The influence of laser treatment on hot corrosion behavior of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

In this study, the effect of laser glazing on the hot corrosion behavior of nanostructured thermal barrier coatings (TBCs) was investigated. To this end, the hot corrosion test of plasma-sprayed and laser-glazed thermal barrier coatings conducted against 45 wt.% Na₂SO₄ + 55 wt.% V₂O₅ molten salt at 910 °C for 30 h in open air atmosphere. The results obtained from hot corrosion test showed that the reaction between Y₂O₃ and the corrosive salt produced YVO₄, leached Y₂O₃ from YSZ and led to the progressive destabilization transformation of YSZ from tetragonal to the monoclinic phase. The lifetimes of the plasma-sprayed TBCs were enhanced approximately twofold by laser glazing. Reducing the reactive specific surface area of the dense glazed layer with the molten salts and improving the stress accommodation through network cracks produced by laser glazing were the main enhancement mechanisms accounting for TBC life extension.     

Effect of molten V2O5 salt on the corrosion behavior of micro- and nano-structured thermal sprayed SYSZ and YSZ coatings

The corrosion resistance of micro-and nano-structured scandia and yttria codoped zirconia (nano-4 mol%SYSZ and micro-8.6SYSZ) and yttria doped zirconia (4YSZ) in the presence of molten vanadium oxide were investigated. To this end, duplex TBCs (thermal barrier coatings), composed of a bond coat (NiCrAlY) and a top coat (4SYSZ or 4YSZ), were deposited on the IN738LC Ni-based supper-alloy by atmospheric plasma spraying (APS). The corrosion studies of plasma sprayed TBCs were conducted in 25 mg V₂O₅ molten salt at 910 °C for different times. The nanostructured coating, as compared to its micro-structured counterpart, in spite of a further reaction with the V₂O₅ salt, showed a higher degradation resistance during the corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones. Finally, the corrosion resistance and degradation mechanism of SYSZ and YSZ coatings were compared with the presence of molten NaVO₃ and V₂O₅ salt, respectively.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Hot-corrosion behaviors of overlay-clad yttria-stabilized zirconia coatings in contact with vanadate–sulfate salts

A dense clad overlay with chemical inertness was achieved on top of the plasma-sprayed YSZ thermal barrier coatings by laser in order to protect them from hot-corrosion attack. The Al₂O₃-clad YSZ coating exhibited good hot-corrosion behavior in contact with salt mixture of vanadium pentoxide (V₂O₅) and sodium sulfate (Na₂SO₄) for a longtime of 100 h at 1173 K. The LaPO₄-clad YSZ coating showed corrosion resistance inferior to the Al₂O₃-clad one. Yttria was leached from YSZ by reaction between Y₂O₃ and V₂O₅, which caused progressive destabilization transformation of YSZ from tetragonal (t) to monoclinic (m) phase. The chemical inertness of the clad layers and the restrained infiltration of the molten corrosive salts by the dense clad layers were primary contributions to improvement of the hot-corrosion resistances.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Hot corrosion behavior of Ba2REAlO5 (RE = Dy,Er, Yb) ceramics by vanadium pentoxide at 900–1000 °C

Hot corrosion behavior of Ba₂REAlO₅ (RE = Dy, Er, Yb) ceramics exposed to V₂O₅ molten salt at 900 °C and 1000 °C was investigated, providing a better understanding of their corrosion resistance as promising thermal barrier coatings. Obvious surface reactions occurred forming continuous, dense reaction layers on the top surfaces of the samples, the types of corrosion products being temperature and time independent. After heat treatment for 4 h and 20 h in V₂O₅ salt at the two temperatures, the corrosion products consisted of REVO₄, Ba₂REV₃O₁₁ and BaAl₂O₄ (RE = Dy, Er, Yb). Prolonged heat treatment and elevated temperature promoted the growth of Ba₂REV₃O₁₁ and REVO₄ grains. The reaction layer had a positive function on suppressing further penetration of the molten salt. The mechanism by which the corrosion reaction occurs is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

Comparison of hot corrosion behaviors of plasma-sprayed nanostructured and conventional YSZ thermal barrier coatings exposure to molten vanadium pentoxide and sodium sulfate

The purpose of the current study was evaluation and comparison of hot corrosion behaviors of plasma-sprayed conventional and nanostructured yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Hot corrosion studies were performed on the surface of coatings in the presence of a molten mixture of V₂O₅+Na₂SO₄ at 1000 °C for 30 h. Results indicated that the hot corrosion mechanisms of conventional and nanostructured YSZ coatings were similar. The reaction between corrosive salt and Y₂O₃ produced YVO₄, leaching Y₂O₃ from YSZ and causing the detrimental phase transformation of zirconia from tetragonal to monoclinic. The nanostructured coating, as compared to its conventional counterpart, in spite of a further reaction with the corrosive salt, showed a higher degradation resistance during the hot corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones.     

Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd,Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO₄ (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO₄, a dense, crack-free reaction layer, mainly composed of Ca₃Ln₇(PO₄)(SiO₄)₅O₂ apatite, CaAl₂Si₂O₈ and MgAl₂O₄, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO₄ (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO₄, SmPO₄ and GdPO₄. GdPO₄ had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO₄.     

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y₂O₃ partially stabilized ZrO₂ (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250 °C for 2 h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250 °C for 8 h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.     

Evolution of hot corrosion resistance of YSZ,Gd2Zr2O7, and Gd2Zr2O7 + YSZ composite thermal barrier coatings in Na2SO4 + V2O5 at 1050 °C

This paper compares the hot corrosion performance of yttria stabilized zirconia (YSZ), Gd₂Zr₂O₇, and YSZ + Gd₂Zr₂O₇ composite coatings in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1050 °C. These YSZ and rare earth zirconate coatings were prepared by atmospheric plasma spray (APS). Chemical interaction is found to be the major corrosive mechanism for the deterioration of these coatings. Characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) indicate that in the case of YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For the Gd₂Zr₂O₇ + YSZ composite coating, by the formation of GdVO₄, the amount of YVO₄ formed on the YSZ + Gd₂Zr₂O₇ composite coating is significantly reduced. Molten salt also reacts with Gd₂Zr₂O₇ to form GdVO₄. Under a temperature of 1050 °C, Gd₂Zr₂O₇ based coatings are more stable, both thermally and chemically, than YSZ, and exhibit a better hot corrosion resistance.     

Hot corrosion behavior of (Gd0.9Sc0.1)2Zr2O7 in V2O5 molten salt at 700–1000 °C

Hot corrosion behavior of (Gd_0.9Sc_0.1)₂Zr₂O₇ ceramic exposed to V₂O₅ molten salt at 700–1000 °C was investigated, providing better understanding of its corrosion resistance as a promising thermal barrier coating. Obvious corrosion reaction occurred between (Gd_0.9Sc_0.1)₂Zr₂O₇ and V₂O₅ molten salt after 4 h heat treatment, corrosion products being temperature dependent. At 700 °C, large amount of Sc₂O₃ doped ZrV₂O₇ and GdVO₄, together with a minor amount of Sc₂O₃-stabilized tetragonal ZrO₂ (t-ZrO₂), formed on the sample surfaces. With the increase of the test temperature, Sc₂O₃ doped ZrV₂O₇ turned to decompose, leading to the formation of more t-ZrO₂. At 900 °C and 1000 °C, the corrosion products were composed of GdVO₄ and t-ZrO₂. The mechanism by which the corrosion reaction occurs is proposed based on phase diagrams and Lewis acid-base rule.     

Plasma-sprayed La2Ce2O7 thermal barrier coatings against calcium–magnesium–alumina–silicate penetration

As one of promising thermal barrier coating (TBC) candidates, La₂Ce₂O₇ (LC) has attracted increasing attention because of its low thermal conductivity and potential capability to be operated above 1250 °C. In this paper, the microstructure evolution and mechanical properties of the plasma-sprayed LC TBC with calcium–magnesium–alumina–silicate (CMAS) glassy deposits at 1250 °C were investigated. Due to chemical reaction between the CMAS deposits and LC coating, a dense sealing layer, mainly composed of Ca₂(La_xCe_1−x)₈(SiO₄)₆O_6−4x and CeO₂, was formed on the coating after heat-treatment at 1250 °C and effectively prevented CMAS from further penetration. The interaction layer had the micro-hardness of ∼10–12 GPa, relatively harder than the LC coating.     

Investigation of reactions between vanadium oxide and plasma-sprayed yttria-stabilized zirconia coatings

The phase evolution occurring during the reaction between corrosive V₂O₅ (T_m = 690 °C) and a plasma-sprayed 7 wt.% Y₂O₃–ZrO₂ (YSZ) coating from 700 to 900 °C has been investigated in situ by X-ray diffraction. The temperature and time of interaction between the V₂O₅ and YSZ coating determines the phases observed. Between 700 and 750 °C, reaction products of ZrV₂O₇ and YVO₄ were observed within minutes of reaching the test temperature. m-ZrO₂ was observed after 220 and 60 min at 700 and 750 °C, respectively. The simultaneous formation of both ZrV₂O₇ and YVO₄ at the beginning of the reaction along with the delay of the m-ZrO₂ formation suggests similar reactivity between both Zr and Y with V₂O₅. The weight percent of the ZrV₂O₇ phase began to diminish after 150 and 60 min at 700 and 750 °C, respectively. For reaction temperatures of 800 and 900 °C, there is a rapid decrease in the amount of t′-ZrO₂ and a rapid increase in the amount of m-ZrO₂ with reaction time. YVO₄ was also observed at these reaction temperatures. SEM and TEM microstructural observations confirmed the phases detected from the in situ XRD experiments. Reactions between YSZ and V₂O₅ suggest that the formation of a liquid phase due to the high solubility of both zirconia and yttria in vanadia is the dominate mechanism that damages the coating. The thermal conductivity of a plasma-sprayed YSZ coating reacted with up to 1 wt.% V₂O₅ did not significantly change due to the small volume affected.     

Molten salt attack on t′ yttria-stabilised zirconia by dissolution and precipitation

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na₂SO₄–30 mol% NaVO₃ on ZrO₂–8 mol% YO_1.5 (8YSZ) at 950 °C was studied by two types of experiments. Sintered compacts were exposed to 25 mg cm^?2 salt dosage for up to 96 h. In the other set of experiments, 10–35 wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25 wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950 °C, followed by slow reactions to form YVO₄ and NaYV₂O₇ at 950 °C. The unreacted Y₂O₃ and monoclinic ZrO₂ precipitate out separately during rapid cooling (～300 °C/min). Slow cooling at ～3 °C/min leads to the formation of ZrOS apart from ZrO₂ and Y₂O₃.     

Characterisation of the microstructure and thermal properties of Nd2Zr2O7 and Nd2Zr2O7/YSZ thermal barrier coatings

The microstructure of following thermal barrier coatings (TBC) was characterised in this paper: monolayer coatings Nd₂Zr₂O₇ and 8YSZ; a double ceramic layered (DCL) coating. Coatings were characterised by thicknesses that did not exceed 300 μm and porosities of approx. 5%. The chemical and phase composition analysis of the DCL layers revealed an external Nd₂Zr₂O₇ ceramic layer approx. 80 μm thick, a transitional zone approx. 120 μm thick and an internal 8YSZ layer 100 μm thick. For the case of the monolayer coating, the Nd₂Zr₂O₇ pyrochlore phase was the only one-phase component. The surface topography of both TBC systems was typical for plasma sprayed coatings, and compressive stress state had a value of approx. 5–10 MPa. Measurements of the thermal parameters, i.e., thermal diffusivity, point to considerably better insulative properties for both new types of layers when compared to the standard 8YSZ layers.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Influence of the deposition energy on the composition and thermal cycling behavior of La2(Zr0.7Ce0.3)2O7 coatings

Lanthanum–zirconium–cerium composite oxide (La₂(Zr_0.7Ce_0.3)₂O₇, LZ7C3) coatings were prepared under different conditions by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphologies, cyclic oxidation behavior of these coatings were studied. Elemental analysis indicates that the coating composition has partially deviated from the stoichiometry of the ingot, and the existence of excess La₂O₃ is also observed. The optimized composition of LZ7C3 coatings could be effectively achieved by the addition of excess CeO₂ into the ingot or by properly controlling the deposition energy. Meanwhile, when the deposition energy is 1.15 × 10⁴–1.30 × 10⁴ J/cm², the coating has a similar X-ray diffraction (XRD) pattern to the ingot, and the thermal cycling life of the coating is also superior to other coatings. The spallation of the coatings occurs either within the ceramic layer approximately 6–10.5 μm above its thermally grown oxide (TGO) layer or at the interface between ceramic layer and bond coat.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Hot corrosion of V2O5-coated NdMgAl11O19 ceramic in air at 950 °C

NdMgAl₁₁O₁₉ ceramic was prepared by solid-state reaction at 1700 °C for 10 h in air, and exhibited a single phase of magnetoplumbite structure. Reaction between molten V₂O₅ and NdMgAl₁₁O₁₉ was investigated at 950 °C using an X-ray diffractometer, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Molten V₂O₅ reacts with NdMgAl₁₁O₁₉ to form α-Al₂O₃, NdVO₄ and MgAl₂O₄ at 950 °C in air. After hot corrosion at 950 °C for 50 h, α-Al₂O₃ is the main corrosion product. The thickness of the corrosion layer gradually increases with increasing corrosion time from 10 to 50 h.