Materials Today: Proceedings xxx (xxxx) xxx

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Materials Today: Proceedings

Study on thermal, mechanical, microstructural properties and failure

a r t i c l e i n f o a b s t r a c t

Article history: This research study summarizes the recent information about the characteristics of multi layer and com-
Received 20 January 2021 posite lanthanum zirconate based thermal barrier coatings (TBCs) that provide high-temperature thermal
Received in revised form 13 February 2021 protection for the jet engines parts. Thus the outcome of lightweight gas turbine components able to
Accepted 23 February 2021
work at 3000° F nearly 1650 °C needs to be built. Such difficulties can be solved using coatings with opti-
Available online xxxx
mized microstructure and chemical composition. Recently, Lanthanum zirconate (La2Zr2O7-LZ) based
pyrochlore oxides (Ra2Zr2O7, Re = rare earth) are now developing TBC material, owing to their fewer ther-
Keywords:
mal conductivity with enhanced stability at elevated operating temperatures. Even though LZ cannot be
Thermal barrier coatings
Lanthanum zirconate
directly replaced for standard YSZ because of its mismatch between the grew of thermal oxide (TGO) film.
Yttria stabilized zirconia When LZ compared to Zirconia partially stabilized with yttria (YSZ), LZ possesses a poor thermal conduc-
Atmospheric plasma spray tivity, lesser coefficient of thermal expansion, lower sintering ability, and lack of toughness than YSZ.
Double ceramic layer Hence LZ comes to be combined with standard TBC YSZ in order to integrate advantages in multilayer
Composite coatings architecture. In this study describes the thermal, mechanical and physical properties and failure analysis
of Multilayer LZ/YSZ were described. It also reviews the characterization of coatings according to differ-
ent criteria and conditions. At last, suggestions for investigation on layered LZ based TBCs applications for
next-generation was indicated.
Ó 2021 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the 3rd International Con-
ference on Materials, Manufacturing and Modelling.

1. Introduction quality of thermal barrier coatings would show high strain resis-
tance, better thermal conductivity and high durability. Thermal
Thermal barrier coatings (TBCs) are now a vital role in the con- barrier coating provides better engine efficiency consequence in
struction of modern turbine systems. Now a day TBCs are widely increasing the temperature of the gas or decreasing the flow of
applied in power plants, marine industry and aerodynamic indus- cooling air. In addition, by reducing material temperatures, the
tries. Implementations of TBCs lead to enhance gas turbine effi- lifespan of turbine blades is enhanced [1,2].
ciency and increased durability of components because of the The standard TBC anatomy consists of four stages of coating
heat insulation offered by TBCs at high operating conditions. A such as ceramic top layer (TC), thermally grew oxide layer (TGO),
comprehensive review is to be undertaken in order to continue bond coating (BC), super alloy base material (BM) or substrate.
developing modern and improved coating systems to reduced The bond coat is made of NiCrAlY, CoCrAlZr and its mixture with
emissions and fuel costs at elevated temperatures. The greater 100–300 lm thickness. Commonly, the plasma spraying technique
used to deposit the ceramic top layer. The bond coat (BC) is grad-
ually oxidized cause to undergo a chemical reaction under elevated
Abbreviations: TBCs, Thermal Barrier Coatings; LZ, Lanthanum Zirconate (La2-
Zr2O7); 8YSZ, 8 wt% Yttria partially stabilized with Zirconia; TGO, Thermally Grown operating temperature to form thermally grown oxides layer coat-
Oxides; SCL, Single Ceramic Layer; DCL, Double Ceramic Layer; JETS, Jet Engine ing with 1–10 lm thickness in amidst of top layer and bond coat-
Thermal Shock Test; FCT, Furnace Cycling Test. ing [1,39]. Alpha-alumina (a-Al2O3) is the major constituent of the
⇑ Corresponding author.
oxide layer to protect the base alloy and bond coat from further
E-mail address: mathanbabb@gmail.com (M. Mathanbabu).

https://doi.org/10.1016/j.matpr.2021.02.672
2214-7853/Ó 2021 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the 3rd International Conference on Materials, Manufacturing and Modelling.

Please cite this article as: M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al., Study on thermal, mechanical, microstructural properties and failure analyses of lanthanum
zirconate based thermal barrier coatings: A review, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2021.02.672
M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al. Materials Today: Proceedings xxx (xxxx) xxx

oxidation. As a result, it can act as a strong oxygen diffusion bar- mixed with specified volume ratios in order to integrate the afore-
rier. Nevertheless, TBCs damage can be caused by the formation mentioned advantages such as thermal insulation performance,
of the TGO layer [8]. the thermal stability of LZ and YSZ. This article presents the follow-
Surface modification in TBCs is done through various tech- ing information from recent literature articles such as SCL, DCL and
niques like atmospheric plasma spray (APS), HVOF spraying, and composite based architecture coating with the dense, porous layer.
Detonation gun deposition (D-GUN) to deposit TBCs. Supply volt- Besides that, the characterization of thermal, physical and mechan-
age, powder flow rate, surface speed standoff, current direction ical properties was discussed. Thermal cycling test and failure
of injection; post spray techniques exist as the significant process analyses under jet engine thermal shock tests, furnace cycling test
parameters to investigate on TBCs system [4]. Thus process param- and erosion test were analyzed. Then performance correlations
eters ought to be optimized by limiting the surface morphology between LZ and YSZ related TBCs were examined. At the end of
defects to influence to control the porosity, thermo-mechanical the study, suggestions for future research for layered LZ based
behavior of TBCs [2]. In order to improve the thermal barrier capa- TBC technologies were proposed.
bility enhancing the spray techniques, adjusting the process
parameters and post treatment technology TBCs are needed.
Yttria stabilized zirconia with 7–8 wt% (7–8% YSZ) is commonly 2. Thermal properties of LZ based TBCs
used as TBCs material. YSZ has comparatively easy to use with high
CTE and low cost. Even though operating temperatures of YSZ is In A good TBC have lower heat conductivity, high melting point,
restricted up to 1200 °C because of phase transformation and sin- better CTE value and low specific heat. The melting point of feed-
tering activity at elevated temperatures. Due to the sintering effect, stock powders is one of the most important criteria for selecting
thermal stress and resistance to strain will be decreased, which can the TBCs. Which is playing a crucial role in achieving good proper-
greatly affect the lifetime of the deposition. In order to develop ties in gas turbine engines for thermal stability in elevated envi-
future advanced gas turbine, the working temperature should be ronments. The melting point of the powders measured by X-ray
increased without compromising above behaviors like low thermal diffraction method, Thermal analysis experiments. Noble TBCs
conductivity, greater durability and improved sintering perfor- always have a high melting point for elevated operating conditions.
mance [37,41]. Specific heat capacity is always low for better TBCs structure. Cp
A variety of ceramic materials are being examined with exclu- value is exactly proportional to the thermal diffusivity [10]. An
sive characteristics of zirconate compositions for the substitution eminent TBCs structure shows low thermal conductivity. Thermal
of YSZ. In contrast, Lanthanum zirconate (LZ) based pyrochlore oxi- conductivity of coating becomes raised due to radiation, porosity,
des (Ra2Zr2O7, Re = rare earth) are now developing TBC material, lattice scattering [11]. To further minimize thermal conductivity,
because of their better thermal conductivity and improved phase doped layer based LZ materials have been enhanced for future
stability and less CTE at high operating temperatures. These attri- research scope [3]. Thermal Expansion Coefficient is evaluated by
butes are very essential for gas turbine components at high operat- the dilatometer approach. Different CTEs at every layer induce a
ing environments. Table 1. Illustrates the characteristics of the change in the volume during durability analysis due to mismatch.
materials of LZ and YSZ [8]. Even though recent investigation reveals doping architecture will
Still, LZ has greatest challenges for the researchers. Because of improve the CTE value of LZ based TBCs.
low performance over traditional YSZ based TBCs in durability Le Wang et al. [12], examined heat conductivity of duplex lay-
analyses. Comparatively, LZ possesses a poor thermal conductivity, ered TBCs architecture is lesser than the single-layered coating.
lower CTE, lower sintering ability, and poor toughness than YSZ. The double layered architecture of YSZ & LZ has outstanding ther-
Even though LZ cannot be directly replaced by YSZ because of its mal insulation ability and lower thermal conductivity owing to
mismatch between the thermally grown oxides (TGO) film [1]. decreased porosity. Their finding revealed that in better thermal
Hence LZ comes to be combined with standard TBC YSZ in order insulation properties enhanced the high thermal stability at ele-
to integrate advantages in multilayer architecture. The multi layer vated conditions. They found that the thermal conductivity of LZ
topcoat architecture based TBC system is believed to be a feasible is 1.2 Wm 1K 1. Which is lower than the ZrO2 is 2.3 Wm 1K 1.
solution to improve thermal strain tolerance. This engineering Panpan Zhang et al. [13], prepared double layer structured TBCs
implication has been proposed for technical analysis in design, with post treatments. They modified the surface by laser remelting
development, and performance evaluation of multi layer deposi- process to change shapes like a grid, dot and striation. Nickel-based
tion techniques. super alloy (K417G) was used as a base material. NiCrAlY powder
Most of the recent compositions are unable to fulfill any of the (AMPERIT 413) and ZrO2-7 weight %Y2O3 (7YSZ) (AMPERIT 827)
aforementioned characteristics, thereby proposing multilayer coat- powder used as coating powder. The aspects of the bond coating
ing and composite coatings [6,31]. In this study, multilayered TBCs were coated by Low temperature HVOF. Then YSZ ceramic top coat
architectures are suggested with minimum densified layers with a was sprayed on the bond coat by APS techniques. According to the
porous scale of 10 percent 20 percent called porous layer as well experimental observations the authors reported that the thermal
as the dense layer [36]. Furthermore LZ and YSZ feedstock powders shock resistances of the dotted coating were nearly double of the
as sprayed coating.
Masayuki Arai et al. [14], deposited on the austenitic stainless
steel as a base material, coating powder CoNiCrAlY for bond coat
Table 1
Properties of LZ and YSZ.
deposited through plasma spray method. Zirconia and polyester
used as topcoat materials with different mixture ratios were
Sl. Materials properties 8YSZ LZ
deposited. They used the atmosphere electric furnace as a post
No
heat treatment process. Authors used to vaporize the polyester
1. Melting point (oC) 2680 2300 inside the top coat to develop a porous TBC layer. The study indi-
2. Maximum operating temperature (oC) 1200 >1300
3. Thermal conductivity (K) (W/m-K) (@ 800 °C) 2.12 1.6
cates that porous TBCs obtained a low thermal conductivity by cre-
4. Thermal expansion coefficient (CTE) (x10 6/K) 11.0 8.9– ating wide open pores in microstructures. This open pore seems to
(@1000 °C) 9.1 have a combination of zirconia powder and polyester in surface
5. Density (g/cm3) 6.07 6.00 morphology. Thermal conductivity has been easily regulated by
6. Specific heat (J/g-K) (@1000 °C) 0.64 0.54
tuning the polyester content of the powder.
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M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al. Materials Today: Proceedings xxx (xxxx) xxx

Xingye Guo et al. [15], examined mono layer LZ and double another reason for TBCs deficiency due to degrading the surface
layer 8YSZ/LZ were deposited on NiCrAlY bond coat using APS caused by mechanical contact among the deposition layer and
technique. The aspects of coatings were Haynes 188 super alloy the impingement of solid particles by eventually removing the
used as the base material. Fig. 1. Illustrates the double ceramic coating layer.
layer (DCL) architectures with porous and dense coatings. The Robert Vassen et al. [9], reported the young’s modulus of lan-
results indicate that the thermal expansion coefficient (CTE) of LZ thanum zirconate (175 ± 10 GPa) is nearly 15 percent less than
(9 to 10  10 6 /K) was lesser than the 8YSZ (10 to 11  10 6 /K). 8YSZ (210 ± 10 GPa).Which is decreases the thermal stresses that
Besides that bond coat possess much greater levels of thermal cause the mismatch of CTE in coatings. The hardness of lanthanum
expansion coefficient (nearly 15  10 6/K). So CTEs variance zirconate (9.9 ± 0.4 GPa) is less than the 8YSZ (13 ± 1 GPa) mea-
amidst of lanthanum zirconate and bond layer has been greater sured in Micro indentation tests. The finding reveals that the hard-
than the amidst of bond layer and 8YSZ. This brings to higher ther- ness is based on surface coating density. KIC of LZ
mal stresses in the LZ layer due to the mismatch. According to the (1.1 ± 0.2 MPa m1/2) is also inferior to 8YSZ (2.0–3.3 MPa m1/2).
experimental observations the authors reported that, the LZ coat- The Authors concluded that the LZ has less hardness, modulus of
ing surface was delaminated after the first 20 cycles in FCT analy- elasticity and less toughness than 8YSZ as described earlier.
sis. However in 8YSZ, no peel-out over the 2000 cycle. In JETs Jing Zhang et al. [17], deposited on the Nickel-based super alloy
study, SCL coating has been completely peeled off in 40 cycles, (Nimonic 263) used as substrate. NiCrAlY (AMDRY 962) used as
but DCL coating after partly peeled out over the edge after the bond coat. LZ and 8YSZ (METCO 204) were used as top coat feed-
2000 cycles. So results showed in SEM analysis, Double layer with stock powder. In this research, Multi-layered ceramic top coatings
porous LZ /8YSZ deposition owns the greatest thermal durability. were decided to make with differing composites for each layer
Benjamin Bernard et al. [16], investigated stainless steel as sub- using the Plasma Spraying technique. The energy dispersive spec-
strate material, bond coat used as Hastelloy X with an aluminized trometer is often used to determine the chemical constitution of
NiAl and 7YSZ used as a top layer. The authors measured the air each surface of TBC layers. The results of the nano indentation test
conduction achieved by SPS, APS and EB-PVD in deposition. show that both Young’s modulus and hardness depend on the 8YSZ
According to experimental analysis investigator reported that the volume ratio. By altering the volume ratio of 8YSZ, they suggest the
heat conductivity of YSZ deposited through SPS was lesser than mechanical characteristics could be modified. Finally author pro-
EB-PVD owing to higher porosity content. The aspects of the coat- posed by using composite coatings, hardness is linearly associated
ing were analyzed by SEM/EDS, XRD, X-ray mapping. They demon- with Young’s modulus of rigidity.
strated the effect of heat conductivity that affects thermal Xingye Guo et al. [18], revealed that DCL based LZ layer peel out
diffusivity. They reported that Some SPS coating provides lower in less number of succession related to mono layer 8YSZ based
thermal conductivity related to APS because of larger void content TBCs coatings. As a result of thermal cycling behavior, DCL based
and reduced porosity. LZ TBCs coatings had greater residual stress, even though fracture
resistance (KIC) has been poorer than the 8YSZ. Results of the
adhesion strength investigation revealed that 8YSZ coating speci-
3. Mechanical properties of LZ based TBCs
mens have greater adhesion strength relative to LZ. As a result of
erosion test, the erosion rate of LZ based coatings was greater than
Higher fracture toughness, higher hardness, lower young’s
the 8YSZ coatings, Because of less critical eroding velocity and frac-
modulus of rigidity, strong coating density are essential factors
ture resistance (KIC) of LZ. Authors suggested that the porous 8YSZ
for choosing the good TBCs. Nano, micro and macro indentation
layer acts as a buffer layer for stress reduction [7,38].
hardness tests are used to calculate the coating hardness. In addi-
Sophie B et al. [20], identified the vertical cracks are helpful in
tion, Elastic property is another important property for designing
TBCs due to better mechanical properties. Dense nano structured
the TBCs. Elastic characteristics depend largely upon the atomic
lanthanum zirconate was developed with crack patterns on stain-
radius of the ingredients [40]. A larger atomic radius gives a higher
less steel used as substrates. Researchers used electron microscopy
modulus of elasticity (E), bulk modulus (K), and modulus of rigid-
(SEM), Vickers indentation, X-ray diffraction for characterization
ity. The atomic radii of LZ are larger than YSZ and other ceramics
study. As a result showed that, multilayered coatings based LZ with
materials. Fracture toughness (KIC) is the ability of material that
narrow rack space and crack gives a high density and better hard-
defines resistance to rupture to maintain fracture against crack
propagation in the material [19]. Erosion is considered to be

Fig. 1. The double ceramic layer (DCL) architectures with porous and dense coatings.

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M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al. Materials Today: Proceedings xxx (xxxx) xxx

ness relative to single layer coatings of equal thickness with huge 4. Microstructural properties of LZ based TBCS
crack spacing and crack opening.
Ramachandran et al. [21], investigated the erosive wear proper- Good TBCs have soundness of coating which means no crack on
ties of LZ and YSZ based TBCs coatings. According to the experi- microstructure, controlled pore morphology, low-sintering activ-
mental investigations, the authors revealed that the degree of ity, and high reflectivity. The sintering effect of the LZ becomes sig-
porosity is the prominent reason influencing the coating erosions, nificant for TBCs performances. The sintering performance of the
hence the high porosity of TBCs coating has improved the erosion. LZ surface is greater than the YSZ surface [8]. Reflectivity is one
In addition, the erosion has been enhanced by improving the veloc- of the important factors to design TBCs. coatings with increased
ity of the abrasive particles. Finally, the authors demonstrated that reflectivity may be used to prevent unwanted heating of the base
microstructure modification is the one of the methods of improv- materials by radiation. Infrared reflectance of TBCs has significant
ing erosion performance. impact for the durability of the coatings called as radiation barri-
Jing Zhang et al. [31], developed the novel LZ/8YSZ composite ers. Which is protect or minimize the internal heating of the coat-
layer based TBCs structures were deposited by plasma spray ing surface and the surface heating of the turbines blade material
method on the Ni based super alloy base materials. Buffer layer under the coatings.
has been incorporated in this structure to reduce residual thermal Azrina Arshad et al. [5], investigated the influence of structural
stress in between the top and bond coating enhanced to reduce properties on TBCs by post treatment process. Surface failures had
CTE mismatch and improve the toughness. The architecture, com- been identified caused by the actions of the lamellar structure.
position, and thickness of the LZ/8YSZ composite coating sample Neodymium-doped yttrium aluminum grenade (Nd-YAG) pulsed
are Fig. 2. Single layer coatings that do not have a buffer layer. laser was used to alter the LZ coating layer. The research revealed
TBC double layer consisting a 8YSZ buffer layer and a fifty percent that the laser glazing technique decreased the microstructure fail-
LZ/fifty percent 8YSZ composite coating. Besides that the fifty per- ure when developing the cracks on the surface by controlling the
cent LZ/fifty percent 8YSZ coating with second buffer layer 25 per- porosity and decreasing surface roughness (nearly 1–3 mm).
cent LZ/75percent 8YSZ to further decrease CTEs mismatch. The Finally, The Authors demonstrated that better resistant to hot cor-
authors demonstrated that the delamination of LZ coating is corre- rosion, good oxidation performance and enhanced thermal dura-
lated to thermal stress and lower fracture resistance of LZ layers bility owing to all of the microstructural enhancements.
and the LZ and YSZ combined feedstock powders have a significant GuanlinLyua et al. [6], studied sintering activity and phase
impact on the consistency of the phase and the mechanical changes behavior in LZ/YSZ based TBCs to manage their application
characteristics. in aircraft engines operating at elevated temperatures. LZ powder
Bo Cheng et al. [40], studied the three types of multilayer LZ/YSZ was mixed with YSZ top coat powder, and YSZ-LZ composite coat-
based TBCs under the identical heat insulation of yttrium based ings were produced in order to control the sintering effect and
zirconia and the various modulus of elasticity (E) of lanthanum Zir- increase the phase stability. The result showed that, the phase sta-
conate. The aspects of TBCs were analyzed by the virtual crack clo- bility and sintering resistance could be enhanced when the LZ
sure technique (VCCT), Thermal gradient mechanical fatigue powder was combined below 50 wt% with YSZ layers working at
(TGMF) test. As per the experimental calculations finding demon- high temperature atmospheres.
strated that the energy release rate (ERR) in between the LZ/YSZ Georg Mauer et al. [22], investigated the performance of thermo
was reduced by reducing the modulus of elasticity that helped to mechanical properties with porosity in the microstructure. Surface
minimize the formation of crack. During the durability analyses, porosity is important for good thermal and mechanical behavior.
they identified high phase stability and the losing of lanthanum The authors used Inconel 738 as Substrate, NiCoCrAlY as bond
oxide in the APS method that could lead to the irregular stoichio- coats, LZ (Metco AMDRY 386) and the 8YSZ feedstock material is
metric ratio in the LZ. Finally, research showed that the reduction being sprayed for the top layer. LZ based TBCs coatings are
in driving force concept by decreasing the modulus of elasticity, equipped by air plasma spray (APS) method operating at lower
which reveals lower modulus of elasticity exhibited high durability power to preventing inhomogeneous evaporation of feedstock
performance than YSZ based TBCs as shown in Fig. 3. powders and non-stoichiometric compositions. They observed that

Fig. 2. YSZ–LZ Composite coatings (a) 50LZ/50YSZ single layer coating; (b) 50LZ/50YSZ coating with buffer layer; (c) 75YSZ/25LZ single layer coating; (D) 50LZ/50YSZ coating
with double buffer layer [6,31].

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M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al. Materials Today: Proceedings xxx (xxxx) xxx

Fig. 3. Relations between the modulus of elasticity and standoff distance for TBCs (a) modulus of elasticity versus standoff distance for LZ coatings; (b) Lifetime versus
modulus of elasticity various types of TBCs [40].

argon and helium are superior to argon and hydrogen for plasma 5. Failure analysis of LZ based TBCs
gas mixtures in order to obtain adequate strain resistance, the
overall porosity has to be at least  20 percent. Thermal cycle is the main cause of TBC failure Effect of thermal
Sang-Won Myoung et al. [23], investigated the surface mor- mismatch between coating layers. This is used to predict the life-
phology of layer based TBCs containing three layer architecture time of TBCs at high operating environment condition [6]. Erosion
in bond and top coats. Then compared with traditional TBCs with referred as secondary cause of failure in TBCs. Other failures due to
a mono layer in each architecture. They observed that the porosity hot corrosion, Calcium-Magnesium-alumina-silicates (CMAS) were
layers changes by the process parameters like standoff distance, happened [34]. Thus prominent TBCs should have Good CMAS
input voltage and powder flow rate. According to the experimental resistance and hot corrosion resistance, erosion resistance and
analysis the author reported that, the porosities in the TBCs top good thermal cycling performance [4].
coats with the single structure were 19.6 percent, while the lay- Xingye Guo et al. [32], investigated that the porous 8YSZ and LZ
ered structure varied from 8.2 percent, suggesting that the porosity layers of DCL coating have improved thermal shock resistance and
could be adjusted to obtain strong thermal and mechanical proper- durability performances compared to SCL based LZ coating during
ties around 8 percent to 19 percent. JETS test and FCT analysis are shown in Fig. 4. According to the
X. Qiao et al. [24], reported the impact of pores on the mechan- experimental investigation authors reported that, the single layer
ical behaviors of APS depositions. Some microscopic defects like LZ were fully peeled off prior to 25 successions. However DCL
micro cracks and pores incorporated in layered 8YSZ TBCs struc- based low densified porous 8YSZ and LZ layers were partly peeled
tures. They noticed porosity was decreased with the rise in thermal out on the boundaries at the end of 2000 successions. Because of
exposure period at elevated temperatures. The authors conclude the porous 8YSZ deposition that offers better strain compatibility
that the porosity of the ceramic layer is decreased by 9.38 percent after too many successions. The authors suggest that the porous
to 5.79 percent after thermal exposure for 200 h. 8YSZ serves as a buffer layer to release stress and enhanced the
L. Wang et al. [25], studied the thermal protection by insulating durability [7,38]. They measured heat conductivity of porous LZ
the TBCs materials have to be improved from pore dimensions, layer was 25 percent less than 8YSZ, the CTE value of LZ was 10
pore molecular weight and pore layer thickness. This is depending percent less than 8YSZ under the similar conditions [7].
on improvement of thickness direction and reduces the space Xiao Li et al. [4], examined the performance of APS-based TBC
among the neighboring pores. The authors used the computational with double layered bond coating by thermal cycling studies. The
micromechanics (CMM) approach to represent the heat transfer aspects of coatings were prepared by Inconel 738 used as a base
behavior of the individual coatings. Thermal analysis was carried material; CoNiCrAlY used as a bond coat, 8YSZ Powder was
out using ANSYS software without deficiencies and with deficien- selected as a ceramic topcoat. The bond coating contains a porosity
cies in the sample. The result shows that pore space, voids, cracks, film coated on the base material and an oxidation film with a scat-
grain boundaries, impurities, coating density enhanced the heat tered a-aluminum oxide adjacent to the YSZ coated on the porosity
conductivity of the components. According to the experimental film [42]. The research indicates that the CTE of the oxidation film
observations the author reported that the thermal insulating effect became smaller than the porosity film and there is no oxidation in
of TBCs improved by developing the number of layers or by reduc- the porosity film. Therefore, the oxidation film is used as a second
ing the gap between the neighboring pores. bond coat amidst the ceramic top coating and the conventional
Robert Vassen et al. [26], compared the single layer YSZ and bond coating. The authors found that the durability of coatings
double layer with zirconate based ceramics coating to improve with porosity film remained 20 percent higher than the oxidation
the quality of TBCs in advanced aerospace turbines. The authors film enhanced by lack of change in stress dispensation.
deal with reducing the radiative heat transfer created by gas Ashofteh et al. [27], deposited YSZ, CoNiCrAlY on base material
pressures and elevated temperatures at high operating condi- authors investigated the single layer, double layer, triple layer
tions. An improved TBC therefore, should not only be resistant architecture based TBCs. Authors used energy dispersive spectrom-
to thermal conduction but to radiation as well. The flow of eter (EDS) and SEM analysis for characterization of microstructural
radiative heat based on optical properties such as absorption morphology. The results showed that, single layer YSZ and triple
coefficient, the scattering factor and the index of refraction of layer nano structured CSZ top layer have low density of oxide layer
the coatings. (TGO) and best results in heat treatment studies.

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M. Mathanbabu, D. Thirumalaikumarasamy, P. Thirumal et al. Materials Today: Proceedings xxx (xxxx) xxx

Fig. 4. SEM and optical microscope images of TBCs after thermal cycling test (a) completely delaminated mono layer LZ coating; (b) edge delaminated double layer coating
with porous 8YSZ and LZ (c) completely delaminated double layer coating with dense 8YSZ and LZ, and (d) mono layer porous 8YSZ coating (scale bar is 50 lm) [3,7].

Ramachandran et al. [29], developed the five layers coating on lifespan relative to SCL 8YSZ based coatings. In order to improve
gas turbine engines to improve the performance and to reduce the thermal barrier capability to further increase the thermal insu-
the melted CMAS penetration. Authors compared the melted CMAS lation impacts, increase the sintering activity and relief the thermal
penetration performance of the Inconel 738 (BM), NiCrAlY (BC), the difference between top coating and metallic coating at elevated
double YSZ/LZ layer and five layers coating with LZ. The results temperature are needed. They suggested that the DCL architecture
demonstrated that five layer deposited samples exhibited remark- is an effective method to combine various standard TBCs powders
able CMAS penetration resistance during thermal cycling perfor- in order to integrate advantages for high operating conditions.
mance. Because of a LZ top coat decreased the penetration of Vaishak Viswanathan et al. [36], developed multilayer architec-
melted CMAS into the YSZ coat. tures based on the YSZ/GDZ (Gadolinum Zirconate), LZ/YSZ under
Yu, Q. M. et al. [30], studied the significance of interface FCT conditions for durability analysis. They used Dense GDZ archi-
microstructure onto erosion failures of TBC. Authors reported that, tecture as one layer of top coat for erosion and CMAS resistant and
particle erosion of plasma sprayed TBCs at high temperature was porous YSZ were used as second layer of top coat for low thermal
simulated through the finite element analysis by optimizing the conductivity, low elastic modulus [40]. They investigated for the
particle parameter such as diameter, initial velocity and position first time introduces YSZ coating with dense vertically cracked
of erosion. The research indicated that the interface microstructure (DVC) in between bond coat and topcoat interface for high fracture
had small impact on the level of infiltration of the top protective toughness. The results showed that the durability analysis shall be
coating. optimized by means of a primary investigation of elastic energies
Jing Zhang et al. [31], suggested the LZ /YSZ multi layer based at failure positions.
TBCs structures with low density coatings. FCT analysis and JETS Dowon Song et al. [38], investigated the performance of
tests showed that the porous 8YSZ and LZ layers of DCL coat have microstructural modification on durability analysis of LZ/8YSZ
improved thermal shock resistance and durability performances based TBCs. They analyzed the LZ and 8YSZ as composite topcoat
compared to SCL based LZ, densified 8YSZ/LZ layers. The authors powders by a combination of various proportions (50:50 &
demonstrated that the delamination of LZ coating is correlated to 25:75) over the different thermal cycling experiments. The authors
thermal stress and lower fracture resistance of LZ layers. reported that the coating with the dual buffer layer exhibited bet-
Michael P. Schmitt et al. [33], investigated the erosion perfor- ter results during the thermal cycling because of the lowered stress
mance by architecture relations for plasma gas flow rate, current on the architectures by reducing thermal mismatch as a result of
density, and feed level on LZ ceramic coating. Authors reported the regular variance of composition and dispersion of stress. This
that greater percentage of erosion in layers with wider gaps and study revealed that the thermal cycling performance enhanced
pores by plasma spray technique. As result showed that the erosion by introducing the buffer layer and monitor the TBCs architecture
rate was enhanced with good porosity and plasma spray technique and it may provide significant surface morphology design for
improves the erosion behavior relative to electron beam physical recent developments TBCs technology.
vapor deposition method [43].
ZhenhuaXu et al. [34], studied hot corrosion impact of LZ based 6. Conclusion & future outlook
thermal barrier coatings. Authors investigated that surface mor-
phology by SEM and X-ray diffraction method. As result showed The following conclusion and research direction for the future
that plasma spray coated LZ based TBCs were subjected to the cor- outlook are identified based on this study.
rosive chemical at elevated temperatures, it was affected. Never-
theless, plasma spray coated 8YSZ based TBCs has outstanding  TBCs with double layered architecture have a longer lifespan
resistance towardsSO2, V2O5. relative to single layered TBCs. The porous 8YSZ and LZ layers
L. Wang et al. [35], investigated the thermal insulation perfor- of DCL coating have improved thermal shock resistance and
mance on three layered TBC at 1000°Celsius and 1200°Celsius. durability performances compared to SCL based LZ coating.
The finding shows that the LZ/8YSZ based coatings have a longer
6
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cial interests or personal relationships that could have appeared
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