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

A two-layer coating with a total thickness of approximately 15 μm was obtained using a combined technology of electrospark deposition (ESD) and high-power impulse magnetron sputtering (HiPIMS), employing HfSi 2 –HfB 2 –MoSi 2 ceramic electrodes/target on a niobium substrate. The formation mechanism, morphology, and structure of the coatings were investigated using glow discharge optical emission spectroscopy (GDOES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that the ESD coating consists of 65 wt. % phases formed through interaction between the electrode and the substrate – namely NbSi 2 and Nb 5 Si 3 – and exhibits a silicon concentration gradient (from 8 to 54 at. %) across the coating thickness, from the substrate toward the surface. The outer amorphous HiPIMS coating is ~5 μm thick. Analysis of structural and phase transformations during heating of the ESD coatings up to 900 °C showed that annealing leads to its separation into two layers: an inner layer composed of dendritic grains of the metastable γ-Nb 5 Si 3 phase and an outer layer based on NbSi 2 . The HiPIMS coating crystallizes sequentially, forming (Hf,Mo)B 2 at 700 °C, MoSi 2 at 800 °C, and Hf 3 Si 2 and HfSi 2 at 900 °C, with the silicon content remaining virtually unchanged. As a result of the two-stage deposition process and subsequent high-temperature annealing, a multilayer protective ceramic coating was obtained, consisting of an outer layer of (Hf,Mo)B 2 –MoSi 2 –HfSi 2 , an intermediate layer of NbSi 2 , and an inner layer of Nb 5 Si 3 , with hardness values of 9.4, 23.3, and 19.4 GPa, respectively. This coating significantly extends the service life of niobium grade Nb-1.