Microstructure and mechanical property of TiN/AlN multilayered coatings: The impact of AlN thickness
Graphical abstract
Introduction
Multilayered coatings of transition metal nitrides have received considerable interest over the past decades, largely because their outstanding properties are important for many applications, including the extreme hardness and the good wear resistance. Controlling the microstructures and bonding at the interfaces is of significant importance for the performances and lifetime of coated tools [1], [2], [3], [4]. Among the coatings, multilayered TiN/AlN ones, which are produced by alternating growth of the two host mononitrides, have attracted the broadest attention. The (Ti,Al)N nitride shows the FCC NaCl-type structure [5], and the Ti1-xAlxN solid solution has been synthesized [6]. On the other hand, the AlN could have a couple of crystal structures: the stable wurtzite structure (w) with a hexagonal symmetry and two metastable phases with a cubic symmetry, B1 (labeled FCC with NaCl prototype) and B3 (ZnS prototype). Interestingly, these crystal structures can be tuned by controlling the thickness of AlN due to the template effect. For instance, the w-AlN can transform to FCC-AlN, forming a coherent interface with FCC-TiN when the thickness of AlN ranges from 0.5 to 2 nm. Such template effect has also been identified in multilayered coatings involving SiC [7], SiO2 [8], Si3N4 [9], and AlON [10].
Because of high hardness, good wear and oxidation resistance, as well as multiple phases of AlN, the AlN/nitride multilayered coating has been the subject of intensive research owing to the hopes its raises for technological applications and also for its fundamental scientific significance, including the TiN/AlN [11], [12], [13], VN/AlN [14], NbN/AlN [15], [16], and CrN/AlN [17], [18], [19], [20]. Experimentally, Li et al. [21] reported that the AlN in the VC/AlN multilayers can be crystallized to a cubic structure when its layer thickness is less than 1 nm. As a consequence, the multilayers are grown coherently with significantly enhanced hardness (maximum of 40.1 GPa). On the other hand, Kim et al. [12], [13], [14] fabricated the epitaxial AlN/TiN(001) superlattices with the AlN layer thickness of not more than 2 nm, and demonstrated the formation of FCC-AlN when the AlN film is thin. Li et al. [22] also prepared a series of AlN/TiN multilayers with different modulation periods, and concluded that the coating reaches the highest hardness when the modulation period is 2 nm due to the alternative strain fields resulting from the coherent growth of FCC-AlN and TiN. Likewise, AlN in the AlN/CrN multilayered coating can exhibit the B1 structure with a high hardness of 40 GPa for the coating when the bilayer period is 3.8 nm [20]. Moreover, Wen et al. [15] recently deposited the NbN/AlN nano-multilayer films on Si(100) substrates by modulating the AlN thickness from 2.2 to 12.2 nm, and found that both the hexagonal wurtzite-AlN(0002) and FCC-AlN(111) are coherently bonded to the FCC-NbN(111). Moreover, the AlN thickness can determine coherency of the FCC-NbN(111)/w-AlN(0002) by minimizing the total energy and forming a strong fiber texture of NbN(111)/AlN(0002).
Theoretically, Chawla et al. [23] combined the finite element (FEM) and density functional theory (DFT) method to investigate the epitaxial stabilization of FCC-AlN layers in the TiN/AlN and CrN/AlN systems. They calculated the strain energy stored in the bulk and predicted that the (111) growth mode is energetically more favorable to stabilize the FCC-AlN than the (100). On the other hand, Chen et al. [11] applied the first-principles method to simulate the growth process of AlN thin film on TiN, and determined that the critical thickness of AlN is 1.95 nm. In addition, they also offered physical insights into the thickness-dependent structural transformation at the atomic scale. Moreover, Stampfl and Freeman [24] investigated structure and stability of rocksalt interfacial systems, AlN/VN, AlN/TiN and VN/TiN, and found that the systems are increasingly less stable with the increase of thickness owing to the lattice mismatch between the host nitrides.
All of these studies, albeit experimentally or theoretically, offer convincing evidence that the critical AlN thickness plays a key role in affecting mechanical properties of the AlN/nitrides multilayered coatings. Detailed knowledge on the role of AlN thickness in TiN/AlN multilayered coatings, particularly under the direct tensile process, is hence essential in elucidating the mechanism and controlling the performance of multilayered coatings. Here, we investigate, by first-principles calculations, how the AlN thickness can have an impact on the mechanical properties of TiN/AlN multilayered coatings. We have chosen purposely the TiN due to its good lattice match with AlN and also because it represents one of the most commonly used nitrides and could be coated onto many engineering materials.
Section snippets
Calculation detail and geometrical models
Calculations were performed using Cambridge Sequential Total Energy Package (CASTEP) within the framework of the DFT. Detailed calculation parameters were reported in the previous work [25]. We adopted cut-off energy of 600 eV and 12×12×12 k-points for bulk TiN and FCC-AlN, which ensured the convergence of total energy to within 1–2 meV/atom. The strain–stress relationships were calculated by incrementally deforming the slabs along the applied strain. Both atomic basis vector orthogonal to the
Bulk properties and relaxed structures
We first verified the accuracy of computational methods by calculating bulk properties of both the TiN and FCC-AlN. With the aforementioned parameters, we calculated the equilibrium lattice constants by minimizing the ground-state total energy with respect to the volume of unit cell. The optimum lattice constant of bulk TiN is calculated to be 4.25 Å, 0.23% larger than the experimental value (4.24 Å) [30], [31], while that of bulk FCC-AlN is 4.07 Å, 0.49% larger than the experimental value (4.05 Å)
Conclusions
We have performed a first-principles study of the multilayered TiAlN(111) coatings, aiming at investigating the effect of the AlN thickness on their microstructures and mechanical properties. After validating the application of our methods by carrying out a series of bulk calculations, we constructed three candidate models, taking into account different AlN layers and full relaxation. We find that the interfacial bonds between Al and N are close to those in the AlN bulk, regardless of the AlN
Acknowledgments
This study was supported in part by the National Science Foundation of China under Grant nos. 11332013, and 11302141. D.Y. thanks Specialized Research Fund for Doctoral Program of Higher Education (New Teachers) under Grant no. 20130181120118. D.Y. and Y. Q. acknowledge the European Commission for financial funding of the research through the EU FP7 MS-2S project and the permission for using the CASTEP code as a registered academic license of CASTEP under the STFC/York/Accelrys agreement (UK).
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