Abstract
Nanoindentation-based fracture toughness measurements of ceramic materials like silicon carbide (SiC) with pyramidal indenters are of significant interest in materials research. A majority of currently used fracture toughness models have been developed for Vickers indenters and are limited to specific crack geometries. The validity of the indentation-cracking method for the fracture toughness measurement of single crystal SiC, the elastic-plastic anisotropy and orientation dependence around the c-axis when indented in the <0001> direction is examined using nanoindentation with different pyramidal indenters. The residual impressions are analyzed using scanning electron microscopy to measure the crack lengths and the validity of existing fracture toughness measurement methods and equations is analyzed. A combination of nanoindentation with different pyramidal indenters to produce a wide range of effective strains and finite element simulation is used to extract flow properties of single crystal SiC in the <0001> direction. It is observed that there is no orientation dependence around the c-axis when SiC-6H is indented in the <0001> direction with a Berkovich indenter, i.e., it is transversely isotropic. It is also found that for a Berkovich indenter, the Jang and Pharr model, which is based on the Lawn model for cone/halfpenny cracks, gives approximately constant values at low loads (<1 N), while at higher loads (>1 N), the Laugier model gives constant fracture toughness values. Finite element analysis using equivalent cones is used along with measured hardness values to estimate the yield strength, the work hardening exponents and the stress–strain curve for single crystal SiC-6H in the <0001> direction.
Highlights
Silicon carbide (SiC) as a material for structural applications has received tremendous interest starting in the 1980s for high temperature, high stress applications like gas turbine engines to increase their efficiency and life span
All nanoindentation experiments were done using the continuous stiffness measurement (CSM) technique [43] which gives the load on the sample and the contact stiffness as a function of the displacement of the indenter into the samples
The hardness and the modulus data were corrected for pile-up behavior by using Scanning electron microscopy (SEM) and interference microscope images of the indents to determine the actual contact area
Summary
Silicon carbide (SiC) as a material for structural applications has received tremendous interest starting in the 1980s for high temperature, high stress applications like gas turbine engines to increase their efficiency and life span. SiC crystallizes in over 200 polytypes [1,2] with different Si–C stacking sequences. The most common polytypes are the 3C (Cubic) or β and 6H (Hexagonal) or α-SiC [3] and have similar mechanical and thermal properties but different electrical and optical properties. SiC has superior creep behavior and wear resistance [5,6] which make it an ideal material for use in wear resistant coatings in a variety of automotive components like pistons, valve heads, Ceramics 2018, 1, 198–210; doi:10.3390/ceramics1010017 www.mdpi.com/journal/ceramics
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