufacturing platforms is needed. On-machine surface measurement (OMSM) will not only allow the assessment of manufactured surfaces just-in-time without transportation and repositioning, but also provide feedback for process optimization and post-process correction with consistent coordinate frame. In the thesis, a single point robust interferometer is integrated onto a diamond turning lathe to establish the metrology-embedded ultra-precision manufacturing platform. To extract a priori information for the subsequent OMSM, a theoretical and experimental study of surface generation was carried out for ultra-precision turning of optical freeform surfaces. With the proposed machining methodology and surface generation simulation, two freeform surfaces (sinusoidal grid and micro-lens arrays) were successfully fabricated using the slow tool servo technique. The machined topography of freeform surfaces was uniformly distributed and in agreement with simulated results. Since it operates in the manufacturing environment, the machine tool effects on the OMSM were comprehensively evaluated, including on-machine vibration test, machine kinematic error mapping and linearity error calibration. A systematic calibration methodology for single point OMSM was proposed. Both theoretical and experimental investigation have been conducted to prove the validity of the proposed calibration methodology and the effectiveness of OMSM. With the aid of OMSM, potential applications were explored to exploit the integration benefits to further enhance the ultra-precision machining performance. OMSM integration will increase the automation level of the manufacturing. As OMSM preserves the coordinate system between the machining and measurement, the process investigation can be carried out in a more deterministic manner. The effect of process parameters on the surface form errors was investigated for ultra-precision cylindrical turning process. An empirical model based on response surface methodology has been established and validated with the experimental results. Moreover, a corrective machining methodology was proposed to further improve the accuracy of diamond turned surfaces with OMSM. According to different correction tasks, corresponding OMSM data processing methods were presented. Profile and surface correction experiments were performed to validate the proposed corrective machining methodology and 40% improvement of surface accuracy was achieved

SELF-ASSEMBLED ALKYLSILANE COATINGS FOR RESORBABLE BIOMEDICAL DEVICES

Magnesium (Mg) and its alloys are excellent candidate materials for resorbable biomedical devices. They are lightweight, biocompatible, osteoinductive and have mechanical properties similar to bone. Most importantly, they can degrade in the body, eliminating the need for device removal surgeries, thus reducing potential complications, patient pain and suffering, and relieving the burden on the healthcare system. The major roadblock for wide implementation of Mg implantable devices is the initial rapid corrosion of the devices, which causes formation of gas pocket around the implant. This can impede healing. One strategy to overcome this problem is use of anticorrosion coatings. We have developed alkylsilane (AS) self-assembled multilayer coatings to control Mg corrosion, increase the biocompatibility of Mg and be further functionalized with bioactive molecules. AS coatings are formed by copolymerization of n- Decyltriethoxysilane and Tetramethoxysilane followed by a dip coating of Mg discs. Some of them were further functionalized with (3-Aminopropyl) triethoxysilane. Structural, chemical and anticorrosive properties of the AS coatings were assessed by numerous characterization techniques. The resulting coatings comprised of highly homogeneous 1 mm thin layers that significantly reduced corrosion in vitro and in vivo. In vitro and in vivo studies demonstrated the coating’s high cytocompatibility and low toxicity. Furthermore, the in vivo studies demonstrated that AS coatings could slow gas bubble formation around implanted Mg devices. Overall, our study demonstrates that AS coatings can be used to control corrosion of resorbable Mg devices

Investigations into a multiplexed fibre interferometer for on-line, nanoscale, surface metrology

Current trends in technology are leading to a need for ever smaller and more complex featured surfaces. The techniques for manufacturing these surfaces are varied but are tied together by one limitation; the lack of useable, on-line metrology instrumentation. Current metrology methods require the removal of a workpiece for characterisation which leads to machining down-time, more intensive labour and generally presents a bottle neck for throughput. In order to establish a new method for on-line metrology at the nanoscale investigation are made into the use of optical fibre interferometry to realise a compact probe that is robust to environmental disturbance. Wavelength tuning is combined with a dispersive element to provide a moveable optical stylus that sweeps the surface. The phase variation caused by the surface topography is then analysed using phase shifting interferometry. A second interferometer is wavelength multiplexed into the optical circuit in order to track the inherent instability of the optical fibre. This is then countered using a closed loop control to servo the path lengths mechanically which additionally counters external vibration on the measurand. The overall stability is found to be limited by polarisation state evolution however. A second method is then investigated and a rapid phase shifting technique is employed in conjunction with an electro-optic phase modulator to overcome the polarisation state evolution. Closed loop servo control is realised with no mechanical movement and a step height artefact is measured. The measurement result shows good correlation with a measurement taken with a commercial white light interferometer

A Tribological assessment of the porous coated anatomic total hip replacement

The tribological performance of internal joint prostheses is a fundamental influence on their longevity. The aim of this study is to characterise the tribological performance of the Porous Coated Anatomic total hip replacement by the analysis of 119 explanted prostheses. Investigations of the friction, wear, surface topography and wear debris were made and related to the joint's clinical performance. The friction of the joints at explant was similar to that of new prostheses. The median total wear volume (419mm(^3)) was found to agree with previous wear studies suggesting the existence of a threshold wear volume which promotes osteolysis. Clinical wear factor for the whole cohort matched that of alternative joint designs. The femoral head finish was shown to degrade but not in proportion to implant duration. The roughness of the UHMWPE liner was shown to fall but no relationship with any head roughness, or temporal, parameter could be distinguished. Simulator studies confirmed that the wear factor of a joint is likely to change over its lifespan. Wear models published previously describing the influence of femoral head roughness on wear could not predict the performance of explanted prostheses. An alternative relationship was observed indicating that head roughness is not as powerful a predictor of wear as previously held. A novel technique for the characterisation of the size distribution of ex vivo and in vitro wear debris was developed. A Low-Angle Laser Light Scattering Particle Analyser was used to size particles continuously over a range from 0.5 to 1000μm. This technique offers considerable unprovement over existing microscope-based methods in terms of the detail of the information and does so with less experimental effort. It was shown to be highly accurate and repeatable in preliminary investigations. Case studies of five tissue samples revealed the potential of this method

Multi-Wavelength Polarising Interferometer for In-Process Metrology

Micro-scale and nano-scale surfaces are now fabricated to serve in many fields: from optics needed in telescope/microscope imaging to semiconductors integrated in electronic devices such as smart phones and micro sensors. The production of these surfaces is inspiring the development of new metrology instrumentation that can not only ensure the quality but also optimise the manufacturing process. However, the state-of-the-art offline metrology instruments suffer from a main limitation, namely the inability to operate in the manufacture environment. The industry evolution requires in-process and metric metrology instrumentation that can provide rich surface information within harsh manufacture environment. The specification of such instruments has to be non-destructive, fast, and highly accurate; such instruments have to be combined with a production line. Interferometers offer non-destructive and parallel fashion measurement with nanometre accuracy. A well-established phase-shift interferometer (PSI) is widely used for high measurement accuracy; however, it has two limitations. Firstly, the height difference between two adjacent points on the sample should be smaller than quarter of wavelength (Λ/4), and secondly, a PSI is slow and not suitable for in-process measurements, if a mechanical scanning is used for phase shifting. In order to utilise PSI for in-process measurements, data capturing at single exposure should be used to overcome the environmental disturbances and advanced phase unwrapping methods need to be employed to extend the measurement range beyond (Λ/4). This research aimed to develop a multi-wavelength polarising phase-shift interferometer (MPI) for surface measurement and to investigate the possibility of its use for in-process metrology applications. The target specifications of the proposed instrument are as follows: a vertical measurement range greater than (Λ/4) (i.e. greater than 1 μm) with the resolution of a single wavelength interferometer (i.e. less than 10 nm). The MPI requires no mechanical scanning to obtain the phase shift with an extended measurement range using a single shot technique. This represents an improvement over the conventional single wavelength interferometer in terms of the measurement range and speed. The methodology followed to achieve this study’s aims included reviewing the literature and implementing proof-of-concept experiments using mechanical and non-mechanical methods to acquire phase-shifted colour interferograms, hence determining algorithms for fringe analysis. Finally a novel MPI setup using polarisation technique and Red-Green-Blue (RGB) illumination source was developed that can be used for in-process measurement with extended range. An acousto-optics tuneable filter (AOTF) was successfully employed to simultaneously provide RGB wavelengths with approximately 2 nm linewidth. Several fringe analyses and phase unwrapping algorithms, such as fringe order and best-match methods, were explored to retrieve areal surfaces. Colour crosstalk between cameras’ pixels was also investigated. It was found that the crosstalk is significant. A mathematical model and AOTF tuning capability were used to achieve minimum crosstalk. A spatial two dimensional image filtration was used to enhance the interferograms, hence signal-to-noise improvement. The proposed MPI has successfully measured samples (from 40 nm to 4 μm) with few nanometres accuracy and with single exposure (less than 0.3 second). This MPI has the potential for use in the measurement of surfaces produced by ultra-fast manufacturing such as roll-to-roll (R2R) manufacturing process. In the R2R process, structured surfaces are fabricated on large-area substrates (on the scale of several metres squared) at high speed exceeding several meters per minute. As such, MPI can be potentially used to measure moving surfaces within the manufacturing environment at speed limited only to the single exposure of the cameras

Edge milled carbon fibre reinforced polymers: surface metrics and mechanical performance

Carbon fibre reinforced thermoset polymer (CFRP) components are becoming increasingly prevalent in aerospace and automotive industries where reduced weight and increased fuel efficiency is required. The manufacturing process typically requires the net shape to be edge trimmed, using a milling process, to achieve final part shape. The cutting process can cause defects on the trimmed edge which, due to the anisotropic nature of the CFRP material, may not be adequately captured by traditional, metallic material based surface quality metrics. More fundamentally, the effect on mechanical performance, in particular flexural strength, is not well understood. The aim of this project is to investigate links between machined edge surfaces and static flexural properties. The effects of machine stiffness and cutting tool design, the effects of tool coating and tool wear, and finally, the effect of machining temperature on the surface quality and subsequent flexural strength are assessed. This is completed through the use of a robust framework to assess materials, machines and tools used in experimentation. Dynamometer data is captured and assessed through an original metric and current state-of-the-art 3D areal metrics are used to assess the machined surface topography. Additionally, scanning electron microscopy (SEM) is used to provide further qualitative data. Chips are collected and analysed, in a first for composite materials, to determine average geometry and changes due to machining variables. Finally, to address the shortcomings of current available metrics, a novel metric to observe sub-surface defects is proposed, validated and used to assess effects of machining variables on edge quality. It has been found that edge quality does alter the mechanical strength of edge trimmed CFRP through static four-point bend analysis. Flexural strength of coupons machined by the 6-axis robotic system is 25.9% greater than the 5-axis gantry. Tool wear and machining at elevated temperatures can reduce flexural strength by 7.1 and 8.7%, respectively. Design of experiment (DoE) and analysis of variance (ANOVA) methods employed to show statistical correlations with machining variables and surface metrics. The edge quality of CFRP, machined using prescribed variables, has been successfully linked to amplitude and volumetric 3D areal metrics (p < 0.05). Cutting mechanisms of different fibre orientations have been successfully characterised through SEM and areal analysis. Analysis of machining chips has confirmed cutting mechanism changes when the CFRP material is pre-heated up to glass transition onset. A novel, validated strategy for measuring sub-surface defects, was able to observe defects in edge trimmed samples, particularly in the 90° fibre region where matrix smearing previously prevented observation of damage