The repeated thermal shock behaviors of a ZrB2-SiC composite heated by electric resistance method

Repeated thermal shocks of ZrB₂-20 vol.% SiC (ZrB₂-SiC) composite was investigated by the electric resistance method with 330 A and 4 V. It was found that the maximum temperature of the specimen center was obtained in 20 s and the specimen was cooled down to room temperature in 10 s due to a coolant system, and then the specimen was heated again. The thermal shock treatment increased the flexural strength with a peak at 20 cycles. Even though 30 cycles showed the minimum strength, it was still higher than that of the unshocked specimen. The increase in the strength was attributed to the formation of the oxide layer and the compression stress that resulted from volume expansion upon conversion of ZrB₂ and SiC, to ZrO₂, B₂O₃ and SiO₂.     

The thermal stability in air of hot-pressed diboride matrix composites for uses at ultra-high temperatures

The resistance to oxidation in ambient air at a temperature up to 1600 °C of two hot-pressed diborides matrix composites, both containing 19.5% v/o SiC and 3 v/o HfN (as sintering aid), was investigated. The diboride matrix was based on HfB₂ or a ZrB₂/HfB₂ mixture (volume ratio ≈ 1). Both the materials were subjected to repeated heating-cooling cycles at 1600 °C, and a 20 h exposure at 1450 °C in flowing dry air. Modest weight gains and limited corrosion depths highlighted a rather good thermal stability. In accordance with the thermo-gravimetric test at 1450 °C, the oxidation kinetics for both the composites superbly fit a para-linear law. The introduction of the SiC particles provided tangible benefits for the resistance to oxidation. One of the oxidation products, a borosilicate glass, sealed pores and coated the exposed faces, greatly limiting the inward transport of oxygen towards the internal oxide/diboride interfaces.     

Investigations on synthesis of ZrB2 and development of new composites with HfB2 and TiSi2

This paper presents the results of experimental investigations carried out on the synthesis of pure ZrB₂ by boron carbide reduction of ZrO₂ and densification with the addition of HfB₂ and TiSi₂. Process parameters and charge composition were optimized to obtain pure ZrB₂ powder. Monolithic ZrB₂ was hot pressed to full density and characterized. Effects of HfB₂ and TiSi₂ addition on densification and properties of ZrB₂ composites were studied. Four compositions namely monolithic ZrB₂, ZrB₂ + 10% TiSi₂, ZrB₂ + 10% TiSi₂ + 10% HfB₂ and ZrB₂ + 10% TiSi₂ + 20% HfB₂ were prepared by hot pressing. Near theoretical density (99.8%) was obtained in the case of monolithic ZrB₂ by hot pressing at 1850 °C and 35 MPa. Addition of 10 wt.% TiSi₂ resulted in an equally high density of 98.9% at a lower temperature (1650 °C) and pressure (20 MPa). Similar densities were obtained for ZrB₂ + HfB₂ mixtures also with TiSi₂ under similar conditions. The hardness of monolithic ZrB₂ was measured as 23.95 GPa which decreased to 19.45 GPa on addition of 10% TiSi₂. With the addition of 10% HfB₂ to this composition, the hardness increased to 23.08 GPa, close to that of monolithic ZrB₂. Increase of HfB₂ content to 20% did not change the hardness value. Fracture toughness of monolithic sample was measured as 3.31 MPa m^1/2, which increased to 6.36 MPa m^1/2 on addition of 10% TiSi₂. With 10% HfB₂ addition the value of K_IC was measured as 6.44 MPa m^1/2, which further improved to 6.59 MPa m^1/2 with higher addition of HfB₂ (20%). Fracture surface of the dense bodies was examined by scanning electron microscope. Intergranular fracture was found to be a predominant mode in all the samples. Crack propagation in composites has shown considerable deflection indicating high fracture toughness. An oxidation study of ZrB₂ composites was carried out at 900 °C in air for 64 h. Specific weight gain vs time plot was obtained and the oxidized surface was examined by XRD and SEM. ZrB₂ composites have shown a much better resistance to oxidation as compared to monolithic ZrB₂. A protective glassy layer was seen on the oxidized surfaces of the composites.     

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.     

Mechanochemical and volume combustion synthesis of ZrB2

Formation of ZrB₂ by volume combustion synthesis (VCS) and mechanochemical process (MCP) from ZrO₂-Mg-B₂O₃ was studied. Production of ZrB₂ by VCS in air occurred with the formation of side products, Zr₂ON₂ and Mg₃B₂O₆ in addition to MgO and ZrB₂. Zr₂ON₂ formation was prevented by conducting VCS experiments under argon. Wet ball milling was applied to the VCS products before leaching for easier removal of Mg₃B₂O₆ phase. MgO and Mg₃B₂O₆ were removed from wet ball-milled products by leaching in 5 M HCl for 2.5 h. In MCP, 30-hour ball milling was found to be sufficient for the formation of ZrB₂ with no minor phase formation. Leaching of MCP products in 1 M HCl for 30 min was sufficient to remove MgO. Complete conversion of ZrO₂ to ZrB₂ did not take place in both production methods, even with excess amounts of Mg and B₂O₃. Therefore, formed ZrB₂ contained residual ZrO₂.     

Effect of α-Al2O3 coatings on the interface of Ni/SiC composites prepared by electrodeposition

This paper describes an investigation on the effect of α-Al₂O₃ coating on the interface between nickel and SiC particle. Uniform, dense and well-adhered α-Al₂O₃ coatings were obtained on the surface of SiC particles by sol–gel technology. The nickel-based composites reinforced with α-Al₂O₃-coated SiC particles (CS_p) or uncoated SiC particles (UCS_p) prepared by composite electrodeposition were heated at 600 °C to study the reactivity and the resulting interfaces. The results showed that the Ni/CS_p composites presented excellent thermal stability without interfacial reaction, while nickel silicide formed in the Ni/UCS_p composites. It indicated that high-temperature interfacial reaction between SiC particles and nickel matrix was efficiently inhibited by the α-Al₂O₃ coatings. Moreover, great differences of the local mechanical properties of interfaces were observed by the nanoindentation characterization.     

Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

A ZrSiO4/SiC oxidation protective coating for carbon/carbon composites

In order to improve the oxidation resistance of carbon/carbon (C/C) composites, a ZrSiO₄ coating on SiC pre-coated C/C composites was prepared by a hydrothermal electrophoretic deposition process. Phase compositions and microstructures of the as-prepared ZrSiO₄/SiC coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The anti-oxidation property and failure mechanism of the multi-layer coating were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to prepare crack-free ZrSiO₄ outer coatings. The multi-layer coating obviously exhibits two-layer structure. The inner layer is composed of SiC phase and the outer layer is composed of ZrSiO₄ phase. The bonding strength between the outer layer coatings and C/C–SiC substrate are 30.38 MPa. The ZrSiO₄/SiC coating displays excellent oxidation resistance and can protect C/C composites from oxidation at 1773 K for 332 h with a mass loss rate of only 0.48 × 10^− 4 g/cm²·h. The mechanical properties of the specimens are 84.36 MPa before oxidation and 68.29 MPa after oxidation. The corresponding high temperature oxidation activation energy of the coated C/C composites at 1573–1773 K is calculated to be 119.8 kJ/mol. The oxidation process is predominantly controlled by the diffusion rate of oxygen through the ZrSiO₄/SiC multi-coating. The failure of the coating is due to the formation of penetrative holes between the SiC bonding layer and the C/C matrix at 1773 K.     

Microstructure and mechanical properties of ZrB2–Nb composite

The high relative density of the ZrB₂-based composite toughened by 25 vol.%Nb (ZN) was hot-pressed at reduced temperatures with low pressure of 30 MPa. Compared with the toughness of 2.3–3.5 MPa m^1/2 and strength of 350 MPa of the monolithic ZrB₂, the toughness and strength of the ZN composite were improved to 6.7 MPa m^1/2 and 773 MPa, respectively, due to the addition of ductile Nb. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. Furthermore, the densification mechanism was analyzed and discussed. The results here pointed to a potential method for improving fracture toughness and strength of ZrB₂-based ceramics.     

Microstructure and mechanical properties of ZrB2–SiC ultrahigh temperature ceramic composite joint using TiZrNiCu filler metal

Abstract

ZrB₂–SiC ceramic composite was brazed by using TiZrNiCu active filler metal. The microstructure and interfacial phenomena of the joints were analysed by means of SEM, energy dispersive X-ray spectroscopy and X-ray diffraction. The joining effect was evaluated by shear strength. The results showed that the reaction products of the ZrB₂–SiC ceramic composite joint were TiC, ZrC, Ti₅Si₃, Zr₂Si, Zr(s,s) and (Ti, Zr)₂ (Ni, Cu), and the microstructure was separately ZrB₂–SiC/Zr(s,s)/Ti₅Si₃+Zr₂Si+TiC+ZrC+(Ti,Zr)₂(Ni,Cu)/Zr(s,s)/ZrB₂–SiC. A conceptual interface evolution model was established to explain the interface evolution mechanism. The maximum shear strength of the brazed joints was 143·5 MPa at the brazing temperature T of 920°C and the holding time t of 10 min.     

High temperature thermo-physical properties and thermal shock behavior of metal–diborides-based composites

The thermo-mechanical properties and thermal shock resistance of two metal diborides composites, HfB₂ with 20 vol.% SiC and HfB₂ with 20 vol.% SiC and 10 vol.% AlN, were investigated. Results showed that the composite HfB₂–SiC–AlN had a higher specific heat capacity than that of composite HfB₂–SiC. However, the occurrence of a Si–Al–O phase in the grain boundaries increased the heat flow resistance at grain boundaries, resulting in decrease thermal diffusivity and thermal conductivity. The calculated thermal shock resistance parameters indicated that the addition of AlN increased the resistance against crack initiation and crack propagation of composite, corresponding to the increase in critical thermal shock temperature difference from 400 to 600 °C.     

Microstructure and mechanical properties of laminated ZrB2-SiC ceramics with ZrO2 interface layers

Laminated ZrB₂-SiC ceramics with ZrO₂ interface layers were successfully prepared by tape casting, laminating and hot pressing. The flexural strength and fracture toughness are 561 ± 20 MPa and 14.4 ± 0.3 MPa m^1/2 for parallel direction, and 432 ± 18 MPa and 5.8 ± 0.3 MPa m^1/2 for perpendicular direction. The fracture toughness for parallel direction is improved significantly compared to monolithic ZrB₂-SiC ceramics. The toughening mechanism was attributed to the deflection and branch of the crack and the new microcracks, which would increase the propagation path and fracture work.     

Thixoforming of AA 2017 aluminum alloy composites

A comparative evaluation has been carried out on the microstructure of aluminum based SiC and Al₂O₃ particle reinforced composites produced by semi-solid direct squeeze forming of composite powder at temperatures of 635-645 °C. The study is focused on the distribution of the reinforcement and the intermetallic phases, the porosity content, the microstructure of the matrix phase, the interfacial state and mechanical properties. The particle size of the reinforcements, the time of the high-energy ball milling procedure for the fabrication of composite powder and the semi-solid forming temperature had a strong influence on the quality of sample in terms of distribution of reinforcement and interfacial interaction. Ball milling improves the interface formation between reinforcement and matrix and influences the remelting behaviour. Increasing ball milling time and decreasing semi-solid forming temperature with isothermal holding time resulted in relatively homogenous microstructures and in a reduced amount of interaction between SiC and metal matrix. Best results were obtained for 5 vol.% SiC_p composites after 3 h ball milling, semi-solid formed at 635 °C and held for 10 min.     

Effect of surface oxidation on thermal shock resistance of ZrB2–SiC–G composite

The thermal shock resistance of a ZrB₂–SiC composite containing flaky graphite was investigated in two different atmospheres – air and vacuum by measuring the retention of the flexural strength after water quenching for the temperature difference ranging from 200 up to 1900 °C. The residual strength values for the samples heated in vacuum gradually decreased with increasing temperature difference. When the temperature difference was above 1200 °C, the individual sample failed upon quenching. In contrast to the samples heated in vacuum, the residual strength values for the samples heated in air increased gradually as the temperature difference increased from 1200 up to 1700 °C; and the residual strength values again decreased for the temperature difference ranging from 1700 up to 1900 °C. These results indicated that the surface oxidation played the positive role in the thermal shock resistance of the ZrB₂–SiC–G composite.     

Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers

This study investigates chemical vapor deposition of C from CH₄ on particulate SiO₂ and subsequent carbothermal conversion of the resultant composite particles to SiC powders. Mass measurements, HR-TEM, SEM and XRD were used to characterize the products at various stages of the processes. It was found that oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere owing to enhanced C uptake. Pyrolytic carbon layers 5-8 nm thick were deposited on SiO₂ particles. The coated powders with high C loadings (40-42.6 wt.% C) were converted to SiC under Ar flow in a temperature range of 1700-1800 K. Almost pure SiC powders containing a mixture of particles and whiskers of ~ 100 nm were synthesized at 1750 K for 45 min and at 1800 K for 30 min using the starting powder with 40 wt.% C. Whisker diameter increased with the C content of the coated powder. It was proposed that SiC whisker was grown by a vapor-solid mechanism. Equilibrium thermodynamic analysis by the method of minimization of Gibbs’ free energy predicted the reaction pathways to SiC and to the product species in the Si-O-C-Ar system. This study demonstrated that either C shell-SiO₂ core powders or SiC powders could be synthesized rapidly in the same reactor.     

Fabrication, mechanical properties and thermal shock resistance of a ZrB2-graphite ceramic

A ZrB₂ ceramic containing 25 vol.% graphite flake (ZrB₂-graphite) ceramic was fabricated by hot-pressing at 1950 °C. It was shown that the fracture toughness was significantly improved due to the addition of graphite flake whereas the flexure strength and hardness were slightly reduced. The increase in fracture toughness was attributed to the presence of soft graphite flake and weaker interface bonding. The decrease in the flexural strength was due to the increase in weakening factors such as the weaker bonding within the ZrB₂-graphite ceramic and lower load transfer since the lower mechanical property of the graphite flake as well as graphite flakes acted as flaws in the ZrB₂-graphite ceramic. The thermal shock resistance was significantly improved and the improvement mechanism in thermal shock resistance was analyzed by Griffith fracture criterion and finite element analysis.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

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.     

The bending strength temperature dependence of the directionally solidified eutectic LaB6-ZrB2 composite

A directionally solidified LaB₆-ZrB₂ eutectic was prepared by the floating zone method based on the crucible-free zone melting of compacted powders. ZrB₂ and LaB₆ powders were used as the initial materials. The bending strength of the composite was evaluated in the temperature range of 25-1600 °C and reached 950 MPa at 1600 °C. Using a residual stress analysis, fracture toughness, and SEM and TEM fracture investigations, the toughening mechanisms under different conditions were studied. We speculate that the strength of the LaB₆-ZrB₂ eutectic at 25-1200 °C is mainly associated with crack deflection, bridge toughening mechanisms and increasing plasticity of the ZrB₂ phase; and at 1200-1600 °C, with the increasing plasticity of the matrix and fibers. By analyzing the dislocation structure of the fibers, the occurrence of strain hardening in the single crystalline ZrB₂ during high-temperature deformation was revealed. The change from the brittle to ductile fracture mode for the directionally solidified LaB₆-ZrB₂ eutectic at near 1600 °C was determined.     

Effects of SiC volume fraction and aluminum particulate size on interfacial reactions in SiC nanoparticulate reinforced aluminum matrix composites

The SiC nanoparticulate reinforced Al-3.0 wt.% Mg composites were fabricated by combining pressureless infiltration with ball-milling and cold-pressing technology at 700 °C for 2 h. The effects of SiC nanoparticulate volume fractions (6%, 10% and 14%) and Al particulate sizes (38 μm and 74 μm) on interfacial reactions were investigated by SEM, TEM and X-ray diffraction. The results show that the MgO at the interface between SiC nanoparticulate and molten Al can provide a barrier for the diffusion of Si, C and Al. Using Al particulate (74 μm) as raw material, the Al₄C₃ phase was not found in the composites containing 6 vol.% and 10 vol.% SiC, but presented in the composites containing 14 vol.% SiC. When SiC content up to 14 vol.%, the products of MgO around SiC nanoparticulate are not enough to provide effective protection from the reaction between SiC and molten Al, therefore the diffusion of Si, C and Al can take place to produce Al₄C₃ and Si phases. Using 38 μm Al particulate as raw material, the fine Al particulate possesses the high reaction activity and can easily be embedded into the gap among the big Mg particulate segregated at the interface, resulting in the appearance of exposure surface of SiCp to the Al and the forming of diffusion channels for the atomics C, Si and Al. So, the formations of Al₄C₃ and Si phases were occurred.