Nonisothermal and Cyclic Oxidation Behavior of Mo-Si-B and Mo-Si-B-Al Alloys

Abstract

A study on nonisothermal and cyclic oxidation behavior of the reaction-hot-pressed 76Mo-14Si-10B, 77Mo-12Si- alloys has been carried out in dry air, and the results have been compared with those of isothermal tests. Nonisothermal studies by thermogravimetric (TG) analysis up to 1300°C have shown a transient mass gain between 700°C and 860°C, followed by a sharp mass loss with increased temperature, with the amount of mass change dependent on the heating rate (5°C/min to 35°C/min). The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of oxide scales formed on the alloys held at 790°C and 820°C for 10 or 20 minutes suggest that the oxidation of a-Mo and Mo 3 Si precedes that of Mo 5 SiB 2 . Thermal cyclic tests involving exposure at 1150°C for 1 hour, followed by either air cooling to room temperature (RT) or furnace cooling to 700°C, 800°C, or 900°C, and the subsequent examination of oxidation products, have confirmed that the formation of B 2 O 3 -SiO 2 scale provides complete and partial protection for the Mo-Si-B and Mo-Si-B-Al alloys, respectively. The results of this study show that oxidation resistance is deteriorated upon Al addition. Residual stress measured by XRD is found to be largely compressive in Mo and in mullite phases of oxide scales. Thermal shock and the mismatch in the coefficients of thermal expansion (CTEs) between the constituent phases of the oxide scale appear to be the main causes of damage.

Lifetime of environmental/thermal barrier coatings deposited on a niobium silicide composite with boron containing M7Si6‐based bond coat

“…The high spallation resistance was probably related to similar values of the coefficient of thermal expansion (CTE) within the E/TBC system. CTE values of (10-11) Â 10 À6 K À1 and (9-11) Â 10 À6 K À1 have been determined for YSZ and GZO topcoats, respectively [46][47][48], and those of the thermally grown oxides chromia and cristobalite are (5.7-10.75) Â 10 À6 K À1 [49,50] and 10.3 Â 10 À6 K À1 , respectively [51,52]. For the M 7 Si 6 -TiNi compound, a silicide similar to the B-containing M 7 Si 6 -based bond coat used in this study, a CTE value of about 12 Â 10 À6 K À1 was measured [53]; and the MASC substrate material was reported to have a CTE of about 10.5 Â 10 À6 K À1 [36].…”

Effect of Si3N4 Addition on Oxidation Resistance of ZrB2-SiC Composites

“…The grain size, microcrack density and SiO 2 area fractions were measured using images obtained from FESEM with the help of an image analyzer. The biaxial residual stress components in the selected phases of the oxide scales were measured by XRD analysis using the sin 2 ψ technique [10][11][12]. The peaks of monoclinic ZrO 2 (111) were considered for this measurement.…”

Mo‐Si‐B Alloys for Ultrahigh‐Temperature Structural Applications

“…Reactive hot‐pressing combines the increased reactivity experienced by a material as it undergoes a phase transformation with the application of pressure to create a dense and strongly bonded material 106. Although only a few researchers107–112 have pursued this processing path, reactive hot pressing has been found to be particularly useful for the manufacture of Mo‐Si‐B‐Al alloys 107–111. For example, Mitra et al107 added Al to their material as a means to scavenge oxygen impurities from their starting materials.…”

“…The oxidation response of Al‐added Mo‐Si‐B alloys has been extensively studied 73, 74, 109–111, 148. Paswan et al investigated both the isothermal109, 110 as well as cyclic and nonisothermal111 response of reaction hot‐pressed Mo‐5Si‐1.4B, Mo‐4.3Si‐1.1B‐1Al and Mo‐4.1Si‐1.2B‐2.6Al alloys. Isothermal experiments conducted between 400 and 800 °C showed a decrease in oxidation resistance with increasing Al content 109.…”