Effect of O2 flow rate on the structure, wettability and tribo-mechanical behaviour of Zr-O-N thin films
Introduction
Stainless steels, in spite of their excellent mechanical properties, may still fail in extreme saline environments due to wear and corrosion. Hence, different routes, such as, change in elemental compositions and different mechanical treatments, have been explored by researchers, to enhance the wear and corrosion resistance of stainless steels [1], [2], [3], [4], [5], [6], [7]. Surface modification techniques such as coatings that enhance hardness and provide an excellent protecting barrier for stainless steels can also be a viable route to improve their surface properties in aggressive environments. Moreover, the physical and mechanical features of the coatings can be customized by controlling the deposition parameters such as the deposition time, temperature and working pressure [5,6]. Among the various available coating techniques, magnetron sputtering has been widely used because of their flexibility and numerous advantages such as, high deposition rate, coating uniformity, the ability to form dense coatings, strong adhesion to substrate, easy control on film thickness and formation of considerably high-purity films [6].
In the last few years, transition metal nitride films, such as, chromium nitride (CrN) titanium nitride (TiN) and zirconium nitride (ZrN) have found their way in several technological applications because of their remarkable properties [7,8]. Among all the mentioned nitride films, ZrN films, demonstrate unique physical properties, such as, high hardness, good thermal stability together with excellent resistance to wear and oxidation, as compared to TiN and CrN films [9,10].
Owing to their good properties, ZrN films have been used in machining processes to enhance tool life, productivity and surface quality [11]. The performance of ZrN films deposited under different sputtering conditions such as applied power, substrate temperature, deposition time/temperature, and gas flow rate have been extensively investigated [11], [12], [13]. Singh et al. [12] reported the effect of N2 flow rate on the microstructure and surface morphology, which are the most important factors governing the performance of the ZrN films. They found that the crystallite size decreased as the nitrogen flow rate increased and the orientation of the highest X-ray diffraction peak changed from (200) to (111). Pichon et al. [13] showed that ZrN films are good reflectors in the IR wavelength range and shows high thermal stability. Aouadi et al. [14] demonstrated that ZrN films exhibited excellent wear properties, better corrosion resistance and improved adhesion to tool steel as compared to TiN.
However, despite their good properties, ZrN films exhibit poor performance in demanding applications such as, machining at higher speeds and elevated temperatures [13]. Therefore, several researchers [15,16] explored ternary nitride systems by adding other elements into Zr-N system. Based on the chemical composition of the deposited films and the process parameters, addition of oxygen (O2) and the formation of new solid solutions were found to enhance the tribo-mechanical properties of ZrN-based films. The metallic oxynitrides, with the presence of nitrogen and O2, show an excellent performance by combining the good properties of the corresponding nitrides and oxides [17].
Researchers have explored and reported various routes to improve the properties of these new oxynitride compounds, which demonstrated excellent mechanical behavior due to significant changes in microstructure, morphology and phase compositions [14], [15], [16], [17], [18], [19]. Deng et al. [18] showed that the surface properties of ZrN films are influenced by the substitution of nitrogen with O2 during the nitridation of ZrN films. Wendel et al. [19] found that the oxynitride films, obtained by a reaction of ZrO2 with different nitrogen content at elevated temperatures, had a high ionic conductivity compared to zirconium oxide. Venkataraj et al. [20] revealed that the Zr-O-N films have a dense structure with high nitrogen content. Moreover, the roughness of the film decreased with an increase in the N2 flow rate. Zr-O-N coatings showed higher wear resistance and lower friction coefficient as compared to Zr-N and Zr-O coatings [21], [22], [23], [24].
Incorporation of O2 into the Zr-N film leads to the formation of a saturated solid solution, resulting in improved hardness. The oxygen content in the Zr-O-N films plays a very important role in governing the mechanical, thermal and tribological properties of these films. However, very limited reports are available, which investigated the effect of oxygen content on the evolution of structure, chemical composition, wettability and the tribo-mechanical properties of Zr-O-N films.
Moreover, even in the available studies, the microstructure, mechanical properties and wettability of the Zr-O-N films as a function of O2 content have been presented without taking into consideration the effect of surface topography and the film texture. Furthermore, only a few studies have reported the effect of O2 incorporation on the wear resistance of Zr-O-N films deposited on steel substrates. Hence, in this study, Zr-N films are incorporated with O2 to form oxynitrides by means of a reactive magnetron sputtering process. The effect of O2 flow rate on microstructure, mechanical properties, wettability and wear resistance of the Zr-O-N films is thoroughly, investigated and discussed, taking into consideration the surface topography and film texture.
Section snippets
Deposition technique of Zr-O-N films
Reactive magnetron sputtering technique (DEPHIS 4, Etupes, France) was used to deposit Zr-O-N films on XC100 steel and Si (100) wafer substrates. A circular Zr target (99.9% purity; diameter = 200 mm; thickness = 6 mm) was used. The XC100 steel substrates were polished using SiC abrasive papers with a grit size of 300, 600, 1000, 1500 and 2000, and subsequently polished with diamond paste to a mirror finish until an average roughness of Ra = 30 nm was obtained. The substrates were cleaned
Crystalline structure and elemental composition analysis of Zr-O-N films
The XRD spectra presented in Fig. 1, clearly divides the Zr-O-N films into two zones/categories. The films deposited at low O2 flow rate (0-5 sccm) are categorized under Zone I, which show a single ZrN phase, while the films deposited at relatively higher O2 flow rate (8-12 sccm) are categorized under Zone II, which show a mixture of nitride and oxide phases.
The XRD spectra in Fig. 1 show a strong dependency of the structure of the Zr-O-N film on its phase compositions. The XRD pattern of ZrN
Conclusions
Zr-O-N thin films were deposited by reactive magnetron sputtering process and the effect of oxygen content on the structure and tribo-mechanical behavior was investigated. The following main conclusions can be drawn from this study: Structural characterization of Zr-O-N films revealed the formation of solid solution with dense structures. Increasing O content in the films leads to a gradual increase of the (200) texture and the appearance of new zirconium oxide phases. The presence of oxygen
CRediT authorship contribution statement
Linda Aissani: Conceptualization, Methodology, Formal analysis, Visualization, Writing – original draft. Mamoun Fellah: Conceptualization, Methodology, Software, Data curation, Writing – original draft, Writing – review & editing. Ahlam Belgroune: Methodology, Formal analysis, Investigation. Aleksei Obrosov: Writing – review & editing. Mohammed Abdul Samad: Writing – review & editing. Akram Alhussein: Conceptualization, Resources, Data curation, Writing – original draft, Writing – review &
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.
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