Strengthening of TiNb alloys surfaces by plasma nitriding

Abstract

TiNb-based alloys are needed as mechanically biocompatible materials, despite having low wear resistance. This topic, not yet well-researched, is being addressed here. Titanium‑niobium alloys with 10, 25 and 35 Nb wt% were DC plasma nitrided at temperatures 600–900 °C. Originally, they featured distinct balances of α, β and α” phases. To probe the heating effects on the bulks, the alloys were thermally treated (TT) at the same nitriding temperatures. They disclose a strong structural and microstructural dependence with the nitriding temperatures and niobium fractions, which intricately rule the mechanical properties of both bulks and layer/substrate systems. The nitriding produces ∼1 μm thick nitride layers (TiN, Ti₂N) at top surfaces, with high hardness (H) and elastic modulus (E). Beneath lie β-rich regions, spread over 20–50 μm thick layers, with embedded α_N precipitates whose size vary inversely with the Nb wt% and heating conditions. These layers formation involve a competing effect between N and Nb, which are α and β stabilizers, respectively. The nitrided Ti10Nb present the highest variations compared to TT substrates: H ∼ 16 GPa (4.7×) and E ∼ 230 GPa (91 %). The H/E ratio, an indicative of tribological resistance, increases in all the nitrided surfaces, while it diminishes in the TT bulks. The overall conclusion is that parameters must be carefully chosen for each TiNb composition. However, under the requirements of low-modulus bulks and wear protective cases for use in bone prostheses, plasma nitridings must be carried out at temperatures near T_β. The Ti25Nb nitrided at 700 °C meets these criteria best.