Influence of iron oxide coating on the tribological behavior of sand grain contacts

Granular materials are often subjected to certain degrees of coating in natural conditions due to weathering and soil–environment interactions and the current literature has suggested that the development of surface coatings has an important influence on the macroscopic response of geological materials. In the present study, a new method was developed to artificially coat sand grains with iron oxides and a series of experiments were performed for surface morphological and mechanical characterization as well as quantification of the tribological behavior of iron oxides coated grains. Through the material characterization tests, it was observed that the artificial iron oxides coating notably altered the roughness and hardness of the grains, which in turn resulted in higher inter-particle friction and lower normal contact stiffness. Clear dependency of the coefficient of friction on the magnitude of normal load and the loading history was observed for the coated specimens, which is speculated to be attributed to asperity breakage, plowing damage and debris build-up of the surfaces as also supported by microscopic images. The test results could also provide, qualitatively, some multi-scale insights into the influence of iron oxide coating on the bulk behavior of natural soils as the increase in inter-particle friction angle can be linked with the observed trend of increased critical state strength.