70 Chapter 4 Corrosion and sliding wear behavior of plasma sprayed nanoceramic coatings for biomedical applications 4.1 INTRODUCTION Presently ceramics are considered as a better alternative for metallic based implants, as they offer a number of advantages over metals and alloys. Besides being biologically inert, ceramics produce very little wear debris and further the compositions and hence the microstructure can be tailored to match the material properties to that of the natural bone. The articulating surfaces on hip, knee and shoulder implants are currently being fabricated using either of the two types of ceramics, alumina or zirconia, that are scratch-resistant and significantly harder than the metal combinations (Hulbert et al; 1993, Jerome Chevalier et al; 2004). These two ceramic materials can also be used on both the ball as well as the socket components of an implant. One of the disadvantages of using ceramics as implants is the material's limited lifespan. Even a small crack or porosity may result in catastrophic failure of ceramic materials and once several batches of implants made of zirconia were withdrawn due to manufacturing defects (Charles et al; 1995).In addition, ceramic implants made of zirconia undergo slow degradation during long term implantation in the human body (Anis Paul et al; 2011, Santos et al; 2004).With orthopedic procedures increasingly being performed on younger patients, surgeons are eagerly awaiting for researchers to develop technological breakthroughs that would extend the life of an implant. In order to develop an implant with high wear resistance and superior fracture toughness, one can profitably try to make use of the advantageous properties of both the ceramics as well as that of metals, as the former exhibits high wear resistance, while the latter is more ductile and mimics the natural bone. This can be achieved by the development of ceramic coatings on metallic substrate which will be more suitable for the manufacturing of new high wear resistant implants. Apart from the enhancement of wear resistance, ceramic coatings are also found to improve the osseointegration and hence the service period of the implant.
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70
Chapter 4
Corrosion and sliding wear behavior of plasma sprayed
nanoceramic coatings for biomedical applications
4.1 INTRODUCTION
Presently ceramics are considered as a better alternative for metallic based
implants, as they offer a number of advantages over metals and alloys. Besides being
biologically inert, ceramics produce very little wear debris and further the
compositions and hence the microstructure can be tailored to match the material
properties to that of the natural bone. The articulating surfaces on hip, knee and
shoulder implants are currently being fabricated using either of the two types of
ceramics, alumina or zirconia, that are scratch-resistant and significantly harder than
the metal combinations (Hulbert et al; 1993, Jerome Chevalier et al; 2004). These two
ceramic materials can also be used on both the ball as well as the socket components
of an implant. One of the disadvantages of using ceramics as implants is the material's
limited lifespan. Even a small crack or porosity may result in catastrophic failure of
ceramic materials and once several batches of implants made of zirconia were
withdrawn due to manufacturing defects (Charles et al; 1995).In addition, ceramic
implants made of zirconia undergo slow degradation during long term implantation in
the human body (Anis Paul et al; 2011, Santos et al; 2004).With orthopedic
procedures increasingly being performed on younger patients, surgeons are eagerly
awaiting for researchers to develop technological breakthroughs that would extend the
life of an implant.
In order to develop an implant with high wear resistance and superior fracture
toughness, one can profitably try to make use of the advantageous properties of both
the ceramics as well as that of metals, as the former exhibits high wear resistance,
while the latter is more ductile and mimics the natural bone. This can be achieved by
the development of ceramic coatings on metallic substrate which will be more suitable
for the manufacturing of new high wear resistant implants. Apart from the
enhancement of wear resistance, ceramic coatings are also found to improve the
osseointegration and hence the service period of the implant.
71
The works of Kabacoff et al and Gell et al showed that the nanostructured
Al2O3-13TiO2 coatings made on some of the mechanical parts such as periscope
piston rod etc used in navy and submarine gave rise to better indentation crack
resistance, adhesion strength, spallation and wear resistance (Gell et al; 2001
Kabacoff et al; 2002). With regard to biomaterials, nano features on the surface
results in superior bone adhesion and strength, as it mimics the porous bone surface. It
is well documented that nanoceramic coatings made using HAP, CNT and TiO2
powders and surfaces tailored with nano topography using different techniques exhibit
excellent biocompatibility (Gadow et al; 2010, Anup kumar keshri et al; 2009,
Webster et al; 2001). It is important to note that the same composites of alumina-
titania namely viz; Al2O3-13TiO2 has the same effect in the field of bioimplants too.
The more recent and significant work of Soo et al, performed on the femoral head
made out of Al2O3-13TiO2 clearly demonstrated that this composition possesses
improved mechanical properties such as hardness, fracture toughness and wear
resistance (Soo Whon Lee et al; 2003). The above conclusions were arrived at by
varying the composition of TiO2.
Though extensive research has been carried out on alumina-titania composite
coatings for other applications, only few attempts have been made on the
development of wear resistant nano Al2O3-13TiO2 based coatings especially using
plasma spray technique for biomedical applications. The work carried out by Richard
et al on commercially pure titanium (Cp titanium) and Ti-13Nb-13Zr (Ti1313)
biomedical alloy, demonstrated that there is an enhancement in fretting wear
resistance of both the above materials, when coated with nano Al2O3-13TiO2 and
ZrO2 powders using plasma spray technique (Richard et al; 2009).
The major drawbacks of plasma spray coating are their inherent porosity and
cracks formed on the coatings. In addition, in thermal barrier coatings, the mismatch
in thermal expansion coefficient between the substrate and coating leads to frequent
failure of the ceramic coatings. Geis et al have observed that the adhesion of the
coatings can be increased considerably by modifying the surface texture or having
suitable bond coat (Geis et al; 2004). NiCrAlY alloy is the commonly used bond coat
for alumina/ zirconia coatings on super alloys. A bond coat is selected in such a way
that the bond coat has low thermal mismatch with the substrate. It is important to note
72
that a suitable bond coating for the ceramics such as alumina and zirconia coated on
biomedical alloys such as titanium alloys, Mg and its alloys as well as other alloys
have not been tried so far. Recent studies have shown that a bilayered coating
consisting of ZrN/Zr on AZ91 Mg alloy exhibited superior corrosion behavior
(YunchangXina et al; 2009). Chang et al have deposited Al2O3 on ZrO2 coated AISI
440C substrate and observed improvement in wear and corrosion resistance when
compared with that of the bare substrate (Chang et al; 2004). Bang-Yen Chow et al,
have performed HAP coating on Ti-6Al-4V alloy using ZrO2 as an intermediate layer
and obtained remarkable improvement in the adhesion strength of the coating (Bang-
Yen Chow et al; 2002) .
The above studies prompted us to make an attempt to deposit Al2O3-13TiO2
and ZrO2 nanoceramic coatings on the Ti-13Nb-13Zr alloy using plasma spray
technique and evaluate their wear and corrosion properties in simulated body
conditions. Further, an effort was made to develop a bilayered coating using both the
above mentioned powders. This chapter describes procedures adopted to develop
thick ceramic coatings and their corrosion and wear behaviors.
4.2 EXPERIMENTAL TECHNIQUES
Atmospheric plasma spraying equipment mainly consists of two electrodes
namely cathode and the anode. Tungsten filament acts as cathode while copper as
anode. When a primary gas especially, argon is allowed to pass between the above
two electrodes, they get ionized and transform into plasma. The plasma generated will
have a high temperature of around 10,000º K to 15,000º K. When the powder is fed
into the plasma flame, it absorbs the heat energy from the plasma flame and gets
melted and finally accelerated towards the substrate thereby forming a coating.
4.2.1 FEED POWDER
Reconstituted nanostructured alumina-titania (Al2O3-13TiO2,), which will be
referred as AT hereafter and 7wt% yttria stabilized zirconia (YSZ) powders were
procured from Inframat Corporation, USA. The procured AT powder consisted of
trace amount of additives such as ZrO2 and CeO2 which were incorporated into the
powder so as to reduce the sintering temperature and enhance the densification
73
process. The composition and the size of the powders used are given in Tables 4.1 and
4.2. The morphologies of both the nano structured AT and 7 wt% YSZ powders are
shown in figure 4.1(a, b). All the coatings were performed on the Ti-13Nb-13Zr alloy
specimens of dimensions of 35 x 20 x 4 mm3.
Table 4.1 Specification of Al2O3-13 wt%TiO2 powder
Al2O3 : TiO2 wt ratio 87 : 13
CeO2 content, wt % 6 – 8 %
ZrO2 content, wt % 8 – 10 %
Powder grain size (ave.) 50 - 500 nm
Agglomerated powder size (ave.) 30 µm
Tap density 2.0 g/cm3
Table 4.2 Specification of 7wt% ZrO2 powder
Y2O3 : ZrO2 wt ratio 7 : 93
Agglomerated powder size 15-150 µm
Tap density 1.4-1.7 g/cm3
(a) (b)
Figure 4.1 SEM Images of (a) AT powder (b) 7 wt% YSZ powder
4.2.2 COATING DEPOSITION
Coating of the reconstituted nanostructured YSZ, AT powders were carried
out using 9 MB Metco Plasma spray system (80 kW) at constant carrier gas flow rate.
74
The process parameters were varied by changing the current from 450 to 600 A and
the voltage from 50 to 55 V. The optimized parameters which yielded thick coating
without spallation that were obtained after repeated trials are given in the Table 4.3.
Bilayered coatings were performed using these optimized parameters, that is, initially
zirconia was coated using the parameters furnished in column A in Table 4.3 and over
this coating, Al2O3-13%TiO2 was coated using the parameters given in the Column B
in Table 4.3.
Table 4.3 Optimized plasma spray parameters used for plasma spraying AT and
YSZ powders
Parameters YSZ coating (A) AT coating(B)
Plasma Current (A) 450 600
Plasma Voltage (V) 55 50
Ar gas flow pressure(NLPM) 42 42
H2 gas flow pressure(NLPM) 8 9
Carrier gas flow (psi) 58 58
Number of spray passes (No) 8 4
Spray distance(cm) 20 20
4.2.3 POWDERS AND COATING CHARACTERIZATION
The morphologies of the starting powders and the coatings were observed by
JEOL JSM-6360 scanning electron microscope (SEM). For cross-sectional SEM
studies, the samples were mounted using bakelite powder followed by grinding and
polishing with emery papers of different grit sizes, ranging from 120 µm to 1600 µm
and mirror finished by 1 µm diamond paste. Phase analysis of the starting powders
and the coatings were performed using Philips 3121 X-ray diffractrometer with CuKα
radiation. The current and voltage were set at 40kV and 20 mA respectively and the
data were collected in the 2θ range from 10ο to 90
ο in a step scan mode with a step of
2ο/min.The phase analysis of the coatings was also carried out using Raman
spectrometer (DILOR-JOBIN-YVON-SPEX). Microhardness measurements were
performed on the cross-section (Y-Z) of the coated samples using micro Vickers
hardness tester. A load of 200g was applied for 15 s and the microhardness values
were measured at five different places across the cross-section and its standard
75
deviation is reported. The thicknesses of the plasma sprayed AT, YSZ and BL-1 are
~200 m, ~200 m and ~200 m respectively. A bilayered coating with 400 m
(200m of YSZ and 200m of AT) was also plasma sprayed (BL-2). The average
surface roughness of the coatings was measured using Talysurf FTS 50 profilometer
(Taylor Hobson Make). Porosity measurements were carried out on the SEM images
using MATERIAL-PRO Software attached with optical microscope. The system is
used to obtain a digitized image of the object. The total area captured by the objective
of the microscope can be measured using MATERIAL-PRO software. Hence the area
covered by the pores is separately measured and the porosity is determined.
4.2.4 ELECTROCHEMICAL CORROSION BEHAVIOR OF COATINGS
Potentiodynamic polarization experiments were carried out on the bare
substrate Ti-13Nb-13Zr alloy (1cm x 1cm) as well as the plasma sprayed coatings in
simulated body fluid (Hank‟s solution) to evaluate their corrosion resistance. All the
potential measurements were made with reference to a saturated calomel electrode
(SCE). A Platinum foil was used as counter electrode and an electrochemical interface
(Gill AC, ACM make) was used for conducting the experiments. For the
electrochemical measurements, the substrate was placed in a Teflon holder, with a 6
mm diameter window and exposed to the solution. Open circuit potential (OCP)–time
measurements were carried out for an hour to achieve a steady open-circuit potential,
which was measured as the corrosion potential. On attaining a constant potential,
potentiodynamic polarization was started from an initial potential of 250 mV below
the OCP. The scan rate used was 0.166 mVs-1
as per ASTM F2129 standards. The
experiments were repeated thrice to check the repeatability. Electrochemical
Impedance Spectroscopy (EIS) measurements were performed with the same
experimental setup used for potentiodynamic polarization studies. The frequency
response analyzer and potentiostat were driven by Z plot software. Impedance
measurements were carried out with a frequency sweep ranging from 10,000 HZ to 1
HZ. The software enables to obtain the best fit from the acquired data, which in turn
led to smooth and reliable curves.
76
4.2.5 SCRATCH TEST
Scratch test was performed using a commercial micro scratch tester (DUCOM,
India) to evaluate the scratch resistance of the coatings following the procedure
adopted by Sung Park et al (Sung Park et al; 2008). A spherical Rockwell C diamond
stylus of 200 µm radius was used to produce the scratch. The test was carried out in
ramp loading mode with the load varying from 20N- 200 N. The loading was varied
in steps of 2 N/mm and the stylus scanned the coating surface perpendicularly at a
speed of 0.5 mm/s. The total length of the scratch scar was 10 mm.
4.2.6 WEAR TEST
The sliding wear test was carried out using reciprocating wear tester TR-285M
machine (DUCOM, India). A ball made of Al2O3 with the diameter of 5.2 mm and
the coated substrate of dimension 35x20x4mm3 was used as a flat in this study. The
wear testing experiments were performed as mentioned in Table 2.3. The parameters
of wear testing are shown in Table 4.4.
Table 4.4 parameters for wear testing
Load applied 10 N
Frequency 2 Hz
Temperature 37º C
Sample dimensions 25mmx20mmx4 mm
Diameter of Alumina ball 5.2 mm
No. of Cycles 105 cycles
Environment Hank‟s solution
4.3 RESULTS AND DISCUSSION
4.3.1 MICROHARDNESS AND MICROSTRUCTURE OF PLASMA SPRAYED
COATINGS
4.3.1.1 MICROHARDNESS OF THE PLASMA SPRAY COATINGS
The micro Vickers hardness values for all the coatings were found to be
substantially higher when compared to that of the uncoated Ti-13Nb-13Zr alloy. The
77
BL coating exhibited significantly higher microhardness value (1096 ± 10 HV) which
is ~ 1.3 times that of AT (820 ± 22 HV) and ~ 1.8 times that of YSZ coatings (617 ±
25 HV). Figure 4.2 shows the variations in hardness of bilayered coating.The increase
in the microhardness of the BL coating is due to the densely packed AT layer over the
ZrO2 coating. Inspite of the fact that same processing parameters were used to coat
AT on YSZ, unlike in single AT coating which consisted of bimodal structure and
some porosity (Figure 4.3), in the present case the top AT coating was found to be
completely melted and adhered well with the ZrO2 layer. Thus the hardness of the BL
coating was considerably higher than the AT or YSZ coatings. The hardness values
measured across the cross section along X- direction are given in Table 4.5.
Table 4.5 Hardness of the plasma sprayed coatings
Coating Hardness (HV)0.2 Hardness (GPa)
Ti-13Nb-13Zr alloy 196 ± 3 1.96± 0.03
ZrO2 617 ± 25 6.17± 0.25
Al2O3-13TiO2 820 ± 22 8.20± 0.22
Bilayered 1096 ± 10 10.96± 0.10
0
200
400
600
800
1000
1200
20 40 60 150 200 Substrate
Depth in micron
Ha
rdn
es
s (
Hv
)
Fig 4.2 Hardness variation of bilayered coating
78
4.3.1.2 MICROSTRUCTURAL ANALYSIS OF AT COATINGS
The SEM images of the surface of the plasma sprayed AT coatings and EDAX
spectra of the same are shown in Figure 4.4(a-c). The SEM micrographs of AT
coating consisted of fully melted (FM), some partially melted (PM) and few unmelted
(UM) particles (Figure 4.4(a)). The porosity in the AT coating as calculated from the
SEM image is found to be 0.6%. The SEM micrographs of AT coatings reveal the
presence of the two regions, one with melted particles of micron size (FM, PM) and
some unmelted of nano size (UM). This kind of microstructure of the coatings with
two distinct regions is often referred to as a bimodal microstructure by several authors
(Eun Pil Song et al; 2006, Kabacoff et al; 2002, Youwang et al; 2000, Ramachandran
et al;1998, Venkataraman et al; 2006). The unmelted region consisted of smaller
particles mostly in the nanosize range (250nm-500 nm) (Figure 4.4(b)). The formation
of this kind of bimodal microstructure (melted and unmelted particles) is attributed to
the fact that the heat transfer within the agglomerated particles is lower compared to
the dense feed stock. With the exposure to the plasma flame, the outer shell of the
agglomerated nano particles is melted and rapidly solidifies upon falling on the
substrate to form micron sized zones, while the inner core will retain its nanosized
structure (Eun Pil Song et al; 2006).The resoldified regions will thus consist of fully
melted alumina with titanium ions and unsoldified regions will have unmelted
alumina (Jordon et al; 2001). The volume percentage of the unmelted particles present
in the AT coating was found to be 56% as obtained from the clemex vision software
attached with optical microscope.
The EDAX spectral analysis of the coatings at various regions is given in
Table 4.6. From the EDAX spectra, it is clearly evident that both the partially melted
(Region 1) and completed melted (Region 2) regions consist of more or less equal
concentrations of alumina and titania. (Figure 4.4(c)). In addition, from the high
resolution SEM image of the partially melted region (Figure 4.5), two kinds of
microstructures were observed i.e liquid phase sintered region and a solid phase
sintered region. Dongsheng Wang et al have made similar observations on coating
these ceramic powders on mild steel substrate. (Dongsheng Wang et al; 2009). As the
melting point of TiO2 is lower compared to that of Al2O3, the existence of the liquid
phase sintered region can be attributed to the selective melting of TiO2 nanoparticles
79
during plasma spraying, whereas, the solid phase sintered regions would possess
nanoparticles of Al2O3-TiO2 in unmelted state during plasma spraying.
4.3.1.3 MICROSTRUCURAL ANALYSIS OF YSZ COATINGS
In order to obtain a dense YSZ coating, the voltage was varied from 50 A to
60 A and the current value was so chosen such that it (Table 4.3) did not result in
spallation of the coating. The parameter that yielded dense coating without any
spallation was considered to be the optimized values. The SEM images of YSZ coated
surface revealed the presence two kinds of structures, one is fully melted splats and
the other structure appears to be poorly consolidated by fine particles. In addition,
pores (3.5%) and cracks (Figure 4.6) were also observed. A similar microstructure
was observed by Lin et al and the presence of poorly consolidated particles in the
coating is attributed to the limited growth of the particles due to the rapid melting of
nanostructured powders. Thus the lower momentum of these droplets with fine grains
results in poorly consolidated structure on deposition (Lin et al; 2003).
It is important to mention that it is easier to spray YSZ as the specific heat
capacity (450 J/Kg) and latent heat capacity (750 J/Kg K) of the YSZ are much lower
than that of the AT whose values are 750J/Kg K (Specific heat capacity) and 1500
J/kg K (latent heat) . However, the YSZ coatings consisted of larger number of poorly
consolidated particles, because the carrier gas flow rate was kept constant in both the
cases. The presence of voids in YSZ can be due to its high specific mass. As the
momentum of YSZ is high due to its higher specific mass (6000 kg/m3) than AT
(4000 kg/m3), it does not stay for longer period in the plasma resulting in improper
melting and hence leading to higher void content in the coating.
The SEM micrographs (Figure 4.7) of the BL coating exhibited completely
different microstructures when compared to YSZ and AT coatings. The surface
morphology of the BL coating showed the presence of large number of fully melted
splats with very less porosity (0.01%). In contrast to the AT coating which consisted
of both melted and unmelted particles, in the BL coating, a complete melting of
alumina particles was observed (Sathish et al; 2011). At this juncture it is not clear as
to why the alumina particles were completely melted in the BL coatings when
compared to pure AT coating. In addition, it is clearly evident that AT coating
80
exhibits good adhesion with the already deposited YSZ layer when compared to direct
coating of AT over the bare substrate as there were no clear interface observed
between the substrate and YSZ or YSZ and AT coatings in the SEM micrographs
(Figure 4.8). The variation in the microstructures of the BL and AT coatings is due to
the difference in the surface temperatures. The surface of the preheated YSZ should
have been high than the preheated substrate, while forming BL coating. The high
temperature of the preheated YSZ coating is attributed to its low thermal conductivity.
Thus the BL coating was completely melted on the YSZ surface and this has resulted
in superior adhesion of the same. Similar to what have been observed in the present
study, Sarikaya also have reported the formation of layered coating due to higher
surface temperature leading to higher microhardness and reduced porosity of the
plasma sprayed coatings (Sarikaya, 2005).
The average surface roughness of plasma sprayed coatings was in the range of
6 to 10 µm, while, the bilayer coating exhibited the lowest surface roughness of about
6 µm when compared to AT and YSZ coatings.
Figure 4.3 SEM image of the plasma sprayed AT coating
Unmelted
Fully melted
Partially melted
81
Figure 4.4 SEM images of surface of plasma sprayed AT coating (a) bimodal
nature (b) unmelted region showing nanosize particles (c) a typical EDAX
spectra of plasma sprayed AT coating.
Table 4.6 EDAX data recorded on the surface of plasma sprayed AT coating
Point on the
SEM image
Element wt% ( %Error)
Oxygen Aluminium Titanium
1 (PM) 39.50 (0.61) 51.63 (0.25) 8.87 (0.26)
2 (FM) 37.33 (0.40) 51.89 (0.26) 10.78 (0.29)
3 (PM) 36.85 (0.39) 51.85 (0.26) 11.29 (0.30)
4 (FM) 42.09 (0.46) 45.14 (0.25) 12.77 (0.36)
82
Figure 4.5 High magnification SEM image of partially melted region of AT
coating
Figure 4.6 SEM image of the surface of plasma sprayed YSZ coating
83
Figure 4.7 Surface morphology of plasma sprayed BL coating
Figure 4.8 Cross sectional SEM image of BL coating
Al2 O3 - 13 TIO2
YSZ
84
(a) (b)
Figure 4.9 Cross sectional morphology of plasma sprayed (a) AT coating (b) YSZ
coating
4.3.2 PHASE ANALYSIS BY XRD AND RAMAN SPECTROSCOPY
As plasma spraying of YSZ and AT powders results in various phase
transformations, both XRD and Raman spectroscopic analysis were carried out.
Raman spectroscopy was taken as complementary as it gives hints regarding the
presence of some non stoichiometric phases along with other peaks which are not
identified in the XRD. Figure 4.10 shows the XRD patterns of the nanostructured
YSZ powder and plasma sprayed YSZ coating. The XRD patterns of both the YSZ
powder and the coating were found to be similar. All the peaks observed were
corresponding to only tetragonal zirconia phase and peaks corresponding to cubic
zirconia were absent in XRD analysis. On the other hand, the Raman spectra of
plasma sprayed YSZ coating revealed the presence of cubic zirconia around 247 cm-1
in addition to the peaks at 149 cm-1
(b1g), 191cm-1
, 322 cm-1
(b1g), 466 cm-1
(eg) and
639 cm-1
(eg) corresponding to tetragonal zirconia (Ghosh et al; 2006, Purohit et al;
2006) (Figure 4.12(a)). The crystallite sizes of the YSZ powder as calculated from
Scherrer formula were 26 nm for tetragonal zirconia.
85
The XRD patterns of AT powders, plasma sprayed AT and BL coatings are
shown in Figure 4.11. The XRD pattern of AT powder showed diffraction peaks
corresponding to α-Al2O3, - Al2O3, brookite, anatase and rutile TiO2 (Figure 4.11(a)).
Additional peaks corresponding to CeO2, ZrO2 and baddeleyite TiO2 were also seen
and similar observation has been reported by Sanchez et al (Sanchez et al; 2008). The
average crystallite size calculated from Scherrer formula for AT powder were 52 nm
and 64 nm respectively for α-Al2O3 and rutile TiO2 phases.
Apart from the- Al2O3 and γ- Al2O3, the XRD analysis of the AT coating
revealed the presence of Al2TiO5 (Figure 4.11(b)). Peaks corresponding to Al2TiO5
phase in the AT coating could have resulted from the reaction between Al2O3 and
TiO2 in the plasma flame. The γ- Al2O3 phase observed in the XRD pattern of AT
powder was not noticed in plasma sprayed AT coating and instead a broad hump was
noticed around the same region. The broadened XRD peak in the coating indicates
nucleation of amorphous phases. The presence of γ- phase in the coated surface is due
to the complete melting of the starting powder and also due to the fact that γ-Al2O3
nucleates in preference to α-Al2O3 during rapid solidification of liquid droplets
(Xinhua Lin et al; 2003, Mcpherson et al; 1980, Dubourg eta al; 2007). It has been
reported by several authors that plasma spraying of AT powder results in the
formation of phases such as of γ-Al2O3, α- Al2O3 and very few peaks corresponding to
brookite and rutile TiO2 (Chang-sheng Zhai et al; 2005, Dongsheng Wang et al; 2009,
Sanchez et al; 2008, Ibrahim et al; 2010).As there were no peaks corresponding to
neither brookite nor rutile TiO2 in the XRD of the AT coating, Raman spectroscopy
was performed. Raman analysis did not show the presence of any bands
corresponding to brookite phase, however, it revealed the presence of rutile TiO2 at
263, 438 and 604 cm-1
(Figure 4.12(b)). The crystallite size calculated from Scherrer
formula was approximately 55 nm for α-Al2O3 and 31 nm for γ-Al2O3. This clearly
indicates that there is not much of a grain growth during plasma spraying of AT
powder.
Contradictory to the AT coatings, the XRD pattern (Figure 4.11(c)) of the BL
coating clearly revealed the presence of brookite TiO2 in addition to α-Al2O3, -
86
Al2O3, Al2TiO5 phases and the crystallite size was measured to be 11 nm, 13 nm, 47
nm and 2 nm respectively for α-Al2O3, -Al2O3, Al2TiO5 and brookite TiO2 . It is
believed that brookite being a metastable phase could have resulted from rapid
quenching of the splats.
Raman analysis was performed for the BL coating as the XRD generally do
not clearly distinguish the presence of tetragonal and cubic zirconia due to line
broadening, However, Raman spectra of BL coating (Figure 4.12(c)) showed only one
peak at 639 cm-1
corresponding to tetragonal zirconia and two peaks corresponding to
the mineral phase of corundum-ruby were at 871 cm-1
and 986 cm-1
(Tomaszek et al;
2004).
Figure 4.10 XRD patterns of (a) YSZ powder and (b) YSZ powder and (b) YSZ coating
87
0
5000
10000
15000
20000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.
u.)
0
5000
10000
15000
20000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.
u.)
Figure 4.11 XRD patterns of (a) AT powder, (b) AT coating and (c) bilayer
coating
t- Tetrogonal
c- Cubic
(a)
t t
c t t
- baddeleyite ZrO2
88
0
1000
2000
3000
4000
5000
6000
7000
8000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.u
.)
0
1000
2000
3000
4000
5000
6000
7000
8000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.u
.)
Figure 4.12 Raman spectra of plasma sprayed (a) YSZ coating (b) AT coating
(c) bilayered plasma sprayed coating
4.3.3 POTENTIODYNAMIC ANODIC POLARIZATION STUDIES
The corrosion behavior of the coated and uncoated substrates is discussed
using open circuit potential and polarization curves. Figure 4.13(a-e) shows the
potentiodynamic polarization plots obtained for the bare substrate and the plasma
sprayed coatings in Hank‟s solution. A significant shift in OCP towards the noble
direction and very less corrosion rate was observed for the plasma sprayed BL
coatings (Figure 4.13 (e)) when compared to the AT and YSZ coatings. In addition,
bilayered coating (BL-2) exhibited a stable passivation up to the maximum scanning
potential. The icorr (corrosion current density) was the lowest for the BL coatings (BL-
(b)
(c)
R
R R
R-Rutile
0
2000
4000
6000
8000
10000
12000
0 300 600 900
Raman shift (cm-1)
Inte
nsity
(a.u
.)
0
2000
4000
6000
8000
10000
12000
0 300 600 900
Raman shift (cm-1)
Inte
nsity
(a.u
.)
0
1000
2000
3000
4000
5000
6000
7000
8000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.u
.)
0
1000
2000
3000
4000
5000
6000
7000
8000
0 200 400 600 800 1000
Raman shift (cm-1)
Inte
nsity
(a.u
.) t-tetrogonal zirconia
M- Mineral phase of
corundum Ruby
t
M M
89
1 = 0.025 µAcm-2
and BL-2 = 0.07 nAcm-2
) when compared to that of the AT (1.2
µAcm-2
), YSZ (4.3 µAcm-2
) coatings and the substrate (4.49 µAcm-2
). Similarly, the
corrosion rates were 1.42x10-6
mm yr-1
, 0.0005 mm yr-1
, 0.023 mm yr-1
, 0.086 mm yr-
1 and 0.089 mm yr
-1 for BL-2, BL-1, AT, YSZ and the substrate respectively. From
the above results it is obvious that the BL coating exhibits strong passivating nature
and highest corrosion resistance and amongst all, BL-2 exhibited the highest corrosion
resistance, when compared to the other coatings. The high corrosion resistance of the
BL coating can be explained based on their microstructural features. The micrograph
of the BL coatings consisted of large amount of melted particles and lower porosity
when compared to the AT and YSZ coatings which possessed two different structures
and high porosity. The lower corrosion resistance of AT and YSZ coatings may be
attributed to the presence of high porosity (0.6%) in AT and the presence of poorly
consolidated splats and cracks in YSZ. The equivalent circuits used to fit the Electro
Chemical Impedance Spectroscopy (EIS) parameters along with the schematic of the
microstructures are shown in Figure 4.14(a-c). We presume that a very thin dense
interface layer would have been formed in between these two layers which would
have enhanced the corrosion resistance (Figure 4.14(c)).
The Nyquist plot of bare substrate showed two time constants. The first time
constant is attributed to the resistance between the electrolyte and very thin oxide film
present on the titanium alloy substrate and the second time constant should be
attributed to the substrate alloy. The very low corrosion resistance offered by the
substrate is due to the poor quality of the oxide film. The Nyquist plot of YSZ plasma
sprayed coating shows two time constants and Warburg behaviour. The Warburg
behaviour may be ascribed to the resistance offered due to the corrosion products
blocking the pores/cracks present in the coating. AT plasma sprayed coating also
showed Nyquist plot similar to that of YSZ coating. The Nyquist plot of BL-2 coating
exhibited two time constants and no Warburg component was observed (Figure 4.13
inset). The absence of Warburg component might be due to the presence of lower
porosity in the BL coating.
90
Figure 4.13 Potentiodynamic polarization plots of (a) bare substrate, (b) AT