Room temperature fatigue of ZrB2–SiC ceramic composites

Room temperature time dependent properties of ZrB₂–30 wt%SiC ceramic composite have been studied. Both static slow crack growth and cyclic fatigue deformation have been investigated. While static slow crack growth has been evaluated only in air, three different environments, water, air, and dry air, have been used to study the cyclic fatigue. It was established that under cyclic fatigue the environment plays an important role and humidity significantly facilitate crack growth in ZrB₂–30wt%SiC. The fractography of selected ZrB₂–30wt%SiC samples was performed and it was established that both defects introduced during machining as well as larger defects introduced during the processing served as fracture origins of ceramic composites.     

Guideline of mapping parameters for Raman mapping of ZrB2–SiC ceramic composites

Determination of the collection parameters to create the most accurate two-dimensional Raman maps of ZrB₂–10?wt-%SiC ceramic composites with the shortest experimental time and highest spatial resolution possible is of high importance and interest. In the present paper, the optimisation of the parameters such as collection times, exposure time, scan type, and resolution step size using a Renishaw inVia micro-Raman spectrometer equipped with a motorised XYZ stage for automatic collection of the maps is reported. The authors found that a static scan with 30-s exposure and any resolution better than 0.7?μm produced an adequate high-quality peak intensity and a peak with two-dimensional maps of TO peak of SiC in ZrB₂–10?wt-%SiC ceramic composites within a reasonable amount of time of a few hours of collection.     

Room temperature R-curve and stable crack growth behaviour of ZrB2–SiC ceramic composites

ABSTRACT

R-curve and controlled stable crack growth behaviour of ZrB₂–17vol.-%SiC and ZrB₂–45vol.-%SiC ceramic composites was studied on V-notched samples using four-point bending at room temperature. The rising K_1R behaviour was determined as a function of the crack extension Δa with a crack bridging mechanism being dominant in such behaviour. Significant differences in crack growth rates were found within the same composition of ceramics simply as the crack length varied during crack growth processes. These differences are indicative of the significant influence of microstructural parameters of the ceramics on crack propagation. The peculiarities of stress intensity factor K₁ and the crack growth-specific behaviour in ZrB₂–SiC particulate ceramic composites are discussed.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Oxidation behavior of hot pressed ZrB2–SiC and HfB2–SiC composites

Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB₂–20 (vol.%) SiC (ZS) and HfB₂–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180 °C for ZS, and mass gain till 1100 °C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100 h durations at 1200 or 1300 °C have shown formation of partially and completely stable oxide scales after ～24 h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO₂ or HfO₂ in oxide scales of ZS or HS, respectively, besides B₂O₃–SiO₂. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO₂; and is worse in HS on cyclic oxidation at 1300 °C with air cooling, because of higher thermal residual stresses in its oxide scale.     

A SiC–Si–ZrB2 multiphase oxidation protective ceramic coating for SiC-coated carbon/carbon composites

In order to improve the oxidation protective ability of SiC-coated carbon/carbon (C/C) composites, a SiC–Si–ZrB₂ multiphase ceramic coating was prepared on the surface of SiC-coated C/C composite by the process of pack cementation. The microstructures of the coating were characterized using X-ray diffraction and scanning electron microscopy. The coating was found to be composed of SiC, Si and ZrB₂. The oxidation resistance of the coated specimens was investigated at 1773 K. The results show that the SiC–Si–ZrB₂ can protect C/C against oxidation at 1773 K for more than 386 h. The excellent oxidation protective performance is attributed to the integrity and stability of SiO₂ glass improved by the formation of ZrSiO₄ phase during oxidation. The coated specimens were given thermal shocks between 1773 K and room temperature for 20 times. After thermal shocks, the residual flexural strength of the coated C/C composites was decreased by 16.3%.     

High temperature mechanical properties of laminated ZrB2–SiC based ceramics

Two laminated ZrB₂-SiC based ceramics were prepared by tape casting and subsequent hot pressing, with BN (LZB) and graphite (LZG ) as interface layers. The LZB specimen presents flexural strength of 381 MPa at room temperature and 111 MPa at 1500 °C; while the LZG specimen shows flexural strength of 414 MPa at room temperature and 377 MPa at 1500 °C. In addition, the flexural strength of LZG specimen is always higher than that of the LZB specimen in the temperature range from room temperature to 1500 °C. Such higher strength is attributed to the healing of surface microcracks and pores by the SiO₂ glass phase, producing less glass phase in graphite interface layers at high temperature.     

Brazing of ZrB2–SiC ceramic with amorphous CuTiNiZr filler

In the present work, amorphous CuTiZrNi foils with low melting point of 1133 K were synthesized using a melt-spinning method in argon atmosphere. A ZrB₂–SiC ultra high temperature ceramic was brazed at 1153–1253 K for 900 s. The wetting and spreading characteristics of amorphous alloy filler on the ceramic were studied with the sessile drop method. The reaction products between the ZrB₂–SiC ceramic and the CuTiZrNi filler were systematic studied by X-ray diffraction and scanning electron microscopy. Also, the formation process of interfacial product of joint was discussed. A maximum room temperature three-point bending strength of 210 MPa of the joint was obtained by reasonable processing. The high temperature strength of the joint was also studied, and the joint was found to exhibit a stable strength (240 MPa) at a high temperature of 873 K. The presence of refractory Ti₅Si₃ compound was believed to be contributed to the stable high temperature strength.     

On the oxidation behavior of ZrB2–SiC–VC composites

In this study, near-fully dense ZrB₂–SiC–VC (75-20-5 vol%) composite was manufactured through hot pressing at 1850°C under the pressure of 40 MPa for 60 min. Then the oxidation examination of the composite was carried out under different durations and temperatures. The microstructure and phase evolution after hot pressing and oxidation processes were examined by scanning electron microscopy, and X-ray diffractometry. The VC addition led to the formation of ZrC and VSi₂ phases, which assisted the densification of the composite by removing ZrO₂ from the particles’ surface. The oxides of ZrO₂, SiO₂, ZrSiO₄, V₂O₅, and VO₂ formed distinct layers on the sample during the oxidation at 1700°C for 4 h with a parabolic regimen and activation energy of 177.5 kJ/mol.     

Reactive hot pressing of ZrB2–ZrCx ultra-high temperature ceramic composites with the addition of SiC particulate

Starting with non-stoichiometric Zr–B₄C powder mixture ZrB₂–ZrC matrix composites with SiC particulate addition have been made. It was found that variable amounts (5–25 vol%) of SiC could be incorporated and reactively hot pressed (RHPed) to relative densities of 97–99% at 1400–1500 °C. This technique has the potential to fabricate ZrB₂-based matrices at low temperatures with a variety of reinforcements whose composition and volume fraction are not limited by stoichiometric considerations. The hardness of the composites is in the range of 17–22 GPa.     

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB₂–SiC–ZrC–C_f ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB₂:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB₂–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (C_f: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:C_f components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% C_f had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.     

ZrB2–SiC composites prepared by reactive pulsed electric current sintering

Fully densified ZrB₂–20 vol% SiC composites were produced by reactive pulsed electric current sintering (PECS) of a powder mixture containing ZrH₂, B, SiC and B₄C within a total thermal cycle time of only 50 min. During the combined synthesis and sintering process, the ZrH₂ powder decomposed gradually from ZrH₂ into ZrH_m and finally metal Zr that reacted with elemental B to form the ZrB₂ matrix. Reducing the ZrH₂ particle size by attritor milling significantly enhanced densification and allowed initiation of self-propagating high temperature synthesis (SHS) during PECS. The PECS grades exhibited a slightly textured structure, with ≤17% of the ZrB₂ grains oriented with their (0 0 1) planes perpendicular to the direction of pressure and DC current. Because of the ZrB₂ grain orientation, anisotropic mechanical properties were observed. Ceramics prepared from attritor milled powders and PECS with a pressure applied after 5 min upon reaching 1900 °C achieved excellent flexural strengths of 901–937 MPa. The hardness and fracture toughness were respectively 19.7–19.8 GPa and 4.0–4.7 MPa m^1/2 in the direction parallel and 20.2–21.3 GPa and 3.8–3.9 MPa m^1/2 in the direction perpendicular to the applied pressure.     

In situ studies of oxidation of ZrB2 and ZrB2–SiC composites at high temperatures

High temperature oxidation of ZrB₂ and the effect of SiC on controlling the oxidation of ZrB₂ in ZrB₂–SiC composites were studied in situ, in air, using X-ray diffraction. Oxidation was studied by quantitatively analyzing the crystalline phase changes in the samples, both non-isothermally, as a function of temperature, up to ～1650 °C, as well as isothermally, as a function of time, at ～1300 °C. During the non-isothermal studies, the formation and transformation of intermediate crystalline phases of ZrO₂ were also observed. The change in SiC content, during isothermal oxidation studies of ZrB₂–SiC composites, was similar in the examined temperature range, regardless of sample microstructure and composition. Higher SiC content, however, markedly retarded the oxidation rate of the ZrB₂ phase in the composites. A novel approach to quantify the extent of oxidation by estimating the thickness of the oxidation layer formed during oxidation of ZrB₂ and ZrB₂–SiC composites, based on fractional conversion of ZrB₂ to ZrO₂ in situ, is presented.     

Flexural strength and fracture behavior of ZrB2–SiC ultra-high temperature ceramic composites at 1800 °C

ZrB₂–15 vol.%SiC and ZrB₂–30 vol.%SiC composites with smaller starting particle sizes in which the particle sizes of ZrB₂ and SiC are 2 μm and 0.5 μm, respectively, demonstrated marked plasticity and significant reduction in the flexural strength at 1800 °C. The flexural strengths of these two composites are 112 ± 12 MPa and 48 ± 10 MPa, respectively, and their corresponding strength retentions are 13% and 7%, respectively. Large ZrB₂ grains were commonly observed in the samples containing 15 vol.%SiC, which are always the sites for the crack initiation. Cavities were found in the samples containing 30 vol.%SiC and the grain boundaries are the main sites for the crack and cavity nucleation. To improve ultra-high temperature strength, larger starting particle sizes (ZrB₂ and SiC are 5 μm and 2 μm, respectively) were used for the preparation of ZrB₂–15 vol.%SiC. This sample fractured in an elastic manner up to 1800 °C and showed a very high strength with a value of 217 ± 16 MPa.     

A ZrB2–SiC/SiC oxidation protective dual-layer coating for carbon/carbon composites

In order to improve the oxidation resistance of C/C composites, a ZrB₂–SiC/SiC oxidation protective dual-layer coating was prepared by a pack cementation combined with the slurry paste method. The phase and microstructure of the coating were characterised by X-ray diffraction, scanning electron microscope and energy-dispersive spectrometer analyses. The anti-oxidation and thermal shock resistance of the coating were also investigated. It was found that the ZrB₂–SiC/SiC coating could effectively improve the oxidation resistance of the C/C composites. The weight loss of the coated samples was only 1.8% after oxidation at 1773?K for 18?h in air. The coating endured 20 thermal shock cycles between 1773?K and room temperature with only 4.6% weight loss.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

Stress measurements in ZrB2–SiC composites using Raman spectroscopy and neutron diffraction

Raman spectroscopy and neutron diffraction were used to study the stresses generated in zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics. Dense, hot pressed samples were prepared from ZrB₂ containing 30 vol% α-SiC particles. Raman patterns were acquired from the dispersed SiC particulate phase within the composite and stress values were calculated to be 810 MPa. Neutron diffraction patterns were acquired for the ZrB₂–SiC composite, as well as pure ZrB₂ and SiC powders during cooling from ～1800 °C to room temperature. A residual stress of 775 MPa was calculated as a function of temperature by comparing the lattice parameter values for ZrB₂ and SiC within the composite to those of the individual powders. The temperature at which stresses began to accumulate on cooling was found to be ～1400 °C based on observing the deviation in lattice parameters between pure powder samples and those of the composite.     

Ablation behavior of ZrB2–SiC protective coating for carbon/carbon composites

Ablation behavior of ZrB₂–SiC protective coating for carbon/carbon composites during oxyacetylene flame test at 2500 °C was investigated by analyzing the microstructure differentiation caused by the increasing intensity of ablation from the border to the center of the surface. After ablation, a continuous SiO₂ scale, a porous SiO₂ layer inlaid with fine ZrO₂ nuclei, and a continuous ZrO₂ scale respectively emerged in the border region, the transitional region, and the center region. In order to investigate the ablation microstructure in the initial stage, the sub-layer microstructure was characterized and found to be mainly formed by coral-like structures of ZrO₂, which showed huge difference with the continuous structure of ZrO₂ on the surface layer. A kinetic model concerning the thickness change induced by volatilization and oxidation during ablation was built to explain the different growth mechanisms of the continuous ZrO₂ scale and the coral-like ZrO₂ structure.     

Multi-wall carbon nanotubes (MWCNTs)–SiC composites by laminated technology

In this paper, MWCNTs–SiC composites were prepared by non aqueous tape casting and hot pressing. The dispersion of MWCNTs was investigated and correlated to the Hansen solubility parameters. The microstructure and mechanical properties of the obtained MWCNTs–SiC composites were studied. It was found that the MWCNTs were retained after sintering. An improvement in toughness was resulted with the MWCNTs content as low as 0.25 mass%. The present research provides a facile route for the preparation of ceramic–MWCNTs composites with improved properties.     

Microstructure and mechanical properties of ZrB2–SiC porous ceramic by camphene-based freeze casting

Camphene-based freeze casting technique was adopted to fabricate ZrB₂–SiC porous ceramic with 3-dimensional (3D) pore network. ZrB₂–SiC/camphene slurries (initial solid loading: 20 vol%, 25 vol% and 30 vol%) were prepared for freeze casting. Regardless of initial solid loading, the fabricated sample had dense/porous dual microstructure. The thickness of dense layer was about 200–300 μm. The microstructures of ZrB₂–SiC porous ceramics were significantly influenced by the initial solid loading, which determines the pore size, porosity and mechanical properties of the final products.