Angle resolved scattering as a tribological investigation tool for surface characterization S. Azouigui a , Z. Silvestri a,n , C. Zerrouki b , S. Bouhtiyya a , M.D. Plimmer a , D. Spaltmann c , A. Kovalev c , M. Woydt c , P. Pinot a a Laboratoire Commun de Métrologie LNE-CNAM, 61 rue du Landy, 93210 La Plaine-Saint-Denis, France b SATIE, UMR 8029 CNRS-ENS Cachan-Cnam, 292 rue Saint-Martin, 75003 Paris, France c BAM, Bundesamt für Materialforschung und-prüfung, Unter den Eichen 87, 12205 Berlin, Germany article info Article history: Received 29 September 2014 Received in revised form 23 December 2014 Accepted 27 December 2014 Available online 3 January 2015 Keywords: Surface roughness Angle-resolved scattering Wear Isotropy abstract This paper shows how Angle-Resolved Scattering can reveal wear on engineered surfaces. The samples studied, three discs made of steel 100Cr6H used in gear wheels in the automotive industry, were assessed after they had undergone ball-on-disc tests. Scattering maps recorded for spatial frequencies from 0.1 mm 1 to 2.7 mm 1 show a contrast inversion around 0.36 mm 1 revealing the wear trace. Besides measurements of rms roughness, where mean values are 20 nm and 24 nm depending on the locations considered, stationarity and isotropy, we demonstrate the ability of the technique to reveal localized wear on this type of surface in a rapid, robust and convenient way. We show that the tool trace is influenced by the load magnitude rather than by the number of oscillation cycles and highlight the key role played by surface roughness in how the sample responds to wear tests. & 2014 Elsevier B.V. All rights reserved. 1. Introduction In the automotive and railway industries, great effort is devoted to the design of special steel grades with minimal wear so as to enhance product lifetime and guarantee safe operation [1]. At the same time, the detection of small wear volumes on such surfaces remains a challenge in physical tribology. Although wear is often visible to the naked eye, assessing the wear of engineered surface using robust and accurate standard techniques is difficult because wear processes can occur at the nanometer scale. Contact profilo- metry is one of the most common methods for wear measurement [2]. Several techniques based on it have been developed to provide qualitative and/or quantitative information on surface topography in both two and three dimensions. In addition, for the last two decades, STM and AFM have also been widely used for quantitative surface characterization, the measurements of film thickness and detection of wear [3]. With atomic force and scanning tunneling microscopies (AFM and STM), mechanical profilometry and optical methods (interferometry, scattering, etc.), resolution at the nanometer level can be achieved [4,5]. However, each technique has advantages and drawbacks. The choice of one or several instruments to monitor surface changes due to tribological tests, depends particularly on the type of wear produced [2] (adhesion, abrasion, fatigue with and without lubrication) and the chemical and physical properties of the material in question (hardness, surface structure, and chemical reactivity) [6]. The ability of each method to reveal wear or not depends on the spatial frequency domain covered and the vertical resolution of the instrument. In mechanical profilometers, the spatial frequency domain is limited by the stylus radius (according to the norm ISO 3274 [7], to study smooth surfaces a conical stylus with a tip radius of 2 mm is required), while in scanning probe microscopy, the limit is imposed by the tip radius (typically 6 nm)[5]. AFM is one of the methods able to reach the high spatial frequency domain (typically 10 2 to 3  10 2 mm 1 ), and so detect wear with a vertical resolution better than 0.05 nm. For spatial frequencies around 1 mm 1 , all the aforementioned techniques can be used but light scattering methods, being more robust and easier to implement are preferable. Moreover, contact profilometry and AFM/STM have two common drawbacks. The first is the risk of damage to the stylus and the second one, the possible the apparent smoothing of high spatial frequency defects, as a result of con- volution of the apparatus function with surface profile. As with all measurement techniques at the nanometer level, they are sensitive to vibration (and therefore difficult to use in online measure- ments), environmental conditions (temperature, humidity and air Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear http://dx.doi.org/10.1016/j.wear.2014.12.040 0043-1648/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ33 1 40 27 20 24; fax: þ33 1 58 80 89 00. E-mail addresses: slamkadmi@yahoo.fr (S. Azouigui), zaccaria.silvestri@cnam.fr (Z. Silvestri), chouki.zerrouki@cnam.fr (C. Zerrouki), said.bouhtiyya@cnam.fr (S. Bouhtiyya), mark.plimmer@cnam.fr (M.D. Plimmer), dirk.spaltmann@bam.de (D. Spaltmann), alexander.kovalev@bam.de (A. Kovalev), mathias.woydt@bam.de (M. Woydt), patrick.pinot@cnam.fr (P. Pinot). Wear 326-327 (2015) 58–67