Comparison of wear characteristics and properties of TiN-coated gear cutting hobs

Two types of TiN-coated gear cutting hobs were compared in the hobbing of case hardening steel under production conditions. The coating reduced the wear of the hobs significantly, even though 25% and 50% increased cutting speeds as compared to the uncoated hobs were used. The performance of the two types of TiN coatings denoted by A and B differed markedly. The coating B lasted generally from l. 5 to 2 times longer than A. Mechanical testing of hardness and adhesion by a scratch test gave, however, slightly better values for coating A. Nuclear resonance broadening analysis showed coating A to be essentially stoichiometric TiN, while B showed a gradually increasing nitrogen content towards the surface. Scanning electron micrographs revealed columnar microstructure with coating A while B was almost featureless. The columnar microstructure of coating A is suggested as an explanation for the poor performance of coating A as compared to coating B.