This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Cavitation is caused by rapid movement of objects in a fluid. The resulting steam bubbles are ent... more Cavitation is caused by rapid movement of objects in a fluid. The resulting steam bubbles are entrained by the flowing fluid and collapse in a sudden manner with increasing static pressure [1]. This results in extreme pressure and temperature peaks, which leads to massive damage and removal of the so-called cavitation erosion on surfaces. This occurs, for example, frequently in rotors of centrifugal pumps, water turbines, as well as in control valves and in motors. Furthermore, cavitation can cause noise, vibration, and loss of energy efficiency. Due to the resulting enormous economic effects, an intensive examination of the cavitation erosion is necessary. Current research focusses on the examination of the synergetic mechanisms of mechanical damage and corrosion [2]. Therefore the aim of this thesis consists in the characterization of these synergetic damage mechanisms and the quantification of static and dynamic corrosion processes during cavitation erosion with different steels ...
To minimize the wear and friction of anti-friction coatings, polyurethane and epoxy resins have b... more To minimize the wear and friction of anti-friction coatings, polyurethane and epoxy resins have been modified and qualified both with graphene for mechanical reinforcement and with PTFE as a friction reducer. The production of the anti-friction coatings was optimized in such a way that a very good adhesion and a very homogeneous distribution of the fillers in the polymer matrix were reached. In tribological tests these coatings were evaluated in terms of friction behavior, abrasion resistance and in comparison with commercial anti-friction coatings. The newly developed model-based anti-friction coating systems showed very good tribological and mechanical properties. Thus, the combination of graphene and PTFE as fillers offers the possibility of a significant improvement in load capacity and service life.
For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus co... more For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus could be used to directly control friction conditions. Therefore, different combinations of lubricants and external triggers for tribological influence have already been investigated. We show that when two metallic friction partners are lubricated with ionic liquid mixtures (ILM), consisting of long-chain cation and two different high charge/mass ratio anion containing ILs, the application of an electric impulse induces a permanent change of the frictional response. Such mixtures are able to alter the coefficient of friction (COF) to a greater extent, more accurately and faster than the respective single-component ILs. This change in the frictional properties is presumably due to changes in the externally induced electrical polarization at the surface, which influences the molecular adsorption, the exchange of adsorbed ions and their molecular orientation. The correlation between surface c...
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Water-based lubricants might become an interesting alternative to conventional oil-based lubrican... more Water-based lubricants might become an interesting alternative to conventional oil-based lubricants and help to reduce wear as well as improve the energy efficiency of transport processes. Since pure water is generally a rather poor lubricant due to its low viscosity and corrosiveness, it must be tribologically optimized with suitable additives. Here, we study the friction behavior of alkyl glucopyranosides (AGPs) with varying lengths of the alkyl chain. Sliding experiments show that a significant reduction in the coefficient of friction compared to that of pure water is observed. The extent of friction reduction depends strongly on the concentration and on the shearing conditions. It is assumed that the low coefficients of friction are due to the ability of AGPs to form liquid crystalline phases with an ordered structure in the friction gap. Furthermore, the interaction of the AGPs with the surface forms a wear protection layer (boundary lubrication). The friction properties of the...
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2019
White etching cracks are one of the possible causes of bearing damage, which can lead to the fail... more White etching cracks are one of the possible causes of bearing damage, which can lead to the failure of wind turbines. The mechanisms of hydrogen diffusion in bearings are not yet fully understood, but it is assumed under tribological loading that the lubricant degradation releases hydrogen which diffuses into the contacting surfaces (i.e. steel). This leads to hydrogen-induced cracking and damage to the bearings. In order to better understand these mechanisms, it is necessary to investigate the influence of tribological loads and lubricant composition on hydrogen release. For this purpose, a test setup was developed for the analysis of tribologically induced hydrogen permeation by means of a ball-on-plate test. The diffused hydrogen was measured with an electrochemical three-electrode setup. The results showed a direct correlation between the amount of hydrogen released and the applied normal load. Furthermore, an increased velocity resulted in enhanced diffusion of hydrogen throug...
A coupled diffusion-mechanical finite element simulation model was developed to study the diffusi... more A coupled diffusion-mechanical finite element simulation model was developed to study the diffusion of hydrogen in a cylindrical roller thrust bearing (CRTB). The simulations enabled obtaining qualitative information pertaining to stress-assisted diffusion in tribological loading and under the influence of residual stresses. The mechanical behavior of bearing steel was obtained from experiments and supported by literature. In parallel, rolling-contact fatigue (RCF) tests on CRTBs lubricated with a fully additivated transmission oil were conducted for 25 h and 50 h to investigate their premature failure modes. Post-RCF rheological analysis indicated decrease of the lubricant viscosity due to degradation. Carrier gas hot extraction analysis indicated a significant increase in hydrogen content in the bearings tested for 50 h. Whereas, serial cross-sectional analysis revealed the formation of subsurface White Etching Crack (WEC) networks associated with White Etching Areas (WEA); the cracks breached the surface on multiple positions causing flaking and eventual failure. On the other hand, no signs of damage were observed in the bearings tested for 25 h. The simulations revealed insignificant hydrogen accumulation due to stress-assisted diffusion in comparison to concentration gradient driven diffusion; hydrogen trapping was pronounced in zones undergoing plastic deformation due to the formation of deformation induced trapping sites. However, residual stresses had an evident influence on the subsurface accumulation of hydrogen. A comparison between the zones of elevated hydrogen concentration due to residual stresses and RCF induced subsurface damage yielded a good correlation.
Abstract Cavitation erosion-corrosion causes damage and removal of material especially in rotors ... more Abstract Cavitation erosion-corrosion causes damage and removal of material especially in rotors of centrifugal pumps, water turbines, as well as in control valves and in motors. Based on a dynamic corrosion model the pure mechanical erosion amount of different steel alloys was identified using an indirect cavitation test method combined with an electrochemical cell. We demonstrate that cavitation erosion can be enormously reduced by application of external cathodic potentials. The cavitation rate was in-situ determined using atomic emission spectrometry during the cavitation test. Due to the separately identified pure mechanical and the combined corrosive and synergistic cavitation erosion-corrosion correlation with hardness and arising charge densities during cathodic potentials were found for the mechanical part.
The friction properties of the water-based surfactant system C8 (octyl ß-D-glucopyranoside) are i... more The friction properties of the water-based surfactant system C8 (octyl ß-D-glucopyranoside) are investigated both at the macro and nanoscale in ring-on-plate and AFM friction experiments, respectively. Surface characterization and measurement of the friction gap during sliding, together with the tribological behavior show a strong shear rate dependence of the friction behavior. High shear rates of approximately 10 6 s-1 in the macroscopic friction experiments induce a molecular alignment of the surfactants in the friction gap. This generates an anisotropic viscosity which allows to carry a high load, but exhibits low viscosity in shear direction. When the nanoscale and macroscale friction experiments are normalized to the same shear rate, almost identical frictional behavior is observed in the two regimes.
Abstract RCF tests were conducted on bearings lubricated with three different lubricants: a gearb... more Abstract RCF tests were conducted on bearings lubricated with three different lubricants: a gearbox oil, a MAC fluid and PFPE oil. The decomposition of the MAC fluid and PFPE was investigated by FTIR and NMR. Gearbox oil tests showed flaking and networks of subsurface cracks with partial intercrystalline crack growth. Cross-sectional analysis revealed the formation of WEAs. FEM correlated the damage with stress fields. The results of the MAC lubricated tests showed surface flaking and intercrystalline crack growth in the subsurface. The PFPE lubricated bearings showed no signs of hydrogen induced damage. Post-experimental hydrogen analysis of the bearings indicated an increased hydrogen concentration in the samples lubricated with the gearbox oil and MAC compared to those lubricated with PFPE.
Advanced ceramic materials have proved their superior wear resistance as well as mechanical and c... more Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si 3 N 4 structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si 3 N 4 /MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si 3 N 4 ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.
Dental materials : official publication of the Academy of Dental Materials, Jan 5, 2016
The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched ... more The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched beam method (CNB) was investigated to assess the feasibility of this method for quality assurance and material characterization. CNB tests were performed using four different yttria-stabilized zirconia ceramics under various testing modes and conditions, including displacement-controlled and load-rate-controlled four point bending to assess the influence of slow crack growth and identify most suitable test parameters. For comparison, tests using single-edge V-notch beams (SEVNB) were conducted. It was observed that the CNB method yields well-reproducible results. However, slow crack growth effects significantly affect the measured KIC values, especially when slow loading rates are used. To minimize the effect of slow crack growth, the application of high loading rates is recommended. Despite a certain effort needed for setting up a sample preparation routine, the CNB method is considered...
Abstract This study focuses on the damage mechanisms in silicon nitride rolling elements used in ... more Abstract This study focuses on the damage mechanisms in silicon nitride rolling elements used in hybrid (ceramic–metal) bearings that operate under non-conformal contact. To get an insight into the prevailing damage modes compared to the real application, a rolling contact experiment was designed to mimic the contact conditions. Hertzian contact pressures ranged from 3.0 to 5.9 GPa (500–4150 N). In order to approach pure rolling, the experiments were run without inducing any gross slip. Extensive surface and subsurface damage analysis was performed using conventional ceramography as well as FIB cross-sectioning. Finite element simulations were carried out to illustrate the stress state prevailing under different loading conditions. Surface damage to rollers subjected to contact pressures up to 5.1 GPa (2500 N) was mainly dominated by micro-spalling, which was induced due to the presence of snowflake structures. At the highest applied loads, damage appeared as a combination of macro-cracking and micro-spalling. Crack propagation was attributed to different mechanisms: (a) fatigue-induced fracture and (b) lubricant-driven crack propagation in the subsurface.
Modeling the damage of brittle materials is of great importance considering a variety of structur... more Modeling the damage of brittle materials is of great importance considering a variety of structural components. Prominent examples are high strength engineering ceramics. The present work is concerned with silicon nitride, a material with increasing relevance in industrial applications. In the sense of a hierarchical model structure, effective properties of micromechanical simulations were applied to macroscopic, phenomenological damage models for monotonous and cyclic loading. In the following, both models are introduced and the application of the cyclic damage model to a four point bending test is discussed.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
Cavitation is caused by rapid movement of objects in a fluid. The resulting steam bubbles are ent... more Cavitation is caused by rapid movement of objects in a fluid. The resulting steam bubbles are entrained by the flowing fluid and collapse in a sudden manner with increasing static pressure [1]. This results in extreme pressure and temperature peaks, which leads to massive damage and removal of the so-called cavitation erosion on surfaces. This occurs, for example, frequently in rotors of centrifugal pumps, water turbines, as well as in control valves and in motors. Furthermore, cavitation can cause noise, vibration, and loss of energy efficiency. Due to the resulting enormous economic effects, an intensive examination of the cavitation erosion is necessary. Current research focusses on the examination of the synergetic mechanisms of mechanical damage and corrosion [2]. Therefore the aim of this thesis consists in the characterization of these synergetic damage mechanisms and the quantification of static and dynamic corrosion processes during cavitation erosion with different steels ...
To minimize the wear and friction of anti-friction coatings, polyurethane and epoxy resins have b... more To minimize the wear and friction of anti-friction coatings, polyurethane and epoxy resins have been modified and qualified both with graphene for mechanical reinforcement and with PTFE as a friction reducer. The production of the anti-friction coatings was optimized in such a way that a very good adhesion and a very homogeneous distribution of the fillers in the polymer matrix were reached. In tribological tests these coatings were evaluated in terms of friction behavior, abrasion resistance and in comparison with commercial anti-friction coatings. The newly developed model-based anti-friction coating systems showed very good tribological and mechanical properties. Thus, the combination of graphene and PTFE as fillers offers the possibility of a significant improvement in load capacity and service life.
For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus co... more For mechanical systems in relative motion it would be fascinating if a non-mechanical stimulus could be used to directly control friction conditions. Therefore, different combinations of lubricants and external triggers for tribological influence have already been investigated. We show that when two metallic friction partners are lubricated with ionic liquid mixtures (ILM), consisting of long-chain cation and two different high charge/mass ratio anion containing ILs, the application of an electric impulse induces a permanent change of the frictional response. Such mixtures are able to alter the coefficient of friction (COF) to a greater extent, more accurately and faster than the respective single-component ILs. This change in the frictional properties is presumably due to changes in the externally induced electrical polarization at the surface, which influences the molecular adsorption, the exchange of adsorbed ions and their molecular orientation. The correlation between surface c...
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Water-based lubricants might become an interesting alternative to conventional oil-based lubrican... more Water-based lubricants might become an interesting alternative to conventional oil-based lubricants and help to reduce wear as well as improve the energy efficiency of transport processes. Since pure water is generally a rather poor lubricant due to its low viscosity and corrosiveness, it must be tribologically optimized with suitable additives. Here, we study the friction behavior of alkyl glucopyranosides (AGPs) with varying lengths of the alkyl chain. Sliding experiments show that a significant reduction in the coefficient of friction compared to that of pure water is observed. The extent of friction reduction depends strongly on the concentration and on the shearing conditions. It is assumed that the low coefficients of friction are due to the ability of AGPs to form liquid crystalline phases with an ordered structure in the friction gap. Furthermore, the interaction of the AGPs with the surface forms a wear protection layer (boundary lubrication). The friction properties of the...
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, 2019
White etching cracks are one of the possible causes of bearing damage, which can lead to the fail... more White etching cracks are one of the possible causes of bearing damage, which can lead to the failure of wind turbines. The mechanisms of hydrogen diffusion in bearings are not yet fully understood, but it is assumed under tribological loading that the lubricant degradation releases hydrogen which diffuses into the contacting surfaces (i.e. steel). This leads to hydrogen-induced cracking and damage to the bearings. In order to better understand these mechanisms, it is necessary to investigate the influence of tribological loads and lubricant composition on hydrogen release. For this purpose, a test setup was developed for the analysis of tribologically induced hydrogen permeation by means of a ball-on-plate test. The diffused hydrogen was measured with an electrochemical three-electrode setup. The results showed a direct correlation between the amount of hydrogen released and the applied normal load. Furthermore, an increased velocity resulted in enhanced diffusion of hydrogen throug...
A coupled diffusion-mechanical finite element simulation model was developed to study the diffusi... more A coupled diffusion-mechanical finite element simulation model was developed to study the diffusion of hydrogen in a cylindrical roller thrust bearing (CRTB). The simulations enabled obtaining qualitative information pertaining to stress-assisted diffusion in tribological loading and under the influence of residual stresses. The mechanical behavior of bearing steel was obtained from experiments and supported by literature. In parallel, rolling-contact fatigue (RCF) tests on CRTBs lubricated with a fully additivated transmission oil were conducted for 25 h and 50 h to investigate their premature failure modes. Post-RCF rheological analysis indicated decrease of the lubricant viscosity due to degradation. Carrier gas hot extraction analysis indicated a significant increase in hydrogen content in the bearings tested for 50 h. Whereas, serial cross-sectional analysis revealed the formation of subsurface White Etching Crack (WEC) networks associated with White Etching Areas (WEA); the cracks breached the surface on multiple positions causing flaking and eventual failure. On the other hand, no signs of damage were observed in the bearings tested for 25 h. The simulations revealed insignificant hydrogen accumulation due to stress-assisted diffusion in comparison to concentration gradient driven diffusion; hydrogen trapping was pronounced in zones undergoing plastic deformation due to the formation of deformation induced trapping sites. However, residual stresses had an evident influence on the subsurface accumulation of hydrogen. A comparison between the zones of elevated hydrogen concentration due to residual stresses and RCF induced subsurface damage yielded a good correlation.
Abstract Cavitation erosion-corrosion causes damage and removal of material especially in rotors ... more Abstract Cavitation erosion-corrosion causes damage and removal of material especially in rotors of centrifugal pumps, water turbines, as well as in control valves and in motors. Based on a dynamic corrosion model the pure mechanical erosion amount of different steel alloys was identified using an indirect cavitation test method combined with an electrochemical cell. We demonstrate that cavitation erosion can be enormously reduced by application of external cathodic potentials. The cavitation rate was in-situ determined using atomic emission spectrometry during the cavitation test. Due to the separately identified pure mechanical and the combined corrosive and synergistic cavitation erosion-corrosion correlation with hardness and arising charge densities during cathodic potentials were found for the mechanical part.
The friction properties of the water-based surfactant system C8 (octyl ß-D-glucopyranoside) are i... more The friction properties of the water-based surfactant system C8 (octyl ß-D-glucopyranoside) are investigated both at the macro and nanoscale in ring-on-plate and AFM friction experiments, respectively. Surface characterization and measurement of the friction gap during sliding, together with the tribological behavior show a strong shear rate dependence of the friction behavior. High shear rates of approximately 10 6 s-1 in the macroscopic friction experiments induce a molecular alignment of the surfactants in the friction gap. This generates an anisotropic viscosity which allows to carry a high load, but exhibits low viscosity in shear direction. When the nanoscale and macroscale friction experiments are normalized to the same shear rate, almost identical frictional behavior is observed in the two regimes.
Abstract RCF tests were conducted on bearings lubricated with three different lubricants: a gearb... more Abstract RCF tests were conducted on bearings lubricated with three different lubricants: a gearbox oil, a MAC fluid and PFPE oil. The decomposition of the MAC fluid and PFPE was investigated by FTIR and NMR. Gearbox oil tests showed flaking and networks of subsurface cracks with partial intercrystalline crack growth. Cross-sectional analysis revealed the formation of WEAs. FEM correlated the damage with stress fields. The results of the MAC lubricated tests showed surface flaking and intercrystalline crack growth in the subsurface. The PFPE lubricated bearings showed no signs of hydrogen induced damage. Post-experimental hydrogen analysis of the bearings indicated an increased hydrogen concentration in the samples lubricated with the gearbox oil and MAC compared to those lubricated with PFPE.
Advanced ceramic materials have proved their superior wear resistance as well as mechanical and c... more Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si 3 N 4 structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si 3 N 4 /MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si 3 N 4 ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.
Dental materials : official publication of the Academy of Dental Materials, Jan 5, 2016
The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched ... more The fracture toughness determination of fine-grained zirconia ceramics using the chevron notched beam method (CNB) was investigated to assess the feasibility of this method for quality assurance and material characterization. CNB tests were performed using four different yttria-stabilized zirconia ceramics under various testing modes and conditions, including displacement-controlled and load-rate-controlled four point bending to assess the influence of slow crack growth and identify most suitable test parameters. For comparison, tests using single-edge V-notch beams (SEVNB) were conducted. It was observed that the CNB method yields well-reproducible results. However, slow crack growth effects significantly affect the measured KIC values, especially when slow loading rates are used. To minimize the effect of slow crack growth, the application of high loading rates is recommended. Despite a certain effort needed for setting up a sample preparation routine, the CNB method is considered...
Abstract This study focuses on the damage mechanisms in silicon nitride rolling elements used in ... more Abstract This study focuses on the damage mechanisms in silicon nitride rolling elements used in hybrid (ceramic–metal) bearings that operate under non-conformal contact. To get an insight into the prevailing damage modes compared to the real application, a rolling contact experiment was designed to mimic the contact conditions. Hertzian contact pressures ranged from 3.0 to 5.9 GPa (500–4150 N). In order to approach pure rolling, the experiments were run without inducing any gross slip. Extensive surface and subsurface damage analysis was performed using conventional ceramography as well as FIB cross-sectioning. Finite element simulations were carried out to illustrate the stress state prevailing under different loading conditions. Surface damage to rollers subjected to contact pressures up to 5.1 GPa (2500 N) was mainly dominated by micro-spalling, which was induced due to the presence of snowflake structures. At the highest applied loads, damage appeared as a combination of macro-cracking and micro-spalling. Crack propagation was attributed to different mechanisms: (a) fatigue-induced fracture and (b) lubricant-driven crack propagation in the subsurface.
Modeling the damage of brittle materials is of great importance considering a variety of structur... more Modeling the damage of brittle materials is of great importance considering a variety of structural components. Prominent examples are high strength engineering ceramics. The present work is concerned with silicon nitride, a material with increasing relevance in industrial applications. In the sense of a hierarchical model structure, effective properties of micromechanical simulations were applied to macroscopic, phenomenological damage models for monotonous and cyclic loading. In the following, both models are introduced and the application of the cyclic damage model to a four point bending test is discussed.
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