J. Esteve

Mechanical properties of calcium phosphate coatings deposited by laser ablation

Amorphous calcium phosphate and crystalline hydroxyapatite coatings with different morphologies were deposited onto Ti-6Al-4V substrates by means of the laser ablation technique. The strength of adhesion of the coatings to the substrate and their mode of fracture were evaluated through the scratch test technique and scanning electron microscopy. The effect of wet immersion on the adhesion was also assessed. The mechanisms of failure and the critical load of delamination differ significantly depending on the phase and structure of the coatings. The HA coatings with granular morphology have higher resistance to delamination as compared to HA coatings with columnar morphology. This fact has been related to the absence of stresses for the granular morphology.

Improvement of hardness in plasma polymerized hexamethyldisiloxane coatings by silica-like surface modification

Abstract Plasma polymerized hexamethyldisiloxane (PPHMDSO) thin films have been obtained in a DC glow discharge from the monomer vapor. Such films are widely used as protective coatings, gas barriers, biocompatible layers and many other applications. An increased wear resistance is often achieved by modifying the surface structure and composition of the deposited polymer films. We have studied two different approaches, both of them leading to a reduction in the carbon content of the film, which is commonly referred to as a silica-like (SiO x C y :H) structure: first, the deposition of a thin overlayer by plasma polymerization of a HMDSO/O 2 gas mixture; second, the modification of the existing polymer films by immersion in oxygen plasmas. The structure and composition of the films have been characterized by X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), and Transmission Electron Microscopy (TEM); their tribological properties have been studied by means of the depth-sensing nanoindentation technique. Detailed studies have been conducted on the hardness of films with different surface treatments. The protective performance of the films and their wettability have been studied.

Wear behavior of nanometric CrN/Cr multilayers

Multilayered structures have attracted much attention as a way of improving the mechanical and tribological properties of hard protective coatings. We have previously reported results on nanometric chromium nitride (CrN)/Cr multilayers deposited by r.f. magnetron sputtering. It was observed that these multilayered structures improve the hardness of the CrN single layer coatings. Therefore, the next step has been the evaluation of the tribological behavior of these CrN/Cr multilayers, which is reported in this paper. The multilayers have been evaluated in their adhesion to the substrate, abrasive wear resistance and sliding wear behavior against alumina balls. The influence of different substrates on the tribological behavior of the coatings has also been investigated. It has been observed that the abrasive and sliding wear resistance of the multilayers greatly improves when decreasing the bilayer period thickness for a fixed overall coating thickness. The determining parameter on the sliding wear resistance has resulted to be the hardness, which is also known to be the main parameter on the abrasive wear behavior. The sliding wear mechanisms have been identified as abrasion, tribochemical wear and, in some cases, coating delamination. In the case of coatings deposited onto steel, it is also important the plastic deformation of the substrate during the test and the possible adhesion failure of the coatings.

Influence of thickness on the properties of hydroxyapatite coatings deposited by KrF laser ablation

The growth of hydroxyapatite coatings obtained by KrF excimer laser ablation and their adhesion to a titanium alloy substrate were studied by producing coatings with thicknesses ranging from 170 nm up to 1.5 microm, as a result of different deposition times. The morphology of the coatings consists of grain-like particles and also droplets. During growth the grain-like particles grow in size, partially masking the droplets, and a columnar structure is developed. The thinnest film is mainly composed of amorphous calcium phosphate. The coating 350nm thick already contains hydroxyapatite, whereas thicker coatings present some alpha tricalcium phosphate in addition to hydroxyapatite. The resulting coating to substrate adhesion was evaluated through the scratch test technique. Coatings fail under the scratch test by spallating laterally from the diamond tip and the failure load increases as thickness decreases, until not adhesive but cohesive failure for the thinnest coating is observed.

Tungsten carbide/diamond-like carbon multilayer coatings on steel for tribological applications

Abstract Tungsten carbide/diamond like (W–C/DLC) multilayers have been investigated as low friction coatings on high-speed steel substrates. The coatings are composed of a W–C multilayer base and an upper lubricious DLC layer and they are obtained by reactive r.f. magnetron sputtering from a single target comprising of two equal halves; one-half carbon and the other half tungsten. The whole coating structure was obtained in situ, without any interruption of the sputtering process. Transmission electron microscopy (TEM) and SIMS were used to assess the multilayer structure and XPS, XRD, and Raman spectroscopy was used to analyze its composition. The tribological properties of the coatings in sliding wear were investigated by means of scratch test and ball-on-disc test measurements. It was found that the multilayer W–C base improves the adhesion of the upper DLC layer to steel substrates while maintaining its low friction coefficient.

Multilayered chromium/chromium nitride coatings for use in pressure die-casting

Chromium nitride coatings are known to give reasonable solutions to the requirements of semisolid forming tools and of pressure die-casting of low-melting-point metals and alloys. These hard coatings have good mechanical behavior when working at high temperatures. They show enhanced hardness and good wear and corrosion resistance, as well as reduced adhesion to the molten or semisolid metal. We have developed a related hard coating based on multilayered stacking of CrN and Cr metal layers with bilayer thickness down to 22 nm. This coating is obtained by both RF magnetron sputtering and reactive cathodic-arc physical vapor deposition (PVD) on hardened tool-steel substrates. Multilayered coatings are characterized with respect to their structure, hardness and adhesion, and compared for performance to standard CrN single-layer coatings. The Cr metal inter-layer and the multilayered film structure improve the adhesion of the coating to the steel substrate by reducing the stress and film brittleness, and by better matching of the thermal expansion coefficients. When the bilayer thickness was reduced, a reduction in residual stress, and an increase in hardness and critical load were observed. In particular, our Cr/CrN multilayers with a bilayer thickness less than 60 nm surpass all CrN single-film properties.

Internal stress and strain in heavily boron-doped diamond films grown by microwave plasma and hot filament chemical vapor deposition

The internal stress and strain in boron‐doped diamond films grown by microwave plasma chemical vapor deposition (MWCVD) and hot filament CVD (HFCVD) were studied as a function of boron concentration. The total stress (thermal+intrinsic) was tensile, and the stress and strain increased with boron concentration. The stress and the strain measured in HFCVD samples were greater than those of MWCVD samples at the same boron concentration. The intrinsic tensile stress, 0.84 GPa, calculated by the grain boundary relaxation model, was in good agreement with the experimental value when the boron concentration in the films was below 0.3 at.%. At boron concentrations above 0.3 at.%, the tensile stress was mainly caused by high defect density, and induced by a node‐blocked sliding effect at the grain boundary.

Preparation of BCN thin films by r.f. plasma assisted CVD

Abstract Boron-carbon-nitrogen (BC x N y ) films were grown on silicon substrates heated at 300 C by r.f. plasma assisted chemical vapour deposition from CH 4 -N 2 -B 2 H 6 gas mixtures. Dense and smooth films with different x:y composition ratios were obtained by varying the flow rate of the precursor gases. The analysis by X-ray photoelectron and infrared spectroscopies of the films revealed the formation of an hybrid B-C-N phase. Microhardness measurements performed with a nanoindenter showed that the mechanical properties of the BC x N y films depended on their composition and some of them presented a hardness higher (13 GPa) than that of hexagonal boron nitride films (12 GPa).

Nanometric chromium/chromium carbide multilayers for tribological applications

Abstract Metal/ceramic multilayers with periods in the nanometric range have been proposed as protective coatings due to their improved tribological and mechanical properties as compared to single coatings. We have evaluated nanometric chromium/chromium carbide (Cr/CrC) multilayers as a promising combination for tribological applications. These multilayers were synthesized by r.f. magnetron sputtering from a pure chromium target onto steel and silicon substrates. The multilayer structure was obtained by alternatively changing the sputtering gas composition between pure argon and a reactive mixture Ar/CH4. The total coating thickness was approximately 1.5 μm, and the bilayer period varied from 300 to 20 nm. Secondary ion mass spectrometry and scanning electron microscopy confirmed the periodic multilayered structure. X-ray diffraction allowed us the characterization of the different crystallographic phases formed in the Cr/CrC system. The influence of deposition parameters and period thickness on the coatings microstructure and mechanical properties is presented in this work.

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Abstract Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm3) and sp3 content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Youngs modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Youngs modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm2, producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.