C.A. dos Santos

X-ray diffraction measurements of plasma-nitrided Ti–6Al–4V

Abstract A systematic study was undertaken with samples of commercial Ti–6Al–4V nitrided in a conventional d.c. plasma equipment. As treatment parameters, we have used: nitriding time (from 15 to 360 min); nitriding atmosphere (H 2 –20% N 2 and H 2 –60% N 2 ); total pressure (from 10 mPa to 1 kPa) and cathode temperature (from 673 to 943 K). X-ray diffraction measurements show that the effect of nitriding on the near-surface composition of (α+β)–Ti–6Al–4V is a very complex function of the process parameters. Among these, it appears that the sample temperature plays the more dramatic role. For temperatures below 773 K, E-Ti 2 N and δ-δ-TiN are clearly present only for a long period of treatment in a N/H=3/2 atmosphere. For the N/H=1/4 atmosphere, these nitrides are not present in the XRD pattern of samples treated at 773 K, even for a long treatment time such as 240 min. However, samples treated at 943 K show well-resolved E-Ti 2 N and δ-TiN reflections, even for short treatment times such as 15 min.

Iron nitride and carbonitride phases in a nitrogen implanted carbon steel

Iron nitride and carbonitride phases formed during nitrogen implantation and subsequent thermal annealing of a medium‐carbon steel are investigated by means of conversion electron Mossbauer scattering. The results are compared to previous work on similar systems and also discussed in terms of the mechanical properties of ion implanted steels.

Oscillatory magnetic coupling in Ag/Ni superlattices

Ag/Ni superlattices have been prepared by triode dc sputtering on glass substrates at 100 K. X‐ray diffractometry with the scattering vector perpendicular to the superlattice plane suggests structural coherence and interface sharpness whatever the thickness of the elemental layers. All the samples present (111) textures for both Ag and Ni. Magnetic measurements performed with a superconducting quantum interference device magnetometer indicate in‐plane anisotropy of the magnetization and oscillatory interlayer magnetic coupling. This is confirmed by magnetoresistance measurements.

Composition and thermal evolution of nitrogen implanted steels: a systematic study

Abstract Conversion electron Mossbauer scattering (CEMS) was used to investigate the composition and thermal evolution of nitrogen-implanted steels. A systematic study is performed in terms of the carbon concentration on the steel matrix and implantation dose. It is discussed how the pre-existent interstitial carbon and the implantation dose may influence the formation of surface compounds during nitrogen implantation.

Surface modifications and the mechanical properties of carbon steels implanted with nitrogen

Discusses the relationships between the surface composition and the mechanical properties of carbon steels implanted with nitrogen. Conversion electron and X-ray Mossbauer spectroscopy (CEMS and CXMS) were used to investigate the surface modifications of the implanted samples. Thermal annealing was performed in high vacuum, at temperatures between 250 and 500 degrees C, in order to simulate the temperature increase in real tribological processes. It is shown that the improved performance of nitrogen-implanted carbon steels against wear and fatigue is mainly due to the action of finely dispersed iron carbonitrides ( theta - and epsilon -Fe3(C,N)). It is shown also that the persistence of the protective action at greater depths is associated with the in-diffusion of nitrogen released by thermal decomposition of the compounds formed at the surface.

Some structure and magnetic effects of Ga incorporation on α-FeOOH

The effects of Ga incorporation on goethite were investigated by using X-ray diffractometry (XRD), particle-induced X-ray emission (PIXE), susceptibility measurements and Mossbauer spectroscopy (MS). Samples GaxFe1−xOOH were prepared with x=0.00, 0.01, 0.03, 0.05, 0.10, 0.35 and 0.70. All the samples are single-phase and an accurate observation of the structure refinement procedure makes clear the sample-dependent structural behaviour. A model is proposed in which the main effect of Ga incorporation is to reduce the crystallite dimensions.

Nitriding of H-12 tool steel by direct-current and pulsed plasmas

Abstract Direct-current (DC) and pulsed plasmas were used to nitride samples of AISI H-12 tool steel with treatment time varying from 1 h to 6 h. X-ray diffraction (glancing-angle and θ –2 θ geometry), conversion electron Mossbauer spectroscopy, conversion X-ray Mossbauer spectroscopy and Vickers microhardness were used as analytical techniques. The near-surface compound layer consists of a mixture of γ′-Fe 4 N and e-Fe x (N,C), while the near-diffusion zone compound layer consists of a mixture of γ′-Fe 4 N, e-Fe x (N,C), α″-Fe 16 N 2 and γ-austenite. For a DC plasma the dependence of compound layer thickness on nitriding time obeys a parabolic law, but for a pulsed plasma this dependence does not follow a parabolic law.

Inverted and crossed hysteresis loops in Ag/Ni multilayers

Abstract We report on the observation of inverted magnetic hysteresis loops in Ag/Ni multilayers. The samples were prepared by triode dc sputtering, at 100 K, with nickel layer thickness of about 8A and Ag layer thicknesses ranging from about 5 to 40 A. The phenomenon was observed in all the samples measured at temperatures larger than 90 K.

Carbonitride precipitation by successive implantation of carbon and nitrogen into pure iron

Conversion electron Mossbauer spectroscopy (CEMS) and X-ray photoelectron spectroscopy (XPS) was used to investigate the precipitation of carbonitrides by successive implantation of carbon and nitrogen into pure iron. The authors have analysed two samples, differing by the order of implantation of the two ionic species. Although the as-implanted surface compositions are rather different, thermal annealing of both samples at 300 degrees C results in the formation of the same kind of precipitate, namely epsilon -Fe3(C,N).

Near-surface composition and microhardness profile of plasma nitrided H-12 tool steel

Samples of AISI H-12 tool steel were plasma nitrided at 500°C in a mixture of H2-20% N2 under a total pressure of 6 mbar, by using DC and pulsed glow discharges. The treatment time varied from 1 to 6 h. X-ray diffraction (glancing angle and u-2u geometry), conversion electron Mossbauer spectroscopy, conversion X-ray Mossbauer spectroscopy, electron probe microanalysis, optical micrograph and Vickers microhardness were used as analytical techniques. The obtained results suggest that, under the present experimental conditions: (i) The near-surface compound layer consists of a mixture of g%-Fe4N and o-Fex(N, C); (ii) the near-diffusion zone compound layer consists of a mixture of g%-Fe4N, o-Fex(N, C), aƒ-Fe16N2 and g-austenite; (iii) the dependence of compound layer thickness on nitriding time violates the parabolic behavior and emphasizes effects from cathode sputtering and radiation-enhanced diffusion.