Isothermal Oxidation Performance of Laser Cladding Assisted with Preheat (LCAP) Tribaloy T-800 Composite Coatings Deposited on EN8

It is anticipated that laser cladding assisted with preheat (LCAP)-deposited Tribaloy (T-800) composite coatings enhances resistance to structural degradation upon exposure to elevated-temperature oxidation service environments. The oxidation kinetics of LCAP T-800 composite coatings deposited on EN8 substrate and its mechanisms have not been explored in severe conditions that are similar to operational parameters. The isothermal oxidation behaviour of the T-800 composite coating deposited on EN8 via LCAP was studied at 800 °C in air for up to 120 h (5 × 24 h cycles) and contrasted to that of uncoated samples. The mass gain per unit area of the coating was eight times less than that of the uncoated EN8 substrate. The parabolic rate constant (Kp) for EN8 was 6.72 × 10−12 g2·cm−4·s−1, whilst that for the T-800 composite coating was 8.1 × 10−13 g2·cm−4·s−1. This was attributed to a stable chromium oxide (Cr2O3) layer that formed on the coating surface, thereby preventing further oxidation, whilst the iron oxide film that formed on the EN8 substrate allowed the permeation of the oxygen ions into the oxide. The iron oxide (Fe2O3) film that developed on EN8 spalled, as evidenced by the cracking of oxide when the oxidation time was greater than 72 h, whilst the Cr2O3 film maintained its integrity up to 120 h. A parabolic law was observed by the T-800 composite coating, whilst a paralinear law was reported for EN8 at 800 °C up to 120 h. This coating can be used in turbine parts where temperatures are <800 °C.

Oxygen Permeability and Its Role in Governing Life of YbDS Environmental Barrier Coatings

The growth kinetics of thermally grown oxide (TGO) silica in Yb-disilicate (YbDS) environmental barrier coatings (EBCs) significantly affects the durability of EBCs. The oxygen permeability can control the TGO growth kinetics and thus could play an essential role in determining EBCs life. Therefore; the oxygen permeability constant of YbDS and TGO is systematically evaluated and quantified in terms of thermodynamics using defect reactions and the parabolic rate constant (kp); respectively. Dry oxygen and wet oxygen conditions as well as different temperatures; partial pressures and top coat modifiers are investigated. The results offer evidence that the oxygen permeability constant for the YbDS top coat is an order of magnitude higher than for the TGO. As such; the TGO hinders the oxidant diffusion stronger; proving to be the diffusion rate controlling layer. Moreover; water vapor strongly increases the oxygen permeability with defect reactions playing a key role. It is suggested that the mass transfer through the top coat is primarily by outward ytterbium ion diffusion and inward oxygen ion movement; with the latter being dominant; particularly in wet environments. The effect of top coat modifiers on oxidant permeation is composition sensitive and seems to be related to their interaction with oxygen ions and their mobility.

Discontinuities in Oxidation Kinetics: A New Model and its Application to Cr–Si-Base Alloys

Some alloys such as many Cr-based systems show mass gain discontinuities during thermogravimetric measurements which strongly affect the oxidation kinetics. The behaviour cannot be described by the current models available in the literature. Thus, a novel $$k_\mathrm{para}$$ k para –$$k_\mathrm{lin}$$ k lin -P-model was developed to describe oxidation kinetics during the isothermal exposure of materials which show such behaviour. Beside the parabolic rate constant $$k_\mathrm{para}$$ k para and the linear mass loss constant $$k_\mathrm{lin}$$ k lin , the P-value and $$f_P$$ f P are introduced to take into account spontaneous rapid mass gains due to local oxide scale failure. The parameter P serves as a measure for the mass gain due to discontinuous events and $$f_P$$ f P is the frequency of such events. The both parameters can be related to oxide scale detachment and growth stresses. The application of the model is demonstrated for the oxidation of Cr–Si-based alloys in synthetic air at $$1200^{\circ }\hbox {C}$$ 1200 ∘ C for 100 h. For these alloys, the origin of the mass gain discontinuities is discussed and the meaning of P and $$f_P$$ f P is explained in more detail. Using this newly developed model, an insight into growth and nitridation resistance of oxide scales as well as scale adhesion is gained.

Effect of Deposition Conditions on Oxide Parameters of Silicon

In this study, the effect of deposition conditions and the temperature thermal treatment on the oxide parameters of two structures of silicon layers were investigated. The study present the evolution of boron profiles following a dry thermal oxidation in poly-Si/SiO2/c-Si films deposited at 520°C and 605°C temperatures and thermally oxidized in dry oxygen at respectively temperature 840°C, 945°C and 1050°C for tr=1h33’duration. The results show that the deposition conditions and the temperature treatment make a very important impact on the obtained films, which affect the redistribution and localization of dopants. It has been observed that the obtained value of the linear and the parabolic rate constant, the diffusion coefficient and the oxidation thickness are higher in the films deposited at Td = 520°C than those deposited at Td = 605°C. Also, the X-ray diffraction is strongly affected by the oxide thickness deposited between poly-silicon layers and crystalline substrates.

Effect of Deposition Conditions on Oxide Parameters of Silicon

In this study, the effect of deposition conditions and the temperature thermal treatment on the oxide parameters of two structures of silicon layers were investigated. The study present the evolution of in situ boron profiles following a dry thermal oxidation in poly-Si/SiO2/c-Si films deposited at 520°C and 605°C temperatures and thermally oxidized in dry oxygen at respectively temperature 840°C, 945°C and 1050°C for duration tr=1h33’. The results show that the deposition conditions and the temperature treatment make a very important impact on the obtained films, which affect the redistribution and localization of dopants. It has been observed that the obtained value of the linear and the parabolic rate constant, the diffusion coefficient and the oxidation thickness are higher in the films deposited at Td = 520°C than those deposited at Td = 605°C. Also, the X-ray diffraction is strongly affected by the oxide thickness deposited between poly-silicon layers and crystalline substrates.

Phase Analysis and Microstructural Investigations of Ce2Zr2O7 for High-Temperature Coatings on Ni-Base Superalloy Substrates

Crystalline cerium-zirconate (CZ) powders were synthesized via solution-assisted combustion synthesis route and calcined at 850°C for 8 h to obtain coarse crystalline powders. SEM (scanning electron microscopy) characterization studies were done to evaluate the morphology of the powders. XRD analysis of the resulting powder confirmed the presence of crystalline α-Ce2Zr2O7 along with a Ce2Zr2O8 phase having a disordered fluorite cubic lattice. Phase composition, lattice parameters, and the atomic positions were also investigated. Refinement of XRD data was done to quantify the amount of α-Ce2Zr2O7 and Ce2Zr2O8 phases. Ni-base superalloy Inconel 625 was chosen as the coating substrate, and the powders were coated using an air plasma spraying (APS). A thermally grown oxide (TGO)/Al2O3 layer was observed owing to the high temperature of the substrate as well as the diffusion of bond coat material into the substrate. Coated samples were characterized by SEM to study the surface morphology, coating thickness, and interface microstructures. The thickness of the coated sample was found to be 400 μm. Thermal cycling test of the coated sample was carried out at 750°C for 50 h to evaluate the thermal shock resistance of the coating as well as the spalling behavior of the coating. Preliminary oxidation tests were carried out for 50 h at 750°C to evaluate the oxide growth by measuring weight gain of the oxide layers formed. Oxide growth signifies the gradual increment of layers over a period with a parabolic rate constant of about Kp= 1.18 × 10–3 mg2 cm–4 h–1.

The Effect of Ultrasonic Treatment on the Oxidation Resistance and Microstructure of Fe80Cr20 Alloy Powder at High Temperature Oxidation Process

Effect of ultrasonic treatment on the oxidation kinetics and microstructure of Fe80Cr20alloy is investigated. The aim of this study is to improve the surface morphology and oxidation resistance of the Fe80Cr20alloy powder as a raw material. Ultrasonic technique is very promise technology and not yet fully explored. The Fe80Cr20material was treated by ultrasonic technique (UT) at various times of 3.5 h, 4.5 h, 5 h and followed by cold compaction with force of 13 Newton. The oxidation test was carried out using tube furnace under argon gas environment with 5 cycles, which has been considered. It was found that the UT has improved the surface morphology of the material effectively. Ultrasonic was revealed that the good interparticle bonding when experimented with ultrasonic treatment at 4.5 h. In the oxidation test, the lowest mass gain of 68 mg/cm2was found at the oxidation time of 100 h. Meanwhile, the lowest parabolic rate constant of 6.8198 x 10-14g2cm-4s-1was revealed when ultrasonic for 4.5 h sample.

Effect of Ti Addition on Oxidation Behavior and Area Specific Resistance of Fe-20Cr Alloys for SOFC Interconnects

In this work, the oxidation behavior of Fe-20wt.%Cr alloys with different titanium contents: 0, 0.5, and 1 wt.% are studied as a function of time in air atmosphere. The samples were isothermally oxidized at 700°C for 24, 48, and 96 h in a box furnace. The area specific resistance of oxides formed at the alloys surface during oxidation is measured by four-point probe methods at 700°C for 24 h. For Ti containing alloys, surface morphology observation by SEM shows that a few of TiO2 particles formed on the top of Cr2O3 scales. Continous TiO2 layer was not formed at the alloys surface after oxidation. XRD analysis on the oxide scales of Fe-20Cr-Ti alloys confirms that Cr2O3 and TiO2 oxide formed on the alloys. Ti addition into the alloys increases the oxidation rates of alloys at the initial stages. Oxidation behavior of Fe-20Cr-0.5Ti and Fe-20Cr-1Ti alloys showed two regimes. The parabolic rate constant, kp (in gr2/cm4s) were 1.57 x 10-13 and 3.08 x 10-13 respectively for initial stage of oxidation then changed to-9 x 10-15 and-3 x 10-14 respectively for the remainder of the test. ASR measurement shows that the presence of Ti in the alloys decreases the electrical resistance up to 60%. Ti addition into Fe-Cr alloys affect the oxide growth rate and increase the conductivity of Cr2O3 scales.

High Temperature Oxidation Behaviour of Detonation-Gun-Sprayed Cr3C2-NiCr-CeO2 Coatings on Inconel-718 at 900°C

The high temperature oxidation behavior of detonation-gun sprayed Cr3C2-NiCr coatings with and without 0.4 wt. % CeO2 additive on Ni-based superalloy inconel-718 is comparatively discussed in the present study. Oxidation studies were carried out at 900°C for 100 cycles in air under cyclic heating and cooling conditions on bare and coated superalloys. The thermo-gravimetric technique was used to establish kinetics of oxidation. X-ray diffraction, SEM/EDAX and X-ray mapping techniques were used to analyze the oxidation products of bare and coated samples. The results indicate that Cr3C2-NiCr-CeO2 coated specimen showed better oxidation resistance. The overall weight gain and parabolic rate constant of Cr3C2-NiCr-CeO2 coated specimen was found to be lowest in the present study signifying that the addition of CeO2 in Cr3C2-NiCr powder has contributed to the development of adherent and dense oxide scale on the coating at elevated temperature.