Abstract
Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (HV), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (HV > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying HV of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.
Highlights
Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary
We used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm
We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials
Summary
The BDD crystals D2 and D4 were synthesized at 5.5 GPa and at ~1300 and ~1380 °C, respectively, by the temperature gradient growth method in a China-type cubic high-pressure apparatus at State Key Laboratory of Superhard Materials of Jilin University. The boron concentration gradient was estimated from Raman spectrum (Figure S3) and lattice parameters (Figure S2) measured by synchrotron X-ray diffraction at sector 16-BM-D of the Advanced Photon Source (APS), Argonne National Laboratory. Synchrotron-IR measurements were conducted at beamline U2A of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. Hardened steel was used as a standard before and after diamond measurements (Figure S6). GHz-ultrasonic interferometry, in conjunction with the newly developed optical contact micrometer for high-precision length measurements, was conducted at Northwestern University
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