Nickel-Zinc Ferrite

Nickel-zinc ferrite is a moderately hard (600-800 HV), brittle spinel ceramic with a density of 4.5-5.0 g/cm³. It is sintered to near-theoretical density, though residual porosity (1-5%) is common. Grain size measurement is the most critical analysis, as it directly determines the high-frequency magnetic performance. Porosity distribution and secondary phase identification are also important. The material is magnetic, which may affect handling during preparation.

Use a precision wafering saw with a thin diamond blade (0.15-0.3 mm). Cutting speed: 100-200 RPM with low feed rate. The brittle ceramic requires gentle cutting to prevent fracture and chipping. Continuous coolant flow is essential. Standard abrasive cut-off wheels are acceptable for larger pieces but may cause more edge damage. For toroidal or ring-shaped ferrite cores, mount on a support fixture to prevent rolling during cutting. Leave 1-2 mm allowance for grinding.

Cold mounting with castable epoxy is preferred. Use a low-viscosity epoxy and vacuum impregnation to fill residual porosity. Compression mounting is possible since ferrites are relatively tough compared to other ceramics, but cold mounting provides better pore filling and avoids any risk of microcracking from mounting pressure. Edge-retaining epoxy formulations are recommended for accurate grain size measurement at the sample edge.

The moderate-to-high hardness (600-800 HV) requires diamond grinding media for best results. SiC paper is acceptable but wears more quickly. Disc speed: 200-300 RPM. Apply moderate pressure (15-25 N per 30 mm sample). The porosity requires attention; excessive pressure can cause grain pullout near pores.

Grinding sequence:

• 30μm diamond disc: Remove sectioning damage (30-60 seconds). Moderate pressure.
• 15μm diamond disc: Refinement (30-60 seconds).
• 9μm diamond disc: Final grinding (30-60 seconds). Reduce pressure to minimize subsurface damage.

Alternatively, SiC paper: 320, 600, 1200 grit with water lubrication. Rotate specimen 90° between steps. The magnetic nature of the sample may cause it to be attracted to steel platens; ensure the specimen is securely mounted.

Use firm, napless cloths to prevent grain pullout. The porous structure requires careful technique to avoid creating artifacts.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a hard napless pad with moderate pressure (15-25 N per 30 mm sample).
• 3μm diamond: 3-5 minutes on a napless pad (15-20 N).
• 1μm diamond: 2-4 minutes on a napless pad (12-18 N).

Final polishing:

• 0.05μm colloidal silica: 3-5 minutes on a soft pad with light pressure (10-15 N). Vibratory polishing (4-8 hours) produces the best surface for grain boundary analysis and porosity measurement.

Ferrite grain boundaries are best revealed by thermal etching. Chemical etching is an alternative when thermal etching equipment is not available.

Thermal Etching - Primary method:

• Conditions: Heat polished sample to 1200-1350°C in air for 15-30 minutes (approximately 100-150°C below the sintering temperature). Cool slowly.
• Reveals: Grain boundaries by thermal grooving. Excellent for grain size measurement per ASTM E112. Also reveals secondary phases and intergranular porosity.
• Note: Temperature must remain below the sintering temperature to prevent grain growth during etching.

Dilute HCl (Chemical Etching) - Alternative:

• Composition: 5-10 ml HCl in 90-95 ml H₂O
• Application: Immerse for 15-60 seconds. More aggressive than thermal etching; monitor carefully to avoid grain pullout.
• Reveals: Grain boundaries and secondary phases. May preferentially attack certain grain orientations.
• Rinse: Water, then ethanol. Dry with warm air.