Oxidation protective silicide coating on Mo-Si-B alloys

Abstract

A MoSi 2 coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo 5 SiB 2 (T 2 ) and Mo solid solution (Mo ss ) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi 2 . The MoSi 2 layer is completely transformed into Mo 5 Si 3 (T 1 ) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T 1 phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T 1 layer reduces thermal stresses generated around the phases. Evolution of T 1 layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi 2 vs T 2 using MoSi 2 /T 2 diffusion couples. Layers of T 1 and MoB ϩ T 1 were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T 2 single crystals show a diffusion structure similar to that of MoSi 2 /T 2 diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi 2 layer is proposed.

Characterization and oxidation behavior of silicide coating on multiphase Mo–Si–B alloy

“…The excellent high temperature strength and oxidation resistance qualify MoSi 2 as protective coating for Mo-based alloys and consistent with studies by Ito et al [21,26] on a Mo-9Si-18B (at.%) alloy and Sakidja et al [22] on a Mo-14.2Si-9.6B at.% alloy. Two significant challenges for future high temperature application still remain: (i) thermal stresses generated by CTE mismatch at the coating-substrate or coating-oxide interface, and the large CTE anisotropy of T1 may induce cracks during process.…”

“…The halide-activated pack cementation process was employed to deposit silicon and/or boron onto the substrate samples and is similar to the process described previously [21]. The components in pack cementation include: the master alloy (i.e., a powder of the element or elements to be deposited on the surface of the parts, such as Si and/or B), a halide activator (e.g.…”

“…One approach for developing an oxidation resistant coating is to deposit a silicon-rich phase, such as the MoSi 2 -or T1-based alloy, onto the surface of the Mo ss -based alloy. The halide-activated pack cementation (HAPC) process has been successfully employed to deposit MoSi 2 onto a two-phase T2/ Mo(ss) eutectic alloy by Ito et al [21], and Mo-3Si-1B (wt.%) alloy by Sakidja et al [22,23]. Although the steady-state oxidation rate of the coated alloy was nearly equal to that of MoSi 2 for up to 50 h at 1300-1500 C, the MoSi 2 layer was partially transformed to T1 phase, and the long-term effect of this microstructural change on the oxidation resistance of the coating was not investigated.…”

Phase continuity in high temperature Mo–Si–B alloys: A FIB-Tomography Study

“…The a-Mo phase is known to be fairly ductile while both T2 and A15 phases are brittle [1]. The T2-phase reveals reasonable oxidation resistance [5]. Thus, at temperatures above 1000 C the diffusion of Si and B to the surface is decisive for the formation of a dense, protective borosilicate glass surface layer which limits the diffusion pathways of oxygen from the surface into the bulk [6].…”

Crack Growth Behavior in a Two-Phase Mo-Si-B Alloy

“…Within the subject of Mo-based alloys, there have been several recent studies on the microstructure and properties of single-and multiphase alloys in the Mo-Si-B system [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. These have included i) establishing ternary phase equilibria [4][5][6][7] and understanding how they change with quaternary alloying [4,8,9], ii) detailed characterization of the resulting microstructures [8,9], iii) understanding and developing strategies for improving hightemperature oxidation resistance [11][12][13][14][15], iv) characterizing the compression and tension response as a function of temperature and strain rate of several ternary and quaternary alloy compositions, some in single crystal forms but most as polycrystals [16][17][18][19][20][21][22][23][24][25][26][27][28]…”