Physical, chemical and biological properties of micro-arc deposited calcium phosphate coatings on titanium and zirconium-niobium alloy

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i 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.wiley-vch.de/home/muw

Physical, chemical and biological properties of micro-arcdeposited calcium phosphate coatings on titanium andzirconium-niobium alloy

Physikalische, chemische und biologische Eigenschaften vonCalciumphosphatbeschichtungen auf Titan und Zirkonium, hergestelltdurch Mikro-Lichtbogenabscheidung

E.V. Legostaeva1, K.S. Kulyashova1, E.G. Komarova1, M. Epple2, Y.P. Sharkeev1,3, I.A. Khlusov3

A comparative investigation of the physical, chemical and biological properties of micro-arcdeposited calcium phosphate coatings on titanium and zirconium-niobium substrates was per-formed. Calcium phosphate coatings on titanium have a higher surface density, porosity andpore size, and a more homogeneous surface topography. Under the same conditions, calciumphosphate coatings on zirconium-niobium have a relief topography, but their surface density,porosity and pore size were all smaller. X-ray diffraction of the coatings showed that the coat-ings on titanium were X-ray amorphous whereas the coatings on zirconium-niobium consistedof a mixture of crystalline CaZr4(PO4)6, ZrP2O7, and ZrO2. These differences are due to differentelectrical and thermophysical characteristics of substrates and passivating films on their surfa-ces. The coatings were shown to be biocompatible by in-vitro cell culture experiments.

Keywords: micro-arc oxidation / calcium phosphate / coatings /

Eine vergleichende Untersuchung der physikalischen, chemischen und biologischen Eigenschaf-ten von Mikro-Lichtbogen-abgeschiedenen Calciumphosphatbeschichtungen auf Titan und einerZirkonium-Niob-Legierung wurde durchgef�hrt. Die Beschichtungen auf Titan wiesen eine h�he-re Dichte, Porosit�t und gr�ßere Poren und eine homogenere Oberfl�chentopographie auf. Dieunter den gleichen Bedingungen abgeschiedenen Beschichtungen auf Zirkonium-Niob hatteneine Relieftopographie mit geringerer Dichte, Porosit�t und Porengr�ße. R�ntgenbeugung zeigte,dass die Beschichtungen auf Titan r�ntgenamorph waren, w�hrend die Beschichtungen auf Zir-konium-Niob aus einer Mischung der kristallinen Phasen CaZr4(PO4)6, ZrP2O7 und ZrO2 bestanden.Diese Unterschiede k�nnen auf die unterschiedlichen elektrischen und thermophysikalischen Ei-genschaften der Substrate und der Passivschichten auf ihrer Oberfl�che zur�ckgef�hrt werden.Die Beschichtungen waren im in-vitro-Zellkulturexperiment biokompatibel.

Schl�sselw�rter: Lichtbogenbeschichtung / Calciumphosphat / Beschichtungen /

1 Introduction

Micro-arc oxidation in aqueous electrolyte solutions, also knownas plasma electrolyte oxidation or microplasma method, is a con-venient and effective technique to generate bioactive coatings on

metals like titanium [1–8]. The coating is formed by high-temper-ature chemical processes in the local zone of a microplasma cre-ated by micro-arc discharges from a high voltage source. The dis-charges lead to oxidation of the substrate and transform the ultra-dispersed phase from an electrolyte into a coating. The type ofsubstrate, the process parameters and the electrolyte compo-nents all influence the chemical composition, the structure andthe properties of the coatings. Calcium phosphate coatings are ofinterest to coat metallic biomaterials [5–8], especially titaniumalloys. In addition, zirconium alloyed with niobium is a promis-ing implant material because it is harder than titanium and isalso biocompatible [9].

Here we present the results a of comparative investigation ofthe physical and chemical properties of calcium phosphate coat-ings deposited by micro-arc deposition on titanium and on a zir-conium-niobium alloy.

1 Institute of Strength Physics and Materials Science of SB RAS, Tomsk,Russia

2 Institute of Inorganic Chemistry and Center for Nanointegration Duis-burg-Essen (CeNIDE); Duisburg-Essen University, Essen, Germany

3 Scientific Educational Center “Biocompatible Materials and Bioengin-eering” at ISPMS of SB RAS, NR TPU and SSMU, Tomsk, Russia

Corresponding author: Y.P. Sharkeev, Institute of Strength Physics andMaterials Science of SB RAS, 2/1 Akademicheskii pr., 634021, Tomsk,RussiaE-mail: [email protected]

DOI 10.1002/mawe.201300107 Mat.-wiss. u. Werkstofftech. 2013, 44, No. 2–3

Mat.-wiss. u. Werkstofftech. 2013, 44, No. 2–3 Physical, chemical and biological properties of micro-arc deposited calcium phosphate coatings

2 Experimental

Commercially pure titanium (99.58 Ti, 0.12 O, 0.18 Fe, 0.07 C,0.04 N, 0.01 H wt%) and zirconium-niobium alloy (96.54 Zr,1.01 Nb, 0.32 Mo, 0.02 Si, 0.1 W, 0.29 Fe, 0.88 Ti, 0.1 O wt%)were used as substrates. The size of the samples was10.10.1 mm3. Samples were prepared with silicon-carbide paperof 120, 480, 600, 1200 grit, respectively. Then samples were ultra-sonically cleaned during 10 min in distilled water (Elmasonic,Germany). Roughness of the samples was approximately 0.6–0.8lm. The previously described Micro-Arc-3.0 method was used[5]. The setup consists of a pulse power source, a computer tocontrol the deposition process, a galvanic bath with water coolingand the electrodes. The calcium phosphate coatings were depos-ited from an aqueous solution prepared from 20 wt% phosphoricacid, 6 wt% dissolved hydroxyapatite, and 9 wt% dissolved cal-cium carbonate in the anode regime [5]. The calcium phosphatecoating was formed in pulse mode with the following param-eters: Pulse time 100 ls, pulse frequency 50 Hz, deposition timefrom 1 to 10 min, voltage from 150 to 400 V. The current density

was calculated as i ¼ I

S, where I is the amplitude current, meas-

ured during the deposition process of the coating, and S is thesquare of the sample conducting the current. Here both sides ofthe sample were taken into account. In our case the square was200 mm2.

The surface morphology was examined by scanning electronmicroscopy (SEM; Philips SEM 515 and Zeiss EVO 50 XVP)equipped with energy-dispersive X-ray spectroscopy (EDAX,ECON IV). To measure the size of the structural elements (spher-ulites and pores), the secant method was applied [10, 11]. Theporosity was measured by SEM images. We used the secantmethod, in which the porosity is calculated according to P =P

lP

L100%, where L is the length of a secant randomly put on the

SEM images, l is the total length of those parts of a secant whichfall on pores. N is the number of secants. The results were obtain-ed with N = 50.

The phase composition was determined by X-ray diffraction(XRD, Bruker D8 Advance) in the angular range 2� = 5–90 8 witha scan step 0.010 with Cu Ka radiation (k = 1.5405 �). IR spectrawere measured with a Fourier Transform Infrared Spectrometer(BIO RAD FTS 175) from 400–4500 cm–1 as KBr pellet. The sur-face roughness Ra was estimated with a profilometer-296 (Rus-sia).

The surface density was calculated according to the formula

� ¼ mS;

where m is the mass of the coating, calculated as a differencebetween the mass of the sample before and after deposition ofthe coating, S is the area of the coating calculated for two sides ofthe sample. The area of the coating was 2 cm2.

To measure the adhesion strength of the coatings to the sub-strate, two cylinders were glued to both sides of the samples withcoating by Loctite Hysol 9514. They were fixed in grips in orderto be tested under tension (Instron-1185, Great Britain). Theadhesion strength is the maximum stress required to tear the cyl-inder off the calcium phosphate coating. It was measured as

A = F/S, where F is the breakout force and S is the area of separa-tion [5].

For biological tests, prenatal stromal stem cells from a humanlung (Stem Cells Bank Ltd., Tomsk, Russia) were used. Aliquotsof CD34–CD44+ adherent cells are maintaining diploid karyotypeand oncological safety during ex vivo passages. The cells werefree from viral (HIV, hepatitis, herpes etc.), bacterial (syphilis,mycoplasma, chlamydiae, etc.) and fungous agents. After thaw-ing, a cell viability of 91–93% was determined according to theISO 10993-5 test with 0.4% trypan blue.

Cells were cultivated on the coated samples in 90% DMEM/F12 (1:1) (Gibko, USA), 10% fetal bovine serum (Sigma, USA),supplemented with 280 mg L–1 L-glutamine (“Biolot”, RussianFederation) and 50 mg L–1 of gentamicin (Invitrogen, UK). Thesamples were placed each into one well (1.77 cm2) of a 24-wellplate (Costar, USA). A cellular suspension with 5 N 104 viable cellsper 1 mL of the culture medium was added. The samples wereremoved after 4 days of cultivation in a humidified atmosphereof 95% air and 5% CO2 at 37 8C.

To judge the morphology of cells adhered to the calcium phos-phate surfaces by SEM study, the samples with adherent cellswere treated with 2.5% glutaric dialdehyde solution for 24 h,fixed in a 1% osmium tetroxide solution for 30 min and thenwashed twice in PBS (pH = 7.2–7.4). The samples were dehy-drated in a graded series of alcohols (30%, 50%, 70%, 90%,100%) for 15 min with each alcohol concentration and thenrinsed twice with 100% acetone for 15 min.

To determine a functional activity of cells adhered to calciumphosphate surfaces, the samples with adherent cells were driedon air, were fixed for 30 s in formalin and were stained with alka-line phosphatase (ALP) by cytochemical techniques. Naphtol AS-BI phosphate (C18H15NO6P, m.w. 452.21) (Lachema, CzechRepublic) and fast blue PP salt (C15H15N3O3.BF4, m.w. 372.10)(Lachema, Czech Republic) were used.

Blue sites of cytoplasmic enzymatic activity were cellular ALPstaining criteria.

3 Results and discussion

3.1 Formation of calcium-phosphate coatings

Fig. 1 shows the current densities for different voltage values as afunction of time intervals of micro-arc deposition of the calciumphosphate coatings on titanium and zirconium-niobium surfa-ces. Coatings on titanium and zirconium-niobium surface wereformed at a voltage of 150 V. First, the current density was 0.1 Amm–2, but it decreased to zero in 1.5 min, indicating that a dielec-tric coating had formed, Fig. 1b. In the case of zirconium-niobium, the current density was 0.15 A mm–2, and the time ofcoating deposition was 2 min, Fig. 1b.

At an increased voltage up to 200–300 V, the initial currentdensity was 0.4–1 A mm–2 and 0.5–1.25 A mm–2 for titanium andzirconium-niobium, respectively. Then it decreased and becamealmost constant after 1.5 to 3 min. The effective current densitieswere 0.2–0.35 A mm–2 for titanium and 0.2–50.5 A mm–2 for zir-conium-niobium. The time for calcium phosphate coating for-mation increased up to 10 min and 5 min for titanium and zirco-nium-niobium, respectively. An increase of the voltage up to

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400 V for titanium and 350 V for zirconium-niobium was accom-panied by a transition of the micro-arc discharge to arc dischargewhich led to the destruction of the coatings.

SEM analysis showed that the calcium phosphate coatingdeposited at 150 V on titanium and zirconium-niobium con-tained porous spherulites, Fig. 2a and 2d. The pore size dependedon the size of micro-arc discharges in the electrolyte which inturn were related to the current density and voltage. For tita-nium, the increase of the voltage from 150 to 350 V led anincrease of the spherulite size from 5 to 30 lm and to pores of 2to 12 lm diameter, Fig. 3. The porosity varied within 20–25% at200–250 V and decreased to 18% when the voltage increased to400 V.

For zirconium-niobium, the size of the structural elementsalso decreased when the voltage was increased to 200–250 V,Fig. 2e. However, the average sizes of spherulites and pores onlya fifth compared to the coating on titanium, Fig. 3a–c. The sur-face porosity of the coating on zirconium-niobium was only halfof the coating on titanium (10–15%). A further increase of thevoltage up to 300 V led to coatings on the zirconium-niobiumsurface with a relief structure with ridges and cavities, Fig. 2f.Note that we considered only the surface porosity according toSEM image analysis. In fact, the bulk porosity may differ fromthe surface porosity, and the coatings may have cavities whichcannot be fully quantified by the optical images, Fig. 4.

Fig. 5 illustrates the dependence of the physical and mechani-cal characteristics (thickness, surface density, roughness, adhe-sion strength) of the coatings on titanium and zirconium-niobium surfaces on the micro-arc voltage. An almost lineargrowth of the coating thickness occurred both on titanium andzirconium-niobium as the micro-arc voltage increased. The coat-ing thickness both for titanium and zirconium-niobiumincreased from 10 to 95 lm when the voltage increased from 150to 300 V (Fig. 5a). This can be ascribed to the increasing currentdensity on the substrate surface, and, as a consequence, the dep-osition rate of the coating increases. It was mentioned above thattransformation of the micro-arc discharges into arc dischargesoccurred if the voltage reached 400 V. This resulted in a consider-

able growth of the coating to a thickness of more than 100 lm,Fig. 5a.

A similar behaviour was found for the surface density of thecoatings. It increased from 1.75 to 21.5 mg cm2 for titanium andfrom 0.3 to 11.5 mg cm2 for zirconium-niobium as the voltageincreased from 150 to 350 V. A further increase of the voltage upto 400 V led to the destruction of the coating so that the surfacedensity decreased to 19 mg cm2, Fig. 5b. Fig. 5c illustrates thedependence of the coating roughness on the micro-arc voltage. Itincreases from 1.9 to 6 lm for the coating on titanium, Fig. 5c.In the case of zirconium, the roughness parameter monotoni-cally grew from 0.6 to 4.5 lm with the voltage increasing from150 to 250 V, and then it suddenly jumped to 11 lm. This is con-nected with the formation of a relief structure alternated by theridges and cavities as the voltage increased to 300 V, Fig. 2f. It isknown that a roughness between 2.5 lm and 5 lm provides opti-mum range adhesion of the stromal stem cells and their differ-entiation into the bone tissue [12].

Fig. 5d shows the adhesion strength of the coating as a func-tion of the micro-arc voltage. The maximum adhesion strengthwas 27 MPa for titanium and 23 MPa for zirconium-niobium forcoatings deposed at 150 V because they had the lowest thicknessand roughness. The increase of the micro-arc voltage led to adecrease of the adhesion strength down to 2–4 MPa which isinsufficient for a practical application [13].

Thus, the variation of the micro-arc process voltage allowedthe preparation of coatings on titanium and zirconium-niobiumsurfaces with different physical and mechanical characteristics.Calcium phosphate coatings on titanium have a higher surfacedensity, porosity and pore size, but their surface topography ismore homogeneous. Under the same conditions, the calciumphosphate coatings on zirconium-niobium have a relief topogra-phy, but their surface density, porosity and pore size are smalleras compared to the coatings on titanium.

It was proposed that the properties of micro-arc coatings canbe explained by different electrolyte composition and by physicaland chemical reactions on the metal-electrolyte interface [6, 8].In addition, these differences can be caused by different physical

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Figure 1. Current densities for different voltage (the step size in voltage was 50 V) as a function of time of micro-arc deposition of the calciumphosphate coatings on titanium (a) and zirconium-niobium (b) surfaces.

Bild 1. Stromdichten bei unterschiedlicher Spannung (Schrittweite in der Spannung von 50 V) als Funktion der Zeit bei der Lichtbogenabschei-dung der Calciumphosphat-Beschichtungen auf Titan (a) und Zirkonium-Niob (b).


and chemical characteristics of a substrate material and oxidefilms on a metal surface. Table 1 shows a number of electro- andthermophysical and thermodynamic characteristics of substratematerials (Ti, Zr, Nb) and oxide films (TiO2, ZrO2, Nb2O5).

The micro-arc process of the zirconium-niobium alloyed withniobium probably begins in the micro-areas containing particles

of b-Nb, despite their small quantity in this alloy. The thermalconductivity of b-Nb particles (k = 54.5 W m–1 K–1) is considerablyhigher than the conductivity of the basic component of zirco-nium (k = 16.8 W m–1 K–1). Furthermore, niobium has a rathersmall electric resistance in comparison with zirconium and tita-nium. Besides, there is always an oxide film on a metal (alloy)

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Figure 2. SEM images of calcium phosphate coatings deposited on titanium (a, b, c) and zirconium-niobium (d, e, f) by micro-arc oxidation atvoltages of a) 150 V, b) 200 V, c) 400 V, d) 150 V, e) 250 V, and f) 300 V.

Bild 2. Rasterelektronenmikroskopische Aufnahmen der Calciumphosphat-Beschichtungen auf Titan (a, b, c) und Zirkonium-Niob (d, e, f) durchLichtbogenabscheidung bei Spannungen von a) 150 V, b) 200 V, c) 400 V, d) 150 V, e) 250 V, und f) 300 V.


surface which in case of titanium and zirconium-niobium hassemi-conducting properties (n-type conductivity) as Ti, Zr andNb are transition metals [14–16].

It is known that the electric conductivity of oxide semiconduc-tors is determined by the fact that the ions of the same metalhave at least two differently valence states and that the conductiv-ity is connected with an electron exchange between these states[17]. Niobium oxide, Nb2O5, has a rather narrow band gap (1.6eV). The band gap in ZrO2 is 6 eV, i. e. it is an isolator. Technically

pure titanium contains small amounts of impurities (e.g., iron,silicon, oxygen, carbon, nitrogen; less than 0.5 wt%) that canlead to the formation of precipitates (intermetallics, carbides oftitanium) [14]. At the same time, the thermal conductivity ofintermetallics and carbides is significantly smaller than that fortitanium. As a result, the oxidation of titanium proceeds moreintensively in a metallic grain than in precipitates as in the caseof zirconium alloyed with niobium. Besides, the band gap inTiO2 is 3 eV, i. e. between ZrO2 and Nb2O5. It also influences the

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Figure 3. Dependence of the spherulite size (a), the pore size (b), and the surface porosity (c)of the calcium phosphate coating on the voltage for both substrates: Titanium and zirco-nium-niobium.

Bild 3. Abh�ngigkeit der Spherulitgr�ße (a), der Porengr�ße (b) und der Porosit�t (c) der Cal-ciumphosphat-Beschichtung von der Spannung f�r beide Substrate: Titan und Zirkonium-Niob.

Table 1. Electronic, thermophysical and thermodynamic characteristics of substrate material (Ti, Zr, Nb) and oxide films (TiO2, ZrO2, Nb2O5) [15–17]

Tabelle 1. Elektronische, thermophysikalische und thermodynamische Eigenschaften der Substrate (Ti, Zr, Nb) und der passivierenden Oxid-schichten (TiO2, ZrO2, Nb2O5) [14–16].

Characteristics of metals Ti Zr Nb

Density / g cm–3 4.5 6.45 8.57Melting temperature / K 1941 2125 2773Temperature of polymorphic transition a$ b / K 1155 1136 –Thermal conductivity at 300 K / W m–1 K–1 15.5 16.8 54.5Linear coefficient of thermal expansion at 293 K /�106 K–1 8.5 5.6 6.5Electrical resistivity /610–6 � m 0.55 0.41 0.15Work function / eV 4.09 3.84 4.01

Characteristics of oxide films TiO2 ZrO2 Nb2O5

Band gap / eV 3 6 1.6Relative dielectric constant 30–100 18–21 11–40


homogeneity of calcium phosphate coatings. Thus, it is possiblethat the differences in electro- and thermophysical properties ofzirconium-niobium and titanium influence the relief structure,the homogeneity, the porosity, and the crystallinity of micro-arccoatings.

3.2 Phase composition and elemental composition ofcalcium phosphate coatings

Figs. 6–7 show typical X-ray diffractograms of micro-arc depos-ited calcium phosphate coatings on titanium and zirconium-niobium. The coatings are X-ray amorphous, Fig. 6a–b. There

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Figure 4. Optical images of cross-sections of the micro-arc-deposited calcium phosphate coatings on titanium at voltages of a) 200 V, b) 300 V,c) 400 V, and on zirconium-niobium at voltages of d) 200 V, e) 250 V, and f) 300 V.

Bild 4. Mikroskopische Darstellung der Querschnitte der Lichtbogen-abgeschiedenen Calciumphosphat-Beschichtungen auf Titan bei den Span-nungen a) 200 V, b) 300 V und c) 400 V, und auf Zirkonium-Niob bei den Spannungen d) 200 V, e) 250 V und f) 300 V.


are weak peaks of the titanium substrate, Fig. 6a. Peaks cor-responding to calcium phosphate and oxide compounds, i. e.CaTi4(PO4)6, b-Ca2P2O7, TiP2O7, TiO2 (anatase), were observedwhen the coating thickness on titanium reached 100 lm at a volt-age of 300 or 400 V. After annealing of the coatings at 1073 K for1 h, the crystalline phases CaTi4(PO4)6, calcium pyrophosphate b-Ca2P2O7, titanium pyrophosphate TiP2O7, and titanium dioxideTiO2 (anatase) were formed, Fig. 6 c, d. On zirconium-niobium,we detected CaZr4(PO4)6, ZrP2O7, ZrO2, and peaks of the sub-strate (zirconium), Fig. 7.

Fig. 8 shows IR-spectra of calcium phosphate coatings on tita-nium and zirconium-niobium. The absorption bands of OH-

groups at 3550–3200 cm–1, 1650–1620 cm–1 and at 660–630 cm–1

and strong bands at 1130–1030 cm–1 and 960–930 cm–1, whichbelong to antisymmetric and symmetric fluctuations of P-Ophosphate bonds, respectively, as well as bands at 600–520 cm–1,corresponding to triple-degenerated deformation oscillations ofO-P-O bonds in the phosphate group were observed in thesespectra. Absorption bands for the coatings on titanium (withoutannealing) were diffuse, Fig. 8a. A broad band of phosphatevibrations (1250–800 cm–1) was observed. There was no such

behaviour for the annealed calcium phosphate coatings on tita-nium and for coatings on zirconium-niobium surface, Fig. 8b, c.

3.3 Biological properties of calcium phosphate coatings

The reaction of stromal stem cells to the coated samples wasstudied in comparison to the uncoated substrates. Scanning elec-tron microscopy showed that the cells well adhered to the cal-cium phosphate coatings on titanium and on zirconium-niobium.

The cellular morphology was presented by cells with singlepseudopodia and fibroblast-like cells, Fig. 9. Only rounded cellswere on the surface of uncoated substrates.

Cells stained with ALP were detected on the surfaces with thehelp of an Olympus GX-71 optical microscope. The number ofALP-positive cells in 1 mm2 of calcium phosphate coating on tita-nium (104 € 14) and on zirconium-niobium (74 € 3) exceededthat on uncoated titanium (20 € 3). Statistical differences withthe Ti substrate alone was established according to Student’s t-test (p < 0,004). Thus, calcium phosphate coatings promote amorphofunctional maturation of human stromal cells.

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Figure 5. Dependence of thickness (a), surface density (b), roughness (c), and adhesion strength (d) of calcium phosphate coatings on titaniumand zirconium-niobium on the voltage.

Bild 5. Die Abh�ngigkeit von Dicke (a), Dichte (b), Rauhigkeit (c) und Haftfestigkeit (d) der Calciumphosphat-Beschichtungen auf Titan und Zir-konium-Niob von der Spannung.

Mat.-wiss. u. Werkstofftech. 2013, 44, No. 2–3 Physical, chemical and biological properties of micro-arc deposited calcium phosphate coatings 195


Figure 6. X-ray powder diffractograms of calcium phosphate coatings on titanium directly after depositing (a and b) and after annealing at1073 K (c and d) at voltages of 200 V (a and c), and 300 V (b and d); * CaTi4(PO4)6, F TiP2O7, 9 b-Ca2P2O7, 0 TiO2 (anatase).

Bild 6. R�ntgenpulverdiffraktogramme der Calciumphosphat-Beschichtungen auf Titan direkt nach der Beschichtung (a und b) und nach demErhitzen auf 1073 K (c und d) mit den Spannungen: 200 V (a und c) und 300 V) (b und d); * CaTi4(PO4)6, F TiP2O7, 9 b-Ca2P2O7, 0 TiO2 (Anatas).

Figure 7. X-ray powder diffractograms of micro-arc deposited calcium phosphate coatings on zirconium-niobium at voltages of a) 200 V, b) 250V; * CaZr4(PO4)6 (rhombohedral), F ZrP2O7 (cubic), 9 Zr (hexagonal), 0 ZrO2 (tetragonal).

Bild 7. R�ntgenpulverdiffraktogramme der Calciumphosphat-Beschichtungen auf Zirkonium-Niob bei Spannungen von a) 200 V, b) 250 V;* CaZr4(PO4)6 (rhomboedrisch), F ZrP2O7 (cubic), 9 Zr (hexagonal), 0 ZrO2 (tetragonal).


4 Conclusions

A comparative investigation of the physical, chemical and biolog-ical properties of micro-arc deposited calcium phosphate coat-ings on titanium and zirconium-niobium was performed. Themain results can be summarized as follows: A variation of themicro-arc voltage permits to vary the properties of the coating.The increase of the micro-arc voltage leads to a linear growth of

the structure elements sizes of coating (spherulites and pores) aswell as of the thickness, surface density and roughness para-meter, but to a decrease of the adhesion strength. The optimumvoltage of the micro-arc coating deposition is 200-250 V both fortitanium and zirconium-niobium substrates. Calcium phosphatecoatings on zirconium-niobium substrate have a relief topogra-phy, but a smaller thickness, porosity, and pore size than the coat-ings on titanium. The surface topography on zirconium-niobium is more homogeneous in comparison with the coatingson titanium. The adhesion strength of the coatings to the tita-nium substrate is 1.5–2 times higher than to zirconium-niobiumones. Micro-arc coatings on titanium substrate are X-ray amor-phous whereas the coatings on zirconium-niobium consisted ofthe crystalline phases CaZr4(PO4)6, ZrP2O7, and ZrO2. The coat-ings were biocompatible and promoted a morphofunctionalmaturation of human stromal cells as shown by in-vitro cell cul-ture experiments.

Acknowledgements

The work has been partially financially supported by the Programof Presidium of RAS, project No. 5.27, Russian Foundation for

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Figure. 8. IR spectra of the calcium phosphate coatings on titanium(a, c) and zirconium-niobium (b).

Bild 8. IR-Spektren der Calciumphosphat-Beschichtungen auf Titan (a,c) und auf Zirkonium-Niob (c).

Figure 9. SEM images of stromal stem cells on calcium phosphatecoatings on titanium (a) and on zirconium-niobium (b).

Bild 9. Rasterelektronenmikroskopische Aufnahmen von Stammzellenauf Calciumphosphat-Beschichtungen auf Titan (a) und Zirkonium-Niob (b).


Basic Research, grant No. 12-03-00903-a, Russian Federal Pro-gram “Kadry”, contract No. 8036, and the German Federal Minis-try of Education and Research, BMBF project No. RUS 11/024.

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Publications, Oxford 1984.[17] C. N. R. Rao, B. Raveau, Transition Metal Oxides: Structure,

Properties and Synthesis of Ceramic Oxides, Wiley-Inter-science 1998.

Received in final form: December 9th 2012 T 107

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