Strengthening of Ti[sbnd]Nb 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 αN 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 Ti[sbnd]Nb 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 Tβ. The Ti25Nb nitrided at 700 °C meets these criteria best.