Sintering Of ZrB2 Research Articles

Synthesis of equimolar solid solution of zirconium and hafnium diborides by vacuum-thermal routes

We explored the Zr₁₋ₓHfₓB₂ system with first-principles cluster expansion and thermodynamic analysis of possible reactions targeting the synthesis in vacuum-thermal conditions. The results suggest that the system may have a slight tendency to ordering but at high temperatures the formation of solid solutions is driven by the entropy of mixing. We realized the synthesis of an equimolar composition (x = 0.5) by sintering of ZrB2 and HfB2 and reduction from oxides by boron with varying homogenization of the reacting mixtures. The temperature of the complete transformation strongly depended on the reactants homogeneity and was 1900 °C for more coarse raw materials and below 1700 °C for a fine hydrothermal mixture of oxides. The possibility of producing non-equimolar compositions of the solid solutions by the same method was also experimentally demonstrated.

Research on experimental synthesis and sintering of ZrB2

Research on experimental synthesis and sintering of ZrB2

Numerical simulation of heat transfer during spark plasma sintering of zirconium diboride

Temperature and electric current distributions in the spark plasma sintering process of a ZrB2 sample were simulated using finite element method. The main source of heating in spark plasma sintering is perceived as the Joule heating effect, which is a result of electric current distribution in the setup. Two sets of governing equations including the electric charge and energy conversion are utilized to obtain the temperature distribution. The acquired results for the sintering process were compared with Al2O3 sintered counterpart propounding two different mechanisms of heating proceeding. Al2O3 is an electrical insulator; therefore, the operating electric current concentrates in the graphite die close to the sample, whereas the ZrB2 is an electric conductor resulting in a more uniform current distribution. In the non–conductive sample, heat is generally generated in the graphite die and conducted to the sample. Both Joule heating effect and thermal conduction from the die to the sample are involved in the sintering of ZrB2. The temperature distribution in electric resistant materials is more uniform than electric conductor cases. This is attributed to the different dominant heating mechanism of materials.

Effect of ZrB2 Particle Size on Pressureless Sintering of ZrB2 - ß-Sic Composites

Zirconium diboride is an ultra high temperature ceramic material that leads this emerging class of materials because of its distinct combination of properties, including high melting temperature (> 3000 °C) and the lowest theoretical density (6.09 g·cm-3) among the borides. This combination of properties makes ZrB2 candidate for airframe leading edges on sharp-bodied reentry vehicles. In this work, the effect of particle size of ZrB2 on the pressureless sintering of ZrB2-SiC composites was studied, using ZrB2 powder with average particle size of 2.6 and 14.2µm. Four different vol% concentration of ß-SiC (0, 10, 20 and 30 vol%) were added to as-received and planetary milled ZrB2 powder. Samples were pressureless sintered at 2050 °C/1h in argon atmosphere. The reduction of initial ZrB2 particle size led to composites with better results of densification, mechanical properties and oxidation resistance regardless ß-SiC addition, showing relative densities around 92.5 %Theoretical Density (Td) and flexural strength and microhardness around 260 MPa and 17.5 GPa, respectively. Composites processed with as-received ZrB2 powder showed increasing in densification and flexural strength with the SiC content increasing. Relative density varied from 74.7 to 90.8 %TD and flexural strength from 102 to 241 MPa, for 0 and 30 vol% of SiC, respectively.

ZrB2\u2013SiC based composites for thermal protection by reaction sintering of ZrO2+B4C+Si

Synthesis and sintering of ZrB2–SiC based composites have been carried out in a single-step pressureless reaction sintering (PLRS) of ZrO2, B4C, and Si. Y2O3 and Al2O3 were used as sintering additives. The effect of ratios of ZrO2/B4C, ZrO2/Si, and sintering additives (Y2O3 and Al2O3), was studied by sintering at different temperatures between 1500 and 1680 °C in argon atmosphere. ZrB2, SiC, and YAG phases were identified in the sintered compacts. Density as high as 4.2 g/cm3, micro hardness of 12.7 GPa, and flexural strength of 117.6 MPa were obtained for PLRS composites. Filler material was also prepared by PLRS for tungsten inert gas (TIG) welding of the ZrB2–SiC based composites. The shear strength of the weld was 63.5 MPa. The PLRS ZrB2–SiC composites exhibited: (i) resistance to oxidation and thermal shock upon exposure to plasma flame at 2700 °C for 600 s, (ii) thermal protection for Cf–SiC composites upon exposure to oxy-propane flame at 2300 °C for 600 s.

Hot Pressing Sintering of ZrB<sub>2</sub> with β-SiC Addition

Zirconium diboride (ZrB2) is a material of particular interest because of the excellent and unique property combination of high melting point, high electrical and thermal conductivity. In this work, the effect of addition of beta-silicon carbide (β-SiC) on hot pressing sintering of ZrB2 was investigated. Four compositions were studied with 0, 10, 20 e 30 vol% of SiC. ZrB2 powder and mixtures were prepared by planetary milling with SiC spheres during 4 h. Samples were sintered at 1850 °C/1h with a pressure of 20 MPa in argon atmosphere. β-SiC has undergone phase transformation to α-SiC during sintering. The addition of SiC increased densification with increasing of SiC content. The total densification of sample was 96.8 % of theoretical density for sample with 30 vol% of SiC and Vickers hardness was 19.9 ± 0.3 GPa.

Pressureless sintering of ZrB2–SiC composite laminates using boron and carbon as sintering aids

A processing method common to composite ceramics with very different ZrB2/SiC ratios was developed in order to exploit ZrB2–SiC laminates comprising alternate layers with different compositions for thermal protection systems of re-entry vehicles. Ceramic laminates were made using SiC, ZrB2 and composites with a SiC/ZrB2 ratio ranging from 100 vol.-%SiC to 100 vol.-%ZrB2. The preparation was performed by tape casting of a slurry, layer stacking, debinding and pressureless sintering. Boron and carbon proved to be suitable sintering aids for SiC laminates as well as for composite laminates containing SiC. In the case of composites with a ZrB2 matrix, the second phase of SiC acted as a sintering aid. This process obtains very similar densification for specimens with very different compositions (ranging from 100%SiC to 80ZrB2–20SiC). The stiffness of ZrB2–SiC laminates increased with the ZrB2 content increase, while the bending strength was not affected by the ZrB2/SiC ratio.

Low-Temperature Hot Pressing of ZrB2-Based Ceramics with ZrSi2 Additives

Densification of zirconium diborides (ZrB2) was examined by hot pressing at different temperatures ranging from 1350°C to 1500°C with sintering additives of zirconium disilicide (ZrSi2), ranging from 10 vol% to 40 vol%. The effect of the amount of ZrSi2 additives on the densification of ZrB2 was assessed. The sinterability of ZrB2 with ZrSi2 additives was significantly improved, and this improvement was enhanced with the increasing ZrSi2 added. The relative density exceeding 94% was obtained for the studied ZrB2 ceramics with ZrSi2 additives at a temperature equal to or above 1450°C, depending on the amount of additive. With 10 vol% ZrSi2 additions, the relative density increased from 79.3% at 1350°C to 96.5% at 1500°C. With additions equal to or above 20 vol% ZrSi2, a relative density exceeding 96% was obtained at 1450°C, and the nearly fully dense ZrB2 compacts were obtained at 1500°C.

Spark plasma sintering of ZrB2- and HfB2-based Ultra High Temperature Ceramics prepared by SHS

The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication of fully dense MB2-SiC and MB2-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B4C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities ≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than, those related to analogous products synthesized by alternative, less rapid, methods.