Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Assessment of abrasive powder behaviour during impact-abrasive wear of PCD elements D. Gomon, F. Auriemma, M. Antonov * Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia ABSTRACT One of the most suitable renewable energy sources is geothermal energy (providing heating as well as electricity). In order to achieve a suitable depth of drilling (several kilometres), it is required to increase the wear resistance, durability and reliability of key components of the deep drills. The effect of abrasive powders on impact energy transmission and/or damping during wear of Polycrystalline Diamond (PCD) cutting elements was evaluated due to its highest resistance in abrasive conditions. The characteristic features of wear mechanism are presented and discussion is supported by SEM images and EDS maps. The strength of silica sand, quartzite, granite, basalt, marble, limestone, pumice is compared to the force transmitted through the contact zone, damping characteristics and stiffness of abrasive particles. It was found that the laboratory impact-abrasive device enables to initiate damage, characteristic for specific abrasive powder, imitating drilling of such mineral. The mechanism of wear of PCD elements in impact-abrasive conditions depends on the strength and shape of abrasive particles as well as on their behaviour during impact (impact energy damping). The wear of PCD elements in the impact-abrasive conditions was close to zero and their use in new generations of deep drills is expected. 1. Introduction Over the course of the last 20 years a dramatic increase in use of natural resources has been observed. The extraction of natural re- sources, such as oil, coil and gas, has been conducted at an alarming rate, due to economic and technological demands. Some of the dramatic predictions are expecting a complete depletion of natural resources within the next 50 years. As a result, the European Union Energy Strategy, as well as other international agreements, aim to achieve more competitive, secure and sustainable energy system by tackling a long-term greenhouse gas reduction target. The most suitable renew- able energy across the globe arguably is the geothermal energy, which not only provides heat in its raw form of heated beds and fluids, but also as electricity through steam power stations [1–4]. The minimum depth to benefit from geothermal sources is at 150–200 (m) where tempera- tures of 6–8 °C could be reached; the conventional high-temperature wells are located already at depths of 500–2000 (m) providing tem- peratures of 250–350 °C, whereas the maximum attainable depths of 3500–5000 (m) can provide temperature range of 400–600 °C [5]. Reaching these extreme depths is both economically desirable yet highly complex endeavour. Multiple factors separately or all together have an impact on the successful drilling process, such as design and material of the drill bit, hardness and abrasiveness of the drilling for- mation, depth of the drilling chamber, stability of the drill among others [6,7]. To reach any of the aforementioned depths, a thorough understanding of the drilling bit's material and/or media to be drilled and its effect on the drilling bit must be obtained. This information can provide necessary data to predict and simulate the reachable stresses exerted onto the drilling bits in different media and its lifetime. The most commonly used materials for drilling bits is the poly- crystalline diamond (PCD) and cemented carbides (various WC-Co grades with or without additives). As a result, the polycrystalline dia- mond compact (PDC) bits are widely used for drilling in soft and medium bedrocks, due to their superior abrasion- and wear-resisting characteristics [8–11]. However, these characteristics dramatically de- crease when PDC bits are employed on much stronger hard-rock stratum. During the drilling process, the drilling bit can interact with highly inhomogeneous and irregular formations of rocks. The changes within the layers of rock, as well as the presence of highly pressurized voids with liquids or gasses, result in extremely unstable and un- predictable impact-abrasive loading of the drill bit [12]. PCD's rela- tively low fracture toughness can lead to premature failure of the cut- ting tool [13–16]. Moreover, the evolution of the stress fields on the cutting edge of the drill bit due to abrasive wear combined with the pre- existing intrinsic manufacturing and thermal stresses [8,11,17–19] may lead to fracture. The PDC fabrication process involves utilization of a metal binder cobalt, which infiltrates the diamond crystals from the cemented car- bide substrate. However, the cobalt acts not only as a binder, but also as a catalyst of diamond to graphite transformation [9,20,21] Thus, the https://doi.org/10.1016/j.wear.2019.03.024 Received 10 September 2018; Accepted 25 March 2019 * Corresponding author. E-mail address: Maksim.Antonov@taltech.ee (M. Antonov). Wear 426–427 (2019) 151–161 0043-1648/ © 2019 Elsevier B.V. All rights reserved. T