Hardness of Polycrystalline Wurtzite Boron Nitride (wBN) Compacts

Abstract

Wurtzite boron nitride (wBN), due to its superior properties and many potential practical and scientific applications, such as ideal machining/cutting/milling ferrous and carbide materials, especially as an ideal dielectric substrate material for optical, electronic, and 2-D graphene-based devices, has recently attracted much attention from both academic and industrial fields. Despite decades of research, there is an ongoing debate about if the single-phase wBN is harder than diamond because of the difficulty to make pure wBN material. Here we report the successful synthesis of pure single-phase polycrystalline wurtzite-type boron nitride (wBN) bulk material by using wBN powder as a starting material with a well-controlled process under ultra-high pressure and high temperature. The cubic boron nitride (cBN) was also successfully prepared for the first time from wBN starting material for comparison and verification. The X-ray diffraction (XRD) and TEM clearly confirmed that a pure single-phase wBN compact was produced. The microstructure and mechanical properties including Vickers hardness, fracture toughness, and thermal stability for the pure single-phase wBN was first evaluated.

Figure 1

(a) Transformation pressure-temperature diagram of wBN (Red circle: cBN, Black rhombus: wBN, Green triangle: wBN + cBN). (b) XRD patterns of the starting wBN, and the synthesis products synthesized at 20 GPa and 1,150 °C, 1,250 °C and 1,850 °C, respectively. (c) XRD patterns of hBN and wBN.

Figure 2

(a) SEM micrography of the initial wBN powder. (b) wBN compact SEM micrograph synthesized by 20 GPa and 1,150 °C. (c) wBN + cBN compact electron microscope photo synthesized by 20 GPa and 1,250 °C. (d) cBN compact SEM micrograph synthesized by 20 GPa and 1,850 °C. Insert: The enlarged SEM micrograph of (d). (e) In samples synthesized at 20 GPa and different synthesis temperatures of 1,150 °C, 1,250 °C and 1,850 °C, the hardness is changed as a function of the load force.

Figure 3

(a,b) Indentation of wBN compact at the loading force of 4.9N and 9.8N. (c,d) Indentation of wBN + cBN compact at the loading force of 9.8N and 29.4 N. (e,f) Indentation of cBN compact at the loading force of 29.4N and 49N. (g–i) TGA results of samples synthesized at 20 GPa and 1,850 °C, 1,250 °C and 1,150 °C, respectively.

Figure 4

(a) TEM micrograph of initial wBN powder. (b) TEM micrograph of wBN compact synthesized at 20 GPa and 1,150 °C.

Figure 5

(a) Selected area electron diffraction pattern of initial wBN powder. (b) Selected area electron diffraction pattern of wBN compact synthesized at 20 GPa and 1,150 °C.