Download files
Abstract
Electrophoretic deposition (EPD) of Ti on low carbon steel followed by heat treatment, which was done at 600-1400 degree celsius for 1 h or 9 h under Ar (sintering) and 5% H2-N2 (nitridation), were used for surface hardening. The coarse Ti particles (≤50 μm) were suspended in ethanol using the charging agent poly(diallyldimethylammonium chloride) (PDADMAC).
EPD: The present work demonstrated the importance of assessing the electrophoretic mobilities of both the suspensions and solutions since the latter plays a critical role in interpretation. Algebraic uncoupling of these data plus determination of the deposit yield as a function of charging agent addition allowed discrimination between the three mechanistic stages of the electrokinetics of the process: (1) surface saturation; (2) compression of the diffuse layer, growth of a polymer-rich layer, and/or competition between the mobility of Ti and PDADMAC; and (3) little or no decrease in electrophoretic mobility of Ti, establishment of a polymer-rich layer, and/or dominance of the mobility of the PDADMAC over that of Ti.
Heat treatment: Although oxygen contamination of the furnace atmosphere was a concern, development of the relevant Fe-Ti-O isothermal sections allowed: (a) unambiguous interpretation of the results, and (b) consideration of the importance of liquid formation.
The Ti-based coatings and steel substrates were mutually exclusive in terms of their phase distributions and microstructures at all temperatures. A tentative Onion Skin model was used to explain the cross-sectional phase distribution in the coating of the samples. A thermal expansion mismatch model was used to describe coating delamination and vertical crack formation, propagation, and branching.
The optimal coating was obtained at 1400 degree celsius for 9 h in Ar. The coating consisted of ulvöspinel (Fe2TiO4), wüstite (FeO), and ferrite (αFe). The coating microstructure and its phase assemblage were dominated by the liquid formation reactions: (a) FeO (liq) + TiO (sol) → αFe (sol) + TiO2 (sol), (b) 2FeO (liq) + TiO2 (sol) → Fe2TiO4 (sol), and (c) FeO (liq) → FeO (sol). The reaction path was explained by the Fe-Ti-O isothermal section. Improved contact between the coating and steel resulted from wetting of the steel substrate by the liquid FeO.