BSECV-293; No.

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www.elsevier.es/bsecv

Microstructure and properties of YSZ-Al2 O3

functional ceramic thermal barrier coatings for
military applications

Mari Ramesh a , Krishnaswamy Marimuthu b , Palanisamy Karuppuswamy a ,

LakshmiNarasimhan Rajeshkumar c,∗
a Department of Mechanical Engineering, Sri Ramakrishna Engineering College, Coimbatore, 641022 TN, India
b Department of Mechanical Engineering, Coimbatore Institute of Technology, Coimbatore, 641014 TN, India
c Department of Mechanical Engineering, KPR Institute of Engineering and Technology, Coimbatore, 641407 TN, India

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

Article history: Current study renders its focus on the investigation of functionally graded Yttria Stabilized
Received 28 September 2020 Zirconia (YSZ) and alumina (Al2 O3 ) thermal barrier coatings (TBC) deposited on EN steel
Accepted 8 June 2021 substrates used in gun barrel through atmospheric plasma spray (APS) process. A bond
Available online xxx coat and the top coats was made with two distinct compositions using YSZ-Al2 O3 in 75:25
and 50:50 weight ratio through APS process with two carrier gas concentrations of 3 and 4
Keywords: standard cubic feet per hour (scfh). Coatings were characterized with thermal testing, X-ray
Thermal barrier coating Diffraction (XRD) and Field Emission Scanning Electron Microscope examination (FESEM).
Yttria Stabilized Zirconia Surface roughness test, wear and scratch test of the uncoated and coated steel specimens
Alumina was also measured. YSZ-Al2 O3 coated EN36C steel possessed greater insulation performance
Atmospheric plasma spray under a carrier gas flow rate of 3 scfh compared to all other specimens. EN36C steel coated
Thermal testing with YSZ-Al2 O3 in the ratio of 75:25 has better wear and scratch resistance indicating lesser
Wear and scratch tests deformation compared to other materials.
© 2021 SECV. Published by Elsevier España, S.L.U. This is an open access article under the
CC BY license (http://creativecommons.org/licenses/by/4.0/).

Microestructura y propiedades de los recubrimientos de barrera térmica

cerámica funcional YSZ-Al2 O3 para aplicaciones militares

r e s u m e n

Palabras clave: El estudio actual se centra en la investigación de recubrimientos de barrera térmica (TBC) de
Recubrimiento de barrera térmica zirconia estabilizada con itria (YSZ) y alúmina (Al2 O3 ) de grado funcional depositados en sus-
Zirconia estabilizada con itria tratos de acero EN utilizados en el cañón de la pistola a través del proceso de pulverización de
Alúmina plasma atmosférico (APS). Se preparó una capa adhesiva y las capas superiores con dos com-
Spray de plasma atmosférico posiciones distintas usando YSZ-Al2 O3 en una relación en peso de 75:25 y 50:50 mediante el
Pruebas térmicas proceso APS con dos concentraciones de gas portador de 3 y 4 scfh. Los recubrimientos se
Pruebas de desgaste y rayado caracterizaron con pruebas térmicas, difracción de rayos X (XRD) y examen de microscopio

∗
Corresponding author.
E-mail address: lrkln27@gmail.com (L. Rajeshkumar).
https://doi.org/10.1016/j.bsecv.2021.06.004
0366-3175/© 2021 SECV. Published by Elsevier España, S.L.U. This is an open access article under the CC BY license (http://creativecommons.
org/licenses/by/4.0/).

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004
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electrónico de barrido de emisión de campo (FESEM). También se midió la prueba de rugosi-

dad de la superficie, la prueba de desgaste y rayado de las muestras de acero revestidas y
sin revestir. El acero EN36C recubierto de YSZ-Al2 O3 poseía un mayor rendimiento de ais-
lamiento bajo un caudal de gas portador de 3 scfh en comparación con todas las demás
muestras. El acero EN36C recubierto con YSZ-Al2 O3 en una proporción de 75:25 tiene una
mejor resistencia al desgaste y al rayado, lo que indica una menor deformación en com-
paración con otros materiales.
© 2021 SECV. Publicado por Elsevier España, S.L.U. Este es un artı́culo Open Access bajo
la licencia CC BY (http://creativecommons.org/licenses/by/4.0/).

continuously by using the weapons like 7.62 mm self-loading

Introduction rifle, 5.56 mm Insas rifle and Light Machine Gun (LMG). During
continuous firing of these weapons, enormous heat is released
Thermal barrier coatings (TBCs) are extensively employed in
from the gun barrels to the area of lower hand guard band.
elevated temperature applications to enhance thermal insu-
Hence, the soldiers were unable to hold the hand guard dur-
lation capability and longevity. TBCs usually comprises 6 to
ing the continuous firing. In order to increase the time of firing
8 wt.% YSZ owing to the better thermal expansion coefficient
and reduce the heat transfer to the lower hand guard band,
of the material, low thermal conductivity and longevity at
TBCs could be provided on the surface of lower hand guard
temperatures lower than 1200 ◦ C. Several researchers pointed
band. The gun barrel is generally made up of EN8 steel and it
that YSZ and Al2 O3 coatings have greater potency for devel-
possesses strength of 100,000 psi (689.5 kPa) to bear gas force
opment of TBCs possessing lower thermal conductivities.
at the time of firing round. It possessed a Rockwell hardness
These TBCs (100–250 ␮m) provides insulation to the parts to
of 25 to 32 HRC to withstand pressure required for propelling
withstand large and prolonged heat loads. Large numbers of
the firing rounds [3].
researchers are focusing on establishing new TBC systems
Xuemei Song et al. (2017) studied the performance of YSZ
with better thermal stability [1]. These TBCs can be utilized
and YSZ-Al2 O3 coating made on stainless steel in terms of
in defense applications for better performance and lifetime
thermal insulation. It is found that YSZ-Al2 O3 coating was
of the component. YSZ is the most commonly used mate-
more stable than individual YSZ coating. The performance of
rial during TBCs in gas turbine and diesel engine applications.
coatings enhanced with increase in temperature up to 1200 ◦ C
Besides this, by using plasma spraying method, only YSZ could
[4]. On high temperature treatment, Y was partially removed
be coated with large thickness when compared with other
from YSZ coating which resulted in decreased stabilization
refractory oxide materials. YSZ is characterized by high ther-
whereas on YSZ-Al2 O3 coating removal of Y was backed up
mal shock resistance, high coefficient of thermal expansion
by Al diffusion thereby forming YSZ-Al2 O3 solid solution [5,6].
(CTE) and low thermal conductivity and alongside its phase
Delon et al. (2018) investigated on the influence of YSZ fibres
instability and corrosion prone owing to exposure to oxy-
on sol-gel coating. It was observed that 60–80% YSZ fibres
gen, YSZ has a limited temperature working range of 1200◦ .
reinforced in the Sol-gel increased the lifetime of the com-
On the other hand, ␣-Al2 O3 is the most stable oxide among
ponents. Microstructural examination revealed that fibrous
various class of alumina. Because of its chemical inertness
YSZ were homogeneously dispersed and were highly adher-
and high hardness it becomes a potential additional coat-
ent to the powder matrix [7]. Baiamonte et al. (2015) studied
ing material in any TBC application. It is also characterized
the properties of micro structured and nanostructured YSZ
with few demerits like low CTE and high thermal conductiv-
coating and stated that micro structured plasma sprayed YSZ
ity [2]. When Al2 O3 is additionally coated with YSZ in TBC,
coating experienced a high densification at higher tempera-
bonding strength, hardness, corrosion and oxidation resis-
tures. Characterization results showed that nanostructured
tance of the coating will be greatly enhanced for almost the
plasma sprayed YSZ preserved their porosity at high tem-
same toughness and elastic modulus of the material coated
perature and both the coating has not suffered any phase
with YSZ alone. When compared with individual YSZ coat-
transformation at higher temperatures [8]. Mohsen Saremi
ings, functionally graded YSZ-Al2 O3 coatings are proven to
et al. (2016) developed YSZ and functionally graded (FG) YSZ–
be thermally stable coatings. Stability of individual YSZ coat-
Al2 O3 TBCs by using material laminates through Atmospheric
ings becomes questionable when the raise in temperature of
Plasma Spraying (APS). Scanning Electron Microscopic (SEM)
coated material removes the yttrium partially from the zir-
examination showed the nanostructure in the coatings and
conium oxide and results in formation monoclinic zirconia
thickness of Thermally Grown Oxide (TGO) for FG YSZ–Al2 O3
(m-ZrO2 ) from metastable tetragonal zirconia (t’-ZrO2 ) due to
laminates was lesser when compared with YSZ coating. It
martensitic phase transformation. On the other hand, when
was stated that YSZ–Al2 O3 laminate coating had higher oxi-
Al2 O3 is added with YSZ for coating, the temperature rise may
dation resistance than YSZ coating [9]. Escarraga et al. (2018)
force the aluminium atoms to diffuse into t’-ZrO2 for stabiliz-
developed eight YSZ/Al2 O3 multi-layer coatings on AISI 304
ing them by the formation of solid solution of YSZ-Al2 O3 thus
stainless steel and investigated its thermal cyclic response. It
reducing the pace of martensitic phase transformation.
was observed that iron oxides were formed in the uncoated
In defence and war fields, while infantry troops are fight-
substrates and the high-temperature oxidation intensity of
ing with enemies in the operational area, they need to fire

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004
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Table 1 – Temperature measurement after rifle firing.

Number of rounds fired Supplier 1 (Temperature in ◦ C) Supplier 2 (Temperature in ◦ C)

30 200 215
60 230 240
90 270 280
120 325 330
150 330 350
180 370 380

Table 2 – Chemical composition of steel materials used.

Element EN 8 EN 19 EN 24 EN 36C
% Composition % Composition % Composition % Composition

Carbon 0.45 0.41 0.43 0.17

Silicon 0.19 0.23 0.27 0.25
Manganese 0.79 0.74 0.52 0.47
Chromium 0.007 1.01 1.17 0.98
Molybdenum 0.002 0.22 0.23 0.17
Nickel 0.009 0 1.37 3.33
Phosphorous 0.026 0.021 0.018 0.025
Sulphur 0.015 0.032 0.027 0.025

the coatings decreased considerably by increasing the bilayers using a thermal imaging camera. It was found that there is a
[10]. Reza Ghasemi et al. (2017) examined the thermal insula- steady increase in temperature while firing the rifles supplied
tion of YSZ TBC (nanostructured) on Nickel based super alloy by both the manufacturers. Values of temperature obtained
(IN-738LC) and compared it with conventional YSZ TBCs. It during firing are listed in Table 1.
was observed that nanostructured YSZ coating has bimodal Therefore, the aim of this study is to overcome the existing
microstructure comprising of nano sized particles and micro problem of heat developed in gun barrel during firing opera-
columnar grains [11]. Kirbiyik et al. (2017) produced double tion by means of coating over the substrate. The potential gun
layered FG CYSZ/Al2 O3 ceramic TBCs through HVOF and APS barrel materials were chosen as EN8, EN19, EN24, EN36C and
processes. It was observed that thermal conductivity of FG FG YSZ-Al2 O3 in the weight ratio of 75:25 and 50:50 were used
CYSZ/Al2 O3 with 8 layers was lower when compared with the as coating materials. Selection of EN8 was based on its com-
double layered CYSZ/Al2 O3 and single layered CYSZ coatings mon usage in many mechanical elements and its wide usage
at any temperature [12]. for making small arms and military weapons. This alloy pos-
From the literature study, it could be witnessed that dif- sesses good tensile strength, wear resistance and toughness.
ferent coatings were tried for various applications but only EN19 was preferred due to its high tensile strength, better duc-
limited works were available with functionally graded YSZ and tility, high shock resistance and good wear resistance. It is
Al2 O3 TBCs for defense related fields like gun barrel appli- utilized for making of engine gear boxes. EN24 was selected
cations. Hence in the present study, various EN grade steel since it has wide range of applications in various machine
materials were chosen as substrate material as they are used elements such as gears, bolts and shafts. It has a hardness
for making of gun barrels. Functionally graded YSZ and Al2 O3 of 248–302 HBN. EN36C was a nickel chromium steel and is
TBCs were developed in the weight ratio of 75:25 (YSZ:Al2 O3 ) commonly used in making of cams and rollers [13–17]. The
and 50:50 on the EN substrates for thermal insulation. Chem- chemical composition of the used EN steels has been deter-
ical composition, microstructural examination and strength mined through Optical Emission Spectrometer (OES) and are
of the coated specimens were carried out by XRD, FESEM, displayed in Table 2.
Energy Dispersive Spectroscopy (EDS), Thermal analysis, sur- Al2 O3 (0.12% of SiO2 , 0.15% of FeO and remaining Al,) and
face roughness test, scratch test and wear test. It was expected YSZ (7.88% of Y, 0.42% of TiO2 , FeO and SiO2 and remaining
that YSZ and Al2 O3 functionally graded TBCs over the EN steel ZrO2 ) were preferred for development of TBCs over the EN sub-
substrates will aid in enhancing the thermal insulation per- strates and the standard properties of these particles are given
formance of the coated material.

Material selection
Table 3 – Properties of Al2 O3 and YSZ powders. [2,18].
In order to identify the temperature values at the time of fir- Property Al2 O3 YSZ
ing of an AK47 type rifle whose gun barrel is made of EN grade Density (g/cm3 ) 3.69 6.02
steel, firing rounds were conducted in the rifle with cylindrical Hardness (HV) 1175 1250
barrel of size 7.62 mm diameter × 39 mm length. Two differ- Elastic Modulus (GPa) 300 205
ent gun barrels supplied by two different manufacturers were Thermal conductivity (W/m-K) 1.8 2.50
taken for analysis. 180 rounds were fired and the tempera- Compressive strength (MPa) 2100 2500
Melting point (◦ C) 2072 2600
ture was measured after the completion of every 30 rounds

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
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Fig. 1 – Schematic illustrations of air plasma spray deposition process.

Table 4 – Coating parameters for YSZ and Al2 O3 .

Substrate/base metal Bond coat/thickness Top coat/thickness Voltage (volts) Current (amps) Carrier gas (Ar) flow rate (scfh)

EN 8 Ni-Cr alloy/50 ␮m YSZ-Al2 O3 200 ␮m 65–70 500 3&4

EN 19 Ni-Cr alloy/50 ␮m YSZ-Al2 O3 200 ␮m 65–70 500 3&4
EN 24 Ni-Cr alloy/50 ␮m YSZ-Al2 O3 200 ␮m 65–70 500 3&4
EN 36C Ni-Cr alloy/50 ␮m YSZ-Al2 O3 200 ␮m 65–70 500 3&4

in Table 3. Al2 O3 was preferred due to its high strength and keeping the flow distance of 2 to 3 inches. This results in rapid
stiffness when compared with other oxide ceramic materi- heating of the ceramic powders and consequently the molten
als and also possesses better dielectric properties, appreciable ceramic powders were accelerated at high velocity towards
hardness, good thermal properties and refractoriness [2,18]. the substrate surface. When the powders come into contact
YSZ was selected due to its high strength and corrosion resis- with the substrate, upon rapid cooling, coating was applied
tance. It is widely used in the coatings, fibre optic ferrules, over the surface. The FG YSZ-Al2 O3 ceramic powder coating
wear parts, solid oxide fuel cells (SOFC) and oxygen sensors. of thickness 200 microns were top coated on the EN series
steels separately with weight proportions 75:25 and 50:50 and
the properties of coated specimens were investigated in detail.
Experimental procedure
The schematic illustration of the APS process is shown in Fig. 1.
The coating parameters used for development of TBCs are
Steels specimens (EN 8, EN 19, EN 24, and EN 36C) with dimen- given in Table 4.
sions as 65 mm diameter and 5 mm thickness were used for Fig. 2(a) and (b) depicts the schematic representation of
the APS process. Nickel chrome powders (10–40 ␮m average bond coat and top coat over the steel substrates using Ni-Al
particle size) were used to provide a bond coat of thickness alloy and FG YSZ-Al2 O3 (in two different weight propor-
50 ␮m on the surface of the specimens. The bond coat has tions). Such coated specimens were expected to possess high
the chemical composition of 9.17% of Aluminium, 22.01% of temperature stability and hence their thermal insulation per-
Chromium, 1.08% of Yttrium and the remaining being Nickel. formance Fig. 2.
The bond coating is allowed to dry for 5–10 min prior to APS Fig. 3 shows the thermal testing apparatus which was
process. Then the specimen is mounted inside the coating used to find the surface temperature of the uncoated and FG
chamber for the process initiation. The FG microsized YSZ ceramic powder coated specimens. Heat was supplied to the
(Avg. particle size 30–70 ␮m) and Al2 O3 (Avg. particle size surface opposite to the exposed surface through an induc-
10–30 ␮m) powders in weight proportions of 75:25 and 50:50 tion heater and was maintained at a constant temperature
were used as coatings materials. Plasma Spray Sulzer Metco of 200 ◦ C. Thermocouple was used in conjunction with the
machine (12e gun model) equipped with 3 MBH Plasma spray heater to find out the surface temperature. IR Thermometer
Gun was used for producing the coating. The flame was sup- was used to confirm the temperature measurement obtained
plied to the chamber using Argon trigger at pressure of 100 in the thermal testing apparatus. It is non-contact temper-
to 120 psi and maximum flow rate could be maintained as ature estimation gadget which identify the infrared vitality
100 scfh. In the current experiment, the carrier gas flow rate radiated, transmitted or reflected and converts into temper-
in argon trigger was kept at two different flow rates of 3 and ature perusing or thermogram. The process was carried out
4 scfh. The ceramic powder mixture was then injected into on the sample for 20 min and the temperature was noted at
a very high temperature plasma flame towards the substrate various time intervals. Uncoated steel substrates and FG YSZ-
through the nozzle at the flow rate of 40 to 45 g.min−1 by Al2 O3 coated steel substrates were then subjected to thermal

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Fig. 2 – Schematic illustration of coatings of YSZ-Al2 O3 (a) 75:25, (b) 50:50.

Fig. 4 – Thermal image of the coated substrate.

Fig. 3 – Thermal testing apparatus.

testing process. The obtained thermal image of the coated Results and discussion
specimen is shown in Fig. 4.
All FG YSZ-Al2 O3 coated specimens (with weight propor- Microstructural examination
tions 75:25 and 50:50) manufactured through two different
XRD analysis is performed on both the FG YSZ-Al2 O3 coated EN
carrier gas concentrations of 3 and 4 scfh were subjected to
steel substrate to identify the phase composition using Cu-K␣
wear and scratch tests. Wear test was conducted to evalu-
radiation ( = 1.545 nm) and shown in Fig. 5a and b. The gen-
ate the wear characteristics of uncoated and coated steels
erator of X-ray was set at 30 mA and 45 kV and the scanning
to determine materials adequacy for application and the
was performed for the diffraction angle 2 varying from 5◦ to
effect of process parameters on the wear performance. Dur-
90◦ . Both 75:25 and 50:50 FG YSZ-Al2 O3 coated EN substrate
ing wear test, a pin was kept in contact against a rotating
displayed the presence of cubic tetragonal crystal structure.
disc under sliding conditions. Experiments were carried out
Similar pattern has been observed in plasma sprayed YSZ-
on a track of mean diameter 80 mm, applied weight of 20 N,
Al2 O3 multilayer TBCs by the early researchers [3,14]. The
speed of 50 rpm and for the time of 15 min. In addition to
formation of such non-equilibrium tetragonal phase was
wear test, scratch test was also performed on the coated
attributed to the higher solidification rate of powders during
steel specimen at three different loads of 1, 2 and 3 kg
the APS process [10]. Hence it could be ascertained that YSZ-
after exposing the specimens to a temperature of 200 ◦ C for
Al2 O3 were deposited on the EN steel substrate through the
1 h.
APS process due to the formation of tetragonal phase.

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Fig. 5 – XRD pattern of YSZ-Al2 O3 (a) 75:25, (b) 50:50.

SEM analysis was performed on the both YSZ-Al2 O3 coated 75:25 YSZ-Al2 O3 and 50:50 YSZ-Al2 O3 coated EN 36C steel
EN steel substrate to observe their surface morphology and are substrate. This could be attributed to the homogenous distri-
shown in Figs. 6 and 7. Elemental identification was assessed bution of the partially molten ceramic particles on the surface
through the EDS attachment with SEM. As observed in fig- of the substrate and such similar behaviour was observed in
ures, the top surface of both the functionally graded YSZ-Al2 O3 case of Ni-YSZ deposited stainless steel by few researchers
coated EN steel substrate appears to be rough and contains [13]. Among the coated samples, 75:25 YSZ-Al2 O3 coated EN
laminar structures. Slight porosity is observed on both the 36C has the higher surface roughness value compared to
coated specimen. In addition, 50:50 YSZ-Al2 O3 appears to be the 50:50 YSZ-Al2 O3 coated EN 36C. This behaviour was due
much denser than the 75:25 YSZ-Al2 O3 coatings. The coating to the presence of more amount YSZ particles in case of
gets denser in 50:50 YSZ-Al2 O3 due to the increase in Al2 O3 , 75:25 YSZ-Al2 O3 coating as the YSZ particles are coarser in
which has a lower melting point of 2325 K as compared to the size (30–70 ␮m). As there is an increase in finer Al2 O3 par-
higher melting point of YSZ which is 2950 K. Similar phenom- ticles in 50:50 YSZ-Al2 O3 coating, a decrease in the surface
ena were also reported in case of plasma sprayed YSZ/Al2 O3 roughness compared to the 75:25 YSZ-Al2 O3 coating could be
TBCs [12]. EDS analysis on the top coat of both the functionally witnessed.
graded coatings ensures the presence of elements such as Zr,
Al, O as observed in Figs. 6 and 7 respectively.
Fig. 8a and b depicts the SEM image EN36C steel cross sec- Thermal testing
tion coated with YSZ-Al2 O3 in 75:25 and 50:50 proportions
which clearly reveals that significant adhesion exist between TBCs are generally exposed to thermal cycling during their ser-
top coat, bond coat and metallic substrate. Low degree of vice. The coefficient of thermal expansion varies between top
porosity could be observed in both the FG coatings which was coat, bond coat and the metallic substrate. Hence expansion
due to the presence of Al2 O3 which is a stoichiometric oxide. mismatch happens at the interface between these materials
The diffusivity of oxygen ions is lesser which results in lesser which will eventually result in development of thermal resid-
permeability. Moreover, the small particle sized Al2 O3 pow- ual stresses. Therefore, failure of such coating will result due
ders contributed to the low porosity of both the FG ceramic to spallation or debonding when exposed to a higher temper-
coatings. ature [14]. Based on this, thermal testing was carried out using
thermal imager for the coated specimens produced with car-
rier gas concentrations of 3 scfh and 4 scfh and the observed
Surface roughness temperature values are plotted as shown in Figs. 9 and 10.
It could be observed from Fig. 9 that the surface temper-
Surface roughness of uncoated and coated EN 36C steel spec- ature increases with increase in the exposure time for both
imens was measured using A SJ-310 Mitutoyo (Japan) surface uncoated and FG ceramic coated steels. The temperature on
roughness tester as per the DIN EN ISO 3274 standard. The the surface of uncoated EN steels increases rapidly when
surface roughness was measured at different points on the compared to the YSZ-Al2 O3 coated steels. Among the coated
top surface of the coat and the average value was considered steels, the 75:25 YSZ-Al2 O3 coated steels has better insula-
finally for analysis. The obtained surface roughness values tion performance than the 50:50 YSZ-Al2 O3 coated steels.
are shown in Table 5. The surface roughness of the uncoated It is observed that a lowest surface temperature of 141.5 ◦ C
EN 36C steel is found to be less than 6 ␮m whereas the is observed in 75:25 YSZ-Al2 O3 coated EN36C steels for
surface roughness value is slightly more than 6 ␮m in the the maximum exposure time of 20 min whereas the highest

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Fig. 6 – SEM and EDS of 75:25 YSZ-Al2 O3 deposited steel substrate.

Fig. 7 – SEM and EDS of 50:50 YSZ-Al2 O3 deposited steel substrate.

Fig. 8 – Cross section of the YSZ-Al2 O3 (a) 75:25, (b) 50:50.

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
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Table 5 – Surface roughness of the uncoated and coated EN substrate.

S.NO Material Avg. surface roughness (Ra) ␮m

1 EN 8 Uncoated 4.53 ± 0.09

2 EN 19 Uncoated 5.45 ± 0.21
3 EN 24 Uncoated 4.55 ± 0.18
4 EN 36C Uncoated 5.71 ± 0.15
5 EN 36C coated YSZ 6.67 ± 0.05
6 EN 36C coated Al2 O3 6.63 ± 0.05
7 EN36C coated YSZ-Al2 O3 (50–50%) 6.25 ± 0.02
8 EN 36C coated YSZ-Al2 O3 (75–25%) 6.31 ± 0.05

Fig. 9 – Surface temperature of the specimens (3 scfh) with respect to time.

surface temperature of 178.7 ◦ C is observed in the uncoated EN porosity in the coatings has greater significance in decreasing
8 steel for the same exposure time. Fig. 10 depicts that surface the thermal conductivity of the coating [15]. Similar study
temperature of both coated and uncoated steels produced was reported earlier which stated that porosity in TBCs offers
under the carrier gas flow rate at 4 scfh, increases upon expo- advantages such as increase in strain endurance and reduc-
sure. Among the coated steels, 75:25 YSZ-Al2 O3 coated steels tion in thermal conductivity [16]. The magnitude of the surface
have better insulation performance than the 50:50 YSZ-Al2 O3 temperature was relatively higher for all the specimens pro-
coated steels. 75:25 YSZ-Al2 O3 coated EN36C steel has shown duced under 4 scfh for all the exposure time compared to the
better performance recording the lowest surface temperature specimens produced with a gas flow rate of 3 scfh. From this, it
(143.5 ◦ C) for the maximum exposure time of 20 min whereas is evident that flow rate of carrier gas has greater significance
the EN 8 steel possess the highest surface temperature for over the inflight particle properties and the trajectory of the
the same exposure time as in the previous case. Presence of particles towards plasma jet. As the gas powder mixtures are

Table 6 – Wear test results.

Material Wear volume rate (mm3 /N.m) Frictional force (N) Coefficient of friction
−6
EN 8 1.59 × 10 13 0.357
EN 19 1.69 × 10−6 13.4 0.387
EN24 1.01 × 10−6 13 0.450
EN 36C 1.02 × 10−6 8.2 0.350
EN 36C (75:25 YSZ-Al2 O3 coated) 1.85 × 10−7 9.6 0.210
EN 36C (50:50 YSZ-Al2 O3 coated) 2.21 × 10−7 9.7 0.220

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Fig. 10 – Surface temperature of the specimens (4 scfh) with respect to time.

externally injected into plasma jet, exchange of heat between

cold particles and the hot plasma jet affects the temperature
circulation between the plasma jet and the ceramic particles.
In addition, parameters such as torch power, composition
of the gas, diameter of the nozzle and momentum of particle
also affects the interaction of the particles with the plasma
jet. Among these, carrier gas flow rate mainly affects momen-
tum of the particles. As the momentum varies from between
low and high in three stages, the particle to be coated may
bounce away from plasma flow after which it penetrates into
plasma jet. When the particle had entered into the plasma
jet, it may enter and cross over the jet or it may continue to Fig. 11 – Macroscopic observation of thermal tested
flow along with the jet. Thus the variation in flow rate of car- YSZ-Al2 O3 deposited EN36C specimen (a) Before exposure
rier gas would modify the particle trajectory below or above and (b) After exposure.
the plasma jet axis. From the thermal performance analysis of
the coatings deposited under 3 and 4 scfh, it can be seen that
the flow rate of 3 scfh provides the optimum particle trajec-
tory (and better spray performance) along with a high coating
particle flow density at higher temperature and velocity of the addition to strength, hardness and toughness, nickel offers
plasma jet. It could be assumed that the centre line of par- corrosion and scaling resistance also at high temperatures
ticle flux and plasma jet axis were coaxial at a flow rate of [20–22]. In order to endorse this, macroscopic observation
3 scfh. Similar studies on variation of plasma spray parame- (Fig. 11) has been performed on the YSZ-Al2 O3 coated EN
ters expressed that the carrier gas flow rate variation greatly 36C specimen (before and after coating) by exposing it to
affected the in-flight particle behaviour [17–19]. a temperature of 200 ◦ C for the time of 20 min. It could be
Altogether, 75:25 YSZ-Al2 O3 coated EN 36C steel fabricated observed that there is no visible spallation of the coat for
under carrier gas with a flow rate of 3 scfh had better insu- the input temperature of 200 ◦ C even after the maximum
lation performance at a temperature of 141.5 ◦ C compared to exposure time of 20 min, which indicates that there is sig-
all other steel substrates selected for the study. More specif- nificant adhesion between top coat, bond coat and metallic
ically, coated EN 36C steel outperformed all other coated EN substrate.
steels due to the presence of higher nickel content (3.33%). In

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004
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Fig. 12 – SEM micrographs of worn out surfaces: (a) EN 8 steel (b) YSZ-Al2 O3 coated EN36C steel.

Fig. 13 – Macroscopic examination of scratch tested (a) 50:50 YSZ-Al2 O3 (b) 75:25 YSZ-Al2 O3 deposited EN36C steel.

Wear and scratch test debonding and splat boundary fracture [27]. In case of poor
bonding between splats, major material removal occurs as a
The obtained wear test results are shown in Table 6. The low- result of splat boundary debonding which consequently end
est wear rate and lowest coefficient of friction was obtained up in high material removal and high wear rate. However,
for 75:25 YSZ-Al2 O3 coated EN 36C steel specimen compared lowest wear rate is obtained for 75:25 YSZ-Al2 O3 coated EN
to all other materials. This could be attributed to the better 36C steel which is evident that there exists a greater cohe-
intersplat bonding and higher micro-hardness of YSZ (1250 sion between the splat boundaries which in turn enhanced
HV) and due to the presence of higher weight fractions of YSZ the wear resistance of the material. Since the 75:25 YSZ-Al2 O3
in 75:25 YSZ-Al2 O3 coating. On the other hand, highest wear coated EN 36C steel has outperformed in the metallurgical
rate was observed for the uncoated EN 8 steel substrate. analysis and wear tests, the coated specimens were alone sub-
SEM analysis of the worn surface of the uncoated EN 8 steel jected to scratch testing.
and 75:25 YSZ-Al2 O3 coated EN 36C steel are shown in Fig. 12(a) The scratch resistance of the both 75:25 YSZ-Al2 O3 and
and (b) respectively. In uncoated EN8 steel, the wear tracks 50:50 YSZ-Al2 O3 coated EN 36C specimen was tested by apply-
are more prominent with the existence of parallel ploughs ing different loads using scratch tester and the scratch tracks
and deep continuous grooves in rotary direction evidencing recorded are shown in Fig. 13. It is inferred that upon scratch-
adhesive and abrasive wear. The worn out surface of the 75:25 ing at different loads, the coating has not been damaged and
YSZ-Al2 O3 coated EN 36C steel revealed lesser deformation the base metallic substrate was not exposed to scratch loads.
with less number of prominent wear tracks. However, surface At a low load of 1 kg, the groove of scratch is shallow with a nar-
is characterized with micro cracks, grooves, plowing marks row width. As the load was increased to 2 kg, fewer cracks with
and darker zone at several places. This is evident that mate- slight deformation could be observed on the scratch edges.
rial removal from the coating has happened through abrasive Further increase in the applied load to 3 kg led to a slight
wear phenomenon, which was also reported consistently by increase in depth and width of the scratch groove for 50:50
several researchers [23–25]. The darker zones on the surface YSZ-Al2 O3 coated steel specimen but the coating has not been
may represent the formation of fine debris compaction during damaged and the base metallic substrate was not exposed.
the wear test and similar behaviour was reported earlier also In general, during scratch testing the indenter tip generates
[26]. Generally, the failure of thermal sprayed ceramic coat- shear stress concentration on the contact surface causing a
ing occurs through three processes as stated by Hawthorne noticeable deformation near the tip. To release this energy and
et al. such as microchipping and plowing, splat boundaries affluence the free energy of the coating, microsized radial and

Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004
BSECV-293; No. of Pages 12
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Fig. 14 – SEM micrographs of scratch tested (a) 50:50 YSZ-Al2 O3 (b) 75:25 YSZ-Al2 O3 deposited EN36C steel.

lateral cracks are instigated by core defects. This phenomenon the study it could be concluded that the lower guard hand
could be noticed for 50:50 YSZ-Al2 O3 coated steel specimen of the AK47 rifle could be coated with 75:25 FG YSZ-Al2 O3 in
and this slight increase in deformation could be attributed to order to reduce the heat transfer at the time of firing rounds.
the presence of minute pores and partially melted YSZ parti-
cles in the coating and the similar phenomenon was reported
in earlier studies [28,29]. In the 75:25 YSZ-Al2 O3 coated EN
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and deformation could be sparsely observed. Hence, 75:25
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Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004
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Please cite this article in press as: M. Ramesh, et al., Microstructure and properties of YSZ-Al2 O3 functional ceramic thermal barrier coatings
for military applications, Bol. Soc. Esp. Cerám. Vidr. (2021), https://doi.org/10.1016/j.bsecv.2021.06.004