J.C. Oliveira

Formation and distribution of brittle structures in friction stir welding of aluminium and copper: influence of process parameters

Abstract Morphological, metallographic and structural analyses of aluminium–copper dissimilar welds produced under different friction stir welding conditions were conducted in order to analyse the mechanisms of intermetallic phases formation, its relation with welding conditions and its consequences in the weld structure and morphology. Under lower heat input conditions, only a thin intermetallic layer distributed along the aluminium/copper interface was depicted inside the nugget. Increasing the heat input promoted material mixing and formation of increasing amounts of intermetallic rich structures. The intermetallic phase content and the homogeneity of the mixed area increased with increasing heat input, evolving from structures containing Al, Cu, CuAl2 and Cu9Al4 to structures predominantly composed of Cu9Al4 and Cu(Al). In order to explain these results, the mechanisms of intermetallic phases formation are discussed, taking into account the process parameters and material flow mechanisms in friction stir welding. Important relations between intermetallic formation and weld surface morphology were also found.

Influence of carbon content on the crystallographic structure of boron carbide films

Abstract Boron carbide thin films were synthesised by laser-assisted chemical vapour deposition (LCVD), using a CO 2 laser beam and boron trichloride and methane as precursors. Boron and carbon contents were measured by electron probe microanalysis (EPMA). Microstructural analysis was carried out by Raman microspectroscopy and glancing-incidence X-ray diffraction (GIXRD) was used to study the crystallographic structure and to determine the lattice parameters of the polycrystalline films. The rhombohedral–hexagonal boron carbide crystal lattice constants were plotted as a function of the carbon content, and the non-linear behaviour observed was interpreted on the basis of the complex structure of boron carbide.

Deposition of boron carbide by laser CVD: a comparison with thermodynamic predictions

Abstract Thin films in the boron-carbon system have been deposited by CO 2 laser chemical vapour deposition (LCVD) on fused silica plates from a gas mixture of BCl 3 , CH 4 , H 2 and Ar. Glancing incidence X-ray diffraction (GIXRD) and electron-probe microanalysis (EPMA) were extensively used for characterisation of structure and chemical composition, respectively. Depending on the process parameters, three different phases were deposited: rhombohedric boron carbide and graphite (stable phases), and a metastable tetragonal boron rich carbide. Thermodynamic calculations were performed for a broad range of gas phase compositions and the data obtained were compared to the experimental results. This comparison showed a generalised lack of carbon (or excess of boron) in all the deposited films and was also used to estimate the deposition temperature.

Structural characterisation of B4C films deposited by laser-assisted CVD

Abstract Boron carbide (B4C) coatings were deposited on fused silica substrates by LCVD with a CO2 laser. A reaction gas mixture of BCl3, CH4 and H2 was used. The coatings were characterised by glancing incidence X-ray diffraction, Raman microprobe spectroscopy and electron probe microanalysis. The influence of gas-phase composition and laser parameters on the phase and chemical composition of the deposits were investigated.

Influence of sputtering conditions on corrosion of sputtered W–Ti–N thin film hard coatings: salt spray tests and image analysis

Abstract The corrosion resistance of sputter-coated W–Ti–N films on high speed steel substrates prepared under different deposition conditions has been compared by varying the film surface roughness, the film thickness and depositing a first W–Ti layer. Salt spray tests have been carried out and the resulting pitting corrosion evaluated by image analysis and by microscopy and X-ray diffraction. The results show that the most important factors influencing corrosion are film compactness and thickness. The detailed mechanism of corrosion is discussed, involving electrolyte penetration with substrate corrosion, the corrosion products either blocking further corrosion or leading to localised film rupture.

Influence of surface features on the adhesion of Staphylococcus epidermidis to Ag–TiCN thin films

Abstract Staphylococcus epidermidis has emerged as one of the major nosocomial pathogens associated with infections of implanted medical devices. The initial adhesion of these organisms to the surface of biomaterials is assumed to be an important stage in their colonization. The main objective of this work is to assess the influence of surface features on the adhesion of S. epidermidis to Ag–TiCN coatings deposited by dc reactive magnetron sputtering. The structural results obtained by x-ray diffraction show that the coatings crystallize in a B1-NaCl crystal structure typical of TiC0.3N0.7. The increase of Ag content promoted the formation of Ag crystalline phases. According to the results obtained with atomic force microscopy, a decrease on the surface roughness of the films from 39 to 7 nm is observed as the Ag content increases from 0 to 15 at.%. Surface energy results show that the increase of Ag promotes an increase in hydrophobicity. Bacterial adhesion and biofilm formation on coatings were assessed by the enumeration of the number of viable cells. The results showed that the surface with lower roughness and higher hydrophobicity leads to greater bacterial adhesion and biofilm formation, highlighting that surface morphology and hydrophobicity rule the colonization of materials.

Laser-assisted CVD of boron carbide at atmospheric pressure

Abstract In a previous study, laser assisted chemical vapour deposition of boron carbide thin films was carried out from a gas mixture of BCl 3 , CH 4 , H 2 and argon at a working pressure of 133 mbar, using a cw CO 2 laser. This paper focuses on the deposition of rhombohedral boron carbide at atmospheric pressure from the same precursor gases. The deposition of single phase boron carbide films was achieved at laser irradiances of 90 W/cm 2 . At higher irradiance values, unlike deposition at lower pressure, the films present a dark central region with whisker-like morphology due to co-deposition of boron carbide and disordered graphite.

Oxygen sensitivity of erbium-doped AlN films probed by site selective spectroscopy

Abstract In the aim of better understand the influence of oxygen on the luminescent properties of Er:AlN films, two samples synthesized by radiofrequency reactive magnetron sputtering, have been analysed by site-selective spectroscopy around 1.55 μm and compared to a reference aluminosilicate sample. The erbium content in the films was estimated to 3.3 at.% and oxygen content ranges from 6.2 at.% to 58.6 at.% (determination by EPMA). Line narrowing study was performed for values 6.2 at.% and 22.5 at.%. Both films were annealed at 1075 K for 1 h. Site-selective spectroscopy was carried out using a Ti:sapphire laser light with typical linewidth of 2 GHz tuned around 980 nm as the excitation source. The samples were cooled down to 1.5 K in a liquid-helium bath cryostat and their 4 I 13/2 ↔ 4 I 15/2 luminescence was detected using a high-sensitivity germanium-cooled detector. It appears that the site distribution of the Er:AlN samples is not continuous as in classical aluminosilicate glass but presents a rupture that suggests the existence of two kinds of sites for erbium: oxygen site and nitrogen site. These observations can easily be interpreted within the framework of the nephelauxetic effect that explains a shift of the site distribution to the longer wavelengths with an increase of boundings bonding covalency.

Influence of Al(Er) interlayer on the mechanical properties of AlN(Er) coatings

Abstract AlN(Er) thin films were deposited by sputtering on M2 steel (AISI) and Si substrates with different thickness of an Al(Er) interlayer. No significant variations were observed in the chemical composition and structure of the AlN(Er) films. The films in all cases presented excess nitrogen in relation to AlN stoichiometry. The diffraction peaks were strongly shifted to lower angles due to the presence of Er. The hardness of the films is approximately 32 GPa and does not change with the thickness of the Al(Er) interlayer. The cohesion and adhesion of the AlN(Er) films were improved with the inclusion of the Al(Er) interlayer; the cohesive and adhesive critical load values increased from 7 and 17 N to 15 and 27 N, respectively.

The influence of Er doping of Al–N sputtered coatings on their mechanical properties

Abstract Thin films of AlN(Er) were deposited on M2 steel substrates using r.f. reactive magnetron sputtering starting from an aluminium target in Ar/N 2 mixtures. Er doping was achieved by partially covering the aluminium target with an adequate number of erbium pieces. All depositions were achieved with a discharge power of 600 W, P N2 / P tot =0.5 and a total pressure of 0.74 Pa. The Al/(N+Al) ratio in the as-deposited films varied between 0.48 and 0.51 while Erbium contents ranging from 0.2 to 3.5 at.% were achieved. X-ray diffraction showed that the AlN(Er) films with Erbium contents up to 1.4 at.% have a crystalline structure with a preferential orientation on the (002) planes. However, as a consequence of lattice distortion and/or defect generation originated by the incorporation of Er atoms in the AlN structure, the films became amorphous for higher Er contents. The highest hardness values were obtained for the crystalline AlN(Er) films (up to 31 GPa at 1.2 at.% of Er) while a significant decrease of the hardness was observed when the films became amorphous (22.5 GPa at 3.5 at.%). The mean critical loads for cohesive failure measured by scratch test were quite similar for all the coatings (varying from 7 to 8 N) revealing the brittle character of the deposited films. The mean critical loads for adhesive failure were in the range of 12 N for low Er content to 22 N for the higher Er content. Concerning the possible use of AlN(Er) films in tools applications, these films have a suitable hardness but a poor adhesion to steel alloys when compared with layers usually used for mechanical protection.