Isothermal Oxidation Experiments Research Articles

HIGH-TEMPERATURE OXIDATION BEHAVIOR OF AN N5 NANOCRYSTALLINE COATING DEPOSITED BY ESD ON A NI-BASED SINGLE-CRYSTAL SUPERALLOY

An N5 nanocrystalline coating was prepared on a Ni-based single-crystal superalloy by electrospark deposition. The morphologies, chemical composition, and phase constitution of the coating were analyzed by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD), respectively. The high-temperature oxidation resistance of the substrate and coating at 1100℃ was tested in a static isothermal oxidation experiment. The results show that the electrospark deposition coating with columnar crystal structure is composed of nanocrystalline; there is no elemental interdiffusion between the substrate and the coating, which effectively avoids decreasing the mechanical properties of the substrate alloy due to the interdiffusion. Migration and aggregation of a reactive element, Ta, in the nanocrystalline coating substantially improves oxide film adhesion

Enhancing anti-oxidation and thermal-radiation performance of the repaired borosilicate glass coating on C/C composites by Sm-doping

To repair the damaged SiC coated C/C composites, a double-layer system including a Sm-doped borosilicate glass external layer and a SiSiC inner layer was prepared by a slurry-based laser cladding technique. Isothermal oxidation experiment and indirect/direct thermal-radiation measurements were performed. The results showed that the absorbance of borosilicate glass to the laser at 900–1200 nm was improved significantly by Sm-doping. Consequently, the repaired coating with a more compact structure and better oxidation resistance was obtained. After oxidation at 1773 K for 10 h, the mass loss of the damaged sample could be reduced by 74.98% with repairing. By increasing laser-absorption and reducing viscosity, the thermal-radiation property of the repaired coating was enhanced to decrease the surface temperature greatly. A repair system with excellent thermal protection performance was achieved.

Structure, mechanical properties, thermal stability and oxidation resistance of arc evaporated CrAlBN coatings

CrAlN coatings are recognized as outstanding protective layers for machining tools owing to their high hardness and exceptional oxidation resistance. However, the thermal stability of CrAlN coatings is relatively inferior. For tailoring the thermal properties of CrAlN coatings and enabling them to serve for severe working conditions, the element of B was introduced into CrAlN coatings in this study. Cr0.42Al0.58N, Cr0.39Al0.58B0.03N and Cr0.35Al0.59B0.06N coatings were deposited by cathodic arc evaporation, and the influence of B-incorporation on the structure, mechanical and thermal properties of CrAlN coatings were studied in detail.X-ray diffraction determinations manifest that the three coatings reveal a face-centered cubic structure. The hardness of Cr0.48Al0.52N is increased from 30.2 ± 0.7 GPa to 35.0 ± 1.3 GPa (Cr0.39Al0.58B0.03N) and 36.4 ± 1.2 GPa (Cr0.35Al0.59B0.06N), respectively, due to solid solution strengthening and grain refinement. Adding B into CrAlN coatings postpones the formation of wurtzite (w-) AlN and hexagonal (h-) Cr2N, thus improving their thermal stability. Additionally, the oxidation resistance of CrAlN coatings is further optimized by B-addition owing to the promoted formation of alpha (α-) Cr2O3. The isothermal oxidation experiment at 1100 °C for 15 h reveals that the oxide thicknesses of Cr0.39Al0.58B0.03N and Cr0.35Al0.59B0.06N coatings are decreased to ~1.6 and 1.8 μm, compared with the ~2.7 μm of Cr0.42Al0.58N coating.

High temperature oxidation behavior and mechanism of a new Ni-Co-based superalloy

The oxidation behavior and mechanism of a new Ni-Co-based superalloy developed for advanced engine turbine disks, were investigated at various temperature for different exposure times. The isothermal oxidation experiments, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and electron probe microanalysis (EPMA) were employed to analyze the oxidation characteristics of the new Ni-Co-based superalloy. The results show that the oxidation kinetics can be described by a parabolic law at each temperature and the oxidation rate constants K at temperature of 800 °C, 900 °C and 1000 °C are 0.015, 0.033 and 0.138 mg2 cm−4 h−1, respectively. The oxidation process is controlled by out-diffusion of metallic elements and in-diffusion of oxygen. Multifarious oxides are formed on the surface of the alloy due to the complex alloying elements. The oxide scale is consisted of three parts: the external oxide layer containing TiO2 and a small amount of Cr2O3; the middle oxide layer consisted of Cr2O3, TiO2, Al2O3 and the spinel phase; the internal oxide layer comprised of Al2O3. With further oxidation, TiO2 accumulates gradually on the external oxide layer because of the large diffusion distance of Ti, while O penetrated into the matrix can be captured by Al to prevent the further diffusion into alloy interior.

Preparation and oxidation kinetics behavior of bulk Cr3C2-20 wt % Ni cermets

In this study, bulk Cr3C2-20 wt % Ni cermets were successfully fabricated by high-energy milling and pressureless sintering in a vacuum furnace. Microstructures, elements distribution, and high temperature oxidation mechanism were researched by SEM, EPMA, and differential thermal analyzer (DTA), respectively. Oxidation kinetics regularity of bulk Cr3C2-20 wt % Ni cermets was investigated at 600–800 °C for the first time. Isothermal cyclic oxidation experiments were studied using the heat-treatment furnace for 100 h. The results indicated that the porosity decreased, while the hardness, bending strength, and fracture toughness increased with an improvement in the vacuum degree. Cr3C2-20 wt % Ni cermets displayed outstanding oxidation resistance and the dynamic oxidation curves followed the parabolic rate law. Besides, the oxidation rate constants increased three orders of magnitudes with an increase in the oxidation temperatures from 600 °C to 800 °C. The mechanism of the oxidation resistance was the generation of the protective and dense oxide layers on the sub-surface of the oxidation specimens, which hindered the diffusion of Cr3+, Ni2+, O2 and effectively protected the substrate from further oxidation.

Oxidation Resistance of Plasma-Sprayed Double-Layered LC/YSZ Coatings with Different Thickness Ratios at High Temperatures

This study aims to enhance the oxidation resistance of ceramic coatings through designing and optimizing microstructure. A series of double-layered lanthanum–cerium oxide (LC) and 8 wt% yttria-partially-stabilized zirconia (YSZ) coatings with different thickness ratios were tailored. The isothermal oxidation experiment and molecular dynamics simulation results suggested that due to the lower oxygen ionic diffusion coefficient, double-layered LC/YSZ coatings exhibited an excellent antioxidation property compared to the single-layer YSZ coating. However, with the increase of LC layer thickness in LC/YSZ coatings, sintering-induced vertical cracks were easily formed within LC layer, which significantly shortened the oxidation resistance of LC/YSZ coatings by a burner rig test at 1225 °C. The equivalent thermal growth oxide thickness manifested that the optimum thickness ratio between LC and YSZ layers was 1:2.

Effect of Y Additions on the Oxidation Behaviour of Novel Refractory High-Entropy Alloy NbMoCrTiAl at 1000 °C in Air

In this work, the influence of 0.5 at.% and 1 at.% Y additions on the high-temperature oxidation resistance of the equiatomic alloy NbMoCrTiAl at 1000 °C in air was investigated. Continuous isothermal and cyclic thermogravimetric experiments were conducted to characterize the oxidation kinetics of the alloys. Various analytical methods such as X-ray diffraction and electron microscopy with energy-dispersive X-ray spectroscopy were used to study the growth, morphology and composition of the oxide scales. It was found that the Y additions led to the formation of Al2Y phase along grain boundaries. The isothermal oxidation experiments of both Y-containing alloys showed lower oxidation rates after a short period of transient oxidation probably due to a reduction of fast-growing and voluminous Nb2O5 oxides within the oxide layer. During the steady-state period, however, oxidation of Al2Y caused local stresses resulting in crack formation and subsequent breakaway oxidation, i.e. pronounce increase in the oxidation rates. The cyclic oxidation experiments revealed lower oxidation rates and thinner, more adherent oxide scales formed on the alloy NbMoCrTiAl1Y compared to NbMoCrTiAl. The increased scale adherence was attributed to the formation of pegs due to the oxidation of Al2Y phase at the oxide/metal interface.

A combined experimental and first-principle study on the effect of plasma surface Ta–W co-alloying on the oxidation behavior of γ-TiAl at 900 °C

Abstract

Investigation of mechanical properties and oxidation resistance of CVD TiB2 ceramic coating on molybdenum

TiB2 anti-oxidation ceramic coatings were successfully synthesised on a Mo substrate by using chemical vapor deposition (CVD). A scratch test was performed to measure the adhesion strength of the TiB2 ceramic coatings to the Mo substrate. The results indicated good adhesion between the two materials; the adhesion strength was 125mN. Furthermore, high-temperature oxidation tests indicated that the CVD TiB2 ceramic coating protected the Mo substrate against oxidation up to 900℃. Isothermal oxidation experiments revealed that the Mo substrate covered with a 13-μm-thick TiB2 ceramic coating was protected against oxidation for a duration of 5h. The oxidation kinetics followed a parabolic law for the first 2.5h and a linear law in the next 2.5h.

Effects of Hf, Y, and Zr on Alumina Scale Growth on NiAlCr and NiAlPt Alloys

The effects of Hf, Y, and Zr additions on the growth of a thermally grown alumina scale formed on NiAlCr and NiAlPt alloys were investigated. Isothermal and thermal cycling oxidation experiments were carried out at 1150 °C, and cross sections of the oxidized samples were characterized using scanning electron microscopy. It was observed that single doping as well as co-doping of Hf, Y, and Zr reduces the rate of alumina scale growth on NiAlCr and NiAlPt alloys, with Hf showing a more significant effect than Y or Zr. The following possible contributing factors to these observations were assessed: (1) the ionic size of the minor alloying elements and (2) the bond strength between the doping elements and oxygen in the oxide grain boundaries in terms of formation and melting enthalpies of oxides. It was concluded that the bond strength between the doping elements and oxygen within the oxide grain boundaries plays an important role in retarding the alumina scale growth.

Formation of C-doped SiO2 coatings on carbon fibers by the sol-dipping process

The carbon-doped silicon dioxide (C-doped SiO2) coatings are prepared on the surface of carbon fibers by a facile sol-dipping method using tetraethoxysilane (TEOS) and diethoxydimethylsilane (DMDES) as precursors. The effect of heat treatment temperature and dipping times on the formation of the C-doped SiO2 coatings is studied. The structure and morphology of C-doped SiO2 coatings are analyzed by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The oxidation resistance of the carbon fibers with and without coatings is investigated by thermogravimetric analysis (TG) and static isothermal oxidation experiments. The results show that the C-doped SiO2 products obtained by pyrolysis of the gel at 900 ℃ displays high crystallinity and low carbon content. The continuous and uniform coatings with the thickness about 200 nm are formed after three times dipping, which improves the initial oxidation temperature of carbon fibers about 110 ℃.

In situ Raman spectroscopy study on the effect of antioxidants on high temperature oxidation properties of TMPTO

Purpose This paper aims to investigate the thermal oxidation characteristics of the unsaturated bonds (C=C) of trimethylolpropane trioleate (TMPTO) and to reveal the high temperature oxidation decay mechanism of unsaturated esters and the nature of the anti-oxidation properties of the additives. Design/methodology/approach Using a DXR laser microscopic Raman spectrometer and Linkam FTIR600 temperature control platform, the isothermal oxidation experiments of TMPTO with or without 1.0 wt. % of different antioxidants were performed. Findings The results indicated that the Raman peaks of =C-H, C=C and -CH2- weaken gradually with prolonged oxidation time, and the corresponding Raman intensities drop rapidly at higher temperatures. The aromatic amine antioxidant can decrease the attenuation of peak intensity, as it significantly reduces the rate constant of C=C thermal oxidation. The hindered phenolic antioxidant has a protective effect during the early stages of oxidation (induction period), but it may accelerate the oxidation of C=C afterwards. Originality/value Research on the structure changes of synthetic esters during oxidation by Raman spectroscopy will be of great importance in promoting the use of Raman spectroscopy to analyze the oxidation of lubricants.

Catalytic effect of iron phases on the oxidation of cellulose-derived synthetic char

Abstract The catalytic influence of iron oxide on the oxidation of synthetic chars as a function of the phase composition was investigated by temperature-programmed measurements in a thermobalance and isothermal oxidation experiments in a fixed-bed reactor. The synthetic solid fuels originated from hydrothermal carbonization of cellulose and subsequent pyrolysis of the obtained hydrochars. Incorporation of iron oxide was either achieved by in situ doping during the hydrothermal carbonization or by tight contact mixing of the chars with iron oxide particles. Temperature-programmed oxidation of the synthetic char doped by tight contact resulted only in a slight decrease of the oxidation temperature. Pyrolysis of the in situ doped chars at 800 °C led to the carbothermal reduction of iron oxide to catalytically inactive iron carbide, and it was not possible to re-oxidize iron carbide by means of an additional pretreatment in 20 % O2 at 350 °C. When pyrolysis of the in situ doped hydrochar was performed at 500 °C, iron oxide was not reduced, and the oxidation of the corresponding char occurred much faster due to the catalytic effect of the iron oxide particles, which had a high degree of contact with the embedding carbon matrix.

Developing the Oxidation Kinetic Model for Magnetite Pellet

Oxidation is a vital phenomenon for magnetite pellets in their excursion through the furnace during induration. One of the pre-requisites for magnetite pellets to achieve homogeneously structured good quality pellets is to have complete oxidation before sintering begins. Partially oxidized magnetite pellets, upon sintering, might result in inhomogeneous structured pellets which could be detrimental to pellet quality. It is necessary to understand the mechanisms responsible for magnetite oxidation, and hence, it is intended in this study to investigate experimentally as well as develop a mathematical model based on oxidation kinetics. Oxidation of pellets is largely influenced by the oxidation kinetics of particles and hence should be studied at particle as well as at pellet scale. The principles of the Grain Model have been adopted to develop the Oxidation Model at pellet scale, whereas the particles’ oxidation follows the Avrami Kinetic Model. Isothermal oxidation experiments performed Thermogravimetric Analyzer showed that oxidation rate of magnetite at pellet scale contained two peaks. They were complemented well by oxidation rates predicted from the model. Further, the pellet was investigated microstructurally at pellet and particle scale to substantiate the findings from the experiments and the model. The oxidation model developed is used to predict the progression of oxidation in the magnetite pellet with respect to the reaction time at three different temperatures (873 K, 973 K, and 1073 K (600 °C, 700 °C, and 800 °C)) and at four levels of oxygen (0.21, 0.30, 0.60, and 1.00 atm) in the oxidizing gas.

Mechanism and kinetics of Cu2O oxidation in chemical looping with oxygen uncoupling

In chemical looping with oxygen uncoupling, oxygen carrier (OC) circulates between the fuel and air reactors to release and absorb O2 repeatedly. In order to assess the re-oxidation characteristic of Cu-based OC in the air reactor from the microscopic mechanism and macroscopic kinetics perspective, DFT calculations and isothermal oxidation experiments were conducted. In DFT calculations, Cu2O(111) surface was chosen as the objective surface to explore the oxygen uptake as well as the atomic transportation pathways, and to determine the rate-limiting steps basing on the energy barrier analyses. It was found that the energy barrier of the surface reaction step (0.96 eV) is smaller than that of the ions diffusion step (1.61 eV). Moreover, the Cu cations outward diffusion occurs more easily than O anions inward diffusion, which confirmed the epitaxial growth characteristic of Cu2O oxidation. The isothermal oxidation experiments were conducted in a thermogravimetric analyzer (TGA), and about 3.5 mg CuO@TiO2-Al2O3 particles within the diameter range of 75–110 µm were tested between 540 and 600 °C, where the internal and external gas diffusion effects were eliminated. Mixtures of 5.2-21.0 vol.% O2 in N2 were adopted as the gas agent for oxidation. Based on the understandings obtained from DFT calculations, a simple mathematical model with unknown parameters of the surface reaction process (mainly the activation energy, Ek) and ions diffusion process (mainly the activation energy, ED) was established to describe the overall oxidation process in TGA experiments. Eventually, these unknown parameters were determined as Ek = 50.5 kJ/mol and Ek = 79.2 kJ/mol via global optimization. With the attained parameters, simulations reproduced the experimental results very well, which demonstrated that this simplification model, where grain is converted almost layer by layer but different from the feature of the shrinking core model is able to accurately describe the overall oxidation process of Cu2O.

Study on oxidation and gas release of active sites after low-temperature pyrolysis of coal

The low-temperature pyrolysis of oxygen-containing functional groups in coal plays a vital role in coal spontaneous combustion. This paper conducted isothermal oxidation experiments on raw and pyrolyzed coal samples to investigate the influence of the content of oxygen-containing functional groups, pyrolysis temperature, oxidation temperature, particle size and pyrolysis-oxidation times on the oxidation characteristics of pyrolyzed coal samples. It attributes the generation of massive active sites that are oxidized once exposed to O2 to the pyrolysis of oxygen-containing functional groups in coal. In addition, the oxidation of active sites releases such gaseous products as CO and CO2 in large quantity and producing oxygen-containing functional groups. For a coal sample with higher content of oxygen-containing functional groups and pyrolysis temperature and smaller particle size, the yield of active sites is greater. The formation law of gaseous products during isothermal oxidation after pyrolysis satisfies the exponential attenuation function P = A + Be−t/K1 + Ce−t/K2, suggesting the two reaction processes during the oxidation of pyrolyzed coal, that is, the oxidation of active sites after coal pyrolysis and the subsequent thermal decomposition with the generation of functional groups. The activation energy required for the former is far lower than that for the latter, and the oxidation of active sites can occur at very low temperatures (below 9 °C). The results are highly applicable to the study on the mechanism of self-heating and spontaneous combustion of coal.

Self-passivating tungsten alloys of the system W-Cr-Y for high temperature applications

Self-passivating tungsten based alloys for the first wall armor of future fusion reactors are expected to provide a major safety advantage compared to pure tungsten in case of a loss-of-coolant accident with simultaneous air ingress, due to the formation of a stable protective scale at high temperatures in presence of oxygen which prevents the formation of volatile and radioactive WO3. This work analyses the oxidation and thermal shock resistance of W-Cr-Y alloys obtained by mechanical alloying followed by HIPing. Alloys with different Cr and Y contents are produced in fully dense form with nanocrystalline or ultrafine-grained microstructure and a dispersion of Y-rich oxide nanoparticles located mainly at the grain boundaries. Isothermal oxidation experiments confirm an excellent oxidation resistance due to the formation of protective oxide scales at the very surface. These layers mainly consist of Cr2O3 and mixed Y-W and Cr-W oxides. The superior oxidation resistance of these alloys is confirmed by tests simulating accident-like conditions. The thermal conductivity of these alloys at 600–1000 °C is 2–3 times higher than standard Ni-base superalloys like Inconel-718. The material also exhibits outstanding thermal shock resistance: 1000 pulses of 0.19 GW/m2 power density and 1 ms duration at 400 °C base temperature resulted in no damage, while an increased power density of 0.38 GW/m2 resulted in the formation of a crack-network and slight surface roughening. An additional thermal treatment at 1550 °C improves slightly the oxidation resistance and significantly the thermal shock resistance of the alloy.

High-temperature oxidation behavior and analysis of impedance spectroscopy of 7YSZ thermal barrier coating prepared by plasma spray-physical vapor deposition

Quasi-columnar structure 7YSZ (yttria stabilized zirconia) thermal barrier coatings (TBCs) are prepared by plasma spray-physical vapor deposition (PS-PVD) onto pretreated and un-pretreated bond coating, respectively. An isothermal oxidation experiment of 7YSZ TBCs is carried out in the atmosphere of 950 °C in order to simulate the high-temperature oxidation process of engine blades. The isothermal oxidation process of 7YSZ thermal barrier coatings is investigated systematically by impedance spectroscopy. The electrochemical physical model and equivalent circuit of columnar 7YSZ coatings are established. Results show that the isothermal oxidation kinetic curve of columnar 7YSZ thermal barrier coatings appears to follow the parabolic law. A pretreatment of bond coating can reduce the growth rate of the thermally grown oxide (TGO) layer, restraining the initiation and propagation of microcracks between YSZ and TGO layers. The oxidation rate constants of 7YSZ coatings with pretreated and un-pretreated bond coating are 0.101 × 10−12 cm2·s−1 and 0.115 × 10−13 cm2·s−1, respectively. Impedance analysis shows that the content of oxygen vacancies decreases and the density increases after the TGO layer is oxidized for 150 h. In addition, shrinkage microcracks formed by sintering during the oxidation process is the main reason for an increase of the capacitance and a decrease of the resistance in the grain boundary of YSZ.

Improvement of high temperature oxidation behavior of Cr3C2-20 wt % Ni cermets by adding 1 wt % Mo

Cr3C2-20 wt % Ni cermets were prepared by Cr, C and Ni mixed powders using high energy milling and reactive sintering method comparing with the one contains 1 wt % Mo addition. Isothermal oxidation experiments were conducted at 800 °C in the static air from 10 h to 100 h using cyclic oxidation method. The high temperature oxidation behavior of Cr3C2-20 wt % Ni cermets effected by 1 wt % Mo addition was investigated. After 100 h oxidation, the oxidation weight increase and parabolic rate constants of the one with Mo decreased from 0.126 to 0.080 mg/cm2 and 4.41 × 10−12 to 1.78 × 10−12 kg2/m4·s, respectively. Generally, two positive effects were found by Mo additive, such as increasing the Cr atom concentration in the Ni and minimizing the formation of NiO during oxidation. The oxidation products of the cermets changed from forming the Cr2O3 and NiCr2O4 to that only forming Cr2O3 after adding molybdenum element. Pure Cr2O3 possessed better and denser crystal structure than that of NiCr2O4. The cermets were protected due to the formation of the dense oxide films on the surfaces of the substrates at high temperature. This novel mechanism was of academic influence and could serve as a guide for the further research on Cr3C2 - 20 wt % Ni cermets.

Investigation of High-Temperature Oxidation in Steam for Ni–Al Alloys Using the Combination of a Hydrogen Sensor and an Oxygen Pump–Sensor

The oxidation rates of Ni–Al alloys in an atmosphere of Ar–12 vol.% H2O were evaluated by a hydrogen sensor and oxygen pump–sensor installed in the latter stage of an oxidizing furnace. Steam oxidation behavior was also investigated. The oxygen pump–sensor was evaluated using Ar–1000 ppm H2 gas. It was found that the H2 gas concentration could be accurately measured as an oxidation current, such that the hydrogen concentration could be measured by the oxygen pump–sensor, and the oxidation rate could be calculated using this hydrogen concentration. From an isothermal oxidation experiment at 1523 K for 14.4 ks, the oxidation rates measured by the hydrogen sensor and the oxygen pump–sensor were almost the same values in each sample. In addition, the mass gain curve was obtained for each sample based on the result of the time change in the oxidation rate. Furthermore, from a cycle oxidation experiment in which the temperature was varied between 1173 and 1523 K it was found that for the Ni–11 at.% Cr–10 at.%Al and Ni–50 at.%Al alloys, during the temperature increase process of the first cycle, a sharp decrease in the oxidation rate was observed around 1373 K. This was considered to be due to the transformation from θ- to α-Al2O3 since the scales formed on both alloys consisted of Al2O3. However, during the temperature increase process of the second cycle, this decrease was not observed in both alloys. Both the hydrogen sensor and the oxygen pump–sensor method could measure small changes in the oxidation rate during the steam oxidation process, so it became clear that the change in the oxidation rate accompanying the phase transformation of the oxide scale could be determined.