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.

A mixture of magnesium, boric oxide and zirconium dioxide were mechanically milled under argon for up to 15 h in a laboratory scale ball mill. X-ray diffraction showed that there was an increasing conversion of ZrO₂ to ZrB₂ with milling time with >98% reaction after 15 h. Differential thermal analysis revealed there were multiple, overlapping reactions all of which seemed to be formation of ZrB₂. The energy evolved decreased with milling time and the sample after 15 h milling showed no thermal reaction. After milling, separation of the ZrB₂ from the coproduct MgO was easily achieved by a mild acid leaching leaving essentially pure ZrB₂ with a crystallite size of ∼75 nm.

Abstract

The reaction of zirconium and boron was investigated as a potential route to form dense monolithic zirconium diboride (ZrB₂) ceramics. Attrition milling of the precursors produced nanosized (less than 100 nm) zirconium metal particles that reacted with boron to form ZrB₂ with an average particle size of less than 100 nm at temperatures as low as 600 °C. Scanning electron microscopy of ZrB₂ compacts heated to 1450 °C and 1650 °C showed average particle sizes of 0.6 μm and 1.0 μm, respectively, suggesting that the fine particle size was maintained during densification. Ceramics with a relative density of ∼99% were produced by hot pressing at 2100 °C. Dense ZrB₂ produced by the reactive hot pressing process had mechanical properties that were comparable to ceramics produced by conventional processes. The four-point flexure strength of ZrB₂ produced in this study was 434 MPa.

Preparation of nano-size ZrB₂ powder by SHS has been investigated. Zr and B elemental powders were mixed with 10–50 wt.% NaCl, and prepared pellets were reacted under argon. Adiabatic temperatures were calculated by HSC software. Increasing NaCl content led to a continuous decrease in adiabatic temperatures and reaction wave velocity. Products were subjected to XRD, SEM and FESEM analyses. Average crystallite size of ZrB₂, which was 303 nm without NaCl, decreased to 32 nm with 40% NaCl addition. Distinct decrease in ZrB₂ particle size was also observed from SEM analyses. 30% NaCl addition was found to be optimum for ensuring a stable SHS reaction and providing the formation of nano-size ZrB₂ particles. It was revealed from particle size distribution measurements that ZrB₂ powder obtained by 30 wt.% NaCl addition contained particles mostly finer than 200 nm. A mechanism, similar to solution-precipitation was proposed for the particle size refining effect of NaCl.

Production of ZrB₂ powder through self-propagating high-temperature synthesis (SHS) from ZrO₂, Mg and H₃BO₃ mixture often leads to incomplete conversion. A new technique, called DSHS (double SHS) has been developed, wherein the reaction product of the first SHS is mixed with calculated amounts of Mg and H₃BO₃ powder and subjected to a second SHS. The ZrB₂ powder produced by DSHS technique yields increased conversion. The NaCl is used as a diluent during SHS to control the particle size of the product. The average particle size of SHS ZrB₂ powder found to be 75–125 nm in range, which decrease to 25–40 nm after DSHS.

The high temperature combustion of the mixture Ta₂O₅ + 5 Mg has been investigated using a microthermocouple in the medium MgO and molten NaCl. The effect of varying the amounts of these two inorganic agents (MgO and NaCl) was examined. It was shown that using MgO as the reaction medium produces the complex oxide Mg_xTa_yO_z as an impurity in the Ta powder, whereas unagglomerated single-phase powder (particle size of 20-90 nm) of Ta was formed in the presence of NaCl. The approximate mechanism for forming the final product is discussed. The distributions of temperature, the rate of heat generation and degree of conversion in the combustion wave of the mixture Ta₂O₅ + 5Mg + NaCl were measured. The sizes of the combustion zones and the kinetics of reaction were derived.

In the present investigation, the ZrB₂ powder is produced by SHS of mixture containing H₃BO_3, ZrO₂ and Mg. The thermal analysis and XRD study reveal the reaction mechanism of ZrB₂ formation by SHS process. Synthesis of H₃BO₃–Mg system results in formation of Mg₃(BO₃)₂ and MgB₄ phases, whereas ZrO₂ is partially reduced to Zr₃O and Zr during synthesis of ZrO₂–Mg system. The reaction between elemental Zr and MgB₄ results in ZrB₂ formation. The particle size of ZrB₂ is found to decrease with the addition of SHS diluent.