Amar Mahiout

Identification of factors affecting the aqueous corrosion properties of (Ti, Al) N-coated steel

Abstract Carbides, borides and nitrides of transition metals are increasingly used as coating materials because of their high hardnesses and excellent wear resistances. However, these coatings can also potentially be used in applications requiring corrosion resistance because of their inherent stability under a variety of aqueous conditions. Thin film coatings based on these materials are generally produced by physical vapour deposition (PVD) methods. Such coatings often exhibit porosity which usually is thought to result from pinholes formed during deposition. If local defects form a direct path between the corrosive environment and the base material, the corrosion protection is lost. In this work the influence of different factors on the corrosion resistance of PVD (Ti, Al) N coatings on type ASP 23 high speed steel was studied. The factors evaluated were the thickness of the coating, the thickness and the type of the intermediate layer, the titanium-to-aluminium ratio of the coating and the number of heterogeneities in the coating. Electrochemical porosity measurements which are essential for the estimation of the corrosion resistance of coated components were made in 1 N sulphuric acid solution at ambient temperature. In these conditions, corrosion of a coated steel part occurs mainly in the base material. The corrosion resistance was also evaluated with immersion tests. The results of this work suggest the dominating role of the inhomogeneities on the corrosion resistance of the PVD (Ti, Al) N and TiN coatings. Preliminary analysis of the nature of the inhomogeneities is made and their possible source is discussed.

Porosity of thin diamond-like carbon films deposited by an arc discharge method

Abstract In an arc-discharge method, diamond-like carbon (DLC) films are deposited by condensation of highly ionized carbon plasma on a substrate. These films, which are virtually hydrogen free, are best characterized as amorphous diamond. As reported previously, a low friction coefficient and a wear coefficient an order of a magnitude lower that that of silicon carbide has been determined in dry sliding contact with hardened carbon steel. These films are also presumably inherently highly corrosion resistant. However, utilization of DLC films in corrosive evironments depends on the degree of porosity. In this work we have determined the porosity of films deposited either with or without enhanced particle filtering, which was accomplished by hindrance plates inside the curved-magnetic-field solenoid. As substrates, discs of plain carbon steel with a diameter of 100 mm were employed. The samples were exposed to a neutral salt spray and the pores were determined using a Ferroxyl test and electrochemical measurements. The microstructural features and defects identified in corrosion and porosity tests were studied in detail using metallographic methods and scanning electron microscopy. On the basis of the results, the possibilities for using DLC films in corrosive environments are discussed.

Arc discharge deposition of stainless steel coatings at different nitrogen pressures

A filtered arc discharge process was employed to deposit stainless steel films using an AISI316 cathode. In this procedure, macroparticles and droplets, which are the most serious drawback of arc deposition processes especially in corrosion applications, are mostly filtered out. Films were deposited in vacuum or in the presence of a nitrogen plasma at different partial pressures. Low carbon steel and silicon single crystals were employed as substrates. Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and X-ray diffraction (XRD) were used to characterize the films. The corrosion properties were examined using electrochemical polarization measurements. The corrosion current density was clearly lower than that of bulk steel, but higher than that of bulk AISI316. Increasing the film thickness and nitrogen content lowered the corrosion current density.

Corrosion resistance of N-, Cr- or Cr + N-implanted AISI 420 stainless steel

Abstract AISI 420 stainless steel samples with a diameter of 25 mm were implanted with nitrogen, with chromium or with chromium and nitrogen. The implantation energy and nominal fluence for Cr were 230 keV and 2 × 10 17 ions cm −2 , and those for N were 50 keV and 2.5 × 10 17 ions cm −2 respectively. In nitrogen implantations, 15 N isotopes were used in order to perform a depth profiling utilizing a resonance of the 15 N(p, αγ ) 12 C reaction. For all three samples and an unimplanted reference sample, anodic polarization diagrams were determined in 1 N H 2 SO 4 solution. The unimplanted sample showed an active peak and a passivation behavior. Chromium implantation decreased the peak current density, but both in the case of the chromium implanted sample and the nitrogen implanted sample, passivation characteristics were unchanged. In the case of the sample implanted with chromium and nitrogen the measured anodic polarization curve exhibited a totally different behavior. The active peak had almost completely disappeared (a reduction of four orders of the magnitude in current density compared with that of the unimplanted sample) and the passivation current density was also greatly decreased. Secondary-ion mass spectroscopy was utilized in analyzing surface films, which revealed a complex structure of a passive layer in the case of the sample implanted with both chromium and nitrogen.