**5. High-energy milling — Reinforcement by dispersing nanoparticles into the matrix**

High-energy milling is a technique fo the processing of powders in solid state involving repeated cycles of deformation, cold welding, fragmentation and cold re-welding of powder particles held in a high-energy ball mill (Suryanarayana, 1998; Suryanarayana, 2001; Koch, 1998). This method uses high-energy milling to form composite powders typically for long milling times (Gomes *et al*, 2001; Costa *el al*, 2002.).

In high-energy milling, the constant ball-powder-ball collisions result in deformations and fractures that define the dispersion of components, homogenization, phases and the final powder microstructure. The nature of these processes depends on the mechanical behavior of the powder components, their equilibrium phase and stress state during milling (Suryanar‐ ayana, 1998 and 2001). High-energy milling can be performed with three different categories of metal powders or alloys, namely: ductile-ductile components, ductile-brittle components and brittle-brittle components.

The influence of particle dispersion with the formation of a second phase, slowing the metal surface movement and its sintering, has been proposed by Kuczynsky and Lavendell, who considered that moving particles act as barriers to the surface advancement. Later, it was discussed that particles are swallowed by the surface movement exerting force and preventing its movement (Sbrockey and Johnson, 1980).

The dispersion of large amounts of ceramic particles by powder metallurgy aims to improve their tribological properties and thus their mechanical properties; therefore, 12% SiC powder with average size of 3 μm was added to 316L steel powders of 5 μm, mixing for 8 hours, uniaxially compressed at 100 MPa and sintered at temperatures of 1100°C for one hour in inert atmosphere, resulting in complete fusion of samples (Patankar, *et al*, 2000).

Another study that used 3% W TaC dispersed in the metal matrix of atomized 316L austenitic stainless steel and processed by powder metallurgy showed higher density values and a significant increase in hardness compared to material without reinforcement (Oliveira, 2008).
