Student Number : 0109917P - PhD thesis - School of Process and Materials Engineering - Faculty of Engineering and the Built Environment / The aim of this project was to characterise new WC-10VC-Co powders, and to deposit WC-10VC-Co thermal spray coatings from these powders for characterisation and testing in adhesion, wear and corrosion tests. Throughout the project, the new powders and coatings were compared to commercial WC-Co powders of the same binder content and commercial WC-Co thermal spray coatings. All the powders i.e WC-10VC-Co and WC-Co powders, were produced by agglomeration (by spray drying) and sintering and characaterised by determining the sizes and size distributions of the powders' particles, the morphology, the flowability and the phase composition and grain size and size distribution of carbide grains. The vanadium carbide in the WC-10VC-Co powders occurred in the solution as the double carbide (V,W)C and the carbides present in the WC-10VC-Co powders were WC and (V,W)C. None of the starting VC was left in the powders. Coatings were deposited using high pressure high velocity oxy-fuel (HP/HVOF) spraying systems, and characterized by determining the microstructures, the phase compositions and the carbide grain sizes, as had been done for the powders. Three types of tests were done on the coatings: adhesion tests, (according to standard SNECMA 14 -008); dry abrasion, wet abrasion and slurry erosion tests; and corrosion tests, in synthetic mione water. Thermal spraying lead to some WC decarburization to W2C and eta phase, and to the formation of amorphous binder. The W2C grains from the WC decarburization formed in the amorphous binder matrix of coatings. All the coatinge were porous, but the new WC-10VC-Co coatings were more porous than the commercial Wc-Co coatings because the spray parameters had only been optimized for the WC-Co coatings. The carbide grains decreased in size by as much as 50% during decomposition. Evidence suggested that the WC grains in the coatings were subjected to different residual stresses that in the powders, probably due to the formation of the amorphous binder. Vanadium carbide in the Wc-10VC-Co coatings occurred as (V,W)C, just as in the powders, with as distribution that was reasonably homogeneous. The apparent hardness of the new Wc-10VC-Co coatings was slightly lower than that of WC-Co coatings of the same cobalt content, due to their higher porosity. The adhesion of the new Wc-10VC-Co coatings was as good as that of the Wc-Co coatings. The dry and wet abrasion resistance of the new Wc-10VC-Co coatings was better that for the Wc-Co coatings of equal Co wt%, on account of the Wc-10VC-Co coatings having a lower binder volume fraction, finer carbide grains, and (V,W)C grains. The (V,W)C grains are harder than WC grains and apparently slowed down the overall abrasion rate. In slurry erosion, the best performance of the Wc-10VC-Co coatings was as good as that of the commercial WC-Co coatings at equal cobalt mass content, due to the higher porosity of the Wc-10VC-Co coatings, apparent faster erosion of the harder but brittle (V,W)C grains, and, from what evidence appreared to suggest, generally slightly poorer erosion resistance of the fine WC grains under the test conditions used. Polishing the slurry erosion test specimens reduced mass losses in slurry erosion by factor of up to 10 compared to the unpolished specimens, and led to better erosion resistance of the WC-10VC-Co coating compared to the WC-12Co coating. The results of the tests done to investigate the corrosion properties of the coatings were conclusive. This is because the effects of cleaning procedures on mass loss after immersion corrosion were not explored, and it appeared, for some coatings, that the corrosion mechanisms in immersion corrosion could not be reproduced in electrochemical testing.

Made available in DSpace on 2016-12-08T17:19:23Z (GMT). No. of bitstreams: 1 Matheus Machado de Souza.pdf: 5533626 bytes, checksum: a5f582284c7919cc899d81e7de2c62c8 (MD5) Previous issue date: 2014-08-25 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The use of coatings based in electrolytic chromium deposition causes damages to the environment, since during the process, hexavalent chromium ions are released and these are extremely toxic to nature and man. However, other technologies have been studied as alternatives to the traditional chromium electroplating process. The thermal spray high velocity oxy-fuel (HVOF) is a technique that has shown the best results considering coating quality, porosity, wear resistance, mechanical and adhesive properties. In this work, a comparative study between the hard chromium electroplated and HVOF sprayed coatings was performed, in a condition which the substrate surface has been rectified. Surface properties were checked in terms of adhesion, hardness and microstructure, while the tribological behavior was obtained by pin on disk wear tests, considering the evaluation of wear resistance, friction coefficient and wear mechanisms. The material used as substrate was the SAE 4140 steel; the coatings sprayed by HVOF were WC-Co-Cr and 316L Stainless Steel. As the conditions studied, no adhesion on substrate of 316L coating was observed. For coatings which obtained satisfactory adherence, the tungsten carbide was harder (1750 HV0,1) than chromium (1050 HV0,1). This result was attributed to the low porosity and high hardness of WC and W2C phases contained into sprayed coating. The coat WC-Co-Cr had a better performance and wear resistance, possibly, due to the formation of a great wear resistant tribofilm and the high hardness of the layer. For chromium, the delamination of a fragile tribofilm aggregated with a big quantity of cracks inside the microstructure might explain the pronounced wear. / A utilização de revestimentos a base de cromo eletrolítico traz prejuízos ao meio ambiente, uma vez que durante o processo são liberados íons de cromo extremamente tóxicos à natureza e ao homem. Outras tecnologias vêm sendo estudadas como alternativas ao processo tradicional de cromagem. A aspersão térmica por oxi- ombustível de alta velocidade (HVOF) é a técnica que tem apresentado os melhores resultados sob as óticas de qualidade de revestimento, porosidade, resistência ao desgaste, propriedades mecânicas e adesivas. Neste trabalho foi realizado um estudo comparativo entre o cromo duro eletrodepositado e revestimentos aspergidos (WC-Co-Cr e o Inox 316L) pela técnica de HVOF em uma condição em que a superfície do substrato (aço SAE 4140) foi retificada. As propriedades de superfície foram verificadas em função da adesão, dureza e microestrutura, enquanto que o comportamento tribológico foi obtido por meio de ensaios de desgaste por deslizamento pino sobre disco, com avaliação da resistência ao desgaste, coeficiente de atrito e mecanismos de desgaste. Verificou-se a não adesão ao substrato do revestimento de Inox 316L para as condições estudadas. Quanto a dureza o carbeto de tungstênio apresentou maior valor (1750 HV0,1), bastante superior ao cromo (1050 HV0,1). Resultado atribuído a baixa porosidade e a elevada dureza das fases WC e W2C contidas no revestimento aspergido. O revestimento de WC-Co-Cr teve um melhor desempenho quanto a resistência ao desgaste, possivelmente, em função da formação de um tribofilme resistente ao desgaste e a elevada dureza da camada. Para o cromo, a delaminação de um tribofilme de natureza frágil somada a uma microestrutura com elevada densidade de trincas foram fatores que puderam explicar seu desgaste mais acentuado.

Mechanical properties of thermal spray deposited coatings are highly influenced by their microstructural characteristics. The objective of this investigation is to evaluate the mechanical properties of thermally sprayed coatings consisted of aluminum and zinc based on the coating microstructure, using an image based computational scheme. Microstructural images of coating samples were subjected to image-based finite element analysis and the results were validated by experimental tests and analytical models. Comparison of the experimental data with FEA was used to explain the microstructural basis of the mechanical characteristics of Al-Zn coatings and the differences between two methods of thermal spray techniques. It was concluded that the cold spraying technique produces higher-quality coatings with less porosity and higher hardness compared to wire arc deposition. An isotropic behavior was observed in the cold sprayed coating. Finally, the electrochemical tests showed that the coating with a higher amount of zinc had better anti-corrosion properties.