Novel Tungsten Carbide Nanocrystalline Composites by Pulsed Laser Deposition
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NOVEL TUNGSTEN CARBIDE NANOCRYSTALLINE COMPOSITES BY PULSED LASER DEPOSITION Ravi K. Venkatesan, A.Kvit, Q.Wei, J.Narayan Department of Materials Science & Engineering, North Carolina State University, Raleigh, NC 27695-7916, USA ABSTRACT We have developed a novel processing technique to fabricate “artifact free” tungsten carbide (WC) nanocomposites. In this method, pulsed laser deposition of WC in conjunction with a few monolayers of nickel aluminide (NiAl) is used to control the grain size of nanocrystalline composites. The grain size of WC was controlled by the thickness of tungsten carbide and the substrate temperature. The role of NiAl is to ensure the nucleation of tungsten carbide islands, and it is also insoluble in WC. Using this approach, we have fabricated nanocomposites of grain sizes ranging from 6 nm to 35 nm. The hardness of the composite increases with the decrease in grain size, following approximately Hall-Petch relationship. The role of NiAl in grain boundary deformation is of particular interest in strengthening the nanocrystalline composites. The potential of this technique to go to even lower grain sizes is discussed. INTRODUCTION Nanocrystalline materials having grain sizes between 1-50 nm exhibit interesting physical and mechanical properties, which are different than that of their bulk counterparts [1-3]. These properties include improved hardness, increased ductility & toughness and reduced elasticity modulus compared to their coarse grained counterparts. At grain size dimensions less than 10 nm, a large volume fraction of atoms are located at the grain boundaries; consequently, the interface provides a very large number of high diffusivity paths. A rough estimate shows that the volume fraction at the grain boundaries is given by 6 δ/D, where, D is the grain size(diameter) and δ ,the width of the grain boundary, thus with δ =1 nm, the volume fraction equals unity for D=6 nm. The yield strength and hardness of nanocrystalline materials increase with decrease in grain size, which is qualitatively described by the Hall-Petch relationship [3-4]. It is also observed that this relationship holds good up to a certain grain size, below which softening is observed with decreasing grain size, thus exhibiting a negative Hall-Petch effect[1-2,4]. In the present investigation, we have used a modified pulsed laser deposition technique to control three-dimensional nucleation or island growth by introducing a few monolayers of insoluble elements with high surface energy. Here, we have used a few monolayers of NiAl during the deposition of WC to control the island growth and grain size. Using this approach, we initially obtained a grain size of 35 nm and have reached a grain size of 6 nm. This grain size refers to the size of the WC grains alone and does not include NiAl, which is present at the WC grain boundaries. The potential is to reach a lower grain size by playing with the parameters. The hardness of the WC films on Si(001) substrates was also investigated using a nanoindentation technique. EXPERIMEN
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