A study of the wear mechanisms of Ti1−xAlxN and Ti1−x−yAlxCryN coated high-speed steel twist drills under dry machining conditions

Recent advances in cutting tool materials and PVD surface coatings are making high-speed dry machining an increasingly viable machining operation in commercial manufacturing sectors. High oxidation resistant coatings, such as TiAlN, are used extensively in global manufacturing for reducing production costs and improving productivity in such aggressive metal-cutting operations. In this investigation, the performance of TiN, Ti1−xAlxN and Ti1−x−yAlxCryN coatings was assessed on Co-HSS twist drills used to machine an automotive grade grey cast iron. As part of the study, two experimental methods were used to characterise the performance of the twist drills, namely, by an audible ‘screech’ and measurement of the progression of outer corner flank wear lands. Both techniques showed that a single layer TiN coating failed to outperform even uncoated twist drills when dry machining grey cast iron under aggressive machining conditions. However, using the measurement of outer corner flank wear technique, the average tool life of uncoated twist drills was increased by factors of 4.5, 6.5 and 6.9 by Ti0.59Al0.41N, Ti0.27Al0.19Cr0.54N and Ti0.21Al0.14Cr0.65N coatings, respectively. In the case of ‘screech’ failure, the average tool life of uncoated twist drills was increased by factors of 3.9, 7.3 and 10.8 by Ti0.59Al0.41N, Ti0.27Al0.19Cr0.54N and Ti0.21Al0.14Cr0.65N coatings, respectively. The performance of twist drills was also assessed using a number of quantitative techniques, namely, measurement of thrust and torque, hole diameter and drill wear. The latter was also observed using scanning electron microscopy (SEM) which showed an early onset of workpiece material transfer at the margins of the uncoated drills. In the case of the PVD coated drills, an improvement in the tribological interaction between the coatings and the grey cast iron workpiece resulted in a significant reduction in material transfer at the drill margins. The above results are discussed in terms of their affect on hole diameter and cutting forces.