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A vehicle flying at hypersonic speeds, i.e. at speeds greater than Mach 4, needs to be able to withstand the heat arising from friction and shock waves, which can reach temperatures of up to 3000oC. The current project focuses on producing thermal protection systems based on ultra high temperature ceramic (UHTC) impregnated carbon-carbon composites. The carbon fibres offer low mass and excellent resistance to thermal shock; their vulnerability is to oxidation above 500oC. The aim of introducing HfB2, a UHTC, as a coating on the fibre tows or as particulate reinforcement into the carbon fibre preform, was to improve this property. The objectives of this project were to: i) identify a low temperature synthesis route for group IV diborides, ii) produce a powder fine enough to reduce the difficulties associated with sintering the refractory diborides, iii) develop sol-gel coating of HfB2 onto carbon fibre tows iv) improve the solid loading of the particulate reinforcement into the carbon fibre preform, which should, in turn, increase the oxidation protection. In order to achieve the above set objectives, fine HfB2 powder was synthesized through a low temperature sol gel and boro/carbothermal reduction process, using a range of different carbon sources. Study of the formation mechanism of HfB2 revealed an intermediate boron sub-oxide and/or active boron formation that yielded HfB2 formation at 1300oC. At higher temperatures the formation of HfB2 could be via intermediate HfC formation and/or B4C formation. Growth mechanism analysis showed that the nucleated particles possessed screw dislocations which indicated that the formation of HfB2 was not only through a substitution reaction, but there could have been an element of a precipitation nucleation mechanism that lead to anisotropic growth under certain conditions. The effect of carbon sources during the boro/carbothermal reduction reaction on the size of the final HfB2 powders was analysed and it was found that a direct relation existed between the size and level of agglomeration of the carbon sources and the resulting HfB2 powders. A powder phenolic resin source led to the finest powder, with particle sizes in the range 30 to 150 nm. SPS sintering of the powder revealed that 99% theoretical density could be achieved without the need for sintering aids at 2200oC. Sol-gel coatings and slurry impregnation of HfB2 on carbon fibres tows was performed using dip coating and a 'squeeze-tube' method respectively. Crack free coatings and non-porous matrix infiltration were successfully achieved. The solid loading of the fine HfB2 into the carbon fibre preform was carried out through impregnation of a HfB2 / phenolic resin/acetone slurry using vacuum impregnation. Although the sub-micron Loughborough (LU) powders were expected to improve the solid loading, compared to the commercially available micron sized powders, due to the slurry made from them having a higher viscosity because of the fine particle size, the solids loading achieved was consequently decreased. Optimisation of the rheology of the slurry with LU HfB2 still requires more work. A comparison of the oxidation and ablation resistance of the Cf-HfB2 composites prepared with both commercial micron sized HfB2 powder and Loughborough sub-micron sized HfB2 powder, each with similar level of solid loading, was carried out using oxyacetylene torch testing. It was found that the composite containing the finer, Loughborough powders suffered a larger erosion volume than the composite with the coarser commercial powders indicating that the former offered worse ablation and oxidation resistance than the latter. A full investigation of the effect of solids loading and particle size, including the option of using mixtures of fine and coarse powders, is still required.

The objective of this master thesis was to investigate an ablative liner for the T-Minus DART booster that will accelerate a dart to Mach 5.2 within five seconds. An oxyacetylene torch test was used to sort out the obviously bad materials. Glass fiber/epoxy, with and without alumina as fire retardant, and carbon fiber/epoxy were selected for further investigation. A sub-scale motor was built to expose the materials for conditions similar to the booster conditions in regard to temperature, chemistry, flow velocity and pressure. The target pressure could not be reached in the sub-scale motor but a polynomial function was fitted to the data in order to extrapolate the data and estimate the ablation rate at 7 MPa. The final design is always based on measurements on full scale motors. This could not be done within this report. Recommendation for future work is to use an insulator of 1.8 mm of carbon fiber/epoxy or 1.3 mm of glass fiber/epoxy/alumina for the sub-scale firings to come.

This thesis deals with the application of simple oxyacetylene welding techniques to the secondary art curriculum. Welded sculptural forms in sheet steel have been the focus of my own work during the past three years and these experiences formed the basis or background of the presentation of these skills to secondary students. An instructional unit was designed to fulfill three primary objectives. First, to familiarize the student with the basic tools and materials used in the construction of welded steel sculpture; second, to introduce the student to the creative concepts involved in the creation of sculptural form; third, to give the student liberal opportunity.to explore the possibilities inherent in the materials.

Orientador: Michelli Leali Costa / Resumo: Dentre os setores de aplicações comerciais de materiais compósitos avançados, destaca-se o setor aeroespacial com larga aplicação desses materiais. Entre as técnicas de união aplicadas aos compósitos termoplásticos está a soldagem com grandes vantagens em relação aos outros processos de uniões tais como, redução de custo, agilidade de processamento, redução de elementos acumuladores de tensão, preparações mais simplórias das superfícies a serem soldadas. Mesmo com essas vantagens e, embora o processo já seja utilizado nos setores industriais, a soldagem de compósitos tem como desafio a melhoria de sua resistência, a aplicação do processo para reparos ou processamentos em campo, além da necessidade de avaliação dos efeitos das condições climáticas em juntas soldadas, as quais podem estar submetidas quando em operação em uma aeronave, por exemplo. Mediante a isso, esse trabalho visa produzir e estudar juntas soldadas por resistência elétrica e por tocha acetilênica em compósito PEI/fibra de vidro. Os compósitos obtidos foram submetidos ao condicionamento higrotérmico seguido de ensaios mecânicos de LSS (Lap Shear Strength) e ENF (End-Notched Flexure). O mecanismo de fratura das juntas soldadas foi avaliado por microscopia. A partir das análises e dos resultados obtidos, foi observado que os valores de resistência ao cisalhamento em modo II dos compósitos soldados por resistência elétrica (3270,9 J/m²) são similares aos descritos na literatura para os compósitos PEI/fibra de vid... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Among the commercial application sectors of advanced composite materials, the aerospace sector with wide application of these materials stands out. Among the joining techniques applied to thermoplastic composites is welding with great advantages over other joining processes such as cost reduction, processing agility, reduction of stress accumulators, little or no preparation required of the surface to be processed. Even with these advantages, and although the process is already used in the industrial sectors, composite welding has the challenge of improving its resistance, the application of the process for field repairs or processing, and the need to evaluate the effects of weather conditions. welded joints, which may be subjected when operating in an aircraft, for example. Therefore, this work aims to produce and study joints welded by electric resistance and acetylene torch in PEI / fiberglass composite. The composites obtained were subjected to hygrothermal conditioning followed by mechanical tests of LSS (Lap Shear Strength) and ENF (End-Notched Flexure). The fracture mechanism of the welded joints was evaluated by microscopy. From the analyzes and results obtained, it was observed that the shear strength values in mode II of the composites welded by electric resistance (3270.9 J / m²) are similar to those described in the literature for PEI / fiberglass composites. However, for the composites welded by the oxyacetylene process, a reduction of approximately 20% in the va... (Complete abstract click electronic access below) / Mestre

Les composites de type Cf/C sont utilisés en tant que pièces structurales dans les propulseurs spatiaux du fait de leurs excellentes propriétés mécaniques dans le domaine des très hautes températures. Néanmoins, l’atmosphère oxydante et corrosive créée lors du décollage des lanceurs et les hauts flux gazeux dégradent ces matériaux. Afin d’améliorer les performances de ces matériaux vis-à-vis de l’oxydation/corrosion, une protection composée de céramiques ultra-réfractaires (dites UHTC) peut être appliquée. Pour une efficacité de protection optimale, des phases UHTC ont été introduites en tant que constituants de la matrice. Dans ces travaux de thèse, la matrice a été réalisée par l’intermédiaire d’un procédé d’élaboration en phase liquide combinant : (i) l’introduction de poudres et (ii) la densification par infiltration réactive d’un métal fondu. La composition de la matrice appartient au système (B;C;Si;Zr). La caractérisation des matériaux après élaboration a permis de comprendre les mécanismes d’infiltration et les réactions permettant de mieux contrôler la composition chimique et la répartition des phases. Des essais sous torche oxyacétylénique ont été utilisés pour se placer dans des conditions proches de l’application visée. La caractérisation post-test des matériaux a permis d’évaluer l’efficacité de la protection dans le cas d’une utilisation unique et également d’une possible réutilisation. Les résultats en oxydation/corrosion ont permis de classer les matériaux en fonction de leur efficacité de protection. / Since many decades, Carbon/Carbon composites are used as structural parts in rocket engines due to their excellent thermomechanical properties. However, under highly oxidizing/corrosive atmosphere and high gas flow rates, carbon suffers from severe oxidation. To improve oxidation resistance of these composites, Ultra High Temperature Ceramics (UHTC) can be used as a protection. To protect the whole composite, the introduction of UHTC as a matrix has been done using a liquid phase process combining: (i) slurry infiltration process and (ii) reactive melt infiltration. Matrix constituents belong to the (B;C;Si;Zr) system. Material characterisation allowed a better understanding of the infiltration mechanisms and of the phase distribution and composition in respect to the processing conditions. To select the best composition, oxyacetylene torch testing has been done to recreate spacecraft launch environmental conditions. Post-test characterisation has been done to evaluate protection efficiency of each matrix composition for single use and possible reuse. Finally, advantages and drawbacks assessment of each composition allowed to highlight the most protective composition and phase distribution.