Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel

The present research involves the deposition of pulsed DC CFUBM sputtered TiN on nickel plated steel discs and electroplated monolayer cBN wheels at seven different target frequencies and ten different bias voltages separately. The coating microstructures and the interaction between TiN and nickel were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Phase detection was carried out using grazing incidence X-ray diffraction (GIXRD) technique. The cohesive and adhesive strengths of nickel layer were assessed by scratch test. After grinding of low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100), the conditions of the uncoated and coated cBN wheels were observed under Stereo Zoom Microscope and SEM. Average column size of TiN was found to decrease with increase in both target frequency and negative bias voltage. The structure of the coating gradually transformed from porous and open columnar (at 0 V bias) to very compact, dense and featureless (at -80 V bias). EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel-titanium intermetallic phases at their interface. The cohesive strength of nickel layer was not effectively enhanced with increase in target frequency, whereas the same was significantly improved with increase in negative bias voltage. Seemingly, TiN coated wheel could not perform better than the uncoated wheel in grinding AISI 1020 steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheels. On the other hand, many fractured grits and some grit pull-out were observed in the uncoated wheel when grinding AISI 52100 steel, whereas almost no pull-out along with much less fractured grits were observed in the wheels coated at bias voltages like -60 V and -90 V. (C) 2010 Elsevier B.V. All rights reserved.