São realizadas análises teóricas e experimentais do atrito nos processos de forjamento a frio, através do ensaio de compressão do anel. Os modelos de atrito de Amonton-Coulomb (μ), atrito interfacial (m) e de Levanov (f) são investigados com auxílio de softwares comerciais de simulação de processos de forjamento. Diferentes condições de lubrificação foram aplicadas para o aço baixa liga 16MnCr5 e a liga de alumínio AA6351 nas condições recozido e encruado. O atrito foi analisado, utilizando os softwares MSC.Superforge e Qform, em função dos resultados obtidos e as condições de lubrificação e estado do material. Valores paramétricos do atrito adquiridos através de correlação teórico-experimental são condizentes com a literatura, indicando a relação do atrito com o material de trabalho, inclusive com seu grau de encruamento. Os modelos de atrito m e f apresentaram maior sensibilidade com relação ao estado do material. / Experimental and theoretical analyses of friction in cold forging process are evaluated by means of the ring compression test. Friction models from Amonton- Coulomb (μ), interfacial friction (m) and Levanov’s model (f) are investigated by aid of numerical simulation software. Different lubricants are applied for low carbon steel alloy 16MnCr5 under annealed and work-hardened conditions. Friction is analyzed by means of lubricant and material conditions using MSC.Superforge and Qform numerical simulation softwares. Acquired frictional parameter values obtained by theoretical-experimental correlation agree with literature, showing friction’s relationship with workpiece material and its hardening level. Friction models m and f presented better sensitivity when concerning the material’s hardening level.

La lubrification eX-Poro-HydroDynamique (XPHD) est un mécanisme de lubrification d’inspiration biomimétique. Il s’agit principalement d’un écoulement dans un milieu poreux, dont la phase solide représentée par des fibres, induit des forces élastiques de compression considérées comme négligeables par rapport aux forces hydrodynamiques générées à l’intérieur du milieu poreux. L’idée essentielle de la lubrification XPHD consiste en remplacement du matériau antifriction et du film de fluide mince, traditionnellement utilisé dans les solutions classiques, par une couche poreuse imbibée d’un fluide qui fournit plus grande capacité de charge. Ce type de lubrification représente une solution technologique totalement nouvelle (en rupture avec la solution classique) qui peut permettre de remplacer les lubrifiants pétroliers, de créer des systèmes tribologiques autolubrifiants et donc plus écologiques et moins coûteux. Ce travail de recherche est donc focalisé sur l’évolution des performances de la lubrification XPHD dans le cadre d’un mouvement tangentiel, adapté à l’étude de butées pour faible et moyenne vitesses de rotation. Dans ce contexte scientifique, une étude approfondie d’un matériau poreux présélectionné (mousse en polyuréthane) fut réalisée avec le but de déterminer les caractéristiques physiques et les paramètres cruciaux pour la lubrification XPHD: la porosité et la perméabilité du matériau poreux. Les modèles théorique et numérique de lubrification XPHD proposés se basent sur l’équation de Darcy-Brinkman et les hypothèses de la lubrification classique, ainsi que l’écoulement à l’intérieur du milieu poreux sont prédit avec une nouvelle forme de l’équation de Reynolds. Le banc d’essais spécialement développé permet d’étudier expérimentalement le mécanisme de lubrification XPHD pour des différents types de dislocateur en combinaison avec des liquides newtoniens et nonnewtoniens. Une description détaillée du banc d’essais et de tous les dispositifs expérimentaux utilisés ainsi que la comparaison des résultats de modélisation et des résultats expérimentaux sont présentés. / The eX-Poro-HydroDynamic (XPHD) lubrication is a new biomimetic inspired lubrication mechanism. It consists of self-sustained fluid films generated within highly compressible porous layers imbibed with liquids, whose solid phase represented by fibers, induces compressive elastic forces considered negligible compared to the hydrodynamic forces generated inside the porous medium. The essential idea of XPHD lubrication is to replace the antifriction material and the thin fluid film, traditionally used for classical sliding motion, with a porous layer imbibed with a fluid that provides a greater load capacity. This type of lubrication represents a completely new technological solution (in rupture with the classical one) that can replace petroleum lubricants, create self-lubricating and therefore more ecological and less expensive tribological systems. This research work is focused on the evolution of XPHD lubrication performances in the context of a tangential movement, adapted to the study of thrust bearing for low and medium rotation speeds. In this scientific context, a thorough study of a preselected porous material (polyurethane foam) was carried out with the aim of determining the physical characteristics and the crucial parameters for XPHD lubrication: the porosity and the permeability of the porous material. The theoretical and numerical models of the XPHD lubrication proposed are based on the Darcy-Brinkman equation and the classical lubrication hypothesis, as well as the flow within the porous media is predicted with a new form of the Reynolds equation. The specially developed test rig is used to investigate experimentally the mechanism of the XPHD lubrication for different types of thrust bearing in combination with Newtonian and non-Newtonian liquids. A detailed description of the test rig and all used experimental devices, as well the comparison between the experimental and numerical results are presented.

Les écoulements biphasiques en milieux poreux sont généralement accompagnés par des phénomènes d'émulsification d'une phase dans l'autre. Les causes peuvent être nombreuses, de la digitation visqueuse aux instabilités purement capillaires. Cette thèse expérimentale a pour objet l'étude d'un mécanisme particulier d'émulsification de l'huile en milieu poreux, ainsi que le transport des gouttes produites dans des milieux confinés. Dans la première partie de cette thèse, l'instabilité gravito-capillaire de Rayleigh-Taylor est revisitée dans un coin formé entre deux plaques de verre centimétriques. La présence d'un gradient de confinement introduit une force capillaire supplémentaire à cette instabilité canonique, susceptible de stabiliser une couche de liquide suspendue au-dessus du vide. Le seuil de stabilité, les longueurs d'onde caractéristiques et les taux de croissance sont bien modélisés par une analyse de stabilité linéaire de l'interface. La caractérisation de cette force capillaire induite par le gradient de confinement nous amène par la suite à l'étude d'une instabilité purement capillaire se produisant lorsqu'un fluide en mouillage très favorable migre vers les régions les plus confinées d'un coin, occupées initialement par un fluide en mouillage moins favorable. Le gradient de confinement introduit alors une force déstabilisante, aboutissant à l'inversion de la position respective des deux phases. Le liquide le moins mouillant est complètement émulsifié et transporté vers les régions les moins confinées sous la forme de gouttelettes. Une analyse de stabilité linéaire de l'interface permet, là encore, de prédire cette sélection de taille. Les taux de croissance mesurés ne sont en revanche pas en accord avec la modélisation, basée sur la loi de Darcy. Leur valeur suggère une localisation de la dissipation visqueuse dans les lignes de contact déplacées durant le développement de l'instabilité, ainsi que dans les films de lubrification également déposés. Ces dynamiques "non-darciennes" nous ont amenés dans une seconde partie de la thèse à l'étude du transport de gouttes d'huile très visqueuses confinées dans de l'eau en mouillage total. Dans cette configuration, la présence de films de lubrification d'eau entre la goutte et le substrat assure la localisation de la dissipation dans les films peu visqueux, favorisant ainsi la mobilité des gouttes. Nous montrons également que la présence de rugosités sur les parois du confinement induit un ralentissement significatif de la vitesse des gouttes, lié à l'amincissement du film de lubrification par ces rugosités. L'interdépendance subtile entre friction visqueuse à l'avant de la goutte et dans son volume est notamment mise en lumière. Dans une dernière partie, nous étudions l'instabilité capillaire se produisant lorsqu'une goutte binaire d'eau et d'alcool est déposée à la surface d'un bain d'huile. L'évaporation majoritaire de l'alcool à la surface de la goutte induit des variations locales de la tension de surface. Des écoulements interfaciaux de Marangoni se produisent, et aboutissent à la déstabilisation spectaculaire de la goutte en étalement. / Biphasic flows in porous media generally lead to the emulsification of one phase into the other. This may be due to several phenomena, such as viscous fingering or pure capillary instabilities. In this experimental thesis, we study a particular emulsifying phenomenon of oil in a model porous medium, as well as the transport of the produced droplets in confined regions. In the first part of the manuscript, the Rayleigh-Taylor instability is revisited in a wedge formed between two centimetric glass plates. The gradient of confinement leads to a capillary force not present in the canonical Rayleigh-Taylor instability. This new force can stabilize liquid layer above air submitted to gravity. The threshold of the instability, the characteristic wavelength and the growth rate are captured by a linear stability analysis of the interface. This characterization of the confinement-induced capillary force drove us to the study of a pure capillary instability occurring when a wetting liquid migrates toward the most confined regions of a wedge, initially filled with a less wetting liquid. The gradient of confinement generates a destabilizing force, leading to the complete inversion of the position of both phases. The less wetting liquid is fully emulsified and the produced droplets are convected towards the less confined regions. A linear stability analysis of the interface here again predicts the characteristic size of the droplets. However, the measured growth rates are not in agreement with the model, based on the Darcy law. This suggests a localization of viscous dissipation in the contact lines displaced during the development of the instability. Another source of viscous dissipation can be in the deposited lubrication films. Those "non-Darcian" dynamics motivated the second part of this thesis, which focuses on the motion of very viscous and non-wetting droplets confined in water. In this configuration, the lubrication film of water between the drop and the substrate ensures the localization of viscous dissipation in those films of low viscosity. This favors the extremely high mobility of the droplets. We also show that wall roughness may induce a thinning of these lubrication films. We shed light on the intricate coupling between viscous friction at the front of the drop and in its bulk. In a last part of this work, we study the capillary instability occurring when a binary droplet of water and alcohol is deposited at the surface of a vegetable oil bath. The dominant evaporation of alcohol at the surface of the drop induces local variations of surface tension. Interfacial Marangoni flows are thus observed, leading to the spectacular destabilization of the spreading droplet

In this thesis, the development of a new type of seal is presented along with the steps taken to attain the conclusive design. The results are presented in this paper as a cross-sectioned CAD-model along with the selection of materials and suggestions for future work. The project was commissioned by the multi-discipline engineering consultancy company Projektengagemang AB in Karlstad, Sweden, and carried out as a master’s thesis at Karlstad University. The commission was due to a demand for a new type of sealing solution, as friction is a problem within the field of micro-pneumatics. The work done consists of a study of different low-friction materials where a 30/70 PTFE/PEEK compound was selected as it displayed promising friction and wear properties. The material was then implemented into a design concluded by employing a product development process. The result rendered a prototype for a new kind of low-friction seal which uses the fluid pressure to achieve a sealing contact load between a piston/rod and the seal, which also was the goal set to achieve at the start of the project. Additionally, a design of experiments study was conducted to settle what design parameters were significant with respect to the contact load. For proceeding, making the concept a product ready for production, additional work is needed in the form of experimental material testing, development of a leakage model to optimize the contact load, determination of temperature and creep behaviour, as well as sufficient field testing. Lastly, a die tool must be designed for manufacturing with injection moulding as well as determining if any further processing is required. / Detta arbete behandlar utvecklingen av en ny sorts tätning samt arbetet som utförts för att nå en slutlig design. Resultaten presenteras i form av en CAD-modell tillsammans med materialval och förslag till fortsatta studier. Projektet utfördes efter förfrågan av konsultconcernen Projektengagemang AB i Karlstad, Sverige och utfördes som ett examensarbete för civilingenjörsexamen i maskinteknik vid Karlstads universitet. Detta gjordes eftersom det finns en efterfrågan på lågfriktionstätningar inom mikropneumatik där friktion är ett rådande problem. Arbetet består av en studie över att antal lågfriktionsmaterial där en komposit med 30/70 PTFE/PEEK valdes på grund av dess lovande nötnings- och friktionsegenskaper. Materialet implementerades sedan i design som togs fram genom en produktutvecklingsprocess. Resultatet är en prototyp för en ny sorts lågfriktionstätning som utnyttjar fluidtryck för att uppnå en tätande effekt mellan tätning och kolv vilket även var målsättningen för projektet. Flerfaktorförsök har under processen utnyttjats för att hitta vilka geometriska parametrar som påverkar kontaktkraften mellan tätning och motliggande yta. För att ta konceptet till en färdig produkt krävs fortsatt arbete innehållande experimentella materialtester, framtagning av en läckagemodell att optimera kontaktkraften mot, studier kring temperatur- och krypberoende samt fälttester för att verifiera funktionen över tid. Slutligen behöver ett verktyg för formsprutning tas fram tillsammans med eventuell ytterligare bearbetning för slutlig tillverkning av produkten.

The question for this work was: How to design and control the guidance of sheet metal molding tool in order to achieve precision over time.The work has been based on the method of product development where the product development is methodically done, individual opinions are eliminated to the greatest possible extent.Several different concepts have been developed, which have since been evaluated in a selection process that has been done by using elimination matrice and criterion weight matrice.A knee joint mechanism has emerged from this selection process. The main advantage of the knee joint mechanism is that it is least affected by the dynamic forces that excite in the molding tools. When the magnitude of the dynamic forces could not be determined, a mechanism emerged that largely eliminates this problem. / Frågeställning för detta arbete var: Hur skall en styrning av ett formverktyg för tunnplåt utformas för att kunna uppnå precision över tid.Arbetet har utgått från metoden om produktframtagning där produktutvecklingen sker metodiskt och individuella åsikter elimineras till största möjliga utsträckning.Flera olika koncept har tagits fram, dessa har sedan har sedan utvärderats i en urvalsprocess som har använts sig av elimineringsmatris samt kriterieviktsmatris. Ur denna urvalsprocess har en knäledsmekanism framkommit. Knäledsmekanismens främsta fördel är att den är minst påverkad av de dynamiska krafterna som exciterar i formverktygen. Då storleken på de dynamiska krafterna inte har kunnat fastställas framkom en mekanism som till stor del eliminerar detta problem.

The use of computational methods in tribology can be a valuable approach to deal with engineering problems, ultimately saving time and resources. In this work, amodel problem and methodology is developed to deal with a common situation found in experiments in tribology, namely a linearly reciprocating block-on-flat drysliding contact. The modelling and simulation of such case would allow a better understanding of the contact pressure distribution, wear and geometry evolutionof the block as it wears out during a test. Initially, the introduction and motivation for this work is presented, followed by a presentation of relevant scientific topics related to this work. Wear modelling of published studies are reviewed next, along with studies available in the literature and the goals for this thesis.The fourth section refers to the methodology used and the built-up of the model problem. In this work the Finite Element Method and Archard’s wear model through COMSOL Multiphysics® and MATLAB® are used to study the proposed contact problem. The construction of the model problem is detailed and the procedure for wear, geometry update and long term predictions, is presented inspired by the literature reviewed. Finally, the results are presented and discussed; wear increment and new geometries evolution are presented in the figures, followed by pressure profile evolution at selected times. The final geometry is also compared for different time steps. At last, conclusions and recommendations for future work are stated.

The main objective for the work described in this PhD thesiswas to formulate a friction model to characterize thefrictional behavior of paper. More specifically, the modelshould explain a phenomenon that is typical for paper grades,viz.: that the level of paper-to-paper friction is dependent onthe direction and the number of previous slides. The modelshould also explain the lubricating effect oflow-molecular-mass lipophilic compounds (LLC) that occur inpaper on paper-to-paper friction. Furthermore, the model shoulddescribe the types of forces that influence paper-to-paperfiction and explain the mechanism by which the LLC decreasepaper-to-paper friction. This thesis consists of a literature review of the basicconcepts of tribology and a summary of the main results andconclusions from four studies on the frictional characteristicsof paper and a study on the friction and adhesion of cellulosesurfaces together with some unpublished material. The purpose of the investigation described in the firstpaper was to explain a phenomenon that is typical for papergrades, viz.: the level of paper-to-paper friction is dependenton the direction and the number of previous slides. Thisbehavior is called“friction hysteresis”by theauthors, and it has its origin in the reorientation of thefibers on the surface of a paper and their alignment relativeto the sliding direction. The second paper describes a study that was aimed atidentifying lubricants that occur natively in paper. Filterpapers were impregnated with model compounds representing woodextractives, i.e. low-molecular-weight lipophilic compounds,which are present in wood, pulp and paper, and thepaper-to-paper friction was determined. The results of thatstudy show that a wood extractive must fulfill severalstructural criteria in order to lubricate a paper surface: Itmust have a hydrophilic group that can attach to the papersurface and a linear hydrocarbon backbone of sufficientlength. Although it is not specifically stated in the second paper,the authors proposed a type of lubrication by which woodextractives decrease paper-to-paper friction that is, ineffect, boundary lubrication. The purpose of the investigationdescribed in the third paper was to clarify whether woodextractives and other low-molecular-mass lipophilic compoundsthat occur in paper can act as boundary lubricants on papersurfaces. The main objective of that study was to investigatethe role of chemical structure of LLC for their orientationrelative to the paper surface, which is an important criterionfor boundary lubrication. Filter papers were impregnated withmagnesium salts of different lipophilic acids, which were usedfor model compounds for the LLC. The deposited layers ofmagnesium salts were characterized by X-ray photoelectronspectroscopy (XPS) and contact angle goniometry and thefriction of the impregnated paper sheets was determined. Theresults show that the degree of lubrication and the resistanceto wear of the layers of a magnesium salt increased withincreasing chain length and increasing degree of linearity ofthe lipophilic acid. Based on the results of that study and ofearlier studies, it is concluded that boundary lubrication isthe type of lubrication by which low-molecular-mass lipophiliccompounds that occur natively in paper decreasepaper-to-paper-friction. In boundary lubrication, surfaces are covered withmonolayers of lubricant molecules that comprise an active headgroup that can attach to the surface, e.g. a carboxyl group,and an inert linear backbone, such as a long saturatedhydrocarbon chain. Such compounds form ordered monolayers onsurfaces, so that the backbone points vertically out of planeof the surface. The friction is then determined by theinteractions between the monolayers, which are weaker than theinteractions between the clean surfaces and this gives a lowerfriction. The fourth paper describes a study on the origin of thedifferences in friction levels between different linerboardsbased on recycled fiber (old corrugated container, OCC). Thesheets were subjected to two extraction stages and analyzedwith respect to surface roughness and their content oflow-molecular-mass lipophilic compounds (LLC). The resultsshowed that a high amount of LLC in the sheets lead to lowfriction, due to lubrication. The fifth paper describes a study that was aimed atdetermining the types of forces that influence the frictionbetween the surfaces of hydrophilic polymers and explaining themechanism by which boundary lubricants decrease the friction.The adhesion and the friction of model systems was measuredwith atomic force microscopy (AFM) using regenerated cellulosefilms and functionalised AFM tips and the effect of fatty acidsand humidity was investigated. The friction significantlyincreased with increasing humidity and that there was a strongcorrelation between the ability of a fatty acid to form ahydrophobic surface and its lubricating performance. Measuredadhesion forces at high humidity were well predicted bytheoretical models that took into account the effect of theLaplace pressure acting in a water meniscus formed aroundcontact regions due to capillary condensation. The resultsindicated that the degree of capillary condensation may beeffectively suppressed by increasing the hydrophobicity of thecontacting surfaces, causing adhesion and friction to decrease.These results suggest that friction between paper surfacesunder ambient conditions is greatly influenced by the degree ofcapillary condensation. Furthermore, lubrication by fatty acidsis achieved by the formation of a vertically oriented,hydrophobic monolayer that can withstand the stresses duringsliding and increase the hydrophobicity of the paper surfaceand thereby suppress capillary condensation. <b>Keywords:</b>Friction, paper-to-paper friction, frictionhysteresis, fibers, orientation, sliding direction, woodextractives, low-molecular-mass lipophilic compounds, boundarylubrication, adhesion, capillary condensation, Laplacepressure, surface forces, JKR theory, gas chromatography-massspectroscopy, X-ray photoelectron spectroscopy, contact angle,atomic force microscopy

Grease-lubricated journal bearings present a common challenge for construction equipment manufacturers in the world. The common design methodology is based on empirical data and has worked very well historically because the market and governments have accepted that bearings in construction equipment need frequent lubrication and exchange of worn parts. Legal and market requirements will soon demand lower environmental impact and increased machine efficiency. These requirements call for better methods of designing grease lubricated journal bearings. The goal of the outlined work was to develop better design methods for grease lubricated journal-bearing design used in heavy-duty construction equipment machines, in order to prolong life and lubrication intervals. The research approach of the project can roughly be divided into three phases: 1. Development of test apparatus and test methods for journal bearing studies. 2. Bench tests of grease lubricated journal bearing design. 3. Verification between bench tests and computer simulations. In the thesis the current state of the art in bearing design for construction equipment is discussed and summarized in the form of design guidelines. The suggested design steps are just a mean to get to the starting point of design. The simple guidelines do however serve a purpose when collected since most published bearing design guidelines are aimed at the bushing material or at continuously rotating bearings. The influence of housing, environment and load cases can not be ignored when designing a bearing. Long term field-testing and experience can not be replaced until better design criteria have been established. Paper A deals with the design of the bearing test apparatus that was built and evaluated. Comparisons between theoretical contact and contact elements in Finite Element program have been made and discussed in paper B. In paper C a replica technique for measuring wear of large field specimens was evaluated. A case study of bearing housing design was performed in paper D utilizing Finite Element program and then validated in paper E in the bearing test apparatus. The influence of grease groove design on bushing life was tested and evaluated in paper F. Wear simulation of a plain bushing has been performed with a Finite Element program and presented in paper G. / QC 20100930

Radial lip seals are used to prevent leakage between machine elements in many industrial applications. During operation, fluid film between seal lip and shaft surface generates a pressure distribution on the lip which is elastically deformed due to hydrodynamic pressure. Surface roughness parameters in terms of moments of height profile distribution (rms roughness, skewness and kurtosis) affect the rate of oil leakage. A computer program is developed for elastohydrodynamic analysis of radial lip seals. Both the fluid mechanics of the lubricating film and the elastic deformation of the lip are taken into consideration to determine the hydrodynamic pressure distribution and the oil flow through the seal lip. The effect of shaft surface roughness on hydrodynamic analysis is taken into account by using average Reynolds equation with flow factors. For non-Gaussian surfaces, the modified flow factors are used to investigate the effects of skewness and kurtosis on the oil leakage. Numerical tests are performed for different skewness, kurtosis and initial seal tightness values. Results show that when a seal is mounted with a high initial tightness, the hydrodynamic pressure developed is not enough to deform the lip to form a fluid film between the shaft and the seal lip. It is observed that for the same rms roughness and skewness, the side flow rate increases as the kurtosis value increases. However, for the same rms roughness and kurtosis values, the side flow rate decreases for all skewness values.

<p>The main objective for the work described in this PhD thesiswas to formulate a friction model to characterize thefrictional behavior of paper. More specifically, the modelshould explain a phenomenon that is typical for paper grades,viz.: that the level of paper-to-paper friction is dependent onthe direction and the number of previous slides. The modelshould also explain the lubricating effect oflow-molecular-mass lipophilic compounds (LLC) that occur inpaper on paper-to-paper friction. Furthermore, the model shoulddescribe the types of forces that influence paper-to-paperfiction and explain the mechanism by which the LLC decreasepaper-to-paper friction.</p><p>This thesis consists of a literature review of the basicconcepts of tribology and a summary of the main results andconclusions from four studies on the frictional characteristicsof paper and a study on the friction and adhesion of cellulosesurfaces together with some unpublished material.</p><p>The purpose of the investigation described in the firstpaper was to explain a phenomenon that is typical for papergrades, viz.: the level of paper-to-paper friction is dependenton the direction and the number of previous slides. Thisbehavior is called“friction hysteresis”by theauthors, and it has its origin in the reorientation of thefibers on the surface of a paper and their alignment relativeto the sliding direction.</p><p>The second paper describes a study that was aimed atidentifying lubricants that occur natively in paper. Filterpapers were impregnated with model compounds representing woodextractives, i.e. low-molecular-weight lipophilic compounds,which are present in wood, pulp and paper, and thepaper-to-paper friction was determined. The results of thatstudy show that a wood extractive must fulfill severalstructural criteria in order to lubricate a paper surface: Itmust have a hydrophilic group that can attach to the papersurface and a linear hydrocarbon backbone of sufficientlength.</p><p>Although it is not specifically stated in the second paper,the authors proposed a type of lubrication by which woodextractives decrease paper-to-paper friction that is, ineffect, boundary lubrication. The purpose of the investigationdescribed in the third paper was to clarify whether woodextractives and other low-molecular-mass lipophilic compoundsthat occur in paper can act as boundary lubricants on papersurfaces. The main objective of that study was to investigatethe role of chemical structure of LLC for their orientationrelative to the paper surface, which is an important criterionfor boundary lubrication. Filter papers were impregnated withmagnesium salts of different lipophilic acids, which were usedfor model compounds for the LLC. The deposited layers ofmagnesium salts were characterized by X-ray photoelectronspectroscopy (XPS) and contact angle goniometry and thefriction of the impregnated paper sheets was determined. Theresults show that the degree of lubrication and the resistanceto wear of the layers of a magnesium salt increased withincreasing chain length and increasing degree of linearity ofthe lipophilic acid. Based on the results of that study and ofearlier studies, it is concluded that boundary lubrication isthe type of lubrication by which low-molecular-mass lipophiliccompounds that occur natively in paper decreasepaper-to-paper-friction.</p><p>In boundary lubrication, surfaces are covered withmonolayers of lubricant molecules that comprise an active headgroup that can attach to the surface, e.g. a carboxyl group,and an inert linear backbone, such as a long saturatedhydrocarbon chain. Such compounds form ordered monolayers onsurfaces, so that the backbone points vertically out of planeof the surface. The friction is then determined by theinteractions between the monolayers, which are weaker than theinteractions between the clean surfaces and this gives a lowerfriction.</p><p>The fourth paper describes a study on the origin of thedifferences in friction levels between different linerboardsbased on recycled fiber (old corrugated container, OCC). Thesheets were subjected to two extraction stages and analyzedwith respect to surface roughness and their content oflow-molecular-mass lipophilic compounds (LLC). The resultsshowed that a high amount of LLC in the sheets lead to lowfriction, due to lubrication.</p><p>The fifth paper describes a study that was aimed atdetermining the types of forces that influence the frictionbetween the surfaces of hydrophilic polymers and explaining themechanism by which boundary lubricants decrease the friction.The adhesion and the friction of model systems was measuredwith atomic force microscopy (AFM) using regenerated cellulosefilms and functionalised AFM tips and the effect of fatty acidsand humidity was investigated. The friction significantlyincreased with increasing humidity and that there was a strongcorrelation between the ability of a fatty acid to form ahydrophobic surface and its lubricating performance. Measuredadhesion forces at high humidity were well predicted bytheoretical models that took into account the effect of theLaplace pressure acting in a water meniscus formed aroundcontact regions due to capillary condensation. The resultsindicated that the degree of capillary condensation may beeffectively suppressed by increasing the hydrophobicity of thecontacting surfaces, causing adhesion and friction to decrease.These results suggest that friction between paper surfacesunder ambient conditions is greatly influenced by the degree ofcapillary condensation. Furthermore, lubrication by fatty acidsis achieved by the formation of a vertically oriented,hydrophobic monolayer that can withstand the stresses duringsliding and increase the hydrophobicity of the paper surfaceand thereby suppress capillary condensation.</p><p><b>Keywords:</b>Friction, paper-to-paper friction, frictionhysteresis, fibers, orientation, sliding direction, woodextractives, low-molecular-mass lipophilic compounds, boundarylubrication, adhesion, capillary condensation, Laplacepressure, surface forces, JKR theory, gas chromatography-massspectroscopy, X-ray photoelectron spectroscopy, contact angle,atomic force microscopy</p>