The influence of V addition on the structure, mechanical properties, and oxidation behaviour of TiAlSiN coatings deposited by DC magnetron sputtering

The influence of V content on the morphology, structure, hardness (H) and reduced Young's modulus (E), adhesion, and oxidation resistance of TiAlSiN coatings is investigated. The coatings were produced by DC reactive magnetron sputtering, with increasing V contents from 0, 4.8 and 11.0 at.%. All coatings exhibit a fcc type structure. The coating with 4.8 at.% of V shows the highest values of H and E, whereas the values are similar for the reference coating and the coating with 11.0 at.% of V. The coatings adhere well to the substrates and show a dense and compact columnar growth extending from the adhesive interlayer to the top surface of the coatings. The dynamic thermal gravimetric oxidation curves reveal that V additions decreases the onset point of oxidation significantly and degrades the oxidation resistance of the coatings. A dual oxide layer is formed on the top surface of the reference coating: an outer porous Ti–Al–O rich layer with plate-like features on the top, which classified to TiO2 (rutile and anatase) and Al2O3 phases, and an inner Ti–Si–O rich layer with Al depletion that identified as mixture of amorphous Si–O and Ti–Si–O protective oxides. The diffusion of V to the top surface governs the oxidation process of the V-containing coatings, i.e. increasing V concentration leads to disrupt the formation of the protective continuous oxide layers easily.This research is sponsored by national funds through FCT e Fundação para a Ciência e a Tecnologia, under the projects: UIDB/00285/2020, SMARTLUBdref. “POCI-01-0145-FEDER-031-807”. MCTool21 project “Manufacturing of cutting tools for the 21st century: from nano-scale material design to numerical process simulation” (reference: POCI-01-0247-FEDER-045940), co-financed by the European Regional Development Fund, through Portugal 2020 (PT2020), and by the Competitiveness and Internationalization Operational Programme (COMPETE 2020).info:eu-repo/semantics/publishedVersio

Effect of Annealing Heat Treatment on the Composition, Morphology, Structure and Mechanical Properties of the W-S-N Coatings

Alloyed-transition metal dichalcogenide (TMD) coatings have been under investigation as multi-environment lubricants for the past few decades. These coatings display very low coefficient of friction properties at elevated temperatures. Studies on the annealing of these low-friction coatings are missing in the literature. For the first time, in this study, the annealing of the W-S-N dry lubricant coatings was carried out to study its effects on the composition, morphology, crystal structure and hardness of the coatings. The W-S-N coatings were deposited by direct current (DC) reactive magnetron sputtering. The analysis was carried out for as-deposited, 200 °C and 400 °C annealed coatings. The as-deposited coatings have N content in the range of 0–25.5 at. %. The coatings are compact and the densification increased with the increase in N-alloying. All the coatings are crystalline except the highest N-alloyed coating which is X-ray amorphous. A maximum hardness of 8.0 GPa was measured for the coating alloyed with 23 at. % N. Annealing did not affect the composition and morphology of the coatings, while some variations were observed in their crystal structure and hardness. The maximum hardness increased from 8 GPa to 9.2 GPa after 400 °C annealing of the 23 at. % N-alloyed coating.This study is sponsored by FEDER National funds FCT under the projects: SMARTLUB ref.“POCI-01-0145-FEDER-031807”, CEMMPRE ref. “UIDB/00285/2020”, On-SURF ref. “POCI-01-0247-FEDER-024521”, “LA/P/0112/2020” and Atrito-0 ref. “POCI-01-0145-FEDER-030446”.info:eu-repo/semantics/publishedVersio

Vacuum Tribological Properties of W-S-N Coatings Synthesized by Direct Current Magnetron Sputtering

This work deals with the investigation of the tribological performance of DC magnetron sputteredW-S-N coatings under vacuum atmosphere, as part of the exploration of multi-environment sliding properties of W-S-N solid lubricants. This study is part of the systematic testing of W-S-N solid lubricants in different environments, especially vacuum, which is often ignored. The trend is to test sliding properties in dry N2 by considering it as replacement of vacuum environment testing. This approach is not appropriate. In this work, a set of coatings was synthesized with N-alloying content in the range of 0–25.5 at.%. A maximum S/W ratio of 1.47 was observed for the pure WSx coating. A maximum hardness of 8.0 GPa was observed for 23 at.% of N-alloying. The coating with the lowest N content (14.6 at.%) displayed the lowest friction, specific wear rate and wear scar depth under vacuum conditions. Despite superior sliding performance at room temperature (35% humidity), 200 C and dry nitrogen conditions, the performance of the WSN12.5 coating deteriorated vacuum environment.This work is sponsored by FEDER National funds FCT under the projects: SMARTLUB ref. “POCI-01-0145-FEDER-031807”, CEMMPRE ref. “UIDB/00285/2020”, On-SURF ref. “POCI-01-0247- FEDER-024521”, “LA/P/0112/2020” and Atrito-0 ref. “POCI-01-0145-FEDER-030446”. Andrey Bondarev acknowledges the support of the project “International Mobility of the scientific workers MSCAIF IV at Czech Technical University in Prague, Grant No: CZ.02.2.69/0.0/0.0/20_079/0017983”.info:eu-repo/semantics/publishedVersio

Ceramic-reinforced HEA matrix composites exhibiting an excellent combination of mechanical properties

CoCrFeNi is a well-studied face centered cubic (fcc) high entropy alloy (HEA) that exhibits excellent ductility but only limited strength. The present study focusses on improving the strength-ductility balance of this HEA by addition of varying amounts of SiC using an arc melting route. Chromium present in the base HEA is found to result in decomposition of SiC during melting. Consequently, interaction of free carbon with chromium results in the in-situ formation of chromium carbide, while free silicon remains in solution in the base HEA and/or interacts with the constituent elements of the base HEA to form silicides. The changes in microstructural phases with increasing amount of SiC are found to follow the sequence: fcc → fcc + eutectic → fcc + chromium carbide platelets → fcc + chromium carbide platelets + silicides → fcc + chromium carbide platelets + silicides + graphite globules/flakes. In comparison to both conventional and high entropy alloys, the resulting composites were found to exhibit a very wide range of mechanical properties (yield strength from 277 MPa with more than 60% elongation to 2522 MPa with 6% elongation). Some of the developed high entropy composites showed an outstanding combination of mechanical properties (yield strength 1200 MPa with 37% elongation) and occupied previously unattainable regions in a yield strength versus elongation map. In addition to their significant elongation, the hardness and yield strength of the HEA composites are found to lie in the same range as those of bulk metallic glasses. It is therefore believed that development of high entropy composites can help in obtaining outstanding combinations of mechanical properties for advanced structural applications.Financial support from the Higher Education Commission of Pakistan (HEC NRPU 6019) is acknowledged. FEDER National funds FCT under the project CEMMPRE, ref. “UIDB/00285/2020” is also acknowledged.info:eu-repo/semantics/publishedVersio

Optimization of sputtered Mo-Se-C coatings for efficient self-adaptation during sliding in diverse environments

Doctoral Thesis in Mechanical Engineering, branch Surface Engineering, submitted to the Faculty of Sciences and Technology of the University of CoimbraCoatings deposited by plasma vapour deposition (PVD) revolutionized the world of tribology and resolved various problems encountered in industrial domains. Magnetron sputtered carbon-alloyed transition metal dichalcogenide (TMD-C) coatings have been widely explored in the past few decades due to their self-adaptive sliding properties. The drawback is that their frictional properties are highly environmental dependent. This substantial variation of the frictional response during sliding in diverse environments, is the main hindrance behind their large-scale application. In the domain of TMD-C coatings, although very less reported, C-alloyed molybdenum diselenide (Mo-Se-C) is a promising candidate as solid lubricant coatings for aerospace and automotive industries, due to an excellent stability in both humid air and vacuum conditions. The present work is aimed at the development, investigation and optimization of magnetron sputtered Mo-Se-C coatings. The goal is to deposit coatings capable of providing highly stable and consistent tribological properties in diverse environments under different conditions, i.e., sliding distances, sliding speeds and contact stresses. In the initial phase of the project, feasibility for development of an efficient self-adaptive lubricant system based on Mo-Se-C coatings was studied. The coatings were deposited via an industrially favourable co-sputtering (separate MoSe2 and C targets) approach, utilizing direct current (DC) power supplies. A detailed parametric study of the deposition parameters and carbon content versus coatings properties was carried out. The tribological testing was done in ambient air and dry N2 atmospheres. These coatings displayed a maximum Se/Mo ratio of 1.88, a compact morphology, an amorphous crystal structure, and a maximum hardness of 4.9 GPa. Small MoSe2 platelets (< 10 nm size) were randomly oriented in an amorphous carbon matrix. The tribological testing in dry N2 and ambient air resulted in the lowest friction coefficient (COF) values of 0.025 and 0.06, respectively. Overall, improved results as compared to literature were achieved. The research was followed by optimization of chemical composition, structure, morphology and mechanical properties of similar coatings deposited by both DC and radio frequency (RF) power supplies in a confocal plasma sputtering unit. The characterization of these optimized coatings showed that the use of low C content as compared to literature recommendations resulted in a Se/Mo ratio of ~2 for DC coatings and ~1.9 for RF coatings. The coatings showed a highly compact morphology, presence of (002) peaks of MoSe2 phase and a hardness increment up to 5.2 GPa. In the post-optimization stage, the sliding behaviour was tested in ambient air, dry N2, and at 200 ⁰C. An in-depth investigation of the chemistry and evolution of the low friction tribolayers with the number of sliding cycles was carried out. In ambient conditions, friction coefficient and specific wear rate decreased with the number of sliding cycles. The coatings outperformed similar coatings presented in initial depositions and the literature. The coverage of the wear tracks with a MoSe2 tribolayer was extended with increasing number of sliding cycles. The measured COF was almost consistent in ambient air, dry N2 and at 200 ⁰C. These results established that the coatings were efficiently capable to be used in diverse operating environments. This was attributed to the fact that the MoSe2 phase was always governing the low friction tribolayer in all environments. Thus, the coatings did not show the chameleon behaviour, disagreeing with the literature on TMD-C coatings. Further, the mechanism and routes behind the formation of low friction tribolayers were also explored. This investigation unfolded that both the transfer layer formed due to the 3rd body inside the contact and the reorientation within the coating matrix contribute to the tribolayer formation. In the last stage, tribological testing of Mo-Se-C coatings was performed, for the first time, in both ambient air and vacuum atmospheres; under various conditions of sliding cycles, sliding speeds and contact stress regimes to confirm their applicability as universal solid lubricants. The testing resulted in no significant variations between the tribological performance (COF: 0.02-0.06). In conclusion, the achieved optimization of self-adaptive behaviour in diverse environments demonstrated that the Mo-Se-C system is a potential universal solid lubricant for low friction in aerospace and terrestrial atmospheres.Os revestimentos depositados por técnicas de deposição física em fase de vapor (PVD) têm revolucionado o mundo da tribologia e resolvido problemas muito diferentes encontrados nos mais variados domínios industriais. Neste contexto, os revestimentos de dicalcogenetos de metais de transição ligados com carbono (TMD-C), depositados por pulverização catódica magnetrão, têm sido largamente explorados nas décadas recentes devido às suas propriedades auto-adaptativas durante o deslizamento. A sua principal desvantagem é que as suas propriedades tribológicas são fortemente dependentes do ambiente em que os revestimentos são utilizados. Esta variação significativa da resposta ao atrito, durante o deslizamento em diferentes ambientes, tem sido o maior obstáculo para a aplicação destes revestimentos numa larga escala industrial. No domínio dos revestimentos TMD-C, embora menos reportados na literatura, os disselenietos de molibdénio ligados com carbono (Mo-Se-C) são candidatos muito promissores para as indústrias automóvel e aerospacial, devido a uma estabilidade excelente em ambos os ambientes húmidos e em vácuo. Este trabalho de investigação tem como objetivo desenvolver, investigar e otimizar revestimentos do tipo Mo-Se-C depositados por pulverização catódica magnetrão. É pretendido depositar revestimentos capazes de providenciar propriedades tribológicas estáveis e consistentes em meios diversos sob diferentes condições de aplicação, i.e. distâncias de deslizamento, velocidades de deslizamento e tensões de contato. Numa primeira fase do projeto, foi estudada a possibilidade de desenvolver um sistema lubrificante auto-adaptativo baseado em revestimentos do tipo Mo-Se-C. Os revestimentos foram depositados utilizando uma aproximação amigável do ponto de vista industrial, consistindo na co-pulverização de alvos separados (MoSe2 e carbono) utilizando fontes de potência de corrente contínua (DC). Para tal foi efetuado um estudo detalhado da influência dos parâmetros de deposição e do teor em carbono nas propriedades dos revestimentos. Os revestimentos foram testados tribologicamente em meios ambientes húmido e seco. Os revestimentos apresentaram uma razão Se/Mo de 1,88 e uma morfologia compacta; a estrutura era amorfa e a dureza máxima alcançada foi de 4,9 GPa. A estrutura consistia em pequenas plaquetas de MoSe2 embebidas numa matriz amorfa de carbono. O teste tribológico em azoto seco e ar ambiente deu valores de coeficiente de atrito (COF) de, respetivamente, 0,025 e 0,06. Em comparação com a literatura, foram alcançados resultados com melhorias acentuadas. A investigação prosseguiu com a otimização da composição química, estrutura, morfologia e propriedades mecânicas de revestimentos semelhantes depositados por ambos os processos em DC e rádio-frequência (RF), num equipamento de pulverização catódica com plasma confocal relativamente aos cátodos. A caraterização dos revestimentos otimizados mostrou que a utilização de baixos teores de C, em comparação com as recomendações encontradas na literatura, resultou numa razão de Se/Mo de ~2, para os revestimentos DC e ~1,9 para os RF. Os revestimentos apresentavam uma morfologia muito compacta, picos (002) da fase MoSe2 e um aumento de dureza para 5,2 GPa. Após esta otimização, o comportamento tribológico foi avaliado em testes em ar ambiente, azoto seco e a 200 ºC. Depois, foi efetuada uma investigação detalhada da evolução das tribocamadas de baixo atrito formadas em função do número de ciclos de deslizamento. Em condições ambiente, os coeficientes de atrito e desgaste diminuíram com o número de ciclos. Estes revestimentos ultrapassaram o desempenho tribológico daqueles depositados anteriormente e os referenciados na literatura. A cobertura da pista de desgaste com uma tribocamada de baixo atrito foi sendo formada progressivamente com a evolução do número de ciclos. Os coeficientes de atrito medidos foram muito similares independentemente das condições de teste, podendo ser concluído que estes revestimentos são adequados para os mais diversos meios de teste. Este comportamento é devido à formação duma camada da fase MoSe2 que governa o deslizamento e providencia um atrito reduzido. Em consequência, estes revestimentos não mostram o tão reclamado “comportamento de camaleão”, em desacordo com o referido na literatura para revestimentos do tipo TMD-C. Para além destes resultados, foram ainda explorados os mecanismos e processos que estão por trás da formação da tribocamada de baixo atrito. Esta investigação revelou que quer a transferência de uma camada do elemento antagonista do par de contato, quer a reorientação dos cristais de MoSe2 dentro da matriz amorfa de carbono, contribuem para a formação da tribocamada. Numa última fase do trabalho, foi efetuado pela primeira vez um estudo tribológico em atmosfera ambiente e em vácuo, variando o número de ciclos de deslizamento, as velocidades de deslizamento e as tensões de contato para confirmar a aplicabilidade dos revestimentos otimizados de Mo-Se-C como lubrificantes sólidos universais. Os testes mostraram não haver variações significativas no desempenho tribológico para todas as condições (COF = 0.02-0.06). Resumindo, a otimização do comportamento auto-adaptativo em diferentes atmosferas, demonstrou claramente que os revestimentos Mo-Se-C são uma potencial solução para baixo atrito em aplicações terrestres e no espaço

On the Microstructural, Mechanical and Tribological Properties of Mo-Se-C Coatings and Their Potential for Friction Reduction against Rubber

Friction and wear contribute to high energetic losses that reduce the efficiency of mechanical systems. However, carbon alloyed transition metal dichalcogenide (TMD-C) coatings possess low friction coefficients in diverse environments and can self-adapt to various sliding conditions. Hence, in this investigation, a semi-industrial magnetron sputtering device, operated in direct current mode (DC), is utilized to deposit several molybdenum-selenium-carbon (Mo-Se-C) coatings with a carbon content up to 60 atomic % (at. %). Then, the carbon content influence on the final properties of the films is analysed using several structural, mechanical and tribological characterization techniques. With an increasing carbon content in the Mo-Se-C films, lower Se/Mo ratio, porosity and roughness appeared, while the hardness and compactness increased. Pin-on-disk (POD) experiments performed in humid air disclosed that the Mo-Se-C vs. nitrile butadiene rubber (NBR) friction is higher than Mo-Se-C vs. steel friction, and the coefficient of friction (CoF) is higher at 25 °C than at 200 °C, for both steel and NBR countersurfaces. In terms of wear, the Mo-Se-C coatings with 51 at. % C showed the lowest specific wear rates of all carbon content films when sliding against steel. The study shows the potential of TMD-based coatings for friction and wear reduction sliding against rubber

Synthesis, Microstructural, and Mechano-Tribological Properties of Self-Lubricating W-S-C(H) Thin Films Deposited by Different RF Magnetron Sputtering Procedures

Carbon-alloyed transition metal dichalcogenide (TMD) coatings have great potential for providing a good tribological response in diverse operating environments. There are di erent ways to synthesize these coatings by magnetron sputtering, with no clear indication of the best possible route for potential upscaling. In this study, tungsten-sulfur-carbon (W-S-C) coatings were deposited by radio frequency (RF) magnetron sputtering via four di erent methods. All coatings were sub-stoichiometric in terms of the S/Wratio, with the bombardment of the growing film with backscattered Ar neutrals being the main mechanism governing the S/Wratio. The crystallinity of the films was dependent on the C and S contents. X-ray photoelectron spectroscopy (XPS) revealed W-S and W-C bonding in all coatings. Raman spectroscopy showed the presence of an a-C phase with predominant sp2 bonding. The hardness of the coatings may be related to the C content and the S/W ratio. A friction coe cient of 0.06–0.08 was achieved during sliding in ambient air by the coatings deposited in non-reactive mode with optimal C contents. The results indicate that sputtering in non-reactive mode should be the method of choice for synthesis of these coatings

On the Microstructural, Mechanical and Tribological Properties of Mo-Se-C Coatings and Their Potential for Friction Reduction against Rubber

Friction and wear contribute to high energetic losses that reduce the efficiency of mechanical systems. However, carbon alloyed transition metal dichalcogenide (TMD-C) coatings possess low friction coefficients in diverse environments and can self-adapt to various sliding conditions. Hence, in this investigation, a semi-industrial magnetron sputtering device, operated in direct current mode (DC), is utilized to deposit several molybdenum-selenium-carbon (Mo-Se-C) coatings with a carbon content up to 60 atomic % (at. %). Then, the carbon content influence on the final properties of the films is analysed using several structural, mechanical and tribological characterization techniques. With an increasing carbon content in the Mo-Se-C films, lower Se/Mo ratio, porosity and roughness appeared, while the hardness and compactness increased. Pin-on-disk (POD) experiments performed in humid air disclosed that the Mo-Se-C vs. nitrile butadiene rubber (NBR) friction is higher than Mo-Se-C vs. steel friction, and the coefficient of friction (CoF) is higher at 25 °C than at 200 °C, for both steel and NBR countersurfaces. In terms of wear, the Mo-Se-C coatings with 51 at. % C showed the lowest specific wear rates of all carbon content films when sliding against steel. The study shows the potential of TMD-based coatings for friction and wear reduction sliding against rubber

Comparative study of DC and RF sputtered MoSe22 coatings containing carbon - An approach to optimize 3 stoichiometry, microstructure, crystallinity and 4 hardness

Low stoichiometry, low crystallinity, low hardness and incongruencies about the reported microstructure have limited the applicability of TMD-C (Transition metal dichalcogenides with carbon) solid-lubricant coatings. In this work, optimized Mo-Se-C coatings were deposited using confocal plasma magnetron sputtering to overcome the above-mentioned issues. Two different approaches were used; MoSe2 target powered by DC (direct current) or RF (radio frequency) magnetron sputtering. Carbon was always added by DC magnetron sputtering. Wavelength dispersive spectroscopy displayed Se/Mo stoichiometry of ~2, values higher than the literature. The Se/Mo ratio for RF-deposited coatings was lower than the DC ones. Scanning electron microscopy showed that irrespective of the low carbon additions, the Mo-Se-C coatings were highly compact with no vestiges of columnar growth due to optimal bombardment of sputtered species. Application of substrate bias further improved the compactness at the expense of lower Se/Mo ratio. X-ray diffraction, transmission electron microscopy and Raman spectroscopy confirmed the presence of MoSe2 crystals, and (002) basal planes. Even very low carbon additions led to an improvement of the hardness of the coatings. The work reports a comparison between RF and DC sputtering of MoSe2 coatings with carbon and provides a guideline to optimize the composition, morphology, structure and mechanical propertie

Synthesis, microstructure and mechanical properties of W-S-C selflubricant thin films deposited by magnetron sputtering

W-S-C thin films were deposited by magnetron sputtering in a semi-industrial deposition unit. Various parameters like the composition, crystallinity, morphology and hardness were studied. A single coating was selected for tribological studies in different testing environments. The tribological response during ambient air testing was governed mainly by carbon-based phase at low normal loads and both carbonbased and WS2 rich tribofilms at higher loads. Very low coefficients of friction (~0.01) were obtained during testing at elevated temperature due to accelerated formation of WS2 rich tribofilms. The tribological response in a vacuum against a steel-based counterbody was unsatisfactory due to the presence of a carbon-phase at the sliding interface. Discrepancies were observed between the tribological response in dry N2 and vacuum environments