Synthesis of Superhard Lightweight Composites and Improvement of Their Properties via Chemical Processing

Abstract

Superhard lightweight composites were prepared by self-propagating high-temperature synthesis (SHS), attrition milling, chemical leaching in concentrated HCl and/or HNO₃ acids and spark plasma sintering (SPS) under N gas pressure. Such processing features were conducted for production of the superhard B₁₃C₂, B_11.72C_3.28, c-BN and c-BC₂N chemical compounds in the B₄C(67 wt%)-Al/WC-Co/Cu-based composite. The materials were studied by X-ray diffraction, scanning electron microscope equipped with energy-dispersive spectrometer system (SEM-EDS) and also with microindentation and nanoindentation of composites. During SHS, the B₄C was partly transformed into boron-rich B₁₃C₂ boron carbide in quantity of ~60 wt%. The c-BC₂N content in the composite was ~12 wt% after heat treatment of SHS-composite under nitrogen gas flow at temperature of 850 °C for 2 h. Subsequently, the disintegrated and attrition-milled powdered SHS-composite was chemically leached, and as result, the soft Al-containing compounds were reduced from ~27 wt% to ~4 wt%. During SPS, the boron-rich B₁₃C₂ boron carbide was transformed into carbon-rich B_11.72C_3.28 boron carbide in quantity of ~87 wt%. The maximal Vickers microhardness of superhard composites was ~4400 HV_1.0 (~46 GPa). The composite has high thermal stability and chemical inertness.