A. Lousa

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.

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.

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.

Cubic boron nitride thin films by tuned r.f. magnetron sputtering

Abstract Cubic boron nitride (c-BN) thin films were deposited on silicon by tuned substrate r.f. magnetron sputtering. A total gas flow of 3 seem (90% Ar and 10% N 2 ) was introduced in a vacuum chamber at a pressure of 1 × 10 −3 Torr. Films were deposited at two r.f. target powers (360 and 500 W) developing d.c. potentials of −220 and −350 V respectively. The substrate holder, heated to 350 °C, was biased from −40 to −95 V and its influence on the cubic content in the films was studied. The c-BN percentage in the films was between 20 and 90%, as calculated by FTIR measurements. The stress effects were also analyzed by SEM. The grain sizes of the c-BN and h-BN crystals, measured by TEM, were about 30 and 20 nm, respectively.

Mechanical strength improvement of electrical discharge machined cemented carbides through PVD (TiN, TiAlN) coatings

Abstract Electrical discharge machining (EDM) is an alternative shaping route for manufacturing complex component shapes of hard and brittle materials as WC–Co cemented carbides (hardmetals). However, it results in a heat-affected surface layer with poor surface integrity that often leads to mechanical degradation of these materials. This work focuses on assessing the feasibility of physical vapor deposition (PVD) of hard coatings as a technique for improving surface integrity and mechanical strength of EDMed hardmetals. In doing so, the influence of PVD of TiN and TiAlN coatings on the flexural strength of cemented carbides substrates shaped using multi-pass sequential EDM was investigated. For comparison purposes, coating effects on a reference surface condition attained through grinding and polishing using diamond as abrasive were also evaluated. Experimental results show that hard coating deposition markedly decreases the lessening effect of EDM on the fracture resistance of hardmetals. However, no flexural strength differences between coated and uncoated abrasive-machined samples are discerned. The mechanical characterization studies were complemented by detailed fractographic examination and the results were analyzed by linear-elastic fracture mechanics. It is concluded that, although coated EDMed cemented carbides exhibit an effectively larger critical defect size, a mechanical strength improvement is finally achieved because an overcompensating effect ascribed to beneficial changes on intrinsic severity (nature) of the critical flaw and residual stress state at the surface.

Amorphous SixC1−x films: an example of materials presenting low indentation hardness and high wear resistance

Abstract Amorphous SixC1−x films, with x ranging from 0 to 0.4, were grown by high-energy ion beam deposition. The mechanical properties of these materials have been studied: indentation hardness, elastic modulus, wear resistance and scratch damage. It has been shown that the wear resistance and indentation behavior could not necessarily be correlated. Although these films do not show as high a hardness as other amorphous carbon films, their excellent wear resistance and thermal stability, as well as their very good adherence, make them suitable as protective coatings.

Effect of ion bombardment on the properties of B4C thin films deposited by RF sputtering

Abstract Boron carbide is a good material for very hard coatings mechanical applications. Sintered bulk B 4 C is one of the hardest known materials (40 GPa), with a high Youngs modulus and a very high chemical and thermal stability. In B 4 C film deposition, ion bombardment during film growth can deeply affect the material properties. Films were deposited by tuned RF magnetron sputtering from a sintered B 4 C target, under different conditions of ion bombardment, corresponding to substrate biases varying from +15 to −80 V. Homogeneous and stochiometric B 4 C films were obtained. Their mechanical properties: microhardness, Youngs modulus, internal stress and adhesion have been measured by the dynamical nanoindentation method, by the beam bending method and by the microscratch method, respectively. As ion energy is increased, the stress of the films and the critical load increases, while both microhardness and Youngs modulus have maximum values of 30 and 350 GPa respectively, for a bombarding ion energy of 50 eV.

Silicon Thin Film with Columnar Structure Formed by RF Diode Sputtering

The morphology of amorphous silicon (a-Si) thin films prepared by rf diode sputtering was examined. The self-bias potential drops sharply with deceasing argon gas pressure below 30 mTorr and strongly influences the morphology of the sputtered a-Si films. In particular, a film deposited at a pressure of 20 mTorr has a very highly oriented columnar structure and each column has a dome-shaped top. The experimental results show that the self-bias potential is an important determinant of sputtered a-Si film microstructure.