Development of Metal Full-Filling Method Joining Ceramic Shaft to Metal Sleeve for High Performance

Toyota Motor Corporation has mass-produced turbochager with sillicon nitride ceramic rotors. A moment of inertia was reduced by 60% using ceramic rotor which improved turbochager response. The ceramic rotor was joined to metal shaft by new method which compensated problems in both shrink fitting and active brazing methods. They are generals for mechanical and chemical techniques, respectively. There still exist the following disadvantages. It is quite severe to controll the clearance of shrink fitting to obtain the reliability of the joint. The shaft may be loosened at high temperature with a small shrink-fit interference. The large shrink-fit interference could result in a failure of ceramic shaft due to large stress. Those may require a machinig accuracy with micron meter order of surface roughness which, leads to high cost. In the case of joining by active brazing where the ceramic shaft is directly bonded with the brazing metal, the cracks may produce in ceramic shaft due to stress generated by the difference of thermal expansion coefficients of ceramic and metal. Therefore, some buffer layers are required to reduce the residual and thermal stress between the metal and ceramic shaft. Those disadvantage can be solved by new method which used buffer layer formed by full-filling a ductile metal into the clearance ceramic shaft and metal cylinder. The bonding is carried out by the compressive stress due to the difference of thermal shrinkage between the metal cylinder and ceramic shaft during cooling from melting point of the full-filling ductile metal to room temperature. This method is based on a mechanical joining without chemical bonding between silicon nitride and the ductile metal. The joining strength can be calculated by a melting point of the full-filling metal, the thermal expansion coefficient and the high temperature strength of the metal cylinder. Among several materials, BAg-8 and Incoloy 903 were selected for a full-filling and cylinder metals. The torsional strength was about 49Nm at 500°C which was quite sufficient value for the present purpose,