10.1111/jace.15820, vol 101, pp. 5524 5533, 2018 ......structure, and the feasibility for generation...

Revised version published as: Shuting Chen, Chee Kiang Ivan Tan, Sze Yu Tan, Shifeng Guo, Lei Zhang, and Kui Yao, “Potassium sodium niobate (KNN)-based lead-free piezoelectric ceramic coatings on steel structure by thermal spray method,” Journal of the American Ceramic Society, DOI: 10.1111/jace.15820, Vol 101, pp. 5524–5533, 2018.

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Potassium sodium niobate (KNN)-based lead-free piezoelectric ceramic

coatings on steel structure by thermal spray method

Shuting Chen, Chee Kiang Ivan Tan, Sze Yu Tan, Shifeng Guo, Lei Zhang, and Kui Yao*

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology

and Research), 2 Fusionopolis Way, Innovis, Singapore 138634

Abstract

For the first time, potassium sodium niobate (KNN)-based lead-free piezoelectric ceramic

coating with strong piezoelectric response was fabricated on stainless steel substrates by

thermal spray process, after introducing NiCrAlY and yttria-stabilized zirconia (YSZ)

intermediate layers. A large effective piezoelectric coefficient (d33) of 125 pm/V was obtained

with the thermal-sprayed KNN-based ceramic coating on the steel substrates. The mechanisms

of improving the structure and enhancing the properties of the KNN-based piezoelectric

ceramic coatings by introducing the intermediate layers were analyzed. Ultrasonic transducers

were designed and fabricated from the KNN-based coatings directly formed on a steel plate

structure, and the feasibility for generation and detection of ultrasonic waves for structural

health monitoring using the thermal-sprayed lead-free piezoelectric ceramic coating was

demonstrated.

*Author to whom correspondence should be addressed. E-mail: [email protected]

mailto:[email protected]


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Introduction

The fabrication of piezoelectric ceramic coatings is often restricted on ceramic substrates due

to the requirement of high temperature processing in an oxidizing atmosphere. However, there

have been many demands for producing piezoelectric sensors and transducers on metallic

substrates.1, 2 As many engineering structures are made of metals, achieving large area

piezoelectric coatings on metallic structures is valuable for realizing integrated ultrasonic

transducers for structural health monitoring. Making ultrasonic transducers directly from the

piezoelectric coatings instead of installing discrete ultrasonic transducers have many

advantages, including lightweight, low profile, improved consistency and enhanced acoustic

coupling between the ultrasonic transducers and the engineering structures, which are very

attractive for achieving in-situ structural health monitoring in aerospace and marine offshore

industries.

Studies have been conducted on the deposition of piezoelectric films on various types of

metallic substrates, including steel,3, 4, 5 copper,2, 6 and nickel7. Lead Zirconate Titanate (PZT)-

based piezoelectric thin films were deposited on copper and nickel substrates by chemical

solution deposition.2, 6, 7 PZT and potassium-sodium niobate (KNN)-based thin films were

deposited on steel substrates by physical vapor deposition (PVD)3, 4, 5 and aerosol deposition.8,

9 Energy harvesting transducers based on the piezoelectric thin films on metallic substrates

have been reported.3, 5, 10 However, there are substantial technical constraints existing in

fabrication of large-area piezoelectric films for practical applications on metallic structures.

For the chemical solution deposition and PVD methods, the thickness of the piezoelectric films

is typically below a few micrometers, with relatively small coating area and requiring flat

substrates with a smooth surface. The aerosol deposition method enables deposition of thicker

piezoelectric films (10 to 20 µm). However, due to the requirement of vacuum chamber


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condition, it is challenging to make piezoelectric coating on large metallic structures with high

throughput by aerosol deposition.

An effective method of making ceramic coatings on metallic structures is thermal spray, which

is used to form wear, corrosion and oxidation resistant barrier coatings in industry. 11, 12 During

thermal spray, the ceramic particles are heated to molten or semi-molten state by a high

temperature torch and are accelerated and impinged on substrates, where coatings are formed

by rapid solidification. It enables high throughput and cost-effective ceramic coating over a

large area on a wide variety of substrates. Besides the applications in forming protective

coatings, thermal spray process has been applied for the fabrication of some electronic

components and sensor devices made of ceramics, 11, 12 including strain gauge13 and humidity

sensors.14 Research efforts have also been made for deposition of PZT-based piezoelectric

coatings by thermal spray.15, 16, 17, 18 However, because of the severe thermal decomposition of

the PZT-based materials, the coatings lost the chemical stoichiometry and exhibited substantial

amount of secondary phases.15, 16 In addition, physical defects such as high porosity, cracks

and delamination were observed in the thermal-sprayed PZT coatings. As a result, it is not

successful in obtaining PZT-based coatings by thermal spray process with practically useful

piezoelectric properties. Moreover, the significant amount of toxic and volatile lead in the PZT-

based materials also limits its application in thermal spray process due to the concern of health

hazard and environmental pollution.

With the extensive efforts and significant progress on developing lead-free piezoelectric

ceramics with competitive performance properties,19, 20, 21, 22, 23, 24 25, 26 they are regarded as the

promising candidates to replace the market-dominant PZT-based materials for various

applications.27, 28 Recently, our group first reported thermal-sprayed KNN-based and bismuth


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sodium titanate (BNT)-based lead-free piezoelectric ceramic coating.29, 30, 31 The thermal

sprayed coatings exhibited single phase of perovskite structure and strong piezoelectric

response, with effective piezoelectric coefficient (d33) up to 112 pm/V.29, 31 The lead-free

KNN-based or BNT-based ceramic compositions are environmentally friendly and thus

suitable for wide implementation in large-area coating applications.

The thermal-sprayed piezoelectric ceramic coatings in our previous works were all deposited

on alumina ceramic substrates, which are chemically stable at high temperature, and possess

thermal expansion coefficient comparable to those of the lead-free ceramic coatings. The

feasibility of depositing lead-free piezoelectric ceramic coating on metallic structures by

thermal spray process has not been studied. There are many potential problems to be solved for

producing thermal-sprayed ceramic coatings on metal substrates, including the oxidation of the

metallic substrate at the high temperature, and stress-induced structural failures caused by the

significant thermal mismatch.

In this study, we succeed in obtaining KNN-based lead-free piezoelectric ceramic coating on

stainless steel substrates by thermal spray method, after introducing two intermediate layers,

NiCrAlY as bond coat layer and yttria-stabilized zirconia (YSZ) as thermal barrier layer. The

mechanisms of improving the structure and enhancing the properties of the KNN-based

piezoelectric ceramic coating by introducing the intermediate layers are analyzed. The

feasibility of generating and sensing ultrasonic waves with the thermal-sprayed piezoelectric

coating for structure health monitoring is demonstrated.


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Experimental

K2CO3 (99.5 %), Na2CO3 (99.0 %), Nb2O5 (99.9 %), Li2CO3 (99.999 %), Ta2O5 (99.85 %),

Sb2O5 (99.998 %) powders (Alfa Aesar, Karlsruhe, Germany) were used as the starting

materials to produce the piezoelectric ceramic powder with stoichiometric composition of

(K0.44Na0.52Li0.04)(Nb0.84Ta0.10Sb0.06)O3 (KNN-LiTaSb). 10 mol% excess K and Na was

introduced in consideration of the volatile loss at high temperature during the thermal spray

processing, based on our previous study on thermal-sprayed KNN-based materials.29,3132 .33

According to our findings, volatile loss of alkaline composition in KNN mainly occurs before

the formation of the crystalline perovskite phase, and the volatile loss from the perovskite phase

is suppressed once the crystalline phase is formed.32 Thus the 10 mol% excess K and Na

addition was introduced to form an alkaline rich composition before perovskite phase

formation, and the excessive alkaline compositions not entering the perovskite phase would be

lost much more significantly than from the perovskite phase.

The weighed materials were mixed with ethanol and zirconia balls in a planetary ball mill for

24 hours. Then the slurry was dried and crushed before it was calcined at 1000 oC for 5 hours

in an alumina crucible. The calcined ceramic powder was crushed and then fractionated to

achieve the desired distribution of particle size for thermal spray process.

Stainless steel (SS316) plates with a dimension of 50 mm x 50 mm x 2.7 mm were used as

substrates for the study of structure and properties of the thermal sprayed coatings. Stainless

steel SS316 was chosen as the substrate material because it is one of the most commonly used

steel grades in engineering structures. Some of the steel plates were coated with two

intermediate layers: a 67%Ni-22%Cr-10%Al-1%Y (NiCrAlY) bond coat layer with a thickness

of 50 µm, and a 8 mol% yttria-stabilized zirconia (YSZ) thermal barrier layer with a thickness


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of 300 µm, both of which were deposited by an atmospheric plasma spraying (APS) system.

An electrode layer of Ag-Pd (70/30) was deposited on the substrates by screen-printing. The

KNN-LiTaSb powder was sprayed onto the substrates using the same APS system to form

coatings with thickness of 120 µm. The plasma spray process was conducted at ambient

pressure using Ar plasma with plasma power of 19 kW. To enhance the crystallinity and

electrical properties of the KNN-LiTaSb coating, post-spray heat treatment was performed at

1100 oC for 30 mins and 800 oC for 30 mins for the samples with and without the two

intermediate layers.

Crystal structure of the thermal sprayed KNN-LiTaSb coatings was analyzed by X-ray

diffraction (XRD) (D8-ADVANCED, Bruker AXS GmbH, Karlsruhe, Germany). Morphology

and composition of the coatings were studied by a Field Emission Scanning Electron

Microscopes (FE-SEM) (JEOL 7600, Tokyo, Japan) with energy-dispersive X-ray

spectroscopy (Aztec Synergy, Oxford). For electrical characterization, patterns of top

electrodes were deposited on the thermal sprayed KNN-LiTaSb coatings by brushing Ag paste

with the aid of shadow masks. Dielectric properties of the KNN-LiTaSb coatings were

measured by an impedance analyzer (HP4294A, Agilent Technologies Inc, USA).

Polarization-electric field (P-E) hysteresis loops were characterized by a standard ferroelectric

testing unit (Precision Premier II, Radiant Technologies Inc, USA) at 10 Hz. After electrical

poling, the effective piezoelectric coefficient d33 was measured using a laser scanning

vibrometer (OFV-3001-SF6, PolyTech GmbH, Germany).

In-situ ultrasonic transducers including both an actuator and sensor were made of the thermal-

sprayed KNN-LiTaSb coating on a stainless steel (SS316) plate with a dimension of 200 mm

x 50 mm x 2.7 mm. The intermediate layers, electrode and KNN-LiTaSb coatings were


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deposited using the same method as described above. In the ultrasonic testing, the actuator was

excited by a function generator (AFG 3000, Tektronix) connected through a power amplifier

(4010, NF Electronic Instruments), and the signal output from the sensor was recorded by an

oscilloscope (TDS 3034C, Tektronix).

Results and discussion

Since serious surface oxidation of the steel plates occurred and their conductivity was lost in

the oxidizing atmosphere at high temperature, as required for processing the piezoelectric

coatings, a layer of Ag-Pd (70/30) was applied on the steel plates as the bottom electrode. To

enhance the structural integrity, two intermediate layers, consisting of a NiCrAlY metallic layer

and a YSZ ceramic layer, were introduced between the steel plate and the Ag-Pd layer. The

structure and properties of the KNN-LiTaSb coatings with and without the two intermediate

layers were studied comparatively in this study. The samples with and without the intermediate

layers are named as Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb and Steel/Ag-Pd/KNN-LiTaSb,

respectively.

Before the deposition of KNN-LiTaSb coatings, the electrical resistance of the Ag-Pd electrode

layers after heat-treated at different temperatures was studied. Two types of structures,

Steel/Ag-Pd and Steel/NiCrAlY/YSZ/Ag-Pd, were heat-treated at different temperatures in the

range of 800 oC to 1100 oC for 30 minutes in a furnace, after which the electrical resistance of

the Ag-Pd electrode layers was measured by a digital multimeter with probe distance of around

10 mm. As listed in Table 1, for the structure of Steel/Ag-Pd, a significant increase of electrical

resistance was found after heat treatment of 850 oC or above. Partial delamination of the Ag-

Pd electrode layer due to the severe oxidation of steel was observed. In contrast, for the

structure of Steel/NiCrAlY/YSZ/Ag-Pd, no substantial increase of electrical resistance was


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found after heat treatment even up to 1100 oC, which is the desired temperature for heat

treatment of the KNN-LiTaSb coatings. Based on the observations above, the condition of the

post-spray heat treatment of KNN-LiTaSb coatings was determined as 800 oC for 30 minutes

for Steel/Ag-Pd/KNN-LiTaSb, and 1100 oC for 30 minutes for Steel/NiCrAlY/YSZ/Ag-

Pd/KNN-LiTaSb.

Table 1. Electrical resistance of Ag-Pd electrode layer after heat treatment at different temperatures

Structure Heat treatment temperature

800 oC 850 oC 900 oC 950 oC 1000 oC 1100 oC

Steel/Ag-Pd < 0.3 Ω 10 MΩ 17 MΩ > 100 MΩ > 100 MΩ > 100 MΩ

Steel/NiCrAlY/YSZ/Ag-Pd < 0.3 Ω < 0.3 Ω < 0.3 Ω < 0.3 Ω < 0.3 Ω < 0.3 Ω

Figure 1 shows the XRD patterns of the heat-treated KNN-LiTaSb coatings of the two types of

structures. The KNN-LiTaSb coatings of both samples exhibited single phase of perovskite

structure. The full-width at half-maximum (FWHM) of the (101)/(110) diffraction peaks in the

XRD patterns of the KNN-LiTaSb coatings with and without the two intermediate layers are

0.388o and 0.450o, respectively. The much smaller FWHM of the KNN-LiTaSb phase in

Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb indicates its higher crystallinity, resulting from the

heat treatment at a higher temperature.

Figure 1. XRD patterns of (top) KNN-LiTaSb coating in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb

(heat treated at 1100 oC), and (bottom) KNN-LiTaSb coating in Steel/Ag-Pd/KNN-LiTaSb (heat

treated at 800 oC).


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The surface and cross-sectional morphology of the two samples are presented in Figure 2.

Cubic-shaped grains with an average size of a few micrometers were found at the KNN-LiTaSb

surface in both structures, as shown in Figures 2(a)-(b) and 2(d)-(e). The cubic-shaped grains

are similar to the grains typically observed in KNN-based bulk ceramics.19 The grain size of

the KNN-LiTaSb coating in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb is comparable to that of

the KNN-LiTaSb coating on alumina substrates after heat treatment in the same condition.29, 31

The grain size of the KNN-LiTaSb coating in Steel/Ag-Pd/KNN-LiTaSb is relatively smaller

due to its lower crystallinity as revealed in the XRD results in Figure 1. No significant cracks

were observed on the surface of the KNN-LiTaSb coating for both samples. Cross-sectional

inspection by SEM clearly revealed the layered structure of the two samples, as shown in

Figures 2(c) and 2(f). Both of the two structures exhibited strong adhesion without crack or

delamination between the layers. The Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb sample

exhibited a denser structure, probably due to the enhanced grain growth-induced densification

during the heat treatment at the higher temperature.

Figure 2. SEM images of Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb, heat-treated at 1100 oC: (a) and (b)

surface, (c) cross-section; Steel/Ag-Pd/KNN-LiTaSb, heat-treated at 800 oC: (d) and (e) surface; (f) cross-

section.


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A zoom-in perspective SEM image on the cross-sectional morphology of YSZ/Ag-Pd/KNN-

LiTaSb layers of Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb structure is presented in Figure

3(a). Solid cohesion among the KNN-LiTaSb layer and the underneath layers was observed,

suggesting that the thermal spray process was conducted under an appropriate condition,

wherein the KNN-LiTaSb powder was effectively heated to molten or semi-molten state.

Figure 3(b) presents the Energy Dispersive X-Ray Spectroscopy (EDX) line scan of the

compositional profile acquired along the line indicated in Figure 3(a). It was revealed that no

significant diffusion of elements among the layers occurred during thermal spray process and

the subsequent heat treatment. This can be attributed to the rapid cooling of the molten or semi-

molten KNN-LiTaSb particles during the thermal spray process, as well as the good thermal

stability of the Ag-Pd electrode and the YSZ layer.

Figure 3. (a) The cross-sectional SEM image of YSZ/Ag-Pd/KNN-LiTaSb layers in Steel/NiCrAlY/YSZ/Ag-

Pd/KNN-LiTaSb structure. (b) EDX line scan results acquired along the line indicated in the SEM image.

The dielectric and ferroelectric properties of the two KNN-LiTaSb coatings are compared in

Figures 4 and 5, respectively. The dielectric constant and loss of KNN-LiTaSb coating in

Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb are 443 and 0.020, respectively (room temperature

and 1 kHz). The KNN-LiTaSb coating in Steel/Ag-Pd/KNN-LiTaSb showed considerably

higher dielectric loss (0.139 at room temperature, 1 kHz) and highly frequency dependent


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dielectric constant, probably because of its lower density and lower crystallinity. The KNN-

LiTaSb coating in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb also exhibited a more saturated

P-E hysteresis loop with significantly higher remnant polarization.

Figure 4. Dielectric properties of KNN-LiTaSb coatings in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb after heat

treatment at 1100 oC (red lines), and in Steel/Ag-Pd/KNN-LiTaSb after heat treatment at 800 oC (black lines).

Figure 5. P-E hysteresis loops of the KNN-LiTaSb coatings in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb after

heat treatment at 1100 oC (red), and in Steel/Ag-Pd/KNN-LiTaSb after heat treatment at 800 oC (black).

The effective piezoelectric coefficient (d33) of the KNN-LiTaSb coatings was measured using

a laser scanning vibrometer, which is a reliable and efficient method in determining the

piezoelectric longitudinal strain coefficient for piezoelectric coatings on a substrate.34 During

the testing, a unipolar AC signal of 20 V amplitude at 1 kHz was applied to the sample. Figure

6 presents a three-dimensional drawing of the vibration of the KNN-LiTaSb coatings of the

two samples under the electric driving. The central protruding portion is the electrically excited


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area under the top electrode, and the surrounding flat area is the KNN-LiTaSb coating without

the top electrode. As the displacement profile of the top electrode area is not flat, the average

displacement values were used in determining the dilatation of the KNN-LiTaSb coatings. The

effective piezoelectric coefficients (d33) of the KNN-LiTaSb coating in the

Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb structure was measured as 125 pm/V, which is

consistent with the effective d33 of the KNN-LiTaSb coating as we obtained previously on

alumina substrates after heat treatment in the same condition.29, 31 The effective d33 of the KNN-

LiTaSb in the Steel/Ag-Pd/KNN-LiTaSb structure was 45 pm/V. The lower effective d33 is

mainly due to the lower crystallinity of the KNN-LiTaSb coating as revealed by the broader

FWHM in XRD and the much smaller grains observed by FESEM. It should be noted that the

intrinsic d33 of the KNN-LiTaSb coatings should be much higher than the measured effective

d33, as the out-of-plane displacement of the KNN-LiTaSb coatings was constrained by the in-

plane clamping effect of the substrate.34

Figure 6. Three-dimensional drawings of the vibration data of the KNN-LiTaSb coatings under

electric driving: (a) Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb, heat-treated at 1100 oC, and (b)

Steel/Ag-Pd/KNN-LiTaSb, heat-treated at 800 oC.


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The high effective d33 of the KNN-LiTaSb coating in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-

LiTaSb sample is attributable to the introduction of the intermediate layers which enables good

structural integrity after heat treatment at 1100 oC. Our analysis below revealed that the

beneficial effects of the intermediate layers are mainly in two aspects: acting as buffer layers

to reduce internal stress, and forming barrier layers to eliminate oxygen diffusion and prevent

oxidation of steel.

Previous work in the literature on piezoelectric ceramic films on metallic substrates revealed

that, substantial residue stress develops in the structure due to the significant difference of

coefficients of thermal expansion (CTE) between the ceramic and metal, leading to defects or

even structural failure.1, 35 In addition, it is widely recognized that residual stress has significant

effect on the structure and performance of thermal sprayed coatings.36 During the thermal spray

process, high residual stress develops due to the rapid cooling of the molten or semi-molten

ceramic particles after they impinge upon the metallic substrate. Moreover, the heating and

cooling during heat treatment of the piezoelectric ceramic coating also cause significant

internal stresses in the structure. Introducing intermediate layers with appropriate thermal

properties could potentially mitigate the CTE mismatch issue and the induced structural failure.

However, there is limited study on appropriate intermediate layers that could be introduced for

the reduction of internal stress of piezoelectric ceramic films on metallic substrates.

As shown in Table 2, the CTE of KNN material is significantly lower than that of steel substrate,

while the CTE of the two intermediate layers are between those of KNN-based material and

steel, suggesting that the intermediate layers could be beneficial to reducing the internal stress

of the structure.


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Table 2. Thermal and mechanical properties of materials in different layers

Material Coefficient of thermal

expansion (10-6/oC)

Young’s modulus

(GPa)

KNN 8 66

Ag-Pd 15 145

YSZ 10 38

NiCrAlY 15 137

Steel 17 200

Numerical simulation was conducted by ANSYS to calculate the internal stress from thermal

mismatch of the two types of structures during heat treatment. In the simulation, the thickness,

thermal and elastic properties of all the layers in the structures were taking into account, and

the stress developed in each of the individual layers when the structures cooled from 1100 oC

to room temperature was calculated. As shown in Figure 7, for Steel/Ag-Pd/KNN-LiTaSb,

there is a sharp increase of compressive stress from the steel substrate to the KNN-LiTaSb

coating, with the highest stress of more than -1100 MPa at the interface of Ag-Pd and KNN-

LiTaSb coating. In contrast, stress of Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb showed a

gradual increase from the steel substrate to KNN-LiTaSb, with a much lower stress level

compared to its counterpart of Steel/Ag-Pd/KNN-LiTaSb. This analysis indicates the effect of

the intermediate layers in reducing the thermal mismatch stress in the structure.


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Figure 7. Numerical simulation analysis on thermal mismatch stress at the layers of different

structures. Solid line: Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb, and dashed line: Steel/Ag-Pd/KNN-

LiTaSb.

Besides the effect of reducing residual stress, the intermediate layer of NiCrAlY plays an

important role in eliminating the diffusion of oxygen from outer layers to the steel substrate.

It has been reported that aluminum in NiCrAlY reacts with oxygen diffused from outer layers

and forms a layer of thermally grown oxide (TGO), which is mainly α-Al2O3.37 The TGO layer

effectively eliminates further diffusion of oxygen towards the metal substrate.37 Steel

substrates with NiCrAlY and YSZ intermediate layers were reported to be able to stand high

temperature of 1200 oC.38

To examine the possible formation of TGO layer in this study, the compositional profile of the

Steel/NiCrAlY/YSZ region in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb sample (after heat

treatment at 1100 oC) was investigated by EDX. As shown in Figure 8, concentrated Al and O

were found at the interface of YSZ and NiCrAlY, indicating the formation of TGO layer. The

thickness of the TGO layer is estimated to be a few micrometers. In addition, minor peaks of

Al and O were observed at the interface of NiCrAlY and steel substrate, suggesting the

formation of inner grown TGO, which is also reported in the interface of NiCrAlY-based

coating and steel substrate after high temperature treatment.39 The TGO layers can effectively

eliminate the diffusion of oxygen towards the steel substrate, and thus prevents significant

oxidation of steel at high temperature. This enables the good structural integrity at the substrate

interface and prevents the formation of scale cracking defect which is typically observed as a

result of high-temperature oxidation of steel. As for Steel/Ag-Pd/KNN-LiTaSb, serious

delamination was observed at the interface of steel and Ag-Pd after heat treatment above 850


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oC, which is mainly resulted from the high internal stress and the oxidation of steel caused by

the high temperature.

Figure 8. SEM image and EDX line scan results of the cross-section of the region of

Steel/NiCrAlY/YSZ in Steel/NiCrAlY/YSZ/Ag-Pd/KNN-LiTaSb sample. EDX data were acquired

along the horizontal line indicated in the SEM image.

To evaluate its potential in structural health monitoring applications, the thermal-sprayed

KNN-LiTaSb coating was applied on a steel plate structure, and the capability of generation

and detection of ultrasonic waves from the actuator and sensor made of the coating was studied.

Composition and thickness of all other layers are the same as Steel/NiCrAlY/YSZ/Ag-

Pd/KNN-LiTaSb as described above. The fundamental symmetric S0 Lamb wave mode was

selected because it is commonly used for damage detection by ultrasonic non-destructive

testing due to its long propagation distance and good sensitivity to defects such as cracks.40

Based on the phase velocity dispersion curve of the structure as presented in Figure 9(a), S0

mode Lamb wave with a frequency of 416 kHz was selected, which is just below the cut-off


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frequency of higher wave modes. The calculated wavelength of the S0 mode Lamb wave at 416

kHz is approximately 12 mm.

A photo of the ultrasonic transducers including the actuator and sensor made of the thermal-

sprayed KNN-LiTaSb coating on the steel structure is presented in Figure 9(b). The KNN-

LiTaSb coating was deposited on two areas of the steel plate: one is used for the actuator and

the other for the sensor to generate and detect the ultrasonic wave, respectively. In order to

selectively generate and detect the S0 mode Lamb wave at 416 kHz, silver top electrode layers

of the transducers were fabricated in a comb shape, wherein the gap between the central lines

of two adjacent electrode fingers was 12 mm. The actuator was excited by 5 cycles of sine

wave of 50 V at 416 kHz. The signal detected by the sensor was processed by digital band-pass

filtering in the range of 350 kHz to 450 kHz. Figure 9(c) shows a representative time domain

signal received by the sensor, wherein a good signal-to-noise ratio is observed. The voltage

amplitude of the signal is around 100 mV. The propagation velocity of the ultrasonic wave

calculated based on the time of flight is approximately 4493 m/s, which is consistent with the

theoretically calculated value (4369 m/s). In the case of defects formed in the propagation path

of the ultrasonic wave, there will be changes of ultrasonic signal, such as reduction of amplitude

and change of waveform. By appropriate signal processing and analysis, the existence and even

the location and dimension of the defect could be obtained.


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Figure 9. (a) Phase velocity dispersion curves of the structure for ultrasonic testing evaluation. The

selected Lamb wave mode is indicated by the arrow. (b) Photo of a prototype of ultrasonic transducers

made of thermal-sprayed KNN-LiTaSb coating on a steel plate. The transducers are designed to

selectively generate and detect ultrasonic Lamb wave in the selected frequency of 416 kHz. (c)

Typical sensor signal in detection of the Lamb wave.

In comparison with our previous report on direct-write ultrasonic transducers made of

poly(vinylidene fluoride/trifluoroethylene) (P(VDF/TrFE)) polymer coatings integrated on

metallic structures for structural health monitoring, 41 the voltage signal output obtained with

the thermal-sprayed KNN-LiTaSb coating in this work is two orders of magnitude higher. The

significantly larger output signal from the KNN-LiTaSb ceramic transducers is mainly

attributed to its much higher effective d33 coefficient (125 pm/V, vs -18 pm/V of P(VDF/TrFE))

and higher Young’s modulus (66 GPa, vs 4.7 GPa of P(VDF/TrFE)). The electromechanical

coupling coefficient of KNN-based ceramics is thus significantly higher than that of

piezoelectric polymers. These key material performance parameters of the KNN-LiTaSb

coating enable generation of stronger ultrasonic wave and detection of ultrasonic wave with

higher sensitivity. The demonstration of generation and detection of ultrasonic wave shows

that the thermal-sprayed KNN-LiTaSb lead-free piezoelectric ceramic coatings on steel plates

are very promising for structural health monitoring of metallic structures.

Conclusions


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For the first time, KNN-based lead-free piezoelectric ceramic coating with strong piezoelectric

response on stainless steel substrates was achieved by thermal spray process, after introducing

two intermediate layers, NiCrAlY and YSZ. It was found that the intermediate layers could

reduce the internal stress of the structure by mitigating thermal mismatch among the multiple

layers, eliminate steel oxidation by forming oxygen barrier layers, and hence improve the

sustainability of the structure at high processing temperature and enhance the piezoelectric

performance property. After post-spray treatment at 1100 C, a large effective piezoelectric

coefficient (d33) of 125 pm/V, measured over macro scale by a laser scanning vibrometer under

substrate clamping effect, was achieved with the thermal-sprayed KNN-LiTaSb ceramic

coating on the steel substrate. Ultrasonic transducers were designed and fabricated from the

KNN-LiTaSb coatings directly formed on a steel plate structure, and the feasibility for

generation and detection of ultrasonic waves for structural health monitoring using the thermal-

sprayed lead-free piezoelectric ceramic coating was demonstrated.

Acknowledgements

This project is partially supported by Singapore Maritime Institute under the Asset Integrity &

Risk Management (AIM) R&D Programme, Project ID: SMI-2015-OF-01 (IMRE/15-9P1115),

and under the Maritime Sustainability R&D Programme, Project ID: SMI-2015-MA-07

(IMRE/16-7P1125).

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