Oxidation behavior of tungsten and germanium-alloyed molybdenum disilicide coatings

Abstract

A multicomponentcoating has been developedto protect Nb-base alloys from high-temperatureoxidation. A solid solution of molybdenumand tungsten disilicide, (Mo,W)Si 2 provided the best results. This alloy compositionwas shown to support a slow-growingprotectivesilica scale in service. Germanium additionsmade during the coating process helped to improvethe cyclic oxidation resistanceby increasingthe thermal expansioncoefficientof the silica scale. Germanium also helped to avoid "pesting"(acceleratedlow temperatureoxidation)by providing better sealant coverage at low temperatures. The results of cyclic oxidationtests performedon coated Nb coupons at 900°C, 1370°C, and 1540oC in air are presented. The coated Nb successfullypassed 200 one-hour cyclic oxidationtests at 1370oC and 60-h at 1540°C. Neither generalized acceleratedlow temperatureoxidation or "pest" oxidation occurred when the coatings were exposed to isothermalconditions of up to 200-h in air in the temperaturerange 500-700oC.

Oxidation Resistant Coatings for Ultrahigh Temperature Refractory Mo‐Base Alloys

“…During the silicon-pack cementation process coating process, a steep chemical potential gradient of Si across the vapor/substrate interface produces a flux of Si into the substrate that uniformly enriches the local Si concentration and results in the synthesis of a MoSi 2 coating layer. [21][22][23] A BSE image of an as-Si packed sample is shown in Figure 3 (a). Underneath the MoSi 2 layer, there is a Mo-rich silicide layer of Mo 5 Si 3 (T 1 ) phase and dispersoids of the MoB phase.…”

“…Briefly, the pack cementation process involves the elevated temperature deposition of Si or Si þ B, carried by a volatile metal-halide vapor species to the substrate embedded in a mixed powder pack containing powder of the deposition element, a halide salt activator, and an inert filler [24][25][26] . The detailed procedure of pack cementation process is well-defined elsewhere [21][22][23] . The powder mixture composed of 70 wt% Al 2 O 3 , 25 wt% total of Si and B powders, and 5 wt% NaF were loaded in an alumina crucible together with the clean alloy pieces followed by sealing with Al 2 O 3 slurry bond.…”

Low-Temperature Cyclic Oxidation Behavior of MoSi<SUB>2</SUB>/SiC Nanocomposite Coating Formed on Mo Substrate

“…After some incubation period, spalling of the scale was observed with the subsequent growth of the dual MoO 3 and amorphous SiO 2 scale in the spalled regions. Mueller et al 14) and Cockeram et al 15,16) reported that germanium additions improved the pest oxidation resistance of MoSi 2 coating, but a fairly rapid rate of low-temperature oxidation was observed and that the MoSi 2 coatings formed by a NaF-activated pack siliconizing process or by the application of superficial alkali-salt layers did not pest or undergo the accelerated oxidation for 2000 $ 2500 h in air at 500 C since the sodium-rich by-product layer produced by the NaF-activated pack passivated the MoSi 2 coating by forming a fast growing Na-silicate scale. Fitzer 17) reported that the pest reaction was also avoided by applying a solid MoO 3 layer to MoSi 2 at 300 to 700 C in an oxidizing atmosphere.…”

Effect of Copper Addition on the Niobium Disilicide Coatings by Pack Cementation

“…Owing to low cost, simplicity and practicability, pack cementation process has been extensively studied and widely used to form different diffusion coatings. [5][6][7][8][9][13][14][15][16][17][18] In the present paper NbSi 2 coatings were formed on pure niobium by halide-activated pack cementation process. Emphasis was placed on the effect of copper addition in pack mixture on the growth rate, morphology, microstructure and phase composition of the coating.…”

“…[1][2][3] To restrain the catastrophic oxidation of niobium and niobium alloys at high temperatures, silicide and aluminide coatings have been developed by different techniques. [4][5][6][7][8][9][10][11][12] Pack cementation process is an in situ chemical vapor deposition technique carried out at high temperatures by embedding the substrate in a pack mixture consisting of fine powders of the depositing material (e.g. Al, Si, etc.…”