Abstract: The formation mechanism of the Ca–P coating on the porous nanocrystalline Ti-6Al-4V alloy is presented. The Ca–P compounds were cathodically deposited at different potential (from –0.5 to –10 V vs. open circuit potential), using a solution mixture of Ca (NO3)2 + (NH4)2HPO4 + HCl. Depending of the deposition potential, the atomic ratio of Ca/P in deposits is in the range from 0.25 to 1.71, which indicates that the coating composition corresponds in some cases to hydroxyapatite.
The Ca–P particles penetrate preferentially the pores inside, which improve bonding of the bioceramic layer to the metallic substrate. Increasing the cathodic deposition potential results in changes of the Ca–P morphology from thin porous, through cracked up to thick 90 μm continous coating. The porosity of the Ca–P decreases with increasing cathodic deposition potential. It is proposed the electric field enhancement mechanism of the electrolytic ions flow and Ca–P growth on the surface irregularities, such as pores and surrounding hillocks.
Abstract: The French Atomic Energy Commission has developed a High Temperature Electrolyzer having an original coaxial architecture. Important elements are spring-like interconnects which allow to cope with thermal dilatation. Specifications of these components are low-cost, long term electrical conductivity achieved by chemical and mechanical stability. Potential candidates are iron-based alloys on which thin silver-coating may be deposited. In this paper we study the interaction of such systems with a mixture of hot vapour and hydrogen that is representative of the electrolyzer environment. Oxidation kinetic is measured by thermogravimetry. The oxide layer composition and morphology of tested samples are then investigated using Energy Dispersive Spectroscopy (EDS) and Auger Electron Spectroscopy (AES). These analyses demonstrate that hydrogen content strongly impacts the oxidation mechanism.
Abstract: Materials utilizing dentin adhesive technology were developed to provide more effective seal of a root canal. The combined use of new bondable root filling materials and self-adhesive sealers may increase the fracture resistance of filled canals. The null hypothesis of a positive influence on tooth biomechanics by a modern root canal filling material was tested in this study. In order to determine the strength characteristics of the analyzed endodontically treated human tooth the FEM (finite element method) was applied. Lower premolar was used to create a three dimensional model. Its canal was obturated using vertical compaction technique, gutta-percha and resin sealer, access opening was filled with composite resins. Strain analysis of the several elements of the tooth restoration does not allow attributing positive influence of contemporary obturating materials on mechanical properties of root canal dentin. Hypothetically, such an influence would have a filling material with comparable to dentin modulus of elasticity and flexural strength. It was also stated that loading the tooth with 250 N had an effect on increasing the tensions on the interface between filling material – sealer – canal wall. The stresses exceeded 4 MPa, reaching in extreme areas 10 MPa, what practically outweigh adhesion strength of modern root canal sealers. This phenomenon may provoke detachment of a filling from canal walls and therefore led to bacterial leakage. Results of this biomechanical analysis of the endodontically treated human premolar are valuable for a proper evaluation of mechanical properties of modern endodontic filling materials.
Abstract: Using the specially designed ultrasonic cavitation facility enabling to measure the hydrogen permeation, the experimental evidence have been provided for hydrogen to ingress in the metal subjected to cavitation in the 3% NaCl solution under the open circuit conditions. Increase in the vibration amplitude resulted in the increase in amplitude of the electrochemical voltage and current pulses and in the hydrogen permeation rate, as well. The cavitation induced modification of hydrogen transport through the membrane has been stated on the base of different appearance of the hydrogen permeation transients at cavitation in comparison with those recorded for the unstressed membrane. The hydrogen behavior has been affected by the cavitation induced dynamic stresses, metal hardening and stress relaxation due to microcracking of the metal.
Abstract: Purpose of this paper is presentation of forms of hydrogen degradation in steel along with pointing out methods for hydrogen degradation prevention. The paper outlines influence of nitrided layer on 34CrAlNi7-10 steel to its susceptibility to hydrogen degradation. Investigation was carried out with the use of slow strain tensile rate test (SSRT). Fracture surfaces after SSRT test were examined with scanning electron microscope (SEM) to reveal a mode and mechanism of cracking.
Abstract: The paper presents results of research and failure analysis undertaken to determine failure causes of a steam turbine casing. After 130,000 hours of service the crack in a outer shell of the turbine casing was found. The inner shell of the casing was made of cast steel grade G21CrMoV5-7, and the outer shell of grade G20CrMo4-5. Following research were performed in order to determine causes of the casing failure: chemical analysis; microstructure examinations with the use of light microscope, scanning electron microscope (SEM); mechanical properties examinations using the Charpy impact test, and Vickers hardness test; fracture mode evaluation with SEM.
Abstract: Coatings of metal and ceramic composite were applied on the steel specimens using the subsonic process of flame spraying. The specimens were then subjected to both cold and hot plastic working by rolling and also cold pressing by the hydraulic press. Plastic working is an alternative to machining, as the method of finishing of coats applied by flame spraying. The article presents the findings of the research into the possibility of using plastic working (hot and cold rolling and pressing) to obtain the corrosion properties of the flame sprayed Ni-Al alloy coatings and Ni-Al-Al2O3 composite coatings. The alloy coatings had a single-phase structure, of the maximum 10% aluminium solubility in the crystal lattice of nickel, whereas in the composite coatings the volume content of non-metallic material Al2O3 was 15% and 30 %. After finishing the adhesion reduction, cracks on the surface and cross-sections of coatings was not observed. The largest value of strain hardening of alloy coating Ni-5%Al was stated after pressing. The composite coatings obtain by flame spraying be characterized by big surface roughness (Ra = 13.3 µm). The plastic working caused decrease surface roughness. Minimum value of Ra parameter was observed after hot rolling. It was found that maximal roughness was presented after pressing. The corrosion tests were performed in 0.01 M H2SO4 solution by potentiokinetic technique. The article presents the effect of dispersion phase of Al2O3 on corrosion properties of composite coatings on the nickel base. The corrosion rate was dependent on method of plastic working. The increasing drafts resulted in rise corrosion current density and decrease in value of corrosion potential.
Abstract: During power transient conditions in nuclear reactors, uranium oxide pellets expand and crack due to the increase in temperature and their poor thermal conductivity. Moreover, the cladding undergoes creep because of the external pressure, and its diameter shortens. These antagonistic phenomena lead to the establishment of a contact between the pellet and the cladding, called the pellet-cladding interaction. The synergistic effect of the hoop tensile stress and strain imposed on the cladding by fuel thermal expansion and corrosion by iodine released from the UO2 fuel as a fission product at the same time can lead to Iodine-induced Stress Corrosion Cracking (I-SCC) of the Zircaloy-4 cladding. I-SCC failures of zirconium alloys are usually described in three steps: initiation of cracks, intergranular subcritical propagation, and critical propagation with a brittle transgranular propagation mode . Transgranular propagation occurs as soon as the stress intensity factor overshoots a threshold value KI,SCC. It is the critical step and leads to the final ductile failure of the cladding. Transgranular cracks propagate by cleavage-like fracture on basal planes of the hexagonal lattice and fluting; it is the result of a competition between a plastic accommodation of the applied strain and the brittle fracture of basal planes by iodine assisted cleavage.
Abstract: The NiTi shape memory alloy passivated for 90 min by autoclaving has been studied towards corrosion performance in the Tyrode’s simulated body fluid using open circuit potential and EIS measurements. The surface morphology and thickness of the oxide layer was determined by XRR. The HREM was used to observe the cross-section of the thin foil and to confirm the amorphous state of the TiO2 layer and its thickness. Electrochemical measurements revealed a good corrosion resistance at the beginning of long-term (20 days) immersion. It was found that with the increase of immersion time, the corrosion resistance of the surface deteriorated after nearly 1 day of immersion due to occurence of pitting corrosion. The EIS method was used to detailed study on the electrolyte | passive layer interfacial properties. Equivalent electrical circuit for the pitting corrosion on the passivated NiTi alloy has been applied.