Advances in Materials Science and Engineering

Volume 2013 (2013), Article ID 790318, 13 pages

http://dx.doi.org/10.1155/2013/790318

Research Article

Evaluation of Normal and Nanolayer Composite Thermal Barrier Coatings in Fused Vanadate-Sulfate Salts at 1000°C

Mohammadreza Daroonparvar, Muhamad Azizi Mat Yajid, Noordin Mohd. Yusof, Hamid Reza Bakhsheshi-Rad, Mohammad Sakhawat Hussain, and Esah Hamzah

Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia

Received 21 May 2013; Accepted 28 July 2013

Academic Editor: Rui Vilar

Copyright © 2013 Mohammadreza Daroonparvar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Hot corrosion behavior of yttria stabilized zirconia (YSZ), YSZ/normal Al₂O₃, and YSZ/nano-Al₂O₃ coatings was investigated in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1000°C. Microstructural characterization showed that the creation of hot corrosion products containing YVO₄ crystals and monoclinic ZrO₂ is primarily related to the reaction between NaVO₃ and Y₂O₃ during hot corrosion. The lowest amount of hot corrosion products was observed in YSZ as an inner layer of YSZ/nano-Al₂O₃ coating. Hence, it can be concluded that the presence of nanostructured Al₂O₃ layer over the conventional YSZ coating can considerably reduce the infiltration of molten corrosive salts into the YSZ layer during hot corrosion which is mainly related to the compactness of nanostructured alumina layer (including nanoregions) in comparison with normal alumina layer.

1. Introduction

Industrial gas turbine blades work at high temperatures. The efficiency and durability of gas turbine engines can be significantly modified by increasing their operating temperatures [1]. The efficiency and durability of turbine blades can be increased using high strength materials and protective coatings against high temperature oxidation and corrosion [2]. Thermal barrier coating (TBC) is usually used to reduce the substrate temperature [1].

TBC was first applied on aircraft engine parts in 1960. However, this coating had several problems such as structural instability of ZrO₂ and poor bonding between the substrate and the ceramic coating (TBC) [2]. These problems were solved between 1970 and 1980 using

(1)yttria stabilized zirconia (YSZ) as a thermal barrier layer due to low thermal conductivity and high stability at elevated temperatures,(2)metallic bond coat (MCrAlY) (M = Ni, Co, or a mixture of these two) which was employed to improve the attachment between ceramic top coat and the substrate (Ni-based superalloy) and to provide protection for the alloy from additional oxidation [2].

On the other hand, YSZ coating is prone to hot corrosion when it is exposed to fused corrosive salts such as V₂O₅ and Na₂SO₄ at higher temperatures as these molten salts can diffuse into the YSZ layer and react easily with Y₂O₃ (stabilizer component of ZrO₂). Therefore, the phase transformation of tetragonal zirconia to monoclinic zirconia followed by the formation of YVO₄ crystals can occur during hot corrosion [3] as YVO₄ crystals, and monoclinic zirconia as hot corrosion products will cause the separation of the ceramic layer from the bond coat [4, 5]. Hence, YSZ coating as the main component of TBC system (base metal/MCrAlY/YSZ) must be maintained during service at elevated temperatures. It is interesting to note that Al₂O₃ is one of the strongest, cheapest, and hardest ceramics. Also this ceramic oxide is highly resistant to chemical changes in most applications. In addition, Al₂O₃ showed high resistance against corrosive and reductive atmospheres and liquid metals [6]. According to previous investigations [7], the characteristics of ZrO₂ as the main component of TBC systems consist of

(1)high coefficient of thermal expansion (CTE),(2)low thermal conductivity (TC).

On the other hand, the coefficient of thermal expansion of Al₂O₃ resembles CTE of ZrO₂ at elevated temperatures. Thermal conductivity of Al₂O₃ can be significantly reduced at high temperatures as TC of Al₂O₃ resembles TC of ZrO₂ in a TBC system during service [7, 8]. In the meantime, the oxygen diffusivity from the crystalline structure of Al₂O₃ is lower in comparison to that of ZrO₂ [9].

Heating conditions, along with the coating microstructure, have been cited in many reports as playing a pivotal role in providing TBC coatings with resistance against thermal shock, oxidation, and hot corrosion. Therefore, the efficiency and durability of TBC coatings can be manipulated by controlling the microstructure [10].

Many researchers have proved that resistance against the thermal shocks, oxidation, and hot corrosion depended principally on the coating structure (TBC) [8, 10].

Under this condition, it can be speculated that the nanosized Al₂O₃ layer with ultrafine grained structure over the YSZ coating can remarkably reduce the penetration of molten salts and oxygen into the YSZ layer, significantly preventing the formation of hot corrosion products in the YSZ layer. On the other hand, due primarily to the compact nature of the nanostructure, less voids and pores should appear in the nanostructured Al₂O₃ layer.

A substantial problem presents itself during the production of nanostructured ceramic coatings from the nanopowders. The problem is feeding the nano-powders into the plasma. Nanopowders adhere to the walls of the feeding system, making it extremely difficult to move them towards the plasma torch due to their high specific area and low mass. In order to overcome this problem, reconstitution of the nanoparticles into micrometer sized granules, a process known as granulation treatment, is necessary. The majority of investigators have demonstrated that the most favorable granule size is in the range of 10 μm–110 μm [11–14]. In order to obtain a dense nanoceramic coating by plasma spraying, granulated nanopowders must have excellent flow ability and high apparent density [14].

The main objectives of this paper are to produce a nanostructured ceramic coating from the granulated powders and to compare the hot corrosion resistance of YSZ/nano-Al₂O₃ coating with YSZ/normal Al₂O₃ and normal YSZ coatings in the presence of molten mixture of 45% Na₂SO₄ + 55% V₂O₅ at 1000°C. It is anticipated that nanostructured Al₂O₃ coating could considerably lessen hot corrosion products in the YSZ as inner layer of YSZ/nano-Al₂O₃ coating.

2. Experimental Procedures

2.1. As-Received Materials

Nickel-based superalloy (Inconel 738) squares of 25 × 25 × 6 mm were employed as substrates. Amdry 962 (Ni-22Cr-10Al-1Y, −106 + 52 μm), was used as bond coat. Metco 204 NS-G (ZrO₂-8%Y₂O₃, −106 + 11 μm), Amdry 6062 (normal α-Al₂O₃ with high purity, −80 + 15 μm), and Inframat LLC 0802 (nano-α-Al₂O₃ with high purity, 80 nm) were used as TBC or top coat.

2.2. Granulation Treatment of the Nano-Al₂O₃ Powders

Nano-Al₂O₃ powders with an average particle size nominally less than 80 nm and polyvinyl alcohol (PVA as a binder) were used as starting materials. In this method, 50 g of PVA was dissolved in 80 mL of distilled water at 200°C using a magnetic stirrer. At the same time, the nano-Al₂O₃ particles were dispersed in distilled water by using an ultrasonic machine for 30 min at 60°C. Then, the dispersed nano-Al₂O₃ solution was added to the PVA solution with the aid of a magnetic stirrer at 250°C for 45 min. The water from the solution was removed using a rotary evaporator in order to avoid phase segregation [12].

These granulated powders were dried using a normal electric furnace at 200°C for 145 min. As mentioned previously, the required particle size for atmospheric plasma spraying was approximately 100 μm [14]. Hence, these agglomerated powders were sieved through 150 μm, 100 μm, and 50 μm meshes in order to obtain an adequate shape and suitable size for plasma spraying. The final particle size of the granulated nano-Al₂O₃ powders used for atmospheric plasma spraying was about 100 μm.

2.3. Air Plasma Sprayed Coatings

In this research, at first, NiCrAlY coating was sprayed on the substrates (Inconel 738) with a thickness of  μm and followed by three types of ceramic top coatings which were deposited on the NiCrAlY layer using air plasma spray (APS) method. The characteristics of the top coatings are as follows.

(a)Normal YSZ layer with a thickness () of  μm.(b)Layer composite of YSZ ( μm)/normal Al₂O₃ ( μm) coating.(c)Layer composite of YSZ ( μm)/nano-Al₂O₃ ( μm) coating.

The APS method was performed using a Sulzer-Metco 3MB plasma spray system. Table 1 shows the parameters of APS method.

Table 1: The parameters of atmospheric plasma spraying (APS).

Parameter
NiCrAlY
YSZ
Normal Al₂O₃
Granulated nano-Al₂O₃ powders

Current (A)
450
550
500
500

Voltage (V)
50
70
70
50

Primary gas, Ar (L/min)
85
38
38
85

Secondary gas, H₂ (L/min)
15
17
17
15

Powder feed rate (g/min)
15
35
30
25

Spray distance (cm)
15
7.5
9
10

2.4. Hot Corrosion Test at 1000°C

V₂O₅ and Na₂SO₄ are principal corrosive compounds at hot corrosion process. Accordingly, a mixture of 55% V₂O₅ + 45% Na₂SO₄ powders of 99.99% purity was used as corrosive salt [5, 10, 15]. The physical specifications of each salt are presented in Table 2. Taking care to stay within 3-4 mm from the edge, the coating surface was covered by 30 mg/cm² of powdery mixture, forming an even film of corrosive material (in order to prevent the effects of edge corrosion), as seen in Figure 1. The samples were then set in a normal electric furnace with an air atmosphere of 1000°C for 4 hr and then cooled down until ambient temperature was reached inside the furnace (in order to prevent the thermal shocks), as shown in Figure 2. Also, the coatings were inspected after every 4 hr of hot corrosion cycles [10, 16]. If there were any cracks or separation in the coating edge, the hot corrosion test was stopped. The temperature of the hot corrosion test and the concentration of corrosive salts were, respectively, selected according to zirconia phase transformation, melting point of corrosive salts, and accomplishing a fast experiment.

Table 2: The physical specifications of corrosive salts.

Type of corrosive salt
Melting point (°C)
Density (g/cm²)
Purity (%)

Na₂SO₄
884
2.7
99.98

V₂O₅
690
3.3
99.99

Figure 1: Taking care to stay within 3-4 mm from the edge, the coating surface was covered by 30 mg/cm² of powdery mixture, forming an even film of corrosive material (in order to prevent the effects of edge corrosion).

Figure 2: Schematic curve of hot corrosion cycles at 1000°C.

2.5. Microstructural Characterization

The surface and the cross-section of the coatings before and after hot corrosion test were explored using field emission scanning electron microscopy (FESEM, Hitachi S-4160) and scanning electron microscopy (SEM, Japan, EM15) equipped with energy dispersive spectrometer (EDS). In order to determine the type of formed phases on the YSZ layer of TBCs after hot corrosion test, XRD was conducted (Siemens-D500) by using Cu Kα line generated at 40 kV and 35 mA.

3. Results and Discussion

3.1. Nano-Al₂O₃ Powders after Granulation

Figure 3(a) shows the surface morphology of nano-Al₂O₃ powders after granulation. It can be observed that, after granulation, particle size has increased. It can be seen that there are several nano-Al₂O₃ grains (Figure 3(b)) in a granulated particle which is suitable for plasma spraying.

Figure 3: FESEM images of nano-Al₂O₃ powders after granulation with different magnifications (a) ×100  and (b) ×20.0 K.

3.2. Microstructural Characterization of Coatings before Hot Corrosion Test

Figure 4 shows the cross-section of the as-sprayed coatings. It can be seen that all layers of the as-sprayed coatings have lamellar structure (Figures 4(a), 4(b), and 4(c)). This specification belongs to the air plasma sprayed coatings [1]. It can be observed that normal Al₂O₃ and YSZ layers have formed a normal-layer composite coating (Figure 4(a)), and also nano-Al₂O₃ and YSZ layers have formed a nanolayer composite coating (Figure 4(c)) on the bond coat. Figures 5 and 6 indicate the surface of the as-sprayed coatings at different magnifications. It can be said that nano-Al₂O₃ layer (Figures 5(a), 5(b), and 6(a)) has the lowest number of pinholes and microcracks compared to that of normal Al₂O₃ layer (Figures 5(c), 5(d), and 6(c)). On the other hand, the YSZ layer has more pinholes and micro-cracks (Figures 5(e), 5(f), and 6(b)). Figure 7 also shows that nanostructured Al₂O₃ layer has been made from ultrafine particles. It is expected that nano-Al₂O₃ layer over the YSZ layer could considerably reduce O₂ diffusion and infiltration of corrosive molten salts into the YSZ layer at elevated temperatures. This phenomenon may be related to the compactness of the nanostructured Al₂O₃ layer (Figures 6(a) and 7(a)).

(a)

(b)

(c)

Figure 4: Cross-section of the as-sprayed coatings.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5: Surface morphology of the as-sprayed coatings: (a, b) nanostructured Al₂O₃ layer, (c, d) normal Al₂O₃ layer;  and (e, f) normal YSZ layer.

(a)

(b)

(c)

Figure 6: Surface morphology of the as-sprayed coatings with high magnification: (a) nanostructured Al₂O₃ layer, (b) normal YSZ layer, and (c) normal Al₂O₃ layer.

Figure 7: FESEM images of surface of the as-sprayed nanostructured Al₂O₃ layer made from ultrafine particles: (a) ×5.0 K  and (b) ×50.0 K.

3.2.1. X-Ray Diffraction Analysis of Outer Surface of TBCs before Hot Corrosion Test

Figure 8 shows the XRD analysis of outer surface of TBCs after air plasma spraying. Figure 8(a) exhibits that the as-sprayed normal YSZ coating is mainly composed of tetragonal zirconia () phase. It is observed that nano-(Figure 8(b)) and normal (Figure 8(c)) Al₂O₃ coatings are mainly composed of α-Al₂O₃ (rhombohedral) and γ-Al₂O₃ (cubic) phases after spraying.

(a)

(b)

(c)

Figure 8: XRD patterns of (a) normal YSZ layer, (b) nano-Al₂O₃ coating, and (c) normal Al₂O₃ coating after air plasma spraying.

3.3. Microstructural Characterization of Coatings after Hot Corrosion Test

Figure 9 shows the cross-section of the coatings after hot corrosion test at 1000°C. Figures 9(a) and 9(b) clearly show a wide crack in YSZ coating (after 12 h or 3 cycles of hot corrosion test) which is a direct result of the formation of monoclinic ZrO₂ and YVO₄ large crystals. Although the spallation of normal Al₂O₃ layer of  YSZ/normal Al₂O₃ coating occurred after 44 h (11 cycles of hot corrosion test), YSZ as inner layer was free of cracks or delamination (Figure 9(c)). Also, in YSZ/nano-Al₂O₃ coating, the spallation of nano-Al₂O₃ layer was observed after 52 h of hot corrosion test (after 13 cycles of hot corrosion test), but cracking or delamination in YSZ as inner layer was not seen (Figure 9(d)). It is seen that the YSZ layer as the main component of TBC systems has been maintained in layered composite coatings compared to conventional YSZ coating after hot corrosion tests.

Figure 9: Cross-section of the coatings after hot corrosion testing at 1000°C. (a, b) Conventional YSZ coating, (c) YSZ as inner layer of YSZ/normal Al₂O₃ coating, and (d) YSZ as inner layer of YSZ/nano-Al₂O₃ coating.

Figure 10 demonstrates XRD patterns obtained from the normal YSZ layer, YSZ as inner layer of YSZ/normal Al₂O₃ coating and YSZ as inner layer of YSZ/nano-Al₂O₃ coating after the hot corrosion test using the 45% Na₂SO₄ + 55% V₂O₅ salt mixture at 1000°C. According to XRD patterns, the YSZ layer of coatings, as the main component of TBC system, contains not only tetragonal zirconia but also additional new phases which may be regarded as hot corrosion products; monoclinic zirconia and tetragonal YVO₄ composition, are also present. As mentioned, YVO₄ crystals and monoclinic zirconia as hot corrosion products were accompanied by a rapid local volume increase, finally causing the separation of the ceramic layer from the bond coat. Therefore, the formation and growth of hot corrosion products in YSZ layer are believed to be detrimental to the durability of TBC systems at elevated temperatures. So, they must be reduced in the YSZ layer.

(a)

(b)

(c)

Figure 10: XRD patterns of (a) normal YSZ layer, (b) YSZ as inner layer of YSZ/normal Al₂O₃ coating, and (c) YSZ as inner layer of YSZ/nano-Al₂O₃ coating after hot corrosion testing.

As shown in Figure 10, the intensity peak of hot corrosion product phases in YSZ as inner layer of YSZ/nano-Al₂O₃ coating has been considerably reduced in comparison to normal YSZ coating and YSZ as inner layer of YSZ/normal Al₂O₃ coating after hot corrosion test.

It can be seen that the intensity peak of YVO₄ crystals in normal YSZ coating is much higher compared to that of YSZ as inner layer of YSZ/nano-Al₂O₃ and YSZ/normal Al₂O₃ coatings, respectively. In other words, by measuring the surface areas covered by the hot corrosion products, for all three coatings, the hot corrosion resistance of TBCs can be qualitatively measured. As seen in Figure 11(a), the hot corrosion product of YVO₄ covers almost the entire surface which may be attributed to the inhomogenously distributed microcracks and open pores inside the conventional YSZ layer (Figures 5(e), 5(f), and 6(b)); in Figure 11(c), an estimated 45% of the area is covered by YVO₄, and, in Figure 11(e), this figure is reduced to about 20% of the surface area. In this regard, Figures 11(a) and 11(b) were taken after three 4 h hot corrosion cycles, Figures 11(c) and 11(d) were taken after eleven 4 h hot corrosion cycles, and finally Figures 11(e) and 11(f) were taken after thirteen 4 h hot corrosion cycles. So, it can be seen that the nanostructured Al₂O₃ layer over the YSZ coating would significantly suppress the formation and growth of YVO₄ crystals in YSZ layer which must be preserved (Figure 9(d)) during service.

Figure 11: SEM micrographs of YVO₄ crystals on the surface of YSZ coatings: (a, b) conventional YSZ; (c, d) YSZ as inner layer of YSZ/normal Al₂O₃ coating; and (e, f) YSZ as inner layer of YSZ/nano-Al₂O₃ coating after hot corrosion testing.

The unstable phase of monoclinic zirconia expressed as a percentage is very important in hot corrosion process. Equation (1) is applied in order to determine monoclinic phase volume fraction [10]:

where is the intensity peak of tetragonal zirconia at (101) plane, is the intensity peak of monoclinic zirconia at (111) plane, and is the intensity peak of monoclinic zirconia at (111) plane, in XRD plots (see Figure 10) after hot corrosion test. The percentage of monoclinic phase volume was derived using (1). For normal YSZ coating, YSZ as inner layer of YSZ/normal Al₂O₃ coating, and YSZ as inner layer of YSZ/nano-Al₂O₃ coating, this parameter was calculated to be 66%, 21%, and 15%, respectively. Clearly, normal YSZ in terms of the volume fraction of monoclinic zirconia is the greatest, while the least is that of YSZ layer in YSZ/nano-Al₂O₃ coating.

The premature deterioration of conventional YSZ layer from the bond coat is primarily related to the more formation and propagation of hot corrosion products specifically YVO₄ crystals in the normal YSZ coating during hot corrosion, as shown in Figures 10, 11(a), and 11(b).

As shown in Figure 11(a), porous and destroyed surface with many micro-cracks and needle-like crystals (as first hot corrosion product) can be seen on the surface of normal YSZ coating. YVO₄ initially has a dendritic shape at the start of the hot corrosion process following the nucleation and growth mechanism; this shape changes to a rod/flat (Figure 11(a)) or needle- (Figure 11(b)) like shape as the hot corrosion process proceeds.

The normal alumina layer spalled at the interface of YSZ/normal Al₂O₃; hence, Figure 11(c) shows the surface of YSZ as inner layer of YSZ/normal Al₂O₃ coating after 44 h of hot corrosion test. Also, the nano-Al₂O₃ layer spalled at the interface of YSZ/nano-Al₂O₃ coating after 52 h of hot corrosion test; therefore Figure 11(e) indicates the surface of YSZ as inner layer of YSZ/nano-Al₂O₃ coating after hot corrosion test.

The detrimental crystals were rod/needle or flat shaped in usual YSZ with large size (85 μm) and thicker (2.5 μm) (Figures 11(b) and 12(a)), thin rod crystals (1.5 μm) with medium size (50 μm) in YSZ/normal Al₂O₃ coating (Figures 11(d) and 12(b)), and thinner rod crystals (0.5 μm) with low number and small size (10 μm) in YSZ/nano-Al₂O₃ coating (Figures 11(f) and 12(c)).

(a)

(b)

(c)

Figure 12: FESEM micrographs of YVO₄ crystals on the surface of YSZ coatings: (a) thicker rod/flat crystals in conventional YSZ (b) thin rod/flat crystals in YSZ as inner layer of YSZ/normal Al₂O₃ coating and (c) thinner rod/flat or needle crystals in YSZ as inner layer of YSZ/nano-Al₂O₃ coating after hot corrosion testing.

It is seen that the formation and propagation of rod/flat- or needle-shaped crystals are much less in YSZ/nano-Al₂O₃ coating. This is because nanostructured Al₂O₃ layer with less pinholes and micro-cracks [17, 18] can considerably prevent the infiltration of molten salts into the YSZ layer in comparison with normal Al₂O₃ and conventional YSZ coatings. In this regard, it can be said that nanostructured Al₂O₃ should have less pores and voids due to the compactness of the nanostructure (Figures 5(a), 6(a), and 7(a)).

Energy dispersive spectrometer (EDS) analysis (see Figure 13) demonstrated that the rod crystals were mainly composed of yttrium, vanadium, and oxygen. Also, XRD analysis identified these crystals as YVO₄ (tetragonal) (Figure 10). This result is in agreement with previous investigations [3, 5]. As mentioned earlier, exposing these porous areas to phase analysis demonstrates that tetragonal zirconia, in large amounts, had been transformed to monoclinic phase (as the second hot corrosion product) on the surface of the conventional YSZ top layer due to the depletion of yttria (as stabilizer component of zirconia). It can be said that a Lewis acid-base mechanism is driving the reactions between vanadium compounds and ceramic oxides. These ceramic oxides have a stronger basicity, and acid vanadium compounds react with them more readily. This can be explained since the basicity of yttrium oxide and zirconium dioxide follows the order: Y₂O₃ > ZrO₂, indicating that molten NaVO₃ has the tendency to react with Y₂O₃ more easily [19].

Figure 13: EDS spectrum from the rod/flat or needle crystals of YVO₄ on the surface of YSZ layer of TBCs.

On the whole, according to above results, it can be said that the nanostructured Al₂O₃ layer over the YSZ coating can act as a strong barrier for the diffusion of fused corrosive materials into the YSZ layer and would considerably lessen the hot corrosion products formation in the normal YSZ layer at elevated temperatures.

3.4. Hot Corrosion Mechanism of YSZ Coating at Elevated Temperatures

The hot corrosion behavior and failure mechanism of TBC in the present research consist of the following steps that are consistent with previous observations [20, 21].

At first, V₂O₅ (with melting point of 690°C) will react with Na₂SO₄ (with melting point of 884°C) during thermal exposure at elevated temperature (1000°C), and then NaVO₃ (with melting point of 610°C) will be formed (Reaction (2)):

NaVO₃ will then react with the stabilizer component (Y₂O₃) of tetragonal ZrO₂ to form YVO₄ crystals and monoclinic ZrO₂  (Reaction (3)):

It can be said that the monoclinic ZrO₂ is as an unstable phase. This phase will be transformed to tetragonal ZrO₂ at 1000°C, and this phase will again be converted to monoclinic zirconia during normal cooling [22]. This phenomenon is accompanied by 3–5% local volume expansion and finally will cause the separation of YSZ top layer from the bond coat [10]. On the other hand, Na₂O can react with V₂O₅ to directly produce NaVO₃   (Reaction (4)):

Also, it had been reported that [10] V₂O₅ could react directly with Y₂O₃ (stabilizer component of zirconia) to form monoclinic ZrO₂ and YVO₄ crystals as hot corrosion products (Reaction (5)):

The hot corrosion behavior of plasma sprayed Al₂O₃ and ZrO₂ coatings in molten Na₂SO₄ has been investigated by Chen et al. [23]. Their research showed that NaAlO₂ could be formed on the surface of Al₂O₃ particles (Reactions (6) and (7)) since the hot corrosion rate of Al₂O₃ coating in molten Na₂SO₄ was much lower in comparison to that of ZrO₂ coating. In this research, NaAlO₂ was barely detected by XRD analysis (see Figure 10), so it can be speculated that the Al₂O₃ layer could be protected by NaAlO₂ compound during the primary cycles of hot corrosion test:

In some references [24, 25], Na₂SO₄ has been known as an accelerator factor of chemical reactions during hot corrosion. On the other hand, NaVO₃ compound with relatively low melting point (630°C) can considerably increase the phase transformation of tetragonal ZrO₂ to monoclinic ZrO₂ during hot corrosion due to stabilizer depletion (Y₂O₃).

Ramachandra and other investigators reported that the reaction between the molten Na₂SO₄ and YSZ coating could not occur to any considerable extent, as the solidification of molten salt inside the porosities of ceramic coating and stress creation can be one of the main factors of degradation of YSZ coating during hot corrosion [4]. Hence, it can be concluded that the infiltration of molten corrosive materials into the YSZ layer can be a major deterioration mechanism of thermal barrier coatings during hot corrosion process.

It was reported that molten NaVO₃ also increases the mobility of atoms, enhancing the depletion of Y₂O₃ from YSZ and the growth of YVO₄ crystals [22, 24]. On the other hand, the presence of high V concentration as seen on the coating surfaces tends to attract the Y³⁺ in the lattice of YSZ which has the mobility to migrate preferentially toward the reaction interface. It can be inferred that the percentage of hot corrosion products will be increased with increases in the reaction time (multiple hot corrosion cycles) which can finally extend the corroded regions. It can be supposed that the best nucleation locations for the formation of new heterogeneous YVO₄ crystals are previously formed YVO₄ crystals, as this case resembles the role of grain boundaries in the heterogeneous nucleation and growth process [16].

As mentioned previously, phase transformation of tetragonal zirconia to monoclinic zirconia is accompanied by local volume increase which generates compressive stresses. According to previous observation [10], YVO₄ crystals could impose additional compressive stresses to the YSZ layer.

It can be said that inhomogeneities, pores, and micro-cracks play a principal role in the molten salts infiltration into the coating during hot corrosion (see Figure 14). It was found that nanostructured YSZ coatings including fully molten parts and nanozones are able to reduce oxygen and corrosive molten salts infiltration into the coating at elevated temperatures [10, 26].

Figure 14: Schematic illustration of corrosive molten salts infiltration into the YSZ layer of different thermal barrier coating systems during hot corrosion process at elevated temperatures.

In this research, dense nano-Al₂O₃ layer significantly prevented the diffusion of molten salts into the YSZ layer; therefore the amount of monoclinic ZrO₂ and YVO₄ crystals was substantially reduced in YSZ/nano-Al₂O₃ coating in comparison with conventional YSZ and YSZ/normal Al₂O₃ coatings after hot corrosion test.

Formation of large rod/flat or needle-shaped YVO₄ crystals with an average length of 85 μm in conventional YSZ coating was observed. These crystals grow outward from the surface (see Figure 15) and cause additional stresses in the coating. The spallation of normal YSZ occurred at the NiCrAlY/YSZ interface (Figures 9(a) and 9(b)) due to those additional stresses in the coating.

Figure 15: Outward growth of YVO₄ crystals in conventional YSZ coating.

In other words, separation of normal YSZ coating from the bond coat is as a direct result of the formation of monoclinic ZrO₂ and YVO₄ large crystals (Figures 11(a) and 11(b)) in YSZ layer during hot corrosion. But in the YSZ/normal Al₂O₃ coating, a small amount of molten salts penetrated through the alumina layer towards the YSZ (Figures 11(c) and 14) and reacted with Y₂O₃ (stabilizer of ZrO₂) at the YSZ/normal Al₂O₃ interface. Although monoclinic ZrO₂ had been decreased to 21%, the presence of YVO₄ crystals on the YSZ as inner layer had a significant role in the spallation of normal Al₂O₃ coating from the YSZ layer, as the medium length of YVO₄ crystals was about 50 μm. On the other hand, in the YSZ/nano-Al₂O₃ coating, the least amount of molten salts infiltrated through nanoalumina layer towards the YSZ layer and reacted with YSZ at the interface of YSZ/nano-Al₂O₃ (see Figure 14). In can be said that the YVO₄ small crystals did not play a substantial role in the spallation of nano-Al₂O₃ layer from the YSZ because of the short length of YVO₄ crystals, which was about 10 μm. However, the spallation of nano-Al₂O₃ layer is related to the formation of monoclinic ZrO₂ (15%) at the interface of YSZ/nano-Al₂O₃ coating during hot corrosion test. The aforementioned results show that YSZ/nano-Al₂O₃ coating has better corrosion behavior in comparison with other coatings at elevated temperatures.

4. Conclusions

(1)Generation of granulated (sprayable) nano-Al₂O₃ powders is followed by production of nanostructured Al₂O₃ layer over the YSZ coating using air plasma spraying method. (2)Monoclinic ZrO₂ and YVO₄ crystals (as hot corrosion products) were generally formed on the YSZ layer during hot corrosion which are mainly related to the reaction of molten salts containing Y₂O₅ and NaVO₃ with Y₂O₃ (stabilizer component of ZrO₂). Hot corrosion products finally led to the spallation of the TBC from the bond coat.(3)The average length and number of rod crystals of YVO₄ in YSZ as the inner layer of YSZ/nano-Al₂O₃ coating had been substantially reduced in comparison to those of YSZ/normal Al₂O₃ and conventional YSZ coatings after hot corrosion test. On the other hand, the volume fraction of monoclinic zirconia in YSZ as inner layer of YSZ/nano-Al₂O₃ coating was much lower compared to that of the other coatings.(4)According to the aforementioned results, it can be concluded that the packness and homogeneity of nanostructured Al₂O₃ layer caused the reduction of hot corrosion products formation in the YSZ layer. In other words, the dense nanostructured Al₂O₃ layer with lower pinholes and microcracks significantly prevented the diffusion of molten salts into the YSZ layer; therefore the amount of monoclinic ZrO₂ and YVO₄ crystals weas substantially reduced at the YSZ/nano-Al₂O₃ interface after 52 h of hot corrosion test compared to those of conventional YSZ and YSZ/normal Al₂O₃ coatings.

Acknowledgments

The work is financed by the Ministry of Higher Education of Malaysia and Research Management Center of UTM (postdoctoral part). The authors also would like to acknowledge the Ministry of Higher Education of Malaysia and Universiti Teknologi Malaysia (UTM) for providing research facilities and financial support under the Grant Q.J130000.252 4.02H55.

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