Controlling Oxygen Content by Varying Oxygen Partial Pressure in ChromiumOxynitride Thin Films Prepared by Pulsed Laser Deposition

Kazuma Suzuki+1, Toshiyuki Endo+2, Teruhisa Fukushima+2, Aoi Sato+2, Tsuneo Suzuki,Tadachika Nakayama, Hisayuki Suematsu and Koichi Niihara

Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka 940-2188, Japan

Chromium oxynitride Cr(N,O) thin films were prepared by pulsed laser deposition in a highly reactive atmosphere, which consisted of pureoxygen gas and nitrogen plasma from a radio-frequency radical source. In order to control the oxygen content in the thin films, oxygen partialpressure (PO2

) in the chamber was varied from 5 to 10 © 10¹5 Pa under a fixed total pressure of 1.5 © 10¹2 Pa. The thin films were thencharacterized by X-ray diffraction, infrared spectroscopy, electron energy loss spectroscopy and nano-indentation testing. It was found that theoxygen content of the thin films changed from 0 to 62mol% with increasing PO2

. The thin films with only the NaCl-type (B1) Cr(N,O) phasewere prepared under PO2

< 7.5 © 10¹5 Pa. The chromium content of the B1 phase decreased from 47mol%, which was close to thestoichiometric composition of CrN, to 40mol% when the oxygen content was increased. The hardness increased up to 32GPa with increasingPO2

up to 7.5 © 10¹5 Pa. [doi:10.2320/matertrans.M2013047]

(Received February 4, 2013; Accepted April 2, 2013; Published June 25, 2013)

Keywords: chromium, nitride, oxynitride, hardness, laser ablation

1. Introduction

Hard coatings have been applied on cutting tools, autoparts and mold tools to improve their characteristics whichinclude high hardness, wear resistivity, chemical stability,oxidation resistivity and so on. Diamond-like carbon (DLC)and transition metal nitride have been widely used as the hardcoating materials. In particular, chromium nitride (CrN) hasexcellent properties such as oxidation resistivity and chemicalstability.1,2) However, CrN thin films typically have lowhardness compared to other materials such as titanium nitride(TiN) and titanium aluminum nitride ((Ti,Al)N), which islimiting the use of CrN coatings.

In our previous work, chromium oxynitride (Cr(N,O)) thinfilms were prepared by pulsed laser deposition (PLD).3,4)

PLD is a deposition method excelling sputtering and arc ionplating in composition control of thin films. Inumaru et al.reported that PLD could prepare epitaxial thin films toevaluate the characteristics which include the electronic andthe magnetic properties.5) Cr(N,O) had a NaCl-type (B1)structure similar to that of CrN. The hardness of Cr(N,O) thinfilms increase with increasing the oxygen content and themaximum value exceed 30GPa.4) Materials with addedfourth elements to Cr(N,O) for improving hardness have alsobeen studied.6,7) For example, chromium magnesium oxy-nitride ((Cr,Mg)(N,O)) thin films exhibit hardness values ashigh as 35GPa.6) In addition, Urgen et al. has described theeffect of oxygen content on the tribological behaviour of Cr­N­O coatings,8) and then I. M. Ross et al. has used Cr­N­Olayers in multilayer films for enhancement of the oxidationand tribological performance.9)

Since the hardness of Cr(N,O) increases with increasingthe oxygen content, precise oxygen content control and itsmeasurement are required for identification of the solubilitylimit and elucidation of the hardening mechanism. Until now,

Cr(N,O) thin films were prepared by depositing Cr vapor inN2 or NH3 ambient gas with residual oxygen.3,4) However,the oxygen content control was difficult in this method. In thecurrent work, an intuitive method using modification of theoxygen partial pressure (PO2

) for the precise oxygen contentcontrol is proposed. A highly reactive atmosphere of O2

mixed with N radicals is used in this method. Chromiumoxynitiride thin films prepared by reactive magnetronspattering in N2/O2 reactive gases and effect of PO2

onchromium oxynitride thin films were studied.10­12) However,preparation by PLD using O2 reactive gas has not beenreported. Our goal in this study is to develop a highly precisecontrol method of oxygen content and to obtain themaximum hardness in Cr(N,O) thin films.

2. Experimental

Figure 1 shows a schematic illustration of an apparatusused for preparing thin films. Ablation plasma was producedby irradiating an Nd: yttrium aluminum garnet laser (355 nm)onto a Cr target (99.9% purity). The laser was electro-optically Q-switched by a Pockels cell to produce intensepulses of a short duration (7 ns). The laser energy density was

Fig. 1 Schematic illustration of the apparatus used for preparing thin films.

+1Corresponding author, E-mail: paper-craft@etigo.nagaokaut.ac.jp. Grad-uate Student, Nagaoka University of Technology

+2Graduate Student, Nagaoka University of Technology

Materials Transactions, Vol. 54, No. 7 (2013) pp. 1140 to 1144©2013 The Japan Institute of Metals and Materials

1.7 J cm¹2 and the deposition time was 5 h at a laser-pulserepetition rate of 10Hz. The deposition surface area was1 cm2 on a single-crystal (100)-oriented silicon substrateplaced at a distance of 45mm from the target. The substratetemperature was controlled at 973K using an infrared lampheater. The deposited film thickness was approximately100 nm in this condition.

The chamber was first evacuated to a pressure of2.5 © 10¹5 Pa using a rotary pump and a turbo molecularpump, and the chamber was then filled with oxygen gas(>99.99995 vol% purity). The value of the vacuum gaugeat this time was defined as PO2

. After O2 gas introduction,nitrogen plasma (>99.99995 vol% purity) from an RF radicalsource was supplied. Both continuous pumping and intro-duction of O2 gas and N plasma were carried out during thedeposition. In order to change the oxygen content of the thinfilms, PO2

was varied with changing the oxygen gas flow rateusing a variable leak valve. The thin films were preparedunder a fixed total pressure of 1.5 © 10¹2 Pa.

For composition analysis of the thin films, Rutherfordbackscattering spectroscopy (RBS) and electron energy lossspectroscopy (EELS) were utilized. In the EELS spectra,since O K-edge and Cr L-edge are very close, only oxygenand nitrogen contents were measured by EELS. Cation andanion contents were calculated from RBS spectra. By thesetwo spectroscopy results, compositions of thin films wereprecisely determined. The oxygen content was defined as x.The crystal structures of the thin films were studied by X-raydiffraction (XRD) using Cu K¡ radiation (0.154 nm) in theBragg­Brentano configuration. The chemical bonding statewas estimated by Fourier transform infrared spectroscopy(FT-IR). The FT-IR spectrum for each sample was obtainedafter taking into account of the absorbance of the Si substrate.The film thickness measured in a scanning electronspectroscope (SEM) was approximately 100 nm. The hard-ness of the thin films (HIT) was measured by nano-indentationtesting under a load of 0.07mN using a Berkovich indenter.The indentation depth was around 10 nm. The load wasdetermined not to exceed the indentation depth more than 1/8of the films thickness. The microstructures of the thin filmswere observed using a field emission transmission electronmicroscope (FE-TEM) with a 200 kV acceleration voltage.The TEM samples were made two methods, i.e., thinning bya focused ion beam (FIB) apparatus and by scratching witha diamond pen. The former was used for cross sectionalobservations in wide area to find initial growth layer amongthe thin films. The latter was chosen for plan viewobservations and compositional analysis to prevent the ionbeam damage.

3. Results and Discussion

3.1 Oxygen contentFrom the results of RBS and EELS measurements, it was

found that the thin films contained chromium, nitrogen andoxygen. Figure 2 shows the oxygen content of the thinfilms as plotted against PO2

. As PO2increased, x increased

monotonically from 0 to 62mol%. The oxygen content ofthe Cr(N,O) thin film was thus successfully controlled byappropriately adjusting PO2

. Figure 3 shows the composition

of the thin films. In the thin film without oxygen, thechromium content was 47mol%, which was close to thestoichiometric composition of CrN. In contrast, the chromi-um content of the thin films became closer to that of Cr2O3

when the oxygen content was increased. As far as we know,there is no report on the chromium content in the Cr(N,O)thin films due to the increasing of the oxygen content. As wewill describe later, the thin films have a B1 structure. Hence itis indicated that vacancy was formed in the Cr site by thereplacement of N with O.

3.2 Phase identificationFigure 4 shows XRD patterns of the thin films. Peak

positions and relative intensities for CrN and Cr2O3 inInternational Centre for Diffraction Data (ICDD) are alsoincluded for comparison. It was found that all samplesincluded a B1 phase based on CrN. In addition, only thesample formed with PO2

= 10 © 10¹5 Pa included the Cr2O3

phase. The peaks due to the B1 structure became broad withincreasing PO2

. Crystallite size effect and lattice strain areconsidered as a major cause for broadening of XRD peaks.However, as we will describe in the next paragraph,

Fig. 2 The oxygen content in the thin films as a function of oxygen partialpressure.

Fig. 3 The composition of the thin films.

Controlling Oxygen Content in Chromium Oxynitride Thin Films 1141

crystallite size of the Cr(N,O) thin films did not change withincreasing PO2

. Hence it is suggested that the cause for thisbroadening is variations in the oxygen content in eachcrystallite. Figure 5 shows the lattice constants of the B1phase of each thin film calculated from the peak positionsfrom the (111) and the (200) reflection. The lattice constantdecreased with increasing PO2

. The decrease in the latticeconstant indicates solution of oxygen in the B1 phase.Transition metal oxides and nitrides which have the totalvalence similar to that of CrN include a lot of vacancies (ex.TiO,13) VO13) and TiN14)). These lattice constants changewith increasing number of vacancies in the cation site.In Fig. 3, it is confirmed that the number of chromiumvacancies increase with increasing the oxygen content. Hence

it is considered that the cause for the decreasing of the latticeconstant is formation of vacancies in the Cr site. Figure 6shows the FT-IR spectra of the thin films. Reference data forCrN and Cr2O3 are also included.15) The absorption spectraof the samples formed with PO2

less than or equal to7.5 © 10¹5 Pa showed a broad peak mainly at 390­430 cm¹1

due to the Cr­N bond. Absorption peaks due to Cr2O3 werenot observed. On the other hand, the absorption spectra ofthe samples formed with PO2

greater than or equal to8.5 © 10¹5 Pa show sharp peaks at 550 cm¹1, which is dueto the Cr­O bond in the Cr2O3 crystal. From the results inFigs. 4 and 6, it was found that the thin films with only theB1-Cr(N,O) phase were prepared under PO2

less than orequal to 7.5 © 10¹5 Pa.

3.3 Microstructure and hardnessFigure 7 shows a bright field image (BFI) of the TEM

sample prepared by focused ion beam (FIB) processing tokeep the TEM sample thickness being less than 100 nm.According to this image, an initial growth layer was notobserved. Figure 8 shows BFI, dark field images (DFI) andselected area diffraction patterns (SAD) for the prepared thinfilms. It was observed that all spots in the SAD patternfrom all samples can be indexed for reflection from the B1structure. The DFI were taken from the 200 diffraction of theSAD. From these DFI, it was found that crystallite size of thethin films was approximately 100 nm and did not change withincreasing PO2

. Rawal et al.12) reported that crystallite size ofCr(N,O) thin films prepared by reactive magnetron spatteringdecreased with increasing oxygen partial pressure in theatmosphere. In our method, variation of PO2

(10¹5 Pa order)is far lower than the total pressure (1.5 © 10¹2 Pa). Hence itis considered that varying PO2

do not affect crystal growthconfiguration of the Cr(N,O). Figure 9 shows the indentationhardness data for the thin films as a function of PO2

. The

Fig. 4 XRD patterns of the thin films prepared at different oxygen partialpressures.

Fig. 5 The lattice constants of thin films calculated from the XRD patterns.The solid and hollow symbols indicate the lattice constants calculatedfrom the diffraction of 111 and 200, respectively. These values werecalibrated from the diffraction of Si substrate.

Fig. 6 FT-IR spectra of the thin films prepared at different oxygen partialpressures.

K. Suzuki et al.1142

results of phase identification by XRD and FT-IR are alsoshown at the top of Fig. 9. The hardness of the thin filmsincreased with increasing PO2

up to 7.5 © 10¹5 Pa. Above7.5 © 10¹5 Pa, the hardness decreased. The thin films showeda maximum hardness value of 32GPa, which was similarto the previous results.4) The maximum value was foundaround the solubility limit.

In general metallic materials, yield stress is related tocrystallite size through Hall­Petch relationship. This relation-ship serves to demonstrate that the yield stress increases withdecreasing crystallite size. In Cr(N,O), the crystallite size didnot change with increasing the oxygen content. Hence it wasfound that high hardness was not achieved by the Hall­Petch

relationship. The decreasing of hardness for PO2above

7.5 © 10¹5 Pa related to the crystal structure of the thin films.In this zone, Cr2O3, which have the lower hardness valuethan that of CrN, exist as the second phase. It is consideredthat the hardness decreased due to increase of the volumefraction of Cr2O3.

4. Conclusions

From the above results, it was found that the oxygencontent in the Cr(N,O) thin films was successfully controlledby varying PO2

in a pulsed laser ablation process using a RFradical source. The thin films had oxygen content up to

Fig. 7 Bright field image of the sample prepared by focused ion beamprocessing. This thin film was prepared without O2.

Fig. 8 Bright field (left) and dark field (right) under excitation of the fcc (200) reflection of the thin films prepared (a) without O2,(b) at PO2

= 5.0 © 10¹5 Pa, (c) at PO2= 7.5 © 10¹5 Pa and (d) at PO2

= 8.5 © 10¹5 Pa.

Fig. 9 Nano-indentation hardness of the thin films as a function of oxygenpartial pressure.

Controlling Oxygen Content in Chromium Oxynitride Thin Films 1143

62mol%. All the samples contained a phase with a B1structure. In the thin films deposited under high PO2

(²8.5 © 10¹5 Pa), Cr2O3 existed as a second phase. Thechromium content of the B1 phase decreased from 47mol%,which was close to the stoichiometric composition of CrN, to40mol% when the oxygen content was increased. From TEMobservations, it was found that crystallite size of the thinfilms did not change with increasing PO2

. The hardness of thethin films increased with increasing PO2

, and then decreasedfor PO2

² 8.5 © 10¹5 Pa. The thin films showed a maximumhardness value of 32GPa around the solubility limit.

Acknowledgement

This work was supported by Grant-in-Aid for ScientificResearch 22686069.

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