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Ceramic engine valves

The interest for ceramic materials for high temperature automotive engine applications is due to their excellent thermo-mechanical characteristics in comparison with traditional metallic materials.

The objective of the work was to develop a low-cost forming and sintering process to produce near-net shape ceramic valves, thus requiring very low finishing operations and significantly minimizing material waste.

Ceracom activity was devoted to the main issues as follows:
1. Optimize the process for forming near net shaped valves;
2. Characterize microstructural and mechanical properties of these valves in cooperation with CRF; and
3. Produce 200 valve mock-ups/rods for grinding and evaluation at TRW.

Process Optimization and Characterization consisted of:
1. Selection of the best starting materials(taking into account the requirements of a cost effective and high volume production);
2. Development of an innovative pressure-injection molding process to produce near-net shape parts via a thermoseting feedstock;
3. Optimization of a proper pressure-less sintering route to obtain cost-competitive, real scale components with adequate final density and mechanical properties.

Material Selection:
Silicon Nitride (Si3N4) offers the best mechanical and thermal properties for engine applications. Si3N4 has in fact excellent thermal shock resistance and relatively high fracture toughness; in the last decade a lot of development work has been conducted in order to improve the strength and the resilience and different industrial applications have been successful, especially for gas turbines applications.

Other materials, such as silicon carbide (SiC)exhibit excellent strength and creep resistance, but its fracture toughness is still so low that it cannot be used in dynamically loaded parts. Another promising ceramic, partially stabilized zirconia, has high strength at room temperature as well as excellent fracture toughness, but its density is too high and it losses its strength at moderate temperatures, so that its application for rotating and moving engine parts is not feasible.

Finally, SiAlON, a promising ceramic material with good toughness and strength, was investigated, but its availability and price are still fairly lacking in comparison with more conventional silicon nitride.

Between available technical ceramic materials, silicon nitride was thus chosen to replace conventional steels and Ni-based alloys for the exhaust valves application.

Process Selection:
The most promising forming process for ceramic parts is injection molding; in this process the ceramic powder is mixed with a thermoplastic polymer to obtain a feedstock mixture. Such feedstock is injected into a mold through a typical plastic injection molding equipment. After the production of a "green" part, the parts are debinded and then sintered. Conventionally, these are very time-consuming processes.

In order to avoid such long and complicated debinding and firing cycles and at the same time guarantee a high green density, a new feedstock was developed based on a thermosetting binder, and combined with a special injection molding process. Such type of binder, even if in low content can provide green bodies with relatively high mechanical strength and high green density. A rapid debinding process in air was also developed, resulting in a simpler and considerably shorter process than that using conventional injection molding binders. This is an important, exclusive advantage for mass production of cost-effective and near-net-shape ceramic parts.

Repeated iterations of this process resulted in numerous improvements to avoid discontinuities in the "greenbody" valves, leading to cracking. Feedstock homogeneity, tape casting improvements, feedstock drying and mold design all contributed to the goal of crack-free valves.

The sintering process was carried out in pressure-less conditions, under inert (N2) atmosphere, without using expensive HIP equipment. Different temperature programs have been tested, in order to improve the final density and the mechanical properties of silicon nitride as function of its sintering cycle.

Prototype manufacturing and testing:
Ceracom produced 406 valves, 234 of which were sintered at CRF and machined at TRW.

Silicon Nitride valve samples were produced using low cost forming and sintering process. These samples have been characterized in order to asses their reliability; in terms of final density and microstructure the pressure-less silicon nitride valves seem to be promising.

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