TiCN-Ni Cermet

TiCN-Ni cermets are ultra-hard (91-93 HRA, ~1500 HV) materials similar to cemented carbides but based on titanium carbonitride instead of tungsten carbide. Preparation follows the same principles as WC-Co: diamond grinding is mandatory, and the goal is a flat surface revealing the core-rim grain structure with sharp phase boundaries. The Ni binder is softer than the TiCN grains, creating similar relief challenges to WC-Co. Cermets are more brittle than WC-Co, so chipping at edges is more likely.

Use a diamond wafering saw with a thin resin-bonded diamond blade. Cutting speed: 100-200 RPM with very low feed rate and continuous coolant. The material is extremely hard and brittle; aggressive cutting causes edge chipping that may propagate as cracks into the sample. For small inserts, mount first in epoxy and then grind to the cross-section plane rather than attempting to cut the thin insert directly. Leave 1-2 mm allowance for grinding.

Hot compression mounting or cold mounting with epoxy are both suitable. The cermet is thermally stable at all mounting temperatures. For porosity analysis, vacuum impregnation with epoxy is recommended to distinguish porosity from grain pullout. Multiple small inserts can be mounted together. Cold mounting with a hard epoxy is preferred if edge examination is needed.

Diamond grinding is MANDATORY. SiC papers are ineffective against TiCN (~3000 HV). Use rigid diamond grinding discs.

Grinding sequence:

• 70 μm diamond disc: Remove sectioning damage (60-120 seconds). Moderate pressure (25-40 N). Material removes slowly.
• 40 μm diamond disc: Remove previous scratches (60-120 seconds). Maintain moderate pressure (25-35 N).
• 15 μm diamond disc: Fine grinding (60-120 seconds). Reduce pressure (20-30 N).

Disc speed: 250-300 RPM. Use complementary rotation. The cermet is more brittle than WC-Co; monitor for edge chipping and micro-cracking. Thorough ultrasonic cleaning between steps.

Diamond polishing is required throughout. The objective is revealing the core-rim grain structure with minimal relief.

Diamond polishing sequence:

• 6 μm diamond: 5-10 minutes on a hard composite or napless pad with firm pressure (25-35 N). The hard TiCN phase polishes slowly.
• 3 μm diamond: 5-10 minutes on a medium-hard synthetic pad (20-30 N). Monitor for binder smearing.
• 1 μm diamond: 3-5 minutes on a synthetic pad (15-25 N). The core-rim structure should begin to be visible in reflected light.

Final polishing:

• 0.25 μm diamond or 0.05 μm colloidal alumina: 2-5 minutes on a soft pad. Use colloidal alumina rather than colloidal silica. For highest quality, vibratory polishing with 0.05 μm alumina for 2-4 hours.

The core-rim grain structure is visible in the as-polished condition under optical microscopy because the core and rim phases have different reflectivity.

The core-rim structure is partially visible in the as-polished condition due to reflectivity differences. Etching enhances the contrast between core, rim, and binder phases.

Murakami's Reagent (Chemical Etching) - For binder etching:

• Composition: 10 g K3Fe(CN)6, 10 g KOH, 100 ml water
• Application: Immerse for 1-5 minutes at room temperature.
• Reveals: Ni binder phase etched (darkened), core and rim phases unetched. Binder distribution becomes clearly visible. Porosity vs. binder is distinguishable.
• Rinse: Water, then ethanol. Dry with warm air.

Villella's Reagent (Chemical Etching) - For phase differentiation:

• Composition: 1 g picric acid, 5 ml HCl, 100 ml ethanol
• Application: Immerse for 15-60 seconds.
• Reveals: Core and rim phases with enhanced contrast. The Ti-rich core typically etches lighter than the W/Mo-enriched rim. Excellent for core-rim structure analysis and rim thickness measurement.

HF-HNO3 Solution (Chemical Etching) - For carbide phase detail:

• Composition: 1-2 ml HF, 3-5 ml HNO3, 95 ml water
• Application: Immerse for 10-30 seconds. Caution: HF is extremely dangerous.
• Reveals: Selective attack of different carbide/carbonitride phases. Enhanced contrast between TiCN core and (W,Mo)-enriched rim.
• Safety: HF is acutely toxic. Use calcium gluconate gel as antidote on hand. Full PPE mandatory. Work in fume hood.