Papers by Keyword: Tribology

Abstract: Various hydrogels such as polyvinyl alcohol (PVA) and poly (2-hydroxyethyl methacrylate) were assessed as articular cartilage replacement in the past. In the current study, the biphasic, biocompatible, and mechanically tunable Polyacrylamide-Alginate (PAAm-Alg) hybrid hydrogel was evaluated with different parameters such as load, speed and lubrication to study friction and wear performance of the material. Dried mass loss of hydrogel with lubrication was in the range of 2-9 wt %. The average friction coefficient of hydrogel under bovine serum lubrication was measured as 0.03, which is fairly close to native cartilage tissue. SEM studies revealed adhesion as the dominant wear mechanism due to excessive plastic deformation, independent of load and speed condition while fatigue wear mechanism was more noticeable under 1.3MPa applied pressure. However, the lubrication has dramatically decreased the wear rate and as a result, it was difficult to distinguish the worn surface of hydrogel samples from the unworn surface and just a light depression was observed on the samples, which is promising for load-bearing applications such as cartilage replacement.

Abstract: This paper reports comparative studies on effects of interlayer on mechanical properties of diamond-like carbon (DLC) coatings. Two interlayers, TiC/Ti and CrC/Cr, were deposited and studied. The DLC coatings were prepared by using an unbalanced magnetron sputtering system. The chemical composition, micro-structure, constituted phases, and fundamental mechanical and tribological properties were evaluated. The results showed that the two amorphous (a-) DLC coatings were obtained. The a-DLC coating with the TiC/Ti interlayer showed higher adhesion, hardness and wear resistance than the one with the CrC/Cr interlayer.

Abstract: Cutting tools made of polycrystalline diamond (PCD) are used for machining of aluminum alloys, fiber-reinforced plastic composites and wood. Compared to cemented carbide tools with geometrically defined cutting edges, PCD tools offer significant advantages with respect to tool life. High demands regarding the cutting edge roughness and the quality of the rake and the flank face usually require a grinding process with diamond grinding wheels. The PCD grinding process, however, is characterized by low material removal rates and high grinding wheel wear. The material removal rate and the grinding wheel wear, in turn, highly depend on the process state variables process force and process temperature. However, the relationship between these process state variables and the process input variables is largely unknown. This work provides a contribution to the closure of this knowledge gap by means of an adapted friction law. A single grain friction test stand using the pin-disk principle was developed, which enables a measurement of the friction force and the contact zone temperature for normal forces and relative speeds that are common in PCD grinding. During the experiments, the specification of the PCD disc, the cross-sectional area of the friction sample made of monocrystalline diamond as well as the process parameters normal force and relative speed were varied. In addition, the tests were carried out without lubrication as well as with a minimum lubrication. A high correlation between the contact force and the coefficient of friction was determined. This relationship was mathematically formulated in a friction law. In addition, a direct influence of the contact force and the relative velocity on the contact zone temperature was identified. The knowledge gained leads to an improved understanding of the PCD grinding process and thus enables a more efficient grinding process design.

Abstract: Oscillating forming processes offer outstanding opportunities for lightweight applications and economic advantages. Gears, especially splines, are industrially produced by oscillating ram movements. The oscillations enable the precise manufacturing of the gearing on the tubes. At the same time, they also enlarge the sliding distance in each stroke. Therefore, tool wear has to be considered carefully. Observations of oscillating gear forming processes reveal a startling phenomenon: Calibration regions of tools show the first wear marks during the manufacturing process, although the maximum contact stress is located in the forming area. According to Archard’s equation, the maximum wear is expected to be located in the area of maximum contact normal stress. In this paper wear prediction based on numerical simulations and a modified Archard ́s equation is presented. The implemented wear factor is of particular influence. Adequate values for this factor are derived from gear forming processes with tools made out of the steel 1.2379. Splines made of case hardening steel (16MnCrS5) can be produced with these tools, but wear is already evolving when laboratory lots are produced. Wear behavior and wear volume are recorded and used to determine the wear factor. The resulting values for wear factors in Archard’s equation indicate that changing contact and lubrication conditions during the pre-and the backstroke of oscillating processes have to be taken into account in wear modelling. This is achieved by a decomposition of the wear index into material and lubrication related factors. The extended model allows for a local wear forecast in processes with oscillating ram movements for which Archard’s classic wear model does not give plausible results.

Abstract: The trend in metal forming tribology is to develop new tribo-systems including new lubricants, tool materials and tool coatings in order to substitute environmentally hazardous lubricants by environmentally friendly tribo-systems. In preliminary testing the limits of lubrication of new tribo-systems for sheet forming production, it is advantageous to use dedicated simulative tribo-tests. This paper studies the influence of tool coatings on deep drawing operations using the Bending Under Tension (BUT) test and also under more severe tribological conditions by adopting the Strip Reduction Test (SRT) to replicate industrial ironing of deep drawn, stainless steel parts. Non-hazardous tribo-systems in form of a double layer Diamond-like coated tool applied under dry condition or with an environmentally friendly lubricant were investigated via emulating industrial process conditions in laboratory tests. Experiments revealed that the double layer coating worked successfully, i.e. with no sign of galling, when it was used with environmentally friendly lubricants, whereas the results were more prone to galling under dry condition.

Abstract: An upsetting-ball ironing test has been developed to investigate the lubricating performance of coatings in multi-stage cold forging. By using this test, the lubricating performance of a zinc phosphate free coating called “dry in-place coating” was evaluated and improved, and now the dry in-place coating is used worldwide due to its high anti-galling ability and low environmental impact. In this study, galling generation mechanism in the upsetting-ball ironing test is investigated by using the point tracking function in FEM simulation. Scratches on ironed surface are generated at the starting point of ironing and the width and depth of scratches increase gradually with the increasing ironing stroke. It is revealed that all points on a scratch have been in contact with the same point of the ball. The lubrication coating on the billet surface peels off locally with the onset of ironing and some contamination particles enter the interface between the billet and the ball and thus cause scratches. Galling takes place at the ironing stroke of around 10 mm when the billet of 14 mm in diameter and 32 mm in height is upset to 45 percent reduction in height and then ironed by the ball of 10 mm in diameter. It is found that all points on the ironed surface at the starting position of galling are not on the initial billet surface but come from the inside of the billet. These points come out at the ironing stroke of 4 mm due to the dividing flow in the surface layer. It is concluded that galling in the upsetting-ball ironing test is generated by the extremely large surface expansion.

Abstract: The lubricating performance of the zinc phosphate coating employed generally in cold forging is evaluated with Bowden-Leben sliding test by changing friction conditions such as the coating thickness, specimen temperature, the tool surface roughness, contact pressure and sliding speed. A specimen for the friction test is prepared from the inner surface of an extruded square cup and the residual thickness of the lubrication coating on the specimen is controlled by using the surface expansion in forward extrusion of the cup. Experimental results showed that the specimen temperature has the strongest influence on the friction coefficient. With an increase of the specimen temperature, the friction coefficient gradually decreases until 473K, and then increases sharply. With an increase of the tool surface roughness, the friction coefficient increases slightly. Friction coefficient is formulated as a function of the specimen temperature and the tool surface roughness. The anti-galling ability of the coating is affected by the residual thickness of lubrication coating, the specimen temperature and the tool surface roughness.

Abstract: Over the last few years lightweight construction became increasingly important in modern cars. Motivated by reducing greenhouse gas emission the car industry is currently working on different approaches to decrease the weight of structural body parts. In this regard, a reduced sheet thickness of these components and therefore a reduced overall weight can be achieved by using high-strength steels. Hot stamping has been established as a suitable manufacturing process for these steel grades, in which a hot austenitic blank is formed and quenched simultaneously. The high strength of the formed parts is realized by the phase transformation of an austenitic to a martensitic structure during hot stamping. Due to the alternating thermo-mechanical loads, which occur during forming and quenching, the hot stamping tools are highly stressed. In addition, when the blank slides over the surface of its counterpart, a substantial adhesive wear occurs, which is the predominant wear mechanism in hot stamping. The aim of this study is, to increase the wear resistance of the tools by modifying the surface. In this context, the chemical affinity between the interacting components need to be reduced in order to decrease the adhesive wear on the hot stamping tool, which is possible by alloying the base material. For this reason, the wear development is investigated for samples alloyed with different materials with a modified pin-on-disc test. This experimental setup enables a continuous contact of the tool with the blank and thermal alternating stress of the pin. The contact area is investigated with a laser-scanning microscope to qualify the tool surface before and after the experiments by measuring the tool topographies. The results of an unalloyed and alloyed tool will be compared with each other to evaluate the wear behavior. In order to quantify the amount of wear the wear volume will be calculated with an algorithm of the software WinSam. The experiments will be carried out under process like conditions to ensure transferability to the real hot forming process.

Abstract: Cold forging processes enable the economical production of high quality components like joints, shafts and gears. The manufactured parts are characterized by improved properties such as hardness, surface quality and fatigue strength. For manufacturing components using cold forging, a comprehensive knowledge regarding the cold forging procedure and its process parameters is needed. One important influencing factor, which needs to be analyzed to use the potential of this kind of processes, is the tribological system, especially the used lubricant. The tribological conditions significantly influence the material flow and thus the workpiece quality. Furthermore, resource efficient and environmentally benign metal forming processes became very important within the last decade. The present study evaluates the resulting tribological conditions and their differences for various cold forging lubricants with and without a zinc phosphate based lubricant carrier. The lubricants are based on molybdenum disulphide, polymers, or both inorganic salts and waxes. The tribological conditions of the different lubricants are investigated using the Double-Cup-Extrusion-Test (DCET) as a laboratory friction test.

Abstract: TiCN thin coatings with various different carbon contents were deposited using cathode arc evaporation of pure titanium in a mixture of N₂ and C₂H₂ gasses at a constant pressure of 1.5 Pa. The analyses show a transition from a stoichiometric to a non-stoichiometric coating structure with an increasing C₂H₂ content. Moreover, the increase in the acetylene in the gas mixture leads to a decrease in the crystal phase from pure polycrystalline to pure amorphous. Nanohardness changes from 30.4 to 4.4 GPa and the cohesive failure of the coatings is in the range of 61 - 72 N. The tribology is estimated by the Ball-on-Disc method and an Si₃N₄ ball as the counterpart. The measured coefficient of friction is in the range of 0.2 - 0.56.