Showing papers by "Ludvik Martinu published in 2010"

Corrosion performance and mechanical stability of 316L/DLC coating system: Role of interlayers

Abstract: In the present work, the corrosion performance and mechanical stability of diamond-like carbon (DLC) coatings were investigated in the context of their biomedical applications. DLC was prepared by radio-frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) onto medical grade 316L stainless steel. Interlayers of amorphous hydrogenated silicon-based materials such as a-Si, a-SiNx, a-SiCx, and a-SiCxNy, and a nitrided interlayer, were studied in order to optimize its adhesion strength. Potentiodynamic polarization tests were performed to evaluate the corrosion performance of the 316L/DLC coating system. Electrochemical impedance spectroscopy (EIS) was used to determine the stability of the coating system during long-term tests of exposure to a simulated body fluid solution. The evolution of EIS spectra was monitored during two years of immersion in Ringer's solution. In addition to providing the best adhesion, the a-SiNx interlayer was found to significantly improve the corrosion resistance of the DLC system since it is highly impervious to the liquid. This is demonstrated by a two-order of magnitude improvement in the corrosion current density compared to the DLC with the nitrided interlayer. The a-SiNx interlayer substantially enhances the mechanical stability of the DLC coating system in the simulated body fluid environment, indicated by a slight reduction (less than 20%) in the adhesion strength and fivefold increase in the charge transfer resistance after two years of immersion. Moreover, Si-doped DLC coatings show improved corrosion barrier properties, due to the formation of a passive silicon oxide film at the electrode/electrolyte interface.

Dynamics of reactive high-power impulse magnetron sputtering discharge studied by time- and space-resolved optical emission spectroscopy and fast imaging

Abstract: Time- and space-resolved optical emission spectroscopy and fast imaging were used for the investigation of the plasma dynamics of high-power impulse magnetron sputtering discharges. 200 μs pulses with a 50 Hz repetition frequency were applied to a Cr target in Ar, N2, and N2/Ar mixtures and in a pressure range from 0.7 to 2.66 Pa. The power density peaked at 2.2–6 kW cm−2. Evidence of dominating self-sputtering was found for all investigated conditions. Up to four different discharge phases within each pulse were identified: (i) the ignition phase, (ii) the high-current metal-dominated phase, (iii) the transient phase, and (iv) the low-current gas-dominated phase. The emission of working gas excited by fast electrons penetrating the space in-between the electrodes during the ignition phase spread far outwards from the target at a speed of 24 km s−1 in 1.3 Pa of Ar and at 7.5 km s−1 in 1.3 Pa of N2. The dense metal plasma created next to the target propagated in the reactor at a speed ranging from 0.7 to 3...

Plasma-Enhanced Chemical Vapor Deposition of Functional Coatings

Abstract: Publisher Summary Functional coating systems can be fabricated by different deposition techniques. Among the above processes, plasma-enhanced chemical vapour deposition (PECVD) has been employed industrially in microelectronics for several decades, and it has now penetrated into a large number of other sectors. Due to fundamental and applied research and the development of new instrumentation tools, recent advances in plasma processing, and in PECVD, have greatly increased the interest in PECVD for the fabrication of different coating systems. Its industrial use has been significantly broadened. Not only can PECVD provide materials with functional characteristics similar to those obtained by their PVD and nonvacuum counterparts, but the PECVD processes can frequently address numerous novel aspects of functional coating fabrication. This chapter reviews the advances in PECVD. Plasma-based technologies are increasingly used for the fabrication of thin films and coatings for numerous applications ranging from optics and optoelectronics to aerospace, automotive, biomedical, microelectronics, and others. The present understanding of plasma–surface interactions that are the cornerstone for tailoring the materials' functional characteristics, in particular their optical, mechanical, electrical, tribological, protective, and other properties are described based on knowledge of fundamental physical and chemical processes in the active plasma environments. The state of the art in PECVD by the description of the performance of different coating systems and thin film architectures suitable for industrial-scale applications is illustrated.

Corrosion and tribo-corrosion behavior of a-SiCx:H, a-SiNx:H and a-SiCxNy:H coatings on SS301 substrate

Abstract: Amorphous hydrogenated silicon carbide (a-SiC x :H), silicon nitride (a-SiN x :H) and silicon carbonitride (a-SiC x N y :H) coatings were prepared by plasma enhanced chemical vapor deposition (PECVD) and investigated as attractive candidates for corrosion and tribo-corrosion enhancement of stainless steel 301 (SS301). The deposition process has first been optimized in order to obtain the best mechanical properties. All coatings prepared under such optimized conditions exhibited significantly enhanced tribological behavior. They substantially improved the wear resistance of the SS301 substrate, while reducing the wear rate by a factor of ~ 47, 33 and 15, for a-SiC x :H, a-SiN x :H and a-SiC x N y :H coatings, respectively. Corrosion tests in 1% NaCl solution revealed that the coated samples possessed substantially better corrosion resistance than the bare substrate. Specifically, the a-SiC x :H coating exhibited the highest corrosion resistance, with a corrosion current ( i corr ) of 7.4 × 10 − 12 A/cm 2 , compared to 2.5 × 10 − 8 A/cm 2 for the bare SS301 substrate. It was observed that the a-SiC x N y :H coating performed very differently under dry and wet tribological conditions. We show that the combination of high wear and corrosion resistance for the a-SiC x :H coating results in its superior performance in tribo-corrosion test, making it a particularly attractive candidate for the applications in which wear and corrosion act simultaneously.

Structural, mechanical, tribological, and corrosion properties of a-SiC:H coatings prepared by PECVD

Abstract: Amorphous hydrogenated silicon-carbide (a-SiC:H) coatings appear very attractive due to their superior optical and mechanical properties and chemical inertness. In the present work a-SiC:H coatings were prepared by PECVD on stainless steel 301 (SS301) and Ti–6Al–4V (TiAlV) substrates at a temperature of 300 °C, using different SiH4/CH4 precursor ratios. We systematically studied such mechanical properties as hardness, reduced Young's modulus and elastic recovery using depth-sensing indentation, their tribological characteristics such as coefficient of friction and wear coefficient using the pin-on-disc method, as well as their corrosion and tribo-corrosion behaviors. a-SiC:H films (∼ 3 μm thick) prepared on SS301 and TiAlV at optimal deposition conditions exhibited a hardness of 23.5 GPa, reduced Young's modulus of 160 GPa, and elastic rebound of 73%. They showed a friction coefficient of ∼ 0.35, and a wear rate of 10 × 10−6 mm3/Nm. These values are low compared to ∼ 0.85 and ∼ 0.5, 240 × 10−6 mm3/Nm and 700 × 10−6 mm3/Nm for SS301 and TiAlV, respectively. The films exhibited a very high corrosion and tribo-corrosion resistance on both metallic substrates. The coating behavior is correlated with the microstructure and composition, determined by complementary characterization techniques including ERD, FTIR and Raman analyses.

Mechanical, tribological and erosion behaviour of super-elastic hard Ti–Si–C coatings prepared by PECVD

Abstract: Titaniun carbide (TiC) based coatings prepared by low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) are investigated as attractive candidates for wear resistance, and particularly for protection against solid particle erosion. In the present work, we incorporated silicon (Si) as an alloying element to TiC, to obtain ternary nanostructured Ti–Si–C films. The incorporation of Si in TiC resulted in significant microstructural, mechanical and tribological modifications. By controlling the Si content in the films, we observed a transition between films consisting of fine nano-sized TiC crystallites (nc-TiC) embedded in an amorphous C:H matrix (a-C:H) to a microstructure formed by nc-TiC encapsulated in a-SiC/a-C:H matrix. This allowed one to selectively control the main mechanical characteristics, namely the hardness ( H ), the Young's modulus ( E ), and the friction coefficient ( μ ), in the range of 14–32 GPa, 140–240 GPa, and 0.16–0.6, respectively. For films prepared under optimized conditions, high elastic strain to failure and high resistance to plastic deformation of the Ti–Si–C films, expressed by H/E and H 3 /E 2 ratios, resulted in an 8 fold increase of the erosion resistance at an impact angle of 90° compared to a bare steel substrate. Erosion resistance at 30° increased by a factor of 22 compared to bare substrate due to a simultaneous combination of high H and low μ . Taking into consideration the severe erosion test conditions and the Ti–Si–C film thickness of less than 5 μm in this work, further improvement is expected for thicker films.

Corrosion and mechanical properties of duplex-treated 301 stainless steel

Abstract: Plasma nitriding is a widely used technique for increasing the surface hardness of stainless steels, and consequently, for improving their tribological properties. It is also used to create an interface between soft stainless steel substrates and hard coatings to improve adhesion. This paper reports on the mechanical and corrosion properties of AISI301 stainless steel (SS) after a duplex treatment consisting of plasma nitriding followed by deposition of Cr bond coat and CrSiN top layer by magnetron sputtering. Mechanical properties of the deposited films, such as hardness ( H ) and reduced Young's modulus ( E r ), were measured using depth-sensing indentation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to evaluate resistance to localized and to general corrosion, respectively. The corrosion behavior has been correlated with the microstructure and composition of the surface layers, determined by complementary characterization techniques, including XRD, SEM, and EDS. The CrSiN layers exhibited an H value of 24 GPa, whereas the nitrided layer was shown to present a gradual increase of H from 5 GPa (in the nitrogen-free SS matrix) to almost 14 GPa at the surface. The electrochemical measurements showed that the nitriding temperature is a critical parameter for defining the corrosion properties of the duplex-treated SS. At a relatively high temperature (723 K), the nitrided layer exhibited poor corrosion resistance due to the precipitation of chromium nitride compounds and the depletion of Cr in the iron matrix. This, in turn, leads to poor corrosion performance of the duplex-treated SS since pores and defects in the CrSiN film were potential sites for pitting. At relatively low nitriding temperature (573 K), the nitrided interface exhibited excellent corrosion resistance due to the formation of a compound-free diffusion layer. This is found to favor passivation of the material at the electrode/electrolyte interface of the duplex-treated SS.

Atom-by-atom simulations of chemical vapor deposition of nanoporous hydrogenated silicon nitride

Abstract: Amorphous hydrogenated silicon nitride (SiNH) materials prepared by plasma-enhanced chemical vapor deposition (PECVD) are of high interest because of their suitability for diverse applications including optical coatings, gas/vapor permeation barriers, corrosion resistant, and protective coatings and numerous others. In addition, they are very suitable for structurally graded systems such as those with a graded refractive index. In parallel, modeling the PECVD process of SiN(H) of an a priori given SiN(H) ratio by atomistic calculations represents a challenge due to: (1) different (and far from constant) sticking coefficients of individual elements, and (2) expected formation of N2 (and H2) gas molecules. In the present work, we report molecular-dynamics simulations of particle-by-particle deposition process of SiNH films from SiHx and N radicals. We observe formation of a mixed zone (damaged layer) in the initial stages of film growth, and (under certain conditions) formation of nanopores in the film bulk...

Time- and Species-Resolved Plasma Imaging as a New Diagnostic Approach for HiPIMS Discharge Characterization

Abstract: We present a novel approach in fast imaging of high-power impulse magnetron sputtering (HiPIMS) discharges in which bandpass optical interference filters are used to isolate the optical emission signal originating from different species populating the plasma. In this paper, we describe the methodology of the proposed diagnostics and discuss its application. In particular, we demonstrate the use of this technique for the time-resolved analysis of HiPIMS discharges operated with a chromium cathode in argon at 4 Pa. Two optical filters were designed and fabricated: (i) one for neutral chromium emission lines (400-540 nm); and (ii) the other one for neutral working gas emission lines and bands (above 750 nm). The introduction of such filters is used to distinguish different phases of the discharge and to reveal numerous plasma effects including background gas excitations during the discharge ignition, gas shock waves, and expansion of metal-rich plasmas.

From Passive to Active: Future Optical Security Devices

Abstract: Nowadays, passive devices offer good but limited anti-counterfeiting protection. We demonstrate design and fabrication of new color shifting security devices based on implementing an active electrochromic material, thus offering new optical features and enhanced protection.