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1.Introduction
The friction behaviour of automotive brakes is determined by the character of the active surfaces of the disc and pad and third bodies between these surfaces.This tribo system is very complex and variable, and still incompletely explored and understood. Earlier investigations have mainly involved tribofilm formation on the disc surface, leaving the pad surface relatively unexplored [1–5]. The present investigation aims at increasing the understanding of this tribosystem, specifically the character of the pad surface.A major reason for the lack of publications on the surface characteristics of brake pads, is the fact that the analysis is a difficult task to perform. The composition of the pad, the rough surface structure and the differences in mechanical properties of the different constituents all constitute obstacles for different measurement techniques.One of the most commonly used techniques to study surface structures is scanning electron microscopy（SEM）.The SEM is very useful for surface analysis of brake pads, but it has its obvious drawbacks. Most SEMrsquo;s require electrically conductive samples, which is seldom true for all pad components. The microscope chamber is hardly ever suitable for holding a whole brake pad and the porous structure of the pad leads to vacuum problems. All these factors force us to use small cut-out samples of the pad, a method that refuses repeated studies of the same area after different stages of wear. In order to study the same area after running at different braking conditions, non-destructive surface analysisis required. Surface profilometers can often hold a whole pad. Optical profilometers are generally fully non-destructive, but require samples with high reflectance and relatively smooth surfaces. Brake pads have rough surfaces and generally also low reflectance. In the present investigation, a white-light interferometer has been used to measure surface topography. When it comes to the difficult pad surfaces, this technique has an advantage over other optical profilometers, such as focusing laser instruments. Data from points on the surface that does not sufficiently reflect light are not included in the measurement, only valid data are presented. A focusing laser returns data from all measurement locations, also those where the laser has not even found the surface. Stylus profilometers are naturally completely insensitive to reflectance. However,the very soft and rough surfaces of brake pads cause major problems.In the following, the brake tribosystem will be discussed on two different size scales, the macroscopic level, including nominal contact conditions and material properties and the microscopic level, including contact geometry, surface composition and mechanical properties of the contact areas. The present investigation comprises surface characterisation of the brake pads using white-light optical interferometry, SEM (including high resolution FEG SEM), energy dispersive X-ray analysis(EDX) and nanoindentation. Previous investigations have also involved light optical microscopy (LOM) and macro photography [6] and video recorded high magnification in situ studies of the contact of pad material against a glass disc [7].
2.Macroscopic description of the brake system
2.1 Nominal contact conditions
The nominal contact conditions in a regular car disc brake system may be exemplified by those of the front brake of a Volvo 850 (or S 70). This is one of the most widely used vehicles in Sweden, and more importantly, its brake system constitutes the platform for all studies in this paper.
This car is fitted with a single piston calliper, gripping around a grey cast iron disc with a diameter of 280mm, see Fig. 1. It uses 4000 mm2 organic brake pads, clamping the disc with up to 25 kN. This clamping force corresponds to a brake line pressure of 100 bar. During normal driving conditions, the clamping force seldom exceeds 10 kN. In the present investigation, the normal force never exceeded 6 kN, corresponding to a nominal pressure on the pads of 1.5 MPa. This is a fairly low pressure, from a materials science point of view. A typical hardness for grey cast iron is 3 GPa, 2000times higher.
The frictional heat developed in an automotive brake is quite substantial. During a hard braking with a regular family car, the power dissipation on each front brake pad may exceed 80 kW. As will be shown later, the area of real contact is confined within ca. 20% of the nominal contact area, and the thickness of the deformed surface layer is Ͻ1 micro;m. The total deformed volume is thus less than 1mm3 and the power dissipation per volume becomes 80 TW/m3.This equals dissipating the power of 80 nuclear reactors in 1 dm3.
Europeans tend to like brakes with high performance, as compared to Americans who prefer high comfort. This is reflected in the choice of pad materials on the two continents. In the USA, average friction coefficients of ca. 0.4 is typical, while the coefficient of friction on European cars often exceeds 0.5.
The test rig is based on the Volvo 850 front wheel suspension; a front left corner, including the complete disc brake, wheel, spring strut and lower wishbone, asdetailed in Ref. [8]. The wheel is driven by an electrical motor using the original drive shaft. The speed and braking line pressure can be varied independently in the ranges 0–4 rpm and 0–30 bar, respectively. All tests are performed to imitate low speed range driving conditions, with relatively mild braking. Unless the running-in process itself is studied, all pads and discs are run-in tore present steady state conditions, before starting the actual tests.
2.2.1. Composition of organic brake pads
The lining materials of automotive brakes are usually composites formed by hot compact
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