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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5568_Библиотеки_им_академика_М_И_Перельмана
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Cold-spray coatings on magnesium and its alloys 387
(a) (b)
Figure 14.5 High-magnification images of coating porosity in coatings sprayed (a) using
10 mm powder and (b) using 5 mm powder.
From Ref. Spencer & Zhang (2011).
Bulk 316SS
–10µm(60%) + –22µm(40%)
0.00
–0.25
–10µm
–22µm
Potential vs. SHE, V
–0.50
–0.75
–9
10–810–710–610
10
Current density, A cm
–5
10–410–310–210
–2
–1
Figure 14.6 Anodic polarization behaviour of 316SS cold-spray coatings using different
powder particles sizes.
From Ref. Spencer & Zhang (2011).
14.3.2 Wear protection coatings
During cold-spray deposition, the high kinetic energy of particles results in significant
plastic strain through impact, whi ch produces substantial work hardening. As a result,
in most cases cold-sprayed coatings are expected to have a significantly higher hardness than the equivalent annealed bulk material (Borchers, Gartner, Stoltenhoff,
Assadi, & Kreye, 2003; Klinkov, Kosarev, & Rein, 2005; Stoltenhoff et al., 2002).
Figure 14.7 shows the micro-hardness of the cold-spray coatings as a function of
Al
content (Spencer et al., 2009). With increasing Al2O3additions in the Al and
2O3
6061Al coatings, micro-hardness increases slightly for both the as-sprayed and heattreated coatings. The decrease in hardness after post-spray heat treatment is attributed
388 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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250
200
6061AI-AI
AI-AI
2O3
AI-AI
2O3
mixtures on AZ91
2O3
mixtures on AZ91
mixtures on AZ91, HT 400°C 2h
150
0.2
HV
100
50
0
0
10 20 30 40
Volume percent AI
AZ91 substrate as-cast
50
60 70 80
2O3
Figure 14.7 Vickers microhardness of cold spray as a function of Al2O3content.
From Ref. Spencer et al. (2009).
to the annealing effect of impact-induced cold work in the coating. Unlike microhardness, increasing the Al
tance, as shown in Figure 14.8. The wear rate of the Ale25% Al
addition level has significant effect on the wear resis-
2O3
and 50% Al2O
2O3
composite coatings is reduced by roughly two decades compared with the pure Al
coating. The latter has a very close wear rate to the bulk T6 AZ91E alloy, the 356.0
Al alloy and the Ale12 wt.% Si alloy under the same testing condit ions. With the addition of 75% Al
the wear rate decreases by over five decades.
2O3
3
–1
10
–2
10
–3
10
–1
m
2
–4
10
–5
10
–6
Wear rate, mm
10
–7
10
–8
10
Cold-spray AI HT
400°C 2hr
Vol.% Al2O3 in cold-spray coatings
0% 25% 50% 75%
Pure AI, as-
sprayed
6061AI, as-
sprayed
356.0 Al
Al-12%Si
AZ91 T6 Mg
Bulk cast alloys
Figure 14.8 Wear volume per unit wear distance of coatings compared to bulk alloys.
Ref. Spencer et al. (2009).
Cold-spray coatings on magnesium and its alloys 389
Spencer and co-workers also investigated the worn surface of the cold-sprayed
coatings (Spencer et al., 2009). Typical wear tracks observed in scanning electron
microscopy from the cold-sprayed pure Al and AleAl
composite coatings are
2O3
shown in Figure 14.9. The smeared appearance of the worn surface of CS pure Al
coating characterizes adhesive wear, with profiles showing evidence of ploughing
and extrusion of the worn material outside the wear track. The wear track from the
Ale50% Al
cold-sprayed composite coating is shown in Figure 14.9(b). In addi-
2O3
tion to the adhesive wear feature that is similar to that of pure Al coating, there is
also some evidence of abrasive wear. The worn surface of the Ale75% Al
2O3
coating
as shown in Figure 14.9(c) is characterized by regular markings and lack of smearing,
which are characteristic of abrasive wear. This indicates that a transition from adhesive
to abrasive wear occurred with increasing Al
additions in the cold-spray coatings.
2O3
This transition is also reflected by a significant reduction in wear rate of the coatings.
When 6061Al was used as the matrix material, a similar transition in wear behaviour
with increase in the Al
25 vol.% Al
the Al
2O3
in the source powder (w12% deposited). Furthermore, increasing
2O3
content did not lead to any further drop in the wear rate in the 6061Al
content was also observed, but this transition occurred at
2O3
(a) (b)
(c)
Figure 14.9 (a) SEM micrograph of the wear track on a cold-sprayed Al coating. (b) SEM
micrograph of the wear track on a cold-sprayed Ale50% Al
the wear track on a cold-sprayed Ale75% Al
From Ref. Spencer et al. (2009).
2O3
coating.
coating. (c) SEM micrograph of
2O3
390 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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coatings, as shown in Figure 14.8. This implies that the higher strength of 6061Al coatings enables a transition from adhesive to abrasive wear with a smaller fraction of
Al
. Thus, to achieve the same wear resistance, less volume fraction of reinforce-
2O3
ment is required in cold-sprayed composite coatings.
14.3.3 Functional coating for biomedical applications
The good biocompatibility and similar mechanical properties of Mg alloys to human
bones are attractive for biomedical applications. The density of magnesium is
1.74 g/cm
modulus of pure Mg is 45 GPa, which is compatible to the elastic modulus of human
bone, which varies from 40 to 57 GPa (Feng & Han, 2010; Razavi, Fathi, & Meratian,
2010). Hence, applications of Mg to hard-tissue engineering would greatly reduce the
possibility of stress shielding, preventing bone resorption. It is particularly promising
in development of biodegradable ortho paedic implants (Li, Gao, & Wang, 2004).
However, the major disadvantage of Mg alloys is the high corrosion rate in the human
body, which becomes the bottleneck problem limiting the medical application of
Mg-based implants due to the electrolytic aqueous environment of the chloride-rich
body fluid (pH ranges between 7.4 and 7.6). There are also two serious consequences
of the rapid corrosion rate of Mg implants. The first is the rapid evolution of subcutaneous hydrogen gas bubbles, which are produced at too high a rate for the surrounding
tissues to handle (Wen et al., 2001). These bubbles usually appear within the first week
after surgery and can easily be treated by drawing off the gas using a subcutaneous
needle. The second consequence of the high corrosion rate is the loss of mechanical
integrity of the Mg implant being used in the load-bearing application. The rapid
decrease in mechanical properties resulting from exposure to the body fluid environment means that the implant is unable to provide the necessary support for the healing
bone tissue. Generally, the implant would be expected to maintain its mechanical
integrity for 12 to 18 weeks while the healing process takes place and then slowly
degrade while natural bone tissues replace the implant (Witte et al., 2005). As a consequence, surface modifications and treatments can play the critical role in governing the
degradation rate of the implant made of Mg alloys.
(Yang, Cui, & Lee, 2011), anodization (Hiromoto et al., 2008) and thermal-spray coatings (Zeng, Dietzel, Witte, Hort, & Blawert, 2008), have been developed to change the
surface characteristics of magnesium alloys as biomaterials. Compared to other technologies, cold spray can produce dense and thick metallic or composite coatings on
magnesium alloys at low temperatures, as discussed above. However, the disadvantage
of using an Al coating on the AZ91D Mg alloy substrate for a possible biomedical
implant application is the negative effect of Al
ing tissues during subsequent corrosion. To the authors’ knowledge, there is little
information on evaluation of cold-sprayed coatings for biomedical applications.
Qiu, Zhang, and Grondahl (2013) produced Ti and TieHA composite coatings on
Ti substrate using cold-spray technology. During the deposition Al powders were
incorporated as porogen, which were consequently removed through alkaline leaching.
3
, which is slightly lower than natural bone (1.8e2.1 g/cm3). The elastic
Numerous surface modification techniques, such as chemical conversion coatings
3þ
ions being released into the surround-
Cold-spray coatings on magnesium and its alloys 391
(a)
100 µm
(c)
0.5 mm
(b)
(d)
21000
18000
15000
12000
9000
Counts
6000
3000
3000
2700
2400
2100
1800
1500
Counts
1200
002
0
0.00
1.00 2.00 3.00 4.00 5.00
001
Ti
900
Ti
600
O
300
0
0.00
1.00 2.00 3.00 4.00 5.00
Ti
Ti
6.00 7.00 8.00 9.00 10.00
keV
Ti
Ca
Ti
Ca
P
6.00 7.00 8.00 9.00 10.00
keV
Figure 14.10 Porous coatings. (a) Surface profile of HA-free porous coating in SEM. (b) EDX
spectrum from the coating surface of (a). (c) Surface profile of HA-CR-10 porous coating in
SEM. (d) EDX spectrum from the coating surface of (c).
From Ref. Qiu et al. (2013).
Figure 14.10 shows the secondary electron ima ges and EDX (energy dispersive X-ray
diffraction) spectrum collected from the coating surface. The structure is composed of
fused Ti particles of approximately 30 mm and clearly displays a porous morphology.
The pore size ranges from 50 to 150 mm and the macroporosity is approximately 50%
for pure Ti coatings (Figure 14.10a) and 60% for Ti-HA (HA-CR-10) composit e
coating (Figure 14.10c).
The elemental mapping of Ti, Ca, P and O on the coating surface is shown in
Figure 14.11. It can be clearly seen that the Ca, P and O areas are overl apped, indi-
cating the presence of HA particles embedded in the coating. Importantly, it can be
observed from the elemental maps that the HA particles are uniformly distributed
across the sample surface.
The surface morphology and the chemical composition of the HA-free and
HA-containing coatings before and after 2 weeks’ immersion in SBF was evaluated
by Qiu et al. (2013) in SEM/EDX. The HA-free coating displayed no noticeable
morphological or topological change after immersion in SBF. In contrast, the surface
morphology of the HA-containing sample changed from large smooth fused particles
before SBF immersion (Figure 14.12a) to large amounts of clusters of a fine precipitate
after immersion (Figure 14.12b). The elemental mapping of these fine precipitate clusters (arrowed in the figure) in a local area of the HA-containing coating after SBF
immersion is also shown in Figure 14.12. It can be clearly seen that the precipitate
clusters are overlapped with high concentration regions of Ca, P and O. The efficiency
392 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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Figure 14.11 Elemental mapping on the coating surface of an HA-CR-10 as-annealed. Top left:
back scatter electron image. Bottom left: overlapped map with Ti (red) and Ca (green). (Ti), (Ca),
(O) and (P) show individual elemental maps for elements indicated.
Ref. Qiu et al. (2013).
of Ca2þmineralization (decrease in Ca2þconcentration of SBF after 2 weeks’ immersion per volume of the coating) of different coatings is summarized in Figure 14.13.
It was found that all HA-containing coatings exhibited an order of magnitude higher
efficiency than the HA-free coating. This is consistent with the SEM observations.
In addition, the efficiency of the HA-containing coatings increased significantly
(p < 0.01) when changing the HA content from 5 to 10 vol. % and displayed an efficiency proportional to the vol. % in the feedstock powder. These findings suggest that
the HA particles are exposed to the coating surface of the HA/Ti skeleton where they
can act as nucleation sites.
It is important to evaluate if the coatings release any constitutes that are cytotoxic to
cells responsible for bone remodelling. In Figure 14.14, TCP (tissue culture plastic)
showed significantly higher cell viability at both time-points (0 and 48 h). However,
viable cells were detected on all surfaces after 48 h at similar levels. This indicates
that the coating samples are not cytotoxic. However, longer-term in vitro and in
vivo studies are needed to further test the biocompatibility of the coatings and their effect on a ll the cell types present in the wound site.
14.4 Bonding mechanism of cold-sprayed coatings
on magnesium and its alloys
Another major concern of surface coating is the adhesive strength between coating and
substrate, which is controlled by the actual bonding mechanisms. Because the nature of
cold spray is a process of powder particle consolidation, there are two types of
Cold-spray coatings on magnesium and its alloys 393
Figure 14.12 The surface morphology of HA-CR-10 porous coating in SEM (a) before and (b)
after 2-week immersion in SBF. Middle left: secondary electron image of a local area in (b) with
mineral nodules formed in SBF arrowed; bottom left: overlapped map of mineral nodules with Ti
(red) and Ca (green). (Ti), (Ca), (O) and (P) show individual elemental maps for elements
indicated of mineral nodules.
From Ref. Qiu et al. (2013).
interface existing in cold-sprayed coatings. One is the bonding between particles
within the coating and another is the bonding between the coating and the substrate.
14.4.1 Bonding between particles within the coating
Aluminium is one of the popular spray materials in cold-spray processes due to the
light weight and good ductility. A number of studies (Balani , Agarwal, Seal, &
Karthikeyan, 2005; Borchers, Gartner, Stoltenhoff, Assadi, & Kreye, 2004; Choi
et al., 2007; Van Steenkiste & Smith, 2004; Van Steenkiste, Smith, & Teets, 2002;
Wang, Birbilis, & Zhang, 2011) have been dedicated to revealing the microstructure
of cold-sprayed aluminium coatings. Wang and Zhang (Wang et al., 2011) investigated the interfacial structure between particles within the CS pure Al coating.
Figure 14.15a shows a backscattered electron image on the etched cross-section of
Al deposition on Mg substrate. The spherical Al particles were elongated splats
with clear particle boundaries, revealing severely plastic deformation. Some porosity
394 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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**
**
**
**
5
)
3
4
*
**
3
2
1
Mineralization efficiency (ppm / mm
2+
Ca
0
HA-free
Figure 14.13 The efficiency of Ca2þmineralization of different porous coating samples after
2-week immersion in SBF relative to coating volume. Statistical significance was evaluated with
Tukey’s multiple comparison tests. ** p < 0.01. * p < 0.05.
Ref. Qiu et al. (2013).
can be observed within the coating, and this seems to be exaggerated by the attack of
etching. Image analysis of the unetched samples shows the porosity within 1.5 vol.%
(Wang et al., 2010). Figure 14.15b shows a typical TEM micrograph of the interface
between Al particles within the coating. Severe plastic deformation resulting from the
impact between particles led to microstructure variation near the particle boundary,
which can be characterized in terms of the following features. In region A, the tangled
structure with interlaced thin strips of material is observed. Thus the particle boundaries are mechanically interlocked to each other. These strips can be explained from
the melt jetting due to the adiabatic shear instability on the surface of particles by
high-velocity impact. Therefore, the extent of mechanical interlocking depends on
the particle size and velocity, which influence the surface contact area and kinetic energy generated during the impact process. In region B, the lamellar microbands (MBs)
presented with elongated sub-micron grains are observed. The formation of MBs
resulted from the crystallographic slip on a dominant slip system and then propagated
through multiple- or cross-slip events because of stress concentration, accompanied
with the produce of a large amount of dislocations between the slip planes.
Figure 14.16a shows the details of the lamellar structure at the particle interface
(which correspond to region B in Figure 14.15b). The elongated sub-micron grains
within the MBs contain dislocation cells that are separated by dense dislocation walls
HA-NC-5 HA-CR-5 HA-NC-10 HA-CR-10
r
Cold-spray coatings on magnesium and its alloys 395
**
**
**
**
0.8
0.6
**
**
**
**
**
Time 0
Time 48h
0.4
**
**
*****
Corrected absorbance
0.2
0.0
TCP
Figure 14.14 Absorbance of MTT reagent for selected coating samples at the zero and 48-h
time points. Pure Ti and TCP served as control samples. Statistical significance was evaluated
with Tukey’s multiple comparison tests. ** p < 0.01. * p < 0.05.
Ref. Qiu et al. (2013).
*
HA-NC-10 HA-CR-10 HA-free Ti block
(a) (b)
Figure 14.15 (a) Backscatter SEM micrograph of the etched cross-section. (b) TEM micrograph
of the particle boundary.
From Ref. Wang et al. (2011).
(DDWs). After annihilation of the DDWs, subgrains with clear boundaries can be
found in the circled area. Inside, these subgrains are almost free of dislocations. In
addition, dislocation arrays (DAs) piling up at subgrain boundaries are also observed.
It is believed that the dislocation arrays were formed during subsequent deformation
after the formation of the subgrains. The grain refinement effect on the particle boundaries can be further confirmed by the microstructure with associated selected area
diffraction pattern, as shown in Figure 14.16b. The mechanism of grain refinement
related to CS is similar to that in SMAT (surface mechanical attrition treatment) process (Wu et al., 2002). The formation of dislocation tangles and high-density
396 Surface Modification of Magnesium and its Alloys for Biomedical Applications
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(a) (b)
Figure 14.16 TEM micrographs of (a) elongated microbands with dense dislocation walls,
dislocation arrays and newly formed subgrain and (b) refined sub-micron grains with selected
area diffraction pattern.
From Ref. Wang et al. (2011).
dislocation walls within the coarse grains occur via severe plastic deformation at high
strain rate, dividing grains into smaller ones separated by dislocations walls. The rotation of subgrains to accommodate further deformation leads to the formation of highly
misoriented sub-micrometer scaled grains.
14.4.2 Bonding between coating and substrate
The deposition of CS coatings begins with co llisions between particles and substrate
surface. Upon impact with substrate, particles undergo severe plastic deformation and
become flattened splats (King, Zahiri, & Jahedi, 2008, 2009). Localized deformation
characteristics that concentrate at the outer layer of particles are revealed through
microstructure analysis along the particle boundaries in cold-sprayed aluminium coatings (Kang, Park, et al., 2012; Wang et al., 2011). On the other hand, the particle
impact also has a significant penning effect on the surface of substrate and induces
the subsequent deformation of substrate surface. Through observing the interfacial
microstructure, previous studies have revealed intimate contact between coldsprayed coatings and substrate (Price, Shipway, Mccartney, Calla, & Zhang, 2007),
and along the interface there is also mixing of coating and substrate materials due to
the jetting of materials resulting from the impact of particles, which accounts for the
mechanical interlocking (Hussain, Mccartney, Shipway, & Zhang, 2009; Schmidt,
Gartner, Assadi, & Kreye, 2006). Therefore, the interfacial reaction between particles
and substrate plays an important role in facilitating successful adhesion of coating
materials.
14.4.2.1 Deformation at particleesubstrate interface
Wang and co-workers (Wang, Birbilis, & Zhang, 2014) used a fast sweep-spray
method to deposit individual Al particles on the surface of Mg substrate. Thus, the
interfacial structure between individual particles and the substrate can be investigated
in TEM (Wang, Qiu, Xiong, Birbilis, & Zhang, 2014). Figure 14.17a shows how a
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