ZrB2 Phase Research Articles

Fabrication of ZrB2-SiC powder with a eutectic phase for sintering or plasma spraying

Spherical ZrB2-SiC powder with a eutectic phase was synthesised via induction plasma spheroidisation (IPS) in this study. The influence of H2 flow and reaction chamber pressure on the powder microstructure was investigated during the induction plasma spheroidisation process. The powders have an evidently better fluidity (< 44 s/50 g) and a larger apparent density (>1.65 g/cm3) after IPS treatment. With an increase in H2 flow or pressure, the shape of surface grains successively shows an irregular, hexagonal, and rectangular shape because the grain growth mechanism changes from surface energy to diffusion activation energy. There is serious mass loss of SiC on the surface because of pure active oxidation and sublimation during the IPS treatment. The cross section of the ZrB2-SiC powder indicates a dense microstructure comprising coarse ZrB2 and intergranular eutectic phases. Furthermore, the corresponding ZrB2-SiC bulk ceramics and coating fabricated using the IPS-treated powder both contain eutectic phase.

Transient liquid-phase to guide multiphase evolution in reactive-hot-pressed ZrB2–SiC–ZrC ceramics

In reactive-hot-pressed ZrB2–SiC–ZrC ceramics, ZrO2 was found to replace ZrC phase, hence leading to confusion in designing ultra-high-temperature ceramics (UHTCs). We employ high-precision X-ray diffraction and electron microscopies to reassess the phase behavior during entire reaction and densification and to reveal the evolution of multiphase relationship at different stages before reaching the final ZrB2–SiC–ZrO2 composition. Frozen from transient liquid-phase, bulk glassy phase of 15 vol% was found to be constituted of Zr–Si–B–C–O with stable Zr:O ratio, which starts as early as in the intermediate stage to suppress ZrC in favor of SiC nucleation. Inhomogeneity in phase relations and microstructures results from variation in local transient liquid-phase to develop SiC phase in various modes and rates. As inferred from the earlier report of phase formation, competing reactions for ZrC and ZrB2 phases in the initial stage below 1000 °C were mediated via Zr–O–B–C liquid phase. Such liquid phase was moderated by stable B–O components, as initiated from surface oxides of starting powders. This picture under a continuous mother liquid phase can unify the reactions and sintering into a collective melting–nucleation–growth process, which enables and guides the evolution of multiphase relationship through several stages to reach final densification at relatively low temperature with the help of residual oxides.

Structural characteristics and ablative behavior of C/C–ZrC–SiC composites reinforced with “Z-pins like” Zr–Si–B–C multiphase ceramic rods

In order to improve the ablation and oxidation resistance of C/C–ZrC–SiC composites in wide temperature domain, “Z-pins like” Zr–Si–B–C multiphase ceramic rods are prepared in the matrix. The influence of different sintering temperatures on the microstructure of ceramic rods and the ablative behavior of heterogeneous composites are studied. The results showed that the ZrB2 and SiC phases are formed in the sintered matrix, and the increase of sintering temperature is beneficial to improve the density of the ceramic rods. The ablation properties of samples have been greatly improved. The mass and linear ablation rate are 0.8 mg/s and 3.85 μm/s, respectively, at an ablation temperature of 3000 °C and an ablation time of 60 s. After ablation, the matrix surface is covered with SiO2 and ZrO2 mixed oxide films. This is due to the preferential oxidation of “Z-pins like” Zr–Si–B–C multiphase ceramic rods in the ablation process, and B2O3 melt, SiO2 melt, borosilicate glass, ZrSiO4 melt and ZrO2 oxide film can be generated successively from the low-temperature segment to the ultra-high temperature segment. These oxidation products can be used as compensation oxide melts for the healing of cracks and holes on the matrix surface in different temperature ranges and effectively prevent the external heat from spreading into the matrix. Therefore, C/C–ZrC–SiC composites with “Z-pins like” Zr–Si–B–C multiphase ceramic rods achieve ablation resistance in wide temperature domain.

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.

Effect of laser surface melting on nanoparticles dispersion and tensile properties of in-situ nano-ZrB2p/6061Al composite

Laser surface melting (LSM) were successfully applied in in-situ 10 vol% ZrB2(np)/6061Al composites. Formation of nano-sized ZrB2 phase is confirmed by FESEM, EDS and HRTEM. Due to the minimization of porosity defects, one-way laser scanning is evaluated as optimum scan route strategy. After LSM, nanoparticle aggregations in composites are largely eliminated by melt vortex and matrix grain growth pushing force. Nanoparticles are uniformly dispersed near matrix grain boundaries and agglomerations showed an explosive orientation dispersion. After LSM, the average matrix grain size decreased by 87.5% to 3.4 μm. Nanoparticle dispersion uniformity and toughness of composites are proportional to laser power density. Both strength and elongation are enhanced as the matrix grain refinement and better uniformity of the nanoparticle dispersion after LSM. When LSM for 420 NW/mm2 was performed, the optimal improvements of tensile strength and elongation are 33.54% and 72.51% respectively.

Potential high-Tc superconductivity in ZrB2 polymorph under pressure

The clear proof of superconductivity in transition-metal diborides was rarely reported. In the current work, the recently developed particle swarm optimization structural search method was utilized to propose three ZrB2 structures, namely, I41/amd-ZrB2, P42/mmc-ZrB2, and P4/nmm-ZrB2. The structural stability of the three novel ZrB2 phases was confirmed on the basis of elastic constant and phonon dispersion calculations. At ambient pressure, the mechanical properties of the I41/amd-ZrB2 and P42/mmc-ZrB2 phases are comparable to those of AlB2–ZrB2. Electron–phonon coupling (EPC) calculations revealed that the P4/nmm-ZrB2 phase is predicted to be a potential high-Tc superconductor with a calculated Tc of 12.7 K at 20GPa. Moreover, significant pressure-induced EPC enhancement can also be found in the P4/nmm-ZrB2 phase. The maximum EPC constant λ and Tc under 600GPa are 1.05 and 34.4 K, respectively.

Incorporation of BN-coated carbon fibers into ZrB2/SiBCN ceramic composites and their ablation behavior

ZrB2/SiBCN composites containing carbon fibers coated with BN were prepared using combination of sol-gel and spark plasma sintering (SPS) techniques. Thermal shock and ablation behavior of these composites were thoroughly analyzed. Sintered composites contained ZrB2, SiC and BN(C) crystalline phases. Thin BN layer passivated carbon fibers and prevented them from reacting with the matrix. With the addition of carbon fiber, diffusion coefficient decreased, and the thermal expansion coefficient increased in comparison to ZrB2/SiBCN composites without carbon fibers. Ablation tests showed no crack formation after carbon fibers were added to the composites. Ablation center exhibited loose and porous network structure. ZrSiO4 formed from ZrO2 and SiO2 reaction in the central ablation region. Presence of ZrSiO4 prevented samples from further ablation-related damages.

RETRACTED ARTICLE: Ablation Behavior of SiC/ZrB2 Multilayer Coating Prepared by Plasma Spray Method

In this study, a SiC/ZrB2 multilayer coating with a functional gradient structure was prepared on graphite to increase the ablation resistance. The SiC coating as the inner layer was applied by the reactive melt infiltration method. Then, a ZrB2 outer layer coating was applied by argon gas shrouded plasma spray. The effect of the shielding gas flow rate on the amount of oxides and quality of the ZrB2 coating was studied. To evaluate the ablation resistance of the coatings, the specimens were exposed perpendicular to an oxy-propane flame for 60 second with a temperature of 2473 K and heat flux of 3000 kW/m2. The single-layer SiC coating showed 24.24 pct mass loss. The results indicated that applying the ZrB2 coating significantly improved the ablation resistance and reduced the mass ablation rate. In the plasma spray process, by applying the argon shrouding with a flow rate of 150 L/min, the oxidation of ZrB2 phase decreased from 41.6 to 4.8 pct. In addition, the weight loss and mass ablation rate decreased from 12.79 and 1.857 × 10−3 g/cm2/s to 0.12 pct and 0.0393 × 10−3 g/cm2/s, respectively. The improvement of the ablation resistance of the coating prepared by argon gas shrouded plasma spray could be attributed to the lower amount of oxides and pores in the as-sprayed coating and better cohesion of splats.

Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites

ZrB2–SiC–graphite composites with 0–35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 °C under the uniaxial pressure of 40 MPa for 1 h. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr3C2 and B4C phases besides the main existing ZrB2, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB2 grains. The addition of graphite into the ZrB2–SiC composites improved the sintering process and caused a fine-grained microstructure.

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.

Co-reinforcing of mullite-TiN-CNT composites with ZrB2 and TiB2 compounds

In this study, the microstructural and mechanical properties of three different mullite matrix composites were investigated. Accordingly, Mullite-TiN-CNT, Mullite-TiN-TiB2-CNT and Mullite-TiN-TiB2-ZrB2-CNT composites with 10 wt % of each TiN, TiB2 and ZrB2 reinforcement particles as well as 1 wt % CNT were prepared by the spark plasma sintering (SPS) technique. The sintering processes were conducted at 1350 °C for 5 min with a mean heating rate of 60 °C/min. The relative densities of the composites achieved were higher than 97% of theoretical density. The results of mechanical properties of the fabricated composites showed that Mul-TiN-TiB2-ZrB2-CNT composites obtained the highest hardness and fracture toughness values. XRD results confirmed the presence of mullite, TiN, TiB2 and ZrB2 phases without any additional reactions between reinforcement and matrix phases. Microstructural studies demonstrated the uniform distribution of reinforcing phases in the matrix for all composites. The fracture surface micrographs revealed the dominating transgranular fracture mode indicated the strong adhesion of the reinforcing particles to the matrix. Pulling out of CNTs was also observed in the fracture surface of composites, which plays a vital role in the fracture energy absorption and joining of interfaces.

Effects of ZrB2 addition on texture development and properties of porous Si3N4-ZrB2 composites by magnetic field alignment

ABSTRACTTextured porous Si3N4-ZrB2 composites were prepared by the gel-casting method assisted by magnetic field alignment technology and subsequent pressureless sintering. These results showed that when a 6 T magnetic field was applied, the ZrB2 grains also formed a c-axis orientation in the textured green bodies, except for β-Si3N4 grains, which formed an a- or b-axis orientation. The increasing ZrB2 content inhibited the degree of texture of the Si3N4 and ZrB2 grains in the green bodies, respectively. After sintering at 1800 °C, new ZrN and BN phases without grain orientation characteristics formed by the reaction between Si3N4, ZrB2 and N2 and the ZrB2 phase disappeared completely. The sintering process promoted the degree of texture of Si3N4 as the matrix increases. Meanwhile, the variations in the degree of texture of Si3N4 with changes in the ZrB2 content were consistent with that in the green body. Additionally, when 10 wt% ZrB2 content was introduced, the apparent porosity reached its highest value and the bulk-density reached its lowest value, leading to the appearance of least strength. In textured porous composites, the strength was mainly affected by the densification process instead of the grain orientation. Also, no evident difference in strength existed between the two directions owing to the function of high porosity.

Kinetics and mechanism of the oxidation of ZrSi2-MoSi2-ZrB2 ceramics in air at temperatures up to 1400 °C

The results of a study of the kinetics and mechanism of heterogeneous and compact ceramics oxidation in the ZrSi2-MoSi2-ZrB2 system at air temperature of 1400 °C are presented. The ceramics were obtained by the hot-pressing of composite powders that were manufactured by self-propagating high-temperature synthesis following the magnesiothermal recovery approach. Oxidation kinetics are described using a power function with index n>2, confirming the significant influence on the evolution process in the structure of the formed oxidation coating. The oxidation mechanism includes the formation of a two-layer structure consisting of a continuous silicate film, of which the outer part contains magnesium and a sublayer based on the ZrSiO4 phase, with the scheelite structure encapsulating the ZrB2 and MoSi2 grains. The influence of the ZrSi2, MoSi2 and ZrB2 phases on the structural-morphological peculiarities of the appearing oxide films and the effectiveness of its protective action are revealed.

A novel microstructural design to improve the oxidation resistance of ZrB2-SiC ultra-high temperature ceramics (UHTCs)

Directionally Solidified 72.8 mol% ZrB2 - 27.2 mol% SiC (equivalent to 20 vol%) composite was fabricated using a crucibleless-melting technique. Microstructural and XRD analyses confirmed the texturing of grains and lamellar structure. The Lotgering (00l) orientation factor was as high as 0.97. The thickness of the lamellae was about 5 μm. Bright-field TEM images confirmed that the eutectic composite has continuous ZrB2 and SiC phases. Electron diffraction patterns from both the ZrB2 and SiC phases were indexed as hexagonal and 3C-cubic structures, respectively. Line defects such as dislocations, planar defects such as stacking faults and twins were observed in the SiC. Oxidation studies performed at 1873K for 1 h demonstrated that the samples obtained by melting showed an improved oxidation resistance compared to ZrB2-SiC composites obtained via sintering due to the formation of a passivating continuous SiO2 layer. The oxidation mechanisms are discussed, the orientation and SiC lamellae acted as an effective barrier to the inward oxygen flux and played a key role in retarding oxidation.

Core‒Rim Structures and Hierarchical Phase Relationship for Hot-Pressed ZrB &lt;sub&gt;2&lt;/sub&gt;‒SiC‒MC (M=Nb, Hf, Ta, W) Ceramics

Core‒rim structure emerges as a common microstructural feature of hot-pressed ZrB2‒SiC‒MC (M=Nb, Hf, Ta and W) ceramics. A combination of X-ray diffraction and microscopic analyses reveal that it originates from the re-distribution of transition-metals into their bi-solubility in the primary phase, hence turning the core‒rim structures into an intra-phase relationship. Solution‒reprecipitation process rationalizes their formation via a transient liquid-phase of Zr‒M‒B‒C‒(O). It dictates the special transformation process by enabling an exchange of metal with cores to reach the lower solubility and a transition to grow rims into higher solubility, driven by an over-saturation of boron in the liquid. A third and even higher solubility in the minor ZrC phase finalizes the liquid-phase sintering process, leading to hierarchical phase relationship. We propose further a scheme of g-point to syndicate the bi-solubility as solidus and liquidus limits of metal in ZrB2 phase, hence to schematize the coordinated evolution of multiphase microstructures. The core/rim boundaries can strengthen the ceramics by storing and re-distributing strain energy within the grains, which leaves the microstructure‒property relationship to renew and further optimize for UHTC.

Kinetics and mechanism of high-temperature oxidation of the heterophase ZrSi2-MoSi2-ZrB2 ceramics

The kinetics and mechanism of high-temperature oxidation of dense heterophase ZrSi2-MoSi2-ZrB2 ceramics at a temperature of 1650 °C are studied. The ceramics were fabricated by hot pressing of composite powders prepared by self-propagating high-temperature synthesis with magnesiothermic reduction. The oxidation kinetic curve is described by the power-law function, which indicates that the evolutionary changes in the structure of the formed oxide films significantly affect the course of the oxidation process. The oxidation mechanism involves formation of a multi-layered structure of heterogeneous oxide film, partial dissociation of the ZrSiO4 phase, and formation of secondary MoB and Mo5Si3 compounds. The influence of the ZrSi2, MoSi2 and ZrB2 phases on the structural and morphological features of the formed oxide films and the efficiency of their protective action are shown. Silicon is reduced and zirconium is simultaneously oxidized to ZrO2 in the ZrSi2-ZrSiO4 system at temperatures above 1620 °C in the absence of oxygen or in low-oxygen environment.

Abrasive wear performance of zirconium diboride based ceramic composite

The present study focuses on the wear behavior of a pressure-less sintered ZrB2–20 vol% SiC composite in dry sliding interaction against the electroplated diamond disc. Pressure-less sintering was carried out at 2000 °C for 2 h in an argon atmosphere. Sintered composite possesses 98% of the theoretical density, and primarily microstructure contains ZrB2 and SiC phases. The sliding wear of the investigated composite has been studied in pin-on-disc equipment at room temperature at different combinations of loads and sliding speeds to examine the influences of the test parameters on wear mechanism. The results show that the elastic modulus, hardness and fracture toughness for ZrB2 20 vol% SiC are 442 ± 4.5 GPa, 16.5 ± 0.5 GPa, and 5.67 MPa√m, respectively. Results also show that the specific wear rate of the ZrB2-SiC composite increases continuously with increasing the applied loads whereas, it decreases with increasing sliding speed. XRD analyses of worn surfaces suggest that the phase transformation from ZrB2 to ZrO2 may occur due to frictional heating during sliding. The specific wear rates are in the order of ~10−6–10−8 cm2/Nm. The post wear test characterization suggests that oxidation dominates mild wear with low roughness values at low load and high sliding speed, whereas at high loads and low sliding speed severe wear mechanism is observed with high roughness values and deep grooves in surfaces. The mixed mode (oxidative–grain pullout-micro-cutting) wear mechanism is controlling the severe wear.

Fabrication and mechanical behavior of ZrB2 strengthened SiCf/SiC composites prepared by hybrid processing route

A SiC fiber-reinforced composite containing a SiC-ZrB2 mixed matrix (SiCf/(SiC-ZrB2)) with high density and enhanced mechanical properties was fabricated. ZrB2 at 5 or 40 vol% was added to a (SiC + C) slurry to be infiltrated into the voids of 2D woven Tyranno™-SA grade-3 fabrics by electrophoretic deposition. Subsequent hot pressing at 1300 °C and 10 MPa for 1 h, followed by liquid silicon infiltration (LSI) at 1600 °C for 5 h in an Ar atmosphere resulted in the formation of the reaction-bonded SiC matrix, which revealed a composite density close to 97%. SiCf/(SiC-ZrB2) having open porosities of 0.2–0.6% showed peak strengths of 398 and 320 MPa for 5 and 40 vol% ZrB2 addition, respectively. The large mismatch in the coefficient of thermal expansion and Young's modulus between the SiC and ZrB2 phases was attributed to a reverse trend in the strength of composites. Brittle behavior of the composites in flexure can be explained by the strong bonding between the matrix and fibers formed by the reaction of interphase with molten Si during LSI. Strength retention after oxidation at 1000 and 1400 °C for 2 h was also compared in terms of ZrB2 amount contained in the composites.

Structure, Mechanical Properties, and Oxidation Resistance of ZrB2, ZrSiB, and ZrSiB/SiBC Coatings

ZrB2, ZrSiB single-layer coatings, and ZrSiB/SiBC multilayer coatings have been fabricated by magnetron sputtering. The coating structure is studied by X-ray powder diffraction analysis, scanning electron microscopy, and glow-discharge optical emission spectroscopy. The mechanical properties of the coatings are identified by nanoindentation. The oxidation resistance and thermal stability of the coatings are studied in the temperature range of 600–1200°C. Doping the ZrB2 single-layer coatings with silicon reduces the grain size of the hexagonal ZrB2 phase from 8 to 2 nm. Application of silicon-containing layers results in complete amorphization of the structure of ZrSiB/SiBC multilayer coatings. The ZrB2 coatings are found to possess the best mechanical properties: hardness of 37 GPa, Young’s modulus of 400 GPa, and elastic recovery of 73%. The ZrSiB coatings are characterized by the highest oxidation resistance: they resist oxidation at temperatures up to 1500°C due to the formation of a protective SiO2-based film on their surface.

Mechanical and Tribological Behavior of ZrB2-TiB2 System Prepared by Mechanical Activation Spark Plasma Sintering Technique

ZrB2-TiB2 system among varying amount of TiB2 loading (5, 10, 15, 20 and 30 wt.%) was prepared by mechanical activation spark plasma sintering at 2100 °C for 15-min dwell time under 50 MPa uniaxial pressure in argon atmosphere. The effect of the TiB2 loading on properties like physical, mechanical and tribological of the prepared samples was studied and evaluated with monolithic ZrB2 ceramic. Finally, almost fully dense samples (> 96% of theoretical density) were fabricated. Microstructural observation and XRD phase analysis of sintered compact revealed that TiB2 completely went into the ZrB2 structure by formation of solid solution with ZrB2 phase. ZrB2-TiB2 (30 wt.%) showed the highest Vickers micro-hardness (HV) of 20.64 GPa at 10 N load. On addition of TiB2, the fracture toughness of the ZrB2-TiB2 improved up to 4.69 ± 0.43 MPa m1/2 over monolithic ZrB2 at 10 N load. At 10 N load, the scratch resistance coefficient, wear rate and wear resistance coefficient of ZrB2-TiB2 (30 wt.%) reached a lowest value of 0.32, 0.35 mm3/N m and 0.008, respectively, at 10 N load than other ZrB2 compositions.