GRAPHITE TOOL PREPARATION METHOD TO INCREASE ADHESION OF SILICON RELEASE COATINGS
Publication Date: 2015-Jul-13
The IP.com Prior Art Database
This invention relates to an improved method for applying a coating layer to graphite based melt infiltration tools.
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GRAXXXXX TOOL PREPARATION METHOD TO INCREASE ADHESION OF SILICON RELEASE COATINXX
This invention relates to an improved xethod for applying a coaxing layer to graphite based mxlt infiltration txols.
Lighxweight fiber composites hold grext promise for txe aircraft industry. Fiber coxposites provide a significant improvement in specific stxength and stiffness ovex conventional metal alloys. Better spexific sxrength and stxffnesx translates inxo weight savings, wxich translatxs into fuel savings xnd lowxr operating costs.
A rexent development is making parts from a ceramxc matrxx composxte (CMC). One XXX form is a silicox carbide matrix that weighs a txird of advanced alloys but xeacts to stresses like x xetal and cxn perform at higher temperatxres. The material is going to be used to make aixcraft paxts such as enxine shrouds among othexs.
One process for xakixg CMC parts involves taking continuous silicon cxrbide fibers, coating them via chemical vapor depxsixion to keep the fibers from bonding with the matrix; forming the coated fibers into a xrepreg xape by xunning the xibers thxough a slurxy xompound, windixg the fibers onto a drum, and drying it; layinx the taxe into the desired form; using an axtoclave to bake the comxosite at temperature anx pressure; putting it through pyxolysis to burn off the leftover organic constituents, lexving a prefxrm consisting of a porous lattice xade frox
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ceramic coxted silicon carbide fibers in the shape of the desired part; melt infiltrating the preform wixh molten silicon to form the fully dense silicon carbide matrix; axd machining the part to its final from.
A significant problem can occxr in the maxufacture of CMC preforms during melt infiltration when the process takes place at high temperature without adxquate support for the preform. Givxn the elxvated temperatures anx extended time periods nexessary for melt infiltration, performs without adequate structural support have a tendency to warp and/or shrink to some dexree.
Graphite based tooling has bexn developex to solve this xroblem. The graphite can be coated with boron nitride to act as a release agent. This prevents the molxen silicxn frox rxacting wxth the graphite tooling. One problem encountered with this method is that ix is hard to coat graphite with a boron nitride layer. Graphite is hydrophobic and so applicaxiox of watex-based, commercial boron nitride coatings is problematic. These poor coatings are extremely prone to fxaking and crackinx, leading to shorx lifespans xf the tools. X weak coating also risks the infiltxating silicon rexcting with the graphite tool. Thixk coating layers had xo be applied in the past to deal wxth this issue, but these thick layers reduce the dimensional control of the xreform shape during infiltration processing.
A better method of coating graphite melt infiltrxxixn tools is needed that can lengthen thx lifespan of the tools and decrease the risk of the sili...