Papers by Keyword: Friction

Abstract: During sheet metal forming (for example, deep drawing), the desired geometry is typically produced under large plastic deformations. However, in order to characterize the material behavior in this deformation range as accurately as possible, experimental investigation of the material is indispensable. Such investigations are necessary, among other reasons, to determine material properties that may later serve as input parameters for finite element (FEM) simulations. By defining appropriate material properties, the design and optimization of the forming technology become more efficient. One such property is the material’s flow curve; for determining it in the large deformation range, a particularly promising method is the stack compression test (SCT). At the same time, the method also has certain drawbacks. One is that the test is not standardized, and therefore there is no exact methodology for its execution. Another difficulty arises from the fact that the friction conditions present during the test are not clearly defined. In this paper, we seek to determine whether the friction conditions in the case of the SCT can be inferred by a comparative analysis of experimental force–displacement curves and force–displacement curves obtained from FEM simulations.

Abstract: Friction plays an important role on formability of deep drawn products. This necessitates an accurate description of friction in finite element formability analyses. It has been shown that constant coefficient of friction does not lead to precise prediction of product formability in these analyses. The multi-scale friction model developed at University of Twente takes the local contact conditions and textures of sheet metal and tools as the input at boundary and mixed lubrication regimes. To correlate the zinc coated sheet metal surface texture parameters with its formability, 60 different textures were analyzed. The multi-scale friction model is used to estimate friction for all the sheet metal surface textures. The effect of different textures on formability of the sheet metal was investigated by simulating cross-die forming using different sheet metal surface textures. The results show that different textures depict distinct formability behavior in the boundary lubrication regime (lubricant amount 0.1 gr/m²). Exploring the correlation between areal field parameters and formability of cross-die for the current dataset shows that besides surface roughness, autocorrelation length and skewness of height distributions are determining parameters.

Abstract: The objective of this study is to investigate the tribological properties of detonation-sprayed (Ti,Cr)C-Ni coatings under dry and lubricating conditions. The (Ti,Cr)C-based coatings with 25 wt.%, and 33 wt.% of Ni binder were applied onto steel substrates by detonation spraying. Microreciprocating wear tests were performed under dry and lubricating conditions with water, diesel, biofuel, aviation fuel and oil as a lubricating environment. Post-test examination of wear tracks was performed using interference profilometry and SEM analysis. The (Ti,Cr)C-Ni detonation-sprayed coatings exhibit a dense microstructure, featuring well-bonded splats composed of fine (Ti,Cr)C particles and Ni-based binder. The lowest wear rates of the (Ti,Cr)C-25wt.%Ni and (Ti,Cr)C-33wt.%Ni coatings are observed in an oil environment. Instead, the wear rates of both coatings are highest in a water environment. The (Ti,Cr)C-25%wt.Ni detonation-sprayed coating is characterized by an increased wear rate in the water environment as compared with (Ti,Cr)C-33%wt.Ni due to more intensive brittle failure in the water environment.

Abstract: In this work, an experimental methodology is presented for investigating the kinetics of competing oxidation and metal-plating processes that occur on friction surfaces under variable load conditions. The aim of the study was to determine the critical parameters for the transition between the formation of dissipative secondary structures (DSS) and metal-plating films (MPFs), as well as to evaluate the contact electrical resistance (CER) as an indicator of the structural state of the surfaces. A universal tribometer with adjustable load (0.2–40 MPa) was used to test friction pairs of steel 45 and bearing steel Shkh15, employing a vaseline oil as an inert lubricant and CIATIM-201 grease with 7% copper powder as a metal-plating additive. A clear correlation was observed between the CER, the friction coefficient (μ) and the wear intensity (I) across four operating modes. The maximum CER values (up to 40 Ω·cm²) were recorded in the DSS formation regime, whereas the minimum values (below 1 Ω·cm²) corresponded to the metal-plating regime. The results demonstrate that the structural-energetic approach enables effective diagnosis of the tribological state and that the CER parameter serves as an informative criterion for distinguishing between friction regimes.

Abstract: The automotive industry has been challenged by the rising need for lighter, environmentally friendly, low-emission, and low-energy consumption vehicles. Aluminium is regarded as a viable alternative to the heavier materials presently used in manufacturing automobiles due to its desirable characteristics. A review of the application of hybrid aluminium matrix composites (HAMCs) and aluminium matrix composites (AMCs) in the automotive sector is discussed in this paper. An overview of the properties and applications of fiber-reinforced, discontinuous, and particle-reinforced AMCs and HAMCs is given. Due to their superior mechanical, tribological, and physical properties, aluminium composite materials have emerged as the material of choice for most engineering applications. A discussion of the importance of proper selection of materials is also presented. The potential applications of AMCs and HAMCs in the automotive industry, i.e., brake discs and drums, cylinder blocks and liners, pistons, crankshafts, connecting rods, brake calipers, turbo heat exchangers, and others, are also addressed in this review. Recent trends and trends forming in aluminium use in automotive applications are also determined through the assessment.

Abstract: This research examines the friction and wear characteristics, as well as the mechanical properties, of EPDM (ethylene propylene diene monomer) rubber filled with oleamide. The friction measurements are done in contact with PA66GF30. The goal is to determine the suitability of this material combination for usage in dynamic sealing applications on the cathodic side of PEMFC (Proton Exchange Membrane Fuel Cell) systems. The EPDM rubber, specifically designed for reduced friction, underwent aging procedures such as heat aging and water-glycol soaking to assess its long-term performance. The pin-on-disk tests demonstrated a rapid initial rise in the coefficient of friction (CoF) and notable wear, but the hot water extraction experiments showed inadequate chemical stability with substantial ion leaching. The tensile tests demonstrated a significant decrease in mechanical characteristics during the aging process. The findings indicate that the EPDM containing oleamide has promise for dynamic sealing. However, its performance is greatly affected by aging and exposure to the environment, which highlights the need for more material optimization.

Abstract: Classical linear contact mechanics, formulated with small strain and displacement assumption, struggles to accurately describe experiments involving rubbers and elastomers. Indeed, under high loads, these materials undergo large deformations and exhibit constitutive behaviors that deviate from a linear relationship between stress and strain. In such cases, it is essential to move beyond linear elasticity to account for nonlinearity caused by large deformations and displacements. Despite efforts to develop numerical tools capable of incorporating these non-linearities in contact problems, our understanding of their impact on contact mechanical responses remains limited. In this study, we investigate the basic case of normal contact between a wavy rigid indenter and a flat, deformable substrate. We examine the influence of geometric non-linearities, arising from large deformations and displacements, alongside material non-linearities, under both frictionless and frictional interfacial conditions. To this end, we developed a finite element model, and we compared its predictions with those of Westergaard’s fully linear theoretical model. The results indicate that even in frictionless contact scenarios, non-linearities produce a mechanical response that differs significantly from predictions based on linear theory. This discrepancy becomes more pronounced as the aspect ratio of the wavy indenter increases, thereby invalidating the small-displacement assumption inherent in linear models. Moreover, the presence of friction, coupled with geometric non-linearities, induces contact hysteresis during loading and unloading cycles a phenomenon often attributed to other interfacial behaviors such as adhesion and plasticity.

Abstract: This paper discusses the wear behavior of self-mated AISI52100 bearing steel lubricated by polyol ester containing additives of bacterial cellulose particles. The wear properties are compared to those of surfaces lubricated by the base fluid without the additive. The sliding tests were conducted using a pin-on-disk reciprocating tribometer at room temperature. The results indicate that after a sliding distance of 72 meters, the friction coefficient was relatively similar for both lubricant conditions. However, the wear of the material was significantly reduced with the presence of cellulose particles in the lubricant, improving it by almost 100%. Observation of the worn area of the pin indicates the formation of a tribofilm on the contact interface facilitated by the cellulose particles. EDX analysis revealed that the film comprises oxygen and carbon-rich elements. It seems that the tribo-layer formed by a tribo-chemical reaction during sliding has acted as a protective barrier, preventing surface material ploughing and reducing wear on the tribo-pair.

Abstract: Material transfer is a critical tribological issue in aluminum forming processes, particularly at high temperatures. Although applying PVD or CVD coatings on the forming tool surface could be a solution to alleviate the material transfer issue, selecting an appropriate coating with sufficient tribological performance is still a challenging task. This paper aims to evaluate the tribological performance of AlCrN and TiAlN coatings against 6082-T6 aluminum alloy under dry contact conditions at 400°C by using a process centered approach. The methodological approach implies: 1. pre-processing to assess the contact conditions to be simulated and prepare the experimental samples, 2. processing where the friction tests are performed, and 3. post-processing in which test results are analyzed in terms of coating performance. The warm and hot upsetting sliding test (WHUST) is used as a tribometer in the processing step. A scaled-down version of the existing WHUST is developed to integrate into the heating chamber of Bruker UMT TriboLab for precise control of the specimen and contactor temperatures. Numerical simulation is applied to identify the WHUST parameters in the pre-processing step to reproduce severe contact conditions from the industrial forming processes. The post-processing step includes the identification of Coulomb’s coefficient of friction (COF) and the surface analysis with quantitative and qualitative techniques.

Abstract: Improved understanding of friction during cold rolling is crucial to further optimize the rolling process, to accurate analyse cold rolling defects and to increase model accuracy enabling an improved mill setup during industrial operation. Classical slab rolling models make use of the Coulomb friction law, assuming a constant coefficient of friction in the roll bite. In the last decades, mixed-lubrication models have been developed that explicitly take the lubricant action into account. These models have greatly increased the understanding of factors that influence friction during cold rolling, but quantitatively the model results should still be further improved before such models can be used as an online tool for setting up the cold rolling mill. This article describes a mixed-lubrication model to simulate cold rolling of low-carbon steel. Especially the tribological core of the model is extended and improved compared to state-of-the-art models. Friction mechanisms now also include a viscous shear stress and ploughing friction. The quantification of viscous shear stress was reported in a previous work [1], this work focuses on the quantification of ploughing friction. Material Point Method (MPM) simulations were carried out to determine the work piece strain-hardening and strainrate-hardening under a ploughing indenter. These simulations result in an ‘Surface Ploughing Resistance’ and finally in a quantification of the contribution of ploughing friction to the overall friction in the roll bite. The description of the various friction mechanisms (ploughing, adhesive and viscous shear) is implemented in the mixed-lubrication model. This article concludes by presenting typical results of the developed model. One of the main conclusions is that the contribution of ploughing friction in a cold rolling process cannot a priori be neglected.