Carbide Grades and Coatings for Gun Drills: Selection Guide

The cutting edge of a gun drill travels at 60–120 m/min through steel, generating localized temperatures above 800°C while being cooled by high-pressure oil on the other side of the same edge. The carbide grade and coating that survive this environment are as specific to the workpiece material as the drill geometry itself.

Overview ​

Gun drill performance is determined by three interacting material choices: the carbide grade of the cutting tip, the coating applied to it, and the guide pad material. Selecting the wrong combination produces rapid flank wear, edge chipping, poor surface finish, or catastrophic tool failure.

ISO Carbide Classification for Gun Drills ​

The ISO 513 classification system groups carbide grades by workpiece material type and application:

For gun drilling, the typical grade range is P25–P40 for steels and K15–K30 for cast irons. The sealed cutting environment and continuous chip contact demand tougher grades than those used for conventional turning or milling.

How ISO Numbers Work ​

The number following the letter indicates the hardness-toughness balance:

Gun drilling typically operates in the middle to upper range because the cutting edge is fully engaged in the bore and cannot be visually inspected during cutting. A tougher grade reduces the risk of unpredictable edge failure.

Carbide Grade Selection by Workpiece Material ​

Material-Specific Grade Recommendations ​

Selection Logic ​

Grain Size and Cobalt Content ​

Grain Size Classification ​

Grain Size Distribution in the Market ​

Most production gun drills use submicron grades (0.5–0.8 µm), which provide the best balance of wear resistance and edge toughness for the 5–25 mm diameter range. Ultrafine grades (0.2–0.5 µm) are increasingly used for high-performance applications and difficult materials.

Cobalt Content vs. Properties ​

The relationship is inverse: higher cobalt content increases toughness (resistance to chipping and breakage) but reduces hardness and wear resistance. The optimum for gun drilling is typically 8–11% cobalt, depending on the application.

Typical Gun Drill Rod Grades ​

Coating Technologies for Gun Drills ​

PVD Coatings ​

Physical vapor deposition (PVD) is the dominant coating technology for gun drills. The low deposition temperature (~400°C) does not affect the carbide substrate properties, and the thin coating (2–4 µm) maintains the sharp cutting edge geometry.

CVD Coatings ​

Chemical vapor deposition (CVD) produces thicker (10–20 µm) multi-layer coatings at high temperature (~1,000°C). CVD is less common for gun drills because the high deposition temperature can reduce substrate toughness, and the thicker coating rounds the cutting edge. However, CVD is used for large-diameter BTA drills and heavy roughing applications.

TiAlN vs. AlTiN: The Key Distinction ​

The higher aluminum content in AlTiN forms a denser, more stable aluminum oxide (Al₂O₃) layer on the coating surface at cutting temperatures. This oxide layer acts as a thermal barrier, protecting the carbide substrate from heat and reducing chemical diffusion wear.

Material-Specific Coating Selection ​

Gun Drill Body Materials ​

The gun drill body (the shank behind the carbide tip) is typically made from steel, not carbide. This distinction is important for both cost and performance.

Body Material Options ​

For diameters below 3 mm, solid carbide gun drills are common because the steel body would lack sufficient torsional stiffness. For diameters above 5 mm, a brazed carbide tip on a steel body is the standard configuration.

Welded vs. Brazed Carbide Tips ​

Guide Pad Materials ​

Guide pads in gun drills support the tool in the bore and determine surface finish and straightness. The guide pad material must have excellent wear resistance and low friction against the bore wall.

The guide pads should wear at approximately the same rate as the cutting edge so that all elements reach end of life simultaneously. If the guide pads wear faster than the cutting edge, the drill will lose bore support and produce tapered or wandering holes. If the cutting edge wears faster, the drilling force increases and the guide pads may gall against the bore wall.

Guide Pad Grade Selection ​

Summary ​

FAQ ​

What is the most common carbide grade for gun drills? ​

The most common carbide grade for general-purpose gun drills is in the submicron range (0.5–0.8 µm grain size) with 8–10% cobalt content, corresponding to ISO P25–P30 for steels. This provides a balanced combination of wear resistance and toughness suitable for the majority of production gun drilling applications. Commercial designations include IC508, IC808, and RD10UF.

What coating is best for gun drilling stainless steel? ​

AlTiN (aluminum-rich titanium aluminum nitride) is the best coating for stainless steel gun drilling. The high aluminum content (approximately 65%) forms a stable aluminum oxide layer at cutting temperatures that provides thermal protection and reduces chemical diffusion wear. For severe galling or built-up edge problems, AlTiCrN (adding chromium) provides additional anti-seizure properties.

How does grain size affect gun drill performance? ​

Finer grain sizes (0.2–0.5 µm) provide higher hardness and wear resistance, enabling better surface finish and longer tool life in abrasive materials. Coarser grain sizes (1.0–3.0 µm) provide higher toughness and resistance to chipping and breakage. For gun drilling, submicron grades (0.5–0.8 µm) offer the best balance. Ultrafine grades (0.2–0.5 µm) are preferred for difficult materials such as hardened steels, titanium alloys, and nickel-based superalloys.

Why are PVD coatings preferred over CVD for gun drills? ​

PVD coatings are preferred for gun drills for three reasons: (1) the low deposition temperature (~400°C) does not affect the carbide substrate properties; (2) the thin coating (2–4 µm) maintains the sharp cutting edge geometry required for efficient cutting; and (3) the smooth coating surface reduces friction and built-up edge formation. CVD coatings (10–20 µm thick, deposited at ~1,000°C) can reduce substrate toughness and round the cutting edge, making them less suitable for the small-diameter, precision cutting edges of gun drills.

What is the difference between TiAlN and AlTiN coatings? ​

TiAlN (titanium-rich, ~50% aluminum) and AlTiN (aluminum-rich, ~65% aluminum) differ primarily in aluminum content. The higher aluminum content of AlTiN forms a denser, more stable Al₂O₃ layer at high temperatures, giving it superior high-temperature hardness and oxidation resistance. AlTiN can operate at up to 1,000–1,100°C compared to 800–900°C for TiAlN. AlTiN is preferred for high-speed drilling of stainless steels, nickel alloys, and titanium. TiAlN is sufficient for standard steel drilling at moderate speeds.

How many times can a gun drill be resharpened with the correct carbide grade? ​

With the correct carbide grade, a gun drill can typically be resharpened 5–10 times before the body is too short for the required drilling depth. The resharpening removes approximately 1–3 mm of carbide tip length each time. The substrate grade affects resharpening potential: tougher grades (higher cobalt) are more resistant to grinding damage and edge chipping during resharpening. A high-quality submicron grade with 10% cobalt will typically survive more regrinds than a harder grade with 6% cobalt.

What cobalt content is best for gun drilling alloy steels? ​

For alloy steels (4140, 4340) in the 25–40 HRC range, 8–10% cobalt provides the best balance. This gives sufficient wear resistance for reasonable tool life while maintaining the edge toughness needed to resist chipping during the continuous cut. For higher hardness (> 40 HRC), reduce cobalt to 6–8% and use an ultrafine grain grade for maximum wear resistance.

Can the same carbide grade be used for cutting tip and guide pads? ​

Not always. The cutting tip and guide pads have different wear mechanisms: the cutting tip wears primarily from abrasion and diffusion at the cutting edge, while the guide pads wear from sliding friction against the machined bore surface. When both components must wear at similar rates, the same grade may be appropriate. However, for abrasive materials (cast iron, aluminum-silicon alloys), a harder grade in the guide pads extends pad life and maintains bore support. For gummy materials (low-carbon steel, stainless steel), a tougher grade or cermet in the pads reduces galling.

What coating is recommended for gun drilling titanium alloys? ​

For titanium alloys (Ti6Al4V), AlTiCrN or AlTiN coatings are recommended. Titanium has low thermal conductivity (7 W/m·K), which concentrates heat at the cutting edge, and is chemically reactive, causing built-up edge and galling. AlTiCrN adds chromium for anti-seizure properties and can operate at the high edge temperatures generated in titanium drilling. AlTiSiN provides even higher temperature resistance for more demanding conditions. AlTiN is a good lower-cost alternative for moderate-speed titanium drilling.

How do I select between coated and uncoated carbide gun drills for aluminum? ​

For aluminum alloys, uncoated carbide with a sharp, polished cutting edge is the standard choice. Aluminum has a strong chemical affinity for many coating materials, particularly TiAlN and AlTiN, causing built-up edge. If a coating is required (for high-volume production or abrasive aluminum alloys), DLC (diamond-like carbon) or TiB₂ coatings provide low friction and anti-seizure properties without chemical reaction. Some manufacturers offer polished or burnished carbide edges that perform similarly to coated tools for aluminum.

Carbide grade and coating selection should be validated through application-specific testing. The recommendations in this article represent general guidelines as of 2026. Consult with tooling suppliers (Sandvik Coromant, ISCAR, Kennametal, Seco Tools) for material-specific grade recommendations and test results.