Effect of Bionic Neutral Complexing Agents on the Friction and Wear Behavior of Natural Granite

Currently, rock breakage and removal in rock engineering is mainly performed by using tools, and thus the wear of tools is inevitable. When encountering rocks with high hardness and abrasiveness such as granite, tool wear is very severe, thereby causing low rock-breaking efficiency. In fact, there exists organisms in nature which secrete or metabolize organic acids to induce mechanical weakening in rocks, and this phenomenon is called bioerosion. In this study, inspired by bioerosion, three bionic neutral complexing agents were prepared using three organic acids, citric acid, tartaric acid, and oxalic acid, and referred to as CA, TA and OA, respectively, and their effects on the friction and wear behavior of granite (a typical hard rock) were investigated using two-body wear tests in a reciprocating horizontal tribometer. A normal force of 2 N, reciprocating amplitude of 2 mm, and frequency of 2 Hz were used, and tests up to 5 000 cycles were conducted in a ball-on-flat configuration with deionized water bath lubrication. Natural granite was prepared as flat rock samples through cutting, embedding, grinding and polishing. H13 steel ball with a diameter of 10 mm was used as counterpart. A total of 15 flat rock samples were used to do wear tests. 3 polished rock samples were randomly selected for wear testing as “original” samples. The rest of the polished rock samples were divided into four groups with 3 samples in each group. Each group was corroded for 3 min with either the as-prepared neutral complexing agents or hydrochloric acid solution in same concentration (which was used as control agent). Thus, four groups of corroded rock samples were obtained, which were referred to as “CA treated” samples, “TA treated” samples, “OA treated” samples and “HCl treated” samples, respectively. The surface morphologies of samples were characterized by laser confocal scanning microscopy, while the surface hardness was measured using Vickers Indenter. The wear of flat and ball samples was evaluated by wear volume. Results showed the as-prepared bionic neutral complexing agents have no obvious corrosiveness to H13 steel, but they all had the potential to corrode the granite. Compared with the hydrochloric acid solution, the three neutral complexing agents had more significant corrosiveness to the granite, manifested by obvious crack expansion and corrosion pits on the surfaces of the CA, TA, and OA treated samples, especially the OA treated samples. As a result, the connection between rock mineral particles was weakened and then the mechanical properties of granite surface decreased. Accordingly, under the same conditions, more significant friction and wear occurred on the surfaces of the CA, TA, and OA treated samples than on the surface of the original rock sample, which was characterized with increased friction coefficient and wear volume. And more obvious ploughing effect and delamination traces appeared on the worn surfaces of the corroded rock samples. Likewise, the friction and wear on the surface of the OA treated sample was the most severe. The observations suggest that the wear of granite under the given condition was dominated by abrasive wear, which was aggravated by the 3 min-corrosion treatment in the bionic neutral complexing agents, especially the OA agent. Clearly, the complexation of organic acids with rock minerals had more obvious influence on the tribological behavior of granite than the acid-attack of strong inorganic acids such as hydrochloric acid. Due to lower molecular weight and simpler molecule structure, oxalic acid was more likely to induce the dissolution of mineral particles on granite surface through complexation and then facilitated the tribo-removal of rock materials, as compared to citric acid and tartaric acid.