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Process Technique and Quality Assurance

C/C-SiC brake disc (Ø 720 mm) for high-speed trains with in situ joined load transmission elements.

The LSI process, developed at DLR, can be subdivided into three main process steps:

1. CFRP preform manufacturing
   Manufacture of a near neat shape CFRP (carbon fibre reinforced plastic) preform based on commercially available, high temperature precursors (e.g. phenolic resins) and carbon fibres (2 D fabrics, short fibres) via common technologies, like  autoclave technique, resin transfer moulding (RTM) or warm pressing. 
2. Pyrolysis
   Thermal treatment of the CFRP preform (Tmax. > 900 °C, inert gas) and transformation to a porous C/C (carbon fibre reinforced carbon) preform, characterized by a translaminar micro-crack system. 
3. Siliconization
   Infiltration of the porous C/C preform with melted silicon (T max. > 1420 °C, vacuum) and build up of the SiC matrix by chemical reaction of Si with C.

Compared to other CMC manufacture procedures, like CVI and LPI/PIP, the LSI process offers significant economic and technological advantages: 

• Short process times
• No additional densification cycles necessary 
• Low cost raw materials 
• No fiber coating necessary 
• No pressure needed for high temperature processes 
• Almost no design limits (wall thickness, component size and - geometry). 
• Near neat shape manufacture of complex parts due to low, reproducible shrinking rates and in situ joining technologies
• Tailoring of material properties possible in a wide range 

The LSI process, well known for the manufacture of C/C-SiC materials and structures, has been transferred to the manufacture of novel, biomorphic SiSiC materials and structural components. Either commercially available, wooden preforms, like medium density fiber boards (MDF), or wood based composites, developed and manufactured at DLR, are used as raw materials. After the pyrolysis of the wooden preforms a highly porous C preform is obtained, which is infiltrated with melted SI in the last process step. Thereby the carbon is converted almost completely to SiC and the remaining porosity is filled with SI leading to a dense SiSiC material. Due to the fact that the shrinkage occurs exclusively during pyrolysis and there is no change in geometry during siliconization, very filigree, complex shaped and even large sized structures can be manufactured in near neat shape technology.
Additionally, very complex structures can be realized with a special in situ joining technology.

At DLR, the basis for the reliable and reproducible manufacture of C/C-SiC materials and components is an extensive quality assurance system, consisting of the income inspection of all raw materials, a detailed job traveller for each individual part, the investigation of materials and components as well as the documentation of all relevant process parameters after each manufacturing step. For the investigation and qualification of components, defined test procedures for non-destructive testing (air and water-coupled ultrasonic, computer tomography, lock in thermography, photogrammetry) are available. 

Current development focuses on: 

• Development of new preform technologies for the reproducible manufacture of thin-walled, highly loaded structural components in near neat shape technology. 
• Further development of the LSI process for the near neat shape manufacture of structural components based on biomorphic SiSiC materials. 
• Integration of computer tomography in the quality assurance system.