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.     

Hot corrosion behavior of CoWSi/WSi2 coating exposed to Na2SO4+NaCl salt at 900 °C

In this study, the high temperature hot corrosion behavior of a CoWSi/WSi₂ composite coating was investigated. Hot corrosion studies were performed on CoWSi/WSi₂ coated nickel specimens after exposure to a molten Na₂SO₄+NaCl salt environment at 900 °C under cyclic conditions. Thermogravimetric technique was used to establish the kinetics of corrosion. XRD and SEM/EDS techniques were used to analyze the corrosion products. The oxide scale formed on the coating surface was complex and the hot corrosion resistance of coating may be attributed to the formation of oxides and spinels of silicon, cobalt and tungsten. Also, NaCl accelerated the degeneration of the coating because of producing the volatile CoCl₂ and thereby oxygen and sulfur could easily penetrate into the coatings and caused the formation of internal oxide and sulfide.     

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.     

Evolution of hot corrosion resistance of conventional CSZ and MoSi2 self-healing thermal barrier coatings in Na2SO4+V2O5 at 950 °C

ZrO₂-based hot corrosion-resistant thermal barrier coatings (TBCs) with MoSi₂+Al₂O₃ have gained increasing attention. In this research, a novel dual-layer TBC (CSZ: ZrO₂-25 wt% CeO₂-2.5 wt% Y₂O₃/MAC: MoSi₂ + Al₂O₃ + CSZ) was developed, and its hot corrosion was compared to a single-layer CSZ. The atmospheric plasma spray (APS) process was utilized to apply CSZ/MAC and CSZ TBCs on NiCrAlY, as a bond coat to nickel-based superalloy (IN738LC). Different investigations, including hot corrosion test, field emission scanning electron microscopy (FESEM/EDS), and X-ray diffraction (XRD) analyses, were used to reveal why the MAC overlayer improves the CSZ hot corrosion behavior. A medium of Na₂SO₄-55 wt% V₂O₅ was used to analyze the hot corrosion; a temperature of 950 °C for 2 h was considered in every single cycle. The results exposed that there is a big difference between the hot corrosion resistance of the dual-layer CSZ/MAC TBC in comparison with the single-layer CSZ. Based on the FESEM analysis, this can be related to the very low diffusion of Na₂SO₄-55 wt% V₂O₅ into the dual-layer TBC where the infiltration of aggressive molten salt was diminished. According to the XRD results, two reasons are leading to the degradation of the aforementioned TBCs: (i) the tetragonal to the monoclinic transformation of ZrO₂ and (ii) the formation of hot corrosion products, i.e., CeVO₄ and YVO₄ crystals.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

Hot corrosion behavior of YSZ/ZrW2O8 composites exposed to V2O5 at 700 °C and 850 °C in air

The hot corrosion behavior of YSZ/ZrW₂O₈ composites as a promising thermal barrier coating system exposed to V₂O₅ at 700 °C and 850 °C was investigated in order to better understand the influence of the incorporated ZrW₂O₈ with isotropic negative thermal expansion performance on the corrosion resistance. Results indicate that the ZrW₂O₈ incorporation could retard the degradation of YSZ from V₂O₅ attack and the corrosion process is significantly related to the inclusion content and the temperature. The corrosion resistance could be determined by the incorporation content, while the reaction products are only temperature dependent. At 700 °C, ZrV₂O₇, YVO₄ and m-ZrO₂ were the main corrosion products, while ZrW₂O₈ recrystallized under the acidic environment provided by V₂O₅. At 850 °C, ZrW₂O₈ decomposed and only WO₃, YVO₄ and m-ZrO₂ could be detected as final corrosion products. The corrosion mechanisms of YSZ/ZrW₂O₈ composites at 700 °C and 850 °C were discussed based on the phase diagrams and Lewis acid-base rule as well as the volume compensation of the positive and negative expansion ceramics.     

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

Effects of sintering additives on hot corrosion behavior of γ-Y2Si2O7 ceramics in Na2SO4+V2O5 molten salt

Dense γ-Y₂Si₂O₇ ceramics were prepared by moulding and sintering of pure γ-Y₂Si₂O₇ powders synthesized by adding various additives. Effects of sintering additives on hot corrosion behavior of γ-Y₂Si₂O₇ ceramics in Na₂SO₄ + V₂O₅ molten salts were systematically investigated. Chemical kinetics of corrosion process was also calculated to illuminate the influence of different additives on chemical stability. Results showed that corrosion reaction started at grain boundary due to the loose microscopic network. When the intercrystalline glass phase was completely etched, the molten salts began to contact Y₂Si₂O₇, forming NaY₉Si₆O₂₆ and YVO₄. Compared with Li₂O and MgO, intercrystalline glass phase formed by Al₂O₃ additive had the most compact microscopic network structure, leading to the best chemical stability. Apparent activation energy for hot corrosion reaction of γ-Y₂Si₂O₇ ceramics in pure Na₂SO₄, Na₂SO₄ + 5 wt% V₂O₅, Na₂SO₄ + 10 wt% V₂O₅, Na₂SO₄ + 15 wt% V₂O₅ molten salts was calculated to be 408.16, 373.60, 310.62, and 249.63 kJ/mol, respectively.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.     

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.     

Anisotropic corrosion of Ti2AlC and Ti3AlC2 in supercritical water at 500 ºC

Textured and untextured M_n+1AX_n compounds, Ti₂AlC and Ti₃AlC₂, namely MAX phases have been synthesized and examined with respect to their corrosion resistance in static supercritical water at 500 °C. The textured ceramics were obtained by hot forging process at high temperatures. Both X-ray diffraction and SEM analysis revealed well alignment of c-plane of MAX phases parallel to the hot-forging surface. Better corrosion resistance on the surface perpendicular to the hot-forged direction was verified by SEM. On the other hand, the side surfaces of the samples showed thick oxidation layers and abundant cracks. The (00l) faces consist of strongly bonded Ti₃C₂ and Ti₂C layers in Ti₃AlC₂ and Ti₂AlC, respectively, hence exhibit higher resistance to water corrosion. On the contrary, the side surfaces where most of weakly bonded interlayers of these hexagonal phases were exposed tend to be easily corroded especially through Al-layers. The corrosion process involved a phase transition of oxidized product, i.e. TiO₂ from anatase to rutile phase, which gave rise to the formation of cracks due to accompanied volume changes.     

Phase and microstructure evolution of Sc2O3–CeO2 –ZrO2ceramics in Na2SO4 + V2O5 molten salts

In this study, the destabilization resistance of Sc₂O₃ and CeO₂ co-stabilized ZrO₂ (SCZ) ceramics was tested in Na₂SO₄ + V₂O₅ molten salts at 750°C–1100 °C. The phase structure and microstructure evolution of the samples during the hot corrosion testing were analyzed with X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). Results showed that the destabilization of SCZ ceramics at 750 °C was the result of the chemical reaction with V₂O₅ to produce m-ZrO₂ and CeVO₄, and little ScVO₄ was detected in the Sc₂O₃-rich SCZ ceramics. The primary corrosion products at 900 °C and 1100 °C were CeO₂ and m-ZrO₂ due to the mineralization effect. The Sc₂O₃-rich SCZ ceramics exhibited excellent degradation resistance and phase stability owing to the enhanced bond strength and the decreased size misfit between Zr⁴⁺ and Sc³⁺. The destabilization mechanism of SCZ ceramic under hot corrosion was also discussed.     

Effects of molten Na2SO4 on wet oxidation behavior of SiCf/SiC composites above 1200 ℃

For SiC_f/SiC composites used in the hot section components of turbine engines, corrosion under high temperature oxidative environment has become potential threat to the service life of the materials. Here, the corrosion behaviors of SiC_f/SiC composites exposed to wet-oxygen and Na₂SO₄-wet-oxygen environments were discussed respectively. In wet-oxygen environment, the oxide scale formed on SiC_f/SiC surface is thin and porous, leading to slight oxygen infiltration into SiC_f/SiC and thus slow weight gain. However, in Na₂SO₄-wet-oxygen environment, molten Na₂SO₄ can affect the phase transformation of SiO₂. With the temperature rising to 1400 ℃, the weight gain rates of SiC_f/SiC begin to decrease and Na₂SO₄ melt penetrates into the composites, which can bring more serious corrosion on SiC_f/SiC. This work can offer important guidance to improve the corrosion resistance of SiC_f/SiC composites.     

Thermal corrosion behavior of Yb4Hf3O12 ceramics exposed to calcium-ferrum-alumina-silicate (CFAS) at 1400 °C

In current work, the interaction between representative CFAS deposit (33CaO-10FeO_1.5-13AlO_1.5-44SiO₂) and Yb₄Hf₃O₁₂ ceramics at 1400 °C was investigated. Results indicated that the Yb₄Hf₃O₁₂ ceramics are of high resistance to infiltration of CFAS melt. Microstructure characterization revealed that Yb₄Hf₃O₁₂ reacted with CFAS to form a continuous reaction layer mainly composed of Yb-Ca-Si apatite, which inhibits CFAS further infiltration. Before the formation of the reaction layer, CFAS melts underwent a crystallization process at high temperatures, precipitating CaYbFeAlSi-garnet, which raised the viscosity of CFAS and thus inhibited the fluidity of CFAS.     

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.     

Calcium–magnesium–aluminosilicate corrosion behaviors of rare-earth disilicates at 1400 °C

Environmental barrier coatings (EBCs) are used to prevent oxidation of underlying ceramic matrix composite (CMC) structural components in gas turbines. When the siliceous minerals deposit on the surface of EBCs, a glassy melt of calcium–magnesium–aluminosilicate (CMAS) will be formed, leading to the EBCs degradation. In this study, seven rare-earth disilicates (RE₂Si₂O₇, RE = Yb, Lu, La, Gd, Eu, Sc, and Y) were fabricated to analyze their CMAS corrosion behaviors. The results indicated that the RE₂Si₂O₇ could react with the CMAS in the temperature range of 1250–1350 °C. Reaction zones formed at the interfaces. For the Yb₂Si₂O₇, Lu₂Si₂O₇, La₂Si₂O₇, Eu₂Si₂O₇ and Gd₂Si₂O₇, the reaction zones dissolved into the molten CMAS and separated from the RE₂Si₂O₇. As for the Sc₂Si₂O₇ and Y₂Si₂O₇, the reaction zones could stay at the interface. They could effectively block the molten CMAS to penetrate into the RE₂Si₂O₇ and protect them from CMAS corrosion.     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Interactions of Na2SO4 and yttrium-silicate environmental barrier coatings at 825°C

Yttrium-silicates (Y₂Si₂O₇ and Y₂SiO₅) are candidate environmental barrier coating (EBC) materials for silicon carbide ceramic matrix composites (SiC-CMCs). These materials’ high-temperature, high-velocity steam, and siliceous debris resistance are well studied. However, Na₂SO₄-induced hot corrosion mechanisms are less understood. Free-standing atmospheric plasma sprayed Y₂Si₂O₇ and Y₂SiO₅ coupons were exposed to 2.5 mg/cm² of Na₂SO₄ at 825°C in 0.1% SO₂-O₂ (g). Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and inductively coupled plasma-optical emission spectrometry were used to identify a previously unknown damage mechanism. Water-soluble Y and Na-Y sulfates and oxysulfates formed in reaction with Na₂SO₄, causing significant damage to the yttrium-silicate EBCs materials.     

Hot corrosion behavior of Y4Al2O9 ceramics for thermal barrier coatings exposed to calcium-magnesium-alumina-silicate at 1250 °C

The degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) attack has become increasingly dramatic. Y₄Al₂O₉ ceramic, a new potential TBC candidate, has received an increasing attention. In this study, porous Y₄Al₂O₉ ceramic pellets, instead of actual TBCs, are used to investigate the CMAS corrosion resistance at 1250 °C. Results indicate that Y₄Al₂O₉ reacts with CMAS melt to form an impervious sealing layer mainly containing Ca-Y-Si apatite, which could mitigate CMAS further penetration. Once the sealing layer formed, further reaction would occur above the layer accompanying by the recession of sealing layer. This process is probably related to a solid state diffusion.     

The corrosion behaviors of thermal barrier material of M-YTaO4 attacked by CMAS at 1250 °C

The corrosion of YSZ TBCs attacked by calcium–magnesium–aluminosilicate (CMAS) is a serious problem. Yttrium tantalite (YTaO₄), a new kind of potential thermal barrier ceramic material, was expected to replace the YSZ to manufacture the TBCs because of its great thermophysical characteristics. In this study, porous YTaO₄ ceramic pellets, instead of actual TBCs, were used to investigate the CMAS corrosion resistance at 1250 °C. Results indicated that CMAS couldn't cover the whole surface of YTaO₄ pellets homogeneously because of low wettability between liquid CMAS and YTaO₄, in addition, there was almost no reaction layer after 4 h reaction. The XRD results showed that M-YTaO₄, M′-YTaO₄, Ca₂Ta₂O₇ and Y₂Si₂O₇ were the main four phases after reaction and there was no phase containing the elements of Mg and Al. Compared with YSZ TBCs, this new kind of potential thermal barrier ceramic material showed well resistance to CMAS corrosion.