Nano-multilayered self-adaptive hard PVD coatings for dry high performance machining

In this research, quaternary nitride nano-multilayered coatings of the form TiAlCrN/MexN were comprehensively characterized. Based on this research principles which can be applied for guiding coating design were developed using the concept of self-adaptability.;The concept of self-adaptability as applied in this research is defined as the ability of a system to provide an improved response to an external stimulus such as friction, temperature, stress and strain. Self-adaptability plays a role in the formation of tribe-oxides on the tool surface under the elevated temperatures associated with dry high performance machining and the tribe-oxides formed in this case work as either liquid or solid lubricants in the cutting zone depending on their respective melting points. The tribo-ceramics (Al-O, W-O, Nb-O and Cr-O) formed during cutting process, were found to be extremely beneficial for an improvement on tool performance. They provided a synergetic action which served to protect the cutting tool by (1) lubricating the cutting zone to reduce friction, (2) insulating the substrate surface from oxidation as well as thermal attack, and (3) dissipate the energy during friction to reduce cutting edge and surface damage. However, due to high stress generating during high performance machining the liquid tribe-oxide lubricants did not provide any significant improvement in tool life. This was attributed to an inability to retain them in the cutting zone due to high contact pressures. Also these liquid phase formations on the surface were found to cause the spallation of the coating layers.;The adaptability of the coating also affects the chip formation process over the life of the tool. Among the parameters of the chips used to characterize wear behavior of a cutting tool, one of the key factors was the contact length between the chip and the tool rake face. The ability to maintain an optimal contact length was a major factor for achieving a long tool life. Too short of a contact length results in a short tool life because of excessively high stress concentration on the cutting edges. It was found that slight seizure was needed within the running-in stage to obtain a certain contact length and the ability to continuously provide excellent lubricity had a significant contribution to reducing further growth in seizure intensity.;Typically saw-tooth chips were generated except when a new sharp cutting edge with good lubricity was used. In this case continuous chips were observed. The cross-sections of saw-tooth chips revealed four deformation zones, i.e., white layer zone, friction zone, primary shear zone and a less deformed zone as a result of the combined effect of strain hardening, thermal softening, quenching phenomena and saw-tooth chip formation. The chip temperature estimation indicated that the cutting tool experienced a temperature of approximately 850 to 1100°C or above at a cutting speed of 220 m/min to 300 m/min. The study of chip formation further confirms that lubricity, ability to dissipate energy and thermal stability are the most important properties for coated tools to achieve a long tool life.;Comprehensive studies were performed on the following aspects: the tribological properties of the coatings at elevated temperatures, tool life, and cutting forces, tribe-oxide formation, wear mechanisms and wear progression, chip characteristics and the mechanisms of chip formation. The techniques applied for comprehensive characterization of the coatings were mainly Scanning Electron Microscope/Energy Dispersive Spectrometer (SEM/EDS) and X-ray Photoelectron Spectroscopy (XPS).;Among the adaptive coatings investigated in this research, the nano-multilayered TiAlCrN/NbN coating achieved excellent tool performance due to its enhanced self-adaptive properties under elevated temperatures. The tool life achieved with the TiAlCrN/NbN coated tool was increased by more than four times as compared to the best commercial available nano-crystalline coating TiAlCrN. The TiAlCrN/NbN coated tools are able to yield an acceptable tool life even when the cutting speed reached 400 m/min and hence makes a great contribution to productivity improvement.;A coated tool should be treated together as the coating controls the tool life while both the structure and the properties of the substrate material have a great impact on the performance of the cutting tool as well. A fine-grained substrate material possesses the combined properties of both high toughness and high hardness and had a significant contribution to tool performance especially for the severe cutting conditions under which the substrate was gradually exposed at the cutting edge as wear progressed.;Overall in the on-going effort to improve wear resistance of a hard coating it was found that between the two ways to improve tool performance, i.e. hardness improvement as represented by superhard nano-composite coatings such as a TiAlN/Si3N4 coating and adaptability improvement by adaptive nano-laminated TiAlCrN/MexN coatings, the latter was more beneficial for tool life enhancement under the proposed cutting conditions.