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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5326_Библиотеки_им_академика_М_И_Перельмана
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Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys 199
coatings, the ceramic layer grows inwards and outwards. Thus, the coating is an integral part of the metal substrate and should exhibit good adhesion.
Generally, orthopaedic implants are subjected to cyclic loading. The coating should
have good mechanical properties such as hardness, tensile strength and elastic modulus
that enable them to withstand repetitive loads or strains. Hence, the fatigue strength of
the coating material should be taken into consideration. Due to their brittle nature, ceramics may be susceptible to cracking under loading conditions. Even fine cracks in
the coating could allow the body fluid to penetrate through and initiate substrate corrosion. This could lead to localized corrosion and then spallation of the coating, eventually affecting the mechanical integrity of the implant. This could be avoided by using a
thin layer of biodegradable polymer top coat, as described earlier.
Wear is another important property to be considered for implant coatings. Wear and
abrasion can act in synergy with corrosion and could lead to dimensional instability of
the implant (Richard, Kowandy, Landoulsi, Geetha, & Ramasawmy, 2010). Furthermore, wear debris can cause aseptic loosening due to osteolysis (Kowandy, Mazouz,
& Richard, 2006). Although ceramics, in general, have high wear resistance, it has
been reported that surface roughness of ceramics affects the wear properties; that is,
ceramics with a rough surface cause excessive wear (Monasky & Taylor, 1971). Nanostructured coatings could overcome this issue, and in fact they have attractive physical
and mechanical properties for biomedical applications.
In summary, a coating material for biodegradable implants requires various properties; however, the key requirements are biocompatibility, controlled degradation rate,
good adhesion and wear resistance, as shown in Figure 8.1. Although certa in PEO
coatings that are phosphate- and silicate-based satisfy all the key requirements,
tailoring of the PEO coatings would be required to further enhance the biocompatibility and achieve desirable results.
Biocompatibility
Key coating
Degradation
Figure 8.1 Key coating requirements for biodegradable magnesium-based implants.
requirements
Wear resistance
Adhesion
200 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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8.4 Controlling coating composition, microstructure
and properties for biomedical applications
The degradation rate of a PEO-coated Mg biomedical device is determined mainly by
microstructure and composition of PEO coating; this section is devoted to the recent
investigations in this contex t. An extended review was published elsewhere, focusing
on strategies to improve the corrosion resistance of micro-arc oxidation-coated magnesium alloys for degradable implants (Sankara Narayanan, Park, & Lee, 2014).
The interested reader can find there an extended range of references, especially
regarding the influence of electrolyte composition, including additives and particles
and processing parameters on microstructure and performance.
8.4.1 Energy input
The formation of PEO coatings is caused by dielectric discharges and microstructure,
and properties of PEO coating can be controlled by the energy of discharges. The electric energy (mainly adjustable by voltage and current) is thereby converted into thermal
energy. Generally, the energy of a discharge is sufficient enough to melt the coating
material so that a highly crystalline oxide coating is obtained after the discharges
are stopped and the coating material is quenched (Van et al., 1977). The energy of a
discharge can be controlled by the parameters of power supply as well as electrolytes
used. Pulsed DC and AC are preferred because they allow better control of the energy
input by the discharges and they reduce the likelihood of damaging the coating by severe burning. Thus, pulse frequency, pulse length and duty cycle are additional options
to control energy input and as a consequence the microstructure, but so far they have
not been studied much for biomedical applications of PEO coatings.
PEO processing is normally performed in either constant voltage or constant current
mode, resulting in specific time dependences. In constant voltage mode, the total energy input is often decreasing with treatment time. While the coating is growing in
thickness, its resistance is increasing; therefore, the current is decreasing if the voltage
is fixed. Contrary, in constant current mode, the total energy input is normally
increasing with time because the voltage is increasing to compensate for the increasing
resistance of the coating to maintain the selected current. However, the total energy
input is not directly related to the energy in one single discharge. In the beginning
of the process, normally a large number of small discharges occur; in the later stages,
the total number is reduced but the size and lifetime is increasing, supposing that the
energy transferred by a single discharge event at the end of the process is higher than at
the beginning (Bala Srinivasan, Blawert, St€ormer, & Dietzel, 2010).
Bala Srinivasan, Liang, Blawert, St€ormer, and Dietze (2009) applied different cur-
rent density to control the microstructure and properties of PEO coatings. DC currents
of 15 mA/cm
ness, microstructure, morphology, phase composition and corrosion resistance were
characterized. The thickness along with the roughness of PEO coatings increased
with increasing current density. PEO coating with very large discharge pores
(Figure 8.2) was formed with 150 mA/cm
2
, 75 mA/cm2and 150 mA/cm2were applied and the thickness, rough-
2
applied current; however, the density of
Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys 201
(a) (b)
(c)
Figure 8.2 Scanning electron micrographs showing the surface morphologies of the specimens
PEO coated at different current densities for 15 min (A: 15 mA/cm
150 mA/cm
Reprinted from Bala Srinivasan et al. (2009) with permission from Elsevier.
2
).
2
, B: 75 mA /cm2and C:
discharge channels appeared to decrease with increasing applied current. This showed
that high current application is responsible for strong discharges. Such strong discharges leave large pores, which were observed in cross-sections of the coatings. However, shorter treatment under higher currents formed compact coatings. Prolonged
treatment at higher currents may cause repeated discharges at the same location and
thereby formation of rather highly defective coating. Formation of large discharge
pores, on the other hand, increased the roughness of the coating. Impedance and potentiodynamic polarization measurements of the coated Mg alloys showed that coatings
with denser and more compact morphology are more protective in the aggressive chloride solution. Large discharge pores act as channels for corrosive media to reach the
underlying substrate easily and thereby weaken the corrosion protection property of
PEO coatings. On the other hand, a certain energy level is required to incorporate
all components of the electrolyte into the final coating. At a low current density, the
coating is mainly composed of MgO, and it requires at least a medium current density
to produce the Mg
SiO4phase as well; the content of the latter increases with
2
increasing current density. A similar effect is observed when the energy input is
controlled by the final voltage (Bala Srinivasan et al., 2010). At the same current density of 15 mA/cm
amounts of the Mg
2
, at least a final voltage of 440 V was required to produce larger
SiO4phase instead of MgO. Therefore, it is important to find
2
the right balance between the destructive and constructive energy of discharges to produce microstructures that have the desired degradation properties.
Gu et al. (2011) formed PEO coatings on a biomedical Mg-Ca alloy in alkaline sil-
icate solution at fixed voltages (300e400 V), 700 Hz, 30% duty cycle and 600 s processing time. The microstructure of the coating changed significantly with applied
202 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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voltage (Figure 8.3). With increasing applied voltage, the pore size and the thickness of
the PEO layer increased. When 300 V was applied (Figure 8.3(a)), a porous oxide
coating with an average coating thickness of 4.5 mm and a pore diameter of 0.4 mm
was formed on the alloy surface. As the applied voltage was increased to 360 V
(Figure 8.3(b)), the thickness and pore size of the coating increased to 6 mm and
1.2 mm, respectively. While increasing the voltage to 400 V, the resulting coating
became still thicker while the pore size increased slightly in diameter. The presence
of microchannels in the cross-section (marked by an arrow in Figure 8.3(c)) of the
coating obtained at 400 V revealed the presence of strong discharges. Strong evidence
for the formation of high-energy discharges also came from the evolution of high
dielectric breakdown current and the formation of large discharge channels when
breakdown is caused by high-voltage pulses (Sah, 2012; Sah et al., 2011). Thus,
increasing applied voltage results in high-energy dielectric discharges, and increased
amounts of molten product may be erupted from and rapidly deposited around the
discharge channels. As a result of this series of reactions, the pore size and the thickness of the layer increased with higher applied voltage. At the same time, the trapping
Figure 8.3 The surface and cross-sectional morphologies of (a) 300 V, (b) 360 V and (c) 400 V
MAO-treated MgeCa alloy samples. (Insets) EDS results.
Reprinted from Gu et al. (2011) with permission from Elsevier.
Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys 203
of gas bubbles in the growing oxide layer might be respon sible for the micropores inside the PEO layer (Guo & An, 2005).
The microstructure that develops at different voltages has a strong influence on
the properties of the coating (Gu et al., 2011). Figure 8.4 presents the variations in
pH and the volume of hydrogen evolved as a function of immersion time. Hydrogen
evolves much more slowly from PEO-treated than untreated Mg-Ca alloy specimens
(a)
22
20
)pH value
2
18
16
14
12
10
8
6
4
Hydrogen evolution volume(mL/cm
2
0
01020
Immersion time (d)
Untreated
300V
360V
400V
30 40 50
(b)
11. 5
11. 0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
01020304050
Immersion time (d)
Untreated
300V
360V
400V
Figure 8.4 (a) The hydrogen evolution volume and (b) change in pH value of Hank’s solution
containing untreated and MAO-treated MgeCa alloy samples as a function of immersion time.
Reprinted from Gu et al. (2011) with permission from Elsevier.
204 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(1.694 ml /cm2/day). Among the three specimens that were PEO treated at different
applied voltages, the one treated at 360 V exhibited the slowest hydrogen evolution
rate (0.007 ml cm
lution rate (0.108 ml cm
(0.045 ml cm
2
/day ), whereas the one treated at 300 V showed the highest evo-
2
2
/day). The c hange in the pH value of the Hank’s solution indicated
/day), followed by the 400 V PEO-treated specimen
a tendency similar to that of the hydrogen evolution results. The variation in the
corrosion rates of the PEO-treated specimen is attributed to changes i n the microstructure of the samples obtained at different applied voltages. On the one hand,
the change in corrosion resistance of the PEO coatings depends on the coating thickness. The 300-V treated PEO coating showed the poorest corrosion resistance, which
is due mainly to its low thickness (4.5 mm). On the other hand, the integrity of an
MAO coating also influences the corrosion resistance. The 400-V treated PEO
coating has more micropores and larger discharge channels inside the coating,
permitting more corrosive medium to be absorbed into the PEO c oating, reducing
its corrosion resistance. Thus, the specimen treated at 360 V has the best combination of thickness and integrity. The surface morphology observed after immersion
in a corrosive medium confirmed that PEO coating formed at 360 V has the best protective ability. The corrosion b ehaviour of PEO-coated alloy si gnificantly influenced
the cell proliferation. The most protective coating also showed the best cell proliferation and bioactivity.
The compactness of coating formed at higher voltages may also depend on the electrolyte, pulse frequency, duty cycle or type of supply (bipolar pulses) used. Lin, Tan,
Zhang, et al. (2013) reported PEO coating formation on ZK60 Mg alloy in alkaline
silicate solution containing fluoride at different constant voltages of 230, 300, 370
and 450 V using 1000 Hz pulse frequency and 40% duty cycle. Thicker and denser
coatings were formed at higher voltages, contrary to the result of Gu et al. (2011) discussed above. The formation of a dense coating at 450 V is explained by the rather
long melt quench time and reduced gas evolution. However, the role of electrolytes
in PEO is discussed in the next section. It should be noted that the compact PEO
coating formed at 450 V showed the best corrosion protection in Hank’s solution
and, because of its highest degradation resistance, also the least cytotoxicity and haemolytic ratio.
Summarizing, it is obvious that the energy input by the discharges is a very important parameter to control the microstructure and properties of PEO coatings, because it
determines what phases will form, how quick the coating grows and how much damage to the coating is generated. The interaction of the processing parameters (voltage,
current, pulse form, frequency, length, duty cycle) are very complex, and even the
electrolyte interacts by determining the breakdown voltage; thus, there is no clear indication of the best processing window.
8.4.2 Electrolyte composition
Like the applied current and voltage during PEO processing, electrolyte composition
also controls the microstructure of PEO coatings. The first evidence that electrolyte
species are incorporated in PEO coating, unlike classical anodic films, came from
Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys 205
the formation of Cd2Nb2O7on a cadmium anode in an aqueous solution of potassium
niobate (McNeill & Nordbloom, 1958). Later, the mechanistic study by Monfort et al.
(2007) using sequential PEO of Al in silicate and phosphate electrolytes showed that
electrolyte species are incorporated in the coating due to short-circuit transport caused
by dielectric breakdown. Therefore, the composition of PEO coatings should mainly
depend on electrolyte and substrate composition.
Generally, a large number of electrolytes are available for PEO processing of magnesium alloys. Strong alkaline solutions are preferred, but under certain conditions
even acidic solutions can be used. However, they can be divided into two main groups
according to the main coating-forming addition in the electrolyte. In that sense, phosphate- and silicate-based electrolytes are available, to which various additions are
possible, such as hydroxides, fluorides, borates and aluminates (to name just a few).
Hydroxides are normally used to adjust the pH value and conductivity of the electrolyte solution. Additionally, the concentration of all these additions has an influence on
the surface modification that can be obtained, by controlling the breakdown voltage
but also determining the amount of possible conversion products added to the coating.
Thus, there is a direct influence on coating composition, morphology and structure,
and thus on the proper ties of the coating as well. Altogether, a large number of studies
of electrolyte influence on microstructure and properties were performed, but just a
few can be presented here. A more complete review can be found elsewhere (Sankara
Narayanan et al., 2014).
Duan, Yan, and Wang (2007) formed PEO coating on AZ91D magnesium alloy in
three different electrolytes e namely, alkaline silicate-phosphate, alkaline silicateborate and alkaline silicate-borate-fluoride (Figure 8.5). Depending on the different
electrolytes, characteristic microstructures of PEO coatings were observed. Large
discharge pores with bigger voids near the coating/substrate interface were developed
in alkaline silicate-phosphate coating, while the surface morphology of coatings developed in the latter two electrolytes remains the same with improved cross-sectional
morphology. The inner region of the coating formed in alkaline silicate-boratefluoride was denser than that formed in alkaline silicate-borate electrolyte. The corrosion performance of the coatings produced in the three electrolytes is closely related to
the microstructure. The coating with denser coating/substrate interface formed in alkaline silicate-borate-fl uoride showed the best corrosion protection. The formation of an
inner dense layer in fluoride-containing electrolyte may be due to fluoride-assisted suppression of localized galvanic corrosion of the substrate during the coating process
(Nemcova, Skeldon, Thompson, & Pacal, 2013). A coating with dense and uniform
microstructures has a higher potential to show promising corrosion protection of the
substrate in body fluid as well.
Not only the composition of the electrolyte but also the concentration of the electrolyte is effective in controlling the microstructure of PEO coatings. Zhang, Zhang,
et al. (2012) studied the effect of silicate concentration on PEO of AZ91HP Mg alloy
in a sodium hydroxide and phytic acid solution. The latter is an acid suitable to form ing
insoluble salts of magnesium, which would be incorporated into the coating; it has a
variety of beneficial effects for human health, including as an anti-cancer agent, inhibitor for renal stone development and anti-oxidation agent. With increasing
206 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 8.5 Morphologies of PEO films on magnesium alloy AZ91D, respectively, treated in
solutions containing phosphate (a), borate (c) and both borate and fluoride (e); (b), (d) and (f)
were their cross-sectional morphologies.
Reprinted from Duan et al. (2007) with permission from Elsevier.
concentration of silicate in the electrolyte, the growth of thicker MgO-based PEO
coating was observed. Unfortunately, the size of discharge channels, which are potential defects of the coating, was also increasing. Thus, the coating formed in the electrolyte containing 10 g/l silicate possessed the best combination of thickness and
compactness, showing the least corrosion current in 3.5% NaCl solution when potentiodynamic polarization was carried out.
Sah et al. (2010) formed MgAl
line aluminate solution (0.15 mol$dm
concentrations of Na
. The concentration of phosphate in alkaline aluminate elec-
3PO4
-based PEO coatings on AZ80 Mg alloy in alka-
2O4
3
K2Al2O4þ 0.01 mol$dm3KOH) with different
trolyte significantly controlled the discharge characteristics and the microstructure of
the coatings developed. The coatings formed in the phosphate-free electrolyte were
Plasma electrolytic oxidation/micro-arc oxidation of magnesium and its alloys 207
highly porous and very thin. The addition of phosphate to the electrolyte accelerated
the growth of the coating and at the same time increased defects, such as cracks and
large discharge pores in the coating. EDS (energy dispersive X-ray spectroscopy)
mapping showed that phosphor was mainly present in the inner half of the coating.
An optimized concentration of 0.05 mol$dm
3
phosphate in alkaline aluminate solution
enabled formation of a uniform and rather dense coating. This coating showed the
lowest anodic current during anodic polarization in 0.5 M NaCl solution.
However, calcium and phosphor are likely the two most interesting elements for
biomedical applications. The easiest way to control the ratio of the two elements in
PEO coatings is by variation of the concentration of the two elements in the electrolyte.
Different sources were successfully used, such as sodium hexametaphosphate and calcium hypophosphite (Yao, Li, & Jiang, 2009) or sodium phosphate and calcium hydroxide (Srinivasan, Liang, Blawert, St€ormer, & Dietzel, 2010). The latter results
suggest that the incorporation of calcium into the layer occurs mainly via formation
of an amorphous phase, while phosphate is incorporated in crystalline (Mg
3PO4
and together with Ca in the amorphous phase. In contrast, although using a similar
electrolyte, it was not possible to incorporate Ca into PEO coatings while using a
DC power supply (Gao, Yerokhin, & Matthews, 2013). This stresses the importance
of the combination of electrolyte and electrical processing parameters to control the
composition and properties of the PEO coatings.
However, there are more options available than a simple replacement of standard
NaOH or KOH electrolyte additions by Ca(OH)
8 g/L KF$2H
O þ 1 g/L Sr(OH)2) was introduced to an MAO coating to improve the
2
. Thus, strontium (3 g/L (NaPO3)6þ
2
bioactivity of the coating (Lin et al., 2014). The Sr-P coated ZK60 alloy had a better
corrosion resistance than the only P-coated reference, which was mainly attributed to
the superior inner barrier layer of the Sr-P coating. The long-term protective ability of
Sr-P coating was better than that of the P coating, probably owing to its greater thickness, superior inner barrier layer and superior apatite formation ability.
Summarizing, one can state that biospecific modifications of PEO coatings via electrolyte modifications have just started, and further improvements can be expected in
the future.
)
8.4.3 Particle additions
To improve the tribological properties and growth rate of PEO coatings, but also to add
new functionalities, several studies were devoted to the incorporation of particles and the
production of composite PEO coatings. Different types of particles, including metals, ceramics and polymers, were successfully incorporated in PEO coatings directly from the
treatment electrolytes. There are several reports of particle incorporation in PEO coatings
on Al alloys (Jin et al., 2006; Lee, Jo, et al., 2011; Malyshev & Zorin, 2007; Matykina,
Arrabal, Monfort, et al., 2008; Matykina, Arrabal, Skeldon, & Thompson, 2008;
Matykina, Arrabal, Skeldon, & Thompson, 2009; Wu, Qin, Guo, & Xie, 2008). Howev-
er, the interest in particle incorporation in PEO coatings on Mg alloys is also increasing
(Arrabal, Matykina, Skeldon, et al., 2008a; Arrabal, Matykina, et al., 2008b; Blawert
et al., 2012; Guo et al., 2009; Lee, Shin, Namgung, Yoo, & Shin, 2011; Lim,
208 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Ryu, & Hong, 2012; Lu, submitted for publication; Necula, Fratila-Apachitei, Berkani,
Apachitei, & Duszczyk, 2009; Song, Sun, & Liu, 2012; Wang et al., 2009).
The first report of zirconia nanoparticle incorporation in PEO coatings on Mg alloy was
made by Arrabal, Matykina, et al. (2008a,b). They found that the addition of zirconia to
the electrolyte generally enhanced coating growth. Basically, a two-layered PEO coating
was formed under the given condition and zirconia was preferentially present in the outer
coatinglayer (Figure8.6). The preferential presence of zirconiawas explainedby the local
heating of microdischarges, under which the zirconia reacts with magnesium species to
form Mg
2Zr5O12
in the outer coating layer. The presence of both zirconia and Mg2Zr5O
showed that both inert as well as reactive incorporation of zirconia occurs in PEO coating
12
Figure 8.6 (a) Scanning electron micrograph (backscattered electrons) of a cross-section of
magnesium following AC PEO treatment for 2400 s at 200 mA/cm
Na
SiO3$5H2O/0.05 M KOH with addition of 10 g/L of monoclinic zirconia. (b) Detail of
2
region between the inner and outer coating layers. EDX elemental maps are shown for
magnesium, oxygen, silicon and zirconium of the cross-section of (a).
Reprinted from Arrabal, Matykina, et al. (2008a,b) with permission from Elsevier.
2
(rms) in 0.025 M
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