Wear Behavior of Plasma Oxidized CoCrMo Alloy under Dry and Simulated Body Fluid Conditions

Journal of Bionic Engineering 11 (2014) 303–310

Wear Behavior of Plasma Oxidized CoCrMo Alloy under Dry and Simulated Body Fluid Conditions Ayhan Çelik1, Mevra Aslan1, Ali Fatih Yetim2, Özgü Bayrak1 1. Department of Mechanical Engineering, Engineering Faculty, Atatürk University, Erzurum 25240, Turkey 2. Department of Mechanical Engineering, Engineering and Architecture Faculty, Erzurum Technical University, Erzurum 25700, Turkey

Abstract In this study, CoCrMo alloy was oxidized in plasma environment at the temperatures of 600 ˚C to 800 ˚C for 1 h to 5 h with 100% O2 gas and its tribological behavior was investigated. After the plasma oxidizing process, the compound and diffusion layers were formed on the surface. XRD results show that Cr2O3, α-Co and ε-Co phases diffracted from the modified layers after plasma oxidizing. The untreated and treated CoCrMo samples were subjected to wear tests both in dry and simulated body fluid conditions, and normal loads of 2 N and 10 N were used. For the sliding wear test, alumina balls were used as counter materials. It was observed that the wear resistance of CoCrMo alloy was increased after the plasma oxidizing process. The lowest wear rate was obtained from the samples that were oxidized at 800 ˚C for 5 h. It was detected that both wear environment and load have significant effects on the wear behavior of this alloy, and the wear resistance of oxidized CoCrMo alloy is higher when oxide-based counterface is used. The wear rates of both untreated and plasma oxidized samples increase under high loads. Keywords: plasma oxidation, wear, CoCrMo, simulated body fluid, implant material Copyright © 2014, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(14)60035-4

1 Introduction Implant materials are natural or synthetic materials, which are used to perform the functions of living tissues in the human body, and they are in continuous contact with body fluids[1]. Therefore, these materials, especially the ones used in hard tissues, should have desirable mechanical strength, biocompatibility and structural biostability in physiologic environments[2–4]. CoCrMo alloys are one of the most important implant materials for orthopedic applications and generally used in load bearing implants due to the superior mechanical properties. Although they have very high corrosion and fatigue resistance, their tribological properties are not sufficient for the parts that work in contact, which restricts their lifetimes and the areas of usage. Due to the fact that tribological properties are related to surface features, the wear and friction properties of CoCrMo alloy have been tried to be improved using different surface treatments[5–8]. Thermochemical surface treatments belong to the most versatile processes of alloys Corresponding author: Ali Fatih Yetim E-mail: [email protected]

and that allow the improvement of the performance of components with respect to wear, fatigue and corrosion[9,10]. For instance, CoCrMo alloy was plasma nitrided at different process temperatures, time periods, and the effect of process parameters on the surface hardness and wear rate of the alloy was examined. The wear resistances of nitrided samples were increased with the increasing temperature and process time[11]. Tsai et al.[12] observed that with the plasma oxidizing treatment on the Si substrates, the wear resistance was increased and the corrosion resistance was improved at the high temperatures. Accordingly, plasma oxidizing process can improve the wear resistance of alloy without degrading the corrosion resistance. The implant materials must withstand the body environment. For this reason, simulated body fluids are usually used in the experiments for testing implant materials[13]. The Co-Cr alloy has been used for making the stems of prosthesis for heavily loaded joints, as wear resistance is very important for these alloys. The cobalt alloys have relatively poor wear resistance and to

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resolve this, the surface treatments have been applied to improve the wear resistance as well as the hardness of the surface[14]. After the plasma diffusion treatments, the composed layers formed on the surface cause lower adhesive contact and therefore, wear resistance was increased[15]. An important point in applying the surface treatments is not to impact the corrosion resistance in a negative manner. The thermal treatments applied to CoCrMo alloys may change the microstructure of the alloy. Therefore, the mechanical and electrochemical properties of the alloy may also change[16]. Dong and Shen[17] studied the effects of boronizing thermochemical surface treatment of CoCrMo alloy. They found that boronized CoCrMo alloy showed superior oxidation resistance. Alsaran et al.[18] analyzed wear resistance of different oxidation processes (anodic oxidation, thermal oxidation and plasma oxidation) applied to implant materials, and they found that after all the oxidation processes the wear resistances of oxidized samples were better than that of the untreated samples. But, there few reports in the literature about plasma oxidization of CoCrMo alloys. The main purpose of this study is to improve the tribological properties of CoCrMo alloy that are considered insufficient, without degrading the corrosion resistance of the alloy. For this purpose, CoCrMo alloy has been oxidized at different treatment conditions. Then, the changes in wear and friction behaviors in terms of oxidizing parameters (temperatures and times), wear environments (both in dry and simulated body fluid conditions) and applied normal loads were studied. The structural and mechanical properties of specimens were investigated using scanning electron microscopy, X-ray diffraction, microhardness tester and pin-on-disc tribotester.

2 Experimental details The specimens used in the experiments were medical grade forged CoCrMo alloy (ISO 5832-12, ASTM F1537), 16 mm in diameter and 5 mm in thickness, with a chemical composition (wt%) of 27% Cr, 6% Mo, 0.62% Mn, 0.67% Si, 0.22% Ni, 0.37% Fe, 0.057% C and balance Co. The specimens were grinded by 220 mesh to 1200 mesh emery paper and then polished with 1 µm grain size alumina powder. After polishing, the specimens were cleaned with alcohol. For plasma oxidizing, the specimens were placed as cathode into

plasma oxidizing chamber which was evacuated to 2.5 Pa. The temperature was monitored by a thermocouple connected to the specimen through the cathode. Prior to the process, the specimens were cleaned to remove surface contaminations by hydrogen sputtering for 15 min under a voltage of 500 V and a pressure of 5×102 Pa. Then, plasma oxidizing was performed in gas mixture of 100% O2 at 600 ˚C and 800 ˚C for 1 h to 5 h. After the plasma oxidizing, metallographic and microhardness measurements were taken on the oxidized specimens. The 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 15 s. Rigaku X-Ray diffractometer operated at 30 kV and 30 mA with Cu-Kα radiation was used for phase analysis. The modified diffusion layer thickness was also investigated using a JEOL6400 scanning electron microscopy. Surface roughness values of the untreated and treated samples were measured by using Mahr PRN-10 profilometer. Wear experiments were performed using pin-ondisc tribo-tester with 6 mm diameter alumina (Al2O3) ball as the pin. Experiments were carried out in both dry and Simulated Body Fluid (SBF) environments using normal loads of 2 N and 10 N. In the wear tests with SBF, solution suggested by Kokubo and Tamada[19] was used and its content was given in Table 1. The friction force was monitored continuously by means of a force transducer. Unlubricated wear tests with a sliding distance of 141 m were carried out at room temperature and a relative humidity of about 50%. In order to calculate the wear volume, the profiles were recorded before and after the wear experiments by using Mahr PRN-10 profilometer. The wear rates were evaluated by measuring the surface profiles across the sliding track. The scanning electron microscope was used to examine the worn surfaces. Table 1 SBF content used in wear tests[16] Ion Na+ K+

Ion consantration in SBF (mM) 142.0 5.0

Mg2+

1.5

Ca2+

2.5

Cl−

147.8

HCO3−

4.2

HPO42−

1.0

SO42−

0.5

pH

7.4

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4 Results and discussion 3.1 Characterization of the modified layers The XRD patterns of untreated and plasma oxidized CoCrMo alloys at the different process conditions are shown in Fig. 1. It was seen that the untreated CoCrMo alloy consists of mainly fcc-α and a small amount of hcp-ε cobalt phases. It was determined that the peaks of α phase were shifted to higher angles after plasma oxidizing. The amount of shifting increased with the process time and temperature. This data showed that some of the oxygen atoms were located in interstitial positions of the fcc-α lattice and they distorted the lattice. After the oxidizing process, it was observed that the chromium oxide phases were dominant in the microstructure of specimens because of the fact that chromium atoms have higher affinity to oxygen atoms than to cobalt atoms. At low temperature (600 ˚C), the required activation energy of atoms to form oxide is less than that of at high temperature. For that reason, the amount and intensity of oxide phases increased at the specimens that were treated at 800 ˚C. In addition, it was observed that intensity of ε phases at low process temperature increased with the process time, but at high process temperature ε phase completely disappeared. The SEM micrographs of the untreated and plasma oxidized specimens are shown in Fig. 2. It was observed that the surface roughness increased with increasing treatment temperature and time after the plasma oxidizing process. For treated samples, the lowest surface roughness value was measured from treated samples at 600 ˚C for 1 h, and the highest surface roughness value was measured from the treated samples at 800 ˚C for 5 h. The cause of the increase in the surface roughness was thought to be related to the Cr2O3 phase which grew in columnar form on the base material surface. The cross-sectional SEM images of the plasma oxidized samples are shown in Fig. 3. A continuous and evident compound layer formation was observed on the surface. It can be observed that the microstructure of diffusion region is rather distinct from the substrate. After the oxidizing process, while the lowest surface layer thickness was measured from the oxidized samples at 600 ˚C for 1h, the highest surface layer thickness was obtained from the oxidized samples at 800 ˚C for 5 h. As the process time and temperature increased, compound and diffusion layer thicknesses also increased with in-

Fig. 1 XRD results of untreated and treated CoCrMo alloy at different process conditions.

Fig. 2 Surface morphology of CoCrMo alloy. (a) Untreated; (b) 600 ˚C, 1 h; (c) 800 ˚C, 5 h.

Fig. 3 The cross-section SEM micrographs of plasma oxidized CoCrMo alloy. (a) 600 ˚C, 1 h; (b) 600 ˚C, 5 h; (c) 800˚C, 1 h; (d) 800 ˚C, 5 h.

creasing diffusion process (Table 2). It was seen that the oxide film that formed on the surface, was considerably intense, and was columnar structured and separated by an explicit line from the diffusion layer (Fig. 3). It was determined that the grains in the diffusion layer are smaller when compared to main alloy grains. A typical microstructure image of CoCrMo alloy was observed beneath the diffusion layer. The shear traces, which were

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formed with plastic deformation, were encountered because the used CoCrMo alloy was forged. The surface hardness values measured from the untreated and plasma oxidized samples are given in Table 2. The microhardness of untreated CoCrMo alloy was measured as approximately 440 HV0.1, the highest hardness value was measured from the samples oxidized at 800 ˚C for 5 h as 1650 HV0.1. After oxidation, it was believed that the surface hardness was increased due to the diffusion layer and Cr2O3 phase formed on the surface. It was determined that as the process time and temperature increased, the microhardness values was increased, because thicker oxide layer and diffusion layer were formed on the surface with increasing diffusion. Additionally, oxide phase intensity was increased when the oxidation process performed at high temperatures and longer times that contributed to the increase of the hardness values. The interstitial oxygen atoms which were diffused in fcc α-Co structure caused distortion on the crystal lattice of α-Co phase and revealing of a stress region strength increase. This stress field may interact with the dislocations and this situation may prevent dislocation movement. Thus, the microhardness values increased[20]. Oxygen atoms can reach deeper from the surface because the plasma oxidizing process is a diffusion process and diffusion ability of oxygen atom increases at the higher process temperature and time. The microhardness values decreased gradually towards the base material because of the formation of diffusion layer.

Table 2 Surface hardness, layer thickness and surface roughness values of untreated and plasma oxidized CoCrMo alloy at different process parameters Process Parameters

Surface Compound Diffusion hardness (HV0.1) Layer (µm) layer (µm)

Mean surface roughness (µm)

Untreated

400 to 440

–

–

0.10 to 0.12

600 ˚C, 1 h

625 to 650

2 to 3

25

0.32 to 0.35

600 ˚C, 5 h

800 to 825

5 to 6

30

0.36 to 0.40

800 ˚C, 1 h

1425 to 1450

8 to 10

35

0.50 to 0.55

800 ˚C, 5 h

1600 to 1650

12 to 15

50

0.75 to 0.80

Table 3 Tribological test results Process parameters

Mean friction coefficient Dry

Wear rate (×10−6 mm3·Nm−1)

SBF

Dry

SBF

2N

10 N 2 N

10 N

2N

10 N

2N

10 N

Untreated

0.39

0.45 0.53 0.39

1.75

3.96

2.25

4.75

600 ˚C, 1 h

0.41

0.35 0.45 0.42

1.65

3.71

2.00

4.61

600 ˚C, 5 h

0.36

0.32 0.47 0.52

1.13

2.92

1.79

4.02

800 ˚C, 1 h

0.43

0.38 0.48 0.41

1.16

2.85

1.52

3.76

800 ˚C, 5 h

0.58

0.51 0.51 0.55

0.62

2.41

0.92

2.98

3.2 Tribological properties The results obtained after wear tests of untreated and plasma oxidized CoCrMo alloy, are given in Table 3. The wear tests performed both in dry condition and SBF solution. The friction coefficients versus time are shown in Fig. 4 and Fig. 5, respectively. In dry conditions (Fig. 4), a run-in-period behavior was seen for both untreated and plasma oxidized samples. It was seen that the friction coefficient reached to maximum at the early stages of the sliding and then decreased and stabilized after about 100 s. In Fig. 4, it was

Fig. 4 Friction coefficient vs. time under dry conditions.

Çelik et al.: Wear Behavior of Plasma Oxidized CoCrMo Alloy under Dry and Simulated Body Fluid Conditions

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Fig. 5 Friction coefficient vs. time under SBF conditions.

seen that the friction coefficient value was about 0.48 under lower normal load (2 N) for the untreated sample. In the case of oxidized samples, it is possible to say that the layer thickness and surface roughness were thought to be the most effective factors on the friction coefficient values. At the normal load of 2 N, the lowest friction coefficient value was measured from the samples oxidized at 600 ˚C for 5 h, the samples oxidized at 800 ˚C for 5 h showed the highest friction coefficient values. Despite the fact that the surface roughness at the low process times and temperatures was lower than that at the high process times and temperatures. It was observed that the friction coefficient values commonly increased with process temperature. On the other hand, the lowest friction coefficient was obtained from plasma oxidized samples at 600 ˚C for 5 h and the highest friction coefficient was obtained from plasma oxidized samples at 800 ˚C for 5 h for the samples worn in dry condition and at the normal load of 10 N. It can be said that the increase of normal load caused slight reduction on friction coefficient when the effect of normal loads on friction coefficient are compared. Friction coefficient versus time of the wear tests in SBF condition is shown in Fig. 5. Contrary to the dry test results, there was no run-in-period behavior under SBF condition. Friction coefficient values rose from zero to about 0.55 after 60 s and then stabilized for the rest of the sliding. In the case of wear tests in SBF solution, it can be said that the aqueous media induced higher friction coefficient values than the dry conditions. This behavior was attributed to that SBF solution may hinder

the reaction of further oxidation[21]. It was observed that the friction coefficient curves under load of 10 N were more stable than load of 2 N. It was thought that the wear debris produced during the sliding caused higher and unstable friction behavior at the lower normal load. With application of increasing normal load, oxide layer occasionally broke down and caused abrasive effect on the relatively thin oxide film (600 ˚C, 1 h) formed samples. Therefore, a sudden increase in the friction coefficient was observed. Fig. 6 and Fig. 7 represent wear rates of untreated and plasma oxidized samples under dry and SBF conditions. The plasma oxidation treatment increased the wear resistance of CoCrMo alloy with increasing process time and temperatures. The lowest wear rate obtained from plasma oxidized specimens at 800 ˚C for 5 h under both dry and SBF conditions. The most important reasons for the increase in the wear resistance in oxidized samples were hard Cr2O3 phase formed in the compound layer. As the oxidation time and temperature increased, intensity of Cr2O3 phase and surface microhardness increased and the wear rate decreased. Because alumina ball was used, oxide-oxide wear occurred and this reduced the possibility of adhesion between the pin and the substrate. On the other hand, it was observed that the wear rates increased with the increase of normal load. With the increasing wear load, fracture probability of oxide film on the surface was also increased. Increasing wear load on the specimens oxidized at lower process time and temperatures caused the breaking of the hard but relatively thin film and formed abrasive effect.

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In Fig. 8 and in Fig. 9, the SEM images of wear tracks of specimens are illustrated. In Figs. 8a and 9a, plate-like debris and excessive plastic deformation in the wear scars were obvious, which confirmed that the untreated CoCrMo alloy showed adhesive wear under both dry and SBF conditions. When the wear scars of untreated and oxidized specimen were compared, it can be observed that the width of wear scars extensively decreased after plasma oxidizing treatment and continued to decrease as the treatment time increased. Shallow wear scars were formed on plasma oxidized specimens at 800 ˚C for 5 h (Fig. 8d and Fig. 9d). The diffusion layer and hard Cr2O3 layer that were formed on the oxidized sample surface decreased the amount of plastic deformation, which caused smaller contact area. Thus, formation of micro-welding which caused adhesive wear in the contact point of pin and oxidized sample surface was prevented. As the process time and temperatures increased, the compound and diffusion layer thicknesses increased. 10 N

2N

2.5 2.0 1.5 1.0 0.5 60 060 1 080 5 080 1 05

0.0 Untreated

Untreated

60 060 1 080 5 080 1 05

3.0

60 060 1 080 5 080 1 05

Wear rate (1×10−6mm3·Nm−1)

3.5

60 060 1 080 5 080 1 05

Wear rate (1×10−6mm3·Nm−1)

4.0

Therefore, wear rates significantly decreased. As a sufficiently thick compound layer prevents the pin to reach to the substrate and that causes the wear to occur in the hard compound layer. Thus, the possibility of excessive adhesion and plastic deformation was reduced. Moreover, the diffusion layer thickness is important in terms of supporting the compound layer. As the compound layer thickness increased, load bearing capacity of the compound layer increased as well. On the contrary, sliding on the thin compound layer could break the layer and may cause the increment of abrasive effect and wear rate. Figs. 8b and 9b reveal that the thin oxide film on the surface was removed and pin reached to the substrate during sliding when the samples were plasma oxidized at 600 ˚C for 1 h, and were worn under normal load of 10 N. In addition, it was seen that the trace characteristics were similar to that of the untreated specimen. On the other hand, it was identified that the wear track produced under load of 2 N was quite narrow and superficial in depth compared to that produced under 10 N (Figs. 8c and 9c).

Oxidation parameters

Fig. 6 Wear rates under dry conditions.

Fig. 8 SEM micrograph of the wear tracks on the specimens under dry wear conditions. (a) Untreated (10 N); (b) 600˚C, 1h (10 N); (c) 600 ˚C, 1 h (2 N); (d) 800˚C, 5 h (10 N).

Fig. 7 Wear rates under SBF conditions.

Fig. 9 SEM micrograph of the wear tracks on the specimens under SBF conditions. (a) Untreated (10 N); (b) 600˚C, 1h (10 N); (c) 600 ˚C, 1 h (2 N); (d) 800˚C, 5 h (10 N).

Çelik et al.: Wear Behavior of Plasma Oxidized CoCrMo Alloy under Dry and Simulated Body Fluid Conditions

Some pits on the sample surface were also seen in Fig. 9 after wear tests performed in SBF. It may be due to corrosive effects of SBF environment.

xyapitate scaffold for tissue engineering. Journal of Bionic Engineering, 2008, 5, 1–8. [3]

Acknowledgements This research is part of the BAP (Ataturk University Scientific Research Projects) project supported by grant no. BAP2009/266. Authors would like to thank to Dr. Sinan Ingec for his precious contribution.

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4 Conclusions In this study, the tribological properties of plasma oxidized CoCrMo alloy under both in dry and SBF conditions were investigated and the following conclusions were drawn. ● Plasma oxidizing of CoCrMo alloy at different treatment temperatures and times can produce Cr2O3 phase. The untreated CoCrMo alloy consists of fcc-α and a small amount of hcp-ε cobalt parent phases. The ε phase intensity increases with the time for the treatments performed at 600 ˚C, and it almost disappears at high temperatures. ● The applied surface treatment can increase the surface microhardness of CoCrMo alloy. ● As the process time and temperature increase, the compound and diffusion layer thicknesses increase. ● The surface roughness of the alloy increase with the process temperature and time. The lowest friction coefficient is obtained on samples oxidized at the 600 ˚C for 5 h. That shows the layer thickness and surface roughness are the two most effective factors at the measured friction coefficient values. ● The relatively poor wear resistance of CoCrMo alloy increases with the applied plasma oxidizing process. The lowest wear rate is obtained from oxidized samples at 800 ˚C for 5 h. The wear environments used in the wear tests have a significant impact on the wear properties, and it is detected that the wear resistance of the oxidized CoCrMo alloy is high under dry wear conditions. The wear rates of both untreated and oxidized samples increase with increasing normal load in the wear tests.

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