Ž .Surface and Coatings Technology 136 2001 211]216

Effects of carbon ion pre-implantation on the mechanicalproperties of ta-C coatings on Ti-6Al-4V

H.H. Tong1, O.R. Monteiro, I.G. BrownU

Lawrence Berkeley National Laboratory, Uni ersity of California, Berkeley, CA 94720, USA

Abstract

We report on our investigations of the effects of carbon ion implantation into Ti-6Al-4V as a pre-treatment process prior todeposition of ta-C coatings using a filtered cathodic arc plasma method. A transition layer is formed that can be either TiC orTiCqC, providing an excellent interface for highly adherent ta-C coatings. We find that a smoothly-graded interface leads togood film]substrate adhesion, while ta-C films formed without a graded interface show poor adhesion. The film microhardnessand wear performance are related to the implantation parameters via the thickness of the transition layer. Carbon ionimplantation provides an effective pre-treatment method for ta-C coated Ti-6Al-4V substrate material. Q 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Ion implantation; ta-C; Biomaterial coatings; Cathodic arc deposition; Ion mixing; Diamond-like carbon

1. Introduction

Ž .Hydrogenerated diamond-like carbon a-C:H coat-ings on metal substrates such as Ti-6Al-4V alloy havebeen used for biomedical applications because of theirhigh hardness, high wear resistance, low coefficient offriction, biological compatibility and chemical inert-ness. However, the coatings are difficult to deposit withthe very strong adhesion required. Various methodshave been explored to improve the film]substrate ad-hesion, such as, e.g. by forming a TirTiCra-C:H func-tionally graded structure prepared by chemical vapor

Ž .deposition CVD or plasma source ion implantationŽ . w xPSII 1]3 .

Non-hydrogenated, tetrahedrally-bonded amorphousŽ .carbon ta-C thin films can be formed by filtered

U Corresponding author. Tel.: q1-510-486-4174; fax: q1-510-486-4374.

Ž .E-mail address: igbrown@lbl.gov I.G. Brown .1Visiting Scientist from Southwestern Institute of Physics, P.O.

Box 432, Chengdu 610041, Sichuan, PR China.

w xcathodic arc plasma deposition 4]7 . It has been shownthat this coating material has the same biological com-

w xpatibility 8 and other desirable characteristics of a-C:H, with the additional advantage of being extremelyhard, with hardness values approaching that of pure

w xdiamond 4,6,7,9,10 . Non-hydrogenated ta-C thin filmsdeposited in this way could thus be expected to makeexcellent biomaterial coatings, e.g. as a final step in themanufacture of orthopedic implants. Unfortunately, itis frequently the case that these very hard ta-C coat-ings also tend to delaminate from the substrate, abehavior which is generally associated with the highinternal stress of the film, as is their hardness. In orderto be able to make use of these particularly hard andwear-resistant films, techniques are needed for enhanc-ing the film]substrate adhesion strength so that de-lamination does not occur.

In the work described here, energetic carbon ionimplantation of the Ti-6Al-4V substrate material wascarried out prior to ta-C formation by filtered cathodicarc plasma deposition. As a comparison, ta-C coatingson Ti-6Al-4V substrates were also formed by metal

0257-8972r01r$ - see front matter Q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 1 0 5 8 - 6

( )H.H. Tong et al. r Surface and Coatings Technology 136 2001 211]216212

Table 1qC ion pre-implantation and ta-C film deposition parameters

qSample C dose and energy Deposition bias and number of pulses

17 y2A 4=10 cm @ 50 keV y100 V for 5000 plasma pulses17 y2 17 y2B 2=10 cm @ 50 keVq2=10 cm @ 30 keV y100 V for 5000 plasma pulses17 y2C 1=10 cm @ 50 keV y100 V for 5000 plasma pulses17 y2 Ž .D 1=10 cm @ 50 keV 0 V substrate grounded for 5000 plasma pulses

Ž .E No implantation done y2 kV for 500 plasma pulsesqy100 V for 4500 plasma pulses

plasma immersion ion implantation and depositionŽ . w xMepiiid 11]14 in a carbon plasma, both with andwithout a preliminary phase of high deposition ionenergy. The microhardness, adhesion strength and fric-tion coefficient of the coatings were measured. Herewe outline the processing methods used and describethe results of our characterization.

2. Experimental

The substrate material used for the work describedhere was annealed Ti-6Al-4V alloy. Disk-shaped sam-ples 25 mm in diameter and 5 mm thick were machinedfrom plate material. The samples were polished to amirror finish using 0.03-mm alumina paste and ultra-sonically cleaned in acetone followed by alcohol for 15min.

Carbon ion implantation was carried out using aŽ .metal vapor vacuum arc Mevva ion implantation facil-

w xity that has been described in detail elsewhere 15]17 .Briefly, a vacuum arc ion source was used to form a Cq

ion beam of energy several tens of keV in a repetitivelypulsed mode; beam pulses were 250 ms wide and therepetition rate was several pulses per second. The ionbeam was formed with an initial diameter of 10 cm asdetermined by the source extraction electrodes, andimplantation was done in a broad-beam, line-of-sightmode. The substrate to be implanted was located on awater-cooled target holder approximately 65 cm down-stream, where the beam profile covered the substratedisk more-or-less uniformly. The Ti-6Al-4V disk sam-ples were subjected to sputter-cleaning by 15 keV Cq

bombardment for 15 min prior to the actual implanta-tion. The carbon ion implantation energy and doseparameters for the four samples A]D are shown inTable 1. The implantation energy was 50 keV and thedose 1, 2 or 4=1017 cmy2 , as indicated, except forsample B which was a dual-energy implant: 2=1017

cmy2 at 50 keV followed by 2=1017 cmy2 at 30 keV.Following carbon ion implantation, the samples were

moved to another vacuum chamber for ta-C coatingusing the Mepiiid technique. This cathodic arc plasma

w xmethod has been described elsewhere 11]14 . Briefly,Žthe substrate is immersed in the plasma stream in this

.case, carbon produced by a repetitively pulsed ca-

Ž . w xthodic arc also called vacuum arc plasma gun 18 .The carbon plasma is ‘filtered’ to remove all non-plasma

Žparticulate contamination microscopic debris from the.carbon cathode by passing it through a 908 quarter-

torus solenoidal ‘magnetic duct’. A carbon film isformed on the substrate by deposition from the filteredcarbon plasma stream. The energy of the depositingcarbon ions is controlled via a high-frequency repetitivepulse-bias that is applied to the substrate in the pres-ence of the plasma. The ability to control the iondeposition energy is an important feature, and allowssubstantial control of the film properties. In particular,

Ž .by optimizing the ion energy approx. 100]150 eV oneŽcan form ta-C films with a very high fraction up to

. 385% of diamond-like sp bonding. In the presentwork, the Ti-6Al-4V disk samples were subjected tosputter-cleaning in the Mepiiid chamber by an Arglow-discharge plasma for 15 min prior to deposition.The deposition parameters for the five samples coated,A]E, are shown in Table 1. The substrate bias was

Žeither zero or y100 V corresponding to an ion deposi-tion energy of 20 or 120 eV, respectively, since thecathodic arc carbon plasma has an inherent streaming

.energy of ;20 eV , or, for sample E, y2 kV for theearly phase of the Mepiiid process and then reduced toy100 V for the bulk of the deposition. The plasmasource was operated with pulses of duration 5 ms at arepetition rate of 1 pulse per second. The substratebiasing used negative-going pulses of duration 2 ms and

Ž .duty cycle 25% 2 ms on-time and 6 ms off-time , andwas gated on only during the 5 ms plasma pulse. Forour operating parameters and geometry the film depo-

˚sition rate was approximately 0.2 A per pulse, for a˚final thickness of approximately 1000 A for all of the

films.The processed samples were characterized for hard-

ness, film-substrate adhesion and wear. The Vicker’smicrohardness was measured at various loads using aMicromet tester. The adhesion of the ta-C coatings wasestimated by a Vicker’s indenter with a load 200 g andevaluated semi-quantitatively by a thermal quench

w x Ž .method 19 . A pin-on-disc tribometer ALEX-1 fittedwith a 5-mm diameter SiC ball was used to determinethe wear and friction properties of the samples in thelaboratory ambient, at a load of 1.96 N and at aconstant speed of 0.24 mrs. Film microstructure was

( )H.H. Tong et al. r Surface and Coatings Technology 136 2001 211]216 213

characterized by glancing-angle X-ray diffraction usingthe CuKa line at 40 kV. Rutherford backscattering

Ž . qspectrometry RBS with 2.0 MeV He ions was usedto measure the carbon depth profiles; as expected forthis diagnostic, however, quantitative measurement oflow-atomic-mass species in higher-atomic-mass sub-strate material is difficult, and the results can be takenonly semi-quantitatively. The carbon depth profiles werealso estimated using the dynamic TRIM Monte-Carlo

w xcode, T-Dyn 20,21 .

3. Results and discussion

The Vickers microhardness for various loads is shownin Fig. 1. The microhardness for all of the coated

Ž .samples at 5 g load HV from approx. 600 to 800 isŽgreater than for the uncoated sample HV approx.

. Ž .470 . The microhardness increase ;50"25% at 5 gŽload decreases to zero i.e. microhardness equal to that

.of the substrate material at 20 g load, since for thehigher loads the indenter is basically probing the sub-strate. The thickness of the coating is only approxi-mately 100 nm while the indentation depth of theMicromet tester is of order a micrometer and increaseswith load. Sample A showed the greatest hardnessincrease, consistent with it having the thickest hard

Žlayer i.e. the combination of the surface ta-C coating.and the ion implanted sub-surface region .

The interface toughness was estimated qualitativelyw xon the basis of an indentation method 22,23 . Due to

the thinness of the films the toughness analysis modelfor this method could not be rigorously applied here,but nevertheless the method could still be used as anindicator of comparative trends. The Vicker’s indenterwith a load of 200 g was used to produce microindenta-tions in the ta-C coatings of all of the samples. SampleD was the only one to show visible circular and radialcracks at the indentation mark, as can be seen in theoptical micrograph of Fig. 2. Thus, sample D } with

Fig. 1. Vicker’s microhardness at various loads.

Ž .Fig. 2. Optical micrograph =400 of sample D showing indentationmarks with visible cracks.

Ž .no bias voltage low ion energy during the ta-C deposi-tion } was the sample with the lowest interface tough-ness, as indeed expected for the case of a sharp inter-face with no atomic mixing.

A thermal quench method described in Fan et al.w x Ž19 was used to determine the adhesion strength morecorrectly, shear stress at the interface at the point of

.film delamination . In this method the sample tempera-Ž .ture is rapidly lowered from room temperature 297 K

Ž .to liquid nitrogen temperature 77 K and the conditionof the coating subsequently observed. If coating de-lamination occurs, then the adhesion strength is esti-mated to be comparable with or less than the thermalstress caused by the quench. Fig. 3 shows an opticalmicrograph of sample D after such a quench, withlateral debonding of the coating evident. The othersamples did not display visible detachment of the coat-ings in the quench. The thermal stress induced by a

Ž .Fig. 3. Optical micrograph =50 of sample D after thermal quench,with visible detachment of the coating.

( )H.H. Tong et al. r Surface and Coatings Technology 136 2001 211]216214

Fig. 4. Thermal expansion coefficients of diamond and Ti-6Al-4V asa function of temperature.

quench from temperature T to T can be calculated2 1from the relationship

Ž . Ž . Ž .s sE a ya dTr 1yn 1Hth s f

where E is Young’s modulus and n is Poisson’s ratiofor the film, and a , a are the thermal expansionf scoefficients for the film and the Ti-6Al-4V substrate,

w xrespectively. We take Es600 GPa 7 , and approxi-mate n by 0.07, its value for diamond. The thermalexpansion coefficient a for ta-C is close to that off

w xdiamond 24 . a and a are shown in Fig. 4; they canf sw xbe fitted by polynomials 19,25 . Using this method we

estimate that the value of s for coating D to thethTi-6Al-4V substrate material is less than approximately0.75 GPa, while the other samples have an adhesionstrength greater than 0.75 GPa.

The coefficient of friction results obtained from pin-on-disk tests for samples A, B, C and E are shown inFig. 5. After an initial short period with values ofapproximately 0.15, the coefficient of friction decreasesto approximately 0.1 for an extended number of revolu-tions, until it starts to increase, reaching values greaterthan 0.3 before the tests were interrupted. A compar-ison between the performance of the different samplesindicated that sample A was the one that took thelongest before a significant increase in friction coeffi-cient was detected. After the tests were stopped thetrack surfaces were examined using optical microscopy.No visible debris could be found. The lack of debrisassociated with the relatively gentle increase in theobserved friction coefficient suggests that the coatingsdid not fail by delamination, but by wear of the coating.

Concentration depth profiles calculated using theT-DYN code for carbon ion implantation with threedifferent parameter sets are shown in Fig. 6. Fig. 6a isfor a total dose of 4=1017 cmy2 and energy of 50 keV;Fig. 6b for 2=1017 cmy2 and energy of 50 keV, fol-lowed by 2=1017 cmy2 and energy of 30 keV; and Fig.

Fig. 5. Friction coefficients in laboratory ambient from pin-on-disctests.

6c for 1=1017 cmy2 and energy of 50 keV. TheseŽ .implantation conditions dose and energy are those

used in preparation of samples A, B and C, respec-tively. In all cases, the carbon content at the surface isbelow 20%, although at depths greater than 50 nm itcan reach a concentration of almost 50%. According to

w xa recent investigation 26 , C implantation in Ti-6Al-4Vresulting in atomic concentration below 33% will leadto only Ti]C bonds. C]C bonds are only detected at

w xconcentrations greater than 33% 26 . Thus, while there

Fig. 6. Carbon concentration profile in Ti-6Al-4V substrate calcu-lated by the T-DYN code after implantation at different parametersas shown.

( )H.H. Tong et al. r Surface and Coatings Technology 136 2001 211]216 215

Ž .Fig. 7. Carbon concentration profiles: a calculated by T-DYN;Ž .Mepiiid with a bias voltage of y2 kV, 25% duty cycle; b from RBS

measurements.

may be formation of carbon clusters deep in samples Aand B, at and near the surface we expect primarily TiCŽ .in addition to the matrix Ti-6Al-4V .

In contrast to the implanted samples using directbeam implantation, in sample E a high energy implan-

Ž .tation but not as high as for the beam implantationwas carried out using PIII to intermix the film and thesubstrate. In this case, a smooth, gradual C-profile isobtained, as seen in Fig. 7, where the calculated andmeasured C atomic fraction are shown respectively in

w xFig. 7a,b. In this case, according to reference 26 , thereis a transition from a region containing only Ti]Cbonds to another containing Ti]C and C]C bonds, andeventually to the film itself, containing only C]C bonds.Glancing angle X-ray diffraction of the samples pre-

Ž .pared by direct beam implantation at high energy andŽ .by PIII at low energy confirmed the presence of a TiC

phase, also.Finally, we comment on the observed wear behavior

of the different samples. The low-friction regime, whichappears to last up to 2000 cycles for all samples, isassociated with the wear of the ta-C. Following this, thefriction coefficient of sample E increases rapidly to 0.3within 1000 cycles. This region of fast increase is most

Ž .likely due to wear of the mixed interface, which isnarrow compared to those in the samples prepared bydirect beam-implantation. In samples A, B and C, agentle increase in friction coefficient from 0.1 to 0.2lasts another 2000 cycles, and in this region we postu-late wearing of the deep implanted regions of theTi-6Al-4V where there is a significant fraction of car-bon. The differences between samples A, B and C aredirectly related to the amount of implanted carbon in

these samples: the sample with highest dose at deeperŽ .regions sample A is the one whose friction coefficient

takes the longest to reach the fast-increase region.

4. Conclusions

We have deposited ta-C coatings on Ti-6Al-4V byfiltered cathodic arc deposition, following pre-implanta-tion of carbon. The pre-implantation was carried outeither by direct beam implantation at high ion energyŽ .50 keV or by carbon plasma immersion ion implanta-

Ž .tion at low energy either 100 eV or 2 keV . Adhesionof the coating to the substrate was greater than 0.75GPa for all cases in which pre-implantation had beencarried out at relatively low energy by the plasmaimmersion process. With no plasma immersion, evenwith high energy beam implantation, the film failed theadhesion tests, indicating adhesion strength below 0.75GPa. We take these results as indicative of the needfor a graded interface as opposed to simply a deepimplant. Wear tests indicated the presence of two dif-ferent regimes, one consisting of wear of the ta-C filmitself followed by wear of the transition layer. In thecase of the pre-implantation carried out using PIII, thissecond phase is of short duration because the transi-tion layer is thin, whereas in the case of beam-implan-tation the second phase is long because the layer isthick. The results of our measurements indicate thedesirability of an atomically mixed interface zonebetween the ta-C film and the Ti-6Al-4V substrate. Amore extensive experimental investigation spanning awider parameter range is needed to fully quantify themechanical properties of the system as a function ofthe processing variables.

Acknowledgements

We are indebted to Bob MacGill for technical assis-tance with the experimental facilities, and to GeraYushkov for carrying out the implantations. One of usŽ .H.H.T. was supported at LBNL by an IAEA Fellow-ship, for which he would like to express his apprecia-tion. This work was supported in part by the US De-partment of Energy, Office of Energy Research, underContract Number DE-AC03-76SF00098.

References

w x1 A.A. Voevodin, M.A. Capano, S.J.P. Laube, M.S. Donley, J.S.Ž .Zabinski, Thin Solid Films 298 1997 107.

w x Ž .2 K. Baba, R. Hatada, Surf. Coat. Technol. 103r104 1998 235.w x3 G.F. Yin, J.M. Luo, C.Q. Zhjang, H.H. Tong, L.L. Mu, Y.F.

Ž .Huo, Thin Solid Films 345 1999 67.w x4 S. Anders, A. Anders, I.G. Brown et al., Surf. Coat. Technol.

Ž .68r69 1994 388.

( )H.H. Tong et al. r Surface and Coatings Technology 136 2001 211]216216

w x5 B.K. Tay, X. Shi, L.K. Cheah, D.I. Flynn, Thin Solid FilmsŽ .308]309 1997 199.

w x6 M.P. Delplancke-Ogletree, O.R. Monteiro, Surf. Coat. Tech-Ž .nol. 108r109 1998 484.

w x Ž .7 Y. Lifshitz, Diamond Relat. Mater. 8 1999 1659.w x8 M.I. Ignatius, N. Sawhney, A. Gupta, B. Thibadeau, O.R.

Ž .Monteiro, I.G. Brown, J. Biomed. Mat. Res. 40 1998 264.w x9 G.M. Pharr, D.L. Callahan, S.D. McAdams et al., Appl. Phys.

Ž .Lett. 68 1996 779.w x10 S. Anders, I.G. Brown, C.S. Bhatia, D.B. Bogy, Data Storage

Ž .October 1997 31]38.w x11 I.G. Brown, A. Anders, S. Anders et al., Nucl. Instrum. Meth-

Ž .ods B 80r81 1993 1281.w x Ž .12 A. Anders, Surf. Coat. Technol. 93 1997 158.w x Ž .13 I.G. Brown, Annu. Rev. Mater. Sci. 28 1998 243.w x14 O.R. Monteiro, Nucl. Instrum. Methods Phys. Res. B 148

Ž .1999 12.w x15 I.G. Brown, M.R. Dickinson, J.E. Galvin, X. Godechot, R.A.

Ž .MacGill, Surf. Coat. Technol. 51 1992 529.w x Ž .16 I.G. Brown, Rev. Sci. Instrum. 65 1994 3061.

w x17 I.G. Brown, A. Anders, M.R. Dickinson, R.A. MacGill, O.R.Ž .Monteiro, Surf. Coat. Technol. 112 1999 271.

w x18 R.A. MacGill, M.R. Dickinson, A. Anders, O.R. Monteiro, I.G.Ž .Brown, Rev. Sci. Instrum. 69 1998 801.

w x Ž .19 Q.H. Fan, A. Fernandes, E. Pereira, J. Appl. Phys. 83 19985588.

w x Ž .20 J.F. Ziegler, J.P. Biersack, U. Littmark, in: J.F. Ziegler Ed. ,The Stopping and Range of Ions in Solids, 1, Pergamon, NewYork, 1985.

w x21 J.P. Biersack, S. Berg, C. Nender, Nucl. Instrum. Methods BŽ .59r60 1991 21.

w x22 S.S. Chiang, D.B. Marshall, A.G. Evans, in: J. Pask, A.G. EvansŽ .Eds. , Surfaces and Interfaces in Ceramic and Ceramic]MetalSystems, Plenum, New York, 1981, p. 603.

w x Ž .23 D.B. Evans, A.G. Evans, J. Appl. Phys. 24 1984 209.w x24 Y.S. Touloukian, Thermalophysical Properties of Materials, 12,

Plenum, New York, 1975, p. 1272.w x Ž .25 I.A. Blech, P. Wood, J. Vac. Sci. Technol. A 11 1993 728.w x26 J.L. Viviente, A. Garcıa, F. Alonso, I. Braceras, J.I. Onate,´ ˜

Ž .Appl. Surf. Sci. 144r145 1999 249.