The optimization of mechanical properties of Si(3)N(4) ceramics using experimental designs.

The objective of this research is to optimize the hardness, fracture toughness, room- and high-temperature strength of $\rm Si\sb3N\sb4$ ceramics in $\rm Si\sb3N\sb4$-$\rm Y\sb2O\sb3$-$\rm MgO-Al\sb2O\sb3$-CaO system. Two series of experiments were carried out to investigate the effects of 7 factors, each at 2 or 3 levels, on the mechanical properties of silicon nitride ceramics using experimental design methods, instead of the one-factor-at-a-time approaches. Data analysis is based on a recent Bayesian variable selection methodology. Regression equations for predicting the weight loss, density, Z value, hardness, fracture toughness, room and high temperature strength of this material have been obtained. The effect of individual factors and their interaction on the response variables can be understood. Gas-pressure sintered silicon nitride ceramics can be made with a weight loss of 2.5%, density of 99% (3.23 g/cm$\sp3),$ hardness of 15 GPa, a toughness of 7 MPa$\cdot$m$\sp{1/2}$ (Vickers indentation method), and fracture strengths of 1150 MPa at 25$\sp\circ$C and 550 MPa at 1200$\sp\circ$C. The optimum combination of mechanical properties is obtained with ${mol\%(Y\sb2O\sb3)}\over{mol\%(MgO)}$ = $2\over3$, ${mol\%(Y\sb2O\sb3)}\over{mol\%(Al\sb2O\sb3)}$ = $1\over4$, CaO contents ranging from 0 to 1 mol%, and $\rm Si\sb3N\sb4$ contents at 85 mol%.Ph.D.Applied SciencesMaterials sciencePure SciencesStatisticsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131140/2/9825386.pd