J. Martan

New experimental device for high-temperature normal spectral emissivity measurements of coatings

A new experimental device for normal spectral emissivity measurements of coatings in the infrared spectral range from 1.38 μm to 26 μm and in the temperature range from 550 K to 1250 K is presented. A Fourier transform infrared spectrometer (FTIR) is used for the detection of sample and blackbody spectral radiation. Sample heating is achieved by a fiber laser with a scanning head. Surface temperature is measured by two methods. The first method uses an infrared camera and a reference coating with known effective emissivity, the second method is based on the combination of Christiansen wavelength with contact and noncontact surface temperature measurement. Application of the method is shown on the example of a high-temperature high-emissivity coating. Experimental results obtained with this apparatus are compared with the results performed by a direct method of Laboratoire National d’Essais (LNE) in France. The differences in the spectra are analyzed.

Two-detector measurement system of pulse photothermal radiometry for the investigation of the thermal properties of thin films

Pulsed photothermal radiometry, a method developed in previous work for thin film thermal effusivity measurements, is now further developed for the determination of thin film thermal conductivity and volumetric specific heat. The present setup consists of a nanosecond laser source and two infrared (IR) detectors for temperature response measurement. The two detectors have different sizes and frequency bandwidths enabling accurate measurement of the surface temperature both in very short (ns) and long (μs) times after the laser pulse. This enables measurement of the apparent effusivity of both the thin film and substrate. The position in time of the transition region between the film and substrate effusivity is essential for recalculation of the thermal conductivity and volumetric specific heat from the measured thermal effusivity. The presented experimental system was applied to the investigation of the thermal properties of TiO2 and Si-B-C-N thin films. The different thermal conductivity of the TiO2 film...

Thermal Characterization of Tungsten Thin Films by Pulsed Photothermal Radiometry

Thermal conductivity and thermal interface resistance of tungsten thin films were investigated by means of pulsed photothermal radiometry. The experimental system based on a nanosecond pulsed laser and a high-speed IR photodetector is presented. Calibration of the IR detector is described. The thermal properties of the samples are identified by comparison with an analytical solution of the heat transfer equation for layered samples already presented in the literature. The experimental system enables investigation of micron and sub-micron thick metallic films. The investigated films were deposited by magnetron sputtering on iron substrates using two different deposition conditions. The measured thermal conductivity values ranged from 40 to 62 W.m−1.K−1 and thermal contact resistances from 0.05 to 1.1 10−8 m2.K.W−1.

Precise nanosecond time resolved infrared radiometry measurements of laser induced silicon phase change and melting front propagation

Nanosecond laser melting of silicon was investigated by infrared (IR) radiometry and time resolved reflectivity (TRR) methods. IR radiometry revealed interesting signal evolutions and their qualitative changes during different stages of the melting process. The observed signals represent melting front propagation, liquid phase overheating, and melt duration. Comparison of IR and TRR methods is shown. The physical origin of the IR signal shapes is discussed in detail.

Coupled photo-thermal and time resolved reflectivity methods to original investigation of laser/material nanosecond interaction

A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale. Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a melting pool by in situ experiments. It should be the same for the surface temperature. A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR) and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV, Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel 15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of melting, melting duration and the threshold of plasma formation are directly deduced.

Method for emissivity measurement of semitransparent coatings at ambient temperature

Coatings deposited on a material surface are effective way of changing its surface properties. For increasing or decreasing radiation heat transfer, coatings with high or low emissivity are used. Measurement of spectral emissivity is a fundamental step to effective use of coatings for this application. Up to now the measurement methods are focused on bulk samples and mainly opaque ones. Here we present a method enabling measurement of emissivity of semitransparent coating itself, although it is deposited on a substrate. The method is based on measurement of transmittance and reflectance using an integration sphere system and Fourier transform infrared (FTIR) spectrometer for samples with two different coating thicknesses deposited on transparent substrates. Measured transmittance of the coating indicates spectral regions of potential emissivity differences using different substrates. From all the measured values, spectral emissivity can be characterized for different coating thicknesses. The spectral range of the method is from 2 μm to 20 μm. The measurement is done at ambient temperature enabling measurement of samples sensitive to heating like biomedical or nanocoatings. The method was validated on known bulk samples and an example of semitransparent coating measurement is shown on high-temperature high-emissivity coating.

Shifted laser surface texturing for bearings applications

The laser surface texturing (LST) technologies, based on creation of micro-pattern with pre-defined geometry can positively influence both the friction and wear of tribo-elements. In practice, the integration of LST technology is often limited due to its slowness. The new method, so called shifted laser surface texturing (sLST) with increased process speed was developed to make the technology more attractive for the industrial application. In the paper, the texture created by sLST technology was applied onto the steel samples and Al-Sn-Si surface of sliding bearings. Both block-on-ring (ASTM G-77) laboratory tests of steel samples and high-loaded working application tests on Al-Sn-Si bearings surface were carried out to evaluate the influence of texture on tribological behaviour. The ASTM G-77 laboratory tests showed a positive effect of the texture on friction behaviour. Under the high-loaded testing conditions, the positive effect was observed in initial stages of the tests, decreasing the torque of textured bearings compared to the untreated one. Lately, the texture was worn out and have no influence on the overall wear of the bearings. Based on the above mentioned observations, the use of alternative bearing material with higher hardness or application of protective layer over the created texture was suggested to exploit the texture benefits.

Time resolved optical methods for investigation of phase transformations in materials exposed to nanosecond laser pulses

Infrared (IR) radiometry and time resolved reflectivity (TRR) methods can be used for investigation of laser pulse effects on materials in nanosecond time scale. The methods in combination are capable to quantify object temperature and detect phase transformations in the solid state, melting and plasma formation from vapour. Measurements with different laser pulse energy densities provide threshold of the transformation. The melt duration can be also determined. The experimental system is described. It contains KrF excimer laser with homogenizer and variable attenuator, fast IR detector for radiometry, continuous probing laser with Si photodiode for reflectivity measurement and UV detector for pump laser pulse reflection measurement. The system was applied to investigation of responses to laser light of silicon and different pure metals and alloys. The range of energy densities used was 1-5500 mJ.cm-2 and measurements were done with temporal resolution of 6 ns for radiometry and 1 ns for reflectivity.