Info

Fig. 10.18. CVI Residual Porosity

mechanical and thermal properties, particularly after matrix cracking occurs in an oxidizing atmosphere at elevated temperatures. The other big disadvantage is that long processing times, often greater than 100 h, and multiple machining cycles result in high costs.

10.10 Directed Metal Oxidation (DMO)22

Directed metal oxidation, or reactive melt infiltration, uses liquid aluminum that is allowed to react with air (oxygen) to form alumina (Al2O3), or with nitrogen to form aluminum nitride (AlN). A fiber preform, which has been precoated with a BN interfacial coating and a SiC protective over-coating, is brought in contact with molten aluminum alloy held in a suitable container, along with air or nitrogen, for the growth of an alumina or aluminum nitride matrix according to the following reactions:

When the liquid aluminum is heated to 1650-2200° F in air, the alumina matrix grows by a complex process involving the dissolution of oxygen and the precipitation of alumina. Wicking of the molten aluminum along interconnected microscopic channels in the reaction product sustains the reaction, promoting outward growth from the original metal surface. To form near net shaped parts, the preform surface is coated on all surfaces, except the surface in contact with the molten metal. The coating is a gas permeable barrier layer that is applied by either spraying or dipping. Since the barrier layer is not wet by the molten aluminum, an impervious barrier is formed when the growth front comes in contact with the barrier. The process flow for fabricating a part by directed metal oxidation is shown in Fig. 10.19.

Important process parameters include alloy composition, growth temperature, oxygen partial pressure, and the presence of fillers, which can be used to control grain size and act as nucleation sites for the reaction. Magnesium, added as a minor alloying element, forms a thin layer of magnesia at the molten interface that prevents the formation of a dense protective scale of alumina that would otherwise stop the in-depth diffusion of oxygen. The matrix slowly builds up (~ 1 in./day) within the fiber preform and fills in the space between the fibers. Since the reaction product is not continuous and contains microscopic channels, the liquid metal wicks to the surface and reacts with the gaseous atmosphere.

For discontinuous composites, preforming can be conducted using conventional ceramic processes such as slip casting, pressing, and injection molding. For continuous fiber composites, fabric lay-up, weaving, braiding, or filament winding can all be used. Since thin layers of both carbon and boron nitride oxidize rapidly during matrix growth of alumina based composites, a duplex

Preform Fabrication

CVD Interfacial/ Over Coatings

Preform Fabrication

Growth Barrier

CVD Interfacial/ Over Coatings

Directed Metal Oxidation

Growth Barrier

Al2O3 Matrix Composite

Growth Direction

Al2O3 Matrix Composite

Molten Al

Directed Metal Oxidation

Fig. 10.19. Directed Metal Oxidation Process coating of BN/SiC is used to protect the fiber and provide the weak interface necessary. The over-coating of SiC (3-4 ^m) protects the thin inner BN layer (0.2-0.5 ^m) from attack by the molten aluminum during processing.22

This process is relatively low cost with near net shape capabilities, and complex shaped parts can be fabricated. Only small dimensional changes occur during processing since the matrix fills the pores within the preform without disturbing the reinforcement. A disadvantage is the presence of residual aluminum phase 5-10%) in the matrix that must be removed if the part is to be used above the melting point of aluminum (1220°F).11 The residual metal can be leached away by an acid treatment that leaves behind open porosity with a residual metal content of ~ 1%.

Matrix cracking also occurs due to the thermal expansion mismatch between the alumina matrix and the protective SiC over-coating. This cracking and the residual porosity from the removed aluminum phase reduces the strength and thermal conductivity. This process is conducted commercially as the proprietary Dimox process, which stands for directed metal oxidation.

10.11 Liquid Silicon Infiltration (LSI)11

Liquid silicon, or one of its lower melting point alloys, is used to infiltrate a fiberous preform to form a silicon carbide matrix. The fibers must contain

Was this article helpful?

0 0

Post a comment