Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: I, Mechanical Characterization

Abstract

In this Part I of a two-part study of Hertzian indentation in silicon nitride we characterize irreversible contact damage as a function of microstructure. Three controlled silicon nitride microstructures are examined, representing a progression toward greater long-crack toughness: fine (F), bimodal with predominantly equiaxed ␣ grains; medium (M), bimodal with mostly ␤ grains of intermediate size; and coarse (C), with almost exclusively elongated ␤ grains. An effect of increasing the microstructural heterogeneity in this sequence is to suppress ring cracking around the indenter, ultimately to a degree beyond that expected from increased toughness alone. Along with the crack suppression is a parallel tendency to enhanced damage accumulation beneath the indenter, such that the contact in the coarsest microstructure is predominantly quasi-plastic. The characterization of damage includes the following: determination of indentation stress-strain curves, to measure the level of quasi-plasticity; measurement of threshold loads for the initiation of ring cracking and subsurface yield, to quantify the competing damage processes; and measurement of characteristic dimensions of the ensuing cracks and deformation zones in their well-developed stages. These quantitative results are considered in terms of formal contact mechanics, along with finite element modeling to generate the essentially elastic-plastic fields in the different silicon nitride structures. This contact mechanics description serves also as the basis for subsequent analysis of strength degradation in Part II. Implications concerning microstructural design of silicon nitride ceramics for specific applications, notably bearings, are considered.

Effect of sintering atmosphere on the mechanical properties of liquid-phase-sintered SiC

“…Earlier work has clearly demonstrated that these microstructural ingredients produce ''quasi-ductility'' in ceramics, where shear-faulting occurs along the weak, elongated interfaces beneath the indenter. 3,4,25 This is also consistent with the relatively lower hardness in Ar-LPS SiC (H=17.6 AE 0.3 GPa), compared with N 2 -LPS SiC (H=20.7 AE 0.6 GPa). The same microstructural ingredients also enhance the efficacy of the toughening mechanism crack-wake bridging, making those ceramics tougher.…”

Effect of temperature on the pre-creep mechanical properties of silicon nitride

“…These constitutive equations are based on a critical shear condition for yield but incorporating a strain-hardening characteristic to allow for local elastic constraints on the sliding shear faults that are responsible for quasi-plasticity in ceramic materials. 6,18,[22][23][24] These uniaxial equations are then generalized to multiaxial behavior using the von Mises yield criterion. The application of this type of treatment, typically used in metallic materials, to quasi-plastic ceramics has been justified by Fischer-Cripps and Lawn, 25,26 and proved to be useful for predicting the main features of Hertzian stress-strain curves and the evolving deformation zone geometries, despite the fact that the underlying deformation mechanism is shear faulting instead of dislocation sliding.…”

“…However, this intensive mechanical characterization has been limited to either room temperature [4][5][6] or high temperatures within the creep regime. [7][8][9][10] The information available at intermediate temperatures is still very scarce -indeed limited to elastic property evaluations by dynamic measurements [11][12][13][14][15][16] -despite the fact that it is within this temperature range where silicon nitride has to work in most practical applications.…”

Textured silicon nitride: processing and anisotropic properties

“…Lee et al [192,193] studied the effect of the microstructure on the Hertzian contact damage of Si 3 N 4 ceramics. They showed that there is competition between brittle (ring or cone fracture) and quasi-plastic damage (distributed shear-activated microfaulting) modes.…”