Microstructure and tribology characteristics of head and chest exoskeleton of Procambarusclarkii

Abstract: Procambarusclarkiihas remarkable ability to burrow and move efficiently underground through a range of harsh and paddy soil environments. To investigate the friction and wear behavior of Procambarusclarkii,its head and chest exoskeleton were chosen as the object of this study. The content and presence of inorganic elements in exoskeleton were analyzed by X-ray fluorescence spectrometer (AxiosMAX, PANalytical, Netherland) and X-ray diffractometer. The microstructure of the exoskeleton of the head and chest was observed by field emission scanning electron microscopy (NovaNano-SEM450, FEI, USA). The hardness and elastic modulus of the thoracic and lateral exoskeleton were measured by nanoindentation system(NanoindenterG200-MTSNanoInstruments). The JLTB-02 friction and wear tester (JLTB-02J<ech, Korea)was used to carry out friction and wear test. The tribological characteristics of the exoskeleton were investigated. Meanwhile, the wear morphology of the wear marks was observed by field emission scanning electron microscopy. The test result showed that its head and chest exoskeleton contain abundant calcium, and most of them are present in a form of amorphous structure and with a small amount of calcium carbonate. The surface of head and chest exoskeleton had a concave and convex hull bristle microstructure. Its exoskeleton was composed of upper epidermis, outer epidermis and inner epidermis. The upper epidermis was made up of thin waxy layers. Calcium salt deposits and crustacean protein fibers constituted the outer and inner epidermis, and the outer epidermis and the inner epidermis were in the form of twistedplywood. The needle-like calcium salt was unevenly distributed in a spiral splint, similar to a bone structure. The hardness of the exoskeleton was 0.503 GPa and the elastic modulus was 18.019 GPa. The twistedplywood had good scalability. After the load was unloaded, the twistedplywood could be well restored. There were many abrasive grains in the furrow at the wear location of the specimen. This showed that the friction behavior is abrasive wear. The friction factor showed a leaping change, the minimum friction factor was less than 0.1, and the maximum value was around 0.8. Many pores were distributed over the calcium salt deposits and the twisted-plywood. The main function of these pores was to deliver nutrients. When the Si3N4 ceramic ball and the exoskeleton rubbed against each other, the continuous sliding friction behavior was interrupted and mitigated by these pores. As the sliding friction behavior continued, the chitin in the spiral plywood was quickly worn away. The needle-like calcium salt was exposed to the friction surface. A non-smooth characteristic of the wear surface geometry was formed on the friction interface. Small free abrasive particles couldeasily roll on the non-smooth surface with less resistance. Therefore, the friction was reduced and the friction factor showed a jumping change. This research can provide a bio-inspired basis for the innovative design of agricultural composite materials and anti-friction surface of soil-engaging components. In addition, the experiment data can provide theoretical basis for the study of the tribology properties of soil animal exoskeleton in the future by observing the microstructure and analyzing the friction properties.