Fabrication and Testing of Ceramic Turbine Wheels

Abstract

Silicon nitride (Si3N4) radial inflow turbine wheels were fabricated by injection molding at Toyota Central R & D Labs. The wheel was 142mm in outer diameter and had 14 blades. The radial wheel was too bulky to manufacture as a homogeneous and defect-free body in one piece. It was divided into two pieces for injection molding and put together into one body by a cold isostatic pressing step after binder removal. Spin testing was executed at room temperature and the resulting photographs taken from three directions provided useful information about the fracture mode of the wheels. The maximum burst speed of the wheels at room temperature was 98,900 rpm, which corresponded to 145% of the design speed. In a hot rig test, one of the wheels survived 75,000rpm for ten minutes at a turbine inlet temperature of 1,050°C.

Dense and near-net-shape fabrication of Si3N4 ceramics

“…In the case of one body forming, the sintered bodies presented internal cracks as well as others were damaged by surface crack generation and broke down during removal binder. Takatori et al [1] and Shimizu et al [25] concluded that one body forming of the large radial wheel was not practical using the current injection molding method. Therefore, the processes of fabrication of radial wheel were attempted dividing the hub of the wheel (the thickest section) in two axially symmetrical parts to reduce the thickness about in half [87].…”

“…Considering that one of the must difficult components to form by conventional ceramic forming processing has been the radial turbine wheel, the development of the forming technique for a large, complex-shaped component as well as the development of a high strength and refractory material to achieve a turbine inlet temperature (TIT) of 1350 • C is required [1,25,39,[82][83][84]86]. Takatori et al [1] used for fabrication of radial turbine wheel Si 3 N 4 with additions of 5 wt% Y 2 O 3 and 5 wt% MgAl 2 O 4 (spinel) as additives. Using this composition, several small components for reciprocating engines were fabricated by injection molding.…”

“…As the liquid binder is depleted in long cycles the fine Si 3 N 4 particles tend to rearrange into denser packing configurations under capillary forces, leading to shrinkage. The part exterior of the ceramic body is at a more advance stage of binder removal where no rearrangement of particles can occur and the internal shrinkage produces interior crack formation observable only by radiography [1,28,82,83].…”

“…Specifically, several key variables on the properties of injection molded parts have been studied such as particle size distribution of the starting powder, binder composition, powder solid loading, and compounding shear level [1,39,[82][83][84].…”

“…Structural ceramics based Si 3 N 4 have been explored since the late 1960s [1] emphasizing Si 3 N 4 based materials primarily for use in high temperature, structural applications such as heat engines. Taking into account their unique combination of properties, silicon nitride and related materials have probably become the most thoroughly characterized non-oxide ceramics with wide applications including heat exchangers, turbine and automotive engine components, valves and cam roller followers for gasoline and diesel engines and radomes on missiles as well as insulators, electronic substrates, high Tc superconductors, tool bits, wear surfaces, to name a few [2,[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”