Papken Eh. Hovsepian

Industrial scale manufactured superlattice hard PVD coatings

Abstract Superlattice hard PVD coatings exhibit high hardness values up to 60 GPa, wear resistance, and excellent protection against corrosion depending on the choice of the material partners involved. TiAlN/CrN, TiAlN/VN, TiAlYN/VN, TiAlN/ZrN, and CrN/NbN superlattice coatings with typical periods λ of 3–4 nm have been produced economically under production conditions in industrial sized coating equipment. Some of the coatings are characterised by high compressive stresses up to 10 GPa. In such cases the utilisation of the combined cathodic arc/unbalanced magnetron deposition method (arc bond sputtering technology) in combination with the introduction of a lower stressed monolithically grown base layer provides a sufficiently high bonding strength at the interface. The major deposition steps are: (1) pump down and preheating; (2) bombardment of the substrate surface with multi-ionised metal ions generated in a steered cathodic arc discharge; (3) deposition of a base layer (0·1–0·2 μm) using the unbalanced magnetron; (4) deposition of the superlattice coating with simultaneously operated unbalanced magnetrons or a combination of simultaneously operated unbalanced magnetrons and steered cathodic arc sources.

The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and growth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputtering (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerated 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The coating and defects were then examined in planar view and cross-section by scanning electron microscopic analyses and the results were compared with those prior to the electrochemical measurement. It is found that the overall coating/substrate corrosion process is closely related to the deleterious effect of the macroparticles and growth defects in the PVD coatings. It is further demonstrated that for through thickness macroparticle inclusions, corrosion initiates by galvanic or crevice corrosion between the defect and the coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate interface. On the basis of the experimental findings and the macroparticle formation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed

Chromium nitride/niobium nitride superlattice coatings deposited by combined cathodic-arc/unbalanced magnetron technique

CrN/NbN superlattice coatings have been developed as an attempt to replace electroplated chromium in some applications. The coatings have been deposited by a combined cathodic-arc/unbalanced magnetron technique in an industrial-size physical vapour deposition (PVD) coater. The investigations have been focused on the question of maximum hardness, adhesion and tribological performance of the coatings deposited at 400 degrees C as a function of the nitrogen content in the films. All CrN/NbN superlattice coatings produced exhibit a single-phase face-centred cubic structure and {200} preferred orientation. The superlattice period of the coatings varies in the range Delta = 3.4-7.4 nm depending on the N-2 flow rate. Under the sputtering conditions used, it was possible for stoichiometric CrN/NbN coatings to be deposited only in a narrow N-2 flow rate range of around 160 seem. Both stoichiometric and sub-stoichiometric coatings showed maximum hardness values of Hk = 3580 and Hk = 3600, respectively. CrN/NbN superlattice coatings outperformed electroplated chromium by factor of 13 in dry sliding conditions. However, both coatings show similar abrasive wear in the range of 0.63 mu m N-1. Stoichiometric CrN/NbN superlattice coatings possess high oxidation resistance in the range 820-850 degrees C

Corrosion performance of CrN/NbN superlattice coatings deposited by the combined cathodic arc unbalanced magnetron technique

Abstract Potentiodynamic polarisation experiments have been used to evaluate the corrosion performance of novel CrN/NbN superlattice coatings in comparison with a commercial CrN coating and a 304L control specimen. In addition, electrochemical impedance has been used to determine the effective surface area of the coatings by capacitance measurements. Results indicate that the superlattice coatings have significantly improved barrier properties evidenced by increases in pitting potentials of up to 500 mV compared with 304L. Furthermore, the impedance evidence indicates that CrN has comparatively large pores while the superlattice coatings have extensive, but diffuse and fine-scale, surface porosity confirming that the corrosion performance improvement is most likely due to microstructural enhancements.

Wear associated with growth defects in combined cathodic arc/unbalanced magnetron sputtered CrN/NbN superlattice coatings during erosion in alkaline slurry

The erosive wear associated with growth defects was studied in deaerated 0.5 M (Na2CO3-NaHCO3) buffer solutions containing 150-200 mum alumina particles on a rotating cylinder erosion-corrosion system for PVD CrN/NbN superlattice coatings grown by the are bond sputtering (ABS) process on mild steel BS6323. Corrosion was minimised by selective control of the samples potential, according to potentiodynamic polarisation, during the slurry erosive wear test. The morphology of the coatings, particularly of the defects, was examined in planar and cross-section views by means of scanning electron microscopy before and after the test. It is found that wear of the coating is typically preferential to the defects, progressing from the particle exterior top to the interior with erosion time, while the coating matrix (areas free of such defects) is largely intact, after 24 h prolonged exposure to erosion

ZrN coatings deposited by high power impulse magnetron sputtering and cathodic arc techniques

Zirconium nitride (ZrN) coatings were deposited on 1 μm finish high speed steel and 316L stainless steel test coupons. Cathodic Arc (CA) and High Power Impulse Magnetron Sputtering (HIPIMS) + Unbalanced Magnetron Sputtering (UBM) techniques were utilized to deposit coatings. CA plasmas are known to be rich in metal and gas ions of the depositing species as well as macroparticles (droplets) emitted from the arc sports. Combining HIPIMS technique with UBM in the same deposition process facilitated increased ion bombardment on the depositing species during coating growth maintaining high deposition rate. Prior to coating deposition, substrates were pretreated with Zr+ rich plasma, for both arc deposited and HIPIMS deposited coatings, which led to a very high scratch adhesion value (LC2) of 100 N. Characterization results revealed the overall thickness of the coatings in the range of 2.5 μm with hardness in the range of 30–40 GPa depending on the deposition technique. Cross-sectional transmission electron mic...

TiAlCN/VCN nanolayer coatings suitable for machining of Al and Ti alloys deposited by combined high power impulse magnetron sputtering/unbalanced magnetron sputtering

Abstract Nanoscale multilayer TiAlCN/VCN coatings with bilayer thickness Δ=2·2 nm were deposited by the combined high power impulse magnetron sputtering (HIPIMS)/unbalanced magnetron sputtering technology. The V+ HIPIMS etching produced atomically clean interface and V implantation resulting in enhanced coating adhesion LC=58 N. High resolution transmission electron microscopy and electron energy loss spectroscopy revealed that in TiAlCN/VCN, carbon forms a lateral phase between the nanolayers, producing low shear strength interfaces. This results in a well defined nanometre scale layer by layer wear mechanism, which is the key for prevention of tribofilm and consequently thick built up layer formation. In dry milling of Al 7010-T 7651 alloy, TiAlCN/VCN outperformed state of the art diamond like carbon, Cr/WC/a-CH coated and uncoated end mills by factors of 4 and 8 respectively. In turning of Ti alloys (Ti0·90Al0·06V0·04) the TiAlCN/VCN coated cemented carbide inserts produced two to three times more components (orthopaedic implants), as compared to uncoated tools.

Oxidation and fatigue behaviour of γ-TiAl coated with HIPIMS CrAlYN/CrN nanoscale multilayer coatings and EB-PVD thermal barrier coatings

Abstract CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl substrate material using high power impulse magnetron sputtering technology. The nitride coating provided effective oxidation protection to γ-TiAl at 850 °C for exposure time periods exceeding 2 000 cycles of 1 h dwell time at high temperature in air. High oxidation resistance was also observed at 900 °C. After exposure to air at 850 °C for 300 h, coated tension specimens exhibited a reduction in fatigue strength of about 70 MPa compared to the bare γ-TiAl alloy. Zirconia topcoats produced by electron-beam physical vapour deposition were well adherent to the CrAlYN/CrN coating with an oxy-nitride overcoat. When thermally cycled at 900 °C, the lifetime of this thermal barrier coating system on γ-TiAl exceeded 1 000 1 h cycles. Below cracks in the nitride coating, the substrate was oxidised and protrusions of an outer oxide scale with columnar structure formed.

Target poisoning during CrN deposition by mixed high power impulse magnetron sputtering and unbalanced magnetron sputtering technique

Target poisoning phenomenon in reactive sputtering is well-known and has been studied in depth over the years. There is a clear agreement that this effect has a strong link on the quality, composition, properties, and pronouncedly on the deposition rate of physical vapor deposition coatings. With the introduction of ionized physical vapor deposition techniques such as the relatively novel high power impulse magnetron sputtering (HIPIMS), which have highly ionized plasmas of the depositing species (metal and gas ions), target poisoning phenomenon is highly contested and thus has been left wide open for discussion. Particularly, there have been contradicting reports on the presence of prominent hysteresis curves for reactive sputtering by HIPIMS. More work is needed to understand it, which in turn will enable reader to simplify the coating deposition utilizing HIPIMS. This work focuses on the study of chromium (Cr) targets when operated reactively in argon + nitrogen atmosphere and in different ionizing con...

On the structure and composition of nanoscale TiAlN/VN multilayers

The chemical and physical structure of a TiAlN/VN multilayer, of average layer thickness 3.4 ± 0.4 nm, was characterized using a spherical aberration-corrected STEM, utilizing a nominal 0.1-nm beam, by HAADF and EELS. The interface between layers was shown to be rough, with local thickness variations evident in layer thickness. Chemical mixing between layers was identified, consistent with numerical modelling of the deposition flux and layer growth. The implications of the compositional modulation are discussed.