Tensile and fracture process of the TiN/VN interface from first principles

Abstract

We have conducted a comprehensive first-principles investigation of tensile and fracture process of the TiN/VN interface by focusing on the strain–stress relationships, the ideal tensile strength and bonding breaking process. The deformation analysis reveals that all bonds are extended at the initial stage until the critical point of fracture is reached. The fracture occurs between the second and third layers on the VN side in both (111) and (001) interfaces and the interaction of potential bonds of facture weakens gradually with the increment of strain during the tensile process. By using several analytical techniques, we identify a charge transfer in both the TiN and VN layers near to interface at the points of fracture during the tensile process, which plays a crucial role in affecting the fracture process. We further demonstrate that the fracture of the TiN/VN interface occurs within the VN and that adhesion energy cannot be simply used to represent tensile strength.

Introduction

Transition–metal–nitride multilayered coatings have received a considerable amount of interest over the past decades, largely because their properties are applicable for various industrial applications [1], [2], including good wear resistance, high thermal and chemical stability. A representative example is that the coatings such as TiN/VN and Ti_xV_1−xN can exhibit a high hardness of over 50 GPa and thus hold substantial promise for dry milling, drilling and turning [3], [4], [5]. Moreover, multilayered TiN/VN coatings show significantly enhanced friction and wear performance as compared to their mononitride coatings TiN and VN and the TiAlN/CrN [6], [7], [8]. All the improved properties, including post-processing behavior, rely critically on specific atomic structure, local chemistry, and local bonding at interface [9].

To understand origins behind mechanical properties of multilayered coatings, it is of fundamental significance to investigate tensile behavior because it is strongly connected to the onset of fracture and dislocation nucleation. In particular, multilayered coatings can often downgrade properties of materials during tensile process. However, few works have been conducted on the deformation mechanism of the multilayered coatings. For instance, Rzepiejewska-Malyska et al. [10] found, by the application of in-situ nano-indentation in scanning electron microscopy, a mixture of two modes where TiN deforms through grain boundary sliding and CrN through densification and material flow. Latella et al. [8] used in-situ tensile tests to introduce strains in coatings, and found that the stressed coatings show a mix of cracks perpendicular to tensile axis. Despite the efforts, it remains difficult to extract sufficient information on the process and ultimate geometry particularly at the atomic scale because of complex in directly visualizing variation of interfacial bondings in multilayered coatings.

One way out to complement the experiments so as to develop a general knowledge of the technologically relevant multilayered coatings is through atomistic simulation. To this end, density functional theory (DFT) calculations have often been applied [11]. Liao et al. [12] established a relationship between mechanical property and electronic structure for ternary M₂AC (M=Ti, V, Cr, A=Al, Si, P, S) carbides from first principles. Stampfl et al. [13] investigated formation, atomic and electronic structure of metal–nitride interfaces and found that the growth of TiN on VN is energetically favorable. Zhang et al. [14], [15] investigated the tensile and fracture process of the Al/TiN interface, and found that the Al/TiN(001) interface has a smaller work of adhesion and larger tensile strength than the Al/TiN(111) interface. Moreover, Zhang et al. [16] investigated superhard mechanism of AlN/TiN system and attributed the hardness enhancement to formation of semicoherent interface. Considering the relevance of fracture process in affecting mechanical property of nano-multilayers, there have been few investigations of such process particularly at the atomic scale, although mechanical strength of single-phase metals or ceramics has been investigated [17], [18], [19], [20], [21], [22], [23]. Here, we conduct a systematic first-principles investigation of ideal tensile stress and fracture behaviors of the TiN/VN interface. To our knowledge, this is a first systematic report on tensile fracture of the TiN/VN interface. We identify an upper limit to the true yield stress and establish a correlation between the fracture properties of multilayers and their electronic structures.

The rest of this paper is structured as follows. Section 2 describes the computational method and details in this study. The major results of this paper are presented in Section 3, where the results of tensile simulation of TiN/VN interface are provided and the strain–stress relationship is discussed. The TiN/VN interfaces considered are divided into two groups, based on the orientations. The results on (111)-oriented interface are presented in Section 4 and those on (001)-oriented one in Section 5. We provide concluding remarks in Section 6.