Model Analysis Of Combustion Synthesis Fusion Casting And Microstructure Of Boride Eutectic Ceramics

With excellent mechanical properties, nice high-temperature performance, finemicrostructrues, unique phase interfaces and strong binding forces of in-situ compositeceramics, boride eutectic ceramics has attracted much attention from researchers allover the world. ZrB2-SiC those with mesh size of160~80and80~40were chosen andheated to melt, and boride eutectic ceramics were synthesed through combustionsynthesis method that using zhe recation between Al and Zr(NO3)4. The phase andmicrostructure of the obtained eutectic ceramics were analysed by XRD, SEM and EDS.Based on the thermodynamic theory, the influence of reactant ratio of Al andZr(NO3)4to reacting pressure and the adiabatic temperature was calculated, in all theZrB2-SiC, LaB6-ZrB2and LaB6-HfB2combustion synthesis systems. Temperature canbe controlled by the reactant ratio. To validate the theoretical calculated temperature,both temperatures in the open environment and closed environment were tested andcombustion wave rate was calculated. Compared with the theoretical results, themeasured temperature in the open environment was600K lower and that in the closedenvironment is200K lower.To make a better understanding of the mechanism of the eutectic formation, bothprocess of melting, mixing of fluid and solidification of the composite pelletizations inthe melting and casting process during combustion sythesis were calculated andsimulated by ANSYS (Mechanical APDL in ANSYS Workbench). Meltingandsolidification were simulated by ANSYS, and results showed that particles with adiameter of40μm needed a time of1.26×10-4s to melt.1D diffusion equation analysisshowed that melting of50%at ranges of0.5~30μm needed (2.5×10-4-0.9)s. Total timeof solidification was13seconds and much less in a μm scale. SEM analysis showed thatthe ZrB2-SiC particles displayed a clear boundary and structure was regular with twophases growing alternatively, which proved that composite particles help in casting.The effects of centrifugation-aided combustion synthesis was studied in theZrB2-SiC system at a adiabatic temperature of3600K and3800K. Results of scanningelectron microscope showed that in3600K there were large areas of two phase meltingand in3800K, eutectic areas were large and eutectic structure was close. Seperation ofZrB2-SiC melting and other components in the combustion was successfully achieved.In both LaB6-ZrB2and LaB6-HfB2systems, microstructure analysis showed thatthere were large areas where particles hadn’t melted, and in others a mixture of laminarand fiber eutectic structures was obtained.