Effect of chromium nitride coating on the corrosion and wearresistance of stainless steel

Jacek Jagielski a, A.S. Khanna b,), Jacek Kucinski c, D.S. Mishra d, P. Racolta e,Piran Sioshansi f, Eric Tobin g, J. Thereska h, Vladimir Uglov i, T. Vilaithong j,

J. Viviente k, Si-Ze Yang l, Anton Zalar m

a Department of Microstructural Research, Institute of Electronic Materials Technology, Warsaw, Polandb Department of Corrosion Science and Engineering, Indian Institute of Technology, Powai, Mumbai 400 076, India

c Material Research Laboratory, Institute of Atomic Energy, Swierk, Polandd Department of Physics, Indian Institute of Technology, Powai, Mumbai 400 076, India

e Cyclotron Laboratory, National Institute of Physics and Nuclear Engineering, Bucharest, Romaniaf Radio-Med, Belford, MA, USA

g Spire, Belford, MA, USAh Industrial Application and Chemistry Section, DiÕision of Physical and Chemical Science, Vienna, Austria

i Physics of Ion Plasma Modification of Solids Research Laboratory, Department of Solid State Physics, Belarussian State UniÕersity,Minsk, Belarus

j Department of Physics, Faculty of Science, Chiang Mai UniÕersity, Chiang Mai, Thailandk Surface Technology Department, INASMET-Centro Technologico Materials, Camino de Portuetxe, San Sebastian, Spain

l Institute of Physics, Chinese Academy of Sciences, Beijing, People’s Republic of Chinam Department of Surface and Thin Film Analysis, Institute of Surface Engineering and Optoelectronics, TesloÕa, SloÕenia

Abstract

Ž .Ion beam-assisted deposition IBAD technique has been used to deposit chromium nitride on the stainless steelsubstrates. Characterization and performance of this coating was carried out by a round-robin test at various laboratories.About 300 samples, prepared at Spire, USA, were distributed to 10 different laboratories. Characterization of coated

Ž . Ž .materials, carried out using Rutherford backscattering RBS , X-ray diffraction XRD , glancing X-ray diffraction, AugerŽ . Ž .electron spectroscopy AES and X-ray photoelectron spectroscopy XPS , confirmed a well-adherent layer of chromium

nitride with composition and thickness of Cr N and 1.2 mm, respectively. The performance of the coating was tested by0.8 0.2

tribological tests: dynamic micro-indentation, micro-hardness, wear and friction, and corrosion tests. The coating was foundto be hard with good wear resistance and had enhanced resistance to corrosion in acid media.

Keywords: Chromium nitride; Corrosion; Wear

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1. Introduction

Surface modification by coatings has become anessential step to improve the surface properties suchas wear, corrosion and oxidation. Various conven-tional techniques are utilized for depositing the de-sired material on to the substrate to achieve thesurface modification. Ion beam-based techniques,such as ion implantation, ion plating, etc., are knownfor a long time. Ion implantation results in a verythin modified layer, not fully adequate for demand-ing tribological environments. To get around this

Ž .problem, ion beam-assisted deposition IBAD hasbeen developed. In this paper, results from inter-laboratory tests, carried out to characterize and eval-uate the performance of chromium nitride coatingsdeposited on stainless steel using IBAD, are pre-sented. This technique has been reviewed by Smidtw x1 for the purpose of understanding the role ofadding energetic ions in the conventional physical

Ž .vapor deposition PVD processes. IBAD, also calledion beam-enhanced deposition, or dynamic ion beam

w xmixing or ion vapor deposition 2–5 , is presentlyw xbeing introduced to the industry 6–8 . It has been

recognized that the presence of energetic ion has asynergistic effect on thin film growth. There areseveral aspects of film growth that can be benefi-cially influenced by ion bombardment during thin

Ž .film deposition, including: i film nucleation andŽ . w x Ž . w x Ž .growth; ii adhesion 9 ; iii internal stress 10 ; iv

Ž . Ž .surface morphology; v density; and vi composi-tion.

Ion bombardment during deposition has also beenshown to produce quite drastic changes. For exam-

w xple, Bunshah 11 has reported that ion bombardmentchanges a normally columnar morphology of metalcoatings into a denser isotropic structure.

IBAD consists of depositing simultaneously thedesired material using vacuum evaporation and bom-bardment of energetic ion beam. This techniquecombines the advantages of a vacuum coatingmethod, electron beam evaporation and ion implanta-tion. In contrast to plasma-based physical vapor de-position techniques, the actual coating process isde-coupled from the energy-input process. Thismakes IBAD highly versatile and flexible. IBAD hastwo basic parameters, the evaporation rate and inten-sity of the energetic ions, which are independent

from each other and can be varied over a wide range.As a result, it is highly controllable and reproducible.Due to these features, IBAD constitutes an excellentaddition to other PVD methods and provides coatingsystems with special features, which, in some cases,cannot be obtained by other techniques. For instance,a comparison of IBAD and other PVD techniques fordepositing corrosion protection layers on tempera-ture-sensitive substrates under very low process tem-peratures showed that the IBAD films generally hada higher structural density and adhesion to the sub-strate than the corresponding PVD films. This re-sulted in a superior corrosion–protection perfor-

w xmance 12 . The ion beam provides the energy,which is necessary to grow films with suitable struc-tures and sufficient adhesion. For a flexible IBAD

Ž .system, a very wide range of ion intensities currentand energies should be available. The energies, whichare suitable for optical thin films, are quite differentfrom those useful for tribologically protective coat-ings.

2. Experimental

Stainless steel coupons of 20-mm diameter and2-mm thickness were cut from a controlled lot of anAISI type 304 stainless steel bar. These sampleswere then polished up to a mirror finish using aseries of standard metallurgical polishing steps. Dia-mond paste was used as a last step. A total of 300polished samples were then loaded into a planetarysystem for rotation and translation to receive a uni-form chromium nitride coating.

IBAD system of Spire, USA was used for thispurpose. Experimental set-up and mechanism of theprocess is shown in Fig. 1. The equipment consistsof a chamber with two independent systems, one fordeposition of Cr by electron beam evaporation andthe other for bombarding the sample surface withintense beam of N ions at an operating voltage of 1V. The base pressure of the chamber was maintainedat 5=10y7 Torr. Nitrogen pressure was maintainedat 10y5 Torr. The temperature of the substrate wascontrolled at 1508C, to obtain the best coating perfor-mance. Ion current density used was 0.06 mArcm2

and the evaporation rate of Cr was 3–5 Ars, whichwas deposited at an angle of 908.

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Fig. 1. Experimental set-up and mechanism of IBAD technique.

The chromium nitride-coated samples, preparedby IBAD were distributed among the participatinglaboratories sponsored by the International AtomicEnergy Agency ‘‘Coordinated Research Programme’’on ‘‘Modification of Materials by Ion Treatment forIndustrial Applications’’. Each participant was en-couraged to use the available analytical technique tocharacterize the coating and to use appropriate test-ing method to evaluate the performance of the coat-ing. The motivation was to provide standard samplesfor inter-laboratory tests to validate the characteriza-

Fig. 2. Backscattered proton spectrum of IB-assisted CrN film onŽ .SS E s1.711 MeV .p

tion techniques and provide identical samples forcomparison of performance testing at each labora-tory.

Chromium nitride was chosen as the coating ofchoice for several reasons:

1. It is a fully developed and optimized coatingoffered by Spire with extensive laboratory testingbehind it.

2. It is a hard, wear-resistant coating for challengingindustrial applications.

Fig. 3. RBS spectrum of ion beam-assisted CrN film on SSŽ .E s3 MeV .A

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3. It is chemically inert and is an ideal coating forcorrosion testing.

The samples were characterized by RutherfordŽ .backscattering RBS , Auger electron spectroscopy

Ž . Ž .AES , atomic force microscopy AFM , X-rayŽ .photoelectron spectroscopy XPS , X-ray diffraction

Ž .XRD and glancing XRD methods and their perfor-mance was evaluated by tribological tests, such asmicro-hardness, dynamic micro-indentation, impact,scratch, dry sliding, wear and friction, and corrosiontests.

3. Results

3.1. Characterization of chromium nitride coatings

3.1.1. RBS analysisThe composition and thickness of the chromium

nitride film on stainless steel samples were analysedby RBS method. The experiments were carried outusing a 3-MV TANDETRON at the Surface andProfile Measurement Laboratory, Hyderabad, India.The ion beam of a-particle and protons were gener-ated using the duoplasmatron and sputter ion sources,respectively. Ion beams of high energy, collimated to2-mm diameter, were impinged normally on thetargets, placed inside a scattering chamber. A silicon

Žsurface barrier detector with a resolution of ;11.keV for 5.5 MeV a-particles mounted at a backward

angle of 1688 was used to detect the backscattered

Fig. 4. AES sputter depth profile of the CrN-coated SS.

Ž . Ž . Ž .Fig. 5. Photoelectron spectra of a Cr 2p, b N 1s, c O 1s, andŽ .d C 1s measured in the CrN coating after 60 min ion sputteringtime in depth of about 70 nm.

Fig. 6. XRD of CrN coated SS.

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Ž . Ž .Fig. 7. XRD analysis of a AISI 304 and b CrN-coated AISI304.

particles. Backscattered spectra were acquired byPC-based multi-channel analyser.

The backscattered spectrum of the chromium ni-14 Ž . 14tride film acquired using N p,p N resonance

reaction at 1.74 MeV is shown in Fig. 2. The firststep in this spectrum is due to Cr in the chromiumnitride film, the second due to Fe in the backingmaterial, whereas, the peak corresponds to N in thefilm.

Fig. 8. Glancing X-ray analysis of CrN coated SS.

Fig. 9. Scanning electron micrographs of CrN-coated SS 304.

The cross-section of the resonance reaction at1.74 MeV has been reported to be 145 mbrsr. Inorder to estimate the thickness, the film was profiledby increasing the energy of the beam from 1.74 MeVonwards in steps of 20 keV.

The width of this resonance reaction is about 4–6keV. Thus, the reaction yield at the resonance issensitive to detector resolution, ion energy and thick-ness of the film. Therefore, this energy was notconsidered suitable for determining the composition.

Table 1Chemical composition of various elements present in coating

Element Wt.% At.% %S.E.

Fe 5.47 5.11 2.1Cr 94.53 94.89 0.32

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Fig. 10. AFM of polycrystalline CrN coating at two different scanŽ . 2 Ž . 2areas: a 5=5 mm and b 2.5=2.5 mm .

However, the cross-section of the resonance reactionŽ .is almost constant ;95 mbrsr in the energy re-

14 Ž . 14gion of 1.6–1.7 MeV. Hence, N p,p N reso-nance reaction at different proton energies in thisregion was utilized to estimate the composition of

Ž .the film. The composition of the film CrrN ratio,estimated using the formula:

Y rY sN s rN sCr N Cr Cr N N

where Yscounts under the N peak or Cr step, andss respective scattering cross-section.

The composition of the chromium nitride filmwas found to be Cr N .0.75 0.25

The backscattered spectrum of the chromium ni-tride film obtained using 3 MeV a-particles is shownin Fig. 3. The step observed at the high energy andcorresponds to Cr in the chromium nitride filmwhereas the peak can be attributed to the presence ofan interfacial thin film of Cr. However, the backscat-tered signal due to N is not discernible.

Fig. 11. Friction curves obtained from CrN-coated samples.

The thickness of the film calculated by taking intoconsideration the energy separation between the frontand rear edges of the Cr step in the spectrum de-picted in Fig. 3 is ;1.3 mm. The stopping powerrequired for estimating the thickness has been calcu-lated using TRIM’95. The thickness of the film as

14 Ž . 14determined by profiling using N p,p N reactionis in close agreement with this value.

The interfacial chromium is not clearly distin-Ž .guishable while using proton beam Fig. 2 . This

film calculated to be ;1000 A is deposited toenhance the adhesion of chromium nitride. This study

14 Ž . 14shows that while N p,p N is suitable for deter-mining composition of films containing low z ele-ments, RBS provides valuable information regardingthe film thickness and interfacial regions.

3.1.2. AESAES was used to determine the composition and

depth profile of chromium nitride coating. The sam-ple was analysed using a PHI 545A scanning Augermicroprobe with a base pressure in the vacuumchamber below 1.3=10y7 Pa. Static primary elec-tron beam of 3 keV with 1 mA ion current density

Table 2Process parameters for sample preparation

Ball material Ball diameter Applied load Track radius Speed Linear speed Temperature Relative humidity

AISI 52100 steel 10.0 mm 100 g 9 mm 53 rpm 5.0 cmrs 258C 60%

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and a diameter of about 40 mm was used. Thesample was ion sputtered with 3 keV Arq ion beam,rastered across an area larger than 5=5 mm, with asputtering rate of about 5 nmrmin. The Auger peak-

Ž . Ž . Žto-peak heights of C 272 eV , N 379 eV , O 510. Ž . Ž . Ž .eV , Cr 529 eV , Fe 703 eV and Ni 848 eV as a

function of the sputtering time were quantified byapplying the following relative elemental sensitivityfactors: S s0.32, S s0.50, S s0.32, S s0.2,1N O Cr Fe

and S s0.29. The sensitivity factors were gainedNi

from the measurements of standards.Fig. 4 shows the AES depth profile of the as-de-

posited chromium nitride coating on the stainlesssteel. The elemental composition of the coating bydesign does not correspond to the stoichiometric CrNwith the atomic ratio CrrNs1r1. The maximumconcentration of the N at the coatingrsubstrate inter-face is about 27 at.% and is gradually decreased upto the coating’s surface where the concentration isabout 22 at.%. The chromium rich chromium nitridecoating with this composition has excellent proper-ties and has been developed based on many years ofresearch. The average concentration of C in thecoating was about 1 at.% and the surface of thechromium nitride film was slightly oxidized.

3.1.3. XPSXPS analysis on chromium nitride-coated stain-

less steel substrate was carried out on a Perkin-Elmer5300 spectrometer with a hemispherical electron en-ergy analyser using Mg Ka irradiation. The CrNcoating was sputtered using a differentially pumped3 keV Ar ion gun at an incidence angle of 458C. Thesample was ion sputtered over an area of 5=5 mm,with a sputtering rate of about 1.2 nmrmin. The sizeof an analysed area was about 1 mm2. The data

Table 3Wear areas obtained by optical laser profilometry from wornchromium nitride coatings

2Ž .Chromium nitride coatings Wear tracks area mm

Sample 1 0.0000225Sample 2 0.0000225Sample 3 0.0000500Sample 4 0.0000500Sample 5 0.0000525Sample 6 0.0000525

Fig. 12. Friction curves obtained from AISI 52100 steel, CrNŽ .sample 1 , TINALOX and Ti-DLC-coated samples.

acquisition was performed by an Apollo workstationinterfaced with the spectrometer. The peak areas ofCr 2p, N 1s, O 1s and C 1s were registered and theatomic concentration was calculated through a properarea calculation and using correspondent sensitivityfactors.

Contrary to the AES results, the XPS analysisalso showed presence of oxygen besides carbon in

Ž .the coating Fig. 5 . In the XPS spectra, the corre-sponding peaks of Cr 2p at 574.2 eV, N 1s at 396.6

Ž . Ž .eV Fig. 5b O 1s at 529.5 eV Fig. 5c and C 1s atŽ .281.9 eV Fig. 5d are characteristics for chromium

nitride, oxide and carbide, respectively. XPS sputterdepth profiling also revealed the following composi-tion of the coating: 75 at.% Cr, 16.0 at.% N, 5 at.%O and 4 at.% C.

3.1.4. XRD analysisThe XRD of chromium nitride-coated SS 304 is

given in Fig. 6. The amorphous pattern can be

Table 4Worn areas measured by optical profilometry

2Ž .MaterialsrCoatings Wear track areas mm

AISI 52100 steel 0.0001025Ž .Chromium nitride sample 1 0.0000225

TINALOX 0.0000475TI-DLC 0.0000275

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Table 5Dynamic micro-indentation results for chromium nitride and its substrate material

Chromium nitride coating Steel substrate2 2Vickers hardness at 10 mN 16749 Nrmm 3728 Nrmm

Relative elastic recovery 35.96% 8.48%Ž .Total work J 0.75 1.51Ž .Elastic work % 46.59 10.35Ž .Plastic work % 53.41 89.65

Ž .Young modulus GPa 201 1652 2 2Ž .Plastic hardness Nrmm 14373 Nrmm 2549 Nrmm

observed very clearly from the XRD spectrum. TheŽ .first peak that appeared at about 438 2u is g-Fe

Ž . Ž .111 . The second peak at about 50.58 is g-Fe 200 .Ž .The third peak at about 748 is g-Fe 220 . The fourth

Ž .peak at about 908 is g-Fe 222 . The amorphouspeak at about 428 is of CrN peak.

XRD of AISI 304 SS and CrN-coated AISI 304SS are also shown in Fig. 7a and b, which reveal thatchromium nitride coating consists mainly of Cr N2

Ž .phase 28 at.% 0f N with some quantity of pure Cr.The presence of CrN phase cannot be determinedunambiguously because CrN diffraction lines coin-cide with diffraction lines of Cr N and substrate2Ž .g- Fe, Cr, Ni .

3.1.5. Glancing X-ray analysisThe results of glancing X-ray analysis of

chromium nitride-coated stainless steel samples aregiven in Fig. 8. These results confirm the coating as

Ž .Cr N with grain size of 5"2 nm in 002 plane.2

Grain size was calculated using XRD by means ofw xthe Scherrer formula 13 . It also confirms that the

thickness of the coating is equal to around 1.2 mm,which is also confirmed by RBS method.

3.1.6. SEMrEDAXrAFMSurface of chromium nitride-coated SS 304 was

analysed by scanning electron microscopy. The coat-ing was found uniform and dense. Some voids were

Ž .also present in the coating Fig. 9 . The compositionof the chromium nitride layer as measured by EDAXis given in Table 1.

The AFM measurements were performed on aTopo-Metrix TMX 2000 Explorer in ambient condi-tions. In this equipment, the cantilever with the tipscans over the fixed sample by means of piezoelec-

tric drives. AFM provides a topography image andŽ .quantitative topography information Fig. 10 . The

most frequently used roughness parameter is R ,a

which is the arithmetic mean of the deviations inheight from the profile mean value. The root mean

Ž .square RMS roughness values based on the powerspectral density, which is defined as the squaremagnitude of the Fourier transform of the surfaceprofile. The values of the roughness parameters de-termined are: R s5.91 nm and RMSs7.53 nm.a

3.2. Performance of chromium nitride coating

3.2.1. Tribological and dynamic micro-indentationtest

Tribological and micro-indentation tests were per-formed on chromium nitride-coated SS substrate.

Fig. 13. Loading and unloading curves from substrate and CrNcoating after micro-indentation experiments.

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Fig. 14. Dry sliding analysis for CrN-coated and uncoated AISISS304.

The ball-on-disk friction and wear tests were per-formed on a TRIBOMETEER FALEX ISC 320. Thesamples were identically prepared. General condi-tions of the test for all the samples are specified inTable 2.

The friction curves for all chromium nitride sam-ples are presented in Fig. 11, where a dispersion ofresults can be observed.

The wear tracks after an UBM OPTICAL PRO-FILOMETRY SYSTEM examined the tests, and theworn area was calculated for all the samples. Theresults are indicated in Table 3. For comparison,results from a set of coatings, including TINALOXand a Ti-doped DLC coating are also given.TINALOX is a trade name of CemeCon for a coat-ing produced by magnetron sputtering with a compo-sition of TiAlN. This coating, according to CemeCon

ŽFig. 15. Wear trend for SS and CrN coating. loads0.5 kg,.Õs0.5 mrs, sliding distances250 m .

ŽFig. 16. Wear trend for SS and CrN coating. loads1.5 kg,.Õs0.5 mrs, sliding distances250 m .

information, is an ultra-hard coating, specially devel-oped for metals with a hardness of 3300 HV0.05.The coating withstands temperatures up to 8008Cand, as a result of its low thermal conductivity,protects the tool against damage due to high temper-atures. The results are presented in Fig. 12 and Table4.

Dynamic micro-indentation tests were carried outwith the FISHERSCOPE H100 MICROHARDNESSTEST SYSTEM with load increments until a maxi-mum load of 10 mN was achieved and finally de-creasing the load. At each loading and unloading

Žsequence, the vertical displacement indentation.depth of the indentor into the material is recorded.

The results obtained are given in Table 5. Theloading and unloading curves for both the CrN coat-

Ž .ing lower curves and the corresponding substrateŽ .material upper curves are presented in Fig. 13. In

this figure, it is clearly seen that the elastic recoveryand hardness values for the coating are significantlyhigher than the base material.

Dry sliding test on chromium nitride-coated anduncoated AISI 304 steel samples is presented in Fig.14. In this figure, it is clearly seen that the friction

Ž .coefficient of chromium nitride coating ms0.4 is

Table 6Wear rate and coefficient of friction for SS and CrN coated SS

Sample Coefficient of Wear rateŽ .friction mmrmin

Stainless steel 0.54 2.24CrN-coated SS 0.47 1.75

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1.5 times less than that of AISI 304 steel substrate atthe load of 0.7 N.

Wear and Friction tests were carried out on pin-on-disk wear machine, as per ASTM standard G99-90. The apparatus consist of a cylindrical pin,which is pressed vertically against a rotating disk. Adesired load is applied to the pin while it is sliding.

The bottom surface of the pin and the face of thedisk together from a wear couple. En 32 steel is thedisc material. The CrN-coated stainless steel samplecut into a size of 0.5 mm2 was mounted horizontallyon the cylindrical pin. The experiment is carried outfor a particular duration for which a time setter isprovided. A linear variable differential transducer

Ž . Ž .Fig. 17. Surface profiles using 3D Tayler–Hobson Profilometer for a as-deposited and b nitrogen-implanted chromium nitride-coatedstainless steel specimens.

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Fig. 18. The loading–unloading curves for three indents madewith maximum load of 2 mN.

Ž .LVDT sensor converts the displacement of the pininto an electronic signal, which, when connected to acomputer terminal through a software, gives a graph-ical plot of a wear vs. time. The frictional force canalso be obtained through a load cell. The speed ofthe rotating disk can be varied when desired.

The results are obtained in the form of a plot of‘‘Displacement in microns vs. Time in minute’’ and‘‘Frictional force vs. Time’’. The test was carried

Ž .out at a load of 0.5 kg 5 N for the sliding distanceof 250 m at a sliding velocity of 0.5 mrs. The plotsof displacement vs. time are shown in Figs. 15 and16 for a load of 0.5 and 1.5 kg, respectively. Ini-tially, some scatter in the data was observed but lateron the overall trends were found to be uniform.

The slope of displacement vs. time gives the wearrate of the coatings in microns per minute. From theplots of frictional force vs. time, coefficient of fric-tion can be calculated as given in Table 6.

Wear and friction tests were also carried out onnitrogen implanted chromium nitride-coated samples.The as-deposited chromium nitride layers were nitro-

Table 7Result of hardness and modulus test of coated material at differentload

Ž . Ž .Load Hardness H GPa Load modulus E GPa

2 mN 18.4"1.2 234"125 mN 19.6"1.5 198"9.5

Table 8Results of microhardness test of chromium nitride coated anduncoated 304 SS sampleAv-HK saverage Knoop hardness; Av-HVsaverage Vickershardness; Dev sdeviation.

Sample Av-HK Dev Av-HV Dev2Ž .surface kgrmm

Chromium nitride-coated 1013.6 59.8 988.9 74.4Uncoated 230.9 8.9 213.4 6.6

gen implanted up to a dose of 5=1016 Nqrcm2,2

1=1017 Nqrcm2, 2=1017 Nqrcm2 by 100 keV2 2Ž q.molecular nitrogen ion N beam. The test was2

carried out using a ball-on-flat Tribotester. The 6.5-mm balls were used as a counter sample. The testwas carried at a load of 5 N in air up to 300oscillations. The values of friction coefficient werefound to be 0.76 to 0.8 for the as-deposited low- andmedium-dose nitrogen-implanted samples. For thehighest nitrogen implanted dose, the friction coeffi-cient value dropped down to 0.54.

The extent of wear was determined using a 3DTaylor–Hobson Profilometer. The typical surfaceprofiles obtained for as-deposited and high-dose ni-trogen-implanted samples are shown in Fig. 17. It isinteresting to note that striking reduction of wearwas observed after nitrogen implantation.

3.2.2. Hardness and modulusThe hardness and modulus of the chromium ni-

tride-coated samples were measured using an ultra-low load depth sensing force transducer for theDigital Instruments Nanoscope III, developed byHysitron. The Hysitron Triboscope has a load resolu-tion of 100 nN and displacement resolution of 0.2

Table 9Results of scratch and adhesion tests for SS and CrN coated SS

Loading Scratching Adhesionrate speed strength

7 2Ž . Ž . Ž .kgrmin mmrmin =10 Nrm

4 5 1203 4 1304 3 125

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Ž . Ž .Fig. 19. top Microphotographs of the tracks made with diamond stylus in scratch test. The images show the early part of the tracks. aŽ . 17 q 2 Ž .As-deposited CrN layer, b CrN layer implanted with 140 keV N q ions up to a dose of 2=10 N rcm . middle Microphotographs of2 2

Ž . Ž .the tracks made with diamond stylus in scratch test. The images show the middle part of the tracks. a As-deposited CrN layer, b CrNq 17 q 2 Ž .layer implanted with 140 keV N ions up to a dose of 2=10 N rcm . bottom Microphotographs of the tracks made with diamond2 2

Ž . Ž .stylus in scratch test. The images show the end part of the tracks. a As-deposited CrN layer, b CrN layer implanted with 140 keVNqions up to a dose of 2=1017 Nqrcm2.2 2

59

nm. Indentations were made using a Berkovich dia-mond tip to a maximum load of 2 and 5 mN. Theloading–unloading behavior was analysed using the

w xmethod of Oliver and Pharr 14 The load displace-ment curves of the 2-mN load for three indents areshown in Fig. 18. Result of hardness and modulus isgiven in Table 7.

While the hardness is approximately the same forboth loads, the modulus is different. The reason isthat the indents made with 5 mN are strongly influ-enced by the substrate. The measured hardness, how-ever, seems to be the real hardness of the film.

Microhardness test on chromium nitride-coatedand uncoated SS 304 samples were performed by theMXT-a Digital Microhardness using a load of 5 g.The result is shown in Table 8.

3.2.3. Adhesion and scratch testThe adhesion test on chromium nitride-coated SS

304 substrate was carried out by using ScratchingTester equipped with a diamond tip and sound sen-sor. The adhesion strength was given by the outputdata of the Scratching Tester, and not from calcula-tion using a formula. The data are shown in Table 9.

The adhesion of the deposited layers on AISI316L steel coated with chromium nitride, was alsomeasured using a scratch test apparatus. Diamondstylus having 120 mm in diameter spherical tip wasused as a counter sample. The stylus was mountedon the lever having the stiffness of 45.739 mNrmm.

The first part of the test was devoted to thedetermination of the critical load, i.e., the load caus-ing the layer damage. The speed of translation wasset to 0.1 mmrs. The critical force value was deter-mined from the length of the crack from the trackbeginning up to the place in which the first traces ofdamage were observed. The tests were done onuntreated samples and chromium nitride layers priorand after nitrogen implantations. During the testsperformed in increasing load mode, tensile typeHertzian cracks were observed for loads exceeding

Ž .700 mN Fig. 19 . This type of damage suggests thatthe CrN layer was not delaminated from the sur-0.4

face of the sample. The final depth of the trackŽ .equals to several micrometers typically 3.5–4 mm

Ž .Fig. 20 . This value exceeds the total thickness ofthe deposited layer. Nevertheless, even at the end of

Fig. 20. Surface micro-topography of the end part of scratch trackfor CrN coated SS.

the track the chromium nitride layer remains bondedŽ Ž ..to the substrate Fig. 19 bottom . One can thus

conclude, that the soft austenitic substrate becamecontorted, however, due to the strong adhesion, thechromium nitride layers bends following the sub-strate deformations. Only slight differences were ob-served between as-deposited and nitrogen ion-im-planted chromium nitride layers. At the beginningŽ Ž .. Ž Ž ..Fig. 19 top and in the middle Fig. 19 middle ,the cracks seem to be less in numbers and shallowerin the case of nitrogen implanted layers. The differ-

Žence disappears at the end of the cracks Fig. 19Ž ..bottom .

ŽThe tests in multi-pass mode constant load 700mN, speed of translation 0.05 mmrs, up to 100

.scans over the same area revealed good adhesionand low wear of the deposited layers. In all testsperformed on as-deposited CrN layers neither delam-ination, neither complete wear of the deposited lay-ers was observed. Typical images of tracks after 1,10, 100 and 150 passes are shown in Fig. 21.

It is interesting to note that after the constitutionof the wear track occurring during the first few

Ž .passes less than 10 the tracks remain almost un-changed during the test. Fig. 22 presents the mi-

60

Ž . Ž . Ž . Ž .Fig. 21. Microphotographs of the tracks made with diamond stylus in scratch test. The images show the middle part of the tracks. a 1 pass b 10 passes, c 100 passes, d 150passes.

61

Ž .Fig. 22. Microphotographs of the tracks made with diamond stylus in scratch test. The images show the middle part of the tracks. a 10Ž .pass, b 100 passes.

crophotographs taken from the CrN sample used inmulti-pass test after 10 and 100 passes. Both imagesare almost identical, neither width nor the depth ofthe tracks changed significantly. The results of pro-filometric analysis performed on the CrN samplesafter 10 and 150 passes are shown in Fig. 23a and b,respectively. In both cases, the depth of the worntrack roughly equals 1 mm.

CrN layers revealed very good adhesion to thesubstrate. The critical load in scratch test required tocause layer-cracking equals to about 700 mN in testusing 120-mm-diameter diamond stylus.

3.2.4. Impact testingImpact test of the coated material was carried out

by using Impactor-I. Results of impact test onchromium nitride-coated SS 304 samples using 0.035J impact energy are given by the following.

Measuring condition:

Impacting energy: 0.035 J for each impactingFrequency: fs2.5 timesrs Total impacting

numbers: 200Frequency: fs3 timesrs Total impacting

numbers: 3000

62

Ž . Ž .Fig. 23. a Profilometric analysis of the tracks made in multipass test after 10 passes. b Profilometric analysis of the tracks made inmultipass test after 150 passes.

After impacting, the chromium nitride coatings oftwo samples were not broken or peeled off, confirm-ing a good impact resistance of the coating.

3.2.5. Corrosion testCorrosion tests were carried out in 1 N H SO2 4

solution for the base SS 304 and chromium nitride-

63

Ž . Ž .Fig. 24. Anodic polarization curve of a SS 304, and b CrNcoated SS 304.

coated SS samples by Potentiodynamic scan. Bothsamples were scanned from y0.2 to 1 V with a scanrate of 1 mVrs. A standard three-electrode set-upwas used for corrosion test of coated and uncoatedmaterial. Coated material was used as a workingelectrode and platinum strip was used as a counter

Ž .electrode. Saturated calomel electrode SCE wasused as a reference electrode for measuring thepotential of coated material. The anodic polarizationcurves are given in Fig. 24a and b. I value wascorr

measured by extrapolating tafel plots and corrosionŽ .rate CR in mryr was calculated from the following

formula.

CR in mryrs0.471 I .corr

The following inference can be obtained about thecorrosion behavior of the coated and uncoated speci-

Ž .mens Table 10 .It is clear that E and I values are muchcorr corr

lower for chromium nitride-coated stainless steelspecimen. These analyses confirm that chromiumnitride-coated SS shows lower corrosion rate thanthe base SS 304.

4. Discussion

Scanning electron micrographs of chromium ni-tride-coated stainless steel substrate showed that thechromium nitride coating is uniform and dense. Thethickness of the chromium nitride coating on SSspecimens was calculated to be 1.2 mm by RBSmethod, which was also confirmed by glancing X-raymethod.

The XPS analysis of the chromium nitride coatingon stainless steel sample showed that the chromiumnitride coating was composed mostly of Cr N, and2

potentially CrN in an elemental Cr matrix, and smallquantities of chromium carbide and oxide. The over-all ratio of Cr to N was 75"5% Cr to 25"5% N.AES and XPS sputter depth profile analyses of thechromium nitride coating on stainless steel has shownthat the average atomic composition was close to

Ž .Cr N also confirmed by RBS method in the0.80 0.20

sub-surface region; and close to Cr N near the0.73 0.27

substratercoating interface. The coating also con-tained small quantities of carbide and oxide. Theroughness parameters of the coated specimens mea-sured with the AFM method are: R s5.91 nm anda

RMSs7.53 nm. The hardness of the coated sampleobtained using a maximum load of 2 mN is 18.4"

1.2 GPa and the modulus is 234"12 GPa. Thechromium nitride coating consists mainly of Cr N2

phase as confirmed by glancing XRD analysis, withgrain size of 5"2 mm. The low grain size is a resultof low temperature coating deposition process whichcharacterizes the IBAD process.

Table 10Results of anodic polarization tests of SS 304 and chromium nitride SS 304 in 1 N H SO on SS specimen2 4

2 2Ž . Ž . Ž . Ž . Ž .Sample E V Area cm E mV I Arcm Corrosion rate mryrocp corr corr

y4SS 304 y0.527 1 508.4 3.51=10 4.084y7Chromium nitride-coated 0.245 0.42 150.1 1.0=10 0.001

64

Stainless steel specimens coated with chromiumnitride were also subjected to various performancetests. Several mechanical tests were carried out onchromium nitride-coated and uncoated AISI 304 steelsample. Chromium nitride coatings are not knownfor low friction; however, dry sliding wear testshowed that the friction coefficient of stainless steelimproves by 1.5 times by chromium nitride coating.Other tests such as hardness, scratch and impact andcorrosion test also showed significant improvementon tribological properties. Knoop hardness and Vick-ers hardness of the coated material are higher thanthe base material. It is interesting to note that corro-sion rate was 4000 times less than the base alloy.The adhesion of the coating to the substrate provedto be excellent as tested in various indentation testsand scratch tests, which shows that layer composi-tion corresponds to CrN in all analysed layers0.4

within 10% accuracy.

Acknowledgements

The authors thank IAEA for creating a coordina-tion group to carry out these studies. Thanks are dueto Spire USA for preparing chromium nitride-coatedsamples. Also, thanks are due to Dr. K.G. Prasad,

co-investigator of the project from the Indian side,who carried out detailed RBS work at surface andprofile measurements laboratory at Hyderabad.

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