Bogusz D. Stępak

Corrosion resistance of the AISI 304, 316 and 321 stainless steel surfaces modified by laser

The article presents the analysis results of the influence of laser fluence on physical and chemical structure and corrosion resistance of stainless steel surfaces modified by irradiating with nanosecond-pulsed laser. The study was carried out for AISI 304, AISI 316 and AISI 321 substrates using Yb:glass fiber laser. All measurements were made for samples irradiated in a broad range of accumulated fluence (10÷400 J/cm2). The electrochemical composition (by EDX) and surface morphology (by SEM) of the prepared surfaces were carried out. Finally, corrosion resistance was analyzed by a potentiodynamic electrochemical test. The obtained results showed very high corrosion resistance for samples made by fluency of values lower than 100 J/cm2. In this case, higher values of corrosion potentials and breakdown potentials were observed. A correlation between corrosion phenomena, the range of laser power (fluence) and the results of chemical and structural tests were also found.

Rapid fabrication of microdevices by controlling the PDMS curing conditions during replication of a laser-prototyped mould

This paper presents a method that enables fast and low-cost fabrication of microchannels with oval cross-section. The procedure is based on formation of a concave meniscus at the interface between an initially cured PDMS and a polymeric mould fabricated using excimer laser. In this technique, the mould is not filled with uncured PDMS. The replica is formed by expanding gas trapped within the structures of the mould during thermal curing. A second shaping factor is connected with surface phenomena at the interface between the mould, gas and partially cured PDMS. The final shape of the meniscus is determined when the PDMS reaches the high cure extent. The microchannels with oval cross-section are obtained by using a completely cured PDMS replica as a mould in an analogical second fabrication step. As a result an all-in-PDMS chip can be produced. The cross-section of channels can be controlled by changing the curing conditions. We investigated the influence of the initial PDMS curing time and pressure during final curing on the geometry of the created microchannels. The fabricated microstructures are characterized by constant depth and high quality of the surface.

Excimer laser-induced incubation of poly(L-lactide)

The irradiation of polylactide by KrF excimer laser with subthreshold fluence results in modification of its properties via photochemical reactions. A common approach is to modify chemical composition of polymer surface by UV irradiation, for example, in order to improve their wetting properties. In this paper, authors present a possibility of bulk modification of poly(L-lactide) which is related to photofragmentation and creation of new terminal groups. The irradiation results in decrease of molecular weight and increase of polydispersity. The appearance of new terminal groups is responsible for enhancement of absorption in UV-C range. The intensity of chemical composition changes introduced by UV irradiation can be precisely dosed thanks to a pulse character of laser source. Modifications can be controlled during the process by the analysis of energy transmitted through a polymer sheet. The distribution of absorption coefficient changes along with the depth of irradiated polymer and its correlation with polydispersity was discussed. Presented technique can be used for selective and controllable modification of hydrolytic degradation time of biodegradable polyesters utilized in biomedical applications.

Fabrication of Fresnel microlens with excimer laser contour ablation

Laser micromachining systems based on excimer lasers are usually oriented to work with mask projection regime because of the low pulse repetition rate as well as large beam aperture of the laser source. In case of fabricating of the complex 3D structures, this approach introduces a number of limitations. Alternative solution might be usage of direct writing laser mode. Some examples of the so called contour ablation approach for fabricating microlenses with an absolutely monotonically changing cross-sectional profile are presented in the literature. Based on this idea and introducing new variables like automatic mask selection as well as optimizing process algorithms led us to obtain more versatile method for shape approximation. Hence, there were fabricated structures with cross-sectional profiles described as functions that are monotonic on specified intervals such as Fresnel microlenses. In this paper we describe approximation of process parameters for obtaining desired cross-sectional profiles and finally fabrication of few exemplary microlenses. All structures were characterized by a digital optical microscopy and compared to the given profiles. The accuracy of reproduction of the desired structures at the level of single microns was achieved.

Effect of CO2 laser micromachining on physicochemical properties of poly(L-lactide)

In this paper, we present some examples of micromachining of poly(L-lactide) with a CO2 laser and an analysis of changes in material properties in the heat affected HAZ induced by the fluence well above the ablation threshold. The complexity of the processes of decomposition implies the need for simultaneous use of many selective analytical techniques which complement each other to give a full image of the changes. Introduced changes were characterized using Differential Scanning Calorimetry (DSC), Gel Permeation Chromatography (GPC), X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). It turns out that CO2 laser processing of poly(L-lactide) mainly induces surface changes. However, oxidation of the surface was not observed. We recorded a bimodal distribution and some reduction in the molecular weight. Infrared spectroscopy in turn revealed the existence of absorption bands, characteristic for the vinyl groups (RCH=CH2). The appearance of these bands indicates that the decomposition of the polymer occurred, among others, by means of the cis-elimination reaction.

Direct selective metallization of AlN ceramics induced by laser radiation

Aluminum nitride (AlN) ceramics has a unique characteristic, namely the ability to form conductive structures on its surface directly by laser-induced decomposition of the base material. Various research has been carried out on obtaining low-ohmic structures depending on process parameters such as the laser power, overlap of subsequent pulses and the type of shielding gas (air, nitrogen and argon). This paper focuses on explaining which factors have the greatest impact on the resistance (resistivity) value of obtained structures. In order to explain the effect of the laser fluence (below and above the ablation threshold of aluminum nitride) on the chemical structure of the conductive layers, qualitative EDX analyses were performed. Optimization of the process allowed obtaining a resistivity of the conductive layers at a level of ρ = 0.64·10-6 Ω·m, with a thickness of aluminum up to 10 μm (sheet resistance RS = 10 mΩ/Sr). This technology can be useful in making printed circuit boards (PCB), various types of sensors as well as radio-frequency identification (RFID) and Lab-On-a-Chip (LOC) structures. This technology can also be useful for the production of metamaterials.

Resonator structures on AlN ceramics surface treated by laser radiation

In this paper a method for producing resonant structures using laser micromachining is presented. In the spot of laser beam impact on AlN ceramics surface a conductive aluminum layer is formed. Compilation of process parameters allows for the fabrication of structures with resistance at Rs ~ 0.01Ω/Rs. It has been also found out that the maximum value of resistance for which spiral resonator structures manifest their unique properties is at the level of Rs = 1.43 Ω. Furthermore, the occurrence of mutual capacity which value is dependent on the arrangement of individual SR structures with respect to each other was observed and examined. Based on satisfactory results for SR structures, it has been attempted to produce a resonant structures dedicated to the THz range based on the process of direct metallization of AlN ceramics surface. As a result, the Split Ring Resonator structure whose properties were verified by using the THz -TDS method was manufactured. In case of the field E perpendicular to SRR structure and one resonance area for 0.50 THz with field E parallel to the structure, two characteristic resonant dips for 0.22 THz and 0.46 THz were obtained. The studies confirmed that the method of direct metallization of AlN ceramics allows to produce resonant structures in the THz range.

Degradation of poly(L-lactide) under femtosecond laser treatment

In this paper, for the first time, we present an analysis of changes of physicochemical properties of poly(L-lactide) induced by the femtosecond laser. Introduced changes were characterized using Differential Scanning Calorimetry, Gel Permeation Chromatography, X-ray Photoelectron Spectroscopy and Fourier Transform Infrared spectroscopy. We have noted that even for these process parameters for which no thermal ablation effects occurred, we observed some changes in material properties. In GPC image we recorded an increase in polydispersity index and some reduction of the molecular weight. The FTIR spectra show a reduction in the number of both C=O and C−O−C bonds in the polymer as well as the appearance of new bands. By using the XPS it was determined that processing with femtosecond laser cause a small oxidation of the surface layer. Decay of the spectra indicates the possibility of carboxyl (−COOH) and hydroxyl (−OH) groups present in the modified polymer. Although the observed changes are relatively small compared to long pulse duration lasers or UV lasers, they cannot be neglected in biopolymer applications for tissue engineering.

Rapid fabrication of microdevices using laser direct writing and replica moulding technique

This paper presents a method that enables fast and low-cost fabrication of microchannels with oval cross-section. The procedure is based on formation of a concave meniscus at the interface between an initially cured PDMS and a polymeric mould fabricated using excimer laser. The replica is formed by expanding gas trapped within the structures of the mould during thermal curing. A second shaping factor is connected with surface phenomena at the interface between the mould, gas and partially cured PDMS. The final shape of the meniscus is determined when the PDMS reaches the high cure extent.

The influence of ArF excimer laser micromachining on physicochemical properties of bioresorbable poly(L-lactide)

The main advantage of laser processing is a non-contact character of material removal and high precision attainable thanks to low laser beam dimensions. This technique enables forming a complex, submillimeter geometrical shapes such as vascular stents which cannot be manufactured using traditional techniques e.g. injection moulding or mechanical treatment. In the domain of nanosecond laser sources, an ArF excimer laser appears as a good candidate for laser micromachining of bioresorbable polymers such as poly(L-lactide). Due to long pulse duration, however, there is a risk of heat diffusion and accumulation in the material. In addition, due to short wavelength (193 nm) photochemical process can modify the chemical composition of ablated surfaces. The motivation for this research was to evaluate the influence of laser micromachining on physicochemical properties of poly(L-lactide). We performed calorimetric analysis of laser machined samples by using differential scanning calorimetry (DSC). It allowed us to find the optimal process parameters for heat affected zone (HAZ) reduction. The chemical composition of the ablated surface was investigated by FTIR in attenuated total reflectance (ATR) mode.