R. Colaço

Corrosion behaviour of steels after laser surface melting

Abstract Laser surface melting (LSM) were carried out in three types of stainless steels — austenitic, martensitic and ferritic — in order to improve pitting corrosion resistance. After the LSM, most of the steels present a shift in the pitting potential to more noble values, a longer passivity stage and lower passive current density than the substrate. These improvements are achieved by means of a modified surface layer, more homogeneous with a fine cellular dendritic structure free of large precipitates. Nevertheless, the corrosion resistance depends critically on the laser processing parameters, particularly for the cases of ferritic and martensitic stainless steels. Therefore, care must be taken in the choice of the laser-processing parameters that leads to optimal properties in each material.

Production of glassy metallic layers by laser surface treatment

Abstract This work aims to investigate if glassy surface layers can be obtained when glass forming alloys are submitted to laser surface treatment techniques. Three types of alloys, with different glass forming abilities, were investigated: Zr-, Mg- and Al-based alloys. X-ray diffraction analysis shows that vitreous phases can be formed in the Zr- and Mg-based alloys, when the treatments are made using laser scanning speeds larger than 2 m/s.

A model for the abrasive wear of metallic matrix particle-reinforced materials

In the present paper a model for the abrasive wear of metallic matrix reinforced materials is presented. The model is based on a generalisation to multiphase materials of the Rabinowicz equation, by considering separately the contributions of the reinforcement particles and of the matrix to material loss. The predictions of the model are discussed and compared with experimental results obtained on metallic matrix particle-reinforced materials subjected to abrasive wear tests. The model enables experimental results to be fitted and, moreover, explains apparently contradictory results previously presented in the literature.

Abrasive wear of metallic matrix reinforced materials

Abstract It is presented an investigation of the abrasive wear behaviour of metallic matrix reinforced materials (MMRMs). Micro-scale wear tests were performed on Fe–Cr–C/NbC composite materials with different volume fractions of reinforcement particles, produced by variable powder feed rate laser cladding. The obtained results show that, depending on the conditions on which the wear tests were made, the wear resistance of the material can decrease continuously with the volume fraction of reinforcement particles or, moreover, can present a non-monotonous variation showing a maximum for a volume fraction of reinforcement particles between 20 and 30%. The identification of abrasive wear mechanisms lead for the formulation of a theoretical model for the wear of this type of materials. The model enables for a qualitative understand of experimental results and explains contradictory results that have been presented in the literature.

Adsorption of albumin and sodium hyaluronate on UHMWPE: a QCM-D and AFM study.

The biotribological properties of artificial joints, in particular the efficiency of the lubrication, strongly determine their lifetime. The most commonly used artificial joints combine a metallic or ceramic part articulating against a ultra high molecular weight polyethylene (UHMWPE) counterface, and are lubricated by the periprosthetic fluid. This fluid contains several macromolecules, namely albumin and sodium hyaluronate (NaHA), that are known to be involved in the lubrication process. There are several studies in the literature concerning the interaction of the referred macromolecules with ceramic or metallic prosthetic materials. However, to our knowledge, information about their binding to the polymeric surface is practically inexistent. The objective of this work is to contribute to clarify the role played by albumin and NaHA on the biolubrication process, through the investigation of their interaction with the UHMWPE surface. The study involves adsorption measurements using a quartz crystal microbalance with dissipation (QCM-D), the characterization of the adsorbed films by atomic force microscopy (AFM) and wettability determinations. Albumin was found to adsorb strongly and extensively to the polymer, while NaHA led to a very low adsorption. In both cases rigid films were obtained, but with different morphology and porosity. The high binding affinity of the protein to the polymer was demonstrated both by the results of the fittings to Langmuir and Freundlich models and by the values of the adhesion forces determined by AFM. In the simultaneous adsorption of albumin and NaHA, protein adsorption is predominant and determines the surface properties.

Phase selection during laser surface melting of martensitic stainless tool steels

Laser surface melting (LSM) of tool steels allows for the complete dissolution of large brittle carbides, leading to homogeneous and extremely fine microstructures. Due to its characteristics, LSM allows improvement of the performance of tool steels by increasing their resistance to erosive and abrasive wear. Nevertheless, when DIN X42Cr13 and DIN X100Cr18 martensitic stainless steels are submitted to LSM, considerable amounts of austenite and {delta}-ferrite formed during the first stage of solidification can be retained in metastable condition at room temperature by mechanisms which are not yet fully understood. The purpose of the present work is to establish the influence of solidification conditions on the primary solidification mode of these two martensitic stainless tool steels, aimed to optimize the LSM operating conditions. Accordingly, samples of DIN X40Cr13 and DIN X100Cr18 were submitted to LSM with a wide range of solidification speeds. The microstructures were analyzed in order to identify the primary solidification mode. The experimental results were compared with theoretical predictions, based on comparison of the dendrite tip temperatures of austenite and {delta}-ferrite as function of the solidification speed.

Comparison of two hydrogel formulations for drug release in ophthalmic lenses.

In the present work two types of polymers were investigated as drug releasing contact lens materials: a poly-hydroxyethylmethacrylate (pHEMA) based hydrogel and a silicone hydrogel. The silicone hydrogel resulted from the addition of TRIS, a hydrophobic monomer containing silicon (3-tris(trimethylsilyloxy)silylpropyl 2-methylprop-2-enoate), to pHEMA. Both hydrogels were loaded with an antibiotic (levofloxacin) and an antiseptic (chlorhexidine) by soaking in the drug solutions. The hydrogel properties were determined to be within the range demanded for lens materials. The release profiles of both drugs from the hydrogels were obtained and eventual drug/polymer interactions were assessed with the help of Raman spectra. A mathematical model, developed to mimic the eye conditions, was applied to the experimental results in order to predict the in vivo efficacy of the studied systems. The release profiles were compared with those resulting from the application of commercial eyedrops. The pHEMA based hydrogel demonstrated to be the best material to achieve a controlled release of levofloxacin. In the case of chlorhexidine, the silicone hydrogel seems to lead to better results. In both cases, our results suggest that these materials are adequate for the preparation of daily disposable therapeutic contact lenses.

Adhesion forces in liquid media: Effect of surface topography and wettability

This work was motivated by the unexpected values of adhesion forces measured between an atomic force microscopy tip and the hydrophobic surface of ultra-high-molecular-weight polyethylene. Two types of samples with different roughness but similar wettability were tested. Adhesion forces of similar magnitude were obtained in air and in polar liquids (water and Hanks Balanced Salt Solution, a saline solution) with the rougher sample. In contrast, the adhesion forces measured on the smoother sample in air were much higher than those measured in water or in the aqueous solution. Those experimental results suggested the presence of nanobubbles at the interface between the rough sample and the polar liquids. The existence of the nanobubbles was further confirmed by the images of the interface obtained in noncontact tapping mode. The adhesion forces measured in a nonpolar liquid (hexadecane) were small and of the same order of magnitude for both samples and their values were in good agreement with the predictions of the London-Hamaker approach for the van der Waals interactions. Finally, we correlate the appearance of nanobubbles with surface topography. The conclusion of this work is that adhesion forces measured in aqueous media may be strongly affected by the presence of nanobubbles if the surface presents topographical accidents.

Simulation of Phase Transformations in Steel Parts Produced by Laser Powder Deposition

Multilayer laser powder deposition is being used for the rapid manufacturing of fully dense near net shape components in a wide variety of materials. In this process parts are built by overlapping consecutive layers of a laser melted material. As a result of this overlapping, the material in each layer will undergo successive thermal cycles as new layers are deposited. Despite their short duration, these thermal cycles can activate solid-state transformations that lead to progressive modification of the microstructure and properties of the material. Since the thermal history of the material in the deposited part will differ from point to point, the part will present a complex and heterogeneous microstructure, and properties that differ from point to point. Given that the microstructure and property distribution in steel parts produced by laser powder deposition can only be predicted by modelling, a three-dimensional thermo-kinetic finite element model of laser powder deposition of tool steels was developed. In the present work this model was applied to the study of the influence of substrate size on the microstructure and properties of a six-layer wall of AISI 420 tool steel. The results show that the temperature field depends significantly on the size of the substrate, leading to distinct microstructures and properties in the final part. Introduction Laser powder deposition (LPD) [1-4] is a very promising technique for the rapid manufacture of fully dense steel components. Although the advantages of this flexible manufacturing process are widely recognised [1-5], there are still some factors restraining its wide acceptance by industry. Controlling and tailoring the material properties of the final part is one such factor. These properties are significantly affected by the solid-state transformations that may occur during deposition, induced by the consecutive thermal cycles created by successive layer overlapping. The lack of detailed understanding of these transformations and of their influence on the final properties of the deposited part may lead to irreproducible results. In order to ensure that the final part possesses an appropriate microstructure, one must know the effect of the processing parameters and part build-up strategy on the thermal cycle and microstructural changes. This knowledge cannot be obtained by trial and error, due to the large number of processing parameters that must be considered. Also, the results of such an approach might not be directly applicable to all cases because the specific geometry of individual parts strongly affects the thermal field in the part and its time evolution. A more satisfactory approach relies on computer aided engineering techniques, such as finite element analysis [6-9]. This approach requires a model describing the time evolution of the thermal field in the part during the deposition process, as well as a detailed knowledge of the solid-state phase transformations that occur in laser processed steels. The latter information is available in the work of several authors [10-15] on laser processed tool steels. In particular, Colaço et. al [11] showed that, due to their fast solidification and cooling rates, laser processed tool steels may contain

Laser Surface Treatment of Tool Steels

Laser surface treatment (LST) is a promising technique to improve the wear and corrosion resistance of materials. In die case of tool steels, laser surface treatment is carried out preferably in the liquid state to allow for complete dissolution of alloy carbides. In this paper, the main requirements for materials used in different types of tools and the advantages of using surface engineered materials for these applications are presented. The application of laser melting to the treatment of tool steels is exemplified for AISI 420 and 440C Cr steels and sintered AISI T15 HSS. Usually, the laser melted layers contain martensite, retained austenite and carbides. In steels containing large proportions of ferrite-forming alloying elements δ-ferrite may also be observed. The laser treatment of sintered steel leads to the elimination of residual porosity. The proportion of retained austenite in laser surface melted steels is much higher than in conventionally treated steels. However, the hardness of the steel is high because the austenite is strengthened by solid solution, dislocations and the small grain size. The high volume fraction of retained austenite usually prohibits the application of tool steels in the laser treated condition. Austenite may be eliminated by double or triple tempering treatments at temperatures in the range 550 to 650 °C. During tempering, carbides precipitate within austenite and martensite, and austenite transforms to martensite. Strong secondary hardening is often observed and the temperature of the secondary hardening peak of laser surface melted (LSM) steels is higher than after conventional heat treatment.