ICMCTF2013 Session G4-2+E: Coatings for Machining Advanced Materials and for use in Advanced Manufacturing Methods

PVD, CVD, and TD coatings have been successfully used for years to enhance the productivity of many forming tools. The wide variety of products available in the marketplace have enabled users to select an option based upon its own preferences and objectives because virtually all the available products provided enhanced performance when used in applications forming low carbon steel materials.

However the rapidly increasing trend of increased use advanced high strength steel materials (AHSS), has resulted in greater challenges for coatings to produce acceptable levels of productivity in these new, demanding applications. These AHSS materials, with common tensile strength levels of 780 MPa to 980 MPa and soon to be 1180 MPa, produce forming conditions containing high mechanical fatigue forces, severe work hardening conditions, and high levels of friction between the workpiece and the coated tool surface. Some AHSS alloys are hot formed and quenched to produce the desired mechanical properties, and these applications present thermal challenges for potential coating solutions along with chemical reactivity with an aluminum coating on the strip aimed protecting the steel from oxidation.

This paper will provide an overview of the challenges presented for potential coating solutions by the increasing use of AHSS materials, both by cold forming and hot stamping. Guidelines will be provided regarding the characteristics of potential coating solutions require in order to successfully perform in AHSS forming applications. Common existing coating solutions will be compared in terms of these key characteristics. Further, recommendations will be made for what would make the optimum coating solution. Case study results will also be provided.

A wide range of industrial applications, current and anticipated, drive the process of miniaturization of thermal and chemical devices. In a range of applications, metal-based microsystems enjoy advantages of performance, cost, or both, over silicon-based counterparts. Examples include microchannel heat exchangers, miniature gas chromatograph sensors, and others. The critical bottleneck restricting the use of metal-based microsystems in actual applications has often been the lack of effective and economical manufacturing methods for metal-based microparts. In this talk, our activities in manufacturing of metal-based microparts through several microscale replication strategies, such as compression molding, roll molding, etc., will be summarized. Our efforts in engineering surfaces of fabrication tools through conformal coating deposition will be illustrated. Our recent attempts to test for coating/substrate interfacial failures will be described.

The presence of microscopic roughness in surface and cutting edge may lead to deterioration in the sharpness and durability of commercial cutting tools. Thin film metallic glasses (TFMGs) have been reported to have some unique properties such as smooth surface. In this work, a novel thin film metallic glass was deposited on stainless steels cutting tool for sharpness improvements. For comparison studies, conventional hard coatings including diamond-like carbon and metal nitride coatings were also prepared. These coated cutting tools were analyzed with various material characterizations and evaluated by the blade sharpness index test. The characterization results of sharpness and durability of these coated cutting tools will be discussed in present talk.

The hardness of protective tool coatings is an important factor for high speed metal machining during which the temperature may exceed 1000 ºC at the cutting edge. Coatings that can retain their hardness at elevated temperatures is therefore of high interest. Ti-Al-N coatings exhibit a hardness increase at ~900 ºC due to spinodal decomposition into coherent nanometer-sized cubic (c)-AlN and c-TiN domains. We have shown that by adding Cr to this material system metastable monolithic c-Ti_0.31Cr_0.07Al_0.62N coatings exhibit an age hardening process between 850 and 1000 ºC due to spinodal decomposition into coherent Ti- and Al-rich c-Ti-Cr-Al-N domains. At higher temperatures the domain size increases to around 20-40 nm where Cr relocates to Ti-rich domains whereupon the Al-enriched c-Ti-Cr-Al-N domains become Cr-depleted and consequently c-Al-N transforms into hexagonal (h)-Al-N. The hardness decrease associated with this transformation was less pronounced compared to ternary Ti-Al-N. The reason is a lowered driving force for relaxation of the coherent domain boundaries allowing the coatings to stay in a semi-coherent stressed state at higher temperatures. Another successful measure to enhance the hardness at elevated temperatures of Ti-Al-N has been reported through a Ti-Al-N/Ti-N multilayer growth concept where the coating architecture affects the decomposition. Our study is based on a combination of these two approaches. Theoretical results based on first principle calculations of the free energy in c-Ti-Cr-Al-N is coupled to experimental results obtained with nanoindentation, X-ray diffraction, analytical transmission electron microscopy and differential scanning calorimetry.

Reactive Plasma Deposition (RPD) is a new method which enables to coat various types of thin films such as metal oxides as functional coatings and nitrides as hard coatings without metal particles. Furthermore, RPD system compared to conventional sputtering or cathodic arc technique is expected to have highly ionized and excited plasma. The kinetic energies of the atoms, radicals and ions in chamber and the ionization ratio in the plasma are expected to change as a function of the output power of plasma gun, and the control of these factors is important to grasp the characteristics of the deposition of the films.

To investigate the characteristics of the deposition of the films by RPD method and their dependence on the output power of the plasma gun, we deposited titanium nitride (TiN) thin films onto cemented carbides by changing the output power and target-substrate distance (TSD). The crystallographic properties were characterized by X-ray diffraction pattern (XRD) and the cross-sectional morphologies were observed by Scanning Electron Microscope (SEM).

Each TiN film showed NaCl-type structure in several deposition rates. However, compressive stresses in the films were increased and (111) peak of XRD patterns were broadened by increasing the output power of plasma gun. The cross-sectional morphologies changed from columnar structure to granular as the output power increased. The ratio of ion current per deposition rate at each TSD decreased as the output power increased.

Even with the same deposition rate without bias voltage, the film characteristics changed into granular structure with lower crystallinity and compressive stresses in the films were increased. Energetic condition of the plasma was shifted in higher level as drawn in the structure zone diagram reported by A. Anders^[1]. In RPD method, the increase of the output power of plasma gun led to the increase of joule flux into anode-crucible from the cathode. The kinetic energies of evaporated atoms, radicals and ions became higher by thermal excitation. From the result of the measurement of the ion current, the ionization ratio was decreased as the output power increased.

In RPD method, by increasing the output power, the structures of the films became granular with lower crystallinity and the compressive stresses in the films were increased. The kinetic energies of the evaporated atoms, radicals and ions were one of the main factors of the characteristics changes. The ionization ratio, as another important factor, was decreased conversely.