Joerg Hermann

Laser treatment of tribological DLC films

The friction and wear reduction in applications that allow only a minimal use of liquid lubricants is done with solid lubricant films or with protective coatings, such as diamond-like carbon (DLC). Further improvements are possible if the geometries of the contact surfaces are modified in a controlled way, as we have already demonstrated it for TiN and TiCN. In this work, the possibilities to generate patterned DLC coated low wear tribological surfaces by means of laser processing were investigated. In the first approach, a two step method was used: steel substrates were laser patterned and subsequently DLC films were deposited on them. The second considered approach was the laser processing of coated surfaces. DLC films were irradiated with laser pulses of different durations and energy densities (100 fs, 800 nm, <4 J/cm2; 150 ns, 1064 nm, <10 J/cm2) and the treated spots were examined using optical microscopy, SEM, AFM and Raman spectroscopy. The graphitisation of a-C:H films under both fs- and ns- regimes was shown as well as a film-peeling phenomenon during the ablation process. Microstructured and DLC coated surfaces obtained in the former approach were used for preliminary tribological tests (oscillation-friction-wear method). The results showed that the friction coefficient did not increase, as compared with the unstructured and DLC coated surfaces, and that the structure pores trapped the debris particles produced when the DLC film eventually broke.

Femtosecond laser ablation of materials

Polycrystalline SiGe is attracting more and more attention in micro and optoelectronics devices both at industrial and university level. Research on both devices and material growth techniques continues at a very rapid pace in the scientific world. Low cost production techniques, capable to produce such alloys with uniform and controlled grain size, becomes of particular attention. Excimer laser crystallization has proved to be a valuable how thermal budget technique for amorphous silicon crystallization. Its main advantages are the high process quality and reproducibility joint to the possibility of tailoring the grain sizes both in small selected regions and in large areas. This technique is here applied for producing poly-SiGe alloys from amorphous SiGe films deposited on glass.

Spectroscopic analyses during femtosecond laser ablation of hydroxyapatite

The plasma generated by ablation of hydroxyapatite Ca10(PO4)6(OH)2 with ultrashort laser pulses was studied in order to get a deeper insight in the fundamental mechanisms of the interaction of femtosecond laser pulses with the biocompatible target material. First, the propagation of the luminous plasma plume was observed using a fast ICCD camera while time- and space-resolved emission spectroscopy was employed to measure the plasma composition and its evolution as a function of time and space. It is shown that the kinetic energies of ablated atoms and ions are one order of magnitude smaller than those generated by hydroxypatite nanosecond laser ablation. Rather low temperatures of about 3500 and 2500 K are deduced from the relative intensity of spectral lines.

Plasma study in laser ablation process for deposition

In order to get a better inside in the reactive Pulsed Laser Deposition of nitride thin films, we performed time- and space-resolved plasma diagnostics during ablation of Ti, Al and C targets in low pressure nitrogen containing atmospheres using pulsed nanosecond UV lasers. In the case of carbon, thin films of CxNy were deposited on silicon substrates and characterized by Rutherford Backscattering Spectroscopy and Nuclear Reaction Analysis. With respect to irradiation of metal targets, during which a dense and highly ionized plasma was induced for laser intensities >= 100 MWcm-2, much higher values >= 1 GWcm-2 were necessary to induce significant plasma ionization on carbon. To increase the plasma reactivity in the case of carbon ablation, a radiofrequency discharge was added to excite and preionize the ambient gas. From correlation between the plasma characteristics and thin film analyses, conclusions could be made about the CxNy deposition process.

Laser ablation in a reactive atmosphere: application to the synthesis and deposition performance of titanium carbide thin films

We report the synthesis and deposition of Ti carbide thin lay- ers by multipulse excimer laser ablation of Ti targets in CH4 at low am- bient pressure (in the microbar range). The layers deposited on single- crystalline Si wafers are characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, photoelectron spectroscopy, and spectroscopic ellipsometry. We obtained deposition rates in the range 0.2 to 0.3 A/pulse for a target-collector separation distance of 12.5 mm. The deposition parameters were found to be in good agreement with the predictions of the theoretical model based on the assumption of the adiabatic expansion of the plasma in the ambient gas.

Direct synthesis of metal nitride by CO2 or XeCl laser-plasma

The present work deals with a new nitriding method applied to titanium: the Ti surface nitriding is carried out by direct laser irradiation in the presence of ambient nitrogen. The experimental procedure is performed in a chamber containing N2 gas, allowing plasma study by emission spectroscopy. Two pulsed laser types are used, a TEA-CO2 ((lambda) equals 10.6 micrometers ) and a XeCl excimer ((lambda) equals 308 nm) in order to compare the laser- material coupling influence on the layer synthesis process. The laser beam is focused perpendicularly to the Ti samples. Different experimental conditions are achieved to investigate the influence of laser and gas parameters on the process. Using the CO2 laser, a N2 plasma is created on the Ti surface. With the XeCl excimer laser, a Ti plasma on the sample appears. After treatment, the surface state of the samples is studied and chemical analysis of the targets are carried out. The TiN synthesis is evidenced. Presence of oxinitride in the compound and native surface oxygen reduction by hydrogen plasma are examined.

Analyses of femtosecond laser ablation of Ti, Zr, and Hf

Femtosecond laser ablation of Ti, Zr and Hf has been investigated by means of in-situ plasma diagnostics. Fast plasma imaging with the aid of an intensified charged coupled device (ICCD) camera was used to characterize the plasma plume expansion on a nanosecond time scale. Time- and space-resolved optical emission spectroscopy was employed to perform time-of-flight measurements of ions and neutral atoms. It is shown that two plasma components with different expansion velocities are generated by the ultra-short laser ablation process. The expansion behavior of these two components has been analyzed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ultra-short laser ablation.

Femtosecond ablation applied to deep-drilling of hard metals

Mechanisms responsible for the limitation of the aspect ratio obtained by deep drilling of hard metals are investigated in the present work. Cemented carbide targets have been irradiated with laser pulses of 100 fs duration and 100 μJ maximum energy delivered by a Ti:sapphire laser system. The experiments are carried out in different gas environments (vacuum, air, helium up to atmospheric pressure) with incident laser fluences ranging from 1 to 20 Jcm-2. During deep drilling, the laser-induced ablation plume is characterized by means of in-situ plasma diagnostics. Fast imaging is used to observe the expansion behavior of the plasma plume whereas time- and space-resolved emission spectroscopy is employed to analyze the plasma composition. After irradiation, the laser-produced craters were examined by optical microscopy. A correlation between the ablation plume characteristics and the morphological changes of the mciro-holes is established. The results indicate that nanoclusters, that present a significant part of the ablated material, are responsbile for the alteration of the crater shape in the high laser fluence regime.

Structure changes in steels and hard metal induced by nanosecond and femtosecond laser processing

Investigations on the occurrence of structure and hardness changes (for two sorts of steel and for a hard metal substrate) in the immediate vicinity of laser induced craters are presented in this work. Experiments with femtosecond pulses were performed in air with a Ti:sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5 and 10 J/cm2. Series of microcraters were induced with 100 to 5,000 laser pulses per hole. Experiments with similar fluences, but 10 to 40 pules per hole, were performed on the same materials using a Nd:YAG delivering 100 ns pulese. After laser irradiation, cuts were made through the processed samples and the changes occurred in the crystalline structure of the target materials were evidenced by metallographical analysis of the resulting cross-sections. Hardness measurements were performed in points situated in the immediate vicinity of the laser-induced pores. Affected zones in the material surrounding laser induced pores were always found in the ns-regime, however with different properties for various laser parameters. In the fs-regime, zones of modified materials were also found and in such zones a significant hardness increasing was evidenced; the limit of the low fluences regime, where no structure changes occurred, was found to be slightly above 2 J/cm2.

Laser-induced explosive boiling during nanosecond laser ablation of silicon

The surface morphology of single crystal (100) Si wafers irradiated by 266 nm and 1064 nm laser pulses emitted by a solid state Nd:YAG laser has been investigated. The morphology of the bottom of craters remained flat and almost featureless after 266 nm single or multipulse laser irradiation up to the maximum fluence of 18 J/cm2 used in this study. The rims of the craters showed signs of radial liquid flow but it was apparent that the vaporization process was confined to the surface region. A different morphology was observed on the bottom of the craters formed by the 1064 nm wavelength laser pulses. Because this wavelength is absorbed in volume, (alpha) <104cm-1, a rather thick liquid Si pool formed at the surface. For laser fluences higher than 3-5 J/cm2 evidence of boiling sites were observed on the bottom of the crater, especially for multipulse irradiation. An evolution of surface morphology, from waves towards deep cavity was observed with the increase of pulse number. By analyzing the cavity formation mechanisms, their density and shape, we suggest that they were induced by heterogeneous boiling and not homogeneous boiling.