Lanthanum- and silicon-incorporated calcium phosphate coatings formed by microarc oxidation E. G. Komarova, M. B. Sedelnikova, Y. P. Sharkeev, M. V. Chaikina, and E. A. Kazanceva
Citation: AIP Conference Proceedings 1909, 020087 (2017); View online: https://doi.org/10.1063/1.5013768 View Table of Contents: http://aip.scitation.org/toc/apc/1909/1 Published by the American Institute of Physics
Lanthanum- and Silicon-Incorporated Calcium Phosphate Coatings Formed by Microarc Oxidation E. G. Komarova1,a), M. B. Sedelnikova1, Y. P. Sharkeev1, M. V. Chaikina2, and E. A. Kazanceva3 2
1 Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634055 Russia Institute of Solid State Chemistry and Mechanochemistry SB RAS, Novosibirsk, 630128 Russia 3 National Research Tomsk State University, Tomsk, 634050 Russia a)
Corresponding author:
[email protected]
Abstract. The structure, phase and elemental compositions of amorphous-crystalline lanthanum- and silicon-incorporated calcium phosphate coatings on titanium substrate deposited by microarc oxidation under different pulsed voltages of 200–350 V were investigated. The increase in the oxidation voltage leads to a linear increase of the coating thickness, surface roughness, surface porosity, and average sizes of structural elements. With increasing pulsed voltage, the amount of the crystalline CaHPO4 phase in the amorphous-crystalline coatings increases from 8 to 57 wt %. It was established that the coatings consist of nanocrystalline phases: β-Ca2P2O7, CaHPO4 and TiO2 (anatase). The coatings with the maximum La and Si content to 0.4 at %, and Ca/P ratio to 0.7 were formed at a voltage of 350 V.
INTRODUCTION An actual problem of implantology is to prevent the implant failure induced by periimplantitis as a result of bacteria migration in the area surrounding the implants. To prevent such complications, it is advisable to use a dopant in the coating composition, such as lanthanum (La) that has antimicrobial and antithrombocyte properties [1]. The presence of silicon (Si) in the coatings has direct effects on the biomineralization process. Many reports have suggested that Si would be essential for metabolic processes associated with the growth and skeletal development and integrity of the extracellular matrix [2]. Microarc oxidation (MAO) is a perspective technology that allows the formation of bioactive calcium phosphate (CaP) coatings modified with such elements as La and Si on the titanium surface, with a wide range of physical and chemical properties, different crystallinity degrees, thickness, roughness and porosity [3–5]. The aim of this work is to investigate the formation of structure, phase and elemental compositions of amorphous-crystalline La- and Si-incorporated calcium phosphate (La–Si–CaP) coatings deposited by MAO in an electrolyte based on hydroxyapatite (HA) with isomorphic simultaneous cationic and anionic substitutions by the La3+ and SiO44–-group, respectively.
MATERIALS AND METHODS The deposition of CaP coatings on Ti substrates of size 10 × 10 × 1 mm3 was carried out by MAO using the MicroArc-3.0 technological complex with a pulsed power source [4]. The electrolyte consists of phosphoric acid (20% H3PO4 aqueous solution), calcium carbonate (CaCO3), and substituted HA in the cationic sublattice by La3+ ions and anionic sublattice by SiO44–-group (Ca9.5La0.5(PO4)5.5(SiO4)0.5(OH)2), as was described in the previous work [5]. The coatings were deposited in the anodic potentiostatic mode with the following parameters: pulse duration 100 µs, frequency 50 Hz, process duration 10 min, and oxidation voltages varied in the range of 200–350 V. The morphology and microstructure of the coatings were observed by scanning electron microscopy (SEM, LEO EVO 50 Zeiss) and transmission electron microscopy (TEM, JEOL JEM-2010) equipped by energy-dispersive X-ray analyzers (EDX, INCA) in the “Nanotech” Center of ISPMS SB RAS (Tomsk). The sizes of structural elements were measured by the secant method using SEM images. The porosity was calculated by the formula: P
Proceedings of the International Conference on Advanced Materials with Hierarchical Structure for New Technologies and Reliable Structures 2017 (AMHS’17) AIP Conf. Proc. 1909, 020087-1–020087-4; https://doi.org/10.1063/1.5013768 Published by AIP Publishing. 978-0-7354-1601-7/$30.00
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(%) = Σl/ΣL × 100, where L is the full length of secants on the SEM images, and l is the length of the secants within the pores [4]. The phase composition of the coatings was determined by X-ray diffraction (XRD) using a DRON-07 diffractometer with CoKα radiation. To quantify the volume fraction of the crystalline phase in the amorphouscrystalline coatings according to XRD, the equation proposed by Huang [5] was used: Vcr = Icr/(Icr + Ia), where Icr and Ia are the integral intensity of the reflections from the crystalline and amorphous phases in the angular range 2θ = 10°–90°, respectively. The coating surface roughness was measured with a Profilometer-296 via the average roughness parameter (Ra). Fourier transform infrared (FTIR) spectroscopy was performed in the reflection mode using an Alpha Bruker FTIR spectrophotometer in the wave number range of 3000–500 cm−1.
RESULTS AND DISCUSSION The SEM results show that the surface morphology of La–Si–CaP coatings deposited under different oxidation voltages is presented by spheroidal elements (spheres) with open pores (Figs. 1a, 1b). The La–Si–CaP coating formed at the voltage 200 V is homogeneous across thickness of 50 µm and has the roughness Ra of 3.0–3.5 µm (Figs. 1c and 2a). The voltage increase to 350 V leads to an increase in the intensity of microarc discharges. In this case, the thickness, roughness, surface porosity and average sizes of pores and spheres increase linearly in the ranges of 50–130 µm, 3–8 µm, 18–24%, 4.5–8.0 µm and 16–26 µm, respectively (Figs. 1b, 1d, and 2). In addition, the increase in voltage leads to a partial destruction of structural elements on the coating surface, the formation of platelet-shaped crystals in destroyed hemispheres (Fig. 1b) and the formation of macropores with sizes of 15–30 µm (Fig. 1d). The increase in the thickness and porosity of the coatings is usually accompanied by an increase of residual stresses that reduce the adhesion strength of the coating to the substrate from 24 to 14 MPa (Fig. 2c). Figure 3a shows FTIR spectra of La–Si–CaP coatings that are characterized by the presence of intense absorption bands corresponding to asymmetric and symmetric oscillations of the P–O phosphate bond with the maximum absorption in the region 1130–930 cm–1 and absorption bands of OH–groups of adsorbed water at 1650– 1620 cm–1 and 3550–3200 cm–1. A shoulder in the region 800–730 cm–1 also indicates the presence of oscillations of O–H bonds of acid phosphate such as НРО4 groups and P–O–P pyrophosphate bridge bonds [6]. The XRD patterns (Fig. 3b) show the mainly X-ray amorphous state of the La–Si–CaP coatings produced under the voltages 200–250 V with a small amount (less than 8 ± 2 wt %) of the crystalline phase of monetite CaHPO4. The coatings formed at higher voltages 300–350 V have the amorphous-crystalline structure with the amount of the crystalline phase CaHPO4 to 57 ± 3 wt % and a small content of the β-Ca2P2O7 crystalline phase. We suppose that the formation of the crystalline phases in the coatings during MAO occurs by the following mechanism [7]: 200° C 400 ° C CaHPO 4 ⋅ 2H 2 O → CaHPO 4 → Ca 2 P2 O 7 .
(a)
(b)
(c)
(d)
FIGURE 1. SEM images of top (a, b) and cross-sectional La–Si–CaP coatings deposited under different oxidation voltages: (a, c) 200 V; (b, d) 300 V
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26
24
6
60
5
Ra
40
4 200
250
300
350
3
lpore
7
24
6
22
5
20
lsphere
4
18
10 8
10
4 2
200
250
Voltage (V)
(a)
6
δΑ
14
14
350
12
16
2
300
14
18
12
250
16
20
16 200
18
P
22
3
Voltage (V)
20
300
350
Adhesion strength (MPa)
8
26
7
80
20
28
28
Porosity (%)
100
30
8
Sphere size (µm)
D
9
Pore size (µm)
Thickness (µm)
120
9
Roughness (µm)
140
0
Voltage (V)
(b)
(c)
FIGURE 2. Thickness D and roughness Ra (a), average sizes of pores lpore and spheres lsphere (b), porosity P and adhesion strength δA (c) of La–Si–CaP coatings versus oxidation voltage
С-O
O-H
∆
O-H
∆
P-O P-O
3000
2500
2000
1500
∆
(240)
P-O
− CaHPO4 ∆ − β-Ca2P2O7 (322) (312)
P-O
(113)
200 V 250 V 300 V
(220) (111) (112) (130) (102)
Like other acid calcium phosphates, monetite exhibits osseoinductive properties, dissolves easily in body fluids and transforms into HA at subsequent mineralization [6]. The XRD results are in agreement with the SEM results (Fig. 1b) which demonstrate that the platelet-shaped crystals are typical of monetite. The presence of the amorphous structure and freely soluble crystalline monetite and pyrophosphate in the coating may indicate a high rate of the coating bioresorption, which was confirmed in previous biological tests [8]. The EDX analysis shows that the elemental composition of the La–Si–CaP coatings includes the following elements: Са (4.3–11.4 at %), Р (14.3–21.2 at %), О (52.0–73.4 at %), Ti (8.1–17.8 at %), La and Si (≤0.4 at %). This result confirms the data of EDX grey-level mapping of the La–Si–CaP coatings reported in the previous work [6]. With the voltage increase to 350 V, the La amount in the coating composition increases, but the Si content is constant. We suppose that it may be due to the increased reactivity and more intensive deposition of La3+ ions during MAO compared to SiO44– ions. An increased voltage and consequently temperature in the microplasma discharge area can lead to the intensification of Ca2+ ion deposition from the electrolyte and formation of the CaHPO4 crystalline phase (Fig. 3b). As a result, the Ca content in the coating increases, and the Ca/P atomic ratio increases from 0.2 to 0.7. Figure 4 shows bright field (BF) and dark field (DF) TEM images with selected area diffraction (SAD) patterns for fragments of La–Si–CaP coatings. It is seen that the coatings have nanocrystalline structure, which is confirmed by SAD patterns with numerous pinpoint reflexes corresponding to different phases (Figs. 4b, 4e) and DF TEM images illustrating individual particles or crystallites (shown by the arrows in Figs. 4c, 4f). The SAD analysis shows the presence of the following phases in the coatings: β-Ca2P2O7 with tetragonal lattice, CaHPO4 with triclinic lattice, TiO2 (anatase) with tetragonal lattice, and Nb2O5 with monoclinic lattice. DF TEM images exhibit crystallites of the β-Ca2P2O7 phase in the (226) reflex with sizes less than 10 nm (Fig. 4c) and crystallites of the CaHPO4 phase in the (010) reflex with sizes 10–80 nm (Fig. 4f). The shape of the crystallites is close to equiaxial.
∆
350 V
∆
300 V 200 V
P-O P-O 1000 -1
500
10
Wave number (cm )
(a)
20
30
40
50
2θ (degree)
60
70
80
90
(b)
FIGURE 3. FTIR spectra (a) and XRD patterns (b) of La–Si–CaP coatings deposited under different oxidation voltages
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(а)
(b)
(c)
(d)
(e)
(f)
FIGURE 4. Bright field TEM (a, d) and dark field TEM (b, e) images and SAD patterns (c, f) for particles of La–Si–CaP coatings deposited under the oxidation voltages 200 V (a–c) and 300 V (d–f)
CONCLUSIONS The structure, phase and elemental compositions of amorphous-crystalline La–Si–CaP coatings on titanium substrate deposited by MAO under different pulsed voltages in the range of 200–350 V were investigated. The increase in the voltage leads to a linear increase of the coating thickness, surface roughness, surface porosity, and average sizes of structural elements. With increasing pulsed voltage, the amount of the crystalline phase CaHPO4 in the amorphous-crystalline coatings increases from 8 to 57 wt %. It was established that the coatings consist of nanocrystalline phases: β-Ca2P2O7, CaHPO4, and TiO2 (anatase). The coatings with the maximum La and Si content to 0.4 at %, and Ca/P ratio to 0.7 were produced at 350 V.
ACKNOWLEDGMENTS The work was partially financially supported by RFBR (Grant No. 16-33-50008 mol_nr) and the Fundamental Research Program of SB RAS (Project No. III.23.2.5).
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