201 (2006) 1401–1408www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Microstructure of thick chromium–nitride coating synthesized usingplasma assisted MOCVD technique

Arup Dasgupta a,⁎,1, P. Antony Premkumar b,2, Falix Lawrence c, L. Houben d, P. Kuppusami a,M. Luysberg d, K.S. Nagaraja b, V.S. Raghunathan a

a Physical Metallurgy Section, Materials Characterisation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam – 603102, Indiab Department of Chemistry, Loyola College, Chennai – 600034, India

c Reprocessing Research and Development Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, Indiad Institüt für Festkörperforschung, Forschungszentrum Jülich, D52425, Germany

Received 29 July 2005; accepted in revised form 2 February 2006Available online 29 March 2006

Abstract

Thick and hard chromium nitride (CrN) films have been developed by plasma assisted metal organic chemical vapor deposition (PAMOCVD)technique. The coating has been characterized using X-ray diffraction, scanning electron microscopy and analytical and conventional transmissionelectron microscopy. The coating exhibits two phenomena on different length scales: (i) formation of nanocrystals, and (ii) the formation of theglobular structure on a micron scale. These globules are thought to be clusters of nanocrystalline CrN. Cross-sectional transmission electronmicroscopy of the film has revealed an extremely complex microstructure. However, the film has been found to have uniform Cr incorporation.The coating cross-section has shown bands of varying contrast. It has been shown that adhesion of the coating can be improved by corrugating thesubstrate surface.© 2006 Elsevier B.V. All rights reserved.

Keywords: Plasma assisted metal organic chemical vapor deposition (MOCVD); Clusters; Thick films; Hard coatings; Nitrides; Surface morphology; X-raydiffraction

1. Introduction

Plasma assisted metal organic chemical vapor deposition(PAMOCVD) of chromium nitride hard coatings is new and hasbeen recently reported [1,2]. Hardfacing or deposition of a hardcoating is commonly practiced to improve the chemical andmechanical properties like surface hardness, wear resistance,corrosion resistance and fatigue resistance of materials. Metallicoxides, nitrides, carbides and carbonitrides are some of thechoices as hardfacing materials. Among the nitrides, chromiumnitride (CrN or Cr2N) can attain hardness up to 2400 HV [3].They also have excellent thermal stability up to ∼996K [4]. It

⁎ Corresponding author. Tel.: +91 44 27480 306; fax: +91 44 27480 381.E-mail address: arup@igcar.ernet.in (A. Dasgupta).

1 On leave to: Institüt für Photovoltaik, Forschungszentrum Jülich GmbH,52428 Jülich, Germany.2 Present address: Physikalische Chemie I, Fakultät für Chemie, Universität

Bielefeld, D-33501 Bielefeld, Germany.

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.02.005

has lower coefficient of friction and superior oxidation resistanceas compared to TiN [5]. In industry, chromium nitride coatingsare generally synthesized by a physical vapor deposition (PVD)technique [5,6]. However, PVD processes are line of sight onesand impose restriction on the geometry of the work-piece to becoated. Other popular techniques include tuftriding, gasnitriding and plasma nitriding [7] of a pre-deposited chromiumlayer. Tuftriding method uses a toxic cyanide fused salt, whileconventional gas nitriding uses hazardous ammonia gas atrelatively high temperatures (∼1073KC). Plasma nitriding,although an environment-friendly diffusion controlled process,has shown slow kinetics [7].

On the other hand, plasma assisted chemical vapordeposition (PACVD) of titanium nitrides and carbides areindustrially acclaimed processes. The introduction of plasmabrings in numerous advantages, such as: (i) low processingtemperature (473 – 873K) as compared to a conventionalthermal CVD (1273K and above), (ii) ability to form uniform

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deposition over large area on metallic as well as ceramicsubstrates, (iii) deposition on any geometry that is exposed tothe plasma, and (iv) low substrate distortion. Attempts havebeen made to combine the PACVD and MOCVD processes fordepositing thin films of various nitrides and oxides such asYBa2Cu3O7−x [8], NbN [9], TiN/BON [10], carbonitrides of Tiand Zr [11] and metal containing amorphous hydrogenatedcarbon (Me–C:H) [12]. The process is popularly referred to asplasma assisted metal–organic chemical vapor deposition(PAMOCVD) technique.

In earlier reports [1,2], we have described the synthesis ofhard (>1000 VHN (Vicker's Hardness Number)) and thinnanocrystalline CrN coatings using PAMOCVD technique. Adc plasma was used for cracking the precursors of a metalorganic compound, namely chromium(III) acetylacetonate (Cr(O2C5H7)3, henceforth to be referred as Cr(acac)3) and nitrogengas. In these reports, the usefulness of a critical amount of anadditive, namely ammonium bifluoride, together with anoptimum hydrogen dilution of the precursors, has beenhighlighted. The coating thickness reported was in the rangeof 2.8–7.1 μm with a maximum deposition rate of about0.9 μmh−1. But, thicker CrN coatings are required for extendedlife of most moving components, which cannot be replacedeasily. However lattice mismatch and difference in thermalexpansion coefficients between the coating and the substrate aresome of the major challenges in the growth of thick films [13].Modification of the substrate surface can help in overcomingthese problems to some extent but then the microstructure of thecoating plays an important role.

In this paper, we report on the microstructural investigationsof thick CrN coating deposited on stainless steel (SS) substratesusing the PAMOCVD technique. Attempts have been made tounderstand the complicated microstructure of the coating forwhich no literature evidence exists.

2. Experimental

We used a PAMOCVD apparatus for the growth of the CrNcoatings and the details are reported elsewhere [1,2]. In thistechnique, vapor of Cr(acac)3 (99.4% purity) was used as thesolid precursor for Cr. An additive, 5 wt.% of ammoniumbifluoride (NH4FHF: 99.9% purity), was added to Cr(acac)3 inthe solid state. This mixture will henceforth be referred to as‘precursor’, which is then heated to yield vapors. The carrier gasfor these vapors was nitrogen. Plasma was generated using alaboratory built dc power supply (0–1000 V). Polished andultrasonically cleaned SS316L(N) grade stainless steel (Cr:17.2, Ni: 12, Mo: 2.3, Mn: 1.6, Si: 0.29, Co: 0.13, Cu: 0.09, C:0.028 and N: 0.078 wt.% [14]) substrate was placed on thecathode plate and was heated using an external heater. Apartfrom the carrier gas, H2 and N2 gases (99.9% purity) mixed inan optimum ratio of 1.05 were also flown into the depositionchamber. All the depositions have been carried out at sub-atmospheric pressures, typically 0.7 kPa and at a substratetemperature of 823K (±5K). The details of the depositionprocess and characterization techniques used are discussedbelow.

2.1. Choice of deposition parameters

For the synthesis of thick films, it is important to attain highgrowth rates. The deposition rate obtained for this CrN coatingwas 4.4 μmh−1. This is a four-fold increase in the growth rateover our earlier reported value of 0.9 μmh−1 for thin (2.8–7.1 μm) CrN coatings [1]. The faster growth was achieved by(a) increasing the chamber pressure from 0.2 to 0.7 kPa and by(b) reducing the weight percentage of NH4FHF from 10% to5%. Increase of chamber pressure increases the partial pressureof the vapors of the precursor and hence their presence in thedeposition chamber. Thus, at any instant of time, higheramounts of growth species are available at the deposition site,leading to an increase in the growth rate. On the other hand,NH4FHF additive has a positive effect on crystallinity andnegative effect on growth rate [1]. Therefore, a lower amount(5 wt.%) of NH4FHF was used. It needs to be mentioned herethat, the flow-rate of the sublimated precursors (Cr(acac)3+5 wt.% NH4FHF) is controlled by temperature instead of hightemperature flow-meter. Since this may have implications onthe deposition process itself, it is explained in details below.

2.2. Control of deposition process

The method of controlling the deposition process isrepresented in Fig. 1. It shows the evolution of precursortemperature (TP) during the entire deposition process from startto stop, represented by temperatures Tstart and Tstop, respectively.The idea for such a control is derived from the thermogravi-metric (TG) analysis of the precursor, which is shown in theinset. The TG analysis has been carried out using 12 mg of thesolid precursor (Cr(acac)3+5 wt.% NF4FHF). and the analysiswas carried out by heating the precursor mixture in an Heatmosphere from room temperature (RT=300 K) to 545 K at therate of 10K/min while continuouslymeasuring the weight loss ofthe solid due volatilization. The TG viewgraph reveals thatvaporization begins as early as 373 K and continues until about540 K. A part of this temperature range (373–511 K) has been

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Measured Values: Peak Center FWHM Grain size#1: CrN (111) 36.8 3.75 ~3nm #2: CrN (220) 63.7 4.28 ~4nm

Fig. 2. XRD analysis of the thick CrN coating.

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used for deposition of the film and is represented as ‘DepositionRegime’ in the inset. The thermogram shows that the weight lossof precursor increaseswith the rise of temperature.Assuming thatno chemical reactions take place during sublimation, the weightof the precursor in the vapor or gas phase will behave similarly.Therefore, in a constant volume as the deposition chamber, thepartial pressure of the precursor in the gas phase is expected toincrease with temperature. In other words, temperature can beused as a parameter to control the amount of vapor that flows intothe chamber. Moreover, at any constant TP, the amount ofprecursor in the gas phase or its partial pressurewill decreasewithincrease in time, as the precursor gets consumed.

Using this idea, TP was manually controlled so as to maintaina constant partial pressure of the precursors in the depositionchamber. The read-out of chamber pressure was used as thefeedback for this purpose. At any TP, one waits until thechamber pressure begins to drop and the corrective step is takenby increasing the TP. This manual variation of TP throughout thetime of deposition has been plotted in Fig. 1. However, the lead-time between the drop in the chamber pressure from its desiredvalue and the time required to recover it by increasing TP, whichis a characteristic of the system's response time, results influctuations of the process conditions. These fluctuations arebelieved to be responsible for some of the film characteristics aswill be explained later.

It may also be mentioned here that, though not very accurate,the procedure has been successfully used as an alternate for hightemperature flow controller, which are otherwise extremelyexpensive. Sublimation of pure Cr(acac)3 occurs in thetemperature range 423–523 K [1]. Interestingly, addition ofNH4FHF, which has a much lower sublimation temperature(348–448 K [1]), helps in widening the temperature range overwhich the Cr(acac)3+5 wt.% NH4FHF mixture sublimates i.e.373–540 K, facilitating easier control of the precursor flow rate.

2.3. Characterization techniques

X-ray diffraction (XRD) analysis on the coatings werecarried out with a Philips PW-1730 diffractometer with Cu Kα

radiation (wavelength, λ=1.5418 Å) in θ–2θ configuration.Hardness of the coatings was measured with a Vicker's microhardness tester (Leitz Miniload-2). The scanning electronmicroscope (SEM) images of the coating were taken with aPhilips XL30 ESEM apparatus using 20 keV energy. Thetransmission electron microscopy (TEM) analysis of the cross-section of the coating was carried out with a Philips CM20 FEGinstrument using an acceleration voltage of 200 kV. The cross-section sample was prepared from a delaminated thick CrN filmsandwiched between two silicon wafers. A section of thesandwich was mounted on a Ti specimen carrier. The mountedspecimen was then ground, polished and ion milled at low angleof incidence (8°) to obtain a large thin viewing region.

Microchemical analysis and conventional TEM analysis ofthe coating were carried out with a Philips CM200 system usingan accelerating voltage of 200 kV, with a super ultra thinwindow EDS (energy dispersive X-ray spectroscopy) detectorand a DX-4 analyzer.

3. Results and discussion

The results of various characterizations of the coatingdeposited on SS substrates using above-mentioned depositionconditions are discussed in the following sections.

3.1. Crystallography of the coating

Fig. 2 shows the XRD pattern of the coating. There are twomajor peaks centered at 2θ values of 36.8° and 63.7° whichhave been marked in the figure. These peaks have beenidentified as (111) (#1) and (220) (#2) peaks of CrN, althoughthese peak positions reported in JCPDS (file #110065) are at37.5° and 63.5°, respectively. The shift of (111)CrN peakposition from JCPDS value may be assigned to strain-relatedeffects along this plane. Assignment of other low intensitypeaks has not been attempted because of noisy backgroundsignal. It is also seen that both the peaks #1 and #2 are broad innature with FWHM (full width at half maximum) values of3.75° and 4.28°, respectively. Using the FWHM values fromboth these peaks, the grain sizes have been evaluated usingScherrer's formula and were found to be around 3–4 nm. Aninaccuracy in measurement is attributed to errors associated inusing this formula for such small grain sizes, whose shape anddegree of strain are not clearly known. However, these values ofthe grain sizes suggest that the coating is composed ofnanocrystalline CrN. This grain size is nearly one-third of thatfor the thin (7 μm) CrN films [1] deposited under optimizedconditions. As discussed in the previous section, the growth rateof the coating is nearly four times higher, mainly because of thehigher chamber pressure. It is expected that the incomingspecies will have shorter effective surface diffusion lengths onthe substrate surface at the higher chamber pressure due toenhanced probability of two-particle process, thereby increasingthe probability of nucleation. This in turn restricts the growth ofCrN crystallites. In addition to this, NH4FHF additive also playsa crucial role. It was indicated in our earlier report that it ishelpful in preferentially etching out any amorphous nuclei of

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CrN that form during the deposition process. Therefore, use ofless amount of this halide (5% instead of 10%) would haveundermined this process, resulting in a smaller grain size.

3.2. Thickness and morphology of the CrN coating

Fig. 3 shows the SEM image of the cross-section of CrNcoating deposited on stainless steel substrate using thePAMOCVD process described above. The figure also showsthe Vicker's indentation marks using a 100 g load across thesubstrate and the coating. It is noted that these impressions onthe coating bear poor contrast. Besides, bands of varyingcontrast parallel to the substrate surface are also seen. From thefigure, the coating thickness is measured to be about 80 μm.Fairly large (30 μm diagonal lengths) indentation marksgenerated using 100 g load are seen on the substrate alongwith plastic flow lines across the edges and these are attributedto a soft substrate (200 VHN). The coating is much harder ascan be inferred from the much shorter diagonal lengths of theindentation marks 1–4, in the figure. The average hardnessmeasured along the cross-section of the coating was 2127 VHN(±50 VHN), demonstrating a very hard coating with nearlyuniform hardness. The surface hardness of the coating wasmeasured using 200 g load and was found to be 2227 VHN(±50 VHN). These values are similar to those of CrN coatingsdeposited by other techniques, viz., 1400–2270 VHN [18–20]].The bands of varying contrast observed across the cross-sectionof the coating may be attributed to varying density, particle sizeor chemical composition of the film. This variation possiblyresults from varying partial pressure of the precursor in thedeposition chamber due to ‘not so accurate’ manual control ofthe precursor flow rate, as explained in Section 2.1.

Fig. 4(a) shows the SEM image of the coating surface at lowmagnification. A unique globular surface morphology isobserved. Globules of varying sizes (5–20 μm) can be seen,although no definite evidence exists to explain the formation ofa globular microstructure.

In order to investigate this morphology in detail, SEM imageat higher magnification is shown in Fig. 4(b). This figure showsthat each globule reveals an internal microstructure, whichappears as polygonal particles measuring about 2–4 μm.

Recalling that the crystallite size measured from XRD in Fig.2 is only about 3–4 nm, the polygonal particles may beconsidered as aggregates or clusters of nanocrystalline CrN. Itneeds to be mentioned here that the length scale in this case (2–4 μm) is much larger than the usual nanoclusters (5–50 nm)reported in the literature [15,16]. However, recently 0.2–0.4 μmclusters of nanocrystalline ZnO thin films deposited on glasssubstrates has been reported by Gao et al. [17]. These authorshave reported that each cluster was made up of ZnOnanocrystals measuring about 30–50 nm.

3.3. Transmission electron microscopy and microchemistry ofthe CrN coating

Fig. 5(a) shows the schematic of the specimen mounted forcross-section TEM studies. Fig. 5(b) shows the TEM brightfield micrograph of cross-section of the freestanding film thatdelaminated from the substrate. The specimen was prepared bysandwiching the film between two crystalline Si wafers (Fig 5(a)) and then sectioning and thinning the specimen for TEMpurposes. The section that was closer to the SS substrate ontowhich the coating was deposited is marked as “Substrate side”in the figure. Fig. 5(b) shows several layers representingdifferent kinds of microstructures and gives a microscopicevidence of the non-uniform process parameters, discussedabove. Near the “Substrate side”, a dense distribution of nucleiis observed up to a thickness of about 160 nm and marked as A(the corresponding region is also marked in Fig. 5(a)). Duringthe next stage of growth, a very fine porous structure in thenanometer scale with voids elongated in the growth direction is

Fig. 5. (a) Schematic of the TEM specimen mounting; (b) cross-sectional TEM micrograph of the CrN coating revealing a very complex microstructure; (c) EDXspectra from the regions marked as A, B, C and D.

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seen. The regions following this are almost featureless and arepossibly amorphous in region D. however region C shows 1- to2-nm-sized crystallites. It may be mentioned that total areaviewed in the micrograph corresponds to 1.5×1.7 μm2. Sinceno sharp interface has been observed in this micrograph, it maybe assumed to represent the typical internal microstructurewithin a cluster seen in Fig. 4(b). Therefore, the micrographindicates a variation in crystallinity of the film, which will beinvestigated further in a following section.

In order to understand this microstructure, microchemicalanalysis of the free standing film corresponding to the regionsmarked as ‘A’, ‘B’, ‘C’ and ‘D’ in Fig. 5(a) have been carriedout using EDS detector attached to the TEM and shown in Fig. 5(b). It is observed that characteristic X-ray emission linescorresponding to Cr–Kα, Cr–Kβ, Cr–Lα, and N–K are presentin all the four regions. Signals resulting from the Si backing forthe TEM specimen appear at about 1.68 keV. Additional signalfrom Fe–Kα is observed in the EDS spectra only from region‘A’. Now, EDS is less sensitive to low z (atomic number)elements, such as C, N and O, and hence the N–K signal mayalso have contributions from C and O. It may be recalled herethat while the presence of crystalline phase of CrN has beenestablished, crystalline chromium carbides or oxides could notbe ascertained from the XRD analysis (Fig. 1). Yet their

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presence cannot be ruled out since the Cr(acac)3 precursor (Cr(O2C5H7)3) contains 15 C and 6 O atoms, per Cr atom.However, it is believed that majority of C is carried away fromthe reactive region by H in the form of hydrocarbon gases suchas methane, ethane etc., and O is either carried away as O2 gasor if adsorbed, is preferentially etched out by the fluoride [1].Therefore, it may be inferred that Cr incorporation is uniformfor all the regions and that N must also be present. But thechemical homogeneity with regard to the actual amounts of Cand O in the coating cannot be commented upon. Since theanalysis corresponds to only the freestanding film (SS substrateexcluded) presence of Fe in region ‘A’ may be attributed to Featoms that diffused from the SS substrate during deposition.Moreover, a different chemical identity of the nanoparticlesmarked as ‘C’ could not be established as it had almost the sameelemental composition as the rest of the film. It may bementioned that, in the absence of proper quantification for thelow z elements (C, N and O), it was not possible to determinethe stoichiometry of the film.

Fig. 6(a) shows the bright field TEM micrograph of a part ofthe thick CrN coating that has peeled off from the stainless steelsubstrate. The micrograph reveals spheroidal particles measur-ing about 250 nm in diameter. Fig. 6(b) shows the selected areadiffraction (SAD) pattern taken on one such particle. The SAD

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l particle seen in (a); (c) analysis of the diffraction spots seen in (b); (d) DF TEMrectangle in (a) and imaged using the (10-1-1) plane of Cr2N; (e) DF TEM

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pattern reveals weak diffraction rings and some diffractionspots. The diffraction rings have been indexed for the variousplanes of CrN and marked on the figure. The diffraction spotshave also been indexed and shown separately in Fig. 6(c). Bycomparing the interplanar spacing (d) and the interplanar angle(90°), these have been analyzed as originating from the [1–11]zone axis of Cr2N. It may be mentioned here that for an idealhexagonal close packed structure (c/a=1.633), the interplanarangle (between those planes that are marked in the figure) is 98°[21]. On the other hand, for hexagonal Cr2N [ICSD patternidentification number: 067400], the c/a ratio is 1.07 and theinterplanar angle is ∼85°. However, in the present case, the c/aratio was calculated to be ∼1.5. Such a deviation in c/a ratio ispossible in a non-equilibrium deposition process as CVD. Fig. 6(d) shows the dark field image of a crystal using a diffractionspot marked as (10-1-1) plane of Cr2N in Fig 6(c). This crystalis found to measure about 70 nm. Fig. 6(e) shows the TEM darkfield image of a ∼70 nm thin CrN film deposited directly ontocarbon coated Cu grids under similar deposition condition as thethick CrN. Its SAD pattern is shown in the inset. From thisfigure a large number of tiny crystallites can be seen in the filmwhich gives rise to sharp diffraction rings in the SAD pattern.

From the weakening of the SAD rings in the case of the thickcoating as compared to the thin film, it is clear that the degree ofcrystallinity weakens as the film grows. This result is in supportfor that obtained from the cross-sectional TEM (Fig. 5(a)),which has also revealed that the degree of crystallinitydeteriorates with growth. Thus, it may be conjectured that theSS substrate favorably influences the initial nucleation.However, as the film grows, these nuclei fail to grow possiblydue to unfavorable growth conditions, such high arrival rate ofprecursors or low relaxation time. As a result, the volumefraction of CrN crystallites in the film is low resulting in weakdiffraction rings in Fig. 6(b). The broadening observed in thesediffractions is due to small crystallite dimensions while highbackground intensity is indicative of the increase in disorder ofthe material. These results match with the XRD pattern shownin Fig. 2 where too broad peaks with low intensity have beenobserved. Since the crystallites in the thick film are of the orderof 3–4 nm (Fig. 2), the spheroidal particles seen in Fig. 6(a) canonly be agglomerates or clusters of these crystallites andamorphous tissue. The reason for formation of these clusters isyet to be understood. From the analysis of the Fig. 6(c) and (d),it must be mentioned that CrN is not the only phase present butoccasional Cr2N also forms in this material, which appear asembedded within the CrN clusters.

3.4. Improving the adhesion of the CrN coating

The thick CrN coating is also associated with problems ofadhesion, when deposited over areas larger than 1 cm2. It tendsto delaminate during cooling. This is attributed to the largedifference in the thermal expansion coefficient of the SSsubstrate and CrN coating (substrate's thermal expansioncoefficient is nearly 6 times more than that of the coating).Delamination from substrate is often visualized in thick films[13]. However, attempts have been made to avoid this problem

by modification of the substrate surface, as will be discussedbelow.

The surface of the SS substrate was roughened using grit(SiC particles) blasting. Fig. 7(a) shows the SEM micrograph ofthe cross-section of the coating deposited on these roughenedsubstrates under similar conditions as in case of the smoothsubstrates, discussed earlier. Just as in Fig. 3, this micrographalso reveals a banded structure. The main observations from thismicrograph are as follows: (1) almost cent percent coverage ofthe corrugated substrate surface, (2) coating surface follows thesubstrate morphology almost exactly, and (3) the bands alsofollow the substrate surface curvatures. It is also observed thateven micron-sized crevices on the surface created by grit blastare filled by the coating. The banded microstructure may bereasoned as before, to a varying density of the film. The coatingshown in the figure measures about 20 μm in thickness forwhich no adherence problems has been encountered duringcooling from 823K (deposition temperature). The superioradherence may be understood as follows: as the substrate iscooled, high stress levels develop at the coating–substrateinterface due to strain from thermal mismatch. This forces theatoms to move away from their preferred site on the substratesurface, thereby weakening or even breaking the bondingbetween the coating and substrate resulting in delamination ofthe film. But when the substrate is corrugated and containscrevices, atomic movement is restricted, thereby improvingadhesion. Fig. 7(b) represents the SEM micrograph of thesurface of the coating. The microstructure shows globularparticles, which have earlier been identified as agglomerates ofclusters of nanocrystalline CrN. Thus, use of a corrugated

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substrate surface help in improving the adherence of the coatingon stainless steel substrate.

4. Conclusion

CrN coatings of about 80 μm have been successfullyprepared by PAMOCVD technique on stainless steel substrates.On smooth substrates, these coatings have poor adhesion, whichmay be improved by using roughened surfaces. The degree ofcrystallinity of CrN has been found to decrease as the filmgrows. CrN is nanocrystalline (3–4 nm) and tends toagglomerate into spheroidal particles in the thick films givinga globular appearance of the coating surface. The complexcross-sectional microstructure of the coating is believed to be aresult of varying density of the film possibly arising out of theprocess control steps.

Acknowledgements

The authors are grateful to Dr. Baldev Raj, Director IGCARand Dr. S.L. Mannan, Director Materials And Metallurgy Groupfor their keen interest and support for this research work. Theauthors are thankful to Dr. Friedhelm Finger and Dr. M.Vijayalakshmi for useful discussions.

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