Interface Stabilization of W-N Coatings by Chromium Alloying
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Interface Stabilization of W-N Coatings by Chromium Alloying 0. Knotek*, F. L6ffler*, A. Barimani ** *University of Technology (IWKB), Templergraben 55, 5 100 Aachen. Germany ** Gottlieb Guihring KG, Herderstr. 50-54, W-7470 Albstadt 1. Germany
Abstract New metastable materials can be deposited using low temperature physical vapour deposition (PVD) techniques. During reactive sputtering, the atoms condensing in an intermixed state attempt to achieve a stable configuration. Due to low mobility of the adatoms, equilibrium phases cannot form and metastable structures are observed. Reactive sputtering can be used to deposit films with different stoichiometries and structures in the W-N system. The metastable phases a-W, B-W, W2 N and WN 1-x are obtained. All coatings are, however, thermally unstable. At temperatures above 570 °C, all phases are transformed into the a-W modification. By alloying chromium to the coatings in the W-N system, It Is possible to stabilize all tungsten and tungsten nitride modifications as well as the interface layer between the substrate and the thin films.
1. Introduction Physical vapour deposition (PVD) techniques can be used to obtain new metastable materials. The formation of non-equilibrium states In sputtered thin films in the W-N system Is also conceivable. Thin films with differing stoichiometries can be deposited by varying the dominant parameters of the magnetron sputtering process (reactive gas pressure, substrate temperature, substrate bias) and by alloying chromium to the coatings. The paper discusses the thin film phase diagrams and the properties of the new interface material within the W-Cr-N system. The resulting composite coatings are subjected to metallographic, EPMA, X-ray and electron microscope analysis. The thermal and phase stability of the coatings are investigated In a high vacuum test.
2. Experimental Techniques Thin coatings in the W-Cr-N system were deposited using Leybold-Heraeus Z400 magnetron sputtering plant In the DC and RF reactive mode with nitrogen as the reactive atmosphere. Three different targets were used: pure tungsten, tungsten-chromium W 0 .8 7 Cro.13 and tungsten-chromium W 0 .9 9 Cro. 0 1 . Coating composition was modified simply by altering the process Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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parameters partial nitrogen pressure, substrate temperature and substrate bias. The substrate materials employed were sintered ISO group M15 carbides, high speed steel (S 6-5-2) and austenitic steel (X 5 Cr Ni 18 9). Test methods comprised X-ray diffraction analysis, electron probe microanalysis (EPMA), scanning electron microscopy (SEM), microhardness testing and scratch tests. Information on the thermal and phase stability of the compounds was obtained from vacuum furnace annealing tests at 600 ... 1000 °C over periods of 3 to 24 hours in a residual atmosphere of less than 10-4 mbar and from high-temperature X-ray diffraction analysis.
3. Results Since the phases in the W-N system are unstable in the equilibrium stat
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