Nickel-titanium and stainless steel are the most commonly used alloys for orthodontic treatments. Even though both are known to be resistant to corrosion, there are circumstances that can lead to undesired situations, like localized types of corrosion attack, wear during sliding of an archwire though brackets and breakdowns due to iatrogenic causes. The aim of this research was to analyse the influence of environmental effects on the corrosion and tribocorrosion properties of NiTi and stainless steel dental alloys. The effects of pH and fluorides on the electrochemical properties were studied using the cyclic potentiodynamic technique. The migration of ions from the alloy into saliva during exposure to saliva with and without the presence of wear was analysed using ICP-MS analyses. Auger spectroscopy was used to study the formation of a passive oxide layer on different dental alloys. It was found that lowering the pH preferentially affects the corrosion susceptibility of NiTi alloys, whereas stainless steel dental archwires are prone to local types of corrosion. The NiTi alloy is not affected by smaller increases of fluoride ions up to 0.024 M, while at 0.076 M (simulating the use of toothpaste) the properties are affected. A leaching test during wear-assisted corrosion showed that the concentrations of Ni ions released into the saliva exceeded the limit value of 0.5 %g/cm2/week. The oxide films on the NiTi and stainless steel alloys after the tribocorrosion experiment were thicker than those exposed to saliva only.
COBISS.SI-ID: 2283111
In the present work the friction and anti-galling properties of the h-BN in a steel-aluminium contact have been investigated, focusing on the possibility of replacing graphite with the h-BN in the aluminium forming operations. Extruded AA2014 alloy was used. Investigation included the effect of h-BN powder size and concentration on the level and stability of the coefficient of friction, galling resistance and the quality of the Al surface. Tribological and anti-galling properties at room and elevated temperature of 400 °C have been carried out on the load-scanner test rig. Graphite was used as a reference solid lubricant. In the case of the h-BN powder three different particle sizes were used, i.e., 0.5 µm, 5 µm and 30 µm, added to NGL class-2 industrial grease in the concentrations of 5%, 10% and 20%. Results show that white h-BN, as a solid lubricant, is capable of successfully replacing graphite and providing “clean” surface, but only under mild contact conditions. It has similar lamellar structure but inferior load-carrying capacity, with its lubrication and anti-galling properties very much depending on the powder size, concentration and temperature. Contrary to the graphite tribological properties of h-BN deteriorate with increased concentration but improve with temperature and powder size. Best results are obtained for concentration of 5% and 30 µm powder size.
COBISS.SI-ID: 1316522
Understanding the repassivation kinetics is very important for understanding corrosion and protection of metals. Passive films are constantly changing and adapting to the environment. Moreover, large spread of reaction rates governing them and particularly their small dimensions (on the order of nm) make their analysis challenging. In the present study, the repassivation kinetics is studied using the electrochemical noise measurements on 304 stainless steel samples in 0.5 M sodium chloride solution under open circuit potential. A part of the study will consists of determination and analysis of creation and migration of cathodic and anodic sites during the damage of passive film. For this purpose the coupled multi electrode array (CMEA) will be used. The primary advantage of CMEAs over planar electrodes is that each electrode is separately addressable, enabling spatial and temporal informationto be obtained simultaneously. In the present case, CMEA is made of5x5 array of stainless steel wires (0.8 mm diameter). All wires are interconnected through multichannel ZRA and the current exchange between all possible pairs can be monitored in real-time. On the other hand, no potential is impressed on the array or sample (freely corroding conditions). This is a distinct difference in comparison to numerous other repassivation studies doneunder potentiostatic conditions. A special scratching device will be used to obtain reproducible scratches while being able to vary length and speed. The aim of the study is to present the possibility of the use of coupled multielectrode arrays, for detection of different corrosion processes by monitoring the spatio-temporal evolution of corrosion and detection of cathodic or anodic sites. Different spectroscopic techniques (SEM, X-ray microtomography and metallography) will be applied at the end of exposure of electrode arrays to identify modes of corrosion and type of corrosion products.
COBISS.SI-ID: 2145639
In biomedical applications, NiTi alloys are used mainly because of their favourable shape memory and superelastic properties. However, in many applications the tribocorrosion properties of these alloys can be of critical concern. For this reason the electrochemical and tribocorrosion properties of superelastic NiTi sheet and orthodontic archwire were studied, taking into account their microstructures and the effect of different surface finishes. Inthe case of the electrochemical tests, samples were tested in artificial saliva, whereas in the tribocorrosion tests the experiments were performed in ambient air, distilled water, and artificial saliva, the latter as a corrosivemedium. In these tests, the total wear rate of the alloy samples was determined, together with the corresponding chemical and tribological contributions. It was confirmed that the microstructure of the investigated alloys had a significant effect on the measured electrochemical and tribocorrosion properties.
COBISS.SI-ID: 1982311
In this work the possibility of replacing graphite with h-BN in Al-forming lubricants was investigated, including the influence of h-BN powder size and concentration on its tribological performance and the aluminium%s surface quality. The results of the investigation show that h-BN, as a solid lubricant, is capable of successfully replacing graphite in Al-forming processes, while at the same time maintaining a clean surface without staining. The study has shown that the tribological performance, including the friction and wear, the lubrication-film stability and the surface quality, very much depend on the powder size and the concentration. Under the investigated contact conditions, the best performance was obtained for 30-micro m h-BN powder when added to the grease in a concentration between 10 and 20 %.
COBISS.SI-ID: 1076394