Micro Thin Film Thermal Sensors Embedded Into Polycrystalline Cubic Boron Nitride (PCBN) for Advanced Machining Study

Monitoring and control of thermomechanical parameters in tooling materials are imperative for improving the fundamental understanding, reliability and workpiece quality of material removal processes. Polycrystalline cubic boron nitride (PCBN) tools are being used heavily in machining of low carbon steel and superalloys. These processes are very sensitive to variation in local machining conditions and there lacks a thorough understanding of fundamental thermomechanical phenomena which can lead to abrupt tool failures. Existing sensors for monitoring machining conditions are not suitable for precision process control as they are either destructively embedded and/or do not possess the necessary spatial and temporal resolution to monitor temperature during machining effectively. This paper presents a novel approach to obtain temperature data from a close distance to the tool cutting edge. An array of 9 micro thin film thermocouples, fabricated using standard microfabrication methods, has been embedded into a PCBN cutting tool using a diffusion bonding technique. Scanning electron microscopy (SEM) was performed to examine material interactions at the bonding interface and determine optimal bonding parameters. The sensors were statically and dynamically characterized using a tube furnace and rapid laser heating, respectively. They exhibit good linearity, sensitivity and very fast response time. The instrumented PCBN inserts were applied in machining experiments. Being embedded into the tool at a total distance of only 260 μm from the cutting edge, the micro sensors enabled the detection of local cutting temperature changes caused by interrupted machining. The data obtained during cutting demonstrate the functionality of the tool-embedded micro thermal sensors and their value for fast, accurate and reliable monitoring and control of machining processes.