Gerard Sliwinski

Characterisation of the laser-clad stellite layers for protective coatings

Abstract Stellite layers produced for protective coatings in a direct process by laser remelting of the powder SF6 on plates of a high-temperature resistive alloy in Ar environment are investigated experimentally. Microphotographs and EDS analysis reveal a metallurgical bond between the substrate and coating as well as the structural homogeneity along the depth of the layer which is characterised by a chemical composition close to that of the powder. The laser-clad layers show a dendritic and fine-grained structure with a minor presence of impurities. For the coatings an improvement of mechanical properties in comparison to the base material such as the behaviour in wear resistance comparable to that of TiN and values of microhardness up to approximately 1000 HV are observed. On the other hand, corrosion properties depended on the defects and appeared on the surface as a result of the laser process parameters.

Improvement of self-regeneration of gas mixtures in a convection-cooled 1.2 kW laser

Degradation of gas mixtures and methods of its reduction in lasers were studied with reference to the DC-excited, transverse-flow CW laser of MLT 1200 type. The long-term output characteristics and their dependence on the mixtures composition determined in the experiment gave a hint concerning the thorough theoretical analysis of decomposition in laser mixtures. The observed optimal concentration of lies within the range and that of is about 2 - 3% for the laser under consideration. In the theoretical analyses of available data we focus our attention on the roles played by the gas pressure, the mixture composition and the growth of the afterglow region in the reduction of working gas degradation. The equilibrium conversion decreases with increasing gas pressure. A 50% reduction in is obtained when the pressure increases from 13.33 to 46.66 hPa. The observed optimal composition corresponds to low reactivity of the laser plasma with a small oxygen content. The optimal period for the laser gas mixture to reside in the afterglow region is larger by a factor of 1000 than the discharge residence time. The relatively small value of the equilibrium conversion in the MLT 1200 laser results from relatively high concentrations of atomic and electronically excited species. The formation of atomic oxygen is inhibited by reactions with electronically excited molecules, especially . Only very small concentrations of and , which are the most harmful reaction products, result according to our calculations for the MLT 1200 system.

Threshold and saturation properties of a solid-state XeF (C-A) excimer laser

A dielectric laser cavity of 1 cm length has been optimized for high gain (7 cm−1) operation which is achieved in XeF doped Ar crystals. Mode structures on the C-A spectral distribution around 540 nm and far field transverse mode patterns are reported. Photchemical gain burning is observed in the spectral mode structures. The dependence of the laser threshold on pump energy, pumped length and on cavity losses is studied. XeF densities of 7×1017 cm−3 and distributed losses of 1.2 cm−1 are derived. The measured quantum efficiency of 14% and the saturation behaviour are consistently described. Losses by transient aborption and two photon absorption are discussed.

Spectroscopy of XeF in Ar and Ne matrices

Spectroscopic constants for the B 2Σ+ and C 2Π charge transfer states of XeF in Ar and Ne matrices are derived from vibrational progressions in excitation and emission spectra. Polarization effects on the Te values are discussed and Rittner potentials are fitted to the constants. Emissions at 389 and 411 nm in Ne and Ar, respectively, are attributed to a distorted B state denoted as B* with a strong red shift of Te and an about 30% larger ωe. B* is assigned to a XeF center with an additional F atom which can be a precursor to XeF2 according to a comparison with Xe2F spectra and the concentration dependence of the intensities. The B* vibrational bands display two well resolved fine structure progressions with a common ωe of ∼60 cm−1 in Ar and Ne which is close to the maximum matrix phonon density and one with 30 cm−1 in Ne and 15 cm−1 in Ar. A reversible change in the Ne fine structure pattern with temperature can be correlated to a face‐centered‐cubic–hexagonal close‐packed (fcc–hcp) phase transition.

UV-spectroscopy and band structure of Ti:YAlO3

Abstract Exciting Ti 3+ :YAlO 3 and Ti 4+ :YAlO 3 crystals in the UV spectral range at 155 nm wavelength, the typical Ti 3+ fluorescence in the orange spectral range, which is rather weak for Ti 4+ -doped YAlO 3 , and another emission around 420 nm wavelength are observed. The excitation spectra of these emissions are measured and interpreted in a band model for the Ti:YAlO 3 system.

Laser investigations at 269 nm for XeF(D-X) in Ne crystals

From line narrowing in amplified spontaneous emission at the D-X transition (269nm) of XeF in solid Ne a gain coefficient of 3.4 cm−1 has been derived and ground-state losses of 2.8 cm−1 have been determined by variation of the absorption length. A dielectric laser cavity has been optimized with the reflectivities R1=100% and R2=70% for 1 cm long crystals; laser action has been achieved.

Diagnostic instrument for measurements of a high power CO2 laser beam

A measuring device based on a rotating pinhole method was designed and developed at IF-FM for recording the spatial structure of high power cs CO2 laser beams in the near and far field region. The main performance parameters of the device are presented together with the measurement results concerning the beam propagation characteristics of the 1.2 kW technological laser.

Influence of the base preheating on cracking of the laser-cladded coatings

Coatings of stellite SF6 are prepared on the chromium steel base by means of a direct laser cladding. A 1.2 kW CO2 laser is applied and the original material is delivered into the processing zone coaxially with the laser beam. The samples are produced with and without a controlled preheating of the substrate and are investigated by means of metallographic techniques. It is found that the observed micro-cracking susceptibility decreases markedly with increase of the base preheating temperature up to 750 K and the crack-free coatings are produced for preheating around 950 K. The substrate-coating interface reveals a metallic bond and the microstructure is characterized by a fine-grained, dendritic structure. The nearly constant concentration dependence of Fe, Co, Cr and Ni on the distance from interface indicates on homogeneous chemical composition of the produced coatings.

Experimental study of a laser processing head with integrated jet of metal powder for rapid prototyping and production of protective coatings

A controllable, homogeneous stream of metal powder particles is remelted by a focused, coaxial CO2 laser beam at the outlet of a multi-stream nozzle and applied for production of the protective coatings and volumetric structures for prototyping. For the stellite SF6 layers the optical, chemical, and metallographic tests show the fine-grained and chemically homogeneous structures with only a minor presence ofthe Cr/C precipitates and local micro-defects.

Ablation of ceramics with ultraviolet, visible, and infrared nanosecond laser pulses

Laser ablation of alumina (Al2O3), aluminum nitride (AIN) and silicon nitride (Si3N4) ceramics by 6 ns Nd:YAG laser operating at wavelengths of 1064 nm, 532 nm and 355 nm is studied. For all materials the maximum ablation rate is observed at wavelength 1064 nm, and the smallest one for irradiation at 532 nm in the case of Si3N4 and AIN. The numerical model, based on the heat-transfer equation describes the temperature distribution in the material and the evaporation process. The calculated depths of the drilled holes are in agreement with experiment.