When carbon steel is directly exposed in 99%H-2-1%CH4 gas mixture for plasma enhanced CVD deposition of diamond coatings, an intermediate graphite layer is preferentially formed on the substrate surface, and the substrate is subjected to a severe internal carburization attack. The diamond coatings formed easily delaminate from the steel substrate. Hot dip aluminizing treatment of the carbon steel has markedly promoted diamond nucleation, growth and interfacial adhesion. The formation of graphite intermediate phase on the substrate surface is effectively inhibited and the substrate carburization is also suppressed. The possible mechanism of this transition is discussed based on a series of analytical techniques. (C) 2014 Elsevier B.V. All rights reserved.