Corrosion behaviors of TiN and Ti-Si-N (with 2.9 at.% and 5.0 at.% Si) coatings by electrochemical impedance spectroscopy

Highlights

• Ti-Si-N with 5.0 at.% Si exhibited nanocomposite structure.

• The addition of Si increased corrosion resistance greatly.

• Nanocomposite structure could effectively retard the electrolyte penetration.

Abstract

TiN and Ti-Si-N (2.9 at.% and 5.0 at.% Si) coatings were fabricated on the surface of polished Si (100) wafers and stainless steel substrates by cathodic arc ion plating. The XRD results showed that the preferred orientation of the TiN crystals changed from TiN (111) to TiN (200) with an increasing silicon content. The TiN coating exhibited a polycrystalline structure with an average grain size of approximately 18 nm, while the Ti-Si-N coatings exhibited a nanocomposite structure of TiN nanocrystals embedded in amorphous Si₃N₄. In electrochemical impedance spectroscopy (EIS) tests, the amorphous Si₃N₄ phase distributed on the boundaries of the Ti-Si-N (5.0 at.% Si) coating and acted as an effective barrier layer against the penetration of the corrosion solution, which resulted in better corrosion resistance compared with TiN and the lower-Si-content Ti-Si-N coating (2.9 at.% Si). Immersion tests and surface morphology observations of the samples after corrosion were also carried out to investigate the corrosion mechanism and behaviors of TiN and Ti-Si-N composite coatings.

Introduction

The Ti-Si-N composite coating has been widely explored as a good candidate for increasing the lifetime of cutting tools because of its high hardness, low friction coefficient and excellent oxidation resistance [1], [2], [3], [4]. The silicon content exerts a tremendous influence on the structure and mechanical properties of the Ti-Si-N coatings [5], [6], [7], [8], [9]. According to the thermodynamic phase diagram, Ti-Si-N coatings will form a solid solution structure when the silicon content is lower than 3 at.% and will transform into a nanocomposite structure when the silicon content falls in the range of 3 to 12 at.% [10], [11]. However, attention has mainly been paid to the mechanical performance and microstructure of Ti-Si-N composite coatings, with little research devoted to a quantitative analysis of their corrosion properties and mechanism.

EIS is a perturbative characterization of the dynamics of an electrochemical performance and has been successfully applied in evaluating the corrosion resistance of thin films exposed to a corrosion medium. The modeling results of EIS spectra can provide abundant quantitative information on the corrosion performance of coatings such as the polarization resistance, porosity resistance and capacitance [12]. The corrosion resistances of CrN, TiN and a CrN/TiN multilayer have been evaluated quantitatively utilizing EIS, and it was found that CrN coatings with equiaxed grains exhibit better corrosion performance than columnar TiN coatings [13]. In addition, an upward protective efficiency was observed in a CrN/TiN multilayer with a decreasing CrN inner layer thickness, approaching a maximum of 99.01% at a thickness ratio of 1:9 [14]. With regard to the evaluation of the corrosion behaviors of nanocomposite coatings, C. L. Chang and V. E. Selvi used EIS to compare the performances of Ti-Si-N and TiN coatings and reported that Ti-Si-N exhibited a better corrosion resistance in 3.5 wt.% NaCl solution [15], [16]. Ha Yoo et al. [17] also found a better corrosion resistance of Ti-Al-Si-N nanocomposite coatings using EIS compared to a conventional Ti-Al-N columnar coating, resulting from the refinement of the microstructure with Si incorporation. Nevertheless, few papers have concentrated on the corrosion mechanism and long-term corrosion behaviors of Ti-Si-N nanocomposite coatings with different Si contents.

In this paper, the corrosion resistances of TiN and Ti-Si-N were investigated by utilizing potentiodynamic tests and EIS in 3.5 wt.% NaCl solution. To further explore the corrosion mechanism and behaviors of Ti-Si-N coatings, immersion tests were carried out for different times ranging from 0 to 758 h. Finally, the effects of the silicon content on the corrosion mechanism of Ti-Si-N coatings are examined according to the morphologies and EDS results of the coatings after the immersion tests.