Silicon Nitride Materials for Engine Applications

Abstract

The various performance profiles of silicon nitride engine components lead to the development of different Si₃N₄-qualities for this application. Dense silicon nitride components can be produced by sintering Si₃N₄-powder compacts (SSN) or by post-densifying RBSN (HIP-RBSN). This paper first describes the correlation between processing parameters and the corresponding microstructure and second the optimization of mechanical properties of both, SSN and HIPRBSN, by using statistical methods.

The key issue for getting a high-strength silicon nitride material is a nearly defect free fine-grained microstructure with a certain grain size distribution and aspect ratio. By carefully controlling the interaction of all processing steps a 4-point-bending strength of 1400 MPa could be realized. By using cheaper silicon starting powders and the HIP technology for post densification of RBSN a strength level of 900 MPa has been reached.

The performance of shaping by injection molding is mainly controlled by the selection of the organic vehicle. The often conflicting parameters, rheological and dewaxing behaviour, are discussed for the Si₃N₄ polyoxymethylen (POM) system. For silicon with sintering aids as a starting material and the slip casting technology the rate of hydrolysis, the viscosity and the surface charge of the suspended powder particles are described as depending on the pH and the moulding behaviour.

Using the above mentioned routes different engine components as turbocharger rotors, precombustion chambers, valve guides and valves have been produced. Their performance in test rigs and in road tests from cooperations with different automotive companies is discussed.