What Drill Bit Coatings Are
Pick up a drill bit and the first thing you notice is the color. Bare metal silver, black, gold, bronze, sometimes an almost oily rainbow sheen. These aren't paint jobs or aesthetic choices. Each color represents a different surface treatment, a thin layer of material bonded to the steel that changes how the bit behaves when it contacts wood, metal, or plastic.
The coatings exist because high-speed steel, the material most bits are made from, has limitations. It works, but it generates friction, it oxidizes, it wears. A few microns of the right coating can reduce friction by 40%, extend bit life by several hundred percent, and allow higher cutting speeds without burning the material.
Black Oxide
The simplest and oldest coating. Black oxide forms when steel is heated in the presence of oxidizing salts, creating a thin layer of magnetite on the surface. The process has been used since the 19th century for protecting steel tools from rust.
On drill bits, black oxide serves two purposes. It provides mild corrosion resistance, which matters for bits stored in damp shop environments. More importantly, it helps retain cutting oil. The magnetite layer is slightly porous at a microscopic level, creating surface texture that holds lubricant better than bare steel.
The coating thickness measures around 1-2 microns. You can see through it if you look closely at a black oxide bit under good light. The base steel shows through in areas of high wear. This is expected behavior, not a defect.
Black oxide doesn't make bits harder or more heat resistant. It doesn't reduce friction by itself. What it does is create better lubrication retention, which indirectly reduces heat and extends life. The performance improvement is modest, maybe 20-30% over uncoated bits in similar materials.
The black color fades to gray as bits wear. This happens because the coating is so thin that normal abrasion removes it in high-contact areas. A bit that's half black and half gray has simply worn through the coating where it contacts the material most.
Titanium Nitride
Gold-colored bits. The coating that made drill bits look like jewelry when it appeared in the consumer market in the 1980s. Titanium nitride had been used in industrial tooling since the 1960s, but manufacturing costs kept it out of general-purpose bits for decades.
The coating forms through physical vapor deposition. Bits are placed in a vacuum chamber. Titanium metal is vaporized and reacts with nitrogen gas, creating a ceramic compound that deposits on the bit surface. The process happens at high temperature, around 450-600 degrees Celsius, and requires several hours.
Titanium nitride is genuinely hard. On the Vickers hardness scale, it measures around 2,400, compared to high-speed steel at 800-900. This extreme hardness reduces friction dramatically. A titanium nitride coated bit generates less heat during cutting because the hard surface creates less resistance as it moves through material.
The gold color comes from the way the titanium nitride crystal structure reflects light. It's not paint or plating. The color is intrinsic to the material. Coating thickness runs 2-4 microns, thick enough to be durable but thin enough that sharpening removes it from the cutting edges.
That last point matters. When you sharpen a titanium nitride bit, you're grinding away the coating on the cutting surfaces. The flutes retain their coating, but the edges revert to bare steel. The bit still works, but some of the friction reduction disappears.
Performance gains over uncoated bits are substantial. In testing, titanium nitride bits last 3-5 times longer in steel and even longer in aluminum or plastic. The heat reduction allows higher drilling speeds without burning wood. The coating also has low adhesion to most materials, meaning chips don't stick as readily.
Titanium Carbonitride
The bronze or brownish-gold bits. Titanium carbonitride adds carbon to the titanium nitride formula, creating a coating that's harder and more wear-resistant than plain titanium nitride.
The manufacturing process is similar to titanium nitride but includes a carbon source in the deposition chamber, typically methane gas. The resulting coating has a more complex crystal structure that achieves hardness around 3,000 on the Vickers scale.
The color difference is subtle but observable. Titanium carbonitride has a darker, more bronze appearance compared to the brighter gold of titanium nitride. The distinction isn't always clear, and manufacturers sometimes use the terms interchangeably on consumer products.
The practical difference in bit performance is modest. Titanium carbonitride bits last slightly longer than titanium nitride in abrasive materials. The improvement is measurable in industrial testing but may not be obvious in typical workshop use. Both coatings provide similar friction reduction and heat resistance.
Cost is higher than titanium nitride because the process is more complex. Whether the performance gain justifies the cost depends on the application and how much the bit gets used.
Titanium Aluminum Nitride
A coating designed specifically for high-temperature applications. Titanium aluminum nitride has a distinctive violet or purple color that makes it immediately recognizable.
The aluminum addition changes the coating's thermal properties. Where titanium nitride starts to break down around 600 degrees Celsius, titanium aluminum nitride remains stable past 800 degrees. This makes it valuable for drilling hard metals where friction generates extreme heat.
The coating forms an aluminum oxide layer when heated, which provides additional thermal protection. This self-healing property means the coating actually becomes more protective under the conditions that would degrade other coatings.
Hardness measures around 3,300 Vickers, the highest of the common bit coatings. The combination of hardness and heat resistance makes these bits extremely long-lasting in demanding applications.
You see titanium aluminum nitride primarily on premium bits designed for metalworking. It's overkill for wood drilling where temperatures never reach levels that stress simpler coatings. The purple color has become associated with high-end tooling, though the performance advantage only appears in specific, high-stress conditions.
Bright Finish vs Coatings
Not all silver-colored bits are uncoated. A "bright finish" bit has been polished to a mirror-like surface but has no additional coating. The polishing creates a smooth surface that reduces friction compared to rough, unpolished steel, but not as much as actual coatings.
The manufacturing difference is mechanical rather than chemical. Polishing removes surface irregularities through abrasion. Coating adds material. A polished bit weighs slightly less than when it started. A coated bit weighs slightly more.
Bright finish bits occupy a middle ground in both cost and performance. They're cheaper than coated bits because polishing is simpler than vapor deposition. They perform better than raw, unfinished steel but not as well as chemically treated surfaces.
Some manufacturers apply black oxide over bright finish. Others leave bits with a dull, unpolished surface under black oxide. The base finish affects how well the coating adheres and how the bit performs after the coating wears through.
Cobalt Confusion
Here's where terminology gets messy. "Cobalt" bits aren't coated. Cobalt is alloyed into the steel itself, creating a different base material rather than a surface treatment.
Cobalt steel contains 5-8% cobalt mixed throughout the metal. This changes the steel's properties at a fundamental level. It increases heat resistance and hardness compared to standard high-speed steel. Cobalt bits can tolerate higher temperatures before the cutting edge softens.
The confusion arises because cobalt bits are often silver or have a dull gray appearance, leading people to think they have no coating when really they're made from a different material entirely. Some cobalt bits also receive coatings like titanium nitride, combining the benefits of both treatments.
Cobalt bits cost significantly more than high-speed steel because cobalt is expensive. The performance difference shows up primarily in hard materials like stainless steel or cast iron. For drilling wood or soft metals, the cobalt content provides minimal advantage.
You can't tell if a bit is cobalt by looking at it unless it's marked. The color varies by finish. The only reliable way to identify cobalt content is manufacturer labeling or, if you're really committed, spectrographic analysis.
Diamond Coatings
The newest category, and genuinely different from the others. Diamond coatings use actual diamond particles, not diamond-like carbon or cubic structures. The particles are micron-sized and embedded in a thin matrix bonded to the bit surface.
These coatings appear in two forms. Chemical vapor deposition creates a continuous diamond layer at extreme cost, used primarily in industrial applications. Electroplated diamond coatings suspend diamond particles in a metal matrix, more common in consumer bits designed for abrasive materials.
The hardness exceeds all other coatings by a wide margin. Diamond sits at the top of the hardness scale. This makes diamond-coated bits exceptional for tile, glass, stone, and other materials that destroy conventional coatings.
The limitation is heat sensitivity. Diamond oxidizes at temperatures above 700 degrees Celsius in the presence of oxygen. This means diamond-coated bits work well at low speeds with water cooling but can be damaged by the heat generated in dry, high-speed drilling. They're specialized tools for specific materials, not general-purpose bits.
What Happens When Coatings Wear
All coatings are consumable. The question is how quickly they wear and what happens when they're gone.
Thin coatings like black oxide wear through first in the areas of highest friction - the cutting edges and the areas where the flutes contact the hole wall. The bit continues working, just with gradually decreasing performance as more coating disappears.
Harder coatings like titanium nitride last longer but still wear. The pattern is similar: cutting edges lose coating first, flutes retain it longest. A partially worn titanium nitride bit performs somewhere between fully coated and uncoated, depending on how much coating remains.
Sharpening accelerates coating loss on cutting edges but leaves flutes intact. A sharpened bit may look gold or black on most of its surface while having bare steel edges. This is normal and doesn't prevent the bit from working effectively.
Some wear is catastrophic rather than gradual. If a coating chips off in large flakes, usually due to impact or excessive heat, the bit may perform worse than if it had no coating at all. The exposed edges where coating broke away can catch and bind. This happens more with improperly applied coatings than with quality manufacturing.
The Chemistry Behind Color
The different colors of coated bits aren't arbitrary. They reflect the actual chemistry of the coating material and how it interacts with light.
Titanium nitride's gold color comes from its band gap, the energy difference between electron states in the material. The coating absorbs blue and violet light while reflecting yellow and red, creating the characteristic gold appearance.
Titanium carbonitride's bronze color results from the carbon changing the crystal structure enough to shift which wavelengths get absorbed versus reflected. The effect is subtle because the chemical difference is subtle.
Titanium aluminum nitride's purple color appears because the aluminum addition creates a different absorption pattern, pulling the reflected color toward violet and reducing yellow reflection.
Black oxide is black because magnetite absorbs most visible light across the spectrum. The slight color variation you see in different black oxide bits reflects differences in coating thickness and crystal structure.
These color relationships are consistent across manufacturers. You can identify coating type by color with reasonable accuracy, though some premium bits use proprietary coating combinations that don't fit the standard categories.
Manufacturing Cost and Price Reality
The price differences between coated bits reflect genuine manufacturing cost differences. Black oxide is cheap because it's a simple hot chemical bath. Titanium nitride requires expensive vacuum equipment and multiple processing steps. More exotic coatings require even more sophisticated equipment.
Consumer bit pricing often deviates from these cost relationships. Marketing plays a role. A titanium nitride bit might cost three times what an equivalent black oxide bit should cost based on manufacturing expenses, but the manufacturer charges five times as much because consumers associate gold bits with premium quality.
At the industrial level, pricing tracks more closely to actual costs. Professional-grade bits show smaller price multiples between coating types because industrial buyers care more about cost per hole than appearance.
The practical question is whether coating performance justifies cost. For occasional use, basic black oxide bits work fine. For production work or drilling hard materials, premium coatings pay for themselves in reduced bit replacement. For most workshop applications, the middle ground of titanium nitride provides good value.
Observable Performance Differences
Laboratory testing shows clear performance differences between coatings. Titanium nitride bits last 300-500% longer than uncoated bits in controlled tests. Titanium aluminum nitride extends life even further in high-heat conditions.
Real-world results vary more widely. A weekend woodworker might never wear out any bit enough to notice coating differences. A contractor drilling dozens of holes daily sees dramatic differences in bit life and performance.
The materials being drilled matter enormously. Coating benefits appear most clearly in metal drilling where friction and heat are highest. In softwood, coating differences are subtle. In hard materials like composite decking or engineered lumber, coatings show their value.
Temperature is the key variable. Coatings reduce friction, which reduces heat, which prevents the bit's cutting edge from losing temper. Once steel overheats enough to soften, the bit dulls rapidly regardless of coating. Coatings extend the time before this happens, but they can't prevent it if drilling conditions generate excessive heat.
When Coatings Don't Matter
Some drilling applications negate coating advantages. Very low speeds generate so little friction that coating benefits disappear. Drilling with constant lubrication reduces heat enough that coating thermal properties become irrelevant.
Large bits, roughly 1/2 inch and up, see less benefit from coatings because the cutting geometry and lower rotational speeds create different heat and friction conditions. Coating still helps, but the performance multiple decreases.
Bits designed for specific materials, like spade bits for rough carpentry or masonry bits for concrete, often use no coating or minimal coating because their cutting geometry and intended materials don't generate conditions where coatings provide significant benefits.
Understanding what drill bit coatings actually are - the chemistry, the manufacturing processes, the real versus marketed benefits - provides context for choosing bits and understanding their performance over time. The colors and marketing language make the category seem more mysterious than it is. At base, coatings are thin layers of hard material designed to reduce friction and heat, applied through various industrial processes that determine both their properties and their cost.