Effect of Surface Modifications on the Hard Turning Performance of Al2O3/TiCN Mixed Ceramic Cutting Tools

Abstract

Hard turning process involves machining of materials of very high hardness, and thus, the
process is associated with high machining forces resulting in the generation of a large amount of heat. The prevailing machining conditions during hard turning cause high tool wear. Thus, various surface modification techniques have been adopted to reduce the friction during the hard turning process resulting in the reduction of tool wear and thereby, increasing the cutting tool durability. Further, the alumina based mixed ceramic cutting tools are gaining popularity during hard turning process due to their economic nature as well as superior chemical and thermal stability. In this context, the present work focuses on investigating the effect of surface modifications mainly surface coatings and texturing on machining performance of Al2O3/TiCN based mixed ceramic cutting tools during turning of hardened AISI 52100 steel having 62 HRC hardness under dry cutting environment. The present work involves a total of five stages of investigation. Initially, the effect of coating thickness of mono-layered AlCrN and multi-layered AlTiN/TiN coatings have been investigated during the first and the second stages respectively from which it was revealed that the coating thickness significantly impacts the hard turning performance of Al2O3/TiCN based mixed ceramic cutting tool with 3 μm coating thickness for AlCrN and 4 μm coating thickness for AlTiN based coating exhibiting the best machining performance. These results have also been validated using 3D finite element numerical simulation performed with the help of Deform 3D software. The chip thickness, chip width and coefficient of friction increased with the increase in coating thickness. However, the performance of coated tools is more dependent on the coating/substrate adhesion strength of the coatings with better adhesion corresponding to superior machining performance.
The third stage compares the performance of mono-layered AlCrN and multi-layered
AlTiN/TiN PVD coatings of optimum thickness as obtained in the first two stages of
investigation. The performance of the cutting tools was evaluated by tool wear and correlated in terms of material’s build-up at the tool-chip interface, friction, chip sliding velocity and cutting zone temperature. The results revealed increment of material’s build-up with feed rate whereas it decreased with the increase of cutting speed and chip sliding velocity. The sliding velocity increased with the increase of cutting speed resulting in lower contact time between the tool and the chip, and thus, the adhesion reduced with an increment of cutting speed. On the other hand, an increase of contact area in combination with higher cutting zone temperature and lower sliding velocities resulted in an increase of material’s build-up with feed rate. AlTiN coated cutting tools exhibited superior anti-oxidation, anti-adhesion and antiabrasive properties as compared to AlCrN coated and uncoated cutting tools. However, the AlCrN coating exhibited superior machining behaviour at higher cutting speeds indicating the suitability of the coating at elevated machining speeds.
The fourth stage compares the machining performance of coatings deposited by cathodic arc evaporation (CAE) and DC reactive magnetron sputtering (DCRMS) process with different
structures and deposition layers. Mono-layered AlCrN and multi-layered AlTiN were
deposited using CAE process whereas nano-structured TiAlSiN/TiSiN/TiAlN coating was
deposited using DCRMS process. The coated tools resulted in improved machining
performance when compared to uncoated cutting tool. The CAE process resulted in surface
defects like droplets and pores whereas the sputtering process generated droplet free surface that helped in superior performance of TiAlSiN/TiSiN/TiAlN coating when compared with AlTiN and AlCrN coatings. Moreover, nanostructure in TiAlSiN/TiSiN/TiAlN coating
prevented coating flaking or peeling and ensured superior anti-abrasive behaviour due to its
amorphous structure.
In the last stage of investigation, micro scale textures were fabricated on the rake surface of Al2O3/TiCN composite ceramic cutting tools using Wire-EDM process. The performance of
cutting tools was evaluated by tool-chip contact length, tool wear, machining forces, cutting
temperature, frictional coefficient and chip morphology. Further, a 3D finite element model
has been used to study the temperature and stress distribution in the cutting tools. Application of textures obstructing the chip flow direction resulted in uniform stress distribution, and higher groove sliding area ensured maximum reduction of machining forces, material adhesion, cutting and tool temperature. However, the textures parallel to the chip flow direction resulted in bending and curling of chips on the rake face causing increase of chiptool contact length and material adhesion resulting in increase of friction as compared to the tools with non-parallel textures. The textured tool with textures inclined at an angle to the chip flow direction exhibited the best machining performance as compared to other textured cutting tools. From the present work, it can be concluded that the surface modification techniques significantly improve the hard machining performance of Al2O3/TiCN mixed ceramic cutting tools.