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Table 7.1 Existing calcium orthophosphates and their major properties (Dorozhkin, 2011b)
Ca/P molar
ratio Compound Formula
Solubility at
25
C, Llog(Ks)
Solubility
at 25C, g/L
158 Surface Modification of Magnesium and its Alloys for Biomedical Applications
pH stability range
in aqueous
solutions at 25C
0.5 Monocalcium
phosphate
monohydrate
(MCPM)
0.5 Monocalcium
phosphate
anhydrous
(MCPA or MCP)
1.0 Dicalcium
phosphate
dihydrate
(DCPD), mineral
brushite
1.0 Dicalcium
phosphate
anhydrous
(DCPA or DCP),
mineral monetite
1.33 Octacalcium
phosphate (OCP)
1.5 a-Tricalcium
phosphate
(a-TCP)
1.5 b-Tricalcium
phosphate
(b-TCP)
Ca(H
2PO4)2$H2
Ca(H
2PO4)2
CaHPO
CaHPO
4
4
O 1.14 w18 0.0e2.0
1.14 w17
c
$2H2O 6.59 w0.088 2.0e6.0
6.90 w0.048
c
Ca8(HPO4)2(PO4)4$5H2O 96.6 w0.0081 5.5e7.0
a-Ca
(PO4)
3
b-Ca3(PO4)
2
2
25.5 w0.0025
28.9 w0.0005
a
a
1.2e2.2 Amorphous calcium
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phosphates (ACP)
1.5e1.67 Calcium-deficient
hydroxyapatite
(CDHA or Ca-def
e
HA)
1.67 Hydroxyapatite
CaxHy(PO4)z$nH2O,
n ¼ 3e4.5; 15e20% H
Ca
(HPO4)
10x
(PO4)
6x
(OH)
x
2x
(0 < x < 1)
Ca
(PO4)6(OH)
10
2
bb
O
2
w85 w0.0094 6.5e9.5
116.8 w0.0003 9.5e12
w5e12
d
(HA, HAp, or
OHAp)
1.67 Fluorapatite (FA or
Ca
(PO4)6F
10
2
120.0 w0.0002 7e12
FAp)
1.67 Oxyapatite (OA,
OAp, or OXA)
Ca
(PO4)6O w69 w0.087
f
,
10
a
mineral
voelckerite
2.0 Tetracalcium
Ca
(PO4)2O38e44 w0.0007
4
a
phosphate (TTCP
or TetCP),
mineral
hilgenstockite
a
These compounds cannot be precipitated from aqueous solutions.
b
Cannot be measured precisely. However, the following values were found: 25.7 0.1 (pH ¼ 7.40), 29.9 0.1 (pH ¼ 6.00), 32.7 0.1 (pH ¼ 5.28). The comparative extent of dissolution in
acidic buffer is: ACP >> a-TCP >> b-TCP > CDHA >> HA > FA.
c
Stable at temperatures above 100C.
d
Always metastable.
e
Occasionally, it is called “precipitated HA (PHA).”
f
Existence of OA remains questionable.
Surface modification of magnesium and its biodegradable alloys 159
160 Surface Modification of Magnesium and its Alloys for Biomedical Applications
Rojaee, Fathi, & Raeissi, 2013a; Rojaee et al., 2013b; Song et al., 2013; Sreekanth &
Rameshbabu, 2012; Tomozawa & Hiromoto, 2011a, 2011b, 2011c; Tomozawa,
Hiromoto, & Harada, 2010; Wang, Wei, et al., 2009; Wang et al., 2013; Wang,
Guan, et al., 2011; Wang, Guan, Wang, Ren, & Wang, 2010; Wen et al., 2009; Wu
et al., 2010; Xiao, Yu, et al., 2013; Xiao, Zhu, Su, & Li, 2013; Xu, Pan, et al., 2009;
Yang, Jiao, Cui, et al., 2008; Yanovska, Kuznetsov, Stanislavov, Danilchenko, &
Sukhodub, 2012; Zhang, Li, et al., 2012; Zhang, Zeng, et al., 2010), ethanol (Chai
et al., 2012; Cui et al., 2013; Gan et al., 2013; Gao et al., 2011; Geng, Tan, Jin,
Yang, & Yang, 2009; Grubac, Metikos-Hukovic, & Babic, 2013; Guan et al., 2012;
Hiromoto et al., 2008; Hu et al., 2010; Iskandar, Aslani, & Liu, 2013; Jo et al.,
2011, 2013; Kannan, 2013; Kim et al., 2014; Li, Wang, Yan, & Lu, 2012a, 2012b;
Liu, Hu, Ding, & Wang, 2011a; Liu, Tang, Wang, Hu, & Li, 2013; Onoki &
Yamamoto, 2010; Rojaee et al., 2013b; Song, Zhang, Li, Zhao, & Zhang, 2010;
Wang, Tan, et al., 2011; Wang et al., 2013; Waterman et al., 2011; Xu, Zhang,
et al., 2009, Xu, Zhang, & Yang, 2012; Yang, Jiao, Cui, et al., 2008; Yanovska
et al., 2012; Zhang, Ma, Chen, & Wei, 2013), mixtures thereof (Meng et al., 2011),
trichloroethylene (Gray-Munro & Strong, 2009), aqueous solution of Na
2CO3
(Gray-
Munro & Strong, 2009), a mixture of 4% nitric acid þ 96% ethylene glycol (Guan
et al., 2012), or distilled water (Abdal-hay et al., 2013; Guan et al., 2012; Hahn
et al., 2011; Li et al., 2012a, 2012b; Gray-Munro & Strong, 2009; Rojaee et al.,
2013b; Sreekanth & Rameshbabu, 2012; Wang et al., 2013; Wu et al., 2010; Yang
et al., 2008; Zhang, Zhang, & Wei, 2008, Zhang, Zeng, et al., 2010).
In addition, various types of physical modifications of the metallic surface are
used. Examples comprise physical grinding and /or polishing (Abdal-hay et al.,
2013; Chai et al., 2012; Chen, Birbilis, & Abbott, 2011, 2012; Cui et al., 2013; Du
et al., 2011; Gan et al., 2013; Gao et al., 2011; Geng et al., 2009; Grubacetal.,
2013; Guan et al., 2012; Hahn et al., 2011; Hiromoto & Tomozawa, 2010, 2011;
Hiromoto et al., 2008; Hiromoto et al., 2013; Hu et al., 2010; Iskandar et al., 2013;
Jo et al., 2011, 2013; Kannan, 2013; Kim et al., 2014; Li et al., 2012a, 2012b; Liu
et al., 2011a; Liu, Hu, Ding, & Wang, 2011b; Liu, Tang, Li, & Hu, 2014; Meng
et al., 2011; Gray-Munro & Strong, 2009; Noorakma et al., 2013; Ohtsu et al.,
2013; Onoki & Yamamoto, 2010; Rojaee et al., 2013a; Song, Sha n, & Han, 2008;
Song et al., 2010, 2013; Sre ekanth & Rameshbabu, 2012; Tomozawa & Hiromoto,
2011a, 2011b, 2011c; Tomozawa et al., 2010; Wang et al., 2010; Wang, Wei,
et al., 2009, Wang et al., 2013; Wang, Guan, et al., 2011, Wang, Tan, et al., 2011;
Waterman et al., 2011; Wen et al., 2009; Wu et al., 2010; Xiao, Yu, et al., 2013;
Xiao, Zhu, et al., 2013; Xu, Pan, et al., 20 09; Xu, Zhang, et al., 2009; Xu et al.,
2012; Yang, Jiao, Cui, et al., 2008; Yanovska et al., 2012; Zhang, Dai, et al.,
2012; Zhang et al., 2013; Zhang, Li, et al., 2012; Zhang et al., 2008; Zhang, Zheng,
et al., 2010), drying (Abdal-hay et al., 2013; Gan et al., 2013; Geng et al., 2009; Jo
et al., 2013; Kim et al., 2014; Li et al., 2012a, 2012b; Liu et al., 2013; Onok i &
Yamamoto, 2010; Rojaee et al., 2013a, 2013b; Song et al., 2013; Wang et al.,
2010; Wang, Guan, et al., 2011; Wang, Tan, et al., 2011; Waterman et al., 2011;
Wen et al., 2009; Xu, Pan, et al., 2009; Xu, Zhang, et al., 2009; Yang, Jiao, Cui,
et al., 2008; Yanovska et al., 2012; Zhang, Ma, et al., 2013; Zhang, Zeng, et al.,
Surface modification of magnesium and its biodegradable alloys 161
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2010), heat treatment (Geng et al., 2009; Hu et al., 2010; Gray-Munro & Strong,
2009), and/or autoclaving (Hiromoto et al., 2008).
Furthermore, prior deposition of calcium orthophosphates, the surface of Mg, and
its alloys might be chemically treated (e.g., activated) (Chen, Birbilis, et al., 2012;
Du et al., 2011; Wang, Guan, et al., 2011; Wang et al., 2010; Wang, Tan, et al.,
2011; Wen et al., 2009), alkaline treated (Du et al., 2011; Guan et al., 2012; GrayMunro & Strong, 2009; Wang, Wei, et al., 2009; Xu et al., 2012), anodized (Hiromoto
et al., 2008; Zhang, Ma, et al., 2013), chemically polished (Hiromoto & Tomozawa,
2010, 2011; Hiromoto et al., 2013; Ohtsu et al., 2013; Tomozawa & Hiromoto,
2011a, 2011b, 2011c; Tomozawa et al., 2010), electrochemically polished (Meng
et al., 2011), etched (Rojaee et al., 2013b; Song et al., 2013), passivated (Cui et al.,
2013; Grubac et al., 2013; Kannan & Orr, 2011; Waterman et al., 2011), or prephosph-
atized (Chai et al., 2012; Du et al., 2011; Zhang, Ma, et al., 2013), among others (Geng
et al., 2009).
More to the point, prior deposition of calcium orthophosphates, the surface of
Mg, and its alloys might be coated by an interlayer of another compound, such as
poly(ε-caprolactone) (Jo et al., 2013), nicotinic acid (Song et al., 2013), Mg(OH)
(Waterman et al., 2011), MgF2(Jo et al., 2011; Lalk et al., 2013), Ca(OH)2(Waterman,
Birbilis, Dias, Woodfi eld, & Staiger, 2012), or titania (Tang, Xin, & Wang, 2013)to
enhance the corrosion resistance and coating flexibility. Precalcified coatings might be
applied as well (Liu et al., 2013; Song et al., 2013). All types of the aforementioned
treatments are usually performed by dipping, spraying, rinsing, and/or soaking,
depending on both the quality requirements and the limitations of the product to be
coated. Finally, the surface of Mg and its biodegradable alloys might be sterilized prior
to deposition of calcium orthophosphates (Cui et al., 2013).
As seen from the amount of available references, grinding and/or polishing of Mg
and its biodegradable alloys appear to be the most popular surface pretreatment techniques, followed by cleaning and/or degreasing.
In addition, after calcium orthophospha te coatings have been deposited, various
types of postdeposition treatments might also be necessary to improve their properties.
For example, postdeposition heat treatment (annealing) of calcium orthophosphates
leads to conversion of the deposited amorphous (ACP) and nonapatite phases, such
as dicalcium phosphate dihydrate (DCPD), into either hydroxyapatite (HA) (Yang,
Jiao, Cui, et al., 2008)orCa
(Xu et al., 2012) (this depends on the Ca/P ratio)
2P2O7
with simultaneous increasing of coating crystallinity, enhancing corrosion resistance,
as well as reducing the residual stress. Furthermore, for the same purposes, chemical
treatment of the coated samples in either aqueous alkaline solutions (Chen et al., 2011;
Grubac et al., 2013; Guan et al., 2012; Jamesh, Kumar, & Narayanan, 2012; Kannan &
Orr, 2011; Song et al., 2008; Su, Li, & Lian, 2012; Wen et al., 2009; Zhang, Li, et al.,
2012) or phosphate-buffered saline (PBS) (Wu et al., 2010; Zhang, Dai, et al., 2012;
Zhang, Dai, et al., 2013) may be used (Figure 7.4). In addition, the coated samples may
be kept in boiling water (Yang, Jiao, Cui, et al., 2008). Finally, to get even better protection, other compounds, such as stearic acid (Zhang, Li, et al., 2012), may be
adsorbed on or deposited over the calcium orthophosphate coatings to form
biocomposites.
2
162 Surface Modification of Magnesium and its Alloys for Biomedical Applications
700
600
500
400
300
200
100
0
22 27 32 37 42 47 52 57 62
400
350
300
250
200
150
Lin (counts) Lin (counts)
100
50
0
20
25 30 35 40 45 50 55 6560
Before alkaline treatment
DCPD
θ / (°)
2
After alkaline treatment
CDHA
θ / (°)
2
Figure 7.4 X-ray diffraction patterns of calcium orthophosphate coated AZ91 Mg alloy
before (top) and after (bottom) an alkaline treatment. One can see that the initially deposited
coating of DCPD was transformed into that of CDHA. DCPD, dicalcium phosphate dihydrate;
CDHA, calcium-deficient hydroxyapatite.
Reprinted from Kannan and Orr (2011), with permission.
7.4 Deposition techniques
7.4.1 A brief historical background
Very briefly, the biomedical history of biodegradable Mg metallic implants started in
1878, when the physician Edward C. Huse used Mg wires as ligatures to stop the
bleeding vessels of three human patients: once in a radial artery and twice in the operation for varicocele. He already observed that corrosion of Mg was slower in vivo and
that the period until complete degradation was dependent on the size of the Mg wire
used (Huse, 1878). The history of biomedical applications of calcium orthophosphates
is much longer, because attempts to treat rickets (rachitis) by these compounds have
been known since at least 1797 (Sen & Jun, 1797). Nevertheless, in spite of such
long biomedical histories for both major constituents, they coexisted independently
Surface modification of magnesium and its biodegradable alloys 163
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until 2007, when the earliest papers on the protective calcium orthophosphate coatings
on Mg were published (Cortés, Lopez, & Mantovani, 2007; Wang et al., 2007). There-
fore, the subject of this review appears to be new. Nevertheless, the substantial amount
of available publications clearly indicates its importance.
7.4.2 Biomimetic deposition and wet chemical precipitation
From the preparation point of view, a spontaneous precipitation upon exposure to
supersaturated solutions is the simplest way to deposit calcium orthophosphate coatings on Mg and its biodegradable alloys. Depending on the experimental conditions,
the spontaneous precipitation can be divided into biomimetic deposition and wet
chemical precipitation. Biomimetic deposition is carried out from artificially prepared
simulating solutions, such as Hank’s balanced salt solution (HBSS) (Hiromoto et al.,
2008; Kuwahara et al., 2000), simulated body fluid (SBF) (Cortés et al., 2007; Hu
et al., 2010; Liu et al., 2013; Lorenz et al., 2009; Rettig & Virtanen, 2008, 2009),
and their modifications (Waterman et al., 2011, 2012; Yang, Jiao, Yin, Zhang, &
Zhang, 2008; Zhang et al., 2008). Wet chemical deposition is performed from solu-
tions of simpler composition (Chen et al., 2011; Chen, Birbilis, et al., 2012; Cui
et al., 2013; Guan et al., 2012; Hu et al., 2010; Kang et al., 2013; Li et al., 2012a;
Liu et al., 2011a, 2011b; Liu et al., 2013, 2014; Gra y-Munro & Strong, 2009; Ohtsu
et al., 2013; Wang et al., 2007; Wang, Wei, et al., 2009; Wang, Tan, et al., 2011; Xu
et al., 2012; Yang et al., 2011; Yanovska et al., 2012; Zhang, Zeng, et al., 2010; Zhang,
Ma, et al., 2013) and those containing non-biomi metic ions (Chai et al., 2012; Iskandar
et al., 2013; Xu, Pan, et al., 2009; Xu, Zhang, et al., 2009). In addition, chemical depo-
sition might also be performed from nonaqueous solutions, such as alcohol (Rojaee
et al., 2013b).
Interestingly, the aforementioned biomimetic solutions HBSS (Abdal-hay et al.,
2013; Cui et al., 2013; Gan et al., 2013; Geng et al., 2009; Grubac et al., 2013; Iskandar
et al., 2013; Jo et al., 2013; Li et al., 2012a, 2012b; Tang, Xin, & Wang, 2013;
Tomozawa & Hiromoto, 2011b; Wang, Tan, et al., 2011; Wang et al., 2013; Waterman
et al., 2011; Xu, Zhang, et al., 2009; Zhang et al., 2007, Zhang, Zeng, et al., 2010) and
SBF (Chen et al., 2011; Choudhary, Raman, & Nie, 2012; Gao et al., 2011; Guan et al.,
2012; Hahn et al., 2011; Jamesh et al., 2012; Jo et al., 2011; Johnson et al., 2013; Kang
et al., 2013; Kannan, 2012, 2013; Kim et al., 2014; Li et al., 2012a; Liu et al., 2011b,
2014; Meng et al., 2011; Gray-Munro et al., 2009; Noorakma et al., 2013; Pan, Chen,
Wang, & Zhao, 2013; Rojaee et al., 2013a, 2013b; Shi, Qi, Chen, & Shi, 2011; Smith
et al., 2012; Song et al., 2 008, 2010, 2013; Sreekanth & Rameshbabu, 2012; Wang
et al., 2007, 2010; Xiao, Zhu, et al., 2013; Xu, Zhang, et al., 2009; Xu et al., 2012;
Yang et al., 2009; Zhang, Dai, et al., 2012; Zhang, Li, et al., 2012; Zhang, Dai,
et al., 2013; Zhang, Ma, et al., 2013; Zhang et al., 2008; Zong et al., 2011), as wel l
as Dulbecco ’ s Modified Eagle Medium (DMEM) (Cui et al., 2013), minimum essential
medium (MEM) (Chen, Kirkland, et al., 2012), and aqueo us solutions (0.8e3.5 wt.%)
of NaCl (Hiromoto & Tomozawa, 2011; Waterman et al., 2011; Xu et al., 2012; Yang,
Jiao, Cui, et al., 2008) are commonly used to study in vitro corrosion of Mg and its
alloys (Virtanen, 2011; Xin, Hu, & Chu, 2010) and, thus, the protective properties
164 Surface Modification of Magnesium and its Alloys for Biomedical Applications
of calcium orthophosphate coatings (Wang et al., 2012). The latter property can be
measured in vitro by hydrogen evolution, increasing the solut ion pH, or following
the concentration of released Mg
2þ
ions (Figure 7.5)(Jo et al., 2011; Kim et al.,
2014). In vivo corrosion kinetics are much more difficult to measure; nevertheless,
the typical results are presented in Figure 7.6 (Kim et al., 2014).
To continue differentiations between the biomimetic deposition and wet chemical
precipitation, the former experiments are always performed under physiological conditions (temperature and solution pH) , while the latter experiments might be performed
at elevated temperatures and nonphysiological solution pH (Geng et al., 2009). Nevertheless, in all cases, calcium orthophosphates are precipitated and grown on the surface
of Mg and its alloys. Both techniques are simple to set up and perform and are a costeffective way of creating homogeneous coatings on several samples simultaneously. In
addition, they do not require line of sight and thus allow complex shapes to be coated
(Habibovic, Barrere, Blitterswijk, Groot, & Layrolle, 2002; Liu, de Groot, &
Hunziker, 2009). Depending on the Ca/P ratio, temperature, and the solution pH, coat-
ings of DCPD, octacalcium phosphate (OCP), or calcium-deficient hydroxyapatite
(CDHA) might be precipitated on Mg and its alloys (Liu et al., 2014; Yanovska
et al., 2012). For example, variations of ionic composition of the initial solutions
were found to lead to deposition of various phases such as DCPD, DCPD þ CDHA,
and CDHA (Yanovska et al., 2012). This is similarly valid for the temperature: with
temperature increases, the intensity of DCPD diffraction peaks in calcium orthophosphate coatings were found to decrease, while those of CDHA gradually increased (Liu
et al., 2014). In addition, precipitation of b-tricalcium phosphate (b-TCP) on Mg was
detected (Chai et al., 2012; Geng et al., 2009).
Because simulating solutions often contain a number of various ions, ionsubstituted calcium ortho phosphates are always deposited with a biomimetic
approach. For example, amorphous carbonated calcium-magnesium orthophosphate
coatings were formed in SBF (Cortés et al., 2007; Lorenz et al., 2009; Rettig &
Virtanen, 2008, 2009). After incubation in SBF for 5 days, the deposited coatings
had a thickness exceeding 20 mm and appeared to be highly permeable (Rettig &
Virtanen, 2008, 2009; Song et al., 2013). In addition, the presence of highly reactive
Mg results in the formation of Mg-enriched calcium orthophosphates (Gra y-Munro &
Strong, 2009), with simultaneous crystallization kinetics decreasing because Mg
2þ
ions are known inhibitors of apatite nucleation and growth ( Bigi et al., 1993).
Various investigations indicated that prolonged immersion in supersaturated solutions resulted in the formation of thicker coatings, only if the supply of calcium and
orthophosphate ions was plentiful. Under these circumstances, biomimetic deposition
requires regular solution renewal (Cortés et al., 2007; Hiromoto et al., 2008; Hu et al.,
2010; Kuwahara et al., 2000; Liu et al., 2013; Lorenz et al., 2009; Rettig & Virtanen,
2008, 2009; Waterman et al., 2011; Zhang et al., 2008). For example, controlling the
biodegradation rate of Mg by biomimetic apatite coating was investigated (Zhang
et al., 2008). The authors used both single-coated (one immersion for 24 h) and
dual-coated (two immersions for 24 h each) Mg samples in modified SBF, as well
as uncoated Mg as a control, followed by corrosion tests performed in the standard
SBF. The findings demonstrated that two immersions resulted in the formation of
Surface modification of magnesium and its biodegradable alloys 165
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10.0
Bare Mg
9.5
HA coated Mg
9.0
8.5
pH valueH
8.0
7.5
0
40
80 120
Immersion time (h)
30
Bare Mg
HA coated Mg
20
10
gas evolution (mL)
2
0
0 40 80 120 160
Immersion time (h)
400
350
300
Bare Mg
coated Mg
MgF
2
HA / MgF
coated Mg
2
250
200
150
100
Mg ion concentration (ppm)
50
10 20 30 40 50 60 70 80
0
Immersion time (h)
Figure 7.5 In vitro determination of the protective properties of calcium orthophosphate
coatings on Mg and its alloys in SBF (corrosion tests). Variation of pH (top) and hydrogen gas
evolution (middle) for bare and HA-coated Mg. Reprinted from Kim et al. (2014), with
permission. Concentration of released Mg
HA/MgF
-coated Mg samples. Reprinted from Jo et al. (2011), with permission.
2
2þ
ions (bottom) for bare Mg, MgF2-coated Mg, and
166 Surface Modification of Magnesium and its Alloys for Biomedical Applications
2w 4w 6w 8w 12w
Bare Mg
HA-coated
Mg
HA coated Mg
210
180
150
120
Tensile strength (MPa)
90
0
24681012
Time (week)
Bare Mg
Figure 7.6 In vivo determination of the protective properties of calcium orthophosphate
coatings on Mg and its alloys. Top: Optical images of the bare Mg and HA-coated Mg specimens
after 2, 4, 6, 8, and 12 weeks of implantation. Bottom: Ultimate tensile strength of the bare
Mg- and HA-coated Mg specimens after 2, 4, 6, 8, and 12 weeks of implantation.
Reprinted from Kim et al. (2014), with permission.
Surface modification of magnesium and its biodegradable alloys 167
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thicker CDHA coatings, with increased corrosion protection in SBF compared to the
controls. The authors concluded that the degradation rate of Mg could be tailored by
controlling the thickness of apatite coatings (Zhang et al., 2008). Other researchers
demonstrated comparable findings for the corrosion protection of calcium orthophosphate coated AZ31 and AZ91D Mg alloys using both immersion tests in SBF (Yang
et al., 2009) and electrochemical tests (Hu et al., 2010), respectively. In support of this,
still other investigators illustrated the corrosion protection of biomimetic
CDHA-coated pure Mg, with subsequent cell adhesion increases (Keim et al., 2011;
Lorenz et al., 2009). Furthermore, biomimetic precipitation of calcium orthophos-
phates on Mg might be performed under the magnetic field (Xu et al., 2013; Yanovska
et al., 2012). A difference in particle morphology and crystal texture of precipitates in
the north pole and the south pole proximities was observed. In the presence of a magnetic field, an increase of crystallite sizes in (0 2 0) and (0 4 0) planes was observed for
precipitated DCPD, which allowed the authors to vary crystallinity of the coatings
(Yanovska et al., 2012).
The spontaneous precipitation techniques have proven to be popular methods for
coating Mg and its alloys. However, there are reports on the formation of nonuniform
and porous calcium orthophosphate coatings on Mg substrates caused by coating formation occurring around hydrogen bubbles formed on the Mg surface during immersion (Cui et al., 2008).
7.4.3 Solegel preparation and dip coating
Solegel preparation combined with a dip coating technique has been widely investigated to coat Mg and its biodegradable alloys for both corrosion protection and
increased adhesion. The technique involves a substrate immersion (dipping) into a
liquid, which is a concentrated calcium orthophosphate solution with a gel-like texture.
Requirements for the sol preparation are calcium and phosphorus precursors and one
or two solvents, often ethanol (as the only solvent) or water and ethanol (if two solvents are used). The phosphorus precursor, regularly P
or triethylphosphate, is dis-
2O5
solved in ethanol. The selected calcium precursor, most often calcium nitrate, is also
dissolved in either water or ethanol and then both solutions are mixed. The obtained
mixture is then refluxed at various temperatures and solvents are evaporated off until
a more viscous solution is obtained to achieve a solegel. Samples of Mg and its alloys
to be coated are then dipped into the solegel several times to acquire a calcium orthophosphate coating, which is then cured at high temperatures to increase the coating/
substrate adhesion and to accomplish apatitic structures within the applied coatings.
When adapting this coating system for Mg substrates and their alloys, the curing temperatures cannot exceed the melting point of pure Mg (650
C) to avoid affecting the
surface integrity of the Mg substrate. Therefore, curing of solegel coatings on Mg and
its alloys has been reported anywhere in the range of 25e400
C by different authors
(Feil, Frbeth, & Schtze, 2009; Galio et al., 2010; Hu, Li, Zhong, Zhang & Chen, 2009;
Rojaee et al., 2013a; Roy et al., 2011; Tan, Soutar, Annergren, & Liu, 2005; Tang,
Xin, Luo, & Wang, 2013; Tang, Xin, & Wang, 2013).
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