Superhard coatings of CN_x/ZrN multilayers prepared by DC magnetron sputtering

Abstract

CN_x/ZrN multilayers were prepared by a dual-cathode unbalanced DC magnetron sputtering system. The results showed that under proper deposition conditions, the multilayer coatings were dense and fully crystalline with hardness exceeding 40 GPa and elastic modulus over 400 GPa. X-ray diffraction studies showed that there is a strong correlation between coating hardness and the occurrence of (111) texture of ZrN. Some crystalline regions going through the multilayers were clearly observed from the cross-sectional high resolution transmission electron microscopy image. Two extra d-spacings were obtained from the diffraction pattern that do not belong to any known d-spacing for ZrN, but match with two calculated d-spacings for β-C₃N₄.

Introduction

β-C₃N₄ has attracted much attention since 1989 due to the prediction that its mechanical properties are similar to those of diamond 1, 2. Numerous attempts have been made to synthesize this material 3, 4, 5, 6, 7, 8, 9, 10. Plasma-based thin film deposition techniques were most frequently used due to the possibility of obtaining and controlling growth conditions far beyond equilibrium. Some researches reported the growth of amorphous films with embedded crystalline β-C₃N₄ 8, 9.

However, most of the results showed that the nitrogen content of these films was 20-45 at.%, substantially below the stoichiometric composition for β-C₃N₄ (~ 57 at.%). The hardness was mainly in the range of 10–25 GPa. One investigation reported hardness up to 60 GPa [11].

Pseudomorphic stabilization was recently proposed as a means to produce crystalline carbon nitride using CN_x/TiN multilayers as a test case 12, 13. TiN was chosen as a template substrate because of the good lattice match between TiN (111) and β-C₃N₄ (0001). The best CN_x/TiN multilayers are fully crystalline, with hardness up to 55 GPa. In this paper, we explore the same strategy by using ZrN as the sandwiching materials. Note that ZrN (111), (a=0.323 nm) has hexagonal symmetry, the same as β-C₃N₄ (0001), (a=0.644 nm), and the lattice mismatch between these two faces is within 0.5%. We will examine the significance of having ZrN (111) texture in improving the overall hardness of the CN_x/ZrN multilayers.