Influence of bias and in situ cleaning on through cage (TC) or active screen plasma nitrided (ASPN) steels

The effect of a substrate bias and prenitride plasma etching on the nitriding response of four steel substrates is investigated in a two factor two level full factorial experimental design. The steels investigated were P20 (M200, plastic mould steel), H13 (W302, hot work tool steel as received), 4140 (preheat treated nitriding steel) and 1020 (CS1020, bright mild steel). The nitriding response was determined from surface and cross-sectional hardness measurements. Nitrogen depth profile measurements were obtained using glow discharge optical emission spectroscopy. Considering the main effects, the results show that without a worktable bias during the nitriding step there is effectively little or no nitriding response in most of the materials. The prenitride plasma etch did not produce a significant surface hardness response in all steels except H13, where a prior etch substantially increased surface hardness and influenced the hardness depth profile. The bias also significantly increased the nitrogen wt-%. The plasma etch also influenced the near surface nitrogen wt-% concentrations, however the practical implications of this require further investigation

Nanocrystalline hexagonal diamond formed from glassy carbon

Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts

Probing the Atomic Structures of Synthetic Monolayer and Bilayer Hexagonal Boron Nitride Using Electron Microscopy

Monolayer hexagonal boron nitride (h-BN) is a phenomenal two-dimensional material; most of its physical properties rival those of graphene because of their structural similarities. This intriguing material has thus spurred scientists and researchers to develop novel synthetic methods to attain scalability for enabling its practical utilization. When probing the growth behaviors and structural characteristics of h-BN, the use of appropriate characterization techniques is important. In this review, we detail the use of scanning and transmission electron microscopies to investigate the atomic configurations of monolayer and bilayer h-BN grown via chemical vapor deposition. These advanced microscopy techniques have been demonstrated to provide intimate insights to the atomic structures of h-BN, which can be interpreted directly or indirectly using known growth mechanisms and existing theoretical calculations. This review provides a collective understanding of the structural characteristics and defects of synthetic h-BN films and facilitates a better perspective toward the development of new and improved synthesis techniques.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Published versio