Hard and tough (NbTaMoW)Nx high entropy nitride films with sub-stoichiometric nitrogen
Introduction
Currently, refractory metal high entropy alloy (HEA) with body-centered cubic (BCC) structure is potentially used as mechanical protective coatings. However, they suffer from a limited hardness, which restrain the development of HEA films. Yuan [1] fabricated nanostructured NbTaMoW film with a high critical stress intensity of 3.3 MPa*m1/2. Zou et al. [2] reported NbTaMoW film with nanoscale columnar grains exhibiting high temperature stability but a weak fracture toughness from the deformed micro-pillars. Matheus [3] deposited NbTaMoW thin films with a hardness of ~24 GPa and an elastic modulus of ~280 GPa. The mechanical properties of HEA films could be improved by cooperation with N atoms, while retaining the ductility of HEA. The development of high-entropy metal nitride (HEN) films provides an effective way to enhance the mechanical properties of HEA. HEN films are promise as hard protective materials with the enhanced mechanical performance [4], [5], [6]. Most of HEN films are based on Ti/Cr-(Al/Si)-N, like the (AlCrTiZrV)Nx [7], (AlCrTiZrHf)N [8], and (AlCrTiVZr)N [9], et al., and also some HEN films with containing no(Ni, Cu)/weak(Fe, Co) nitride forming elements [10], [11], where consists by multiphase with crystalline nitride or/and metallic alloys. Differentially, the refractory transition metal (TM) in group 4, 5 is prone to form strong nitride with a mixture of covalent and metallic bond [12], the hardness, elastic modulus and toughness of multielement TMN (transition metal nitride) is determined by the valence electron concentration and solid solution structure [13], [14].
Nb, Ta, Mo and W as refractory TM, the formed ternary nitrides are FCC metastable solid solutions structure under N sufficient [15]. The unusual metastability is due to the strong metal-nitrogen bonds in solid state, although the computed positive values of the formation energies indicate the metastable phase can be only stabilized at high temperature [16]. Under N insufficient, whether the structure of (NbTaMoW)N quaternary nitride with different N content is a single or multi-phase remains to be studied. The mechanical properties of HEAs film could be improved by introducing little N atoms to form a composite structure. Yang et al. [17] deposited the Ta-W-N films under N insufficient, the grains was composed by a Ta-W-N solid solution structure with surrounding a W-based amorphous. More important, the unavoidably presented point defects for TMNs (TM=Nb, Ta, Mo, W) resulting in the B1 phase field extends towards a lager TM/N ratio at N insufficient from the ab initio and the deposited film [18], [19], [20], [21], [22], [23]. The defects affect the mechanical properties of films in addition to stabilize FCC structure. Such as, the elastic constants of TaNx can be regulated by the N and Ta vacancies [24], [25]. The vacancies increased the elastic moduli for NbN [19], and stabilizing the mechanically cubic structures of WN [26] and MoN [27]. For the ternary nitrides, Klimashin et al. introduced a nearly 50% vacant N-sublattice in Ta-Mo-N, improving the hardness and fracture toughness to 30 GPa and 7.2 MPa*m1/2 [28]. Kindlund et al. [29] deposited the V0.5Mo0.5Nx films, where the hardness increased from 17 to 26 GPa with increasing vacancy concentration from x = 1.03 to with 0.55. Above all, appropriate composition and structure design in HENs films can relieve the natural tendency brittleness of TMNs and improve the strength of alloys to obtain a protective film with an improved hardness and toughness.
Although some multicomponent refractory transition metal nitrides have been researched, such as (TiNbZrTa)N [30], (MoNbTaVW)N [31], (TiVCrZrHf)N [32], (TiZrHfVNbTa)N [33], generally, the HEN films show a FCC phase structure with promising mechanical properties, there still have a need to understand the structure and mechanical properties of HEN films with a multicomponent of refractory transition metal. In this research, refractory (NbTaMoW)Nx films with a nitrogen flow ratio [fN = N2/(Ar + N2)] of 0–25% were deposited on (1 0 0) silicon substrates by magnetron sputtering. The structure evaluation was characterized by XRD, XPS and Raman, and their mechanical performances were investigated, including hardness, fracture toughness. The multicomponent HENs solid solution films with a single phase structure and appropriate defects are expected to have superior mechanical properties.
Section snippets
Experimental details
(NbTaMoW)Nx films with different stoichiometric ratio (x = N/TMs) were deposited on silicon wafer (1 0 0) by direct-current magnetron sputtering (DCMS) with a base pressure below 4 × 10−3 Pa. A 60 mm × 5 mm NbTaMoW alloy cylindrical plate with equal atomic ratio and a purity of 99.98% was used as target material. Prior to deposition, the substrates were heated to 575 K in Ar+N2 atmosphere and etched in plasma with the ions from an additional discharge. The target power was controlled at 150 W
Chemical composition and structure
Table 1 shows the chemical composition of films varied with fN, the contents of TMs and N are calculated from the EDS results. The compositions of Nb, Ta, Mo and W are near the equimolar ratios, while Nb, Mo is little higher (~0.5–1.5 at%) than Ta and W, which is due to the fact that the sputtering yield of Ar+ to Nb and Mo metal atoms is higher than Ta and W [34], [35]. The x value is in a range of 0–0.72 for fN = 0–25%. The O contents are below 10 at% for these nitride films. The XPS spectra
Conclusions
(NbTaMoW)Nx films were prepared on Si wafer by magnetron sputtering at 575 K. NbTaMoW films containing 0 and around 8 at% N are BCC structure with (1 0 0) preferred orientation, while the film transforms to NaCl FCC structure with larger crystallite size at a wide fN range from 5–25%. The cubic (NbTaMoW)Nx is stabilized in a wide N/TM rang (0.16–0.72) by introducing TMN antisite point defects. The interstitial solid solution film with containing 8 at% N shows an enhanced hardness and elastic
CRediT authorship contribution statement
Hang Li: Conceptualization, Methodology, Software, Investigation, Writing – original draft. Nan Jiang: Validation, Formal analysis, Visualization, Software. Jianliang Li: Resources, Writing – review & editing, Supervision, Data curation, Validation, Formal analysis. Jiewen Huang: Writing – review & editing. Jian Kong: Writing – review & editing. Dangsheng Xiong: Resources, Writing – review & editing, Supervision, Data curation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is financially supported by National Natural Science Foundation of China (No. 51101087); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX20_0277).
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