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)