Preparation of a Self-Lubricating Cu/h-BN Coating on Cemented Carbide

Hindawi Advances in Materials Science and Engineering Volume 2018, Article ID 8150791, 12 pages https://doi.org/10.1155/2018/8150791 Research Article Preparation of a Self-Lubricating Cu/h-BN Coating on Cemented Carbide Tongkun Cao , Zhibin Zhu, and Yajun Liu College of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao 266061, Shandong, China Correspondence should be addressed to Tongkun Cao; Received 23 June 2018; Revised 4 November 2018; Accepted 11 November 2018; Published 6 December 2018 Academic Editor: Massimiliano Barletta Copyright © 2018 Tongkun Cao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In this work, a Cu/h-BN self-lubricating coating was prepared on cemented carbide by electrospark deposition (ESD). The microstructure and properties of the coating were examined. The results showed that no decomposition of or reactions with h-BN occurred. As the h-BN content and capacitance increased, the number of pores and microcracks in the coatings increased. Additionally, as the capacitance increased, the electrode mass loss increased. However, the specimen mass increased first and then decreased. The coating thickness was affected by the capacitance, deposition time, and volume ratio of h-BN to Cu. The results exhibited were consistent over the tests. The self-lubricating coating exhibited excellent tribological behavior under the test conditions, and the worn surface showed features consistent with shear slippage and abrasive wear. 1. Introduction Cemented carbides are composed of tungsten carbide grains as the aggregate and a ductile metal binder matrix such as cobalt or nickel. Cemented carbides have been widely used in turning tools, molds, and mineral applications because of their high hardness, high toughness, high thermal conductivity, low coefficient of thermal expansion, high thermal shock resistance, and good wear resistance [1–5]. Carbide is more expensive per unit mass than other typical tooling materials and steel. Therefore, to justify this investment, it is necessary for the tool to have a long life. Heavy wear can lead to machinery failure, meaning that many engineering components require high wear properties compared to the substrate material to meet demanding operating environments [6]. One approach to improving the wear properties of a surface involves self-lubricating coatings [7, 8]. Selflubrication is the process of becoming lubricated without external factors. Self-lubricating coating is usually fabricated by soft metal and solid lubricants, which have low shear strength. The friction coefficient is proportional to the critical shear stress at the interface [9]. When there is a selflubricating coating on the worn surface, the load is borne by the substrate and the friction is dominated by the self- lubricating coating. Since the shear strength of the selflubricating coating is much smaller than that of the substrate, the friction coefficient can be reduced. So, selflubricating coatings can provide lubrication and reduce friction without any external lubrications. Solid self-lubricating coatings are mainly employed to control friction and wear, especially under harsh application conditions, such as aerospace, high vacuum, high speeds, high pressure, and very low or high temperatures [10, 11]. Under extreme conditions, conventional lubricants cannot provide the desired performance or durability, and in some cases, they cannot be used. At present, PVD (physical vapor deposition) [12, 13] and CVD (chemical vapor deposition) [14] are the most popular and familiar surface technologies to fabricate solid self-lubricating coatings. Additionally, significant attention has been paid to thermal spraying [15] and laser cladding [7, 16] in recent years. However, there is no low-cost preparation method to achieve thick coatings with a strong bonding strength that exhibit good selflubricating performance. Electrospark deposition (ESD) has some distinct advantages, such as minimal damage to the underlying substrate, metallurgical bonding at the interface, no pollution, high energy density, and low cost. When selflubricating coatings are fabricated by ESD, the coatings will 2 achieve a strong bonding strength because of the “cold welding” that occurs between coating and substrate. For soft metals, as matrix will envelope solid lubricants, soft metals and solid lubricants can cooperate with each other to increase the tribology and wear performance. Thus, ESD can be used to achieve thick coatings with a strong bonding strength and good self-lubricating performance in a convenient and cheaper way. However, ESD coating technology has presently focused on coatings such as Ti6Al4V [17] and TiC [18], instead of self-lubricating coatings. Hexagonal BN is a well-known solid lubricant [19]. Hexagonal BN has a lamellar crystalline structure similar to graphite and MoS2. However, hexagonal BN has been considered less effective than other solid lubricants, with the exception for high-temperature applications. Hexagonal BN shows remarkable chemical and thermal stability. For example, h-BN is resistant to breakdown at temperatures up to 1000°C in air, 1400°C in vacuum, and 2800°C in an inert atmosphere. Therefore, h-BN is a good lubricant at high temperatures (up to 900°C, even in an oxidizing atmosphere). Some studies have been focused on h-BN-based self-lubricating coatings. Ni60-hBN self-lubricating coatings [20] have been prepared by laser cladding. Under dry sliding conditions, these coatings exhibited excellent wear resistance compared to a Ti6Al4V alloy at relatively high temperatures (300°C and 600°C). A Co-BN (h) nanocomposite coating was prepared using conventional electrodeposition [21]. The results showed that Co-BN (h) nanocomposite coatings exhibited a higher hardness and a lower friction coefficient for the same conditions. Copper is a soft, malleable, and ductile metal with very high thermal and electrical conductivity [22]. Copper is also a lubricant due to its low shear strength. Along with the selflubricating properties of graphite, MoS2, etc., copper selflubricating composites have been widely used in many industrial applications, such as in brushes, contact strips, and bearing materials [22, 23]. Additionally, copper has been used in a self-lubricating coating as a matrix [24]. In our previous work [9, 25, 26], Cu/h-BN and Cu/CuMoS2 self-lubricating coatings were prepared on steel and high-speed steel, respectively, by ESD. The production process, microstructure, and tribological behaviors of the self-lubricating coatings have been preliminarily investigated, with the results showing that the self-lubricating coatings could reduce the friction coefficient and wear loss. However, cemented carbide is still not used as a substrate for preparing self-lubr (...truncated)