Influence of hexamethyldisilazane vapor on the structure and mechanical properties of TiC-based coatings

Abstract

Silicon-modified titanium carbide (TiC) coatings exhibit excellent properties and are attractive for a variety of protective coating applications. These coatings are often produced by sputtering silicon from elemental or composite targets, which limits both large area compositional uniformity and the ability to grade silicon composition to improve coating adhesion. Here we demonstrate the use of an organosilicon vapor, hexamethyldisilazane (HMDS), as a silicon source during the plasma enhanced magnetron sputtering (PEMS) of TiC-based coatings. Three coatings, with compositionally graded interlayers, were deposited under various HMDS flow rates onto silicon and stainless steel substrates. The growth structures of the coatings were characterized by scanning electron microscopy and atomic force microscopy. The introduction of HMDS under low flow conditions resulted in an increase in root-mean-square surface roughness from 13 to 41 nm, however, the coating was characterized by a similar columnar growth structure as produced without HMDS flow. At higher HMDS flow, columnar growth was disrupted due to the segregation of sufficient amounts of silicon and excess carbon. Coating composition, as determined by energy dispersive X-ray spectroscopy, indicates that the densification of the growth structure occurred at titanium, silicon, and carbon contents of 29, 9, and 57 at.%, respectively. X-ray diffraction analysis demonstrated that all coatings contained rock-salt structure phases (TiC, TiCN) with 〈111〉 out-of-plane texture. Raman spectroscopy and X-ray photoelectron spectroscopy confirmed the presence of amorphous phases, consisting primarily of silicon and excess carbon (a-C:H/a-SiCx:H), in both coatings produced with HMDS. The simultaneous disruption of columnar growth and formation of a composite microstructure consisting of crystalline and amorphous phases at high HMDS flow resulted in improved mechanical properties when compared to the TiC control coating, evidenced by an increase in the H/E ratio from 0.063 to 0.10. The processing methods demonstrated here may potentially be applied to industrial settings for improved compositional control during the introduction of silicon to TiC-based coatings.