The objectives of ATTAIN were:
- the detection cracks, disbonding, and adhesion failure in plasma sprayed coatings
- detection of thickness variations in ceramic, metallic and organic coatings and bulk materials
- high resolution inspection, and measurement, of thermal properties of thin films and substrates for microelectronics components
Static pulsed LITT
Static pulsed LITT in the nanosecond time resolution regime has been shown by both heat flow studies and experimental measurements on iron-zinc coated steels to have a sensitivity to coating thicknesses in the range 1-10 um and to the microstructure of alloy coatings in the near surface region of coatings in this thickness range. Heat flow studies have shown that the laser pulse duration needs to be reduced to the hundred picosecond timescale in order to neglect thermal diffusion effects in metals during the finite pulse duration. The finite pulse duration, and a strong parameter correlation between the optical absorption coefficient and the thermal diffusivity in the analytical expressions for the LITT transient are the major obstacles to a simple analysis of the pulsed LITT transient for the extraction of the thermal diffusivities of materials. A methodology to overcome these effects, based upon the application of an ultrathin (0.1 um) metal overlayer to ceramics, has been demonstrated to result in a thermal transient whose shape is independent of the influence of optical absorption. Further work on the analytical theories of heat flow, and technological developments to produce picosecond timescale LITT, are recommended. Nanosecond pulsed LITT has clearly demonstrated potential as a laboratory based analysis tool for thin coatings, in terms of controlling drift in both microstructure and in thickness as a function of process parameter drift. Standard laser flash diffusivity measurement of bulk thermal diffusivities has been demonstrated using nanosecond scale laser pulses, demonstrating the feasibility of combining this technique with pulsed LITT in a single instrument.
A new thickness measuring device for organic coatings on steel strip and other substrates has been developed, using standard, inexpensive components. The technique has shown promise to, and interest from, Spanish steel industry.
A novel, imaging transient thermography technique has been developed, using a scanned laser beam and a single infrared detector. The technique is applicable to a wide range of coating materials, and the wavelength of the laser can be chosen for optimum absorption by the surface. In most cases, a low power sealed CO2 laser is the best choice (CW power 20-50 W typically). Scanning LITT has been shown to be applicable to ceramic, metallic and organic coatings of thickness in the range of 10 um to several mm, although the technique's particular strength is its applicability to coatings at the thin end of the range. For these coatings, there is at present no other imaging transient thermography instrumentation available that has the speed to detect the fast transients induced in such thin coatings. The scanning LITT instrument is controlled by software that was developed for the purpose and offers data - and image analysis options.
Scanning LITT is of special interest to the turbine manufacturing industry, since many blades are coated with thin plasma sprayed or electrodeposit thermal barrier coatings. However, there are also several other areas of industry, such as coated steel strip production and aerospace, which too have shown strong interest in scanning LITT.
ATTAIN deals with the development of a non-invasive inspection technique, transient thermography. The concept of passive thermography is not new; indeed infrared cameras and sensors have long been used to monitor equipment in operation for incipient failure, to measure the temperature of components in general, to inspect factory buildings for insulation faults and so on. Transient thermography, for surface inspection, was first used in the late 70's and early 80's, but has since then remained very much a laboratory technique without significant use in industry.
The innovation in ATTAIN is the development of transient thermography for industrial applications. Without the technique, the surface of the object under investigation is briefly and gently heated and subsequently observed with an infrared detector. The rate of penetration of the heat into the object is a measure of the heat conduction of the surface and can be interpreted in a number of useful ways. For example, it can show underlying defects in a coating, or can be used to measure thermal and electronic properties in thin films.
ATTAIN focusses on two distinct application areas of transient thermography, each of which has its own specific technical requirement. The first area, Laser Irradiation Transient Thermography (LITT), uses a laser which is used to heat the surface rapidly, followed by the observation of the decaying surface temperature with an infrared detector. This technique can be used in a number of ways; for example, scanning LITT uses a scanned laser and a scanned detector, where the time lag between the laser and the detector is fixed so that differences in the observed temperature must be due to sub-surface defects or changing surface conditions. Scanning LITT is especially useful for the inspection of very thin or very 'fast' coatings (i.e. with a very high thermal diffusivity), and curved objects. In static pulsed LITT, the laser and detector are not scanned over the surface under inspection. In this way, the whole of the surface temperature decay curve is recorded, rather than a single temperature measurement at a fixed time after heating, as in scanning LITT. The temperature curve can then be used to calculate thermodynamic properties of the material under inspection. This is highly relevant for the quantitative characteristics of substrates and thin films for the microelectronics industry.
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