Isothermal Oxidation Resistance Research Articles

Improvement in oxidation behavior of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy superalloys by minor Nb addition

High-entropy superalloys (HESAs) can replace commercial Ni-based superalloys. For high-temperature applications, the oxidation resistance of HESAs must be considered. Herein, the oxidation mechanism of HESA with sufficient minor Nb addition was conducted at 900 °C under atmosphere. With 0.9 at% Nb added in the base metal, oxidation resistance was significantly improved, which was confirmed by a furnace test and thermogravimetry. The isothermal oxidation resistance was enhanced by approximately 66%, owing to the presence of the Nb-rich layer. This improvement was observed and analyzed by backscattering electron images through scanning electron microscopy and wavelength dispersive spectroscopy with a field-emission electron probe microanalyzer. The mechanism of oxide formation was elucidated by X-ray diffraction for various exposure time durations. With Nb minor addition, the microstructures of the present alloy were found to be composed of γ matrix and γ′ precipitates and the mechanical properties were slightly higher than those without Nb.

Microstructural characterization of YSZ-CoNiCrAlY two-layered thermal barrier coating formed on γ-TiAl intermetallic alloy via APS process

With the aid of development in materials science in recent years, γ-TiAl alloys have been extensively utilized to reduce mass density and enhance mechanical characteristics of gas turbine blades used in the last stage of low pressure part. In the current study, the γ-TiAl alloy with chemical composition of Ti-48Al-2Nb–2Cr (at. %) is employed as the substrate material. Two different coatings including a single-layer coating applied via Co-based powder of AMDRY 995C [CoNiCrAlY] and a two-layered TBC coating consisted of a ceramic top coat of YSZ and a metallic bond coat of CoNiCrAlY using APS process at 2000°C. Microstructural analysis as well as the isothermal oxidation resistance of the both obtained coatings at 1050°C is investigated. For the two-layered TBC coating, it is demonstrated that the dominant compounds include ZrO2 oxide with a little amount of Y2O3 oxide. The presence of these oxides plays the essential role in high temperature oxidation resistance of the formed TBC coating on the γ-TiAl alloy. Furthermore, it is revealed that the best high temperature oxidation resistance is obtained by the two-layered TBC coating on the γ-TiAl substrate.

Effect of vacuum heat treatment on the oxidation kinetics of freestanding nanostructured NiCoCrAlY coatings deposited by high-velocity oxy-fuel spraying

In this study, the nanostructured NiCoCrAlY powder feedstock was coated on the substrate via the high-velocity oxy-fuel spraying process. Freestanding nanostructured NiCoCrAlY coatings were then vacuum heat-treated at 1100 °C for 1, 2, and 5 h under a pressure of 10−3 Pa. The microstructure examinations of the nanostructured NiCoCrAlY coatings before and after vacuum heat treatment were investigated using x-ray diffraction and the field-emission scanning electron microscope equipped with electron dispersed spectroscopy analysis. In order to measure the high-temperature oxidation resistance, the freestanding coatings were evaluated using the isothermal oxidation test at 1000 and 1100 °C, respectively, under the air atmosphere. The obtained results showed that homogeneous and adherent α-Al2O3 was successfully formed on the surface of the heat-treated coating during isothermal oxidation. Results also indicated that the heat-treated NiCoCrAlY coating had a higher isothermal and cyclic oxidation resistance. As a consequence, a lower oxide growth rate was achieved by controlling diffusion due to the finer and denser structure of Al2O3 on the heat-treated nanostructured coating for 2 h after the oxidation test.

Improvement of high-temperature oxidation resistance of γ-TiAl intermetallic alloy by YSZ-NiCoCrAlY coating using APS process

As an important class of high temperature structural materials, γ-TiAl intermetallic alloys have demonstrated unique mechanical characteristics which can be considered as essential payoffs for industrial gas turbines and other applications. In the present investigation, the γ-TiAl alloy with chemical composition of Ti-48Al-2Nb–2Cr (at%) is put to use as the substrate material. Two different coatings are then employed namely as a single-layer coating applied via Ni-based powder of AMDRY 365-2 [NiCoCrAlY] and a two-layered TBC coating consisted of a ceramic top coat of Yttria-stabilized zirconia (YSZ) and a metallic bond coat of NiCoCrAlY using atmospheric plasma spraying (APS) process at A comprehensive microstructural analysis as well as the isothermal oxidation resistance is performed for the both coated intermetallic samples at It is found that for the two-layered thermal barrier coating (TBC) coating, there are the dominant compounds which include ZrO2 oxide with a little amount of Y2O3 oxide. It is indicated that the presence of these oxides has a substantial influence on the high temperature oxidation resistance of the formed TBC coating on the γ-TiAl alloy. Moreover, it is shown that the best high temperature oxidation resistance can be acquired via the two-layered TBC coating on the intermetallic substrate of γ-TiAl.

High-temperature oxidation of Ti–Al–Si alloys prepared by powder metallurgy

The Ti–Al–Si alloys are promising materials for high-temperature use in automotive, aerospace or cosmic industry. The main advantages of these alloys are their low density (approximately 4 g/cm3), good oxidation resistance, and mechanical properties at elevated temperatures. Addition of silicon into the Ti–Al alloys improves the high-temperature behaviour and improves compactness and adhesion of the oxide layer. The resistance against oxidation can be effectively improved also by an appropriate technology of preparation. In this work, the high-temperature cyclic and isothermal oxidation resistance of the Ti–Al–Si alloys are described. The effect of powder metallurgy production route (reactive sintering, mechanical alloying, Spark Plasma Sintering) on high-temperature behaviour was compared. Cyclic and isothermal oxidation tests were carried out at 800 °C and 1000 °C, well above the air-operating limit for TiAl. It was confirmed, that the Ti–Al–Si alloys are resistant at temperature 800 °C, where only very thin oxide layer was formed. Mechanical alloying followed by Spark Plasma Sintering improved the high-temperature behaviour of these alloys, the oxide layer was even thinner.

Oxide Dispersion Strengthened Bond Coats with Higher Alumina Content: Oxidation Resistance and Influence on Thermal Barrier Coating Lifetime

The oxidation resistance of the bond coat in thermal barrier coating systems has significant influence on thermal cycling performance of the protective coating. In this study, the influence of varying the alumina content of plasma-sprayed oxide dispersion strengthened bond coats with CoNiCrAlY matrix material on the oxidation resistance was analysed by thermogravimetric analysis, SEM and TEM. Yttrium ions at the alumina scale grain boundaries and the grain size in the scale appear as major factors influencing oxidation properties. The ODS material with 2, 10 and 30 wt% alumina content was applied in TBC systems as an additional thin bond coat. The thermal cycling performance of those advanced TBC systems, in burner rig tests, was evaluated with respect to the ODS material properties. Thermal cycling behaviour is in good correlation with the isothermal oxidation resistance. All results indicate that TBC systems with 10 wt% alumina content in the ODS bond coat have a superior thermal cycling performance, as compared to ODS bond coats with lower or higher alumina content.

Boronizing of the silicide coating on a multicomponent Nb[sbnd]Ti[sbnd]Si based alloy as protection against corrosion at high temperatures

In this work, the preparation, high temperature oxidation and hot corrosion behavior of multilayered composite coatings prepared on NbTiSi based alloy have been reported. By boronizing of the SiY co-deposition silicide coating, part of (Nb,X)Si2 layer was transformed into (Nb,X)B2 by a replacement reaction between the active B atom and (Nb,X)Si2. Isothermal oxidation resistance tests showed that the as-prepared coating can provide good oxidation resistance both at an intermediate temperature of 850 °C and a high oxidation temperature of 1250 °C. The oxidation kinetics of the coating followed the parabolic law at both temperatures. After 100 h of exposure at 850 and 1250 °C, the mass gains are 1.35 and 4.05 mg/cm2 respectively. The good oxidation resistance of the composite coating should be attributed to the formation of the dense glass-like scale. At 900 °C, the Na2SO4-25 wt% NaCl molten salts can cause a serious damage to the composite coating which showed a big mass gain and a thick corroded scale after hot corrosion for 100 h.

Role of grain boundary engineered microstructure on high temperature steam oxidation behaviour of Ni based superalloy alloy 617

The effect of grain boundary engineered microstructure on isothermal and cyclic high temperature oxidation of Ni based alloy 617 in steam comprising of 20% H2O vapour is studied. The grain boundary character and network in alloy 617 was modified through thermomechanical processing by subjecting material to cold rolling and annealing. The isothermal oxidation resistance improved significantly after the thermomechanical processing and is attributed to the increase in twin boundary fraction and disruption of random high angle grain boundary network. Weight loss due to spallation and volatilization of chromium oxide is observed in the thermo-mechanically processed specimen during cyclic oxidation. It is attributed to reduction in grain size and non-uniform grain size distribution following thermomechanical processing.

Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings

Ti-Al-based intermetallics are promising candidates as coating materials for thermal protection systems in aerospace vehicles; they can operate just below the temperatures where ceramics are commonly used, and their main advantage is the fact that they are lighter than most other alloys, such as MCrAlY. Therefore, Ti-Al-Si alloy coatings with five compositions were manufactured by spraying pure Ti and Al-12 wt.% Si powders using warm spray process. Two-stage hot pressing at 600 and 1000 °C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical composition, and phase composition of the as-deposited and hot-pressed coatings were investigated using SEM, EDS, and XRD. Applying of hot pressing enabled the formation of dense coatings with porosity around 0.5% and hard Ti5(Si,Al)3 silicide precipitates. It was found that the Ti5(Si,Al)3 silicides existed in two types of morphologies, i.e., as large particles connected together and as small isolated particles dispersed in the matrix. Furthermore, the produced coatings exhibited good isothermal and cyclic oxidation resistance at a temperature of 750 °C for 100 h.

Effect of Si content on microstructure, mechanical and oxidation properties of hot pressed Mo-Ti-Si alloys

Mo-Ti-Si based materials are promising candidates for high-temperature applications beyond the capability limits of nickel based superalloys. In the present investigation, two alloy compositions with different Si content namely Mo-40Ti-30Si (in at.%) (alloy 1) and Mo-40Ti-10Si (alloy 2) were prepared in the form of plates by adopting mechanical alloying and subsequent reactive hot pressing at 1600 °C. The densities of the alloys were found to be much lower than the conventional alloys used for high-temperature applications. The detailed microstructural characterization using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD) analysis indicated that alloy 1 was composed with Ti(Mo)5Si3 matrix and Ti(Mo)3Si precipitates. Alloy 2 consisted with three phase microstructure containing α-Mo(Ti)ss, β-Ti(Mo)ss and Mo(Ti)3Si. Micro-hardness values of 1400 H V and 850 H V respectively for alloy 1 and alloy 2 were noticed, however, alloy 1 showed the tendencies of micro-crack formation from the corners of the Vickers indentations. Alloy 1 showed a superior isothermal oxidation resistance at 900 °C, 1000 °C, and 1300 °C for prolonged duration due to formation of a thin duplex SiO2 (TiO2) oxide scale. However, the formation of a porous oxide layer led to evaporation of MoO3, showing drastic weight loss of alloy 2 at all three tested temperatures. Enrichment of Si was done using pack siliconizing process forming a (Mo, Ti)Si2 layer of thickness 10 μm and 30 μm on the surfaces of alloys 1 and 2 respectively at 900 °C. The silicide coated alloy 1 showed further improvement in the oxidation behaviour, and enrichment of Si on alloy 2 surface imparted a drastic improvement in the resistance against oxidation in air at 1300 °C for the prolonged exposure times of 70 h.

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50 nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400 µm, and the outer two layers are about 200 µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773 K for 361 h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80 mm) after erosion at 1873 K for 130 h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.

Oxidation protection and mechanism of the HfB2-SiC-Si/SiC coatings modified by in-situ strengthening of SiC whiskers for C/C composites

To improve the oxidation resistance and alleviate the thermal stress of the HfB2-SiC-Si/SiC coatings for C/C composites, in-situ formed SiC whiskers (SiCw) were introduced into the HfB2-SiC-Si/SiC coatings via chemical vapor deposition (CVD). Effects of SiCw on isothermal oxidation and thermal shock resistance for the HfB2-SiC-Si/SiC coatings were investigated. Results showed that the SiCw-HfB2-SiC-Si/SiC coatings exhibited excellent oxidation resistance for C/C composites with only 0.88% weight loss after oxidation for 468 h at 1500 °C, which was markedly superior to 4.86% weight loss for coatings without SiCw. Meanwhile, after 50 times thermal cycling, the weight loss of the SiCw-HfB2-SiC-Si/SiC coated samples was 4.48%, which showed an obvious decrease compared with that of the HfB2-SiC-Si/SiC coated samples. The SiCw-HfB2-SiC-Si/SiC coatings exhibited excellent adhesion to the C/C substrate and had no penetrating cracks after oxidation. The improved performance of the SiCw-HfB2-SiC-Si/SiC coatings could be ascribed to the SiCw, which effectively relieved CTE mismatch and remarkably suppressed the cracks through toughening mechanisms including whiskers pull-out and bridging strengthening. The above results were confirmed by thermal analysis based on the finite element method, which demonstrated that SiCw could effectively alleviate thermal stress generated by temperature variation. Furthermore, the SiCw-HfB2-SiC-Si/SiC coating can provide a promising fail-safe mechanism during the high temperature oxidation by the formation of HfSiO4 and SiO2, which can deflect cracks and heal imperfections.

Isothermal and cyclic oxidation resistance of pack siliconized Mo–Si–B alloy

Oxidation behaviour of MoSi2 coated Mo–9Si–8B–0.75Y (at.%) alloy has been investigated at three critical temperatures including 750, 900 and 1400°C in static air. Thermogravimetric analysis (TGA) data indicates a remarkable improvement in the oxidation resistance of the silicide coated alloy in both isothermal and cyclic oxidation tests. The cross-sectional scanning electron microscopy and energy dispersive spectroscopic analysis reveal the occurrence of internal oxidation particularly at the crack fronts formed in the outer MoSi2 layer during thermal cycling. The dominant oxidation mechanisms at 750–900°C and 1400°C are identified. Development of MoB inner layer further improves the oxidation resistance of the silicide coated alloy.

High-temperature oxidation behaviour of novel Co-Al-W-Ta-B-(Mo, Hf, Nb) alloys with a coherent γ/γ'–dominant microstructure

In this work, 2at% Mo, 2at% Nb and 2at% Hf were substituted for the same amount of W into a Co-9Al-9W-2Ta-0.02B alloy (hereafter referred as to 2Mo, 2Nb and 2Hf alloys, respectively, while the original alloy is denoted as 0Me alloy). The effect of the Mo, Hf and Nb additions on the isothermal oxidation resistance, oxide scale evolution and failure mechanism, of the Co-9Al-9W-2Ta-0.02B alloy when exposed at 800°C and 900°C for 100h was investigated. It was found the Mo, Hf and Nb additions degraded the oxidation resistance of the Co-9Al-9W-2Ta-0.02B alloy, while the 2Mo alloy always displayed the poorest oxidation resistance, resulted from heavy spallation of the oxide scale. An oxide scale composed of an outer Co3O4+CoO layer, a middle complex oxide layer enriched with Al, W and Ta, and a γ/needle-like Co3W zone adhering to the γ/γ' substrate was gradually formed; moreover, a continuous or discontinuous Al2O3 layer and dispersive Al2O3 dots or slices were observed within the γ/needle-like Co3W zone, depending on the oxidation temperature and added elements (Mo, Hf and Nb). The formation of volatile MoO3 in the oxide scale of the 2Mo alloy enhance the exfoliation of the oxide products, resulting in severe spallation and poor oxidation resistance.

Isothermal Oxidation of Aluminized Coatings on High-Entropy Alloys

The isothermal oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy is analyzed and the microstructural evolution of the oxide layer is studied. The limited aluminum, about 3.6 at %, leads to the non-continuous alumina. The present alloy is insufficient for severe circumstances only due to chromium oxide that is 10 μm after 1173 K for 360 h. Thus, the aluminized high-entropy alloys (HEAs) are further prepared by the industrial packing cementation process at 1273 K and 1323 K. The aluminizing coating is 50 μm at 1273 K after 5 h. The coating growth is controlled by the diffusion of aluminum. The interdiffusion zone reveals two regions that are the Ti-, Co-, Ni-rich area and the Fe-, Cr-rich area. The oxidation resistance of aluminizing HEA improves outstandingly, and sustains at 1173 K and 1273 K for 441 h without any spallation. The alumina at the surface and the stable interface contribute to the performance of this Al0.2Co1.5CrFeNi1.5Ti0.3 alloy.

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO4, and YSZ-20%LaPO4 coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO4 was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO4 phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000°C for 120h. After 120h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.

HfC nanowires to improve the toughness and oxidation resistance of Si-Mo-Cr/SiC coating for C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites, a dense HfC nanowire-toughened Si-Mo-Cr/SiC multilayer coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, thermal shock and isothermal oxidation resistance of the coating were investigated. HfC nanowires could improve the toughness of the coating and suppress the coating cracking. After incorporating HfC nanowires in the coating, both of the thermal shock and isothermal oxidation resistance of the coating were obviously improved. The multilayer coating with HfC nanowires could effectively protect C/C composites at 1773K for 270h, whose weight loss is only 0.19%. The good oxidation resistance is mainly attributed to the formation of a compound glass layer containing SiO2 and Cr2O3.

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm2. The parabolic rate constant, kp (mg2/cm4·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al2O3 layer between the outer TiO2 layer and the inner (TiO2-rich, Al2O3-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.

Oxidation resistance and mechanical properties of HfC nanowire-toughened ultra-high temperature ceramic coating for SiC-coated C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites, a dense HfC nanowire-toughened ultra-high temperature ceramic multiphase coating was prepared on SiC-coated C/C composites by chemical vapor deposition (CVD) and pack cementation. The microstructure, mechanical and oxidation resistance properties of the coating were investigated. The results show that the HfC nanowires in the coating could suppress the cracking of the coating and then improve the toughness of the coating. The flexural property, thermal shock and isothermal oxidation resistance of the coating were all improved due to the incorporation of HfC nanowires.

Microstructure and Isothermal Oxidation Resistance of Thermal Barrier Coatings Deposited by LPPS, CVD and PS-PVD Methods on Inconel 617 Nickel Superalloy

The paper presents results of research into thermal barrier coatings characterized by higher oxidation resistance. Bondcoats were formed by overaluminizing of the MeCrAlY coating, deposited by low pressure plasma spraying (LPPS). The outer ceramic layer of yttrium oxide stabilized zirconia oxide was deposited by plasma spray physical vapour deposition (PS-PVD). For comparison purposes additionally formed were MeCrAlY bondcoats, which were not subsequently aluminized. The research showed that during CVD overaluminizing there was formed an additional layer built of the β-NiAl phase, which protects the base material from oxidation. Preserved below it increased chromium content ensures resistance to hot corrosion. The outer layer was characterized by columnar structure, similar to that obtained in the EB-PVD process. The isothermal oxidation tests showed that thickness of the TGO layer into overaluminized bondcoat significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats.