T. Ujvári

Surface treatment of polyethylene by fast atom beams

Abstract Ultra-high molecular-weight polyethylene (UHMWPE) was treated by fast atom beams (FAB) obtained from He, Ar, H 2 and N 2 with about 1 kV accelerating voltage and estimated fluence of 10 17 particles cm −2 . The modified surface layers were characterised by X-ray photoelectron spectroscopy (XPS or ESCA) and dynamic microhardness measurements. Each applied FAB treatment results in the increase of the bulk plasmon loss energy ( E L ) of the C 1s peak. This implies the formation of graphitic-type material and/or hydrogenated amorphous carbon (or carbon nitride in case of treatment by N atoms) in the modified surface layer. FAB treatment by N atoms leads to the incorporation of ≈35 at.% of N, which is about three times higher than the value obtained previously after low-kilo-electron-volt N 2 + ion-beam treatment. The related C 1s peak shows that the overwhelming portion of C is bound to N, while the N 1s peak reflects that N is present in at least three chemical bonding states. Angle-dependent XPS studies of the nitrogen-FAB-treated UHMWPE reveal the presence of a N-rich subsurface layer with a topmost layer containing less N. This is in agreement with the calculated depth distribution of N atoms. Each applied FAB treatment leads to significant increase in the surface microhardness.

The effect of the carbon matrix on the mechanical properties of nanocomposite films containing nickel nanoparticles

The correlation of structural electronic and mechanical properties of carbon–nickel composite thin films has been investigated. The films were deposited on oxidized silicon substrates by dc magnetron sputtering of Ni and C targets in argon at different temperatures between 25 and 800 °C. Composition variation was achieved by variation of the power of the Ni target with constant power on the C target. Structural investigations were performed by transmission electron microscopy (including high resolution) both in plan view and cross section. The nanocomposite films consisted of metallic nanocrystals embedded in a carbon matrix. The carbon matrix was disordered or graphite-like carbon; the crystalline phase consisted of Ni3C or Ni nanoparticles, depending on the deposition temperature. The temperature coefficient of resistivity measurements at low temperature confirmed the various structures of the carbon matrix. The samples in which the prevailing matrix was disordered carbon show the tunneling effect and samples with graphite-like carbon matrix show metallic behavior. The hardness of the films varies between 2 GPa (hardness of Ni) and 13 GPa depending on the deposition temperature, but is independent of the Ni content. The highest hardness of ~11–13 GPa and modulus of elasticity of ~120 GPa were obtained when the crystalline Ni3C nanoparticles were separated by a 2–3 nm thin carbon matrix consisting of amorphous and graphite-like carbon phases. In these films the hardness to modulus of elasticity ratio (H/E) is ~0.1.

Composition and chemical structure characteristics of CNx layers prepared by different plasma assisted techniques

CNx layers were grown on Si (100) wafers and cleaved NaCl (100) slices by reactive sputtering of graphite in DC nitrogen plasma, by DC and RF magnetron sputtering of graphite in N2 or N2+Ar mixture, and from C-containing precursors in nitrogen by plasma-enhanced chemical vapour deposition (PECVD). The chemical structure of the deposited layers was characterised by XPS and FT-IR spectroscopy. DC plasma deposition resulted in layers with high N content, close to CN stoichiometry (x≈1), as determined by XPS. The CNx layers deposited by DC or RF magnetron sputtering contained 22–32 at.% N (x≈0.3–0.5). For layers prepared by PECVD, N-content varying between 20 and 50 at.% was characteristic. The broad and asymmetric C1s and N1s XPS spectral lines manifested several chemical bonding states of the constituents. The proportions of the line components varied with the preparation conditions. Significant differences were also observed in the 1100–1700 cm−1 region of the FT-IR spectra. Assignment of the two major N1s photoelectron peak-components was proposed based on the established correlation between the FT-IR results, composition and binding energy values measured for magnetron-deposited CNx phases as follows: the N1 at 398.3 eV BE to NC double bonds and N2 at 400.2 eV BE to NC single bonds.

Surface modification of polyethylene by nitrogen PIII: Surface chemical and nanomechanical properties

Ultra-high molecular weight polyethylene (UHMWPE) was surface treated by nitrogen plasma immersion ion implantation (PIII), with the main aim of improving its wear resistance. Accelerating voltages (U) between 15 and 30 kV, fluences (F) between 1×1017 and 3×1017 cm-2 and fluence rates (FR) between 3×1013 and 7×1013 cm-2 s-1 have been applied. XPS was used to characterise the surface chemical composition and structure. Changes induced in the surface mechanical properties like hardness (H), reduced modulus (E) and in the tribological property of volume loss upon uniform wear test (V) were studied by nanoindentation and multipass wear measurements. The evolution of surface topography was followed by measuring the mean roughness (Ra). The macroscopic temperature (T) developed during the PIII-treatment was also studied. Incorporation of N and O took place into the surface layer. With the increase of U the surface N-content tended to decrease. The bulk plasmon loss energy of the C 1s peak increased from 20 eV up to about 25 eV, suggesting densification and the formation of amorphous hydrogenated carbon nitride-like layer. H, T and Ra increased, and V decreased upon PIII treatment, while E either decreased or increased depending on the actual process parameter set applied. In the parameter range studied Hmax, Emax and Ra,max values have been observed at Umax, Fmax and FRmin. Vmin and Tmax have been observed at Umax, Fmin and FRmax, suggesting that the thermal effect is a dominant factor in determining the extent of reduction in the wear rate.

Effect of plasma parameters on the structure of CNx layers deposited by DC magnetron sputtering

Abstract CNx layers were grown on polished Si(100) wafers by reactive DC magnetron sputtering of a high purity graphite target by nitrogen ions. The deposited layers were characterized by XPS and FT-IR spectroscopy. The ‘as-prepared’ CNx layers contained approximately 20–40 at.% nitrogen measured by XPS. The large width and asymmetric shape of the C1s and N1s lines manifested several chemical bonding states of the constituents. The two major components of the N1s lines were assigned to the CN double bonds (N1 at 398.2 eV B.E.) and to the CN single bonds (N3 at 400.6 eV B.E.). The proportions of these line-components varied significantly with the preparation conditions and showed a correlation with the plasma parameters (electron density, ion current density and electron temperature) of the magnetron, as measured by a Langmuir-probe. The N3/N1 ratio increased with decreasing target-to-substrate distance. Significant differences were also observed in the 1100–1700 cm−1 region of the FT-IR spectra. For layers grown in the high electron-density plasma, a major increase in the intensity ratio of IR band at 1300 cm−1 to that at 1530 cm−1 was observed, which can be connected to the increase of the ratio of the sp3 type N to the sp2 type one in the CN clusters.

Structure and Properties of Carbon Based Nanocomposite Films

DC co-sputtered Carbon-Nickel and Carbon-Nitride-Nickel thin films were investigated by transmission electron microscopy, XPS and nanoindentation to clarify the influence of Nitrogen and Ni additions on the structure formation and mechanical properties of the films. The films were deposited by magnetron sputtering in argon or nitrogen plasma at temperatures from 25 to 800°C onto NaCl and Si+SiO2 substrates. The microstructures of the films can be described as nanocomposites, built from Ni or Ni3C nanocrystals in a carbon/CNx matrix. The mechanical properties of the films were found to be dependent on the substrate temperature during deposition along with the changes of the structure. The highest nanohardness of 14 GPa was measured for the film grown at 200°C, while low values (2 GPa) were obtained for high temperature deposition. The change of the hardness is thought to be primarily the consequence of morphological and phase changes of the films in the temperature range 200-800°C.