A comparative study of CrN, ZrN, NbN and TaN layers as cobalt diffusion barriers for CVD diamond deposition on WC–Co tools

Abstract

Chromium, zirconium, niobium and tantalum nitrides layers have been sputtered onto WC–12 wt.% Co substrates as diffusion barriers and buffer layers for improving performances of diamond surface coating. X-ray diffraction shows under specific reactive sputtering conditions, only MN (M = Cr, Zr, Nb, Ta) type phase exits. Their electric resistivity has been measured on samples deposited onto silica under the same conditions and related to those published. Surface and transverse scanning electron microscopy shows a dense columnar morphology. Thermo chemical computing proves the stability of those nitrides against Co, hydrogen and methane up to 1150 K (877 °C). A computed diagram of nitrogen partial pressure is given for their carburization with methane showing the highest stability for ZrN and TaN. Diamond deposition for 5 h up to 1153 K (880 °C) highlights a different behaviour for each of those materials. Auger Electron Spectroscopy (AES) profiles show a massive diffusion of cobalt through the decomposed CrN layer, a transformation of TaN and NbN into carbide without diffusion of cobalt while ZrN is outstandingly well preserved.