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Role of Rare‐Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si 3 N 4 /BN Fibrous Monolith Ceramics
Author(s) -
Karlsdottir Sigrun N.,
Halloran John W.
Publication year - 2006
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2006.00911.x
Subject(s) - flexural strength , materials science , oxide , lanthanum , lanthanum oxide , ceramic , ytterbium , composite material , mineralogy , chemistry , metallurgy , inorganic chemistry , doping , optoelectronics
The rare‐earth oxides ytterbium oxide (Yb 2 O 3 ) and lanthanum oxide (La 2 O 3 ) were used as additives in fibrous monolithic (FM) Si 3 N 4 /BN composites to study their individual effect on flexural strength and oxidation behavior of the composite. Two compositions were prepared: 20 vol% BN/80 vol% Si 3 N 4 with additives for the Si 3 N 4 and BN being either 8 wt%Yb 2 O 3 or 8 wt% La 2 O 3 . Four‐point flexural testing and static oxidation experiments at 1400°C in dry air for 10 h were performed. The material with Yb 2 O 3 showed a high flexural strength, graceful failure, and comparable strength to reported Si 3 N 4 /BN FMs with 6 wt% Y 2 O 3 and 2 wt% Al 2 O 3 . The material with La 2 O 3 showed lower flexural strength and brittle failure in the majority of the samples; this was believed to be related to the hydration of La 2 O 3 from the rare‐earth apatite phase, La 5 Si 3 O 12 N, resulting in lanthanum hydrate crystals on the side surfaces of the samples and disintegration of the material. The surface of the FMLA sample after oxidation showed severe oxidation. In contrast, the oxidation test of the FMs with Yb 2 O 3 revealed a thin oxide scale containing small Yb 2 Si 2 O 7 on the Si 3 N 4 cells but large Yb 2 Si 2 O 7 on the BN cell boundary. Also, microscopic analysis showed ∼100 μm recession in the BN cell boundary and an ∼4 μm oxide scale on Si 3 N 4 cells.