Materials Science and Engineering A 425 (2006) 213–218 Chemical and structural nature of tribo-surface of aluminium–SiC composites at nanometre and micrometre length scales K.M. Shorowordi a,∗ , A.S.M.A. Haseeb a , J.P. Celis b a Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka 1000, Bangladesh b Department of Metallurgy and Materials Engineering (MTM), Catholic University of Leuven, Kasteelpark Arenberg 44, Heverlee 3001, Belgium Received 14 September 2005; received in revised form 3 March 2006; accepted 20 March 2006 Abstract The worn surface of Al–SiC metal matrix composites (MMC) sliding against phenolic brake pad at a linear sliding speed of 1.62 m s -1 under contact pressures of 0.75–3.00 MPa in a pin-on-disc apparatus was investigated. XPS was used to extract information from the top few nanometres of the worn surface, while scanning electron microscopy and energy dispersive X-ray microanalysis (SEM–EDX) provided information from within a few micrometres. Results reveal that the surface of Al–SiC undergoes significant chemical and physical changes during wear. The tribo-surface on Al–SiC is converted into a mixture that contains the constituents of Al–SiC and the phenolic pad counter body as well as oxygen from atmosphere. The worn surface obtained in the present study is suggested to consist of a relatively finely mixed top layer of a few m in thickness. The topmost few nanometres of this finely mixed layer is totally oxidized. In addition to the continuous top layer, a thick mechanically mixed layer (MML) termed as massive MML also forms. Its thickness and coverage is found to be load dependent. The massive MML is found to be stratified at places; it contains defects and is rather heterogeneous at the micrometre scale. The characteristics of the modified surface are discussed and a schematic model for the MML is proposed. © 2006 Elsevier B.V. All rights reserved. Keywords: Al–SiC MMC; Phenolic brake pad; Wear; Mechanically mixed layer 1. Introduction The development of metal matrix composites (MMC) having a high strength-to-weight ratio is of great importance partic- ularly in the aerospace and automotive sectors where weight saving leading to lower fuel consumption is a prime considera- tion. The tribological properties of Al–MMC reinforced by hard ceramic particles have also been found to be quite attractive. They have the potential to replace conventional tribomaterials in many applications. Improved tribological properties coupled with high strength-to-weight ratio of Al–MMC already led their applications to automotive brake rotor, piston, etc. A considerable amount of work has been done on the tribol- ogy of Al–MMC, particularly on the Al–SiC system. In these studies, mainly ferrous materials have been used as counter body ∗ Corresponding author at: Institute of Appropriate Technology (IAT), BUET, Dhaka 1000, Bangladesh. Tel.: +880 2 9662365; fax: +880 2 8613046. E-mail address: kmshorowordi@iat.buet.ac.bd (K.M. Shorowordi). [1–8]. A few studies involving brake pad material as counter body have also appeared recently [9–12]. One important char- acteristic of the tribology of Al–MMC is that its worn surface is covered by a layer which contains the constituents of both MMC and the counter body. This layer has been termed as mechan- ically mixed layer (MML). It was postulated that the presence of hard ceramic particles in MMC helps to create conditions favourable for the formation of MML [8]. The presence of the mechanically mixed layer was suggested to control greatly the wear rate and friction coefficient of Al–MMC [1–10]. It has been shown by specially designed experiments [2] that under condi- tions where MML is not allowed to form, the wear rate of MMC is quite high. The nature of MML has been investigated in a number of studies [2,3,8] mainly by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX). These studies suggested that MML forms as a result of turbulent mixing of the constituents of the counter body and MMC. Another mechanism based on debris burial has also been suggested by others [5]. A couple of schematic models have 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.03.058