Desgaste é um dos três principais problemas industriais comumente encontrados, que levam à substituição de componentes e equipamentos em engenharia. A possibilidade de se recuperar esses componentes, ou prepará-Ios para suportar melhor as condições de trabalho, é constantemente estudada e novas técnicas são propostas, sempre no sentido de aumentar a vida útil e reduzir custos operacionais. Esta é a razão para o aumento da importância dos sistemas de revestimentos. O cromo duro eletrolftico tem sido muito utilizado para conferir resistência ao desgaste elou à corrosão, porém as características do processo de deposição eletrolítica do mesmo, com a presença do cromo hexavalente, altamente cancerígeno, têm resultado numa limitação crescente ao seu uso. Devido a isso se tomou necessário o desenvolvimento de novos revestimentos alternativos menos agressivos. Os substitutos mais promissores são os \"cermets\" depositados pelo processo HVOF, que foram produzidos no presente trabalho e comparados com o cromo duro eletrolítico quanto à resistência ao desgaste. Os revestimentos obtidos foram caracterizados por meio de ensaios micrográficos óticos, eletrônicos, raios-x, densidade e ensaios de desgaste dos seguintes tipos: roda de borracha e areia a seco, ensaio abrasivo para carbonetos e micro-abrasivo. O revestimento Cr3 Cr2-NiCr apresentou desempenho geral inferior ao do cromo duro. Entretanto no caso do desgaste micro-abrasivo, onde o mecanismo de desgaste apresentado no mesmo foi misto adesivo-abrasivo, ao contrário dos demais que tiveram características abrasivas, sua resistência ao desgaste foi superior a dos demais. O revestimento constituído de WC-CoCr apresentou o melhor desempenho entre todos os avaliados, o que o indica como um candidato altamente promissor à substituição do cromo / Wear is one of the three main industrial problems, that causes the substitution of components and equipments in engineering. The possibility to recover those components or to prepare them to better support the work conditions is constantly studied and new techniques are proposed, a/ways seeking to increase the life span and to reduce operational eosts. This is the reason why the coating systems are important. The hard ehrome coatings has been traditionally used to improve wear resistance and/or eorrosion resistance. However the eharacteristics of the electroplated deposition proeess, with the presence of the hexavalente ehrome, highly cancerous, result in a growing limitation to its use. Due to that it beeome necessary the development of new less aggressive alternative eoatings. The most promising substitutes are the \"cermets\" deposited by the HVOF process that were produced in the present work and compared with electroplated hard ehrome coatings for wear resistance. The obtained coatings were characterized through optical and electronic mierographics analysis , x-ray, density and submitted to the following types of wear tests: Dry Sand I rubber wheel, Wear test for cemented carbides and mieroabrasive wear test. The coating Cr3Cr2-NiCr presented smaller general performance in comparison with the hard ehrome. However, in the case of the miero-abrasive wear, where the wear mechanism presented was mixed adhesive-abrasive, unlike the others that had only abrasive eharacteristics, the resistance to the wear was superior. The WC-CoCr coating presented the best performance overall, what indicates it as a candidate to the hard ehrome substitution

Devido à necessidade de obtenção de uma superfície resistente ao desgaste e a oxidação, os cilindros hidráulicos são tradicionalmente revestidos com cromo duro eletrodepositado. Todavia, este tipo de revestimento apresenta uma redução gradual do coeficiente de atrito, interferindo diretamente na lubrificação da haste, ocasionando danos aos elementos de vedação e promovendo consequentes vazamentos. Outro apelo contrário à utilização do processo de revestimento com cromo é a presença, em elevados níveis, de cromo hexavalente Cr+6, apresentando alto poder cancerígeno e de contaminação ambiental. Atualmente, o processo de aspersão térmica em alta velocidade (HVOF - High Velocity Oxi-Fuel) se caracteriza como uma técnica de revestimento alternativa ao cromo duro, utilizando materiais compósitos (metal-cerâmico), os quais propiciam baixas taxas de desgaste. Este trabalho tem como objetivo avaliar o desempenho de revestimentos aplicados em hastes de cilindros hidráulicos revestidas com WC-CoCr pelo processo HVOF, em comparação ao revestimento de cromo duro eletrodepositado. As técnicas de caracterização empregadas foram divididas em duas etapas, sendo que a primeira foi: análise microestrutural, ensaios de dureza e desgaste, medições de rugosidade e perfilometria, em corpos de prova planos e, na segunda etapa, a análise dos revestimentos adquiridos através de uma bancada de testes de hastes de cilindros hidráulicos, onde a caracterização foi obtida através de, medições específicas dos principais parâmetros de rugosidades aplicadas a superfícies de vedação, como Rz, Rmáx e Rmr, desgaste visuais das hastes e vedações e medições de vazamento de óleo. Ao final, os resultados mostram que o revestimento aspergido, indica características competitivas em relação à eletrodeposição de cromo, podendo ser aplicado em hastes de cilindros hidráulicos com algumas vantagens, como por exemplo, maior resistência ao desgaste, menor coeficiente de atrito, menores taxas de vazamento e melhores condições finais de superfícies. / To obtain a surface that is resistant to wear and oxidation, hydraulic cylinders are typically coated with in hard chromium through the process of electroplating process. However, this type of coating displays a gradual reduction of the friction coefficient as the lubrication directly interferes with the shaft mechanism, which in turn causes damage to the sealing elements and promotes subsequent leakage. Another disadvantage in using the electroplated hard chromium process is the presence of high level hexavalent chromium Cr+6 which is not only carcinogenic, but also extremely contaminating to the environment. Currently, the alternative process of high-speed thermal spraying (HVOF - High Velocity Oxy-Fuel), as opposed to hard chrome coating, uses composite materials (metalceramic) possessing low wear rates. This study aims to evaluate and compare the performance of hydraulic cylinder rods coated with WC-CoCr in the HVOF process, and those coated with electroplated hard chromium. The characterization techniques employed were divided into two steps, the first of which was: microstructural analysis, testing the hardness, wear, roughness and profilometry and on specimens plans and, in the second step, analysis of the coating obtained through a bench test rods of hydraulic cylinders, where the characterization was through achieved , specific measurement of the main parameters of roughness applied to sealing surfaces, as Rz, Rmáx and Rmr, wear visual of the rods and seals and measurements of oil leakage. At the end, the results show that the sprayed coating, indicate competitive characteristics in relation to the electrodeposition of chromium, which can be applied in rods of hydraulic cylinders with some advantages, such as improved wear resistance, coefficient friction low, lower rates of leakage and condition final better surface.

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

Metal foams and wire meshes are open cell structures with low weight and density, high permeability and high thermal conductivity which make them attractive for a wide range of industrial applications involving fluid flow and heat transfer. In this study, the effect of natural convection, radiation and heat transfer enhancement of metal foams and wire meshes of 10 and 40 PPI (pores per inch) heat exchangers were examined and compared for different heat exchanger orientation, coolant flow rate and atmosphere temperature. Thermal spray coating processes were also used in development of a new class of high temperature stainless steel heat exchangers. Stainless steel wire mesh heat exchangers were prototyped by connecting the tube to the wire mesh using wire arc thermal spray coating. Thermal spray coating provided efficient connections between the wire mesh and the tubes’ outer surface, and has potential to replace expensive brazing or other metal connection techniques.

Two new mechanisms of droplet splashing and breakup during impact have been identified and analyzed. One is the internal rupture of spreading droplet film through formation of holes, and the other is the splashing of droplet due to its freezing during spreading. The mechanism of film rupture was investigated by two different methods. In the first method, circular water films were produced by directing a 1 mm diameter water jet onto a flat, horizontal plate for 10 ms. In the second method, films were produced by making 0.6 mm water droplets impact a solid surface mounted on the rim of a rotating flywheel. Substrate wettability was varied over a wide range, including superhydrophobic. In both cases, the tendency to film rupture first increased and then decreased with contact angle. A thermodynamic stability analysis predicted this behavior by showing that films would be stable at very small or very large contact angle, but unstable in between. Film rupture was also found to be promoted by increasing surface roughness or decreasing film thickness. To study the effect of solidification, the impact of molten tin droplets (0.6 mm diameter) on solid surfaces was observed for a range of impact velocities (10 to 30 m/s), substrate temperatures (25 to 200°C) and substrate materials (stainless steel, aluminum and glass) using the rotating flywheel apparatus. Droplets splashed extensively on a cold surface but on a hot surface there was no splashing. Splashing could be completely suppressed by either increasing the substrate temperature or reducing its thermal diffusivity. An analytical model was developed to predict this splashing behavior. The above two theories of freezing-induced splashing and film rupture were combined to predict the morphology of splats typically observed in a thermal spray process. A dimensionless solidification parameter, which takes into account factors such as the droplet diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, was developed. Predictions from the model were compared with a wide range of experimental data and found to agree well.

Metal foams and wire meshes are open cell structures with low weight and density, high permeability and high thermal conductivity which make them attractive for a wide range of industrial applications involving fluid flow and heat transfer. In this study, the effect of natural convection, radiation and heat transfer enhancement of metal foams and wire meshes of 10 and 40 PPI (pores per inch) heat exchangers were examined and compared for different heat exchanger orientation, coolant flow rate and atmosphere temperature. Thermal spray coating processes were also used in development of a new class of high temperature stainless steel heat exchangers. Stainless steel wire mesh heat exchangers were prototyped by connecting the tube to the wire mesh using wire arc thermal spray coating. Thermal spray coating provided efficient connections between the wire mesh and the tubes’ outer surface, and has potential to replace expensive brazing or other metal connection techniques.

Thermal spray is a well established technology that is commonly used in the aerospace and automotive industries. This thesis reports on the effect that substrate surface chemistry, morphology and temperature has on the morphology of PEEK single splats on aluminium substrates. PEEK single splats were deposited by HVAF and plasma spraying on aluminium substrates with 6 different pretreatments. Substrates were either sprayed at room temperature, or 323°C, and a subset of substrates was held at incremental temperatures up to 363°C. HVAF deposited splats on room temperature substrates showed sensitivity to surface chemistry, with increased circularity and area associated with low levels of hydroxide and chemisorbed water on the aluminium surface. Substrates held at 323°C were more sensitive to substrate morphology, where rough surfaces resulted in decreased circularity and area apparently independent of surface chemistry. Substrate temperature trials revealed a significant step in the results, equating to greater circularity, and lower splat area, perimeter and Feret diameter. This step occurred between 123°C and 163°C, the two points bracketing the glass transition temperature of PEEK (143°C). This result was due to the relaxation of splats deposited on surfaces above 143°C, whilst splats on cooler substrates quench through the glass transition and do not relax. PEEK splats deposited by plasma spray on room temperature and 323°C substrates showed sensitivity to the amount of hydroxide and chemisorbed water present on the aluminium substrates, with low levels resulting in more circular and larger area splats. Plasma splats did not show the same temperature effects as HVAF splats, thought to be due to the more molten state of plasma splats upon impact compared to the HVAF splats. The primary conclusions reached were that plasma sprayed polymers were sensitive to surface chemistry, and that as such the surface chemistry of a substrate should be considered when forming plasma spray polymer coatings. It was also concluded that the kinetic energy of particles in HVAF thermal spray contributed significantly to the thermal energy of a particle on impact, allowing for improved splat properties without overheating the particles in flight. Finally it was concluded that substrate temperature is far more important for HVAF thermal spray of polymers than plasma spray of polymers, but that it improves splat properties for both techniques.

Thermal spray is a well established technology that is commonly used in the aerospace and automotive industries. This thesis reports on the effect that substrate surface chemistry, morphology and temperature has on the morphology of PEEK single splats on aluminium substrates. PEEK single splats were deposited by HVAF and plasma spraying on aluminium substrates with 6 different pretreatments. Substrates were either sprayed at room temperature, or 323°C, and a subset of substrates was held at incremental temperatures up to 363°C. HVAF deposited splats on room temperature substrates showed sensitivity to surface chemistry, with increased circularity and area associated with low levels of hydroxide and chemisorbed water on the aluminium surface. Substrates held at 323°C were more sensitive to substrate morphology, where rough surfaces resulted in decreased circularity and area apparently independent of surface chemistry. Substrate temperature trials revealed a significant step in the results, equating to greater circularity, and lower splat area, perimeter and Feret diameter. This step occurred between 123°C and 163°C, the two points bracketing the glass transition temperature of PEEK (143°C). This result was due to the relaxation of splats deposited on surfaces above 143°C, whilst splats on cooler substrates quench through the glass transition and do not relax. PEEK splats deposited by plasma spray on room temperature and 323°C substrates showed sensitivity to the amount of hydroxide and chemisorbed water present on the aluminium substrates, with low levels resulting in more circular and larger area splats. Plasma splats did not show the same temperature effects as HVAF splats, thought to be due to the more molten state of plasma splats upon impact compared to the HVAF splats. The primary conclusions reached were that plasma sprayed polymers were sensitive to surface chemistry, and that as such the surface chemistry of a substrate should be considered when forming plasma spray polymer coatings. It was also concluded that the kinetic energy of particles in HVAF thermal spray contributed significantly to the thermal energy of a particle on impact, allowing for improved splat properties without overheating the particles in flight. Finally it was concluded that substrate temperature is far more important for HVAF thermal spray of polymers than plasma spray of polymers, but that it improves splat properties for both techniques.

Thermal spray is a well established technology that is commonly used in the aerospace and automotive industries. This thesis reports on the effect that substrate surface chemistry, morphology and temperature has on the morphology of PEEK single splats on aluminium substrates. PEEK single splats were deposited by HVAF and plasma spraying on aluminium substrates with 6 different pretreatments. Substrates were either sprayed at room temperature, or 323°C, and a subset of substrates was held at incremental temperatures up to 363°C. HVAF deposited splats on room temperature substrates showed sensitivity to surface chemistry, with increased circularity and area associated with low levels of hydroxide and chemisorbed water on the aluminium surface. Substrates held at 323°C were more sensitive to substrate morphology, where rough surfaces resulted in decreased circularity and area apparently independent of surface chemistry. Substrate temperature trials revealed a significant step in the results, equating to greater circularity, and lower splat area, perimeter and Feret diameter. This step occurred between 123°C and 163°C, the two points bracketing the glass transition temperature of PEEK (143°C). This result was due to the relaxation of splats deposited on surfaces above 143°C, whilst splats on cooler substrates quench through the glass transition and do not relax. PEEK splats deposited by plasma spray on room temperature and 323°C substrates showed sensitivity to the amount of hydroxide and chemisorbed water present on the aluminium substrates, with low levels resulting in more circular and larger area splats. Plasma splats did not show the same temperature effects as HVAF splats, thought to be due to the more molten state of plasma splats upon impact compared to the HVAF splats. The primary conclusions reached were that plasma sprayed polymers were sensitive to surface chemistry, and that as such the surface chemistry of a substrate should be considered when forming plasma spray polymer coatings. It was also concluded that the kinetic energy of particles in HVAF thermal spray contributed significantly to the thermal energy of a particle on impact, allowing for improved splat properties without overheating the particles in flight. Finally it was concluded that substrate temperature is far more important for HVAF thermal spray of polymers than plasma spray of polymers, but that it improves splat properties for both techniques.

Thermal spray is a well established technology that is commonly used in the aerospace and automotive industries. This thesis reports on the effect that substrate surface chemistry, morphology and temperature has on the morphology of PEEK single splats on aluminium substrates. PEEK single splats were deposited by HVAF and plasma spraying on aluminium substrates with 6 different pretreatments. Substrates were either sprayed at room temperature, or 323°C, and a subset of substrates was held at incremental temperatures up to 363°C. HVAF deposited splats on room temperature substrates showed sensitivity to surface chemistry, with increased circularity and area associated with low levels of hydroxide and chemisorbed water on the aluminium surface. Substrates held at 323°C were more sensitive to substrate morphology, where rough surfaces resulted in decreased circularity and area apparently independent of surface chemistry. Substrate temperature trials revealed a significant step in the results, equating to greater circularity, and lower splat area, perimeter and Feret diameter. This step occurred between 123°C and 163°C, the two points bracketing the glass transition temperature of PEEK (143°C). This result was due to the relaxation of splats deposited on surfaces above 143°C, whilst splats on cooler substrates quench through the glass transition and do not relax. PEEK splats deposited by plasma spray on room temperature and 323°C substrates showed sensitivity to the amount of hydroxide and chemisorbed water present on the aluminium substrates, with low levels resulting in more circular and larger area splats. Plasma splats did not show the same temperature effects as HVAF splats, thought to be due to the more molten state of plasma splats upon impact compared to the HVAF splats. The primary conclusions reached were that plasma sprayed polymers were sensitive to surface chemistry, and that as such the surface chemistry of a substrate should be considered when forming plasma spray polymer coatings. It was also concluded that the kinetic energy of particles in HVAF thermal spray contributed significantly to the thermal energy of a particle on impact, allowing for improved splat properties without overheating the particles in flight. Finally it was concluded that substrate temperature is far more important for HVAF thermal spray of polymers than plasma spray of polymers, but that it improves splat properties for both techniques.