Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites

The impact of SiAlON on densification behavior and microstructure of the ZrB₂-SiC composite was investigated. ZrB₂, SiC, and SiAlON were used as the initial materials to produce ZrB₂-SiC composite by hot pressing at 1900 °C. A fully dense composite was obtained having ~99.9% relative density. High-resolution X-ray diffraction (HRXRD) assessment verified the in-situ formation of ZrC, and the presence of residual carbon, SiAlON, and ZrB₂ and SiC phases in the as-sintered ceramic. Furthermore, the thermodynamic calculations confirmed the results attained by HRXRD. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized for the microstructural investigation. SEM fractographs indicated the impact of SiAlON on the hindering of grain growth and the formation of flaky phases (graphitized carbon or solidified liquid phase) at the grain boundaries. TEM studies revealed the presence of a transparent glassy phase at the particle interfaces. A significant impact of liquid phase sintering was also affirmed in the clean interfaces.     

Influence of graphite nano-flakes on densification and mechanical properties of hot-pressed ZrB2–SiC composite

Hot pressed monolithic ZrB₂ ceramic (Z), ZrB₂–20 vol% SiC composite (ZS₂₀) and ZrB₂–20 vol% SiC–10 vol% nano-graphite composite (ZS₂₀G_n10) were investigated to determine the influence of graphite nano-flakes on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB₂–SiC composites. Hot pressing at 1850 °C for 60 min under 20 MPa resulted in a fully dense ZS₂₀G_n10 composite (relative density: 99.6%). The results disclosed that the grain growth of ZrB₂ matrix was efficiently hindered by SiC particles as well as graphite nano-flakes. The fracture toughness of ZS₂₀G_n10 composite (7.1 MPa m^1/2) was essentially improved by incorporating the reinforcements into the ZrB₂ matrix, which was greater than that of Z ceramic (1.8 MPa m^1/2) and ZS₂₀ composite (3.8 MPa m^1/2). The fractographical observations revealed that some graphite nano-flakes were kept in the ZS₂₀G_n10 microstructure, besides SiC grains, which led to toughening of the composite through graphite nano-flakes pull out. Other toughening mechanisms such as crack deflection and branching as well as crack bridging, due to the thermal residual stresses in the interfaces, were also observed in the polished surface.     

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.     

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.     

Effects of graphite nano-flakes on thermal and microstructural properties of TiB2–SiC composites

In the last decades, the production of ultra-high temperature composites with improved thermo-mechanical properties has attracted much attention. This study focuses on the effect of graphite nano-flakes addition on the microstructure, densification, and thermal characteristics of TiB₂–25 vol% SiC composite. The samples were manufactured through spark plasma sintering process under the sintering conditions of 1800 °C/7 min/40 MPa. Scanning electron microscopy images demonstrated a homogenous dispersion of graphite flakes within the TiB₂–SiC composite causing a betterment in the densification process. The thermal diffusivity of the specimens was gained via the laser flash technique. The addition of graphite nano-flakes as a dopant in TiB₂–SiC did not change the thermal diffusivity. Consequently, the remarkable thermal conductivity of TiB₂–SiC remained intact. It seems that the finer grains and more interfaces obstruct the heat flow in TiB₂–SiC–graphite composites. Adding a small amount of graphite nano-flakes enhances the densification of the mentioned composite by preventing the grain growth.     

Oxidation resistance of hot-pressed SiC–BN composites

The oxidation resistance of SiC–BN composites with different BN content hot-pressed from Si₃N₄, B₄C and C was investigated. The oxidized products of SiC and BN were identified to be SiO₂, C and B₂O₃, N₂. SiO₂ and B₂O₃ could further form a borosilicate glass which covered the surfaces of the samples and withstood oxidation because of its flowability and self-healing. The oxidation resistance of the SiC–BN composites in static air atmosphere deteriorated with the increase of temperature as well as of the BN content.     

ZrB2–SiC laminated ceramic composites

The oxidation behavior for ZrB₂–20 vol% SiC (ZS20) and ZrB₂–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB₂–30 vol% SiC (ZS30)/ZrB₂–25 vol% SiC (ZS25)/ZrB₂–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m^1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics.     

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.     

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.     

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.     

Effect of SiC–graphite–Al-metal addition on low- and ultra-low cement bauxite castables

Eight batches of low- and ultra-low cement castables were prepared from calcined Chinese bauxite and high alumina cement (HAC). The effect of alumina-cement replacement by SiC, graphite and aluminum metal on the sinterability and properties of these castables was investigated. Physical properties such as bulk density and apparent porosity of hydrated and sintered castables were studied. The sintered castables were also characterized for their solid phase compositions and microstructure using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. In the castables containing SiC, new phases such as mullite (3Al₂O₃·2SiO₂), SiC, and quartz (SiO₂) were formed at the expense of calcium aluminate phases (i.e. CA and CA₂; the main cement phases). Generally, the bulk density of the control castable sample was the highest among all prepared samples, while the batches containing graphite showed the lowest bulk density. The presence of Al-metal reduced the oxidation of SiC and consequently increased the densification of the castables compared with castables containing graphite only. Cold crushing strength (CCS) of the hydrated specimens i.e. green castables, decreased as the additives contents increased at the expense of HAC which is responsible for the bonding at room temperature. The highest CCS value of the sintered castable was obtained for the sample containing 6 wt.% SiC, 3 wt.% CA and 0.5 wt.% Al-metal.     

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.     

The effect of a graphite addition on oxidation of ZrB2–SiC in air at 1500 °C

Dense ZrB₂ containing 15 vol.% SiC and 15 vol.% graphite was exposed to flowing air at 1500 °C. A layered scale structure developed that consisted of (1) a uniform SiO₂-rich layer on the surface, (2) a layer of ZrO₂ and SiO₂, (3) a layer of ZrO₂ (4) a partially oxidized layer composed of porous ZrB₂, ZrO₂, and graphite, and (5) unaffected ZrB₂–SiC–C. A thermodynamic model based on volatility diagrams and consistent with the experimental observations was constructed to explain the development of the layered structure. Oxidation behavior was consistent with passive oxidation and formation of a protective surface layer. Analysis indicated that it may not be possible to form a protective surface layer without actively oxidizing SiC and producing a porous partially oxidized layer between the outer protective layer and the underlying unoxidized material.     

Effect of La2O3 addition on long-term oxidation kinetics of ZrB2–SiC and HfB2–SiC ultra-high temperature ceramics

Long-term oxidation kinetics of SiC-reinforced UHTCs and La₂O₃-doped UHTCs over an intermediate temperature range (1400–1600 °C) reveal partially protective behavior for the former characterized by an oxidation kinetic exponent 1 < n < 2. In addition, unstable oxidation behavior was observed in HfB₂-based UHTCs due to the presence of SiC agglomerates. On the other hand, La₂O₃-doped UHTCs were found to be protective over the whole temperature range studied (n = 2), in particular at 1600 °C, where oxidation kinetic exponents as high as 8 were observed as a consequence of formation of new oxidation protective particles, MeO_xC_y, where Me is Zr, Hf or Si. Adsorption of oxygen-containing species formed protective MeO_xC_y phases, which enhanced the thermal stability of the oxide scale as well as providing protection against oxidation for long exposure times at 1600 °C.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Effect of Al addition on the single and cyclic ablation properties of C/C–SiC composites

This article focuses on the damage behavior and mechanism of aluminum addition on reactive melt infiltrated C/C–SiC composites in single and cyclic ablation environments. Plasma ablation tests were performed on C/C–SiC composites containing 20 wt % and 40 wt % aluminum respectively. Coupled with TMA, XRD, SEM and EDS, the results showed that composites with 40 wt % Al had better ablation resistance during the cyclic ablation, while the composites with 20 wt % Al had excellent ablation damage resistance during a single ablation. This difference was due to higher number of microcracks formed inside the composites containing 40 wt % Al than 20 wt % Al, the lower specimen surface temperature during ablation, and the thermal stresses can be released by pore crack expansion during gas reciprocal loading. While in the single continuous loading of gas, the 20 wt % Al composite formed a protective oxide layer with smaller pores and fewer gas and oxygen entry channels, resulting in good resistance to ablation.     

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.     

Solid solution formation during spark plasma sintering of ZrB2–TiC–graphite composites

Densification and mechanical behavior of graphite-free and graphite-doped ZrB₂–TiC composites were investigated. Spark plasma sintering was used to achieve near fully-dense composites. Microstructural and phase analysis were carried out via scanning electron microscopy and X-ray diffraction spectroscopy, to illustrate the sintering and toughening mechanisms in the fabricated samples. Results indicated that 1 wt% graphite nano-flakes can improve the hardness of the composite. However, 3% drop in relative density and ~6% decrease in indentation fracture toughness were observed. The formation of TiB₂ and ZrC was verified in both TiC-contained composites, although B₄C was recognized as the byproduct of reactive sintering in graphite-doped composite. Moreover, the microstructural analysis and the peak shifts in XRD pattern indicated the formation of a solid solution between the ZrB₂ and TiB₂ phases. Higher hardness of the graphite-doped sample was also attributed to the formation of B₄C as a superhard interfacial phase. Toughening mechanisms as well as possible chemical reactions which result in the in-situ formed reinforcement phases were also discussed.     

Effects of Sc2O3 sintering aid for the densification and mechanical properties of SiC–ZrB2 composites

This study examined the effects of a Sc₂O₃ sintering aid on the density, microstructure and mechanical properties of SiC–5 vol% ZrB₂ composites prepared by hot-pressing. Microstructural studies showed that the addition of Sc₂O₃ not only caused a decrease in the hot-pressing temperature from 1950 to 1750 °C by liquid phase sintering, but also resulted in the formation of crystalline Sc₄Zr₃O₁₂ at the grain boundaries via a reaction with ZrO₂ on the surface of the ZrB₂ powder. The addition of Sc₂O₃ produced a fine-grained microstructure with a 43% (430→615 MPa) and 20% (3.6→4.3 MPa m^1/2) increase in flexural strength and fracture toughness, respectively, compared to the SiC–ZrB₂ composite without Sc₂O₃.     

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.