Acoustic emission (AE) was used to monitor the machining process and tool condition during turning of AISI 4340 steel with uncoated, two-layer (TiC/Al2O3) coated, and three-layer (TiC/Al2O3/TiN) coated cemented WC-Co tools. The experiments were performed at four different feedrates and constant cutting speed and depth of cut. The variation of the AE signal with cutting time is interpreted in light of the dominant mechanisms, rates, and patterns of wear and the contact friction conditions at the tool/workpiece and roof/chip interfaces. Correlations between intrinsic frequencies and AE sources are identified by examining the root-mean-square, dominant amplitude, type, and count rate of the AE signals. It is shown that AE frequencies ill the range of 50-100 kHz are primarily due to plastic deformation in the near-surface tool regions and the primary, secondary, and tertiary shear zones of the workpiece, whereas cracking leading to coating delamination and WC grain pull-out generates frequencies in the range of 170-200 kHz. The tool life estimated from the root-mean-square of the AE signal is shown to be in good agreement with that determined from measurements of the maximum wearland width on the tool nose. The obtained results demonstrate that AE is an effective technique for in-process wear monitoring and wear mechanism identification of multi-layer ceramic coated tools.
