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Atmospheric plasma sprayed silica–hydroxyapatite coatings on magnesium alloy substrates
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 6005–6012 www.elsevier.com/locate/ceramint
Atmospheric plasma sprayed silica–hydroxyapatite coatings on magnesium alloy substrates Erzsébet Sára Bogyaa,b, Zoltán Károlyc, Réka Barabásd,n a
Department of Chemical Engineering, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, 11 Arany Janos St, RO-400028 Cluj-Napoca, Romania b MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla sq. 1, H-6720 Szeged, Hungary c Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry, Hungarian Academy of Sciences, Magyar tudósok krt. No. 2, H-1117 Budapest, Hungary d Department of Chemistry and Chemical Engineering of the Hungarian Line of Study, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, 11 Arany Janos St, RO-400028 Cluj-Napoca, Romania Received 4 December 2014; received in revised form 22 December 2014; accepted 7 January 2015 Available online 14 January 2015
Abstract Silica-doped hydroxyapatite as a bioactive coating presents some advantages compared to the pure one, such as: increased in vivo bioactivity and early bone ingrowth. The aim of this study is to obtain a deposition of silica-doped hydroxyapatite on magnesium alloy plates by atmospheric plasma spraying. The coating material was prepared by a precipitation method with sodium silicate addition as a source of silica, and various methods were used to characterize it. Spraying conditions including powder feed rate and current values were varied. The coating properties were defined by determining the purity, phase composition, morphology and corrosion protection of the HAP–Si deposits on the magnesium plates. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Silica–hydroxyapatite; Plasma spraying; Magnesium alloy; Bio-protective coating
1. Introduction Hydroxyapatite (HAP), Ca10(PO4)6(OH)2 is a major bone component. For this reason, it has been mostly used as a biomaterial for over 20 years in medicine and dentistry [1]. Although their mechanical properties are insufficient to use them as load-bearing implants, HAP ceramics can be applied as coatings on metallic implants. One way to enhance the mechanical strength of hydroxyapatite is by obtaining substituted apatites, which resemble the chemical composition and structure of the mineral phase in bones. From this necessity a new generation of silicon substituted hydroxyapatite (HAP–Si) bioactive material has emerged. It has been demonstrated that hydroxyapatite modified with the inclusion of small concentrations of silicon improves n
Corresponding author. E-mail address: [email protected] (R. Barabás).
http://dx.doi.org/10.1016/j.ceramint.2015.01.041 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
the in vivo bioactivity [2] and benefits the early bone ingrowth and repair through the incorporation of Si [3–5]. This means that the implant is able to chemically join the bone through a strong “bioactive bond”. HAP–Si was found to have increased surface adhesion compared to un-substituted HAP [6,7]. In this way, the osteointegration and the good performance of the implant are ensured [7]. Another important factor in implant design, which has to be taken into consideration, is the implant substrate material. Titanium (Ti) and its alloys are the most commonly used metallic materials, but stainless steel, cobalt–chromium alloys, ceramics and polymer composites were also studied [8]. Each has its advantages and limitations, too. For instance, the density, elastic modulus and yield strength of magnesium are closer to the bone tissue than that of the conventional implants. Due to these excellent mechanical properties, as well as for their biocompatibility, magnesium and its alloys have attracted ever-increasing attention for load-bearing biocompatible implant
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applications [9]. Moreover, Mg is also an essential element for bone metabolism that promotes the formation of new bone tissue [10]; being a cofactor for many enzymes, it stabilizes the structures of DNA and RNA [11]. The major drawback of using magnesium in many engineering applications is its low corrosion resistance, which may manifest in pitting corrosion [12], especially in electrolytic, aqueous environments. The corrosion behavior of the magnesium substrates was briefly studied in different media, such as NaCl solution, simulated body fluid and Hanks' balanced salt solution, using unpolarized and polarized [13] techniques. Several approaches are applied to reduce the corrosion rate or even stop the corrosion of magnesium by using alloying elements and protective coatings; of course, these processes must lead to a non-toxic, biologically compatible material. The most effective approach in order to improve the corrosion resistance of magnesium alloys is probably by covering their surfaces with an additional layer [14]. Coatings may induce biodegradation at a controlled rate, thus offering a limited barrier function. Coatings on magnesium and magnesium alloys can be manufactured through different methods [15] and all these were thoroughly studied in the past. Calcium phosphates represent a big promise for they can be used as biomedical coatings on magnesium substrates [16]. The literature of the last couple of years enumerates several methods for hydroxyapatite deposition on magnesium/Mg alloys: sol–gel coating [17], electrodeposition [18,19], a cathodic arc PVD technique [20], chemical solution deposition [21–23], dipping and immersion [24,25], cold spray deposition [26], plasma electrolytic oxidation [27,28], and a transonic particle acceleration deposition process [29]. However, the available literature presenting hydroxyapatite coatings deposition with atmospheric plasma spraying on magnesium alloy substrates is incomplete. Plasma spraying is used to produce almost all commercially available HAP coatings for orthopedic and dental implants [7,30,31]. The major drawback of this process is that when HAP is plasma sprayed, it may transform into other crystalline phases of calcium phosphate, such as α- or β-tricalcium phosphate, tetracalcium phosphate or calcium oxide; the crystallinity of HAP may also be lowered due to the rapid solidification. These alterations in chemistry and crystallinity often interfere with the novel bioactive properties of HAP, as well as with its adhesion to the implant [26]. The amorphous material and other phases are significantly more soluble than hydroxyapatite, therefore these are preferentially dissolved in vitro [32]. The advantage of this method is that plasma spraying is a well-established tool to deposit HAP on metallic implants [7]; also, many studies were performed to investigate the microstructure, phase transformation, influence of post-treatment on the properties of sprayed HAP in vitro and in vivo [33,34]. The aim of this work is to obtain a deposition of silica doped hydroxyapatite bio-compound on magnesium alloy substrate material using a plasma spraying technique. The novelty of the work relies on the application of plasma spraying to achieve HAP–Si coatings on magnesium alloy, which considering the studied literature, was barely applied before.
2. Material and methods 2.1. Material preparation and characterization Silica doped hydroxyapatite with 1.5 wt% silica content was prepared by precipitation, under controlled conditions, as described in one of our previous article [35]. The following materials were used: calcium nitrate tetrahydrate, diammonium hydrogen phosphate, 25% ammonia solution (Merck, Germany), sodium silicate (Lach:ner, Czech Republic). The reaction time was 20 h. After filtration, the resulting material was dried for 24 h at 105 1C. The heat treatment was performed at 1000 1C. Various methods were used to characterize the calcined material. The characteristic vibrations of the material were identified by infrared spectroscopy (Bruker Vertex 70 FT-IR); the morphological structure was studied with a Philips XL30 scanning electron microscope; and the particle size distribution was determined with a SALD-7101 powder and particle size analyzer (Shimadzu), with an ultrasonic dispersion time of 10 s. In order to identify the phase composition and content, Xray powder diffraction measurements were made using CuKα radiation (typically 40 kV, 40 mA) and a Siemens D5000 diffractometer. The XRD data were obtained over 2θ range of 20–801 at 0.041 step size. The evaluation of the diffraction patterns was obtained by full profile fitting techniques. 2.2. Plasma spraying process The coated substrates were magnesium plates (52 mm 30 mm 2.5 mm), which were grit blasted by corundum, prior to spraying, in order to enhance roughness and adhesion of the coating. After that, the specimens were ultrasonically cleaned in water and rinsed with acetone. The composition of the magnesium plates was determined by an XRF analyzer (NITON, XL3t 900). The atmospheric plasma spraying was carried out with a commercial plasma spray gun (Metco 9MB). The main operating conditions – including the applied voltage, current, gas flow rates and powder feed rate – are summarized in Table 1. The applied conditions were determined in preliminary tests that are not detailed here. During the main tests, only the feeding rate was varied, by which we could investigate the effect of the specific energy on the crystalline phase of the coating. 2.3. Coating heat treatment Heat treatment on the sprayed coatings was also employed in order to investigate its effect on possible post-phase transformation. It was carried out in two different ways: i) at 600 1C in air for one and three hours; ii) hydrothermal treatment at 450 1C in water vapor enriched media for 1 h. 2.4. Coating characterization The coatings were characterized by X-ray diffraction, using CuKα radiation 2θ range 10–801 at 0.011 step size to determine
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the phase composition. The surface morphology and deposit thickness were studied with scanning electron microscopy. Potentiodynamic polarization tests were conducted in order to investigate the electrochemical corrosion behavior of the uncoated magnesium alloy and HAP–Si coated magnesium alloy specimens (A ¼ 1 cm2). A Zahner Im6e potentiostat interfaced with a computer was used to carry out corrosion measurements. The magnesium alloy specimen formed the working electrode, a saturated calomel electrode (SCE) was the reference electrode and a Pt wire served as the counterelectrode. The parameters for conducting the potentiodynamic scan in order to calculate the corrosion rate by plotting the Tafel plot are listed in Table 2. A 3.5% NaCl solution was applied as an electrolyte. The initial delay refers to the stabilization of the immersed specimen in NaCl solution [36].
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Fig. 1. IR spectra of silica doped hydroxyapatite.
3. Results and discussions 3.1. Characterization of materials 3.1.1. Infra-red spectroscopy Fig. 1 The IR spectrum of the calcined HAP–Si 1.5% shows characteristics of absorption bands typical of hydroxyapatites. The intense bands at 1094, 1034 and 963, 472 cm 1 correspond to P–O υ3 and υ2 modes. The doublet at 604–565 cm 1 is attributed to the O–P–O bending mode. The band at 630 cm 1 corresponds to the hydroxyl group vibrational mode. The band characteristic of adsorbed water (3570 cm 1) is also present [37]. The low intensity pick at 837 cm 1 can be related to the presence of SiO4 groups in the apatite structure [5]. 4 3.1.2. XRD measurements The X-ray diffraction patterns of silica-doped hydroxyapatite can be seen in Fig. 2. The diffraction peaks are typical for the apatite structure. The material is well crystallized, showing many Table 1 Operating conditions of plasma spraying. Parameter
Value
Voltage (V) Current (A) Plasma gas (dm3 min 1)
50–80 250–380 Ar–42.5 H2–2.1 Ar–11–19 1–12.5 5–10 100–300
Carrier gas (dm3 min 1) Powder feed rate (g min 1) Spraying distance (cm) Substrate pre-heating temperature (1C)
Table 2 Parameters for conducting the potentiodynamic scan. Parameter
Value
Initial potential Final potential Scan rate Initial delay
2 V 1.2 V 1 mV/s 2h
Fig. 2. XRD pattern of HAP-1.5 wt% Si.
sharp X-ray diffraction peaks, having a hexagonal symmetry, with a crystallite size of 17 nm. The material does not contain any other secondary phases. The calculated cell parameters are: a¼ b¼ 9.418 Ǻ, c¼ 6.892 Ǻ, slightly higher values than those of pure HAP (a¼ b¼ 9.41339 Ǻ, c¼ 6.88181 Ǻ [38]). The small changes in the crystal structure can be the result of the incorporation of a slightly larger silicate ion – instead of a phosphate ion –, and the loss of some of the OH groups [38]. 3.1.3. Average particle size and particle size distribution The particle size distribution (PSD) can be defined as: NðDÞ ¼ N`ðDÞ ΔD
ð1Þ
where N(D) is the average number of particles per unit volume in the size interval D 7 0.5ΔD. In this formulation, D represents the median diameter of each size class. The measure of particle size D can be expressed in a variety of ways, such as particle volume, projected-area, or diameter. N0 (D) represents the differential frequency distribution per unit size [39]. The calculated mean particle size – determined by the distribution of the differential number (A) and volume size (B) – is presented in Table 3, and the distribution is shown in Fig. 3. The differential volume size distribution shows that the particle amount is distributed in two particle size ranges: one of 0.01–0.0145 μm, representing 46.4% of the total amount
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(single particles), and the other of 0.6–130 μm, being 53.6% (agglomerated particles). The comparison of number and volume size distribution demonstrates that, although the majority of the particles have low particle diameter, the agglomeration tendency is high. As can be seen in Fig. 3, nano-sized hydroxyapatite particles possess a high aggregation tendency, and the large majority of the particles are assembled in micro-sized conglomerations. 3.1.4. Scanning electron microscopy Fig. 4 The SEM image of the feedstock powder supports the aggregation of the particles. It can also be observed that the fine particles tend to aggregate in a spherical shaped mass. 3.2. Plasma spraying
Fig. 4. SEM image of the feedstock powder.
coating containing phases was determined from the XRD spectra (see Table 5). It can be observed, that the crystallite size increased to a value of approximately 100 nm.
The chemical composition of the magnesium plate is shown in Table 4. It also contains 1.2% of other elements, such as: Fe, Zr, Ti, Ni, and Pb. Table 5 presents the spraying parameters and the phase composition of the coating determined by X-ray diffraction. The following parameters were not varied: the substrate preheating temperature was Z 200 1C, U ¼ 50 V, the plasma gas mixture consisted of argon and hydrogen, with 42.5 and 2.1 cm3 min 1 feed rate, respectively. The carrier gas was argon and the flow rate was set to 19 cm3 min 1. The spraying distance was 10 cm. Coating phase composition was studied with X-ray diffraction (Fig. 5). As can be observed in Table 5, a high decomposition of the material occurs and the quantitative phase composition is also presented. The crystallite size of the Table 3 Mean particle size for silica–hydroxyapatite. Differential distribution type
Mean particle size (mm)
Standard deviation (mm)
Number Volume size
0.015 0.948
0.180 1.426
Fig. 5. XRD patterns of silica–hydroxyapatite before and after plasma spraying (for plasma spraying conditions No. 4, Table 5).
The exposure of the HAP–Si powder particles to high temperature during the plasma spraying process leads to the dehydroxylation and decomposition of the material.
Fig. 3. Differential number (A) and volume size (B) distribution for HAP–Si 1.5%.
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The dehydroxylation of HAP occurs above 800 1C; above 1050 1C HAP decomposes to β-tricalcium phosphate and tetra-calcium phosphate; finally, above 1120 1C β-tricalcium phosphate is converted to α-tricalcium phosphate [40]. The optimal spraying parameters were defined in order to obtain parameters resulting in high coating purity. Low material feed rate results in a higher decomposition rate, which can be explained by the higher heat transfer per unit material. Table 5 supports the conclusion that the optimal feed rate is around 5 g min 1, when the applied current is 300 A. Good biocompatibility, however, requires that the coating material contains a highly crystalline form of hydroxyapatite. The other forms of transitional phases and amorphous calcium phosphate have different biological responses and show enhanced solubility in human blood serum and simulated body fluid [41,42]. It is also known that the thermal decomposition of the apatite can be reversed with heat treatment. Post-heat treatment of the coating can promote the phase transformation of amorphous calcium phosphate into crystalline hydroxyapatite, and it can also result in great uniformity of the coating composition. In addition, it improves the bonding strength by further diffusion [43]. When water comes into contact with these amorphous phases, water molecules take part in and promote the transformation of unstable phases into stable hydroxyapatite. The reaction is suggested by Cao et al. [44], as follows: watervapor
Ca4 P2 O9 þ 2Ca3 ðPO4 Þ2 þ H 2 O 3Ca3 ðPO4 Þ2 þ H 2 O þ CaO
-
watervapor
-
2Ca5 ðPO4 Þ3 ðOHÞ
2Ca5 ðPO4 Þ3 ðOHÞ
into other phosphates. The XRD result showed that the quantity of HAP increased significantly after heat treatment, while αtricalcium phosphate, β-tricalcium phosphate, tetracalcium phosphate, and CaO phases were almost undetectable. Tsui et al. [51] found that heat treatment in air at 700 1C for 1 h proved to be effective in increasing the crystallinity, regaining the OH ion and removing other non-HAP compounds, although it caused a significant decrease in the degree of adhesion (interfacial fracture toughness). Lu et al. [52] state that excessive high temperatures and long holding times are unfavorable due to the decrease in the structural integrity of HAP; also, heat treatment at 600 1C is sufficient to maintain hydroxyl levels within the coating and to achieve a complete crystallization of the amorphous phase. Because the magnesium substrate is heat sensitive and the melting point is around 650 1C, the post-heat treatment was carried out below this temperature. Two approaches were applied: i) heat treatment in air at 600 1C for one and three hours, respectively; ii) furnace treatment in water vapor enriched media at 450 1C for 1 h. Both treatments led to the increase of the apatite phase (an increase of 10–20%, maximal hydroxyapatite content of 85%), but calcium-orthophosphate and lime were also present. We found that the heat treatment duration does not influence significantly the final phase composition. Supplementary investigations are needed in order to increase the HAP content in the coatings. A hydrothermal post-treatment approach is suggested, as a potential method, which could lead to higher HAP content. In the following sections, results are presented for coatings treated in air for 1 h at 600 1C.
ð2Þ ð3Þ
Various post-spray heat treatments have been reported, including heat treatment in air [45,46], laser treatment [47], hot isostatic pressing [48], electric polarization treatment [49], and water vapor treatment [44]. Dong et al. [50] found that controlled post-spray heat treatment, for e.g. 600 1C for 6 h, effectively and preferentially transformed the amorphous calcium phosphate into hydroxyapatite, rather than
3.3. Characterization of coating 3.3.1. Scanning electron microscopy Fig. 6(A) and (B) shows the SEM images reflecting the surface morphology and HAP–Si coating structure. The SEM micrographs clearly indicate that a dense HAP–Si coating is deposited on the magnesium plates. Based on this microstructure, it was found that HAP–Si particles were well bonded to each other. It is possible, that the high impact velocity of the particles in conjunction with the heat from the substrate has caused the particle bonding. The dense and rough surfaces of the coatings are mainly composed of well-flattened splats and partially melted round shaped particles. Some micro-cracks are visible. Post-heat treatment not only increases the HAP content of the coating, but also releases
Table 4 Chemical composition of the magnesium substrates.
Mg Al Zn Mn Fe
Weight percent
Error (%)
93.313 3.181 2.322 0.764 0.164
0.381 0.371 0.047 0.054 0.026
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Table 5 Coating phase composition obtained by plasma spraying for different spraying parameters. No.
1. 2. 3. 4.
Current
A 300 250 300 300
Powder feed rate
g min 1 4.8 7.6 3.3 3.1
Phase composition Ca5(PO4)3OH
Crystallite size
Ca-orthophosphate
Crystallite size
Whitlockite
Crystallite size
CaO
% 80 50 50 35
nm 110 110 110 110
% 5 30 25 40
nm 110 110 110 110
% 15 15 15 10
nm 110 110 110 71
%
10 15
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Fig. 6. SEM images of HAP–Si coating on magnesium plates before (A) and after post-heat treatment (B) and cross section image (C).
the residual stress between the coating and the substrate; moreover, it also reduces the micro-cracks and pores in the coating. The SEM images of the coatings after heat treatment (see Fig. 6(B)) support this theory, the majority of the micro-cracks have disappeared, and a much smoother surface is visible. Fig. 6(C) shows the cross-sectional view of the coating. The film thickness is ranged around 100 μm. Sun et al. [53] report that the coating thickness is completely depending on the location of implantation, cellular environment, cleanliness of implant, and coating characteristics. For example, a thickness of 50–75 μm has
been preferred by most manufacturers of commercially used orthopedic implants [53], but for dental implants the optimal coating thickness can be totally different. 3.3.2. Corrosion measurements The Tafel extrapolation method was used to determine the corrosion parameters such as corrosion potential (ECorr) and corrosion current density (ICorr) from potentiodynamic curves (see Fig. 7). The results of these corrosion parameters are shown in Table 6.
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spraying, for the sake of obtaining bio-protective coatings on magnesium substrates. The spraying conditions were varied in order to obtain a stable coating, and they were also further developed to establish the optimal plasma spraying parameters. Quality and coating thickness were determined by scanning electron microscopy. Finally, corrosion measurements proved that postheat treated silica–hydroxyapatite coatings assure high corrosion protection for the magnesium substrate. In conclusion, it can be stated that the thoroughly studied plasma spraying method can be applied to obtain high quality, stable, dense silica–hydroxyapatite deposition on magnesium alloy substrates. Fig. 7. Potentiodynamic polarization curves in 3.5% NaCl solution for uncoated and HAP–Si 1.5% coated Mg plates before and after post-heat treatment.
Table 6 Corrosion parameters – corrosion potential and corrosion current density – for uncoated and HAP–Si 1.5% coated Mg plates before and after post-heat treatment. Sample
Corrosion potential (ECorr) V
Mg plate 1.425 HAP–Si coating – before post-heat 1.371 treatment HAP–Si coating – after post-heat 1.215 treatment
Current density (ICorr) A 8.22 10 6 7.09 10 7 1.09 10–7
The chance of corrosion in a material depends on the corrosion current density (ICorr) with a given potential; materials with higher value of ICorr are more prone to corrode. The shifting of corrosion potential towards more positive values indicates a higher corrosion resistance of the coated alloys. The corrosion current density of the coated samples was more than one order of magnitude lower than that of the Mg alloy, confirming that the HAP–Si coating on the magnesium alloy plate promoted its corrosion resistance. The same pattern was reported for hydroxyapatite–chitosan composite coatings on AZ31 Mg alloy [54]. The results indicate that post-heat treatment of the plasma sprayed coatings increases the corrosion resistance of the sprayed Mg plates. The lower corrosion protection of the HAP–Si coating before post-heat treatment can be explained by the presence of a higher number of cracks. These cracks accelerate the dissolution of the coating, because they act as active sites for pitting corrosion [55]. 4. Conclusions Silica–hydroxyapatite was successfully deposited as protection coating on magnesium plates. The silica was added to the hydroxyapatite because of its known property to increase in vivo bioactivity and early bone ingrowth. The novelty of this paper lies in the application of a thoroughly studied process, namely plasma
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 6005–6012 www.elsevier.com/locate/ceramint
Atmospheric plasma sprayed silica–hydroxyapatite coatings on magnesium alloy substrates Erzsébet Sára Bogyaa,b, Zoltán Károlyc, Réka Barabásd,n a
Department of Chemical Engineering, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, 11 Arany Janos St, RO-400028 Cluj-Napoca, Romania b MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla sq. 1, H-6720 Szeged, Hungary c Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry, Hungarian Academy of Sciences, Magyar tudósok krt. No. 2, H-1117 Budapest, Hungary d Department of Chemistry and Chemical Engineering of the Hungarian Line of Study, Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, 11 Arany Janos St, RO-400028 Cluj-Napoca, Romania Received 4 December 2014; received in revised form 22 December 2014; accepted 7 January 2015 Available online 14 January 2015
Abstract Silica-doped hydroxyapatite as a bioactive coating presents some advantages compared to the pure one, such as: increased in vivo bioactivity and early bone ingrowth. The aim of this study is to obtain a deposition of silica-doped hydroxyapatite on magnesium alloy plates by atmospheric plasma spraying. The coating material was prepared by a precipitation method with sodium silicate addition as a source of silica, and various methods were used to characterize it. Spraying conditions including powder feed rate and current values were varied. The coating properties were defined by determining the purity, phase composition, morphology and corrosion protection of the HAP–Si deposits on the magnesium plates. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Silica–hydroxyapatite; Plasma spraying; Magnesium alloy; Bio-protective coating
1. Introduction Hydroxyapatite (HAP), Ca10(PO4)6(OH)2 is a major bone component. For this reason, it has been mostly used as a biomaterial for over 20 years in medicine and dentistry [1]. Although their mechanical properties are insufficient to use them as load-bearing implants, HAP ceramics can be applied as coatings on metallic implants. One way to enhance the mechanical strength of hydroxyapatite is by obtaining substituted apatites, which resemble the chemical composition and structure of the mineral phase in bones. From this necessity a new generation of silicon substituted hydroxyapatite (HAP–Si) bioactive material has emerged. It has been demonstrated that hydroxyapatite modified with the inclusion of small concentrations of silicon improves n
Corresponding author. E-mail address: [email protected] (R. Barabás).
http://dx.doi.org/10.1016/j.ceramint.2015.01.041 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
the in vivo bioactivity [2] and benefits the early bone ingrowth and repair through the incorporation of Si [3–5]. This means that the implant is able to chemically join the bone through a strong “bioactive bond”. HAP–Si was found to have increased surface adhesion compared to un-substituted HAP [6,7]. In this way, the osteointegration and the good performance of the implant are ensured [7]. Another important factor in implant design, which has to be taken into consideration, is the implant substrate material. Titanium (Ti) and its alloys are the most commonly used metallic materials, but stainless steel, cobalt–chromium alloys, ceramics and polymer composites were also studied [8]. Each has its advantages and limitations, too. For instance, the density, elastic modulus and yield strength of magnesium are closer to the bone tissue than that of the conventional implants. Due to these excellent mechanical properties, as well as for their biocompatibility, magnesium and its alloys have attracted ever-increasing attention for load-bearing biocompatible implant
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applications [9]. Moreover, Mg is also an essential element for bone metabolism that promotes the formation of new bone tissue [10]; being a cofactor for many enzymes, it stabilizes the structures of DNA and RNA [11]. The major drawback of using magnesium in many engineering applications is its low corrosion resistance, which may manifest in pitting corrosion [12], especially in electrolytic, aqueous environments. The corrosion behavior of the magnesium substrates was briefly studied in different media, such as NaCl solution, simulated body fluid and Hanks' balanced salt solution, using unpolarized and polarized [13] techniques. Several approaches are applied to reduce the corrosion rate or even stop the corrosion of magnesium by using alloying elements and protective coatings; of course, these processes must lead to a non-toxic, biologically compatible material. The most effective approach in order to improve the corrosion resistance of magnesium alloys is probably by covering their surfaces with an additional layer [14]. Coatings may induce biodegradation at a controlled rate, thus offering a limited barrier function. Coatings on magnesium and magnesium alloys can be manufactured through different methods [15] and all these were thoroughly studied in the past. Calcium phosphates represent a big promise for they can be used as biomedical coatings on magnesium substrates [16]. The literature of the last couple of years enumerates several methods for hydroxyapatite deposition on magnesium/Mg alloys: sol–gel coating [17], electrodeposition [18,19], a cathodic arc PVD technique [20], chemical solution deposition [21–23], dipping and immersion [24,25], cold spray deposition [26], plasma electrolytic oxidation [27,28], and a transonic particle acceleration deposition process [29]. However, the available literature presenting hydroxyapatite coatings deposition with atmospheric plasma spraying on magnesium alloy substrates is incomplete. Plasma spraying is used to produce almost all commercially available HAP coatings for orthopedic and dental implants [7,30,31]. The major drawback of this process is that when HAP is plasma sprayed, it may transform into other crystalline phases of calcium phosphate, such as α- or β-tricalcium phosphate, tetracalcium phosphate or calcium oxide; the crystallinity of HAP may also be lowered due to the rapid solidification. These alterations in chemistry and crystallinity often interfere with the novel bioactive properties of HAP, as well as with its adhesion to the implant [26]. The amorphous material and other phases are significantly more soluble than hydroxyapatite, therefore these are preferentially dissolved in vitro [32]. The advantage of this method is that plasma spraying is a well-established tool to deposit HAP on metallic implants [7]; also, many studies were performed to investigate the microstructure, phase transformation, influence of post-treatment on the properties of sprayed HAP in vitro and in vivo [33,34]. The aim of this work is to obtain a deposition of silica doped hydroxyapatite bio-compound on magnesium alloy substrate material using a plasma spraying technique. The novelty of the work relies on the application of plasma spraying to achieve HAP–Si coatings on magnesium alloy, which considering the studied literature, was barely applied before.
2. Material and methods 2.1. Material preparation and characterization Silica doped hydroxyapatite with 1.5 wt% silica content was prepared by precipitation, under controlled conditions, as described in one of our previous article [35]. The following materials were used: calcium nitrate tetrahydrate, diammonium hydrogen phosphate, 25% ammonia solution (Merck, Germany), sodium silicate (Lach:ner, Czech Republic). The reaction time was 20 h. After filtration, the resulting material was dried for 24 h at 105 1C. The heat treatment was performed at 1000 1C. Various methods were used to characterize the calcined material. The characteristic vibrations of the material were identified by infrared spectroscopy (Bruker Vertex 70 FT-IR); the morphological structure was studied with a Philips XL30 scanning electron microscope; and the particle size distribution was determined with a SALD-7101 powder and particle size analyzer (Shimadzu), with an ultrasonic dispersion time of 10 s. In order to identify the phase composition and content, Xray powder diffraction measurements were made using CuKα radiation (typically 40 kV, 40 mA) and a Siemens D5000 diffractometer. The XRD data were obtained over 2θ range of 20–801 at 0.041 step size. The evaluation of the diffraction patterns was obtained by full profile fitting techniques. 2.2. Plasma spraying process The coated substrates were magnesium plates (52 mm 30 mm 2.5 mm), which were grit blasted by corundum, prior to spraying, in order to enhance roughness and adhesion of the coating. After that, the specimens were ultrasonically cleaned in water and rinsed with acetone. The composition of the magnesium plates was determined by an XRF analyzer (NITON, XL3t 900). The atmospheric plasma spraying was carried out with a commercial plasma spray gun (Metco 9MB). The main operating conditions – including the applied voltage, current, gas flow rates and powder feed rate – are summarized in Table 1. The applied conditions were determined in preliminary tests that are not detailed here. During the main tests, only the feeding rate was varied, by which we could investigate the effect of the specific energy on the crystalline phase of the coating. 2.3. Coating heat treatment Heat treatment on the sprayed coatings was also employed in order to investigate its effect on possible post-phase transformation. It was carried out in two different ways: i) at 600 1C in air for one and three hours; ii) hydrothermal treatment at 450 1C in water vapor enriched media for 1 h. 2.4. Coating characterization The coatings were characterized by X-ray diffraction, using CuKα radiation 2θ range 10–801 at 0.011 step size to determine
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the phase composition. The surface morphology and deposit thickness were studied with scanning electron microscopy. Potentiodynamic polarization tests were conducted in order to investigate the electrochemical corrosion behavior of the uncoated magnesium alloy and HAP–Si coated magnesium alloy specimens (A ¼ 1 cm2). A Zahner Im6e potentiostat interfaced with a computer was used to carry out corrosion measurements. The magnesium alloy specimen formed the working electrode, a saturated calomel electrode (SCE) was the reference electrode and a Pt wire served as the counterelectrode. The parameters for conducting the potentiodynamic scan in order to calculate the corrosion rate by plotting the Tafel plot are listed in Table 2. A 3.5% NaCl solution was applied as an electrolyte. The initial delay refers to the stabilization of the immersed specimen in NaCl solution [36].
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Fig. 1. IR spectra of silica doped hydroxyapatite.
3. Results and discussions 3.1. Characterization of materials 3.1.1. Infra-red spectroscopy Fig. 1 The IR spectrum of the calcined HAP–Si 1.5% shows characteristics of absorption bands typical of hydroxyapatites. The intense bands at 1094, 1034 and 963, 472 cm 1 correspond to P–O υ3 and υ2 modes. The doublet at 604–565 cm 1 is attributed to the O–P–O bending mode. The band at 630 cm 1 corresponds to the hydroxyl group vibrational mode. The band characteristic of adsorbed water (3570 cm 1) is also present [37]. The low intensity pick at 837 cm 1 can be related to the presence of SiO4 groups in the apatite structure [5]. 4 3.1.2. XRD measurements The X-ray diffraction patterns of silica-doped hydroxyapatite can be seen in Fig. 2. The diffraction peaks are typical for the apatite structure. The material is well crystallized, showing many Table 1 Operating conditions of plasma spraying. Parameter
Value
Voltage (V) Current (A) Plasma gas (dm3 min 1)
50–80 250–380 Ar–42.5 H2–2.1 Ar–11–19 1–12.5 5–10 100–300
Carrier gas (dm3 min 1) Powder feed rate (g min 1) Spraying distance (cm) Substrate pre-heating temperature (1C)
Table 2 Parameters for conducting the potentiodynamic scan. Parameter
Value
Initial potential Final potential Scan rate Initial delay
2 V 1.2 V 1 mV/s 2h
Fig. 2. XRD pattern of HAP-1.5 wt% Si.
sharp X-ray diffraction peaks, having a hexagonal symmetry, with a crystallite size of 17 nm. The material does not contain any other secondary phases. The calculated cell parameters are: a¼ b¼ 9.418 Ǻ, c¼ 6.892 Ǻ, slightly higher values than those of pure HAP (a¼ b¼ 9.41339 Ǻ, c¼ 6.88181 Ǻ [38]). The small changes in the crystal structure can be the result of the incorporation of a slightly larger silicate ion – instead of a phosphate ion –, and the loss of some of the OH groups [38]. 3.1.3. Average particle size and particle size distribution The particle size distribution (PSD) can be defined as: NðDÞ ¼ N`ðDÞ ΔD
ð1Þ
where N(D) is the average number of particles per unit volume in the size interval D 7 0.5ΔD. In this formulation, D represents the median diameter of each size class. The measure of particle size D can be expressed in a variety of ways, such as particle volume, projected-area, or diameter. N0 (D) represents the differential frequency distribution per unit size [39]. The calculated mean particle size – determined by the distribution of the differential number (A) and volume size (B) – is presented in Table 3, and the distribution is shown in Fig. 3. The differential volume size distribution shows that the particle amount is distributed in two particle size ranges: one of 0.01–0.0145 μm, representing 46.4% of the total amount
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(single particles), and the other of 0.6–130 μm, being 53.6% (agglomerated particles). The comparison of number and volume size distribution demonstrates that, although the majority of the particles have low particle diameter, the agglomeration tendency is high. As can be seen in Fig. 3, nano-sized hydroxyapatite particles possess a high aggregation tendency, and the large majority of the particles are assembled in micro-sized conglomerations. 3.1.4. Scanning electron microscopy Fig. 4 The SEM image of the feedstock powder supports the aggregation of the particles. It can also be observed that the fine particles tend to aggregate in a spherical shaped mass. 3.2. Plasma spraying
Fig. 4. SEM image of the feedstock powder.
coating containing phases was determined from the XRD spectra (see Table 5). It can be observed, that the crystallite size increased to a value of approximately 100 nm.
The chemical composition of the magnesium plate is shown in Table 4. It also contains 1.2% of other elements, such as: Fe, Zr, Ti, Ni, and Pb. Table 5 presents the spraying parameters and the phase composition of the coating determined by X-ray diffraction. The following parameters were not varied: the substrate preheating temperature was Z 200 1C, U ¼ 50 V, the plasma gas mixture consisted of argon and hydrogen, with 42.5 and 2.1 cm3 min 1 feed rate, respectively. The carrier gas was argon and the flow rate was set to 19 cm3 min 1. The spraying distance was 10 cm. Coating phase composition was studied with X-ray diffraction (Fig. 5). As can be observed in Table 5, a high decomposition of the material occurs and the quantitative phase composition is also presented. The crystallite size of the Table 3 Mean particle size for silica–hydroxyapatite. Differential distribution type
Mean particle size (mm)
Standard deviation (mm)
Number Volume size
0.015 0.948
0.180 1.426
Fig. 5. XRD patterns of silica–hydroxyapatite before and after plasma spraying (for plasma spraying conditions No. 4, Table 5).
The exposure of the HAP–Si powder particles to high temperature during the plasma spraying process leads to the dehydroxylation and decomposition of the material.
Fig. 3. Differential number (A) and volume size (B) distribution for HAP–Si 1.5%.
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The dehydroxylation of HAP occurs above 800 1C; above 1050 1C HAP decomposes to β-tricalcium phosphate and tetra-calcium phosphate; finally, above 1120 1C β-tricalcium phosphate is converted to α-tricalcium phosphate [40]. The optimal spraying parameters were defined in order to obtain parameters resulting in high coating purity. Low material feed rate results in a higher decomposition rate, which can be explained by the higher heat transfer per unit material. Table 5 supports the conclusion that the optimal feed rate is around 5 g min 1, when the applied current is 300 A. Good biocompatibility, however, requires that the coating material contains a highly crystalline form of hydroxyapatite. The other forms of transitional phases and amorphous calcium phosphate have different biological responses and show enhanced solubility in human blood serum and simulated body fluid [41,42]. It is also known that the thermal decomposition of the apatite can be reversed with heat treatment. Post-heat treatment of the coating can promote the phase transformation of amorphous calcium phosphate into crystalline hydroxyapatite, and it can also result in great uniformity of the coating composition. In addition, it improves the bonding strength by further diffusion [43]. When water comes into contact with these amorphous phases, water molecules take part in and promote the transformation of unstable phases into stable hydroxyapatite. The reaction is suggested by Cao et al. [44], as follows: watervapor
Ca4 P2 O9 þ 2Ca3 ðPO4 Þ2 þ H 2 O 3Ca3 ðPO4 Þ2 þ H 2 O þ CaO
-
watervapor
-
2Ca5 ðPO4 Þ3 ðOHÞ
2Ca5 ðPO4 Þ3 ðOHÞ
into other phosphates. The XRD result showed that the quantity of HAP increased significantly after heat treatment, while αtricalcium phosphate, β-tricalcium phosphate, tetracalcium phosphate, and CaO phases were almost undetectable. Tsui et al. [51] found that heat treatment in air at 700 1C for 1 h proved to be effective in increasing the crystallinity, regaining the OH ion and removing other non-HAP compounds, although it caused a significant decrease in the degree of adhesion (interfacial fracture toughness). Lu et al. [52] state that excessive high temperatures and long holding times are unfavorable due to the decrease in the structural integrity of HAP; also, heat treatment at 600 1C is sufficient to maintain hydroxyl levels within the coating and to achieve a complete crystallization of the amorphous phase. Because the magnesium substrate is heat sensitive and the melting point is around 650 1C, the post-heat treatment was carried out below this temperature. Two approaches were applied: i) heat treatment in air at 600 1C for one and three hours, respectively; ii) furnace treatment in water vapor enriched media at 450 1C for 1 h. Both treatments led to the increase of the apatite phase (an increase of 10–20%, maximal hydroxyapatite content of 85%), but calcium-orthophosphate and lime were also present. We found that the heat treatment duration does not influence significantly the final phase composition. Supplementary investigations are needed in order to increase the HAP content in the coatings. A hydrothermal post-treatment approach is suggested, as a potential method, which could lead to higher HAP content. In the following sections, results are presented for coatings treated in air for 1 h at 600 1C.
ð2Þ ð3Þ
Various post-spray heat treatments have been reported, including heat treatment in air [45,46], laser treatment [47], hot isostatic pressing [48], electric polarization treatment [49], and water vapor treatment [44]. Dong et al. [50] found that controlled post-spray heat treatment, for e.g. 600 1C for 6 h, effectively and preferentially transformed the amorphous calcium phosphate into hydroxyapatite, rather than
3.3. Characterization of coating 3.3.1. Scanning electron microscopy Fig. 6(A) and (B) shows the SEM images reflecting the surface morphology and HAP–Si coating structure. The SEM micrographs clearly indicate that a dense HAP–Si coating is deposited on the magnesium plates. Based on this microstructure, it was found that HAP–Si particles were well bonded to each other. It is possible, that the high impact velocity of the particles in conjunction with the heat from the substrate has caused the particle bonding. The dense and rough surfaces of the coatings are mainly composed of well-flattened splats and partially melted round shaped particles. Some micro-cracks are visible. Post-heat treatment not only increases the HAP content of the coating, but also releases
Table 4 Chemical composition of the magnesium substrates.
Mg Al Zn Mn Fe
Weight percent
Error (%)
93.313 3.181 2.322 0.764 0.164
0.381 0.371 0.047 0.054 0.026
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Table 5 Coating phase composition obtained by plasma spraying for different spraying parameters. No.
1. 2. 3. 4.
Current
A 300 250 300 300
Powder feed rate
g min 1 4.8 7.6 3.3 3.1
Phase composition Ca5(PO4)3OH
Crystallite size
Ca-orthophosphate
Crystallite size
Whitlockite
Crystallite size
CaO
% 80 50 50 35
nm 110 110 110 110
% 5 30 25 40
nm 110 110 110 110
% 15 15 15 10
nm 110 110 110 71
%
10 15
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Fig. 6. SEM images of HAP–Si coating on magnesium plates before (A) and after post-heat treatment (B) and cross section image (C).
the residual stress between the coating and the substrate; moreover, it also reduces the micro-cracks and pores in the coating. The SEM images of the coatings after heat treatment (see Fig. 6(B)) support this theory, the majority of the micro-cracks have disappeared, and a much smoother surface is visible. Fig. 6(C) shows the cross-sectional view of the coating. The film thickness is ranged around 100 μm. Sun et al. [53] report that the coating thickness is completely depending on the location of implantation, cellular environment, cleanliness of implant, and coating characteristics. For example, a thickness of 50–75 μm has
been preferred by most manufacturers of commercially used orthopedic implants [53], but for dental implants the optimal coating thickness can be totally different. 3.3.2. Corrosion measurements The Tafel extrapolation method was used to determine the corrosion parameters such as corrosion potential (ECorr) and corrosion current density (ICorr) from potentiodynamic curves (see Fig. 7). The results of these corrosion parameters are shown in Table 6.
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spraying, for the sake of obtaining bio-protective coatings on magnesium substrates. The spraying conditions were varied in order to obtain a stable coating, and they were also further developed to establish the optimal plasma spraying parameters. Quality and coating thickness were determined by scanning electron microscopy. Finally, corrosion measurements proved that postheat treated silica–hydroxyapatite coatings assure high corrosion protection for the magnesium substrate. In conclusion, it can be stated that the thoroughly studied plasma spraying method can be applied to obtain high quality, stable, dense silica–hydroxyapatite deposition on magnesium alloy substrates. Fig. 7. Potentiodynamic polarization curves in 3.5% NaCl solution for uncoated and HAP–Si 1.5% coated Mg plates before and after post-heat treatment.
Table 6 Corrosion parameters – corrosion potential and corrosion current density – for uncoated and HAP–Si 1.5% coated Mg plates before and after post-heat treatment. Sample
Corrosion potential (ECorr) V
Mg plate 1.425 HAP–Si coating – before post-heat 1.371 treatment HAP–Si coating – after post-heat 1.215 treatment
Current density (ICorr) A 8.22 10 6 7.09 10 7 1.09 10–7
The chance of corrosion in a material depends on the corrosion current density (ICorr) with a given potential; materials with higher value of ICorr are more prone to corrode. The shifting of corrosion potential towards more positive values indicates a higher corrosion resistance of the coated alloys. The corrosion current density of the coated samples was more than one order of magnitude lower than that of the Mg alloy, confirming that the HAP–Si coating on the magnesium alloy plate promoted its corrosion resistance. The same pattern was reported for hydroxyapatite–chitosan composite coatings on AZ31 Mg alloy [54]. The results indicate that post-heat treatment of the plasma sprayed coatings increases the corrosion resistance of the sprayed Mg plates. The lower corrosion protection of the HAP–Si coating before post-heat treatment can be explained by the presence of a higher number of cracks. These cracks accelerate the dissolution of the coating, because they act as active sites for pitting corrosion [55]. 4. Conclusions Silica–hydroxyapatite was successfully deposited as protection coating on magnesium plates. The silica was added to the hydroxyapatite because of its known property to increase in vivo bioactivity and early bone ingrowth. The novelty of this paper lies in the application of a thoroughly studied process, namely plasma
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