SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2002; 33: 100–107 Use of scanning force microscopy studies with combined friction, stiffness and thermal diffusivity contrasts for microscopic characterization of automotive brake pads M. Munz, * E. Schulz and H. Sturm Federal Institute for Materials Research and Testing (BAM), Lab. VI.21, Unter den Eichen 87, D-12205 Berlin, Germany Received 27 September 2000; Revised 30 May 2001; Accepted 2 July 2001 A scanning force microscopy (SFM) study encompassing a thermal and various mechanical contrasts was performed on a sample related to automotive brake pads. The sample consisted of polybutadiene (PBD) and the solid lubricant antimony sulphide (Sb 2 S 3 ), both dispersed in a polymeric matrix of phenolic resin. Via this highly simplified composition, the assignment of contrasts to the ingredients was assured. The mechanical measurements, namely the stiffness measurements via force modulation microscopy (FMM) and the friction measurements via lateral force microscopy (LFM) and harmonically modulated lateral force microscopy (HM-LFM), were carried out in one combined experiment (using the same silicon tip). Owing to the available stiffness data, the results of the lateral force measurements are discussed in terms of the size of the mechanical contact radius. Compared with LFM, HM-LFM is less prone to topographic artefacts and shows enhanced contrasts. The additional phase image indicates deviations from perfect elastic behaviour. Consistently with the FMM measurement, the HM-LFM phase image displays a distinct phase shift on PBD. For performing thermal measurements, a probe made up of Wollaston was employed. The resulting images are interpreted in terms of the thermal diffusivity and the thermal conductance of the tip–sample contact. Beyond the mechanical properties of the material under investigation, the influence of the bridging water layer also has to be taken into account. Copyright  2002 John Wiley & Sons, Ltd. KEYWORDS: antimony sulphide; polybutadiene; phenolic resin; SFM/AFM; friction; thermal diffusivity INTRODUCTION The numerous components of friction pads for automotive brake systems can be classified roughly as abrasives, solid lubricants, reinforcing fibres, space fillers and the binder resin. The types and relative amounts of ingredients of the brake pad have to be matched to several technical design criteria such as noise propensity, brake-induced vibration, reliable strength, wear resistance, as well as high and stable friction force. 1 These requirements have to be met for a wide range of temperatures of the frictional interface, of revolution frequencies and of loading cycles. In the literature only a small number of studies are reported, shedding some light on the tribomechanical mechanisms and on the role that particular additives play therein for ensuring the performance criteria. In particular, the importance of solid lubricants and abrasives was studied. Metal sulphides such as lead sulphide (PbS) and antimony sulphide (Sb 2 S 3 ) are supposed to modify and stabilize the friction coefficient. 2 During operation of the braking system, Ł Correspondence to: M. Munz, Federal Institute for Materials Research and Testing (BAM), Lab. VI.21, Unter den Eichen 87, D-12205 Berlin, Germany. E-mail: martin.munz@bam.de Contract/grant sponsor: Deutsche Forschungsgemeinschaft (DFG). these solid lubricants are expected to build up a stable friction film, and an abrasive such as zirconium silicate (ZrSiO 4 ) or aluminium oxide (Al 2 O 3 ) is added to remove thermally decomposed layers of the friction film. 1 Thus, the braking performance is governed by a complex interplay of the action of particulate additives, chemical reactions, thermal decomposition of organic ingredients and wear mechanisms. Using pad-on-disc-type tests, dynamometer tests or in-vehicle tests, both commercial brake pads and simplified formulations were investigated. Besides the macroscopic simulation of the operational conditions of technical systems, a more fundamental approach can be devised. From the microscopic point of view, the surface of the rotating grey cast-iron disc can be modelled as an ensemble of asperities covering a broad range of surface energies, stiffness values, heights and widths. By scanning a single asperity over the surface of a brake pad of well-known composition, one can get some basic information concerning the mechanical and thermal interaction between the asperity and the particulate constituents of the brake pad. This kind of elementary single-asperity experiment can be performed by using a scanning force microscope (atomic force micro- scope), tooled up for measuring several material properties. The resulting data may allow a more detailed description DOI: 10.1002/sia.1171 Copyright  2002 John Wiley & Sons, Ltd.