Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

Abstract

The creep behavior of a commercial grade of Si 3 N 4 was studied at 1350°and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ؎ 22.9 kJ/mol, and compression, 489.2 ؎ 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple-junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ϳ18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution-precipitation of Si 3 N 4 . Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution-precipitation creep of Si 3 N 4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.

Mecanismos de deformación plástica en policristales cerámicos con fases secundarias vítreas

“…Como ejemplo, la figura 2 muestra este hecho en materiales a base de nitruro de silicio, deformados en compresión y en tracción (7,(15)(16)(17); un comportamiento análogo ha sido identificado en cerámicos a base de SiC (18) o de Al 2 O 3 (19) con fases secundarias. En cambio, los materiales monofásicos exhiben el mismo comportamiento en compresión y en tracción (14).…”

“…Así, la mayor parte de los estudios atribuyen la fluencia en compresión del nitruro de silicio al deslizamiento de fronteras de grano acomodado por disolución-precipitación, si bien existen menciones a acomodación por fluencia viscosa (7). En tracción, en cambio, la deformación se produce generalmente por cavitación acomodado por el deslizamiento relativo de los granos (7,16,17) o por un mecanismo combinado (7,21). Esta afirmación está apoyada por la observación sistemática de que la fracción volumétrica de cavidades aumenta linealmente con la deformación (7,(15)(16)(17)20).…”

Prediction of tensile power law creep constants from compression and bend data for ZrB2–20 vol% SiC composites at 1800 °C

“…Observation of the microstructure showed that, unlike compression, creep in tension is controlled by cavity formation. At 1.8% strain, the volume fraction of cavities in compression was only 18% of that in tension [10]. In general, polycrystalline materials with a viscous grain boundary phase exhibit larger creep strains in tension compared to compression [11].…”

“…For example, silicon nitride was subjected to compressive and tensile creep stresses ranging between 10 and 300 MPa at temperatures between 1350 and 1400 ℃ [10]. Over the entire stress range, compressive creep rates showed a linear dependence with stress.…”

Cavitación durante la fluencia de policristales de Si₃N₄

“…La fluencia de estos compuestos en compresión y en tracción ha sido investigada en un gran número de estudios (1)(2)(3)(4)(5), que han puesto de manifiesto que la fluencia estacionaria del nitruro de silicio está controlada por cavitación en tracción (6) y por disolución -reprecipitación en compresión (7).…”