Oxidation behaviour of zirconium diboride–silicon carbide ceramic composites under low oxygen partial pressure

The oxidation behaviour of ZrB₂-based ceramics under low oxygen partial pressure ranged from 0.5 to 1.5 kPa was investigated. Low oxygen partial pressure was found to have remarkable effect on phase composition of the surface and the structures of oxide scale. And the thickness and microstructures of oxide scale was characterized by using scanning electron microscopy and X-ray diffraction analysis. The results indicate that the oxidation mechanism of ZrB₂-based ceramics changes under low oxygen partial pressure, and the oxidation resistance increases with the reduction of oxygen partial pressure.     

Microstructural evolution of coating-modified 3D C/SiC composites after annealing in wet oxygen at different temperatures

Two types of coating-modified 3D C/SiC, coated with CVD SiC/SiC/SiC (type I) and CVD SiC/amorphous-BC/SiC (type II), are subjected to a 14 vol.% H₂O/8 vol.% O₂/78 vol.% Ar atmosphere at 700, 1000 and 1200 °C up to 100 h. Microstructure and corrosion behaviour are investigated using a variety of characterization techniques. The type II shows a better oxidation resistance than type I during annealing at relatively low temperatures. Nevertheless, residual strength of the type I annealed above 1000 °C is enhanced by healing of many micron-sized defects. Interfacial bond strength of the composites is reasonably improved after annealing.     

Oxidation of β-SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering

The oxidation of β-Si_6−zAl_zO_zN_8−zs (z = 1, 2, and 3) prepared by a combination of combustion synthesis (CS) and spark plasma sintering (SPS) was investigated. The oxidation experiments were conducted at temperatures of 1000, 1200, and 1400 °C in air for 100 h (360 ks). Their oxidation kinetics follow a parabolic rate law, and the deviation from that increases with a decrease in the z value. The results of XRD and EPMA show that the oxide formed on β-SiAlONs (z = 1 and 2) consists of silica and mullite, and on β-SiAlON (z = 3) of only mullite.     

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

In this investigation, Cr₂O₃ and MnCr₂O₄ were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr₂O₄ exhibits much better resistance to carbonaceous attack than Cr₂O₃. The carburization rate of MnCr₂O₄ decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr₂O₃. The increased resistance of MnCr₂O₄ to carburization is attributed to the dissolution of MnO into Mn-Cr-O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn-Cr-O spinel. The thermodynamic equations defining the carburization stability of MnCr₂O₄ and Cr₂O₃ are modified.     

Dynamic oxidation of ultra-high temperature ZrB2–SiC under high enthalpy supersonic flows

The dynamic response to oxidation of a hot-pressed ZrB₂ + 15 vol%. SiC ceramic composite was studied under aero-thermal heating using a high enthalpy supersonic flow of a N₂/O₂ gas mixture in plasma wind tunnel. Microstructural features of the reaction scale developed upon oxidation were analyzed and correlated to test conditions through Computational Fluid Dynamics simulations. The significant heat flux and temperature gradients of the sample’s surface exposed to the highly energized N₂/O₂ gas stream led to the formation and evolution of distinct layered oxide sub-scales. The diffusion of oxidants through silica-rich 3D glassy network was proposed as the rate governing factor for oxidation.     

Comparison of thermal and ablation behaviors of C/SiC composites and C/ZrB2-SiC composites

A comparison was presented of the thermal and ablation behaviors of two carbon fiber reinforced ceramic-matrix composites (one with a SiC matrix and the other with a ZrB₂-SiC matrix). The C/SiC composite possessed a lower thermal conductivity (TC) and a higher emissivity in comparison to the C/ZrB₂-SiC composite. The two composites exhibited the good ablation-resistive properties with no obvious erosion rate after the arc-heated wind tunnel ablation tests. The surface of the C/SiC composite appeared to be coarse and had many rounded protrusions while a denser and more homogeneous glass oxide scale was formed for the C/ZrB₂-SiC composite. The maximum surface and back side temperatures of the C/ZrB₂-SiC composite were about 50 °C lower than those of the C/SiC composite, respectively, which was mainly attributed to the evaporation of the B₂O₃ as well as its higher TC.     

Preparation and oxidation protection of CVD SiC/a-BC/SiC coatings for 3D C/SiC composites

An amorphous boron carbide (a-BC) coating was prepared by LPCVD process from BCl₃-CH₄-H₂-Ar system. XPS result showed that the boron concentration was 15.0 at.%, and carbon was 82.0 at.%. One third of boron was distributed to a bonding with carbon and 37.0 at.% was dissolved in graphite lattice. A multiple-layered structure of CVD SiC/a-BC/SiC was coated on 3D C/SiC composites. Oxidation tests were conducted at 700, 1000, and 1200 °C in 14 vol.% H₂O/8 vol.% O₂/78 vol.% Ar atmosphere up to 100 h. The 3D C/SiC composites with the modified coating system had a good oxidation resistance. This resulted in the high strength retained ratio of the composites even after the oxidation.     

Effect of ozone adsorption on the oxidation behaviour of ZrB2–SiC ceramic composites

Ceramics of ZrB₂–20 vol.% SiC were prepared by hot pressing method, and ozone (O₃) was adsorbed on the surface of the ceramics. Then the as-adsorbed ceramics were oxidized in air and the effect of ozone adsorption on the oxidation behaviour of the ceramic composites was analyzed. The experimental results indicate that adsorption of ozone promotes the oxidation of the ceramic composites, especially for the SiC. In addition, more silica glass formed promotes the formation and crystal growth of zircon.     

Oxidation of zirconium diboride-silicon carbide ceramics under an oxygen partial pressure of 200 Pa: Formation of zircon

The formation and evolution of zircon during oxidation of ZrB₂-20 vol.% SiC ceramics under a low oxygen partial pressure of 200 Pa is studied. The formation mechanism of zircon is proposed according to experimental results and thermodynamic consideration. And the main reason to the formation of zircon can be attributed to the active oxidation of SiC. Two steps can be divided for the formation and evolution of zircon: (1) nucleation from silica glass; and (2) crystal growth into prism like particles. Furthermore, the emergence of zircon significantly improves the oxidation resistance performance.     

Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air

The oxidation behaviour of nanometre and micrometre sized Al₂O₃–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800–1000 °C.     

The effect of zirconium carbide on ablation of carbon/carbon composites under an oxyacetylene flame

Ablation of zirconium carbide (ZrC) modified carbon/carbon (C/C) composites was tested by an oxyacetylene torch. The formation of zirconia from the oxidation of ZrC improves the ablation resistance of the C/C composites because of the evaporation at elevated temperature, which absorbs heat from the flame and reduces the erosive attack to carbon. Zirconia also acts as an accelerator for carbon oxidation as it reacts with carbon during the ablation, increasing the mechanical breakage rate of the fibres. However, the effect of mechanical breakage is inferior in the ablation of the composites. The heterogeneous reactions control the ablation of the composites.     

Model of oxidation of SiC microparticles at high temperature

The oxidation kinetics of SiC microparticles has been studied from both experimental data and theoretical aspects based on recent research work. The effects of temperature, especially the particle size on the oxidation kinetics are explained not only qualitatively but also quantitatively based on our theory and model. The results show that our new model and the calculated results can reach a good agreement. It may also be seen that the important role of particle size in oxidation of SiC powder, especially in the nano era for SiC material.     

Reaction of modified SUS316 with tellurium under low oxygen potentials

Corrosion behavior of modified SUS316 has been studied under different oxygen potentials with tellurium vapor. Under the condition of oxygen potential ?−500 kJ/mol and tellurium vapor pressure of 8.9 × 10⁻⁴ atm, duplex corrosion layers formed on the surface of specimens, which mainly consisted of chromium telluride, iron telluride and chromium oxide. The thickness of corrosion layer was increased with decreasing the oxygen potential. Furthermore, the amount of oxygen uptake in sample corroded with tellurium vapor was much greater than that in sample oxidized without tellurium vapor.     

ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C

In this paper, we study the oxidation behaviour of a ZrB₂/Al-doped SiC composite at 1500 °C. The composite was prepared by hot-pressing the mixture of ZrB₂ and polymer-derived SiC(Al). The oxidation behaviour was studied by measuring the weight change as a function of oxidation time and by observing the structure of the oxide layer. It is shown that the ZrB₂–SiC(Al) exhibits different oxidation behaviour and improved oxidation resistance as compared to the conventional ZrB₂–SiC without Al-doping. The improvement in oxidation resistance is attributed to that Al-doping could increase the bond strength of the Si–O and suppress the active oxidation of SiC.     

Oxidation mechanism and resistance of ZrB2–SiC composites

The oxidation mechanism of ZrB₂–SiC composites was investigated based on a combination of theory and experiments. The oxidation reactions, microstructure evolution, scale stability and temperature limit were examined in our research and a good correspondence was obtained between theoretical predictions and experimental results. Microstructure evolution and stability are significantly dependent on both temperature and composition. SiO₂ is thermochemically stable below 1800 °C and will lose its protective properties at temperatures above 2300 °C. The temperature limit for ZrB₂–SiC composites is strongly dependent on the vapor pressure of the gaseous products and volume content of ZrB₂.     

Influence of preparation temperature on the oxidation resistance and mechanical properties of C/SiC coated C/C composites

A C/SiC oxidation resistance coating was prepared on carbon/carbon (C/C) composites by slurry and pack cementation. The microstructure, oxidation resistance and mechanical properties of C/SiC coating prepared from 1773 to 2573 K were investigated. With the increase of the preparation temperature, the oxidation resistance of C/SiC coating increases, however, the flexure strength decreases gradually. The preparation of C/SiC coating on C/C composites results in the fracture behavior of C/C composites changing from pseudo-plastic to brittle failure model. The decrease of flexure strength is mainly attributed to the decrease of C/C matrix’ flexure strength at high temperature.     

Influence of SiC nanowires on the properties of SiC coating for C/C composites between room temperature and 1500 °C

To prevent carbon/carbon (C/C) composites from oxidation between room temperature and 1500 °C, a dense SiC nanowire-toughened SiC oxidation resistant coating was prepared by a two-step technique composed of chemical vapor deposition and pack cementation. SiC nanowires could effectively baffle the propagation of the microcracks and avoid the formation of the through-thickness microcracks in the original coating. The results indicated that, after introducing SiC nanowires, the coefficient of thermal expansion of the coating was decreased between 100 and 1500 °C, and the oxidation protective ability for the coated C/C composites was improved largely between room temperature and 1500 °C.     

Degradation of CVD-SiC coated C/SiC composites exposed to the monopropellant combustion products based on Hydroxylammonium nitrate (HAN)

C/SiC composites with CVD-SiC coating were tested under hot fire in a HAN-propellant thruster. The degradation of the composite was determined from weight change and characterization of the surface variations. According to the responses to the corrosive combustion gases, two different ablation modes of C/SiC were discussed. Attributed to the relatively moderate temperature, the water vapor invasion to the silica film via passive-oxidation on the composite was limited, which produced an even, glassy, and honeycombed morphology on surface and about 1% weight gain.     

The role and effect of residual stress on pore generation during anodization of aluminium thin films

The role and effect of residual stress on pore generation of anodized aluminium oxide (AAO) have been investigated into anodizing the various-residual-stresses aluminium films. The plane stresses were characterised by X-ray diffraction with sin²ψ method. The pore density roughly linearly increased with residual stress from 64.6 (−132.5 MPa) to 90.5 pores/μm² (135.9 MPa). However, the average pore size around 40 nm was not changed significantly except for the rougher film. The tensile residual stress lessened the compressive oxide growth stress to reduce AAO plastic deformation for higher pore density. The findings provide new foundations for realizing AAO films on silicon.     

Effects of temperature and the incorporation of W on the oxidation of ZrB2 ceramics

The effects of tungsten additions and temperature on the oxidation behavior of nominally pure ZrB₂ and ZrB₂ containing 4, 6 or 8 mol% of W after oxidation at temperatures ranging from 800 to 1600 °C were investigated. For pure ZrB₂, the protective liquid/glassy layer covering the surface as a result of oxidation was evaporated above 1500 °C. For (Zr,W)B₂ specimens, the liquid/glassy layer was present after exposure up to 1600 °C. The higher stability of the liquid/glassy phase in the W-containing compositions was attributed to the presence of tungsten in the liquid/glassy phase, resulting in improved oxidation resistance for ZrB₂ samples containing W.