The development of thermal barrier coatings (TBCs) has been undoubtedly the most critical advancement in materials technology for modern gas turbine engines. TBCs are widely used in gas turbine engines for both power-generation and propulsion applications. Metallic oxidation-resistant coatings (ORCs) are also widely employed as a stand-alone protective coating or bond coat for TBCs in many high-temperature applications. Among the widely studied durability issues in these high-temperature protective coatings, one critical challenge that received greater attention in recent years is their resistance to high-temperature degradation due to corrosive deposits arising from fuel impurities and CMAS (calcium-magnesium-alumino-silicate) sand deposits from air ingestion. The presence of vanadium, sulfur, phosphorus, sodium and calcium impurities in alternative fuels warrants a clear understanding of high-temperature materials degradation for the development of fuel-flexible gas turbine engines. Degradation due to CMAS is a critical problem for gas turbine components operating in a dust-laden environment. In this study, high-temperature degradation due to aggressive deposits such as V2O5, P2O5, Na2SO4, NaVO3, CaSO4 and a laboratory-synthesized CMAS sand for free-standing air plasma sprayed (APS) yttria stabilized zirconia (YSZ), the topcoat of the TBC system, and APS CoNiCrAlY, the bond coat of the TBC system or a stand-alone ORC, is examined. Phase transformations and microstructural development were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. This study demonstrated that the V2O5 melt degrades the APS YSZ through the formation of ZrV2O7 and YVO4 at temperatures below 747°C and above 747°C, respectively. Formation of YVO4 leads to the depletion of the Y2O3 stabilizer and the deleterious transformation of the YSZ to the monoclinic ZrO2 phase. The investigation on the YSZ degradation by Na2SO4 and a Na2SO4 + V2O5 mixture (50-50 mol. %) demonstrated that Na2SO4 itself did not degrade the YSZ, however, in the presence of V2O5, Na2SO4 formed vanadates such as NaVO3 that degraded the YSZ through YVO4 formation at temperature as low as 700°C. The APS YSZ was found to react with the P2O5 melt by forming ZrP2O7 at all temperatures. This interaction led to the depletion of ZrO2 in the YSZ (i.e., enrichment of Y2O3 in t' -YSZ) and promoted the formation of the fluorite-cubic ZrO2 phase. Above 1250°C, CMAS deposits were observed to readily infiltrate and significantly dissolve the YSZ coating via thermochemical interactions. Upon cooling, zirconia reprecipitated with a spherical morphology and a composition that depended on the local melt chemistry. The molten CMAS attack destabilized the YSZ through the detrimental phase transformation (t - > t - > f + m). Free standing APS CoNiCrAlY was also prone to degradation by corrosive molten deposits. The V2O5 melt degraded the APS CoNiCrAlY through various reactions involving acidic dissolution of the protective oxide scale, which yielded substitutional-solid solution vanadates such as (Co,Ni)3(VO4)2 and (Cr,Al)VO4. The molten P2O5, on the other hand, was found to consume the bond coat constituents significantly via reactions that formed both Ni/Co rich phosphates and Cr/Al rich phosphates. Sulfate deposits such as Na2SO4, when tested in encapsulation, damaged the CoNiCrAlY by Type I acidic fluxing hot corrosion mechanisms at 1000°C that resulted in accelerated oxidation and sulfidation. The formation of a protective continuous Al2O3 oxide scale by preoxidation treatment significantly delayed the hot corrosion of CoNiCrAlY by sulfates. However, CoNiCrAlY in both as-sprayed and preoxidized condition suffered a significant damage by CaSO4 deposits via a basic fluxing mechanism that yielded CaCrO4 and CaAl2O4. The CMAS melt also dissolved the protective Al2O3 oxide scale developed on CoNiCrAlY by forming anorthite platelets and spinel oxides. Based on the detailed investigation on degradation of the APS YSZ and CoNiCrAlY by various corrosive deposits, an experimental attempt was carried out to mitigate the melt-induced deposit attack. Experimental results from this study demonstrate, for the first time, that an oxide overlay produced by electrophoretic deposition (EPD) can effectively perform as an environmental barrier overlay for APS TBCs. The EPD protective overlay has a uniform and easily-controllable thickness, uniformly distributed closed pores and tailored chemistry. The EPD Al2O3 and MgO overlays were successful in protecting the APS YSZ TBCs against CMAS attack and hot corrosion attack (e.g., sulfate and vanadate), respectively. Furnace thermal cyclic oxidation testing of overlay-modified TBCs on bond-coated superalloy also demonstrated the good adhesive durability of the EPD Al2O3 overlay.

This dissertation includes theoretical and applied components that address the effect of coatings on rates of mineral reactions that occur in acid mine drainage (AMD) environments. The two major projects investigated how diffusion-limited transport of reactants through pore spaces in coatings on mineral grains affects the reaction rate of the underlying mineral. The first project considered the growth of gypsum coatings on the surface of dissolving limestone in anoxic limestone drains (ALD), which reduces the neutralization rate of the dissolving limestone and the subsequent effectiveness of this treatment. The second project investigated the conditions where iron oxyhydroxide coatings form on oxidizing pyrite and the potential strategies to prevent "runaway" AMD by reducing the rate of acid production to the point that the acid can be neutralized by the surrounding rocks. In both studies, experiments were conducted to measure reaction rates for the underlying minerals, as coatings grew thicker. These experimental data were fit to a diffusion model to estimate diffusion coefficients of reactants through pore spaces in coatings. These models are extrapolated to longer times to predict the behavior of the coated grains under field conditions. The experimental results indicate that management practices can be improved for ALDs and mine waste piles. For example, supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite. AMD can be prevented if the rate of alkalinity addition to mine waste piles is faster than acid is produced by pyrite oxidation. The diffusion model developed in this study predicts when iron oxyhydroxide coatings will become thick enough that alkalinity from the surroundings is sufficient to neutralize acid produced by coated pyrite oxidation and additional alkalinity is no longer required. / Ph. D.

Ice accretion on various surfaces can cause destructive effect of our lives, from cars, aircrafts, to infrastructure, power line, cooling and transportation systems. There are plenty of methods to overcome the icing problems including electrical, thermal and mechanical process to remove already accumulated ice on the surfaces and to reduce the risk of further operation. But all these process required substantial amount of energy and high cost of operation. To save the global energy and to improvement the safety issue in many infrastructure and transportation systems we have to introduce some passive anti-icing coating known as ice-phobic coating to reduce the ice-formation and ice adhesion onto the surface. Ice-phobic coatings mostly devoted to utilizing lotus-leaf-inspired superhydrophobic coatings. These surfaces show promising behavior due to the low contact area between the impacting water droplets and the surface. In this present study we investigate systematically the influence of chemical composition and functionality as well as structure of surfaces on wetting properties and later on icing behavior of surfaces. Robust anti-icing coating has been prepared by using modified silica particles as a particles film. Polymer brushes were synthesized on flat, particle surfaces by using Surface initiated ATRP. We have also investigated the effect of anti-icing behavior on the surfaces by varying surface chemistry and textures by using different sizes of particles. This approach is based on the reducing ice accumulation on the surfaces by reducing contact angle hysteresis. This is achieved by introducing nano to micro structured rough surfaces with varying surface chemistry on different substrates. Freezing and melting dynamics of water has been investigated on different surfaces by water vapour condensation in a high humidity (80%) condition ranging from super hydrophilic to super hydrophobic surfaces below the freezing point of water. Kinetics of frost formation and ice adhesion strength measurements were also performed for all samples. All these experiments were carried out in a custom humidity and temperature controlled chamber. We prepared a superhydrophobic surface by using Poly dimethyl siloxane (PDMS) modified fumed silica which display very low ice-adhesion strength almost 10 times lower than the unmodified surface. Also it has self-cleaning behavior after melting of ice since whole ice layer was folded out from the surface to remove the ice during melting. Systematic investigation of the effect of three parameters as surface energy, surface textures (structure, geometry and roughness) and mechanical properties of polymers (soft and stiff) on icing behavior has also been reported.

Cancer metastasis directly accounts for an estimated 90% of all cancer related deaths and is correlated with the presence of malignant cells in systemic circulation. This observed relationship has prompted efforts to develop a fluid biopsy, with the goal of detecting these rare cells in patient peripheral blood as surrogate markers for metastatic disease as a partial replacement or supplement to tissue biopsies. Numerous platforms have been designed, yet these have generally failed to support a reliable fluid biopsy due to poor performance parameters such as low throughput, low purity of enriched antigen positive cells, and insufficiently low detection thresholds to detect poor expressed surface markers of target cell populations. This work describes the development of a rapid cell sorting technology called Antigen Specific Lysis (ASL) based on photo-crosslinked polymer encapsulation to isolate tumor cells in suspension. In the first study, we characterize the chemical and structural properties of the surface-initiated polymer films formed directly on mammalian cell surfaces. Coated populations are shown to remain highly viable after coating formation. Biomolecular transport is examined though film coatings on cellular substrates using fluorescent, time-resolved confocal microscopy and diffusivity estimates are generated for these materials. In the next study, a lysis-based cell isolation platform is described in which marker positive cells can be specifically coated in a heterogeneous cell suspension. Anionic surfactants lyse virtually 100% of uncoated cells while fully encapsulated cells remain protected, and are then easily collected by centrifugation. We report that purified cells are released from polymeric coatings to yield viable and functional populations. We monitor cell response throughout the isolation process by multiple techniques, and report viability >80% after the sorting process. Lastly, we examine the response of process yield on the level of photoinitiator loading on target populations. Streptavidin-fluorochrome loading was quantitatively assessed on a panel of markers, both epithelial and mesenchymal, on representative model breast and lung cancer cells. We report that ASL is fundamentally capable of achieving 50-60% yield which is promising for fluid biopsy applications. Finally, both EpCAM and metastatic targeting strategies are then compared to covalently biotinylated samples to inform future robust targeting strategies.

Thesis (MScIng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: Vesiculated beads consist of aerated microvoids encapsulated in a solid spherical continuous polymeric shell. The difference in refractive index between the voids and polymer granules causes effective scattering of incident light on the particles, presenting it with a white appearance. The size of these beads generally range in the region of 0.5 – 40 μm, making it suitable for use as pigment extender in the surface coatings or paint industry. Currently, titanium dioxide pigment is predominantly used as opacifying agent in paint formulations, but due to the high cost associated in purchasing this pigment, as well as fluctuation in import prices, paint manufacturers are looking for alternative products to replace or at least partially replace this pigment. As an alternative, opaque vesiculated polymer particles can be produced locally at a cheaper price and in existing vessels available in the paint industry. Approximately five years ago a paint company in Mexico and member of the Nova Club, started research in developing vesiculated beads for production in their factories. However, it was found extremely difficult to scale-up the production to industrial size, since the system was very sensitive to process variables. A local paint company and member of the Nova Club acquired this technology and continued further research in developing vesiculated beads on large scale in existing Cowles disperser systems found in the paint industry. The beads consist mainly of an organic phase comprising of unsaturated carboxylated polyester and styrene. A polyamine is also added to assist the formation of vesicles in the organic phase. This phase is slowly added under agitation to an aqueous phase consisting of deionised water, a thickener and colloid stabilisers to form an oil-inwater emulsion. Agitation is continued for a specified period of time, also known as the emulsification period, to allow sufficient time for the organic globules to break-up to smaller particle sizes. These globules are subsequently catalysed with a freeradical initiator and redox activator and left static overnight to allow formation of the solid beads. To determine the most important process parameters during production of vesiculated beads, a fully integrated laboratory scale Cowles reactor system was designed and constructed, geometrically analogous to the vessels found in the paint industry. The system measures and controls production temperature, mixing speed and component addition rates. Production runs were performed where various process parameters were varied to investigate the effect on properties, which include average particle size and particle size distribution, pH, viscosity and opacity. The most important process parameters that were found to play a significant role include production temperature, organic phase addition rate, emulsification time, the Cowles impeller diameter and mixing speed. Production runs were performed in geometrically similar 5l and 20l vessels on the laboratory-scale system to investigate the effect of scale-up. A model presented by Klein et al. (1996) was used as basis for describing the average particle size as a function of mixing speed, impeller diameter, vessel diameter and emulsification time. The applicability of this model was tested on average particle size data obtained from industrial scale runs performed on the plants and proved to be reasonably accurate. / AFRIKAANSE OPSOMMING: Sferiese polimeerpartikels met klein lugholtes vasgevang in ‘n harde omhulsel word al jare in die verf industrie aangewend as pigment. Weens die verskil in brekingsindeks tussen die soliede polimeerpartikel en die vasgevange lugholtes, word invallende lig versprei op so ‘n manier dat die partikels ondeursigtig (of wit) voorkom. Hierdie partikels kan geproduseer word met deursneë wat strek van 0.5 – 40 μm, wat dit geskik maak vir gebruik in verf formulasies. Tans word titaandioksied poeier hoofsaaklik gebruik in verf as pigment, maar weens die hoë koste van die invoer en aankoop van hierdie produk, het verfmaatskappye begin soek na goedkoper alternatiewe. Aangesien hierdie ondeursigtige polimeerpartikels plaaslik goedkoper vervaardig kan word in bestaande mengvate beskikbaar in verf aanlegte, dien dit as moontlike plaasvervanger. Ongeveer vyf jaar gelede het ‘n Mexikaanse verfmaatskappy, wat lid is van die Nova Klub, navorsing begin doen om hierdie polimeerpartikels in hul fabrieke te produseer. Dit was egter vir hulle onmoontlik om die produksie op te skaal na industriële vervaardiging aangesien die proses baie sensitief was vir produksieveranderlikes. Sekere eienskappe soos die gemiddelde partikelgrootte, partikelverspreiding, pH, viskositeit en deursigtigheid van die partikels kon nie van lot tot lot herhaal word nie en verdere navorsing is gestaak. ‘n Plaaslike verfmaatskappy (ook lid van die Nova Klub) het die tegnologie oorgeneem en die proses verder ontwikkel. Die proses is aangepas sodat “Cowles” mengers, wat wydverspreid in die verf industrie beskikbaar is, gebruik kan word om dit te vervaardig. Die partikels bestaan hoofsaaklik uit ‘n organiese fase wat ‘n onversadigde gekarboksileerde poliëster en stireen insluit. ‘n Poli-amien word ook bygevoeg en is verantwoordelik vir die vorming van die lugholtes in die partikels. Hierdie fase word stadig onder menging by ‘n tweede water fase, bestaande uit gedeïoniseerde water, ‘n verdikker en kolloïdale stabiliseerders gevoeg om ‘n olie-in-water emulsie te vorm. Menging word voortgesit vir ‘n bepaalde emulsifiseringsperiode om die oliedruppels verder op te breek. Gevolglik word hierdie druppels gekataliseer met ‘n vry-radikaal inisieerder en redoksaktiveerder en oornag staties gelos om vorming van die soliede partikels toe te laat. Aangesien eienskappe van die polimeerpartikels so sensitief is vir prosesveranderlikes, is besluit om aanvanklik ‘n ten volle geïntegreerde laboratorium skaal “Cowles” reaktorsisteem te ontwerp en bou. Hierdie sisteem is geometries gelykvormig aan die mengvate wat in verffabrieke gevind word. Die produksietemperatuur, stuwergrootte, mengspoed en materiaal toevoertempo kan effektief gemeet, verstel en beheer word. Eksperimentele lopies is gedoen en die effek van verskeie produksieveranderlikes op eienskappe is ondersoek. Die belangrikste veranderlikes wat die proses beïnvloed, is die emulsifiseringstemperatuur, die toevoertempo van die organiese fase, emulsifiseringsperiode, stuwerdeursnit en mengspoed. Eksperimentele lopies is gedoen op twee geometriese gelykvormige mengvate (5l en 20l kapasiteit) om die effek van opskaling op eienskappe te ondersoek. ‘n Model wat deur Klein et al. (1996) voorgestel is, is as basis gebruik om die gemiddelde partikelgrootte te bepaal as ‘n funksie van mengspoed, stuwerdeursnit, mengvat deursnit en emulsifiseringstyd. Hierdie model is getoets op partikelgrootte data wat verkry is van groot industriële skaal lopies uitgevoer in die fabrieke onder bekende produksie kondisies en daar is gevind dat hierdie model bevredigend gebruik kan word om die gemiddelde partikelgrootte te voorspel.

The replacement of chromate conversion coatings for zinc coated components has been necessitated by the materials finishing industries due to the inherent toxicity issues with Cr(VI) and the legislative enforcement of WEEE and ELV Directives by the European Union. Current replacements are based on non-chromate , Cr(III) systems, these may be perceived by some to be problematic as they still contain chromium . Molybdate based conversion coatings have long been viewed by many researchers to be a viable non-chromium alternative due to their low toxicity. An extensive literature review of the research carried out in the last 20 years was carried out, highlighting areas of interest for improving the corrosion resistance of the coatings studied. These were, primarily, the synergesis that exists with molybdate and phosphate compounds for corrosion resistance and the incorporation of nanoparticle silica into treatment solution. Also discovered was the importance of the acid used to adjust treatment solution pH, immersion time, oxidising agent additions and the incorporation of rare earth metal species. Silicate sealant layers were also highlighted as a post treatment. Molybdate-based coatings were formed on commercial electrodeposited acid zinc surfaces. Many treatment conditions were investigated, and initially performance analysed using DC Linear Polarisation Resistance (LPR) trials. Subsequently, the highest performing coatings were subjected to the more aggressive, industry standard, ASTM B 117 Neutral Salt Spray (NSS) corrosion test. The highest performing molybdate coatings were found to have an average LPR of ~ 9 000 Ω. cm2, in contrast to ~ 12 000 Ω. cm2 for the Cr(VI) based reference. NSS results were amongst the highest performing for molybdate based coatings documented, at 24 h until 5% white rust, however remained inferior to Cr(VI) coatings, which lasted 120 h. The highest performing coatings were characterised using FEG-SEM, Cryofracture EDXA and site specific AES. These techniques revealed that the enhanced molybdate coatings had a columnar structure that was around 300 nm thick, with pores that appeared to expose the substrate. AES showed this type of coating to have a mixed Mo, P and Zn oxide surface. Corrosion initiation was also studied; this can be thought of as an investigation to determine the point(s) of weakness or the mechanism that causes coating failure. Coatings were immersed in 5 % wt/ vol NaCl(aq) until they showed any surface change. Initial signs of corrosion were deemed to be any appearance of pitting or discolouration of the film, not a voluminous corrosion product. Untreated Zn, Cr(VI) and simple molybdate coatings were studied as well as enhanced molybdate coatings. There were clear differences in the way the coatings behaved at the onset of corrosion. Cr(VI) coatings delaminated, leaving an area of decreased Cr concentration. The enhanced molybdate coatings failed by the appearance of localised pores of ~ 70 µm in diameter. Substrate exposure was indisputably the reason for coating failure in chloride environments. In light of the work carried out in the present thesis the outlook for the use of molybdate as a potential replacement for chromate for the conversion coating of electrodeposited zinc surfaces is a positive one.

The aim of this project was to, based on a given idea, develop and evaluate a method for measuring residual stress in thin PVD coatings. AlCrN was deposited, by PVD, on thin circular samples of stainless steel foil and the radius of the emerged curvature was measured using an optical profilometer. From the radius data the residual stress in the coating of each sample was calculated. The foil samples examined were of two different thicknesses, 0.3 mm and 0.5 mm. With the parameters of the project the foils of 0.3 mm were found most suitable. Furthermore, the method was compared to an already established method where depositions are made on thicker substrates, which are then ground to an appropriate thickness. A correlation factor between the two methods was calculated and found to be 0.91 ± 0.28. Finally, the possibility of adapting the method in running production was investigated. Cost and time analyses were conducted and both supported the applicability of the method. / Projektets syfte var att utveckla och utvärdera en metod för att mäta restspänningar i tunna PVD-beläggningar utifrån en redan befintlig idé. Tunna cirkulära prover av rostfritt stål belades med AlCrN genom PVD och radien på den utböjning som uppstod mättes med en optisk profilometer. Restspänningen bestämdes sedan utifrån den uppmätta radien. Folier av två olika tjocklekar, 0,3 mm och 0,5 mm, utvärderades. Med de processparametrar som användes i projektet visades att folien med tjocklek 0,3 mm var den bäst lämpade. Metoden jämfördes även med en etablerad metod där tjocka substrat beläggs och sedan slipas ned till lämplig tjocklek. En korrelationsfaktor som relaterar de två metoderna bestämdes till 0,91 ± 0,28. Slutligen undersöktes möjligheterna att använda metoden i löpande produktion. Kostnads- och tidsanalyser utfördes och resultaten stödjer metodens användbarhet.

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