Which type of particles is often used in MPI for better detection?

In Magnetic Particle Inspection (MPI), iron oxide particles are commonly used because they possess magnetic properties that make them ideal for detecting surface and near-surface discontinuities in ferromagnetic materials. Iron oxide particles can easily become magnetized in the presence of a magnetic field, allowing them to cluster at areas where the magnetic field is disrupted by a flaw in the material, such as cracks or voids.

These particles are typically available in both dry and wet forms, with wet methods providing a more thorough examination since the suspension allows for better coverage and penetration into the surface imperfections. The magnetic nature of iron oxide is crucial, as it enables effective visualization of defects through a contrast against the background when the particles are attracted to them.

The other types of particles listed do not share the same beneficial characteristics for MPI. For instance, strontium and silicon particles do not have the requisite magnetic properties, and copper particles are not ferromagnetic, thus making them unsuitable for effective detection during MPI.