New restrictions and regulations regarding internal combustion engines introduced severe mechanical and thermo-mechanical loads on engine parts, mostly on piston rings, leading to high wear and premature damage, thus causing oil leakage into the combustion chamber and increasing emissions. One of the most viable solutions to overcome these issues is the use of coatings to change surface properties. Coatings produced by High Power Impulse Magnetron Sputtering (HiPIMS) and Direct Current Magnetron Sputtering (dcMS) have shown promising results to form dense coatings with high hardness and high wear resistance. Those properties can be further enhanced by using a periodic structure in the form of multilayers of different materials. In this study, we proposed a hybrid process with HiPIMS and dcMS to produce multilayer coatings of CrN and CrAlN. Different target combinations and negative substrate bias were studied. Furthermore, the base layer influence and an evaluation of the periodicity effects were presented. Structure and morphology of the coatings were characterized with X-Ray Diffraction, Scanning Electron Microscopy and Atomic Force Microscopy. Meanwhile, instrumented nanohardness, corrosion and wear tests were performed to characterize mechanical, electrochemical and tribological properties. Results showed that the choice of target combination and bias values are fundamental for enhancing mechanical and tribological properties. On the other hand, we found no evidence of superlattice hardening when changing periodicity but an increase in corrosion resistance when reducing periodicity was found. The combination of targets, substrate bias, base layer and periodicity presented here showed promising results on wear tests, especially when compared with the current coating under the same engine conditions. / Novas restrições e regulamentações de motores de combustão interna introduziram severas cargas mecânicas e termomecânicas no motor, principalmente nos anéis de pistão, o que leva a um alto desgaste e ao dano prematuro dos mesmos e causa vazamentos de óleo na câmara, aumentando as emissões de poluentes. Uma das soluções mais viáveis para solucionar este problema é o uso de recobrimentos para alterar as propriedades da superfície. Recobrimentos produzidos pelas técnicas High Power Impulse Magnetron Sputtering (HiPIMS) e Direct Current Magnetron Sputtering (dcMS) mostraram resultados promissores na formação de filmes densos com elevada dureza e resistência ao desgaste. Essas propriedades podem ser melhoradas com a utilização de estruturas periódicas na forma de multicamadas. Nesse estudo, foi proposto um processo híbrido de HiPIMS e dcMS para produzir recobrimentos multicamadas de CrN/CrAlN. Diferentes combinações de alvos e de valores de polarização negativa do substrato (bias) foram estudadas. Além disso, foram apresentadas a influência da camada base e a avaliação dos efeitos da mudança de periodicidade. Estrutura e morfologia dos recobrimentos foram caracterizadas por Difração de Raios-X, Microscopia Eletrônica de Varredura e Microscopia de Força Atômica. As caracterizações mecânica, eletroquímica e tribológica foram realizadas com ensaios de Nanodureza Instrumentada, Corrosão e Desgaste. Os resultados mostram que a escolha da combinação de alvos e de valores de bias são fundamentais para a melhoria das propriedades mecânicas e tribológicas. Por outro lado, não foram encontradas evidências de endurecimento devido ao efeito de super-redes com a variação de periodicidade, mas um aumento da resistência à corrosão foi evidenciado. A combinação de alvos, bias, camada base e periodicidade apresentadas aqui mostraram resultados promissores em testes de desgaste, principalmente quando comparadas com os recobrimentos utilizados comercialmente nas mesmas condições de trabalho do motor.

This thesis explores the feasibility of high power impulse magnetron sputtering (HiPIMS) to synthesize hydrogenated amorphous carbon (a-C:H) thin films in Ar-hydrocarbon ambient and the relationship between process parameters, gas phase composition and film properties. To this purpose a stable process based on HiPIMS and direct current magnetron sputtering (DCMS) has been developed. Four series of amorphous carbon thin films were deposited by hybrid HiPIMS-DCMS and pure DCMS processes at 15 mTorr pressure using different Ar-acetylene compositions and a substrate bias from 0 to -350 V. The effect of Ar-acetylene compositions and depositions processes on the film properties was investigated by characterizing the films using scanning electron microscopy (SEM), x-ray reflectometry (XRR), nanoindentation and elastic recoil detection analysis (ERDA). Moreover the process characterization was done by recording the optical emission spectrum and current and voltage waveforms of the hybrid HiPIMS-DCMS discharge. The characterization of the films revealed that the hybrid HiPIMS-DCMS process is a powerful tool for controlling the amorphous carbon film properties such as density, deposition rate, hardness and hydrogen content.

In this study we contribute towards establishing the process-microstructure relationships in thin films grown off-normally by ionized physical vapor deposition. High power impulse magnetron sputtering (HiPIMS) is used at various peak target powers and deposition rates to grow copper (Cu) and chromium (Cr) films from a cathode placed at an angle 90 degrees with respect to the substrate normal. Films are also deposited by direct current magnetron sputtering (DCMS), for reference. Scanning electron microscopy is employed to investigate column tilting and deposition rate while X-ray diffraction techniques are utilized to study crystal structure and grain tilting. It is demonstrated that the columnar structure of Cu tilts less with respect to the substrate normal as the peak target power increases, which has been shown to correspond to a higher ionization degree of the sputtered material [1]. One explanation for this is that the trajectories of the ions are deflected towards the substrate and therefore deposited closer to the normal, as has been suggested in the literature (see e.g. [2]). Energetic bombardment by ions might also increase surface mobility, which further raises the columns. It is also concluded that the change in tilting is not caused by a lower deposition rate obtained when employing HiPIMS. The same is not seen for Cr, where all deposited films exhibit the same tilting angle. When the column tilting of Cu and Cr is compared a large difference is observed, where the columns of Cr are closer to the substrate normal. The reasons for this difference are discussed in light of nucleation and growth characteristics in the two materials. X-ray diffraction analysis reveals that Cu films exhibit an (111) fiber texture. Comparison of films grown by DCMS and HiPIMS shows that in the HiPIMS cases the grains are closer to the surface normal and better oriented with each other. In the case of Cr both DCMS and HiPIMS grown films are (110) biaxially aligned.

O PVD (Physical Vapor Deposition- Deposição física na fase de vapor) é um meio utilizado para a produção de recobrimentos e empregado em grande escala industrial. É um processo de deposição atômica no qual o material é vaporizado de alvo sólido por sputtering e posteriormente condensado sobre a peça a ser revestida na forma de filme. O processo ocorre em uma câmara de vácuo, na presença de plasma, e por diferença de potencial os íons, na forma pura ou combinados com átomos de hidrogênio ou carbono, são movidos para a superfície do substrato. Uma técnica relativamente nova de sputtering é o HiPIMS (High Power Impulse Magnetron Sputtering) que utiliza impulsos de energia extremamente altas com densidade de potência possibilitando filmes com melhores performances e mais densos. No presente trabalho filmes de nitreto de cromo (CrN) foram depositados por duas técnicas de magnetron sputtering, HiPIMS e DCMS (Direct Current Magnetron Sputtering), variando frequência de pulso em 400 Hz, 450 Hz e 500 Hz para o HiPIMS e a tensão de polarização em 0 V, -20 V, -40V, -60V, - 100 V e -140 V para os dois processos. Foram obtidos filmes com maior dureza, menor rugosidade para HiPIMS, no entanto DCMS apresentou maior taxa de deposição. O aumento da frequência nos filmes HiPIMS, assim como o aumento da tensão de polarização negativa possibilitaram filmes com morfologia mais densa e homogênea. Este fato também foi observado com o aumento do valor de bias nos filmes depositados por DCMS. Os valores de dureza obtidos (17 ± 2 para DCMS e 26 ± 1 para HiPIMS) são superiores aos reportados na literatura e podem estar relacionados ao efeito de \"multicamadas\" obtido pela oscilação do substrato. / PVD (Physical Vapor Deposition) is a process used for coatings deposition and it is used on a large industrial scale. It is an atomic deposition process in which the material is vaporized from solid target by sputtering and then condensed onto the part to be coated in film form. The process occurs in a vacuum chamber in the presence of plasma, and by potential difference the ions in pure form or combined with hydrogen or carbon atoms are moved to the surface of the substrate. A relatively new sputtering technique is the HiPIMS (High Power Impulse Magnetron Sputtering) which uses extremely high energy pulses with power density to enable higher performance and denser films. In the present work, chromium nitride (CrN) films were deposited by two magnetron sputtering techniques, HiPIMS and DCMS (Direct Current Magnetron Sputtering), varying the pulse frequency at 400 Hz, 450 Hz and 500 Hz for the HiPIMS and the bias at 0 V, -20 V, -40 V, -60 V, -100 V and -140 V for both processes. It was obtained films with high hardness, less roughness for HiPIMS, however DCMS presented a higher rate of deposition. The increase of the frequency in the HiPIMS films, as well as the increase of the negative polarization voltage, allowed films with denser and homogeneous morphology. This fact was also observed with the increase of the value in the films deposited by DCMS. The hardness values obtained (17 ± 2 for DCMS and 26 ± 1 for HiPIMS) were higher than those reported in the literature and may be related to the \"multilayer\" effect obtained by substrate oscillation.

Les oxydes conducteurs transparents (TCO) sont une classe importante de matériaux possédant de nombreux domaines d’application, telles que les revêtements à faible émissivité ou comme électrodes transparentes pour les panneaux photovoltaïques et écrans plats. Parmi les matériaux TCO possibles, le ZnO dopé à l'Al (AZO) est couramment étudié notamment à cause de son coût relativement faible et de l'abondance en matières premières des éléments qui le compose. Les films minces d'AZO sont généralement produits par l’intermédiaire de procédés de dépôts physiques en phase vapeur, tels que la pulvérisation cathodique magnétron. Cependant, l’une des limitations de ces techniques repose sur l’homogénéité des films en utilisant la pulvérisation magnétron réactive (DCMS). Ce problème d'homogénéité peut être lié au bombardement du film en croissance par des ions négatifs d'oxygène, lesquels peuvent induire la présence de défauts accepteurs supplémentaires et la formation de phases secondaires isolantes. Dans ce travail, les films d'AZO sont déposés par pulvérisation cathodique magnétron en régime d’impulsions haute puissance (HiPIMS), un procédé dans laquelle des densités de courant instantanées élevées sont obtenues par l’intermédiaire de courtes impulsions de faible rapport cyclique. Dans la première partie de cette thèse, la possibilité d'améliorer l'homogénéité des films AZO par HiPIMS est démontrée. Cette amélioration peut être liée à un important taux de pulvérisation instantanée pendant chaque impulsion HiPIMS, de sorte que le processus puisse avoir lieu en régime métallique. Ceci permet de réduire l’impact du bombardement en ions d'oxygène pendant la croissance du film, évitant ainsi la formation de phases secondaires. Un autre problème de l'AZO est la stabilité des propriétés intrinsèques dans des environnements humides. Pour évaluer ce problème, la dégradation des propriétés électriques après une procédure de vieillissement a été étudiée pour des films déposés à la fois par DCMS et HiPIMS. Une méthode a été proposée pour restaurer les propriétés des films qui consiste en un recuit à basse température sous atmosphère de N2. L'amélioration des propriétés électriques des films peut être liée à un processus de diffusion dans lequel l’eau est évacuée hors des films. Ensuite, l'influence de la température du substrat sur les propriétés des films d'AZO déposés par HiPIMS a été étudiée. Les propriétés électriques, optiques et structurelles ont été améliorées avec l'augmentation de la température du substrat jusqu'à 600°C. Cette amélioration peut s’expliquer par l'augmentation de la qualité cristalline et le recuit des défauts. Dans une dernière partie, le dépôt de films d'AZO sur des substrats flexibles de PET a été étudié. Les films se développent sous la forme d'une couche épaisse, poreuse et composés de colonnes orientées de manière préférentielle selon l’axe c au-dessus d’une fine couche de départ. L’analyse de la résistance électrique après déformation mécanique a permis de déterminer une relation de proportionnalité inverse ou la résistance électrique augmente avec la diminution du rayon de courbure, cette dernière étant moins prononcée pour des films plus épais / Transparent conducting oxides (TCOs) are an important class of materials with many applications such as low emissivity coatings, or transparent electrodes for photovoltaics and flat panel displays. Among the possible TCO materials, Al-doped ZnO (AZO) is studied due to its relatively low cost and abundance of the raw materials. Thin films of AZO are commonly produced using physical vapour deposition techniques such as magnetron sputtering. However, there is a problem with the homogeneity of the films using reactive direct current magnetron sputtering (DCMS). This homogeneity problem can be related to the bombardment of the growing film with negative oxygen ions, that can cause additional acceptor defects and the formation of insulating secondary phases. In this work AZO films are deposited by high power impulse magnetron sputtering (HiPIMS), a technique in which high instantaneous current densities are achieved by short pulses of low duty cycle. In the first part of this thesis, the possibility to improve the homogeneity of the deposited AZO films by using HiPIMS is demonstrated. This improvement can be related to the high instantaneous sputtering rate during the HiPIMS pulses, so the process can take place in the metal mode. This allows for a lower oxygen ion bombardment of the growing film, which can help to avoid the formation of secondary phases. Another problem of AZO is the stability of the properties in humid environments. To assess this problem, the degradation of the electrical properties after an aging procedure was investigated for films deposited by both DCMS and by HiPIMS. A method was proposed, to restore the properties of the films, using a low temperature annealing under N2 atmosphere. The improvement of the electrical properties of the films could be related to a diffusion process, where water is diffusing out of the films. Then, the influence of the substrate temperature on the properties of AZO films deposited by HiPIMS was studied. The electrical, optical and structural properties were found to improve with increasing substrate temperature up to 600°C. This improvement can be mostly explained by the increase in crystalline quality and the annealing of defects. Finally, the deposition of AZO films on flexible PET substrates was investigated. The films are growing as a thick porous layer of preferentially c-axis oriented columns on top of a thin dense seed layer. The evolution of the sheet resistance of the films after bending the films with different radii was studied. There is an increase in the sheet resistance of the films with decreasing bending radius, that is less pronounced for thicker films

O PVD (Physical Vapor Deposition- Deposição física na fase de vapor) é um meio utilizado para a produção de recobrimentos e empregado em grande escala industrial. É um processo de deposição atômica no qual o material é vaporizado de alvo sólido por sputtering e posteriormente condensado sobre a peça a ser revestida na forma de filme. O processo ocorre em uma câmara de vácuo, na presença de plasma, e por diferença de potencial os íons, na forma pura ou combinados com átomos de hidrogênio ou carbono, são movidos para a superfície do substrato. Uma técnica relativamente nova de sputtering é o HiPIMS (High Power Impulse Magnetron Sputtering) que utiliza impulsos de energia extremamente altas com densidade de potência possibilitando filmes com melhores performances e mais densos. No presente trabalho filmes de nitreto de cromo (CrN) foram depositados por duas técnicas de magnetron sputtering, HiPIMS e DCMS (Direct Current Magnetron Sputtering), variando frequência de pulso em 400 Hz, 450 Hz e 500 Hz para o HiPIMS e a tensão de polarização em 0 V, -20 V, -40V, -60V, - 100 V e -140 V para os dois processos. Foram obtidos filmes com maior dureza, menor rugosidade para HiPIMS, no entanto DCMS apresentou maior taxa de deposição. O aumento da frequência nos filmes HiPIMS, assim como o aumento da tensão de polarização negativa possibilitaram filmes com morfologia mais densa e homogênea. Este fato também foi observado com o aumento do valor de bias nos filmes depositados por DCMS. Os valores de dureza obtidos (17 ± 2 para DCMS e 26 ± 1 para HiPIMS) são superiores aos reportados na literatura e podem estar relacionados ao efeito de \"multicamadas\" obtido pela oscilação do substrato. / PVD (Physical Vapor Deposition) is a process used for coatings deposition and it is used on a large industrial scale. It is an atomic deposition process in which the material is vaporized from solid target by sputtering and then condensed onto the part to be coated in film form. The process occurs in a vacuum chamber in the presence of plasma, and by potential difference the ions in pure form or combined with hydrogen or carbon atoms are moved to the surface of the substrate. A relatively new sputtering technique is the HiPIMS (High Power Impulse Magnetron Sputtering) which uses extremely high energy pulses with power density to enable higher performance and denser films. In the present work, chromium nitride (CrN) films were deposited by two magnetron sputtering techniques, HiPIMS and DCMS (Direct Current Magnetron Sputtering), varying the pulse frequency at 400 Hz, 450 Hz and 500 Hz for the HiPIMS and the bias at 0 V, -20 V, -40 V, -60 V, -100 V and -140 V for both processes. It was obtained films with high hardness, less roughness for HiPIMS, however DCMS presented a higher rate of deposition. The increase of the frequency in the HiPIMS films, as well as the increase of the negative polarization voltage, allowed films with denser and homogeneous morphology. This fact was also observed with the increase of the value in the films deposited by DCMS. The hardness values obtained (17 ± 2 for DCMS and 26 ± 1 for HiPIMS) were higher than those reported in the literature and may be related to the \"multilayer\" effect obtained by substrate oscillation.

Dans le cadre des recherches menées sur l'amélioration de la résistance à l'oxydation des gaines de combustible en conditions accidentelles, des revêtements Cr-Al-C et Cr2AlC ont été développés dans ce travail. Dans la première partie, nous avons étudié le procédé HiPIMS afin de comprendre l'effet de différents paramètres de dépôt sur le plasma et les propriétés des films obtenus. Il en ressort que malgré un bombardement ionique plus intense, un apport supplémentaire d'énergie est requis pour obtenir un revêtement cristallin. Des recuits à partir de 500°C sous argon de systèmes Cr-Al-C tels que déposés permettent ainsi une cristallisation partielle des revêtements en Cr2AlC à une température suffisamment basse pour être compatible avec la métallurgie des alliages de zirconium. Dans un second temps, l'évaluation du comportement à haute température de ces deux types de revêtements, recuits ou non, a révélé un effet protecteur contre l'oxydation rapide du zirconium jusqu'à 1200°C en atmosphère oxydante grâce à la formation d'une couche d'oxyde continue. Cette couche est constituée d'un mélange d'alumine α et de chromine pour le revêtement de Cr-Al-C tandis que seule l'alumine α est présente pour le revêtement Cr2AlC dans les premiers instants de l'oxydation. Ensuite, en raison de l'appauvrissement en Al, les revêtements se dégradent en formant une couche intermédiaire résiduelle de carbure Cr7C3 servant de réservoir de Cr jusqu'à complète oxydation. Ces résultats ont également montré la perte d'une partie du réservoir d'Al par diffusion dans les alliages Zr. Une architecture multicouche a été développée pour limiter cette diffusion et ainsi prolonger la durée de vie du revêtement. L'ajout d'un intercalaire en Mo pour bloquer la diffusion d'Al dans le substrat s'est avéré peu concluant, le molybdène s'évaporant à haute température. Les systèmes base Cr-Al-C revêtus chrome, présentent quant à eux, un comportement amélioré par rapport aux revêtements monocouches. / This study aims to improve oxidation resistance of nuclear fuel claddings in accident conditions. In this context, Cr-Al-C and Cr2AlC coatings deposition and their behavior were studied. Firstly, we investigated the influence of HiPIMS process parameters on the properties of the plasma and the deposited films. Despite more intense ionic fluxes due to the HiPIMS process, coatings do not crystallize without an additional energy supply. Partially crystallized Cr2AlC thin films were obtained by a 500°C annealing of as-deposited Cr-Al-C coatings. This two-step process is a viable solution to protect nuclear claddings with Cr2AlC coating while maintaining the metallurgical properties of the zirconium-based substrates. Secondly, the assessment of the oxidation resistance of as-deposited and annealed coatings revealed significant protective effect against rapid oxidation under dry and wet air at high temperatures (up to 1200°C) owing to the formation of a continuous oxide layer. During the first stages of oxidation, this layer is made of α Al2O3 and Cr2O3 for as-deposited coating while only α-Al2O3 is present for the annealed one. Because of Al depletion, coatings later deteriorate and form a residual and porous intermediate chromium carbide (Cr7C3) layer which further fully oxidizes. It was shown that the inward diffusion of Al with Zr also accelerates the coating deterioration. To improve the oxidation resistance of these coatings, multilayered architectures were developed. Adding a molybdenum interlayer as diffusion barrier globally decreased the oxidation resistance of the coating. In contrast, topping Cr-Al-C and Cr2AlC with a Cr layer improved oxidation behavior over single-layer coatings.

Thins films play a critical role in many technical fields today, such as optics, semiconductor, and tool manufacturing. A widely used method for thin film deposition is magnetron sputtering, with Direct-Current Magnetron Sputtering (DCMS) being the most conventional approach. DCMS is very capable at depositing pure metallic (e.g. Al, Ti) or non-metallic (e.g. C, B) films, but encounters problems when depositing compound films, such as oxides or nitrides, in a process known as reactive DCMS (R-DCMS). The process tends to drift into either metallic mode, characterized by high deposition rate but lacking compound formation, or poisoned mode, where compound forms, but with significantly lower deposition rate. Ideally, the process should be maintained in the transition region in-between the two, but this mode is very unstable and further complicated by a hysteresis effect when switching between metallic and poisoned modes. An alternative approach is High-Power Impulse Magnetron Sputtering (HiPIMS) which has been shown to mitigate or remove the instability and hysteresis seen in R-DCMS, as well as improving film density, adhesion and film uniformity on complex geometries. In many cases however, feedback control is needed to stabilize the transition region in reactive HiPIMS (R-HiPIMS). This thesis presents a novel, non-invasive control method for R-HiPIMS using Plasma Emission Monitoring (PEM) to automatically adjust the HiPIMS pulse repetition rate to maintain the delicate transition region. The method was tested on growth of titanium oxide (TiOx) using an industrially relevant deposition system. It was bench-marked against deposition in poisoned mode, and against a control method based on peak discharge current. Results are promising, with higher deposition rate than poisoned mode deposition, and better process stability than peak current control. Challenges remain in making the controller more robust to changes in plasma characteristics due to moving parts, in this case a moving substrate carrier.

<p>In this study, six dc Magnetron Sputtered (dcMS) CrN hard coatings were deposited on pretreated High Speed Steel (HSS) to achieve different interface architectures. The aim was to correlate the interfacial microstructure to the adhesion of the coatings. The substrates were pretreatment using the Ionized Physical Vapor Deposition (IPVD) method High Power Impulse Magnetron Sputtering (HIPIMS) using a Cr target in an inert atmosphere varying the substrate bias ($U_b$) between 0 V and 1100 V at ambient temperature as well as at a substrate temperature of 400$^\circ$C. The deposition parameters were chosen to show how kinetically induced diffusion, etching and implantation changes the interface chemistry and structure and to investigate their effect on the adhesion on the film. At elevated temperatures, the diffusion will be thermally driven. Annealing of the deposited samples were, therefore, performed at 900 K in an Ar atmosphere. The films were characterized employing XRD, HR-TEM, A-STEM and by scratch test measurements to see how the the interface microstructure can be correlated to the adhesion of the coating. The study shows that a sputter cleaned substrate surface with well preserved crystal structure of the substrate enhances the adhesion of the coating by promotion of local epitaxial growth. However, annealing was also shown to have a large effect on the adhesion enhancement by allowing for interdiffusion in the interface region and due to promotion of interface strain relaxation. Implantation of target material on the other hand had limited influence on the adhesion compared to the clean oxide free surfaces. The low adhesion improvement when gradually changing the chemical composition at the interface is assumed to stem from that the radiation induced defects and strain diminished the positive effect of this gradient.</p>

In this study, six dc Magnetron Sputtered (dcMS) CrN hard coatings were deposited on pretreated High Speed Steel (HSS) to achieve different interface architectures. The aim was to correlate the interfacial microstructure to the adhesion of the coatings. The substrates were pretreatment using the Ionized Physical Vapor Deposition (IPVD) method High Power Impulse Magnetron Sputtering (HIPIMS) using a Cr target in an inert atmosphere varying the substrate bias ($U_b$) between 0 V and 1100 V at ambient temperature as well as at a substrate temperature of 400$^\circ$C. The deposition parameters were chosen to show how kinetically induced diffusion, etching and implantation changes the interface chemistry and structure and to investigate their effect on the adhesion on the film. At elevated temperatures, the diffusion will be thermally driven. Annealing of the deposited samples were, therefore, performed at 900 K in an Ar atmosphere. The films were characterized employing XRD, HR-TEM, A-STEM and by scratch test measurements to see how the the interface microstructure can be correlated to the adhesion of the coating. The study shows that a sputter cleaned substrate surface with well preserved crystal structure of the substrate enhances the adhesion of the coating by promotion of local epitaxial growth. However, annealing was also shown to have a large effect on the adhesion enhancement by allowing for interdiffusion in the interface region and due to promotion of interface strain relaxation. Implantation of target material on the other hand had limited influence on the adhesion compared to the clean oxide free surfaces. The low adhesion improvement when gradually changing the chemical composition at the interface is assumed to stem from that the radiation induced defects and strain diminished the positive effect of this gradient.