Thermal expansion studies of electron beam evaporated yttria films on Inconel-718 substrates

Accepted Manuscript Thermal expansion studies of electron beam evaporated yttria films on Inconel-718 substrates

A.M. Kamalan Kirubaharan, P. Kuppusami, T. Dharini PII: DOI: Reference:

S0257-8972(18)31017-X doi:10.1016/j.surfcoat.2018.09.034 SCT 23807

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

17 April 2018 24 August 2018 14 September 2018

Please cite this article as: A.M. Kamalan Kirubaharan, P. Kuppusami, T. Dharini , Thermal expansion studies of electron beam evaporated yttria films on Inconel-718 substrates. Sct (2018), doi:10.1016/j.surfcoat.2018.09.034

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ACCEPTED MANUSCRIPT Thermal Expansion Studies of Electron Beam Evaporated Yttria Films on Inconel-718 Substrates A.M. Kamalan Kirubaharan, P. Kuppusami*, T. Dharini a

Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology, Chennai-600119, India

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*Corresponding author, email: [email protected]

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Abstract In this paper, phase pure cubic yttrium oxide (Y2O3) films deposited by electron beam physical vapor deposition technique on Inconel-718 substrate at different substrate temperatures (773-973 K) are investigated. The structure and phase evolution in the films of Y2O3 as a

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function of substrate temperature have been evaluated by X-ray diffraction. Atomic force microscopy and scanning electron microscopy were used to examine the surface roughness and

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morphology of the as-deposited Y2O3 films. The adhesion behavior of the Y2O3 film with Inconel-718 substrate was investigated using scratch indentation testing. The thermal expansion

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coefficient (TEC) of Inconel-718 substrate and Y2O3 film coated on Inconel-718 substrate were determined by high temperature X-ray diffraction (HTXRD) measurement in the temperature

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range of 298-1273 K with temperature interval of 100 K. The linear TECs of Inconel-718 and Y2O3 film at 1273 K were found to be 1.22 × 10-5 K-1 and 7.02 × 10-6 K-1, respectively. Thermal

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stability of the Y2O3 coated Inconel-718 substrate tested at 1273 K in air over a period of 100 h is also discussed.

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Keywords: Yttria, High temperature X-ray diffraction, Thermal expansion coefficient, Thermal

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stress. 1. Introduction

High temperature oxidation is one of the major material degradation problems which affects the reliability and safety of the components used in power plants, petroleum and gas turbine industries [1,2]. Inconel-718 is a nickel-chromium based superalloy exhibiting excellent resistance to chemicals used in high temperature applications [3]. However, during long-term high temperature exposure, chromium depletion from the alloy leads to stress corrosion cracking (SCC) or inter-granular corrosion (IGC) which leads to failure of the alloy components [4]. Development of ceramic diffusion barrier coatings on alloy surface can potentially delay or 1

ACCEPTED MANUSCRIPT reduce the high temperature oxidation of alloy components and improve the component life [5]. The deposition of ceramic coatings such as yttria (Y2O3), alumina (Al2O3) and zirconia (ZrO2) on superalloy enhances oxidation resistance and thus protects the alloy components under aggressive environments [6]. Among these oxides, Y2O3 has good thermal stability up to its melting point (~2622 K),

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excellent thermal shock resistance and high corrosion resistance against chemical attacks at high

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temperatures [7,8]. Y2O3 is also known to possess good plasma-etch and erosion resistance,

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particularly in plasmas containing halogen species [9]. In addition, yttrium oxide exhibits excellent electrical insulation and resistant to many reactive molten metals. Yttrium compounds provide good coating adherence and exhibit the formation of a thin reactive zone which acts as a

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diffusion barrier [10]. Therefore, Y2O3 coating is used to protect thermal barrier coatings from calcium-magnesium-aluminosilicate (CMAS) attack [11]. The high temperature phase

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stabilization in zirconia has been routinely achieved by the addition of yttria, and the yttria stabilized zirconia (YSZ) is highly recommended as a topcoat material for thermal barrier

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applications due to low thermal conductivity [12]. Due to greater thermodynamic stability, Y2O3 coating has been proposed as a candidate material for uranium melting at 1573 K [13]. High

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adhesion strength, less porosity and relatively dense structure of the Y2O3 coatings on metallic thermal cycling [14,15].

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substrates are highly demanded in applications where components are subjected to frequent

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Understanding the thermal behavior of the materials is essential in a number of applications, in particular, thermal expansion coefficients (TEC) and phase transitions at high

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temperatures [16]. The calculation of thermal expansion coefficient involves the measurement of two physical quantities namely elongation (or displacement) and temperature [17]. Absolute methods and relative techniques are the two types of techniques available to measure the TEC of materials. The absolute methods use X-rays, neutrons or electrons to obtain a diffraction pattern from the crystal lattice planes to measure the TEC. Relative techniques such as mechanical dilatometry [18,19], optical interferometry [20], and high-resolution microscopes [21] which require reference material have also been used to quantify thermal expansion. However, absolute methods, especially HTXRD technique is suitable for flat samples of thin dimension and do not require sample preparation. Also, HTXRD is suitable for anisotropic materials where the 2

ACCEPTED MANUSCRIPT variation in thermal expansion with different directions within the crystal lattice can also be determined [22]. The TEC can be determined from the change in the d-spacing of the crystal planes of a sample as a function of temperature. Maneesha et al. [23] studied the TEC of Y2O3 film deposited on Si (100) substrates using the PLD technique. The TEC was found to be higher than that of bulk (9.3 × 10-6 K-1) [24] and is

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reported to vary from 5.37 to 8.39 × 10-6 K-1 with increase in the temperature from 300 to 1373

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K. The TEC of a coating has been found to depend on the residual porosity of the deposited Y2O3 film [25]. Swamy et al. [16] studied the structural transitions and thermal expansion

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behaviour of cubic-Y2O3 powder from room temperature to its melting point. Shui et al. [26] reported that the presence of residual porosity in the film increases the

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TEC mismatch. Therefore, the choice of deposition method for Y2O3 coating also plays an important role in the performance of the coating. The Y2O3 coatings can be developed by a

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variety of deposition techniques including sol-gel, pulsed laser deposition, chemical vapor deposition, electron beam evaporation, plasma-spraying etc. [27]. However, it is difficult to

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produce a Y2O3 coating of thicknesses exceeding a few micrometres by pulsed laser deposition [28] or magnetron sputtering [29] without cracking due to the difference in thermal expansion

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mismatch between the substrate and the coating. Sol-gel derived Y2O3 coatings suffer from cracking with respect to film thickness as well as due to thermal expansion mismatch [14]. The

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Y2O3 coating developed by plasma spraying has been reported to suffer from a high degree of porosity and cracks and thus offers a poor corrosion resistance [25]. Among physical vapor

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deposition methods, electron beam physical vapor deposition (EBPVD) of oxide materials offers advantages such as large area deposition, film uniformity, less porosity, and relatively higher

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deposition rate.

In the present study, Y2O3 films deposited on Inconel-718 substrates by EBPVD method in the substrate temperature range of 773-973 K have been investigated. The surface morphology, surface roughness and adhesion of the Y2O3 film with Inconel-718 substrate have been reported. A comparative study on the thermal expansion coefficients of Y2O3 coating and Inconel-718 by in-situ high temperature X-ray diffraction (HTXRD) in the temperature range of 298-1273 K has been carried out. The residual thermal stress associated with the film has also been discussed. 3

ACCEPTED MANUSCRIPT 2. Experimental Procedure Y2O3 films were prepared by three source electron beam evaporator system (MEB600, Plassys, France). Y2O3 pellets were prepared by uniaxial pressing of Y2O3 powders (99.99 % purity, Alfa Aesar) followed by sintering at 1723 K, 6 h. These pellets were used as source material for producing a Y2O3 film. Inconel-718 substrates were cut into a size of 10 mm × 10

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mm × 1 mm and metallographically prepared using SiC grit emery sheets in the range 400-2000

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mesh followed by diamond polishing. The well polished substrates were first cleaned using soap

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solution followed by ultrasonic cleaning in distilled water and ethanol. The dried Inconel-718 substrates were mounted in the sample holder of the EBPVD system for deposition.

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The chamber was evacuated using rotary and turbomolecular pumps to pressure of about 2 × 10-4 Pa. The Y2O3 films were deposited on Inconel-718 substrates by evaporating the sintered

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Y2O3 pellet at an accelerating voltage of 8 kV and an emission current of 70-90 mA. The chamber pressure maintained during the process was 5 × 10-3 Pa and the distance between the

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source and the substrate holder was kept at 250 mm. The deposition rate was controlled at 1 Å/s using a quartz crystal thickness monitor (INFICON IC6 controller). The Y2O3 films (thickness of

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about 2 µm) were prepared by varying substrate temperatures from 773 K to 973 K using a resistive heater. The temperature was measured using a chromel-alumel thermocouple and it was

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controlled to an accuracy of ± 0.5 K.

The thickness of the films was measured using Dektak Stylus profiler (Bruker, USA).

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The crystallinity and phase analysis of Y2O3 films were carried out using a grazing incident Xray diffractometer (Rigaku SmartLab, Japan) with Cu-Kα radiation generated at 30 kV and 100

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mA. The patterns were recorded with the incident angle of X-rays set at 0.3°, scan speed of 2 seconds per step and step width of 0.02°. The diffraction profiles obtained from the Y2O3 films were fitted with pseudo-Voigt (pV) function. The peak position (2θ) and full width at half maximum (β) were obtained from the fitted curves. Since the total peak broadening (βtot) has three independent contributions namely (i) crystallite size (βsize), (ii) lattice strain (βstrain) and (iii) instrumentation (βins), the βins was corrected by subtracting the diffraction profile obtained from the standard defect free silicon sample from βtot. Hence, the total broadening was due to βstrain and βsize.

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ACCEPTED MANUSCRIPT The crystallite size present in the films was calculated using the following WilliamsonHall relation (Eq. 1) [30]. 𝛽𝑐𝑜𝑠𝜃 𝜆

1

=

𝐷𝑣

2𝑠𝑖𝑛𝜃

+ 2𝜀 (

𝜆

)

(1)

The reflections corresponding to (222), (400), (411), (332), (431), (440), (611), (622) and (662) were used to calculate size and strain using Williamson-Hall relation. The plot of 2𝑠𝑖𝑛𝜃 𝜆

) gives the value of crystallite size and strain from y-intercept and slope,

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𝜆

) versus(

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𝛽𝑐𝑜𝑠𝜃

(

respectively [31].

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Quantitative analysis of texture is normally carried out using pole figure analysis and is used for metallurgical processing of uranium rods [32] and in the characterize epitaxial films of

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Al:ZnO [33]. However, in the present work, qualitative analysis of the texture is used to understand the growth texture in Y2O3 films using texture coefficient. Texture coefficients (TC)

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of (222) and (400) reflections were determined from the peak intensities of Y2O3 films. XRD reflections of (222), (400), (431), (440) and (622) were used for calculating Tc. The TC for any

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plane or reflection can be determined qualitatively by substituting the normalized peak intensity values using the following relation (Eq. 2) [34]: {𝐼𝑚 (ℎ𝑘𝑙)⁄𝐼𝑂 (ℎ𝑘𝑙)}

(2)

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𝑇𝐶 =

(1⁄𝑛) ∑{𝐼𝑚 (ℎ𝑘𝑙)⁄𝐼𝑂 (ℎ𝑘𝑙)}

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where n, the number of peaks; Im, the measured peak intensities of reflections of Y2O3 films; and I0, the respective peak intensities corresponding to the bulk Y2O3 data using JCPDS card No. 41-

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1105.

The surface topography and surface roughness of the as-deposited films were analyzed by atomic force microscope (NTEGRA PRIMA Modular Mode, Ireland) in the semi-contact mode.

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The scan range selected for all the as-deposited films was about 5 µm × 5 µm. The surface morphology of the Y2O3 films was analyzed using a Carl Zeiss (SUPRA 55, Germany) scanning electron microscopy (SEM). The scratch test was used to assess thin film adhesion on the substrate and the test was carried out using Revtest (CSM, Switzerland) instrument. In the present work, a Rockwell diamond indenter with a spherical tip radius of 100 µm was drawn across the coated surface under an increasing load from 1 N to 10 N along the scratch length of 3 mm with a speed of 3 mm/min. Thermal stability of the Y2O3 film was carried out by annealing the Y2O3 coated Inconel-718 sample at 1273 K in air for 1 h, 2 h, 4 h and 100 h. 5

ACCEPTED MANUSCRIPT In order to study the thermal expansion behaviour of Y2O3 film and Inconel-718 substrate, their lattice thermal expansions were investigated using a high temperature X-ray diffractometer (HTXRD), using Rigaku SmartLab X-ray diffractometer (Cu Kα = 1.5418 Å) equipped with Anton Paar DHS 1100 dome stage containing a heater attachment. The dome stage was made up of graphite, and is attached with a rotary pump to attain base pressure of 0.1 mbar. This vacuum condition has helped to investigate the samples to avoid oxidation and other

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contamination, if any, at high temperature. The diffraction data were recorded from 298 K to

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1273 K in steps of 100 K. The heating rate was maintained constant at 20° per min and the

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holding time was set for 10 minutes at a temperature interval of 100 K. The pattern was recorded from 20° to 100° (2θ) with a step width of 0.02° and 2 seconds per step in Bragg-Brentano

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geometry. Before carrying out the HTXRD study, the sample was heat treated at 1273 K in air for 1 h in a tubular furnace to avoid any internal stresses associated with the film during film

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deposition [35,36]. From the HTXRD data, the changes in the lattice parameter and phase stability were monitored with a temperature interval of 100 K. The variation in the lattice

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parameters as a function of temperature was calculated from the XRD pattern using ‘Unit Cell Program’ [37].

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From the lattice parameter data, average thermal expansion coefficient values were calculated from difference in the lattice parameter with respect to temperature increment using

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the following Equation 3 [23,38]:

da

Average linear thermal expansion coefficient,αL = (a 1 )×( dTT) K-1

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RT

(3)

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where aT and aRT are the lattice parameter values at temperature (T) and room temperature (RT) respectively.

3. Results and Discussion 3.1. Deposition of Y2O3 Films by EBPVD XRD pattern of the sintered Y2O3 pellet used for the preparation of Y2O3 films using the EBPVD system is shown in Fig.1. XRD pattern shows all the reflections corresponding to a cubic phase of Y2O3. These patterns were indexed using the JCPDS card No. # 41-1105. The lattice parameter was calculated using “Unit Cell Program” software and found to be 10.570 ± 0.003 Å which is in close agreement with the value reported in the JCPDS card data (10.604 Å). 6

ACCEPTED MANUSCRIPT The Y2O3 films of thicknesses of about 2 µm have been deposited at the substrate temperatures of 773 K, 873 K and 973 K. The type of microstructures obtained in these films can be categorized in terms of film growth model which correlates to the ratio (Ts/Tm) of the substrate temperature (Ts) to the melting temperature (Tm) of Y2O3. This ratio has been found to be 0.21, 0.25, and 0.29, respectively for the above substrate temperatures. In the present work, the characteristic microstructure of the Y2O3 films lies in the range of 0.2 < Ts/Tm<0.3 and the

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films are expected to have zone 1 structures, according to the structure zone model reported

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elsewhere [39].

At zone 1 structure, the grain size is small due to the lower adatom mobility on the substrate surface [40]. It is also reported that such type of coatings could yield high lateral

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strength [41]. Attempts have also been made to develop Y2O3 films deposited at temperatures < 773 K. However, there was very poor adhesion of the film with the substrate and the films

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tended to peel off due to lower adatom mobility and poor mechanical locking of adsorbed atoms

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with the substrate at low substrate temperatures.

Figure 2 shows the XRD patterns of Y2O3 films deposited at different substrate

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temperatures from 773 to 973 K. It is seen that all the films are polycrystalline in nature and have monoclinic and cubic phases of Y2O3. Unlike bulk material, the formation of the small volume

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fraction of monoclinic phase along with stable cubic phase in thin films was due to the formation of a large amount of stress during thin film deposition [42]. In addition, the film deposited by

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EBPVD generally produces films with oxygen deficiency leading sub-stoichiometry in the lattice system [43] due to oxygen depletion by vacuum pumping system during evaporation. When the

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deposition temperature is low, a significant amount of strain is produced during deposition as compared to higher substrate temperature (Table 1). Maneesha et al. [23] reported that initial crystallization processes could lead to significant amount of stress in the film which led to the formation of a small fraction of m-Y2O3. The volume fraction of monoclinic phase was qualitatively calculated using the peak intensities of all monoclinic (Im) and cubic phases (Ic) by applying the following equation (Eq. 4) [44].

𝑉𝑚 =

(4)

𝐼𝑚 𝐼𝑚 + 𝐼𝑐

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ACCEPTED MANUSCRIPT The volume fraction of monoclinic phase decreased from 8.5 to 1.3 percent which can be clearly seen from the decrease in the intensity of (402̅) reflection of the monoclinic phase as the substrate temperature was increased from 773 K to 973 K. The crystallite size was calculated using Williamson-Hall plot [30] and the values are shown in Table 1. The crystallite size increased from 9 ± 1 nm to 17 ± 1 nm as a result of increased adatom (adsorbed atoms) mobility. According to the nucleation theories, nucleation barrier (energy barrier) has been used to explain

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the size and density of the crystallites of the film [45]. The increased crystallite size occurs at higher substrate temperature due to less nucleation barrier. Also, the increased adatom mobility

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of the depositing species on the substrate increases with increasing substrate temperature and promotes lower nucleation barrier resulting in the increased size of the crystallites.

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The lattice parameter of the films was found to be slightly higher than the bulk lattice parameter value of Y2O3 (10.604 Å) indicating a compressive stress in the as-deposited films

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[40,46]. The thermal expansion coefficient (TEC) of Inconel-718 substrate is higher than that of the TEC of c-Y2O3, which leads to a difference in the thermal expansion during heating process.

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As a result of which a compressive stress is generated in the film. It is also noticed that the film deposited at high substrate temperature has lower strain than the film deposited at lower substrate

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temperature. This could be attributed to the increased crystallite size which causes stress relaxation with increasing substrate temperature.

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Substrate Temperature (K) 773 873 973

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Table 1. Lattice parameter, strain and crystallite size of Y2O3 films at different substrate temperature. Lattice Strain (× 10-3) (± 0.4) 3.8 3.1 2.5

Lattice Parameter (Å) (± 0.004) 10.626 10.626 10.633

Crystallite Size (nm) (± 1 nm) 9 15 17

In order to assess the preferred orientation of the as-deposited films qualitatively, texture coefficient for (222) and (400) reflection was calculated [28]. It is noted that the texture coefficient of the (222) reflection for the films decreased from 0.37 to 0.16 as the substrate temperature increased from 773 K to 973 K. But, the texture coefficient for the (400) reflections increased from 2.2 to 4.3. In the case of bulk cubic Y2O3, the atoms prefer to align in (222) plane due to lower surface free energy of the (222) plane. But for thin films in the current work, the 8

ACCEPTED MANUSCRIPT atoms tended to align in (400) plane. Similar observation was observed by Gaboriaud et al. [47] where the change in the preferred orientation is strongly related to the oxygen partial pressure in the deposition chamber. This oxygen concentration could alter the growth crystallography direction of the Y2O3 film. Based on the above XRD results (Fig. 2), the Y2O3 films deposited at 973 K was

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considered for further study due to lower content of monoclinic phase and low strain (Table 1) as

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compared to the films deposited at 773 K and 873 K. It is also pointed out that in most of the PVD systems, the maximum substrate temperature that can be operated is restricted to  1073 K

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in order to avoid degassing and chamber heating. Therefore, in the present investigation, the deposition of Y2O3 has been restricted to 973 K. Also, the deposition of these films at

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temperatures exceeding 1073 K may induce inter-diffusion effects across the film-substrate interface.

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3.2 AFM analysis

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The AFM topographic images of the as-deposited and heat treated Y2O3 films on Inconel718 substrates are shown in Fig. 3. The root mean square roughness (RMS) value of Y2O3 was

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found to be 7 ± 1 nm for all the films and remained constant at various locations. From the roughness value, it is observed that the film is smooth and the particles are uniformly distributed

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on the surface. At the substrate temperature of 773 K, the particle size is very small and could not be distinguished from the 2D image. At the substrate temperature of 873 K, the particles are

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discrete and appear to be elliptical in shape and change to spherical shape when the substrate temperature is increased to 973 K.

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The porosity in the film is reduced as the particle grows due to the increased substrate temperature. From the AFM image, the average particle size was found to be 170 ± 10 nm, 180 ± 10 nm and 220 ± 10 nm for the film deposited at 873 K, 973 K and the film heat treated at 1273 K for 1 h, respectively. The increasing trend in the particle size with substrate temperature observed by AFM is in good agreement with the crystallite size data obtained from XRD analysis. It is also noted that during the heat treatment, the distribution of the particles are very uniform with less porosity and the size of the particle increases only very slightly because of the restricted mobility of Y2O3 particles on the substrate surface owing to its high melting point (~2698 K). 9

ACCEPTED MANUSCRIPT 3.2. Adhesive behavior of Y2O3 films Fig. 4 shows a typical scratch track of Y2O3 film deposited on Inconel-718 substrate at 973 K. A Rockwell diamond indenter was used to scratch the film with an increasing load from 1 N to 10 N over a distance of 3 mm on the film surface. The qualitative measure of film- substrate adhesion and adhesion strength could be determined from the critical loads (Lc). From the

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significant variation in the acoustic signal, the first critical load (Lc1) due to cohesive failure was

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determined to be 5.5 N. It occurs as a result of crack formation within the film. The second critical load (Lc2), is measured when there is an adhesive failure between the film and the

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substrate and Lc2 was found to be at 6.1 N. The later failure could result in the spallation of the film from the substrate. Since the adhesive failure (Lc2) has occurred at higher load, it confirms

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that the film is well adherent with the substrate.

It is seen from the Fig.4 that the friction coefficient gradually increases once the cohesive

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failure is significant and saturates when the indenter scratches the bare Inconel-718 substrate after the occurrence of the adhesive failure. SEM image shown in the inset of Fig. 4 reveals a

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progressive scratch track with no severe chipping and spallation of the film until the distance of 1.5 mm in accordance with a smooth variation of friction coefficient up to Lc2. The measurement

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of the adhesion properties from the scratch test provides useful information in assessing the

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quality of coatings, particularly for tribological applications. 3.3. In-situ HTXRD study of Inconel-718

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Before carrying out the HTXRD experiment, the Inconel-718 specimen was annealed at 1273 K for 1 h in air in a tubular furnace to remove any residual stress associated with the alloy

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during processing. In-situ high temperature XRD patterns of Inconel-718 have been recorded from 298 to 1273 K with a temperature interval of 100 K (Fig. 5). XRD pattern shows reflections of (111), (200), (220) and (311) planes which correspond to face-centered cubic (space group = Fm3m) crystal structure of Ni-Cr-Fe system (JCPDS card # 33-0945). The lattice parameter of Inconel-718 increases from 3.600 to 3.645 Å with increasing temperature from 298 K to 1273 K (Fig. 7). The lattice parameter values of Inconel-718 were fitted using second order polynomial function (coefficient of determination, R-square close to 1). The corrected lattice constant was calculated as per the following equation (Eq. 5): 10

ACCEPTED MANUSCRIPT acorrected = 3.594 × 10-10 + 3.856 × 10-15 × T + 2.224 × 10-19 × T2

(5)

where a is the lattice parameter and T is the absolute temperature. From the Eq. 5, it is possible to calculate the average linear thermal expansion coefficient (αL) from the corrected lattice parameter by differentiating the Eq. 3. The following Eq. 6 defines αL: Average linear thermal expansion coefficient, αL= 1.069 × 10-5 + 1.233 × 10-9 × T

(6)

a function of temperature.

αL-average (10-5 K-1) (± 0.01) 1.11 1.12 1.13 1.14 1.15 1.16 1.18 1.19 1.20 1.21 1.22

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a corrected (± 0.001) (Å) 3.606 3.609 3.613 3.617 3.621 3.626 3.630 3.634 3.638 3.642 3.647

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298 373 473 573 673 773 873 973 1073 1173 1273

a (± 0.001) (Å) 3.600 3.612 3.614 3.618 3.621 3.629 3.633 3.634 3.640 3.641 3.645

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T (K)

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Table 2: The lattice parameter and average thermal expansion coefficient data of Inconel-718 as

It is noticed that the average thermal expansion of Inconel-718 increases linearly from

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1.11 ± 0.01 × 10-5 K-1 to 1.22 ± 0.01 × 10-5 K-1 over the temperature range used in the present work (Table 2). The average thermal expansion coefficient value of Inconel-718 at room

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temperature is comparable to that of Inconel-600 (1.14 × 10-5K-1) [48] measured by XRD and slightly lower than that of Inconel-718 (1.26 × 10-5 K-1) [49], Inconel-690 (1.33 × 10-5 K-1)

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measured by XRD [50] and pure nickel (1.29 × 10-5 K-1) by dilatometry measurement [51]. The small variation in the reported values of TEC is due to compositional variation in the alloys and minor errors in the measurements of temperature and elongation obtained from these two techniques [52]. 3.4. In-situ HTXRD study of Y2O3 film The Y2O3 film deposited on Inconel-718 at the substrate temperature of 973 K was chosen for HTXRD study because of the good crystalline quality of the film, lesser lattice strain and good adhesion strength. Before carrying out the HTXRD experiments, the Y2O3 film 11

ACCEPTED MANUSCRIPT deposited at 973 K was post annealed at 1273 K for 1 h in air in a tubular furnace to avoid residual stress contribution in the determination of the TEC values. Fig. 6 shows the HTXRD patterns of the Y2O3 film in the temperature range of 298-1273 K. XRD patterns show that the film is polycrystalline in nature and the reflections corresponding to (200), (222), (400), (411), (420) and (622) planes of cubic phase of Y2O3 (JCPDS file no: 41-1105). In addition, there is a slight increase in the intensity of the peaks with increasing heat treatment temperature indicating

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the growth of crystallites with temperature.

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Fig.6 also shows the absence of monoclinic phase in the film heat treated at 1273 K for 1

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h in a tubular furnace. The monoclinic Y2O3 was reported to be a high pressure phase of Y2O3 which could form as a result of a large amount of stress accumulation during thin film deposition

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[53,54]. The monoclinic phase was not present possibly because of the significant reduction in the residual stress associated with the film due to pre-heat treatment. Maneesha et al. [23] studied

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HTXRD of pulsed laser deposited Y2O3 film deposited on Si (100) substrate and reported that stress relaxation of the crystallites could be the reason for the absence of monoclinic phase [16].

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The crystallite size was determined from Williamson-Hall relation and found to vary in the range 40-50 nm over the temperature range of 298-1273 K. Since the as-deposited film was annealed in

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air at 1273 K for 1 h, no significant increase in the crystallite size was observed. HTXRD pattern showed a leftward shift due to the lattice expansion during heat treatment.

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The lattice parameter values are found to increase linearly from 10.664 ± 0.002 Å to 10.735 ± 0.002 Å with the increase in the temperature from 298 to 1273 K (Fig. 7). A second

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order polynomial was used to fit the lattice parameter data (coefficient of determination, R = 0. 988) and the corrected lattice parameter was calculated from Equation 7. (7)

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acorrected = 10.641 × 10-10 + 7.355 × 10-15 × T + 5.108 × 10-19 × T2

The average linear TEC (αL) of Y2O3 film using Eq. 8 was calculated by differentiating Eq. 7 and then applying to Eq. 3. Average linear thermal expansion coefficient, αL= 6.897 × 10-6 + 9.578 × 10-11 × T

(8)

The calculated TEC data for Y2O3 film on Inconel-718 obtained from Eq. 8 are listed in Table 3. The TEC values are found to vary from 6.92 ± 0.01 to 7.02 ± 0.01 × 10-6 K-1 with the increase in the temperature from 298 to 1273 K (Table 3), and are slightly lower that of the bulk Y2O3 value (~7.5 × 10-6 K-1) [24,55]. The lattice parameters and the average TEC values of both yttria thin film and Inconel-718 are plotted in Fig. 7 and 8, respectively. As expected, Inconel12

ACCEPTED MANUSCRIPT 718 alloy, being more metallic shows a steady increase in TEC values compared to that of yttria as a function of temperature. Table 3: The lattice parameter, average thermal expansion coefficient and strain of Y2O3 film deposited on Inconel-718 as a function of temperature.

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Lattice strain for (222) plane (× 10-3) 2.63 2.53 2.41 2.28 2.16 2.03 1.91 1.78 1.66 1.53 1.41

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αL-average (10-6 K-1) (± 0.01) 6.92 6.93 6.94 6.95 6.96 6.97 6.98 6.99 7.00 7.01 7.02

a corrected (± 0.002) (Å) 10.663 10.669 10.676 10.684 10.691 10.698 10.706 10.713 10.721 10.728 10.736

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298 373 473 573 673 773 873 973 1073 1173 1273

A (± 0.002) (Å) 10.664 10.669 10.675 10.683 10.691 10.700 10.708 10.711 10.720 10.730 10.735

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T (K)

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Since the thermal expansion is dependent on the bond strength of the materials, Y2O3 with higher bond strength and high melting point has shown lesser TEC than that of Inconel-718

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[56]. However, it must be pointed out that the thermal expansion behavior of Y2O3 film is complex and not as simple as that of bulk Inconel-718because, the thin films are associated with

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stress, defects as well as smaller crystallite sizes and they are further constrained by the substrate and interdiffusion of film with the substrates. These factors would seriously affect the thermal

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expansion of the Y2O3 film at higher operating temperatures exceeding 1273 K. The lattice strain (ε) corresponding to (222) reflection of cubic yttria was calculated in

Table 3.

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the temperature range of investigation using the following Eq. 9, and the values are shown in the

𝛽

Lattice strain, ε = 4𝑡𝑎𝑛𝜃

(9)

where β is the full width at half maximum of (222) reflection and θ is the Bragg angle. The lattice strain is found to decrease with increasing temperature as a result of increased crystallite sizes [57].

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ACCEPTED MANUSCRIPT It is very useful to understand the thermal stress associated within the film, in particular, for high temperature applications. The in-plane thermal stress (σ) was calculated qualitatively [58,59] using the following Eq. 10: 𝜎=

𝐸𝑌2 𝑂3 (𝛼𝑌2 𝑂3 −𝛼𝑠 )△𝑇

(10)

1−𝑌2 𝑂3

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where 𝐸𝑌2 𝑂3 is the Young’s modulus (150 GPa) [60] of the bulk Y2O3, 𝛼𝑌2 𝑂3 is the thermal

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expansion coefficient of cubic Y2O3 film and αs is the thermal expansion coefficient of Inconel718 substrate, △T is the temperature difference (700°C - RT) and 𝑌2 𝑂3 is the Poisson’s ratio

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(0.298) of bulk Y2O3 [60] . Assuming that the mechanical constants of the bulk material are equal to that of the film, the thermal stress of the film at room temperature was calculated and is

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found to be -602 ± 15 MPa. The obtained compressive stress value is in close agreement with the earlier reported values [59] .

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Fig. 9 shows the SEM images of the Y2O3 film in the as-deposited condition at 973 K and after the HTXRD study. The low magnification image of the as-deposited film (Fig. 9 (a)) shows

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a smooth, uniform, and crack-free surface. No cracks were seen at low magnified images of the film heat treated for HTXRD study in the vacuum. From the magnified image of the as-deposited

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film (Fig. 9 (c)), it is noticed that the as-deposited films are porous and the particles are spherical and uniformly distributed. The porosity is covered to some extent after HTXRD study (10 (d)).

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During the heat treatment, the Y2O3 particles tend to grow and cover the pores and voids present in the as-deposited film. This can be clearly seen in the SEM images. Ghosh et al. [61] reported

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that such type of porous grain microstructure allows plastic deformation to accommodate the

coating.

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generated compressive stress during heat treatment and improve the adhesion property of the

Thermal stability of the Y2O3 film was carried out by annealing the Y2O3 coated Inconel718 sample at 1273 K in air for 1 h, 2 h, 4 h and 100 h and the microstructural changes were studied. Fig. 10 shows the XRD patterns for the yttria films deposited at 973 K annealed at 1273 K in air for different durations. All the patterns show reflections corresponding to cubic yttria which match with JCPDS card data indicating crystallographic stability of the film. The crystallite size corresponding to (400) plane was found to be 15 ± 1 nm for the film annealed up to 4 h and increased to 30 ± 1 nm for the film annealed at 100 h. The prolonged annealing in the 14

ACCEPTED MANUSCRIPT air promotes the growth of crystallites in the yttria film. The relative intensity of substrate peak also increased as the annealing duration increased indicating the formation of cracks and interdiffusion in the coatings. It is noted that, the RMS surface roughness of the yttria film annealed at 1 h and 100 h are 10 ± 1 nm and 6 ± 1 nm, respectively. The decrease in surface roughness could be due to the

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effect of grain coalescence during heat treatment process resulting in smooth surface. The film

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did not peel off even after 100 h of annealing suggesting the fact that the Y2O3 film developed

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through EBPVD could be useful for high temperature applications. 4. Conclusions

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Y2O3 films with a thickness of ~2µm have been deposited on Inconel-718 in the substrate temperature range of 773-973 K by electron beam physical vapor deposition technique. In-situ

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high temperature X-ray diffraction was performed from 298 to 1273 K to study the thermal expansion behavior of Inconel-718 and Y2O3 coated Inconel-718 substrate. The crystallographic stability of the yttria film deposited on Inconel-718 was further tested at 1273 K. The following

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major conclusions are drawn:

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(i) The Y2O3 films deposited on Inconel-718 in the temperature range 773-973 K indicated the formation of cubic phase of Y2O3 as a major phase along with minor phase of

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monoclinic Y2O3. The formation of monoclinic phase at lower substrate temperature could be correlated with higher residual stress and oxygen deficiency in the as-deposited films. The

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monoclinic phase was found to be removed upon annealing the film at 1273 K in air. The calculation of texture coefficient has indicated a change of preferred orientation from (222) to

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(400) for cubic Y2O3 films with increasing substrate temperature. (ii) AFM analysis indicated an increase in particle size with increasing substrate temperature as a result of higher adatom mobility. The analysis further showed a uniform and smooth surface of the film prepared by EBPVD method. (iii) Good adherence of the film with the Inconel-718 substrate was confirmed by scratch test. The test showed higher values of critical loads of 5.5 and 6.1 N for cohesive failure and adhesive failure, respectively. This was also supported by microstructural examination of the scratch track by scanning electron microscope.

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ACCEPTED MANUSCRIPT (iv) HTXRD studies showed that the Inconel-718 has higher thermal expansion coefficient (1.22 ± 0.01 × 10-5 K-1) than that of Y2O3 film (7.02 ± 0.01 × 10-6 K-1) and increases linearly in the temperature range of 298 - 1273 K. The thermal stress associated with the Y2O3 film was found to be compressive in nature due to the difference in the thermal expansion coefficients of the Inconel-718 substrate and Y2O3 film.

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(v) The thermal stability of the Y2O3 film tested at 1273 K in air over 100 h has revealed

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no delamination of the film from the substrate.

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(vi) In brief, the current he investigation on the structural analysis, thermal expansion, and thermal stability of the yttria films has demonstrated the suitability of these films for high

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temperature applications Acknowledgements

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The authors acknowledge Pro-Chancellor and Pro-Vice Chancellor, Sathyabama Institute of Science and Technology, Chennai for providing the facilities to carry out this research work.

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The authors also acknowledge UGC-DAE CSR (CSR-KN/CRS-49/2013-14/648) for funding this

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research work.

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ACCEPTED MANUSCRIPT List of Figures Figure 1. XRD pattern of a Y2O3 pellet used for the preparation of Y2O3 films. Figure 2. XRD pattern of the Y2O3 films deposited on Inconel-718 substrates at different substrate temperatures. Figure 3. AFM images of Y2O3 films at (a) 773 K, (b) 873 K (c) 973 K and (d) Y2O3 film deposited at 973 K and further heat treated at 1273 K in air for 1 h.

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Figure 4. Scratch test of the Y2O3 film deposited on Inconel 718 at the substrate temperature of

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973 K.

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Figure 5. HTXRD profiles of Inconel-718 in the temperature range 298-1273 K. Figure 6. HTXRD profiles of Y2O3 film in the temperature range 298-1273 K.

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Figure 7. Lattice parameters of Inconel-718 and Y2O3 film on Inconel-718 substrate as a function of temperature.

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Figure 8. Thermal expansion coefficient versus temperature of Inconel-718 and Y2O3 film. Figure 9. SEM image of Y2O3 film deposited on Inconel 718 (a) as-deposited at 973 K, (b)

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HTXRD tested sample, (c) and (d) are the magnified images of (a) and (b), respectively.

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Figure 10. Y2O3 films annealed at 1273 K in air for 1 h, 2 h, 4 h and 100 h.

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 Optimization of substrate temperature on the deposition of c-Y2O3 films by EBPVD technique.  HTXRD studies of Inconel-718 and c-Y2O3 film from RT to 1273 K.  Comparative study of thermal expansion coefficients of Inconel-718 and cY2O3 film.  Adhesion property of the c-Y2O3 film on Inconel-718 was tested.  Stability of the Y2O3 film on Inconel-718 was tested at 1273 K in air for 100 h.

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Figure 10