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Oxidation-induced healing in laser-processed thermal barrier coatings
Thin Solid Films 688 (2019) 137481
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Oxidation-induced healing in laser-processed thermal barrier coatings J.J. Gu, S.S. Joshi, Y.-S. Ho, B.W. Wei, T.Y. Huang, J. Lee, D. Berman, N.B. Dahotre, S.M. Aouadi
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T
Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal barrier coatings Laser processing YSZ Self-healing
The use of columnar structures in thermal barrier coatings (TBCs) enables producing materials with lower thermal conductivities that are also able to accommodate thermal expansion during temperature cycling events. However, open porosity leads to the failure of TBCs because of the rapid oxidation of the coating-substrate interface. Here we propose to create an oxidation-induced sealing process to mitigate any oxidation or potential chemical attack. This is achieved by pre-mixing the top surface with a carbide phase (TiC or SiC) that oxidizes in contact with air to seal the surface through volume expansion. Laser processing was used as a technique to fabricate YSZ-Al2O3-TiC or YSZ-Al2O3-SiC layers on 316 stainless steel substrates to understand the viability of this process as well as this coating design in creating self-healing thermal barrier coatings. Nanocomposite selfhealing coatings and single-phase yttria-stabilized zirconia (YSZ) reference layers were produced using a laser fluence of 17.0 J/mm2 (800 W), 19.1 J/mm2 (900 W), and 25.5 J/mm2 (1200 W) to optimize process conditions. These coatings were subsequently annealed at 720 °C for 12 h to ensure complete oxidation of the TiC(SiC) phase in the former coating. X-ray diffraction and cross-sectional energy dispersive x-ray spectroscopy (EDS) elemental mapping confirmed the creation of the desired columnar structure to accommodate thermal stresses and that a post-annealing treatment was required to achieve complete oxidation of the TiC(SiC) phase. The migration of TiO2 phase to the crack site was demonstrated using cross-sectional scanning electron microscopy (SEM) in tandem with elemental mapping via energy dispersive spectroscopy. The suggested optimum laser fluence to create a viable self-healing composite was found to be 17.0 J/mm2 since higher values resulted in significant interdiffusion between the substrate and the coating as indicated by cross-sectional SEM/EDS. Laser processing was demonstrated to be a viable technique that has the potential to create a self-healing layer that seals YSZbased underlayers from diffusion.
1. Introduction Thermal barrier coatings (TBCs) are ceramic-based materials that provide thermal protection to alloy substrates in aerospace and landbased gas-turbine engines [1–4]. TBCs limit heat access to the turbine blades thus enabling engine operation at higher gas inlet temperatures and enhancing their efficiency and lifetime [5]. Yttria stabilized zirconia (YSZ) is traditionally used as a TBC top-coat layer [3,4,6]. To decrease the thermal conductivity and to accommodate thermal expansion during thermal fluctuations, YSZ top-coats are grown as porous structures by electron beam physical vapor deposition (EB-PVD) or by air plasma spray (APS) [7,8]. In the EB-PVD process, a typical YSZ coating consists of quasi-crystalline columns with an average diameter of 2–3 μm at the substrate surface that increases to 10–20 μm at the coating tip. Inter-columnar gaps are of a few nanometers wide close to the substrate and can be as large as 1 μm in width at the coating tip [8]. Top-coats fabricated by APS contain lamellar stacking microstructures,
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splat cracks, and porosity although not columnar in their morphology [9]. Though the introduction of porosity is beneficial in reducing the thermal conductivity of the coatings, it provides a pathway for the diffusion of oxygen and other chemical species through the top-coat layer leading to oxidation and corrosion of the underlying materials (bond coats and/or substrates) [10–12]. Additionally, surface porosity accelerates the diffusion of molten calcium–magnesium-alumino-silicates (CMAS) from mixtures of dust, sand, and ash. CMAS is known to react with YSZ leading to the destabilization of the whole structure [13–17]. A possible solution to prevent potential oxidation and/or chemical attack is to isolate the porous structure by creating a dense overlayer. This may be achieved by introducing an impermeable or a sacrificial layer on the surface of the material [18–20]. A sacrificial layer enables the elimination of absorbed masses through slow but steady material delamination. For this, the top layer of the YSZ coating is doped with rare earth elements, such as CeO2, Sc2O3, In2O3, Ta2O5 and Re2O3, to
Corresponding author. E-mail address: [email protected] (S.M. Aouadi).
https://doi.org/10.1016/j.tsf.2019.137481 Received 20 June 2019; Received in revised form 25 July 2019; Accepted 2 August 2019 Available online 03 August 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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create thermodynamically stable nanoscale ordered phases or highly defective localized lattice structures that hinder the mobility of atoms, ions, and phonons [15]. Meanwhile, impermeable layers remain intact and demonstrate low wetting characteristics achieved through minimization of their surface energy. Previous studies demonstrated improved sealing of the columnar structures using Pd, SiO2, Ta2O5, CaZrO3, MgAlO4, and SiOC [18]. Unfortunately, the addition of dense ceramic structures often leads to the formation of defects such as cracks, which still remain as vulnerable places for the diffusion of chemical species leading to TBC failure [21]. Here we propose a new concept to limit the diffusion of oxygen or other chemical species through the porosity of YSZ-based thermal barrier coatings by creating a self-healing dense protective overlayer. The concept of self-healing used in the proposed design involves repair of the preformed cracks via phase transformation during the annealing process [22,23]. Laser glazing was used to produce a two-layer structure that consists of a YSZ underlayer and a self-healing impermeable overlayer [24]. Laser processing has been reported in the literature as a suitable technique for synthesizing ceramic coatings on metallic substrates [25,26]. It provides distinct advantages such as speed, reproducibility, a controllable processing-structure relationship, and a strong coating-substrate bond [24]. We precisely tuned the laser processing recipe to achieve excellent adhesion to the substrate while maintaining a good morphology. To aid the stability and impermeability of the designed ceramic structures, we further eliminated any pre-formed cracks or process-inherent porosity in the top layer by carefully designing the top layer whose self-healing capability is activated by a post-processing thermal treatment step. To achieve this, TiC or SiC phases were intentionally embedded in the top layer of the YSZ matrix as healing agents capable of undergoing an oxidative reaction resulting in filling porosity that is inherent to the process [27,28]. Upon annealing, it was hoped that the initially uniformly distributed SiC or TiC powders will migrate to grain boundaries and convert into oxide structures while preserving the columnar structure of the underlayer. In addition, Al2O3 was added to create a three-phase composite since it will help decrease the mobility of oxygen in the TBC, which is hoped to significantly decrease the oxidation of the bond coat. Al2O3 is also known to have good high temperature corrosion resistance and good mechanical properties. The protective self-healing overlayer is hypothesized to have a very minimal effect on the thermal properties of the overall TBC coating because (1) it will be produced as a much thinner overlayer compared to the YSZ underlayer and (2) it is made up of oxides that have low thermal conductivities.
Table 2 Laser parameters. Power (W)
Beam diameter (mm)
Scanning speed (mm/ s)
Laser fluence (J/ mm2)
800 900 1200
0.6 0.6 0.6
100 100 100
17.0 19.1 25.5
The powder was laid down on the substrate in a non-compact form. Laser processing was carried out by melting the pre-coated powder using a continuous wave Nd:YAG laser with a wavelength of 1064 nm. The laser beam diameter and scanning speed were both kept constant at 0.6 mm and 100 mm/s, respectively. Tuning of the process parameters was performed by varying the laser power in the range from 800 W to 1200 W. The laser parameters employed in the current work along with the laser fluence generated on the sample surface are shown in Table 2. All the laser processing experiments were performed using Ar as cover gas. 2.2. Design of 2-layer coatings After tuning of the laser process, this technique was used to produce two-layer systems. The bottom layer consisted of a relatively thick single phase YSZ layer while the top layer included alumina and TiC or SiC powders (1 and 2 or 1 and 3). The thickness of the YSZ underlayer was in the 50 to 100 μm range. 2.3. Material characterization
2. Experimental details
After laser processing, samples were cut into 20 mm × 20 mm squares using a slow speed oil-cooled diamond wafer saw. Selected square samples were annealed at 720 °C for 10 h. The cross-sectional surface of each sample was polished. Ion milling (Gatan 682, Pleasanton, CA) was used to etch the cross-sectional surface of the samples to facilitate the investigation of the crystal structure. X-ray diffractometry (XRD – Rigaku Ultima III), with a Cu Kα radiation source and a scanning rate of 3°/min, was used to explore phase and composition evolution of the self-healing layer post-deposition and post-annealing. Finally, an FEI Nova 200 Nanolab dual-beam focused ion beam (FIB)/scanning electron microscope (FESEM) with Pt GIS, Omniprobe Nanomanipulator, EDS and electron backscatter diffraction operated at 30 keV was used to observe the morphology of the samples and to create elemental maps for selected regions of the coating (top as well as cross-sectional views).
2.1. Optimization of the laser process
3. Results and discussion
YSZ, Al2O3, and TiC powders (purity of 99.99%) having average particle sizes of 20 nm, 50 nm, and 40 nm, respectively, were purchased from SkySpring Nanomaterials, Inc. (Houston, Texas). Three sets of samples with compositions highlighted in Table 1 were produced. These compositions were based on the findings by Ouyang et al. [27,28] who optimized the composition of similar self-healing coatings produced by APS. The powders were mechanically mixed in a ball milling apparatus and were subsequently deposited on 40 mm × 40 mm × 6 mm 316 stainless steel (SS) substrates.
3.1. Tuning of the laser processing recipe Before the incorporation of the phases that produce oxidation-induced healing, we evaluated the efficiency of laser processing for the design of YSZ coatings. Fig. 1 displays topview SEM micrographs of (a) a laser-processed YSZ coating and (b) an annealed YSZ coating, as well as XRD spectra before and after laser processing as well as post-annealing. The SEM micrographs revealed that during growth, YSZ developed cellular columns with an average diameter of 10 ± 2 μm. No major changes were observed as a result of annealing. The XRD spectra suggest that the pre-coated YSZ sample consisted primarily of cubic ZrO2 (c-ZrO2 - PDF Card No.99–000-0162) with very small content of monoclinic ZrO2 (PDF Card No.98–000-0105), as evidenced by weak contributions at 28.19° and 32.75°. The desired c-ZrO2 phase remained as the only one present in the coatings post laser processing. The XRD spectra did not change substantially upon annealing at 720 °C for 10 h. The c-ZrO2 peaks became sharper after laser processing (peaks with full
Table 1 Composition of the TBC coatings. Composition (wt%)
YSZ
Al2O3
TiC
SiC
#1 #2 #3
100 56 70
0 38 10
0 0 20
0 6 0
2
Thin Solid Films 688 (2019) 137481
J.J. Gu, et al.
Fig. 1. Top-view SEM figures of: (a) laser-processed YSZ coating, (b) annealed YSZ coating. (c) XRD spectra of as-sprayed YSZ coating, laser-processed YSZ coating, and annealed YSZ coating.
investigated the nature of the adaptive reactions of TiC in aluminabased composites via indentation-induced crack-healing studies. The authors concluded that healing occurred by virtue of the formation of the rutile phase. Crack filling in ceramics requires that the volume of the reaction product, oxidation in the present case, is larger than that of the starting material, preferably by > 50% [28]. The crystallographic volume of the newly formed TiO2 phase is 54% larger than that of the original TiC phase, which is a desired attribute for crack filling [29]. In addition, this system satisfies another important requirement in terms of achieving strength recovery since the reported cohesion strength between the constituent phases is very large [28]. A comparative analysis of XRD spectra for samples deposited with different laser powers before (Fig. 2g) and after (Fig. 2h) annealing further confirmed our conclusions. Upon laser processing (Fig. 2g), the XRD spectra revealed the partial oxidation of the TiC phase since peaks corresponding to both the TiC and TiO2 phases were observed. In addition, the relative strength of the peak at 43° increased dramatically for samples processed using larger laser fluence (power) values as revealed by Fig. 2g. The peak at 43° corresponds to an overlap of contributions from Al2O3 and Fe. Our hypothesis is that the observed increase in peak intensity with an increase in laser fluence (power) was due to the more significant interdiffusion between the substrate and the coating, which was promoted by the heat generated by the laser beam. It has been reported that the phenomenon of dilution occurs during laser processing [24]. Dilution causes melting of a portion of the substrate material near the
width at half-maximum values of 0.56 ± 0.02° and 0.22 ± 0.02° before and after laser processing, respectively - average values from all peaks) suggesting the occurrence of grain growth (sintering of YSZ grains) as a result of the heating process. Next, we employed the laser process to deposit YSZ-TiC-Al2O3 selfhealing composites (Fig. 2). Shown in Fig. 2 are topview micrographs for samples synthesized with different laser powers suggesting that YSZ-Al2O3-TiC coatings developed cellular columns similar to those observed for YSZ (Fig. 1). The features observed in Fig. 2 are in the 5 to 10 μm range that contain smaller features (< 1 μm). Grain refinement was expected for an immiscible three-phase system compared to a single-phase system [29]. The morphology of the laser processed coatings produced using a laser fluence of 19.1 J/mm2 (900 W) and laser fluence of 25.5 J/mm2 (1200 W) was similar to that of samples created using a fluence of 17.0 J/mm2 (800 W), as shown in Fig. 2(c) and 2(e), correspondingly. However, upon annealing, the porous structure was no longer observed due to the outward diffusion of newly formed oxides that seal the surface, as shown in Figs. 2(d) and 2(f). These results suggest that the use of a higher laser fluence (power) resulted in a more compact structure and fusion or coalescence of the columns being covered by the formed oxide phase migrating to the surface. The porosity, revealed by cross-sectional image software analysis, was found to be ~ 10.5%, 9.4%, and 7.2% for samples produced with a laser fluence of 17.0 J/ mm2, 19.1 J/mm2, and 25.5 J/mm2, respectively. Yoshioka et al. [28] 3
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Fig. 2. Top-view SEM figures of laser-processed YSZ-Al2O3-TiC coatings produced using a processing power of 800 W (a and b), 900 W (c and d), and 1200 W (e and f) before and after annealing. XRD curves of (g) laser-processed YSZ-Al2O3-TiC coating, and (h) annealed YSZ-Al2O3-TiC coating.
coatings and that the oxidation/healing temperature will increase for larger grain sizes due to slower kinetics.
interface between the precursor layer and the substrate. Dilution is essential to form a metallurgical bond between the coating and substrate. Moreover, dilution also leads to the infiltration of substrate elements into the coating which is critical during the synthesis of ceramic coatings on metallic substrates. However, excessive dilution is undesirable to avoid degradation of coating properties. The TiC peak completely disappeared once the laser processed coatings were annealed at 720 °C for 10 h (Fig. 2h). The TiO2 peak was the strongest for the sample produced at a low laser fluence of 17.0 J/mm2 (800 W), which seems to be counter-intuitive. This may be a result of the formation of larger crystallites of TiO2 compared to the samples that were laser processed using higher laser powers. The cross-section of the laser-processed composite samples was evaluated before and after annealing for samples produced using a laser fluence of 17.0 J/mm2 (Fig. 3). This figure also includes elemental maps to investigate the potential of healing of the as-processed porous structure. These maps revealed that all of the elements were uniformly distributed throughout the coating post laser processing. Upon annealing, however, the elemental spectra suggested that the Al2O3 and TiO2 phases migrated to the inter-columnar areas. These observations suggest that annealing was a necessary step to completely seal the pores in the laser-processed structures. It is hypothesized that the annealing step will be required for any initial powder sizes used to create these
3.2. Design of 2-layer coatings We employed the tuned recipe for the design and production of a 2layer system. Specifically, our coatings were created by depositing a YSZ layer followed by a self-healing layer. We extended our study to investigate the universality of the process by creating a YSZ-Al2O3-SiC overlayer. SiC and TiC are the two phases that are used in oxidationbased self-healing materials [30]. Fig. 4 provides cross-sectional micrographs of selected as-produced coatings before and after annealing. The cross-sectional micrographs revealed that annealing of the YSZ samples leads to grain growth followed by the disappearance of the columnar structures. Meanwhile, the presence of TiC-Al2O3 and SiCAl2O3 additives in YSZ helped limit grain growth and prevented densification of the YSZ. Notably, the healing occurs in two-steps. During the laser glazing process, the added carbide and oxide phases form a composite that inhibit grain growth and form a more refined columnar structure. Flash temperatures during this process are estimated to be in the 1500 to 1600 °C range [23]. Post-processing annealing, which is conducted at lower temperatures (650 °C for SiC and 720 °C for TiC) was found to be 4
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Fig. 2. (continued)
adhesion between this phase and the coating matrix [31]. These two systems offer contrast and flexibility in terms of the design of selfhealing overlayers for TBCs. Their effectiveness as TBCs will be investigated in a follow-up study.
a necessary step to completely oxidize the carbide phases and form TiO2 or SiO2 to heal cracks in the coatings. A relatively long annealing time was perhaps required to allow these phases to diffuse to the porous areas. The energy dispersive x-ray spectroscopy (EDS) maps of the cross-sections of the coatings indicate segregation of the healing phases throughout the films to provide better structural stability of the grain boundaries (Fig. 5). For the three-phase composite system studied here, the healing agent (the carbide) formed an amorphous oxide (SiO2 and TiO2) during the early stages of the annealing process. The amorphous nature of this phase generates a viscous behavior leading to liquid-like spreading of the reaction product into cracks that are in its vicinity. Notably, SiO2 seems to be more viscous as it was able to diffuse into the porous regions of the YSZ underlayer (Fig. 5(b)). In contrast, TiO2 remained in the self-healing overlayer and did not flow to the YSZ single phase region as shown in Fig. 5(a). The contrast between both systems is attributed to the lower viscosity of the SiO2 phase and the work of
4. Conclusions YSZ-Al2O3-TiC(SiC) and YSZ reference layers were produced on SS316 substrates using laser cladding to understand the viability of this technique to create a self-healing overcoat on TBCs. The laser fluence was varied between 17.0 and 25.0 J/mm2. A fluence of 17.0 J/mm2 was found to minimize the interdiffusion between the coating and the substrate and was selected for further studies. Laser processing alone was found to partially oxidize the TiC(SiC) and the kinetics associated with the process was not sufficient to seal/heal the self-healing layer. Annealing of the composite coating at 720 °C for 12 h was found to achieve complete oxidation of the TiC phase and the subsequent 5
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Fig. 3. Cross-sectional elemental mapping micrographs of: (top) laser-processed YSZ-Al2O3-TiC coating, and (bottom) annealed YSZ-Al2O3-TiC coating. Processing Power = 800 W.
Fig. 4. Top-view SEM figures of: (a) laser-processed YSZ coating, (b) laser-processed YSZ-Al2O3-TiC coating, (c) laser-processed YSZ-Al2O3-SiC coating. Crosssectional SEM figures of: (d) laser-processed YSZ coating, (g) annealed YSZ coating, (e) laser-processed YSZ-Al2O3-TiC coating, (h) annealed YSZ-Al2O3-TiC coating, (f) laser-processed YSZ-Al2O3-SiC coating, (i) annealed YSZ-Al2O3-SiC coating. 6
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Fig. 5. Cross-sectional elemental mapping figures of (a) laser-processed YSZ-Al2O3-TiC coating and (b) laser-processed YSZ-Al2O3-SiC coating.
migration of the newly formed TiO2 phase to the crack site to form a strong bond to the YSZ matrix. A two-layer coating with a thermally insulating YSZ underlayer and a dense self-healing layer was successfully produced. Future studies will include investigating other potentially efficient self-healing materials at elevated temperatures (Ti, Cr, Zr, Nb, Hf, TiC, TiN, Cr3C2, Cr2N, ZrN, NbC, and NbN) [31] and conducting systematic studies on the response of these newly developed coatings to thermal cycling and CMAS attack.
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Acknowledgments The authors acknowledge funding from the National Science Foundation, USA (NSF REU program – award 1461048). This work was performed in part at the University of North Texas's Materials Research Facility. The authors would also like to acknowledge discussions with Drs. Anindya Ghoshal and Michael Warlock of the Army Research Laboratory that pertain to thermal barrier coatings. This work was performed in part at the University of North Texas's Materials Research Facility: A shared research facility for multi-dimensional fabrication and characterization. References [1] A.F. Renteria, B. Saruhan, U. Schulz, H.-J. Raetzer-Scheibe, J. Haug, A. Wiedenmann, Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs, Surf. Coat. Technol. 201 (2006) 2611–2620. [2] M. Stiger, N. Yanar, M. Topping, F. Pettit, G. Meier, Thermal barrier coatings for the 21st century, Z. Metallk. 90 (1999) 1069–1078. [3] D.R. Clarke, S.R. Phillpot, Thermal barrier coating materials, Mater. Today 8 (2005) 22–29. [4] X. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur.
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Oxidation-induced healing in laser-processed thermal barrier coatings J.J. Gu, S.S. Joshi, Y.-S. Ho, B.W. Wei, T.Y. Huang, J. Lee, D. Berman, N.B. Dahotre, S.M. Aouadi
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T
Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal barrier coatings Laser processing YSZ Self-healing
The use of columnar structures in thermal barrier coatings (TBCs) enables producing materials with lower thermal conductivities that are also able to accommodate thermal expansion during temperature cycling events. However, open porosity leads to the failure of TBCs because of the rapid oxidation of the coating-substrate interface. Here we propose to create an oxidation-induced sealing process to mitigate any oxidation or potential chemical attack. This is achieved by pre-mixing the top surface with a carbide phase (TiC or SiC) that oxidizes in contact with air to seal the surface through volume expansion. Laser processing was used as a technique to fabricate YSZ-Al2O3-TiC or YSZ-Al2O3-SiC layers on 316 stainless steel substrates to understand the viability of this process as well as this coating design in creating self-healing thermal barrier coatings. Nanocomposite selfhealing coatings and single-phase yttria-stabilized zirconia (YSZ) reference layers were produced using a laser fluence of 17.0 J/mm2 (800 W), 19.1 J/mm2 (900 W), and 25.5 J/mm2 (1200 W) to optimize process conditions. These coatings were subsequently annealed at 720 °C for 12 h to ensure complete oxidation of the TiC(SiC) phase in the former coating. X-ray diffraction and cross-sectional energy dispersive x-ray spectroscopy (EDS) elemental mapping confirmed the creation of the desired columnar structure to accommodate thermal stresses and that a post-annealing treatment was required to achieve complete oxidation of the TiC(SiC) phase. The migration of TiO2 phase to the crack site was demonstrated using cross-sectional scanning electron microscopy (SEM) in tandem with elemental mapping via energy dispersive spectroscopy. The suggested optimum laser fluence to create a viable self-healing composite was found to be 17.0 J/mm2 since higher values resulted in significant interdiffusion between the substrate and the coating as indicated by cross-sectional SEM/EDS. Laser processing was demonstrated to be a viable technique that has the potential to create a self-healing layer that seals YSZbased underlayers from diffusion.
1. Introduction Thermal barrier coatings (TBCs) are ceramic-based materials that provide thermal protection to alloy substrates in aerospace and landbased gas-turbine engines [1–4]. TBCs limit heat access to the turbine blades thus enabling engine operation at higher gas inlet temperatures and enhancing their efficiency and lifetime [5]. Yttria stabilized zirconia (YSZ) is traditionally used as a TBC top-coat layer [3,4,6]. To decrease the thermal conductivity and to accommodate thermal expansion during thermal fluctuations, YSZ top-coats are grown as porous structures by electron beam physical vapor deposition (EB-PVD) or by air plasma spray (APS) [7,8]. In the EB-PVD process, a typical YSZ coating consists of quasi-crystalline columns with an average diameter of 2–3 μm at the substrate surface that increases to 10–20 μm at the coating tip. Inter-columnar gaps are of a few nanometers wide close to the substrate and can be as large as 1 μm in width at the coating tip [8]. Top-coats fabricated by APS contain lamellar stacking microstructures,
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splat cracks, and porosity although not columnar in their morphology [9]. Though the introduction of porosity is beneficial in reducing the thermal conductivity of the coatings, it provides a pathway for the diffusion of oxygen and other chemical species through the top-coat layer leading to oxidation and corrosion of the underlying materials (bond coats and/or substrates) [10–12]. Additionally, surface porosity accelerates the diffusion of molten calcium–magnesium-alumino-silicates (CMAS) from mixtures of dust, sand, and ash. CMAS is known to react with YSZ leading to the destabilization of the whole structure [13–17]. A possible solution to prevent potential oxidation and/or chemical attack is to isolate the porous structure by creating a dense overlayer. This may be achieved by introducing an impermeable or a sacrificial layer on the surface of the material [18–20]. A sacrificial layer enables the elimination of absorbed masses through slow but steady material delamination. For this, the top layer of the YSZ coating is doped with rare earth elements, such as CeO2, Sc2O3, In2O3, Ta2O5 and Re2O3, to
Corresponding author. E-mail address: [email protected] (S.M. Aouadi).
https://doi.org/10.1016/j.tsf.2019.137481 Received 20 June 2019; Received in revised form 25 July 2019; Accepted 2 August 2019 Available online 03 August 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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create thermodynamically stable nanoscale ordered phases or highly defective localized lattice structures that hinder the mobility of atoms, ions, and phonons [15]. Meanwhile, impermeable layers remain intact and demonstrate low wetting characteristics achieved through minimization of their surface energy. Previous studies demonstrated improved sealing of the columnar structures using Pd, SiO2, Ta2O5, CaZrO3, MgAlO4, and SiOC [18]. Unfortunately, the addition of dense ceramic structures often leads to the formation of defects such as cracks, which still remain as vulnerable places for the diffusion of chemical species leading to TBC failure [21]. Here we propose a new concept to limit the diffusion of oxygen or other chemical species through the porosity of YSZ-based thermal barrier coatings by creating a self-healing dense protective overlayer. The concept of self-healing used in the proposed design involves repair of the preformed cracks via phase transformation during the annealing process [22,23]. Laser glazing was used to produce a two-layer structure that consists of a YSZ underlayer and a self-healing impermeable overlayer [24]. Laser processing has been reported in the literature as a suitable technique for synthesizing ceramic coatings on metallic substrates [25,26]. It provides distinct advantages such as speed, reproducibility, a controllable processing-structure relationship, and a strong coating-substrate bond [24]. We precisely tuned the laser processing recipe to achieve excellent adhesion to the substrate while maintaining a good morphology. To aid the stability and impermeability of the designed ceramic structures, we further eliminated any pre-formed cracks or process-inherent porosity in the top layer by carefully designing the top layer whose self-healing capability is activated by a post-processing thermal treatment step. To achieve this, TiC or SiC phases were intentionally embedded in the top layer of the YSZ matrix as healing agents capable of undergoing an oxidative reaction resulting in filling porosity that is inherent to the process [27,28]. Upon annealing, it was hoped that the initially uniformly distributed SiC or TiC powders will migrate to grain boundaries and convert into oxide structures while preserving the columnar structure of the underlayer. In addition, Al2O3 was added to create a three-phase composite since it will help decrease the mobility of oxygen in the TBC, which is hoped to significantly decrease the oxidation of the bond coat. Al2O3 is also known to have good high temperature corrosion resistance and good mechanical properties. The protective self-healing overlayer is hypothesized to have a very minimal effect on the thermal properties of the overall TBC coating because (1) it will be produced as a much thinner overlayer compared to the YSZ underlayer and (2) it is made up of oxides that have low thermal conductivities.
Table 2 Laser parameters. Power (W)
Beam diameter (mm)
Scanning speed (mm/ s)
Laser fluence (J/ mm2)
800 900 1200
0.6 0.6 0.6
100 100 100
17.0 19.1 25.5
The powder was laid down on the substrate in a non-compact form. Laser processing was carried out by melting the pre-coated powder using a continuous wave Nd:YAG laser with a wavelength of 1064 nm. The laser beam diameter and scanning speed were both kept constant at 0.6 mm and 100 mm/s, respectively. Tuning of the process parameters was performed by varying the laser power in the range from 800 W to 1200 W. The laser parameters employed in the current work along with the laser fluence generated on the sample surface are shown in Table 2. All the laser processing experiments were performed using Ar as cover gas. 2.2. Design of 2-layer coatings After tuning of the laser process, this technique was used to produce two-layer systems. The bottom layer consisted of a relatively thick single phase YSZ layer while the top layer included alumina and TiC or SiC powders (1 and 2 or 1 and 3). The thickness of the YSZ underlayer was in the 50 to 100 μm range. 2.3. Material characterization
2. Experimental details
After laser processing, samples were cut into 20 mm × 20 mm squares using a slow speed oil-cooled diamond wafer saw. Selected square samples were annealed at 720 °C for 10 h. The cross-sectional surface of each sample was polished. Ion milling (Gatan 682, Pleasanton, CA) was used to etch the cross-sectional surface of the samples to facilitate the investigation of the crystal structure. X-ray diffractometry (XRD – Rigaku Ultima III), with a Cu Kα radiation source and a scanning rate of 3°/min, was used to explore phase and composition evolution of the self-healing layer post-deposition and post-annealing. Finally, an FEI Nova 200 Nanolab dual-beam focused ion beam (FIB)/scanning electron microscope (FESEM) with Pt GIS, Omniprobe Nanomanipulator, EDS and electron backscatter diffraction operated at 30 keV was used to observe the morphology of the samples and to create elemental maps for selected regions of the coating (top as well as cross-sectional views).
2.1. Optimization of the laser process
3. Results and discussion
YSZ, Al2O3, and TiC powders (purity of 99.99%) having average particle sizes of 20 nm, 50 nm, and 40 nm, respectively, were purchased from SkySpring Nanomaterials, Inc. (Houston, Texas). Three sets of samples with compositions highlighted in Table 1 were produced. These compositions were based on the findings by Ouyang et al. [27,28] who optimized the composition of similar self-healing coatings produced by APS. The powders were mechanically mixed in a ball milling apparatus and were subsequently deposited on 40 mm × 40 mm × 6 mm 316 stainless steel (SS) substrates.
3.1. Tuning of the laser processing recipe Before the incorporation of the phases that produce oxidation-induced healing, we evaluated the efficiency of laser processing for the design of YSZ coatings. Fig. 1 displays topview SEM micrographs of (a) a laser-processed YSZ coating and (b) an annealed YSZ coating, as well as XRD spectra before and after laser processing as well as post-annealing. The SEM micrographs revealed that during growth, YSZ developed cellular columns with an average diameter of 10 ± 2 μm. No major changes were observed as a result of annealing. The XRD spectra suggest that the pre-coated YSZ sample consisted primarily of cubic ZrO2 (c-ZrO2 - PDF Card No.99–000-0162) with very small content of monoclinic ZrO2 (PDF Card No.98–000-0105), as evidenced by weak contributions at 28.19° and 32.75°. The desired c-ZrO2 phase remained as the only one present in the coatings post laser processing. The XRD spectra did not change substantially upon annealing at 720 °C for 10 h. The c-ZrO2 peaks became sharper after laser processing (peaks with full
Table 1 Composition of the TBC coatings. Composition (wt%)
YSZ
Al2O3
TiC
SiC
#1 #2 #3
100 56 70
0 38 10
0 0 20
0 6 0
2
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Fig. 1. Top-view SEM figures of: (a) laser-processed YSZ coating, (b) annealed YSZ coating. (c) XRD spectra of as-sprayed YSZ coating, laser-processed YSZ coating, and annealed YSZ coating.
investigated the nature of the adaptive reactions of TiC in aluminabased composites via indentation-induced crack-healing studies. The authors concluded that healing occurred by virtue of the formation of the rutile phase. Crack filling in ceramics requires that the volume of the reaction product, oxidation in the present case, is larger than that of the starting material, preferably by > 50% [28]. The crystallographic volume of the newly formed TiO2 phase is 54% larger than that of the original TiC phase, which is a desired attribute for crack filling [29]. In addition, this system satisfies another important requirement in terms of achieving strength recovery since the reported cohesion strength between the constituent phases is very large [28]. A comparative analysis of XRD spectra for samples deposited with different laser powers before (Fig. 2g) and after (Fig. 2h) annealing further confirmed our conclusions. Upon laser processing (Fig. 2g), the XRD spectra revealed the partial oxidation of the TiC phase since peaks corresponding to both the TiC and TiO2 phases were observed. In addition, the relative strength of the peak at 43° increased dramatically for samples processed using larger laser fluence (power) values as revealed by Fig. 2g. The peak at 43° corresponds to an overlap of contributions from Al2O3 and Fe. Our hypothesis is that the observed increase in peak intensity with an increase in laser fluence (power) was due to the more significant interdiffusion between the substrate and the coating, which was promoted by the heat generated by the laser beam. It has been reported that the phenomenon of dilution occurs during laser processing [24]. Dilution causes melting of a portion of the substrate material near the
width at half-maximum values of 0.56 ± 0.02° and 0.22 ± 0.02° before and after laser processing, respectively - average values from all peaks) suggesting the occurrence of grain growth (sintering of YSZ grains) as a result of the heating process. Next, we employed the laser process to deposit YSZ-TiC-Al2O3 selfhealing composites (Fig. 2). Shown in Fig. 2 are topview micrographs for samples synthesized with different laser powers suggesting that YSZ-Al2O3-TiC coatings developed cellular columns similar to those observed for YSZ (Fig. 1). The features observed in Fig. 2 are in the 5 to 10 μm range that contain smaller features (< 1 μm). Grain refinement was expected for an immiscible three-phase system compared to a single-phase system [29]. The morphology of the laser processed coatings produced using a laser fluence of 19.1 J/mm2 (900 W) and laser fluence of 25.5 J/mm2 (1200 W) was similar to that of samples created using a fluence of 17.0 J/mm2 (800 W), as shown in Fig. 2(c) and 2(e), correspondingly. However, upon annealing, the porous structure was no longer observed due to the outward diffusion of newly formed oxides that seal the surface, as shown in Figs. 2(d) and 2(f). These results suggest that the use of a higher laser fluence (power) resulted in a more compact structure and fusion or coalescence of the columns being covered by the formed oxide phase migrating to the surface. The porosity, revealed by cross-sectional image software analysis, was found to be ~ 10.5%, 9.4%, and 7.2% for samples produced with a laser fluence of 17.0 J/ mm2, 19.1 J/mm2, and 25.5 J/mm2, respectively. Yoshioka et al. [28] 3
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Fig. 2. Top-view SEM figures of laser-processed YSZ-Al2O3-TiC coatings produced using a processing power of 800 W (a and b), 900 W (c and d), and 1200 W (e and f) before and after annealing. XRD curves of (g) laser-processed YSZ-Al2O3-TiC coating, and (h) annealed YSZ-Al2O3-TiC coating.
coatings and that the oxidation/healing temperature will increase for larger grain sizes due to slower kinetics.
interface between the precursor layer and the substrate. Dilution is essential to form a metallurgical bond between the coating and substrate. Moreover, dilution also leads to the infiltration of substrate elements into the coating which is critical during the synthesis of ceramic coatings on metallic substrates. However, excessive dilution is undesirable to avoid degradation of coating properties. The TiC peak completely disappeared once the laser processed coatings were annealed at 720 °C for 10 h (Fig. 2h). The TiO2 peak was the strongest for the sample produced at a low laser fluence of 17.0 J/mm2 (800 W), which seems to be counter-intuitive. This may be a result of the formation of larger crystallites of TiO2 compared to the samples that were laser processed using higher laser powers. The cross-section of the laser-processed composite samples was evaluated before and after annealing for samples produced using a laser fluence of 17.0 J/mm2 (Fig. 3). This figure also includes elemental maps to investigate the potential of healing of the as-processed porous structure. These maps revealed that all of the elements were uniformly distributed throughout the coating post laser processing. Upon annealing, however, the elemental spectra suggested that the Al2O3 and TiO2 phases migrated to the inter-columnar areas. These observations suggest that annealing was a necessary step to completely seal the pores in the laser-processed structures. It is hypothesized that the annealing step will be required for any initial powder sizes used to create these
3.2. Design of 2-layer coatings We employed the tuned recipe for the design and production of a 2layer system. Specifically, our coatings were created by depositing a YSZ layer followed by a self-healing layer. We extended our study to investigate the universality of the process by creating a YSZ-Al2O3-SiC overlayer. SiC and TiC are the two phases that are used in oxidationbased self-healing materials [30]. Fig. 4 provides cross-sectional micrographs of selected as-produced coatings before and after annealing. The cross-sectional micrographs revealed that annealing of the YSZ samples leads to grain growth followed by the disappearance of the columnar structures. Meanwhile, the presence of TiC-Al2O3 and SiCAl2O3 additives in YSZ helped limit grain growth and prevented densification of the YSZ. Notably, the healing occurs in two-steps. During the laser glazing process, the added carbide and oxide phases form a composite that inhibit grain growth and form a more refined columnar structure. Flash temperatures during this process are estimated to be in the 1500 to 1600 °C range [23]. Post-processing annealing, which is conducted at lower temperatures (650 °C for SiC and 720 °C for TiC) was found to be 4
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Fig. 2. (continued)
adhesion between this phase and the coating matrix [31]. These two systems offer contrast and flexibility in terms of the design of selfhealing overlayers for TBCs. Their effectiveness as TBCs will be investigated in a follow-up study.
a necessary step to completely oxidize the carbide phases and form TiO2 or SiO2 to heal cracks in the coatings. A relatively long annealing time was perhaps required to allow these phases to diffuse to the porous areas. The energy dispersive x-ray spectroscopy (EDS) maps of the cross-sections of the coatings indicate segregation of the healing phases throughout the films to provide better structural stability of the grain boundaries (Fig. 5). For the three-phase composite system studied here, the healing agent (the carbide) formed an amorphous oxide (SiO2 and TiO2) during the early stages of the annealing process. The amorphous nature of this phase generates a viscous behavior leading to liquid-like spreading of the reaction product into cracks that are in its vicinity. Notably, SiO2 seems to be more viscous as it was able to diffuse into the porous regions of the YSZ underlayer (Fig. 5(b)). In contrast, TiO2 remained in the self-healing overlayer and did not flow to the YSZ single phase region as shown in Fig. 5(a). The contrast between both systems is attributed to the lower viscosity of the SiO2 phase and the work of
4. Conclusions YSZ-Al2O3-TiC(SiC) and YSZ reference layers were produced on SS316 substrates using laser cladding to understand the viability of this technique to create a self-healing overcoat on TBCs. The laser fluence was varied between 17.0 and 25.0 J/mm2. A fluence of 17.0 J/mm2 was found to minimize the interdiffusion between the coating and the substrate and was selected for further studies. Laser processing alone was found to partially oxidize the TiC(SiC) and the kinetics associated with the process was not sufficient to seal/heal the self-healing layer. Annealing of the composite coating at 720 °C for 12 h was found to achieve complete oxidation of the TiC phase and the subsequent 5
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Fig. 3. Cross-sectional elemental mapping micrographs of: (top) laser-processed YSZ-Al2O3-TiC coating, and (bottom) annealed YSZ-Al2O3-TiC coating. Processing Power = 800 W.
Fig. 4. Top-view SEM figures of: (a) laser-processed YSZ coating, (b) laser-processed YSZ-Al2O3-TiC coating, (c) laser-processed YSZ-Al2O3-SiC coating. Crosssectional SEM figures of: (d) laser-processed YSZ coating, (g) annealed YSZ coating, (e) laser-processed YSZ-Al2O3-TiC coating, (h) annealed YSZ-Al2O3-TiC coating, (f) laser-processed YSZ-Al2O3-SiC coating, (i) annealed YSZ-Al2O3-SiC coating. 6
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Fig. 5. Cross-sectional elemental mapping figures of (a) laser-processed YSZ-Al2O3-TiC coating and (b) laser-processed YSZ-Al2O3-SiC coating.
migration of the newly formed TiO2 phase to the crack site to form a strong bond to the YSZ matrix. A two-layer coating with a thermally insulating YSZ underlayer and a dense self-healing layer was successfully produced. Future studies will include investigating other potentially efficient self-healing materials at elevated temperatures (Ti, Cr, Zr, Nb, Hf, TiC, TiN, Cr3C2, Cr2N, ZrN, NbC, and NbN) [31] and conducting systematic studies on the response of these newly developed coatings to thermal cycling and CMAS attack.
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