Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Lately, we have witnessed a growing concern about determining energy consumption and pollutant emissions. Large part of the energy spent is directly related to the friction and wear. Likewise, there a growing demand for production of mechanical systems of high efficiency. To develop systems that are more economical, noiseless and not harmful to the environment, there is a limit imposed by materials and conventional surface treatment. Thus, critical tribological contacts and possible new materials have been intensively investigated. Solid lubrication and solid lubricants are emerging as a promising alternative to control friction and wear in mechanical systems modern. The use of multifunctional coatings have been studied and used in modern mechanical systems, so that the final properties depend on the system composed of substrate - coating - environment. This study aims to optimize the thickness of multifunctional coatings with potential for use in mechanical systems of high efficiency, particularly on soft substrates. The aim is an optimization using the principles of contact mechanics, together with strength criteria to analyse stress and strain. Efficient techniques to characterize the coatings, and powerful simulation tools in the stress analysis of multifunctional coatings were used. The results showed that the thickness of the coating plays an important role on coating properties. / Ultimamente, tem-se presenciado uma preocupação cada vez mais determinante quanto ao consumo energético e emissões de poluentes. Grande parte da energia gasta está relacionada diretamente com o atrito e o desgaste. Há igualmente uma demanda crescente por produção de sistemas mecânicos de alta eficiência, mas, também, para desenvolver sistemas cada vez mais econômicos, silenciosos e não prejudiciais ao meio ambiente, havendo um limite imposto por materiais e tratamento de superfícies convencionais. Assim, a tribologia de contatos críticos e possíveis novos materiais estão sendo amplamente pesquisados. A lubrificação sólida e lubrificantes sólidos estão aparecendo como uma alternativa promissora para controlar o atrito e o desgaste nos sistemas mecânicos modernos. A utilização de revestimentos multifuncionais tem sido estudada e aplicada nos sistemas mecânicos modernos, de modo que as propriedades finais resultantes sejam dependentes do sistema substrato-revestimento-ambiente. O presente trabalho busca a otimização de tribo revestimentos multifuncionais com potencial para a aplicação em sistemas mecânicos de alta eficiência, particularmente sobre substratos macios. Busca-se essa otimização por meio da análise de tensões e deformações, as quais são compreendidas através do princípio da mecânica do contato, juntamente com os critérios de resistência. Usam-se técnicas de caracterização de revestimentos amplamente usadas e eficientes, como também ferramentas de simulação poderosas na análise de tensões de revestimentos multifuncionais. Os resultados demonstram que a espessura do revestimento tem um papel importante na origem das propriedades do revestimento. / Doutor em Engenharia Mecânica

The titanium alloy, Ti-6Al-4V, is vastly studied and used in many applications because it has a transformation microstructure, which can be tailored for apt properties that are consistent up to 500°C. Compared to conventional steels, this alloy favours certain applications due to its high specific strength, hardenability, corrosion resistance, biocompatibility and weldability. Its weldability makes the alloy a good candidate for additive manufacturing (AM). Ti-6Al-4V parts are widely built by the AM process of electron beam melting (EBM). However, heat transfer remains crucial in EBM process. The high intensity localized, moving, electron beam heat source and the rapid self-cooling are critical, especially in thin parts/ sections. When thin sections are built by the EBM process, there will be microstructural variation in their build direction, which can lead to the variation of their mechanical properties. It is necessary to understand the microstructure and mechanical properties of thin sections when they are used as functional parts in various applications in aerospace, automotive, medical, etc. industries. The microstructure, tribological behaviour and mechanical properties of Ti-6Al-4V, as-built EBM thin plates were studied by means of various hardness, scratch and tensile testing. The hardness and scratch tests were performed on the thin plates to correlate the microstructural variation. In-situ micro tensile test was performed inside the scanning electron microscope (SEM), to see the sample’s deformation behaviour. Microstructural characterization revealed equiaxed grains in the transverse section and the longitudinal surface exhibited columnar grains elongated along the build direction. The size of the equiaxed grains are found to vary across the thickness of the plate. The indentation and scratch hardness also vary in correlation with the varying grain size across the plate’s thickness. The micro tensile results reveal that the tensile properties of the thin plate are comparable to that of its bulk Ti-6Al-4V counterpart.

Tungsten carbide (WC) based metallic matrix coatings sprayed using high velocity air fuel (HVAF) technique have attracted increasing attention as they show excellent tribological performance in various wear conditions. In the HVAF method, particles’ in-flight temperature and velocity can be influenced by the process variables including nozzle configuration as well as feedstock particle size range. On the other hand, the chemistry of metallic binder can also play a key role in determining both properties and performance of these coatings. In this thesis, characteristics and tribological behaviour of HVAF-sprayed WC-based coatings were investigated employing four different nozzle configurations (4L2, 4L4, 5L2and 5L4), and three different feedstock particle sizes of WC-CoCr feedstock powder (5/20, 5/30 and 15/45 μm). Also, characteristics and performance of coatings processed with four different WC-based feedstocks comprising alternative binders to traditionally used CoCr (namely CoCr, NiMoCrFeCo,FeNiCrMoCu and FeCrAl) were investigated. Characteristics of the coatings were explored by conducting scanning electron microscopy observations, microindentation testing and X-ray diffraction analysis. Performance of the coatings was evaluated by conducting sliding wear, dry jet erosion wear and sand rubber wheel abrasion wear testing. Moreover, material removal mechanisms in the coatings subjected to the above tests were investigated through post wear analysis. X-ray diffraction analysis showed that no considerable phase change compared to the starting feedstock was observed in any of the coatings sprayed by HVAF technique. It was revealed that decrease in powder particle size range can result in an improvement in microstructural characteristics, such as homogeneity and density, as well as hardness of the coatings. Besides, it can lead to a substantial improvement in wear performance of the coatings. It was shown that using various nozzle configurations does not result in any considerable change in characteristics or performance of the HVAF-sprayed WC-CoCr coatings. It was further shown that, by decreasing particle sizes from coarse to medium or fine, a significant difference can be observed. While wear mechanisms for medium and fine feedstock coatings were dominated by ploughing and fracture of individual carbide grains, for the coarse feedstock coatings ploughing and grooving along with the local removal of coating material were observed. Hardness values for all the four coatings with different binder chemistries were in a narrow range of 1100 – 1300 HV0.3. WC-NiMoCrFeCo and WC-FeNiCrMoCu coatings showed better or comparable sliding wear performance compared to WC-CoCr coating (as reference). WC-FeNiCrMoCu and WC-FeCrAl coatings showed comparable performance under erosion conditions while all the three binder alternatives yielded slightly inferior coating performance under abrasion wear, compared to the reference coating. While ploughing was the common wear mechanism in all the four coatings, pitting was noted in coatings with CoCr and FeNiCrMoCu binders in case of sliding wear. / Populärvetenskaplig Sammanfattning Cermet-beläggningar består av WC-partiklar inbäddade i ett metallbindemedel,sprutat med High Velocity Air Fuel (HVAF) uppvisar utmärkt tribologiskprestanda under olika slitage förhållanden. Med HVAF-tekniken kan partiklarnasflygtemperatur och hastighet påverkas av konfigurationen av den utrustning somanvänds såväl som partikelstorleksintervallet för råmaterialet. Därtill kan kemin hos det metalliska bindemedlet spela en nyckelroll för beläggningarnas egenskaper och prestanda. I denna avhandling undersöktes egenskaper och tribologiskt beteende hos HVAF-besprutade WC-baserade beläggningar. Undersökningen gjordes genom att använda fyra olika uppsättningar av ändra sprutparametrar förHVAF-processen där fyra olika munstyckskonfigurationer (4L2, 4L4, 5L2 och5L4) och tre olika partikelstorleksintervaller av WC-CoCr råvarupulver (5/20,5/30 och 15/45 μm). Studieegenskaper och prestanda för alla deponerade beläggningar, påverkan av processvariabler (olika munstyckskonfigurationer och olika partikelstorlekar) undersöktes. Dessutom undersöktes egenskaper och prestanda för tre olika WC-baserade råvaror med alternativa bindemedel till CoCr (NiMoCrFeCo, FeNiCrMoCu och FeCrAl) och jämfördes med WC-CoCrbeläggningsom referens. Beläggningens egenskaper undersöktes genom att genomföra SEM-analys, mikroindragningstest och röntgenanalys. Beläggningens prestanda utvärderades genom att utföra glidförslitning, erosionsslitage under torra förhållanden och test med torr sand/gummihjulanordning. Vidare undersöktes mekanismer för materialavlägsning i beläggningarna med ovanstående tester genom analys efter slitage. Ingen avsevärd fasförändring observerades för alla beläggningar som besprutades med HVAF-teknik. Det avslöjades att minskning av pulvrets genomsnittliga partikelstorlek resulterade i en förbättring av mikrostrukturella egenskaper, såsom homogenitet och densitet, samt beläggningarnas hårdhetsvärde. Dessutom leder det till en avsevärd förbättring av beläggningens slitageförmåga. Det visades att användning av olika munstycken till HVAF-processen inte resulterar i en avsevärd förändring i egenskaper eller prestanda hos WC-CoCr-beläggningarna. När det gäller förslitningsmekanismer visades det att genom att minska partikelstorleken från grov till medium eller fin sågs en avsevärd skillnad. För beläggningar besprutade med fina och medelstora partiklar dominerades förslitningsmekanismer av plöjningsslitage och sprickor av enskilda hårdmetallkorn. För grova beläggningar observerades plöjningsslitage och spårning tillsammans med avlägsnande av material. Ingen signifikant skillnad i mikrostruktur eller fasförändring observerades i alla beläggningar med alternativa bindemedel såväl som WC-CoCr som referensbeläggning. Hårdhetsvärdet för alla de fyra beläggningarna låg inom x intervallet 1100 - 1300 HV0.3. NiMoCrFeCo och FeNiCrMoCu visade bättre eller jämförbar glidförmåga med referensbeläggningen. FeNiCrMoCu och FeCrAl visade jämförbara prestanda under erosionsförhållanden och alla de tre undersökta beläggningsmaterialen visade något sämre prestanda under nötningsslitage jämfört med referensbeläggningen. Medan plöjningsslitage var den vanliga förslitningsmekanismen i alla de fyra beläggningarna noterades gropning i beläggningarna CoCr och FeNiCrMoCu vid glidförslitning.