Effect of abrasive particle size distribution on the wear rate and wear mode in micro-scale abrasive wear tests
Introduction
In micro-scale abrasion wear tests, a normal load forces the specimen against a sphere in the presence of an abrasive slurry and wear is analyzed based on the evolution of the diameter of the worn crater as a function of time [1], [2], [3]. This type of test is conducted with abrasives with average particle size usually below 10 μm, which, in addition to the selected normal loads, make this type of abrasive test suitable for the analysis of small volumes.
Many works have been dedicated not only to the use of micro-abrasive wear tests in the tribological analysis of engineering materials, but also to a better understanding of the test itself. One of the key issues in this test is the identification of the wear modes at the specimen, which are usually classified into grooving abrasion and rolling abrasion [4], [5], [6], [7], [8], [9]. Grooving abrasion refers to the condition in which the abrasive particles slide against the specimen, while rolling abrasion is the wear mode associated with the rolling of the particles in the gap between the sphere and the specimen. Both modes can occur simultaneously and, depending on the test conditions, grooving abrasion may be observed in one area of the worn crater and rolling abrasion in another [4], [5], [6], [7], [8], [9]. Moreover, Cozza et al. [10] have defined ‘micro-rolling abrasion’ as a localized mixture of both wear modes, namely for the case where particles rolled along grooves formed previously.
Several wear mode studies are based on the mechanics of particle motion. More specifically, loads and constraints at an abrasive particle are evaluated in order to understand the conditions that would result in its rotation in the gap between the two bodies in contact [4], [5], [6], [11], [12]. Following these ideas, Adachi and Hutchings [4], [5] developed a wear mode map for micro-abrasive wear tests, in which it is possible to predict the occurrence of grooving or rolling abrasion based on (i) the ratio between the hardness of the specimen Hs and the hardness of the sphere Hb and (ii) a parameter called severity of contact, which depends on the applied load W, the interaction area A and the volume fraction of abrasive particles υ, as well as Hs and Hb. This map reflects the tendency for grooving abrasion to increase as the separation h between the surface of the specimen and the surface of the ball decreases, or, in other words, as the severity of contact increases. According to those authors [4], [5], separation could be calculated based on Eq. (1), in which, besides the terms previously defined, c is a constant of proportionality and H′ is dependent only on Hs and Hb. This model was defined assuming spherical abrasive particles with diameter D.
More recently, some works in the literature [7], [9], [13] have highlighted the fact that the severity of contact may decrease during a given micro-abrasive test, due to the fact that tests conducted with constant normal load are associated with a continuous decrease in contact pressure due to the increase in crater area. Lower contact pressures are associated with lower forces applied to each particle.
The transition between grooving and rolling abrasion was also analyzed by Trezona et al. [6], considering the effect of the parameters selected during micro-abrasive tests. In particular, those authors studied the effect of load, slurry concentration and abrasive material. A nonlinear behavior was observed when wear volume was plotted as a function of the volume fraction of abrasive particles, i.e. as a function of slurry concentration. In this case, maxima in wear volume were observed in curves obtained for different normal loads. Furthermore, at low slurry concentrations, similar wear volumes were obtained for three applied normal loads and a continuous decrease in wear volume with the decrease in the volume fraction of abrasive particles was observed at this portion of the graph.
Despite the significant amount of work dedicated to the study of the wear rates and modes in micro-abrasive wear tests, most of the analyses conducted so far are based on the average value of a given particle size distribution. Thus, at this point, it is not clear if the wear rate in micro-abrasive tests may be affected by the particle size distribution. Similar questionings are possible in terms of the wear modes, including the possibility of considering particle size distribution as one of the reasons for micro-rolling abrasion.
In this work, micro-abrasive tests were conducted with different silicon carbide powders, in order to evaluate the effects of particle size distribution. Each powder was prepared by mixing different mass fractions of two original powders, one with average particle size of 2 μm and another with average particle size of 6 μm.
Section snippets
Experimental procedure
Micro-abrasive wear tests were conducted in a test rig with fixed-ball configuration (TE 66 from Plint and Partners, Wokingham, UK). The sphere had a diameter of 25.4 mm and was made of AISI 52100 steel. The specimens were manufactured on ASTM 1020 carbon steel, presenting a testing area of 30×30 mm2. Prior to the tests, all specimens were sanded up to 1200 SiC paper, followed by polishing to a mirror finish with 1 μm alumina paste.
In order to prepare the abrasive slurries, the silicon carbide
Results and discussion
Fig. 2 presents a typical topographic analysis of the geometry of the worn craters. This particular example refers to a crater obtained with 100L slurry, i.e. a slurry prepared entirely with the original powder with larger average particle size (Fig. 1). Fig. 2 indicates that the craters generated during the tests were spherical, with diameter of 25.4 mm, confirming the possibility of using the equations for wear volume and wear coefficient available in the literature [3], [6].
Fig. 3 presents
Conclusions
In this work, micro-abrasive wear tests were conducted with slurries presenting different particle size distributions. Each slurry was prepared by mixing two original powders with different average particle sizes. Minima in wear volume and wear rate were observed for the slurries presenting 50% in mass of the large powder and 50% in mass of the small powder, as a result of the variation in the number of active abrasive particles in each case. This variation was also identified as the reason for
Acknowledgments
The authors would like to thank the financial support of the Brazilian agency Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); the Partnerships Program for Education and Training (PAEC) of the Organization of American States (OAS) and the National Institute of Surface Engineering, one of the CNPq National Institutes of Science and Tecnology (INCTs).
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