Wear resistance of coatings in high speed gear hobbing

Coating technology is one means of achieving a crucial enhancement in tool performance, especially in hobs that were among the first tools to be coated on a large scale. Nevertheless only few detailed analysis of wear mechanism have been done on field machines. The bifunctional coatings (combination of a tough, hard and refractory coating and of a self lubricating coating possessing a good thermochemical and abrasion resistance but a lower hardness) are very interesting since it is difficult to get a simple coating showing all these characteristics. The use of bilayer coatings raises several problems especially for dry and high speed cutting. Therefore, in order to investigate the behaviour of these bifunctional coatings, hobs have been coated by physical vapour deposition (PVD) methods. After the elaboration of a procedure for hobs testing, field tests have been performed. Results of tool life tests and investigations on tool wear mechanisms for different coated hobs are presented and discussed. The interesting performance in high speed gear hobbing of sintered high speed steel (HSS) hobs (ASP2052) combined with a (Ti,Al)N+MoS2 coating is particularly underlined.