Cyclic-Oxidation Resistance of Protective Silicide Layers on Titanium

Abstract

Titanium was powder siliconized and gas nitrided, in order to improve its cyclic-oxidation resistance. Siliconizing was performed in a pure-silicon powder at temperatures in the range of 800-1100 • C for 3-48 h. Gas nitriding was carried out in pure N 2 at 1100 • C/12 h. Cyclic-oxidation experiments with the siliconized and nitrided samples were conducted in air at 850 and 950 • C for up to 560 h. It was found that the siliconized layers grew according to the parabolic law with the activation energy for siliconizing E S being 47.2 kJ mol −1 . Powder siliconizing at 900-1100 • C/3 h produced multi-phase layers, in which Ti 5 Si 3 silicide predominated The siliconizing temperature of 800 • C/3 h appeared to be insufficient, because it led to a non-uniform surface layer with a slight protective effect. The nitrided layers were composed of titanium nitride TiN and α-Ti(N) intestitial solid solution. Measurement of the oxidation kinetics revealed that the titanium siliconized at 900-1100 • C/3 h oxidized much more slowly than pure Ti, Ti-6Al-4V alloy and nitrided titanium. Microstructural investigation revealed the complex substructure of the scales on the siliconized samples which was composed of rutile+silica, rutile and nitrogen-rich sub-layers. The mechanism of high-temperature cyclic oxidation of the siliconized and nitrided titanium is discussed.

Oxidation of nitride layers formed on Ti-6Al-4V alloys by gas nitriding

“…On the other hand, titanium exhibits an allotropic transition between a low temperature hcp α-Ti and a high temperature bcc β-Ti at 882°C. According to Vojt ch et al [8], a 20 µm-thick TiN layer and a 50 µm-thick α-Ti(N) layer were obtained when pure titanium was gas-nitrided at 1100°C for 12 h in atmospheric nitrogen. Their nitriding temperature was well above the β-transus of pure titanium (882°C).…”

Intermetallic protective coatings on titanium

“…1. It was shown in our previous work [5] that the protective effect of the powder siliconized layers is much better as compared to commonly used nitrided layers. A protective effect of the layer prepared by the liquid phase alloying is illustrated in Fig.…”

“…It was shown [5] that the protective effect of the siliconized layers is due to an incorporation of silicon into the scales which are formed during oxidation. Silicon is known to reduce the transport rate of oxygen in rutile and the porosity of scales.…”

High-temperature and electrochemical oxidation of transition metal silicides

“…The papers [67][68][69][70][71] show that siliconizing of titanium greatly improves its resistance to isothermal and cyclic oxidation since a Ti 5 Si 3 layer forms on the surface. The oxidation kinetics of the silicide coating is parabolic.…”