Titanium Nitride/Carbon Coatings on Graphite Fibers

Abstract

Titanium nitride (TiN) coatings were applied to graphite‐fiber tows by dip coating desized (900°C in nitrogen) fibers in hexane solutions containing 0.5, 1, or 2 equivalent wt% TiN as the precursor, [Ti(NMe2)x(μ–NCHMe2)4−2x]n. The precursor was characterized by thermogravimetric analysis (TGA), chemical analysis, and nuclear magnetic resonance. Bulk samples of precursor were heated to selected temperatures in argon. Based on microstructural and chemical analyses, two pyrolysis temperatures— 900°C and 1200°C— were selected for more‐detailed studies predicated on fiber oxidation‐resistance behavior. At 900°C, the bulk material exhibited a powder X‐ray diffractometry (XRD) pattern consistent with microcrystalline TiN, although some free carbon remained. XRD analysis of the samples pyrolyzed to 1200°C indicated a higher degree of crystallinity and some carbon incorporation in the TiN lattice; e.g., a TiCN solid solution forms, as supported by chemical and TGA analyses. Fibers coated with precursor were heated at a rate of 10°C/min in argon to selected temperatures, followed by a hold for 1 h, and the resulting coatings were characterized by scanning electron microscopy, X‐ray photoemission spectroscopy, and oxidation resistance at 700°C for 2 h in air. Coated fibers pyrolyzed to 900° and 1200°C were studied in detail. Fibers heat treated at 900°C exhibited better oxidation resistance than fibers pretreated at 1200°C, as determined by mass loss. Uncoated fiber tows fully oxidized after 80 min. The oxidation‐resistance studies on fiber tows pretreated at 900°C and coated with 0.5, 1, and 2 equivalent wt% TiN indicated that the sample with a coating of 1 wt%, with thicknesses of 0.1–0.2 μm, was the most stable. The coatings of 0.5 equivalent wt% TiN were irregular, whereas the coatings of 2 equivalent wt% TiN exhibited process‐related cracking, leading to poor oxidation resistance.