Surface and Coatings Technology 183 (2004) 45–50

0257-8972/04/$ - see front matter � 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2003.09.064

Influence of nitrogen on the structure and mechanical properties ofr.f.-sputtered Cr–B–N thin films

Min Zhou * , M. Nose , K. Nogia, ,1 b a,c

Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japana

Department of Industrial Art, Takaoka National College, Takaoka 933-8588, Japanb

Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japanc

Received 24 September 2002; accepted in revised form 29 September 2003

Abstract

Cr–B–N thin films were synthesized by r.f.-plasma assisted magnetron sputtering apparatus. The influence of nitrogen on thephase composition and mechanical properties of the Cr–B–N thin film is evaluated in this study. XRD results indicate that aphase transformation from CrB to h-BN and b-Cr N occurred in the Cr–B–N films with increasing nitrogen partial pressures.2 2

The residual stress in the Cr–B–N films is compressive. Although it increases with increasing nitrogen partial pressure, the valueis much smaller than that of the conventional sputtered films. The hardness of the films decreases due to the formation of soft h-BN phase in the Cr–B–N films. The anti-oxidation property becomes worse with increasing nitrogen partial pressure; however, itis noted that the lowest beginning temperature of oxidation is still approximately 860 8C, which is higher than the typical workingtemperature of contact area for high-speed cutting tools.� 2003 Elsevier B.V. All rights reserved.

Keywords: Chromium boron nitrides; Thin film; Nitrogen partial pressure; r.f.-plasma assisted magnetron sputtering

1. Introduction

Thin film coating techniques have increased theirimportance in the deposition of wear resistant coatingsfor tribological applications. The process of thin filmcoatings has proved to be remarkably effective for anincrease of lifetime for cutting tools or moulds. Thehardness of boride coatings is high due to a higherdegree of covalent bonding w1x. Boride coatings are alsowell known as stable hard refractory materials with ahigh resistance to corrosion and oxidation. Therefore,boride coatings are promising candidates for manyengineering applications such as machine tool, moldingdie and diffusion barrier. However, the multiphase sys-tems, such as Ti–B–N w2,3x and Zr–B–N w4,5x, havebeen well studied. They show many similarities withcomposite materials and often display better propertiesthan single phase materials. However, until now, there

*Corresponding author. Tel.: q1-510-683-7557; fax: q1-510-683-7065.

E-mail address: min.zhou@readrite.com (M. Zhou).Present address: Read-Rite Corporation, 44100 Osgood Road,1

Fremont, CA 94539, USA.

are very few published reports about Cr–B–N thin filmsw6–8x.As a part of a research program concentrating on

syntheses of new hard thin films with high-temperaturestability and better mechanical properties by a newlydeveloped r.f.-plasma assisted magnetron sputteringapparatus, Cr–B–N thin films were synthesized andinvestigated in this study. In this paper, the influence ofnitrogen partial pressure (P ) on the structure andN2

properties, such as residual stress and hardness, of Cr–B–N thin films was investigated.

2. Experimental details

2.1. Film preparation

Fig. 1 shows the schematic diagram of the r.f.-plasmaassisted planar magnetron sputtering apparatus used inthe present study (MPS-2000-HC3, ULVAC Co., Japan).Two r.f. generators, operating at 13.56 MHz, power thetarget and helix coil located just above the target,independently. Using the helix coil, an additional highdensity inductively coupled r.f. discharge is generated

46 M. Zhou et al. / Surface and Coatings Technology 183 (2004) 45–50

Fig. 1. Schematic diagram of the deposition apparatus.

Table 1Experimental conditions

Apparatus MPS-2000-HC3 (ULVAC Japan Ltd)Target CrB (99.99%)2

Target diameter 51 mmr.f. power of target 100 Wr.f. power of coil 50 WTarget–substrate distance 180 mmGas Ar (99.9999%), N (99.9999%)2

Gas flow rate Ar: 9 sccm, N : 0–10 sccm2

Base pressure 10 Pay5

Sputtering pressure 0.28–0.37 PaSubstrate temperature during the deposition 310 K

just in front of the magnetron target. The major role ofthis additional r.f. discharge is twofold: (1) to activatethe reactions between evaporating metal species andreactive gas to enhance the formation of compoundfilms; (2) to modify the growth kinetics and physicalproperties of the deposits through the change of the filmmorphology w9x. Owing to this high ion density plasma,coatings deposited by the inductively coupled plasmaassisted sputtering can be expected to show bettermechanical properties with denser microstructures thancoatings formed by conventional deposition methodw10x. Because this apparatus is designed to synthesizenew materials by co-sputtering two targets at the sametime, the target–substrate distance of this apparatus (180mm) is much longer than that of the planar magnetrontype or the conventional sputtering machine (-100mm) w11x in order to avoid the contamination of targetby each other during the sputtering. In this study,however, only one pure CrB (99.99%) target was used.2

Silicon wafer, quartz glass T4040 and glass ceramicPEG3130C plates were used as substrates. Cleaning wasperformed with acetone and propanol in an ultrasonic

bath before the deposition experiment. In order toinvestigate the influence of the nitrogen partial pressureon the structure and properties of the Cr–B–N thinfilms, the ratio of the nitrogen to argon gas flow rateswas changed. The total pressure and gas flow rate duringdeposition were measured. The partial pressure basedon these measurement data was calculated. The detailedexperimental conditions are given in Table 1. Thedeposition rate is approximately 2.5 nmymin. The thick-ness of films, determined using the surface profilometerDektak-II by partly masking the substrate surface andmeasuring the height of the step, is selected mainly tobe approximately 500 nm for most tests.

2.2. Film analysis

The crystalline structure of films was identified byX-ray diffractometer using Cu Ka radiation with a thinfilm goniometer (M03X, Mac Science Co., Japan).Scans were made in glancing incidence mode (Seeman–Bohlin mode) with an incident beam angle of 48. Theatomic ratio of chromium to boron in the depositedfilms was determined by electron probe microanalysismeasurement (JXA-8600, JEOL, Japan) with the ZAFcorrection method.

2.3. Nanoindentation

The hardness was chosen as the main mechanicalproperty criterion in this study. It is well known that thedata of thin film mechanical properties such as hardnessand toughness are dependent on the measurement toolsuch as indentor and also on the theory the researcheruses to explain the curve. Now there is not a wellaccepted theory or a measurement process for toughnessmeasurement of thin films. Therefore, it is very difficultto compare the toughness data from different researchgroups. In the case of hardness, the situation is different.Now theory for hardness measurement of thin film iswell established and the tool for measuring hardness isalso commercialized. Hardness data are easy to comparewith other researchers’ work because most data formsimilar tool using the same theory. Most papers in the

47M. Zhou et al. / Surface and Coatings Technology 183 (2004) 45–50

Fig. 2. Chemical compositions of the Cr–B–N thin films.

field of hard coatings choose hardness as the mainmechanical property criteria instead of toughness.Indentation experiments were performed using a Tri-

boscope (Hysitron Inc., USA), a quantitative depth-sensing nanoidentation system that is interfaced with anatomic force microscope to provide in situ imaging. Thetriangular Berkovich diamond pyramid indenter wasused in this study. For each sample, six peak loads wereinvestigated from 500 to 3000 mN at an increment of500 mN. Five indentations were made at each load andresults showed here represent the averages for the group.

2.4. Residual stress

The film stress was evaluated by a surface profile androughness measuring machine (FORM TALYSURFSERIES S4, Rank Taylor Hobson Ltd, England). Thechanging of 40=5=0.5 mm glass ceramic PEG3130C3

plate’s curvature caused by the deposited film wasinvestigated by this machine. Then the film stress, s,was calculated by the following equation derived fromStoney equation w12x:

2ETss 4d (1)23(1yn)L t

where E is the Young’s modulus of the substrate, n isthe Poisson’s ratio of the substrate, T is the thickness ofthe substrate, L is the length of the substrate, t is thethickness of the film (t<T) and d is the deformationat the center of substrate after deposition.

2.5. Oxidation of films

The weight gain due to the oxidation of the films wasmeasured by the thermogravimetry. A quartz glass sub-

strate was used in order to prevent substrate oxidationand the reaction between the film material and thesubstrate. The samples were heated in air. The temper-ature increased from room temperature to 1473 K at arate of 5 Kymin.

3. Results and discussion

The influence of the nitrogen on the composition ofCr–B–N thin films is shown in Fig. 2. The chemicalcompositions of the Cr–B–N thin films were measuredby EPMA using bulk CrB material as standard. It isevident that, with an increase in nitrogen partial pressure(P ) from 0 to 0.07 Pa, nitrogen content in the filmsN2

increases sharply at the cost of chromium in the films.Inside the sputtering chamber, the nitrogen is introducedinto the chamber just above the target, which meansthat the nitrogen will react with Cr and B on the targetsurface. Therefore, the sputtering rates of Cr and B arechanged by the addition of nitrogen. The deposition ratechanging with the addition of nitrogen is shown in Fig.3.X-ray diffraction patterns of Cr–B–N thin films in

glancing incidence mode with varying nitrogen partialpressures is shown in Fig. 4, with the peak positionsmarked for the b-Cr N, CrN and BN positions w13x,2

respectively. When there is no nitrogen introduced, thefilm consists of the CrB phase. Although the broad2

diffraction peak may indicate that the film is amorphous,they actually consist of very small grains in the nano-meter range (;10 nm) w14x. When a small amount ofnitrogen is introduced and P is very low (;0.02 Pa),N2

X-ray diffraction patterns display a strong (0 0 2) peakof h-BN phase. This may be explained by the fact thatboron nitride has a greater negative free energy of

48 M. Zhou et al. / Surface and Coatings Technology 183 (2004) 45–50

Fig. 3. The deposition rate of the CrB2 target as a function of nitrogengas flow rate (sccm).

Fig. 5. Residual stress of Cr–B–N films as a function of nitrogenpartial pressure (Pa).

Fig. 4. X-ray diffraction patterns of Cr–B–N thin films in glancing incidence mode with varying nitrogen partial pressures (Pa).

formation than chromium nitride and chromium boridew15x, which means boron combines easily with nitrogen.In addition, a strong (1 1 0) peak of b-Cr N phase is2

also detected. However, the (1 0 1) peak of CrB phase2

reduced to a very low intensity due to combination ofN with B and Cr. With increasing P to 0.07 Pa, onlyN2

peaks of h-BN and b-Cr N phases could be detected.2

The (1 0 1) peak of CrB phase disappears, which may2

indicate that the amount of CrB phase is too small to2

be detected. With further increasing P to 0.2 Pa, aN2

new (1 1 1) peak of the CrN phase is detected but inlow intensity. The peaks of h-BN and b-Cr N phases2

are also reduced to low intensity, which maybe inter-preted by high structure disorder of the thin films.

The residual stress found in Cr–B–N films is com-pressive, as shown in Fig. 5. However, the compressivestress in these films is rather small in comparison withthe compressive stress in hard coatings prepared by theconventional sputtering technique. As we discussed in

49M. Zhou et al. / Surface and Coatings Technology 183 (2004) 45–50

Fig. 6. Hardness and Young’s modulus of Cr–B–N films as a functionof nitrogen partial pressure (Pa).

Fig. 7. Oxidation resistance of Cr–B–N thin films as a function ofnitrogen partial pressure (Pa).

the previous section, the target–substrate distance ofthis apparatus is much longer than that of normalsputtering machine, and the working gas pressure is notlow. Another reason is that coatings deposited by theinductively coupled plasma assisted sputtering can beexpected to show denser microstructures than coatingsformed by conventional deposition method owing to thehigh ion density plasma w10x. Therefore, the atomicpeening effect in this study is not as obvious as that inthe conventional sputtering technique. Consequently,compressive stress in these films is smaller than that inhard coatings prepared by the conventional sputteringtechnique.The compressive stress in these films increases when

the nitrogen partial pressure is increased. The reasonsfor this are complicated and can be explained by thefollowing: (1) It is well known that nitrogen inclusionsin materials especially in the interstitial site will enhancethe compress stress w16x. (2) As shown in Fig. 5, thecompressive stress in the Cr–B–N films increases sharp-ly when only small amounts of nitrogen are introducedand the nitrogen partial pressures is low. This may beinterpreted by the phase transformation from CrB to h-2

BN and b-Cr N occurring in the Cr–B–N films that2

cause the volume of thin film expansion and increasethe compressive stress in the films.

The hardness and Young’s modulus computed fromthe Oliver–Pharr method w17x is also shown in Fig. 6.Both hardness and Young’s modulus of the filmsdecrease sharply when just a very small amount ofnitrogen is introduced. It is believed that this decreaseis caused by the formation of soft h-BN phase (hardness:2–10 GPa, Young’s modulus: 70–160 GPa w18x) in theCr–B–N films. As nitrogen partial pressure increasesfrom 0.02 to 0.07 Pa, the hardness and Young’s modulusof the films decrease continuously because the amountof the soft h-BN phase in the films increases (Fig. 2).However, when nitrogen partial pressure is larger than0.07 Pa, the proportion of the soft h-BN is almostsaturated (Fig. 2). As a result, the hardness and Young’smodulus of the Cr–B–N films also basically remainconstant, which is similar to those of h-BN films.The influence of the nitrogen on the oxidation resis-

tance of the Cr–B–N thin films is shown in Fig. 7. Thebeginning temperature of oxidation is determined in thethermalgravity data where the weight gain begins toincrease obviously. Although the oxidation resistancebecomes worse with increasing nitrogen partial pressure,it is noted that the lowest beginning temperature ofoxidation is still approximately 860 8C, which is higherthan the typical working temperature for high-speedcutting tools w19x. These results indicated that theoxidation resistance deteriorates with increasing nitrogen

50 M. Zhou et al. / Surface and Coatings Technology 183 (2004) 45–50

partial pressure due to the formation of chromiumnitrides in the Cr–B–N thin films. Chromium nitridesare always oxidized under 800 8C, which is much lowerthan the CrB2 and h-BN.

4. Summary

Cr–B–N thin films were synthesized by r.f.-plasmaassisted magnetron sputtering. The influence of nitrogenon the phase composition and mechanical properties ofthe Cr–B–N thin film was evaluated in this study. XRDresults indicate that a phase transformation from CrB2

to h-BN and b-Cr N occurs in the Cr–B–N films with2

increasing nitrogen partial pressures. The residual stressin the Cr–B–N films is compressive. Although itincreases with an increase in nitrogen partial pressure,the value is much smaller than that of the conventionalsputtering films. The hardness of the films decreasesdue to the formation of soft h-BN phase in the Cr–B–N films. The anti-oxidation property becomes worsewith increasing nitrogen partial pressure; however, it isnoted that the lowest beginning temperature of oxidationis still approximately 860 8C, which is higher than thetypical working temperature for high-speed cutting tools.

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