Full Length ArticleSputter-deposited TaCuN films: Structure, tribological and biomedical properties
Graphical abstract
Introduction
Tooth deformity can cause a series of oral problems which adversely affect the psychological growth of adolescents. Orthodontics is an effective way to improve periodontal health and enhance individual self-confidence [[1], [2], [3]]. Until now, stainless steel has been widely used in surgical instruments such as forceps, scalpels and scissors owing to its non-toxicity and biocompatibility [4]. In addition, its low cost and high rigidity make stainless steel an ideal material for brackets and arch wires in orthodontic treatment [[5], [6], [7]]. To correct teeth effectively and accurately, the friction between the arch wire and the bracket generally needs to be as little as possible. Typically, 12–60% of the force applied to moving a tooth is dissipated in the form of friction, and greater friction can cause teeth not to move in the designated direction or at the desired speed during the correction process [[8], [9]]. Unfortunately, there is a high friction coefficient between a stainless steel bracket and stainless steel arch wire [[5], [10], [11]], which reduce the efficiency of treatment [12]. In addition, periodontal health is another factor that must be considered during the orthodontic process. Stainless steel tends to adhere to salivary proteins [[13], [14]], which causes an increase in plaque and streptococcus on the teeth and, ultimately, dental caries [[15], [16], [17]]. There is therefore an urgent need to identify materials with antibacterial, anti-inflammatory properties and reliable tribological performance.
Recently, films made of transition metal nitrides (TMNs) such as titanium nitride (TiN), tantalum nitride (TaN) and zirconium nitride (ZrN) have attracted attention in the field of materials protection due to their good mechanical [18] wear resistance [[19], [20]] and biocompatibility characteristics [21]. Among these materials, TaN film especially has excellent blood compatibility and resistance to microbiologically induced corrosion [22]. In addition, in a previous study it was found that depositing TaN film on a Ti substrate can enhance the adhesion, proliferation and differentiation of osteoblast, which indicates that TaN film has great potential for future use in clinical applications [23]. Unfortunately, TaN does not demonstrate intrinsic antibacterial ability, which limits its application as a biomedical material. However, introducing antibacterial agents (such as Ag, Cu and Zn) is an efficient way of enhancing the antibacterial ability of TaN film [[24], [25], [26]]. Among these metals, group IB metal nanoparticles (Au, Ag, Cu) have been known to exhibit a broad spectrum against bacteria and limited toxicity towards mammalian cells and have thus been widely employed to fight infections within the medical device field [[27], [28]]. Cao et al. have found that metallic Ag that emerged on the surface of Ti substrates by plasma ion immersion implantation shows pronounced antibacterial efficiency [29]. Metallic Cu atoms also exhibit antibacterial ability by inhibiting the transcription of bacterial DNA and have limited toxicity to mammalian cells [[30], [31]]. Therefore, the introduction of IB metals into TaN is expected to be an effective method of enhancing the antibacterial capacity of TaN.
Regarding their tribological performance, it has been reported that TMN films have high friction coefficients (0.6–0.7) in air [[32], [33]]. Fortunately, introducing IB metals (such as Ag and Cu) into TMNs to construct nanocomposite structures has been proven to greatly optimize the tribological properties of TMNs and reduce their friction coefficients in a variety of environments. In a medium-to-high temperature environment (≥350 ℃), VN/Ag nanocomposite film reacts with oxygen during the friction process to form AgVOx (Magnéli phases) with slip planes. The formation of these oxides results in a lower shear strength, they can serve as a lubricant phase to reduce the friction coefficient [34]. In addition, incorporating ductile metal atoms into a TMN lattice to form a solid solution can also cause the friction coefficient of TMNs in air to decrease, as the solute ductile metal atoms induce self-oxidation of the film surface, resulting in more lubricated products on the worn surface [20]. Both the introduction of IB metals and the incorporation of ductile metals can be used as strategies to optimize TMN films to reduce friction coefficients in air. However, it is not clear whether the films corresponding to these strategies can improve the tribological properties of stainless steel substrates in saliva. It is therefore of interest to assess which structure is more effective at improving the tribological performance of stainless steel and, in particular, which provides a better lubrication mechanism in the saliva environment.
In this work, TaN was chosen as the research object, and Cu atoms were incorporated into the TaN, due to the fact that Cu can act against gram-negative bacteria [35]. To determine how existing forms of Cu in TaN films affect the mechanical and tribological properties of these films, Cu-free TaN, TaCuN solid solution and TaN/Cu nano-composite films were each deposited. An interesting result observed in this study is that the wettability of TaN can be tuned through the incorporation of a small number of solute Cu atoms, while a robust hydrophobic surface can be obtained through the formation of a TaCuN solid solution structure. It was also revealed that solute Cu atoms not only can improve the hardness and toughness of TaN but can also tailor the surface’s chemical composition, forming Cu2O and CuO groups on the surface. Density functional theory (DFT) calculations were performed to confirm that the hydrophobic features of Cu2O groups induce the tuning of wettability via the destruction of the hydrogen-bonding network of water molecules next to the surface [[36], [37]]. It is well known that the teeth and other parts of the oral and maxillofacial region are hydrophobic and that the adsorption of salivary lubrication film on hydrophobic interfaces is extremely stable [38]. Therefore, due to its surface hydrophobic properties and excellent mechanical properties, TaCuN solid solution film exhibits outstanding lubrication ability and wear resistance in the saliva environment. More importantly, the biological experiments conducted in this study show that TaCuN solid solution film is non-toxic, and also shows remarkable antibacterial properties and biocompatibility. These findings indicate that TaCuN solid solution film has strong mechanical strength and can exhibit excellent protection ability in the salivary environment. Ultimately, this material system provides a new method for the surface modification of orthodontic materials and has the great potential for oral environment applications.
Section snippets
Sample preparation
The TaN and Cu contained TaN films were deposited by reactive co-sputtering. The 25 × 25 × 0.5 mm3 Si (1 0 0) wafers (for structural and mechanical characterizations) and the Ø10 × 1 mm 304 steel circular sheets (for tribological and biological experiments) were used as the substrate. All the substrates were ultrasonically cleaned in subsequent baths of acetone, alcohol and de-ionized water for 20 min, respectively, and then blown dry with dry N2. Water-cooled Ø60 mm Ta and Cu targets with
The structure of TaN with different Cu statuses
The content and existing forms of IB metals such as Cu or Ag usually have a strong influence on the structure and crystallinity of TMNs [[20], [40]]. In this study, in order to eliminate the influence of Cu content on the films’ structures and properties, we fixed the Cu target power, prepared TaN/Cu films with a Cu content of 4.2 at.% (confirmed by XPS) and performed in-situ annealing at 350 ℃ to obtain different pure forms of Cu. At the same time, we also prepared Cu-free TaN film for the
Conclusion
In this study, TaN, TaCuN solid solution and TaN/Cu nanocomposite films were prepared by magnetron sputtering, and the effects of different structures and of the introduction of Cu atoms on the mechanical and tribological behavior of the films were tested. The measured hardness of the TaCuN solid solution film, at 17.6 GPa, was 1.1 times and 2.4 times higher than that of the TaN and TaN/Cu films, respectively. Meanwhile, the friction coefficient of the TaCuN solid solution film in the salivary
CRediT authorship contribution statement
Dongsheng Yu: Writing – original draft, Formal analysis, Investigation. Kaifei Miao: Resources, Data curation, Writing – original draft. Ying Li: Investigation, Data curation. Xingfu Bao: Validation, Formal analysis. Min Hu: Supervision, Methodology. Kan Zhang: Project administration, Writing - review & editing, Funding acquisition.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos. 51972139, 51602122).
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