Modificações estruturais e mecânicas de superfície em ligas titânio-nióbio por imersão em plasma

Abstract

In the last decades, Ti-Nb alloys have received increasing attention due to interesting properties such as low density, good biocompatibility, shape memory effect, low elastic modulus, and absence of toxic or allergenic effects to the human body. However, these materials present low wear resistance. In this context, the DC plasma nitriding was employed in previously solubilized Ti-Nb alloys containing 10, 25 and 35 %wt niobium. The treatments were carried out at 600, 700, 800 e 900 oC under 60% N2 and 40% H2 atmosphere for two hours. Additional samples, masked with alumina sheets, were annealed (thermally treated - TT) in the same plasma chamber in order to probe the temperature effects on the structure and mechanical properties of the bulks. The employed characterization methods were X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and nanoindentation. The alloys disclosed a strong structural and microstructural dependence with the nitriding temperatures and niobium fractions, which intricately ruled the mechanical properties of both bulks and layer/substrate systems. The overall tendency was the decrease of hardness and increase of elastic modulus due to the α/β ratios and the presence of ω and martensitic phases. Annealings at 700 °C, slightly above the transus for the Ti25Nb and Ti35Nb (T~680 and 630 oC, respectively) yielded lower elastic moduli than the onlysolubilized conditions. The nitriding produced nitride layers (TiN, Ti2N) with high hardness and elastic modulus, with thicknesses up to 1 m. The Ti10Nb alloy presented the best results, whose surface hardness values were two- to fourfold higher than the TT substrate due to a 4 m thick N layer underneath the compound one. Also observed were the reduction of hardness at a region right below the nitrided layer in the Ti25Nb nit92 sample in comparison to the TT substrate, due to a synergistic nitrogen diffusion and heating interaction. The parameter H/Er (hardness / reduced modulus), which is an indicative of the material´s wear resistance, increased in all the nitrided surfaces as compared to the only-TT ones. Again, the 10%wt niobium samples showed the highest values. Regarding elastic modulus, the nitrided Ti25Nb alloy reached the overall lowest values, but with thin nitride layers and low H/Er. It is concluded that the formation of effective protection layers by plasma-based methods requires a careful choice of parameters in order to ensure the necessary Ti-Nb bulk properties for biomedical applications.