ICMCTF2001 Session G4-1: Hard and Hybrid Coatings for Cutting and Forming Tools, and Surface Engineered Components

Wear and failure modes of multi-point cutting tools, broaches, bandsaws, and circular saws, can be different to single point tools owing to the mechanics of chip formation and the restriction of chip flow due to the gullet. Full product testing, and evaluating the cutting tool prior to and after surface treatment, is time consuming and expensive. Therefore there is a need for a time-compression technique for testing coatings applied to multi point cutting tools.

The paper describes a test method and results obtained from a single tooth test suitable for evaluating surface treated single tooth cutting edge samples. @paragraph)The result should be of immense value to both coating centres and product manufactures.

Conventionally grown columnar PVD hard coatings usually fail to protect both mild and stainless steels against corrosive attack in chloride media. The main reason for this is the formation of voids and pinholes along the grain boundaries of typically columnar grains in the coating, which produces through-holes and therefore electrochemical dissolution of the substrate material. Extensive experiments with multilayered structured PVD coatings have shown that PVD coatings may protect mild steel against corrosion in 3 % NaCl and against corrosion in salt spray, exhibiting in the potentiodynamic polarisation tests corrosion currents Ic < 5 x 10 -7 A cm -2 at a potential of 800 mV and in the salt spray tests a successful exposure time of 300 hours compared with 200 hours for electroplated 25 µm thick hard chromium layers.

The strict realisation of a multilayered architecture of the coating is paramount to achieve this result utilising the combined steered arc/unbalanced magnetron deposition process. The coating process is initiated by a low energetic ion implantation process employing (EI = 2.4 keV) Nb3 + ions which forms a dense amorphised 5-8 nm thick Nb film on top of the substrate surface. The coating process is continued by depositing a 1-3 µm thick metallic Nb layer. The coating process is completed by the deposition of a 4.5 µm multilayer system consisting of a 0.3 µm CrN base layer and a CrN/NbN superlattice exhibiting a period ? = 3.2 nm. Both the metallic Nb and the superlattice structured CrN/NbN layers have been deposited by unbalanced magnetron sputtering in an industrial sized multitarget PVD coating unit.

XRD analysis using both glancing angle and Bragg-Brentano geometry revealed a high compressive stress of –6.5 GPa and a {200} texture in the CrN/NbN multilayer being typical for direct deposition on steel substrate and for non-coherent growth of the CrN/NbN superlattice resulting from continued re-nucleation during film deposition. The coating was found to be extremely smooth (Ra = 0.033 µm) indicating a small amount growth defects. Finally XTEM analysis of the coating showed dense microstructures in all three regions consisting of a Nb, CrN and CrN/NbN superlattice coating.