INVESTIGATION OF DIAMOND COATINGS ON IRON BASED MATERIALS BY MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

Diamond thin films on Fe based materials (ferrous alloys) for the purpose of improving their surface properties such as wear and corrosion resistance have been investigated. There are main barriers restricting the quality and adhesion of diamond coatings on Fe based materials. Firstly, the incubation time of diamond nucleation is long due to the high solubility of carbon in Fe. Secondly, graphite soot forms before diamond nucleation due to the catalytic effect of Fe for formation of graphitic carbon. Thirdly, high internal stress remains at the interface which is induced by the large difference in the thermal expansion coefficients of diamond and most of the Fe based materials. Surface modification and interlayers are two important approaches to overcome these problems. In this work, the effect of Cr content in Fe-Cr alloys on diamond nucleation and growth is being studied in order to clarify the mechanisms of Cr in diamond deposition. Furthermore, in order to enhance the adhesion and quality of diamond coatings, Al based interlayers are being investigated on ferrous alloys. Fe-Cr alloys (with 20~80 wt.% Cr) were exposed to a CH4-H2 mixture in a microwave plasma enhanced chemical vapor deposition (MPCVD) reactor. Severe metal dusting and carburization were observed on the alloys with low Cr content and diamond did not nucleate on those alloys until a graphite intermediate layer had been formed, which takes a long incubation time. Increasing Cr concentration in the Fe-Cr alloys promotes the formation of a Cr carbide buffer layer, which inhibits metal dusting and the formation of graphite soot. Consequently, diamond nucleation and growth can be greatly enhanced, and continuous diamond films with enhanced adhesion have been deposited on the Fe-80Cr alloys. Al based interlayers including Al and Al/AlN interlayers were deposited on ferrous alloys (SS316 and Kovar: FeNiCo) to enhance diamond deposition. The deposition was carried out in a microwave plasma chemical vapor deposition (MPCVD) reactor using a CH4-H2 mixture. The obtained samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, synchrotron-based X-ray absorption spectroscopy (XAS) and indentation testing. The results show that a single Al layer can effectively suppress the formation of graphite at the interface and the inward diffusion of carbon into Fe based substrates, and thereby enhances diamond nucleation and growth. The dual layers of Al/AlN can further enhance the adhesion of diamond coatings comparing with the single Al interlayer

Revestimentos multicamada de diamante CVD micro/nanocristalino para biotribologia

Doutoramento em Ciência e Engenharia de MateriaisIn the present work multilayered micro/nanocrystalline (MCD/NCD) diamond coatings were developed by Hot Filament Chemical Vapour Deposition (HFCVD). The aim was to minimize the surface roughness with a top NCD layer, to maximize adhesion onto the Si3N4 ceramic substrates with a starting MCD coating and to improve the mechanical resistance by the presence of MCD/NCD interfaces in these composite coatings. This set of features assures high wear resistance and low friction coefficients which, combined to diamond biocompatibility, set this material as ideal for biotribological applications. The deposition parameters of MCD were optimized using the Taguchi method, and two varieties of NCD were used: NCD-1, grown in a methane rich gas phase, and NCD-2 where a third gas, Argon, was added to the gas mixture. The best combination of surface pre-treatments in the Si3N4 substrates is obtained by polishing the substrates with a 15 μm diamond slurry, further dry etching with CF4 plasma for 10 minutes and final ultrasonic seeding in a diamond powder suspension in ethanol for 1 hour. The interfaces of the multilayered CVD diamond films were characterized with high detail using HRTEM, STEM-EDX and EELS. The results show that at the transition from MCD to NCD a thin precursor graphitic film is formed. On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, as a result of the richer atomic hydrogen content and of the higher substrate temperature for MCD deposition. At those transitions, WC nanoparticles were found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate. In order to study the adhesion and mechanical resistance of the diamond coatings, indentation and particle jet blasting tests were conducted, as well as tribological experiments with homologous pairs. Indentation tests proved the superior behaviour of the multilayered coatings that attained a load of 800 N without delamination, when compared to the mono and bilayered ones. The multilayered diamond coatings also reveal the best solid particle erosion resistance, due to the MCD/NCD interfaces that act as crack deflectors. These results were confirmed by an analytical model on the stress field distribution based on the von Mises criterion. Regarding the tribological testing under dry sliding, multilayered coatings also exhibit the highest critical load values (200N for Multilayers with NCD-2). Low friction coefficient values in the range μ=0.02- 0.09 and wear coefficient values in the order of ~10-7 mm3 N-1 m-1 were obtained for the ball and flat specimens indicating a mild wear regime. Under lubrication with physiological fluids (HBSS e FBS), lower wear coefficient values ~10-9-10-8 mm3 N-1 m-1) were achieved, governed by the initial surface roughness and the effective contact pressure.No presente trabalho desenvolveram-se revestimentos de diamante micro/nanocristalino (MCD/NCD) em multicamadas obtidos por deposição química em fase vapor (CVD) assistida por filamento quente. Pretendeu-se minimizar a rugosidade através de um camada superficial de NCD, maximizar a adesão com um filme inicial de MCD sobre substratos cerâmicos de nitreto de silício (Si3N4) e incrementar a resistência mecânica pela presença de interfaces MCD/NCD nestes revestimentos compósitos. Este conjunto de características garante elevada resistência ao desgaste e baixo coeficiente de atrito, o que somado à biocompatibilidade do diamante, configuram este material como ideal para aplicações em biotribologia. Os parâmetros de deposição do MCD foram otimizados usando o método de Taguchi e utilizaram-se duas variedades de NCD: NCD-1 crescido numa atmosfera com sobressaturação de metano e NCD-2 crescido na presença de árgon. A melhor combinação de pré-tratamentos nos substratos de Si3N4 consiste num polimento com suspensão de diamante (15 μm), seguido de ataque por plasma de CF4 durante 10 minutos e riscagem em suspensão de pó de diamante em etanol durante 1 hora. As interfaces das multicamadas de diamante foram caracterizadas em detalhe por HRTEM, STEM-EDX e EELS. Os resultados mostram que na transição de diamante MCD para NCD ocorre a formação de um filme fino de carbono amorfo, inexistente na transição de NCD para MCD, como resultado da maior percentagem de hidrogénio atómico na mistura de gases e do incremento da temperatura do substrato para a deposição de MCD. Uma característica comum nas interfaces nos dois tipos de NCD é a presença de partículas esféricas de carboneto de tungsténio, devido à contaminação pelos filamentos, estando também presentes na interface entre a camada de MCD e o substrato de nitreto de silício. A adesão e resistência mecânica dos filmes de diamante foram avaliadas por ensaios de indentação, erosão com partículas de carboneto de silício e ensaios tribológicos em movimento recíproco, com pares próprios. Por indentação verificou-se que as multicamadas suportam uma carga de 800N, sem delaminação, valor superior ao atingido pelas mono- e bicamadas. Nos ensaios de erosão, as multicamadas apresentaram igualmente melhor comportamento, devido à ação das interfaces MCD/NCD como defletoras das fissuras, sendo estes resultados confirmados por uma análise de distribuição de tensões de von Mises. As multicamadas apresentam também as cargas críticas de delaminação máximas nos ensaios tribológicos a seco (200 N para multicamadas com NCD-2). Os valores do coeficiente de atrito variam na gama μ=0.02-0.09, para coeficientes de desgaste ~10-7 mm3 N-1 m-1 para a esfera e placa, indicando um regime de desgaste moderado. Sob lubrificação de líquidos fisiológicos (HBSS e FBS) descem para ~10-9-10-8 mm3 N-1 m-1, valores determinados pela rugosidade de partida e pelo regime de pressão de contato efetiva

Adhesion Enhancement of Diamond Coating on WC-Co Substrates Through Interlayer Design

Diamond coating with sufficient adhesion on WC-Co cutting tools is expected to significantly increase their cutting performance. However, the adhesion is always limited by the formation of graphitic soot in the interface due to the catalytic effect of Co on graphite formation. Moreover, the low nucleation density and the high thermal stress in the coatings also result in poor adhesion. The introduction of interlayer is one of the available approaches to enhance the coating-substrate interfacial adhesion. The goal of this project is to improve the adhesion through the optimization of interlayer design. The Al2O3 and Ta mono-interlayer, Al-Al2O3, Al-AlN, Al2O3-Ta and Al-Ta duplex interlayer systems have been developed in this study. These interlayer materials were prepared using a magnetron sputtering method, and diamond coating were deposited on them using microwave plasma enhanced chemical vapor deposition. In addition, different diamond seeding conditions have been studied to increase the diamond nucleation density. Grazing incident X-ray diffraction was carried out to determine the phase components in the Al-Al2O3 and Al-AlN interlayers. Raman spectroscopy and scanning electron microscopy were used to evaluate the quality, morphology and microstructure of the deposited diamond coatings. Rockwell C indentation testing was performed to evaluate the adhesion of the coatings. To elucidate the coating failure mechanism, the compositions around the delaminated spots of diamond coatings after indentation were identified by Energy-dispersive X-ray spectroscopy. To evaluate the tribological properties of the diamond coatings, the diamond coated WC-Co sheets were rubbed against steel and alumina balls respectively. The results show that continuous diamond coatings were achieved on Al2O3, Al-Al2O3, Al-AlN and Al-Ta interlayered substrates, whereas a graphite layer was still formed with the Ta monolayer or Al2O3-Ta duplex layer accompanied by an easy spallation of diamond coatings. The Al interlayer has played an important role in obtaining high purity diamond by in-situ forming an alumina barrier layer. Especially, the diamond coating deposited with an Al-AlN interlayer exhibits superior interfacial adhesion in comparison with all the other interlayers. Meanwhile, seeding with nano-diamond particles is more efficient than micro-diamond particles for improving the diamond nucleation density on Al-AlN interlayered substrates. Furthermore, the diamond coated WC-Co sheets possess lower coefficient of friction and wear rate than bare sheets when rubbing against either steel or alumina balls

Nano to micrometric grain sized CVD diamond for turning hard and abrasive materials

Doutoramento em Ciência e Engenharia de MateriaisO presente trabalho consistiu no desenvolvimento de ferramentas de corte de diamante CVD (Chemical Vapour Deposition) obtido na forma de revestimento em materiais cerâmicos à base de nitreto de silício monolítico (Si3N4) ou compósitos nitreto de silício-nitreto de titânio (Si3N4-TiN). A adição de TiN acima de 23 vol.% conferiu conductividade eléctrica ao compósito, na ordem de 1×10-1 W−1.cm-1, possibilitando a sua maquinagem por electroerosão. Duas técnicas foram utilizadas para o crescimento dos filmes de diamante: deposição química em fase vapor por plasma gerado por microondas, MPCVD (Microwave Plasma Chemical Vapour Deposition), e por filamento quente, HFCVD (Hot Filament Chemical Vapour Deposition). Previamente os substratos cerâmicos sofreram uma preparação superficial por diversos métodos: rectificação por mós diamantadas; polimento com suspensão de diamante (15μm); ataque da superfície por plasma de CF4; riscagem manual ou por ultra-sons com pó de diamante (0.5-1.0 μm). A caracterização das ferramentas revestidas envolveu: o estudo da qualidade e tensões residuais dos filmes de diamante a partir da difracção dos raios X e espectroscopia Raman; a análise da respectiva microestrutura e medida da espessura por microscopia electrónica de varrimento (SEM); a determinação dos valores de rugosidade dos filmes por microscopia de força atómica (AFM); e a avaliação da adesão dos filmes aos substratos por indentação com penetrador Brale. Foram obtidos filmes com granulometria que variaram da gama do diamante nanométrico (< 100 nm) até ao micrométrico convencional (3-12 μm), com consequências na rugosidade superficial do filme. Os filmes de diamante CVD apresentaram espessuras de 15 a 150μm. Os revestimentos apresentaram elevada adesão ao substrato, sendo que o melhor resultado foi atingido pelo diamante micrométrico, suportando um limite de carga aplicada de até 1600 N. O estudo do comportamento em serviço das ferramentas foi efectuado na operação de torneamento de metal duro (WC-Co) e de eléctrodos de grafite, com medição de forças de corte em tempo real por meio de um dinamómetro. Os ensaios foram realizados num torno CNC, em ambiente industrial, na empresa Durit (Albergaria-a-Velha), produtora de metal duro. Os modos de desgaste das ferramentas foram avaliados por meio de observação em microscopia óptica e electrónica de varrimento e o grau de acabamento da superfície maquinada por rugosimetria. A influência destes parâmetros foi estudada em termos das forças envolvidas em operações de torneamento, desgaste das ferramentas e do acabamento conferido à peça maquinada. Os melhores resultados do torneamento de metal duro foram atingidos pelas ferramentas com geometria de aresta em quina-viva, recobertas com os filmes de diamante de 100-200 nm de tamanho de grão, correspondentes às menores forças de corte (<150N), melhor qualidade da peça maquinada (rugosidade aritmética igual a 0,2 μm) e menor desgaste (flanco igual a 110μm). No torneamento de eléctrodos de grafite, as forças de corte foram baixas (< 20N), sendo que o principal modo de desgaste foi a formação de cratera na superfície de ataque (valor máximo igual a 22 μm). O fio da aresta de corte permaneceu inalterado (devido ao mínimo desgaste de flanco), sendo que as diferentes granulometrias do diamante não tiveram influência significativa no comportamento geral das ferramentas.This work consisted on the development of CVD (Chemical Vapour Deposition) diamond cutting tools directly deposited on monolithic silicon nitride (Si3N4) based ceramics and silicon nitride-titanium nitride composites (Si3N4-TiN). A TiN content higher than 23 vol.% confers electric conductivity to the composite in the order of 1×10-1 W−1.cm-1, making possible its machinability by means of electrodischarge machining. Two techniques were used for diamond growth: Microwave Plasma Chemical Vapour Deposition (MPCVD) and Hot Filament Chemical Vapour Deposition (HFCVD). The substrate pre-treatment steps prior to diamond deposition were: grinding with diamond wheels; polishing with diamond suspension (15μm); chemical etching with CF4 plasma; manual scratching or ultrasonic bath scratching with diamond powder (0.5-1.0 μm) for seeding purposes. The diamond cutting tools characterization involved: study of the quality and the residual stress of the films by X ray diffraction and Raman spectroscopy; analysis of respective film microstructure and measurement of film thickness by scanning electron microscopy (SEM); quantification of film surface roughness by atomic force microscopy (AFM); evaluation of adhesion strength of the thin films to Si3N4 substrate by the indentation technique with a Brale indenter. The grain size of the films varied from nanometric (< 100 nm) to conventional micrometric (3-12 μm), therefore giving different surface roughness. The CVD diamond film thickness was in the range of (15-150 μm). The diamond films presented a high adhesion level to the Si3N4 ceramic substrates, the best results being achieved by the micrometric grain sized film, which undergo a normal load of until 1600N. The study of the cutting tool behaviour was performed in turning operations of hardmetal (WC-Co) and graphite electrodes, by real-time acquisition of the cutting forces using a dynamometer. The turning operations were carried out in a CNC lathe, at industrial environment of a hardmetal producer company, Durit (Albergaria-a-Velha). The wear modes of the tested cutting tools were analysed by optical and electronic microscopy observations and the finishing quality of the machined workpiece was measured by surface roughness measurements. The influence of these parameters was studied in terms of the cutting forces developed during turning operations, of tool wear and of the finishing quality of the machined workpieces. The best results attained in hardmetal turning were achieved by the cutting tools with sharp edges, covered with diamond films of 100-200 nm of grain size, which presented the lowest cutting forces (<150N), the best workpiece surface quality (Ra=0.2μm) and the lowest flank wear (110μm). In graphite turning, the cutting forces were very low (<20N) and the main wear mode was the crater one on the rake face (maximum value of 22μm). The cutting edge remained almost intact (due to the minimum flank wear) while the different diamond grain sizes did not have a significant influence on the overall cutting behaviour

Tribossistemas de revestimentos multicamada de diamante CVD micro/nanocristalino

Doutoramento em Ciência e Engenharia de MateriaisA combinação das características do diamante microcristalino (MCD) e nanocristalino (NCD), tais como elevada adesão do MCD e a baixa rugosidade superficial e baixo coeficiente de atrito do NCD, é ideal para aplicações tribológicas exigentes. Deste modo, o presente trabalho centrou–se no desenvolvimento de revestimentos em multicamada MCD/NCD. Filmes com dez camadas foram depositados em amostras de cerâmicos de Si3N4 pela técnica de deposição química em fase vapor assistida por filamento quente (HFCVD). A microestrutura, qualidade do diamante e adesão foram investigadas usando técnicas como SEM, AFM, espectroscopia Raman, DRX, indentação Brale e perfilometria ótica 3D. Diversas geometrias para aplicações distintas foram revestidas: discos e esferas para testes tribológicos à escala laboratorial, e para testes em serviço, anéis de empanques mecânicos e pastilhas de corte para torneamento. Nos ensaios tribológicos esfera–sobre–plano em movimento recíproco, sob 10–90% de humidade relativa (RH), os valores médios dos coeficientes de atrito máximo e em estado estacionário são de 0,32 e 0,09, respetivamente. Em relação aos coeficientes de desgaste, observou–se um valor mínimo de cerca de 5,2×10–8 mm3N–1m–1 para valores intermédios de 20–25% de RH. A humidade relativa tem um forte efeito sobre o valor da carga crítica que triplica a partir de 40 N a 10% RH para 120 N a 90% de RH. No intervalo de temperaturas 50–100 ° C, as cargas críticas são semelhantes às obtidas em condições de baixa RH (~10–25%). A vida útil das ferramentas com revestimento de dez camadas alternadas MCD/NCD e 24 μm de espessura total no torneamento de um compósito de matriz metálica Al– 15 vol% Al2O3 (Al–MMC) é melhor do que a maioria das ferramentas de diamante CVD encontradas na literatura, e semelhante à maioria das ferramentas de diamante policristalino (PCD). A formação de cratera ocorre por desgaste sucessivo das várias camadas, atrasando a delaminação total do revestimento de diamante do substrato, ao contrário do que acontece com os revestimentos monocamada. Os anéis de empanque testados com biodiesel apresentaram coeficientes de desgaste (4,1x10–10 mm3N–1m–1) duas ordens de grandeza menores do que em ensaios esfera–sobre–plano em movimento recíproco (k = 5,0x10–8 mm3N–1m–1), mas não foi possível obter vedação completa devido a sobreaquecimento do fluido. Esta condição foi obtida com água sob pressão, para condições P.V na gama 0,72–5,3 MPa.ms–1. Um coeficiente de atrito em estado estacionário de ~ 0,04 e um valor de coeficiente de desgaste de 6,0x10–10 mm3N–1m–1, característico de um regime desgaste ultra–suave, revelam o alto desempenho deste tribossistema.The combination of the characteristics of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) varieties, such as high adhesion of MCD and low surface roughness and low friction coefficient of NCD, is ideal for highly–demanding tribological applications. The main objective of this study was thus the development of multilayered MCD/NCD coatings for such purpose. Single layer and tenfold multilayer coatings were grown onto Si3N4 ceramic samples by the hot–filament CVD (HFCVD) process and their microstructure, diamond quality and adhesion were investigated using SEM, AFM, Raman spectroscopy, XRD, Brale indentation and 3D optical profilometry. Several geometries for distinct applications were then coated: discs and balls for lab–scale tribological testing, mechanical seal rings and cutting inserts for in–service testing. For the ball–on–flat reciprocating tests in the 10–90% relative humidity (RH) range the average values of the maximum and steady–state friction coefficients are 0.32 and 0.09, respectively. Regarding the wear coefficient of the discs, a valley–shaped evolution is observed within the same RH range, with a minimum of about 5.2×10–8 mm3N–1m–1. Humidity has a strong effect on the value of the critical load that triples from 40 N at 10% RH to 120 N at 90% RH. In the 50–100 °C range the critical loads are similar to those attained under dry conditions ( 25% RH). The tool life of a 24 μm thick tenfold multilayered MCD/NCD coated insert in the turning of an Al–15 vol.% Al2O3 metal matrix composite (Al–MMC) is better than most reported CVD diamond systems, behaving as well as most PCD tools. Crater wear occurs by successive wear of the layers, delaying total delamination of the diamond coating from the substrate, unlike what would happen with monolayer coatings. Under biodiesel lubrication seal rings present wear coefficients (4.1x10–10 mm3N–1m–1) two orders of magnitude lower than the reciprocating sliding ball–on–flat experiments (k = 5.0x10–8 mm3N–1m–1), but no full sealing was possible due to overheating of the fluid. This condition was only attained with pressurized water, for P.V conditions in the range 0.72–5.3 MPa.ms–1. A steady state friction coefficient value of ~0.04 and a wear coefficient value of 6.0x10–10 mm3N–1m–1, characteristic of an ultra–mild wear regime, reveal the high performance of this tribosystem

Status and applications of diamond and diamond-like materials: An emerging technology

Recent discoveries that make possible the growth of crystalline diamond by chemical vapor deposition offer the potential for a wide variety of new applications. This report takes a broad look at the state of the technology following from these discoveries in relation to other allied materials, such as high-pressure diamond and cubic boron nitride. Most of the potential defense, space, and commercial applications are related to diamond's hardness, but some utilize other aspects such as optical or electronic properties. The growth processes are reviewed, and techniques for characterizing the resulting materials' properties are discussed. Crystalline diamond is emphasized, but other diamond-like materials (silicon carbide, amorphous carbon containing hydrogen) are also examined. Scientific, technical, and economic problem areas that could impede the rapid exploitation of these materials are identified. Recommendations are presented covering broad areas of research and development

The critical raw materials in cutting tools for machining applications: a review

A variety of cutting tool materials are used for the contact mode mechanical machining of components under extreme conditions of stress, temperature and/or corrosion, including operations such as drilling, milling turning and so on. These demanding conditions impose a seriously high strain rate (an order of magnitude higher than forming), and this limits the useful life of cutting tools, especially single-point cutting tools. Tungsten carbide is the most popularly used cutting tool material, and unfortunately its main ingredients of W and Co are at high risk in terms of material supply and are listed among critical raw materials (CRMs) for EU, for which sustainable use should be addressed. This paper highlights the evolution and the trend of use of CRMs) in cutting tools for mechanical machining through a timely review. The focus of this review and its motivation was driven by the four following themes: (i) the discussion of newly emerging hybrid machining processes offering performance enhancements and longevity in terms of tool life (laser and cryogenic incorporation); (ii) the development and synthesis of new CRM substitutes to minimise the use of tungsten; (iii) the improvement of the recycling of worn tools; and (iv) the accelerated use of modelling and simulation to design long-lasting tools in the Industry-4.0 framework, circular economy and cyber secure manufacturing. It may be noted that the scope of this paper is not to represent a completely exhaustive document concerning cutting tools for mechanical processing, but to raise awareness and pave the way for innovative thinking on the use of critical materials in mechanical processing tools with the aim of developing smart, timely control strategies and mitigation measures to suppress the use of CRMs

Investigação das propriedades de filmes de diamante depositados por TMCVD

Mestrado em Física AplicadaO diamante possui um conjunto de propriedades que tornam a sua aplicação na industria e no mercado de consumo, altamente apetecível. O interesse em revestimentos de diamante disparou após o desenvolvimento da tecnologia de deposição química na fase de vapor [Chemical Vapor Deposition (CVD)], a qual possibilitou o revestimento de uma série de materiais diferentes. Contudo, a dificuldade de depositar diamante de elevada qualidade é tão grande como a grandeza das suas propriedades. Os filmes de diamante depositados utilizando sistemas convencionais de CVD, onde o metano e o hidrogénio são utilizados como precursores, tende a apresentar elevadas rugosidades superficiais, devido principalmente ao crescimento colunar dos cristais e ao não crescimento homogéneo. Uma das grandes limitações de aplicação é mesmo a elevada rugosidade dos filmes, não permitindo a sua utilização maciça nos campos da micro electrónica e biomedicina, por exemplo. Investigadores do Centro de Tecnologia Mecânica e Automação, da Universidade de Aveiro, desenvolveram um processo de deposição por modulação temporal do hidrocarboneto precursor [Time-modulated CVD (TMCVD)], dando origem a filmes nanocristalinos de baixa rugosidade, mais duro e de melhor qualidade. O processo chave da tecnologia TMCVD é a modulação temporal do metano durante o crescimento do filme. No método convencional, o CH4 é mantido a um fluxo constante ao longo do crescimento. No TMCVD, é esperado que ocorra nucleações de segunda ordem, com o aumento da concentração de metano no precursor. É dado assim origem a um filme crescido por multicamadas, que envolve uma nucleação inicial, seguido de uma fase de crescimento e de uma nova nucleação, podendo o processo ser continuado de forma cíclica. A caracterização dos filmes crescidos por TMCVD, pode abrir as portas para uma série de aplicações até agora barrada pelas limitações dos filmes convencionais, tais como ferramentas de cirurgia, implantes humanos, por exemplo válvulas cardíacas. Neste trabalho, foi efectuado estudos comparativos das propriedades de filmes convencionais e modelados. As propriedades eléctricas de filmes convencionais e modelados foram também estudas. No entanto será necessário proceder a mais investigação, para uma total compreensão das consequências do novo processo de deposição proposto. Métodos de optimização foram também aplicados. ABSTRACT: Diamond has many extreme properties that make it suitable for many industrial and consumer applications. The interest in diamond coatings boosted ever since the realization that diamond films can be deposited onto a range of materials using a process called Chemical Vapor Deposition (CVD). However, the difficulty in depositing truly superior diamond coatings is nearly as extreme as the material’s outstanding properties. Diamond films deposited using conventional CVD systems, where methane and hydrogen are used as the precursor gases, tend to exhibit high surface roughness mainly due to the columnar growth of the non-orientated polycrystalline diamond films. One of the major limitations of the wide scale use of diamond coatings has been the high roughness of the films. As a result, this has severely hampered diamond coatings’ widespread use in microelectronics, biomedical, and optical applications. At the “Centre for Mechanical Technology and Automation”, University of Aveiro, the researchers have developed a novel time-modulated CVD (TMCVD) process for producing smooth/nano-crystalline, harder and better quality diamond films. The key distinctive feature of the TMCVD process is that it modulates methane flow during film growth. In conventional diamond CVD, the CH4 flow is generally kept constant for the complete growth process. In TMCVD, it is expected that secondary nucleation processes occur during the high/low CH4 pulse cycles. This can effectively result in the formation of a diamond film involving nucleation stage, diamond growth, and secondary nucleation, and the cycle is repeated. The successful and effective characterization of the time-modulated films can see such film coatings used in a number of new and interesting applications, such as surgical tools, human implants, e.g. heart valves. In this work, we perform comparative studies of the properties of conventional and time-modulated films. Electrical properties are also investigated in conventional and modulated films, although further research is needed to fully understand the consequences of the new proposed deposition process. Optimization methods are also performed

Si-DLC coatings optimized for low wear and low friction applications.

Diamond-like carbon (DLC) has excellent properties for the use as tribological coating such as high hardness, high wear resistance and a low friction coefficient. Nanolayer systems of hydrogenated diamond-like carbon (a-C:H) and silicon doped hydrogenated diamond-like carbon (Si-DLC or a-C:H:Si) have been studied. The objectives of this work are to develop layer combinations which allow combining low abrasive wear, low friction properties, and improved temperature stability for the coatings.A literature survey on sputtering, plasma enhanced chemical vapor deposition (PECVD) and carbon based materials with a focus on a-C:H and Si-DLC coatings is given. This survey includes a brief overview of the 60 years history of DLC coating.In the second part of the thesis, the experimental set up for the layer preparation by a magnetron based deposition method and a basic description of the process are presented. Using sputtering from a solid SiC target as a source of the Si for the Si-DLC instead of the commonly used PECVD process with Si-containing precursors Si-DLC/a-C:H nanolayers were deposited at high deposition rates.For the coatings, the influence of the acetylene gas flow, the bias voltage, and the hydrogen and silicon concentration are discussed. Undoped a-C:H coatings with high indentation hardness above 40 GPa and very low abrasive wear rates of 0.6 x 10[-15] m[3]/(Nm) and low hydrogen content of about 11 at.% were deposited. Si-DLC/a-C:H nanolayer coatings with high hardness of 20 to 30 GPa and a high temperature stability up to 500 &deg;C were prepared. For these nanolayer films low friction coefficients of 0.06 to 0.11 and high abrasive wear rates of >2.5 x 10[-15] m3/(Nm) were achieved at high silicon contents above 15 at.%. On the other hand nanolayer coatings with low silicon contents of less than 10 at.% showed low wear rates below 1.7 x 10[-15] m3/(Nm) combined with higher friction coefficients of 0.12 to 0.15. In order to combine the low abrasive wear rate and the low friction coefficient at first a Si-DLC/a-C:H layer with low Si-concentration followed by a second layer with high Si-concentration was deposited. Due to the low surface free energy of Si-DLC coatings of 30 to 35 mN/m the wetting behavior for some lubricants may deteriorate. In this case a further optimization of the nanolayer system is required