M. Fiset

Electroformed pure iron as a new biomaterial for degradable stents: In vitro degradation and preliminary cell viability studies ☆

In the search for a metallic material showing moderate and uniform degradation for application as degradable cardiovascular stents, electroformed iron (E-Fe) was evaluated by in vitro degradation and cell viability tests. Static immersion and dynamic degradation were used to evaluate degradation rate and mechanism, while cell viability assay was used to assess cytotoxicity. The results were compared with those of iron fabricated by casting and thermomechanical treatment previously investigated as a stent material. Electroformed iron showed faster degradation than iron fabricated by casting (0.25 vs. 0.14 mm year(-1)), with a uniform degradation mechanism. Cell viability results showed that E-Fe did not result in a decrease in metabolic activity when exposed to primary rat smooth muscle cells. However, it caused a decrease in cell proliferation activity which could be beneficial for the inhibition of in-stent restenosis.

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M7C3 carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M7C3 and V6C5 carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M23C6 carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M7C3 carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.

The effect of sand concentration on the erosion of materials by a slurry jet

Abstract Slurry erosion tests were performed to evaluate the effect of sand concentration on the erosion rate. By keeping the slurry velocity constant, different particle fluxes were obtained by varying the sand concentration in the slurry. The erosion tests were done with an erosion machine which gave a narrow slurry jet with a velocity of 17 m s−1. The slurry jet was always oriented toward the surface at a normal incidence angle. It was observed that the erosion rate decreased according to a power law of the sand concentration in the slurry. This effect can be attributed to the flow conditions near the surface giving rise to rebounding particles which protect the surface from incident particles. The scale of this effect also depends on the incidence angle of individual sand particles which are forced to deviate near the surface. Because there is a distribution of incidence angles near the surface, the relation between the protection effect and the incidence angle can explain partially the erosion profile of the eroded surface.

Degradation Behaviour of Metallic Biomaterials for Degradable Stents

The short-term need of scaffolding function of stent and the prevention of potential longterm complication of permanently implanted stent have directed to the original idea of biodegradable stent. Selecting and developing materials showing appropriate mechanical and degradation properties are key steps for the development of this new class of medical devices. Therefore, the study of their in vitro degradation behaviour is mandatory for the selection of potential candidate materials suited in vivo. In this work, the degradation behaviour of current studied biodegradable metals including three magnesium alloys (Mg, AM60B and AZ91D), pure iron and Fe-35Mn was investigated. The tests were performed in a simulated blood plasma solution at 37±0.1 oC, using three different methods; potentiodynamic polarization, static immersion, and dynamic test in a test-bench which mimics the flow condition in human coronary artery. Degradation rate was determined as ion release rate measured by using atomic adsorption spectroscopy (AAS) and also estimated from weight loss and corrosion current. Surface morphology and chemical composition of corroded specimens were analyzed by using SEM/EDS. The three degradation methods provide consistent results in corrosion tendency, where Mg showed the highest corrosion rate followed by AZ91D, AM60B, Fe-35Mn and iron. Potentiodynamic polarization gives a rapid estimation of corrosion behaviour and rate. Static immersion test shows the effect of time on the degradation rate and behaviour. Dynamic test provides the closest approach to the environment after stent implantation and its results show the effect of the flow on the materials degradation. In conclusion, the three investigated methods can be applied for screening, selecting and validating materials for degradable stent application before going further to in vivo assessments.

Influence of structural parameters on the slurry erosion resistance of squeeze-cast metal matrix composites☆

Abstract Metal matrix composite specimens were made by squeeze casting, where 5083 aluminum alloy infiltrates a fiber preform. The ceramic fibers consisted of Al 2 O 3 (47%) and SiO 2 (53%). The use of different preforms resulted in specimens with fiber volume fractions of 10%, 15% and 20%. Erosion tests were performed with a slurry jet having a velocity of 15 m s −1 and a sand concentration of 10 wt.%. Different size distributions of silica sand particles were chosen as abrasive. The results are presented in the form of erosion profiles of different specimens. Although the erosion resistance increases with fiber volume fraction, the erosion profile variations are negligible. Due to the trajectory deviation of abrasive particles near the surface, the erosion conditions (particle velocity, local impact angle) are greatly modified for different abrasive particle sizes. Large abrasive causes fiber fracture which results in poor protection of the matrix by the fibers. This results in gradual erosion of the surface with no relief. For small abrasive particles, the erosion of the matrix is more important than that of the fibers, resulting in a rough eroded surface, especially in the region where the local impact angle is small. These results are interpreted in terms of the structural parameters of the composites.

Fundamental investigations of electrical conductor fretting fatigue

Fretting is known to be the main factor leading to conductor individual wire breaks under aeolian vibration in the vicinity of a clamp. In this paper, previous studies on overhead electrical conductor bending fatigue are summarized. Results obtained with several conductor types and clamps are compared. A general fretting analysis as well as testing procedure are suggested. Influence of the main mechanical parameters on the occurrence of several types of degradation processes is discussed.

Fretting fatigue in electrical transmission lines

Abstract One of the worst problems in overhead electrical transmission line fatigue is fretting, which can lead to wire failure or even conductor breakage at suspension clamps. However, fretting behaviour is extremely complicated owing to its synthetic geometry and loading condition. The tests concern bending fatigue on the overhead electrical conductor DRAKE. The imposed amplitude ranges from 0.60 to 0.80 mm. The static axial load was set at 25% RTS (Rated Tensile Strength). The tests were performed (including the static test) with a frequency of 10 Hz for any number of cycles. Post-test studies of the fretting scars were carried out by OM and SEM. Plastic flow, local wear and fretting cracking were observed at different conductor locations. Fretting cracking depends strongly on fretting regime, and the later propagation on the external load. Fretting regime and external load are determined by numerous factors such as contact type and position. Through metallographic examinations, it has been demonstrated that fretting is most likely to lead to cracks in the vicinity of the keeper contact edges, and probable propagation occurs at the upper half-section for the outer contact. The wire failure is found precisely in this zone and it fits our analysis well.

Microstructure and wear resistance of CP titanium laser alloyed with a mixture of reactive gases

Abstract Laser processing is a promising technique for alloying and synthesis of wear resistant layers. In this work, commercially pure titanium was laser alloyed with various proportions of nitrogen and carbon monoxide in order to produce a composite surface layer. The following gas mixtures were selected: 100% N 2 , 67% N 2 + 33% CO, 50% N 2 + 50% CO, 33% N 2 + 67% CO and 100% CO. The microstructure, roughness and composition of coated specimens were characterized. The presence of Ti(C,N,O) in the surface layer was assessed by X-ray diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The abrasive wear resistance of coatings was determined and related to their surface hardness. The abrasive wear resistance of the laser alloyed coating was substantially improved compared to untreated titanium and appears to be superior when composed of Ti(C,N,O) prepared from a mixture of gases.

Effect of lubricant in electrical conductor fretting fatigue

Bending fatigue tests have been performed on the lubricated electrical conductor ZEBRA held with two different spacer clamps. A constant axial load of 25% RTS (rated tensile strength) was imposed on all specimens. After each test, fretting patterns have been studied for each aluminium layer. OM and SEM metallographic examinations have been carried out to study fretting cracking behaviour. Compared with a similar unlubricated conductor, results show that the lubricant plays an important role in preventing contact wear and in retarding fretting cracking. Further tests on single lubricated and unlubricated aluminium wires show that influence of lubricant on fretting fatigue strongly depends upon the contact conditions (normal load and slip amplitude).

Single wire fretting fatigue tests for electrical conductor bending fatigue evaluation

Abstract Overhead electrical conductors are often subjected to aeolian vibrations which may induce fretting fatigue damage of individual aluminium wires in suspension clamp regions. Many bending fatigue tests have been performed on electrical conductors. Depending on the test conditions, wire fracture may be found to occur in the external as well as internal layers. Individual wire fretting fatigue is very difficult to predict due to a conductor complex structure and dynamic mechanical behaviour. The main objective of this work is to present experimental results obtained from tests on single wires under conditions simulating a typical conductor-clamp contact. A fretting fatigue test bench specifically designed for such simulation has been used on single H19 aluminium wires. They have been subjected to an initial minimal axial stress of 59 MPa. At the fretting point, a transverse compressive load of 130 N to 4000 N has been imposed, as well as an alternating displacement of 100 to 900 μm displacement amplitude. Cycling frequency has been kept at 10 Hz and test duration went up to 1.6 × 10 7 cycles. All tests were performed in the stick-slip regime occurring in the plastically deformed contact zone. No global slip was allowed. Subsequent examination of the fretting scars at the contact surface and through the cross-section have been carried out by optical microscopy and scanning electron microscopy. The mechanical parameters influence is studied and comparison with results from complete conductor fatigue tests is discussed.