Effects of plasma nitriding on mechanical and tribological properties of CoCrMo alloy

Effects of plasma nitriding on mechanical and tribological properties of CoCrMo alloy

Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 2433 – 2438 www.elsevier.com/locate/surfcoat Effects of plasma ni...

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Available online at www.sciencedirect.com

Surface & Coatings Technology 202 (2008) 2433 – 2438 www.elsevier.com/locate/surfcoat

Effects of plasma nitriding on mechanical and tribological properties of CoCrMo alloy A. Çelik, Ö. Bayrak, A. Alsaran, İ. Kaymaz, A.F. Yetim ⁎ Department of Mechanical Engineering, Ataturk University, 25240 Erzurum, Turkey Available online 25 August 2007

Abstract Surface treatments of orthopedic materials are commonly used to enhance mechanical and tribological properties. One of the well-known orthopedic materials, wrought CoCrMo alloy was nitrided at various temperatures and time periods at a gas mixture of 75%N2–25%Ar to achieve these enhancements. SEM, XRD, pin-on-disc tribotester, and microhardness tester have been used to examine the treated materials. Analyses confirm the formation of nitride layers and the significant effect of plasma nitriding on wear rate and surface hardness. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma nitriding; CoCrMo; Orthopedic material

1. Introduction The requirements for a biomaterial are excellent mechanical properties like tensile strength, fatigue strength etc, high corrosion and wear resistance and biocompatibility. Most common materials used as biomaterials are: metals, ceramics, polymers and composites [1]. Metallic orthopedic medical implants have been manufactured mainly from stainless steel, titanium alloys and cobaltbased alloys. One of the well-known cobalt-based metallic biomaterial is CoCrMo alloy. The two basic elements of the CoCr alloys form a solid solution of up to 65%Co. The molybdenum is added to produce finer grains which results in higher strengths after casting or forging. The chromium enhances corrosion resistance as well as solid solution strengthening of the alloy [2]. The majority of orthopedic implants made of CoCrMo are castings of this alloy. However, castings may have some handicaps such as coarse grain size, dendritic structure, casting defects and lower tensile and fatigue strength than forged alloys. These drawbacks can be avoided by manufacturing medical implants using forged CoCrMo alloys. Potential toxicity of Co and Cr ions is a cause of concern. Therefore, applying surface treatments to CoCrMo alloys can be useful to minimize this effect. Since plasma assisted thermo ⁎ Corresponding author. Tel.: +90 442 2314860; fax: +90 4422360957. E-mail address: [email protected] (A.F. Yetim). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.08.030

chemical surface treatments improve wear, corrosion resistance and fatigue strength [3], plasma nitriding of forged CoCrMo alloy would be beneficial. Even though there is not much study about plasma nitriding of CoCrMo alloy, performed studies showed that nitriding improved the mechanical properties of CoCrMo alloy [4–6]. It is also known that this alloy is available with either carbide strengthening or nitrogen solid-solution strengthening [7]. The aim of this study is to determine the nitriding behavior of forged CoCrMo alloy. To achieve this goal, plasma nitriding treatments have been applied at various temperatures and time periods. Structural and tribological properties have been investigated using SEM, XRD, microhardness and pin-on-disc tribotester. 2. Experimental details Medical grade forged CoCrMo alloy (ISO 5832-12, ASTM F1537COND.A) has been used in the experiments with a chemical composition of 27%Cr, 6%Mo, 0.62%Mn, 0.67%Si, 0.22%Ni. 0.37%Fe, 0.057%C and balance Co (wt.%). The specimens were cut from cylindrical bars with diameters of 16 mm and thickness of 5 mm. The specimens were grinded by 220–1200 mesh emery papers, and then electropolished at a voltage of 5 V in a solution of 150 ml water, 10 ml HCl and 20 ml H2O2 at room temperature. After cleaning with alcohol, the specimens were placed into a holder which is the cathode of

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Fig. 1. XRD results of forged CoCrMo alloy after plasma nitriding at 600 °C for 1, 2 and 4 h.

plasma nitriding chamber and the chamber was evacuated to 2.5 Pa. The temperature was monitored by a thermocouple connected to the specimen through the cathode. Prior to the process, to remove surface contaminations, the specimens were subjected to cleaning by hydrogen sputtering for 30 min under a voltage of 500 V and a pressure of 5x102 Pa. Then, the plasma nitriding was performed in gas mixture of 75%N2–25%Ar, with a constant pressure of 5 × 102 Pa and process temperatures of 600–800 °C and process times of 1–4 h. Treatments were

conducted using direct current and voltages between 400 and 480 V depending on desired temperature. Ar gas has been used in order to reach high temperatures since auxiliary heating hasn’t been applied. After the nitriding process, the metallographic and microhardness examinations conducted from the cross sections of the specimens. Surface hardness and modified layer thickness were measured by using a Buehler Omnimet-MHT1600-4980T instrument at a constant load of 100 g and a loading time of

Fig. 2. XRD results of forged CoCrMo alloy after plasma nitriding at 600, 700 and 800 °C for 4 h.

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15 s. XRD-Rigaku operated at 30 kV and 30 mA with CuKα radiation was used for XRD analysis. The modified diffusion layer thickness was also investigated using SEM-Jeol6400. The morphology of the modified layer was examined using molecular imaging picoscan AFM-STM system. The dry wear tests were carried out on Teer-POD-2 pin-ondisc tester, using a 5 mm diameter WC-Co ball as the pin. The friction force was monitored continuously by means of a force transducer. The dry wear tests with a sliding distance of 141 m were carried out at room temperature (≈ 18 °C) and a relative humidity of about 50%, a sliding speed of 0.078 ms− 1, normal load of 3 N and a wear track diameter of 7 mm. To calculate the wear volume and rate, the profiles were recorded before and after the wear tests by a profilometer-Mitutuyo. Then, from the superimposed profiles, the wear volume and rate were calculated. The worn regions after the wear tests were examined using SEM.

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layer was measured about 30 μm and the thickness of the second layer was measured about 10 μm. A coarse grained structure can be seen beneath these layers. Nitrogen atoms take interstitial places among the parent α structure and form surface layers by expending the lattice. The possibility of CrN formation increases with increasing treatment temperature and therefore these layers dissolve. While SEM image of the specimen treated at 600 °C clearly shows two different layers, a

3. Results and discussion 3.1. XRD analysis The XRD patterns of the untreated specimen and other specimens treated with different parameters are shown in Figs. 1, 2. The untreated specimen consists of mainly fcc-α (some sources refer this phase as γ) and a small amount of hcp-ɛ cobalt parent phases. Distinctive phases of the treated specimens which are CrN, Cr2N and σ, depend on the treatment parameters. It is observed that after nitriding treatment, ɛ phase disappears and σ-CrCo phase occurs at the nearly same angle. Formation of a few oxide phases can be also observed at the XRD results. Since our plasma nitriding system is not designed for ultra vacuum, oxygen may originate from several sources such as the residual vacuum, sputtering from surface of the specimen, gas leaks or out-gassing. As can be seen in Fig. 1, σ-CrCo phase forms around 47.1 degree at the treatment temperature of 600 °C and the amount of this phase increases with treatment time at this temperature. On the other hand, σ phase disappears with increasing treatment temperature. Lanning and Wei observed σ, α, ɛ and Cr2N phases but not Co2N. They also observed that amount of Cr2N phase increases at higher temperatures. They attributed the formation of σ phase to higher concentration of Co or higher temperature [8]. However, formation of σ phase is essentially related to Co/Cr ratio since σ is a chromium rich phase. It has been observed that the intensities of Cr2N phase increase as the time and temperature increase. This may due to decomposition of Cr atoms from σ-CrCo phase, thus Cr atoms may bond with N atoms and form Cr2N. In addition to these, Co2N formation at the specimens treated at 600 °C should be noted. 3.2. Microstructure and morphology The cross sectional SEM micrographs of the plasma nitrided specimens are given in Fig. 3. A double layered structure formed at lower temperature. It is believed that upper layer is Co2N and the one below is σ phase. The thickness of the first

Fig. 3. SEM micrograph of the plasma nitrided specimens for 4 h. (a) 600 °C (b) 700 °C (c) 800 °C.

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Fig. 4. Surface morphology of forged CoCrMo alloy. (a) untreated (b) plasma nitrided at 700 °C-1 h.

fine grained and widened CrN layer (like a diffusion layer) occurs instead of a new layer at 700 °C. In addition to these, a layer which is integrated with the substrate material can be observed at 800 °C. AFM images of the untreated and treated specimens are given in Fig. 4. It is observed that the nitriding treatment causes a rougher surface. Surface roughnesses of the untreated specimens were about 0.10 μm. Surface roughnesses of the as-treated specimens were measured between 0.23 and 0.45 μm. It has been also observed that surface roughness increases with

increasing treatment time and temperature due to ion bombardment. 3.3. Microhardness The change of microhardness values of the surface layers with treatment temperature and time is given in Fig. 5. The plasma nitrided specimens have higher surface hardness values in respect of the untreated ones. The microhardness of the untreated specimens was measured as 400–440HV0.1. All the

Fig. 5. Surface hardness results of forged CoCrMo alloy after plasma nitriding.

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Fig. 6. Wear rate and friction coefficient results of forged CoCrMo alloy after plasma nitriding.

treated specimens show high hardness values in the modified layer, and then the hardness decrease to the substrate values. However, the hardness values descent with increasing displacement into the substrate. After the treatment, the minimum and the maximum hardness values have been measured as 800–

830HV0.1 (at 600 °C-1 h) and 2000–2050HV0.1 (at 800 °C-4 h), respectively. The microhardness of the surface increased 2–5 times as depending on the process parameters. The microhardness value of the plasma nitrided surface depends on the thickness of the modified layer. It is known that, at the plasma

Fig. 7. SEM micrographs of wear tracks. (a) untreated (b) plasma nitrided at 600 °C-1 h.

Fig. 8. SEM micrographs of wear tracks. (a) plasma nitrided at 700 °C-2 h (b) plasma nitrided at 800 °C-4 h.

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nitriding processes, as the treatment time increases, the thickness of case depth increases [9]. It should be noted that even an hour of treatment at 600 °C almost doubles the microhardness. Along with that, the microhardness values of the specimen treated at 600 °C-4 h remains unchanged for the 10 and 20 μm measurements which indicate that the layer shown in Fig. 3 (30 μm) is homogenous.

specimen nitrided at 800 °C. In addition, the surface of the specimen nitrided at 800 °C appears like etched. It was observed that the canals occurred in the wear track of the specimen nitrided at 600 °C-1 h, because the layer on the surface of this specimen does not have enough thickness. These kinds of valleys occur as a result of debris removed from the surface during the wear test.

3.4. Friction and wear

4. Conclusions

According to the nitriding parameters, the relation between wear rate and friction coefficient of untreated and as-treated specimens is given in Fig. 6. The wear rate decreased compared to the untreated specimen after nitriding. While the specimens nitrided at 600 °C have lower friction coefficient values, the specimens nitrided at 700 °C and 800 °C have higher friction coefficient values than the untreated specimen. A significant variation was not observed in the friction coefficient value of the specimens nitrided at 600 °C and 800 °C for different treatment times. On the other hand, the friction coefficient values of the specimens nitrided at 700 °C showed an increase with increasing treatment time. It is observed that friction coefficient values of nitrided specimens stay constant between 0.5 and 0.6 until the nitride/oxide surface layer breaks down during the test as in the case of some specimens. If this breakage happens, friction coefficient values leap up. As seen in Fig. 6, as the treatment temperature increased, the wear rate clearly decreased for every treatment times. Anomalous wear rates for some specimens are related to the oxide layer formed on the surface. Chromium rich oxide film formed on the surface is a continuous layer and if its thickness, intensity and adhesion are well enough, it causes an increase at the wear resistance of the specimens. For example, because nitride/oxide layer thickness is thin for the specimen nitrided for 1 h, the pin easily reaches to the substrate. The specimens nitrided for 2 h have high wear rates because the decomposition of the layer during the wear test causes abrasive effect owing to poor adhesion of the layer. In the case of treatment time of 4 h, the wear resistance of the alloy increased, because layer has an enough thickness and a good adhesion. In Figs. 7, 8, the wear tracks of the treated at different process parameters and the untreated specimens are given. The wear mechanism is mostly adhesive but it may transform to abrasive if nitride/oxide layer breaks down and creates debris. It was observed that the widths of the wear tracks evidently decreased after the nitriding treatment. While the canals occur in the wear track of the untreated specimens, it is seen that a transfer film occurs for the nitrided specimens. The amount of abrasive debris is low and ploughing occurs near the wear track for the

CoCrMo alloy orthopedic material has been nitrided using plasma nitriding process. A double layered structure has been observed at the treatment temperature of 600 °C but a diffusion layer like structure has been observed at the treatment temperatures of 700 °C and 800 °C. Surface roughness, microhardness values and wear resistance increased with increasing treatment time and temperatures. Friction coefficient values decreased for the specimen treated at 600 °C, but increased for the others. Breakage of nitride/oxide layers changed the wear mechanism to abrasive from adhesive for some specimens and caused either higher wear rates or higher friction coefficients. The improvements are ascribed to nitrogen diffusion and hard nitride formation at the surface. CrN, Cr2N and a small amount of Cr2O3 phases observed for every process parameter. Along with those, Co2N and σ-CrCo phases detected at the specimens treated at 600 °C. Acknowledgement This research is part of the TUBITAK (The Scientific and Technical Research Council of Turkey) project supported by grant no. 106M066. References [1] J.B. Park, The biomedical engineering handbook, in: Joseph D. Bronzino (Ed.), Section IV — Biomaterials, 2nd Ed, CRC Press LLC, Florida, 2000. [2] J.B. Park, Y.K. Kim, The biomedical engineering handbook, in: Joseph D. Bronzino (Ed.), 37.3 — CoCr Alloys, 2nd Ed, CRC Press LLC, Florida, 2000. [3] A. Alsaran, I. Kaymaz, A. Çelik, F. Yetim, M. Karakan, Surf. Coat. Technol. 186 (2004) 333. [4] R. Wei, T. Brooker, C. Rincon, J. Arps, Surf. Coat. Technol. 186 (2004) 305. [5] O. Öztürk, U. Türkan, A.E. Eroğlu, Surf. Coat. Technol. 200 (2006) 5687. [6] J.M. Williams, L. Riester, R. Pandey, A.W. Eberhart, Surf. Coat. Technol. 88 (1996) 132. [7] P.J. Andersen, ASM Metal Handbook, ASM International, vol. 13, 1994, p. 1680. [8] B.R. Lanning, R. Wei, Surf. Coat. Technol. 186 (2004) 314. [9] A. Alsaran, A. Çelik, C. Çelik, Surf. Coat. Technol. 160 (2002) 219.