Ricardo D. Torres

Performance and Surface Integrity of Ti6Al4V After Sinking EDM with Special Graphite Electrodes

Titanium and its alloys have high chemical reactivity with most of the cutting tools. This makes it difficult to work with these alloys using conventional machining processes. Electrical discharge machining (EDM) emerges as an alternative technique to machining these materials. In this work, it is investigated the performance of three special grades of graphite as electrodes when ED-Machining Ti6Al4V samples under three different regimes. The main influences of electrical parameters are discussed for the samples material removal rate, volumetric relative wear and surface roughness. The samples surfaces were evaluated using SEM images, microhardness measurements, and x-ray diffraction. It was found that the best results for samples material removal rate, surface roughness, and volumetric relative wear were obtained for the graphite electrode with 10-μm particle size and negative polarity. For all samples machined by EDM and characterized by x-ray (XRD), it was identified the presence of titanium carbides. For the finish EDM regimes, the recast layer presents an increased amount of titanium carbides compared to semi-finish and rough regimes.

The effect of nitriding on adhesion and mechanical properties of electroless Ni–P coating on AISI 4140 steel

In this work, electroless nickel has been deposited onto steel substrates by a selfcatalytic process, which involves temperature and time to create a Ni–P layer with a uniform thickness. The major drawback of electroless nickel is the interdiffusion post heat treatment (PHT) that is necessary to create a metallurgical bond between the steel substrate and the Ni–P deposit. The PHT results in softening of the steel. The aim of this ongoing research is to investigate the effect of introducing a nitrided layer prior to deposition of Ni–P layer, as an attempt to lessen the softening of the steel. To this end, Ni–P coatings were obtained over nitrided and non-nitrided steel substrates. The PHT was done at 400°C or 610°C. The nitrided layer, indeed, avoided the softening of the steel substrate. In addition, the nitrided layer improves the adhesion of the Ni–P coating even when the PHT was done at 400°C.

Influence of the annealing treatment on the tribocorrosion properties of Ca and P containing TiO2 produced by plasma electrolytic oxidation

Titanium is widely used as implant material; however, regardless of the excellent properties such as low density, corrosion resistance and biocompatibility, it usually presents poor tribological behaviour. The plasma electrolytic oxidation (PEO) technique produces a high hardness ceramic layer on the titanium surface, by the interaction of anodic oxide growth and plasma channel shock caused by the dielectric breakdown at high voltages taking place in an aqueous electrolyte. The characteristics of the oxide layer, such as the roughness, thickness, porosity, crystallinity and chemical composition, can be tailored changing the PEO parameters or by a posterior annealing treatment at temperatures above 400 °C. This work aims to evaluate the influence of the PEO voltage and annealing treatment on the tribocorrosion properties. Cp–titanium were submitted to PEO treatment at 300 and 400 V for 1 min in an electrolyte containing Ca and P. The annealing treatment was carried out at 600 °C for 1 h following slow cooling at furnace. Surface properties were evaluated by X-ray diffraction, scanning electron microscopy (SEM/EDS) and profilometry. Tribocorrosion was evaluated using a linear reciprocating ball on flat tribometer with a three-electrode electrochemical cell coupled to potentiostat/galvanostat in Phosphate Buffered Saline solution. Results show that PEO layer increases the tribocorrosion resistance of bare titanium significantly. The tribocorrosion resistance is also increased by the presence of rutile phase through voltage or posterior heat treatment.

Mechanical and tribological properties of Ca/P doped titanium dioxide layer produced by plasma electrolytic oxidation: Effects of applied voltage and heat treatment

ABSTRACT Plasma electrolytic oxidation (PEO) is a technique that produces a hard oxide layer on the titanium surface where its properties can be tailored by changing the process parameters or by a posterior heat treatment (HT). In this work, a TiO2 layer with different crystallinity was produced by PEO with different applied voltages (250 to 400 V) and post-HT at 600°C. Our aim was to evaluate the influence of the PEO voltage and HT on the mechanical and tribological properties of anodized Ti. There is an increase in pore size, oxide thickness, and Ca/P ratio for the oxide layer with the applied voltage during the PEO process. X-ray diffraction (XRD) results indicated an increase in the crystalline rutile phase in the oxide layer with voltage and HT. Nanoindentation shows an increase in the oxide hardness and elastic modulus with increased voltage and HT, leading to an improvement in the wear resistance.