Tribologia - Finnish Journal of Tribology
An open access journal publishing both practical and scientific articles related to tribology: wear, friction and lubrication. Benefits to Authors: No publication fees Authors remain as copyright owners Peer reviewed publishing process Prompt publishing Free printed copy for the corresponding author The flyer of the journal can be downloaded in pdf-form by clicking the picture below:
The authors of the publication retain the copyrights and the author can publish the article also in other media, such as in her/his PhD thesis or open access databases, such as ResearchGate. The license of the published metadata is Creative Commons CC0 1.0 Universal (CC0 1.0).
Modelling of spur gear contact using a local adaptive finite element mesh
The gear drive is a critical part of a power transmission system. Modelling and simulating of the gear pair with finite element method (FEM) take important role as a part of the design process to estimate stresses, deformations, and damage risks in various operating conditions. A parameterized calculation model for the analysis of stresses in the gear contact and the gear root was developed. A local adaptive FE mesh was used, where a dense FE mesh zone around the contact point moves along the line of action to speed up the computation. The adaptive FE mesh, the rotation of the gear pair, and the accurate surface profile based on gear hobbing process were created in Matlab environment to obtain a good control of the flank profile and the meshing parameters. These were integrated with a commercial FEM software to calculate deformations and stresses. The developed FE mesh approach was validated successfully against analytical Hertzian theory. In addition, the developed spur gear model was compared to the gear standard ISO 6336 and a commercial gear calculation software resulting in relatively good correspondence with the maximum contact pressure and the maximum tooth root stresses.
Copyright (c) 2016 J. Lahtivirta, A. Lehtovaara
Validation of a new tribological test bench for lightweight hydraulic components
The presented paper deals with tribological contacts in “lightweight” hydraulic pumps. One approach for the mass reduction of hydraulic systems is the material substitution. According to this strategy, steel-components are replaced with polymeric material or hybrid design components. The consequences of this material substitution are tribological changes of the contact pairings. Hence, there is research necessary on the tribological behavior of the specific contacts between polymers or hybrid components. For the characterization of these tribological contacts, a test bench was developed. With this test bench the analysis of the tribological behavior of the contact between cylinder and control plate in the axial piston variable pump is possible. A description of the modeling and a validation of the test bench are presented in the this paper. The result is, the test bench indicates good measurement result quality. In addition a cooling system is necessary to guarantee a constant test fluid temperature.
The role of frictional work in tribological behavior of polyamide 66 composites containing rice bran ceramics particles or glass beads
In this study, we investigated tribological behavior of polyamide 66 (PA66) composites containing rice bran ceramics (RBC) particles or glass beads (GBs) under a wide range of contact pressures and sliding velocities, and discussed the role of the frictional work in the behavior. The volume fractions of the fillers were 8 and 26 vol%. The friction coefficients of pure PA66 showed the increasing tendency with the Pv values. In contrast, those of the PA66/RBC composites showed approximately no change, and those of the PA66/GB composites slightly decreased with increasing Pv value. The surface temperatures of the materials increased with increasing frictional work. The maximum temperature of pure PA66 and the PA66 composite with 26 vol% RBC particles exceeded 80 °C, which were higher than the glass transition temperature of PA66 resin. Pure PA66 exhibited a decrease in specific wear rate with increasing frictional work even as a change of the wear mode. This change was considered to be caused by softening of the resin because of high frictional work. In contrast, the specific wear rates of the PA66 composites with the RBC particles or GBs were much lower (<0.5× 10−8 mm2/N) compared with pure PA66 at low frictional work (<1.3 MJ/(m2∙s)). At high frictional work (>1.4 MJ/(m2∙s)), the PA66 composites with the RBC particles showed relatively high specific wear rates (0.6–1.0 × 10−8 mm2/N) because of high surface temperature.
Testing new tribo-systems for sheet metal forming of advanced high strength steels and stainless steels
Testing of new tribo-systems in sheet metal forming has become an important issue due to new legislation, which forces industry to replace current, hazardous lubricants. The present paper summarizes the work done in a recent PhD project at the Technical University of Denmark on the development of a methodology for off-line testing of new tribo-systems for advanced high strength steels and stainless steels. The methodology is presented and applied to an industrial case, where different tribo-systems are tested. A universal sheet tribotester has been developed, which can run automatically repetitive Bending Under Tension tests. The overall results show that the methodology ensures satisfactory agreement between laboratory tests and production tests, although disagreement can occur, if tribological conditions are not the same in the two cases.
Empirical evaluation of spin-on-glass-layers on steel surfaces by wear tests
Dies and moulds with high precision surfaces are being used in various branches, e.g. in the plastic industry where the surface finish are conventionally performed by manual polishers. With ever increasing demands of shorter lead times and reduced costs, efforts have been made to automate this finishing process. This paper presents an empirical study performed to test durability properties of SOG (spin-on-glass)-layers on steel surfaces. The results showed that the thin coating last longer than the thicker ones, and that the harder coatings withstood wear significantly better than the steel reference samples thus motivating further investigations.
Effect of abrasive properties on the high-stress three-body abrasion of steels and hard metals
Especially in tunneling, the abrasiveness of rock is an important property, which can easily be determined by several methods developed for the purpose. With this in mind, it is rather surprising that the effects of different rock types on the wear mechanisms of engineering materials have not been too widely studied. In this paper, high stress three-body abrasive tests were conducted with four different abrasives with a relatively large (2-10 mm) particle size. As test materials, three different steels and three hard metals were used. The tests clearly showed that material type has an influence on how different abrasive and material properties affect the abrasive wear mechanisms and severity. For example with hard metals, the most important property of the abrasives is their crushability, as only small abrasive particles are able to properly attack the binder phase and cause high wear rates. On the other hand, it seems that the abrasiveness of rock is not the dominating property determining the severity of wear in the current test conditions for any of the tested materials. In fact, with steels no single abrasive property could be shown to clearly govern the abrasive wear processes. In any case, when using the determined abrasiveness values in wear estimations, the contact conditions in the method used for determining the abrasiveness values should be as similar as possible with the end application.
Wear properties of thermally sprayed tungsten-carbide coatings in paper machine environments
Thermally sprayed tungsten-carbide (WC) coatings have proven to be one of the most wear resistant coatings available and a respectable replacement for hard-chromium coatings. They are used in paper machine parts such as calender rolls. However, improved lifetime and performance are continuing considerations, as well as finding more economical alternatives. This study researched the wear phenomena of tungsten-carbide coatings in a paper machine environment. To achieve this, five different feedstock materials and coatings manufactured from these were compared by electron microscopy as well as dry abrasion-, high-speed slurry abrasion- and cavitation erosion tests. Improvements in ductility by changing the matrix material were found, while changing the particle strength had no effect on the behavior of the coatings. The findings suggest further research on altering the matrix of the feedstock could lead to overall improvements in coating quality and component lifetime.
Friction and wear studies of some PEEK materials
The friction and wear behavior of several types of PEEK polymers and composites were studied. The influence of carbon fiber, lubricant and thermally conductive fillers were evaluated, as well as the effects of contact load and temperature. The tests were done using a reciprocating ball-on-disc set-up. The materials were tested under the load of 5 N and 15 N, at room temperature, 80°C, 120°C and 150°C. The difference between the materials was substantial, with a friction coefficient varying between 0.03 and 0.3 for the different materials at 120°C. PEEK with carbon fiber filler showed an improvement in both friction and wear compared to unfilled PEEK. When adding lubricant, PTFE, to the composite the friction and wear were improved even more. PEEK with thermally conductive filler on the other hand had both highest friction and wear. Increasing the temperature slightly decreased both friction and wear for most of the PEEK materials. At 150°C, only the composite with PTFE lubricant had a low friction and wear.
Erosion wear of glass fibre reinforced vinyl ester
This study evaluates the slurry-erosion wear of glass fibre reinforced vinyl ester composites (VE-FRP) using a high speed slurry-pot type wear tester. The wear rates of VE-FRP were compared using different abrasives, namely quartz, chromite, copper ore, zinc concentrate, and tailings. Furthermore, the effect of abrasive particle size and slurry concentration on the VE-FRP wear was studied. The erosion wear results of VE-FRP were compared to natural rubber (NR) and bromobutyl rubber (BIIR) as well as to few common thermoplastics, such as polypropylene (PP) and polyvinyl chloride (PVC). Moreover, the failure characteristics of VE-FRP were analyzed. The results demonstrated that coarse quartz produced the largest wear rates on VE-FRP samples, while the zinc concentrate showed the lowest wear. Minor changes in the abrasive particle size had no effect on the wear results, only when the particle size was markedly raised, the wear started to increase. When comparing the wear rates of different materials, it was concluded that with all abrasive types, tested rubbers and thermoplastics had lower wear rates than VE-FRP.
Particle size and surface texturing effects on friction of magnetorheological fluids
MR fluids are produced by dispersing micron-sized ferrimagnetic particles in oil. Generally, the presence of foreign particles in a lubricant affects the sliding behavior of the lubricated surface. Accordingly, the dispersed particles in MR fluids should significantly affect the lubrication properties of the fluids. In this study, we investigated the lubrication properties of MR fluids through sliding tests and in situ observations, while focusing on the behavior of the dispersed particles. MR fluids containing dispersed particles of various diameters were tested. The results of the sliding tests suggested that the MR fluid with the smallest particles exhibited the best lubrication characteristics. Surface observations showed the presence of grooves on the lubricated surfaces after wear. It is assumed that the grooves are formed by the abrasive action of the dispersed particles and that they affect the behavior of the particles. In situ observations proved that, in the case of flat surfaces, the dispersed particles remained stationary during sliding. On the other hand, in case of grooved surfaces, they moved along the grooves. Given these results, it can be surmised that the grooves on the worn surface changed the behavior of the particles and improved the lubricity of MR fluids.
High speed slurry-pot erosion wear testing with large abrasive particles
One of the testing methods used to simulate slurry erosion in laboratory conditions is the slurry-pot method. In this work, a novel high speed slurry-pot type erosion wear tester was constructed for testing of materials used in mining and other mineral handling applications. In the tester, the samples are attached to a vertical rotating shaft on four levels in a pin mill configuration. High speeds up to 20 m/s at the sample tip can be achieved also with large abrasive size up to 10 mm. In the tests, the equipment proved to be functional and durable even with the high loads created by the high speeds and large abrasive sizes. There are, however, variations in the slurry concentrations inside the pot during testing, leading to different wear rates at the different sample levels. Therefore, a sample rotation test method was developed. By rotating the samples evenly through all sample levels, the overall deviations between samples will be minimized. Furthermore, with the sample rotation method up to eight materials can be tested simultaneously. The slurry-pot is suitable for testing various materials, such as steels and rubbers.
Cavitation estimates by orbit prediction of a journal bearing – Finite element modelling and experimental studies
The paper presents a two-sided approach to establish understanding of the cavitation phenomenon in dynamically loaded journal bearings, more specifically the engine bearings of large two-stroke marine diesel engines. One disadvantage of the journal bearing is the converging-diverging geometry making it prone to cavitation which again affects the load carrying capacity of the bearing. In combustion engines the journal bearing plays a vital role especially as main and crosshead bearings transmitting the combustion forces. Those forces vary highly during one combustion cycle which is further influencing the load carrying capacity and ultimately the chances of fatal shaft-sleeve contact. By solving Reynolds equation numerically using finite elements and incorporating a cavitation algorithm, the dynamic coefficients can be used to establish the journal orbit for a given bearing and load pattern. Validation of the results is done against the Ruston and Hornsby 6VEB-X Mk III engine. Besides the numerical investigations a cavitation test rig has been developed. With this rig it is possible to generate cavitation under controlled conditions in terms of load/eccentricity and rotational speed. The development of cavitation in time in terms of position and distribution can be visually recorded.
Abrasion resistance of selected commercially available polymer materials
The aim of this investigation was to examine several types of commercially available polymers for potential application in machine elements that are exposed to abrasion. Selected materials were evaluated on a rubber and steel wheel abrasion test rig according to the standard ASTM G65, using as abrasive quartz sand (SiO2) as abrasive, with a particle grain size in the range of 0.8 – 1.6 mm. Tests were performed under dry conditions and at room temperature. Applied load was 50 N, within testing times of 1½, 3, 4½, and 6 hours respectively. Results of testing revealed influence on wear behavior of polymer hardness as well as influence of abrasive particle embedment.
Fretting wear behaviour of MoS2 dry film lubricant
Dry film lubricants (DFL) are used as palliative coatings to prevent fretting wear. In this work fretting tests are carried out on coated Ti6Al4V cylinders on coated flat samples under dry sliding conditions, using an amplitude of 300 µm, 2.5 Hz frequency and 575 N normal loads. During the tests the coefficient of friction (CoF) was monitored, with tests being terminated when the coefficient of friction reached 0.7. Wear scars were analysed by profilometry and SEM to elucidate wear mechanisms. Results show that CoF initially increases rapidly to 0.4, this is then followed by a plateau region that finishes in a sudden step decrease in CoF following which CoF rises steadily. This behaviour is shown to be characteristic and interrupted tests are presented to allow elucidation of the wear scar at different stages in the lifetime and thus aid an understanding of the mechanisms of degradation which control the tribological behaviour.
An unconventional approach in polymer wear: online vision system
Tribology which deals with interaction of materials has robust link with surface morphology of contacting bodies. Adequate information on surface state during the interaction is essential for understanding the constructive/destructive nature of wear, especially for polymer composites. Such information can be acquired using recently developed advanced electronic imaging systems to study the morphological changes of moving surface. In the present research, polymer composite is tested against steel counterface under combined rolling/sliding condition. A high speed camera system in conjunction with a reflected light bright field optical microscope is used to acquire contact surface micrographs during testing. The acquired real-time micrographs are post processed in order to quantitatively estimate the area occupied by specific surface scars. Image processing techniques such as blur estimation, segmentation by local thresholding and grey scale granulometry were used to have a quantitative estimate of the surface state. The area of segmented regions from the values of grey scale granulometry validates the self-healing mechanism complemented by resin back transfer phenomena. It is also evident from this study that a substantial contact area should be investigated in wear analysis. The present research clearly points out that a combination of quantitative and qualitative analysis of real-time micrographs is indispensable for wear surface investigations.
Overview of recent dissertations in the field in Finland
The public examination of the doctoral dissertation of Mona Mahboob Kanafi, M.Sc. (Eng.), Rocky road – surface roughness impacts on rubber friction, was held on 7 April 2017 at the Aalto University School of Engineering. MSc Janne Juoksukangas’s doctoral dissertation Modelling and Experimental Analysis of Fretting Fatigue in Complete and Bolted Contacts was publicly examined at the Faculty of Engineering Sciences of Tampere University of Technology (TUT) on Wednesday, 24 May 2017. The public examination of the doctoral dissertation of András Kriston, M.Sc. (Eng.), Micro contact analysis of rubber-ice interaction during frictional processes, was held on 30 June 2017 at the Aalto University School of Engineering.
Focus on friction and wear - The 40th anniversary seminar of the Finnish Society for Tribology in Tampere
The Finnish Society for Tribology was founded 31.8.1977 to promote the research, technical development and education of tribology in Finland. The society publishes the quarterly journal Tribology and arranges conferences, seminars and special events on tribology. The society celebrated its 40 years existence by arranging an international commemorative seminar in the Finnish Labour Museum Werstas located in the former buildings of Finlayson factories in Tampere.
Kenneth Holmberg from VTT Technical Research Centre of Finland Ltd won the highest international award in the tribology field
On 20 December, the Tribology Trust presented VTT's Kenneth Holmberg with the most highly prized award in the field, the Tribology Gold Medal Award, for his long-standing, major achievements in material and friction research. The award was presented by Her Majesty’s Ambassador to Finland in a ceremony at the British Embassy in Helsinki on 20 December 2017.
The year’s 2017 end is at hand, meaning it’s time to summarize the highlights of the year. Towards the end of the year, we tribologists had two great reasons to celebrate. On 31st August 2017, The Finnish Society for Tribology reached the cornerstone of 40 years. The 40th anniversary was commemorated through an international seminar, after which the festivities continued with celebratory dinner and speeches. The day offered many interesting presentations, discussions and possibilities for networking. The readers of Finnish Journal of Tribology are provided with an overview of the 40th Anniversary seminar. Just before the Christmas, professor Kenneth Holmberg from VTT Technical Research Centre of Finland Ltd received the highest international award in the tribology field, The Tribology Gold Medal Award. Finnish Journal of Tribology wants to congratulate Kenneth for this great achievement! This end-of-year issue therefore reviews the award declaration for the readers. I hope the New Year will be as prosperous for the tribologists as the ending one. Elina Huttunen-Saarivirta Editor-in-Chief of Finnish Journal of Tribology
Copyright (c) 2017 Elina Huttunen-Saarivirta
Film thickness measurements in a running hydrostatic unit using ultrasound
The present paper describes the measurement of the oil film thickness in the range of a few micrometers in an operating hydrostatic unit using ultrasound. The investigated tribological system consists of a rotating steel cylinder block and a stationary bi-metal valve plate under static and hydrodynamic lubrication. The film thickness has been recorded in a wide range of operating conditions, pressure between 100 to 300 bar and rotational speed between 500 and 3000 rpm, to support a deeper understanding of the system. Temperature sensors were implemented next to the ultrasound sensors to compensate the ultrasound signal amplitude and phase change due to temperature dependent acoustic impedances. To confirm the results, especially the presence of deliberate zero-film conditions, wear profiles of the running surface were taken. The ultrasound technique also allows the real-time observation of film thickness oscillations with shaft and piston frequency. Steady-state measurements confirm the system behavior observed in transient operation and zero-film conditions with respect to hardware configuration were detected. The findings will be utilized to support current product development activities and to validate and improve simulation models used for film thickness predictions.
Investigation of physicochemical and tribological properties of TiO2 nano-lubricant oil of different concentrations
Nano-lubricants provide enhanced tribological properties in several applications like machines and engines. The presence of nanoparticles in the lubricating oil effects its wear, friction, thermal, chemical and physical properties in many ways. This article reports effect of TiO2 nanoparticles suspended in servo system lubricating oil prepared by sonication process without adding any surfactant. Four different volume concentrations (0.2%, 0.4%, 0.6% and 0.8%) of TiO2 nanoparticles in the base lubricating oil are used for analysis. Calorific value, flash point, viscosity are the physicochemical properties of the TiO2 nano-lubricant investigated at different Volume Concentrations (VC). Wear scar diameter and coefficient of friction are the tribological properties analyzed for the prepared nano-lubricant at different VCs. Calorific value and flash point of the nano-lubricant was found to be decreasing with increasing VCs compared to base lubricant oil. Viscosity, on the other hand was almost same as the base lubricant oil for the proposed VCs. Wear scar diameter and friction coefficient was found to increase with increase in VC of TiO2 nanoparticles in the lubricant oil. It is concluded that, to obtain better results agglomeration of nanoparticles has to be avoided which can be achieved with the use of surfactant.
Copyright (c) 2017 Mohan Kumar, Asif Afzal, Ramis M.K
Multiphysics modelling of a hybrid magnetic bearing (HMB) for calculating power loss and temperature with different loss minimization strategies
This paper represents a multiphysics modelling for calculating loss and temperature of a hybrid magnetic bearing (HMB) using finite element method (FEM). It also addresses the different loss minimization strategies for the HMB. The main sources of losses are identified as eddy current loss in permanent magnets, flywheel and copper loss in electromagnet. Due to these losses, the temperature distribution in different portions of HMB is computed using coupled field analysis. To minimize the eddy current loss, slits are fabricated in flywheel plate instead of a solid flywheel. The improvement of the control current is investigated by providing a coating of different metal, like copper, brass and stainless steel on the flywheel. A zero bias current (ZBC) scheme has been introduced where no bias current is required to levitate the rotor or to avoid singularity due to external disturbances, thus reducing the copper loss.
Chemically modified PTFE particles as solid lubricant additive for the fixation on substrate surfaces
During the irradiation of high molecular weight poly(tetrafluoroethylene) (PTFE) in presence of oxygen perfluoroalkyl(peroxy) radicals and functional groups are formed which allow chemical coupling reactions (cc = chemical compatibilized) with oils and plastics. Contrary to the well-known inert properties of perfluorinated organic compounds high-effective additives can be produced by radiation modification and subsequent chemical compatibilization of such substances with olefinically unsaturated groups of oils/lubricants. By the irradiation process high molecular weight PTFE degrades by C-C and C-F bond scission to lower molecular weight PTFE microparticles which possess perfluoroalkyl(peroxy) radicals. These radicals are used for the chemical compatibilization reaction with olefinically unsaturated groups base oils. The tribological properties and the dispersion stability of the resulting oil-PTFE-cc (cc = chemical compatibilized) dispersions are significantly enhanced in comparison to physical mixture of oil and PTFE micropowder. These oil-PTFE-cc-dispersions show primarily anti-wear (AW) properties. The use of reactive groups (e.g. phosphite groups) in the oil gives the dispersions extreme pressure (EP) properties additionally. This article demonstrates the usefulness of the oil-PTFE-cc-dispersions in rolling bearings using phosphite-modified PTFE products as additive in lubricants (FE-8 test). The investigations are completed by the examination of roller elements by SEM/EDX analysis. A model is shown to explain the effect of phosphite groups on oxidic/hydrolytic metal surfaces.
Effect of finish rolling and quench stop temperatures on impact-abrasive wear resistance of 0.35 % carbon direct-quenched steel
Novel high-hardness medium-carbon martensitic laboratory steel has been produced and tested for abrasive wear resistance. Different finish rolling temperatures (FRT) combined with either direct quenching (DQ) or interrupted quenching to 250 °C was applied to vary the content of retained austenite and hardness. The steel carbon content was set to 0.35 % to obtain a surface hardness of approximately 600 HB. Lowering the finish rolling temperature in the range 920–780 °C, i.e. into the non-recrystallization regime resulted in a more elongated prior austenite grain structure, which increased the hardness of the DQ variants without any significant loss of Charpy-V impact toughness. Although increasing the degree of autotempering by raising the quench stop temperature reduces the hardness of the martensitic microstructure, it was found that proper quenching stop temperature could be utilized to achieve balanced toughness and hardness properties. Impact-abrasive wear resistance as measured in impeller-tumbler tests with natural granite as the abrasive demonstrated that wear resistance increased with increasing surface hardness.
Copyright (c) 2017 Oskari Haiko, Ilkka Miettunen, David Porter, Niko Ojala, Vilma Ratia, Vuokko Heino, Anu Kemppainen
The comparative study of polymers for sliding pairs with UNMT (Universal Nano/Micro Tester)
In the article the results of comparative research of eight types of polymers, used in sliding pairs, are presented. For the selected polymers mechanical properties and then also their tribological characteristics were examined with UNMT (UNMT - Universal Nano/Micro Tester) to select the best material. Friction coefficient and wear measurements were performed with reciprocating ball-on-disk pair of UNMT under dry lubrication conditions. The usefulness of UNMT was confirmed as a quick and precise method. Presented results showed also that the best antifriction and anti-wear properties were observed for polymer with small hardness and elasticity module.
Dissertation: Behavior of martensitic wear resistant steels in abrasion and impact wear testing conditions
The doctoral dissertation of Vilma Ratia in the field of materials engineering entitled “Behavior of Martensitic Wear Resistant Steels in Abrasion and Impact Wear Testing Conditions” was publicly examined at the Faculty of Engineering Sciences of Tampere University of Technology (TUT) on 6th of November, 2015. The opponents were Professor Mikael Olsson (Dalarna University, Sweden) and Dr. Mikko Uusitalo (Valmet Technologies, Inc.). Professor Veli-Tapani Kuokkala from the Department of Materials Science at TUT acted as the Chairman.
Copyright (c) 2016 Elina Huttunen-Saarivirta
Water-based boundary lubrication with biomolecule additives on diamond-like carbon and stainless steel surfaces
Doctoral dissertation - short version to be published in Tribologia - Finnish Journal of Tribology
Static friction measurements on steel against uncoated and coated cast iron
Static friction is a phenomenon we may mainly consider as related to frictional joints within static mechanics. The step from static friction to tribological phenomena is, however, rather short, since at the onset of sliding in a mechanical contact, the static friction determines the initial resistance against motion. Static friction furthermore plays a role in contacts subjected to traction and fretting. Although being a phenomenon of short duration, the tribological phenomena during the transition from static friction to sliding friction may be of great importance for the operational life of the contact surfaces, particularly if the procedure is repeated for a sufficient number of times. The present paper describes the principles of static friction measurements, details of the employed static friction tribometer and the results of measurements with unlubricated and lubricated sliding couples consisting of steel against uncoated and coated cast iron.
Copyright (c) 2016 P. Andersson, L. Kilpi, K. Holmberg, A. Vaajoki, V. Oksanen