Improvements of tribological properties of CrNiMo and CrCoMo alloys by nitrogen plasma immersion ion implantation

Improvements of tribological properties of CrNiMo and CrCoMo alloys by nitrogen plasma immersion ion implantation

Surface & Coatings Technology 200 (2005) 594 – 597 www.elsevier.com/locate/surfcoat Improvements of tribological properties of CrNiMo and CrCoMo allo...

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Surface & Coatings Technology 200 (2005) 594 – 597 www.elsevier.com/locate/surfcoat

Improvements of tribological properties of CrNiMo and CrCoMo alloys by nitrogen plasma immersion ion implantation M. Uedaa,T, L.A. Bernia, R.M. Castroa, H. Reutherb, C.M. Lepienskic, P.C. Soares Jrc a

Associated Laboratory of Plasma, National Institute for Space Research, San Jose dos Campos, S. Paulo, Brazil b Institute of Ion Beam Physics and Materials Research, Center Rossendorf, Dresden, Germany c Department of Physics, Federal University of Parana´, Parana´, Brazil Available online 8 March 2005

Abstract Alloys made of CrCoMo and CrNiMo are commonly used materials with UHMWPE (Ultra High Molecular Weight Polyethylene) as joint couple components in the orthopedic prosthesis. We have applied the plasma immersion ion implantation (PIII) to the samples made of these alloys to enhance their tribological properties and hence to make it possible to extend considerably their lifetime as joint components when implanted in humans. As a result, we obtained CrCoMo surface with 70% improvement in hardness and 10% in modulus of elasticity and CrNiMo surface with 250% improvement in hardness and practically no change in modulus. Peak nitrogen concentrations as high as 40% and 30% were achieved by nitrogen PIII for CrCoMo and CrNiMo, respectively. Formation of gN phases in CrNiMo sample was clearly seen by XRD. D 2005 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Tribological properties; Prosthesis

1. Introduction The market requirement for high performance prosthesis components has been one of the major motivation to carry out research on ion implantation of knee and hip joint biomedical components, mainly with nitrogen ions. Despite some possible drawbacks concerning the long term systemic effects of cobalt and chromium release in the body, CrCoMo alloy continues to be one of the preferred material for joint couple with UHMWPE (Ultra High Molecular Weight Polyethylene) acetabular. Also CrNiMo steel alloy is a commonly used material with UHMWPE as a joint couple component in the orthopedic industries, despite similar problems concerning Cr, Ni or Mo release. We have applied the plasma immersion ion implantation (PIII) technique [1] to the samples made of these materials to enhance their tribological properties and hence to make it possible to extend considerably their lifetime as joint components implanted in humans. T Corresponding author. E-mail address: [email protected] (M. Ueda). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.01.072

PIII has been under development since the late 80’s throughout the world, to allow a widespread use of surface modification by high energy ion bombardment which is restricted to small and planar targets if ion beams are used for that purpose [2]. PIII is quite suitable for the enhancement of tribological properties of three dimensionally shaped components. Considering the fact that the biomedical components as knee joints and femoral heads have three dimensional shapes, it seems naturally appropriate to use the PIII processing for the improvement of the performance of those parts.

2. PIII and surface analysis equipments The experimental system used for PIII treatment of samples made of CrCoMo and CrNiMo alloys is described elsewhere [3,4]. It has been improved recently with respect to its vacuum pumping system which was exchanged from a diffusion to a turbo-molecular pump. A new hard tube pulser RUP-5 replaced the older RUP-4, and now we have available a maximum power of 10 kW

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SPUTTERING TIME (s) Fig. 1. AES profiles of the elemental concentrations for CrCoMo sample treated by PIII.

with pulses of up to 60 kV implantation voltage, up to 2.5 kHz frequency and 300 As maximum duration, making it possible to produce faster, cleaner, higher voltage treatments compared to the previous years. Gas injections can be controlled, either by needle valve or gas flow meters, and in some cases combined injection of different gases (as H2 and N2) can be performed with desired mixing rates. The 100 l stainless steel chamber’s volume can be extended by 30% more and it is water refrigerated at the outside wall to avoid excessive heating by secondary electron bombardment. The samples used presented the following composition:1) CrCoMo: 1.44% Si; 32.47% Cr; 62.75% Co; 3.34% Mo, and 2) CrNiMo: 0.66% Si; 20.50% Cr; 2.04% Mn; 61.75% Fe; 13.61% Ni; 1.44% Mo. Results are in atomic percent and was determined by EDX technique. For the surface analysis of the treated samples, we used: a) For AES (Auger Electron Spectroscopy) analysis, an equipment made by FISONS Instruments Surface Science, model MICROLAB 310-F; b) For hardness and elastic modulus profiles, a triboindenter from HYSITRON Incorporated. X-ray data were taken with a Philips 3410 diffractometer in the standard 2u mode and with Ni-filtered CuKa radiation source.

3. Experimental results and discussion For CrCoMo sample, the AES data from Fig. 1 show that a nitrogen concentration percentage superior to 40% was

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The temperatures of the samples during the PIII treatment were continuously monitored by an infrared pyrometer from RAYTEC. The condition of PIII treatments for the samples of CrCoMo and CrNiMo alloys for the present experiments were typically: 12 kV peak voltage, 1.5 kHz repetition frequency, 60 As pulse duration, total processing time of 6 h, using nitrogen + hydrogen glow discharge plasma with p=210 3 mbar. A sample temperature of about 380 8C was measured. The exposed surface of CrCoMo and CrNiMo samples with 1.5 cm diameter and 3 mm thickness was polished down to a mirror finish, cleaned in acetone bath and subjected to argon PIII at 5 kV for cleaning and then to nitrogen PIII.

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M. Ueda et al. / Surface & Coatings Technology 200 (2005) 594–597

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Fig. 3. (a) Hardness profiles of CrCoMo samples with and without PIII treatment. (b) Hardness profiles of CrNiMo samples with and without PIII treatment.

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other elements were measured up to 600 nm when the AES was aborted because of the time limit of the operator. We should emphasize that the oxygen contamination is restricted to depths smaller than 30 nm in this case. We also obtained substantial increase in hardness for samples of CrCoMo and CrNiMo treated by PIII. In Fig. 3(a) and (b) it is possible to see the results on hardness profile of CrCoMo and CrNiMo, respectively. In the case of CrCoMo sample, the hardness increased by 70% near 400 nm while for CrNiMo, it increased by 250% at the same depth. Notice that the polishing process of the surface of these materials itself was responsible for a substantial increase of the hardness near the surface. However, the change in hardness for the implanted samples was undoubtedly much larger. The irradiated sample had a behavior

achieved. The oxygen contamination was restricted to the layers immediately below the surface while the nitrogen penetration was much deeper. Unfortunately, we could not get the exact nitrogen implantation profile with depth because of the large roughness on the sample surface. The measurements are stopped around 6000 2 (or at an equivalent sputtering time of 6000 s of the Ar beam). In Fig. 2 we show the AES result for the CrNiMo sample implanted with nitrogen under the PIII conditions cited above. Here, there was also an excellent implantation and diffusion of nitrogen. CrNiMo sample temperature of orders of 380 8C was measured on the sample support, during the referred treatment. The peak nitrogen concentration reached 30% and it decreased smoothly to about 20% in depths around 500 nm. The implantation profiles of nitrogen and

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2θ (degrees) Fig. 4. (a) X-ray diffraction of untreated and 6h PIII treated CrCoMo samples. (b) X-ray diffraction of the untreated and 6h treated CrNiMo samples.

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similar to a hard film over a soft substrate. In this case, for depths higher than the thickness, the hardness is due to a composition of hardness of the film with hardness of the substrate. The hardness at some depth does not correspond to the depth of that specific region but a composed effect of harness from surface to that penetration depth. The modulus of elasticity for CrCoMo, also measured with the triboindenter, showed an increase of about 20% while for CrNiMo no perceptible change was seen (not shown here). Next, we show the results from X-ray diffraction for these two materials used in the prosthesis components. In Fig. 4(a) it is shown the XRD result for CrCoMo samples, one without any treatment and another with 6 h nitrogen + hydrogen PIII treatment. The PIII processing conditions were the same as ones cited above. From this result we can see that the treatment changed the intensity ratios of the Bragg reflections at 478 and at 518, both present in the XRD from standard CrCoMo alloy sample. We could speculate that the reflection at 478 has been broadened somehow also but no new peaks are seen from the treated sample. In contrast with this case, we obtained a completely new phase for the PIII treated sample of CrNiMo, as can be seen in Fig. 4(b). The new, broadened peak near 428 and one at around 478 are typical of gN phase. This is the expanded austenite phase commonly seen in other austenitic steels treated by low pressure plasma nitriding or nitrogen PIII [5–7]. The intense peaks of normal Bragg diffractions from the untreated CrNiMo are now much reduced and of the same order of the ones from the new gN phase. This result corroborates the ones obtained by AES and nanoindentation which showed significant enrichment of nitrogen and increase of hardness in PIII treated CrNiMo samples.

4. Conclusion Two metal alloys, CrCoMo and CrNiMo, used commonly for femoral heads in orthopedic prosthesis were treated by nitrogen + hydrogen PIII, for the improvement of their tribological properties. The major objective here was to

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reduce the wear in the surface of these materials to reach longer lifetime of the heads, in vivo. In fact, improvements in hardness were obtained for both materials but it was most significant in CrNiMo samples, reaching 250%. This increase was due to the formation of the gN phase in this steel, as a result of injection of a high concentration of nitrogen by implantation and thermal diffusion [8]. We also obtained CrCoMo sample surface with 70% improvement in hardness and 10% in modulus of elasticity after nitrogen implantation, both measured by nanoindentation probing. For CrNiMo samples, we observed no increase in modulus, after the ion implantation. Depth profiles of implanted samples by Auger Electron Spectroscopy (AES) indicated peak nitrogen concentrations of as high as 40% and 30% for CrCoMo and CrNiMo alloys, respectively. Measurements of wear in PIII treated CrCoMo and CrNiMo (not presented in this paper) showed excellent wear behavior [9].

Acknowledgements This work is supported by FAPESP and CNPq, Brazil.

References [1] Andre´ Anders (Ed.), Handbook of Plasma Immersion Ion Implantation and Deposition, 1st edition, J. Wiley & Sons, Inc., Toronto, 2000. [2] J.R. Conrad, J.R. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [3] M. Ueda, L.A. Berni, R.M. Castro, Surf. Coat. Technol. (in press). [4] M. Ueda, L.A. Berni, R.M. Castro, et al., Surf. Coat. Technol. 156 (2002) 71. [5] M. Ueda, G.F. Gomes, E. Abramof, H. Reuther, Nucl. Instrum. Methods B 206 (2003) 749. [6] X.B. Tian, Z.M. Zeng, T. Zhang, B.Y. Tang, P.K. Chu, Thin Solid Films 366 (2000) 150. [7] B. Larish, U. Brusky, H.-J. Spies, Surf. Coat. Technol. 116–119 (1999) 205. [8] W. Moller, S. Parascandola, O. Kruse, R. Gunzel, E. Richter, Surf. Coat. Technol. 116–119 (1999) 1. [9] M. Ueda, et al., J. Appl. Phys. submitted for publication.