Measuring Porosity Using Sound Velocity in Phosphate Bonded Silicon Nitride

Phosphate-bonded silicon nitride produced by cold iso-static pressing (CIP) retains many of the desirable characteristics of hot iso-static pressed (HIP) silicon nitride including a low dielectric constant, low thermal expansion and the ability to operate in a high temperature environment, yet is considerably more cost effective due to reduced processing costs. An important difference is that unlike the HIP silicon nitride, the CIP phosphate-bonded silicon nitride cannot be produced in the fully dense state. Components that are CIP, using a US Navy patented process, at pressures of 60Ksi often have porosities between 15% and 20%. Many components where extreme high strength is not required can utilize this material even with its inherent porosity. In applications where performance engineering is involved the porosity is extremely important due to its effect on dielectric and mechanical properties. An effort has been made to characterize porosity in phosphate-bonded silicon nitride by correlating the velocity of a compressive wave in this material to a porosity value. This method allows for the determination of porosity in a small volume of the sample and can be used to quantify porosity variations in a component unlike bulk methods such as the Archimedes method which gives an average value for the entire component.</p