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The interface fatigue strength of coated tools with nano-crystalline diamond (NCD) coatings affects their milling performance. Depending on the developed cutting loads and temperatures, the repetitive impacts on the tool deteriorate progressively the coating-substrate interface strength, thus influencing the effective NCD film adhesion. Aiming at a comprehensive investigation of this effect, NCD coated cemented carbide inserts with improved or insufficient adhesion were manufactured. Hereupon, a parameter characterizing the coatings' interface adhesion strength, the shear failure stress (SFLS) was estimated by inclined impact tests and FEM calculations at ambient and elevated temperatures. This metric is significantly affected, among others, by the compressive residual stresses developed in the NCD film during cooling after its deposition and the coating operational temperature. A correlation between the determined SFLS at the investigated adhesion levels and the FEM calculated maximum shear stress in the coating interface during milling AA 7075T6 contributed to the explanation of the wear evolution on the employed NCD coated tools. Prime novelty statement The submitted paper aims at investigating the effect of the interface fatigue strength of NCD coated hardmetal inserts on their cutting performance in milling. The fatigue strength of the NCD coating-substrate interface is a dominant mechanical property for the NCD coated tool life. In the submitted work, NCD coated cemented carbide inserts with different adhesion qualities were manufactured. The interface fatigue strength at temperatures up to 400 °C was quantified using a critical shear failure stress (SFLS). This was attained by conducting appropriate FEM supported evaluation of the inclined impact test results at ambient and elevated temperatures. A correlation between the determined SFLS at various adhesion levels and the FEM calculated maximum shear stress in the film interface during milling AA 7075T6 contributed to the explanation of the wear evolution on the employed NCD coated tools. This is a new and original contribution to the field of testing NCD coatings and their effective application in cutting. The submitted paper is not being considered for publication elsewhere.

The effect of brittleness and fatigue of mono- and multi-layer PVD films on coated tools cutting performance is introduced. Cemented carbide inserts were coated to the same overall film thickness with various numbers of layers. Nanoindentations were conducted to evaluate the hardness of the diverse coating structures. The film brittleness and fatigue were characterized by nano- and macro-impact tests respectively. The coated inserts' wear behaviour was investigated in milling hardened steel. The attained results revealed the coatings' brittleness and fatigue endurance enhancement by increasing the number of film's layers. This increase leads simultaneously to the coated tool life improvement.

The effect of two different treatments of cemented carbide substrates, prior to the deposition of a nanocrystalline diamond (NCD) coating, on the film interface fatigue strength was investigated at ambient and elevated temperatures. The first substrate treatment of the cemented carbide substrate was a selective chemical Co-etching and the second one the deposition of a Cr-adhesive layer. Inclined impact tests at 25 °C and 300 °C were performed on the NCD coated specimens. The related imprints were evaluated by confocal microscopy measurements and EDX micro-analyses. The thermal residual stresses developed in the film structure at various temperatures were estimated by Finite Element Method (FEM) calculations. A fatigue damage in the NCD coating interface region was induced by the repetitive impacts. After this damage, the compressive residual stresses in the NCD film are released leading to its lifting from the substrate (bulge formation) and subsequent coating failure. The NCD film-substrate interface fatigue behavior is significantly affected by the test temperature. Based on the attained results at diverse substrate treatments, Woehler-like diagrams were developed for monitoring the fatigue failure of NCD coating interface area at 25 °C and 300 °C. The interfacial fatigue strength worsens as the impact test temperature grows in both examined substrate treatment cases. Moreover, Co-etched substrates compared to coated ones by an adhesive Cr-interlayer possess higher interfacial strength at ambient and elevated temperatures. These phenomena were investigated and related explanations are described in the paper.