E. Martı́nez

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.

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.

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.

Mechanical properties of plasma deposited polymer coatings

Thin polymeric films have been obtained by plasma polymerisation of dimethylsiloxane (DMSO). Such films are widely used as protective coatings on optical reflective surfaces and decorative parts. Thin films of polydimethylsiloxane (PDMS) have been deposited by a DC glow discharge plasma from DMSO vapour on pre-treated and untreated polycarbonate (PC) substrates, with or without previous evaporation of an additional aluminium film. A subsequent process of mineralisation by immersion in an air plasma has been applied in order to improve the PDMS films wear resistance. Polycarbonate substrates have been treated in a similar way aiming at improved adhesion of Al films. The present paper is specially focused on the surface mechanical properties of these films. These have been measured by nanoindentation and microscratch techniques and the improvement of the surface hardness and scratch resistance of PDMS films and PC substrates has been assessed.

Micromechanical and microtribological properties of BCN thin films near the B4C composition deposited by r.f. magnetron sputtering

Ternary materials with compositions in the B-C-N system offer properties of great interest. In particular, mechanical and tribological properties are expected to be excellent, as they can combine some of the specific properties of BN, B 4 C and C 3 N 4 . In this paper, BCN thin films deposited by r.f. magnetron sputtering are characterized by their micromechanical and microtribological behavior. BCN coatings with different composition were obtained by varying the N 2 /Ar proportion in the sputtering gas. Hardness and elastic modulus of the coatings were measured by nanoindentation. The adhesion and friction coefficient against diamond have been evaluated by microscratch and the coatings have been characterized in their wear behavior at the nanometric scale. These mechanical and tribological properties have been related to film composition and structure, which have been studied in a previous work. It is found that the measured wear resistance at the nanometric scale is directly related to the coating microhardness rather than friction behavior or adhesion of the coating to the substrate, which are the determinant factors in the macroscopic scale wear behavior.

MICROMECHANICAL PROPERTIES OF BN AND B-C-N COATINGS OBTAINED BY R.F. PLASMA-ASSISTED CVD

Abstract We have obtained highly transparent and hard BN films in a capacitively coupled r.f. plasma-assisted CVD reactor from three different gas mixtures: B2H6–H2–NH3, B2H6–N2 and B2H6–N2–Ar. It was found that the films were smooth, dense, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. The microhardness, friction coefficient and adhesion of these coatings were measured by nanoindentation and microscratching. BCxNy films were also prepared in the same plasma-assisted CVD reactor from B2H6–N2–CH4 gas mixtures. The carbon content in the films was varied by using different CH4 flow rates. These films had a less ordered structure. The mechanical properties of these films had been compared to those of hexagonal BN films. Microhardness measurements showed that there is a correlation between film composition and hardness of the BCN films.