201 (2006) 4317–4322www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Tribological behavior of hafnium diboride thin films

A. Chatterjee a, S. Jayaraman a,1, J.E. Gerbi a, N. Kumar a, J.R. Abelson a,⁎,P. Bellon a, A.A. Polycarpou b, J.P. Chevalier c

a Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, 1304 W. Green Street, Urbana, IL- 61801, USAb Department of Mechanical and Industrial Engineering, University of Illinois at Urbana Champaign, 1206 W. Green Street, Urbana, IL- 61801, USA

c Centre d'Etudes de Chimie Métallurgique, 15, rue Georges Urbain, Vitry-sur-Seine 94407, France

Available online 18 September 2006

Abstract

Transition metal diborides and their coatings offer an excellent combination of high hardness, high chemical stability and high thermalconductivity, thus they are excellent candidates for a wide range of tribological applications. In this work, stoichiometric hafnium diboride filmswere grown by chemical vapor deposition from a single-source, heteroatom-free precursor Hf(BH4)4 under conditions that afford highly conformaland smooth films. HfB2 films of thickness ∼0.6 μm deposited on steel substrates were subjected to pin-on-disk wear testing against a counter facedisc of AISI 440C martensitic stainless steel. Based on wear measurements, both as-deposited (X-ray amorphous) and annealed (nanocrystalline)samples showed very high wear resistance compared to uncoated samples. For the annealed samples, SEM analysis indicated the formation of awear resistant composite body in the wear scar, even at depths far exceeding the film thickness, which appears to dramatically improve wearresistance. No mild-to-severe wear transition was observed which indicates that mild wear occurred throughout the wear test. This ensemble ofresults, when considered in the light of high contact pressures (∼700 MPa) used in the study, makes the HfB2 material potentially attractive forwear-resistant applications.© 2006 Elsevier B.V. All rights reserved.

Keywords: Hafnium diboride; Pin-on-disk test; Chemical vapor deposition

1. Introduction

Hard coatings with low friction coefficient, high corrosionresistance and low wear rates have been developed in recentyears to increase the lifetime of machine-components [1].Common applications include Micro Electro-Mechanical Sys-tems (MEMS) and magnetic storage devices, drawing dies andpunches used in metal cutting and forming. As currently avail-able tool coatings are very hard, wear due to abrasion or erosionis limited. Instead, most light and ductile metals have a strongtendency to adhere to counterfaces in sliding contacts [2]. As aconsequence, in addition to high hardness and toughness, anoptimized coating for metal cutting should also possess highchemical stability and high thermal conductivity [3].

⁎ Corresponding author. Tel.: +1 217 333 7258; fax: +1 217 333 2736.E-mail address: abelson@mrl.uiuc.edu (J.R. Abelson).

1 Presently at Intel Corp., Santa Clara, CA.

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.08.086

Metal diborides offer an excellent combination of highhardness, high chemical stability and thermal conductivity. Inparticular, hafnium diboride (HfB2) has a melting temperatureof 3250 °C, a bulk hardness of 29 GPa and good corrosionresistance, all of which make it an excellent candidate for wearresistant coatings [4,5]. In spite of such attractive combinationof properties, relatively few reports of transition metal boridethin films compared to transition metal nitrides could be found.This can be traced to the processing difficulties associated withdepositing these films. Magnetron sputtering is a straightfor-ward method to deposit such films, but the control ofstoichiometry is a non-trivial issue [1]. Using a single sourceprecursor, Jayaraman et al. [6] have reported that dense,stoichiometric, and conformal HfB2 coatings can be grown bychemical vapor deposition (CVD) at low temperature. Uponannealing for 1 h at 700 °C, these X-ray amorphous coatingstransformed to nanocrystalline ones, resulting in hardness of40 GPa and elastic modulus of 430 GPa. An important issue forthe potential use of these coatings is their wear resistance. Wear

Fig. 1. Wear testing geometry.

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of hard coatings is generally not abrasive as long as the counter-face material does not contain any hard phases. However, if thework material adheres strongly to the coating it could result insevere cohesive delamination or detachment of the coating [7].Many authors have reported highly desirable tribologicalproperties such as high wear resistance, high critical loads fordelamination and decohesion as well as high hardness for PVD-grown Ti(BN) class of thin films (TiB2, TiBN, TiBCN, TiAlBN)[8–12].

The present work takes advantage of the aforementionedsingle-source CVD to grow HfB2 coatings on mild steel andstainless steel half-sphere substrates. Pin-on-disc wear testswere performed and the results reported here demonstrate thatthe coatings, especially annealed ones, lead to very significantreduction of wear rates. SEM is also used to characterize wearscars.

2. Experimental procedure

2.1. Film deposition and growth

Thin film deposition of HfB2 was performed by thermalCVD of Hf(BH4)4 in an ultra-high vacuum (UHV) chamber asdescribed in Ref. [13]. The growth temperature was kept at

Fig. 2. Wear vs time for HfB2 deposited on mild steel against 440C martensitic staidiboride film.

250 °C. Two types of steel were chosen as substrates, mild steeland 440C martensitic stainless steel. Half spheres (diameter6 mm) of both steels were first degreased in an ultrasonic bathusing acetone, isopropyl alcohol and de-ionized water. Thesehalf-spheres were then attached to a 2 cm×1.2 cm n-type Siwafer using a ceramic paste and cured at 200 °C in N2 atmos-phere. The samples were heated resistively by passing currentthrough the Si substrate. The precursor pressure inside thechamber was maintained at 0.15 mTorr. The growth rate at thistemperature was 12 nm/min on flat substrates. Based on thisgrowth rate, the film thickness on the half-sphere was estimatedto be 600 nm. The as-deposited film is primarily X-ray amor-phous as described in [13]. Samples annealed at 700 °C for 1 hwere however clearly nanocrystalline.

2.2. Wear testing: Pin on disk tester

In order to evaluate the wear behavior of these films, theywere tested against a counterface disk of AISI 440C martensiticstainless steel in rotating sliding motion on a standard pin-on-disk wear tester from Koehler Instruments Co. The flat side ofthe coated half-spheres was attached to standard 6 mm cy-lindrical pins using an adhesive. These pins were then held atfixed position while the counterface disk rotated at a constantvelocity of 0.05 m/s. The geometry is shown in Fig. 1. It ensuresthat a point (Hertzian) contact is established initially for thewear test, regardless of any small misalignment of the pin.Different ranges of rotational speeds and wear track radii wereused, keeping the sliding velocity constant. The load was alsoheld constant at 1 N. From Hertzian contact theory [14], thecontact pressure was calculated to be in the range of 700–800 MPa, which is similar to the contact pressures that indus-trial coatings are normally subjected to in service. The filmswere worn without any lubrication and in ambient air atmos-phere. The wear depth of uncoated substrate, as-deposited filmand annealed film were in situ monitored by an LVDT trans-ducer as a function of test duration. The wear depth at the end ofthe test was also precisely measured from the wear scar

nless steel: (a) uncoated mild steel, (b) as-deposited diboride film, (c) annealed

Fig. 3. SEM images ofwear scar for annealed films deposited inmild steel: (a) overallscar (clear difference visible between the morphology of worn coating and unworncoating, diameter of the wear scar indicated=0.16 mm), (b) magnified view of thetransition zone between worn and unworn regions.

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diameter, using an SEM. Because of the small load used in thestudy (1 N), no reliable friction coefficient values could berecorded with the present wear tester.

Fig. 4. Wear vs time for HfB2 deposited on 440Cmartensitic stainless steel against 440deposited diboride film, (c) annealed diboride film.

3. SEM and EDS characterization

Coating morphology and wear surface topography werestudied using a JEOL 6060 LV Scanning Electron Microscope.Secondary electron images of the wear scar were obtained atdifferent magnifications. From the wear scar images on thespherical surface, the wear depth was calculated using the ap-proximate formula w=d2 /4d0, where w is the wear depth, d isthe diameter of wear scar and d0 is the diameter of the sphere.SEM-EDS (from Oxford Instruments) was used to characterizethe elements in the wear scar, using the ISIS™ software and Si(Li) X-ray detector.

4. Results

4.1. Wear of HfB2 deposited on mild steel

The wear behavior of uncoated substrate, as-deposited di-boride film and annealed diboride film are shown in Fig. 2.Initial transients were observed for all three samples. The as-deposited film shows mild wear while almost zero wear isobserved in case of annealed samples after the initial transients.In contrast, the uncoated sample shows constant progressivewear. No transition from mild to severe wear was observed forany of the samples. Fig. 3a and b depict SEM images of thewear scars of annealed samples. Even though cracks are visiblein the films before wear, the annealed coatings resist delami-nation as a whole. However the post-wear morphology is dif-ferent compared to the unworn state. At some regions of thewear scar edges, the underlying substrate is clearly visible.There is some uncertainty in the LVDT measurements becauseof the presence of transients, which are due to the sudden onsetof the rotational motion of the pin, which has been fixed rigidlyin point contact against the counterface. Thus the LVDT mea-surements cannot be considered as absolute measures of wear

Cmartensitic stainless steel: (a) uncoated 440Cmartensitic stainless steel, (b) as-

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but do provide useful trends. Accurate values of the final wearare nevertheless obtained by SEM measurements of scar di-mensions. These final wear values are 5.4 μm, 2.3 μm, and1 μm for the uncoated substrate, as-deposited coating, and an-nealed coating, respectively. It is important to note that thesefinal wear values are greater than the film thickness, and thematerial visible on the wear scar after the wear process cor-responds to a tribochemical product superimposed on the ex-posed substrate.

Fig. 5. SEM images of wear scar for films deposited on 440C martensiticstainless steel: (a) overall scar of as-deposited film (coating seen clearlydelaminated, clear boundary between worn/unworn regions, wear scar diameterindicated=0.83 mm), (b) overall scar of annealed film (transition zone betweenworn/unworn zones visible, wear scar diameter indicated=0.25 mm), (c) mag-nified view of transition zone between worn and unworn regions for annealedsample.

4.2. Wear of HfB2 deposited on 440C martensitic stainless steel

As can be seen in Fig. 4, the overall trend in wear behaviorfor HfB2 films deposited on 440C martensitic stainless steel wassimilar to that for films deposited on mild steel. The as-depo-sited film resists wear for 10 min after which there is a suddenjump in the wear which corresponds to delamination of the filmleading to exposure of substrate, as confirmed in Fig. 5a. Thewear after this sudden change corresponds to wear of under-lying 440C martensitic stainless steel substrate. The annealedfilm (Fig. 5b), on the other hand, resisted wear for almost 1 h, inspite of cracks visible in the unworn state. As seen in Fig. 5b,the film worn off only at some locations and was never com-pletely delaminated as in the as-deposited case (Fig. 5a). Thefinal wear values as determined by SEM are 34 μm, 27 μm, and2.5 μm for the uncoated substrate, the as-deposited coating, andthe annealed coating, respectively.

4.3. SEM-EDS analysis

The regions in the wear scar for the 440C martensiticstainless steel substrate were subjected to SEM-EDS analysis.While no Hf was detected in the wear scar of the as-depositedcoating, very significant amounts of Hf were detected in thewear scar of the annealed coating (Fig. 6a). Hf elemental map-ping as shown in Fig. 6c, reveals the distribution of Hf in thewear scar. Note the very good correspondence between the lightgray regions in the secondary electron SEM image (Fig. 6b)with the ones containing significant amount of Hf in the ele-mental map. The morphology of the wear track periphery in thesecondary electron SEM image in Fig. 6b is distinct from themorphology of the unworn film (see Fig. 5c). Besides Hf, theSEM-EDS spectrum (Fig. 6a) indicate the presence of Fe andCr, as well as a small oxygen peak at≈550 eV. These results arediscussed in the next section.

5. Discussion and conclusions

For both mild steel and 440C martensitic stainless steelsubstrates, the HfB2 coatings provide appreciable reductions inwear. The present results also indicate that the annealed coatingsprovide better wear resistance than the as-deposited coatings.Two factors may contribute to this difference. Firstly, annealing,which induces a transformation from X-ray amorphous to na-nocrystalline [6], leads to a significant increase in hardness,from 25 GPa to 40 GPa, and thus it is reasonable to expect anincrease in wear resistance brought about by this annealing. Inthis context, it is interesting to note that Mitterer [1] reported thatZr–B–N coatings that are nanocrystalline lead to a reduction ofwear coefficient compared to amorphous coatings. Secondly,sudden and global delamination was observed in the as-depositedcoating but not in the annealed coatings. This difference suggeststhat annealing may have improved the adhesion of the coating tothe substrate. While this first set of results demonstrated clearlythe potential of HfB2 as a protective coating for wear-resistantapplications, additional improvements can be obtained byoptimizing the growth and annealing conditions. Recent results

Fig. 6. EDS results of annealed HfB2 wear scar (films deposited on 440C martensitic stainless steel): (a) X-ray spectrum showing the presence of elements in theoverall scar, (b) secondary electron image of transition zone between worn and unworn region, (c) Hf elemental mapping of transition zone.

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[15] indicate that in situ nitridation of the substrates prior to thegrowth of the coatings leads to a much-improved resistance todelamination for the as-deposited coatings. Adhesion tests toevaluate the strength of the interface between film and substrateneeds are currently under progress.

For the 440C martensitic stainless steel substrate, the mor-phology of the wear scar for the as-deposited and annealedcoatings was different. The wear mechanism in as-depositedfilms appears to be brittle in nature. For annealed films, however,SEM images (Figs. 5b,c, 6b) and the SEM-EDS Hf elementalmap (Fig. 6c) suggest that wear has lead to the formation of acomposite layer containing diboride particles. The matrix of thecomposite is mostly comprised of Fe and Cr. SEM imagessuggest that this material belongs to the stainless steel substrate,although some of it may result from chemical transfer from themartensitic steel used a counterface onto the wear scar. Whilethis steel-based material could be oxidized, in particular near tothe surface, no massive formation of oxide was observed here, asattested by the small oxygen peak in the EDS spectrum, Fig. 6a.Rigney [16] had postulated that transfer and mechanical mixingin metallic materials results in a change in the chemical com-position of the near surface material. Rainforth [17] has indeedobserved formation of surface layers (also known as tribolayersor transfer layers) in ceramics and modifications of wear beha-

vior. The same author proposed that such surface layers arise dueto compaction of wear debris or from reaction of the substratewith the environment. A result of our study is that the wearresistance provided by annealed films persists even when thewear depths exceeds the initial film thickness (of the order of600 nm). The present analysis suggests that the formation of aHfB2-steel composite layer contributed to provide this surprisingwear resistance. Further work is needed to confirm this analysis.

Acknowledgements

The authors are grateful to the National Science Foundationand CNRS, France for the support of this research under grantnumber NSF DMR-0354060 through a NSF-CNRS cooperativeagreement. Compositional and structural analyses of the filmswere carried out in the Center for Microanalysis of Materials,University of Illinois, which is partially supported by the U.S.Department of Energy under grant DEFG02-91-ER45439.

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