Correlation of Processing and Sintering Variables with the Strength and Radiography of Silicon Nitride

A sintered Si{sub 3}N{sub 4}-SiO{sub 2}-Y{sub 2}O{sub 3} composition (6Y) was developed that reached four-point flexural average strength/standard deviation values of 857/36, 544/33, and 462/59 MPa at room temperature, 1,200{degree}, and 1,370{degree}C, respectively. These strengths represented improvements of 56, 38, and 21% over baseline properties at the three test temperatures. At room temperature the standard deviation was reduced by over a factor of three. These accomplishments were realized by the iterative utilization of conventional x-ray radiography to characterize structural (density) uniformity as affected by systematic changes in powder processing and sintering parameters. Accompanying the improvement in mechanical properties was a change in the type of flaw-causing failure from a pore to a large, columnar {beta}-Si{sub 3}N{sub 4} grain, typically 40 to 80 {mu}m long, 10 to 30 {mu}m wide, and with an aspect ratio of 5:1.