Direct Ink Write of Carbon fiber loaded Silicon Nitride

Silicon nitride, with its low dielectric constant and capacity for high thermal loads, is a candidate material for many high temperature applications. In the work reported here, the effect of two different sintering aids and/or the addition of carbon fibers on ink printability and densification was studied. Rheology, varying printing parameters, SEM microstructural studies and mechanical properties were used to quantify differences as a result of composition changes.

40 vol% to 47.5 vol% colloidal suspensions were additively manufactured via direct ink write (DIW), varying the amount of carbon fiber from 0 vol% to 10 vol% and pressurelessly sintered. Rheology measurements showed that with the addition of carbon fiber, viscosity decreased even with the overall increase in the solids vol% of the colloidal suspension. With increasing vol% of carbon fibers, alignment of those fibers decreases. The samples manufactured with 5 w% alumina and 5w% yttria resulted in a higher relative density versus their 5 w% ytterbia counterparts. XRD verified the transition of alpha phase silicon nitride into beta phase silicon nitride and showed the formation of silicon carbide at the interface between the silicon nitride and carbon fiber. The decrease in density resulted in a corresponding decrease in flexural strength. However, both 3 vol% carbon fiber compositions showed an increase in flexural strength compared to the 0 vol% carbon fiber compositions of both sintering aid packages. Elastic modulus was also affected, in that the decrease in elastic modulus from the 0 vol% carbon fiber to 3 vol% carbon fiber was much lower compared to the decrease from 3 vol% to 6 vol% carbon fiber despite the greater decrease in density.