Surface characterization of wear particles

Wear particles are three-dimensional objects. Recent advances, accelerated by the application of computer technology, allow numerical characterization of particle shape in two dimensions. However, three-dimensional (3D) chara cterization of wear particle surface topography is still largely an unresol ved problem. There are two issues associated with this problem, i.e., the a cquisition of accurate wear particle surface topography data and the numeri cal description of 3D surface features. The usually small size of wear part icles restricts the techniques that could be used to acquire accurate data from the particle surface. Surface profilometers, e.g., Talysurf, the instr uments traditionally used in surface topography imaging, cannot be used bec ause of the small size of particles. The limitation of an atomic force micr oscope is its relatively small vertical range, while the horizontal resolut ion of laser confocal or interferometric microscopes is too low to obtain a ccurate particle surface topography data. The application of a combination of SEM and stereoscopy techniques seems to alleviate this problem. 3D surfa ce topography data obtained using this technique can be processed and prese nted in many different ways. The usefulness of various methods of surface d ata representation in visualization and numerical characterization of wear particle surfaces is discussed. One of the major difficulties associated wi th the characterization of surface topographies is the accurate description of surface spatial properties, i.e., their anisotropy and directionality. Recently, a specially modified Hurst Orientation Transform (HOT), to suit w ear particle surface data, has been developed and applied to characterize t he surface topography of particles. The Hurst coefficients are related to f ractal dimensions and are a measure of surface roughness, i.e., a rougher s urface is represented by lower Hurst coefficients. It was found that the mo dified HOT can be applied to reveal the surface anisotropy of wear particle s. Although none of the other methods developed so far allow such a thoroug h characterization of wear particle surfaces as does the modified HOT, this method still does not provide a full description of the surface topography . Therefore, it appears that a totally different approach is needed in orde r to make a fundamental breakthrough in the characterization of wear partic le surfaces. Since many of the complex structures observed in nature can be described and modelled by a combination of simple mathematical rules, it m ay be possible to describe the surface of a particle by a set of such rules . In our first attempt, a Partitioned Iterated Function System (PIFS) was a pplied to encode the wear particle surface topography information. This inf ormation can then be used to calculate the relevant surface descriptors. In this paper, an overview of recent advances and developments in the numeric al characterization of wear particle surfaces is presented. (C) 1999 Elsevi er Science S.A. All rights reserved.