ABSTRACT

Nowadays, it is usual to improve the adhesion between ceramic coatings and steel substrates, especially TiN coatings, by introducing a titanium interlayer that promotes a decrease of the compressive stresses between the ceramic film and the metallic substrate [1,2]. However, Helmersson et al. [1], concluded that the increase of the substrate temperature above 400°C during presputtering and sputtering could reduce significantly the adhesion due to the diffusion of carbon of the substrate to the TiN film with the formation of TiC at the interface. This carbide has a more covalent character than TiN, which is responsible for a ductility decrease. Nevertheless, these authors did not take into consideration the reactivity of titanium with substrate surface oxides, only highlighting the role of FeO in the increase of structural matching between the substrate and the TiN coating. In the work of Tang et al. [3], the influence of Ti and Cr interlayers with different thicknesses on friction and wear behaviours of TiC coatings was studied. Among the coatings with an interlayer, those with 50 and 500 nm Cr or 50 nm Ti exhibited less delamination than those with a 500 nm Ti interlayer and their hardness and elastic moduli were significantly higher, thus enhancing their wear and friction behaviours. However, the high chemical reactivity of titanium, that presupposes a good adhesion, does not always give rise to an adhesion improvement [4, 5]. The titanium reactive interlayer only improves adhesion, if stable and non-fragile reaction products are formed at the interface. The titanium reacts with the oxygen from the substrate surface forming a family of oxides ranging from TiO to T1O2, depending on its activity [6 ]. Among these oxides, the TiO is the most stable [7] and promotes a strong and ductile bonding with the steel substrate due to its more metallic character. To obtain the desired oxide, titanium with an element of dilution could replace the titanium interlayer. The composition of this interlayer should be carefully chosen because the titanium activity will dictate the reaction products formed. The aluminium is an interesting element for dilution because it forms solid solutions or stable intermetallic compounds with Ti [8 ], leading to a decrease of titanium activity and/or formation of Ti3AlC compounds with the carbon from the substrate (Ti3AlC compounds have a more metallic character than TiC). Both these factors contribute to adhesion improvement. Lii et al. [9] reported that TiAl was very promising for use as an interlayer to enhance the adhesion of TiAIN coatings to high-speed steel substrates. Single TiAl coatings adhere very well to steel substrates and the adhesion between TiAl and TiAIN was guaranteed by the nitrogen diffusion. However, the influence of the TiAl interlayer was not compared to a pure Ti one which according to the above explanation might also lead to the same results.