Nitriding of Zirconia by Irradiation with Nd:YAG Laser Beam
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When many ceramic materials absorb high power Nd:YAG laser beams, melting and/or evaporation are usually observed. Chemical changes such as reduction or reaction with atmosphere are also observed. Since the chemical reaction occurs under non-equilibrium conditions with rapid heating and quenching, unpredictable phases are often obtained. In the previous work, the authors have examined the possibility of applying a Nd:YAG laser in order to modify ceramic surfaces [2]. It was found that the surface of yttrium stabilized zirconia (YSZ) changes into ZrN by irradiation with pulsed Nd:YAG laser beam in nitrogen containing atmospheres [2]. The nitriding of a YSZ ceramic surface into ZrN occurred not only in nitrogen or ammonia but also in air. This reaction is an example of obtaining a material which is unpredictable from the viewpoint of equilibrium thermodynamics. Yoshioka et al. [3] reported that the color of yttrium stabilized zirconia changed into black by irradiation of Nd:YAG laser. However, the formation of ZrN was not reported. The formation of TiN by YAG laser irradiation into Ti[N(CH 3)2 ]4 liquid was reported by Narula et al. [4]. The direct nitriding of oxide ceramics by laser irradiation has not been yet reported. The objective of this paper is to determine the conditions for the formation of ZrN on YSZ surface and to characterize its properties.
561 Mat. Res. Soc. Symp. Proc. Vol. 397 0 1996 Materials Research Society
EXPERIMENTAL Materials Partially stabilized zirconia powder (Chichibu Cement Co., 95ZrO 2-5Y20 3) was pressed into pellets 50 mm in diameter and 5 mm in thickness. These pellets were sintered at 1450'C for 4h in air on an A120 3 plate. Samples with >97% theoretical density were obtained. The main phase of sintered material was tetragonal ZrO 2. Weak X-ray diffraction peaks from the monoclinic phase were also observed. The sintered material was cut into rectangular plates of 10 by 20 by 3 mm 3. Irradiation with YAG Laser The sample with a flattened surface was placed into the holder with a glass window shown in Fig. 1(a). The atmosphere in this holder was maintained by flowing the gases (air, 02, He, N2, NH 3 or N 2 containing 6 vol% of H 2) at a rate of 200 cc/min. The Nd:YAG laser beam was irradiated onto the sample through the window. The commercially available Q switched pulsed Nd:YAG laser generator (Miyachi Technos ML-2101A) was used with precise X-Y-Z sample stage controlled by the microprocessor. (a) Laser Beam , glass window I The standard Nd:YAG beam conditions Gas Inlet employed were 600 V of lamp voltage, 500 [tsec air,N ,He,O of pulse length, 1.9 J/pulse of laser energy, 800 NH [tm beam diameter and 2 pulse/sec rate. Four (4) pulses per point were irradiated. The beam was irradiated over the entire flattened surface with 1 mm separation as shown in Fig. 1(b). Microstructure and Phase Determination (b)
The change in the surface crystal structure by the YAG irradiation was determined using a conventional X-ray diffraction (XRD) method by positioning the sample plate directly into
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