Preparation and characterization of CVD-TiN-coated carbon fibers for applications in metal matrix composites

Abstract

Aluminum matrix composites reinforced with carbon fibers (CF/Al-MMC) are promising materials for lightweight applications. So far the application has been hindered by a lack of wettability of carbon fibers by aluminum alloys and detrimental reactions like aluminum carbide (Al4C3) formation. Protective coatings prepared by chemical vapor deposition (CVD) could be a suitable way to overcome these problems. In this work carbon fiber based 3D-textile preforms were coated with titanium nitride (TiN). Coating thickness, homogeneity, layer structure and composition were determined. Furthermore oxidation resistance and tensile strength of coated fibers were investigated. The wettability by molten aluminum combined with an interface analysis is examined on TiN-coated graphite substrates. Finally the quality of MMC parts prepared by gas pressure infiltration (GPI) method is evaluated. The CVD process was performed with a gas mixture of TiCl4, N2 and H2. A homogeneous coating was achieved at moderate deposition temperatures in the range from 800° to 850°C and by proper adjustment of further process parameters as TiCl4 concentration and total pressure. A very thin TiN layer with a thickness between 30nm and 35nm improves the oxidation resistance and results in a moderate decrease of the tensile strength. TiN improves the wettability and acts as a protective coating. However because of the long infiltration time in the GPI process, interface reactions occur. SEM investigations of TiN coated 3D-textile preforms infiltrated with AlSi9Cu3 alloy (226D) show a completely dense composite with a strong reduction of the Al4C3 formation. TiN offers a good protective effect if the contact time with the melt is not too long.