Silicon carbide fiber oxidation behavior in the presence of boron nitride

Abstract

The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O2 at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO2 due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.

Investigations into the thermal stability of sol‐gel‐derived glasses as models for thermally grown oxides

“…The peaks present in the spectra shown in Figure 9 correspond with FTIR spectra reported by Bruckner and Navarro collected from binary borosilicate glasses synthesized from the melt containing 39 mol% B 2 O 3 , balance SiO 2 , 40 and with FTIR spectra reported by Tenney and Wong collected from 0.6 µm thick binary borosilicate glass films containing 15‐36 mol% B 2 O 3 , balance SiO 2 , vapor deposited onto Si wafers 41 . Structural investigations into thermally grown borosilicate oxides are sparse 10,42 . Quemard et al reported FTIR peaks at 3220, 1472, 1196, 784, and 548 cm −1 collected from an oxide drop thermally grown on the surface of a composite reinforced with Hi‐Nicalon SiC fibers in a matrix containing layers consisting of SiC, B 4 C, and SiC nanocrystals interspersed in B 4 C; however, the spectrum was not presented 10 .…”

“…Boria volatility from sol‐gel‐derived borosilicate glass exposure to the high‐temperature conditions was determined by post‐exposure characterization of the glass surface chemistry by XPS. Structural properties of the sol‐gel‐derived silica and borosilicate glasses are compared with those of pure silica and binary borosilicate glasses synthesized from the melt, 40 vapor deposition, 41 or thermally grown 10,42 . Results from the characterization of the sol‐gel‐derived glasses for weight change, composition, structure, and morphology of this present study will be shown to directly relate to understanding the thermal behavior of the sol‐gel‐derived glass coatings applied onto SiC substrates investigated in a companion study 39 …”

“…41 Structural investigations into thermally grown borosilicate oxides are sparse. 10,42 F I G U R E 6 Macrographs of the freestanding glass (FSG) for silica glass A, borosilicate glass B synthesized with the B 2 O 3 precursor, and borosilicate glass C synthesized with the trimethoxyboroxine precursor following isothermal thermogravimetric analysis (TGA) exposures in dry O 2 for 100 h at 800°C (top row) and 1200°C (bottom row) [Color figure can be viewed at wileyonlinelibrary.com] T A B L E 4 Bulk retained carbon (C) concentration measured by Combustion IR spectrometry in silica glass A and borosilicate glass D following isothermal thermogravimetric analysis exposures at 1200°C for 100 h in dry O 2 . Starting retained carbon concentrations are provided in Table 2 Glass type Quemard et al reported FTIR peaks at 3220, 1472, 1196, 784, and 548 cm −1 collected from an oxide drop thermally grown on the surface of a composite reinforced with Hi-Nicalon SiC fibers in a matrix containing layers consisting of SiC, B 4 C, and SiC nanocrystals interspersed in B 4 C; however, the spectrum was not presented.…”

Na2SO4 deposit‐induced hot corrosion of BN‐coated α‐SiC

“…Molecular oxygen transport rates are faster through silicates than through pure SiO 2 . [8][9][10][11] The formation of low-melting silicates on SiC CMC components during service is a concern, as B 2 O 3 is present from the oxidation of SiC/BN/SiC CMC oxidation and Na 2 O species are present in marine environments.…”

“…The oxidation of SiC with B-containing materials increases SiC oxidation rates by several orders of magnitude due to the formation of a borosilicate glass. 11,12 Additionally, B-containing species in monolithic BN and borosilicate layers are known to produce various volatile gaseous boron hydroxide species of the form H x B y O z (g). 11,[13][14][15][16][17] This causes thermally grown borosilicate layers to become more SiO 2 -rich over time as B 2 O 3 levels decrease due to volatilization.…”