Zirconium diboride based materials are of considerable interest to the industry because of their high melting point, high strength, high corrosion resistance, and high hardness. The processing technique is needed to attain these favorable properties as the actual component. It is known that their strength and corrosion resistance are very adversely affected by excessive porosity in sintered bodies. But it is difficult to obtain fully dense body, although $ZrB_2$ powder is routinely hot-pressed at 2000℃ or even lower temperature, with densification aids.
In order to obtain dense $ZrB_2$ ceramics, in the present study, the effect of the addition of Fe on the pressureless and hot press sintering behavior has been investigated. Also, the reaction behavior, microstructure and mechanical property of the $ZrB_2$ ceramics by reactive melt infiltration have been investigated.
Pressureless sintering was performed at 1900∼2200℃ under Ar atmosphere with vibratory-milled $ZrB_2$ powder. The addition of Fe upto 1wt% was effective to increase the sintered density. However it was rather difficult to obtain the relative density of higher than 90 percent. It was predicted that sintering was enhanced by the mass transport of Fe in a liquid state at the sintering temperature. The Zr-Fe-B compound in liquid phase was observed from the EDS and WDS analysis. Hot press sintering was performed at 1600∼1700℃ for 1hr under Ar atmosphere. It was possible to obtain 95% relative density of $ZrB_2$ specimen, which is higher density than that obtained by pressureless sintering. It can be found that high pressure causes for particles to rearrange easily and make them dense.
After the preparation of preforms by addition of B, $B_4C$ or C as the reactant to $ZrB_2$, the fused Zr of Si was infiltrated into thr preforms. Reaction behavior, microstructure and mechanical property between infiltrated metals and reactants, B, $B_4C$ or C were investigated. $ZrB_2$/Zr and $ZrB_2$/ZrC/Zr composites have been prepared by addition of B and $B_4C$ as the reactant and reactive infiltration of Zr was taken place under vacuum at 1900℃ for 1hr. In addition, the $ZrB_2$/SiC/Si composites also have been prepared by adding C as a reactant and then Si was applied to make reactive infiltration at 1600℃. It was possible to obtain the high dense composites from both procedures at the wide range of reactant concentration. As the reactant concentration increases the content of solid phase of $ZrB_2$, ZrC and SiC increases, thereby the content of liquid phase decreses. When the B was added as reactant, the average particle size of $ZrB_2$ was propertionally increased with the additional content of B. This may be due to the heat of reaction increase upon the reaction Zr and B that might result in the increase of grain growth rate. On the other hand, the particle size of $ZrB_2$ slowly decresed with the addition of $B_4C$ as the reactant and this fact can be regarded as inhibition of the $ZrB_2$ grain growth due to the increase of ZrC generated from the increase of $B_4C$.
The fracture strength of $ZrB_2$/Zr and $ZrB_2$/ZrC/Zr composites is approximately 550 MPa ∼ 400 MPa and the fracture toughness is 11.5 MPa·$m^{\frac{1}{2}}$ ~ 8 MPa·$m^{\frac{1}{2}}$ which is more excellent than the value of conventional sintered ceramic materials. Two phases of $ZrB_2$ and Zr have been observed from the reactive infiltration of fused Zr at 1900℃ after adding 5 ∼ 30vol.% of B to $ZrB_2$. It was also observed that three phases of $ZrB_2$, ZrC and Zr coexisted when the reactive infiltration of fused Zr was taken place after adding $B_4C$ to $ZrB_2$. Meanwhile three phases of $ZrB_2$, siC and Si were observed from the reactive infiltration of fused si after adding C to $ZrB_2$.
