Pin-on-Disc Testing of Low-Metallic Friction Material Sliding Against HVOF Coated Cast Iron: Modelling of the Contact Temperature Evolution

Tribology Letters, Aug 2017

Pin-on-disc (PoD) testing is widely used to investigate the sliding behaviour of materials and relevant wear mechanisms under different tribological conditions. The approach has been also profitably applied to the characterization of materials for brake systems to obtain specific information on the wear mechanisms. In the present study, the transient thermal analysis of a pin made with a friction material dry sliding against HVOF coated and uncoated pearlitic cast iron disc in a PoD apparatus was investigated by means of a finite element analysis together with experimental measurements. The aim of the investigation was to model the surface contact temperature in this sliding system to highlight the role of the different surface conditions, i.e., coated and uncoated, on the evolution of the pin and disc temperatures during sliding. In addition, we propose a simplified analytical equation for estimating the average temperature rise in the contact region during sliding, by extending the Kennedy approach in order to be able to provide a quick evaluation of the contact temperature for this kind of couplings, what is very helpful when characterizing a large number of systems in different contact conditions.

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Pin-on-Disc Testing of Low-Metallic Friction Material Sliding Against HVOF Coated Cast Iron: Modelling of the Contact Temperature Evolution

Tribol Lett Pin-on-Disc Testing of Low-Metallic Friction Material Sliding Against HVOF Coated Cast Iron: Modelling of the Contact Temperature Evolution Matteo Federici 0 1 Giovanni Straffelini 0 1 Stefano Gialanella 0 1 0 Department of Industrial Engineering, University of Trento , Trento , Italy 1 & Giovanni Straffelini Pin-on-disc (PoD) testing is widely used to investigate the sliding behaviour of materials and relevant wear mechanisms under different tribological conditions. The approach has been also profitably applied to the characterization of materials for brake systems to obtain specific information on the wear mechanisms. In the present study, the transient thermal analysis of a pin made with a friction material dry sliding against HVOF coated and uncoated pearlitic cast iron disc in a PoD apparatus was investigated by means of a finite element analysis together with experimental measurements. The aim of the investigation was to model the surface contact temperature in this sliding system to highlight the role of the different surface conditions, i.e., coated and uncoated, on the evolution of the pin and disc temperatures during sliding. In addition, we propose a simplified analytical equation for estimating the average temperature rise in the contact region during sliding, by extending the Kennedy approach in order to be able to provide a quick evaluation of the contact temperature for this kind of couplings, what is very helpful when characterizing a large number of systems in different contact conditions. Pin-on-disc testing; Contact temperature analysis; Friction material; FE modelling 1 Introduction Pin-on-disc (PoD) tribological tests are commonly used to investigate the wear behaviour of materials in contact with a sliding motion. The approach is particularly suited to study the relationships existing among wear mechanisms and such parameters like contact pressure, sliding velocity, environmental conditions [ 1, 2 ]. Several studies report on the PoD results, concerning investigations on materials for vehicular brake systems for different transportation fields, like road vehicles [ 3–7 ] and trains [ 8, 9 ]. As automotive brakes are concerned, pads are made of friction materials, comprising a large number of organic and inorganic components, pressed against a rotating disc, typically made of pearlitic cast iron [ 10 ]. Dynamometer and road tests are mandatory to obtain design-oriented information and for product certification. However, plain PoD testing is very useful to obtain focused information on the wear mechanisms and on their role on the tribological behaviour of real systems [ 6, 7, 11–14 ]. Moreover, considering the complexity of the formulation of friction materials for brake pads, it is paramount to have a reliable selection tool for the development of novel compositions [ 10, 15 ]. As it is well known, the tribological response of friction materials sliding against cast iron is mainly determined by the characteristics of the friction layer and of its components, i.e., the so-called primary and secondary plateaus [ 16, 17 ]. Metallic fibres and hard particles typically act as primary plateaus against which the wear fragments accumulate to form the secondary plateaus. Therefore, wear fragments originate either from a direct wearing out of the friction material and tribo-oxidation of the counterface cast iron or from the damage of the friction layer that forms in between the two mating surfaces [ 7, 12, 18, 19 ]. The compactness of the secondary plateaus present in the friction layer is strongly determined by the pin–disc contact temperature during sliding [ 6, 7, 12, 17, 20 ]. As shown by Stott and coworkers [ 21, 22 ], wear debris may sinter together to form compact and dense layers under the effect of the high local compressive pressures. Of course, the compactness of the secondary plateaus increases with temperature. Therefore, an evaluation of the contact temperature is paramount to understand and to explain the main wear mechanisms [ 5, 7, 20 ]. In this regard, it has to be noticed that the local temperature at the friction plateaus, where sliding is really confined, is higher than the average surface contact temperature (or ‘‘bulk’’ temperature according to the nomenclature proposed by Ashby et al. [23]). This parameter is not easy to handle, since it is very difficult to know the actual thermal properties of the friction layers, given its different composition with respect to the base friction material [ 12, 24–26 ], and its possible fluctuations during the tribological test. The extension of the contact plateaus is also difficult to evaluate a priori as well as their thickness. Contact plateaus range in between 20 and 60% of the nominal area of contact [ 16, 17, 27 ]; their thickness ranges from some micrometres up to a few tens of micrometres [ 7, 8, 16, 17 ]. In view of these geometrical parameters, the average temperature at the contac (...truncated)


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Matteo Federici, Giovanni Straffelini, Stefano Gialanella. Pin-on-Disc Testing of Low-Metallic Friction Material Sliding Against HVOF Coated Cast Iron: Modelling of the Contact Temperature Evolution, Tribology Letters, 2017, pp. 121, Volume 65, Issue 4, DOI: 10.1007/s11249-017-0904-y