Crystallization and Amorphization of Sic-Ceramic PVD Coatings After Laser Treatment
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CRYSTALLIZATION AND AMORPHIZATION OF SiC-CERAMIC PVD COATINGS AFTER LASER TREATMENT 0. Knotek, F. Loffler Institut rurWerkstoffkunde, Technical University Aachen, Templergraben 55, D-5100 Aachen, Federal Republic of Germany Abstract Over the last ten years, surface coating engineering has become a key technology throughout the world, especially due to the successes achieved in the optics and microelectronics sectors. First advances in the wear protection field were attained with coated carbides, indicating a substantial technical and economic potential in these areas. The development of wear protective coatings often reaches the limits of what is technically possible. For this reason, processes incorporating two different methods and combining the specific advantages of each are increasingly being developed for the deposition of certain coatings. In the study presented here, PVD (Physical Vapour Deposition) technology was combined with laser surface treatment technology to produce amorphous and crystalline SiC coating phases. The objectives of the study were to establish the respective advantages of crystalline and amorphous phases. The amorphous ceramic coating exhibits excellent corrosion resistance, whereas the crystalline phases generally attain higher adhesion and superior stability at high temperatures. There are therefore technical applications for both phases.
Method The combined process should be seen against the background of the high reactivity of silicon with the metal substrates, which makes it impossible to induce phase transformations in SiC coatings by heat treatment in a vacuum furnace. A silicate-substrate reaction can be avoided only by means of surface heating in a depth range only slightly less than the coating thickness. Since temperatures of about 1000 'C are required, the only feasible heat source for treating specimens in this way is a laser beam. A suitable laser type is selected according to its attainable laser power and operating mode. For extremely high energy densities over extremely short periods, the choice is between the Nd-YAG and the CO 2 laser. Both types of laser are used for surface treatment of workpieces in production engineering applications, the Nd-YAG being a solid-state and the CO2 a gas laser. Input power for both types of laser was selected in the 1 kW range. The Nd-YAG laser was operated in the pulsed and the CO 2 laser in the continuous mode, so that results in the different modes could be compared. There were three variation parameters for the Nd-YAG laser, as indicated in Fig. 1. Track shift is determined by the movements of the laser and the timing of the pulses; the focal position (def) indicates the distance from the focal point of the laser and the feed rate describes the relative motion between the specimen and the laser. An increase in any one of these three parameters entails a decrease in the energy input per unit area. Mat. Res. Soc. Symp. Proc. Vol. 230. ýc1992 Materials Research Society
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choice of the laser - parameters laS
-•Nd - YAG laser --
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