Microstructure and Oxidation Behavior of ZrB_(2)-SiC Ceramics Fabricated by Tape Casting and Reactive Melt Infiltration

ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2) and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2) phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2) oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2) is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2) particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2) increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2) in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.

Oxidation behavior of Zr-containing Ti_2AlNb-based alloy at 800°C

The oxidation behavior of Ti?22Al?(27?x)Nb?xZr (x=0, 1, 6) alloys at 800 °C for exposure time up to 100 h was examined. It is shown that oxidation rate of experimental alloys obeys the parabolic kinetics. Ti?22Al?26Nb?1Zr alloy demonstrates more excellent oxidation resistance than the other two alloys. The main oxidation products are TiO2, Al2O3 and AlNbO4 phases for all these alloys. For the Ti?22Al?26Nb?1Zr alloy, Zr addition can modify the growth mechanism of oxide scale, which can effectively hinder the diffusion of oxygen. Whereas, reaction of Zr with oxygen leads to the formation of ZrO2 precipitates for the Ti?22Al?21Nb?6Zr alloy, which promotes the oxygen ingress into the substrate. Meanwhile, oxidation affected zones, including internal-oxidation layer and oxygen-enriched zone, are present beneath the outmost oxide scale. The difference in these zones is derived from the phase constitution in the starting Ti?22Al?(27?x)Nb?xZr (x=0, 1, 6) alloys.

High temperature oxidation behavior of electroconductive TiN/O′-Sialon ceramics prepared from high titania slag-based mixture

The oxidation behavior of electroconductive TiN/O′-Sialon ceramics prepared using high titania slag as main starting material was studied at 1 200-1 300 °C in air. The isothermal and non-isothermal oxidation processes were investigated by DTA-TG. Phase compositions and morphologies of the oxidized products were analyzed by XRD, SEM and EDS. The results indicate that the oxidation of TiN and O′-Sialon occurs at about 500 °C and 1 050 °C, respectively. After oxidation at 1 200-1 300 °C, a protective scale that consists of Fe2MgTi3O10, SiO2 and TiO2 is formed on the surface of the materials, which effectively prevents the oxidation process. The formation of a protective scale is relative to TiN content and apparent porosity of the samples, the amount of SiO2 and amorphous phase in the oxidation product. At the initial oxidation stage, the oxidation kinetics of the materials follows perfectly the linear law with the apparent activation energy of 1.574×105 J/mol, and at the late-mid stage, the oxidation of the samples obeys the parabolic law with the apparent activation energy of 2.693×105 J/mol. With the increase of TiN content, mass gain of the materials increases significantly.

Preparation and oxidation behavior of a novel CeO_2-modified chromizing coating

By using CeO2 particles instead of part of Al2O3 particles as filler, the CeO2 was successfully entrapped into the outer layer of the chromizing coatings on the as-deposited nanocrystalline （NC） and microcrystalline （MC） Ni films using a conventional pack-cementation method at 800 °C. For comparison, chromizing was also performed under the same condition on MC Ni film using Al2O3 as filler without CeO2 particles. SEM/EDX and TEM results indicate that the refinement of Ni grain and CeO2 entrapped into the chromizing coatings refine the grain of the chromizing coating. Oxidation at 900 °C indicates that compared with the CeO2-free chromizing coating, the CeO2-dispersed chromizing coating exhibits an increased oxidation resistance. For the CeO2-dispersed chromizing coating, the refinement of Ni grain size significantly decreases the transient-oxidation scaling rate of the chromizing coatings. Together with this, the CeO2-dispersed chromizing coating formed on NC Ni exhibits a better oxidation resistance.

Oxidation behavior of Hastelloy C-2000 superalloy at 800 °C and 1000 °C

The oxidation behavior of Hastelloy C-2000 alloy was investigated in air at 800 °C and 1000 °C for 100 h, respectively. Oxidation kinetics and oxide scales morphologies were examined by mass gain measurement, scanning electron microscopy（SEM）, X-ray diffraction（XRD） and X-ray photoelectron spectroscopy（XPS）. The oxidation behavior of the alloy approximately follows a parabolic rate law. Moreover, annealing twins defect structure in matrix deteriorates the oxidation resistance of alloy due to the improvement of diffusion rates for alloying elements and oxygen atoms. At 800 °C, the microstructure is primarily composed of Ni O and Cr1.3Fe0.7O6 and the initial annealing twins structure is visible and Mo-rich phases are emerged to approach boundary of oxide scales. At 1000 °C, however, the morphology microstructure of oxide scales consists of oxide particle with fine Cr oxides and large Ni oxides by inlaying each other, whilst Mo-rich phases hardly appear closing to the interface of oxide scales.

Effect of Nb on plasticity and oxidation behavior of TiAlNb intermetallic compound by density functional theory

Based on the pseudo potential plane-wave method of density functional theory （DFT）, Ti1-xNbxAk （x=0, 0.062 5, 0.083 3, 0.125, 0.250） crystals＇ geometry structure, elastic constants, electronic structure and Mulliken populations were calculated, and the effects of doping on the geometric structure, electronic structure and bond strength were systematically analyzed. The results show that the influence of Nb on the geometric structure is little in terms of the plasticity, and with the increase of Nb content, the covalent bond strength remarkably reduces, and Ti-Al, Nb-M （M=Ti, Al） and other hybrid bonds enhance; meanwhile, the peak district increases and the pseudo-energy gap first decreases and then increases, the overall band structure narrows, the covalent bond and direction of bonds reduce. The population analysis also shows that the results are consistent with the electronic structure analysis. The density of states of TiAINb shows that Nb doping can enhance the activity of Al and benefit the form of Al2O3 film. All the calculations reveal that the room temperature plasticity and the antioxidation properties of the compounds can be improved with the Nb content of 8.33%-12.5% （mole fraction）.

Oxidation Behavior of C/C-SiC Gradient Matrix Composites

Oxidation behavior of C/C-SiC gradient matrix composites and C/C composites were compared in stationary air. The results show that oxidation threshold of C-SiC materials increases with the amount of SiC particles in the codeposition matrix. Oxidation rate of C/C-SiC gradient matrix composites is significantly lower than that of C/C material. The micro-oxidation process was observed by SEM.

Structure and Oxidation Behavior of a Plasma Sprayed Yttrium Silicates Coated SiC-C/C with a Glass Outer Sealant from 1573K to 1873K

Different compositions of yttrium silicates coatings were deposited on SiC-C/C by plasma spraying and an outer borosilicate glass was applied on the yttrium silicates coatings surfaces. The structure of the multi-layer coatings was characterized by XRD and SEM analyses. High temperature oxidation behavior of the multi-layer coatings coated C/C composites was investigated. Results show that SiC/2SiO2 Y2O3/1.5SiO2 Y203/ SiO2 Y2O3/glass multi-layer coating has better high temperature oxidation resistance, protecting carbon/ carbon composites from oxidation at 1 773 K in air for 164 h with the weight loss of 1.65%. The oxidation weight loss of the coated C/C with time accorded with parabolic rule in the temperature range 1 573 K-1 873 K; and the corresponding oxidation activation energy of the coated carbon/carbon composites is 132.2 kJ/mol.

High Temperature Oxidation Behavior of La_2O_3-Mo_5Si_3/MoSi_2 Composites

The oxidation behavior of 0. 8% La2O3- Mo5Si3/MoSi2 composites at 1200℃ in air was investigated. The results reveal that the oxidation resistance of the material with 0. 8% La2O3 and Mo5Si3 is impaired. The oxidation resistance is decreased with increasing Mo5Si3 content. The mass loss follows a linear law in the initial oxidation. With oxidation time prolonging, a continuous and dense oxidation scale prevents oxygen from diffusing increasing when and leads to mass change a Mo5Si3 content is less than 30%. However, the composite shows ＂PEST＂ with the addition of 40% Mo5Si3. With increasing Mo5Si3 content, the oxidation resistance of 0.8% La2O3- Mo5Si3/MoSi2 decreases. This attributes to the poor oxidation resistance of M05Si3 and the relative density decreasing of 0. 8% La2O3-Mo5Si3/MoSi2 composite.

Oxidation behavior of the Fe-36Al-0.09C-0.09B-0.04Zr alloy at 1250℃

To explore and study the Fe-A1 system alloy presenting exceptional oxidation resistance at high temperature, the Fe-36Al-0.09C-0.09B-0.04Zr alloy was designed and developed. The microstructure and hardness of the backing at 1250℃were analyzed and measured. Thermodynamics and kinetics of the oxidation behavior were also analyzed by X-ray diffraction, scanning electron micros- copy, and energy-dispersive X-ray spectroscopy techniques. The results show that the microstructttre of the Fe-36Al-0.09C-0.09B-0.04Zr alloy is FeAl phase at ambient temperature and is stable at 1250℃. It displays the excellent property of oxidation resistance because the oxide film has only the Al2O3 layer, and its oxidation kinetics curve obeys the parabolic law at 1250℃. The oxidation mechanism at 1250℃ is presumed that in the early oxidation period, the alloy oxidizes to form a large number of Al2O3 and a little Fe2O3, then, the enrichment of Al caused by Fe oxidization combines with O to form Al2O3.

Influence of surface finish and annealing treatment on oxidation behavior of Ti-48Al-8Cr-2Ag alloy

The effect of surface finish and annealing treatment on the oxidation behavior of Ti-48Al-8Cr-2Ag (molar fraction, %) alloy was investigated at 900 and 1 000 ℃, respectively in air. Thermal gravimetric analysis (TGA) was conducted for the characterization of oxidation kinetics. The microstructures of oxide scales were studied by scanning electron microscopy (SEM) and transmission election microscopy (TEM) techniques. Unfavorable effect of the annealing treatment on the oxidation behavior of the coating was also investigated. The results indicate that the oxidation behavior of the alloy is influenced by surface finish and annealing treatment. The oxidation rate of ground sample is lower than that of the polished alloy at 1 000 ℃ in air. The former forms a scale of merely Al2O3, and the latter forms a scale of the mixture of Al2O3 and TiO2. Annealing can improve the formation of TiO2.

High-temperature oxidation behavior of a cast Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy

The oxidation behavior of the Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy at 900℃ was investigated at different oxidation times(5,20,60 and 100 h).The results show that the total weight gain of the alloy after 100 h at 900℃ oxidation is 9.1 g·m^(-2),and the oxidation rate decreases with oxidation time.The oxides on the alloy surface are mainly TiO_(2) and Al_(2)O_(3).At the beginning of oxidation(5 h),the oxide film is relatively complete,thin,and the interface between the oxide layer and the matrix is virtually flat.At the end of oxidation(100 h),the thickness of the oxide film is expanded,cracking and spalling occur,and the spalling form is intra-film spalling.At the same time,oxygen is mainly distributed in the oxide film and the oxygen content in the alloy substrate is reduced,confirming that the TiAl alloy has a certain oxidation stability at 900℃.From the outer surface of the oxide layer to the matrix,the TiO_(2) content increases and the Al_(2)O_(3) content decreases.Oxidation proceeds to completion in this system via the dissolution and diffusion of O atom.

Oxidation behavior of Ni-based superalloy GH4738 under tensile stress

Revealing the oxidation behavior of superalloys is crucial for optimizing material properties and extending service life.This study investigated the oxidation behavior of superalloy GH4738 under stress states at 850℃.High-throughput specimens were fabricated to withstand different stresses at the same time.Isothermal oxidation s amples were analyzed using the mass gain method to obtain oxidation kinetic curves.The results show that the external stress below 200 MPa could improve the oxidation resistance of the GH4738.With tensile stress increasing,the oxide layer becomes thinner,denser and more complete,while internal oxidation decreases.The tensile stress alters the structure of the external oxide layer from a two-layer to a threelayer configuration.The Cr_(2)O_(3) oxide layer inhibits the outward diffusion of Ti,leading to Ti enrichment at the oxide-matrix interface and altering the oxidation mechanism of GH4738.

Evaluation of the Oxidation Reactivity and Behavior of Exhaust Soot Particles from Diesel Engines with Different Emission Levels

The aim of this study was to investigate the oxidation reactivity and behavior of exhaust particulate matter(PM)from diesel engines.PM samples from two diesel engines(1K,CY4102)with different emission levels were collected by a thermophoretic system and a quartz filter.The oxidation reactivity,oxidation behaviors,and physicochemical properties of the PM samples were analyzed using thermogravimetric analysis(TGA),high-resolution transmission electron microscopy(HRTEM),Fourier-transform infrared spectrometry(FTIR),and Raman spectroscopy.The results showed that there was a great difference in the oxidation reactivity of soot particles emitted by the two different diesel engines.A qualitative analysis of the factors influencing oxidation reactivity showed that the nanostructure,degree of graphitization,and relative concentration of aliphatic C—H functional groups were the most important factors,whereas no significant correlation was found between the primary particle size and activation energy of the diesel soot.Based on the oxidation behavior analysis,the diesel soot particles exhibited both internal and surface oxidation modes during the oxidation process.Surface oxidation was dominant during the initial stage,and as oxidation progressed,the mode gradually changed to internal oxidation.Internal oxidation mode of soot particles from the 1K engine was significantly higher than that of CY4102.

Insight into the effect of Ti substitutions on the static oxidation behavior of (Hf,Ti)C at 2500℃

Hf-based carbides are highly desirable candidate materials for oxidizing environments above 2000℃.However,the static oxidation behavior at their potential service temperatures remains unclear.To fill this gap,the static oxidation behavior of(Hf,Ti)C and the effect of Ti substitutions were investigated in air at 2500℃ under an oxygen partial pressure of 4.2 kPa.After oxidation for 2000 s,the thickness of the oxide layer on the surface of(Hf,Ti)C bulk ceramic is reduced by 62.29%compared with that on the HfC monocarbide surface.The dramatic improvement in oxidation resistance is attributed to the unique oxide layer structure consisting of various crystalline oxycarbides,HfO_(2),and carbon.The Ti-rich oxycarbide((Ti,Hf)C_(x)O_(y))dispersed within HfO_(2) formed the major structure of the oxide layer.A coherent boundary with lattice distortion existed at the HfO2/(Ti,Hf)C_(x)O_(y) interface along the(111)crystal plane direction,which served as an effective oxygen diffusion barrier.The Hfrich oxycarbide((Hf,Ti)CxOy)together with(Ti,Hf)C_(x)O_(y),HfO_(2),and precipitated carbon constituted a dense transition layer,ensuring favorable bonding between the oxide layer and the matrix.The Ti content affects the oxidation resistance of(Hf,Ti)C by determining the oxide layer's phase distribution and integrity.

Microstructures and high-temperature oxidation behavior of directionally solidified Nb-Si-based alloys with Re additions

High-temperature oxidation behavior of directionally solidified(DS) Nb-Si-based alloys with Re additions was investigated at 1200 and 1250℃,respectively.Microstructures and high-temperature oxidation behavior of the alloys were characterized.Results show that the microstructures in vertical section of Nb-24Ti-15Si-4Cr-2Al-2Hf-xRe(x=0,1,3;at%) alloys grow parallel to the withdrawal direction and the cross section exhibits bud-like structures.The bud-like structures become finer with more Re additions.The weight gain of the Nb-24Ti-15Si-4Cr-2Al-2Hf-3Re alloy after oxidation at 1200℃ for 100 h is 198.1 mg·cm^(-2),and it is a bit higher at 1250℃.The other two alloys perform somewhat worse.The influence of Re addition on the oxidation resistance at 1250℃ is more significant than that at 1200 ℃.Although Re addition does not benefit obviously the high-temperature oxidation resistance of the DS samples,it does not compromise the oxidation resistance with a certain amount of Re additions in contrast with the alloy without Re addition.

Recovery in oxidation behavior of damaged SiCeZrB_(2)/SiC coating of carbon/carbon composites

Polysiloxane(PSO)was adopted as the matrix of the repair agents,and SiCeZrB_(2)powder was used as the filler,to repair the prefabricated defects on the SiCeZrB_(2)/SiC(SZS)coating of carbon/carbon(C/C)composites.The repair agents were brushed on the defect areas and then underwent preoxidation(PR)or heat-treatment(HR)in a vacuum.The effects of different treatment processes on the chemical composition,microstructure of the repair agents,and the oxidation resistance behavior of the repaired coating were investigated.The repaired agents after both processes were pyrolyzed and generated SiOC ceramics,and they were well combined with the original coating.The thermal stability of PSO after preoxidation is poorer than that after heat-treatment,resulting in a weight loss rate of 5.88%after oxidation at 1500℃for 270 min,while that of the HR coating is only-0.87%,yet both have been great improvement compared with the unrepaired coating.This work provides an effective and simple approach to repairing damaged coatings for high-temperature applications.

Oxidation behavior of boron-containing(Zr,Ti)C_(x)B_(y)solid solution ceramics at 1600℃in air

Multicomponent boron-containing carbide coatings(i.e.,(Zr,Ti)C_(x)B_(y))on a C/C composite show good ablation resistance.However,the high-temperature oxidation behavior of this new type of boron-containing(Zr,Ti)C_(x)B_(y)solid solution ceramics has not been clarified yet.The present work fabricated(Zr,Ti)C_(x)B_(y)solid solution block ceramics by spark plasma sintering,and their oxidation behavior at 1600℃in air(N2–20-vol%O2)was investigated for the first time.The effects of boron on the oxidation resistance of(Zr,Ti)C_(x)B_(y)ceramics were examined.The results indicate that the(Zr,Ti)C_(x)B_(y)ceramics display good oxidation resistance with the parabolic rate law describing the oxidation process.After the trace solution of boron(0.5 wt%)into(Zr,Ti)Cx,the oxidation resistance of carbide ceramics is significantly enhanced,leading to a decrease of 30%in the oxidation rate constant.The formed oxide scale in the(Zr,Ti)C_(x)B_(y)ceramics is dense,and the interlayer shows stronger ability to inhibit inward diffusion of oxygen.In addition,the introduction of boron leads to more negative binding energy of(Zr,Ti)C_(x)B_(y)and improves the oxidation resistance of carbides.

Structure characterization of the oxide film on FGH96 superalloy powders with various oxidation degrees

The structure of the oxide film on FGH96 alloy powders significantly influences the mechanical properties of superalloys.In this study,FGH96 alloy powders with various oxygen contents were investigated using high-resolution transmission electron microscopy and atomic probe technology to elucidate the structure evolution of the oxide film.Energy dispersive spectrometer analysis revealed the presence of two distinct components in the oxide film of the alloy powders:amorphous oxide layer covering the γ matrix and amorphous oxide particles above the carbide.The alloying elements within the oxide layer showed a laminated distribution,with Ni,Co,Cr,and Al/Ti,which was attributed to the decreasing oxygen equilibrium pressure as oxygen diffused from the surface into the γ matrix.On the other hand,Ti enrichment was observed in the oxide particles caused by the oxidation and decomposition of the carbide phase.Comparative analysis of the oxide film with oxygen contents of 140,280,and 340 ppm showed similar element distributions,while the thickness of the oxide film varies approximately at 9,14,and 30 nm,respectively.These findings provide valuable insights into the structural analysis of the oxide film on FGH96 alloy powders.

High temperature oxidation behavior of Ni-based superalloy GH586 in air

In this paper, the isothermal oxidation kinetics and oxidation behavior of GH586 superalloy from 800 to 1000℃ were investigated. The oxide scale morphologies of the surfaces and the cross sections after oxidation were characterized by means of X-ray diffraction （XRD） and scanning electron microscope （SEM） equipped with energy-dispersive spectroscopy （EDS）. The results show that the growth of the oxide scales on the surface of superalloy GH586 obeys a parabolic law with the activa- tion energy of 241.4 kJ.mo1-1 from 800 to 1000℃ The dense oxide scale formed at 800℃ is mainly composed of Cr203, NiCr204 and a small amount of TiO2. At 900℃, the oxide scale is divided into two layers： the outer layer with multiple cracks is mainly composed of ＂Cr203 and TiO2, while the inner is a layer of dense Cr203. Under the oxide scale, aluminum-rich oxides along the grain bound- aries are generated by the internal oxidation. At 1000 ℃for 100 h, cracks throughout the whole oxide film accel- erate the oxidation rate of Ni-based superalloy GH586 and large blocks of TiO2 in the oxide scale are generated, resulting in the spallation of oxide scale.