EFFECT OF FE NI/TI INTERDIFFUSION ON THE MICROSTRUCTURE OF DIRECT-ON ENAMEL ON STEEL/

Cross-sectional transmission electron microscopy coupled with energy d ispersive X-ray analysis was used to study the titanium-opacified enam el overlaid directly on nickel pickled steel at 800-degrees-C for 2.5 min. The titania-opacified enamel (about 100 mum thick) was enriched i n faceted anatase which grew and/or coalesced to form larger crystals. A minor amount of rutile with a rough surface was also formed. Interd iffusion, mainly cations of Fe and Ti, across the metal-enamel interfa ce caused the formation of an intermediate zone (about 10 mum wide) wh ich is basically a glass ceramic with more or less equiaxed crystals, namely, spinel oxide (Fe3O4 alloyed with Ni ion), pseudobrookite (Fe2T iO5) and garnet (silicate with Al, K, Ti and Fe) in the inner area, bu t with prismatic ilmenite (FeTiO3 with varied Fe to Ti stoichiometry) oriented regularly in the outer area. The ilmenite crystal has (0001) habit plane and elongated along [1100] which is approximately parallel to the direction of interdiffusion. Regardless of the alloying of Ni at the interface, the adherence oxide and the co-existing iron phase r emain as spinel and b.c.c. structure, respectively. The effect of P (a dded in the steel or enamel) on the Fe-Ni alloying at the interface is discussed.