Atomic layer deposition of titanium phosphate onto reinforcing fibers using titanium tetrachloride, water, and tris(trimethylsilyl) phosphate as precursors

A thermal atomic layer deposition process with precursors tris(trimethylsilyl) phosphate (TTMSP), titanium tetrachloride (TiCl4), and water was used with various pulse sequences in order to deposit titanium phosphate onto bundles of carbon fibers (diameter of one filament = 7 μm, 6000 filaments per bundle) and flat silicon substrates. Pulse sequence 1, TTMSP/N2/TiCl4/N2, which comprises no water, yields no significant deposition. Pulse sequence 2, TTMSP/N2/H2O/N2/TiCl4/N2, which comprises a water pulse, yields a mixed phosphate/oxide coating and shows a self-limiting character at 200 °C with a growth per cycle of 0.22 nm cycle−1. Wet chemical analysis of the coating revealed a ratio of Ti:P between 3:1 and 2:1 in reasonable agreement with the composition Ti2.4P1O7 obtained from X-ray photoelectron spectroscopy. Thus, the deposited material can approximately be described as a mixture of Ti¾PO4 and TiO2 in a molar ratio of 1:1.5. The coating shifts the temperature of the onset of oxidation—3% weight loss in thermogravimetry—of the carbon fibers from 630 °C (uncoated C-fiber) to 750 °C (with the titanium phosphate coating).