ICMCTF2013 Session G5-1: Coatings, Pre-Treatment, Post-Treatment, and Duplex Technology

Majority of failures in engineering materials such as fatigue fracture, fretting fatigue, wear and corrosion, are very sensitive to the structure and properties of the material surface, and in most cases failures originate from the exterior layers of the work piece. Therefore, it would be considerably effective to apply some technological process to enhance the material properties on the surface of the part.

Gas nitriding is a case hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature (below Ac1, for ferritic steel) in contact with nitrogenous gas, usually ammonia. Temperature and time are two important nitriding processing parameters. The possibility of nitriding duration reduction by performing prior sever plastic deformation can be of a great interest both in technological and economical points of view.

In this study shot peening is applied with particularly severe parameters to generate ultra-fine grains on the surface of a low alloy steel. Three different batches of specimens are prepared. The first is the as-received specimens. Nitriding is performed for the second series. The last series is exposed to severe shot opening and then nitriding at reduced time. The treated specimens are characterized by optical and SEM micro-structural observation, micro hardness, surface roughness and XRD measurement of residual stress. The fatigue limit of treated specimens was experimentally determined and compared with the as-received specimens. Based on the result of this study, a critical discussion on the possibility of nitriding duration reduction by performing prior severe plastic deformation is presented.

Cobalt-based alloys have been widely used in implant components due to their corrosion resistance and wear properties. Two types of cobalt-based alloys are the most common implant materials (the cast and wrought alloys denoted as ASTM F-75 and F-1537, respectively). Once implanted and exposed to the aggressive body environment, CoCrMo alloys tend to corrode over time, releasing Co, Cr and Mo ions into body fluids by electrochemical corrosion or chemical dissolution. Over time the level of metal ions may become clinically significant. In addition, the mechanical properties at the surface of the cobalt-based alloy can be affected by the presence of wear, corrosion fatigue and fretting corrosion.

One alternative to increase the chemical and mechanical properties at the surface of the cobalt alloys is the boriding process. A combination of high corrosion resistance, wear resistance, and high hardness is the basis for the application of boride layers in CoCrMo alloy.

In this study, new data about the growth kinetics and mechanical properties of cobalt boride layers formed at the surface of the ASTM F-75 biomedical alloy have been estimated. The formation of CoB/Co₂B layers was carried out by the powder-pack boriding process at temperatures of 1223-1273 K with different exposure times for each temperature.

A diffusion model evaluated the evolution of the boride layers at the surface of the biomedical alloy, where the boron diffusion coefficients were estimated as a function of the boron concentration limits in each layer, the boride incubation time and the parabolic growth constants. In addition, an expression to estimate the cobalt boride layer thickness was developed, and the calculated results were compared with the experimental data obtained for the CoCrMo borided alloy exposed to 1323 K with 6,8 and 10 h of treatment. The estimation of the theoretical values of the CoB and Co₂B layer thicknesses showed good agreement with the experimental data over the range of exposure times.

Finally, the mechanical characterization of cobalt boride layers was evaluated by Berkovich nanoindentation technique using a constant indentation load of 50 mN. The indentations were performed along the depth of boride layers for the set of experimental conditions of the boriding process. The results showed that the ratio between the hardness and the Young´s modulus could be used to explain the wear behavior of the boride layers. Furthermore, the hardness dissipation parameter (HDP) of the CoB/Co₂B layers was evaluated, whose values above 0.5, denoted the ability of the surface to dissipate a significant part of the energy of mechanical deformation under adhesive wear conditions.

The system studied in a first program of research introduced a strong character of originality in the context. Beside the multilayer coatings and the duplex processes, a new class of systems process /product seems to be able to show with success on scientific/technical panorama, thanks to preliminary results that are quite encouraging. Such class of systems consist in the combination in succession of thick coatings obtained by means of thermal-spraying and of thin films deposited through PVD. If in a preliminary work it has been developed a specific combination (Reactive Plasma Spray-PVD), this does not exhaust the potentialities of such typologies of approach; in principle it would be possible combining all the technological variants of thermal-spraying (Plasma-Spray, Arc-Spray, HVOF, PTA) and of the PVD (arc, sputtering, EB), for the planning and the realization of surface systems, with graded mechanical characteristics.

In this context, first experiments combining HVOF and Arc-PVD, were carried out relating piston pins for sport automotive. The objective is the realization of a multilayered coating resistant to the concentrated loads, with a surface roughness very low.

The Surface Engineering, especially in the field of Aerospace, at present is characterized by important efforts in terms of R&D. These works can be considered a original approach to these applications where the conventional coatings/treatments show their limits.

Boriding considerably enhances the corrosion-erosion resistance of ferrous materials in non-oxidising dilute acids and alkali media. Different attempts to estimate the corrosion resistance of borided steels were performed using alternative techniques instead of immersion corrosion testing. One of the principal tests used is the electrochemical test or one of its variants, according to the procedure method.

In this study, the corrosion resistance of boride layers formed in the AISI 1018 and AISI 304 steels was evaluated by the potentiodynamic polarization and EIS techniques. The boriding of the steel samples was carried out by the powder-pack method at a temperature of 1223 K with 6 h of exposure. Structural examinations at the surface of the borided steels revealed the presence of the Fe₂B layer (AISI 1018 borided steel) with a layer thickness of 164 μm, and a FeB/Fe₂B layer with a total layer thickness of 55 μm in the AISI 304 borided steel.

The potentiodynamic polarization and EIS measurements over the borided as well as non-borided steels were evaluated in a corrosive solution of 0.1 M HCl. All the experiments were performed at open circuit potential (Ecorr) in a electrochemical cell with three-electrodes: a working electrode (the test specimen) attached to an acrylic cylinder leaving an exposed area of 1 cm², the reference and counter electrodes were of Ag/AgCl, and AISI 316 L steel, respectively. The anodic polarization curves were obtained using an AUTOLAB PGSTAT 100 equipment from – 350 mV to 1200 mV at a scan rate of 10 mV/s. Moreover, the EIS data were collected with an IM6 ZAHNER equipment, in which the borided and non-borided samples were exposed for more than 40 days in the HCl solution. The frequency range was from 8 MHz to 1 mHz under excitation of a sinusoidal AC signal of 10 mV amplitude for a required time of exposure. After the electrochemical tests, the samples were analyzed by SEM to identify the corrosion mechanisms at the surface of the borided and non-borided steels.

The results from the Tafel plots showed high corrosion resistance of the steels exposed to the boriding process. In addition, the presence of the FeB/Fe₂B layer at the surface of the AISI 304 borided steel, the chemical composition of the steel, and the low porosity in the boride layer, have a beneficial effect in the values of the corrosion resistance in comparison with those estimated in the AISI 1018 borided steel.

Finally, the EIS data were modelled using an appropriate equivalent electric circuit to establish the electrochemical properties and corrosion behavior of the borided and non-borided steels as a function of the exposed days in the corrosive solution.