Influence of the surface constitution on the as-sintered strength of Si3N4 micro specimens

In order to study the significance of as-sintered surfaces for the strength of Si3N4 micro components, micro bending specimens with dimensions of 240 μm × 240 μm × 1400 μm were prepared from sintered reaction-bonded silicon nitride. The processing parameters sintering temperature and dwell time were varied, and two different types of powder bed were applied during sintering. The characteristic strength σ0 – determined by micro-3-point-bending tests – varied from about 500 to 1200 MPa. Strength values of about 1000 MPa and higher were observed when powder beds were applied which were newly prepared and doped with the sintering additives Y2O3, Al2O3, and MgO. In that case, the mass loss during sintering was negligible or even a small mass gain occurred. Samples sintered in used and undoped powder beds by contrast showed mass loss and an enrichment of the secondary phase YN-melilite (Y2Si3O3N4) at the surface; for these samples σ0 was found to be restricted to about 500–700 MPa. ry to expectations, it was observed that the residual porosity – ranging from less than 1% up to 11% – does not act as a predominant strength-determining factor. Instead, the more or less pronounced formation of surface-near defects, due to the decomposition and volatilization of Si3N4 during sintering, is considered to be decisive for the achievable strength. Mass loss and YN-melilite formation are indicators for defect creating reactions. Due to the increased surface-to-volume ratio of micro components, the affected surface zone in particular determines the overall mechanical properties.