Papers by Keyword: Nitrides

Abstract: The technology of restoring worn-out engine parts using modern techniques for applying wear-resistant diamond-like (DLC) coatings has been analyzed. To simplify the coating technology and reduce the wear of the highly adhesive deposited layer, applying metal-containing adhesive undercoats has been proposed. The camshaft surface wear types have been studied.The dependence of the change in the oil film thickness in the camshaft -follower contact zone on the cam rotation angle at a rotation frequency of 1,500 rpm is given. The contact causes an unsteady friction surface lubrication mode and intensive wear of the material in the contact zone.To apply a wear-resistant coating, ion etching has been proposed, followed by spattering a tungsten layer with a thickness of up to 10 μm on the product surface with subsequent 20 μm thick tungsten carbide-based coating using microwave plasma. Such a gradient structure over the cross-section of the wear-resistant coating allows achieving the best product performance.The values ​​for the change in surface temperature in the cam-follower friction pair contact zone are given. Nitride, carbonitride, and other types of wear-resistant coatings, their recommended thicknesses, and maximum allowable operating temperatures are given. The hardness of various coatings and possible types of their destruction are shown.The reasons for adhesive and cohesive delamination of wear-resistant coatings are shown, and ways to eliminate this phenomenon are proposed.

Abstract: In this study, the effect of a high-temperature annealing process on AlN is investigated. The high-temperature annealing process reduces the screw dislocation density of the AlN film to 2.1x10⁷ cm^-2. The AlN surface is highly flat. Through HRXRD and Raman spectroscopy, the stress mode changes in the sputtered AlN film before and after high-temperature annealing were studied in depth. Based on the HTA-AlN template, a high-quality, high-Al composition AlGaN epitaxial wafer, with a (0002) plane rocking curve FWHM of 246 arcsec , was prepared at 1080°C The growth mode of AlGaN grown directly on the AlN template at low temperature is summarized.

Abstract: This article is devoted to studiing the ion-plasma sprayed coating on the surface of copper-nickel alloy obtained by complex titanium nitride and zirconium nitride treatment. To evaluate the coating microhardness we use the method that provides the obligatory penetration of diamond-pointed pyramid through nitride layer. It is shown that nitride coating microhardness is much higher than the copper-nickel alloy microhardness. Furthermore, surface treatment provides the color of the nitride coating just as gold plated.

Abstract: Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Abstract: The aim of this work is to study the precipitation mechanism of the intermetallic phases present in duplex stainless steels (UNS S32205 and UNS S32750), as well as to find out the most suitable method for detecting and analyzing accurately these secondary phases, particularly Sigma-phase, Chi-phase, nitrides and carbides. The samples were characterized after a solution annealing at 1080^oC followed by an isothermal treatment at 830^oC from 1 min to 9 h, with the purpose of figuring out the mechanism of chi-phase nucleation and nitrides formation in relation with the sigma-phase. The study has two main objectives: 1) to find out the most suitable technique for the detection, identification and quantification of the secondary phases, obtaining the best results with the combination of field emission scanning electron microscopy (FESEM) and backscattered electron detector (BSE) in comparison with the optical microscopy (MO); 2) to study the influence of the chemical composition on the nucleation mechanism of the intermetallic phases. It has been concluded that molybdenum balance content in chi-phase related to sigma phase is close to two, consequently the kinetics of nucleation and growth of these phases is remarkably faster when this alloying element content in the steel is higher. Chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments carried out to the specimen wherein chromium nitrides role is a favorable site for the nucleation of sigma and chi phases.

Abstract: Supersaturated titanium-aluminum nitride (Ti_1-xAl_xN) is a very attractive material for a wide range of applications due to its high oxidation and wear resistance accompanied by high strength, hardness, thermal conductivity and thermal shock resistance. Currently, its applications are limited to coatings obtained by physical or chemical deposition. Bulk materials based on Ti_1-xAl_xN may be fabricated by powder metallurgy approach using powders synthesized by high-energy ball milling (HEBM), which composition corresponds to supersaturated Ti_1-xAl_xN solid solution. In the present study, thermal stability of the supersaturated Ti_1-xAl_xN solid solution was investigated. According to the quasi-binary TiN-AlN phase diagram, constructed using density functional theory (DFT) analysis, the concentration ranges, where decomposition takes place through spinodal decomposition or through nucleation and growth, were determined. Experimental study on thermal stability of solid Ti_1-xAl_xN solution powder was conducted by means of differential scanning calorimetry (DSC), Brunauer-Emmited-Teller (BET) and XRD. The results indicated that spinodal decomposition of Ti_1-xAl_xN starts at 800°C, while at temperature higher than 1300°C regular decomposition (nucleation and growth) is occur.

Abstract: Processes and products of combustion of systems of "sodium azide – aluminum and boron halides " are studied. Halide salts AlF₃, Na₃AlF₆, KBF₄ and NH₄BF₄ are used as precursors of Al and B elements. The measured combustion temperature is in the range from 900 to 1800°C. It is established that the end product of combustion synthesis consists of several phases, not only the target phases (AlN and BN), but also the by-products (Na₃AlF₆, К₂NaAlF₆, NaF, KF), part of which is removed by water washing. The composite powder of AlN-BN is formed only in the case of AlF₃ as precursor of Al; in doing so, it is formed with side, impurity product К₂NaAlF₆ or Na₃AlF₆. In the case of precursor Na₃AlF₆, the washed product of SHS may only be a by-product К₂NaAlF₆ or Na₃AlF₆ or along with the by-product, it may contain AlN or BN, but not their composition. That is explained by different combustion temperature.The structure formation of the composite powder is investigated. The obtained agglomerated nitride nano- and micropowders have an irregular and predominantly acicular shape with an average particle size of 50-170 nm in the agglomerates.

Abstract: The composite powder of AlN-BN is of interest from the point of view of the use for sintering the AlN-BN composite ceramics. The ceramics compared with AlN ceramics would have better thermal shock resistance, less britlleness, better machinability, good heat conductivity and tribological properties. Possibility of obtaining a composite powder of AlN-BN with the application of process of self-propagating high-temperature synthesis (SHS) from powder mixtures of NaN₃ and precursors of aluminum and boron was investigated. Halide salts AlF₃, Na₃AlF₆, KBF₄ and NH₄BF₄ were used as the precursors. It was shown that these mixtures are capable of burning and obtaining agglomerated composite powders from nanosized (50-170 nm) particles.

Abstract: The transparent β-Si₃N₄ ceramic with a whisker-like microstructure was prepared by hot-pressing at 2000 °C for 26 h, with MgSiN₂ as an additive. The resultant material achieves the maximum transmittance of 70 % at the wavelength of about 2.5 μm and the transmittance value keeps higher than 60 % in the range of 700-4500 nm wavelength, which is attributed to the very small amount of the intergranular amorphous phase along with high density. The present transparent β-Si₃N₄ ceramic exhibits an indentation fracture toughness of 7.2±0.3 MPa m^1/2.

Abstract: This paper presents the results of a study on the preparation of nanostructured nitride composition TiN-BN in the boron halide - sodium azide - titanium halide system by the azide SHS technology. SHS-Az technology makes it possible to produce powders having the desired properties, only in one step, without blending separately prepared powders of titanium nitride and boron nitride, and to obtain the final product in the form of micro-and nanosized powder nitride compositions TiN-BN by using complex halides of titanium and boron. Thus, the composite powder TiN and BN obtained from the KBF₄-NaN₃-Na₂TiF₆» and «NH₄BF₄-NaN₃-Na₂TiF₆ systems can be classified as a nanostructured powder (nanopowder).