Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Effects of Al and Al4C3 contents on combustion synthesis of Cr2AlC from Cr2O3-Al-Al4C3 powder compacts

Preparation of the ternary carbide Cr₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the Cr₂O₃-Al-Al₄C₃ powder compact. Effects of the contents of Al and Al₄C₃ on the product composition and combustion behavior were studied by formulating the reactant mixture with a stoichiometric proportion of Cr₂O₃:Al:Al₄C₃ = 3:5x:y, where x and y varied from 1.0 to 1.5. When compared to those of the powder compact with Cr₂O₃:Al:Al₄C₃ = 3:5:1 (i.e., x = y = 1.0), the combustion temperature and reaction front velocity increased with content of Al, but decreased with that of Al₄C₃. Besides Cr₂AlC and Al₂O₃, the final products always contained a secondary phase Cr₇C₃ that was substantially reduced by adopting additional Al and Al₄C₃ in the reactant compacts. For the sample with Cr₂O₃:Al:Al₄C₃ = 3:7.5:1 (x = 1.5), solid state combustion reached a peak temperature of 1245 °C and yielded Cr₂AlC with a trivial amount of Cr₇C₃. Although Cr₇C₃ was lessened by introducing extra Al₄C₃, the increase of Al₄C₃ from y = 1.1 to 1.5 produced almost no further reduction of Cr₇C₃ in the final product. This is partly attributed to the low combustion temperature in the range of 1065-1095 °C for the samples with additional Al₄C₃, and in part, due to the role of Al₄C₃ which might react with Cr to form Cr₇C₃, Cr₂Al, and Cr₂AlC.     

Long-Time Oxidation of Cr2AlC Between 700 and 1,000?°C in Air

Cr₂AlC compounds were synthesized via a powder metallurgical route and their long-term oxidation behavior studied. Oxidation at temperatures between 700 and 1,000?°C for up to 360?days in air resulted in formation of a thin, adherent Al₂O₃ surface layer and a narrow Cr₇C₃ sublayer, accompanied by evaporation of carbon from the Cr₂AlC. Preferential oxidation of Al on the surface suppressed oxidation of the less mobile Cr in Cr₂AlC. In the Al₂O₃ layer, (0.7–8.3)?at.% Cr was incorporated. In the Cr₇C₃ sublayer, Al was either absent or incorporated. When Cr₂AlC oxidized at 850 and 1,000?°C for 30–360?days, metastable θ-Al₂O₃ blades formed on the α-Al₂O₃ layer. However, such blades were scarcely visible when the oxidation was carried out above 1,100?°C, because of the fast θ?→?α-transition. Moreover, the θ-Al₂O₃ were not noticeable during oxidation at 700?°C for 30–360?days, due to a small extent of oxidation.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Oxidation of single crystal PWA 1483 at 950 °C in flowing air

The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10⁻⁶ mg²/(cm⁴ × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO₂ and NiTiO₃ but mostly Cr in Cr₂O₃ and (Ni/Co)Cr₂O₄ besides NiTaO₄. This outer scale is connected to a discontinuous layer of Al₂O₃ and an area of γ′-depletion within the base material.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

Oxidation of Cr2AlC Between 900 and 1200 °C in Air

High-purity, dense Cr₂AlC compounds were synthesized by hot pressing a mixture of CrC _X (x = 0.5) and Al powders. Oxidation at temperatures between 900 and 1200 °C in air for up to 480 h resulted in the formation of a thin Al₂O₃ layer. The consumption of Al to make the Al₂O₃ layer led to the enrichment of Cr immediately below the Al₂O₃ layer, resulting in the formation of an underlying Cr₇C₃ layer. At the same time, carbon escaped from the Cr₂AlC into the air. During the initial stage of oxidation, oxygen diffused inward to form the Al₂O₃ layer, which vastly improved the oxidation resistance of Cr₂AlC from the initial stage of oxidation.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 °C: A detailed microstructural investigation

Early stages of the evolution of Al₂O₃ scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigated by analytical TEM. The samples were oxidized isothermally at 900 °C in dry O₂ or O₂ + 40% H₂O for 1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing α-Al₂O₃ inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of γ-Al₂O₃ in both environments. After 24 h exposure in dry O₂, the γ-Al₂O₃ in the outer oxide layer was partly transformed to α-Al₂O₃ and spinel oxide (Mg_1−xFe_xAl₂O₄). In contrast, the γ-Al₂O₃ in the outer layer was not transformed after 24 h in O₂ + 40% H₂O, showing that water vapour stabilizes γ-Al₂O₃. All oxide scales contained a Cr-rich band, a product of the initial oxidation. The inner α-Al₂O₃ layer is suggested to nucleate on Cr₂O₃ or Cr_2−xFe_xO₃ in the initial oxide.     

Corrosion of Cr2AlC in Ar/1%SO2 Gas Between 900 and 1200?��C

Cr₂AlC compounds were synthesized by a powder metallurgical route and corrosion tested at 900, 1000, 1100 and 1200 °C for up to 150 h under an Ar/1% SO₂ gas atmosphere. The compounds were resistant to corrosion because a thin ??-Al₂O₃ barrier layer quickly formed on the surface which suppressed sulfidation. Virtually no sulfur was detected inside the scale except during the initial corrosion stage. The superior corrosion resistance of Cr₂AlC originated from the high affinity of Al for oxygen to form the thermodynamically stable Al₂O₃. Unlike Al, Cr was not active because Cr was strongly bound to carbon as Cr₂C layers in Cr₂AlC. The small amount of Cr₂O₃ that had formed was dissolved in the Al₂O₃ layer. The corrosion of Cr₂AlC resulted in the formation of an ??-Al₂O₃ layer and an underlying Cr₇C₃ layer.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Variation of microstructure and composition of the Cr2AlC coating prepared by sputtering at 370 and 500 °C

Cr₂AlC coating was deposited at 370 and 500 °C by D.C. magnetron sputtering from an as-synthesized bulk Cr₂AlC target. The phase composition and preferential orientation of the coating were investigated using XRD, and the microstructure of the coating was characterized by TEM. Results indicated that Cr₂AlC coating with a strong (110) preferential orientation could be obtained. The coating microstructure was clearly affected by the deposition temperature. At 370 °C, the deposited coating possessed a triple-layered structure with an α-(Cr, Al)₂O₃ inner layer, an amorphous intermediate layer and a crystalline Cr₂AlC outer layer. However, the coating deposited at 500 °C had a single-layered structure consisting of crystalline Cr₂AlC layer. The growth mechanism of the Cr₂AlC coating at different deposition temperatures is discussed.     

Oxidation behaviour of Ti–Al–C films composed mainly of a Ti2AlC phase

This paper addresses the oxidation behaviour of Ti–Al–C films composed mainly of a Ti₂AlC phase. The films exhibited rather low oxidation rates at 600 and 700 °C, with an oxygen-rich zone or a thin oxide layer appearing on the film surfaces. Much faster oxidation rates were observed at 800 and 900 °C. The Ti₂AlC phase was quickly consumed by oxidation. From the film surface to the inner zone, TiO₂-rich layer, Al₂O₃-rich layer, and TiO₂ + Al₂O₃ mixed layer was observed, respectively. The oxidation mechanism of the Ti–Al–C film is discussed based on the experimental results.     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Early high temperature oxidation behaviour of Ti(C,N)-based cermets in air

Isothermal oxidation behaviour of two Ti(C,N)-based cermets (TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32-x)Ni-xCr, x = 0 and 6.4 wt%) was investigated in air at 800-1100 °C up to 2 h. Mass gains exhibited neither linear nor parabolic law during isothermal oxidation. The oxide scales formed at 800-1100 °C were multi-layered, consisting of NiO outerlayer, NiTiO₃ interlayer and TiO₂-based innerlayer. The internal oxidation zones formed at 1000-1100 °C consisted of Ti-based, Ni-based and Mo-based complex oxides. Cermet with 6.4 wt% Cr exhibited superior oxidation resistance, due to the presences of Cr_0.17Mo_0.83O₂ in TiO₂-based innerlayer of the oxide scale and Cr-rich Ti-based complex oxide in the internal oxidation zone.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Sulphidation/oxidation behaviour of TiAlCr and Al2Au coated Ti45Al8Nb alloy at 750 °C

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al₂Au deposited by physical vapour deposition. The coated alloy was subjected to a H₂/H₂S/H₂O yielding pS₂ - 10⁻¹ Pa and pO₂ - 10⁻¹⁸ Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb - base alloy, Al₂O₃, TiO₂ and Cr₂S₃ developed. For the Al₂Au coated Ti45Al8Nb samples an Al₂O₃ scale containing TiO₂ nodules was observed at the surface.     

Oxidation and corrosion of silicon nitride ceramics with different sintering additives at 1200 and 1500 °C in air, water vapour, SO2 and HCl environments - A comparative study

The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of silicon nitride based ceramics was investigated. Comparative tests were conducted at 1200 and 1500 °C in air, in water vapour, and with the highly corrosive gases HCl and SO₂. Si₃N₄ was prepared with MgO, Al₂O₃, Y₂O₃ and Al₂O₃ + Y₂O₃ sintering additives. Hot pressed discs were tested for a total time of up to 128 h. The electrically conductive ceramic composites Si₃N₄ + TiN and Si₃N₄ + MoSi₂ were also tested under the same conditions. The effects that the different corrosion environments have on the different ceramics are presented. SEM studies of the oxidised ceramics show the direct transformation of Si₃N₄ grains into SiO₂ through a reaction interface layer.     

Type II Hot Corrosion Screening Tests of a Cr<Subscript>2</Subscript>AlC MAX Phase Compound

Low-temperature hot corrosion tests were performed on bulk Cr₂AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na₂SO₄ salt deposits needed to form corrosive mixed (Ni,Co)SO₄–Na₂SO₄ eutectic salts active in Type II hot corrosion. Cr₂AlC samples coated with 20K₂SO₄–80Na₂SO₄ salt were exposed to 300 ppm SO₂ at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (<?1/10) compared to a Ni(Co) superalloy baseline material. Layered Al₂O₃/Cr₂O₃ scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr₇C₃ impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr₂O₃ scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr₂AlC. This indicates some potential for Cr₂AlC as LTHC-resistant coatings for superalloys.