Formation of nanocrystalline microstructure in arc ion plated CrN films

Qi-min WANG ¹, Se-Hun KWON¹, Kwang-Ho KIM^{1, 2}

1. National Core Research Center for Hybrid Materials Solution, Pusan National University,

Busan 609-735, Korea;

2. School of Materials Science and Engineering, Pusan National University, Busan 609-735, Korea

Received 21 April 2010; accepted 10 September 2010

Abstract: Applying negative bias voltages caused significant microstructure changes in arc ion plated CrN films. Nanocrystalline microstructures were obtained by adjusting the negative bias voltage. Structural characterizations of the films were carried out using X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HR-TEM). The results indicated that increasing ion bombardment by applying negative bias voltages resulted in the formation of defects in the CrN films, inducing microstructure evolution from micro-columnar to nanocrystalline. The microhardness and residual stresses of the films were also affected. Based on the experimental results, the evolution mechanisms of the film microstructure and properties were discussed by considering ion bombardment effects.

Key words: CrN; thin films; deposition; microstructure; arc ion plating; ion bombardment

1 Introduction

Compared to other binary transition metal nitride films, CrN films exhibit better oxidation and corrosion resistance, and are expected to possess good wear resistance under severe tribological conditions such as high load and high temperature[1-3]. The films can be synthesized by a variety of methods including magnetron sputtering (MS)[4-5], ion beam assisted deposition (IBAD)[5] and arc ion plating (AIP)[6]. It is found that AIP technology is an outstanding method for synthesizing hard films because of its high deposition rate, flexibility of target arrangements and the merits of producing films of high packing density and good film-substrate adhesion.

The microstructure and mechanical properties of the films by arc ion plating are strongly dependent on the impinging energy of the deposition species, which can be controlled by the substrate bias voltages in the AIP process[7-8]. In previous studies[9-10], it was found that the film growth characteristics, chemical composition and phase structure of AIP CrN films were greatly affected by the application of negative bias voltages. Nanocrystalline microstructures were formed by adjusting negative bias voltage. However, the detailed microstructure of the nanocrystalline CrN films was not investigated. The formation mechanism of nanocrystalline microstructure in the arc ion plated CrN films was not understood. In this study, this point was further focused by analyzing the films microstructure using high-resolution transmission electron microscope (HR-TEM).

2 Experimental

CrN films were deposited on polished single-crystal Si wafers with (111) orientation by an arc ion plating apparatus. An arc cathode of Cr source (99.99%) was installed on one side of the chamber wall. A rotational substrate holder was located at the center of the chamber, with distance of about 350 mm from the arc source to the substrate. The Si substrates were cleaned in an ultrasonic cleaner for 20 min with acetone and alcohol, respectively. The chamber was evacuated to less than 6×10^-3 Pa and the substrates were heated to 300 °C. CrN films were deposited at a working pressure of 0.08 Pa. The inlet flux rates of Ar and N₂ were fixed at 40 and 80 mL/min, respectively. The arc current was fixed at 55 A. The substrate was exposed to bias voltage in a range of 0-250 V. The film thickness was controlled in a range of 1.5-2 μm by varying the deposition time.

Cross-sectional morphologies of the films were obtained using a scanning electron microscope (SEM, Hitachi S-4200). The structure of the films was investigated using an X-ray diffractometer (XRD, D8, ADVANCE). Some specimens were investigated by transmission electron microscopy (TEM, JEOL-2010F) analyses. The cross-sectional TEM samples were prepared using focusing ion beam technique. Relevant high-resolution TEM (HR-TEM) analyses were operated with an accelerating voltage operated at 200 kV.

The hardness of films was evaluated using a micro-hardness tester with Knoop indenter (Matsuzawa, MMT-7) under a load of 0.25 N. To ensure the accuracy of the microhardness measurement, each recorded microhardness value was the mean value of five measurements. The residual stress, σ, in the films was analyzed by measuring the radius of curvature of the Si substrate with an optical profilometer after the film deposition, and the measurements were further processed using Stoney’s equation[11].

3 Results and discussion

SEM analysis revealed that the CrN films synthesized at 0 V exhibited apparent columnar morphology (Fig.1(a)). With increasing bias voltage, transition from columnar microstructure to glassy microstructure was observed. Fig.1(b) shows the typical glassy one with featureless morphology (at -50 V). The morphology film of deposited at bias voltage higher than -50 V is similar to that in Fig.1(b).

Fig.2 shows the XRD patterns of the CrN films. Only CrN phase with cubic rock-salt structure appeared in the AIP CrN films. Firstly, (200) preferred orientation was observed in the film at 0 V. Then the films evolved into (220) textured when the substrate bias voltage was higher than -50 V. At the inflection point (-50 V), the film seemed to be quasi-amorphous. Apparent XRD peak broadening can be detected with the increase of negative bias voltages, which is the effect of decreasing grain size and increasing microstrains. By estimation with Scherrer equation, the CrN films deposited at bias voltage higher than -50 V were nanocrystalline, with grain size less than 20 nm.

Fig.3 shows the typical crystalline HR-TEM image (Fig.3(a)) and two characteristic HR-TEM images of the interfaces between two adjacent columns (Figs.3(b) and (c)) in the CrN films grown at 0 V. The single crystal lattice with [111] lattice fringes and the fast Fourier transformation (FFT) of the lattice fringes revealing the diffraction points of single crystal are shown in Fig.3(a). The grain boundaries between the columnar grains in the CrN films deposited at 0 V were either large-angle grain boundaries (Fig.3(b)) or amorphous ones (Fig.3(c)). No lattice distortions indicating dislocation-like defects were observed.

Fig.4 shows typical HR-TEM images of the AIP CrN film grown at -50 V. The quasi-amorphous CrN film (Fig.2) was actually composed of seriously deformed nanocrystalline clusters and enveloping amorphous matrix. The inserted FFT spectra in top-right and down-left corners in Fig.4(a) demonstrate the seriously deformed character in region A and amorphous character between regions A and B, respectively.  Figs.4(b) and (c) show the typical HR-TEM images in regions A and B at higher magnifications. In Fig.4(b), the dislocation walls and stacking faults were observed. In region B, some discontinuous dislocations and amorphous grain boundary can be detected.

Fig.1 SEM images of cross-section in arc ion plated CrN films deposited at 0 V (a) and -50 V (b)

Fig.2 XRD patterns of AIP CrN film at various negative substrate bias voltages

Fig.3 HR-TEM images of AIP CrN films deposited at 0 V: (a) Lattice image in one column; (b), (c) Interfaces between two adjacent columns (Inset in Fig.3(a) is FFT image from lattice fringes)

Fig.5 shows three kinds of typical HR-TEM images of the AIP CrN film grown at -250 V. Recrystallized nanograins with size of 10-20 nm appeared in an amorphous matrix in Fig.5(a). Much smaller nanograins (2-5 nm) in the amorphous matrix appeared in Fig.5(b). These microstructures shown in Figs.5(a) and (b) preoccupied most part of the film. Some heavily distorted large grains with dislocation walls and stacking faults (Fig.5(c)) also existed in the film.

Fig.6 summarizes the microhardness and residual stress of the AIP CrN films as a function of negative bias voltage. It can be seen that the microhardness exhibited a sharp increase to 30-40 GPa when the bias voltage increased and then rebounded. Correspondingly, the residual stress evolved from tensile to compressive. The absolute values experienced a sharp increase to the maximum stress values of 1.3-2.0 GPa when the bias voltage increased to -50- -100 V and then gradually decreased. The microhardness enhancement and increased compressive residual stress seemed to be related to the formation of nanocrystalline microstructure in the films.

Fig.4 Typical HR-TEM images of AIP CrN film deposited at -50 V, top-right inset showing FFT spectrum in region A and down-left inset showing FFT spectrum in matrix between region A and region B; (b, c) magnified HR-TEM images showing details of typical crystal lattice in region A and region B in Fig.4(a)

As described above, the microstructure of AIP CrN films was greatly affected by applying negative bias voltages. Micro-columnar microstructure was observed in the films deposited at 0 V (Figs.1 and 3). Nanocrystalline microstructure was observed in the films deposited at bias voltages higher than -50 V. In Fig.4, seriously deformed nanocrystalline clusters enveloped by amorphous matrix were observed. In Fig.5, the recrystallized nanograins appeared. The formation of nanocrystalline microstructure in the arc ion plated CrN films can be attributed to the inherent characteristics of ion bombardment induced by applying negative bias voltages. The ion bombardment is a strong non- equilibrium process, in which ions transfer their kinetic energy to the growing film and heat in an atomic level. This process differs significantly from conventional heating because the kinetic energy of bombarding ions is transferred into very small areas of atomic dimensions and is accompanied by an extremely fast cooling. There is a rapid increase in the number of secondary nuclei by increasing ion bombardment on the growing film. Accordingly, the columnar growth is interrupted. Nanocrystalline microstructure or even quasi-amorphous microstructure comes into being (Fig.4 and 5).

Fig.5 Typical HR-TEM images of AIP CrN films grown at -250 V: (a), (b) Some recrystallized grains dispersed in amorphous matrix; (c) Distorted grains

Fig.6 Variations in microhardness and residual stress of AIP CrN films affected by negative substrate bias voltage

Following the Hall-Petch relation, the decrease in crystalline size induces increase in microhardness (Fig.6). At the same time, atomic peening occurs due to ion bombardment by applying negative bias voltage. Compressive stress arises when a growing film is bombarded by ions with energies of tens or hundreds of eV. Defects such as stacking faults, low-angle grains and trapped atoms will also occur, which causes the deformation of the lattice, and produces growth stresses. The growth-defect hardening (dislocations and stacking faults)[12-13] can be obtained. All these factors resulted in microhardness enhancement and increase of compressive residual stress (Fig.1(b)). However, further increasing bombarding ion energy could result in the lattice relaxation and recrystallization. The crystalline defects would be annihilated. According to Davis’s model[14], the growth stresses can be relaxed by the thermal spikes, which results in the release of strain caused by the atomic peening effect. Therefore, the compressive stresses in the films decreased at higher substrate bias voltages. Correspondingly, the rebounding of microhardness occurred (Fig.6).

4 Conclusions

1) Nanocrystalline microstructure in arc ion plated CrN films was observed when the negative bias voltage was larger than -50 V. Seriously deformed nanocrystalline clusters with dislocations and stacking faults, amorphous matrix and recrystallized grains were observed in the nanocrystalline films. Correspondingly, microhardness enhancement and compressive stress were obtained.

2) The formation of nanocrystalline microstructure and microhardness enhancement can be attributed to ion bombardment induced by applying negative bias voltages.

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(Edited by FANG Jing-hua)           

Foundation item: Project(2010-0001-226) supported by the National Core Research Center (NCRC) Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology; Project supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea

Corresponding author: Kwang-Ho KIM; Tel: +82-51-5102391; E-mail: kwhokim@pusan.ac.kr