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<p>In recent years, surface engineering techniques have been developed in order to improve the tribological performance in many industrial applications. In sheet metal forming processes, the usage of liquid lubricants can be decreased by using self lubricated tribo surfaces which will result in more environmentally friendly workshops. In the present work two different concepts, i.e. the deposition of thin organic coatings on the steel sheet and PVD coatings on the tool, have been evaluated. The sheet materials investigated include Zn and 55%Al-Zn metal coated steel sheet, which in general are difficult materials to form under dry conditions since they are sticky and thus have a high tendency to adhere to the tool surface. The PVD coatings include CrN, TiN and various DLC coatings. The work comprises tribo testing and post test characterisation using surface analytical techniques in order to evaluate the tribological properties of the tribo surfaces. The tribological tests of different tribo couples were conducted by using modified scratch testing and ball-on-disc testing. From these test results different friction and wear mechanisms have been identified. </p><p>The deposition of thin organic coatings on the steel sheet metal has been found to be promising in order to control the friction and to avoid metal-metal contact resulting in galling. However, it has been found that the tribological characteristics of organic coated steel sheet are strongly influenced by coating chemical composition, the substrate surface topography and the coating thickness distribution. </p><p>The performance of the PVD coatings depends mainly on the chemical composition and topography of the coated surface. By choosing PVD coatings such as diamond like carbon (DLC) low and stable friction coefficients can be obtained in sliding contact against Zn. Surface irregularities such as droplet-like asperities may cause an initial high friction coefficient. However, after a running in process or by polishing the PVD coating low friction coefficients can be obtained resulting in a stable sliding contact. </p><p>The combination of imaging (optical profilometry, LOM, SEM) and chemical analytical techniques (EDS, AES, ToF-SIMS) gave valuable information concerning the friction and wear properties of the tribo surfaces investigated.</p>

In recent years, surface engineering techniques have been developed in order to improve the tribological performance in many industrial applications. In sheet metal forming processes, the usage of liquid lubricants can be decreased by using self lubricated tribo surfaces which will result in more environmentally friendly workshops. In the present work two different concepts, i.e. the deposition of thin organic coatings on the steel sheet and PVD coatings on the tool, have been evaluated. The sheet materials investigated include Zn and 55%Al-Zn metal coated steel sheet, which in general are difficult materials to form under dry conditions since they are sticky and thus have a high tendency to adhere to the tool surface. The PVD coatings include CrN, TiN and various DLC coatings. The work comprises tribo testing and post test characterisation using surface analytical techniques in order to evaluate the tribological properties of the tribo surfaces. The tribological tests of different tribo couples were conducted by using modified scratch testing and ball-on-disc testing. From these test results different friction and wear mechanisms have been identified. The deposition of thin organic coatings on the steel sheet metal has been found to be promising in order to control the friction and to avoid metal-metal contact resulting in galling. However, it has been found that the tribological characteristics of organic coated steel sheet are strongly influenced by coating chemical composition, the substrate surface topography and the coating thickness distribution. The performance of the PVD coatings depends mainly on the chemical composition and topography of the coated surface. By choosing PVD coatings such as diamond like carbon (DLC) low and stable friction coefficients can be obtained in sliding contact against Zn. Surface irregularities such as droplet-like asperities may cause an initial high friction coefficient. However, after a running in process or by polishing the PVD coating low friction coefficients can be obtained resulting in a stable sliding contact. The combination of imaging (optical profilometry, LOM, SEM) and chemical analytical techniques (EDS, AES, ToF-SIMS) gave valuable information concerning the friction and wear properties of the tribo surfaces investigated.

In sheet metal forming (SMF) of materials such as stainless steels there is a major problem with transfer and accumulation of sheet material to the metal forming tool surface. The problem is known as galling; a sort of severe adhesive wear, which results in severe scratching of produced parts. In this thesis, the overall aim was to gain knowledge of the influence of tool steel microstructure on galling initiation under sliding conditions. It was discovered that material transfer and tool steel damage caused by sheet material flow creating wear-induced galling initiation sites occurred in the early stage of galling. The galling resistance was higher for tool steels with higher matrix hardness due to better resistance to tool steel damage. Initial friction and critical contact pressure to galling was influenced by the strength of the sheet material. Material transfer happened at low pressures and the friction value was high in a case of sheet materials with lower proof strength, possibly due to the sheet contact against the tool steel matrix resulting in high adhesion and quicker tool damage. It was demonstrated that, in addition to hardness of the tool steel matrix and sheet material proof strength, tool steel microstructural features like size, shape, distribution and height of hard phases are important parameters influencing galling. Tool steels comprising homogeneously distributed, small and high hard phases better prevented the contact between sheet material and the tool steel matrix. Thus, a metal to metal contact with high friction was more efficiently avoided, which resulted in better tool performance.

Le grippage dans le cas des aciers inoxydables est un phénomène complexe nécessitant une approche multi-échelle et multi-physique. Les 6 nuances d’aciers inoxydables sélectionnées au cours de cette thèse (316L, 316LN, Nitronic60, AISI660, Uranus45N et 17-4PH) ont été étudiées à l’état de réception et après traitements thermochimique de surface de type S3P. Ces matériaux ont subi un essai de grippage selon la norme ASTM G98 et les échantillons qui en résultent ont été caractérisés selon une approche d’abord surfacique (évolution de la morphologie de la surface, observations MEB…) puis en volume, notamment par le biais de l’approche originale de la microstructure (EBSD, DRX et MET notamment). Cette thèse a ainsi permis d’étudier les différents mécanismes intervenant lors du grippage des aciers inoxydables ainsi que d’étudier l’impact de différentes microstructures et composition chimiques sur ce phénomène. / Galling is a complex phenomenon requiring a multi-scale and multi-physical approach. During this thesis, 6 stainless steel grades (316L, 316LN, Nitronic60, AIS1660, Uranus45N et 17-4PH) are studied, both untreated and after S3P thermochemical heat surface treatment. These materials are tested according to ASTM G98 galling test and resulting samples are then characterized. This characterization is at first realized on the surface (surface morphology evolution, SEM investigations...), then on the bulk material, consisting mainly into a microstructural characterization based on EBSD, XRD and TEM investigations. It results from this thesis a better comprehension of the galling mechanisms in the case of stainless steel. The various effects of material composition, microstructure and material properties in general are also discussed.

La mise en forme à froid des alliages d’aluminium peut être limitée par les transferts de matière sur la surface des outillages pouvant mener au grippage. L’objectif de ce travail de thèse est de contribuer à une meilleure compréhension de ces mécanismes pour permettre à terme d’optimiser les procédés de formage d’un alliage Al-Mg-Si. Une approche couplée expérimentale et numérique est développée afin d’alimenter une base de données d’indicateurs de dégradations des surfaces. L’approche expérimentale développée au cours de ces travaux repose sur l’utilisation de l’essai de compression-translation. Ce dispositif permet de reproduire une large gamme de conditions de contact rencontrées en mise en forme à froid. Une méthode d’analyse des surfaces reposant sur des mesures par profilométrie optique et des analyses au microscope électronique à balayage est développée afin d’évaluer le volume de matière adhérée sur les outils de chaque configuration d’essai. Le comportement rhéologique de l’alliage Al-Mg-Si est caractérisé à l’aide du simulateur thermomécanique GLEEBLE 3500 sur la gamme de températures de 298 à 423 K et à deux vitesses de déformation de 0,1 et 1 s-1. Un modèle de comportement physique reposant sur l’analyse du taux d’écrouissage et l’utilisation du modèle de la contrainte mécanique seuil est développé puis implémenté dans un logiciel éléments finis. Un modèle numérique de l’essai de caractérisation tribologique est développé. Il s’agit d’une analyse thermomécanique utilisant la formulation arbitrairement lagrangienne-eulérienne. Les données locales telles que la pression de contact et la vitesse de glissement calculées par ce modèle permettent de mieux comprendre les mécanismes de collage observés expérimentalement. Enfin, un programme de traitement des résultats numériques est développé afin de réaliser les modifications géométriques de la surface de l’outil en fonction du dépôt relevé expérimentalement. Un modèle d’usure basé sur le travail des forces de frottement est identifié. Ce dernier est un bon indicateur de la tendance au collage et permet une première approximation de ce mécanisme. / Cold forming of aluminium alloys can be limited by a severe material transfer to the die surfaces, compromising the process viability. The purpose of this research work is to contribute to a broader understanding of the galling mechanisms which would further allow the optimisation of the forming processes of an Al-Mg-Si alloy. A methodology combining experimental and numerical approaches is introduced in order to build up a database of surface degradations indicators. The experimental approach is carried out by means of upsetting-sliding tests which reproduce a wide range of contact conditions experienced at the tool/workpiece interface during cold forming operations. To evaluate the amounts of adhered material on the tools of each test configuration, surface topography acquisitions are performed by optical profilometry and coupled to scanning electron microscope analyses. Axisymmetric compression tests are carried out by means of the GLEEBLE 3500 thermomechanical simulator to determine the deformation behaviour of the Al-Mg-Si alloy at high strains, in the temperature range of 298 to 423 K and strain rates of 0.1 and 1 s¡1. A constitutive model based on both the Mechanical Threshold Stress model and the analysis of the work-hardening rate is proposed and then implemented in a finite element code. A finite element simulation of the upsetting-sliding test, which is a thermomechanical analysis using the arbitrary lagrangian-eulerian formulation, is introduced. The local contact variables such as the contact pressure and the sliding velocity are evaluated through this simulation and allow a better understanding of the galling mechanisms observed experimentally. Finally, a post-processing program, which analyses the results of the finite element simulation and updates the tool shape according to the amount of adhered material determined experimentally, is developed. A wear model based on the friction work is introduced. This model is a good indicator of the galling tendency and allows a first approximation of this mechanism.

Over the last decade, the application of ultra-high strength steel as safety components and structural reinforcements in automobile applications has increased due to their favourable high-strength-to-weight ratio. The complex shaped components are widely produced using hot stamping. However, this process encounters problems such as galling and increased wear of the tools due to harsh operating conditions associated to the elevated temperatures. Moreover, quenching is a critical step that affects the hot formed components. Slow cooling rates results in inhomogeneous mechanical properties and increased cycle time. Therefore, fast and homogeneous quenching of the formed components in combination with reduction of wear rates during hot forming are important targets to ensure the quality and efficiency of the process. The use of additive manufacturing (AM) technologies opens up potential solutions for novel tooling concepts. The manufacturing of complex shape cooling channels and integration of high-performance alloys at the surface could benefit the tribological performance in the forming operation. However, the research into high temperature tribological behaviour of AM materials in hot forming applications is very limited. The aim of this work is to study the tribological performance of additively manufactured materials. Two steels were used – a maraging steel and modified H13 tool steel. The hot work tool steel H13 is commonly applied for dies in metal forming processes. In this thesis it was used to study additive manufacturing as the processing route instead of conventional casting. The choice of a maraging steel is motivated by a possible application of high-performance alloys as a top layer on dies. The materials were post-machined and studied in milled, ground and shot-blasted conditions. The different post-machining operations were applied to study the effect of surface finish on the tribological behaviour and also to evaluate different methods of post-machining an AM surface. As fabricated dies are usually manufactured with milled surface. During its use, the dies undergo refurbishment after certain number of cycles and the surface condition is changed to a ground surface. These surface finishes are commonly tested for hot forming applications. The shot blasted operation was chosen as alternative surface finish. The process allows to prepare large sized tools easily and the surface has beneficial compressive stresses. The tribological behaviour of AM steels was studied using a hot strip drawing tribometer during sliding against a conventional Al-Si coated 22MnB5 steel. The workpiece temperature during the tests was 600 and 700°C. The results of the tribological performance of AM materials were compared to conventionally cast tool steel QRO90.The results have shown that the friction behaviour of both maraging and H13 steels at 600°C was stable and similar whereas at 700°C the COF was more unstable and resulted in an earlier failure of the tests due to increased material transfer of Al-Si coating from the workpiece surface.The main wear mechanisms for AM materials were galling and abrasion at both temperatures. Abrasion is more severe for the AM steels in comparison to cast tool steel QRO90. The galling formation on milled and ground surfaces showed similar behaviour to cast steel and it increased with higher workpiece temperatures. The shot-blasted surfaces showed less build-up of transferred material on the surface but folding of asperities and entrapment of Al-Si particles within surface defects generated during shot-blasting.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Interações como a herbivoria há o consumo de partes de tecidos vegetais por insetos herbívoros que promovem impactos no desempenho vegetativo e reprodutivo de plantas hospedeiras. Mecanismos de defesas induzidas como respostas de hipersensibilidade (RH) são observadas em diversas espécies e têm relação direta com a especificidade entre inseto herbívoro e planta hospedeira. No entanto, a capacidade de desenvolver RH varia de acordo com o genótipo e fenótipos de plantas hospedeiras e com distúrbios ambientais como o fogo. Assim, o objetivo geral do nosso estudo foi avaliar o desempenho de Bauhinia brevipes, a capacidade de desenvolver defesas induzidas contra o inseto galhador Schizomyia macrocapillata e as taxas de herbivoria por insetos de vida livre. Avaliamos essas interações no contexto das variações fenotípicas em desenvolver respostas de hipersensibilidade, e de distúrbios ambientais como o fogo. Assim, no primeiro capítulo, avaliamos o desempenho de plantas adultas e da progênie de acordo com os fenótipos de resistência (resistente e susceptível) aos ataques de S. macrocapillata. Para tal, avaliamos o desempenho vegetativo como arquitetura e o desempenho reprodutivo como número de sementes (componente feminino), o número de frutos formados (componente masculino) e a qualidade nutricional (macro e micronutrientes) em indivíduos resistentes e susceptíveis. Além disso, avaliamos o desempenho da prole em três estágios de desenvolvimento: sementes, plântulas e juvenis. Não houve diferenças nos desempenhos vegetativo e reprodutivo das plantas adultas, bem como na relação entre macro e micronutrientes e o fator resistência entre os fenótipos. Além disso, as taxas de germinação e investimentos em porção aérea e subterrânea na prole dos diferentes fenótipos foram similares. Acreditamos que a baixa incidência de S. macrocapillata sobre a população de B. brevipes contribuiu para o desempenho similar entre os fenótipos. Além disso, estratégias como tolerância à herbivoria devem ser analisadas nesse sistema inseto galhador / planta hospedeira. No segundo capítulo, avaliamos como as interações entre herbívoros e plantas hospedeiras, no contexto das defesas induzidas, como RH e defesas mecânicas como o acúmulo de silício em plantas com diferentes respostas ao fogo. Verificamos os indivíduos da população de B. brevipes apresentaram diferentes características quanto à incidência de fogo. As plantas que não rebrotaram apresentaram maiores taxas de herbivoria por insetos mastigadores e menores conteúdos de silício em relação às rebrotas. Por outro lado, as rebrotas foram mais atacadas por S. macrocapillata, mas exibiram maiores quantidades RH que àquelas plantas que não rebrotaram e em relação à estação anterior. Acreditamos que rebrotas de B. brevipes podem ser mais bem defendidas uma vez que alocam mais nutrientes, como nitrogênio para o crescimento e silício que reduz o desempenho de insetos mastigadores. As RH também podem ser aumentadas pela alocação de recursos pós-fogo o que levou às maiores taxas de defesas entre as rebrotas. Este estudo forneceu evidencias sobre o papel do fogo nas interações entre plantas hospedeiras e insetos de diferentes guildas e possíveis alterações no comportamento de insetos mastigadores. / In interactions such as herbivory the consumption of plant tissue by herbivore insects promotes several impacts on vegetative and reproductive performance of host plants. Defense mechanisms induced as hypersensitive response (HR) are observed in several species and is directly related to the specificity of herbivore insects and host plants. However, the ability to develop HR varies according to genotype and phenotype of plant hosts and with environmental disturbances such as fire. Thus, we aimed evaluate the performance of Bauhinia brevipes, the ability to evolve induced defenses against gall insect Schizomyia macrocapillata and herbivory rates by free-living insects. We analyzed these interactions in the context of phenotypic variation of resistance and environmental disturbances as fire. Then, in the first chapter, we evaluated the performance of mature plants and offspring according to the phenotypic resistance (resistant and susceptible) to S. macrocapillata attacks. Thus, we evaluate vegetative performance as architecture and reproductive performance as number of seeds (female component), the number of fruits (male component) and the nutritional quality (macro and micronutrients in leaves) on resistant and susceptible individuals. In addition, we evaluated the performance of the offspring in three stages of development: seeds, seedlings and juveniles. There were no differences in vegetative and reproductive performances of mature plants from resistant and susceptible phenotypes. Besides, there was no relationship between macro and micronutrients and the resistance factor. Germination rates and investments in shoot and root portion in the offspring of different phenotypes were similar. We believe that the low incidence of S. macrocapillata on the population of B. brevipes contributed to the similar performance between phenotypes. Moreover, strategies such as tolerance to herbivory should be analyzed in this gall insect / plant host system. In the second chapter, we evaluate the differences in responses to fire effects in relation to interactions between herbivore insects and host plants in and induced defenses (HR and silicon content). We detected an individual variation in response to fire as resprouting and non-resprouting plants in the population of B. brevipes. Non-resprouting plants showed higher rates of herbivory by chewing insects and lower silicon content in relation to resprouting. Contrasting, resprouting presented greater attack rate by S. macrocapillata, but exhibited larger amounts of HR than non- resprouting. We believe that resprouting of B. brevipes can be better defended, since allocate more nutrients such as nitrogen for growth and silicon which reduces the performance of chewing insects. HR can also be increased by the allocation of resources post-fire, which lead to the greatest defenses rates among resprouting plants. This study provided evidences about the role of fire in the interactions between host plants and insects of different guilds and possible changes in the behavior of chewing insects. / Dissertação (Mestrado)

The main purpose of this project is to investigate different tool steels in terms of their ability to withstand material transfer buildup, so-called galling, occurring in SMF (sheet metal forming) operations. The ability to withstand galling is vital to optimize cost-effectiveness and increase the work tool’s effective operational time. This investigation studies four different tool steels, including a TiN-coating, with the intention of evaluating the microstructures, chemical composition and hardness effect on galling resistance in dry conditions using a slider-on-flatsurface (SOFS) tribo-tester which measures the coefficient of friction during sliding. An OP (optical profilometer) was used to measure the size and geometry of lump growth on the tool and damage on the work sheet. A scanning electron microscope (SEM) was used to identify the interacting tribological mechanisms exhibited at different stages during the slide. The SEM figures confirmed three different types of characteristic patterns exhibited in the tracks after tribo- testing which were categorized as mild adhesive, abrasive and severe adhesive damage. A SEM figure that illustrates a ragged contact surface and an obvious change in the sheet materials plastic behavior is in this report regarded as a sign of severe adhesive contact, the characteristics could possibly be explained by local high temperature and high pressure followed by a sudden pressure drop and creation of hardened welds or solders between the two surfaces which increase the frictional input needed for further advancement. Friction coefficients observed in the initial 100% mild adhesive stage were, μ=0,22-0,26 succeeded by abrasive SEM characteristics often in association with mild adhesive contact and friction values between μ=0,25-0,4 which where sometimes followed by severe adhesive SEM characteristics in 100% of the contact zone with friction values between μ=0,34- 0,9 respectively. The tool material that performed best according to the friction detection criteria was Sv21 closely followed by Sleipner (TiN coated) and Va40 (HRC 63.3). Unfortunately was the friction criteria, a significant raise in friction for defining a sliding length to galling, not adequate for dry conditions due to immediate material transfer succeeded by cyclic changes between partial or 100% abrasive+mild adhesive and severe adhesive contact. The mechanism that change abrasive wear in association with mild adhesive contact, (moderate friction input), to sever adhesive wear, (higher friction input), is dependent on lump shape (lump geometry) and can appear at comparably low speeds 0,04-0,08 [m/s] and low friction energy input (μ=0,34), the magnitude of the change in friction is therefore not always significant and hardly detectable on the friction graph. This was quite unexpected but could be explained by concentration of friction energy rater than the absolute amount. The problem with using friction graphs for galling evaluation was increased even further when a very small lump size and low corresponding rate of material transfer to the tool surface caused a sustainable high raise in friction (μ≈0,3→0,6) on a TiN-coated tool steel called Sleipner. A hardly detectable or similar friction raise for Sv21 and Va40 showed much larger corresponding lump size and rate of material transfer. This means that friction graphs demonstrate a clear problem with quantifying lump size [m3] and rate of  material transfer [m3/s]. Another phenomenon called stick slip behavior, material transfer and lump growth followed by a sudden decrease in lump size and transfer of material back to the work sheet, is also not possible to detect on a friction graph. Because a drop in friction can easily be a change in contact temperature and lump attack angle due to a growing lump and not a decreasing lump.   The conclusion, a friction graph is not suited for galling evaluation and ranking in dry SOFS conditions. A ranking should primarily be based on dimensional OP measurements of the cross section of formed tracks and scratches or preferably by repeated OP measurements of the tool surface during a single test, the last revel the exact lump growth history and true lump growth even in the sliding direction. / civilingenjörsexamen

<p>The main purpose of this project is to investigate different tool steels in terms of their ability to withstand material transfer buildup, so-called galling, occurring in SMF (sheet metal forming) operations. The ability to withstand galling is vital to optimize cost-effectiveness and increase the work tool’s effective operational time. This investigation studies four different tool steels, including a TiN-coating, with the intention of evaluating the microstructures, chemical composition and hardness effect on galling resistance in dry conditions using a slider-on-flatsurface (SOFS) tribo-tester which measures the coefficient of friction during sliding.</p><p>An OP (optical profilometer) was used to measure the size and geometry of lump growth on the tool and damage on the work sheet. A scanning electron microscope (SEM) was used to identify the interacting tribological mechanisms exhibited at different stages during the slide. The SEM figures confirmed three different types of characteristic patterns exhibited in the tracks after tribo- testing which were categorized as mild adhesive, abrasive and severe adhesive damage.</p><p>A SEM figure that illustrates a ragged contact surface and an obvious change in the sheet materials plastic behavior is in this report regarded as a sign of severe adhesive contact, the characteristics could possibly be explained by local high temperature and high pressure followed by a sudden pressure drop and creation of hardened welds or solders between the two surfaces which increase the frictional input needed for further advancement. Friction coefficients observed in the initial 100% mild adhesive stage were, μ=0,22-0,26 succeeded by abrasive SEM characteristics often in association with mild adhesive contact and friction values between μ=0,25-0,4 which where sometimes followed by severe adhesive SEM characteristics in 100% of the contact zone with friction values between μ=0,34- 0,9 respectively. The tool material that performed best according to the friction detection criteria was Sv21 closely followed by Sleipner (TiN coated) and Va40 (HRC 63.3). Unfortunately was the friction criteria, a significant raise in friction for defining a sliding length to galling, not adequate for dry conditions due to immediate material transfer succeeded by cyclic changes between partial or 100% abrasive+mild adhesive and severe adhesive contact. The mechanism that change abrasive wear in association with mild adhesive contact, (moderate friction input), to sever adhesive wear, (higher friction input), is dependent on lump shape (lump geometry) and can appear at comparably low speeds 0,04-0,08 [m/s] and low friction energy input (μ=0,34), the magnitude of the change in friction is therefore not always significant and hardly detectable on the friction graph. This was quite unexpected but could be explained by concentration of friction energy rater than the absolute amount. The problem with using friction graphs for galling evaluation was increased even further when a very small lump size and low corresponding rate of material transfer to the tool surface caused a sustainable high raise in friction (μ≈0,3→0,6) on a TiN-coated tool steel called Sleipner.</p><p>A hardly detectable or similar friction raise for Sv21 and Va40 showed much larger corresponding lump size and rate of material transfer. This means that friction graphs demonstrate a clear problem with quantifying lump size [m3] and rate of  material transfer [m3/s]. Another phenomenon called stick slip behavior, material transfer and lump growth followed by a sudden decrease in lump size and transfer of material back to the work sheet, is also not possible to detect on a friction graph. Because a drop in friction can easily be a change in contact temperature and lump attack angle due to a growing lump and not a decreasing lump.</p><p> </p><p>The conclusion, a friction graph is not suited for galling evaluation and ranking in dry SOFS conditions. A ranking should primarily be based on dimensional OP measurements of the cross section of formed tracks and scratches or preferably by repeated OP measurements of the tool surface during a single test, the last revel the exact lump growth history and true lump growth even in the sliding direction.</p><p> </p> / civilingenjörsexamen