The durability of Thermal Barrier Coatings (TBCs) used on the turbine blades of aircraft and power generation engines has been known to be affected by sand particle ingression comprised of Calcium-Magnesium-Alumina-Silicate (CMAS). Previous studies have shown that these effects present themselves through variations in the thermomechanical and thermochemical properties of the coating. This study investigated the impact of CMAS ingression on the Yttria Stabilized Zirconia Topcoat (YSZ) and Thermally Grown Oxide (TGO) strain in sprayed Thermal Barrier Coating (TBC) samples of varying porosity with and without CMAS ingression. In-Situ Synchrotron X-ray Diffraction measurements were taken on the sample under thermal loading conditions from which the YSZ and TGO peaks were identified and biaxial strain calculations were determined at high temperature. Quantitative strain results are presented for the YSZ and TGO during a thermal cycle. In-plane strain results for YSZ near the TGO interface for a complete thermal cycle are presented, for a 6% porous superdense sample with CMAS infiltration. The outcomes from this study can be used to understand the role of CMAS on the strain tolerance of the TBC coating. It is well known that under engine operational conditions the development of the TGO layer, with large critical stresses, has been linked to failure of the coating. The growth of the TGO manifests as undulations in a series of peaks and troughs. Understanding the mechanics of the oxide layer at these locations provides significant information with respect to the failure mechanisms of the TBC coating. This study investigated the stress at the peak and trough of a TGO undulation for a cycled Dense Vertically Cracked (DVC) plasma sprayed TBC sample through photo-luminescence (PL) spectroscopy. High resolution nanoscale stress maps were taken nondestructively in the undulation of the TGO. Preliminary results from first line mapping of TGO peak and trough scan, at a resolution of 200 nm, have shown a non-uniform TGO stress variation. The results obtained from this study can be used to understand the stress variation in the peak and trough of a DVC sample's TGO undulation and how it contributes to the life of the TBC coating.

Acoustic levitation permits the study of droplet dynamics without the effects of surface interactions present in other techniques such as pendant droplet methods. Despite the complexities of the interactions of the acoustic field with the suspended droplet, acoustic levitation provides distinct advantages of controlling morphology of droplets with nanosuspensions post precipitation. Droplet morphology is controlled by vaporization, deformation and agglomeration of nanoparticles, and therefore their respective timescales are important to control the final shape. The balance of forces acting on the droplet, such as the acoustic pressure and surface tension, determine the geometry of the levitated droplet. Thus, the morphology of the resultant structure can be controlled by manipulating the amplitude of the levitator and the fluid properties of the precursor nanosuspensions. The interface area in colloidal nanosuspensions is very large even at low particle concentrations. The effects of the presence of this interface have large influence in the properties of the solution even at low concentrations. This thesis focuses on the dynamics of particle agglomeration in acoustically levitated evaporating nanofluid droplets leading to shell structure formation. These experiments were performed by suspending 500µm droplets in a pressure node of a standing acoustic wave in a levitator and heating them using a carbon dioxide laser. These radiatively heated functional droplets exhibit three distinct stages, namely, pure evaporation, agglomeration and structure formation. The temporal history of the droplet surface temperature shows two inflection points. Morphology and final precipitation structures of levitated droplets are due to competing mechanisms of particle agglomeration, evaporation and shape deformation. This thesis provides iv a detailed analysis for each process and proposes two important timescales for evaporation and agglomeration that determine the final diameter of the structure formed. It is seen that both agglomeration and evaporation timescales are similar functions of acoustic amplitude (sound pressure level), droplet size, viscosity and density. However it is shown that while the agglomeration timescale decreases with initial particle concentration, the evaporation timescale shows the opposite trend. The final normalized diameter hence can be shown to be dependent solely on the ratio of agglomeration to evaporation timescales for all concentrations and acoustic amplitudes. The experiments were conducted with 10nm silica, 20nm silica, 20nm alumina and 50nm alumina solutions. The structures exhibit various aspect ratios (bowls, rings, spheroids) which depend on the ratio of the deformation timescale (tdef) and the agglomeration timescale (tg). For tdef

Total hip joint prostheses made of CoCrMo heads versus ultra high molecular weight polyethylene (UHMWPE) cups have a limited lifetime, mainly due to the wear of the UHMWPE cups as a result of high friction between the articulating surfaces leading to osteolysis and implant loosening with revision surgery becoming inevitable in more active patients. Tribology plays an important role in developing the design, minimizing wear and reducing friction of hip joint prostheses in order to improve their long-term performance, with good lubricating properties. Metal-on-metal hip resurfacing prostheses have shown significantly lower wear rates compared with conventional metal-on-polyethylene implants and thus osteolysis is potentially reduced leading to increased lifetime of the prosthesis. Nevertheless, excessive wear of metal-on-metal joints leads to metal ion release, causing pseudo-tumours and osteolysis. An alternative approach to such bearings is the use of newly developed carbon fiber-reinforced poly-ether-ether-ketone (CFR PEEK) acetabular cups articulating against ceramic femoral heads due to their better wear resistance compared to UHMWPE. In this study, therefore, friction and lubrication properties of large diameter, as cast, Co-Cr-Mo metal-on-metal hip resurfacing implants with various diameters and clearances have been investigated and compared to those of the newly developed zirconia toughened alumina (ZTA) ceramic femoral heads articulating against carbon fiber reinforced poly-ether-ether-ketone (CFR PEEK) acetabular cups with different diameters and clearances. Friction hip simulator was used to measure frictional torque and then friction factors were calculated along with Sommerfeld numbers leading to Stribeck analysis and hence the lubricating mode was also investigated. This involved using lubricants based on pure bovine serum (BS) and diluted bovine serum (25 vol. %BS+75 vol. %distilled water) with and without carboxymethyl cellulose (CMC) (as gelling agent). Standard Rheometer was used to measure lubricant viscosity ranged from 0.0014 to 0.236 Pas at a shear rate of 3000 . Pure bovine serum, diluted bovine serum without CMC and with CMC (25BS+75DW+0.5gCMC and +1gCMC) showed pseudoplastic flow behaviour up to shear rate of ¿139 above which a Newtonian flow with significant increase in shear stress was observed. The viscosity flow curves for the 25BS+75DW+2gCMC, +3.5gCMC and +5gCMC showed only shear thinning up to a shear rate of 3000 . The shear rate application modified the flow behaviour of bovine serum from a pseudoplastic to a Newtonian flow depending on its purity and CMC content. This will cause a different frictional behaviour depending on joint diameter and clearance, as seen in this work. The experimental data were compared with theoretical iv predictions of the lubricating regimes by calculating theoretical film thickness and lambda ratio. The metal-on-metal Biomet ReCaps showed similar trends of Stribeck curves, i.e. friction factors decreased from ~0.12 to ~0.05 as Sommerfeld numbers increased in the range of viscosities ~0.001-0.04Pas indicating mixed lubrication regimes above which the friction factor increased to ~0.13 at a viscosity of 0.236Pas. The Stribeck analyses suggested mixed lubrication as the dominant mode with the lowest friction factor in the range ~0.09 - ~0.05 at the physiological viscosities of ~0.01 to ~0.04 Pas and that such joints can be used for more active patients as compared to the conventional total hip replacement joints with 28mm diameter. The Stribeck curves for all ZTA ceramic-on-CFR PEEK components illustrated a similar trend with BS fluids showing higher friction factors (in the range 0.22-0.13) than the diluted BS+CMC fluids (in the range 0.24-0.05). The friction tests revealed boundary-mixed lubrication regimes for the ZTA ceramic-on-CFR-PEEK joints. The results, so far, are promising and suggest clearly that the newly developed ZTA ceramic femoral heads articulating against CFR PEEK cups have similar friction and lubrication behaviour at optimum clearances to those of currently used metal-onmetal hip resurfacing implants at the range of viscosities 0.00612 to 0.155Pas. These results clearly suggest that the ZTA ceramic-on-CFR-PEEK joints showed low friction at the physiological viscosities of ~0.01Pas in the range ~0.1-0.05, suggesting that these novel joints may be used as an alternative material choice for the reduction of osteolysis. The result of this investigation has suggested that the optimum clearance for the 52mm diameter MOM Biomet ReCaps could be ~170¿m. However, 48 and 54mm joints showed lower friction due to clearances to be >200¿m. For the 52mm ZTA ceramic-on-CFR-PEEK joints the optimum clearance seems to be ¿ 630¿m radial clearance. These results suggested that increased clearance bearings have the potential to generate low friction and hence no risk of micro- or even macro-motion for the ceramic-on-CFR-PEEK joints. This study found no correlation between theoretical predictions and experimental data for all metal-onmetal and ZTA ceramic-on-CFR PEEK bearings at the physiological viscosity (0.0127Pas). However, at lubricant viscosity of 0.00157Pas, the theoretical prediction of lubrication regime correlated well with the experimental data, both illustrating boundary lubrication. As expected, a decrease in viscosity resulted decrease in the film thickness.

The natural hip joint in healthy people has a very low friction with very little (or no) wear. It works as a dynamically loaded bearing and is subjected to about 1-2 million cycles of loading per year. The applied load is the body weight which is tripled when walking and even higher during other activities such as running and jumping. Unfortunately these joints are not always healthy due to various causes such as fractures or disease leading to severe pain which necessitates joint replacement. Currently, the orthopaedic industries are working towards developing an ideal artificial hip joint with low wear, low friction, good lubrication, better fixation/stability and biocompatibility. Many different designs and materials have been investigated with some promising new implants which can be used depending on patients¿ individual need (large or small joint), activity and age. In this work, two types of artificial hip joints were tested for friction and lubrication studies: Metal-on-Metal (MoM) Biomet hip resurfacing ReCaps with large diameters (>35-60 mm) and different diametral clearances (~ 60-350 µm), and Zirconia Toughened Alumina (ZTA) heads against carbon-fibre-reinforced poly-ether-ether ketone (CFR PEEK) cups with different diameters (>35-60 mm) and diametral clearances (60-1860 µm). Seven serum-based lubricants with different viscosities were used with and without carboxy methyl cellulose (CMC) additions as gelling agent to increase viscosity depending on the CMC content. The maximum load applied was 2000 N for the stance phase with a minimum load of 100 N for the swing phase. A Pro-Sim friction hip simulator was used to investigate the frictional torque generated between the articulating surfaces so as the friction factor can be calculated. Stribeck analysis was then employed to assess the mode of lubrication. For the metal-on-metal hip resurfacing joints, the friction factors were in the range 0.03-0.151 and those for the ZTA ceramic heads versus CFR PEEK cups were in the range 0.006-0.32. Stribeck analyses showed mainly mixed lubrication for both MoM and ZTA ceramic-on-CFR PEEK joints. The experimental results were in agreement with most of the theoretical calculations suggesting mixed lubricating regimes at low viscosities and moving on to fluid film lubrication at higher viscosities. Joints with larger-diameters, lower clearances and lower surface roughness exhibited a higher lambda ratio suggesting improved lubrication. Viscosity flow curves for the serum-based lubricants having viscosity ¿ 0.00524 Pas showed non-linear relationship between viscosity and shear rate indicating non-Newtonian flow with pseudoplastic or shear-thinning characteristic, i.e. viscosity decreased as shear rate increased up to shear rates of ~ 1000 s-1. However, at shear rates greater than 1000 s-1 Newtonian flow became dominant with almost constant viscosity, i.e. a linear relationship between shear stress and shear rate. On the other hand, viscosity flow curves for the lubricants with viscosity ¿ 0.0128 Pas showed non-Newtonian behaviour up to a shear rate of 3000 s-1 with shear-thinning characteristic. / Ministry of Higher Education, Libya

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<p>The superior high-temperature elastic-plastic properties coupled with greater damage tolerance when compared with monolithic ceramics make ceramic matrix composites, CMCs, promising candidates for challenging applications such as engine components, rocket nozzles, cutting tools and nuclear energy reactor core components. Anelastic recovery is the time-dependent back strain observed upon the load removal following creep. In whisker-reinforced CMCs this can be a factor limiting operating conditions. Plastic strain misfit between two phases is thought to be the main driver in terms of the interactions within a percolating network. However, the network deformation mechanisms are still unclear and a previous neutron diffraction study showed an unexpected decrease of peak width after creep contradicting the theoretical predictions.</p> <p>In this contribution, the finite element method (FEM) is applied to a representative volume element (RVE) with proper boundary conditions in order to simulate the creep deformation and hot pressing processes. Three geometries have been generated and studied: a 3D randomly-oriented short-fiber unit cell without fiber to fiber contact, generated by a random sequential adsorption algorithm; 3D regularly aligned single fiber unit cells; and 2D regularly aligned percolating unit cells. Deformation mechanism has been studied from an energy point of view and compared with a modified analytical model. Then a virtual diffraction model has been developed providing a framework to transfer information between the FEM simulations (strain fields) and the diffraction pattern in terms of the peak width (full width at half maximum: <strong><em>FWHM</em></strong>) and peak position as a measure of stress distribution and mean stress state respectively. Furthermore, the coupling effects of external stress, deformation mode, and thermal stress on the diffraction patterns have been studied.</p> <p>The critical importance of a percolating whisker network for the anelastic recovery is demonstrated based on the 3D multi-whisker random unit cell. Whisker bending is shown to be the dominant mechanism over contact effects during the creep deformation of a composite containing a well aligned percolating whisker network based on the 2D unit cell model. Good qualitative agreement was found between our FEM simulations and the analytical model of Wilkinson and Pompe with regards to the maximum recoverable strain and the characteristic relaxation time. The analytical model captures all the critical factors characterizing the strain recovery, e.g., the effect of creep pre-exponent constant, whisker Young’s modulus and aspect ratio. Furthermore, it is found that the deformation from an initial stress-free state inevitably introduces peak broadening of whiskers inside the matrix. Several factors determine the peak-width and -shift, i.e., creep strain, applied stress, aspect ratio and geometry. However, thermal stress from the cooling stages following creep and hot pressing processes shelters this broadening effect and complicates the trends. Wide-ranging peak-width changes from narrowing to broadening are predicted depending on the geometry and applied stress. The peak position is shifted to a lower angle due to this thermal effect. This clearly explains the contradicting phenomena motivating this work and leads to that recommendation that a diffraction source with high angular resolution is needed to detect the subtle change of peak profile during creep.</p> / Doctor of Philosophy (PhD)