Effect of Ringer s Solution on Wear and Friction of Stainless Steel 316L after Plasma Electrolytic Nitrocarburising at Low Voltages

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1 J. Mater. Sci. Technol., 2011, 27(10), Effect of Ringer s Solution on Wear and Friction of Stainless Steel 316L after Plasma Electrolytic Nitrocarburising at Low Voltages N. Afsar Kazerooni, M.E. Bahrololoom, M.H. Shariat, F. Mahzoon and T. Jozaghi Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran [Manuscript received January 3, 2011, in revised form July 28, 2011] A plasma electrolytic nitrocarburising (PEN/C) process was performed on stainless steel 316L to improve the surface properties for using as medical implants. A bath was optimised to reduce the required voltage to 150 volts. Aqueous urea-based solutions with 10% NH 4 Cl were prepared with slightly different amounts of Na 2 CO 3 to optimise the electrolyte composition. The surface and the cross-section morphologies were studied by scanning electron microscopy. The microstructure and the chemical composition of samples were investigated by X-ray diffraction (XRD) and energy dispersive X-ray (EDX) techniques. The microstructure of the outer layer of the coatings was found to be a complex oxide containing Cr and Fe. The wear properties of the samples were examined by using a pin on disk wear test with Ringer s solution and were compared with their wear properties in the ambient atmosphere. The Ringer s solution acted as a lubricant, reducing friction coefficient. Hardness and roughness were also studied. The bath with the composition of 10% NH 4 Cl and 3% Na 2 CO 3 exhibited the best tribological properties. The results showed that the tribological properties of treated samples were improved and the wear mechanism was abrasion of the pin. KEY WORDS: Plasma electrolytic nitrocarburising; Ringer s solution; Tribological properties 1. Introduction Plasma electrolytic nitriding/carburising (PEN/PEC) processes are electrochemical treatments which form a nitrogen and carbon rich layer with desirable mechanical properties and interfacial adhesion performance [1]. Significant studies of different plasma electrolytic techniques have been made by Yerokhin et al. [2] which included AC-pulse plasma electrolytic oxidation (PEO) on aluminium in order to form a thick, hard and adhesive surface layer, and also plasma electrolytic saturation (PES) using different modifications of carburising (PEC) and nitriding (PEN) to form corrosion resistant layers with good mechanical properties. Surface morphology, roughness, microstructure and different properties of steel Corresponding author. Prof.; Tel.: ; address: nazanin.afsar.k@gmail.com (N.A. Kazerooni). treated by using this method have been studied [3]. The electrolyte composition plays a crucial role in the type and content of alloying elements such as C, O, N, B, the applied voltage, the ultimate achieved temperature and also properties of the treated item [3]. Plasma electrolytic boronising has been carried out on stainless steels to improve surface hardness of them [4]. The effect of voltage on plasma electrolytic saturation (PES) has also been investigated [5]. Generally speaking, due to their mechanisms, plasma electrolytic techniques create an exceptional surface structure. By applying a gradually increasing voltage, the current density increases first, then as the continuous plasma envelope is formed around the cathode, the current density drops. This ionized gaseous medium creates electrical discharges. A very thin, corrosion-resistant layer is formed on steel in a short period of time, often in several minutes [2]. Stainless steel 316L is often used for the fabrication of orthopaedic implants.

2 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Table 1 Composition of stainless steel 316L C Cr Ni Mo Mn S Si P Fe < <2 <0.03 <1 <0.045 Bal. Some examples are: hip prosthesis, screws and fixation plates. This vast usage of stainless steel 316L is mainly due to its good resistance to uniform corrosion. Although, passive films form in stainless steel and the steel exhibits relatively high resistance to uniform corrosion, the passive films are highly susceptible to localised forms of corrosion. Thus, the application of stainless steel 316L, as a biomaterial, is limited by its pitting corrosion which is one of the most severe types of localised surface attacks on stainless steels. Some improvement in pitting resistance has been achieved by methods that have focused mainly on the removal of surface inclusions, on the modification of chemical properties and element distribution in the passive film, or on the increase of the Cr/Fe ratio in the film [6]. Surface modification technologies can be used to improve biocompatibility, enhance bone bonding, reduce wear and/or corrosion, etc. They also can reduce the wear and improve frictional behaviour of the surface while maintaining desirable bulk properties of the underlying substrate [7]. Friction and wear are the main problems in joint replacement and their role has been growing with the increase in the expected life of the prostheses, due to the effect of time in wear related phenomena [8]. Combination of wear and corrosion is a major problem for some orthopaedic implants. Different mechanisms responsible for wear/corrosion have been introduced by Dearnley and Aldrich-Smith [9]. The wear behaviour, hardness and roughness of stainless steel 316L can be improved by plasma electrolytic processes. Mahzoon et al. [10] had optimised the composition of a bath for plasma electrolytic nitrocarburising of stainless steel 316L in order to reduce the required voltage; they attempted this process at 180 V successfully with the ultimate temperature of 400 C. They subsequently studied tribological properties of their surface treated steels. Since stainless steel 316L is used for orthopaedic applications, it is interesting to know the effect of a physiological solution on the tribological properties of nitrocarburised stainless steel 316L. Ringer s solution is often used as a physiological solution for in vitro study of biomaterials. Met et al. [11] have analysed the friction behaviour of diamond-coated Ti 6Al 4V samples at room temperature in ambient air and also studied the effect of Ringer s solution on nano-smooth diamond coatings. This physiological solution has also been used by Younesi et. al. who studied hardness, wear and friction properties of nickel free stainless steel and hydroxyapatite biocomposites in air and in a Ringer s solution [12]. One of the most important parameters in plasma electrolytic processes is the applied voltage. Different applied voltages have been used in different studies, i.e V [1], V [2], 120 V [3], 600 V [4], V cathodic and V anodic [5], 400 V [13], 230 and 250 V [14] and 180 V [10] have been reported. The ultimate achieved temperatures have been reported to be C [1], 200 C and 400 C [10 13]. The purpose of the present investigation was to decrease the applied voltage and therefore the consumed energy by modifying the electrolyte composition without deteriorating tribological properties, and to study the effects of some influencing process parameters on the wear behaviour and friction properties of the surface treated 316L stainless steel in a Ringer s solution. 2. Experimental Samples, with dimensions of 20 mm 13 mm 1 mm, prepared from rolled 316L stainless steel sheets, with a composition shown in Table 1, were polished to clean surface contaminations and washed with distilled water and alcohol. After drying, they were used for surface treatment. Three aqueous urea-based solutions were prepared with slightly different compositions (Table 2) in a 2 container. A cylindrical shell of Table 2 Composition of eletrolyte, wt% Samples NH 4Cl Na 2CO stainless steel sheet with 15 cm in internal diameter was immersed in the container and was attached to the positive terminal of a DC power supply as the anode. Each sample, acting as the cathode, was immersed in the solution. The voltage was applied gradually, starting from the lowest possible value and rising gradually to 150 volts. This procedure resulted in a maximum current of 18 A at a voltage of 80 volts. As it was expected, gas liberation and ignitions occurred at the initial stages of raising the voltage. When the voltage reached 80 volts, a continuous plasma envelope was formed and the current was dropped to almost 5 A. By increasing the voltage to 150 volts, the plasma envelope became steady and began to glow with a deep blue colour. A k-type thermocouple was used to measure the temperature which was found to be 400 C. Each sample was treated for 10 min and afterwards it was quenched and water-washed. The surface morphology and the cross-section morphology were studied by scanning electron microscopy (Oxford S-360 model). The samples were mounted and their cross sections were polished and etched in marble or glycergia reagents. The microstructure and chemical

3 908 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Fig. 1 Surface morphologies of sample 1 (a), sample 2 (b), sample 3 (c), and the microstructures of the layer sections and EDX Line Scans of sample 1 (d), sample 2 (e), and sample 3 (f) composition of the samples were also studied by X-ray diffraction (XRD) and energy dispersive X-ray (EDX) techniques. The values of surface roughness (Ra) of samples were obtained by using a Mitutoyo surface roughness instrument. The hardness of the PEN/C layer of the samples was measured by using a microhardness testing instrument with a 10 g and 50 g load for surface and crosssection, respectively, in order to illustrate the variation of hardness vs distance from the surface of the samples. Ten hardness measurements were made for each sample. Wear tests were carried out by using a rotating pin-on-disk wear test instrument with a linear speed of 2 cm/s for 200 metres at 2.5 N force. The width of the pin tracks on samples was 7 mm and was against SAE pins. During the wear test process, a Ringer s solution (used as a similar-to-body environment) was continuously injected to the sample. A test was also carried out in the ambient atmosphere for comparison. Three wear tests were carried out for each specimen. The wear traces resulted after the test and the crosssections of samples, after being mounted, were studied by scanning electron microscopy (SEM). Finally, the friction coefficient factors vs time were recorded for each sample. 3. Results and Discussion 3.1 Surface morphology, XRD and EDX analyses Figure 1(a) to (c) show the surface morphologies and Fig. 1(d) to (f) illustrate the cross-section morphologies of the treated samples, with their respective EDX Line Scans. By adding an adequate amount of NH 4 Cl and Na 2 CO 3, a unique surface structure was formed which could be due to plasma formation and local melting of the metal surface and the subsequent quenching [2]. The excess amount of NH 4 Cl added to the electrolyte in comparison with the previous work [10], has caused a 30 volts decrease in the applied voltage. The outer layer of the coatings was found to be an oxide containing Cr and Fe (Fig. 2), with thickness values of 20 µm for sample 1, 16 µm for sample 2 and 24 µm for sample 3, as shown in Fig. 1. The XRD diagrams also show that the treated samples contained Fe 2 N and Fe 2 C, i.e. expanded austenite. The presence of iron oxide and chromium oxide caused a high surface hardness. Similar results have also been reported by Nie et al. [14]. The EDX analyses, presented in Fig. 3, illustrate that the major element in the surface layer was Cr. However, chromium nitride wasn t found in the surface layer. This might be due to the fact that the surface temperature was lower than 450 C which is the critical temperature for precipitation of chromium nitride. Rapid quenching of the sample after treatment prevented the diffused nitrogen and carbon to precipitate as nitride or carbide on grain boundaries, and preserved them as a solid solution [14]. In 316L stainless steel, the absence of chromium nitride improves the wear resistance [15].

4 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Fig. 2 XRD spectra (a) the untreated sample, (b) sample 1, (c) sample 2, and (d) sample 3 Fig. 3 EDX spectra (a) the untreated sample, (b) sample 1, (c) sample 2, and (d) sample Roughness Figure 4 shows the roughness values of the untreated and treated samples. The roughness values of the PEN/C treated samples were much higher (almost 15 times) than the untreated samples. The effect of sodium carbonate concentration on the roughness values, however, was negligible. Thus, plasma electrolytic processes can offer a significant improvement in surface properties while conventional methods for this purpose, e.g. grit blasting and chemical etching, are not environment friendly. In addition, the surface microcavities formed by plasma electrolytic treatment may act as a lubricant reservoir or as a trap to capture wear debris [16]. 3.3 Microhardness The hardness distribution vs distance from the surface (Fig. 5) indicates that the surface of each treated sample had the highest hardness. The measured hardness for the substrate was 186 HV which is not shown in Fig. 5. The high value of hardness for the treated sample 1 (783 HV) could be due to the

5 910 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Roughness Base metal Sample Fig. 4 Roughness values of the untreated and the treated samples Hardness / HV Sample 1 Sample 2 Sample Distance from edge / m Fig. 5 Microhardness distribution of the treated samples presence of a mixture of chromium oxides, ferrite and expanded austenite with diffused carbon and nitrogen, in the outer layer. This hypothesis was supported by the data obtained from EDX analysis and microscopic inspections. Malinova et al. [17] have studied the effects of nitrocarburising parameters on the microhardness profile of various steels using both experimental techniques and artificial neural network modelling. The increase in hardness as a result of PEN/C treatment has been reported previously by Mahzoon et al. [10]. A hardness of 1200 HK0.01 has also been reported on the surface compound layer of the PEN/C treated 316 stainless steel by Nie et al., which is due to the surface chromite/magnetite-containing compound layer [14]. 3.4 Wear Figures 6 and 7 illustrate the wear tracks of the surface treated samples tested in a Ringer s solution and in the atmosphere, respectively. These wear tracks indicate that the wear resistance of the nitrocarburised samples were much higher than the untreated stainless steels. Furthermore, the wear resistance of the samples tested in a Ringer s solution was higher than the samples tested in the atmosphere. Clearly, wear of the untreated samples occurred in an unlubricated harsh condition which created a rough surface with fine peeled-off fragments. This characterises an adhesive mode of wear which is very common amongst stainless steels [15]. However, for the treated samples, a different mechanism seems to be Fig. 6 Micrographs of the wear tracks of untreated sample (a), sample 1 (b), sample 2 (c), sample 3 (d), carried out in a Ringer s solution

6 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Fig. 7 Micrographs of the wear tracks of untreated sample (a), sample 1 (b), sample 2 (c), sample 3 (d), carried out in the atmosphere Fig. 8 Friction coefficients of untreated sample (a), sample 1 (b), sample 2 (c), sample 3 (d), carried out in a Ringer s solution at work. The mechanism is suggested to be neither due to plastic deformation nor adhesion [10], but is due to the abrasion of the pin. The products of corrosion are detached from the surface and cause abrasion. This detachment exposes the surface to the corrosive media which intensifies the corrosion [18]. It should be remembered that the pin, affected by the wear process, roughens and causes abrasive wear. Effect of Ringer s solution was to decrease wear, since it acted as a lubricant. In addition, the Ringer s solution provided a corrosive environment and consequently the worn out fragments were corrosion products with compositions different from the substrate and its coat- Fig. 9 Friction coefficients of untreated sample (a), sample 1 (b), sample 2 (c), sample 3 (d), carried out in the atmosphere ing. The untreated sample had a weight loss of about 10 3 grams, while the PEN/C treated samples had a weight gain of about 10 4 g. The fragments which were released by the wear test adhered on the surface, leading to a weight gain in Ringer s solution. 3.5 Friction The results of friction tests are presented in Figs. 8 and 9 for the samples tested in a Ringer s solution and in the atmosphere, respectively. Friction coefficients of the samples are shown in Fig. 10. These figures indicate that the PEN/C treatment caused a great de-

7 912 N. Afsar Kazerooni et al.: J. Mater. Sci. Technol., 2011, 27(10), Friction coefficient Base metal In Ringer's solution In atmosphere Sample in comparison with the tests carried out in the atmosphere. The PEN/C treatment in the optimised electrolyte has diminished the friction factor considerably, not only in comparison with the untreated stainless steel, but also in comparison with the previous works done on PEN/C process. In conclusion, given that the tribological properties have improved significantly, the PEN/C suggests a useful process for optimising the material for medical implant uses. Acknowledgement This research was financially supported by the Research Committee of Shiraz University under grant No. 88-GR-ENG-62, which is gratefully appreciated. Fig. 10 Histograms to show the mean values of friction coefficients of the surface treated and the untreated sample crease in the friction coefficient factor. This decrease in coefficient of friction as a result of PEN/C processing, has been reported before [10 14]. The Ringer s solution acted as a lubricant during the wear test, which would cause a more uniform load over the surface. The presence of the Ringer s solution also caused further decrease in the coefficient factor in comparison with the tests that were carried out in the normal atmosphere, and reached a minimum value of about 0.4 for sample 1 in the atmosphere and about 0.15 for sample 1 in the Ringer s solution. This was ten times smaller than the untreated sample. 4. Conclusion The extra addition of NH 4 Cl has further optimised the electrolyte and has caused further decrease in the applied voltage, from 180 to 150 volts, without decreasing the achieved local temperature. This decrease in the applied voltage can lead to an advantageous decrease in the energy consumption during the process. However, it must be noted that any more addition of NH 4 Cl to the bath, can cause adverse effects on the obtained properties. The PEN/C treatment has caused a considerable increase in surface hardness, and the bath with a composition of 3% Na 2 CO 3, amongst others, has provided the best surface hardness properties. The PEN/C process has also improved wear resistance by altering the wear mechanism and boosting surface hardness. In this work, a Ringer s solution was used as a similar-tobody environment. Application of the Ringer s solution results in a weight gain instead of weight loss, which means the fragments of the wear product has formed other compositions and has been attached to the work piece. Exploiting this behaviour in medical implants can be a valuable characteristic. The results of wear tests and friction coefficient measurements in the Ringer s solution points out a great improvement REFERENCES [1 ] A.L. Yerokhin, A. Leyland, C. Tsotsos, A.D. Wilson, X. Nie and A. Matthews: Surf. Coat. Technol., 2001, , [2 ] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews and S.J. Dowey: Surf. Coat. Technol., 1999, 122, 73. [3 ] P. Gupta, G. Tenhundfeld, E.O. Daigle and D. Ryabkov: Surf. Coat. Technol., 2007, 201, [4 ] M.A. Béjar and R. Henriquez: Mater. Design, 2009, 30, [5 ] M. Aliofkhazraei, A. Sabour Rouhaghdama, A. Heydarzadeh and H. Elmkhaha: Mater. Chem. Phys., 2009, 113, 607. [6 ] A. Shahryari, S. Omanovic and J.A. Szpunar: Mater. Sci. Eng. C, 2008, 28, 94. [7 ] H. Liang, B. Shi, A. Fairchild and T. Cale: Vacuum, 2004, 73, 317. [8 ] R. Pietrabissa, M. Raimondi and E.D. Martino: Med. Eng. Phys., 1998, 20, 199. [9 ] P.A. Dearnley and G. Aldrich-Smith: Wear, 2004, 256, 491. [10] F. Mahzoon, M.E. Bahrololoom and S. Javadpour: Surf. Eng., 2009, 25(8), 628. [11] C. Met, L. Vandenbulcke and M.C. Sainte Catherine: Wear, 2003, 255, [12] M. Younesi, M.E. Bahrololoom and H. Fooladfar: J. Mechanical Behav. Biomedical Mater., 2010, 3, 178. [13] M. Aliofkhazraei, C. Morillo, R. Miresmaeili and A. Sabour Rouhaghdam: Surf. Coat. Technol., 2008, 202, [14] X. Nie, C. Tsotsos, A. Wilson, A.L. Yerokhin, A. Leyland and A. Matthews: Surf. Coat. Technol., 2001, 139, 135. [15] G. Jiang, Q.A. Peng, C. Li, Y. Wang, J. Gao, S.Y. Chen, J. Wang and B.L. Shen: Surf. Coat. Technol., 2008, 202, [16] P. Gupta, G. Tenhundfeld, E.O. Daigle and D. Ryabkov: Surf. Coat. Technol., 2007, 201, [17] T. Malinova, S. Malinov and U.N. Pantev: Surf. Coat. Technol., 2001, 135, 258. [18] M.T. Wright, B. Stuart and M.D. Goodman: American Academy of Orthopedic Surgeons Symposium on Implant Wear in Total Joint Replacement: Clinical and Biologic Issues, Material and Design Considerations, Oakbrook, Illinois, USA, October 2000.