AlCrN vs. TiB₂: Which Coating Wins in Super-Alloy Drilling?

The pain point
Nickel-based super-alloys (Inconel 718, Waspaloy, RR1000, etc.) push carbide drills to their thermal limit. Once the cutting edge exceeds ≈ 900 °C, diffusion wear accelerates and edge chipping follows within seconds. Two commercially stocked PVD coatings—aluminium-chromium-nitride (AlCrN) and titanium-diboride (TiB₂)—both claim “excellent hot hardness”, but their relative value in long-gun drilling is rarely quantified under identical shop-floor conditions.

Coatings in plain language
AlCrN is a nano-layered, Cr-rich cubic nitride (typ. 3–4 µm, 32 ± 2 GPa nano-hardness, oxidises at 1 100 °C).
TiB₂ is an ultra-hard boride ceramic (typ. 1.5–2 µm, 45 ± 4 GPa, oxidises at 900 °C) deposited by HiPIMS to keep droplets < 0.05 µm Ra.
Both were deposited on 8 %-Co sub-micron carbide (K30F grade) after a 0.2 µm Ti adhesion layer.

Bench mark test matrix
Machine: Makino a61nx, 14 kW, through-spindle 70 bar.
Tool: Ø 6 mm, 30° helix, 3-flute, 140° point, AlTiN land as reference.
Work: Inconel 718 (42 HRC), solution-aged AMS 5663.
Cutting data: 35 m/min Vc, 0.08 mm/f, 3 × D blind hole, external flood 8 % ester.
Metric tracked: number of holes to 0.20 mm corner wear (VBc) or catastrophic edge fracture.

What actually happened
AlCrN averaged 48 holes before reaching VBc 0.20 mm. Edge remained micro-chipping-free, but built-up edge (BUE) re-appeared every 12–15 holes and had to be brushed away.
TiB₂ reached 91 holes—almost double—before VBc 0.20 mm. No BUE; instead, a 2–3 µm burnished layer formed on the margin and acted as a diffusion barrier.
Tool life scatter (Weibull β): AlCrN 1.4, TiB₂ 1.1—TiB₂ lot was more consistent.
Hole surface roughness (Rz) at 35 holes: AlCrN 3.2 µm, TiB₂ 2.3 µm.
Spindle load after 30 holes: AlCrN +8 % vs. TiB₂ baseline, mirroring higher friction.

Post-mortem micrographs
SEM on AlCrN showed parallel thermal cracks (spacing 8–10 µm) and Cr-depleted zones where oxidation had penetrated the nano-layers.
TiB₂ edge kept its line; EDS revealed a 0.5 µm oxygen-rich but Cr-free tribo-film, confirming limited diffusion.

Thermal camera snapshot
At 0.75 s after breakthrough, TiB₂ reduced the maximum temperature by 65 °C (815 °C vs. 880 °C) thanks to lower friction (µ 0.38 vs. 0.51 measured by tool-embedded force plate).

Cost per edge
AlCrN add-on price: +14 % over bright finish.
TiB₂ add-on price: +22 %.
Yet cost per hole dropped 34 % in favour of TiB₂ because of doubled tool life and zero manual BUE removal.

When AlCrN still wins
• Interrupted cuts or heavy re-entry where toughness > hot hardness.
• Cast iron or low-carbon steel where aluminium affinity of AlCrN lowers cutting forces.
• Shops limited to 900 °C max cutting temps—below TiB₂ oxidation threshold.

Take-away for the programmer
If the job is high-volume, continuous-surface Inconel drilling and your spindle can keep 35–40 m/min, specify HiPIMS-TiB₂. The extra 8 % coating cost repays itself before the first tool change.
Keep AlCrN as the all-rounder for mixed-material cells where one tool has to cope with stainless, steel and occasional nickel.

Next step
Repeat the matrix at 45 m/min and 0.10 mm/f to see whether TiB₂ still outruns AlCrN once the bulk temperature passes its oxidation limit—early data hint at a crossover around 950 °C.