Juan J. Meléndez-Martínez

Creep of silicon nitride

Abstract General features of silicon nitride based ceramics, which may well influence their creep behavior are presented. Then, the most commonly invoked models for the microscopic mechanisms assumed to take place during creep (viscous flow, solution–precipitation, cavitation and shear thickening) are analyzed. Finally, the very numerous macroscopic and microscopic experimental findings about the plastic deformation of silicon nitride based ceramics at high temperatures, such as the fundamental role played by the secondary phases, the essential compressive-tensile asymmetry, and the microstructural evolution accompanying creep are summarized and discussed in terms of those models.

Compressive creep of mullite containing Y2O3

Compressive creep of mullite and mullite containing 5 and 9 wt% Y{sub 2}O{sub 3} has been investigated in the temperature range of 1300-1400 C over stresses between {approx}0.6 and 40 MPa in air. The nominally single-phase mullite deforms by diffusional flow with a stress exponent of 1 (for higher stresses) and an activation energy of 385{+-}20 kJ/mol. It is likely that the rate-controlling diffusing species is oxygen. Creep of the Y{sub 2}O{sub 3}-containing specimens was similar to that of the pure mullite at 1300 C. Near and above the temperature at which melting was observed in DTA, the Y{sub 2}O{sub 3}-containing specimens crept significantly faster than the pure mullite. Models of creep of materials that contain a glass phase can explain most, but not all, of the observed behavior. Creep rates were not significantly affected by partial crystallization of the glass to Y{sub 2}Si{sub 2}O{sub 7}, but the crystallized specimens exhibited cavitation at larger strains.

High temperature mechanical behavior of aluminium titanate–mullite composites

Abstract Compressive deformation tests at high temperatures (1300–1450°C) have been performed on aluminium titanate (undoped and modified with MgO and Fe 2 O 3 )–10 wt.% mullite composites obtained by reaction sintering. Two deformation regimes have been observed, depending on the experimental conditions. At high stresses/strain rates, the samples failed intergranularly at very small failure strains. At low stresses/strain rates, large macroscopic strains were reached without significant microstructural changes. MgO and Fe 2 O 3 additions have a pronounced effect on the brittle-ductile transition. In contrast, they have no effect on the steady-state flow, which is characterized by a stress exponent close to unity and an activation energy of 650 kJ/mol. Experimental results are compatible with a mechanism of grain boundary sliding controlled by cation bulk diffusion.

Bulk Silicon Is Susceptible to Fatigue

It has long been held that bulk silicon is immune from fatigue. We present contrary evidence demonstrating severe fatigue in macroscale cracks produced in cyclic loading of single-crystal silicon with a sphere indenter. The key ingredient is a component of shear stress acting on the cracks during contraction and expansion of the contact circle. This gives rise to frictional sliding at the crack walls, dislodging and ejecting slabs of material and debris onto the silicon surface. The damage expands with continued cycling, leading to progressive degradation of the surface. The results have implications concerning the function of silicon-based devices.

Compressive creep of polycrystalline ZrSiO4

Polycrystalline ZrSiO{sub 4} ceramics were prepared from commercial powder. Silicate-based glass phase was observed at multiple-grain junctions. compressive creep tests were conducted in Ar at 1197-1400{sup o}C. For stresses of {approx}1-120 MPa, steady-state creep occurred by diffusional flow. For stresses of >3 MPa, the steady-state strain rate {dot {var_epsilon}} could be expressed as {dot {var_epsilon}} = A{sigma}{sup 1.1{+-}0.1}exp - [(470 {+-} 40 kJ/mol)/RT], where A is a constant, {sigma} the steady-state stress, R the gas constant, and T the absolute temperature. At 1400{sup o}C and 1 MPa, an increase in the value of n was observed. Electron microscopy revealed no deformation-induced change in the microstructures of any of the specimens, which is consistent with creep by diffusion-controlled grain-boundary sliding. Comparison with literature data indicated that volume diffusion of oxygen controlled the creep rate.

A critical analysis and a recent improvement of the two-dimensional model for solution–precipitation creep: application to silicon nitride ceramics

The step model is one of the most widely used models of solution–precipitation creep in polycrystalline ceramics with secondary glassy phases. However, it leads to unrealistic stress exponent values when two-dimensional step nucleation takes place at the grain boundaries. We present a modification of the original model of step nucleation that avoids such unreasonable values by considering in detail the precipitation (or solution) process. The modified model agrees with reported experimental results for ceramic systems in which it has been accepted that high-temperature plasticity occurs by solution–precipitation.

Creep mechanism of gas-pressure-sintered silicon nitride polycrystals. I. Macroscopic and microscopic experimental study

The creep behaviour and microstructure of two silicon nitride ceramics have been investigated. Compressive creep tests were performed at temperatures between 1450 and 1700°C at stresses between 6 and 90 MPa in an Ar atmosphere. The creep behaviour was characterized by a stress exponent lower than one for both materials, with an average value n ≈ 0.6 over the whole range of stresses and temperatures, and with apparent activation energies between 470 and 530 kJ mol−1. The study of the microstructural evolution revealed the absence of dynamic grain growth and, in some cases, evidence of grain rearrangement. Partial coalescence of cavities was observed only at the highest stress, but this did not result in accelerated creep.

Creep of Al2O3 containing a small volume fraction of SiC-whiskers

Creep experiments performed at low stresses for an alumina-SiC whisker-reinforced composite containing 6 vol.% whiskers agree with the deformation model presented schematically in the deformation map. This is, below the percolation limit creep is controlled by diffusional flow. The stress exponent evolution is consistent with the predictions that the flow under low stress is controlled by pure diffusion while under high stress, flow is controlled by accommodated grain-boundary sliding.

Creep mechanism of gas-pressure-sintered silicon nitride polycrystals II. Deformation mechanism

As reported in the companion paper, part I, the creep behaviour of two silicon nitride ceramics is governed by a stress exponent n = 0.6 ± 0.1. It is known that, under certain conditions, silicon nitride ceramics may undergo a transition n = 1 → n<1 (the ‘shear-thickening transition’) at a well-defined stress σ*. This transition has not been observed, which, together with considerations regarding the value of the transition stress σ*, leads us to reject the shear-thickening phenomenon as an explanation for the creep behaviour of the materials investigated. Instead, it may be understood in terms of a particular grain boundary sliding mechanism accommodated by solution–precipitation.

High temperature mechanical behavior of silicon nitride ceramics

The high-temperature compressive creep behaviour of silicon nitride ceramics with different densification aids has been investigated. The microstructure of the as-received materials consists of randomly oriented rod-shaped β-silicon nitride grains with no remaining a phase. The grains have average dimensions of about 1.5 μm length and 0.5 μm width. Creep experiments were conducted in an argon atmosphere at temperatures ranging from 1450°C to 1700°C and under stresses from 20 to 125 MPa. The stress exponent n and the activation energy Q were measured from stress and temperature changes. Values of n 0) or decreasing (ΔT < 0) temperature changes were found. Although no macroscopic failure was observed after relatively large strains (∼30%), cavities developed along grain boundaries. The creep parameters may be consistent with a shear-thickening behaviour.