Samples of 316L austenitic steel were submitted to a thermochemical treatment which implies surface diffusion of Al and Ti. The technique of pack cementation with NH4Cl as activator was employed. The powder mixture was made of aluminium, titanium, aluminium oxide and ammonium chloride. The same ratio of Al : Ti = 1 : 5 was used in all experiments. The variables were temperature and time. As a function of these parameters, diffusion layers of different thicknesses were obtained. The samples were analyzed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX), X-ray diffraction and Vickers microhardness trials. All layers were formed by diffusion with reaction and present two zones with different structures and compositions and therefore different properties. The Ti3NiAl2N compound was identified by X-ray diffraction. The presence of this compound in the diffusion coatings increases the superficial hardness of the samples.

Pack cementation procedure implies the use of a powder mixture containing the diffusive elements, which in our case are either Ti or Ti+Al, Al2O3 and NHCl as activator. In the case of titanizing the powder mixture contained 77% in weight Ti, while for alumino-titanizing Al/Ti = 1/5 ratio was employed. NH4Cl content was 3% in weight in all cases. Aluminium additions to the powder mixtures led to a decrease of the process temperature. Activation energy for the aluminizing of austenitic 316L steel is 73.87 KJ/mol, much smaller than for the titanizing, 257.86 KJ/mol. Activation energy for alumino-titanizing, in the same conditions, is 146.01 KJ/mol. All diffusion coatings, in the Ti – 316L and Ti+Al – 316L couples are formed of two layers having different structures and compositions. All couples were investigated by optical microscopy, electron microscopy (SEM and EDX), X-ray diffraction and microhardness trials.

Austenitic stainless steel 316L is widely used in implantology due to its biocompatibility, a lower price than titanium and because can be easily mechanically machined. The drawback is due to the fact that toxic nickel and chromium ions are released into human body fluids. Our proposal is to coat 316L austenitic stainless steel with biovitroceramic layers made of oxide system SiO2, B2O3, Na2O, CaO, TiO2, P2O5, K2O, Li2O and MgO by means of an enamelling procedure in order to hinder the release of Ni and Cr ions from the metallic implant surface toward the tissue around the implant. In order to achieve a firm adherence of biovitroceramic layer onto the metal, with an optimal composition for biocompatibility and bioactivity, we have modified the steel surface by a titanizing thermochemical treatment. The adherence of the biovitroceramic layer to the 316L stainless steel with modified surface is very good. The biovitroceramic coating - metallic substrate couple was studied by optical microscopy, electron microscopy (SEM and EDAX), X-ray diffraction analysis and microhardness trials.

In order to manufacture ceramic coatings on metallic substrates with medical applicability, a compromise has to be made between adherence, mechanical resistance and bioactivity. Biovitroceramic layers to satisfy all these requirements are extremely difficulty to develop. The goal of this contribution is to employ a simple technique like enamelling for making biocompatible and bioactive coatings with improved mechanical properties and very good adherence onto metallic substrates made of titanium samples. One important factor for a good adherence is the value of the thermal expansion coefficient of the ceramic coating to be closed to that of the metallic substrate. Once this achieved by establishing an adequate ceramic composition, there remain other factors that contribute to a good adherence such as metallic surface pretreatment. The surface was processed by Al2O3 powder (125 'm) blasting, degreased with acetone in an ultrasonic device and immersion in phosphoric acid. The (Na2O-K2O-Li2O-CaO-MgO-SiO2- B2O3-TiO2-P2O5) biovitroceramic coating – titanium interface was examined by means of optical microscopy and electronic microscopy. The coating adherence to the metallic substrate was evaluated qualitatively by Vickers microhardness tests at the interface.