ICMCTF2011 Session G4/E4: Coatings for Machining Advanced Materials and Advanced Manufacturing Methods

PM (powder-metallurgical) high speed steels are increasingly employed as tool materials for cold work applications due to their high abrasion resistance and toughness. Nevertheless, the machining of these materials is very challenging due to vanadium and iron carbides which lead to high abrasive wear on the cutting edges as well as the high toughness of these materials which leads to adhesive wear.

To meet these challenges nitride nanocomposite coatings seem to be promising. In the present work (Ti,Cr,Al,Si)N coatings were deposited on cemented carbides via MSIP (Magnetron Sputter Ion Plating) and investigated using XRD, SEM and scratch test. Different working points within the nitrogen hysteresis were chosen to optimize the coating’s morphology and mechanical properties.

The developed coating was deposited on ballnose end mills and tested in milling of S790PM, 62HRC (1.3345, M3 class 2) in comparison to two different commercial coatings: (Ti,Al)N deposited via MSIP and (Ti,Cr,Al,Si)N deposited via Arc Ion Plating. As tool life criterion a flank wear width of 100 µm was chosen. It could be shown that, depending on the cutting parameters, the tool life is increased by 20 - 30% by the new developed coating.

The increasing use of Titanium alloys in advanced civil and military aerospace manufacturing has created a demand for continuous improvement in the performance and reliability of round shank tooling. Evolutionary advances in tool design are achieved by optimising the substrate, macro and micro geometry, surface finish and through PVD coating with the latest generation high performance ceramic materials.

In this work Ti6Al4V titanium alloy plate was drilled with 6.5mm diameter carbide drills with conventional soluble oil coolant. Machining parameters were varied between 25-80m/min and feed rates of 0.1-0.25mm/rev. The drill performance was quantified by metal removal rate and compared to an un-coated tool. A force dynamometer was used to measure torque and thrust forces. Hole quality metrics were investigated using an infinite focus microscope (IFM) to measure hole surface roughness and burr height on entry and exit. Different edge treatments of 3-30 microns were applied to the tooling. The as-ground sharp drill lips were modified by a conventional chamfer, which was ground during tool manufacture and a blended radius was applied by nylon abrasive filament brushing (NAF) with a 500 grit SiC peripheral brush. The effect of PVD coating the drills was also investigated, both in the as deposited condition and after post-polishing by aluminium oxide NAF brushing. The torque and thrust data from the drilling tests was compared to a model derived by treating the drill cutting lip as an assembly of concentric single point tools. This practical model used data generated from orthogonal cutting tests using single edge carbide tooling with systematically varied rake, clearance and edge micro-geometries. IFM measurements of the edge micro-geometry were made to determine the detail of the shape factors created by the edge treatments. The effects of the edge offsets a,b and radii R on the cutting forces were investigated during the turning experiments. The results contribute to improved drill design for the optimisation of high performance hole-making in titanium components.

The machining of difficult-to-cut materials as austenitic steels is the focus of investigations for a long time now. One approach to overcome the problems when machining these materials is an appropiate coating system for the cutting tool. Especially the γ-phase of alumina is interesting for cutting operations due to its outstanding characteristics, such as high hot hardness, high thermal stability and low tendency to adhesion.

The deposition of alumina by using the Magnetron Sputter Ion Plating (MSIP)-PVD process faces different challenges: Deposition rates are low and insulating layers on the target surface can cause arcing. The development for deposition of oxides was first done by using radio frequency (r.f.) power supplies. This kind of process results in very low deposition rates, mostly amorphous structures of the coatings and expensive costs for ceramic targets and power supplies. During the last years, the deposition of oxides by the use of pulsed power supplies became more economically.

In the present work a (Ti,Al)N/γ-Al₂O₃-coating is deposited on cemented carbide in an industrial coating unit by means of MSIP. The (Ti,Al)N bond coat was employed to improve the adhesion of γ-Al₂O₃ on the substrate. The objective of this work is to study the wear mechanisms and the cutting performance of aluminium oxide based coated tools in turning, drilling and milling operations of austenitic steels, supplementary in combination with innovative environmental friendly lubricants. Based on the remarkable properties of this coating system the performance of the cutting tools is increasing significantly.

The use of high thermal conductivity copper alloys in plastic injection moulds provides the benefit of rapid moulding cycles through effective heat transfer. However copper alloys are relatively soft and wear rapidly so manufacturers are developing new copper alloys with increased hardness and wear resistance. This wear resistance can be further improved by the deposition of hard coatings such as electroplated chromium, electroless Nickel and PVD coatings. In this paper, the tribological performance of three proprietary high-strength Cu alloys (Ampcoloy 940, Ampcoloy 944 and Ampcoloy 83) coated with PVD CrN and CrAlN coatings has been evaluated. A medium phosphorous content electroless Ni-P (ENi-P) plated layer was also deposited as a pre-treatment to PVD CrN and CrAlN coatings to increase the load support. The effect of this intermediate ENi-P layer was also evaluated. Surface roughness and ultra-microindentation hardness measurements were used to characterise all coated systems in both plated (i.e., with the intermediate ENi-P coating) and standard (i.e. unplated) conditions. Scratch tests were also performed to evaluate the effect of the ENi-P on PVD coating adhesion to Cu alloy substrates. The tribological behaviour of PVD-coated Cu alloy systems was evaluated by pin-on-disc wear tests and ball-on-plate impact tests. Results demonstrate that the ENi-P layer improves the load support for PVD coatings on Cu alloys, thereby improving their tribological performance. However, for PVD-coated Cu alloys in the standard condition, the Cu alloy substrate type plays an important role in the tribological performance of PVD coatings. For instance, PVD CrN coatings were more suited to a certain Cu alloy type whilst CrAlN to the other two types.

In this work, silicon nitride thin films were deposited on Si(100) substrates by rf reactive magnetron sputtering from a Si target in a Ar/N₂ plasma at variable substrate temperatures. In order to reproduce the oxidation conditions of cutting tools in dry machining, the samples were oxidized in ¹⁸O atmosphere at temperatures of 500 and 1000^oC. Afterward, the thin films were characterized by glancing angle X-ray diffraction, X-ray reflectometry, Rutherford backscattering spectrometry, narrow nuclear resonant reaction profiling, nano-hardness and nano-scratch and friction measurements. Si₃N₄ stoichiometric films are obtained. Although the oxide layer thickness increases with temperature, silicon nitride thin films show a high corrosion resistance where the oxide layer thickness achieves a maximum value of 6-7 nm at an oxidation condition of 1 h and 1000^oC. Nanohardness increases with deposition temperature as well as the compound density. Before oxidation, the friction coefficient follows a random behavior. After oxidation at both 500 and 1000^oC temperatures, the friction coefficient follows a clear tendency where lower friction forces are measured when lower normal forces are applied. Such lower friction coefficients are attributed to a higher contribution of the oxide layer in the mechanical system. Finally, the thin oxide film obtained after dry machining at high temperature leads to improve the corrosion resistance and decrease the friction forces and, consequently and due to the maintenance of the hardness even at higher temperatures, the wear resistance is enhanced.