Wear particles are produced when materials rub against each other. It has been identified that wear particles carry substantial information about the wear processes experienced by a material working in a tribological environment. Through careful examination of the wear particles, the wear mechanisms and the cause of wear can be successfully deduced [I]. In addition to the scientific significance, investigation of wear particles is also important in the industrial field, for example, a ferrography technique is now being used for on-line inspection of metallic frictional couples. However, such an analysis and interpretation depending on unreliable visual assessment cammt always guarantee the necessary objectivity and accuracy. Evidently, a numerical method that is independent of subjectivity is needed. A recent development in this direction is the application of fractals to the quantitative characterization of wear particles [2, 3]. Fractal geometry describing non-Euclidean objects [4] has proved to be effective for analysing the chaotic morphology of wear particles. It was further found that the fractal dimensions of wear particles were closely related to the wear behaviour of the bulk material, and can serve as a measure of wear rate [5]. The present letter is a continuation of previous work [5] in which the fractal dimensions of wear particles generated at different loadings were measured by the "slit island method" following sliding wear of polyetheretherketone (PEEK) under unlubricated conditions against steel. The main objective here was to examine the sensitivity of a different fractal characterization technique, i.e. the Richardson method [6]. The materials employed in this investigation were PEEK plates with a reduced viscosity of 0.62 ml gl , kindly supplied by Jilin University, China. The specimens for wear tests were machined from these plates with a geometry of 5 x 10 x 15 mm, resulting in an apparent contact area of about 5 x 10 man 2. Room temperature sliding wear tests were carried out on a pin-on-ring apparatus [7] under different loadings at a constant velocity (v = 0.4 msi ) . The carbon steel rings (0.42-0.45 wt% C, 0.170 .37wt% Si and 0 .50-0.80wt% Mn, HRC 50) had a diameter of 40 mm and an initial surface roughness Ra of 0.1 Nm. When the apparatus had operated for 2 h in steady state, the wear test was