Identification of oxidation mechanisms in silicon nitride ceramics by transmission electron microscopy studies of oxide scales

Abstract

Additive-free HIPSN and Y 2 O 3 + Al 2 O 3 -doped HPSN are oxidized in air in the temperature range from 1300 to 1500 °C. TEM, SEM, EDS, and XRD are used to analyze the composition and microstructure of the oxide scales in order to determine the oxidation mechanisms. HIPSN exhibits excellent resistance to oxidation in air at temperatures up to 1480 °C due to the formation of a protective silica (cristobalite) scale. No formation of Si2N2O and oxygen-enriched /3'-Si 3 N 4 under the silica layer is observed for materials densified without additives. Oxidation rates of additive-containing HPSN are more important due to the formation of a viscous aluminosilicate phase, which easily penetrates along the grain boundaries in the material. Silicon nitride grains in contact with the viscous phase first become enriched in aluminum and oxygen and are then dissolved in the glassy phase. No Si2N2O intermediate layer is formed. The finding of the decisive role of the aluminosilicate in the oxidation process allows one to explain inconsistencies observed in the oxidation kinetics of silicon nitride ceramics. Effects of sintering additives, WC contamination and temperature on the oxidation mechanisms, and structure of oxide scales are discussed.

Oxidation behaviour of Si3N4–TiN ceramics under dry and humid air at high temperature

“…Generally, silicate glasses represent good solvents and transport media for the cationic species present in our composites (Ti, Si, Y, Al) and also for oxygen and nitrogen. 8,9 In the course of oxidation, the intergranular glass is subjected to a gradient in oxygen chemical potential between the outer sample surface and the bulk. This oxygen potential gradient acts as a driving force for the in-diffusion of oxygen and the counter diffusion of nitrogen and cationic species through the glassy phase.…”

Factors inducing degradation of properties after long-term oxidation of Si3N4–MoSi2electroconductive composites

“…28,29,31 In the temperature range 1200-1500°C, MoSi 2 , assisted by oxygen, reacts with Si 3 N 4 39 forming Mo 5 Si 3 and additional Si 2 N 2 O and SiO 2 (as detected by XRD) Sintering aid cations diffuse from the bulk to the reaction interface where they react with silica, forming a glassy silicate phase.…”

“…The thermal expansion mismatch between added particles and Si 3 N 4 may produce, after sintering, compression stresses in the matrix, which may improve the mechanical properties by toughening and strengthening effects. [27][28][29][30][31][32] The oxidation of electroconductive Si 3 N 4 -based composites is also affected by the type and amount of the secondary phases, because TiN, TiB 2 , and MoSi 2 are easyto-oxidize phases. [1][2][3][4][5][6][7][8][9][10][11][12] In particular, MoSi 2 is an excellent structural and electroconductive ceramic that fulfils many of the requirements for high-temperature application, [13][14][15] but its low fracture toughness at room temperature and the brittle-to-ductile transition at about 1000°C have pushed the attention on compositing MoSi 2 with Si 3 N 4 , SiC, AlN, and so forth, to obtain high-performance ceramic composites.…”

Electron microscopy study on the high-temperature oxidation of Si3N4–TiN ceramics: in situ and ex situ investigations

“…For silicon nitride ceramics, the oxidation resistance depends strongly on quality and quantity of the sinter additives and can be tremendously improved by carefully designing the microstructure. [5][6][7] To predict and improve the performance of the much more complicated Si 3 N 4 -TiN materials in hightemperature services, a fundamental knowledge of the oxidation mechanism is needed. In the past, several studies showed that the oxidation mechanism is complex, and multilayered scales form.…”