C. Alves

Thermal behavior of supersolidus bronze powder compacts during heating by hollow cathode discharge

Abstract Porous aluminum–bronze compacts formed from prealloyed metastable (supersolidus) powder were heated either by plasma or by a resistive furnace technique. The plasma heating was performed in two different setups: (i) Hollow Cathode Discharge (HCD) method and (ii) Shielded Cathode Discharge (SCD) method. All experiments were carried out for 12 min in hydrogen at temperatures ranging from 673 to 1323 K. In HCD, for temperatures above 773 K, a fully filled, non-porous region was obtained in the center of the compacts, exhibiting a quite different behavior than that obtained using either SCD or resistive techniques. In these cases, uniformly dispersed condensed micro-regions were detected in samples treated at temperatures higher than 1223 K. These observations are discussed in terms of the heating mechanism, mass transfer and behavior of supersolidus powder particles. A simple physical model for the creation of the central fully filled region obtained by HCD is introduced.

Influência das espécies do plasma na modificação das propriedades superficiais do titânio tratado por plasma de N2 - Ar - O2

The present study correlates plasma active species with titanium surface properties. This was achieved by performing optical emission spectroscopy (OES) analysis of N2+ (391.4 nm) and O (844.6 nm) within a N2-Ar-O2 plasma. The fluxes of gases were established to be 1 and 4 sccm (standard cubic centimeter) for nitrogen and argon, while that of oxygen was varied between 2 and 4 sccm. A non-linearity of the intensity of N2+ (391.4 nm) species with the increase on O2 flux was observed with an intensity peak at 3 sccm. It was also concluded that the active species influence the modification of properties after the plasma treatment. The N2+ (391.4 nm) species were more effective to form nitrides and the surface roughness was bigger when those species presented higher intensity. Surface hardness and surface stress were also higher whenever using higher intensity fluxes of N2+ (391.4 nm). Furthermore, while the N2+ (391.4 nm) specie influenced the dispersive component, the O (844.6 nm) specie influenced the polar component.

Modificação da superfície e das propriedades tribológicas do titânio por carbonitretação a plasma

TiCxNy surfaces were produced by thermochemical treatment achieved by N2 - Ar - CH4 plasma mixture. The influence of methane concentration and C and N atoms inserted into the crystalline lattice of Ti were investigated through the characterization of its tribomechanical properties. Therefore, commercially pure Ti (grade II) samples were carbonitrided in the x N2 - y Ar - z CH4 plasma mixture (where x and y = 4 SCCM and z = 2, 3 and 4 SCCM) at 500 °C, during 1 h. C and N concentration profile after treatments were determinate by NRA. The superficial phases were identified by GIXRD. Pin-on-disk assays were utilized to determine the friction coefficient, whose damaged areas were studied by perfilometry and scanning electron microscopy (SEM) techniques. It was observed that the friction coefficient and the damage mechanism of Ti are intensively dependent of C and N concentration on the surface. However, the concentration profile of those interstitial elements is not directly proportional to the methane gas flow.