Dissertations / Theses on the topic 'Laser Surface Alloying'

A comprehensive, transient three-dimensional model of a single-pass laser surface alloying process has been developed and used to examine the heat, momentum and species transport phenomena. A numerical study is performed in a co-ordinate system moving with the laser at a constant scanning speed. In this model a fixed grid enthalpy-porosity approach is used, which predicts the evolutionary pool development. In this model two extreme cases of alloying element and base metal combinations are considered based on their relative melting points. One extreme case is for an alloying element with its melting point much lower than that of the base metal. In this case the alloying element melts almost instantaneously. Hence it is assumed that the alloying element introduced on the melt pool surface is in the molten state. Thus, while solving the species conservation equation a species flux condition is used on the entire melt pool surface. This case is analysed for aluminium alloying element on iron base metal. The final species distribution in the melt pool as well as in the solidified alloy is predicted. The other extreme case is studied for an alloying element with its melting point relatively higher than that of the base metal. In this case all the alloying element particles on the melt pool surface will not melt. Only those particles which fall in the region on the melt pool surface where the local temperature is higher than the melting point of the alloying element will melt. The particles which fall away from this region are advected into the melt pool, due to a strong Marangoni convection on the melt pool surface. If a particle is advected into the inner region in the melt pool (where the temperature is higher than its melting point), it starts melting and thus the molten species mass gets distributed. Hence, the species flux condition at the entire surface of the melt pool is not valid. The particles are tracked in the melt pool by assuming the alloying particles to be spherical in shape and moving without any relative velocity with the surrounding fluid. Simultaneously, the temperature field inside the spherical particle is solved by assuming its surface temperature to be the local temperature in the melt pool. The amount of particle mass that fuses as it passes through a particular control volume is noted. The same procedure is repeated for a large number of particles initiated at various locations on the pool surface, and a statistical distribution of the species mass source in the entire pool is obtained. This species mass source distribution is then used to solve the species conservation equation. Nickel alloying element on aluminium base metal is used to illustrate this case. The numerical results obtained from the two cases are compared with the available experimental results. A qualitative matching is found between the numerical and experimental results.

A comprehensive, transient three-dimensional model of a single-pass laser surface alloying process has been developed and used to examine the heat, momentum and species transport phenomena. A numerical study is performed in a co-ordinate system moving with the laser at a constant scanning speed. In this model a fixed grid enthalpy-porosity approach is used, which predicts the evolutionary pool development. In this model two extreme cases of alloying element and base metal combinations are considered based on their relative melting points. One extreme case is for an alloying element with its melting point much lower than that of the base metal. In this case the alloying element melts almost instantaneously. Hence it is assumed that the alloying element introduced on the melt pool surface is in the molten state. Thus, while solving the species conservation equation a species flux condition is used on the entire melt pool surface. This case is analysed for aluminium alloying element on iron base metal. The final species distribution in the melt pool as well as in the solidified alloy is predicted. The other extreme case is studied for an alloying element with its melting point relatively higher than that of the base metal. In this case all the alloying element particles on the melt pool surface will not melt. Only those particles which fall in the region on the melt pool surface where the local temperature is higher than the melting point of the alloying element will melt. The particles which fall away from this region are advected into the melt pool, due to a strong Marangoni convection on the melt pool surface. If a particle is advected into the inner region in the melt pool (where the temperature is higher than its melting point), it starts melting and thus the molten species mass gets distributed. Hence, the species flux condition at the entire surface of the melt pool is not valid. The particles are tracked in the melt pool by assuming the alloying particles to be spherical in shape and moving without any relative velocity with the surrounding fluid. Simultaneously, the temperature field inside the spherical particle is solved by assuming its surface temperature to be the local temperature in the melt pool. The amount of particle mass that fuses as it passes through a particular control volume is noted. The same procedure is repeated for a large number of particles initiated at various locations on the pool surface, and a statistical distribution of the species mass source in the entire pool is obtained. This species mass source distribution is then used to solve the species conservation equation. Nickel alloying element on aluminium base metal is used to illustrate this case. The numerical results obtained from the two cases are compared with the available experimental results. A qualitative matching is found between the numerical and experimental results.

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2015
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Laser surface alloying (LSA) é uma poderosa técnica de modificação de camada limite de um componente, cada vez mais reconhecida, usada para aumentar a resistência ao desgaste e à corrosão em componentes de engenharia. Neste trabalho, a região próxima a superfície de uma ferramenta de estampagem automotiva (aço ASTM A681), foi tratada com um laser de fibra Nd: YAG contínuo de comprimento de onda de 1064 nm e com pó de adição WC-Cr-Co, na proporção de 86%, 6% e 8%. Este processo consiste basicamente em fundir uma camada próxima a superfície de um substrato, adicionando simultaneamente partículas de um material de adição na forma de um pó pré ligado, modificando a composição química e microestrutura, assim como tamanho de grão de forma localizada da ferramenta, alterando sua dureza e resistência ao desgaste. Para analisar a resistência ao desgaste para este processo, foram feitas mil estampagens em uma ferramenta tratada por LSA e em outra não tratada e seus resultados comparados entre si. A finalidade do processo LSA é a de aumentar o tempo de vida de uma ferramenta em uma aplicação industrial. As análises da ferramenta tratada apresentaram um resultado significativo quando comparada com uma ferramenta sem o tratamento. Obteve-se uma redução cerca de nove vezes na rugosidade superficial e uma maior resistência ao desgaste. Neste trabalho também foi analisada a influência do aumento da potência do laser no processo de laser surface alloying. Para isto cinco diferentes amostras foram tratadas e analisadas quanto a microdureza, composição química, identificação de fases e ensaios de resistência ao desgaste. Os resultados obtidos neste trabalho foram importantes para concluir que este é um processo muito complexo e deve ser rigorosamente controlado, pois diversos fatores alteraram a estrutura e resistência ao desgaste da camada próxima à superfície tratada.

Abstract : Laser Surface Alloying (LSA) is a powerful boundary limit of a component modification technique, increasingly recognized, used to increase the wear and the corrosion of engineering components resistance. In this thesis a near surface region of an automotive deep drawing tool (steel ASTM A681) was treated with a continuous fiber laser Nd: YAG with wavelength of 1064 nm and with WC-Cr-Co particles in ratio of 86%, 6% and 8%. This process consists basically in melting a substrate's layer near of surface adding simultaneously particles of a filler material in a pre-connected powder shape, altering the microstructure, as well as the grain size in localized form of the tool, altering the hardness and wear resistance. To perform a comparative analysis of this process one thousand deep drawings were made in a tool treated by LSA and in an untreated tool. The behavior's characteristics of these tools have been analyzed and their results compared with each other. The purpose of LSA process is to increase the lifetime of a tool in the industrial application. What could be observed after the analysis was the success of this process, because all the characteristics results showed significant improvement when compared with the untreated tool. As example cite a decrease of about nine times the roughness and significantly reducing of the treated tool wear. In this work the influence of the increase in laser power in the laser surface alloying process was analyzed. Five different samples were treated and analyzed, their hardness, chemical composition, phase identification and wear resistance were investigated. The results obtained of this work were important to conclude that LSA is a very complex process and must be strictly controlled, as there are several factors that can change the structure and wear resistance of the surface treated.

Aluminum (Al) and its alloys are widely used in various technological applications, mainly due to the excellent thermal conductivity, non-magnetic, ecofriendly, easy formability and good recyclability. However due to the inferior corrosion resistance its applications are hampered in various engineering sectors. Besides, the corrosion related failures such as leakage of gas from pipeline, catastrophic breakdown of bridges and fire accidents in processing plants further puts the human life in jeopardy. Within the United States over $ 400 billion dollars per year are spent over research to understand and prevent the corrosion related failures. Recently, the development of transition metal(TM) aluminides (AlxTMy, where, TM = Mo, W, Ta, Nb, Cr, Zr and V) has received the global attention mainly due to high strength at elevated temperatures, light-weight, excellent corrosion and wear resistance. In light of this, surface modification via laser surface alloying (LSA) is a promising engineering approach to mitigate the corrosion and wear problems. In the present study the attempts are made to study the Al-Mo, Al-W, Al-Nb, and Al-Ta systems as a potential corrosion resistant coatings on aluminum. The refractory metal (Mo, W, Nb, Ta) precursor deposit was spray coated separately on aluminum substrate and was subsequently surface alloyed using a continuous wave diode-pumped ytterbium laser at varying laser energy densities. Microstructural analysis was conducted using scanning electron microscopy and further X-ray diffractometry was carried out to evaluate the various phases evolved during laser surface alloying. Corrosion resistance of laser alloyed coatings were evaluated using open circuit potential, cyclic potentiodynamic polarization, electrochemical impedance spectroscopy measurements were performed in 0.6 M NaCl solution (pH:6.9±0.2, 23˚C). Open circuit potential measurements indicate the more stable (steady state) potential values over long periods after laser surface alloying. Cyclic polarization results indicated reduction in the corrosion current density, enhancement in the polarization resistance, and increase in coating/protective efficiency with increase in laser energy density compared to untreated aluminum. Electrochemical impedance spectroscopy measurements also indicated an increase in charge transfer resistance after laser surface alloying of refractory metals on aluminum. Additionally, first principle calculations of thermodynamic, electronic and elastic properties of intermetallics evolved during LSA were also thoroughly investigated to correlate the corrosion performance of intermetallic coatings with these properties. The present study indicates that novel Al-Mo, Al-W, Al-Nb, and Al-Ta intermetallics has a great potential for light weight structural applications with enhanced corrosion resistance.

Orientador: João Batista Fogagnolo
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: Titânio e suas ligas são amplamente utilizados como biomateriais devido a propriedades como alta resistência específica, alta resistência à corrosão e baixo módulo de elasticidade. As ligas de titânio apresentam diferentes estruturas cristalinas, as quais possuem diferentes módulos de elasticidade. Esta característica permite que o módulo de elasticidade seja manipulado através do controle da composição química da peça, o que por sua vez possibilita a fabricação de peças com gradientes de rigidez, abrindo-se um amplo campo de pesquisa e novas aplicações. Este trabalho utiliza a técnica de modificação superficial por laser e adição de nióbio com o objetivo de obter peças de titânio com revestimentos constituídos por fases com menores módulos de elasticidade, ou seja, peças com revestimentos menos rígidos que o substrato. Inicialmente foram caracterizados cordões isolados obtidos com diferentes densidades de energia do feixe de laser, através da variação da potência, do diâmetro e da velocidade de varredura do feixe de laser. Nos cordões isolados, observou-se que a potência do feixe de laser é o parâmetro de maior influência e que densidades de energia intermediárias possibilitaram obter cordões com zonas fundidas com dimensões e formato adequados para a posterior etapa de obtenção de recobrimento contínuo por sobreposição de cordões. As composições dos cordões isolados, apesar de apresentar heterogeneidade, permitiram a estabilização da fase ?. No entanto, uma fração significativa dos cordões apresentou também a fase ? com microestrutura dendrítica, indicando que essa fase foi resultado da solidificação, o que pode ter ocorrido pela contaminação por oxigênio, que é um forte estabilizador dessa fase. Dessa forma, mesmo com a formação da fase ?, não se observou uma redução do módulo de elasticidade. Em todos os casos, a dureza apresentou um significativo aumento, também devido a forte contaminação por oxigênio. A formação de revestimentos contínuos foi obtida através da sobreposição de 50% da largura dos cordões. Os revestimentos contínuos assim obtidos apresentaram segregação de soluto, com fase ? estabilizada nas regiões com maiores teores de nióbio e agulhas de martensita nas regiões com menores teores de nióbio. A contaminação por oxigênio foi reduzida em comparação com os cordões isolados. Os revestimentos contínuos apresentaram aumento de dureza e diminuição do módulo de elasticidade, conforme o objetivo proposto
Abstract: Titanium and its alloys are commonly used as biomaterials due to their properties as high specific strength, high corrosion resistance and low elastic modulus. Titanium alloys show differents crystalline structures, which have different elastic modulus. This characteristic allows the manipulation of the elastic modulus through chemical composition control, which allows the manufacture of pieces with stiffness gradients, spreading a large research field and new applications. This work use the laser surface alloying technique with addition of niobium powder to obtain titanium pieces with coatings composed by phases with lower elastic modulus, in other words, pieces with coatings less stiff than the substrate. Firstly, isolated tracks obtained by different beam laser energy densities, by varying laser beam power, diameter and scan speed were characterized. In this case, the laser beam power was the most important parameter and the intermediaries energy densities allowed to obtain tracks with melted zones with dimensions and shape suitable to obtain a continuous coating by overlapping tracks. The isolated tracks compositions, despite their chemical heterogeneities, showed the stabilization of the ? phase. However, a significant fraction of tracks showed also ? phase with dendritic microstructure, indicating that this phase was the result of solidification, which may have occurred due oxygen contamination, which is a strong ? stabilizer element. Thus, despite the stabilization of the ? phase, a reduction in elastic modulus was not observed. In all the cases the hardness showed a significant increase, also because of the high contamination by oxygen. The continuous coating formation was obtained by overlapping 50% of the tracks width. The continuous coatings obtained showed solution segregation, with ? phase stabilized in the regions with higher niobium content and needle like traces of martensite in the regions with less niobium content. The oxygen contamination was reduced in comparison with isolated tracks. The continuous coatings showed high hardness with a decrease of the elastic modulus, in accordance with the proposed objective
Mestrado
Materiais e Processos de Fabricação
Mestra em Engenharia Mecânica

Des alliages de surface renforces par des carbures mixtes fer-chrome m#7c#3 (m = fe, cr) d'une part, et par du carbure de titane d'autre part, ont ete elabores par irradiation laser sur l'acier inoxydable austenitique aisi 316l (z2 cnd 17-12) afin d'ameliorer sa resistance a l'usure. L'utilisation de differents precurseurs (graphite, carbure de silicium, carbure cr#3c#2 seul ou associe a du chrome, tic submicronique et micronique, melange de poudres de titane et de carbure sic) a permis d'obtenir des revetements presentant une large variete de microstructures et de compositions. Les microduretes correspondantes, comprises entre 350 hv et 900 hv temoignent d'un durcissement significatif de l'acier 316l (200 hv). La confrontation des phases presentes dans les differents alliages de surface avec celles des diagrammes d'equilibre des systemes correspondants a mis en evidence le caractere metastable des carbures m#7c#3 et la bonne stabilite thermique des carbures de titane. La resistance a l'oxydation a haute temperature des divers alliages de surface a ete caracterisee par thermogravimetrie. Les resultats obtenus ont montre le role essentiel des elements chrome et silicium dans le processus d'oxydation. Ainsi, les revetements elabores par irradiation d'un melange de carbure cr#3c#2 et de chrome, et ceux obtenus a partir d'un predepot constitue d'un melange de titane et de carbure de silicium, presentent une excellente resistance a l'oxydation a 1000c, equivalente a celle de l'acier 316l. D'autre part, le maintien prolonge a 1000c des alliages contenant des carbures m#7c#3 conduit a la precipitation des phases d'equilibre ce qui provoque une nette baisse de durete. Aucune modification structurale n'est observee dans les alliages contenant des carbures de titane. Les tests de corrosion intergranulaire effectues ont confirme le role preponderant du chrome sur le comportement a la corrosion. Enfin, les tests d'usure par frottement effectues sur ces differents alliages de surface ont montre que le durcissement obtenu permet une amelioration significative du comportement tribologique de l'acier 316l

L'objectif du travail presente ici est d'augmenter, par un traitement de surface, la tenue a l'oxydation a temperature elevee et la durete d'aciers ferritiques faiblement allies. Nous presentons les resultats obtenus avec les aciers 35ncd16 et z38cdv5 par irradiation laser assurant l'incorporation superficielle et la dissolution de predepots de nitrures (aln, si#3n#4) ou de carbures (sic, cr#3c#2). Nous precisions les conditions d'obtention d'alliages de surface exempts de fissures, d'amas ou de porosites. La durete de ces alliages depend principalement de la formation de martensite et (ou) de la precipitation de carbures ou de nitrures. L'excellente tenue a l'oxydation en conditions isothermes jusqu'a 950c de quelques alliages de surface dans une atmosphere d'oxygene resulte essentiellement de la formation d'une couche d'oxyde de chrome cr#2o#3. Dans les alliages contenant du silicium et relativement peu de chrome, la croissance de cette couche de cr#2o#3 n'intervient qu'apres celle, initiale, d'une tres mince pellicule de silice qui inhibe l'oxydation du fer et rend possible le developpement de la couche protectrice de cr#2o#3. Le bon comportement en cyclage thermique des alliages de surface etudiees sur l'acier z38cdv5 resulterait de la croissance de cr#2o#3 par diffusion anionique. La petite taille de grains de cet oxyde permettrait une bonne relaxation de contraintes thermiques par deformation plastique. Nous avons donc montre que le traitement laser avec ajout peut constituer un procede efficace pour conferer aux aciers peu allies un comportement en oxydation a haute temperature tout a fait comparable a celui des aciers inoxydables avec l'avantage d'une durete plus elevee

[ES] Dentro del sector biomédico, el titanio y sus aleaciones han desplazado a otros materiales como el acero inoxidable 316L y las aleaciones Co-Cr por ofrecer un módulo elástico más cercano al hueso cortical, lo que reduce sustancialmente el efecto de apantallamiento de tensiones. Una manera de disminuir aún más el módulo elástico es estabilizando la fase cúbica beta (bcc) del titanio a temperatura ambiente mediante la adición de elementos como el Nb y Mo, cuya biocompatibilidad ha sido comprobada en numerosos estudios. La ruta convencional para el procesado de estas aleaciones es la fundición y/o forja, pero en esta investigación se ha optado por la pulvimetalurgia ya que el desperdicio de material es mínimo y se reducen las etapas posteriores de tratamientos térmicos y mecanizado, facilitando el procesado en general y reduciendo los costos. Además, el carácter refractario de los elementos estabilizadores beta justifica aún más el empleo de un método de consolidación en estado sólido como la pulvimetalurgia donde no es necesario alcanzar temperaturas tan elevadas. Por tanto, en una primera aproximación se han procesado aleaciones de Ti-xNb-yMo (x = 13, 20, 27, 35 ; y = 12, 10, 8, 6; % en peso) mediante pulvimetalurgia convencional para estudiar el efecto del Nb y Mo en la microestructura y propiedades mecánicas de las aleaciones. A fin de atacar otros problemas inherentes del proceso como la porosidad y la falta de homogeneidad en la microestructura se ha acudido a la mezcla mecánica de polvos de la aleación Ti-35Nb-6Mo, evaluando también el efecto de diferentes agentes controladores de proceso en la molienda. Por último, se ha tratado la superficie de las aleaciones Ti-27Nb-8Mo y Ti-35Nb-6Mo mediante fusión por láser con diferentes parámetros para estudiar la capacidad del proceso de cerrar la porosidad abierta, mejorar la homogeneidad superficial, y evaluar su efecto en la microestructura y propiedades mecánicas de las aleaciones. Pese a aumentar la porosidad en función del contenido de Nb, las aleaciones Ti-27Nb-8Mo y Ti-35Nb-6Mo de las obtenidas por mezcla elemental exhibieron las mejores propiedades en general, con una microestructura casi beta en su totalidad y un módulo de elasticidad de 67 - 74 GPa, que se acerca más al presentado por el hueso cortical en comparación con la aleación comercial Ti-6Al-4V ELI. Por su parte, la mezcla mecánica mejoró considerablemente la homogeneidad química de la aleación Ti-35Nb-6Mo, pero promovió la formación de la fase alfa y deterioró la resistencia y deformación mecánica debido a la ganancia en acritud del polvo y el aumento de la porosidad. No obstante, la microdureza de las aleaciones aumentó significativamente. En cuanto al agente controlador de proceso, el cloruro de sodio (NaCl) exhibió los mejores resultados en términos de rendimiento y distribución de tamaño de partícula, mientras que el ácido esteárico indujo la contaminación del polvo mediante la formación de la fase no deseada TiC. El tratamiento de fusión superficial por láser consiguió cerrar efectivamente la porosidad abierta de las aleaciones y mejorar la homogeneidad microestructural. Adicionalmente, promovió un aumento de la resistencia y la deformación mecánica y una leve disminución del módulo elástico en ambas aleaciones. Por último, la aleación Ti-27Nb-8Mo tratada superficialmente a 1000W y 6,67 mm/s exhibió una microestructura beta casi en su totalidad y las mejores propiedades mecánicas desde un punto de vista biomédico, con una resistencia de 1467 MPa, una deformación de 7% y un módulo de elasticidad de 67 - 72 GPa.
[CA] Dins del sector biomèdic el titani i els seus aliatges han desplaçat a altres materials com l'acer inoxidable 316L i els aliatges Co-Cr per oferir un mòdul elàstic inferior i més pròxim a l'os cortical, la qual cosa redueix substancialment l'efecte d'apantallament de tensions. Una manera de disminuir encara més el mòdul elàstic és estabilitzant la fase cúbica beta; (bcc) del titani a temperatura ambient mitjançant l'addició d'elements altament biocompatibles com el Nb i Mo. La ruta convencional per al processament d'aquests aliatges és la fosa i/o forja, però en aquesta recerca s'ha optat per la pulvimetalurgia ja que el desaprofitament de material és mínim i es redueixen les etapes posteriors de tractament tèrmics i mecanitzat, facilitant el processament en general i reduint els costos. A més, el caràcter refractari dels elements estabilitzadors beta justifica encara més l'ús d'un mètode de consolidació en estat sòlid com la pulvimetalurgia on no és necessari aconseguir temperatures tan elevada. Per tant, en una primera aproximació s'han processat aliatges de Tu-xNb-yMo (x = 13, 20, 27, 35 ; i = 12, 10, 8, 6; % en pes) mitjançant pulvimetalurgia convencional per a estudiar l'efecte del Nb i Mo en la microestructura i propietats mecàniques dels aliatges. A fi d'atacar altres problemes inherents del procés com la porositat i la falta d'homogeneïtat en la microestructura s'ha acudit a la mescla mecànica de pólvores de l'aliatge Tu-35Nb-6Mo, avaluant també l'efecte de diferents agents antiadherentes en la mòltaa. Finalment, s'ha tractat la superfície dels aliatges Tu-27Nb-8Mo i Tu-35Nb-6Mo mitjançant fusió per làser amb diferents paràmetres per a estudiar la capacitat del procés per a tancar la porositat oberta, millorar l'homogeneïtat superficial, i avaluar el seu efecte en la microestructura i propietats mecàniques dels aliatges. Malgrat augmentar la porositat en funció del contingut de Nb, els aliatges Tu-27Nb-8Mo i Tu-35Nb-6Mo van exhibir les millors propietats en general. La mescla mecànica va millorar l'homogeneïtat química de l'aliatge Tu-35Nb-6Mo però va deteriorar les propietats mecàniques amb excepció de la microdureza. El tractament de fusió superficial per làser va aconseguir tancar efectivament la porositat oberta dels aliatges i va millorar l'homogeneïtat microestructural i les propietats mecàniques. Finalment, l'aliatge Tu-27Nb-8Mo tractada superficialment a 1000W i 6,67 mm/s va exhibir una microestructura beta gairebé íntegrament i les millors propietats mecàniques des d'un punt de vista biomèdic.
[EN] Within the biomedical sector, titanium and its alloys have replaced other materials such as 316L stainless steel and Co-Cr alloys due to a lower elastic modulus, closer to the cortical bone, which significantly reduces the stress shielding effect. An alternative to decrease the elastic modulus even more is to stabilize the cubic beta phase (bcc) of titanium at room temperature by adding highly biocompatible elements such as Nb and Mo. These alloys are normally processed by casting and/or forging, but in this work powder metallurgy was conducted due to lower material waste and less subsequent stages of heat treatments and machining, reducing costs significantly. Moreover, the refractory nature of beta stabilizing elements justifies even more the use of a solid-state consolidation method such as powder metallurgy where it is not necessary to reach meting point temperatures. Therefore, in a first approach, Ti-xNb-yMo alloys (x = 13, 20, 27, 35; y = 12, 10, 8, 6;% by weight) were processed by conventional powder metallurgy to study the effect of Nb and Mo in the microstructure and mechanical properties of the alloys. In order to attack other powder metallurgy inherent problems, such as porosity and inhomogeneity in the microstructure, mechanical mixing was carried out for the Ti-35Nb-6Mo powder, evaluating the effect of different non-stick agents on the milling process. Finally, the surface of Ti-27Nb-8Mo and Ti-35Nb-6Mo alloys were treated by laser surface melting under different parameters to reduce open porosity, improve surface homogeneity, and evaluate its effect. in the microstructure and mechanical properties of alloys. Despite increasing porosity with the increase of Nb content, Ti-27Nb-8Mo and Ti-35Nb-6Mo alloys exhibited the best overall properties, consisting of a beta phase microstructure. Mechanical mixing improved the chemical homogeneity of the Ti-35Nb-6Mo alloy but deteriorated the mechanical properties with the exception of microhardness. The laser surface melting treatment effectively closed the open porosity in the alloys and improved the microstructural homogeneity and mechanical properties. Lastly, the Ti-27Nb-8Mo alloy which surface was treated at 1000W and 6.67 mm/s exhibited a beta microstructure and the best mechanical properties from a biomedical point of view. Despite increasing porosity with Nb addition, Ti-27Nb-8Mo and Ti-35Nb-6Mo alloys exhibited the best overall properties, both consisting of a beta phase microstructure and an elastic modulus of 67 - 74 GPa, which is closer to the cortical bone in comparison to the commercial Ti-6Al-4V ELI alloy. On the other hand, the mechanical mixing significantly improved the chemical homogeneity of the Ti-35Nb-6Mo alloy, but induced the formation of alfa phase and deteriorated the resistance and mechanical deformation due to the increase in porosity and the hardening effect produced during the milling process. Therefore, the microhardness increased significantly. Regarding the process control agent, sodium chloride (NaCl) exhibited the best results in terms of powder yield and particle size distribution, while stearic acid induced contamination by the formation of the undesired TiC phase. The laser surface melting treatment was able to effectively close the open porosity of the alloys and improve microstructural homogeneity. Moreover, it promoted an increase in strength and mechanical deformation and a slight decrease in the elastic modulus in both alloys. Finally, the surface-treated Ti-27Nb-8Mo alloy at 1000W and 6.67 mm/s exhibited almost an entirely beta phase microstructure and the best mechanical properties from a biomedical point of view, with a resistance of 1467 MPa, a deformation of 7% and a modulus of elasticity of 67 - 72 GPa.
El trabajo se realizó en el Instituto de Tecnología de Materiales y en el Departamento de Ingeniería Mecánica y de Materiales de la Universitat Politècnica de València, bajo la ayuda del proyecto MAT2014-53764-C3-1-R del Ministerio de Economía y Competitividad de España, y la subvención 2016/020 del programa SANTIAGO GRISOLIA, enmarcada en la convocatoria del 2015 de la Conselleria de Educación, Cultura y Deporte de la Generalitat Valenciana.
Viera Sotillo, M. (2020). Desarrollo de aleaciones ß Ti-Nb-Mo mediante pulvimetalurgia para aplicaciones biomédicas [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/156194
TESIS

Thesis (Ph.D.)--Worcester Polytechnic Institute.
Keywords: composite Pd and Pd/Ag membranes, alloying, Pd/Ag barrier, intermetallic diffusion, bi-metal multi-layer BMML deposition, electroless plating kinetics, high temperature x-ray diffraction, aluminum hydroxide surface grading, porous sintered metal supports, hydrogen separation. Includes bibliographical references (leaves 279-296 ).

M. Tech. Metallurgical Engineering
Martensitic stainless steel (MSS) metal matrix composite coatings that can accommodate most applications were developed using TiC and stellite 6 as reinforcement powders and Nd: YAG solid state laser for surface alloying. The alloyed surfaces were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM/EDS) and x-ray diffraction (XRD). The microstructures of the alloyed zones show even distribution and super saturation of titanium carbide and other complex carbides of chromium (Cr23C6), niobium (Nb6C5), and iron (Fe7C3) in the matrix of the MSS A significant increase in the hardness was achieved by the addition of stellite 6, although with cracks in the alloyed layer. The cracks formation was due to high heat build-up during the laser alloying process. As a result of the addition of TiC powder in the form of premixed ratio, the heat build-up in the work-piece was minimized. This is due to the high absorptance of laser irradiation by the TiC powder. The alloyed zone of the premixed ratio was free from cracks with microhardness increase of more than 150% of that of the substrate. Multiple tracks of 50% and 75% overlap were fabricated. Three-body abrasive wear study of the overlapped coatings revealed the wear mechanism of the laser coatings to be adhesion, mild abrasive and less degree of deformation showing less plastic ploughing and cutting compared to the as-received. The wear mechanism of the as-received is adhesion, severe abrasive and plastic deformation, showing distinct grooves and damaged spots in the form of craters. The wear resistance of the MMC obtained by alloying with the TiC powder was significantly improved with the 75% overlap, about 90% better than the native material.

D. Tech. Chemical and Metallurgica Engineering.
Discusses the development highly corrosion resistant multi-functional materials for automobile applications by using laser surface alloying of aluminium substrate with a combination of metallic and ceramic powdery materials.

碩士
國立成功大學
材料科學及工程學系碩博士班
93
Aluminum alloys have been considered to be one of the most versatile useful materials because of to interesting mechanical properties, such as high strength-weight ratio, high electrical conductivity, good themal conductivity, easy to shape, and relatively inexpensive. However, the low hardness, low melting point of such alloys result in poor friction properties and heat resistance. The YAG laser alloying is one of the efficient method to improve the surface wear resistance. This paper is using YAG laser alloying Ni-based and Co-based powder on the surface of 6061 alloy and then to discuss the wear resistance property, microstructure, composition, and hardness throughout the alloying layers. As experimental result, we can find alloying layer can be divided into four layers, such as surface layer, subsurface layer, middle layer, and interface layer. The main reason to cause the variation of the alloying layer is due to different cooling rate and density. The Ni-based of the surface layer has the Al3Ni, the subsurface layer has Al3Ni2 feather-like microstructure, and the middle layer is AlNi . The Co-based of the surface layer has the Al9Co2, the subsurface layer is Al13Co4, and the middle layer is needle-like Al5Co2,and after laser alloying treatment, the hardness of middle layer of Ni-based and Co-based are 10 times higher than substrate. We can find transition from mild wear to severe wear point from SEM of wear surface and wear debris, and from results, we can find the wear resistance of Ni-based alloy is 8 times better than substrate’s, and Co-based alloy is 5 times better than substrate. And after heat treatment, because the middle layer of Ni-based Al3Ni will segregate to reduce the hardness, it results in that the transition from mild wear to severe wear point reduce to 500m, and so is Co-based alloy, The optimum specimen of wear resistance is Ni-based alloy without heat treatment.

博士
國立成功大學
材料科學及工程學系碩博士班
94
The surface microstructures of 6061 Al-Mg-Si alloy coated with laser surface alloyed (LSA) pure Ni, Ni base and Co base powder and the microstructure, hardness, and sliding wear performance have been investigated. Experimental results show that there are three regions, as region I, region II and region III in LSA Ni base powder treated specimen. Two regions, IV and V are observed in the LSA Co base treated specimen. The Al3Ni and Al3Ni2 compounds appear in the region I and region II, respectively. The Al-Ni-Cr amorphous structure can be observed in the region III. The Al9Co2 particles with a network structure are present in region IV and block-like Al13Co4 and Al7Cr are distributed in region V. The hardness of the LSA Ni and Co base specimens is much higher than the Al-matrix. Compared with the Al-matrix, the LSA Ni and Co base specimens have excellent sliding wear performance. They have lower friction coefficient and wear rate. The critical temperature of sliding wear resistance of LSA specimen is higher than that of Al-matrix by about 50°C.

碩士
國立中正大學
機械工程所
96
Finite element modeling for single and multi-pass laser surface alloying (LSA) process with plasma sprayed coating layer was established. The finite element computation is carried out with SYSWELD commercial FEM packaging. Based on the previous research Al-Si aluminum alloy was chosen as the substrate and the coating powder was commercial iron based powder METCO 42C. Finite element with half-disc shape geometry was established for modeling the single-pass LSA process with axis-symmetric condition, while solid-rectangular and full-disc shaped geometries were established for the multi-pass LSA process. The adopted moving heat source used for modeling the laser power on those geometries was Goldak double ellipsoid heat source. This research was started from establishing an FEM model for single-pass LSA processing on a half-disc shape specimen was established due to its simplicity. Two stages of FEM computation using SYSWELD were carried out, which were thermal-metallurgical and thermal-mechanical computation. After confirming the appropriate temperature distribution between FEM result and previous experimental result, the computation for thermal-mechanical stages was carried out for predicting the residual stress distribution. An experiment for plasma sprayed-LSA process on a solid rectangular shaped specimen was then carried out for multi-pass FEM computation verification purpose. The selected substrate was also an aluminum alloy but with different series and slightly different composition compared with the aluminum in a previous work. Material for coating layer was the same as previous one. The chosen experimental parameters were based on the optimal parameters from previous work. An LSA treated specimens for both single and multi-pass process were then successfully accomplish with acceptable results compared to previous experimental results. The next effort was to apply the established finite element modeling setup for multi-pass LSA process to model the present experimental work which is in solid shape geometry. A small change of the Goldak parameters was done in order to adjust the size of heat source in order to synchronize the temperature distribution with the molten zone of the rectangular shape element. Thermo-Mechanical computation was also performed to predict the maximum residual stress on the geometry. Finally, the FEM simulation of multi-pass plasma sprayed LSA process on a disc-shaped specimen was established based on previous modeling technique. Some modification of Goldak parameters was done in order to ensure a good agreement between computation results and experimental results that were also conducted previously. Extracted data from thermal-mechanical analysis was only the residual stress distribution in the geometry. Similar residual stress distribution can be observed from those three FEM computation that the stress is accumulated on the area interacted with heat source for the very final time and in HAZ.

This thesis describes the novel laser surface alloying of aluminium AA1200 with various combinations of Ni, Ti and SiC powders, using a 4.4kW Rofin Sinar Nd:YAG laser in order to improve its mechanical and tribological properties. The laser alloying parameters were optimized on the breakdown systems of the complex Al-Ni-Ti-SiC system. Various analytical techniques were used to study the microstructures produced. Wear testing was conducted under sliding and abrasion conditions while the fracture mechanisms were investigated using impact tests. Aluminium surfaces reinforced with metal matrix composites and intermetallic phases were achieved. The phases present depended on the composition of the alloying powder mixture. Al reacted with Ni to form Al3Ni and Al3Ni2 intermetallic phases while Ti reacted with Al to form an Al3Ti intermetallic phase. Some of the SiC particles dissociated and reacted with either Al or Ti to form Al4C3, Al4SiC4, TiC or Ti3SiC2 phases. Si reacted with Ti to form a Ti5Si3 phase. An increase in surface hardness was achieved, up to a maximum of 13 times that of aluminium when alloying with 80wt%Ni + 15wt%Ti + 5wt%SiC. The increase in hardness was attributed to the intermetallic phases especially the Al3Ni2 phase. Alloying led to a 4-38% improvement in the wear resistance of the pure aluminium under sliding wear conditions and a 19-82% improvement under three body abrasion wear conditions. The predominant wear mechanisms for both wear types were groove formation by ploughing and cutting action of the abrasive particles, smearing, material pile-up, extensive cracking of the intermetallic phases and fracturing of the embedded SiC particles in the MMCs. Alloying led to a 31-50% decrease in the impact resistance of the pure aluminium. Brittle fracture of the SiC particles and transgranular cracking of the intermetallic phases were observed for the laser alloyed surfaces while ductile fracture was observed for the bulk aluminium.

Yes
The mass production of polymeric parts with functional surfaces requires economically viable manufacturing routes. Injection moulding is a very attractive option however wear and surface damage can be detrimental to the lifespan of replication masters. In this research, the replication of superhydrophobic surfaces is investigated by employing a process chain that integrates surface hardening, laser texturing and injection moulding. Austenitic stainless steel inserts were hardened by low temperature plasma carburising and three different micro and nano scale surface textures were laser fabricated, i.e. submicron triangular LaserInduced Periodic Surface Structures (LIPSS), micro grooves and Lotus-leaf like topographies. Then, a commonly available talc-loaded polypropylene was used to produce 5000 replicas to investigate the evolution of surface textures on both inserts and replicas together with their functional response. Any wear orsurface damage progressively built up on the inserts during the injection moulding process had a clear impact on surface roughness and peak-to-peak topographies of the replicas. In general, the polymer replicas produced with the carburised inserts retained the wetting properties of their textured surfaces for longer periods compared with those produced with untreated replication masters.
European Union’s H2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 675063 (www.laser4fun.eu). The work was also supported by three other H2020 projects, i.e. “HighImpact Injection Moulding Platform for mass-production of 3D and/or large micro-structured surfaces with Antimicrobial, Self-cleaning, Anti-scratch, Anti-squeak and Aesthetic functionalities” (HIMALAIA, No. 766871), “Process Fingerprint for Zero-defect Net-shape Micromanufacturing” (MICROMAN, No. 674801) and “Modular laser based additive manufacturing platform for large scale industrial applications” (MAESTRO, No. 723826). Further support was provided by the UKIERI DST programme “Surface functionalisation for 18/20 Accepted in the journal Tribology – Materials, Surfaces & Interfaces. food, packaging, and healthcare applications”
The full-text of this article will be released for public view at the end of the publisher embargo on 13 Jul 2021.

碩士
國立成功大學
材料科學及工程學系碩博士班
96
The microstructure, Vicker’s microhardness, and electrochemical property on 7075 Al-Zn-Mg-Cu alloy surface coated by Nd: YAG laser surface alloying (LSA) with Ni base and Co base powder have been investigated. The experimental results showed that the Al3Ni, Al3Ni2 dendrites existed in the Ni base alloy layer, and the Al9Co2, Al13Co4, Al11Cr2 dendrites existed in the Co base alloy layer respectively. The microstructure was decided by the cooling rate and distribution of alloy elements in LSA layers. The microhardness was raised to HV 900 for Ni base and Co base specimens, which is nine times to the matrix. The OCP was -0.9 V for Ni base and Co base alloyed layers in 3.5 wt% NaCl solution. The results of potentiodynamic polarization test showed that corrosion potential and protection potential were increased for Ni base and Co base alloyed layers, but the pitting potential was similar with 7075 alloy. The SEM images after PD test showed the aluminum in the Ni base and Co base alloyed layers flow away. The Al-Ni, Al-Co and Al-Cr IMCs were remained on the surface after PD test. Ni base specimen showed the minimum weight loss of 0.08 g at velocity for 900 mm/min and power for 1400 W. And, Co base specimen showed the minimum weight loss of 0.06 g at velocity for 600 mm/min and power for 1400 W.

In the present investigation, surface coatings employing laser surface alloying (LSA) and plasma electrolytic oxidation (PEO) processes have been prepared on Mg alloys. The coatings have been investigated for corrosion and wear behaviour. Two important Mg alloys based on Mg–Al system were selected namely, MRI 230D and AM50 as substrates. LSA coatings have been prepared employing Al and Al2O3 as precursors using different laser scan speeds. PEO coatings were prepared in standard silicate and phosphate based electrolytes employing unipolar, pulsed DC. Hybrid coatings using a combination of the two processes were also produced and investigated for corrosion and wear behaviour. Hybrid coatings of LSA followed by PEO (LSA+PEO) were investigated for effectiveness of sealing the cracks in the LSA coatings by subsequent PEO process and consequent improvement in the corrosion resistance. Hybrid coatings of PEO followed by LSA (PEO+LSA) were prepared with an objective of sealing the pores in the PEO coating LSA treatment. In an attempt to produce more compact PEO coatings, electrolyte containing montmorillonite clay additives was employed for the PEO process of AM50 Mg alloy. The coatings were produced employing different current densities and the effect of current density on the microstructure and corrosion behaviour of coating was investigated. Electrochemical corrosion tests of uncoated and coated alloys were carried out in 3.5 wt.% (0.6M)NaCl, neutral pH, solution with an exposed area of 0.5 cm2 for a time duration of 18.5 h. For the PEO coatings with clay additives, corrosion tests were conducted additionally in 0.5 wt.% (0.08 M) NaCl, neutral pH, solution for a time duration of 226.1 h. Wear behaviour of LSA coatings was analyzed by employing a pin on disc tribo–tester conforming to ASTM G–99 standard at ambient conditions with ground EN32 steel disc of hardness Rc 58 as the counterface. Tests were conducted under dry sliding conditions for a sliding distance of 1.0 km at a sliding velocity of 0.837 m/s employing normal loads of 10, 20, 30 and 40 N. Friction and wear behavior of PEO and PEO+LSA coatings were analyzed at ambient conditions by employing a ball−on−flat linearly oscillating tribometer conforming to ASTM G–133 standard. AISI 52100 steel ball of diameter 6 mm was employed as the friction partner. Wear tests were conducted under dry sliding conditions for a total sliding distance of 100 m at normal loads of 2 N and 5 N with oscillating amplitude of 10 mm and mean sliding speed of 5 mm/s. LSA coatings could not improve the corrosion resistance of MRI 230D Mg alloy. This was attributed to the presence of cracks in the LSA coating, which resulted in the accelerated galvanic corrosion of the substrate. LSA coatings improved the wear resistance at all loads. The improved wear resistance was attributed to β (Mg17Al12) phase and Al2O3 particles in the coating which increased the hardness of the LSA layer. No trend in corrosion and wear resistance with laser scan speed was observed for LSA coatings. PEO coatings improved the corrosion resistance of the MRI 230D Mg alloy significantly. The improved corrosion resistance was attributed to the enhanced barrier protection provided by dense barrier layer formed at the substrate/coating interface and to the insoluble phase constituents in the coatings. PEO coating was effective in improving the wear resistance at low loads/contact pressures. At higher loads, the coating underwent micro–fracture as a result of the porosity in the coatings. Hybrid coatings of LSA followed by PEO (LSA+PEO) in silicate based electrolyte improved the corrosion resistance of LSA coatings. However, the corrosion resistance was not improved to the extent of PEO coatings on as–cast alloy as a result of cracks in the primary coatings, which were not fully sealed by the plasma conversion products. No trend in corrosion resistance with laser scan speed was observed for LSA+PEOcoatings. In hybrid coatings of PEO followed by LSA (PEO+LSA), primary PEO coating was completely melted and mixed with applied precursor to form a single composite LSA layer. The corrosion resistance of the hybrid coatings was observed to be lower than that of the as–cast alloy. The presence of solidification cracks reduced the barrier properties and resulted in the accelerated galvanic corrosion of the substrate similar to LSA coatings. Hybrid (PEO+LSA) coatings exhibited improved wear resistance as compared to as–cast alloy at lower loads as a result of increase in the hardness due to β (Mg17Al12) phase and oxide/ceramic particles in the hybrid layer. At higher loads, hybrid coatings exhibited higher wear rate as compared to as–cast alloy and PEO coatings. This was attributed to three–body abrasive wear as a result of dislodged hard oxide/ceramic particles in the wear tracks. No trend in corrosion and wear resistance with laser scan speed was observed for PEO+LSA coatings. PEO coatings on AM50 Mg alloy by employing clay additives in the electrolyte resulted in the reactive uptake of clay particles producing a predominantly amorphous coating at low current density. Clay additives were effective in improving the compactness of the coating at lower current density. At higher current densities, the porosity of the coatings increased. The clay particles got re–constituted producing increasing amount of crystalline phases with increase in current density. Long term impedance measurements showed that clay addition as well as increased current density employed for the PEO process was not effective in improving the corrosion resistance of the coatings. At low current density, even though the coating with clay additives was more compact, it was deficient in MgO and consisted predominantly of an amorphous phase, which underwent fast dissolution in electrolyte thereby resulting in an early loss of barrier properties. At higher current densities, even though the coatings consisted of increased amount of MgO and crystalline phases, which resist dissolution in the electrolyte, the increased porosity and defective barrier layer resulted in easy permeation of the electrolyte into the substrate/coating interface, which resulted in much earlier loss of barrier properties and inferior corrosion resistance.

In the present investigation, surface coatings employing laser surface alloying (LSA) and plasma electrolytic oxidation (PEO) processes have been prepared on Mg alloys. The coatings have been investigated for corrosion and wear behaviour. Two important Mg alloys based on Mg–Al system were selected namely, MRI 230D and AM50 as substrates. LSA coatings have been prepared employing Al and Al2O3 as precursors using different laser scan speeds. PEO coatings were prepared in standard silicate and phosphate based electrolytes employing unipolar, pulsed DC. Hybrid coatings using a combination of the two processes were also produced and investigated for corrosion and wear behaviour. Hybrid coatings of LSA followed by PEO (LSA+PEO) were investigated for effectiveness of sealing the cracks in the LSA coatings by subsequent PEO process and consequent improvement in the corrosion resistance. Hybrid coatings of PEO followed by LSA (PEO+LSA) were prepared with an objective of sealing the pores in the PEO coating LSA treatment. In an attempt to produce more compact PEO coatings, electrolyte containing montmorillonite clay additives was employed for the PEO process of AM50 Mg alloy. The coatings were produced employing different current densities and the effect of current density on the microstructure and corrosion behaviour of coating was investigated. Electrochemical corrosion tests of uncoated and coated alloys were carried out in 3.5 wt.% (0.6M)NaCl, neutral pH, solution with an exposed area of 0.5 cm2 for a time duration of 18.5 h. For the PEO coatings with clay additives, corrosion tests were conducted additionally in 0.5 wt.% (0.08 M) NaCl, neutral pH, solution for a time duration of 226.1 h. Wear behaviour of LSA coatings was analyzed by employing a pin on disc tribo–tester conforming to ASTM G–99 standard at ambient conditions with ground EN32 steel disc of hardness Rc 58 as the counterface. Tests were conducted under dry sliding conditions for a sliding distance of 1.0 km at a sliding velocity of 0.837 m/s employing normal loads of 10, 20, 30 and 40 N. Friction and wear behavior of PEO and PEO+LSA coatings were analyzed at ambient conditions by employing a ball−on−flat linearly oscillating tribometer conforming to ASTM G–133 standard. AISI 52100 steel ball of diameter 6 mm was employed as the friction partner. Wear tests were conducted under dry sliding conditions for a total sliding distance of 100 m at normal loads of 2 N and 5 N with oscillating amplitude of 10 mm and mean sliding speed of 5 mm/s. LSA coatings could not improve the corrosion resistance of MRI 230D Mg alloy. This was attributed to the presence of cracks in the LSA coating, which resulted in the accelerated galvanic corrosion of the substrate. LSA coatings improved the wear resistance at all loads. The improved wear resistance was attributed to β (Mg17Al12) phase and Al2O3 particles in the coating which increased the hardness of the LSA layer. No trend in corrosion and wear resistance with laser scan speed was observed for LSA coatings. PEO coatings improved the corrosion resistance of the MRI 230D Mg alloy significantly. The improved corrosion resistance was attributed to the enhanced barrier protection provided by dense barrier layer formed at the substrate/coating interface and to the insoluble phase constituents in the coatings. PEO coating was effective in improving the wear resistance at low loads/contact pressures. At higher loads, the coating underwent micro–fracture as a result of the porosity in the coatings. Hybrid coatings of LSA followed by PEO (LSA+PEO) in silicate based electrolyte improved the corrosion resistance of LSA coatings. However, the corrosion resistance was not improved to the extent of PEO coatings on as–cast alloy as a result of cracks in the primary coatings, which were not fully sealed by the plasma conversion products. No trend in corrosion resistance with laser scan speed was observed for LSA+PEOcoatings. In hybrid coatings of PEO followed by LSA (PEO+LSA), primary PEO coating was completely melted and mixed with applied precursor to form a single composite LSA layer. The corrosion resistance of the hybrid coatings was observed to be lower than that of the as–cast alloy. The presence of solidification cracks reduced the barrier properties and resulted in the accelerated galvanic corrosion of the substrate similar to LSA coatings. Hybrid (PEO+LSA) coatings exhibited improved wear resistance as compared to as–cast alloy at lower loads as a result of increase in the hardness due to β (Mg17Al12) phase and oxide/ceramic particles in the hybrid layer. At higher loads, hybrid coatings exhibited higher wear rate as compared to as–cast alloy and PEO coatings. This was attributed to three–body abrasive wear as a result of dislodged hard oxide/ceramic particles in the wear tracks. No trend in corrosion and wear resistance with laser scan speed was observed for PEO+LSA coatings. PEO coatings on AM50 Mg alloy by employing clay additives in the electrolyte resulted in the reactive uptake of clay particles producing a predominantly amorphous coating at low current density. Clay additives were effective in improving the compactness of the coating at lower current density. At higher current densities, the porosity of the coatings increased. The clay particles got re–constituted producing increasing amount of crystalline phases with increase in current density. Long term impedance measurements showed that clay addition as well as increased current density employed for the PEO process was not effective in improving the corrosion resistance of the coatings. At low current density, even though the coating with clay additives was more compact, it was deficient in MgO and consisted predominantly of an amorphous phase, which underwent fast dissolution in electrolyte thereby resulting in an early loss of barrier properties. At higher current densities, even though the coatings consisted of increased amount of MgO and crystalline phases, which resist dissolution in the electrolyte, the increased porosity and defective barrier layer resulted in easy permeation of the electrolyte into the substrate/coating interface, which resulted in much earlier loss of barrier properties and inferior corrosion resistance.