Surface and Coatings Technology 158–159(2002) 630–635

0257-8972/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00321-3

Successive ion implantation of high doses of carbon and nitrogen onsteels

R. Sanchez *, J.A. Garcia , A. Medrano , M. Rico , R. Martınez , R. Rodrıguez , C. Fernandez-a, a a a a a´ ´ ´ ´Ramos , A. Fernandezb b´

Centro de Ingenierıa Avanzada de Superficies de AIN, CySan Cosme y San Damian syn, 31191 Cordovilla, Pamplona, Spaina ´ ´Instituto de Ciencias de Materiales de Sevilla-CSIC-C, Americo Vespucio syn, 41092 Seville, Spainb ´

Abstract

HSS and stainless steel samples were successively implanted with high doses of carbon and nitrogen. Different sequences ofimplantation(C , C qN , N qC and N qC qN ) were carried out in order to achieve C N layers of ultra-high hardness.q q q q q q q q

2 2 2 2 x y

The bonded states of carbon and nitrogen were studied by XPS spectroscopy. The tribological behaviour was measured by meansof ball-on-disc and microhardness tests. The microhardness tests showed that increases in hardness are highly influenced by theimplantation order. None of the sequences tested led to a hardness increase higher than that for simple carbon implantation. Whennitrogen was implanted into carbon-implanted steel, the result was a huge decrease in surface hardness. Ion implantation of highdoses of nitrogen and carbon brought about a decrease in the friction and wear coefficients. The samples for which carbon wasimplanted in the last step showed the lowest friction and wear coefficients.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ion implantation; Microhardness; Wear; Friction; Tribology; X-Ray photoelectron spectroscopy(XPS)

1. Introduction

In 1989, the carbon nitride C N was calculated to be3 4

harder than diamondw1x. The calculations of Sung andSung w2x, based on an analogy to the hardness differ-ences between corresponding phases(a and b) ofSi N and SiC, predicted thata-C N would be harder3 4 3 4

than diamond, whereasb-C N would be softer. The3 4

synthesis of polycrystalline C N film was first reported3 4

in 1992w3x and 1993w4x and is currently under intensiveinvestigation. A variety of techniques have been used toattempt the synthesis of carbon nitride, including dualion beam sputteringw5x, reactive sputteringw6x, pyrolysisof several organic materialsw7x and ion implantationw8x.It is known that when a high dose of C is implantedq

into steel a graphitic carbon layer is created over thesteel surface. With the aim of growing up CN layers,x

N was implanted into a C-implanted layer over twoq

different steels.

*Corresponding author. Tel.:q34-948-421101; fax:q34-948-421100.

E-mail address: rsanchez@ain.es(R. Sanchez).´

2. Experimental

2.1. Sample preparation

SP 30 PM high-speed steel and AISI 304N stainlesssteel samples were mirror-polished using abrasivepapers, diamond paste and, finally, an alumina suspen-sion of 0.05mm.

2.2. Ion implantation

Successive carbon and nitrogen implantations wereperformed using a 200-keV Whickham Ion Beam Sys-tems ion generator at AIN laboratories. Table 1 describesthe different series of successive implantations(C ,q

C qN , N qC and N qC qN ). For each indi-q q q q q q q2 2 2 2

vidual implantation the acceleration voltage was fixedat 100 keV.

2.3. X-Ray photoelectron spectrometry (XPS)

XPS spectra were recorded in a VG-Escalab 210spectrometer working in the constant analyser energy

631R. Sanchez et al. / Surface and Coatings Technology 158 –159 (2002) 630–635´

Table 1Successive ion implantation series for each group of samples

Implantation

First Second Third

Ion Nq2 Cq Nq

2

Energy(keV) 100 100 100Dose(atomsycm )2 3=1018 3=1018 3=1018

C samples No Yes NoCN samples No Yes YesNC samples Yes Yes NoNCN samples Yes Yes Yes

Table 2Results of the XPS analysis

Sample Peak positions in the XPS spectra(eV) O, N and C content(at.%)

O 1s N 1s C 1s Fe 2p3y2 O 1s N 1s C 1s NyCatomicratio

Stainless steelC 532.4 400 284.6 – 8.8 1.1 90.1 0.0122

532.5 – 284.6 – 1.6 0.3 98.1 0.003CN 532.9y530.5 398 284.6 – 16.1 3.2 80.8 0.0396

532.6y530 398.4 284.6 709.3 10.8 2.5 86.7 0.0288NC 532.4 399.3 284.6 – 7.5 2.4 90.2 0.0266

532.4 399 284.6 – 1.7 2.5 95.7 0.0261NCN 531y529.5 397.2 284.6 710.4y717.2 33.7 5.7 60.6 0.0941

531y529.6 397 284.6y282.9 709.2y707.2 52.3 16.1 31.6 0.5095

High-speed steelC 532.3 – 284.6 – 7.8 0 92.2 0

532.6 – 284.6 – 0.9 0.2 98.9 0.002CN 534y531.9 399 284.6 – 11.9 7.2 80.8 0.0891

531.3 398.6 284.6 – 6.8 8.5 84.7 0.1003NC 532.4 – 284.6 7.4 0.7 92 0.0076

532.5 – 284.6y282.9 – 1 0.7 98.3 0.0071NCN 533.7y531.5y 398.7 284.6 710.7 15.9 7.5 76.5 0.098

529.6531.2 398.7 284.6 709.9 13.8 9.5 76.7 0.1238

Original spectra recorded before sputtering; spectra registered after sputtering are in bold.

mode with a pass energy of 50 eV and non-monochro-matised MgK radiation as excitation source. The bind-a

ing energy scale was referenced to the main componentof the C 1s peak at 284.6 eV arising from a mixture ofadventitious and pure carbon. The sputtering conditionswere the same for all samples: Ar ions at a pressureq

of 5=10 mbar and 3.0-keV voltage for 3 min. Fory6

quantification, the XPS spectra were subjected to back-ground subtraction(Shirley background), and sensitivityfactors supplied by the instrument manufacturer wereused.

2.4. Microhardness

A Fischercope H100VP XY-PROG microindenter wasused to measure the microhardnessw9–11x. Sampleswere placed on a computer-controlledX–Y table, whichallows control of the indentation co-ordinates. The

equipment software allows control of all the relevantindentation parameters, such as the depth of indentation,the final load and the number of steps for loading andunloading. A Vickers indenter was used and three finalloads were tested: 2, 5 and 100 mN. For each samplemore than 10 indentations were made to obtain astatistical average.

2.5. Friction and wear

Friction and wear measurements were made employ-ing a FALEX 320 PC ball-on-disk tribometer, with ahumidity controller unit. This equipment is also con-trolled by the PC software, which displays the evolutionof the friction coefficient. The humidity controller allowstests to be carried out at any chosen humidity valuebetween 5 and 95%.The friction coefficient against bearing steel was

determined at different humidity atmospheres(15%,50% and 85% RH). The parameters chosen were: 100g of applied load; hardened 100Cr6 steel balls of 12.5mm in diameter; and a linear speed of approximately0.07 mys.In order to determine the wear coefficientw12x, after

ball-on-disc tests the volume lost in the wear track wasmeasured using a WYCO RST 500 optical profilometer.This equipment measures the surface topography withresolution better than 3 nm. The conditions employedin the ball-on-disk test were 100 g of load and WCballs of 12.5 mm in diameter.

632 R. Sanchez et al. / Surface and Coatings Technology 158 –159 (2002) 630–635´

Fig. 1. Fe 2p peaks for the carbon–nitrogen–carbon implantations onHSS, before and after sputtering.

Fig. 2. N 1s peak for the nitrogen–carbon implantation on HSS, beforeand after sputtering.

3. Results and discussion

The XPS study (Table 2) on the HSS samplesconfirms the implantation of high doses of carbon andnitrogen, which in some cases generated a homogeneousoverlayer of carbon. These carbon overlayers are clearlyevident in the samples without a final nitrogen implan-tation (carbon implantation, and nitrogenqcarbonimplantation). In the case of nitrogen implantation fol-lowed by carbon implantation, the N 1s photoelectronpeak is practically negligible before and after sputtering.Oxygen(532.3–532.6 eV) is always present as a minorsurface contaminant, being easily removed after someminutes of sputter etching. In samples for which nitrogenwas implanted in the final step, the resulting carbonnitride overlayers were very thin(carbon plus nitrogen)or practically absent(nitrogen plus carbon plus nitro-gen). For these samples the N 1s core level peak couldbe detected, while no surface nitrogen appeared for

those samples in which carbon was implanted in the laststep. For the HSS NCN-implanted samples the Fe signal(709.9 eV) is associated with nitride formation, as canbe observed in Fig. 1a,b, while for CN samples thenitrogen is bonded to the surface carbon(Fig. 2a,b).For samples grown on stainless steel, the same behav-

iour as for those grown on HSS was found(Fig. 3).There is a moderately thick carbon overlayer for thelast-step carbon-implanted samples, while for the last-step nitrogen-implanted samples this layer is thin ornon-existent, probably due to some sputtering effectcaused by nitrogen species during implantation. It ispossible to distinguish a small shoulder in the N 1s XPSpeak in the samples implanted with carbon plus nitrogen.This is evidence of C–N bonds on the surface, ashappened for homogeneous samples grown on silicon.Microhardness tests carried out at 2 mN on HSS

implanted samples show, in all cases, lower hardnessthan for the unimplanted samples, as shown in Fig. 4a.The series of samples with an implantation sequencefinishing with carbon show higher hardness than thoseending with nitrogen. The correlative microhardness

633R. Sanchez et al. / Surface and Coatings Technology 158 –159 (2002) 630–635´

Fig. 3. N 1s peak for the nitrogen–carbon implantation on stainless steel, before and after sputtering.

Table 3Surface mechanical properties of the samples

Sample Universal hardness(MPa) Roughness Friction coefficient Wear rate

2 mN 5 mN 100 mN R (nm)a 15% RH 50% RH 85% RH (m yN)2

Stainless steelReference 3100"250 2830"130 2120"80 13.2"2.4 0.70–0.90 0.70–0.90 0.60–0.80 1.34=10y13

C 6500"400 5090"320 2300"120 18.9"5.2 0.26 0.20 0.20 4.47=10y16

CN 1250"90 1540"170 2320"160 25.2"4.3 0.22–0.48 0.45–0.52 0.45–0.50 6.27=10y14

NC 5250"770 3300"290 2070"120 21.7"6.1 0.22 0.16 0.20 4.82=10y16

NCN 3590"590 4280"420 2140"110 23.2"3.9 0.54 0.50–0.55 0.38–0.50 1.61=10y15

High-speed steelReference 10000"760 8970"690 9280"490 29.8"3.9 1.10 0.42–0.48 0.10–0.24 4.66=10y15

C 9400"1400 8350"790 8550"590 33.9"4.7 0.25–0.34 0.20–0.24 0.20–0.26 6.20=10y16

CN 990"180 1660"320 5140"330 36.2"7.1 0.62 0.50–0.60 0.48–0.60 1.33=10y15

NC 5366"650 6800"700 8330"240 35.3"14.0 0.25 0.30–0.24 0.24 6.52=10y16

NCN 3400"1400 2410"210 6170"200 32.2"4.2 0.37 0.37–0.48 0.30–0.38 1.38=10y15

Fig. 4. Load–unload curves at 2 mN of final load for the different implantations on HSS(above) and stainless steel(below).

order of the HSS samples can be summarised as follows:CN (;1.000 Nymm )-NCN (;3.400 Nymm )-NC2 2

(;5.300 Nymm )-C (;9.400 mm)-reference2 2

(;10.000 Nymm ).2

The microhardness results obtained at 2 mN on

stainless steel samples(Fig. 4b) are as follows: CN(;1.200 Nymm )-reference(;3.000 Nymm )-NCN2 2

(;3.500 Nymm )-NC (;5.300 Nymm )-C2 2

(;6.500 mm).2

As shown in Fig. 5a–c and Fig. 6a–c, results of the

634 R. Sanchez et al. / Surface and Coatings Technology 158 –159 (2002) 630–635´

Fig. 5. (a–c) Friction coefficient at different levels of relative humid-ity for the HSS implanted samples.

Fig. 6. (a–c) Friction coefficient at different levels of relative humid-ity for the stainless steel implanted samples.

friction tests carried out show a large decrease in thefriction coefficients for implanted compared to unim-planted samples, mainly in the case of C- and NC-implanted samples. The influence of humidity on thefriction coefficients for the implanted layers is nearlynegligible. On the stainless steel reference samples,humidity slightly affects the friction coefficient(as thehumidity increases, the friction coefficient decreases).

However, humidity has an enormous influence on thefriction coefficient of the HSS reference, decreasingfrom 1.10 at 15% RH to 0.25 at 85% RH.The wear tests results, summarised in Table 3, indicate

large decreases in wear coefficients for all the implantedsamples, which implies a reduction in the volume lostin ball-on-disk tests. This reduction is particularly high

635R. Sanchez et al. / Surface and Coatings Technology 158 –159 (2002) 630–635´

for the C and NC samples. The implantation of N intoq

carbon-implanted layers increases the wear coefficients.

4. Conclusions

Different sequences of high-dose implantation(C ,q

C qN , N qC and N qC qN ) were appliedq q q q q q q2 2 2 2

to HSS and AISI 304N samples.Microhardness tests showed that the final hardness is

highly influenced by the implantation order. The hardestsamples were those with a single carbon implantation,followed by nitrogen plus carbon implantation, whilethe nitrogen implantation produced softer surfaces.For the samples where carbon was implanted in the

last step the following behaviour was found:

● Friction tests showed the lowest friction values, witha friction coefficient below 0.2–0.3.

● The decrease in the wear coefficient with referenceto the unimplanted samples was approximately threeorders of magnitude.

● A graphitic carbon overlayer was formed. In thislayer, nitrogen remains associated with the carbon,but this nitrogen can also diffuse through the carbonoverlayer, forming bonds with the iron.

Acknowledgments

The authors would like to acknowledge the supportreceived from CICYT through Project MAT 98-1733CE,and from European Commission through Project BRITEBE97-4845

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