Surface and Coatings Technology, Volume 229, 25 August 2013, Pages 84-89
W.L. Cheng, Z.F. Zhou, P.W. Shum, K.Y. Li
Advanced Coatings Applied Research Laboratory (ACARL), Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
Closed-field unbalanced magnetron sputtering ion plating (CFUMSIP) is a versatile technique for the preparation of high-quality coatings due to the enhancement of plasma density during deposition. In this study, Cr–Ni–N hard coatings with different Ni contents (0–64 at.%) were deposited onto AISI M2 steel substrates by CFUMSIP in Ar–N2 reactive gas mixtures. The coatings were characterized in terms of their structure and properties by a variety of techniques, such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), micro-indentation, reciprocating wear tester, etc. The surface free energy of the coatings was calculated using the Owens–Wendt geometric mean approach. The experimental results indicated that the microstructure and properties of Cr–Ni–N coatings changed with the incorporation of Ni content at different level. The resultant Cr–Ni–N coatings were consisted of Cr2N, CrN and metallic Ni mixed phases. With the appropriate addition of Ni in the range of 20–40 at.%, the fracture toughness and wear resistance could be improved as compared to the Cr–N binary coating, and the hardness remained relatively high (20–23 GPa). Additionally, both low total surface energy (18–22 mN/m) and low polar component of surface energy (0.1–0.2 mN/m) could be obtained in the Cr–Ni–N coating system. Therefore, the Ni incorporation can provide an effective route to tailor the structure and properties of the Cr–Ni–N coatings. The good combined properties of the Cr–Ni–N coatings demonstrate their potential for applications in plastic injection molding.
In recent years, chromium nitride (CrN) coatings have been considered to be the promising material to benefit quite a number of industrial applications due to their high hardness, as well as good wear and corrosion resistance. On the other hand, nickel (Ni) coatings are widely used as protective layers against oxidation and corrosion. The combination of these two materials is expected to lead to the development of composite coatings with new material properties. In addition, the idea behind the preparation of Cr-Ni-N composite coatings is based on the combination of hard but relatively brittle CrN with a soft and ductile metal (Ni), which should provide a better fracture toughness.
Modern injection molding applications have raised important material requirements in terms of wear, corrosion and oxidation resistance as well as surface chemistry. Due to the environmental awareness as well as the design complexity of molds, the development of new coating materials and technologies is required. In this study, Cr-Ni-N coatings with different compositions have been deposited by the closed field unbalanced magnetron sputter ion plating (CFUMSIP) onto mold steel substrates. This is the first attempt to systematically investigate the effects of Ni addition on the structure and properties of the coating system. Our experiments have confirmed that the versatile coating technique provides an effective route to tailor the structure and properties of the coatings in a wide composition range. The experimental results indicate that the Cr-Ni-N coatings with ~20 at.% Ni exhibit the most promising potential for molding applications due to their excellent combined properties, as compared to other conventional PVD hard coatings.
Due to the superior properties of the new Cr-Ni-N coatings and the environmental friendliness of the coating process, it is anticipated that not only the IC packaging industry, but also the plastic injection molding and compounding industries will benefit from this research. Injection molds are often subjected to corrosion and pitting attack. Hard coatings can serve as a diffusion barrier to the corrosive substances in plastics. Additionally, the hardness of the coatings can protect the molds from the abrasive additives in plastic materials. Furthermore, the hard coatings can have more special properties, such as rejection of polymer materials sticking to the uncoated tools. This anti-sticking property can also allow non-lubricated forming or cutting processes.