1. Introduction
Cemented carbide is a powder metallurgy product that is widely utilized around the world [
1]. Because of its adaptable combination of transverse fracture strength, hardness, toughness, and wear resistance, WC-Co cemented carbide is utilized extensively in many associated industries, including cutting tools for milling metal parts in the machining industry, wear components in molds, and drill parts in mining areas, etc. [
2]. In the field of machining, these tools are known as the teeth of the industry, and WC-Co cemented carbide is often used as their substrate. However, these tools suffer from the challenges of strong friction, huge thermal shock, and high-temperature oxidation in the cutting area during the service process. It is difficult for a single cemented carbide material to meet the processing requirements of difficult-to-machine materials and high-speed cutting [
3]. Although a modest number of additives (VC, Cr
3C
2, Mo
2C, NbC, TaC, etc., and their mixed additives) can be used to improve the mechanical characteristics [
4,
5], the hard coating is still an irreplaceable method to enhance the comprehensive performance of tools. As a result, almost all tool surfaces in current machining applications are prepared with commercial hard coatings by CVD or PVD [
6].
TiN, a binary nitride of transition metal, is still frequently employed as a protective hard coating for bearing, cutting, and forming tools due to its low cost, superior abrasion resistance, and corrosion resistance [
7]. TiN coating on tools can improve the product’s surface quality and extend the tool’s service life. The golden yellow color of the TiN coating makes it useful as a tool wear indication. However, TiN coatings undergo severe oxidation at operating temperatures above 500 °C, resulting in coating failure [
8]. Researchers have enhanced the cutting performance of TiN-based coatings by incorporating additional components (Al, Cr, Si, Y, etc.) into binary TiN coatings to create multi-component coatings (TiAlN [
9], TiCrN [
10], TiSiN [
11], TiAlSiN [
12], etc.) in order to fulfil the demanding processing requirements of difficult-to-machine materials. These multi-component coatings provide outstanding wear resistance, toughness, high-temperature oxidation resistance, and hardness characteristics. However, the addition of doping elements can cause lattice distortion in the crystal, which makes the coating have higher residual stress, resulting in poor film-substrate bonding. It is necessary to deposit a bonding layer between the multi-component coating and the substrate to improve the bonding force, among which TiN is one of the commonly used bonding layers [
13]. In the research of many scholars, the application of the TiN contact layer significantly improves the film-substrate adhesion and cutting performance of the coating. According to Zhong et al., a rich WC(0001) and WC(10
0) cemented carbide substrate is advantageous for forming a coherent or semi-coherent interface between TiN and WC, enhancing the interface structure [
14]. These characteristics improve the coating-substrate system’s capacity to withstand damage from outside forces. As a result, commercial cemented carbide-coated tools, including CVD [
15] and PVD [
16] coatings, are frequently made with TiN serving as a bonding layer.
A good combination of the coating and the substrate is an important guarantee to improve the durability of the coating. The adhesion between the cemented carbide and coatings depends on WC-film bonding, film-substrate interlocking, and WC carbide embedment. Studies by Bouzakis et al. show that mechanical treatment of the substrate can significantly improve the machinability of the coating [
17]. During the CVD coating process, the high-temperature increases the grain size of WC and reduces the residual compressive stress, which in turn leads to annealing and softening of the cemented carbide material. This softening reduces the cutting performance of CVD-coated carbide tools, resulting in high coating deformation and premature fatigue fracture. Skordaris et al. further proposed to improve the performance of the substrate based on the appropriate heat treatment technology to improve the cutting performance of the CVD coated tool [
18]. These studies provide an explanation for the adhesion mechanisms between the cemented carbide and coatings in terms of the film-substrate interlocking and WC carbides embedment, while studies on WC−film bonding are lacking.
First-principles calculations are frequently employed in contemporary scientific research because they provide insight into interface structures at the atomic and electronic levels [
19]. It is an important tool for in-depth explanation of some interface issues. Zhao et al. used first principles to reveal the interface structure and electronic characteristics of wear-resistant coating WC/TiC [
20]. Zavodinsky et al. researched the adhesion in the TiN/ZrN layered systems by density functional theory (DFT) and pseudopotentials methods, which revealed that the interfacial structure greatly affects the adhesion of TiN/ZrN [
21]. Rao et al. revealed the mechanism by which TiN-promoted graphitization transformation of diamond decreases the interface bond strength through TiN/diamond interface property calculations [
22]. Through first-principles calculations, Fan et al. identified the bonding mechanism at the interface of TiC(111)/TiN(111) with various atomic stacking configurations [
23]. All these studies have proved the importance of theoretical calculation in the study of interface structure. This method makes up for the lack of traditional experiments that lack atomic resolution.
Studying the interface characteristics and electrical structure of the film-based interface is important in order to fully utilize TiN film for the interface structure design of the coating in the tool application and improve the overall performance of the multi-component coating. Cemented carbides can be divided into about 20 types according to their microstructure and chemical composition. Among them, hexagonal WC occupies the most prominent position in all hard phases of cemented carbide. More than 80% of all carbide grades contain WC, many of which are pure WC-Co alloys [
1]. Co is distributed in the gaps of WC particles. The Co content of the cemented carbide used for cutting tools is low (≤12 wt.%), and the Co on the surface layer will be selectively etched away during the ion etching process prior to coating deposition [
24]. Therefore, the interface bonding between cemented carbide and TiN film is mainly the bonding between WC and TiN. However, first-principles calculations of the WC/TiN interface have not been reported so far. The bonding strength between WC/TiN depends on their interfacial relationship. In this study, the structure and properties of the interface between WC and TiN were investigated by first principles, which could provide a theoretical basis for explaining their adhesion strength and stability.