A MoSi2/SiC oxidation protective coating for carbon/carbon composites

Alloy Design of Intermetallics for Protective Scale Formation and for Use as Precursors for Complex Ceramic Phase Surfaces

https://hal.archives-ouvertes.fr/hal-01486885/document

Thermal cycling and reactivity of a MoSi2/ZrO2 composite designed for self-healing thermal barrier coatings

Oxidation and interdiffusion behavior of Niobium substrate coated MoSi 2 coating prepared by spark plasma sintering

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo3Si–Mo5SiB2 at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO3 and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO2 phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The oxidation behavior of two MoSi2 variants, one Mo-rich and one Si-rich, and TiSi2 was investigated between 1000 and 1400°C in air, oxygen and an 80/20-Ar/O2 mixture. A protective SiO2 scale develops on MoSi2 in all atmospheres in the temperature range investigated. The SiO2 modification changes around 1300°C from tridymite to cristobalite. This change in SiO2 modification seems to cause an enhanced formation of SiO2 and evaporation of MoO3. The SiO2 grows at the MoSi2-scale interface. In air a two-layer scale grows on TiSi2 between about 1000 and 1200°C with an inner inwards growing fine-grain mixture of SiO2 + TiO2 and an outer outward-growing TiO2 partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N -free atmospheres a scale of a SiO2 matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi2 in air at T > 1200°C. In oxygen the TiO2 precipitates grow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO2 compared with SiO2, these TiO2 needles act as “oxygen pipes,” causing an enhanced oxidation of TiSi2 in front of these needles. The SiO2 scale dissolves about 1–2% TiO2. This doping causes a mixed oxygenand Si transport with the consequence that the SiO2 scale on TiSi2 grows partly by oxygen transport inwards and Si transport outwards. The SiO2 modification is cristobalite over the entire temperature range investigated.

V. Kolarik

Papers

Oxidation of the intermetallics Mosi2 and TiSi2 : A comparison