Papers by Author: M. Popović

Abstract: The effects of nitrogen pre-implantation of AISI C1045 steel substrates on the properties of deposited TiN coatings were investigated. Nitrogen ion implantations were performed at 40 keV, to the fluences from 5x1016 – 5x1017 ions/cm2. On so prepared substrates we deposited 1.3 μm thick TiN layers by reactive sputtering. Structural characterizations of the samples were performed by grazing incidence X-ray diffraction analysis (GXRD), standard X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). Microhardness was measured by Vicker’s method. The obtained results indicate the formation of iron-nitrides in the near surface region of the substrates, more pronounced for higher implanted fluences. The structure of the deposited TiN coatings shows a strong dependence on the pre-implantation of the substrates, which is attributed to the changed local structure at the surface. Ion implantation and deposition of hard TiN coatings induce an increase of the microhardness of this low performance steel of more than eight times.

Abstract: We present a study of the micro-structural changes induced in Cr-N layers by irradiation with argon ions. The layers were deposited by reactive ion sputtering on (100) Si wafers, to a thickness of 240-280 nm, at different nitrogen partial pressures and different substrate temperatures. The samples were subsequently irradiated with 120 keV Ar+, to 1x1015 and 1x1016 ions/cm2. Structural characterization was performed with Rutherford backscattering spectroscopy, x-ray diffraction analysis and transmission electron microscopy, and we also did electrical resistivity measurements on the samples. It has been found that the layers grow in the form of a polycrystalline columnar structure, with a columnar width of a few tens of nm. The layer composition, Cr2N or CrN, strongly depends on the nitrogen partial pressure during deposition. Ion irradiation induces local micro-structural changes, formation of nano-particles and defects, though the structures retain their polycrystalline nature. The induced crystalline defects yield an increase of electrical resistivity after ion irradiation.

Abstract: In this paper we present a study of the formation of TiN thin films during the IBAD process. We have analyzed the effects of process parameters such as Ar+ ion energy, ion incident angle, Ti evaporation rates and partial pressure of N2 on preferred orientation and resistivity of TiN layers. TiN thin films were grown by evaporation of Ti in the presence of N2 and simultaneously bombarded with Ar+ ions. Base pressure in the IBAD chamber was 1⋅10-6 mbar. The partial pressure of Ar during deposition was (3.1 – 6.6)⋅10-6 mbar and partial pressure of N2 was 6.0⋅10-6 - 1.1⋅10-5 mbar. The substrates used were Si (100) wafers. TiN thin layers were deposited to a thickness of 85 – 360 nm at deposition rates of Ti from 0.05 to 0.25nm/s. Argon ion energy was varied from 1.5 to 2.0 keV and the angle of ion beam incidence from 0 to 30o. All samples were analyzed by Rutherford backscattering spectrometry (RBS). The changes in concentration profiles of titanium, nitrogen and silicon were determined with 900 keV He++ ion beam. The RBS spectra were analyzed with the demo version of WiNDF code. We have also used X-ray diffraction (XRD) for phase identification. The resistivity of samples was measured with four-point probe method. The results clearly show that TiN thin layer grows with (111) and (200) preferred orientation, depending on the IBAD deposition parameters. Consequently, the formation of TiN thin layers with wellcontrolled crystalline orientation occurs. Also, it was found that the variations in TiN film resistivity could be mainly attributed to the ion beam induced damage during the IBAD process.

Abstract: Two Aluminium alloys, type AA5182 and AA5182+1.2wt% Cu, have been studied. The second alloy in solution treated condition is 18% stronger than the first one. During ageing at 150°C or 200°C it shows a characteristic fast increase in yield strength during the first minutes of ageing, followed by a 'plateau'. Both materials have been deformed in an ECAP die (4 and 8 passes) at 200°C and the microstructure, hardness and mechanical properties in compression at room temperature have been investigated. Although in none of the two materials a true sub-micron grain size was obtained at 200°C, a fair combination of strength and strain hardening was observed. The AA5182+Cu alloy, when ECAP’ed after a solution treatment and quenching, shows an increase in strength of about 20% compared to the AA5182 reference alloy. A post-ECAP annealing at 200°C does not lead to a further increase in hardness or strength. An analysis of the substructure and the mechanical properties during ECAP led to the conclusion that the precipitates formed during ECAP at 200°C do not directly contribute to the higher strength of alloy AA5182+Cu, but they contribute indirectly by slowing down the recovery.