Segregation phenomena during deposition of functionally graded zirconia-based ceramics with Stellite 21 on a steel substrate

Journal Pre-proof Segregation phenomena during deposition of functionally graded zirconia-based ceramics with Stellite 21 on a steel substrate

Harish Rao, Indumini Jaysekara, Bhaskar Dutta, David Maurice PII:

S0257-8972(19)31260-5

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125270

Reference:

SCT 125270

To appear in:

Surface & Coatings Technology

Received date:

29 October 2019

Revised date:

10 December 2019

Accepted date:

12 December 2019

Please cite this article as: H. Rao, I. Jaysekara, B. Dutta, et al., Segregation phenomena during deposition of functionally graded zirconia-based ceramics with Stellite 21 on a steel substrate, Surface & Coatings Technology (2019), https://doi.org/10.1016/ j.surfcoat.2019.125270

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© 2019 Published by Elsevier.

Journal Pre-proof Segregation phenomena during deposition of Functionally Graded Zirconia -Based Ceramics with Stellite 21 on a Steel Substrate Harish Rao1, Indumini Jaysekara1, Bhaskar Dutta2, David Maurice1 1

National Energy Technology Laboratory, Albany, OR 97321, USA DM3D Technology, LLC, Auburn Hills, MI 48326, USA

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Abstract

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This report describes the co-deposition of ceramic-metal systems based upon yttria-stabilized zirconia, and gadolinium zirconate, with Stellite 21 via laser direct energy deposition (LDED).

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The segregation of un-melted ceramic particles and the depletion of stabilizing dopants observed

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in previous work [1] is replicated. The segregation of un-melted ceramic particles results in a surface coating which prevents further deposition. The depletion of stabilizing dopants from the

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ceramics results in phases which are known to lack stability at the high temperatures expected in

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such applications. We provide a mechanistic explanation for the compositional segregation

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phenomenon, which suggests that the pairing of lanthanide series – based ceramics such as yttria-stabilized zirconia and gadolinium zirconate with transition metal alloys such as Inconel and Stellite may be difficult to effectively utilize in a functionally graded coating.

Keywords: ceramic additive manufacturing; functionally graded materials, high temperature zirconia ceramic, compositional stability; phase stability, thick thermal barrier coatings

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Journal Pre-proof 1 Introduction We previously reported [1] on the use of laser direct energy deposition (LDED), an additive manufacturing process in which focused thermal energy is used to fuse materials by melting as they are being deposited. to produce a thick functionally graded thermal barrier coating (FGTBC) composed of proportionally varied blends of

Inconel 625 and yttria-stabilized zirconia

(YSZ). In that effort the depletion of yttrium from matrix-entrapped YSZ particles, which is

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known to lead to a loss of phase stability, was influenced by the laser source used in the

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deposition. To explore whether this segregation was specific to that material combination, YSZ

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and gadolinium zirconate (Gd2Zr2O7, or GZO) are utilized in combination with Stellite 21 via

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LDED to produce thick, FG-TBCs. The compositional and phase stability of the two doped

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zirconia ceramics, and the impact of laser source and metal-ceramic combinations are reported. Commercially, TBCs are produced by depositing a thin layer (< 150 µm in thickness) of

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intermetallic bond coat (BC) on the metallic substrate on which a monolithic ceramic layer (<

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500 µm in thickness) is deposited via air plasma-spraying (APS) [2–9] or electron beam physical

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vapor deposition (EB-PVD) [10–12]. At this thickness, most TBCs in turbine applications (the most common use of TBCs currently) are capable of reducing the substrate surface temperature between 100 to 300 oC [13–16]. While thicker TBCs would enable higher operating temperatures for an expanded range of components in engines and power-generation, and higher efficiencies and reduced emissions in existing applications, it is a major challenge to produce thick (> 500 µm) TBCs of acceptable quality. Attempts to building thicker monolithic TBCs have introduced decreased thermal shock resistance [17], high shrinkage forces [18], decreased adhesive strength [19], and higher residual stresses [19–23].

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Journal Pre-proof One of the remedies that has been suggested to address the problems associated with thick monolithic TBC is to replace the monolithic ceramic layers with a functionally graded TBC (FG-TBC) [20,24–26]. Functionally graded TBCs are composite structures with continuous or discrete spatial changes in mechanical and thermal properties. By functionally grading the TBC, it is argued that it is possible to bring a gradual transition in thermal and mechanical properties in the TBC and thereby improve its service life [14,20,24,27–30]. However, to date these potential

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advantages remain undemonstrated. Air plasma spray has been widely studied as one of the

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methods to produce FG-TBC [20,21,28,31]. However, the air plasma sprayed TBCs are

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surface and are thus a source of weakness [8,15].

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characterized by high porosity and splat boundaries that mostly run parallel to the substrate

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This deficiency in available deposition technology for depositing thick ceramic layers can be bridged by adopting additive manufacturing (AM) technology. With recent advancement in AM,

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it is now possible to fabricate ceramic components in complex shapes without the need for

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subsequent machining [32,33]. AM makes it possible to produce fully dense monolithic or porous ceramic components in complex shapes which are otherwise not possible with

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conventional manufacturing techniques. Today, ceramic components used for dental and medical applications have been produced on a commercial scale using AM technologies like binder jetting and stereolithography [34–39]. Although these AM technologies can produce FG-ceramic layers, they lack the ability to produce large-sized ceramic components. This is primarily due to the gasses released during high temperature curing of the polymer-derived ceramics, which is associated with shrinkage and prevents fabrication of bulk, large sized ceramic builds [33]. Laser direct energy deposition (LDED) has received major interest in academia and in industry due to its versatility in processing a variety of materials and its application in rapid prototyping, 3

Journal Pre-proof alloying, metallic coatings and component repairs [40,41]. The major advantages of LDED over other laser based additive manufacturing process are: having no restrictions on build size; the ability to build functionally graded materials; and its abilities to build material at higher rates (up to 300 cm3/h) [42,43]. Although LDED can fabricate complex shaped, large components, laser deposition of ceramics is still considered a major challenge due to issues such as relatively high melting temperature of ceramics, extremely high temperature gradients within deposition layers

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leading to thermal transient stresses and residual stress and micro-defects which develop between

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the deposited layers making laser AM of ceramic difficult to impossible [39,44]. The vast

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majority of studies on the AM of materials via LDED are on metals, with very few studies on

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monolithic ceramic materials [41,44–47]. Additional laser deposition work on ceramics has been on laser cladding for hard-facing nickel, titanium and cobalt based super alloys with reinforced

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ceramics [48–54].

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Zirconia based ceramics and in particular, yttria-partially stabilized zirconia (YSZ) with 8 wt.%

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yttria and 92 wt.% zirconia are the most widely studied and commercially used ceramics in producing thermal barrier coatings (TBCs) for high temperature applications. The limited studies

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on laser deposition of zirconia-based TBCs have demonstrated that laser deposition produces coatings that are denser and less porous compared to conventionally produced TBCs via plasmaspray or electron beam vapor deposition [55,56]. Other studies on laser clad zirconia-based ceramics observed vaporization of zirconia, thermal stress induced spalling and poor adherence due to rapid solidification [57,58]. Nevertheless, barring the work of Rao et al. [59], Mumtaz et al. [60] and Savitha et al. [61], there are no available studies on producing thick FG-TBCs via laser deposition. While the above three studies discussed in detail the feasibility of depositing a thick FG-TBCs, only Rao et al.[59] and Savitha et al. [61] noted the diffusion of elements across

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Journal Pre-proof the YSZ-metal alloy interface. Diffusion of stabilizing elements, particularly the yttrium can negatively impact the critical phase stability of YSZ crucial for longevity of the TBCs. [62–66]. Although researchers have been successful in laser depositing monolithic YSZ TBCs with the stable tetragonal prime (t’) phase [56,58], the ability to utilize LDED or other laser based deposition to produce the ‘non-transformable’ tetragonal (t’) prime phase in thick FG-TBCs, has not been demonstrated.

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Gd2Zr2O7 (GZO) is an attractive TBC material due to its low thermal conductivity, high thermal

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expansion coefficient and phase stability up to its melting temperature [67]. Gadolinium

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zirconate thin layers have been successfully deposited by both the APS and EB-PVD techniques

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[68], but there are no reports of the deposition of GZO via LDED. Gadolinium zirconate may

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exhibit a pyrochlore or fluorite phase, but the pyrochlore form of GZO is the target for TBCs, and it exists only within a narrow compositional range. This is analogous to the YSZ system, in

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which the required tetragonal prime phase exists only within a narrow compositional range.

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In our previous study [59], Y depletion from YSZ in a YSZ-In625 combination was observed

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with all tested LDED processing conditions, resulting in “YSZ” particles which were outside that required compositional range. Observation of this phenomenon was limited to that single materials combination. As this compositional degradation has significant ramifications for the feasibility of building graded TBCs using LDED, in this work the matrix was expanded to include other potential FGM-TBC cermet combinations: YSZ-Stellite 21 and GZO-Stellite 21. Understanding the degradation mechanisms of doped zirconia ceramics due to the diffusion of stabilizing elements discussed is crucial to developing strategies for depositing FG-TBCs via laser AM technologies, particularly LDED for big area manufacturing or large components. To that end, both YSZ and gadolinium zirconate (Gd2Zr2O7 or GZO) were utilized in combination 5

Journal Pre-proof with Stellite 21 and deposited via LDED on a steel substrate to produce thick FG-TBCs. This is the first report on the impact of laser source on the critical phase stability of the two doped zirconia ceramics. Further, we provide a mechanistic explanation on the diffusion of stabilizing elements from doped ceramic particles into the surrounding metal matrix during LDED. 2 Experimental procedure

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The FG-TBCs were produced using powders of Stellite 21 (Kennametal Stellite, USA), YSZ

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(Metco 204C-NS premium, Oerlikon-Metco, US Inc), GZO (Saint-Gobain Ceramics and Plastics, Inc.) and Inconel 625 (Micromelt 625, Carpenter Technology Corporation, USA). All

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powders (Figure 1) were produced to be 45 µm to 150 µm, with up to 0.2 wt. % fines present.

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The compositions of the as-received powders are presented in Table 1.

Figure 1: SEM images of the starting particles (a) Stellite 21, (b) YSZ and (c) GZO Substrates were 12.7 mm thick A516 steel plate, upon which a 0.2 mm thick bond coat (BC) of Inconel 625 had been deposited via LDED. Two material combinations, Stellite 21 with yttria stabilized zirconia (St21-YSZ) and Stellite 21 with gadolinium zirconate (St21-GZO), were chosen to produce FG-TBC specimens using CO2 (C) and diode (D) lasers as indicated in Table 2. All specimens were initially deposited with a targeted thickness of 0.2 mm bond coat (BC)

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Journal Pre-proof constituting 100 wt.% Inconel 625. Based on specimen configuration (Max 10 and Max 20), individual specimens were then deposited with three or five layers of cermet, each layer measuring about 0.6 mm in thickness. The metal and ceramic powders were initially blended according to desired proportion and then co-deposited in proportions given in Figure 2. The FGTBC specimens for each material combinations were produced with a maximum of 10 wt.% ceramic (Max 10) and 20 wt.% (Max 20) ceramic as illustrated in Fig. 2c and 2d. The FG-TBC

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specimen combinations were produced by DM3D Technology, MI, USA using a DMD 5000

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machine and a DMD44R machine, for CO2 and diode lasers, and the processing parameters

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including laser power, laser scan speed, step over and powder feed rate used in FG-TBC Max 10

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and Max 20 specimens are provided in table 3 and table 4 respectively.

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The approach employed in depositing the coatings is illustrated in Fig. 2a and 2b. The specific energy E = P/d.v (J/mm2), where P is the laser power (watt), d is the hatchet distance (mm) and v

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is the traverse speed of the laser source (mm/s), is calculated and provided for each layer in

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respectively.

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Table 2. The beam diameter of the CO2 and diode lasers were maintained at 3.5 mm and 3.2 mm

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Figure 2: Schematic illustrating the LDED process of producing FG-TBCs (a) and (b) indicating

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the laser traverse direction, (c) layer configuration in Max 10 coupons and, (d) layer

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configuration in Max 20 coupons. M indicates metal (Stellite 21) and C indicates ceramic (GZO or YSZ)

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Specimens were sliced along the Y-Z plane and then hot-mounted in a conductive epoxy. After

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grinding and polishing, the mounted specimens were imaged using an FEI Inspect F50 Scanning Electron Microscope (SEM), while elemental mapping and microanalysis of the specimens were performed using a JEOL JXA-8530FPlus HyperProbe. Samples for TEM analysis were prepared using the focused ion beam lift-out method using an FEI Helios NanoLab SEM. Transmission electron microscopy (TEM) and associated energy-dispersive X-ray spectroscopy (EDX) analysis of the TBC specimens were done using an FEI Titan 80-200 kV TEM operating at 200 kV and equipped with a ChemiSTEM EDX detection system. High-resolution (HRTEM) images were collected in bright-field mode, while lower resolution scanning (STEM) images were

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Journal Pre-proof collected using a high-angle annular dark-field (HAADF) detector. Electron probe elemental mapping of functionally graded specimens were done using JEOL 8530F Plus Hyper Probe. Table 1: Chemical composition of the starting powder particles Ni

Cr

Fe

Mo

Nb

C

Mn

Si

Al

Other

balance

20-23

5

8-10

3-4.15

0.10

0.50

0.50

0.40

1.40

Stellite 21 (St21)

Co

Cr

C

Ni

Mo

Fe

Si

Other

balance

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0.25

3

5.2

<3.0

<1.5

<0.5

ZrO2

Y2O3

SiO2

TiO2

Al2O3

Fe2O3

Other

balance

7-8.0

0.05

0.05

0.05

Gd2O3

ZrO2

55-70

45-30

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0.5

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GZO

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YSZ

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Inconel 625 (In625)

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Table 2: Specific energy, E, used in depositing various cermet layers in Max 10 (-10-) and Max 20 (-20-)

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coupons. The C indicates deposition with a CO2 laser and the D indicates deposition with a diode laser.

Coupon

BC

StYSZ-10-D StGZO-10-D StYSZ-10-C StGZO-10-C

In625 In625 In625 In625

StYSZ-20-D StGZO-20-D StYSZ-20-C StGZO-20-C

In625 In625 In625 In625

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Layer 1

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125.71 125.71 124.29 124.29 125.71 125.71 124.29 124.29

Specific energy, E (J/mm2) Layer 2 Layer 3 Layer 4 Max 10 coupons 120 114.29 120 114.29 117.86 113.23 117.86 113.23 Max 20 coupons 120 114.29 108.57 120 114.29 108.57 117.86 113.23 108.09 117.86 113.23 108.09

Layer 5

102.86 102.86 103.16 103.16

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Layer

Composition

Expected Power Laser Scan thickness (W) Speed (mm) (mm/min) StYSZ10C and StGZO10C

Powder Feed Rate (g/min)

BC

100% In 625

0.2

1500

1100

2.0

5

Layer 1

99% St 21+ 1% YSZ/GZO

0.6

2900

700

2.0

12

Layer 2

95% St 21+ 5% YSZ/GZO

0.6

2750

700

2.0

12

Layer 3

90% St 21+ 10% YSZ/GZO

0.6

2642

700

2.0

12

1500

1100

2.0

5

0.6

2200

700

1.5

15

0.6

2100

700

1.5

15

0.6

2000

700

1.5

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Step Over (mm)

Powder Feed Rate (g/min)

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Step Over (mm)

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Table 3: Processing parameters for Max 10 specimens

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StYSZ10D and StGZO10D 0.2

Layer 1

99% St 21+ 1% YSZ/GZO

Layer 2

95% St 21+ 5% YSZ/GZO

Layer 3

90% St 21+ 10% YSZ/GZO

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100% In 625

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BC

Layer

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Table 4: Processing parameters for Max 20 specimens Composition

Expected Power Laser Scan Thickness (W) Speed (mm) (mm/min) StYSZ20C and StGZO20C

BC

100% In 625

0.2

1500

1100

2.0

5

Layer 1

99% St 21+ 1% YSZ/GZO

0.6

2900

700

2.0

12

Layer 2

95% St 21+ 5% YSZ/GZO

0.6

2750

700

2.0

12

Layer 3

90% St 21+ 10% YSZ/GZO

0.6

2642

700

2.0

12

Layer 4

85% St 21+ 15% YSZ/GZO

0.6

2522

700

2.0

12

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80% St 21+ 20% YSZ/GZO

0.6

2407

700

2.0

12

StYSZ20D and StGZO20D 100% In 625

0.2

1500

1100

2.0

5

Layer 1

99% St 21+ 1% YSZ/GZO

0.6

2200

700

1.5

15

Layer 2

95% St 21+ 5% YSZ/GZO

0.6

2100

700

1.5

15

Layer 3

90% St 21+ 10% YSZ/GZO

0.6

2000

700

1.5

15

Layer 4

85% St 21+ 15% YSZ/GZO

0.6

1900

700

1.5

15

Layer 5

80% St 21+ 20% YSZ/GZO

0.6

1800

700

1.5

15

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BC

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3.1 Formation of a surface ceramic layer

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3 Results and Discussion

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In our previous work [1], we reported that any layer deposited with 10 wt.% or higher YSZ resulted in the formation of a YSZ layer on that layer’s surface. The surface YSZ layer greatly

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inhibited the deposition of additional cermet layers, resulting in a limitation on the deposition

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thickness. The mechanism of formation of the surface YSZ layer [59] is reproduced here. During LDED, the pre-mixed Inconel 625-YSZ powder particles were blown through a coaxial nozzle. Due to the Bernoulli effect, YSZ powder particles, with lesser apparent density compared to Inconel 625 powder particles, got pushed to edge of the laser beam and were insufficiently exposed to the laser beam, resulting in incomplete melting. These YSZ particles ultimately floated to the surface of the Inconel 625 rich melt pool and remained there. Subsequently during the deposition of the next layer, or adjacent bead, these YSZ particles melted and formed a surface YSZ layer. It is also possible that some molten YSZ segregated to the surface due to the 11

Journal Pre-proof density difference and the poor wetting, or that some YSZ from overspray became trapped between intended layers. The relative contributions of these potential factors have not been determined. Regardless of the mechanism of origin, due to poor wetting between ceramics and metals, this ceramic layer prevented effective bonding with the next, predominantly metal layer. Since the formation of the surface ceramic layer poses a significant barrier to building thick FGTBCs via LDED, we investigate whether this behavior is specific to the Inconel – YSZ system.

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We further initiate efforts in evaluating the impact of processing on this behavior.

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Table 5 presents the average coating thickness and surface features as measured in representative

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Max 10 and Max 20 FG-TBC specimens (refer to Figure 2). The Max 10 specimens were

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deposited with three cermet layers with the top layer consisting of maximum 10 wt.% ceramic.

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The Max 20 specimens were deposited with five cermet layers with the top layer consisting of

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maximum 20 wt.% ceramic.

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Specimen

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Table 5: Average coating thickness and surface features in Max 10 and Max 20 specimens

StYSZ-10-D StGZO-10-D StYSZ-10-C StGZO-10-C StYSZ-20-D StGZO-20-D StYSZ-20-C StGZO-20-C

Average coating thickness (mm) Max 10 2.90 2.72 2.54 2.20 Max 20 4.19 3.91 2.90 2.45

Surface feature

Uneven Uneven Relatively flat Relatively flat Highly uneven Highly uneven Relatively flat Relatively flat

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Journal Pre-proof Although the Max 10 FG-TBC were deposited with a targeted or intended overall coating thickness of 2 mm (including the 0.2 mm thick BC layer), the actual average coating thickness achieved is relatively higher. The difference in targeted coating thickness to actual average thickness is most prominent in Max 10 FG-TBC specimens produced via diode laser. Representative cross-sections of the as received Max 10 FG-TBC coupons are presented in Fig. 3. In general, the diode-produced specimens exhibit a slightly higher apparent coating thickness,

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albeit with rougher surfaces, compared to the CO2 produced specimens. A similar trend in

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coating thickness is observed in specimens produced with 20 wt.% max ceramic (Max 20) as

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seen in Fig. 4. Although there is an apparent increase in coating thickness in FG-TBCs produced

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via diode laser, the top coat of the diode produced specimens display uneven surfaces. The

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surface unevenness further increases with an increase in ceramic content.

Figure 3: Cross-section images of the representative Max 10 FG-TBCs indicating the total deposition thickness and TBC morphology in (a) StYSZ-10-D, (b) StYSZ-10-C, (c) StGZO-10D, and (d) StGZO-10-C specimens. The two dotted horizontal lines show the 2 mm distance from the substrate interface. 13

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Figure 4: Cross-section images of the representative Max 20 FG-TBCs indicating the total

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deposition thickness and TBC morphology in (a) StYSZ-20-D, (b) StYSZ-20-C, (c) StGZO-20-

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D, and (d) StGZO-20-C specimens. The two dotted horizontal lines show the 2 mm distance

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from the substrate interface. The difference in coating thickness between the diode and CO2 produced FG-TBC specimens is

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partially due to better powder capture capability and the higher laser absorptivity of metals under

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the diode laser, which facilitates a quicker solid-to-liquid transition of metal powder [69,70]. Additionally, the YSZ-Stellite 21 material combination exhibits a slightly greater coating thickness compared to GZO-Stellite 21 material combination. From figures 4a and 4b it is evident that the layers within the 2 mm markers exhibit a continuous interface. This thickness should technically include the BC plus layer 1 through layer 3 (with 10 wt.% ceramic), while anything beyond the 2 mm marker should include layer 4 and layer 5 (with 20 wt.% ceramic). Slightly above the 2 mm marker, in both material combinations, the surface turns highly uneven with patches of pure ceramic between asperities.

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Journal Pre-proof In our previous work [59] we noted that once the surface YSZ ceramic layer formed, deposition of subsequent layers was greatly limited. With the Stellite – ceramic pairs used in this trial, weakly bonded top layers were present in all specimens in as-produced conditions. But they were very fragile and spalled off during handling or metallographic preparations. The loss of the last deposited layer was most pronounced in specimens built using the CO2 laser. Therefore, we have

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only included micrographs of the diode produced specimens for discussion.

The formation of the surface ceramic layer is indicated by red arrows in the SEM images

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presented in Fig. 5a (StYSZ-20-D) and Fig. 5b (StGZO-20-D). In addition, several un-melted

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starting ceramic particles, as indicated by white arrows in Fig. 5a and 5b, were also observed

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embedded in the top coat.

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Journal Pre-proof Figure 5: Cross-section SEM images of the top surface showing the surface ceramic layer (indicated by red arrow) and un-melted ceramic particles (indicated by white arrows) in (a) StYSZ-20-D and, (b) StGZO-20-D specimens. Irrespective of the metal-ceramic combination, processing parameters used in this study for depositing individual layers were very similar for the diode and CO2 produced specimens. However the specific energy was increased (refer table 2, SE for layer 3 is 114.29 J/mm2)

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compared to our previous work [1] (72.89 J/mm2 under CO2 and 67.51 J/mm2 under diode

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laser), with the intent of achieving more complete liquification of the ceramics and thus reduce

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the formation of a ceramic coating which prevents the deposition of the next layer of metal-

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ceramic mixture. It is also possible that the resulting increased temperatures could simply lead to

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a greater subsequent time lag between the solidification of any ceramic particles and the solidification of the metal during cooling, which would enable more ceramic particles to reach

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the surface through buoyancy. Once there they would be subject to re-melting during the next

tested.

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laser pass. Irrespective of mechanism, a ceramic coating formed for all processing conditions

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Analogous to our previous work, we conclude that in Stellite 21 – YSZ and Stellite 21 - GZO, a ceramic surface layer starts to form when the ceramic content of the previous-deposited layer equals or exceeds 10 wt.%. This layer prevents further deposition and thus limits coating thickness. While there is some variability in the extent of this effect due to laser source, modification of processing to overcome this phenomenon will be necessary to enable LDED of FG-TBCs.

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Journal Pre-proof 3.2 Diffusion of stabilizing elements from matrix-entrapped ceramic particles The segregation of yttrium from zirconia has been reported earlier and is still a subject which is not completely understood [71,72]. In our recent study on producing Inconel 625-YSZ based thick TBCs via LDED [59], we noted the depletion of yttrium from the re-solidified, matrixentrapped YSZ particles. Since the phase stability of zirconia-based ceramics is dependent upon dopant content, it was appropriate to determine whether the depletion previously reported [1]

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was specific to that material combination. To that end, a survey on the impact of processing

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conditions, laser source and metal-ceramic combination on dopant diffusion in Stellite is

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presented.

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In Figure 6 representative wavelength dispersive spectroscopy (WDS) elemental maps of the

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ceramic components (zirconium, yttrium, gadolinium and oxygen) from selected regions in Max 20 specimens are presented. Irrespective of metal-ceramic combination and laser source, the

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Max 20 specimens showed a dispersion of fine ceramic particles within the Stellite 21 matrix.

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The WDS elemental maps indicate the diffusion of stabilizing dopant elements - yttrium and

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gadolinium - from the matrix entrapped YSZ and GZO particles, respectively. The depletion of yttrium from StYSZ Max 20 specimens (Figs. 6a and 6b) is especially pronounced. The entrapped GZO particles retain considerable gadolinium as evident from the maps in Fig. 6c and 6d, though as will be discussed in more detail later, the retained gadolinium content is insufficient to retain the pyrochlore form of GZO. The diffusion of zirconium into the metal matrix has not been previously reported. Under the parameters used in this study, the rate of zirconium diffusion into the matrix does not appear to be influenced by either laser source or by the specific ceramic material used. Under all

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Journal Pre-proof conditions zirconium readily diffuses out of the ceramic and into the matrix. In certain regions within the Stellite 21 matrix, both zirconium (Zr) and zirconia (ZrO2) were present, analogous to

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that observed in the previous study [1] on the Inconel 625- YSZ system.

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Figure 6: Electron microscopic images and microprobe WDS elemental maps of the matrix-

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entrapped ceramic particles in Stellite 21 matrix: (a) StYSZ-20-D; (b) StYSZ-20-C; (c) StGZO-

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20-D; and (d) StGZO-20-C FG-TBC specimens.

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The WDS maps provide a gross view of elemental diffusion. To examine the diffusion on a finer

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scale, such as how it manifests at an individual particle level, detailed electron dispersive spectroscopy (EDS) of the selected elements was performed using transmission electron

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microscopy (TEM.). Representative results are shown in Figure 7 using Max 10 specimens.

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Journal Pre-proof Figure 7: Compositional analysis and elemental maps of StYSZ-10-D specimens under TEM: (a) STEM image of an YSZ particle entrapped in the Stellite 21 matrix; (b) EDX map of the zirconium; (c) yttrium; (d) oxygen; and the EDX quantification results (normalized wt.%) for regions shown in Fig. 7a. The electron microscopic image in Fig. 7a shows a matrix entrapped YSZ particle in a StYSZ10-D specimen. The table in Fig. 7 provides the EDS elemental wt.% of zirconia, yttrium and

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oxygen at selected regions as indicated Fig. 7a. The elemental maps indicate the depletion of

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yttrium from the entrapped YSZ particle as observed in Fig. 7c. The EDX elemental analysis

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indicates the maximum yttrium content in the entrapped YSZ particle is 1.42 wt.%, which is a

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considerable reduction from the 6.3 wt.% yttrium in a pre-deposition YSZ particle. Since the

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sensitivity of the EDX on the TEM used for elemental quantification is 0.1 wt.%, yttrium was not detected in the Stellite 21 matrix.

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Zirconium also diffused from the matrix-entrapped YSZ particle into the Stellite 21 matrix as

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observed in Fig. 7b. Depletion of zirconium from entrapped YSZ particles into the surrounding

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metal matrix was not observed in our previous study on LDED Inconel 625-YSZ based FGTBCs [1], nor has it been reported elsewhere. In certain regions within the matrix entrapped YSZ particle, for example region 1, high amounts of zirconium were identified. These zirconium-rich regions were deprived of any oxygen content as evident from Fig. 7d. Small traces of zirconium were also observed in the matrix close to the entrapped YSZ particle (region 6, 7). Figure 8 presents a series of EDS maps of a matrix-entrapped YSZ particle in a StYSZ-10-C specimen. The supposedly YSZ particles are mainly rich in zirconium and deprived of any yttrium and oxygen as evident from the EDS elemental analysis in Fig. 8c and 8d. In the specimens produced via CO2 laser there is an apparent complete diffusion of yttrium and oxygen 20

Journal Pre-proof from the entrapped YSZ particles. The entrapped particles are mostly zirconium metal and are not technically YSZ. Again, due to the sensitivity limitations of the equipment, only zirconium

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was detected in the matrix, and not the yttrium lost from the particles.

Figure 8: Compositional analysis and elemental maps of StYSZ-10-C specimens under TEM (a)

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STEM image of an YSZ particle entrapped in the Stellite 21 matrix, (b) EDX map of the

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zirconium, (c) yttrium, (d) oxygen and, (e) the EDX quantification results (normalized wt.%) for regions shown in Fig. 7a.

Compared to the previous study on diode laser-produced Inconel 625-YSZ FG-TBCs, wherein a maximum of 4.13 wt.% of yttrium was retained in Inconel 625 entrapped YSZ particles [59], a maximum of 1.42 wt.% of yttrium was retained in Stellite 21 matrix-entrapped YSZ particles. In specimens produced with a CO2 laser, a maximum of 1.05 wt.% yttrium remained in the Inconel 625 entrapped YSZ particles [59], and 0 wt.% yttrium in Stellite 21 entrapped YSZ particles. In addition to yttrium, zirconium also diffused into the Stellite 21 matrix, which was not observed 21

Journal Pre-proof in the LDED of Inconel 625-YSZ. In the following section a model will be presented to explain this behavior. The specific energy (E) utilized in the deposition of layer 3 in StYSZ-10-C specimen is 113.23 J/mm2, much higher than the 72.89 J/mm2 used in the deposition of the analogous layer 3 in Inconel 625-YSZ FG-TBCs produced via CO2 laser [59]. Similarly, the specific energy (E) for depositing layer 3 in StYSZ-10-D specimen is 114.29 J/mm2, which is again much higher than

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67.51 J/mm2 utilized in depositing Inconel 625-YSZ FG-TBCS via diode laser in our previous

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studies. The higher specific energy (E) used in the present study resulted in the YSZ remaining at

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an elevated local temperature for a longer period of time. This enabled greater diffusion of

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yttrium as well as zirconium into the Stellite 21 matrix.

Figure 9: Compositional analysis and elemental maps of StGZO-10-D specimens under TEM: (a) STEM image of a GZO particle entrapped in the Stellite 21 matrix; (b) EDX map of the zirconium; (c) gadolinium; (d) oxygen; and the EDX quantification results (normalized wt.%) for regions shown in Fig. 9a. 22

Journal Pre-proof The elemental maps of zirconium, gadolinium and oxygen in StGZO-10-D specimens produced via diode laser are presented in figure 9. As gadolinium is present in much higher concentration in the starting powder particles (47.7 wt% to 60.73 wt.%), the gadolinium does not diffuse completely from the GZO particles, as evident in Fig. 9c. The maximum gadolinium wt.% in the highly concentrated region is 22.17 wt.%, which is much lower than that in the starting GZO powder particles. Small traces of gadolinium were observed in multiple regions of the matrix,

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while the amount of zirconium detected in the matrix is comparable to the amount of zirconium

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detected in a matrix with YSZ particles.

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Finally, representative EDS elemental maps of StGZO-10-C FG-TBC specimens produced via

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CO2 laser are presented in figure 10. Figure 10a shows three matrix-entrapped GZO particles.

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As in StGZO-10-D, a region rich in zirconium, lacking oxygen and gadolinium was observed around particles, as observed in Fig. 8b. The gadolinium content (max 28.82 wt.%) in the matrix-

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entrapped GZO particle is far below the gadolinium content of the feedstock powder. The

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gadolinium is mostly concentrated in the center of the entrapped GZO particles along with

specimens.

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oxygen as evident from Fig. 8c and 8d. This is consistent with the results of StGZO-10-D

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Figure 10: Compositional analysis and elemental maps of StGZO-10-C specimens under TEM:

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(a) STEM image of an GZO particle entrapped in the Stellite 21 matrix; (b) EDX map of the zirconium; (c) gadolinium; (d) oxygen; and the EDX quantification results (normalized wt.%) for

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regions shown in Fig. 11a.

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In summary, the WDS and EDS elemental maps of the zirconium, yttrium, gadolinium and

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oxygen elements show the non-agglomerated, homogeneous distribution of ceramic particles within the Stellite 21 matrix. The depletion of dopant elements from matrix entrapped, re-

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solidified ceramic particles is observed in both Stellite 21-GZO and Stellite 21-YSZ- FG-TBC

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specimens. The depletion of yttrium is more prominent in a Stellite 21 matrix than that reported [1] in an Inconel 625 matrix. In both the diode and CO2 laser produced Stellite 21-YSZ-10 FGTBCs, the yttrium content of entrapped ceramic particle is well below the critical level (min 5.51 wt.% yttrium or 7 wt.% Y2O3) needed for YSZ to exhibit the favorable tetragonal prime (t’) phase which has superior phase stability during high temperature operations. Similarly, the gadolinium content in matrix entrapped GZOs in both diode and CO2 laser produced FG-TBC specimens are well below the gadolinium content in starting feedstock powder particles. However, gadolinium depletion was more prominent in diode produced 24

Journal Pre-proof specimens than in those produced under a CO2 laser. The effects of the expected accompanying phase changes, as might be expected based on the proposed phase diagram of Leckie et al.[73], have not been fully investigated for TBC performance. However, such a large deviation from the proven target content is an indication that, as with the YSZ, the changes are not desirable. The pyrochlore form of GZO is the target for TBCs, and it exists only within a narrow compositional range (48 – 56 wt.% Gd according to the Leckie phase diagram.).

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Depletion of yttrium, gadolinium, zirconium and oxygen from YSZ and GZO were only

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observed in matrix-entrapped ceramic particles. In the surface ceramic layers that formed on the

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surfaces of layers with 10 wt.% ceramic, there was no evidence of any depletion of stabilizing

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elements. The WDS elemental maps of the surface YSZ and GZO layers (Figure 11) show a

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homogeneous distribution of elements. The segregated GZO layers in Figures 11c and 11d show differences in contrast due to the higher concentration of Gd in GZO as compared to the

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concentration of Y in YSZ – it is an artifact of the analytical technique.

25

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Figure 11: Microprobe elemental maps of the surface YSZ layer and GZO layer (a) StYSZ-20-D, (b) StYSZ-20-C, (c) StGZO-20-D and (d) StGZO-20-C FG-TBC specimens

26

Journal Pre-proof 3.3 Diffusion during LDED of ceramic-metal blends In building a functionally gradient TBC, a starting point would be to employ metals and ceramics already proven in environments characterized by high temperatures and chemical attack. Metals meeting this description would include the Inconel and Stellite series alloys; superalloys made of transition metals (Co and Ni). As noted previously, the ceramics GZO and YSZ are forefront TBC materials. The observed elemental depletion (Gd, Y, and Zr) of the ceramic particles is so

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pronounced across all materials pairings tested, despite the rapid solidification intrinsic to the

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LDED process, that it poses a significant challenge to building an FG-TBC. As data on the

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diffusion of these elements in transition metals is limited, an effort is made to provide an

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explanation of the rapid diffusion of lanthanides in transition metals like nickel and cobalt.

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For transition metal-transition metal interfaces, interdiffusion commonly occurs only at temperatures above 500 K [73]. For lanthanide metal – transition metal interfaces, interdiffusion

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has been observed at much lower temperatures [74]. Lanthanide and transition metal

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combinations have lower kinetic energy barriers [74], and combinations with lower kinetic

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barriers will result in faster elemental diffusion. The Debye model of lattice heat capacity [75] has been extended to describe low temperature (0 K to 300 K) diffusion behavior [76]. A larger Debye-Waller factor, or equivalently a smaller Debye temperature (not an actual temperature) indicates higher jump rates and thus more rapid diffusion. When there is a large difference in these factors between two elements, the element with the lower Debye temperature will readily diffuse into the element with the higher Debye temperature.

27

Journal Pre-proof A small Debye temperature is characteristic to the lanthanide-series elements, which are commonly used in the ceramics for TBCs. Examples of this can be seen in Table 6, which includes Gd and Y (which exhibits chemical behaviors similar to the lanthanides).Table 6 also includes the Debye temperatures for transition metals likely to serve as bases for alloys used in extreme environments, such as Co (for Stellite), Fe (for stainless steels), and Ni (for Inconel). The possible combinations of elements from Table 6 reflect the probable starting point

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combinations for trying to build FG-TBCs, whether they are monolithic ceramics (GZO, YSZ)

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deposited on transition metal alloys, or functionally graded layers comprised of the ceramics and

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alloys.

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Table 6: Debye temperatures of elements typical of TBCs and substrates [77–79]

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Gd Y Zr Co Ni Fe

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Element

Debye Temperature Low temperature limit at 0 Room temperature K (298K) 182 155 248 214 290 250 460 386 477 345 477 373

Additive manufacturing via LDED is not conducted at low temperatures, but at conditions which result in the ceramic and metal particles undergoing a solid to liquid transition. Thus it is necessary to discuss the extrapolation of this model to high temperatures. Bolmatov at. al. [80] compared the phonon theory of liquid thermodynamics against the Debye model and demonstrated good agreement between the calculated and experimental heat capacity of 21 liquids over a wide range of temperature and pressure. Sears and Shelley [81] derived a simple

28

Journal Pre-proof analytical model for the Debye-Waller factor which suggested that the error in the model is about (3-5)% when applied at a temperature much different from the one at which the phonon density of states was measured. They also presented arguments showing that the anharmonic contribution to the Debye-Waller factor remains at the few-percent level until the temperature exceeds about half the melting point, and that near the melting point the anharmonic shift in calculated value vs experimental value is typically 25%. Consequently, the application and

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extrapolation of the Debye model is sufficiently supported to utilize in explaining the diffusion

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behavior during the co-deposition of these elements.

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The Debye temperatures given in Table 6 imply that gadolinium should diffuse more rapidly in

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transition metals than yttrium. While the data support this, there is a confounding factor to

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consider. The specific energy had been increased (refer table 2, SE for layer 3 is 114.29 J/mm2) compared to our previous work (72.89 J/mm2 under CO2 and 67.51 J/mm2 under diode laser).

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The data are thus inadequate to compare the rates of diffusion of the two.

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The Debye temperatures in Table 6 also suggest that there should be little difference in Y

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diffusion into Stellite (Co) versus Inconel (Ni). As the depletion of Y in Stellite was more pronounced than was seen in Inconel [1], the difference is more reasonably attributed to differences in specific energies than to differences in base metal. Yttrium depletion is more pronounced when deposition is done with the CO2 laser than with the diode laser in both Inconel 625 [59] and Stellite 21 matrices. However, gadolinium depletion was more prominent in diode produced specimens than in those produced under a CO2 laser.

29

Journal Pre-proof The elemental depletion behavior for the previous [1] and current study are summarized in Table 7. There may be both materials and processing (laser source, specific energy) factors influencing the extent of segregation taking place during LDED. Table 7: Summary of elemental depletion during LDED of metal-ceramic pairs

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Stellite 21 - YSZ Y diffusion greater than in [1] due to greater specific energy. Zr depletion from YSZ (not observed in [1]), due to greater specific energy. The depletion of Y from YSZ is more pronounced than when deposited with diode.

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-p Gd depletion from GZO is more pronounced than when deposited with CO2. Zr depletion from GZO due to greater specific energy.

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The depletion of Y from YSZ is less pronounced than when deposited with CO2. Reduced depletion attributed to YSZ having lower absorptivity at diode laser wavelength.

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diode

Stellite 21 – GZO Gd depletion from GZO is less pronounced than when deposited with a diode. Zr depletion from GZO due to greater specific energy.

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Laser/Materials Inconel 625 – YSZ [1] CO2 The depletion of Y from YSZ is more pronounced than when deposited with diode. Greater depletion attributed to YSZ having higher absorptivity at CO2 laser wavelength.

Y diffusion greater than in [1] due to greater specific energy. Zr depletion from YSZ (not observed in [1]), due to greater specific energy. The depletion of Y from YSZ is less pronounced than when deposited with CO2.

Additional studies are underway directed to gain further insight into the driving factors of segregation behaviors during LDED of these material combinations. Control of these factors during processing will be requisite to successful building of functionally graded TBCs using LDED and other AM technologies. Conclusions

30

Journal Pre-proof 1. A 2 mm thick, functionally graded thermal barrier coatings composed of Stellite 21/yttria-stabilized zirconia (YSZ) and Stellite 21/gadolinium zirconate (GZO) was deposited via laser direct energy deposition on a steel substrate. During the deposition of any cermet layer comprising 10 wt% or more of ceramic, a coating of that ceramic formed on the surface. This occurred for all material and laser combinations, and confirms what was observed in our previous study [1] on Inconel 625-YSZ FG-TBCs.

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This ceramic layer inhibited the deposition of additional layers with higher ceramic wt.%

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and ultimately limited FG-TBC thickness.

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2. The depletion of stabilizing dopants from ceramic components into transition metals

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during LDED occurred for all combinations tested. As the resulting ceramic particles are in phases known to be sub-optimal for use in TBCs, this poses a significant challenge to

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the building of FG-TBCs using these materials in this process.

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3. Application of the Debye model suggests that the diffusion of elements common to the ceramics used in TBCs (YSZ and GZO) will occur readily in the transition metal alloys

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(Inconcel and Stellite) commonly used in high temperature components. While such

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pairings seem obvious for building an FG-TBC, this behavior represents a significant challenge to building such coatings using LDED. 4. The magnitude of the elemental segregation during LDED of the tested materials combinations is impacted by both materials and laser inputs. A greater understanding of the interplay of these factors is requisite to enable the AM of FG-TBCs. Acknowledgements This technical effort was performed in support of the National Energy Technology Laboratory’s ongoing gasification research, in the Advance Reaction Systems Program. This research was also 31

Journal Pre-proof supported in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education. The authors would like to thank Keith Collins of NETL (SEM, microbe analysis) and Peter Eschbach of Oregon State University (TEM, STEM, diffraction and EDX analysis) for their invaluable contributions. The authors would also like to thank Arshad Harooni (formerly with DM3D Technology) for his assistance with the deposition

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work and for his critical review of this manuscript, and to James Bennett for insightful

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discussions.

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DISCLAIMER

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This work was funded by the U.S. Department of Energy, National Energy Technology Laboratory, an agency of the United States Government. Neither the United States Government

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nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied,

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or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not

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infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Journal Pre-proof Declaration of competing interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Journal Pre-proof Author Contribution Harish Rao: Conceptualization, Methodology, Investigation, Writing ; Indumini Jayasekara: Conceptualization, Methodology, Investigation, Writing; Bhaskar Dutta: Conceptualization, Investigation, Methodology, Resources; David Maurice: Conceptualization, Methodology,

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Supervision, Writing

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Journal Pre-proof Highlights

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Functionally graded St21/YSZ/GZO thick TBCs deposited via LDED Dopant segregates from YSZ & GZO into St21 matrix during LDED Segregation is influenced by laser specific energy and laser source Gd depletion from GZO higher in diode deposited TBCs compared to CO2 deposited TBCs Y depletion from YSZ higher in CO2 deposited TBCs compared to diode deposited TBCs

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