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Article

Effects of Selective Laser Modification and Al Deposition on the Hot Corrosion Resistance of Ceria and Yttria-Stabilized Zirconia Thermal Barrier Coatings

by
Panpan Zhang
1,2,
Xiaofeng Zhang
2,
Fuhai Li
2,
Zhihui Zhang
1,3,*,
Hong Li
2,
Yueliang Wang
2,
Luquan Ren
1 and
Min Liu
2
1
The Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130022, China
2
National Engineering Laboratory for Modern Materials Surface Engineering Technology, The Key Lab of Guangdong for Modern Surface Engineering Technology, Guangdong Institute of New Materials, Guangzhou 510651, China
3
The State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China
*
Author to whom correspondence should be addressed.
Coatings 2019, 9(6), 353; https://doi.org/10.3390/coatings9060353
Submission received: 3 May 2019 / Revised: 22 May 2019 / Accepted: 28 May 2019 / Published: 30 May 2019
Browse Figures
Versions Notes

Abstract

:
The air-plasma-sprayed ceria and yttria-stabilized zirconia (CYSZ) coating was modified by selective laser remelting and Al deposition to enhance hot corrosion resistance. The dotted coating was obtained after selective laser remelting. Magnetron sputtering was used to deposit an Al film on the dotted coating, and a vacuum heat treatment was subsequently performed to produce a dense α-Al2O3 overlay. Hot corrosion behavior of the following three types of coatings was investigated: plasma-sprayed, dotted, and dotted coatings combined with Al deposition (DA). Hot corrosion behaviors were evaluated in a mixture of 55 wt % V2O5 and 45 wt % Na2SO4 molten salts at 1000 °C for 30 h. The hot corrosion reaction between molten salts and zirconia stabilizers (Y2O3 and CeO2) led to the generation of monoclinic zirconia, YVO4, and CeVO4 plate-shaped crystals, and the mineralization of CeO2. The results indicated that the hot corrosion resistance of the DA coating was the best, and the dotted coating had superior hot corrosion resistance in comparison with the plasma-sprayed coating. The minimal surface roughness and dense dotted units improved the hot corrosion resistance of the dotted coating. The dense α-Al2O3 overlay with chemical inertness effectively inhibited the infiltration of molten salts, which led to the optimal hot corrosion resistance of the DA coating.

1. Introduction

Thermal barrier coatings (TBCs) are finding ever increasing applications in the severe high-temperature environment of gas turbine engines, where they can improve the engine efficiency and decrease undesirable emissions [1,2]. The typical TBC is a duplex material system that is composed of a ceramic topcoat to provide thermal insulation and an anti-oxidation metallic bondcoat. MCrAlY alloys (M: Ni, Co, and Ni + Co) as the typical bondcoat material are widely used due to their compatible thermal expansion coefficient (TEC) with the metallic substrate [3]. Generally, bondcoats are sprayed on the metallic substrate by air plasma spray (APS) [4], vacuum plasma spray (VPS) [5], low pressure plasma spraying (LPPS) [6], and high velocity oxygen fuel (HVOF) [7], etc. The HVOF technique provides the bondcoats with low oxide content and high density. The main role of the topcoat is to insulate the components against heat and exhaust gas flows in the service environment. Topcoats are usually sprayed on the bondcoat by APS [8,9], suspension plasma spraying (SPS) [10], or electron beam physical vapor deposition (EB-PVD) [11,12]. Because of the relatively low costs and high deposition efficiency, APS has been widely accepted in industry [13,14].
In a TBC system, the material used as the ceramic topcoat is expected to meet some basic criteria such as low thermal conductivity, high melting point, chemical inertness, and phase stability [15,16]. The most widely used material of the ceramic topcoat is yttria partially stabilized zirconia (6–8 wt % YSZ) because of its many merits such as low thermal conductivity, relatively high TEC, and adequate fracture toughness [17,18,19]. However, the YSZ also has several demerits, for example, hot corrosion and spallation which may shorten the lifetimes of the TBCs [20,21,22]. By contrast, the ceria and yttria-stabilized zirconia (ZrO2–CeO2–Y2O3, CYSZ) coating appears to show excellent properties in more harsh environments, for example, higher temperatures, corrosive atmospheres, and complex stress conditions [23,24]. In addition, previous results demonstrated that the CYSZ coating not only presented good phase stability at high temperature [23] and higher TEC [24], but also improved the thermal insulation, corrosion, and thermal shock resistance [25,26,27], which was obviously superior to the YSZ coating.
Hot gas that is in contact with the gas turbine can bring many impurities from several sources including the burned fuel, the intake air, and the injected water or steam [28]. These impurities containing sodium, sulfur, and vanadium may lead to hot corrosion in a gas turbine [29]. During the long-time service process, the condensation of combustion products leads to the formation of molten sulfate and vanadate salts, which are extremely corrosive to the TBCs at high temperatures [30,31]. It is well known that plasma-sprayed coatings show the representative characteristics of porous and laminar structures. However, the molten salts can easily penetrate into the coatings owing to the existence of these pores. Therefore, hot corrosion resistance exposed to the low-quality fuel combustion or corrosive environments is one of the most critical issues for the plasma-sprayed TBCs.
Many researchers have been dedicated to improving the resistance to hot corrosion of the TBCs for decades. Ahmadi-Pidani et al. [32] reported that laser glazing could enhance the hot corrosion resistance of plasma-sprayed CYSZ TBCs because the specific reactive area of the dense glazed surface layer was reduced. Yi et al. [33] modified the plasma-sprayed YSZ coatings by laser remelting, and a dense and smooth surface was obtained, which reduced the coat permeability to the molten salts, leading to improved resistance to hot corrosion. Thus, laser remelting is a prospective approach for increasing the resistance to hot corrosion of TBCs, since it is expected to close the pores of the plasma-sprayed coating and produce a dense surface layer. Nonetheless, the traditional laser process treated the whole ceramic topcoat, resulting in the formation of high-level residual stresses and a weak interface between the treated layer and the untreated layer. Chang et al. [34] fabricated peg-nail structured TBCs by selective laser modification, and the results indicated that the thermal shock resistance of the plasma-sprayed TBCs was enhanced by the peg-nail structured laser modification. Zhang et al. [35] compared the thermal shock behaviors of the TBCs with three different surface shapes processed by laser remelting, and concluded that the dotted specimen showed the best properties while the striated and grid specimens exhibited relatively poor thermal shock resistance. To date, the hot corrosion behaviors of TBCs modified by selective laser modification have not been reported.
Previous studies [36] found that when alumina was produced as an outer layer of an YSZ/Al2O3 coating, the hot corrosion resistance was significantly improved due to the lower infiltration of molten salts into the YSZ layer and lower reaction of molten salts with YSZ. Zhang et al. [37] reported that the novel LaPO4/YSZ double ceramic-layer TBCs with LaPO4 as an outer coating revealed high resistance to hot corrosion. Keyvani et al. [38] revealed that the nanostructured Al2O3/YSZ composite coating showed better hot corrosion resistance than that of the conventional YSZ coating. Ahmadi et al. [39] compared the hot corrosion behavior of sprayed and laser-glazed YSZ–Al2O3 composite coatings, and found that the content of the monoclinic zirconia fraction in sprayed coatings was higher than that in laser-glazed composite coating, indicating the enhancement of hot corrosion resistance in the laser glazing process. Therefore, it can be said that a layered TBC containing alumina components is beneficial to resisting hot corrosion. In addition, Al2O3 cannot be dissolved within the ZrO2, and it can merely encircle the ZrO2 particles in the TBC system. In addition, the induced local compressive stresses could suppress the detrimental phase transformation of tetragonal zirconia to monoclinic zirconia [36,40]. Interestingly, a novel method of forming a thin Al2O3 layer on the surface of TBCs was introduced by our research group recently. First of all, magnetron sputtering was used to deposit an Al film on the TBCs, and then a vacuum heat treatment was performed to produce a dense α-Al2O3 overlay [41,42]. It was proved that the TBCs modified by Al deposition were better than the plasma-sprayed TBCs in many properties, such as thermal insulation [42], thermal shock resistance [43,44], solid particle erosion resistance [41,45], and CMAS (CaO–MgO–Al2O3–SiO2) corrosion resistance [46,47]. Therefore, the selective laser remelting combined with Al deposition is an alternative compound technology for improving the hot corrosion resistance of the CYSZ TBCs, but has received less attention.
In this study, the APS processed CYSZ coatings were selectively post-remelted by Neodymium-doped Yttrium Aluminium Garnet (Nd:YAG) laser, and the dotted coating was obtained. Afterwards, magnetron sputtering was used to deposit an Al film on some of the dotted coating, and vacuum heat treatment was performed to produce a dense α-Al2O3 overlay through in-situ reaction of Al and ZrO2. Finally, three types of CYSZ TBCs were produced: plasma-sprayed coating, dotted coating, and dotted coating combined with Al deposition (DA). Then, these three types of coatings were investigated by evaluation of microstructure, phase composition, and hot corrosion behaviors.

2. Materials and Methods

2.1. Materials

Nickel-based superalloy (K417G) was used as the substrate with the size of Φ25.4 mm × 6 mm. A NiCrAlY bondcoat on the superalloy substrate was fabricated by a low-temperature high-velocity oxygen fuel (LT-HVOF) spraying system (K2, GTV Verschleißschutz GmbH, Luckenbach, Germany), and then a CYSZ ceramic coating was sprayed on the bondcoat by an APS system (MF-P1000, GTV Verschleißschutz GmbH). The spraying parameters of the LT-HVOF and APS are listed in Table 1 and Table 2, respectively. The bondcoat material was commercially available NiCrAlY powder (AMPERIT 413, H.C. Starck GmbH, Goslar, Germany) with different particle sizes ranging from 5 to 45 μm, as shown in Figure 1a. The topcoat material was ZrO2–24 wt % CeO2–2.4 wt % Y2O3 (CYSZ) powder (24C, Ganzhou Keying Structural Ceramics Co., Ltd., Ganzhou, China) with particle sizes of 15–120 μm, as shown in Figure 1b.

2.2. Laser Remelting Process

The plasma-sprayed specimens were processed by a Nd:YAG laser, model XL-1000Y with maximum mean power of 1000 W and Gaussian-shaped pulses (Guangzhou Xinglai Laser Technology Co., Ltd., Guangzhou, China). Based on a series of preliminary experiments, the corresponding laser remelting parameters are shown in Table 3. The surface images of the plasma-sprayed and dotted specimens are shown in Figure 2.

2.3. Al Deposition

Direct current circular magnetron sputtering (J-1250, Jinzhou Industrial Coating, Jinzhou, China) was used to deposit an Al film on the dotted coating. The Al target (99.99%) was adopted and the direct current, voltage, and pressure values were set at 3 A, 150 V, and 5 × 10−3 Pa, respectively. Afterwards, the dotted specimens deposited with Al film were put in a closed vacuum tube (about 2 × 10−3 Pa) for heat treatment at 665, 808, and 900 °C for 1 h, respectively. Finally, an α-Al2O3 layer on the dotted coating was obtained through the in-situ reaction of Al and ZrO2.

2.4. Hot Corrosion Tests

Before the hot corrosion tests, the coatings were ultrasonically cleaned with anhydrous ethanol, and then dried. Subsequently, 55 wt % V2O5 and 45 wt % Na2SO4 with a salt concentration of 15 mg/cm2 was mixed as the corrosive salts, which were uniformly spread on the coating surfaces by a glass rod. In order to avoid the edge effect, the distance between the corrosive mixture and the edge was kept at 3 mm. Then, the specimens were put into an electric furnace and isothermally heated at 1000 °C in air atmosphere for 30 h. After cooling to room temperature, the specimens were ultrasonically cleaned again to remove the excessive corrosive salts and surface impurities.

2.5. Characterization

The phase analyses of TBCs were carried out using X-ray diffraction (XRD) (Rigaku SmartLab 9 kW, Cu Kα radiation, Rigaku, Tokyo, Japan) with a rotating anode generator operated at 40 kV and 100 mA. The scanning rate was 4°/min at a 2θ range of 20°–90°. The scanning rate was 0.2°/min at the 2θ range of 27°–33° to identify the monoclinic zirconia phase. The morphology and microstructure of different TBCs were examined by using a scanning electron microscope (SEM) (SU8220, Hitachi, Tokyo, Japan) fitted with energy dispersion spectroscopy (EDS). EDS point analysis was used to analyze local elements and detect the corrosion products. EDS mapping was carried out to analyze the element distribution of TBCs after the hot corrosion test. The SEM pictures were taken at different locations in the cross section of the plasma-sprayed coating and the porosity was measured by ImageJ software (Version 1.52 n). The porosity reported is the average of ten values. The surface roughness was measured by a 3D surface Profiler (DEKTAK XT, Bruker, Karlsruhe, Germany). The surface roughness is the mean value of three individual experimental results at different regions of the coating surface.

3. Results and Discussion

3.1. Microstructure

Figure 3 shows the surface morphology and the cross-sectional images of the plasma-sprayed CYSZ coating. There are many microcracks and partially melted and unmelted particles on the coating surface, as shown in Figure 3a. These typical characteristics of plasma-sprayed coatings caused a rough surface with a surface roughness (Ra) of 6.03 μm, as depicted in Figure 3b. Figure 3c exhibits the cross section of the plasma-sprayed TBC. It is observed that a CYSZ ceramic coating with a thickness of about 200 ± 10 μm and a NiCrAlY bondcoat with a thickness of about 110 ± 10 μm are deposited on the superalloy substrate. Note that many pores and voids are in the CYSZ ceramic coating, and the porosity is about 13.6%, which provide permeable routes for the molten salts. The fracture microstructure of the plasma-sprayed CYSZ ceramic coating shows the typical lamellar structure, as shown in Figure 3d. It can be seen that there are many inter-splat cracks and pores, which is attributed to the random deposition, the incomplete overlap of adjacent splats, and gas entrapment during the process of plasma spraying [48].
The surface morphologies of the dotted unit in the dotted specimens are presented in Figure 4. There are many segmented cracks on the coating surface after laser remelting, as shown in Figure 4a. The formed segmented cracks are mainly attributed to the molten pool with a small size, volume shrinkage, the release of residual stresses during the fast and uneven cooling of molten zirconia, and probably the large and regional temperature gradient [34]. Herein, the dot-like laser-remelted zone can be called as dotted unit. Meanwhile, the defects such as pores, protrusions, and unmelted and partially melted particles in the plasma-sprayed coatings were eliminated, and the surface of the dotted unit was much smoother. Therefore, the dotted unit had a low surface roughness of 2.33 μm (Figure 4b). Figure 4c demonstrates the high magnification surface morphology of the dotted unit. It can be seen that the surface of the dotted unit with fine grains is much denser. This is due to the complete recrystallization of CYSZ ceramic material after the ultrahigh laser heating and cooling.
Figure 5 exhibits the cross-sectional morphologies of the dotted specimen. In the dotted specimen, the dotted unit, remained ceramic coating, and bondcoat were deposited on the superalloy substrate, as depicted in Figure 5a. The microstructure of the dotted unit was much denser than that of the remaining CYSZ ceramic coating. The dense dotted unit with vertical cracks had a thickness of approximately 70 μm. The induced thermal stress caused by the rapid cooling rate (106–108 K/s) and localized temperature gradient after laser treatment led to the formation of vertical cracks [49]. From Figure 5b, the fracture microstructure of the dotted unit presents a structure similar to a columnar structure, a pseudo-columnar structure, which was formed along the heat flow direction owing to the coupling effect of the temperature gradient and solidification rate in laser processing. In fact, it is a columnar structure composed of closely-linked partially melted particles, and a similar structure was also found by others [50].
After Al deposition and vacuum heat treatment, there were still many segmented cracks on the surface of the dotted unit in the DA specimen, as shown in Figure 6a. However, the surface of the dotted unit in the DA specimen was much rougher, with a surface roughness of 2.66 μm (Figure 6b). Figure 6c shows the surface morphology of the dotted unit in the DA specimen at a higher magnification, its surface is covered with many multi-scale grains, and partial cracks are filled. These multi-scale grains are the result of the in-situ reaction of Al and ZrO2 in the course of the vacuum heat treatment [42].
Figure 7 demonstrates the cross-sectional morphologies of the DA specimen. From Figure 7a, it can be seen that there was no obvious difference in the cross section of the dotted unit between the dotted specimen and the DA specimen. Figure 7b shows the upper layer of the dotted unit at high magnification and the corresponding elemental mapping analysis. It was observed that Al and O elements were plentiful in the surface layer of the dotted unit. This demonstrates the existence of Al2O3 in the DA specimen. Figure 7c shows the high magnification image of the Al2O3 overlay. It can be seen that the microstructure of the overlay was dense and the thickness of the Al2O3 overlay was about 1–2 μm.

3.2. Phase Composition

Figure 8 displays the XRD patterns obtained from the plasma-sprayed, dotted, and DA coatings at the 2θ range of 20°–90°. Both the plasma-sprayed and the dotted laser-remelted coatings showed the presence of the non-equilibrium tetragonal zirconia (t’-ZrO2) and cubic zirconia (c-ZrO2), as shown in Figure 8a,b. This is because of the rapid solidification after plasma spraying and laser remelting of CYSZ. Meanwhile, the dotted coating showed a higher peak intensity than the plasma-sprayed coating, which was mainly owing to the pseudo-columnar grain orientation induced by laser remelting. From Figure 8c, the phases of the DA coating were composed of t’-ZrO2, c-ZrO2, a small amount of α-Al2O3, and Al3Zr. The α-Al2O3 and Al3Zr phases were the results of the in-situ reaction of Al and ZrO2 at high temperatures [41,42]. No monoclinic zirconia (m-ZrO2) phase was detected in any coating at the 2θ range of 27°–33°, as shown in Figure 9. It was demonstrated that the harmful phase transformation of tetragonal zirconia (t-ZrO2) to m-ZrO2 was inhibited thanks to the extremely high cooling and solidification rate of plasma spraying and laser remelting. It is worth noting that the (111) diffraction peak showed a shift to a lower diffraction angle after selective laser remelting and Al deposition (Figure 9b,c), compared with the (111) diffraction peak in the plasma-sprayed coating (Figure 9a). According to Bragg’s law:
2 d sin θ = n λ
where d denotes the distance between the diffracting lattice planes, θ is the diffraction angle, n is an integer, and λ represents the wavelength of the X-ray radiation. The displacement of 2θ to lower angles indicates an increase in the lattice plane spacing, which is believed to be caused by residual stresses after the ultrahigh laser heating and cooling. However, the degree of diffraction peak shift in the DA coating (Figure 9c) is more obvious. This is probably due to the increase of oxygen deficiencies induced by the in-situ reaction of Al and ZrO2 in the course of vacuum treatment, which leads to the lattice distortion of ZrO2.

3.3. Characterization of Coatings after Hot Corrosion

3.3.1. Microstructure Observation and Phase Analysis

Figure 10 exhibits the surface morphologies of the plasma-sprayed coating after 30 h hot corrosion. The appearance of the plasma-sprayed coating was almost covered with plate-shaped crystals, indicating the presence of corrosion reactions between the molten salts and the CYSZ coating, as depicted in Figure 10a. Figure 10b shows the corrosion products of the plasma-sprayed coating at higher magnification. As can be seen, there were many plate-shaped crystals stacked on top of each other and a few particle-shaped crystals. Their compositions were confirmed by EDS analysis, as shown in Figure 10c,d. Figure 10c demonstrates that the plate-type crystals (Spot A) consisted of O, Y, Ce, and V elements. Figure 10d demonstrates that particle shaped crystals (Spot B) mainly contained O, Zr, and Ce elements.
The SEM surface morphologies of the dotted coating after the hot corrosion test are presented in Figure 11. The plate-shaped crystals were uniformly distributed on the surface of the dotted unit (Figure 11a,b). Compared with the plasma-sprayed coating, these reduced plate-shaped crystals on the dotted unit signify the enhancement of the hot corrosion resistance after laser remelting. This is mainly because the dense impermeable units reduced the penetration paths of molten salts into the CYSZ coating. From Figure 11c,d, the boundary of the dotted unit showed two distinct morphologies. It is observed that the amount of the plate-shaped crystals within the dotted unit was less than that outside the dotted unit. This indicates that the dense dotted units played important roles in the enhancement of the hot corrosion resistance. Obviously, the hot corrosion behavior outside the dotted unit was similar to that of the plasma-sprayed coating shown in Figure 10. Figure 11e indicates that the plate-type crystals (Spot C) consisted of O, Y, Ce, and V elements. Figure 11f shows that the matrix (Spot D) mainly contained O, Zr, and Ce elements.
Figure 12 demonstrates the SEM surface morphologies of the DA coating after the hot corrosion test. In contrast, these long plate-shaped crystals were scarce on the surface of the dotted unit, which were replaced by short plate-shaped crystals (Figure 12a,b). This indicates that the dense α-Al2O3 overlay can effectively inhibit the reaction between molten salts and CYSZ coating. From Figure 12c, the hot corrosion behavior on both sides of the boundary of the dotted unit was obviously different. The long plate-shaped crystals outside the dotted unit extended to the dotted unit. Although the plasma-sprayed zone was also modified by Al deposition, there were still many long plate-shaped crystals distributed on it, as shown in Figure 12d. Compared with the plasma-sprayed coating without Al modification (Figure 10), the plasma-sprayed zone modified by Al deposition was not fully covered with plate-shaped crystals. This demonstrates that Al deposition was also beneficial to the hot corrosion resistance of the plasma-sprayed zone. Figure 12e indicates that the plate-type crystals (Spot E) consisted of O, Y, Ce, and V elements. Figure 12f demonstrates that particle shaped crystals (Spot F) mainly contained O and Al elements.
The EDS elemental mapping of the cross section of the plasma-sprayed specimen after hot corrosion test is given in Figure 13. It can be seen that vanadium element was mainly distributed in the surface layer. This reveals that the molten salts can easily infiltrate into the CYSZ coating through the defects such as pores, voids, and microcracks in the plasma-sprayed coating. However, aluminum and oxygen elements existed not only in the voids of the CYSZ ceramic coating, but also at the interface between the topcoat and the bondcoat. The third layer between topcoat and bondcoat is the thermally grown oxide (TGO) and is a result of oxygen diffusion through the ceramic coating towards the bondcoat and the oxidation of metallic bondcoat [51,52]. Table 4 lists the total elemental composition of the plasma-sprayed coating after the hot corrosion test. It is evident that the content of O and V was higher, which signifies the poor hot corrosion resistance of the plasma-sprayed coating. The EDS elemental mapping and total elemental composition of the cross section of the dotted unit in the dotted specimen after the hot corrosion test are shown in Figure 14 and Table 4, respectively. We can see that the oxygen and vanadium elements were few in the dense dotted units. This confirms that the dense dotted units can suppress the penetration of molten salts into the CYSZ ceramic coating. The EDS elemental mapping of the cross section of the dotted unit in the DA specimen after the hot corrosion test is shown in Figure 15. It is observed that there were aluminum and oxygen elements on the surface of the dotted unit after the hot corrosion test. The relatively dense overlay was the in-situ synthesized α-Al2O3, which led to the relatively high content of aluminum and oxygen elements in the DA coating as shown in Table 4. Above all, the cerium, yttrium, and vanadium elements were fewer in the ceramic coating of the DA specimen. Therefore, the dense α-Al2O3 layer and laser remelted units effectively inhibited the infiltration of molten salts into the CYSZ ceramic coating, leading to the optimal hot corrosion resistance of the DA specimen.
The XRD patterns of the plasma-sprayed, dotted, and DA coatings after 30 h hot corrosion are shown in Figure 16. For all three types of coatings, the phases were mainly t′-ZrO2, m-ZrO2, YVO4, CeVO4, and a small amount of CeO2 and residual V2O5 after the hot corrosion test. The phases of the plasma-sprayed and dotted coatings were almost the same (Figure 16a,b). In particular, there was a small quantity of α-Al2O3 in the DA coating after the hot corrosion test, shown in Figure 16c. This indicates that the in-situ synthesized alumina overlay did not react with molten salts in the course of the hot corrosion test. Consequently, according to the SEM micrographs and EDS analyses (Figure 10, Figure 11 and Figure 12), it was demonstrated that the plate-type crystals “A”, “C”, and “E” were the mixture of YVO4 and CeVO4, particle shaped crystal “B” was CYSZ, the matrix “D” was CYSZ, and particle shaped crystal “F” was α-Al2O3. The monoclinic ZrO2, YVO4, CeVO4, and CeO2 crystals as hot corrosion products in CYSZ (ZrO2–25% CeO2–2.5% Y2O3) coatings were also reported by other researchers [29,53].

3.3.2. Hot Corrosion Reactions

For further understanding of the hot corrosion mechanism of different coatings in the sulfate–vanadate molten salts, the possible corrosive reactions are discussed [29,54].
Firstly, molten salts (55 wt % V2O5 + 45 wt % Na2SO4) infiltrate into the coating through the defects (pores and microcracks) of the plasma-sprayed coating and the vertical cracks of the dotted and DA coatings. NaVO3 is formed at high temperature according to the following reactions:
Na2SO4 (l) → Na2O (l) + SO3 (g)
Na2O (l) + V2O5 (l) → 2NaVO3 (l)
Subsequently, the NaVO3 reacts with the stabilizer yttrium oxide of CYSZ and leads to the formation of hot corrosion products.
Y2O3 (s) + 2NaVO3 (l) → 2YVO4 (s) + Na2O (l)
Additionally, the vanadium salt can directly react with yttria, resulting in the formation of YVO4 [55].
Y2O3 (s) + V2O5 (l) → 2YVO4 (s)
Park et al. [56] found that pure CeO2 precipitates were formed owing to mineralization of the CeO2 stabilizer in CYSZ coating in the course of the hot corrosion test. This is why the free CeO2 was detected in the XRD patterns of all coatings after the hot corrosion test (Figure 16). Both the free CeO2 precipitates and CeO2 stabilizers in zirconia could react with the V2O5 to generate the CeVO4 [57].
2CeO2 (s) + V2O5 (l) → 2CeVO4 (s) + 1/2O2 (g)
Even though the pure CeO2 (or ZrO2) could not react with NaVO3, CeO2 was leached from the CeO2-stabilized ZrO2 ceramic with the formation of CeVO4 in the presence of V2O5 or NaVO3–V2O5 mixtures [58]. However, if the concentration of Na2SO4 is appreciable and the SO3 partial pressure is high enough to raise the V2O5 activity in the surface salt film to certain levels, the CeO2 may react with NaVO3. The reaction may be presented in the unbalanced formula as [54,57]:
CeO2 (s) + NaVO3 (l) (in Na2SO4) + SO3 (g) → CeVO4 (s)
According to the EDS analyses (Figure 10, Figure 11 and Figure 12) and X-ray diffraction patterns (Figure 16), no Na or Na2O was detected. It is possible that Na2O was evaporated during the hot corrosion test [59,60]. Moreover, it can be seen from the XRD patterns (Figure 16) that there were no chemical reactions between Na2SO4, CYSZ, and α-Al2O3. That is to say, Na2SO4 did not have chemical effects on the CYSZ coating at 1000 °C, which is accordant with the results of other researchers [29,61].
Therefore, the hot corrosion mechanism of the CYSZ TBCs in the present study contained the following steps, which are accordant with the findings of others [29,32,53]: firstly, the molten salts infiltrated into the CYSZ ceramic coating through the defects (pores, voids, and microcracks) of the plasma-sprayed coating and the vertical cracks of the dotted and the DA coatings; secondly, the molten salts reacted with zirconia stabilizers (Y2O3 and CeO2), leading to the generation of YVO4 and CeVO4 plate-shaped crystals and the mineralization of CeO2; thirdly, the depletion of stabilizers resulted in the detrimental phase transformation of t-ZrO2 to m-ZrO2 with the damaging volume expansion; finally, the stresses stemming from phase transformation and formation of hot corrosion products, and the bondcoat oxidation may have led to the failure of the TBCs.

3.3.3. Comparison of Hot Corrosion Resistance of Different Coatings

As mentioned above, the t-ZrO2 phase was transformed to the detrimental m-ZrO2 phase with the depletion of CeO2 and Y2O3 stabilizers. Therefore, the amount of m-ZrO2, as the standard for coating destabilization during the hot corrosion test, can be used to quantitatively estimate the hot corrosion resistance of the plasma-sprayed, dotted, and DA coatings. Figure 17 shows the XRD patterns of all coatings after the 30 h hot corrosion test at the 2θ range of 27°–33°, with the main peak of t′-ZrO2 and m-ZrO2. Therefore, the volume fraction of m-ZrO2 (%m) after the hot corrosion test is measured by the following formula [29,32,54]:
% m = I m ( 1 ¯ 11 ) + I m ( 111 ) I m ( 1 ¯ 11 ) + I m ( 111 ) + I t ( 111 ) × 100
where I denotes the diffraction intensity of the respective lattice planes. The volume fractions of monoclinic ZrO2 (%m) in different coatings after the hot corrosion test are listed in Table 5. It can be seen that the volume fraction of m-ZrO2 in the plasma-sprayed coating was 64.6%, in the dotted coating %m was 56.3%, and in the DA coating %m was 40.5%. According to Figure 9, no m-ZrO2 was detected in any coating before the hot corrosion test. As a result, the resistance to hot corrosion of the plasma-sprayed coating was the worst. Conversely, both selective laser remelting and Al deposition had beneficial effects on the hot corrosion resistance. In comparison with the plasma-sprayed coating, the hot corrosion resistance of the dotted coating increased by 12.8%; and hot corrosion resistance of the DA coating was the best, which increased by 37.3%. For one thing, the laser remelting treatment eliminated the defects such as pores and voids of the plasma-sprayed coating, and hence the dense remelted units were beneficial to restraining the penetration of the molten salts. For another, the surface roughness of the dotted unit (2.33 µm) was less than that of the plasma-sprayed coating (6.03 µm), which led to the reduced contact area between the molten salts and the dotted coating. Therefore, the hot corrosion reaction between the molten salts and the dotted coating decreased. These are the reasons why the dotted coating had superior hot corrosion resistance in comparison with the plasma-sprayed coating. Since the remelted layer with vertical cracks did not completely avoid the contact between the CYSZ coating and the molten salts, the dotted coating had some limitations in the enhancement of the hot corrosion resistance. Even though the surface roughness of the DA coating (2.66 µm) was slightly higher than that of the dotted coating (2.33 µm), the dense α-Al2O3 overlay effectively inhibited the permeability of molten salts into the CYSZ coating and separated the corrosive salts from the CYSZ coating. As a result, the hot corrosion reaction between the molten salts and the CYSZ coating was considerably inhibited, which resulted in the optimal hot corrosion resistance of the DA coating.

4. Conclusions

Hot corrosion behavior of the plasma-sprayed coating, dotted coating, and dotted coating combined with Al deposition was estimated at 1000 °C for 30 h in molten salts (55 wt % V2O5 + 45 wt % Na2SO4). Several main conclusions are summarized as follows:
  • The dotted coating with some discrete and dense dotted units was produced by selective laser remelting. The dotted units showed a dense pseudo-columnar crystal structure and vertical cracks. The direct current circular magnetron sputtering was used to deposit Al film on the dotted coating, and vacuum heat treatment was subsequently performed to produce a dense α-Al2O3 overlay through the in-situ reaction of Al and ZrO2.
  • The reaction between the molten salts and zirconia stabilizers (Y2O3 and CeO2) led to the generation of YVO4 and CeVO4 plate-shaped crystals, and the mineralization of CeO2. The depletion of stabilizers caused the detrimental phase transformation of t-ZrO2 to m-ZrO2. Finally, the stresses resulting from phase transformation and the generation of hot corrosion products, and the bondcoat oxidation caused the failure of CYSZ coatings.
  • The dotted coating had superior heat corrosion resistance in comparison with the plasma-sprayed coating. The minimal surface roughness and the dense dotted units significantly contributed to improving the hot corrosion resistance of the dotted coating. However, the dotted units with vertical cracks did not completely avoid the contact between the molten salts and the CYSZ coating, which further limited the improvement in hot corrosion resistance of the dotted coating.
  • The dense α-Al2O3 overlay with chemical inertness effectively inhibited the infiltration of molten salts into the CYSZ coating and separated the corrosive salts from the CYSZ coating, which considerably reduced the hot corrosion reaction between the CYSZ ceramic coating and the molten salts. The selective laser remelting combined with Al deposition can maximize the hot corrosion resistance of TBCs. Therefore, the DA coating showed the best hot corrosion resistance.

Author Contributions

Conceptualization, P.Z. and Z.Z.; Methodology, P.Z. and X.Z.; Validation, F.L., H.L. and Y.W.; Formal Analysis, X.Z., H.L. and Y.W.; Writing–Original Draft Preparation, P.Z.; Writing–Review and Editing, P.Z. and Z.Z.; Supervision, F.L., H.L., L.R. and M.L.

Funding

This research was funded by the GDAS’ Project of Science and Technology Development (2019GDASYL-0502006, 2019GDASYL-0402006, 2019GDASYL-0402004, 2019GDASYL-0501009, 2018GDASCX-0947, 2017GDASCX-0111, and 2017GDASCX-0202), the Science and Technology Project of Guangdong Province (2017A070702016, 2017B030314122, and 2017A070701027), Natural Science Foundation of Guangdong Province (2016A030312015), and Zhongshan Project of Science and Technology (2017G1FC0008).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Morphologies of powders: (a) NiCrAlY used in bondcoat; (b) CYSZ used in topcoat.
Figure 1. Morphologies of powders: (a) NiCrAlY used in bondcoat; (b) CYSZ used in topcoat.
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Figure 2. Surface images of specimens: (a) plasma-sprayed specimen; (b) dotted specimen.
Figure 2. Surface images of specimens: (a) plasma-sprayed specimen; (b) dotted specimen.
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Figure 3. (a) Surface morphology, (b) surface roughness, (c) polished cross section, and (d) fracture microstructure of CYSZ ceramic coating in the plasma-sprayed specimen.
Figure 3. (a) Surface morphology, (b) surface roughness, (c) polished cross section, and (d) fracture microstructure of CYSZ ceramic coating in the plasma-sprayed specimen.
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Figure 4. (a) Surface morphology, (b) surface roughness, and (c) high magnification surface micrograph of the dotted unit in the dotted specimen.
Figure 4. (a) Surface morphology, (b) surface roughness, and (c) high magnification surface micrograph of the dotted unit in the dotted specimen.
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Figure 5. Cross-sectional morphologies of the dotted specimen: (a) overview of the dotted unit; (b) fracture microstructure of the dotted unit.
Figure 5. Cross-sectional morphologies of the dotted specimen: (a) overview of the dotted unit; (b) fracture microstructure of the dotted unit.
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Figure 6. (a) Surface morphology, (b) surface roughness, and (c) high magnification surface micrograph of the dotted unit in the Al deposition (DA) specimen.
Figure 6. (a) Surface morphology, (b) surface roughness, and (c) high magnification surface micrograph of the dotted unit in the Al deposition (DA) specimen.
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Figure 7. Cross-sectional morphologies of the DA specimen: (a) overview of the dotted unit; (b) high magnification micrograph of the dotted unit; and (c) high magnification micrograph of the overlay.
Figure 7. Cross-sectional morphologies of the DA specimen: (a) overview of the dotted unit; (b) high magnification micrograph of the dotted unit; and (c) high magnification micrograph of the overlay.
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Figure 8. XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings before the hot corrosion test at the 2θ range of 20°–90°.
Figure 8. XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings before the hot corrosion test at the 2θ range of 20°–90°.
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Figure 9. XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings before the hot corrosion test at the 2θ range of 27°–33°.
Figure 9. XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings before the hot corrosion test at the 2θ range of 27°–33°.
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Figure 10. The SEM surface morphology of the plasma-sprayed coating after the hot corrosion test: (a) lower magnification; (b) higher magnification of image (a). EDS analysis of (c) Spot A and (d) Spot B.
Figure 10. The SEM surface morphology of the plasma-sprayed coating after the hot corrosion test: (a) lower magnification; (b) higher magnification of image (a). EDS analysis of (c) Spot A and (d) Spot B.
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Figure 11. The SEM surface morphology of the dotted coating after the hot corrosion test: (a) the dotted unit; (b) higher magnification of image (a); (c)the boundary of the dotted unit; and (d) higher magnification of image (c). EDS analysis of (e) Spot C and (f) Spot D.
Figure 11. The SEM surface morphology of the dotted coating after the hot corrosion test: (a) the dotted unit; (b) higher magnification of image (a); (c)the boundary of the dotted unit; and (d) higher magnification of image (c). EDS analysis of (e) Spot C and (f) Spot D.
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Figure 12. The SEM surface morphology of the DA coating after the hot corrosion test: (a) the dotted unit; (b) higher magnification of image (a); (c) the boundary of the dotted unit; and (d) the plasma-sprayed zone. EDS analysis of (e) Spot E and (f) Spot F.
Figure 12. The SEM surface morphology of the DA coating after the hot corrosion test: (a) the dotted unit; (b) higher magnification of image (a); (c) the boundary of the dotted unit; and (d) the plasma-sprayed zone. EDS analysis of (e) Spot E and (f) Spot F.
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Figure 13. EDS elemental mapping of the cross section of the plasma-sprayed specimen after the hot corrosion test.
Figure 13. EDS elemental mapping of the cross section of the plasma-sprayed specimen after the hot corrosion test.
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Figure 14. EDS elemental mapping of the cross section of the dotted unit in the dotted specimen after the hot corrosion test.
Figure 14. EDS elemental mapping of the cross section of the dotted unit in the dotted specimen after the hot corrosion test.
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Figure 15. EDS elemental mapping of the cross section of the dotted unit in the DA specimen after the hot corrosion test.
Figure 15. EDS elemental mapping of the cross section of the dotted unit in the DA specimen after the hot corrosion test.
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Figure 16. The XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings after the hot corrosion test at the 2θ range of 20°–90°.
Figure 16. The XRD patterns of (a) plasma-sprayed, (b) dotted, and (c) DA coatings after the hot corrosion test at the 2θ range of 20°–90°.
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Figure 17. The XRD patterns of (a) the plasma-sprayed, (b) dotted, and (c) DA coatings after the hot corrosion test at the 2θ range of 27°–33°.
Figure 17. The XRD patterns of (a) the plasma-sprayed, (b) dotted, and (c) DA coatings after the hot corrosion test at the 2θ range of 27°–33°.
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Table
Table 1. Spraying parameters of low-temperature high-velocity oxygen fuel (LT-HVOF) system.
Table 1. Spraying parameters of low-temperature high-velocity oxygen fuel (LT-HVOF) system.
ParametersNiCrAlY
Spray distance (mm)150
Kerosene (L/h)12
Oxygen (L/min)730
Chamber pressure (bar)14
Powder feed rate (g/min)40
Gun speed (mm/min)1000
Overlap distance (mm)6
Table
Table 2. Spraying parameters of air plasma spray (APS) system. CYSZ = ceria and yttria-stabilized zirconia.
Table 2. Spraying parameters of air plasma spray (APS) system. CYSZ = ceria and yttria-stabilized zirconia.
ParametersCYSZ
Current (A)630
Voltage (V)65
Primary gas, Ar (SLPM)45
Secondary gas, H2 (SLPM)9
Carrier gas, Ar (SLPM)4
Spray distance (mm)110
Powder feed rate (g/min)50
Gun speed (mm/min)1000
Overlap distance (mm)6
Table
Table 3. Laser remelting parameters.
Table 3. Laser remelting parameters.
ParametersValue
Pulse energy (J)4
Pulse duration (ms)5
Frequency (Hz)1
Spot diameter (mm)1.2
Table
Table 4. EDS compositions of plasma-sprayed (Figure 13), dotted (Figure 14), and DA (Figure 15) coatings after the hot corrosion test.
Table 4. EDS compositions of plasma-sprayed (Figure 13), dotted (Figure 14), and DA (Figure 15) coatings after the hot corrosion test.
CoatingsComposition (at %)
ZrCeOVNiCrAlY
Plasma-sprayed18.742.3654.471.048.228.415.950.81
Dotted19.862.3453.180.839.058.215.910.62
DA20.081.9955.580.647.887.196.160.48
Table
Table 5. Volume fractions of m-ZrO2 (%m) after the hot corrosion test.
Table 5. Volume fractions of m-ZrO2 (%m) after the hot corrosion test.
SpecimensPlasma-Sprayed CoatingDotted CoatingDA Coating
m-ZrO2 (%m)64.656.340.5

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Zhang, P.; Zhang, X.; Li, F.; Zhang, Z.; Li, H.; Wang, Y.; Ren, L.; Liu, M. Effects of Selective Laser Modification and Al Deposition on the Hot Corrosion Resistance of Ceria and Yttria-Stabilized Zirconia Thermal Barrier Coatings. Coatings 2019, 9, 353. https://doi.org/10.3390/coatings9060353

AMA Style

Zhang P, Zhang X, Li F, Zhang Z, Li H, Wang Y, Ren L, Liu M. Effects of Selective Laser Modification and Al Deposition on the Hot Corrosion Resistance of Ceria and Yttria-Stabilized Zirconia Thermal Barrier Coatings. Coatings. 2019; 9(6):353. https://doi.org/10.3390/coatings9060353

Chicago/Turabian Style

Zhang, Panpan, Xiaofeng Zhang, Fuhai Li, Zhihui Zhang, Hong Li, Yueliang Wang, Luquan Ren, and Min Liu. 2019. "Effects of Selective Laser Modification and Al Deposition on the Hot Corrosion Resistance of Ceria and Yttria-Stabilized Zirconia Thermal Barrier Coatings" Coatings 9, no. 6: 353. https://doi.org/10.3390/coatings9060353

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