Development of Multilayer TiAlN + γ-Al_2O_3 Coatings for Difficult Machining Operations

摘要

The continuous advances in the aircraft and nuclear industry demand the development of new materials that among other properties offer a good resistance at elevated temperatures. For this reason, Ni super alloys were developed from a simple Ni-Cr matrix to multi-element, multi-phase systems. These new super alloys are specially favored for their exceptional thermal resistance and ability to retain mechanical properties at elevated temperatures. They are classified as difficult-to-machine materials due to their high shear strength, work hardening tendency, content of abrasive carbide particles within the microstructure, strong tendency to weld and to form built-up edges. Their low thermal conductivity leads to elevated temperatures during machining. Consequently, their tendency to maintain a high strength at elevated temperatures leads to elevated cutting forces. On this account, the development of an adequate coating for machining Ni alloys is today an important challenge. Xowadays, TiAlN is well known because of its excellent overall cutting performance. However, Al_2O_3 possesses better chemical and thermal properties than TiAlN, especially regarding hot hardness, oxidation resistance, diffusion resistance. Motivated by the advantages offered by PVD-Technology, γ- Al_2O_3 was synthesized by reactive bipolar pulsed magnetron sputtering in previous works. PVD-Processes allow the deposition of insulating coatings such as alumina at reduced temperatures, which implies a lower substrate thermal load. At the same time using PVD-Technology it is possible to produce high compressive stresses in the coatings (high alternating thermal stress resistance), keep sharp cutting edges, synthesize pure structures and deposit various coating combinations such as multilayer TiAlX + γ -Al_2O_3. Therefore, a PVD-multilayer coating TiAlN + γ-Al_2O_3 is very promising for machining of Ni alloys such as Inconel 718. However, due to the very different characteristics of the nitrides and oxides, particularly the good coating cohesion of this coating concept constitutes a challenge. In this work different multilayer combinations were deposited on cemented carbide cutting inserts. In order to improve the adhesion between the nitride and oxide-layers and consequently the coating cohesion, a special gradation was developed. The reactive gas flows (N_2 and O_2) was gradually changed during the coating deposition. The result was a soft gradation consisting of different nitrides and oxides species synthesized in very thin layers. Tribological tests demonstrated that the developed soft gradation offers an important improvement of the coating cohesion and fatigue resistance. For the evaluation of the coating adhesion and cohesion calo-, Rockwell- and scratch-tests were carried out. Mechanical properties were investigated by impact tests.