2,854 research outputs found

    Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

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    The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers

    Effect of Heat-Treatment Time on Bending Properties of Cobolt-Chromium Orthodontic Wires

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    Different protocols appear in the literature with respect to heat-treating cobalt-chromium orthodontic wires. The objective was to determine the effect of variable heat-treatment time and method on the bending properties of CoCr wires. Two tempers of CoCr ‘Elgiloy’ wires (Rocky Mountain Orthodontics), blue (B) and yellow (Y), were heat-treated for different durations. The groups (n=20/group/temper) included: 1) as-received (control); 2) brush-flame; 3) 480oC for 5sec; 4) 480oC for 10min; 5) 480oC for 2hr; and 6) 480oC for 5hr. Wire segments were tested by a three-point bend test. Stiffness/flexural modulus, percent recovery, and force values at select deflections were statistically compared using ANOVA/Bonferroni post-hoc test (p\u3c0.05). A T-test compared the different tempers. Longer heat-treatment (2hr/5hr) increased % recovery, flexural modulus, and force values when compared to the as-received counterparts. Heat treatment for 10 minutes resulted in intermediate increases. Using a brush-flame technique reduced elastic recovery and resulted in greater bending variability. Similar mechanical properties can be achieved in just 2 hours compared to the manufacturer recommended 5 hours of heat-treatment of Elgiloy wires. Ten minutes of heat-treatment, which may be more realistic in a busy orthodontic practice, can increase bending properties 50-75% compared to the 5-hour group. The brush-flame technique is not recommended due to inconsistent heating conditions resulting in varying bending properties

    The application of Taguchi approach to optimise the processing conditions on bonnet polishing of CoCr

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    This paper applied the Taguchi approach to investigate the effects of each polishing parameter and obtain the optimal processing conditions for CoCr alloy polishing. The polishing medium was 1”m diamond paste with Microcloth(polishing cloth). Surface finish parameter Sa was chosen as criterion for optimization. The experimental result indicates that the optimal polishing condition for CoCr alloy polishing is 5deg of precess angle, 800 rpm of head speed, 0.2mm of tool offset and 1.5 bar of tool pressure. With this optimal condition, a confirmatory experiment was conducted. The surface roughness Sa reduced from initial 24nm to 7nm and reduction ratio was 72.5% which was very close to the estimated ratio 64%

    Cell adhesive peptides functionalized on CoCr alloy stimulate endothelialization and prevent thrombogenesis and restenosis

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    Immobilization of bioactive peptide sequences on CoCr surfaces is an effective route to improve endothelialization, which is of great interest for cardiovascular stents. In this work, we explored the effect of physical and covalent immoblization of RGDS, YIGSR and their equimolar combination peptides on endothelial cells (EC) and smooth muscle cell (SMC) adhesion and on thrombogenicity. We extensively investigated using RT-qPCR, the expression by ECs cultured on functionalised CoCr surfaces of different genes. Genes relevant for adhesion (ICAM-1 and VCAM-1), vascularization (VEGFA, VEGFR-1 and VEGFR-2) and anti-thrombogenicity (tPA and eNOS) were over-expressed in the ECs grown to covalently functionalized CoCr surfaces compared to physisorbed and control surfaces. Pro-thrombogenic genes expression (PAI-1 and vWF) decreased over time. Cell co-cultures of ECs/SMCs found that functionalization increased the amount of adhered ECs onto modified surfaces compared to plain CoCr, independently of the used peptide and the strategy of immobilization. SMCs adhered less compared to ECs in all surfaces. All studied peptides showed a lower platelet cell adhesion compared to TCPS. Covalent functionalization of CoCr surfaces with an equimolar combination of RGDS and YIGSR represented prevailing strategy to enhance the early stages of ECs adhesion and proliferation, while preventing SMCs and platelet adhesion.Postprint (author's final draft

    Fretting of CoCrMo and Ti6Al4V Alloys in Modular Prostheses

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    Implantation of a total hip replacements (THR) is an effective intervention in the management of arthritis. Modularity at the taper junction of THR was introduced in order to improve the ease with which the surgeon could modify the length of the taper section and the overall length of the replacement. Cobalt chromium (Co–28Cr–6Mo) and titanium (Ti–6Al–4V) alloys are the most commonly used materials for the device. This study investigates the fretting behaviour of both CoCr–CoCr and CoCr–Ti couplings and analyses their damage mechanisms. A reciprocating tribometer ball on plate fretting contact was instrumented with in situ electrochemistry to characterise the damage inflicted by tribocorrosion on the two couplings. Fretting displacements amplitudes of 10, 25 and 50 mm at an initial contact pressure of 1 GPa were assessed. The results reveal larger metallic volume loss from the CoCr–CoCr alloy compared to the CoCr–Ti alloy, and the open circuit potential indicates a depassivation of the protective oxide layer at displacement amplitudes .25 mm. In conclusion, the damage mechanisms of CoCr–CoCr and CoCr–Ti fretting contacts were identified to be wear and fatigue dominated mechanisms respectively

    Cytological response of palatal epithelium to TiN-coated CoCr alloy denture

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    The aim of this study was to assess the impact of titanium nitride coatings on CoCr alloy metal parts in framework dentures on human palatal epithelium cytology compared to framework dentures made with the same alloy but without titanium nitride coating, and to acrylic dentures. Every prosthetic restoration introduced into the oral cavity and remaining in direct contact with the palate exhibits a varied and harmful effect on the state of the palatal epithelium by disturbing its keratinization. CoCr alloy dentures produce a significantly greater perturbation of keratinization compared to acrylic dentures. There is no evidence showing that a titanium nitride coating of the CoCr alloy plays a protective role in the environment of the oral cavity

    Pelepasan Ion Kobalt dari Aloi CoCr L605 pada Saliva Asam

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    CoCr has high mechanical properties but may release ion in an acidic environment as in the oral cavity. Cobalt (Co) ion can release from CoCr alloy if such material is immersed in acidic environment.  This situation may lead to several consequences including allergies and oral lesions. The aim of this research was to determine the influence of pH degree on acidic saliva to the Co ion release concentration of CoCr L605 alloy. This research used CoCr L605 alloy and artificial saliva. Twelve samples of CoCr L605 metal plates were made into (10x10x1.5) mm size. The samples were divided into three groups; each group immersed in 20 ml of artificial saliva for 14 days, 37o C. The pH variables for the groups are 3, 5 and 7. Co ion released was measured by Atomic Absorption Spectroscopy (AAS). Data analysis was performed by one-way ANOVA (Analysis of Variance). The result showed that the mean of concentration value of Co ion released for group pH of 3, 5 and 7 were (3.0970±0.612)ppm; (2.0250±0.609) ppm; and 0.000 ppm respectively. One-way ANOVA showed that pH degree of acidic saliva significantly influenced by the Co ion release concentration (p<0.05). LSD test showed a significant difference between groups. The conclusion of this research was the less pH degree of saliva, the concentration of Co ion released of CoCr L605 alloy was higher

    A COMPARISON OF THE QUALITY OF DENTAL CROWNS FROM TI-6AL-4V AND COCR ALLOYS MADE WITH SLM TECHNOLOGY

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    At present, dental replacements are more and more often being made with the use of additive manufacturing. In dentistry, selective laser melting (SLM), which enables the manufacturing of dental replacements from the alloys CoCr and Ti-6Al-4V, is the technology most used. With comparable costs for production, Ti-6Al-4V has significantly better biocompatibility, better mechanical properties and lower weight in comparison with an alloy from CoCr. The goal of the submitted study is a comparison of the precision of production of dental crowns from CoCr alloy and Ti-6Al-4V alloy manufactured on an Mlab cusing R machine. For purposes of the study 30 crowns were manufactured from CoCr alloy and 30 from Ti-6Al-4V alloy on an Mlab cusing R machine, with the settings recommended by the manufacturer. After production the crowns were scanned using a Medit Identica dental scanner. The obtained data were subsequently modified and compared with a nominal CAD model in the Volume Graphics VGStudioMAX 2.2 software. The results are deviations on the level of 95% of coverage of the compared work with nominal geometry for all evaluated groups. The obtained deviations were subsequently processed statistically for the purpose of determining the statistical significance of the difference between the materials
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