High rate reactively sputtered TiN coatings on high speed steel drills

Abstract

Abstract TiN can be reactively sputtered at the full metal deposition rate and still achieve excellent properties. The TiN coatings have a (111) preferred orientation and a lattice parameter of 0.424 nm. The coating hardness is 2490 kgf mm-2 for a coating 1–2 μm thick. TiN-coated drills were tested under accelerated machining conditions. The speed and feed were chosen such that an uncoated drill failed in two or three holes while drilling dry a hole 1.25 cm deep in 4130 steel hardened to 235 HB. The speed was 1150 rev min-1 and the feed was 0.21 mm rev-1. Under the same conditions, the TiN-coated drills averaged 150 holes before failure. Cutting fluids either helped or hindered coated cutting tool life. One soluble oil cutting fluid increased the coated tool life to 301 holes whereas another decreased life to only ten holes. For a second set of tool life tests, M-7 high speed steel drills were coated by two different commercially available physical vapor deposition (PVD) processes (A and B) and by the high rate reactive sputtering process. The drills were run at 2300 rev min-1 with a feed of 0.11 mm rev-1 with a water-soluble cutting fluid. The workpiece was 4340 steel hardened to 285 HB, and the hole depth was 1.25 cm. Drills coated by PVD process A had an average life of 530 holes (S = 407); those coated by PVD process B, 398 holes (S = 209); and the reactively sputter-coated drills, 452 holes (S = 96). The smaller standard deviation of the sputter-coated drills means that these drills will be much more reliable in cutting applications than the other two.