Composite-metal stacks like Carbon Fiber Reinforced Plastic – Titanium alloy (CFRP-Ti) stacks are in high demand for various structural components of modern aircraft due to their high strength to weight ratio, good corrosion resistance, high thermal stability and other superior characteristics. Furthermore, CFRP-Ti stack is an eco-friendly material choice due to the growing emphasis towards sustainability and energy efficiency, to minimise fuel consumption. 

Mechanical drilling of CFRP-Ti stacks is inevitable and frequently employed operation for the purpose of rivets and bolts connections needed for component assembly in the aerospace sector. However, exceptional material endurance of CFRP-Ti stacks poses unmatched difficulty during the hole making process, as both CFRP and Ti6Al4V are characterised as difficult-to-machine materials. The distinct nature of stack materials causes rapid tool wear due to synergistic action of abrasive and adhesive wear, yielding poor hole quality, meanwhile compromising the structural integrity of CFRP-Ti stacks. 

Amidst growing productivity demands and environmentally friendly measures, prioritizing sustainability restrict the machining solutions viable for drilling of CFRP-Ti stacks. Dry machining emerges as the best option to abate the ecological hazards of metal-working fluids and their detrimental effect to CFRP matrix. Consequently, excessive cutting forces and temperatures were generated causing accelerated tool wear during the dry drilling process of CFRP-Ti stacks. The tool life is a decisive factor determining the hole quality during the mechanical drilling operation. The tool wear is aggravated from simultaneous wear action from distinct materials, as hard, brittle and heterogenous nature of CFRP fibers causes abrasive wear, whereas Ti6Al4V alloys are responsible for adhesive wear. The complex tribological interaction experienced by the cutting tool with distinct workpiece material entails for fundamental understanding into the tool wear mechanism. Cemented carbide (WC-Co) tools primarily employed in drilling of CFRP-Ti stacks undergo rapid tool wear. As tool material is the sole barrier towards the mechanical/thermal stresses in dry drilling process. The cutting tools are coated with wear resistance coatings such as Diamond-like Carbon (DLC) to transform the tribological contact and improve the tool life.

The main objective of this thesis is to perform detailed tool wear analysis in drilling of CFRP-Ti stacks and emulate the tool wear mechanism by custom-tailored cross-cylinder tribotest using uncoated and DLCs coated tools. Non-hydrogenated DLC coatings were deposited by High Power Impulse Magnetron sputtering (HiPIMS) and arc deposition technology on cemented carbide substrate of different geometries (disc, cylinders & drills). The doctoral dissertation comprises of three parts (covered in 5 articles). The Part-1 (PAPER A&B) covers the preliminary characterisation of as-deposited DLC coatings to understand the effect of substrate surface roughness on the adhesion and tribological performance of DLC coatings. Secondly, the tribological behaviour of different DLC coatings against Ti6Al4V counterbody in pin-on-disc tribotest was analysed at different applied loads, to correlate with titanium machining. 

As well known, traditional tribometer does not involve renewing of the countersurface as movement occurs in the same track. Such tribotest are not effective to mimic the complex contact situation experienced during the drilling of CFRP-Ti stacks. As, no simple yet effective tribotest technique is available to replicate contact situation of complex multi-material stack sequence. Therefore, in Part-2 (PAPER C&D), development and exploitation of a cross-cylinder tribotest technique against multi-material stack of CFRP-Ti workpiece and its individual constituents was performed to emulate wear mechanism of the uncoated and DLCs coated tools during machining of CFRP-Ti stacks. Lastly, in Part-3 (PAPER E), in-depth tool wear analysis of uncoated and DLCs coated drills were analysed in dry drilling of CFRP-Ti stack by successively analysing the tool wear growth after set number of drilled holes.

The results showed that cross-cylinder tribotest is an effective and simple appraisal technique to emulate the complex tool wear mechanism against multi-material of CFRP-Ti stacks. Despite showing difference from actual drilling experiment and need for improvement, cross-cylinder test replicates the contact situation satisfactorily well. The customised cross-cylinder test would guide in tool material selection, optimization of cutting parameters and tribological understanding of new coatings solutions in a facile, economical, and effective way.