The thesis is devoted to the research of the regularities of the influence of refractory compounds (Cr3C2, CrB2, Mo2C, MoSi2, WC, WSi2, W2B5, SiC, HfC, TaC) and technologies of obtaining (hot pressing, pressureless sintering and combined technology) on the formation of structural phase composition and properties of ceramics based on zirconium diboride.
Thermodynamic calculations for predicting the phase composition of composite ceramics based on ZrB2 showed that the addition of carbides (VC, NbC, TiC, HfC, TaC, Mo2C, WC) or silicides (WSi2, MoSi2) leads to interactions between components. In the case of the addition of silicides, the interaction occurs with the formation of a stable mono boride (WB, MoB). The carbide additive interacts with oxides (ZrO2, B2O3) located on the surface of zirconium diboride, which leads to the formation of ZrC and monoboride. The ability to reduce oxides increases in the range VC> NbC> TiC> HfC> TaC> Mo2C> WC.
The results of thermodynamic calculations were confirmed by experimental studies. Composites based on zirconium diboride with the addition of Cr3C2, CrB2, Mo2C, MoSi2, WC, WSi2, W2B5 in the amount of 3–20 vol% were prepared by hot pressing. As a result of studying the structure of composite materials it is shown that the addition of 5 vol.% carbide (Cr3C2, Mo2C, WC) activates the sintering process due to the interaction between the components with the formation of new refractory compounds. The initial carbide additives (Cr3C2, WC, Mo2C), which were present in a charge, are absent in the structure of as-sintered material. Due to the reaction of the interaction of carbide with zirconium oxide, which acts
as an impurity, solid solutions based on zirconium carbide are formed, which are the most thermodynamically stable phases. The addition of silicide compounds (WSi2 or MoSi2), the formation of WB or MoB, SiC and low-melting layers based on SiO2 is
observed. The addition of CrB2 or W2B5 lead to formation, solid solutions based on ZrB2 are formed. Independent of the type of additive there are formation of core-shell structures, where core is ZrB2 and shell solid is solutions based on ZrB2, which is confirmed by energy dispersive spectra (EDS) and X-ray diffraction (XRD).
The flexural strength of “ZrB2 – refractory compounds” at room temperature was 500–600 MPa. At 1800 °С flexural strength of materials decreases to 180 MPa in the case of silicide additives (MoSi2, WSi2), while with the carbide additives (Mo2C, WC) the strength is from 240 to 600 MPa. This difference is related to the structure and phase composition: in materials with silicide additives at the grain boundaries there are low- melting phases, while addition of carbides reduces their number and promotes the formation of more refractory compounds ZrC, MeB (MoB, WB) at grain boundaries.
The dependence of oxidation resistance on the additive amount for composite ceramics of ZrB2 – MeC and ZrB2 – MeB2 systems has been established. The optimum amount of MeC or MeB2 addition for oxidation resistance is ~ 5 vol.%. The lower content does not allow to fully densify ceramics, the higher content (from 5 vol.%) results in the formation of craters and cracks in the oxide scale due to lower oxidation resistance of the additive Cr3C2, CrB2, Mo2C, WC or W2B5. Silicide additives significantly increase the oxidation resistance of ZrB2-based ceramics due to the formation of a stable borosilicate glass on the surface, which ensures the normal operation of the composite at a temperature of 1500 °C for 50 hours. In the same time addition of carbides (Mo2C, WC) or borides (CrB2, W2B5) to zirconium diboride does not form a protective film on the surface of the composites, which decrease operation time to 5 hours at 1500 °C.
The idea of developing ceramics based on zirconium diboride with high oxidation resistance and high temperature flexural strength due to the simultaneous introduction of additives of molybdenum silicide (MoSi2) as the most oxidation resistant component and tungsten carbide addition (WC) which significantly increase high temperature flexural strength. It was found that in the process of obtaining composite materials of the ZrB2 – MoSi2 – WC system there is a chemical interaction between the components, which leads to the formation of new phases (Mo, W) B, (Zr, Mo, W) B2 and the presence of oxides ZrO2 and SiO2. There is the content of 15 vol.% MoSi2 in the composite ZrB2 – MoSi2 – WC provides oxidation resistance (10 mg / cm2), but high temperature flexural strength (145 ± 35 MPa). As a result of reducing the MoSi2 content to 7.5 vol.%, the high temperature flexural strength increases (175 ± 28 MPa), but this leads to a decrease of oxidation resistance (15 mg / cm2). The content of 5 vol.% WC does not lead to complete purification of the material ZrB2 – MoSi2 – WC from oxide phases, and therefore it is not possible to provide the required level of high temperature strength (> 300 MPa).
