Nanocrystalline Ni coatings strengthened with ultrafine particles
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I. INTRODUCTION
THE significant performance enhancement that results from the presence of small amounts of dispersed fine particles in materials is well recognized, i.e., improving strength through interactions with deformation faults[1,2,3] and inhibiting grain growth due to Zener pinning.[4,5] The contributions of nitrides and oxides (formed through reactions between powders and oxygen/nitrogen from the environment during mechanical milling in liquid nitrogen) to the thermal stability of nanocrystalline materials have been realized.[6–9] Strengthening has also been found to result from the presence of ultrafine precipitates in nanostructured coatings.[10,11] Mechanical milling in liquid nitrogen has been employed to synthesize nanocrystalline Ni powders, which can be used as feedstock powders to fabricate nanocrystalline Ni coatings.[12] However, Ni does not form a stable nitride in the nitrogen atmosphere. In principle, the nitrogen atmosphere could result in the formation of hexagonal Ni3N (a ⫽ 2.670 ˚ and c ⫽ 4.306 A ˚ ), but the formation enthalpy of Ni3N at A room temperature is quite low (⌬H298 ⫽ 0.8 kJ/mol[13]), indicating its instability at room temperature. Alternatively, the high hardness and thermal stability of aluminum nitride, (AlN) are well known.[14,15] Therefore, the aim of the present study was to demonstrate another method for introducing the nitride phase, namely, through the intentional addition of AlN to the milling process, and to provide insight into the influence of these AlN particles on the microstructural evolution and mechanical properties of nanocrystalline Ni/ AlN coatings. Furthermore, we demonstrate that the AlN JIANHONG HE, Materials Scientist, and JULIE M. SCHOENUNG, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616-5294. Contact e-mail:[email protected] Manuscript submitted January 10, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
additions need not be in the nanometer scale at the start. They fracture during milling to become finely dispersed in the Ni matrix, in much the same way as a nitride created during milling. II. EXPERIMENTAL PROCEDURE Commercially available, pure Ni powder (Sulzer Metco Inc., Westbury, NY) with a purity ⱖ99.5 wt pct and a nominal particle size of 45 ⫾ 11 m and AlN powder (CERAC Inc., Milwaukee, WI) with a purity of 99 pct and a nominal particle size of 1.97 m were selected for the present study. The Ni powders were blended with AlN powders for 0.5 hours in the amounts of 0, 0.5, and 2 wt pct AlN (or 0, 1.38, and 5.54 vol pct AlN, using specific-gravity values of 3.26[14,15] and 8.902[16] g/cm3 for AlN and Ni, respectively). The blended powders were mechanically milled in a modified Union Process 1-S attritor mill with a grinding-tank capacity of 0.0057 m3. Stainless steel balls of 0.635 cm in diameter were used, with a powder-to-ball mass ratio of 1:20. The powder charge was 1 kg. The mill was operated at 180 r.p.m. for 8 hours, and liquid nitrogen was contin
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