卷 53, 编号 5 (2017)

The structure of alpha- and beta-titanium alloy VT22, its microhardness, surface roughness, wear and corrosion resistance after anodic plasma electrolytic nitrocarburising in an electrolyte containing carbamide and ammonium chloride were investigated. An X-ray diffractometer and a scanning electron microscope were used to characterize the phase composition and structure of the modified surface. Tribological properties of the treated titanium alloy were evaluated using a ball-on-disc tribometer under lubricated testing conditions. The effect of processing temperature on corrosion resistance of the plasma electrolytic nitrocarburising samples was examined by means of potentiodynamic polarization in Ringer’s solution. It was shown that the anodic nitrocarburising provides saturation of the titanium alloy with oxygen, nitrogen and carbon and the formation of TiO₂ with a rutile structure and a nitrogen/carbon solid solution in titanium. The anodic plasma electrolytic nitrocarburising at all treatment temperatures diminishes the wear rate of the titanium alloy samples and the surface roughness. Friction coefficient of treated samples can be reduced by 4.7 times. The anodic nitrocarburising results in an enhanced corrosion resistance since the corrosion potential increases by an order of magnitude.

Titanium alloy implants are widely employed in biomedical devices and components, especially as hard tissue replacements as well as orthopaedic applications, owing to their favourable properties such as high-strength to weight ratio, low density, low Young’s modulus and biocompatibility. However, metallic implants cannot meet all of the clinical requirements. Therefore, in order to increase their clinical success and long term stability in the physiological environment, surface modification is often performed. This review focuses on the latest achievements in the field of surface modification techniques including sol-gel, thermal spray, magnetron sputtering, electrophoretic deposition and micro-arc oxidation of biocompatible calcium phosphates (CaP) based ceramics coatings for metallic implants with emphasis on the structure, morphological characterization, phase transformation and coating composition. A reflection on the results shows that CaP coatings can be grown with the each type of techniques and a stronger fixation can be enhanced with CaP fabrication on metallic implants. Advantages and limitations of the aforementioned techniques of CaP-based coatings from the point of view of the process simplicity as well as the most important challenges of each coating techniques are highlighted. Further, the most promising method for CaP deposition was identified and a specific area for improvement was discussed.

The purpose of this paper is to provide an overview of the impact of the microstructural modifications via severe plastic deformation on the corrosion behavior of magnesium and its alloys, especially when they are considered to be biodegradable materials. Mechanical processing involved in grain refinement modifies textures and residual stresses of materials which have their own impacts on corrosion behavior as reported in a large number of studies. However, the existing literature on the influence of microstructure on corrosion resistance is often contradictory, which discloses a lack of knowledge in this area. In this article the effects and contributions of critical factors such as the grain size, texture, residual stress and second phase distribution, affected by severe plastic deformation, on the magne-sium corrosion behavior is reviewed in order to find a relation between the microstructure and corrosion resistance.

This paper presents the results of the experimental and theoretical studies of the flows of a weakly conducting liquid in a narrow coaxial channel with dielectric walls, in which the liquid is glowing. It is experimentally shown that in addition to visible light radiation, there is also electromagnetic radiation in the radio range and X-ray radiation. Various physical mechanisms that can cause these phenomena are discussed. It is assumed that the main cause of the observed phenomena is associated with the electrization of the liquid during its flow in a channel of a complex shape. The results of the calculations of electrization in the electro-hydrodynamic two-ion model of the medium are given, taking into account the electrochemical processes at the interfaces and convective charge transfer with the formation of strong electric induced fields near the walls.

This paper deals with the elaboration of a stable suspension of TiO₂ nanoparticles and their incorporation by electrophoretic deposition into pores of an anodized 5754 aluminum alloy. The as-synthesized TiO₂ nanopowder was characterized by the X-ray diffraction, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy and IR spectroscopy. During this work, both the transmission electron microscopy and particle analysis showed that the resulting particles had a narrow size distribution with a crystallite size of about 15 nm. The zeta potential and stability of TiO₂ nanoparticles dispersed with poly(acrylic acid) in an aqueous solution were also measured. A porous anodic film was synthesized in the phosphoric acid-base electrolyte and then filled by 15 nm TiO₂ particles via electrophoresis. In addition, the effect of poly(acrylic acid) and pH on the suspension stability has been investigated. It was also demonstrated that by adding glycine in buffered suspension gelating phenomenon can be avoided that inhibits the insertion of nanoparticles inside the pores of an anodic film. It was also noted that an applied electric field greatly influences the electrophoretic deposition process. The field emission gun-scanning electron microscopy observations showed that larger (125 nm in diameter) and linear (6 μm in length) pores are successfully filled in 5 min.

To make use of the full capability of electrochemical micro-machining (EMM), a meticulous research is needed to improve the material removal, surface quality and accuracy by optimizing various EMM process parameters. Keeping this in view, an indigenous development of an EMM machine set-up has been considered to carry out a systematic research for achieving a satisfactory control on the EMM process parameters to meet the micromachining requirements. In this study an EMM machine has been developed and experiments were conducted to study the influence of some of the major process parameters such as the machining voltage, electrolyte concentrations, the pulse-on-time and the machining current on the machining rate and accuracy. The effect of the shape of the tool electrode tips on EMM has been investigated experimentally with 304 stainless steel sheets. The machining rate and the overcut are significantly influenced by the shape of the tool electrode tip.

The birch leaf litter in a DC corona discharge is processed to increase the sorption capacity with respect to the iron ions Fe²⁺ and Fe³⁺. It is shown that the corona treatment increases the sorption capacity of a birch leaf litter with respect to the iron ions. The corona treatment parameters (voltage and time of polarization) are determined at which the maximal sorption capacity of the birch leaves is attained. The thermodynamic parameters of the process are calculated. The obtained values of the activation energy (13.39 kJ/mol for Fe²⁺ ions and 14.56 kJ/mol for Fe³⁺ ions) follow the laws of physical sorption.