Growth and Microstructure‐Dependent Hardness of Directionally Solidified WC–W₂C Eutectoid Ceramics

Abstract

Directionally solidified WC-W 2 C ceramics containing 40 at% carbon, corresponding to the WC-W 2 C eutectoid composition, were produced by laser surface melt processing. The resulting microstructures showed a lamellar-type eutectic/eutectoid microstructure with the WC minor phase embedded in the W 2 C matrix phase. The interlamellar spacing (k) in the eutectoid regions followed the relationship Vk 3.8 = constant, with the smallest spacing of 331 AE 36 nm achieved in the 3.24 mm/s processed sample. The indentation hardness increased with decreasing interlamellar spacing, and a Vickers indentation hardness of 28.5 GPa was achieved in the sample with the smallest interlamellar spacing. The directionally solidified WC-W 2 C materials show enhanced indentation mechanical properties in comparison to previously reported WC-Co composites and WC-based materials.

First-principles calculations of the phase stability and the elastic and mechanical properties of η-phases in the WC–Co system

“…calculated hardness of WC is slightly higher than the previously reported experimental values [37][38][39][40][41][42], whereas the hardness of W 2 C-ε is lower than previous experimental results [16,17,19,39,43].…”

“…In particular, hardness measured by indentation methods strongly depends on the testing load and time [16]; thus, the hardness of a sample can vary. Dense WC sintered bodies are also difficult to obtain by general vacuum sintering, and several studies have reported that carbon-deficient phases, such as W 2 C, that are softer than WC are frequently detected in WC sintered bodies [13,16,17,44]. Therefore, the experimental hardnesses are lower than the calculated hardnesses.…”

“…However, Co can form W x Co y C (η-phases) during powder preparation and sintering. These phases form when C is deficient in Co-rich regions [13][14][15], and degrade the mechanical properties of the resulting composite [10,16,17].…”

Directionally Solidified Boride and Carbide Eutectic Ceramics

“…However, rigorous consideration of interface anisotropy and kinetics of the pattern selection process during solidification can explain deviations from the simplistic volume fraction criterion and variations in morphology with growth rate. 68,69 Also included in Table I is a WC-W 2 C eutectoid ceramic composite that has been explored by Chen, et al 70 During directional solidification from the melt, the system undergoes both eutectic and eutectoid transformations and series of solid-state transformations through various W 2 C polytypes. 65 In the B 4 C-MoB 2 eutectic, the irregular, "Chinese script" microstructure may be attributed to anisotropic interfacial energy.…”

“…9(a). 70 Moreover, the preferred crystal orientation in these WC-W 2 C eutectoids corresponded to close-packed carbon directions (WC: [ 12 10], W 2 C: [ 12 10]) in both phases aligned parallel to each other within the interface place. 9(b) and (c).…”