Effect of reinforcing submicron SiC particles on the wear of electrolytic NiP coatings Part 2: Bi-directional sliding

As-plated and heat-treated electrodeposited NiP and composite NiP-SiC coatings were investigated in bi-directional ball-on-disc sliding tests. All tests were performed under gross slip conditions. Heat treatment decreases the wear volume loss during fretting in ambient air for all coatings investigated. Heat-treated NiP coating has a lower wear volume loss compared to composite NiP-SiC coatings for all sliding tests. The wear rate at the bi-directional sliding test was found to be lower relative to the wear rate at uni-directional sliding test

Effect of reinforcing submicron SiC particles on the wear of electrolytic NiP coatings Part 1. Uni-directional sliding

As-plated and annealed NiP coatings and composite NiP-SiC coatings were investigated in uni-directional ball-on-disc sliding tests. Abrasive wear was noticed in the case of composite NiP coatings containing submicron SiC particles, whereas in NiP coatings oxidational wear was active. The addition of submicron SiC particles not only increases the hardness of these electrolytic coatings but also hinders the formation of an oxide film in the sliding wear track. As a consequence, the wear loss on as-plated NiP coatings is not markedly reduced by the addition of SiC particles. On the contrary, a heat treatment at 420 °C for 1 h decreases the wear loss on both pure NiP and composite NiP-SiC coatings. During that heat treatment, Ni3P precipitates are formed in the NiP matrix and owing to this fact, the hardness of both pure NiP and composite NiP-SiC coatings increases. However, the heat treatment of composite NiP-SiC coatings induces the sensitivity for crack formation in the NiP matrix around these SiC particles. As a result, the pull out of SiC particles in the wear track occurs easily during sliding, and the wear loss of composite NiP-SiC coatings remains above the wear loss on NiP coatings

Electrochemical study of the repassivation of titanium in different artificial saliva solutions

The passive film presented at the dental implant surfaces can be damaged or eventually destroyed during insertion and implantation into hard tissue due to abrasion/wear with bone or other materials. However, when the wear action stops, the surface will tend to regenerate immediately, and a new passive film will be formed. In such conditions we are in the presence of a tribo-electrochemical phenomenon which comprises the analysis of two different processes and of the synergism effects between them. In fact, the mechanisms of mechanical degradation due to wear might be influenced by the presence of a corrosive environments, but the electrochemical behaviour of the material is likely to be modified by the presence of the mechanical solicitation and by the presence of wear debris and/or the formation of tribolayers. Also, the repassivation kinetics of the tribocorrosion system during or after mechanical damage becomes an important issue to be studied. This work deals with the study of the repassivation of titanium when in contact with artificial saliva solutions, after mechanical damage. Samples were immersed in different kinds of artificial saliva solutions (artificial saliva (AS), AS + citric acid, AS + anodic, cathodic or organic inhibitor). After stabilisation the passive film was mechanically disrupted and the open-circuit potential (OCP) was monitored both during the mechanical damage and until the repassivation was completed. Additionally, and in order to evaluate the quality of the passive film, EIS measurements were performed before and after mechanical disruption of the passive film. The effect of pH variation and of electrolyte composition on the repassivation evolution was also investigated. Considering the evolution of the open circuit potential represented in figure 1 the approximation ln(E) = ln(k) + b*ln(t) was used to study the repassivation evolution with the time. As it can be observed, the open circuit potential achieved before and after the mechanical damage varies, which indicates that the nature of the electrolyte influences the properties of the passive film. As indicated by the b values presented in Table 1, after repassivation, the AS + citric acid is the solution that provides better repassivation evolution with the time. In contrast, the AS + cathodic inhibitor is the solution that provides worst repassivation evolution. However, the EIS results suggests that AS solution is that providing the most stable and thick passive film. The results obtained with the AS + cathodic or + organic solution shows that these solutions do not have a good influence on the film growth

Tribocorrosion behaviour of titanium in artificial saliva solutions

Dental implants are used to replace teeth lost due to decay, trauma or periodontal disease. In last years, the demand for such implants has increased exponentially due mostly to the ageing of the population and/or increased reliability of implant therapy. To choose a material for a particular rehabilitation treatment different characteristics must be considered such as mechanical strength, elastic properties, machinability, chemical resistance, etc. There is, however, one aspect that is always of prime importance; namely how the tissue at the implant site responds to the chemical disturbance imposed by the presence of a foreign material. In this work, commercially pure titanium (grade 2) was selected mainly due to its excellent corrosion behaviour and biocompatibility, that make this material one of the most used in dentistry. The tribocorrosion behaviour of the material in conditions simulating mastication, when in contact with artificial saliva solutions was investigated. Corundum was used as counterface material. The tests were performed, at MTM- KULeuven (Belgium), in a fretting machine specially adapted for tribocorrosion experiments. A normal load of 2 N was applied to the specimens and a displacement amplitude of 200µm at a frequency of 1 Hz. The number of cycles was varied between 5.000 and 10.000. In order to simulate different environmental conditions, simple artificial saliva (0.7%KCl + 1,2%NaCl; pH =6), artificial saliva with corrosion catalyst (citric acid), and artificial saliva with anodic, cathodic and organic corrosion inhibitors (sodium nitrite, calcium carbonate or benzotriazole, respectively), was used. The tribocorrosion degradation mechanisms were investigated by Electrochemical Noise technique (TEN), which combines Electrochemical Emission Spectroscopy with fretting tests [1]. After being tested, the surface of the samples was investigated by SEM and EDX. Roughness measurements and wear volume measurements were also performed. The depassivation/repassivation phenomena occurring during the tests were clearly detected, and are discussed. Considering the influence of the corrosion inhibitors, it was possible to observe that the degree of protection varies with the nature of the inhibitor.(undefined

Influence of the structural, physical and chemical characteristics of titanium surface oxide layers on its tribocorrosion behaviour in contact with an artificial saliva solution

Titanium has been widely used for dental implant applications because their attractive properties such as low Young modulus, good fatigue strength, excellent corrosion resistance and biocompatibility. However it cannot meet all clinical requirements. 1-2 When inserted in the oral environment, dental implants are submitted to a complex degradation phenomena occurring due to the interaction of tribological and corrosive phenomena which can result on its failure. In fact, most of the failures are a result of the combined action of mechanical solicitations (sliding or abrasive wear, erosion, impact, fretting or fatigue processes) and chemical solicitations as a consequence of contact with saliva, cells or bacteria. Consequently, dental implants become part of a tribocorrosion system, and the investigation of the tribocorrosion mechanisms in such systems becomes essential. 3 Recent work has shown that wear resistance of titanium might be improved selectively using the appropriate surface treatment, such as plasma oxidation, which is also often used to enhance the osseointegration process. 4-5 The main aim of this paper is to study the influence of the characteristics of the oxide layers obtained by plasma oxidation on the tribocorrosion behaviour of c.p. Ti intended for the fabrication of dental implants.The plasma oxidation treatment were performed at three different temperatures (300, 450 and 530ºC) and a detailed study of surface topography (Fig. 1), thickness, structure and chemical composition of the oxidized layers was performed. Tribocorrosion experiments were performed using a reciprocating motion pin-on-plate tribometer with an alumina pin as a counterface material in the presence of Fusayama solution. The applied normal force used was 3N, the stroke length 1mm and the displacement frequency was 1Hz. During rubbing the friction coefficient was monitored. Electrochemical techniques were used to follow the evolution of the system during sliding. Each experiment involved an in situ surface cleaning by a cathodic polarization at -900mv during 3 minutes, and after that a potentiostat was used to impose a passive potential to titanium samples. This experiment allows in situ and in real time following of the corrosion kinetics through current measurements. It is important to refer that electrochemical impedance spectroscopy was also used before and after rubbing in order to evaluate the protective character of the oxide film. The influence of the thickness of the oxide layer and of surface roughness on the wear-corrosion behaviour of the material is discussed in detail. Results clearly showed that plasma oxidation treatments have a strong influence on the tribocorrosion behaviour of titanium. Both the corrosion and wear performance of the samples are improved by the increase of the processing temperature. In relation to friction profile no significant differences were found between all samples tested (μ remains between 0.35 and 0.45). Results show that electrochemical techniques yield useful results

Alzheimer disease genetic risk factor APOE e4, and cognitive abilities in 111,739 UK Biobank participants

Background: the apolipoprotein (APOE) e4 locus is a genetic risk factor for dementia. Carriers of the e4 allele may be more vulnerable to conditions that are independent risk factors for cognitive decline, such as cardiometabolic diseases. Objective: we tested whether any association with APOE e4 status on cognitive ability was larger in older ages or in those with cardiometabolic diseases. Subjects: UK Biobank includes over 500,000 middle- and older aged adults who have undergone detailed medical and cognitive phenotypic assessment. Around 150,000 currently have genetic data. We examined 111,739 participants with complete genetic and cognitive data. Methods: baseline cognitive data relating to information processing speed, memory and reasoning were used. We tested for interactions with age and with the presence versus absence of type 2 diabetes (T2D), coronary artery disease (CAD) and hypertension. Results: in several instances, APOE e4 dosage interacted with older age and disease presence to affect cognitive scores. When adjusted for potentially confounding variables, there was no APOE e4 effect on the outcome variables. Conclusions: future research in large independent cohorts should continue to investigate this important question, which has potential implications for aetiology related to dementia and cognitive impairment

Tribological behaviour of oral mixed biofilms

The use of dental implants has been increasing even though failures do occur. The presence of wear debris and oral microorganisms can contribute to infections and jeopardize implant integration. The aim of this work was to study the influence of mixed biofilms in the tribological behaviour of commercially pure titanium for dental implants under different concentrations of fluoride. Samples of titanium with two different surface topographies were used. Mixed biofilms of Candida albicans and Streptococcus mutans were formed on both surfaces at 37 °C in a tryptic soy broth containing mucin, peptone, yeast extract and sucrose. After 8 days, biofilm biomass was analysed by crystal violet staining method. Biofilm biomass was significantly higher for the samples with higher roughness. Some samples with biofilms were analysed under friction (using a force of 100 mN) in an artificial saliva solution (Fusayama) without or with different concentrations of fluoride (30 and 227 ppm). It was verified that the coefficient of friction (COF) decreased in the presence of biofilms. Moreover, samples with more biomass (0.4 μm of roughness) presented the lowest values of COF. Concerning the effect of the presence of fluoride, although there were no significant differences on the COF for 30 ppm, for 227 ppm a transition regimen was observed. These results were confirmed by sample observation under scanning electron microscopy. In conclusion, it can be highlighted that biofilm formation on dental implants can significantly affect the tribological behaviour of titanium, namely, the presence of biofilms reduces the release of wear debris

Repassivation of commercially pure Ti in different saliva solutions under tribocorrosion conditions

The surface of dental implants can undergo wear during insertion and implantation into hard tissue, or, in some conditions, during its lifetime. As a consequence, the passive film presented at the implant surface can be damaged or even totally destroyed. However, if the wear action stops the surface might regenerate, giving origin to a new passive film. As all the process occurs in the presence of a chemical aggressive environment, human saliva, the material is under tribocorrosion conditions. It is also possible to introduce the concept of tribo-electrochemistry which may include two main research areas: the tribocorrosion, where the mechanical solicitations in corrosive environments are studied, and the electrochemistry of film free surfaces, where repassivation kinetics is studied after the removal of the protective oxide film on a passive metal. The main aim of this work was to study the repassivation evolution of commercially pure Ti in artificial saliva solutions. Grade 2 commercial pure titanium samples were subjected to a small alternative sliding in a pin-on-plate tribometer against a corundum ball. At the same time, open-circuit potential (OCP) measurements were performed, before, during and after mechanical disruption of the passive film. Also, to obtain a more detailed information on the characteristics of the original and reformed passive film, EIS measurements were done before and after the mechanical damage. All the test were performed in different kinds of artificial saliva solutions (artificial saliva (AS), AS + citric acid, AS + anodic, cathodic or organic inhibitor). Additionally, the effect of pH and electrolyte composition on the repassivation evolution was also investigated. Finally, all samples were characterized using SEM, EDS, and AFM. Surface roughness was also evaluated. Results show that, in some solutions, the open circuit potential, after repassivation, is more noble than that measured before sliding. Also, the repassivation evolution appears to be strongly affected by the electrolyte nature. The AS + citric acid is the solution that provides a better repassivation evolution with the time, however the stability of the passive film takes some time to be acquired. Also, this solution does not provide a very thick film