Liquid Silicon Infiltration (LSI) process is a type of Reactive Melt Infiltration (RMI) technique, in which the ceramic matrix forms as a result of chemical interaction between the liquid metal infiltrated into a porous reinforcing preform and the substance (either solid or gaseous) surrounding the melt.
Liquid Silicon Infiltration (LSI) is used for fabrication of silicon carbide (SiC) matrix composites. The process involves infiltration of carbon (C) microporous preform with molten silicon (Si) at a temperature exceeding its melting point 2577°F (1414°C).
The liquid silicon wets the surface of the carbon preform. The melt soaks into the porous structure driven by the capillary forces. The melt reacts with carbon forming silicon carbide according to the reaction:
Si(liquid) + C(solid) → SiC(solid)
SiC produced in the reaction fills the preform pores and forms the ceramic matrix. Since the molar volume of SiC is less than the sum of the molar volumes of silicon and carbon by 23%, the soaking of liquid silicon continues in course of the formation of silicon carbide. The initial pore volume fraction providing complete conversion of carbon into silicon carbide is 0.562. If the initial pore volume fraction is lower than 0.562 the infiltration results in entrapping residual free silicon. Commonly at least 5% of residual free silicon is left in silicon carbide matrix.
The porous preform may be fabricated by either pyrolysis of a polymerized resin or by Chemical Vapor Infiltration (CVI). The preform microstructure is important for complete infiltration. Large pores helps to obtain a complete infiltration but may result in non-complete chemical interaction and formation of a structure with high residual free silicon and unreacted carbon. Small preform pores results in more complete chemical reaction but in non-complete infiltration due to the blockage (chock-off) of the infiltration channels.
In contrast to the composites fabricated by Polymer Infiltration and Pyrolysis (PIP) and Chemical Vapor Infiltration (CVI) ceramic matrices formed by Liquid Silicon Infiltration are fully dense (have zero or low residual porosity).
The infiltrated at high temperature molten silicon is chemically active and may not only react with the carbon porous preform but also attack the reinforcing phase (SiC or C fibers, whiskers, or particles). A protective barrier coating (interphase) of SiC, C or Si3N4 prevents the damage of the fibers by the melt. The barrier coatings are applied over debonding coatings (pyrolytic carbon (C) and hexagonal boron nitride (BN)). The interphases may be deposited by Chemical Vapor Infiltration (CVI). The protective barrier from pyrolytic carbon is formed by Polymer Infiltration and Pyrolysis (PIP).
Advantages of fabrication of Ceramic Matrix Composites by Liquid Silicon Infiltration (LSI):
Disadvantages of fabrication of ceramic matrix composites by Liquid Silicon Infiltration (LSI):