Preparation and properties of plasma electrolytic oxidation coating on sandblasted pure titanium by a combination treatment

Materials Science and Engineering C 42 (2014) 657–664

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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec

Preparation and properties of plasma electrolytic oxidation coating on sandblasted pure titanium by a combination treatment Hong-Yuan Wang a,b,c, Rui-Fu Zhu a,b,⁎, Yu-Peng Lu a,b,⁎⁎, Gui-Yong Xiao a,b, Xing-Chuan Zhao a,b, Kun He a,d, Y.F. Yuan d, Ying Li e, Xiao-Ni Ma e a

School of Materials Science and Engineering, Shandong University, Ji'nan 250061, China Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Ji'nan 250061, China Jinan Vocational College, Ji'nan 250103, China d Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931, USA e Stomatology Department, Affiliated Jinan Centre Hospital of Shandong University, Ji'nan 250013, China b c

a r t i c l e

i n f o

Article history: Received 6 July 2013 Received in revised form 13 April 2014 Accepted 9 June 2014 Available online 19 June 2014 Keywords: Properties Coating Titanium Combination treatment Sandblasting Plasma electrolytic oxidation

a b s t r a c t Plasma electrolytic oxidation (PEO) is one of the most applicable methods to produce bioceramic coating on a dental implant and sandblasting is a primary technique to modify metal surface properties. This study aims to deposit bioceramic Ca- and P-containing coatings on sandblasted commercially pure titanium by PEO technique to improve its bioactive performance. The time-dependent modified surfaces are characterized in terms of their microstructure, phase, chemical composition, mechanical properties and bioactivities. The results show that the combination-treated coating exhibits better properties than the PEO-treated one, especially in bioactivities, as evidenced by the HA formation after immersion in simulated body fluid (SBF) for 5 days and the cell viability after seeding for 1 or 3 days. The enhancement of the modified surface is attributed to a combination of the mechanical sandblasting and the microplasma oxidation. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Titanium (Ti) and its alloys have excellent properties such as biocompatibility, corrosion resistance and lightweight, and have become the most striking metallic material for the purpose of orthopedic implants nowadays [1,2]. However, Ti together with its native oxide thin film is known to be bio-inert, and the ability of its surface to induce hydroxyapatite (HA) formation is rather poor. To overcome this drawback, many topological and chemical modification techniques have been applied on Ti surfaces to ensure the long-term osseointegration [3–5]. The surface properties of Ti and its alloys, as typical dental implants, play a key role. A roughened surface of Ti was shown to markedly increase osteoinduction, cellular differentiation and mechanical fixation [6,7]. To create the appropriate type of roughness, several techniques were developed, including sandblasting, acid etching and SLA (sandblasting, large-grit, acid-etching) [8]. Sandblasting is

⁎ Correspondence to: R.-F. Zhu, School of Materials Science and Engineering, Shandong University, Ji'nan 250061, China. Tel.: +86 531 86358503; fax: +86 531 86358603. ⁎⁎ Correspondence to: Y.-P. Lu, Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Ji'nan 250061, China. Tel.: +86 531 86358503; fax: +86 531 86358603. E-mail addresses: [email protected] (R.-F. Zhu), [email protected] (Y.-P. Lu).

http://dx.doi.org/10.1016/j.msec.2014.06.005 0928-4931/© 2014 Elsevier B.V. All rights reserved.

a common technique to modify mechanical properties of metal surface and near surface region through micromachining and inducing severe plastic deformation, leaving the treated region with a rough surface and a compressive residual stress state [9]. So it could create a coarse surface to facilitate adhesion of cells and growth of new bone. Nevertheless, the roughened surface is not completely identical and abrasive materials are often embedded into the treated surface, which might hinder osseointegration process [10,11]. The sandblasting method, by itself, might not be effective enough to achieve initial cell fixation and bone formation. Subsequently, additional surface modifications of the implant should be applied [12–15]. Compared with other modifications of producing bioactive coatings, plasma electrolytic oxidation (PEO) has been one of the most applicable methods to deposit bioceramic film on an implant and could provide the possibility for incorporating Ca and P ions, which can further improve HA's inducing ability and even facilitate its crystallization [16–20]. Moreover, PEO can form an oxide coating with a complex geometry, which is of importance for enabling the bone-bonding ability of the implant. However, to achieve a bioactive coating with high-quality performance, this technique demands to combine with other modifications [21–26]. In this study, bioceramic coatings on titanium substrate were synthesized by sandblasting and PEO treatments in a mixture of Ca- and

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P-containing system under varied treating intensities, and the coatings' properties were examined. Since most PEO coatings were formed by sole technique in relevant literatures, the present study aimed at depositing a bioceramic PEO coating on sandblasting-treated titanium and having, thus, synergistic effects of the modified substrate with an oxide layer. These coatings were fabricated by the combination treatment which not only achieved their original morphology, but also possibly improved surface chemistry as well as characteristics due to the multiple functions of mechanical enhancement and electrochemical oxidation. 2. Experimental 2.1. Sample preparation Commercially pure titanium TA2 discs (ZhiRui Metal Material Ltd., China) with a diameter of 8 mm and a thickness of 3 mm were used as the substrates. The surfaces of the discs were polished with #180-600 abrasive papers to a roughness of Ra = 0.25 μm, ultrasonically cleaned in acetone, distilled water respectively, and then dried in air.

2.5. Cell experiments 2.5.1. Cell culture In this survey, the fibroblast cell line L929 was selected to appraise the biological properties of the samples. L929 cells were cultured in Dulbecco's Modified Eagle's Medium (DMEM) with 10% fetal bovine serum (FBS) and 1% (10,000 units) penicillin/streptomycin and then were placed in incubator (37 °C, 5% CO2 and 90% RH). 2.5.2. Cell attachment and adhesion assay The P-group/control and C-group samples of 25 min (surface area: Ф8 × 3 mm3) were firstly placed in 6-well plates and sterilized by ultraviolet light for 2 h. 5 × 104 cells/μl culture medium were seeded on triplicate samples and then they were located in incubator for 3 h. When the cells have been adhered to the sample surface, 1 ml culture medium containing 10% FBS was poured on each sample, which was then kept in incubator for 24–72 h. Afterwards, each sample was removed from the culture medium and washed with phosphate buffer saline (PBS). 2.5% glutaraldehyde was used for fixation of the cells and then the samples were put in the refrigerator for 24 h. Eventually, the samples were washed with PBS, and 60%, 80%, 90% and 100% alcohol, respectively.

2.2. Sandblasting The polished discs were sandblasted by applying a jet of SiC particles in 200 μm size until the surface reached a uniform gray tone. In the treatment, the air pressure was set at 0.9 MPa, with the mass flow rate of 2.15 m/min and the treating time of 45 s; the impact angle was set at 75° with the distance between the nozzle and samples at 15 cm. Afterward, the treated samples were washed with distilled water and dried in warm air.

2.5.3. Cell proliferation assay Cell viability was quantitatively analyzed by using the method of MTT assay, which has been widely used for appraising cytotoxicity and proliferation. After seeding for 24–72 h, the samples were incubated with a MTT solution at 37 °C for 3 h. The formazan product, which was obtained by the reduction of MTT in the mitochondria of viable cells, was measured using a microplate reader [28–35]. 2.6. Characterization

2.3. PEO treatment The PEO treatment was carried out for 10–30 min in a watercooled bath made of stainless steel, which served as a cathode, and the specimens were used as anode during the treatment. Bipolar electric rectangular pulses were applied to the specimens, fed from an AC-type high power supply, and the anodic power voltage was fixed at 500 V, with the current frequency of 400 Hz and the duty cycle ratio of 25%. In the experiments, aqueous solutions of electrolytes were prepared by mixing 30 mmol/l calcium acetate monohydrate ((CH3COO)2Ca·H2O, CA), 10 mmol/l β-glycerophosphate disodium salt pentahydrate (C3H7Na2O6P·5H2O, β-GP) and 8 mmol/l sodium hydroxide (NaOH, CA), and the electrolyte was cooled to prevent heating over 40 °C during the oxidation process. In this study, the PEO coupled with sandblasting pretreatment was defined as the combination treatment. For purposes of comparison, the polished discs of titanium substrates and the discs prepared by sandblasting were treated by PEO process, respectively, to obtain the PEO-treated group (P-group/control-group) and the combinationtreated group (C-group) samples. After the treatment, the coated samples were flushed with distilled water, dried in an air oven, and labeled for analysis. 2.4. Immersion in SBF The simulated body fluid (SBF) was prepared by dissolving the reagent-grade chemicals of NaCl, NaHCO 3, KCl, K 2 HPO 4 ·3H 2O, MgCl 2·6H2 O, CaCl2 and Na 2 SO 4 into distilled water sequentially and buffering at pH 7.40 with Tris (hydroxymethyl) aminomethane and dilute HCl at 37 °C. The ionic concentrations of Na+, K+, Mg2 +, Ca2 +, Cl−, HCO23 −, HPO24 − and SO24 − of SBF were 142.0, 5.0, 1.5, 2.5, 147.8, 4.2, 1.0 and 0.5 mmol/l, respectively [27]. Both the P-group and C-group specimens were soaked in 35 ml of SBF for 5 days to compare the results, and the SBF was refreshed every other day.

The coating morphology was examined by SU-70 field emission SEM (FE-SEM). Because of the low conductivity, the sample was sputtercoated with Pt prior to the SEM. JXA-8800R electron probe microanalyzer (EPMA) with a Link ISIS300 energy spectrum analyzer was used to give the secondary electron (SE) image and the element analysis was conducted by energy dispersed spectroscopy (EDS). The phase analysis of the coatings was carried out using Bruker D8 Advance X-ray diffractometer (XRD), with a scan speed of 4°/min, operated at 40 kV and 40 mA. The thickness was measured through touching the FN sensor on the coatings non-destructively, using Mini Test 600B FN2 thickness meter. The microhardness was determined by indentation test on the HV-1000 Huayin microhardness tester, the load was fixed at 25 g and loading time of 15 s. The weight was tested through a TE214S Sartorius, with an accuracy of 0.1 mg. 3. Results and discussion Treated bioceramic coating patterns were examined using a scanning electron microscope to characterize the mode of topography. Fig. 1 shows the surface morphology evolution of the polished, sandblasted substrates and their coatings formed by PEO and combination treatment, together with EDS spectra of the element contents. All the surfaces of both groups' sample coatings were characterized by micropores in different sizes and shapes, which depended on the treatment mode and conditions, and the elemental composition was detected from different proportions of oxygen, titanium, calcium and phosphorus by EDS. It could be identified clearly in Fig. 1(c) that little fine micropores less than 5 μm distributed randomly on the P-group sample coating, where some agglomerate islands could also be observed. Some particles were deposited beside the micropores on the C-group sample as shown in Fig. 1(d), and these were related to calcium phosphate by XRD test due to the electrophoresis process. With the proceeding of treatment, Fig. 1(e)–(h) shows that the micropore size increased strikingly in both

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Fig. 1. FE-SEM micrographs of the treated surfaces: (a) polishing and (b) sandblasted substrate; P-group samples of (c) 15 min, (e) 20 min and (g) 25 min; C-group samples of (d) 15 min, (f) 20 min and (h) 25 min.

group samples, while the micropore number quickly decreased owning to the remarkable reduction of the microdischarge channels. The variations of micropore size and number of the both groups were also presented in Fig. 2. As the sandblasting roughened Ti substrate and strengthened it in a compressive residual stress state, it was more difficult for oxidation in the C-group samples than in the P-group. When the same treating conditions were applied, the PEO function on the P-group samples was much stronger than that on the C-group, which always led to excessive oxidation. Fig. 1(h) shows that the pores were well separated and homogeneously distributed over the Cgroup sample of 25 min, however, Fig. 1(g) shows the enlarged pore

size or ablated pores on the P-group sample by the superabundant discharging energy of 25 min, as demonstrated in Table 1. The chemical composition of the PEO coatings was examined by EDS, and Ca and P element contents as well as the Ca/P at.% ratios with the treating time were shown in Fig. 3, estimated from the EDS spectra of P-group and C-group samples. It could be observed that the element content of Ca in C-group samples was higher than that in P-group after the initial period. After 20 min, the content of P sharply reduced in the P-group sample, while it increased to the maximum at 25 min in the C-group. For the sample of 25 min in C-group, the content of Ca and P was up to 20.17 and 8.65 at.% respectively, and the Ca/P at.%

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Fig. 2. Variation of the micropore size and number in P-group and C-group sample coatings.

ratio was 2.33, which was higher than 1.67 of typical HA due to the formation of CaTiO3, as shown in the XRD patterns. Fig. 4 shows the XRD patterns of the coatings at different treating periods in the PEO and combination treatments. In the plasma electrolytic oxidation, the temperature and pressure in discharge channels could reach about 20,000 K and 100 MPa [36], and thus promoted the conversion of Ti substrate into titanium oxides of TiO2, which usually exists in two kinds of form: anatase and rutile, however, anatase is the unstable phase and would transform into rutile at 915 °C. It could be observed that the intensities of rutile peaks increased but that of anatase almost decreased with the treating time in the P-group and C-group coatings. The intensity of the diffraction peaks of titanium substrate was also identified in both groups because of the thin coatings or enlarged pores, which could be penetrated by X-rays. Since the Ca–P compounds might exist with low crystallinity or even amorphous phases in the oxide layers, the peaks of Ca3(PO4)2 were only observed in the Cgroup sample of 10 min, as shown in Fig. 4(b). The compressive residual stress made the substrates difficult to oxidize, leading to much of rutile and CaTiO3 presented in the C-group sample of 25 min or 30 min. Moreover, the intensity of CaTiO3 diffraction peaks in the C-group increased with the phase of rutile, indicating the rutile might incorporate Ca2+ and OH− ions into coatings to form CaTiO3. Generally, PEO technical parameters such as applied voltage, frequency, and oxidizing time could influence the thickness of the treated coatings. Fig. 5 shows that the thickness of the P-group coatings increased rapidly with the treating time before 20 min, and hardly decreased after 25 min. However, the thickness of the C-group coatings increased with the treating time all throughout the oxidation, and the variation was in disagreement with P-group because the C-group samples had never been ablated by PEO treatment. Especially, the maximum

Fig. 3. Variation of Ca and P element contents and Ca/P ratio in P-group and C-group coatings.

thickness of the P-group coatings was about 33.2 μm at 20 min, thicker than that of C-group due to the easier oxidation. A microroughness test and a microhardness test examined the treated coatings' mechanical properties respectively, whose indicators include R a and HV0.025 , as shown graphically in Fig. 6. The value of Ra, which was measured by light-section method, denoted the mean microsize of height calculated over the entire measured array, and the measurements of HV0.025 were performed by placing

a)

)

b)

Table 1 Discharging energy during PEO process. Treating time (min)

Group

Discharging energy Power (kW)

Current (A)

10

P C P C P C P C P C

1.14 1.11 0.99 0.76 0.75 0.60 0.57 0.41 0.38 0.22

2.27 2.21 1.98 1.52 1.49 1.20 1.13 0.81 0.75 0.43

15 20 25 30

) Fig. 4. XRD patterns of (a) P-group and (b) C-group samples. (○: rutile, ●: anatase, □: titanium, ■: CaTiO3, ▼: Ca3(PO4)2.)

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Fig. 5. Variation of the thickness of P-group and C-group sample coatings. Fig. 7. Weight increase or loss of P-group and C-group samples.

Vickers indenter on the surfaces to obtain the indentations. Fig. 6 shows that the P-group microhardness increased with the treatment time, and its roughness reduced to a minimum on the treating condition of 20 min. The microhardness of C-group increased from 440 to 780 HV0.025 with the oxidizing time of 10–25 min, and the roughness decreased gradually from 2.9 to 1.6 μm. In contrast, the roughness of polished substrate was 0.25 μm, lower than that of all the treated sample coatings, and its microhardness was 222 HV0.025, also lower than theirs', for ceramic coating hardness is always higher than metals'. The results were in accordance with the SEM observation. It was the micropore that reduced the microhardness due to being easily crushed by indenter, however, it improved the coating microroughness. Fig. 6 demonstrated that the sample of 25 min in C-group has a combination of a lower microroughness and a higher microhardness. Samples' weight during the treatments was measured by the evaluation of dry weight. For the test, the samples were rinsed in distilled water and dried at 20 °C to constant weight. The relative weight was calculated according to the following equation: Rw ¼

W t −W 0  100% W0

where W0 is the starting dry weight and Wt is the dry weight after treatment. The results were summarized in Fig. 7 and the variations of P-group and C-group relative weight together with those after immersion in SBF were shown in Fig. 8. The value of dry weight of sandblasted substrate was compared with that found in untreated one and there was a loss of weight after sandblasting, suggesting

Fig. 6. Variation of microroughness and microhardness of P-group and C-group coatings.

that the substrate was slightly gouged or cut off by the abrasive particles. In the case of sole PEO treatment, the sample weight decreased gradually with an increase of treating time, and the most weight loss was observed at 30 min, implying that the PEO process oxidized and dissolved the Ti substrate in situ, which was therefore characterized by a porous coating. After immersion in SBF for 5 days, the P-group relative weight increased to a maximum at the treatment time of 20 min (Fig. 8). In the case of combination treatment, the coated sample weight decreased as the P-group samples, however, their weight loss was much lower, implying that the substrates were difficult to oxidize after sandblasting and their coatings were thinner than the P-group samples'. Most important of all, Fig. 8 shows that the relative weight of C-group samples after SBF immersion rose to the highest level at the treating time of 25 min, suggesting that under this trial condition more HA or related calcium phosphates were deposited onto the surface, which could exhibit better bioactivities. The weight of HA coating decreased after 25 min due to lower Ca and P ions its PEO layer contained. The surface morphologies of both the group specimens immersed in SBF were shown in Fig. 9. The 25 min P-group sample immersed for 5 days exhibited a rough and porous characteristic of the oxidized layer as shown in Fig. 9(a) and (b). The exposed surface was barely modified and there appeared to be no obvious hydroxyapatite deposition, which was scarcely identified by XRD, while 25 min C-group sample surface was completely covered with HA layer, which could be detected by the XRD pattern as shown in Fig. 9(c). Furthermore, some

Fig. 8. Variation of relative weight of P-group and C-group samples before or after SBF immersion.

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Fig. 9. FE-SEM micrographs of the treated specimens immersed in SBF for 5 days: (a) and (b) P-group sample of 25 min; (c) and (d) C-group sample of 25 min.

micron size particles and cracks could be observed as well, and the formation of cracks might be due to the thickness of the coating and the shrinkage during drying. The XRD peaks indicated that the HA layer consisted of superfine crystalline structure grains. It was thought that the existing phase of CaTiO3 was the key structure factor for the Cgroup coating to possess biological activity [37], because a hydrolysis reaction about CaTiO3 could happen in SBF according to the following equation: 2þ

CaTiO3 þ 2H2 O→Ca

−

þ 2OH þ TiOðOHÞ2 :

The reaction helped to build up a beneficial surface state to HA nucleation by generation of a Ti–OH matrix and increasing ion concentration of OH− and Ca2 + at the surface [38]. On the other hand, in the case of the 25 min C-group sample, abundant Ca and P ions incorporated in the oxide coating were released during the immersion treatment. Some of the Ca, P and increased OH− ions participated in the formation of HA crystallites on the surface of the oxide coating [5]. Therefore, many HA nuclei could be created more sufficiently on the sample surface than the others, and these could contribute to induce HA layer and to be bioactive for the oxide coating. Several rapidly formed HA nuclei might grow micro-sized HA in the SBF immersion, whereas the many slowly formed HA grew into nanostructures as shown in Fig. 9(d). Fig. 10 shows the relative cell viabilities on the sample coatings of 25 min P-group and the 25 min C-group after culture for 24–72 h. At 24 h, statistical analysis reveals that the number of cells attached on the C-group sample coating is apparently higher than that on the Pgroup coating, indicating that initial cell attachment was enhanced by the combination treatment. At 48–72 h culture, the number of cells on the C-group coating is much higher than that on the control group. It demonstrates that cell proliferation on the C-group coating could be significantly improved. The morphologies of cells cultured on the 25 min C-group coating for 72 h are shown in Fig. 11. The cells on the coating appeared to spread out uniformly over the surface, exhibiting flattening on the substratum (Fig. 11(a)). Fig. 11(b) especially displays multiple microvilli and numerous filopodia of cells on the surface, indicating a continuous exchange between the cell surface and the environment.

The long and fine cytoplasmic extensions together with large lamellipodia imply excellent adhesion and homogeneous colonization. The properties of C-group coating might be related to the cell reaction and distribution selectivity, promoting biocompatibility and protein binding as well as osseointegration. 4. Conclusions A PEO based surface modification technique has been developed. It produces a porous, adhesive, Ca- and P-containing coating on the surface of sandblasted titanium substrate in situ to improve its bioactive properties and performance. Compared to sole PEO technique, the combination treatment has exhibited its excellent advantages of creating favorable surface characteristics such as microstructure, phase and element composition, and microroughness and microhardness. It could also promote the formation of hydroxyapatite on the treated coating, as a result of some micro-sized or nanostructure of crystallized hydroxyapatite deposited on the surface in SBF after 5 days of immersion.

Fig. 10. Cell viabilities by MTT after 24–72 h seeding. (*P b 0.05, compared with control group.)

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Fig. 11. Morphologies of cells cultured on the C-group coatings for 72 h: (a) low magnification and (b) high magnification.

Furthermore, the cell viability was significantly enhanced. The PEO technique together with sandblasting might provide another choice to fabricate high-performance coatings on titanium and its alloys and this kind of coating could exhibit an optimum combination of mechanical properties and bioactivity. Acknowledgments The author would like to thank Dr. X. Zhang and Dr. C. C. Jiang of Shandong University for their friendly help to this work. We also would like to acknowledge the financial support provided by the Natural Science Foundation of Shandong Province (Grant No. ZR2009FM019) and the Visiting Scholar Project for Young Backbone Teachers of Shandong Province Colleges and Universities (Grant No. LJRZ(2013)08).

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