Melt Infiltration

As an alternative to sintering, melt infiltration can be used to produce a dense component from a porous preform. The prerequisite is that the base material from which the preform is made has a higher melting point than the material to be infiltrated. Additionally, the melt must wet the base material. The preform and the material to be infiltrated can be heated until the melting point of the infiltrated material is exceeded. The melt is drawn into the pore channels of the preform by capillary forces and completely fills the pore space. After cooling, a dense component is obtained which, if processed correctly, corresponds exactly to the dimensions of the preform.

Ceramics, metals, and metal-ceramic composites can be produced using melt infiltration. The most well-known process is the production of SiSiC ceramics by infiltrating silicon melt into porous preform made of silicon carbide. This so-called LSI process (Liquid Silicon Infiltration) is also used to infiltrate preforms made of carbon or SiC fibers, resulting in CMC components (Ceramic Matrix Composites). Hard materials can also be densified by melt infiltration. For example, metallic binders such as cobalt or nickel can be infiltrated into porous preforms made of tungsten carbide to produce cemented carbides. An example in the field of oxide ceramics is the infiltration of a glass melt into a porous preform made of aluminum oxide. Since the glass melt crystallizes upon cooling, a largely crystalline material is obtained. An example in the field of metals is the infiltration of bronze into porous preforms made of steel.

Design of Microstructure
Infiltration of a Si melt into a C/C preform
© Fraunhofer Center HTL
Infiltration of a Si melt into a C/C preform

Melt infiltration allows for a design of the microstructure that is often not possible with sintering processes. Reactive components can be introduced into the preform before melt infiltration. During infiltration, new phases form through reactions with the melt. For example, in the LSI process, additional carbon can be introduced into the porous preform. This carbon reacts with the silicon melt during infiltration to form additional silicon carbide, which significantly increases the hardness and stiffness of the SiSiC material. The thermal expansion coefficients of the phases involved can be matched so that compressive stresses are deliberately built up in the weaker components. Gradients in the material composition and properties can also be created through reactive melt infiltration processes. The microstructure-property design methods developed at the HTL are helpful here (see material design).

Porous preforms for melt infiltration can be produced using typical powder metallurgical shaping processes, such as dry pressing, injection molding, and extrusion. Green bodies must be debinded before melt infiltration. Since the porous preforms are already in their final form, shaping processes that enable the production of complex shapes are interesting. This includes 3D printing in particular. Similar to sintering, the quality of the preforms directly affects the quality of the infiltrated components. Specific measuring methods are available at the HTL to assess the quality of the preforms.

Product Quality
FE simulation of Si infiltration
© Fraunhofer Center HTL
FE simulation of Si infiltration into a brake disc
© Fraunhofer Center HTL
ThermoOptical Measurement system TOM_ac

The parameters for the melt infiltration process must be carefully optimized in order to obtain products with sufficient quality. This is achieved at the HTL through a combination of in-situ measurements and finite element (FE) simulations. The kinetics of melt infiltration is measured with special thermo-optical measurement methods (TOM) in the furnace atmosphere relevant for the respective process. The increase in weight of the porous preform during infiltration is recorded, and the temperature changes that occur, especially in reactive melt infiltration processes, are determined simultaneously. The wetting properties of the melt with respect to the base material are also recorded with the TOM equipment. The heat transfer properties in the preform before and after infiltration are measured at different temperatures with laser flash technique. All data are combined in an FE model specifically developed for melt infiltration processes. With FE simulation, the melt infiltration process can be simulated and optimized. Particularly, thermal management is essential for success. Thermal effects can occur not only during heating, but also during infiltration and subsequent cooling, which can affect the quality of the infiltrated components, for example by frozen-in thermal stresses. The complexity increases significantly with the size of the components. With the FE model developed at the HTL, the properties determined on small samples can be transferred to large components.

Service Offering:

  • Investigation of specific questions and identification of critical aspects in melt infiltration, such as:
    • Infiltration kinetics, infiltration progress (gravimetric and optical)
    • Influence of temperature and atmosphere
    • Thermodynamic equilibria (reactions with the kiln furniture, byproducts, etc.)
    • Temperature development during infiltration
    • Melting behavior of the phase to be infiltrated
    • Material redistribution, etc.
  • Support in the design and optimization of infiltration processes (t-T profile, set-up, atmosphere) with regard to:
    • Complete infiltration without infiltration defects         
    • Improved profitability
    • Reduced CO2 footprint through increased energy efficiency
    • More efficient occupancy and furnace set-up
  • Provision of adapted furnace curves and improvement of furnace structures
  • Performance of infiltration runs and product characterization