Effect of partial pressure of reactive gas on chromium nitride and chromium oxide deposited by arc ion plating

LI Ming-sheng(李明升)^{1, 2}, FENG Chang-jie(冯长杰)², WANG Fu-hui(王福会)²

1. Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and
Technology Normal University, Nanchang 330013, China;

2. State Key Laboratory for Corrosion and Protection, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The effects of reactive gas partial pressure on droplet formation, deposition rate and change of preferred orientation of CrN and Cr₂O₃ coatings were studied. For CrN coatings, as nitrogen partial pressure increases, the number and size of droplets increases, the deposition rate initially increases obviously and then slowly, and the preferred orientation of CrN changes from high-index plane to low-index one. For Cr₂O₃ coatings, with the increase of oxygen partial pressure, the number and size of droplets decreases, the deposition rate decreases and the (300) becomes the preferred orientation. These differences are ascribed to the formation of CrN (with a lower melting point) and Cr₂O₃ (with a higher melting point) on the surface of Cr target during the deposition of CrN and Cr₂O₃. Complete coatings CrN or Cr₂O₃ film can be formed when reactive gas partial pressure gets up to 0.1 Pa. The optimized N₂ partial pressure for CrN deposition is about 0.1-0.2 Pa in order to suppress the formation of droplets and the suitable O₂ partial pressure for Cr₂O₃ deposition is approximately 0.1 Pa for the attempt to prevent the peel of the coating.

Key words: chromium nitride; chromium oxide; arc ion plating; reactive gas partial pressure

1 Introduction

Arc ion plating (AIP) technique is one of the most perspective PVD methods for industrial manufacture   of ceramic coatings due to the high degree of   ionization in the target material, convenient control    of the parameters and high deposition rate [1,2].   Owing to the thermal stability, excellent wear resistance and good corrosion resistance, chromium nitride coating [3-6] and chromium oxide coating [7,8] are regarded as the prospective alternatives in corrosion-resistance applications and in machining industry. During the process of AIP, The control of reactive gas is critical for the optimization of coating structure and improvement of coating properties. In this study, CrN and Cr₂O₃ coatings were respectively deposited by arc ion plating, the  effects of reactive gas partial pressure on droplet formation,  deposition rate and change of preferred orientation of CrN and Cr₂O₃ coatings were studied, and the cause was discussed.

2 Experimental

The substrate material is the wrought martensite steel 1Cr11Ni2W2MoV, which were cut into 15 mm×10 mm×2 mm and ground to 1 000 grit, followed by ultrasonic cleaning in alcohol and acetone solution.

CrN and Cr₂O₃ coatings were deposited on steel specimens by a homemade reactive multi-arc ion plating facility of DH-4 coating system. The target material used herein was pure Cr. Before coatings were deposited, the substrates were sputter-cleaned using Ar ion under     1 000 V DC bias voltage to remove contaminant layer and to ensure good adhesion of deposited coatings. A mass flow meter was used to adjust the N₂ or O₂ partial pressure from 0 to 0.8 Pa. The other deposition parameters were listed as following: substrate temperature 400 ℃, total pressure 0.8 Pa, arc voltage 20 V, arc current 60 A, pulse bias voltage -450 V and duty cycle 20% and frequency 20 kHz, deposition time 30 min.

The thickness of coatings was gained by measuring the cross-section samples and the deposition rate was calculated via the coating thickness and deposition time. Surface morphologies were gained by scan electron microscopy. The phase structure was determined by X-ray diffraction using Cu K_α radiation.

3 Results and discussion

3.1 Surface morphologies

Surface morphologies of CrN and Cr₂O₃ deposited under different reactive gas partial pressure are shown in Figs.1 and 2. For CrN coatings, the number and size of droplets increased when N₂ partial pressure increased from 0 to 0.8 Pa. While for Cr₂O₃, with the increase of O₂ partial pressure, the number and size of droplets decreased obviously. During the process of reactive sputtering, when reactive gas pressure is high enough, a thin layer of nitride or oxide will be formed on the target surface and which is called “target poisoning” [9]. As shown in Fig.3 the melting point of CrN is far lower than

Fig.1 Surface morphologies of CrN coatings deposited at various N₂ partial pressures: (a) 0.1 Pa; (b) 0.2 Pa; (c) 0.4 Pa; (d) 0.8 Pa

Fig.2 Surface morphologies of Cr₂O₃ coatings deposited at various O₂ partial pressures: (a) 0 Pa; (b) 0.05 Pa; (c) 0.10 Pa; (d) 0.20 Pa

that of pure Cr while Cr₂O₃ has a much higher melting point than pure Cr. It had been reported that the metal with high melting point gave rise to a slighter droplet contamination than the metal with a low melting point [10], which explains the contrary effluence of reactive gas partial on droplet formation of CrN and Cr₂O₃.

Fig.3 Binary phase diagrams of Cr—N (a) and Cr—O (b)

3.2 Deposition rate

Fig.4 shows the deposition rates of CrN and Cr₂O₃ coatings at various reactive gas partial pressure. The results indicated that the deposition rate of CrN initially increased dramatically with increasing nitrogen partial pressure and then kept almost unchanged and the deposition rate of Cr₂O₃ first decreased obviously and then slightly when O₂ partial pressure increased. Similarly the “target poisoning” accounts for the change of deposition rate. The evaporation rate of Cr₂O₃ is much higher than that of pure Cr and the evaporation rate of CrN is far lower than that of pure Cr, which accounts for the initial change of the deposition rate. After a complete thin layer of nitride or oxide film is formed on the target surface, the deposition rate will show little change since the evaporation rate of target material keeps almost

Fig.4 Deposition rate versus reactive gas partial pressure

unchanged.

3.3 Phase structure

Typical X-ray diffraction patterns of CrN and Cr₂O₃ coatings deposited under various reactive gas partial pressures are shown in Fig.5. When no reactive gas was introduced, BBC-Cr coating was obtained. For CrN, when N₂ partial pressure got up to higher than 0.1 Pa, complete CrN of B1 NaCl structure was formed. The (111) and (220) textures were found to be the competitive orientations and the (220) texture became weaker and (111) stronger as the nitrogen partial pressure was raised. The melting point (T_m) of CrN is only about 1 000 ℃ and the deposition temperature is above 0.25 T_m, thus the atoms of formed coating have some ability of diffusion, which is in favor of the growth of the close-packed (111) plane. For Cr₂O₃, with increasing O₂ partial pressure, the peaks of XRD pattern shifted to lower diffraction angle and the (006) texture weakened while the (300) texture increased. The lattice distance paralleled with substrate surface is 1.5%-1.7% bigger than the corresponding lattice distance of bulk Cr₂O₃, signifying a high compressive stress existed in the coating. With increasing O₂ partial pressure, the compressive stress of the Cr₂O₃ coating increased and which accounted for the peel of the coating when O₂ partial pressure exceeded 0.2 Pa. The transformation of (006) texture to (300) is in favor of release of compressive stress.

4 Conclusions

During the process of deposition of CrN or Cr₂O₃ coatings, correspondingly, a thin layer of CrN with a lower melting point or Cr₂O₃ with a higher melting point is formed on the surface of the Cr target. Due to the contrary change of the melting point of the target, the

Fig.5XRD patterns of CrN(a) and Cr₂O₃(b) deposited at various reactive gas partial pressure

droplet formation, the deposition rate and the orientation of CrN and that of Cr₂O₃ exhibits opposite change trend with increasing reactive gas partial pressure. The optimized N₂ partial pressure for CrN deposition is about 0.1-0.2 Pa in order to suppress the formation of droplets and the suitable O₂ partial pressure for Cr₂O₃ is approximately 0.1 Pa for the attempt to prevent the peel of the coatings.

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(Edited by YANG Hua)

Foundation item: Projects (59971052; 50401022) supported by the National Natural Science Foundation of China

Corresponding author: LI Ming-sheng; Tel: +86-791-3801423; E-mail: mshli@163.com