L. Cunha

Microstructure of CrN coatings produced by PVD techniques

Abstract The surface of machine parts and moulds in plastic transformation processes may be exposed to corrosive agents evaporated from the plastics at elevated temperatures and to the abrasive action of hard fibres in the case of reinforced plastics. Nitrides of transition metals can be wear resistant but also resistant to many chemically aggressive environments. The microstructure and mechanical properties of chromium nitride based coatings were studied in order to analyse the potential of this material to be used as a protective coating of the surface of injection moulding or extruding machines. Chromium nitride based hard materials were deposited by reactive magnetron sputtering onto stainless steel substrates in the form of homogeneous coatings. The coating microstructure and morphology were studied by X-ray diffraction (XRD), optical and atomic force microscopy (AFM). The mechanical properties of these coatings were studied by scratch and nano-indentation testing and residual stress measurements. The coatings can be strongly textured, if unbiased during deposition. Optical image analysis revealed the density and size distribution of defects. Atomic force microscopy was used to identify these defects as pinholes, particulates and grains growing with different growth direction. The coating residual stress depends, of course, on the strain within the growing grains and therefore on the deposition conditions. But the residual stress was also influenced by the coating morphology, namely the coating density. The Youngs moduli of CrN-PVD coatings depend on the measuring direction as well as on the coating morphology. Normal to the coating surface a Youngs modulus of about 400 GPa can be found. Parallel to the substrate surface, the E -modulus was very much affected by the existence of voids or loosely packed CrN and varied between 100 and 300 GPa. Some interesting features were found such as the case of open porosity being frequently found on coatings with higher density and lower effective elastic modulus and coatings with lower density and higher effective elastic modulus do not necessarily show open porosity.

Corrosion of CrN and TiAlN coatings in chloride-containing atmospheres

Abstract Chromium and titanium–aluminium nitrides were deposited, by physical vapour deposition techniques, on stainless steel substrates and their corrosion behaviour was studied in two different environments: a gaseous environment containing HCl at a temperature of 350°C and a 1 M HCl aqueous solution environment, at room temperature. X-ray photoelectron spectroscopy was used to study the mechanism of the reactions that occurred in the gaseous environment. This analysis shows an oxidation of the surface caused by the substitution of nitrogen by oxygen. Open circuit potential and potentiodynamic polarisation measurements were performed in the aqueous solution environment. The aqueous corrosion behaviour of the nitride coatings is strongly dependent on the microdefect density of the coating.

Structural, electrical, optical, and mechanical characterizations of decorative ZrOxNy thin films

The main objective of this work is the preparation of decorative zirconium oxynitride, ZrOxNy, thin films by dc reactive magnetron sputtering. Film properties were analyzed as a function of the reactive gas flow and were correlated with the observed structural changes. Measurements showed a systematic decrease in the deposition rate with the increase of the reactive gas flow and revealed three distinct modes: (i) a metallic mode, (ii) a transition mode (subdivided into three zones), and (iii) an oxide mode. The measurements of target potential were also consistent with these changes, revealing a systematic increase from 314to337V. Structural characterization uncovered different behaviors within each of the different zones, with a strong dependence of film texture on the oxygen content. These structural changes were also confirmed by resistivity measurements, whose values ranged from 250to400μΩcm for low gas flows and up to 106μΩcm for the highest flow rates. Color measurements in the films revealed a chan...

Characterisation of chromium nitride films produced by PVD techniques

Chromium nitride thin films have been deposited on stainless steel substrates by r.f. reactive magnetron sputtering. The influence of process parameters such as substrate bias and partial pressure of reactive gas have been investigated. The characterisation of the coatings was performed by X-ray diffraction (XRD), Raman Spectroscopy (RS) and nano-indentation experiments. These studies allow to analyse the influence of deposition parameters in crystal phases, crystal orientation/texture and crystallite size. The relationship between structural defects and their characteristics with deposition conditions will also be taken into account. The presence of oxygen on the coatings surface, due to atmospheric contamination, is analysed by means of Raman spectroscopy. This optical technique can be used for the characterisation of the surface oxides at different stages of oxidation. The changes observed in Raman spectra can be correlated with process parameters. Coatings produced with an unbiased substrate showed higher tendency to surface oxidation. Increasing the nitrogen partial pressure in the working atmosphere produces changes from a hexagonal Cr2N to cubic CrN microstructure. The strain in CrN crystals increases with nitrogen content in working atmosphere. When the Cr2N phase is dominant the hardness has a relative maximum (42.2 GPa), but the highest hardness was obtained for a coating with dominant CrN phase produced with highest nitrogen flow (44.9 GPa).

Corrosion of TiN, (TiAl)N and CrN hard coatings produced by magnetron sputtering

Abstract Metallic components like moulds, dies and machinery can be subjected to intensive degradation during plastic transformation processes, namely when working with fibre filler materials and plastics which release F, S or Cl during transformation. The degradation is attributed to the combined erosive and abrasive wear by the filler material and corrosive attack of agents. This degradation reduces the lifetime of the components considerably and has a direct impact on process productivity and surface finish of the final products. Nitride-based hard coatings like TiN, (TiAl)N, BN, etc. have proved their capability to increase tool lifetime when exposed to abrasive and corrosive environments found in plastic transformation processes (halogenated polymers, acrylics, polyesters, fibre reinforced plastics, etc.). Within the frame of this work we produced TiN, (TiAl)N, CrN hard coatings, with and without a metallic interlayer, by dc and rf reactive magnetron sputtering, with a thickness of about 2 μ m. The aqueous corrosion behaviour of the coatings was studied in saline and acidic environments by potentiodynamic and open circuit potential (OCP) measurements. The oxidation resistance during annealing in air was also studied. In saline (NaCl 9%) and acid (HCl 3.4%) environments we found that a metallic interlayer of Ti or Cr in the case of TiN–(TiAl)N-coated samples and CrN-coated samples, respectively, generally improve the corrosion resistance. Best results for all tested nitride coated samples were obtained for the Ti 0.27 Al 0.73 N coating. The OCP vs. Saturated Calomel Electrode (SCE) (60 min) measurements indicated that most samples were nobler than the un-coated substrate. The mentioned potentials depend on the deposition conditions and the film microstructure. Most of the coatings lose some of their protective capabilities after an high temperature annealing. In contrast to the Ti-based hard coatings, the corrosion resistance of CrN is improved by a 800°C annealing treatment in air.

Tuning of the surface plasmon resonance in TiO2/Au thin films grown by magnetron sputtering: The effect of thermal annealing

Nanocomposites consisting of a dielectric matrix, such as TiO2, with embedded noble metal nanoparticles (NPs) possess specific optical properties due to the surface plasmon resonance (SPR) effect, interesting for several applications. The aim of this work is to demonstrate that these properties are sensitive to the nanostructure of magnetron-sputtered TiO2/Au thin films, which can be tuned by annealing. We study the role of the shape and size distribution of the NPs, as well as the influence of the crystallinity and phase composition of the host matrix on the optical response of the films. All these characteristics can be modified by vacuum annealing treatments of the deposited films. A theoretical interpretation and modeling of the experimental results obtained is presented. The model involves a modified Maxwell-Garnett approach for the effective dielectric function of the composite (describing the SPR effect) and the transfer matrix formalism for multilayer optics. Input data are based on the experiment...

Influence of the chemical and electronic structure on the electrical behavior of zirconium oxynitride films

This work is devoted to the investigation of decorative zirconium oxynitride, ZrOxNy, films prepared by dc reactive magnetron sputtering, using a 17:3 nitrogen-to-oxygen-ratio gas mixture. The color of the films changed from metallic-like, very bright yellow pale, and golden yellow, for low gas mixture flows [from 0 to about 9SCCM (SCCM denotes cubic centimeter per minute at STP)] to red brownish for intermediate gas flows (values up to 12SCCM). Associated to this color change there is a significant decrease of brightness. With further increase of the reactive gas flow, the color of the samples changed from red brownish to dark blue (samples prepared with 13 and 14SCCM). The films deposited with gas flows above 14SCCM showed only apparent colorations due to interference effects. This change in optical behavior from opaque to transparent (characteristic of a transition from metallic to insulating-type materials), promoted by the change in gas flow values, revealed that significant changes were occurring in...

Performance of chromium nitride and titanium nitride coatings during plastic injection moulding

Monolithic coatings of chromium nitride, titanium nitride and multilayer titanium/chromium nitride coatings were produced by r.f. and d.c. reactive magnetron sputtering in order to determine their potential to be used as protective coatings for machinery parts of plastic injection moulding or extruding machines. The tribological and mechanical behaviour of these coatings were studied. Monolithic coatings showed lower wear rates, measured by pin-on-disc experiments, when compared with multilayer coatings. The oxidation of the surface was also lower in monolithic coatings. The performance of the coatings during plastic processing was tested in a dye fitted to an injection moulding machine and using glass reinforced thermoplastic. The wear rates of the nitride-based coatings during plastic processing was more than two orders of magnitude better then some traditional methods of protecting the surfaces such as hardening the steel by heat treatment, electrodepositing hard chromium or nitriding the steel surface. The physical vapour deposition coatings also showed higher corrosion protection during plastic processing tests.

Performance of chromium nitride based coatings under plastic processing conditions

Chromium nitride based coatings were produced in the form of monolithic and multilayer coatings, by DC and RF reactive magnetron sputtering. These coatings were deposited onto stainless steel and tool steel substrates. Chromium nitride coatings have proved to be wear and corrosion resistant. The combination of these characteristics was necessary to protect surfaces during plastic processing. In order to select the best coatings, some mechanical and tribological tests were performed. Hardness and Youngs modulus of the produced coatings were measured by nano-indentation, adhesion was assessed by scratch test experiments and wear was evaluated by performing pin-on-disc experiments. The behaviour of these coatings in chemically aggressive environments was studied by different corrosion experiments. The behaviour of these coatings in thermoplastic processing equipment was also studied. The abrasive and corrosive wear of chromium nitride based coatings was assessed under relatively high pressures and temperatures and was compared with the surface wear of hardened tool steel substrate by heat treatment, nitrided by chemical-heat treatment, or protected by hard chromium coating deposited in a galvanic way. Monolithic coatings were harder, and had higher Young modulus and adhesion when compared with multilayered coatings. Their wear resistance, measured by pin-on-disc test, was between two and three orders of magnitude higher, but multilayer coatings showed significantly better corrosion resistance. In general, monolithic and multilayer coatings have significantly better behaviour when compared with traditional processes of protecting functional surfaces for plastic transformation equipment. The measured wear of the best performance obtained by chromium nitride based coatings can be more than two orders of magnitude lower than nitrided surfaces or hard chromium coated samples.

ZrOxNy decorative thin films prepared by the reactive gas pulsing process

Zirconium oxynitride thin films were deposited by dc reactive magnetron sputtering. A zirconium metallic target was sputtered in an Ar + N2 + O2 atmosphere. Argon and nitrogen flow rates were maintained constant whereas oxygen was pulsed during the deposition, implementing the reactive gas pulsing process (RGPP). A constant pulsing period T = 3?s was used following an exponential periodic signal versus time. The introduction time of oxygen was systematically changed from 17% to 83% of the period. The RGPP allowed the synthesis of ZrOxNy films with tuneable metalloid concentrations adjusting the introduction time of the oxygen. Composition and structural variations associated with mechanical, optical and electrical properties exhibited a smooth transition, from metallic-like characteristics, typical of the fcc-ZrN phase, to semi-conducting behaviour corresponding to a mixture of orthorhombic-Zr3N4(O) and ?-Zr2ON2 crystalline phases.