ANS:A - spherical

For the preparation of porous bearings by powder metallurgy, the preferred particle shape is typically spherical. Spherical particles provide several advantages in powder metallurgy processes:

1. Uniform Packing: Spherical particles tend to pack together more uniformly, allowing for better compaction during the powder metallurgy process. This leads to more consistent density and porosity in the final product.
2. Improved Flowability: Spherical particles exhibit better flow characteristics compared to irregularly shaped particles. This facilitates the handling and feeding of the powder during compaction and molding stages.
3. Reduced Wear: In bearings, spherical particles can distribute loads more evenly, reducing localized stress concentrations and minimizing wear on the bearing surfaces.

While other particle shapes like nodular or irregular particles can be used in powder metallurgy processes, spherical particles are often preferred due to their favorable packing and flow properties, particularly in applications where uniformity of porosity and mechanical properties is crucial, such as porous bearings.