Prakt. Met. Sonderband 30 (1999) 449 
Characterization of composite materials for automotive braking industry. 
Peter Filip and Maurice A. Wright 
Center for Advanced Friction Studies, SIU Carbondale, Illinois, USA 
Abstract 
The tendency to produce faster vehicles with higher power, better aerodynamic properties requires better and 
much more reliable friction materials. In general, an ideal friction material should possess a stable coefficient of 
friction, exhibit low wear, and be environmentally friendly. The emissions of noise, smoke and dangerous 
particles should be avoided. Friction linings represent a “sacrificial component” in friction couple since they 
should wear but not cause wear of the discs or drums of vehicles. Also, it is generally accepted that the ideal 
lining should last for the entire lifetime of vehicle. In spite of above mentioned well known requirements for 
friction materials, the typical development route of new brake linings is based on trial and error methods. For 
instance in Japan, the formulation and testing of new friction materials increased by 300% in the last 10 years. A 
Lötverfahren significant factor in this increased testing of brake materials is the lack of understanding of friction mechanisms 
he related to these complex composite materials. The friction process itself is dependent on materials in contact and 
(kes by, the roughness of the rubbing surfaces. Hence, the relationship of the bulk material to that generated on the 
iebmethod friction surface becomes a necessary requirement in a more efficient and tailored design of friction materials. In 
this work, hybrid friction materials were prepared and their performance was related to the friction films 
developed at the rubbing interface. It was found that the composition and properties of the heterogeneous friction 
layer differed from that of the bulk material. 
Introduction 
So-called hybrid friction materials contain several different kinds of fibers. The properties of hybrid materials 
reflect a compromise between so called semi-metallics (presence of a high volume of metallic fiber in structure) 
and so-called non-asbestos organics (presence of organic materials and fibers). In semi-metallics, friction occurs 
dominantly between disc and metallic component of brake lining. The non-metallic structural constituents 
control the friction process of non-asbestos organics. Semi-mets exhibit low friction coefficient, enhanced judder 
and higher wear at low temperature. Problems with vibration, corrosion, rotor compatibility, and thermal 
properties are also present. Poor high temperature wear, fade and squeal are typical for non-asbestos organics [1, 
2]. The development of friction materials with increased coefficient of friction and lifetime endurance is a 
typical tendency observed worldwide. The aerodynamic shape of vehicles diminishes the air resistance, and the 
increased speed of cars require more efficient braking as exemplified by a higher coefficient of friction and 
lower wear. In addition to the tribological properties, friction materials should be strong enough to withstand 
significant shear loads generated under high temperature and moist environment but, in order to avoid 
thermoelastic instability should not be too strong and rigid. In this work, laboratory prepared specimens were 
subjected to the 90 minutes-long friction assessment and screening tests (FAST). Structure of friction layer of 
selected samples were determined and related to the detected performance (the coefficient of friction and wear). 
Since the FAST is typified by a constant amount of dissipated energy it is possible to compare different materials 
and to investigate the fundamental physical and chemical changes that occur at different temperatures in the 
friction process. However, there is not a simple relationship that connects the behavior of materials tested in 
FAST with that exhibited in real operating conditions. The relationship between the bulk composition, the 
coefficient of friction and wear is not easily predictable because friction layers are formed on friction surfaces. 
Composition of these friction layers differs from the material of the original bulk. Nevertheless, the composition 
of a friction layer strongly depends on the composition of the bulk material and on the ability of lining to interact 
with the metallic disc material