Structure and properties of two-layer coatings in the HfSi₂–HfB₂–MoSi₂ system produced by electrospark deposition and magnetron sputtering

Introduction

The potential for using niobium and its alloys in components operating at high temperatures and in aggressive gaseous environments largely depends on the application of protective coatings [1; 2]. Among the most promising are coatings based on silicides such as NbSi₂ [3–5], MoSi₂ [6–8], NbSi₂–MoSi₂ [9], and NbSi₂–HfSi₂ [10], due to the formation of a SiO₂ film on their surface at elevated temperatures. A characteristic feature of such coatings is the formation of an intermediate diffusion layer of Nb₅Si₃. Since the enthalpy of formation of Nb₅Si₃ (–516.8 kJ/mol) is lower than that of NbSi₂ (–161 kJ/mol) [11; 12], the Nb₅Si₃ layer forms first during thermochemical treatment and is subsequently transformed into the NbSi₂ phase [3–12]. The rate of mutual diffusion between the coating and the substrate – which leads to coating thinning and, consequently, a reduction in performance – can be reduced by introducing diffusion barriers [13–15], modifying the coatings with diffusion inhibitors [16; 17], or forming additional layers of mullite (3Al₂O₃·2SiO₂) [17; 18] or borosilicate glass (B₂O₃·SiO₂) [19; 20]. Multilayer coatings can also be produced by combining several technological approaches in a single process [21; 22] or by employing two-stage processes [2; 17].

In [23], the feasibility of using heterophase electrospark coatings based on MoSi₂–MoB–HfB₂ to improve the performance of the heat-resistant nickel superalloy EP741NP was demonstrated. Enhancement of the oxidation resistance of electrospark coatings through the application of an upper magnetron-sputtered layer has been demonstrated for steels [24] and nickel alloys [25].

The aim of this study is to fabricate two-layer ceramic coatings on Nb-1 niobium using a combination of electrospark deposition (ESD) and high-power impulse magnetron sputtering (HiPIMS), and to investigate the effect of vacuum annealing on their composition, structure, and properties.

Materials and methods

The consumable ceramic electrodes and targets were produced by self-propagating high-temperature synthesis (SHS) from elements (wt. %: 59 Hf, 28 Si, 11 Mo, 2 B), followed by hot pressing of the synthesized products [26]. Both the ESD electrodes and the HiPIMS targets had the following phase composition (wt. %): hafnium silicide (34 HfSi₂), molybdenum silicide (17 MoSi₂), hafnium boride (19 (Hf,Mo)B₂), elemental silicon (21 Si), and hafnium oxide (9 HfO₂) [27]. The ESD electrodes were fabricated as rectangular rods measuring 4×4×50 mm, while the HiPIMS targets were discs with a diameter of 120 mm and a thickness of 10 mm.

The coatings were deposited onto Nb-1 niobium plates measuring 10×10×3 mm. The ESD process was carried out in an argon atmosphere using the Alier-303 Metal unit (Russia–Moldova) under the following parameters: discharge current 120 A, pulse frequency 3200 Hz, and pulse duration 20 µs. HiPIMS deposition was performed using a UVN-2M system equipped with a magnetron and ion source. Target sputtering was performed using the TruPlasma 4002 system (Trumpf, Germany) at an average power of 1 kW, with a peak power of up to 50 kW, peak current of 50 A, pulse frequency of 1 kHz, and pulse duration of 50 µs. The two-layer ESD + HiPIMS coatings were obtained by sequential application of the ESD and HiPIMS processes.

The microstructure and elemental composition were examined using an S-3400N scanning electron microscope (Hitachi High-Technology Corporation, Japan) equipped with a NORAN X-ray System 7 energy-dispersive X-ray spectroscopy (EDS) unit (Thermo Scientific, USA). Metallographic cross-sections were prepared using a Rotopol-21 polishing system (Struers, Denmark). Elemental depth profiling of the coatings was carried out by glow discharge optical emission spectroscopy (GDOES) using a Profiler-2 instrument (Horiba Jobin Yvon, France). Phase composition was determined by X-ray diffraction (XRD) using a DRON-4 automated diffractometer (Burevestnik R&D Center, Russia) with CuK_α radiation over a 2θ range of 10–120°. The obtained X-ray diffraction patterns were analyzed using the JCPDS database.

To analyze the structural and phase transformations occurring in the coatings during heating, high-resolution transmission electron microscopy (HRTEM) and electron diffraction were used. The studies were carried out in the column of a JEM 2100 microscope (JEOL, Japan) during isothermal holding at temperatures of 400, 500, 600, 700, 800, and 900 °C for 20–25 min. The heating rate was 100 °C/min. The elemental compositions of the as-deposited coatings and those cooled from 900 °C were analyzed by EDS using an X-Max80T detector (Oxford Instruments, UK). Lamellae were prepared using focused ion beam (FIB) milling in a Scios DualBeam scanning electron-ion microscope (FEI, USA). Annealing of the coatings was performed in a VSl-16-22-U vacuum furnace (VakETO, Russia) at 900 °C for 30 min under low vacuum.

The hardness (H) and Young’s modulus (E) of the coatings were determined by selective nanoindentation [28] using a NanoHardnessTester (CSM Instruments, Switzerland) with Indentation 3.0 software, in accordance with GOST R 8.748–2011 (ISO 14577).

Results and discussion

Despite the use of the same material as both the electrode and target for coating deposition, the layers formed by electrospark deposition (ESD) and high-power impulse magnetron sputtering (HiPIMS) differ in structure and phase composition. During ESD, due to high temperatures in the interelectrode gap, local melting of the electrode and the treated substrate occurs, resulting in a coating with a ~ 10÷12 thickness (Fig. 1, a). Across the coating thickness, from the surface toward the substrate, an elemental concentration gradient is observed: the niobium content increases from 18 to 85 at. %, while the contents of the electrode-derived elements Hf, Mo, B, O, and silicon decrease from 54 to 8 at. % (Fig. 1, b).

During HiPIMS, the coating is formed by atomic fluxes with a high fraction of ionized species and has a thickness of ~5 μm (Fig. 1, c). The elemental concentrations within the HiPIMS-deposited layer remain constant throughout its thickness, and no substrate material (Nb) is detected in the coating, which is confirmed by the sharp interface (Fig. 1, d).

In sequentially deposited ESD + HiPIMS coatings, no pronounced interaction between the layers is observed (Fig. 1, e). The individual thicknesses of each layer are preserved, forming a two-layer coating with a total thickness of ~15 μm. EDS analysis shows that the niobium concentration in the ESD layer is 27.3 at. % (region 1 in Fig. 1, e, at. %: 8.8 O; 43.6 Si; 27.3 Nb; 4.9 Mo; 15.4 Hf), while in the HiPIMS layer it is only 0.5 at. % (region 2 in Fig. 1, e, at. %: 10.5 O; 55.0 Si; 0.5 Nb; 10.3 Mo; 23.7 Hf). The GDOES profile can be divided into three zones: the first (0–5 μm) corresponds to the magnetron-sputtered layer, the second (approximately 5–14 μm) corresponds to the ESD coating, and the final section corresponds to the substrate (Fig. 1, f).

X-ray diffraction patterns of the coatings are shown in Fig. 2. The formation of NbSi₂ and Nb₅Si₃ phases in the ESD layer – as a result of interfacial diffusion-reactions between the electrode material and the niobium substrate – indicates strong adhesion of the coating to the substrate (Fig. 2, a). The coating also contains (Hf,Mo)B₂, free Si, and HfO₂ phases, consistent with the electrode composition. In the microstructure, HfO₂ appears as bright inclusions (Figs. 1, a, e). After vacuum annealing, all major phases remain present at approximately the same concentrations (Fig. 2, b, Table 1). The Si phase disappears due to its reaction with the substrate, forming NbSi₂. In addition, a Nb₂B₃ boride phase forms in an amount of 5 wt. %.

The HiPIMS coating is X-ray amorphous: the diffractogram shows only substrate reflections and broad amorphous halos (Fig. 2, c). However, after vacuum annealing, crystalline phases such as HfSi₂, MoSi₂, and (Hf,Mo)B₂ are identified (Fig. 2, d). The formation of NbSi₂ is attributed to the diffusion of niobium from the substrate, while the presence of MoO₃ results from residual oxygen impurities in the target material. The phase analysis results are summarized in Table 2, excluding the contribution of the substrate (Nb), as the coating thickness is less than the X-ray penetration depth.

The microstructure of the ESD + HiPIMS coating after vacuum annealing is shown in Fig. 3, a. Within the electrospark-deposited layer, an outer region with a sharply defined boundary forms, corresponding to the NbSi₂ phase (region 2 in Fig. 3, a, at. %: 2.9 O; 64.2 Si; 28.8 Nb; 2.1 Mo; 2.0 Hf). Bright HfO₂ inclusions are observed both in the outer dark layer and in the inner layer adjacent to the substrate. According to EDS data, the inner region contains a lower silicon concentration of 28.1 at. % (region 3, at. %: 7.4 O; 28.1 Si; 48.3 Nb; 4.2 Mo; 12.0 Hf). These results indicate that heat treatment promotes silicon homogenization within the ESD layer and reduces microscale compositional inhomogeneities originating from individual mass transfer events. The magnetron-sputtered layer, which is compositionally and structurally uniform, retains its original composition after crystallization (region 1, at. %: 11.5 O; 52.8 Si; 0.5 Nb; 10.9 Mo; 24.1 Hf) and does not contribute to NbSi₂ formation. The elemental distribution map confirms an increased silicon concentration in the outer ESD layer and a higher niobium content in the inner region (Fig. 3, b).

The initial lamella containing both ESD and HiPIMS layers, used for in situ investigation of structural and phase transformations during heating in the TEM column, is shown in Fig. 4, a. After cooling from 900 °C, a new layer appears at the interface with the HiPIMS coating (Fig. 4, b). According to EDS data, its composition corresponds to the NbSi₂ phase (Fig. 4, c). Electron diffraction analysis confirms this identification: diffraction rings with interplanar spacings d/n = 0.356, 0.218, 0.210, and 0.136 nm correspond to the (101), (111), (112), and (114) planes of the h-NbSi₂ phase. Additional rings with d/n = 0.638, 0.319, 0.240, 0.218, 0.210, and 0.140 nm, corresponding to the (100), (200), (210), (211), (112), and (402) planes, are assigned to the Nb₅Si₃ phase (Fig. 4, d).

No significant structural changes were observed in the inner region of the ESD coating during heating (Figs. 5, a–c). However, after cooling the sample from 900 °C to room temperature, contrast variations appeared in certain areas. These may be attributed either to diffusion-driven elemental redistribution – accompanied by the dissolution of some structural and phase components – or to stress relaxation (Fig. 5, d).

As a result of non-equilibrium crystallization during ESD, γ-Nb₅Si₃ phase grains are observed in the coating, formed in the direction from the substrate toward the surface and resembling dendrites in their morphology. Dendritic growth of the metastable Nb₅Si₃ phase due to non-equilibrium crystallization in Nb–Si alloys has been reported in [29; 30]. A HRTEM image of a silicide grain oriented along the [\(\bar 1\)10] direction, the corresponding electron diffraction pattern, and EDS data are shown in Figs. 5, e–g. The presence of HfO₂ grains, 50–100 nm in size, was also confirmed in the coating. A HRTEM image of an oxide particle oriented along the [02\(\bar 4\)] direction, together with the corresponding diffraction pattern and EDS results, is presented in Figs. 5, h–j.

The HiPIMS coating remains stable up to 600 °C and retains a layered structure (Fig. 6, a). The corresponding electron diffraction pattern displays a broad diffuse ring, indicating that the coating is in an amorphous state. Crystallization begins at 700 °C, as evidenced by the appearance of diffraction rings with interplanar spacings of d/n = 0.355, 0.261, 0.213, and 0.178 nm, corresponding to the (001), (100), (101), and (002) planes of the (Hf,Mo)B₂ phase (Figs. 6, b, f). As the temperature increases to 800 °C, additional reflections emerge at d/n = 0.291, 0.225, and 0.213 nm, attributed to the (101), (110), and (103) planes of the MoSi₂ phase (Figs. 6, c, g).

Further heating to 900 °C leads to the formation of hafnium silicides. Hf₃Si₂, reflections corresponding to the (110), (001), (210), and (211) planes with d/n = 0.493, 0.357, 0.309, and 0.262 nm indicate the presence of Hf₃Si₂, while reflections at 0.355, 0.262, 0.226, and 0.206 nm from the (110), (111), (131), and (061) planes correspond to HfSi₃ (Figs. 6, d, h). After cooling, the coating structure remained unchanged. Four crystalline phases were observed: Hf₃Si₂, HfSi₂, MoSi₂, and (Hf,Mo)B₂.

Table 3 presents the hardness (H) and elastic modulus (E) values for the coatings and the substrate. The hardness of the as-deposited ESD coating is uniform across the thickness and amounts to 18.3 GPa. After annealing, the inner layer retains a hardness of H = 19.4 GPa, indicating the absence of structural transformations in the Nb₅Si₃-based layer. A slight decrease in the elastic modulus may be attributed to an increased Nb content due to diffusion from the substrate into the NbSi₂-based layer. The hardness value of 23.3 GPa in the outer layer is attributed to the inherently higher hardness of the NbSi₂ phase compared to Nb₅Si₃, the absence of free niobium in this region, and alloying with niobium boride additions (Hf,Mo)B₂ and Nb₂B₃. For the HiPIMS coating, the hardness is H = 12.5 GPa, which corresponds to reported values for magnetron-deposited Hf–Si–Mo–B coatings [31]. After vacuum annealing and stress relaxation, the hardness decreases to 9.3 GPa. The increase in substrate hardness from 1.8 to 2.5 GPa following annealing is attributed to silicon diffusion into the interfacial region.

Conclusions

1. A 15 μm-thick two-layer coating was fabricated on a Nb-1 substrate by sequential application of electro-spark deposition (ESD) and high-power impulse magnetron sputtering (HiPIMS) using HfSi₂–HfB₂–MoSi₂ SHS-ceramic electrodes/targets. The ESD layer, ~10÷12 μm thick, consists of ~65 wt. % phases formed as a result of interfacial reactions between the electrode and the substrate, primarily NbSi₂ and Nb₅Si₃ (H = 18.3 GPa, E = 285 GPa). A compositional gradient was observed across the ESD coating, with the Nb content increasing from 18 to 85 at. % and the Si concentration decreasing from 54 to 8 at. % from the surface toward the substrate. The HiPIMS coating has a homogeneous amorphous structure, ~5 μm thick, with H = 12.5 GPa and E = 216 GPa.

2. During heating, an interlayer ~2 μm thick based on NbSi₂ forms at the interface between the ESD and HiPIMS layers (H = 23.3 GPa, E = 292 GPa). The inner ESD layer consists of dendritic grains of the metastable γ-Nb₅Si₃ phase solidified perpendicular to the substrate surface (H = 19.4 GPa, E = 256 GPa). Crystallization of the HiPIMS layer begins at 700 °C with the formation of (Hf,Mo)B₂. Upon further heating to 800 °C, MoSi₂ appears, and at 900 °C, HfSi₂ and Hf₃Si₂ phases are detected. After vacuum annealing, mechanical properties decrease slightly (H = 9.4 GPa, E = 207 GPa), which may be attributed to stress relaxation. Since the silicon content remains unchanged, the HiPIMS layer does not contribute to the formation of the NbSi₂-based interlayer. Thus, heat treatment results in the formation of a multilayer coating with enhanced mechanical properties.