Effect of surface mechanical attrition treatment of titanium using ...

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Research Article. First Online: 11 June 2013. Received: 30 April 2013; Accepted: 25 May 2013 ... Biomaterials Science: An Introduction to Materials in Medicine.
Front. Mater. Sci. 2013, 7(3): 285–294 DOI 10.1007/s11706-013-0208-6

RESEARCH ARTICLE

Effect of surface mechanical attrition treatment of titanium using alumina balls: surface roughness, contact angle and apatite forming ability M. JAMESH1,2,3, T. S. N. SANKARA NARAYANAN (✉)1,4, Paul K. CHU2, Il Song PARK4, and Min Ho LEE (✉)4

1 National Metallurgical Laboratory, Madras Centre, CSIR Madras Complex, Taramani, Chennai 600 113, India 2 Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China 3 School of Physics, University of Sydney, NSW 2006, Australia 4 Department of Dental Biomaterials, School of Dentistry, Chonbuk National University, Jeonju 561-756, South Korea

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2013

ABSTRACT: The effect of surface mechanical attrition treatment (SMAT) of commercially pure titanium (CP-Ti) using 8 mm Ø alumina balls was studied. SMAT induced plastic deformation, increased the surface roughness, reduced the grain size and decreased the contact angle (from 64° to 43°) with a corresponding increase in surface energy (from 32 to 53 mJ/m2). Untreated CP-Ti and those treated using alumina balls for 900 s reveals no apatite growth until the 28th day of immersion whereas those treated for 1800 and 2700 s exhibit apatite growth in selective areas and the extent of growth is increased with increase in immersion time in SBF. The study reveals that SMAT using alumina balls is beneficial in imparting the desired surface characteristics, provided the surface contamination is limited, which would otherwise decrease the apatite forming ability. KEYWORDS: nanostructured material; surface modification; scanning electron microscopy (SEM); surface mechanical attrition treatment (SMAT)

1

Introduction

Titanium and titanium alloys are widely accepted as implant materials, particularly for orthopaedic and osteosynthesis applications, due to their low density, better mechanical properties, excellent biocompatibility and corrosion resistance [1–2]. However, the native passive oxide layer formed on them is not bioactive enough to promote a direct bond with juxtaposed bone. Implants made of titanium and its alloys are inclined to form fibrous Received April 30, 2013; accepted May 25, 2013 E-mail: [email protected] (T.S.N.S.N.), [email protected] (M.H.L.)

tissues at the implant-bone interface, which may translate into a lack of osseointegration and thus increases the possibility of loosening of the implant over a longer period of time [2–4]. Hence, it is evident that the clinical success on the use of these implants is largely determined by their interaction with biological fluids and tissues and, the conditions that would promote an early osseointegration [5]. The rate and quality of osseointegration are related to the surface properties, particularly surface topography, surface chemistry and wettability [6–7]. A variety of surface modification techniques have been used to modify these surface properties to enable a better integration between

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implants and the bone [4]. Shot peening and grid blasting have been used to modify the surface topography of implants such as hip endoprostheses [8–13]. Various ceramic particles such as alumina, titanium oxide, bioactive glass and calcium phosphate are used for blasting to create a suitable surface topography [5,8–13]. Wennerberg et al. [13] using a rabbit model has demonstrated that grit-blasting of Ti implants using TiO2 or Al2O3 particles has drastically increased its biomechanical fixation when compared to the Ti implant with a smooth surface finish. However, the major limitation of shot/grit blasting is the embedding of the blasting material and the difficulty in removing them from the implant surface [5,8–13]. In addition, the chemical heterogeneity of blasted implanted surface could decrease its corrosion resistance in a physiological environment [14]. In some instances, release of these particles into the surrounding tissues could interfere with the osseointegration of the implants. Surface mechanical attrition treatment (SMAT) is a surface modification method, which involves controlled peening using spherical balls. Many researchers have studied the various aspects of SMAT of commercially pure titanium (CP-Ti) [15–21]. Zhu et al. [15] have elaborated the formation mechanism of nanostructured surface (~50 μm thick) on CP-Ti while the strong dependence of thermal conductivity on grain size of CP-Ti was reported by Guo et al. [16]. The formation of nanoporous TiO2 on Ti by a hybrid treatment of SMAT followed by immersion in H2O2 was addressed by Wen et al. [17]. Similarly, the formation of bioactive nanoporous TiO2 film by a hybrid SMAT was reported by Laleh et al. [18]. SMAT induces compressive residual stress and increase the mechanical properties. CPTi processed by SMAT has been shown to possess a high strength [19]. SMAT of CP-Ti has been shown to generate a nanocrystalline surface layer with an enhanced strength, microhardness, corrosion resistance and in vitro cell response [20–21]. The objective of the present study is to evaluate the suitability of SMAT as a surface modification method for CP-Ti for implant applications. The study aims to focus on the effect of SMAT of CP-Ti using alumina balls on the surface roughness, grain size, mean microstrain, hardness, contamination of the treated surface with alumina particles, change in contact angle and surface energy and apatite forming ability.

2

Materials and methods

CP-Ti (grade-2) (chemical composition in weight percent:

N, 0.01 wt.%; C, 0.03 wt.%; H, 0.01 wt.%; Fe, 0.20 wt.%; O, 0.18 wt.%; Ti, balance) discs (65 mm diameter) were cut from 2 mm thick sheets and degreased using acetone. The CP-Ti discs were fixed on the upper side of a cylindrical vessel and subjected to SMAT using 8 mm Ø Al2O3 balls (kept in the bottom of the vessel) for 900, 1800 and 2700 s. The methodology of SMAT has already been described in detail elsewhere [22–23]. All the experiments were performed at a fixed frequency of 50 Hz under vacuum. The microstructure of CP-Ti samples after SMAT was assessed using a Leica DMLM optical microscope. The surface roughness of the CP-Ti samples before and after SMAT was measured using a profilometer (Mitutoyo SJ 301). The grain size and the mean microstrain induced during treatment were determined by X-ray diffraction (XRD) measurement (Model: D-8 Discover, Bruker AXS) using Cu Kα radiation. The microhardness of untreated CPTi and those treated by SMAT, both on the surface as well as on the cross section, was determined using a Leica VMHTMOT microhardness tester at a load of 10 gf applied for 15 s. Seven indentations were made on each sample and the values were averaged out. Contact angle measurements (Phoenix series contact angle analyzer, Korea) of untreated CP-Ti and those processed by SMAT were performed using double distilled water as the contacting medium. Ten measurements were made after placing the droplet of water on sample surface for 10 s under ambient conditions and averaged out. The surface energy (Es) was calculated from the contact angle using the following equation [24–25]: Es ¼ Evl cos

(1)

where Evl is the surface energy between water and air under ambient condition (i.e., 72.8 mJ/m2 at 20°C) for pure water and θ is the static contact angle. The effect of SMAT using alumina balls on the apatite forming ability of CP-Ti was evaluated in simulated body fluid (SBF) under biomimetic conditions. Untreated and treated samples (8 mm  6 mm) were cut and cleaned in deionized water followed by acetone and dried using a stream of compressed air. The SBF solution (similar to those of human extracellular fluid), with inorganic ion concentrations of 142.0 mmol/L Na+, 5.0 mmol/L K+, 1.5 mmol/L Mg2+, 2.5 mmol/L Ca2+, 147.8 mmol/L Cl–, 4.2 mmol/L HCO3– , 1.0 mmol/L HPO24 – and 0.5 mmol/L SO24 – (pH 7.40), was prepared by dissolving reagent grade NaCl, NaHCO3, KCl, K2HPO4$3H2O, MgCl2$6H2O, CaCl2 and Na2SO4 in distilled water. It

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was buffered at pH 7.3 with the addition of trishydroxymethyl aminomethane (THAM) and 1.0 mol/L HCl [26]. The samples were immersed in the SBF solution under static conditions in a constant water bath at 37°C for a period of 7, 14, 21 and 28 d. The surface morphology of the samples was observed using scanning electron microscopy (SEM). Energy dispersive X-ray (EDX) analysis was used to ascertain the chemical nature of the surface.

3

Results and discussion

3.1 Characteristics of CP-Ti after SMAT using alumina balls

During SMAT, impingement of the alumina balls induces plastic deformation on the surface of CP-Ti with a high strain rate during each impact. The microstructure of CP-Ti samples after SMAT, which are taken at the cross section, confirms the increase in the extent of deformation with increase in treatment time (Fig. 1). The surface roughness of untreated CP-Ti and those subjected to SMAT for 900, 1800 and 2700 s are shown in Fig. 2. SMAT increased the surface roughness and the extent of increase in roughness increased with increase in treatment time. Surface roughness of titanium implants is an important factor in determining the rate of osseointegration and biomechanical fixation [27–28]. It has been shown that an early fixation and long-term mechanical stability of prosthesis can be achieved by imparting a rough profile rather than a smooth finish [5]. Wennerberg et al. [13] have used a rabbit model and demonstrated that grit blasting of titanium implants with TiO2 or Al2O3 particles results in a drastic increase in the biomechanical fixation when compared to those with a smooth surface finish. It has been reported that a microtopographic profile with surface roughness in the range from 1 to 10 μm would enable a better mechanical interlocking between the implant surface and the bone [13,28]. However, a major limitation of high surface roughness is the increase in peri-implantitis as well as an increase in ionic leakage [29]. A moderate roughness of 1 to 2 μm may limit these two parameters [30]. A theoretical approach suggests that the ideal surface should be covered with hemispherical pits approximately 1.5 μm in depth and 4 μm in diameter [31]. Based on the requirements of surface roughness to achieve a better mechanical interlocking between the implant surface and the bone, it is evident that SMAT using 8 mm Ø Al2O3 balls appears to be a suitable surface treatment to impart the desired roughness

Fig. 1 Microstructure of CP-Ti after SMAT using 8 mm Ø alumina balls for different treatment time: (a) 900 s; (b) 1800 s; (c) 2700 s.

of titanium for implant applications. The XRD patterns of untreated CP-Ti and those treated using 8 mm Ø balls for 900, 1800 and 2700 s is shown in

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Fig. 2 Surface roughness of untreated CP-Ti and those subjected to SMAT using 8 mm Ø alumina balls for different treatment time.

Fig. 3. It is evident from Fig. 3 that compared to untreated CP-Ti, samples subjected to SMAT exhibit a small shift towards lower diffraction angles indicating residual compression on the treated surface. Broadening of the diffraction peaks observed for SMAT treated samples

Fig. 3 XRD patterns of untreated CP-Ti (a) and those subjected to SMAT using 8 mm Ø alumina balls for 900 s (b), 1800 s (c) and 2700 s (d).

indicates a reduction in crystallite size and an increase in micro-lattice strain imparted during treatment. Similar observations of peak shifting and broadening were also observed earlier during SMAT or other forms of similar surface treatments [23,32]. The appearance of alumina peaks in the XRD patterns of CP-Ti samples subjected to SMAT for 1800 and 2700 s is due to the fragmentation of the alumina ball and subsequent attachment of its fine particles on the surface. A similar behavior is also observed when titanium is shot blasted using ceramic powders such as, TiO2, Al2O3 or hydroxyapatite [5]. The blasting material is often embedded into the implant surface and its residue remains even after ultrasonic cleaning, acid passivation and sterilization [5]. Increase in treatment time has lead to a decrease in the average grain size from 25 to 22 nm whereas it caused an increase in mean microstrain from 0.0042 to 0.0048. The grain size of as-received CP-Ti sample is 15–20 μm. SMAT for 900 s decreased the grain size of CP-Ti to 25 nm. The microhardness of untreated CP-Ti is (1786) HV0.01 while those treated using alumina balls exhibit an increase in hardness. The hardness of CP-Ti samples after SMAT is increased from 320 to 450 VHN0.01 when the treatment time is increased from 900 to 2700 s. This is due to the increase in lattice strain evolved during impingement of the alumina balls on the surface of titanium. This might also be due to the energy stored on the surface during the highenergy impact of the Al2O3 balls on CP-Ti. The surface topography of titanium implant determines its hydrophilic or hydrophobic nature. For implants, a hydrophilic surface is considered to be desirable than a hydrophobic one in view of its better interaction with biological fluids, cells and tissues [33–34]. The shape of the water droplet formed on the surface of untreated CP-Ti and those treated using alumina balls (Fig. 4) make evident of the fact that compared to untreated CP-Ti samples subjected to SMAT using alumina balls exhibit a relatively higher hydrophilicity. The contact angle is decreased from 64° to 43° with a corresponding increase in surface energy from 32 to 53 mJ/m2 as the surface of CP-Ti is changed from a smooth (Ra: (0.150.12) μm for untreated CP-Ti) to

Fig. 4 Shape of the water droplet formed on the surface of untreated CP-Ti and those subjected to SMAT using 8 mm Ø alumina balls for different treatment time: (a) untreated; (b) treated for 900 s; (c) treated for 1800 s; (d) treated for 2700 s.

M. JAMESH et al. Effect of SMAT of titanium using alumina balls: surface roughness, contact angle and ...

rough (Ra: (3.270.98) μm for CP-Ti SMATed for 2700 s) finish with increase in treatment time (Fig. 5). A wide range of contact angles, ranging from 0° (hydrophilic) to 140° (hydrophobic), have been reported for titanium surfaces prepared by various methods [33–35]. Buser et al. [33],

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based on their in vivo animal studies, have reported that a combined treatment of sandblasting and acid etching enabled a hydrophilic surface on titanium and offered a better interaction between the bone and the implant than a regular acid etching treatment. However, plasma and glow discharge treatments though imparted a hydrophilic nature on the surface of titanium, failed to demonstrate a higher osseointegration in dental implants [36–37]. In order to get a better insight on whether the hydrophilic nature of the surface obtained after SMAT using alumina balls is suitable to promote bone bonding, the apatite forming ability is assessed by immersing the treated surface in simulated body fluid. 3.2 Apatite forming ability of untreated CP-Ti and those treated using alumina balls

Fig. 5 Change in contact angle and surface energy of CP-Ti subjected to SMAT using 8 mm Ø alumina balls as a function of treatment time.

SEM images of the surface of untreated CP-Ti and those treated using alumina balls after immersion in SBF for 7, 14, 21 and 28 d are shown in Fig. 6. The morphological features of untreated CP-Ti reveal etching while those treated using alumina balls for 900 s exhibits deformation of their surface induced during treatment and no apatite

Fig. 6 SEM images showing the extent of apatite growth on the surface of untreated CP-Ti and those subjected to SMAT using 8 mm Ø alumina balls after immersion in SBF for 7, 14, 21 and 28 d.

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Fig. 7 EDX analysis performed on CP-Ti samples subjected to SMAT using 8 mm Ø alumina balls for 1800 s and subsequently immersed in SBF for 7 d: (a) entire region; (b) spot analysis on the white patches; (c) spot analysis on the dark particles.

M. JAMESH et al. Effect of SMAT of titanium using alumina balls: surface roughness, contact angle and ...

growth is visible until the 28th day of immersion in both cases. The surface of CP-Ti samples processed by SMAT for 1800 and 2700 s exhibits apatite growth in selective areas and the extent of growth are increased with increase in immersion time in SBF. EDX analysis performed at selected regions (Figs. 7 and 8) provides a good comparison of the extent of apatite growth on samples processed by SMAT for 1800 and 2700 s. For samples processed for 1800 s, the presence of Ca (0.40 at.%) and P (0.63 at.%) on the entire region (Fig. 7(a)) indicates the ability of SMAT to promote apatite growth on the 7th day itself. The grain refinement induced during treatment, decrease in contact angle and increase in surface energy help to improve the bioactivity that is

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observed in terms of apatite growth. Faghihi et al. [38] and Thirugnanam et al. [39] have also reported that a high degree of surface wettability associated with an increase in the surface area would promote bioactivity of titanium. The decrease in contact angle and increase in surface energy following SMAT is likely to impart a negative charge on the surface of CP-Ti, which combines with positively charged calcium ions to form calcium titanate when it is immersed in SBF. When the surface is accumulated with Ca ions, it becomes positively charged and it combines with the negatively charged phosphate ions in SBF to form an amorphous calcium phosphate, which acts as a nucleation point for apatite deposition. Since the SBF is a supersaturated solution of Ca and P ions, the amorphous

Fig. 8 EDX analysis performed on two different region on CP-Ti samples subjected to SMAT using 8 mm Ø alumina balls for 2700 s and subsequently immersed in SBF for 7 d: (a) region where the apatite crystals are formed; (b) region which lacks the formation of apatite crystals.

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calcium phosphate will be a metastable phase and it eventually grows into a crystalline bone-like apatite phase [40]. The presence of Al (26.71 at.%) and O (55.87 at.%) could have originated from the alumina particles incorporated during impingement of the alumina ball (Fig. 7(a)). Spot analysis indicates that the white patches contain Ca and P ions (Fig. 7(b)) while the dark particles are predominantly Al and O (Fig. 7(c)), suggesting the incorporation of Al2O3 particles, which is also confirmed by XRD analysis. For CP-Ti samples processed by SMAT for 2700 s, the Ca (0.31 at.%) and P (0.23 at.%) content (Fig. 8(a)) are relatively lower than those treated for 1800 s. At selected regions, no Ca and P could be identified (Fig. 8(b)) and these regions mainly consists of Al (15.38 at.%) and O (62.55 at.%). These observations suggest that incorporation of alumina particles on the surface of CP-Ti could hinder apatite growth. Since increase in treatment time from 1800 to 2700 s promotes the level of incorporation of alumina particles, which is already confirmed by XRD analysis, it would decrease the apatite forming ability. Hence, the beneficial effect of reduction in grain size, decrease in contact angle and increase in surface energy observed with increase in treatment time from 1800 to 2700 s is nullified by the decrease in apatite forming ability.

contact angle and increase in surface energy observed with increase in treatment time from 1800 to 2700 s is nullified by the decrease in apatite forming ability. The study reveals that peening using alumina balls is beneficial in providing the desired surface roughness, contact angle and surface energy, provided if the contamination of the surface by the fragmented alumina particles is limited, which would otherwise decrease the apatite forming ability.

Acknowledgements MJ and TSNSN express their sincere thanks to Dr. S. Srikanth, Director, National Metallurgical Laboratory, Jamshedpur, for his constant support and encouragement to carry out this research work and permission to publish this paper.

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SMAT using 8 mm Ø alumina balls induced plastic deformation on the surface of CP-Ti with a high strain rate. SMAT increased the surface roughness, decreased the grain size, increased the mean microstrain and increased the hardness. Compared to untreated CP-Ti, samples subjected to SMAT using alumina balls exhibit a relatively higher hydrophilicity. The contact angle is decreased from 64° to 43° with a corresponding increase in surface energy from 32 to 53 mJ/m2 as the surface of CP-Ti is changed from a smooth (Ra: (0.150.12) μm for untreated CP-Ti) to rough (Ra: (3.270.98) μm for CP-Ti SMATed for 2700 s) finish with increase in treatment time. Untreated CP-Ti and those treated using alumina balls for 900 s reveals no apatite growth until the 28th day of immersion whereas those treated for 1800 and 2700 s exhibit apatite growth in selective areas and the extent of growth is increased with increase in immersion time in SBF. Incorporation of alumina particles on the surface of CP-Ti beyond a threshold level would hinder the apatite growth. The beneficial effect of reduction in grain size, decrease in

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