Ceramic Turbocharger Rotor Research Articles

Precision Forming and Machining Technologies for Ceramic-Based Components

Structural ceramics components for industrial use are classified under two categories: one that is originally designed for ceramics (Ceramic Origin), and the other that is originally designed for metals and subsequently replaced with ceramics because of their improved hardness and resistance to both heat and corrosion (Metal Origin). Ceramic insulators for power lines and catalytic substrates used to control automotive emissions in gasoline engines are “Ceramic Origin” components. As ceramics are difficult to machine, a precision mold has been used in the forming process to minimize the machining volume in the case of “Ceramic Origin” components. Meanwhile, ceramic turbo charger rotors and valves for automotive engines are “Metal Origin” components, which not only require durability under severe operating conditions but also severe dimensional accuracy, similar to metal parts. These components have been derived from extensive R&D efforts in materials and process technologies for ceramic gas turbines, which have been implemented in the majority of advanced countries since the 1970s. This paper includes some examples of precision forming and machining technologies for both types of ceramic components developed by NGK Insulators, Ltd., and highlight their issues. Finally, the possibility of new types of ceramic-based components will be introduced.

Stacking faults in deformed α-silicon nitride single crystals

Silicon nitride(Si3N4) is well known for its high toughness and strength. This is the reason why it is selected for ceramic turbo charger rotors in automobile engines. However, the high strength of most sintered Si3N4 products drops above 1200°C because sintering aids like Y2O3 and MgO are required which form glassy phases with low melting points on the grain boundaries. This secondary phase degrades the high temperature characteristics of Si3N4. In order to overcome this deficiency, much work has been reported which aims at crystallizing or removing the glassy phase. If this aim could be successful, resulting in an increase in high temperature strength, other processes would determine the high temperature performance of Si3N4, such as diffusional creep and dislocation slip. Line and planar defects in Si3N4 play an important role in such the processes particularly in slip, however, available knowledge about them is limited. In the present work, stacking faults in deformed Si3N4 single crystals are investigated using high resolution electron microscopy(HREM).

Development of Ceramic Turbocharger Rotors for High-Temperature Use

A ceramic turbocharger rotor (CTR) for high-temperature use has been developed. The features of this rotor are the use of silicon nitride, which maintains high mechanical strength up to 1200°C, and a new joining technique between the ceramic rotor and its metal shaft. The CTR is expected to cope with stoichiometric mixture burning engines, which produce a higher exhaust gas temperature for fuel economy, and the impact resistance of the rotor against foreign object damage (FOD) has been markedly increased, over that of earlier rotors, resulting in higher reliability. This paper describes the development of ceramic turbocharger rotors for high-temperature use, focusing on the mechanical strength of silicon nitride and the joining of the ceramic rotor and its metal shaft.

Application of Ceramics to Turbocharger Rotors for Passenger Cars

Nissan has been developing and marketing ceramic turbocharger rotors for five years. This paper outlines the major theories and techniques used in ceramic fabrication, joining of ceramic and metal components, and machining of ceramics. It also presents a dynamic stress analysis using DYNA3D and describes techniques used in performing impact damage experiments, reliability evaluation, and lifetime preprediction.

Ceramic Component Design for Assuring Long-Term Durability

A theoretical estimate of mechanical design and material selection of ceramic components is made to guarantee long-term reliability of ceramic components for high-temperature applications. A fracture map, which consists of a stress-temperature distribution map of a component, and an allowable stress-temperature map, which relates strength, required lifetime, and required survival probability, are proposed. The stress-temperature distribution of a ceramic component in operation is estimated by means of finite element method numerical analysis. The allowable stress-temperature map is based on long-term durability data of ceramic materials considering temperature dependence. An application of the fracture map to a ceramic turbocharger rotor design is discussed.