I use TiAlN-coated carbide for steel and DLC for aluminum. That combination covers about 80% of what I drill.
The tool material and coating are critical for gun drill performance. Get the combination wrong and the drill wears fast, holes drift, or surfaces come out rough. I have tested enough bad combinations to have a strong opinion on what works.
Tool Material Options
Carbide Grades
Most gun drills use carbide. It offers the best balance of wear resistance and toughness.
- C2 grade (ISO K20-K30). General purpose. Works for cast iron, non-ferrous, and most steels under 30 HRC. This is what I use for 90% of my work.
- C5 grade (ISO P20-P30). Higher hardness for steels above 30 HRC and stainless. Costs about 20% more than C2 but lasts 2x longer in hard materials.
- Micrograin carbide. Finer grain structure gives better edge retention. I use it for finishing passes where surface finish matters.
HSS-Cobalt
HSS-cobalt (M42 grade) still has a place. It costs roughly 40% less than carbide and resists chipping better. I use it in two situations: small shops with low production volumes where the tool spends more time sitting than cutting, and manual machines where spindle runout is > 0.05 mm and carbide would chip on the first pass.
PCD (Polycrystalline Diamond)
PCD-tipped gun drills are expensive — roughly 5x the cost of carbide — but they last 20–50x longer in aluminum and other non-ferrous materials. I only recommend PCD for high-production aluminum jobs where tool-change downtime eats into cycle time.
Tool Material Comparison
| Material | Hardness (HRA) | Toughness | Cost Multiplier | Best Material Match |
|---|---|---|---|---|
| Carbide C2 | 91–93 | Good | 1x | Steel, cast iron, non-ferrous |
| Carbide C5 | 92–94 | Fair | 1.2x | Hard steel, stainless |
| HSS-cobalt | 65–67 | Excellent | 0.6x | Low-production, rough spindles |
| PCD | N/A | Poor | 5x | High-production aluminum |
Coating Types — What Each Does Best
The coating matters more than the base material for most applications. A good coating can extend tool life by 50–300%, depending on the material.
TiN (Titanium Nitride) — Gold
The original PVD coating. TiN reduces friction and provides moderate wear resistance. Maximum operating temperature around 500°C. I use TiN-coated drills for general purpose steel drilling below 30 HRC. It is the cheapest coating option — adds roughly 10–15% to the tool cost.
TiCN (Titanium Carbonitride) — Blue-Gray
Harder than TiN, with better wear resistance. Handles temperatures up to 400°C but has higher friction. Works well in abrasive materials like cast iron and high-silicon aluminum. I reach for TiCN when I am drilling cast iron and need better edge life than TiN provides.
TiAlN (Titanium Aluminum Nitride) — Violet
This is my workhorse coating. The aluminum forms an aluminum oxide layer at high temperatures that acts as a thermal barrier. Effective up to 800°C. TiAlN is ideal for steel drilling above 100 m/min surface speed. It adds about 20–30% to tool cost but extends life by 100–200% compared to uncoated carbide in steels.
AlCrN (Aluminum Chromium Nitride) — Dark Gray
Similar to TiAlN but with better oxidation resistance at extreme temperatures (up to 900°C). AlCrN excels in high-temperature alloys like Inconel and titanium. I switch to AlCrN for anything above 35 HRC or any nickel-based alloy. It costs 30–40% more than TiN but is the only coating that survives in superalloys.
AlTiN (Aluminum Titanium Nitride) — Dark Gray
Higher aluminum content than TiAlN. Even harder, but slightly more brittle. I use AlTiN for very hard materials above 45 HRC — tool steels, die steels, hardened components. The coating is brittle enough that I avoid it for interrupted cuts.
DLC (Diamond-Like Carbon) — Black
DLC has the lowest coefficient of friction of any coating — around 0.1 compared to 0.6 for uncoated carbide. It prevents built-up edge in aluminum, copper, and plastics. Maximum temperature is lower than TiAlN (around 400°C), so it does not work for high-speed steel cutting. For aluminum gun drilling, DLC is the best choice. Tool life is 3–5x uncoated carbide in aluminum 6061.
Coating Comparison Table
| Coating | Max Temp | Hardness (GPa) | Friction Coeff | Cost Add | Best For |
|---|---|---|---|---|---|
| TiN | 500°C | 23 | 0.4 | 10–15% | General steel |
| TiCN | 400°C | 30 | 0.5 | 15–20% | Cast iron, abrasive |
| TiAlN | 800°C | 28 | 0.35 | 20–30% | Steel, high-speed |
| AlCrN | 900°C | 32 | 0.35 | 30–40% | Inconel, titanium |
| AlTiN | 850°C | 35 | 0.3 | 30–40% | Hardened steel > 45 HRC |
| DLC | 400°C | 15 | 0.1 | 25–35% | Aluminum, non-ferrous |
Material-Specific Recommendations
Low-carbon steel (1018, A36). TiN or uncoated carbide. Low speeds around 60 m/min. No need for expensive coatings — the material is forgiving.
Alloy steel (4140, 4340). TiAlN-coated carbide at 70–90 m/min. I get 2x tool life over TiN in 4140.
Stainless steel (304, 316). TiAlN or AlCrN. Stainless work-hardens, so the coating needs to handle the heat. AlCrN gives me 50% longer life than TiAlN in 316.
Aluminum (6061, 7075). DLC or uncoated. DLC-coated drills last 4x longer between sharpenings in 6061. The cost premium pays for itself after 200 holes.
Cast iron (gray, ductile). TiCN on carbide. Cast iron is abrasive — TiCN resists the wear better than TiN or TiAlN.
Titanium (Ti-6Al-4V). AlCrN only. Nothing else survives the combination of heat and work-hardening. I tried TiAlN once and got 12 holes before the coating failed.
Inconel / superalloys. AlCrN, low surface speed (20–30 m/min), and high coolant pressure. The coating is mandatory, not optional.
Cost vs. Benefit — When Coatings Pay Off
I track cost per hole, not tool cost. A $200 TiAlN-coated drill that lasts 500 holes costs $0.40 per hole. A $150 uncoated drill that lasts 150 holes costs $1.00 per hole. The coated tool is cheaper.
| Scenario | Uncoated Life | Coated Life | Cost per Hole Uncoated | Cost per Hole Coated | Savings |
|---|---|---|---|---|---|
| 4140 steel | 80 holes | 250 holes (TiAlN) | $1.88 | $0.80 | 57% |
| 6061 aluminum | 150 holes | 600 holes (DLC) | $0.67 | $0.33 | 50% |
| 304 stainless | 50 holes | 120 holes (AlCrN) | $3.00 | $1.67 | 44% |
The savings multiply on production runs. A 50-hole job might save $20 in tooling. A 10,000-hole job saves thousands.
Key Takeaways
- For steel, always coat it. TiAlN pays for itself in the first 100 holes.
- For aluminum, DLC is worth the premium if you run more than 500 holes per setup.
- For superalloys, coating is not optional — it determines whether the tool cuts at all.
- The coating matters more than the base carbide grade in most applications.
- Buy the best coating you can justify. Cheap coating peels off mid-hole and takes the carbide with it.
- Keep a coating selection chart by the machine. It saves the operator from guessing.