The allotropic transformation of polycrystalline cubic boron nitride structures resulting from the thermal effects of pulsed laser ablation

Abstract

Cubic boron nitride, being only second to diamond for its hardness and possessing superior thermal stability is suitable for a wide range of applications. In particular, the versatility of this material and its polycrystalline cubic boron nitride composite form can be extended by micro processing enabling its use in advanced manufacturing applications. By employing a pulsed laser ablation technique for the micro texturing of this material, this paper presents for the first time the thermal response of polycrystalline cubic boron nitride incorporating a ceramic binder to the laser ablation process. Combined FIB/TEM/EELS/EDX techniques have been used to characterise a cubic boron nitride based material and its Titanium Carbide binder at nanometric resolutions after the surface has been ablated via an Nd Yag laser. Allotropic transformations of the cubic boron nitride into amorphous boron nitride immediately below the ablated surface and into hexagonal boron nitride down to depths exceeding 300nm have been identified in a site exposed to high thermal excitation while the boundaries between boron nitride and the primary binder constituents remained definitive. Importantly, the structural integrity of the studied PCBN remained intact below the regions of phase transformations. Prime novelty statementThe paper presents for the first time an evaluation of laser ablated polycrystalline cubic boron nitride surfaces and substructures at nanometric resolution. A novel procedure for the focus ion beam preparation of circa 100nm thick lamella using multi surface protection coatings and stabilising straps is presented, allowing and HRTEM imaging and EELS analyses of this fragile structure.