Advanced Materials Research Vol. 445

Abstract: Oxidation of chromium carbide coating formed on AISI 1040 steel deposited by thermo-reactive deposition method (TRD) has been realized by two stepped reactions. In the initial part of the reactions in the oxidation process, carbon atoms combined with chromium on the outer part of the coating layer react with the oxygen in air, effectively up to 120 min. After that, the chromium atoms react with oxygen in the air and produce Cr₂O₃ phase on the coating layer. The higher the temperature and the longer the treatment time, the more the Cr₂O₃ phases became. The kinetic study was realized for the reactions of carbon and chromium with oxygen, individually. The kinetic study of oxidation was calculated by weight changing of the coated samples at the temperatures of 973 K, 1073 K and 1273 K up to 720 min. We established that the chromium carbide coated steel are characterized by an insignificant increase in the mass in the oxidation period up to 3.5 h, after which the degree of oxidation increases somewhat. The nature of oxidation kinetics for chromium carbide coated steel varies from some mass degrease in the initial period ( 2 h) in connection with the formation of CO and CO₂ to later mass increase with in connection with the formation of Cr₂O₃ layer. The oxidation resistance of chromium carbide coated steel decrease with an increase in oxidation temperature. The growth rate constant of oxidation of chromium carbide coated steel ranged from 5.13x10^-13 to-9.617x10^-11 g⁴.cm^-2s^-1 in the initial period of oxidation (up to 120 min), while it ranged from 3,163x10^-13 to 2.188 x10^-10 g⁴.cm^-2s^-1 in the second period of oxidation test (over 120 min). The activation energies of oxidation of the chromium carbide coated steel are 185 kJ/mol for the initial period and 215 kJ/mol for the second period.

Abstract: Surface cladding utilizes a high energy input to deposit a layer on substrate surfaces providing protection against wear and corrosion. In this work, TiC particulates were incorporated by melting single tracks in powder preplaced onto AISI 4340 low alloy steel surfaces using a Tungsten Inert Gas (TIG) torch with a range of processing conditions. The effects of energy input and powder content on the melt geometry, microstructure and hardness were investigated. The highest energy input (1680 J/mm) under the TIG torch produced deeper (1.0 mm) and wider melt pools, associated with increased dilution, compared to that processed at the lowest energy (1008 J/mm). The melt microstructure contained partially melted TiC particulates associated with dendritic, cubic and globular type carbides precipitated upon solidification of TiC dissolved in the melt; TiC accumulated more near to the melt-matrix interface and at the track edges. Addition of 0.4, 0.5 and 1.0 mg/mm² TiC gave hardness values in the resolidified melt pools between 750 to over 1100Hv, against a base hardness of 300 Hv; hardness values are higher in tracks processed with a greater TiC addition and reduced energy input.

Abstract: Production of defect-free galvanized steel sheet is considered a major concern for automotive and other critical applications; nevertheless, the occurrence of some defects in the coated sheets is unavoidable. In order to alleviate the problem, we need to know the extent to which the properties of a galvanized sheet are influenced by the presence of a given defect. In this investigation, specimens including any of the two major defects of continuously galvanized steel sheets were selected from a large number of coated samples. The defects, including furnace roll pimples and bare spots, were microstructurally characterized and their influence on corrosion behaviour and mechanical properties of the steel sheet was evaluated. Corrosion resistance was examined via standard salt spray test and Tafel polarization. Tensile test was employed as a measure of mechanical properties of the defective galvanized sheets. The results indicated that the presence of defects had little influence on the tensile properties of the samples, but considerably reduced their corrosion resistance. Based on the results of salt spray tests, pimples reduced corrosion resistance of galvanized sheets 23 % (50 hours) on average and bare spot defects caused reduction in corrosion resistance up to 39%.

Abstract: Niobium nitride (NbN) films were deposited on Nb using pulsed laser deposition (PLD), and the effect of substrate deposition temperature on the preferred orientation, phase, and surface properties of NbN films were explored by x-ray diffraction (XRD) and atomic force microscopy (AFM). It was found that the substrate deposition temperature has a significant influence on properties of the NbN films, leading to a pronounced change in the preferred orientation of the crystal structure and the phase. We find that substrate temperature is a critical factor in determining the phase of the NbN films. For a substrate temperature of 650 °C 850 °C, the NbN film formed in the cubic δ-NbN phase mixed with the β-Nb₂N hexagonal phase. With an increase in substrate temperature, NbN layers became β-Nb₂N single phase. Essentially, films with a mainly β-Nb₂N hexagonal phase were obtained at deposition temperatures above 850 °C. Surface roughness and crystallite sizes of the β-Nb₂N hexagonal phase increased as the deposition temperatures increased.

Abstract: In this research wear mechanism of ADI under different intensity of loading with different hardness have been investigated. To study of wear behavior, a series of austempered specimens with optimum mechanical properties were used for wear tests. Dry sliding wear tests were carried out in pin-on-ring wear tester machine at speed of 0.5 ms-1 and loaded with normal loads of 100,200,300 and 400 N. Scanning electron microscopy for microstructure and wear surface analysis was used. To determine the austenite volume fraction and the percentage of carbon content in austenite, X-ray diffraction analysis was used. Results show that the role of retained austenite at wear properties of ADI is dependent on loading intensity and austenite carbon content.

Abstract: Hydroxyapatite (HA) is the main inorganic component in the human hard tissues, such as bones and teeth. The use of HA in load-bearing orthopedic applications has been extensively studied to achieve a biocompatible coating layer on implant metals. Commercial pure titanium (cp-Ti) is the most commonly used metallic material in the manufacture of orthopedic implants. Uncoated metal surfaces generally show numerous toxic effects to the surrounding tissues. However, coating of the metallic surfaces before implantation may prevent the interference of implant surfaces in biological media and also could accelerate the new bone formation. The present study focused on the coating of HA on cp-Ti implants, using novel biomimetic sol-gel method. The surfaces of cp-titanium substrates were coated with novel biomimetic sol gel (BSG) method under biological conditions (37°C and pH= 7.4). A bone-like apatite with a nanosized and pure HA structure was formed on the substrates under atmospheric pressure. The pre-coating results indicate that the novel biomimetic sol gel method is definitely a promising coating route in case that optimum coating parameters are established in detail.

Abstract: Hydroxyapatite (HA) is the major component of the natural hard tissues such as teeth and bone. It has been studied extensively as a candidate biomaterial for its use in prosthetic applications. However, the main weakness of this material lies in its poor mechanical strength which makes it unsuitable for load-bearing applications. On the other hand zirconia (ZrO₂) powder has been widely studied because of its high strength and fracture toughness and good biocompatibility. Therefore, the addition of zirconia phase into HA will improve the mechanical properties and biocompatibility of HA ceramics. The present study focused on coating of HA-ZrO₂ on commercially pure titanium (cp-Ti) using novel biomimetic sol-gel method. The HA-ZrO₂ coatings produced with BSG method were exhibited highly crystalline and pure structure. The coating thickness of the samples was not significantly influenced by the change in gelatin concentration and volume. It was concluded that the suggested coating method is a useful method to produce a biomimetic coating layer on the cp-Ti sample surfaces.

Abstract: Titanium (Ti) and Ti-alloys are often used in dental and orthopedic applications because of their good mechanical properties and biocompatibility. The advantages of Ti and Ti-alloys are its superior corrosion resistance, high fatigue strength and low elastic modulus which reduce stress shielding. Morover biocompatibility of them can be improved coating with bioceramics such as hydroxyapatite (HA) or other ceramic composites. The hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, H is frequently used as a coating material on the surfaces of Ti-based medical implants to improve the bone fixation and thus the lifetime of the implant is increased. However, the main weakness of HA lies on its poor mechanical strength that makes it unsuitable for load-bearing applications. An attractive way to produce the tougher HA is to use composite powders such as Yttria-Doped Zirconia-Hydroxyapatite (YSZ-HA) consisting of 8 mol% yttria-stabilized tetragonal zirconia (YSZ) so that the apatite phase increases the biocompatibility and zirconia (ZrO₂) phase improves the strength. Y₂O₃ addition into zirconia can stabilize the tetragonal phase at room temperature (YSZ) and the tetragonal phase plays a major role to increase the fracture toughness. In the present study yttria-dopped zirconia powders by using ZrO(NO₃)₂.xH₂O and Y(NO₃)₃.6H₂O were produced to synthesize HA-YSZ composites. In accordance with this purpose, at the first step, Ca (NO₃)₂.4H₂O, (NH₄)₂HPO₄ and YSZ powders were dissolved in simulated body fluids (SBF) to obtain sol. The gelatin solutions with different concentration were added into sol to provide the gelation. Then the surfaces of Ti implants were soaked in this solution. The coating rate of Ti samples was arranged as 14 cm/s and coated implants were sintered at 900°C. Structural analysis of coated powders was obtained by using XRD. Morphological examinations and coating thickness were investigated by SEM. After the sol-gel solution was dried at 80°C, dried-powder was sintered at 900°C. Sintered powders were analyzed by FT-IR to determine any gelatin residue.

Abstract: In this study tungsten inert gas (TIG) surface melting of pre- plasma sprayed WC-14%Co low carbon steel has been studied. Surface melting was performed under different heat inputs by using various TIG parameters including intensity and kind of current. Microstructure and microhardness of surface alloyed specimens were then studied. Eutectic structures containing tungsten-rich carbides were shaped in a matrix including martensite lath, when high heat input was used. Decreasing at heat input affected the microstructure of the alloyed layers, and high amount of faceted tungsten-rich carbides (Fe₃W₃C) were formed in fairly low heat input. Microhardness of melted layers highly improved in comparison with that of the substrate.

Abstract: The present study deals with a low carbon steel containing 0.09%C (weight %). This steel is produced by Trifisoud El-elma-Setif Algeria as wires and used for galvanization. Tensile tests are carried out on the material in as received conditions, i.e before, after deformation and after galvanization. Optical microscope and Secondary Electron Microscope (SEM) are used for microstructure characterization of the material. The results show that the initial microstructure is ferrite-pearlite with 11µm of ferrite grain size. SEM observations show that the galvanization layer is not regular and it thickness varies between 5 and 13µm. It has been found that the mechanical properties are affected by the rate of deformation. The annealing treatment at 530°C for different time follows the Avrami law. The two constants in Avrami law (n, k) are determined. The obtained results are compared with other published works.