ICMCTF2015 Session B1-1: PVD Coatings and Technologies

Owing to high mixing entropies and sluggish diffusion, simple solid-solution and nanocomposite structures are typically formed in high-entropy materials (HEMs) with the equimolar incorporations of multi-principal components. The HEMs present many extraordinary properties including a good mechanical performance, a high thermal stability and an extreme interdiffusion resistance, and are promising for applications to diffusion barriers and hard coatings. Herein, high-entropy alloys (HEAs; unitary Ti to senary AlCrRuTaTiZr with different numbers of metallic elements) and their nitrides (HEANs; (AlCrTaTiZr)N_x, (AlCrTaTiZr)N_xC_y and (AlCrTaTiZr)N_xSi_z) as examples are presented. With more elements, the failure temperature of the HEAs increases from 550 to 900°C, and the activation energy of interdiffusion increases from 110 to 163 kJ/mole. The hardness (H), modulus (E), H/E and H³/E² of the HEANs reach 35 GPa, 270 GPa, > 0.12, and > 0.4 GPa respectively; in deformed regions, the reversible activities of stacking faults are observed, suggesting suppressed dislocation activities and a high resistance to plastic deformation. Mechanistic analyses reveal that severe lattice distortions, a strengthened cohesion, a high packing density, and a nanocomposite structure increase the activation energy of atom motion by 55 kJ/mole and are believed to suppress the interdiffusion kinetics and dislocation-mediated plastic deformation of HEMs.

Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), and resistivity measurements combined with high-resolution cross-sectional transmission electron microscopy (HR-XTEM), and ab-initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T_c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a non-centrosymmetric tetragonal structure. Upon cooling below T_c, reflections intensify following the scaling behavior I α (Τ_c - T)^1/2. Resistivity ρ(T) measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal VN phases.

The VN transport Eliashberg spectral function α²_trF(ℏ ω), the product of the phonon density-of-states F(ℏ ω) and the transport electron/phonon coupling strength α²_tr(ℏ ω), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T > T_c. Free energy contributions due to vibrational entropy, often-neglected in theoretical modeling, are essential in understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

Software for modeling the thickness distribution and microstructure of an Electron Beam Physical Vapor Deposition (EBPVD) process for ceramic thermal barrier coating on complex 3D parts has been developed. To model the coating thickness we use a first order approximation algorithm to calculate the intensity of the vapor as a function of the distance and relative source substrate deposition angle. The model allows complex substrate manipulation over two vapor sources. 3D part models are imported as .STL or other file formats by means of a simple Excel based user interface. The software then calculates the intensity of the vapor from single or multiple point emitters. To validate the basic model parameters an experimental program has been executed. The global cloud intensity can be simulated by a COS(Theta)^N power distribution where Theta is the cone angle from the center line of the emitter and N=2.5. This value yields satisfactory agreement of experiment and model. The model also accurately predicts partial shadowing on a rotating substrate such as e.g. observed with trailing edge masks. The microstructure in selected areas of the parts is modeled with an embedded atom model (EAM) molecular dynamics module. The microstructure for extreme substrate geometries is compared to the actual microstructure obtained by metallographic evaluation. The model for thickness and microstructure is in good agreement with the actual coating properties.

Energetic deposition using cathodic (vacuum) arc plasmas is widely used to produce dense coatings for various applications. The most likely energy of ions is high, namely in the range from about 20 eV (carbon) to about 200 eV (tungsten), and somewhat less when process gas used. The origin of high ion energies has been the subject of research and debate for many decades. Early on, in the 1960s, a potential hump in front of the cathode was suggested to be responsible; however, this hypothesis was increasingly dismissed as all ions, regardless of charge state, seem to have the same velocity.

Research on the ion charge state distributions indicated that charge exchange collisions have an important role for the charge of ions that arrive at a substrate or that are observed at a detector. In this contribution, the role of charge exchange collisions for ion velocities or energies is explored. It is shown that charge exchange collisions allow us to revive the potential hump hypothesis as such collisions remove the apparent conflict between electric field and gas-dynamic acceleration mechanisms.

A great variety of coating materials can be obtained by methods of physical vapor deposition (PVD). Nearly all inorganic coating materials composed of metals, alloys and compounds can be realized. For instance high-quality oxide or nitride coatings can be produced by pulsed magnetron sputtering or by plasma-activated electron beam evaporation. The paper gives an introduction to the latest developments in relevant PVD technologies and presents some results for decorative purposes on metal strips.

Interesting decorative effects were obtained by “Surface colored coatings” like nitride or carbide layers. Such compounds have a high hardness and a characteristic color. Gold-colored and well-adherent titanium nitride coatings with a micro-hardness up to 30 GPa could be deposited onto stainless steel strip by plasma-activated electron beam evaporation. Zirconium - Aluminum nitride layers were deposited by pulsed magnetron sputtering. The color of the layers can be adjusted by the composition of the compound in a wide range.

“Interference colored coatings” can be made by thin transparent, mostly oxide layers like Silica, Zirconia or Titania. The color of the coatings can be adjusted by the layer thickness in a wide range. Pulsed magnetron sputtering is the most common deposition process. Nevertheless Titanium dioxide thin films were deposited on stainless steel strips by high-rate plasma-activated electron beam evaporation. The process is based on generation of titanium vapor and chemical reaction in a pure low-pressure oxygen atmosphere. Refractive index of titanium dioxide coatings was measured in the range of 2.3 to 2.5. This relatively high refractive index causes strong color effects based on thin film interference.

It could be shown that scratch resistance of the coated steel surface was remarkably increased by deposition of thin fused quartz layers. These transparent coatings were prepared by plasma-activated evaporation of Silica. A further possibility to enhance the resistance against environmental impact is the final coating with organic and transparent materials. High hardness can be obtained by electron beam curing of the lacquer. This technology is useful for anti-fingerprint behavior too. On the other hand, by applying the organic coating prior to PVD coating its smoothing effect for the metal strip can be used and therewith an enhancement of the decorative function can be reached.

One of the most perspective development directions of surface engineering is related to hybrid technologies, which best fulfill the expectations of the industry concerning the obtainment of adequate properties of the surface of tools or machine components.

Duplex treatment is combined thermo-chemical treatment as Plasma Nitriding of the tool or components followed by:

- PVD hard coatings deposition

- PECVD low friction coating

The nitrided layer increases the surface hardness and substrate resistance to plastic deformation and in the near-surface zone.

The presentation is illustrated by few typical industrial applications.

Since few years the Duplex treatment has proven successful in improving wear and fatigue resistance. This combination has been introduced successfully since few years for forming dies, aluminum dies casting, and forging dies.

The Duplex treatment is made by adding devices in a PVD equipment to make the nitriding in a coating equipment.

Now to introduce PLASMA NITRIDING +DLC means a Duplex /PECVD equipment we modified our plasma nitriding equipment to add special power supply to make the PECVD. The plasma nitriding process is followed by a low friction coating DLC by PeCVD technology. We get better adhesion that the best Metal / DLC , we have developed 2 different types of DLC one for wear and friction resistance and one for corrosion resistance.

This technology offers many advantages:

- very low running costs compare to the hybrid technology PVD/PECVD

- Possibility to work on finished parts , precision parts which are carburized or full quenched require grinding after carburizing before DLC

Multilayers TiN/AlN, TiN/TaN and TiN/NbN were deposited on silicon and steel substrates by dc magnetron sputtering. Structure and stoichiometry were obtained by X-ray diffraction and Auger Electron Spectroscopy, respectively. Vickers indentations with 5, 10 and 50 gf were observed by Scanning Electron Microscopy. In all cases Vickers Hardness was higher than 50 GPa. Abrasion wear was determined by the crater test. Corrosion resistance was determined by polarization curves in an electrolitycal cell with Ag/AgCl as reference electrode in a 0.01 solution of NaCl and 0.1 M of Na₂SO₄.

Metal containing a-C:H, where metal = Cr, Ti and Ta, were deposited by rf magnetron sputtering on glass, as well as, metal-nitride-coated steel substrates. Coatings were analyzed by Raman Spectroscopy, Auger Electron Spectroscopy and X-ray diffraction. Vickers micro hardness and abrasion wear were measured on coatings deposited on steel. Optical Transmittance and Reflectance in the range 300-2500 nm were carried out on samples deposited on glass.