162 research outputs found
Microstructure research - Material engineering - New surfaces: a short overview of our research topics at the Institute for Functional Materials and the Material Engineering Center Saarland
Direct laser interference patterning for photocatalytically active self-cleaning surfaces
Laser surface structuring has long proven to be a suitable tool to produce surfaces that repel both water and bacteria on actively antibacterial and inert surfaces. As a mostly undesired side effect, the laser induced surface oxidation, that occurs when producing the pattern can also have a significant effect on the behavior of the surface. For this reason, the focus of this work is to employ direct laser interference patterning to not only affect the surface topography of titanium but to simultaneously produce a precisely tailored oxide layer. Using this approach, photocatalytically active surfaces were produced. Those surfaces can produce a variety of radicals when illuminated with UV-A light which are then used to degrade organic compounds like bacteria or organic waste. With this in mind, this work aims to develop a one-step production method for self-cleaning surfaces that uses not only the lasers topographical but also its chemical surface modifications to produce photocatalytically active surfaces with high surface area
Mechanical reliability of dental grade zirconia after laser patterning
The aim of this work is to test the mechanical properties of dental zirconia surfaces patterned with Nd:YAG laser interference (¿¿=¿532¿nm and 10¿ns pulse). The laser treatment produces an alteration of the topography, engraving a periodic striped pattern. Laser-material interaction results mainly in thermal effects producing microcracking, phase transformation and texturization. The role of such microstructural modifications and collateral damage on the integrity and mechanical performances has been assessed.
Laser patterned discs of zirconia doped with 3% mol yttria (3Y-TZP) have been tested before and after a thermal treatment to anneal residual stresses and revert phase transformation. Both groups of samples behave in a similar manner, excluding residual stresses and phase transformation from the origin of properties modification.
Result show that laser patterning induces a minor decrease in mechanical properties and surface integrity of 3Y-TZP surfaces. The biaxial strength decreases as a consequence of the damage induced by laser patterning. Fractographic observations identify preexisting defects enlarged by local laser interaction as the fracture origins. The Hardness and Young modulus of treated surfaces tested with nanoindentation also decrease slightly after laser treatment and this may be attributed to laser-induced microcracking.Peer ReviewedPreprin
Micropatterned 3D-printed PLLA/PLCL bioresorsable stents: degradation and influence of sterilization
Bioresorbable stents (BRS) are cylindrical scaffolds designed to provide a temporary support to the vessel wall while the structure slowly degrades until completely resorbed [1]. Current stent fabrication technology hinders local modification of the surface topography. This work presents a novel solvent-cast direct-write (SC-DW) 3D printing system to manufacture inner patterned BRS. Poly-L-lactic acid (PLLA) and poly(lactic-co-caprolactone) (PLCL) stents were obtained by cylindrical printing onto a Ø 3 mm rotating mandrel (Figure 1a) [2]. The ink consisted in a solution of high Mw PLLA or PLCL copolymer (95:5) in chloroform at 10% w/v and 12.5% w/v, respectively. Steel mandrels were modified by direct laser interference patterning with a femtosecond laser to obtain a linear micropatterning with a periodicity of 10 μm, which was transferred onto stents' luminal surface (Figure 1b). Stents biodegradation was characterized by an accelerated degradation assay in PBS at 50oC over 4 months and characterized in terms of mass loss, SEM, DSC, mechanical tests, GPC and 1 H-NMR. PLLA and PLCL stents underwent bulk degradation, with a sustained decrease in molecular weight and an increase in crystallinity as degradation proceeded. PLCL stents degraded 1.5 times faster than PLLA stents due to higher water penetration in amorphous regions. Finally, two sterilization methods were evaluated: γ-irradiation (8 kGy) and ethylene oxide (EtO). Whereas γ- irradiation induced chain scission and a marked decrease in molecular weight, no structural or chemical alterations were found after EtO sterilization (Figure 1c). In conclusion, customizable PLLA and PLCL BRS were successfully fabricated through SC-DW technique, showing luminal micropatterning for enhanced endothelialization and adequate degradation timeframe for resorption
Growth and thermal stability of TiN/ZrAlN: Effect of internal interfaces
Wear resistant hard films comprised of cubic transition metal nitride (c-TMN) and metastable c-AlN with coherent interfaces have a confined operating envelope governed by the limited thermal stability of metastable phases. However, equilibrium phases (c-TMN and wurtzite(w)-AlN) forming semicoherent interfaces during film growth offer higher thermal stability. We demonstrate this concept for a model multilayer system with TiN and ZrAlN layers where the latter is a nanocomposite of ZrN- and AlN- rich domains. The interfaces between the domains are tuned by changing the AlN crystal structure by varying the multilayer architecture and growth temperature. The interface energy minimization at higher growth temperature leads to formation of semicoherent interfaces between w-AlN and c-TMN during growth of 15 nm thin layers. Ab initio calculations predict higher thermodynamic stability of semicoherent interfaces between c-TMN and w-AlN than isostructural coherent interfaces between c-TMN and c-AlN. The combination of a stable interface structure and confinement of w-AlN to nm-sized domains by its low solubility in c-TMN in a multilayer, results in films with a stable hardness of 34 GPa even after annealing at 1150 °C.Peer ReviewedPostprint (author's final draft
Deposition rate controls nucleation and growth during amorphous/nanocrystalline competition in sputtered Zr-Cr thin films
Dual-phase Zr-based thin films synthesized by magnetron co-sputtering and
showing competitive growth between amorphous and crystalline phases have been
reported recently. In such films, the amorphous phase grows as columns, while
the crystalline phase grows as separated cone-shaped crystalline regions made
of smaller crystallites. In this paper, we investigate this phenomenon and
propose a model for the development of the crystalline regions during thin film
growth. We evidence using X-ray diffraction (XRD), scanning electron microscopy
(SEM) and transmission electron microscopy (TEM), that this competitive
selfseparation also exists in co-sputtered Zr-Cr thin films with Cr contents of
~84-86 at.%, corresponding to the transition between the amorphous and
crystalline compositions, and in the Zr-V system. Then, to assess the
sturdiness of this phenomenon, its existence and geometrical characteristics
are evaluated when varying the film composition and the deposition rate. The
variation of geometrical features, such as the crystalline cone angle, the size
and density of crystallites, is discussed. Is it shown that a variation in the
deposition rate changes the nucleation and growth kinetics of the crystallites.
The surface coverage by the crystalline phase at a given thickness is also
calculated for each deposition rate. Moreover, comparison is made between
Zr-Cr, Zr-V, Zr-Mo and Zr-W dual-phase thin films to compare their nucleation
and growth kinetics
Quantification of the Phase Transformation Kinetics in High Chromium Cast Irons Using Dilatometry and Metallographic Techniques
Tailoring functional properties of Zr-V thin films by competitive self-separation of crystalline and amorphous phases during sputtering
Investigation of grain orientations of melt-textured HTSC with addition of uranium oxide, Y2O3 and Y2BaCuO5
Local grain orientations were studied in melt-textured YBCO samples processed with various amounts of depleted uranuim oxide (DU) and Y 2O3 by means of electron backscatter diffraction (EBSD) analysis. The addition of DU leads to the formation of Ucontaining nanoparticles (Y2Ba4CuUOx) with sizes of around 200 nm, embedded in the superconducting Y-123 matrix. The orientation of the Y 2BaCuO5 (Y-211) particles, which are also present in the YBCO bulk microstructure, is generally random as is the case in other melttextured Y-123 samples. The presence of Y-211 particles, however, also affects the orientation of the Y-123 matrix in these samples
Atom Probe Tomography investigations on grain boundary segregation in polycrystalline Ti(C,N) and Zr(C,N) CVD coatings
Atom Probe Tomography (APT) was used to obtain a direct evidence of chlor segregation and cobalt diffusion at the grain boundaries (GBs) of polycrystalline coatings deposited by moderate temperature chemical vapor deposition (MT-CVD) on a WC-Co cemented carbide substrate. Reasons behind segregations are discussed, and its effects are correlated to the micromechanical properties of Ti(C,N) and Zr(C,N). It is concluded that chlorine segregation is a relevant factor for explaining the low cohesive strength at the GBs of Ti(C,N) leading to intergranular failure during micro-compression testing, while its absence in Zr(C,N) along with Co diffusion contribute to grain boundary strengthening.Peer ReviewedPreprin
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