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Tribological and corrosion behavior of friction stir processed Ti-CaP nanocomposites in simulated body fluid solution
journal of the mechanical behavior of biomedical materials 20 (2013) 90 –97
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Research Paper
Tribological and corrosion behavior of friction stir processed Ti-CaP nanocomposites in simulated body fluid solution Hamidreza Farnousha,, Ashkan Abdi Bastamia, Ali Sadeghib, Jamshid Aghazadeh Mohandesia, Fathollah Moztarzadehb a
Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran Biomedical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran
b
ar t ic l e in f o
abs tra ct
Article history:
In the present study, friction stir processing was utilized to incorporate nano-
Received 21 September 2012
hydroxyapatite particles into Ti–6Al–4V substrates to fabricate Ti-CaP nanocomposite
Received in revised form
surface layer. Microstructures of the stir zone and the fabricated Ti-CaP nanocomposite
30 November 2012
layer were analyzed using optical and scanning electron microscopy, respectively. Micro-
Accepted 4 December 2012
hardness profile and AFM analysis of substrates were then studied. The microhardness of
Available online 20 December 2012
Ti-CaP nanocomposite layer was reached about 386 HV due to the grain refinement and the
Keywords:
distribution of nano-hydroxyapatite particles. Potentiodynamic polarization studies
Biomaterials
showed that the Ti-CaP nanocomposite layer protected effectively the Ti–6Al–4V substrates
Titanium alloys
from corroding in simulated body fluid solution. The tribological properties of the samples
Wear
were studied in both dry and simulated biological conditions. The wear rate and friction
Corrosion
coefficient decreased by friction stir processing on Ti–6Al–4V substrates. From the analysis
Friction stir processing
of plotted graphs of weight loss versus sliding distance, a correlation between wear coefficient and microhardness through thickness was established. The wear mechanisms were also investigated through scanning electron microscopy. It was shown that the major mechanism was abrasive wear. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Due to high strength-to-weight ratio, low Young modulus, resistance to corrosion and biocompatibility, Ti–6Al–4V (Ti64) alloys have been widely used as orthopedic implant materials (Geetha et al., 2009). However, the low surface hardness, high friction coefficient, and poor wear resistance of Ti64 alloys in the body fluid result in the implant loosening (Chen et al., 2012). Moreover, the issue of biocompatibility with respect to Corresponding author. Tel.: þ98 21 64542949; fax: þ98 21 64542941.
E-mail address: [email protected] (H. Farnoush). 1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.12.001
the release of aluminum and vanadium ions in Ti64 alloys and the possibility of its toxic effects has been concerned during the recent years (Bruni et al., 2005). Therefore, a great deal of interest is focused on improving wear and corrosion resistance as well as biocompatibility and hardness of the surface of the implants (Yildiz et al., 2009; Cvijovic´-Alagic et al., 2011; de Souza et al., 2011; Faria et al., 2011; Wang et al., 2012). For this purpose, different surface treatments have been applied such as electrophoretic deposition (Farnoush
journal of the mechanical behavior of biomedical materials 20 (2013) 90 –97
et al., 2012a, b, c), plasma spraying (Laonapakul et al., 2012; Yugeswaran et al., 2012), ion implantation (Byeli et al., 2012), laser processing (Balla et al., 2012; Nag and Banerjee, 2012), electric discharge machining (Harcuba et al., 2012), blasting (Pazos et al., 2010), thermal oxidation (Guleryuz and Cimenoglu, 2005, 2009), micro-arc oxidation (Cimenoglu et al., 2011; Vangolu et al., 2011) and wire brushing (Farnoush et al., 2012a). Friction stir processing (FSP) has been demonstrated as an effective technique for the fabrication of nanocomposites enhancing microstructural modification of the work piece (Mishra and Ma, 2005). In the FSP, a rotating tool was inserted into a single piece of material, for severe plastic deformation, material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogenization (Mishra and Ma, 2005). This technique has been successfully applied in the production of fine grained structures, surface composites, and microstructural modification of castings (Mishra and Ma, 2005). Since the mechanical, chemical, and physical properties such as wear resistance, corrosion resistance and biocompatibility of Ti64 substrates is improved by introducing a Ti-CaP nanocomposite surface layer, friction stir processing can be potentially applied as a surface modification treatment. To the authors’ best knowledge, there is no through work reporting the influence of FSP on the wear and corrosion properties of Ti-CaP nanocomposites. In the current study, an innovative technique was attempted to incorporate nano-hydroxyapatite particles into Ti–6Al–4V substrates to fabricate a Ti-CaP nanocomposite surface layer by using friction stir processing (FSP). The process is conducted completely in the solid state and is performed by plunging a rotating cylindrical tool into a work piece and traversing it. The fabricated nanocomposite layer was then characterized employing microhardness measurements, SEM, XRD, and AFM analyses. As a result of the FSP treatment, wear and corrosion properties of the samples in a bio-simulated environment were discussed. Potentiodynamic corrosion tests were carried out in the corrected-SBF solution. Finally, tribological experiments were performed by using of pin-on-disc tribotester in both dry and corrected-SBF solution at 37 1C.
2.
Materials and methods
Ti–6Al–4V mill-annealed plates and nano-sized HA powders (synthesized by precipitation method (Farnoush et al., 2012a)) were used as substrate and reinforcement particulates, respectively. The as-received substrates (AR) were 3-mm-thick millannealed Ti–6Al–4V plates and the quantometer method was used to determine chemical composition of Al¼ 6.28, V¼ 4.90, Fe¼0.29, Nb¼ 0.05, Mn¼ 0.03. Cr¼ 0.02, Si¼ 0.05, Sno0.05, Moo0.03, Cuo0.02, Zr¼ 0.01, and Ti balance (all in wt%). The whole surface of a 10 cm 2 cm work piece was subjected to friction stir processing (FSP) and the sheets were fixed along the rolling direction. The used tungsten carbide tool was 16 mm in diameter and the tilt angle was 31. The tool rotation was set to be 250 rpm, plunge depth was 1.5 mm and its advancing speed was 16 mm/min. Three grooves with 1 mm depth and 2 mm width were machined on the surface of work piece and separated from each other by a 2 mm distance. Subsequently, nano-sized HA powders were compacted into the grooves and
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FSP was carried out on the prepared sheets under an argon gas shrouding system to prevent the stir zone and tool from oxidizing. The peak temperature experienced during FSP was measured by using non-contact laser thermometer which indicated that the processing temperature was kept lower than 850–900 1C. In fact, FSP parameters (rotating and traversing speeds) should be chosen so that the maximum temperature does not exceed the b-transus temperature (937 1C) of Ti64 alloy in the stir zone (Atapour et al., 2010). Cross and lateral sections of the friction stir processed (FS) samples were mounted, and then mechanically polished, followed by etching in an etchant composed of 2.5% HNO3, 1.5% HF, and 96% distilled water. The phase composition of FS samples was analyzed by XRD (Philips PW 1480) in 2y¼20–1001 range at a step size of 0.021 and a count time of 0.6 s. The hardness profile was obtained using a scanning microhardness tester with a load of 100 g and a dwell time of 10 s. Five Vickers hardness indentations were performed on each specimen. Surface topography and roughness of AR and FS samples were characterized by atomic force microscopy (AFM, DME DS-95-50E) in contact mode. The micro-structural characterization and elemental composition of the FS samples were analyzed by using scanning electron microscope (SEM, Philips XL 30) equipped with an Energy Dispersive Spectrometer (EDS). To investigate electrochemical corrosion behavior of coatings on Ti–6Al–4V with and without FSP, samples were embedded in cold-curing epoxy resin, exposing a surface area of 1 cm2. Potentiodynamic polarization test in a corrected-SBF solution (Kokubo, 1991), open to air at 37 1C and at the physiological pH 7.40, was performed using AutoLab PGstat 30. All potentials were measured with respect to a saturated calomel electrode. Two parallel graphite rods served as the counter electrode for current measurement. From the polarization curve, the corrosion parameters were evaluated by Tafel extrapolation method by Nova v1.6 software. The friction and wear tests of AR and FS samples were carried out at room temperature in both dry and simulated biological conditions using a pin-on-disc tribotester. All wear experiments were conducted with 5.2 mm diameter high-chromium steel pins in sliding distance of 300 m. The wear parameters were selected as a load of 15 N, a constant sliding velocity of 7 cm/s and rotating radius of 10 mm on the disc samples. Wear tests were interrupted at certain intervals (50 m sliding) to determine the progress of wear. At each interval, samples were cleaned in acetone and weighed to an accuracy of 70.1 mg. Each wear test was repeated three times to evaluate the mean value of weight loss. The coefficient of friction between the pin and the disc specimen was determined by measuring the frictional force with a stress sensor. After the wear tests, the worn surfaces of the samples were examined by scanning electron microscopy.
3.
Results and discussion
3.1.
Characterization of FS Ti and FS Ti-CaP samples
The cross sectional view and surface microstructure of friction stir processed layer and base metal are illustrated in Fig. 1 which exhibits the grain refining within the stir zone. During a severe plastic deformation of Ti–6Al–4V alloy, the occurrence of dynamic recrystallization (DRX) is the most
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Fig. 1 – Optical micrographs of (a) cross-sectional view, surface microstructure of (b) stir-zone and (c) base-metal after three passes FSP.
equation for their analyses and studied the effects of the material variation through the thickness and the size of the functionally graded material (FGM) layer inserted between metal and ceramic layers using the finite element method. In our analysis, it is assumed that the microhardness variation of friction stir processed (FSPed) and Ti-CaP nanocomposite layer (H(t)) can be approximated by an idealized power-law equation when measured along the thickness (t): n t ð1Þ þ HB HðtÞ ¼ ðHS HB Þ 1 l
Fig. 2 – Vickers microhardness profile versus the distance from surface of FS Ti and FS Ti-CaP samples.
probable restoration mechanism (Guo et al., 2006). The severity of plastic deformation results in the formation of dislocation tangle zones close to the grain boundaries. Then, annihilation of dislocations leads to sub-grain formation and finally new dislocations-free grains are produced with high misorientations (Hoseini et al., 2012). On the other hand, the grain growth was limited effectively due to severe deformation and short time at elevated temperature in FSP zone. Fig. 2 presents the microhardness profiles of FS Ti and FS Ti-CaP samples versus the distance from surface. An approximately 33% increase in microhardness can be achieved in severe deformed layer. Besides, the thickness of the friction stir processed layer may reach to 160 mm. The grain refinement accompanied by a considerable increment in hardness is observed due to friction stir processing and the distribution of CaP particles along the mentioned depth. Cho and Oden (2000) showed that the material-property variations through the thickness could be hierarchically analyzed. They used power-law
where HS and HB are Vickers microhardnesses at surface and base-metal regions of FSPed samples, l is the thickness of FSPed layer and n is the constant indicating the hardening rate in stir zone. The curves in Fig. 2, show the variation of microhardness through thickness of FSPed layer which was successfully fitted by Eq. (1). The obtained values of n for FS Ti and FS Ti-CaP samples are 0.364 and 0.445, respectively. The higher value of n in FS Ti-CaP sample depicts the more hardening effect caused by introducing CaP particles during FSP. Fig. 3(a) shows SEM micrograph of nanocomposite surface layer fabricated by three passes FSP for FS Ti-CaP sample which indicates that CaP nanoparticles are distributed uniformly in the titanium matrix. Using Clemex image analyzing software, the mean particle size and the average area percentage of the nano-sized CaP particles in the Ti matrix were found to be70 nm and 11%, respectively (Fig. 3b). The EDS spectrum in Fig. 3(c) reveals quantitative results of the average element composition. The calcium to phosphorous atomic ratio (Ca:P¼ 1.21) indicates that the phase is likely to be between octacalcium phosphate (OCP, Ca8(HPO4)2(PO4)4.5H2O) with Ca:P ¼1.3 and dicalcium phosphate anhydrous (DCPA, CaHPO4) with Ca:P¼ 1. Actually, the FSP parameters determine temperature, processing time during severe plastic deformation and consequent partially transformation of HA phase into OCP and DCPA phases. Fig. 4(a) shows the surface topography of Ti-CaP nanocomposite layer. According to the AFM profile along Z axis in Fig. 4(b), the surface layer is composed of nanoparticles ranging from 40 to 70 nm which is in agreement with Fig. 3(b). In addition, the average surface roughness height obtained from roughness
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93
Fig. 3 – SEM micrograph (a) and EDS spectrum (b) for the surface of FS Ti-CaP sample.
Fig. 4 – (a) 3D AFM image and (b) surface topography profile for FS Ti-CaP sample along an arbitrary direction.
analysis was 79.7 nm. The characterization of the surface roughness is very important for biomedical application where an appropriate roughened surface is highly desirable due to its higher effective surface area and promoting tissue and bone ingrowth (Zheng et al., 2011). Fig. 5 illustrates XRD spectrum of Ti-CaP nanocomposite surface layer. The pattern reveals a typical fabricated Ti-CaP layer; no titanium oxide reflection was detected using XRD. Reflections related to HA (JCPDS card # 9-432) were indicative of
its presence in the fabricated composite layers. However, the number of HA reflections and their associated intensities were limited; these are attributed to its relatively low volume fraction in the composite layer. In the spectrum shown in Fig. 5, the peaks appeared at 2y¼ 25.91, 31.81, 32.11 and 32.91 are corresponded to (002), (211), (112) and (300) planes of hydroxyapatite crystal, respectively. Furthermore, the characteristic peaks of Ti (JCPDS card #44-1294) at 2y¼37.41 and 39.31 are apparently distinguishable which are assigned to (002) and (101) planes,
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journal of the mechanical behavior of biomedical materials 20 (2013) 90 –97
Fig. 5 – X-ray diffraction patterns of FS Ti-CaP sample.
respectively. The pattern reveals that the peaks corresponding to HA phase are much broadened which is due to its fine crystallites. The mean crystallite size (d) of HA phase was evaluated from the full-width at half maximum (FWHM) of diffraction peaks, using Scherrer equation (Cullity, 1977) d¼
kl Bs cosy
ð2Þ
where k is Scherrer constant (¼ 0.9), l is the wavelength (lCuKa ¼0.154 nm), y is the diffraction angle, and Bs is the sample broadening related to FWHM. The instrumental peak broadening (Bi) was removed by using an annealed Si powder according to Warren equation (Cullity, 1977) B2s ¼ B2e B2i
ð3Þ
where Be is the FWHM of measured XRD peaks. The mean crystallite size of HA phase was calculated to be 26 nm. The other processing techniques such as laser processing (Roy et al., 2008) which was applied for forming TCP-Ti surface composites are based on liquid phase processing at high temperatures. In this case, it is hard to avoid decomposition of HA into TCP, degradation of substrate, the interfacial reaction between reinforcement and metal-matrix and the formation of some detrimental phases. Moreover, decomposition of HA must be avoided since it results in enhanced in-vitro dissolution (Sridhar et al., 2003). Obviously, if the processing of Ti-CaP surface composite is carried out at temperatures below melting point of substrate, the problems mentioned above can be avoided.
3.2.
Corrosion behavior in simulated body fluid solution
The potentiodynamic polarization curves of the AR, FS Ti and FS Ti-CaP samples in the corrected-SBF solution are depicted in Fig. 6. The corrosion parameters, extracted from polarization curves using Tafel least square fitting method, are listed in Table 1. Additionally, the polarization resistance (Rp) is
Fig. 6 – Potentiodynamic polarization curves of AR, FS Ti and FS Ti-CaP samples in SBF at 37 1C.
estimated by the use of Stern–Geary equation ba bc Rp ¼ 1=Icorr 2:303ðba þ bcÞ (Stern and Geary, 1957). The results corresponding to FS Ti-CaP sample imply a significant decrease in the corrosion current density (Icorr) and corrosion rate, and an increase in the corrosion potential (Ecorr) and linear polarization resistance (Rp). By implementation of friction stir processing, the Ti-CaP nanocomposite layer acts as a barrier to the penetration/transport of chloride ions and water molecules through the coating, transport of ions the coating. In addition, the corrosion rate of FS TiCaP and FS Ti samples are lower as compared with the AR substrate which is caused by rapid formation of stable passive layer due to its fine grained microstructure (Balyanov et al., 2004; Kim et al., 2011; Raducanu et al., 2011; Argade et al., 2012). The reduction of grain size plays an important role in enhancing the activity of electrons at the grain boundaries. In this regard, the surface becomes more electrochemically reactive giving rise to an increase in passivation ability resulting in rapid formation of a mechanically strong and stable passive film (Balyanov et al., 2004; Kim et al., 2011; Argade et al., 2012).
3.3.
Wear behavior
The variation of friction coefficient with sliding distance for AR and FS Ti-CaP samples in dry ambient and SBF environment are presented in Fig. 7. The average friction coefficients of the FS TiCaP were found to be 0.5 and 0.3 for dry ambient and SBF environment, respectively which are smaller than those of the AR sample (i.e. 0.6 and 0.4). Differences in extent of localized plastic deformation at real contact areas may lead to the difference in friction coefficient, Since the FSP surfaces are harder, less plastic deformation and hence lower friction is
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Table 1 – Electrochemical corrosion parameters of AR, FS Ti and FS Ti-CaP samples. Sample
Ecorr (mV)
Icorr (mA/cm2)
ba (V/decade)
bc (V/decade)
Rp (O cm2)
Corrosion rate (mm/year) 102
AR FS Ti FS Ti-CaP
327 144 127
2.76 0.085 0.048
0.23 0.10 0.07
0.28 0.11 0.12
19,925 265,300 390,030
2.4 0.074 0.041
Fig. 7 – Friction coefficient versus sliding distance for FS Ti and FS Ti-CaP samples in (a) dry and (b) biological conditions.
Fig. 9 – Wear resistance (1/K) of AR, FS Ti and FS Ti-CaP samples estimated by Eq. (4).
Fig. 8 – Variation of weight loss of AR, FS Ti and FS Ti-CaP samples with sliding distance.
expected (Niinomi, 2008; Soleymani et al., 2012). Fig. 8 shows the relationship between weight loss and the sliding distance of AR, FS Ti and FS Ti-CaP samples in SBF condition. As seen in Fig. 8 for all samples, the wear weight loss increased with sliding distance, also FSP was found to be beneficial in improving wear resistance under applied load of 15 N. The amount of weight loss of AR sample (26.3 mg) is higher in comparison with FS and FS Ti-CaP samples (12.3 and 11.4 mg) after 300 m sliding. The wear behavior of a material has traditionally been related to hardness by Archard’s equation (Archard, 1953). Archard
described that the volume loss of a material due to wear is inversely proportional to its hardness and linearly proportional to the sliding distance and normal load. However, it makes no assumptions about the hardness variations through the thickness of sample. The experimental results of wear tests carried out in our laboratory are analyzed by the following equation that assess the mean wear coefficient (K) as a function of weight loss per sliding unit ((m/s)i) and microhardness through the thickness of FSPed samples (Hi(t)): K¼
N 1 1X m Hi ðtÞ with rF N i ¼ 1 s i
t¼
m rA
ð4Þ
where r is the density of Ti–6Al–4V alloy (¼ 4.506 g/cm3), F is the applied normal load (¼ 15 N), N (¼ 6) is the times of measurement for each 50 m sliding distance, A is the area of worn track and Hi(t) can be estimated from Eq. (1). The wear resistance (1/K)
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Fig. 10 – SEM micrographs of worn surface morphologies of (a) AR, (b) FS Ti and (c) FS Ti-CaP samples.
is of fundamental importance and provides a valuable parameter of comparison for the severity of the wear process in various tribologic systems. Fig. 9 illustrates the wear resistance of AR, FS Ti and FS Ti-CaP samples. The wear resistance of FSPed samples is increased approximately three times in respect of the AR sample. It is necessary to use the scanning electron microscope to identify the main acting wear mechanisms. The SEM micrographs presented in Fig. 10 show the typical worn surface morphologies of the specimens tested at the same wear conditions. On the worn surfaces of all samples, evidences of abrasive wear can be detected. Continuous sliding marks with plastically deformed grooves and ridges are seen on the wear tracks. However, the extent of plastic deformation or ‘‘plowing’’ is found to be smaller in FSPed samples. The penetration depth depends on the relative hardness of the abrasive with respect to the specimen surface hardness. As the surface hardness of the FS Ti and FS Ti-CaP samples are higher than that of AR Ti (Fig. 2) it is expected that the depth of penetration due to the abrasive wear in the FSPed samples is less. Thus, FSPed samples exhibits significantly higher wear resistance as compared to AR Ti sample (Fig. 9). Long and Rack (1997) reported that adhesive wear is also the principal mechanism of dry wear of Ti–6Al–4V alloy articulated against hardened steel. In the present study, the contribution of adhesive wear to total wear is found to be much smaller. However, the smaller traces of local material removal imply that the severity of adhesion is significantly lower in this alloy.
4.
Conclusions
In the present study, friction stir processing was performed to fabricate Ti-CaP nanocomposites. The FSP was performed at a constant tool rotation rate of 250 rpm, travel speed of 16 mm/ min and plunge depth of 1.2 mm with a tool tilt angle of 31. The microhardness of FS Ti-CaP sample was reached about 386 HV
due to the grain refinement and the incorporation of CaP particles. The electrochemical corrosion behavior of the AR, FS Ti and FS Ti-CaP samples in SBF solution at 37 1C was investigated by means of potentiodynamic polarization test. The results imply that friction stir processing significantly decreases the corrosion current density and corrosion rate, and increases the corrosion potential and linear polarization resistance of FS Ti and FS Ti-CaP samples. According to tribological behavior in simulated biological condition, the wear resistance of FSPed samples is increased approximately three times in respect of the AR sample. From the analysis of weight loss versus sliding distance together with microhardness profile through the thickness, a correlation between wear resistance and hardness was established. The results of wear tests depicted that the depth of abrasive penetration was reduced and subsequently the wear resistance was increased for FSPed samples. According to scanning electron microscopy observations, it was shown that the major wear mechanism was abrasive wear in all samples.
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Research Paper
Tribological and corrosion behavior of friction stir processed Ti-CaP nanocomposites in simulated body fluid solution Hamidreza Farnousha,, Ashkan Abdi Bastamia, Ali Sadeghib, Jamshid Aghazadeh Mohandesia, Fathollah Moztarzadehb a
Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran Biomedical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413 Tehran, Iran
b
ar t ic l e in f o
abs tra ct
Article history:
In the present study, friction stir processing was utilized to incorporate nano-
Received 21 September 2012
hydroxyapatite particles into Ti–6Al–4V substrates to fabricate Ti-CaP nanocomposite
Received in revised form
surface layer. Microstructures of the stir zone and the fabricated Ti-CaP nanocomposite
30 November 2012
layer were analyzed using optical and scanning electron microscopy, respectively. Micro-
Accepted 4 December 2012
hardness profile and AFM analysis of substrates were then studied. The microhardness of
Available online 20 December 2012
Ti-CaP nanocomposite layer was reached about 386 HV due to the grain refinement and the
Keywords:
distribution of nano-hydroxyapatite particles. Potentiodynamic polarization studies
Biomaterials
showed that the Ti-CaP nanocomposite layer protected effectively the Ti–6Al–4V substrates
Titanium alloys
from corroding in simulated body fluid solution. The tribological properties of the samples
Wear
were studied in both dry and simulated biological conditions. The wear rate and friction
Corrosion
coefficient decreased by friction stir processing on Ti–6Al–4V substrates. From the analysis
Friction stir processing
of plotted graphs of weight loss versus sliding distance, a correlation between wear coefficient and microhardness through thickness was established. The wear mechanisms were also investigated through scanning electron microscopy. It was shown that the major mechanism was abrasive wear. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Due to high strength-to-weight ratio, low Young modulus, resistance to corrosion and biocompatibility, Ti–6Al–4V (Ti64) alloys have been widely used as orthopedic implant materials (Geetha et al., 2009). However, the low surface hardness, high friction coefficient, and poor wear resistance of Ti64 alloys in the body fluid result in the implant loosening (Chen et al., 2012). Moreover, the issue of biocompatibility with respect to Corresponding author. Tel.: þ98 21 64542949; fax: þ98 21 64542941.
E-mail address: [email protected] (H. Farnoush). 1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.12.001
the release of aluminum and vanadium ions in Ti64 alloys and the possibility of its toxic effects has been concerned during the recent years (Bruni et al., 2005). Therefore, a great deal of interest is focused on improving wear and corrosion resistance as well as biocompatibility and hardness of the surface of the implants (Yildiz et al., 2009; Cvijovic´-Alagic et al., 2011; de Souza et al., 2011; Faria et al., 2011; Wang et al., 2012). For this purpose, different surface treatments have been applied such as electrophoretic deposition (Farnoush
journal of the mechanical behavior of biomedical materials 20 (2013) 90 –97
et al., 2012a, b, c), plasma spraying (Laonapakul et al., 2012; Yugeswaran et al., 2012), ion implantation (Byeli et al., 2012), laser processing (Balla et al., 2012; Nag and Banerjee, 2012), electric discharge machining (Harcuba et al., 2012), blasting (Pazos et al., 2010), thermal oxidation (Guleryuz and Cimenoglu, 2005, 2009), micro-arc oxidation (Cimenoglu et al., 2011; Vangolu et al., 2011) and wire brushing (Farnoush et al., 2012a). Friction stir processing (FSP) has been demonstrated as an effective technique for the fabrication of nanocomposites enhancing microstructural modification of the work piece (Mishra and Ma, 2005). In the FSP, a rotating tool was inserted into a single piece of material, for severe plastic deformation, material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogenization (Mishra and Ma, 2005). This technique has been successfully applied in the production of fine grained structures, surface composites, and microstructural modification of castings (Mishra and Ma, 2005). Since the mechanical, chemical, and physical properties such as wear resistance, corrosion resistance and biocompatibility of Ti64 substrates is improved by introducing a Ti-CaP nanocomposite surface layer, friction stir processing can be potentially applied as a surface modification treatment. To the authors’ best knowledge, there is no through work reporting the influence of FSP on the wear and corrosion properties of Ti-CaP nanocomposites. In the current study, an innovative technique was attempted to incorporate nano-hydroxyapatite particles into Ti–6Al–4V substrates to fabricate a Ti-CaP nanocomposite surface layer by using friction stir processing (FSP). The process is conducted completely in the solid state and is performed by plunging a rotating cylindrical tool into a work piece and traversing it. The fabricated nanocomposite layer was then characterized employing microhardness measurements, SEM, XRD, and AFM analyses. As a result of the FSP treatment, wear and corrosion properties of the samples in a bio-simulated environment were discussed. Potentiodynamic corrosion tests were carried out in the corrected-SBF solution. Finally, tribological experiments were performed by using of pin-on-disc tribotester in both dry and corrected-SBF solution at 37 1C.
2.
Materials and methods
Ti–6Al–4V mill-annealed plates and nano-sized HA powders (synthesized by precipitation method (Farnoush et al., 2012a)) were used as substrate and reinforcement particulates, respectively. The as-received substrates (AR) were 3-mm-thick millannealed Ti–6Al–4V plates and the quantometer method was used to determine chemical composition of Al¼ 6.28, V¼ 4.90, Fe¼0.29, Nb¼ 0.05, Mn¼ 0.03. Cr¼ 0.02, Si¼ 0.05, Sno0.05, Moo0.03, Cuo0.02, Zr¼ 0.01, and Ti balance (all in wt%). The whole surface of a 10 cm 2 cm work piece was subjected to friction stir processing (FSP) and the sheets were fixed along the rolling direction. The used tungsten carbide tool was 16 mm in diameter and the tilt angle was 31. The tool rotation was set to be 250 rpm, plunge depth was 1.5 mm and its advancing speed was 16 mm/min. Three grooves with 1 mm depth and 2 mm width were machined on the surface of work piece and separated from each other by a 2 mm distance. Subsequently, nano-sized HA powders were compacted into the grooves and
91
FSP was carried out on the prepared sheets under an argon gas shrouding system to prevent the stir zone and tool from oxidizing. The peak temperature experienced during FSP was measured by using non-contact laser thermometer which indicated that the processing temperature was kept lower than 850–900 1C. In fact, FSP parameters (rotating and traversing speeds) should be chosen so that the maximum temperature does not exceed the b-transus temperature (937 1C) of Ti64 alloy in the stir zone (Atapour et al., 2010). Cross and lateral sections of the friction stir processed (FS) samples were mounted, and then mechanically polished, followed by etching in an etchant composed of 2.5% HNO3, 1.5% HF, and 96% distilled water. The phase composition of FS samples was analyzed by XRD (Philips PW 1480) in 2y¼20–1001 range at a step size of 0.021 and a count time of 0.6 s. The hardness profile was obtained using a scanning microhardness tester with a load of 100 g and a dwell time of 10 s. Five Vickers hardness indentations were performed on each specimen. Surface topography and roughness of AR and FS samples were characterized by atomic force microscopy (AFM, DME DS-95-50E) in contact mode. The micro-structural characterization and elemental composition of the FS samples were analyzed by using scanning electron microscope (SEM, Philips XL 30) equipped with an Energy Dispersive Spectrometer (EDS). To investigate electrochemical corrosion behavior of coatings on Ti–6Al–4V with and without FSP, samples were embedded in cold-curing epoxy resin, exposing a surface area of 1 cm2. Potentiodynamic polarization test in a corrected-SBF solution (Kokubo, 1991), open to air at 37 1C and at the physiological pH 7.40, was performed using AutoLab PGstat 30. All potentials were measured with respect to a saturated calomel electrode. Two parallel graphite rods served as the counter electrode for current measurement. From the polarization curve, the corrosion parameters were evaluated by Tafel extrapolation method by Nova v1.6 software. The friction and wear tests of AR and FS samples were carried out at room temperature in both dry and simulated biological conditions using a pin-on-disc tribotester. All wear experiments were conducted with 5.2 mm diameter high-chromium steel pins in sliding distance of 300 m. The wear parameters were selected as a load of 15 N, a constant sliding velocity of 7 cm/s and rotating radius of 10 mm on the disc samples. Wear tests were interrupted at certain intervals (50 m sliding) to determine the progress of wear. At each interval, samples were cleaned in acetone and weighed to an accuracy of 70.1 mg. Each wear test was repeated three times to evaluate the mean value of weight loss. The coefficient of friction between the pin and the disc specimen was determined by measuring the frictional force with a stress sensor. After the wear tests, the worn surfaces of the samples were examined by scanning electron microscopy.
3.
Results and discussion
3.1.
Characterization of FS Ti and FS Ti-CaP samples
The cross sectional view and surface microstructure of friction stir processed layer and base metal are illustrated in Fig. 1 which exhibits the grain refining within the stir zone. During a severe plastic deformation of Ti–6Al–4V alloy, the occurrence of dynamic recrystallization (DRX) is the most
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Fig. 1 – Optical micrographs of (a) cross-sectional view, surface microstructure of (b) stir-zone and (c) base-metal after three passes FSP.
equation for their analyses and studied the effects of the material variation through the thickness and the size of the functionally graded material (FGM) layer inserted between metal and ceramic layers using the finite element method. In our analysis, it is assumed that the microhardness variation of friction stir processed (FSPed) and Ti-CaP nanocomposite layer (H(t)) can be approximated by an idealized power-law equation when measured along the thickness (t): n t ð1Þ þ HB HðtÞ ¼ ðHS HB Þ 1 l
Fig. 2 – Vickers microhardness profile versus the distance from surface of FS Ti and FS Ti-CaP samples.
probable restoration mechanism (Guo et al., 2006). The severity of plastic deformation results in the formation of dislocation tangle zones close to the grain boundaries. Then, annihilation of dislocations leads to sub-grain formation and finally new dislocations-free grains are produced with high misorientations (Hoseini et al., 2012). On the other hand, the grain growth was limited effectively due to severe deformation and short time at elevated temperature in FSP zone. Fig. 2 presents the microhardness profiles of FS Ti and FS Ti-CaP samples versus the distance from surface. An approximately 33% increase in microhardness can be achieved in severe deformed layer. Besides, the thickness of the friction stir processed layer may reach to 160 mm. The grain refinement accompanied by a considerable increment in hardness is observed due to friction stir processing and the distribution of CaP particles along the mentioned depth. Cho and Oden (2000) showed that the material-property variations through the thickness could be hierarchically analyzed. They used power-law
where HS and HB are Vickers microhardnesses at surface and base-metal regions of FSPed samples, l is the thickness of FSPed layer and n is the constant indicating the hardening rate in stir zone. The curves in Fig. 2, show the variation of microhardness through thickness of FSPed layer which was successfully fitted by Eq. (1). The obtained values of n for FS Ti and FS Ti-CaP samples are 0.364 and 0.445, respectively. The higher value of n in FS Ti-CaP sample depicts the more hardening effect caused by introducing CaP particles during FSP. Fig. 3(a) shows SEM micrograph of nanocomposite surface layer fabricated by three passes FSP for FS Ti-CaP sample which indicates that CaP nanoparticles are distributed uniformly in the titanium matrix. Using Clemex image analyzing software, the mean particle size and the average area percentage of the nano-sized CaP particles in the Ti matrix were found to be70 nm and 11%, respectively (Fig. 3b). The EDS spectrum in Fig. 3(c) reveals quantitative results of the average element composition. The calcium to phosphorous atomic ratio (Ca:P¼ 1.21) indicates that the phase is likely to be between octacalcium phosphate (OCP, Ca8(HPO4)2(PO4)4.5H2O) with Ca:P ¼1.3 and dicalcium phosphate anhydrous (DCPA, CaHPO4) with Ca:P¼ 1. Actually, the FSP parameters determine temperature, processing time during severe plastic deformation and consequent partially transformation of HA phase into OCP and DCPA phases. Fig. 4(a) shows the surface topography of Ti-CaP nanocomposite layer. According to the AFM profile along Z axis in Fig. 4(b), the surface layer is composed of nanoparticles ranging from 40 to 70 nm which is in agreement with Fig. 3(b). In addition, the average surface roughness height obtained from roughness
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93
Fig. 3 – SEM micrograph (a) and EDS spectrum (b) for the surface of FS Ti-CaP sample.
Fig. 4 – (a) 3D AFM image and (b) surface topography profile for FS Ti-CaP sample along an arbitrary direction.
analysis was 79.7 nm. The characterization of the surface roughness is very important for biomedical application where an appropriate roughened surface is highly desirable due to its higher effective surface area and promoting tissue and bone ingrowth (Zheng et al., 2011). Fig. 5 illustrates XRD spectrum of Ti-CaP nanocomposite surface layer. The pattern reveals a typical fabricated Ti-CaP layer; no titanium oxide reflection was detected using XRD. Reflections related to HA (JCPDS card # 9-432) were indicative of
its presence in the fabricated composite layers. However, the number of HA reflections and their associated intensities were limited; these are attributed to its relatively low volume fraction in the composite layer. In the spectrum shown in Fig. 5, the peaks appeared at 2y¼ 25.91, 31.81, 32.11 and 32.91 are corresponded to (002), (211), (112) and (300) planes of hydroxyapatite crystal, respectively. Furthermore, the characteristic peaks of Ti (JCPDS card #44-1294) at 2y¼37.41 and 39.31 are apparently distinguishable which are assigned to (002) and (101) planes,
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Fig. 5 – X-ray diffraction patterns of FS Ti-CaP sample.
respectively. The pattern reveals that the peaks corresponding to HA phase are much broadened which is due to its fine crystallites. The mean crystallite size (d) of HA phase was evaluated from the full-width at half maximum (FWHM) of diffraction peaks, using Scherrer equation (Cullity, 1977) d¼
kl Bs cosy
ð2Þ
where k is Scherrer constant (¼ 0.9), l is the wavelength (lCuKa ¼0.154 nm), y is the diffraction angle, and Bs is the sample broadening related to FWHM. The instrumental peak broadening (Bi) was removed by using an annealed Si powder according to Warren equation (Cullity, 1977) B2s ¼ B2e B2i
ð3Þ
where Be is the FWHM of measured XRD peaks. The mean crystallite size of HA phase was calculated to be 26 nm. The other processing techniques such as laser processing (Roy et al., 2008) which was applied for forming TCP-Ti surface composites are based on liquid phase processing at high temperatures. In this case, it is hard to avoid decomposition of HA into TCP, degradation of substrate, the interfacial reaction between reinforcement and metal-matrix and the formation of some detrimental phases. Moreover, decomposition of HA must be avoided since it results in enhanced in-vitro dissolution (Sridhar et al., 2003). Obviously, if the processing of Ti-CaP surface composite is carried out at temperatures below melting point of substrate, the problems mentioned above can be avoided.
3.2.
Corrosion behavior in simulated body fluid solution
The potentiodynamic polarization curves of the AR, FS Ti and FS Ti-CaP samples in the corrected-SBF solution are depicted in Fig. 6. The corrosion parameters, extracted from polarization curves using Tafel least square fitting method, are listed in Table 1. Additionally, the polarization resistance (Rp) is
Fig. 6 – Potentiodynamic polarization curves of AR, FS Ti and FS Ti-CaP samples in SBF at 37 1C.
estimated by the use of Stern–Geary equation ba bc Rp ¼ 1=Icorr 2:303ðba þ bcÞ (Stern and Geary, 1957). The results corresponding to FS Ti-CaP sample imply a significant decrease in the corrosion current density (Icorr) and corrosion rate, and an increase in the corrosion potential (Ecorr) and linear polarization resistance (Rp). By implementation of friction stir processing, the Ti-CaP nanocomposite layer acts as a barrier to the penetration/transport of chloride ions and water molecules through the coating, transport of ions the coating. In addition, the corrosion rate of FS TiCaP and FS Ti samples are lower as compared with the AR substrate which is caused by rapid formation of stable passive layer due to its fine grained microstructure (Balyanov et al., 2004; Kim et al., 2011; Raducanu et al., 2011; Argade et al., 2012). The reduction of grain size plays an important role in enhancing the activity of electrons at the grain boundaries. In this regard, the surface becomes more electrochemically reactive giving rise to an increase in passivation ability resulting in rapid formation of a mechanically strong and stable passive film (Balyanov et al., 2004; Kim et al., 2011; Argade et al., 2012).
3.3.
Wear behavior
The variation of friction coefficient with sliding distance for AR and FS Ti-CaP samples in dry ambient and SBF environment are presented in Fig. 7. The average friction coefficients of the FS TiCaP were found to be 0.5 and 0.3 for dry ambient and SBF environment, respectively which are smaller than those of the AR sample (i.e. 0.6 and 0.4). Differences in extent of localized plastic deformation at real contact areas may lead to the difference in friction coefficient, Since the FSP surfaces are harder, less plastic deformation and hence lower friction is
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Table 1 – Electrochemical corrosion parameters of AR, FS Ti and FS Ti-CaP samples. Sample
Ecorr (mV)
Icorr (mA/cm2)
ba (V/decade)
bc (V/decade)
Rp (O cm2)
Corrosion rate (mm/year) 102
AR FS Ti FS Ti-CaP
327 144 127
2.76 0.085 0.048
0.23 0.10 0.07
0.28 0.11 0.12
19,925 265,300 390,030
2.4 0.074 0.041
Fig. 7 – Friction coefficient versus sliding distance for FS Ti and FS Ti-CaP samples in (a) dry and (b) biological conditions.
Fig. 9 – Wear resistance (1/K) of AR, FS Ti and FS Ti-CaP samples estimated by Eq. (4).
Fig. 8 – Variation of weight loss of AR, FS Ti and FS Ti-CaP samples with sliding distance.
expected (Niinomi, 2008; Soleymani et al., 2012). Fig. 8 shows the relationship between weight loss and the sliding distance of AR, FS Ti and FS Ti-CaP samples in SBF condition. As seen in Fig. 8 for all samples, the wear weight loss increased with sliding distance, also FSP was found to be beneficial in improving wear resistance under applied load of 15 N. The amount of weight loss of AR sample (26.3 mg) is higher in comparison with FS and FS Ti-CaP samples (12.3 and 11.4 mg) after 300 m sliding. The wear behavior of a material has traditionally been related to hardness by Archard’s equation (Archard, 1953). Archard
described that the volume loss of a material due to wear is inversely proportional to its hardness and linearly proportional to the sliding distance and normal load. However, it makes no assumptions about the hardness variations through the thickness of sample. The experimental results of wear tests carried out in our laboratory are analyzed by the following equation that assess the mean wear coefficient (K) as a function of weight loss per sliding unit ((m/s)i) and microhardness through the thickness of FSPed samples (Hi(t)): K¼
N 1 1X m Hi ðtÞ with rF N i ¼ 1 s i
t¼
m rA
ð4Þ
where r is the density of Ti–6Al–4V alloy (¼ 4.506 g/cm3), F is the applied normal load (¼ 15 N), N (¼ 6) is the times of measurement for each 50 m sliding distance, A is the area of worn track and Hi(t) can be estimated from Eq. (1). The wear resistance (1/K)
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Fig. 10 – SEM micrographs of worn surface morphologies of (a) AR, (b) FS Ti and (c) FS Ti-CaP samples.
is of fundamental importance and provides a valuable parameter of comparison for the severity of the wear process in various tribologic systems. Fig. 9 illustrates the wear resistance of AR, FS Ti and FS Ti-CaP samples. The wear resistance of FSPed samples is increased approximately three times in respect of the AR sample. It is necessary to use the scanning electron microscope to identify the main acting wear mechanisms. The SEM micrographs presented in Fig. 10 show the typical worn surface morphologies of the specimens tested at the same wear conditions. On the worn surfaces of all samples, evidences of abrasive wear can be detected. Continuous sliding marks with plastically deformed grooves and ridges are seen on the wear tracks. However, the extent of plastic deformation or ‘‘plowing’’ is found to be smaller in FSPed samples. The penetration depth depends on the relative hardness of the abrasive with respect to the specimen surface hardness. As the surface hardness of the FS Ti and FS Ti-CaP samples are higher than that of AR Ti (Fig. 2) it is expected that the depth of penetration due to the abrasive wear in the FSPed samples is less. Thus, FSPed samples exhibits significantly higher wear resistance as compared to AR Ti sample (Fig. 9). Long and Rack (1997) reported that adhesive wear is also the principal mechanism of dry wear of Ti–6Al–4V alloy articulated against hardened steel. In the present study, the contribution of adhesive wear to total wear is found to be much smaller. However, the smaller traces of local material removal imply that the severity of adhesion is significantly lower in this alloy.
4.
Conclusions
In the present study, friction stir processing was performed to fabricate Ti-CaP nanocomposites. The FSP was performed at a constant tool rotation rate of 250 rpm, travel speed of 16 mm/ min and plunge depth of 1.2 mm with a tool tilt angle of 31. The microhardness of FS Ti-CaP sample was reached about 386 HV
due to the grain refinement and the incorporation of CaP particles. The electrochemical corrosion behavior of the AR, FS Ti and FS Ti-CaP samples in SBF solution at 37 1C was investigated by means of potentiodynamic polarization test. The results imply that friction stir processing significantly decreases the corrosion current density and corrosion rate, and increases the corrosion potential and linear polarization resistance of FS Ti and FS Ti-CaP samples. According to tribological behavior in simulated biological condition, the wear resistance of FSPed samples is increased approximately three times in respect of the AR sample. From the analysis of weight loss versus sliding distance together with microhardness profile through the thickness, a correlation between wear resistance and hardness was established. The results of wear tests depicted that the depth of abrasive penetration was reduced and subsequently the wear resistance was increased for FSPed samples. According to scanning electron microscopy observations, it was shown that the major wear mechanism was abrasive wear in all samples.
r e f e r e n c e s
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