Structure and corrosion properties of multilayer metal/nitride/oxide ceramic coatings formed on austenitic-martensitic steel by magnetron deposition

Abstract

The design of protective coatings preventing the degradation and failure of austenitic steels in humid climate is a relevant issue. In this work, a conventional magnetron sputtering method was utilized in order to fabricate a four-layer Ni/Al-Si-N/Ni/Al-Si-O coating onto austenitic-martensitic steel. At the first stage of deposition, a three-layer coating was formed in an Ar+N2 atmosphere using a mosaic target (Ni or Al-Si), while the second stage included the deposition of the outer ceramic Al-Si-O layer in an Ar+O2 gas mixture. The studies of the structure, hardness, adhesion strength and electrochemical behavior (in 3.5 wt. % NaCl) of the «coating/steel» system were performed by means of X-ray Diffraction, electron microscopy, mechanical (indentation and scratch) tests and electrochemical impedance spectroscopy. The ceramic Al-Si-O layer exhibits an amorphous structure. The adjacent nickel layers have submicrocrystalline columnar grains. The ceramic Al-Si-N layer possesses a nanocrystalline columnar structure based on AlN + Si3N4 phases. Hardness behavior of the multilayer coating varies nonmonotonically due to a contribution of the soft amorphous phase in the upper surface layer and high-strength nanocrystalline phases in the underlying sublayers. It has been revealed that as-deposited multilayer coating decreases a corrosion rate of the substrate. The layered structure of the coating hinders the transport and diffusion of chloride ions to the substrate and decreases the corrosion rate of the substrate. However, a transport of chloride ions into the inner layers of the coating may occur through the microstructural defects preserved in the amorphous Al-Si-O layer. The scheme describing an initiation of corrosion damage and formation of corrosion products under critical corrosion conditions was proposed.