- Quality of coatings varies widely between manufacturers, especially oxide coatings
- Coated drill bits and taps are worth the cost as they increase the life of the tools at a higher ratio than their increased cost
- Coatings increase surface hardness, smoothness, and prevent galling and build up on the cutting edge.
- TiN coatings are good for drill bits or taps
- TiCN is the ideal coating for taps used by farriers
- High performance nano coatings are ideal for carbide drill bits
.Left to right: Steam oxide coating, TiN coating, TiN with Oxide coating, TiCN, and nano coated carbide
One of the more confusing aspect of selecting drill bits and taps is the variety of available coatings since the manufacturers provide little to no actual information about the coatings themselves. The issue is further confused by the wide range of quality of coatings of the same type, despite looking similar.
Most quality tool coatings are applied by a process called physical vapor deposition (PVD) in which the tools are placed in a vacuum chamber and the coating is vaporized by heat then condenses on the tool which is relatively cold, like water condensing on the outside of a cold drink. This process creates a very uniform layer of the material that's usually around 2 microns thick (about 1/50th the thickness of a piece of paper).
As a complicated process, not all coatings are equal. There is a noticeable difference in the performance of coatings, even of the same type, from different manufacturers. The most commonly noted difference is how fast the coating wears.
Oxide coatings are applied differently, and vary widely in their effectiveness based on how they are applied. High quality oxide coatings are created by a precise process of heating the tool in a steam furnace and the coating is actually created from the steel of the tool. It's essentially a much more precise version of how steel becomes black when forging. Some cheaper “oxide” coatings are applied cold and are actually a copper selenium compound that wears off easily.
Tool coatings are utilized to achieve 3 goals:
- Create a surface that's much harder than the material of the tool, so that it lasts longer and holds an edge better. Similar to a knife with an edge that's harder than the spine, this technique allows a tool to hold an edge well but not be as brittle as if the entire tool were as hard as the coating.
- Create a surface that's significantly smoother than its possible to get the surface of the tool. The smoother the surface, the less friction created when cutting. The less friction when cutting the less heat that's created and the easier the tool is to turn, which means longer tool life and less broken tools. Some coatings actually have lubricating properties.
- Prevent material from galling and sticking to the cutting tool. A common cause of cutting tool failure is whats referred to as “built up edge” (BUE). This is created by the material being cut, galling and sticking to the surface of the cutting tool. This mostly happens around the cutting edge and effectively dulls the tool very quickly. Tool coatings are selected to prevent the material from sticking to its surface. For farriers, this is most commonly a problem in aluminum shoes.
Walking down the tool aisle at your local Home Depot, the two most common coatings you'll see are Oxide, which is black, and Titanium Nitride (TiN) which is gold. The cheaper oxide coatings are virtually useless and wear off the cutting edge almost immediately. Good quality oxide coatings are porous, allowing them to hold and carry cutting lubricant better than an uncoated surface. They prevent galling and BUE but are the softest of the coatings. They are also the cheapest.
TiN is a clear step up from even high quality oxide coating. Its around 20% harder, and is significantly smoother and resistant to BUE. In industrial applications though, its typically only seen on tools meant for use in Aluminum since the demands are lower, and higher grade coatings are used on tools meant for use in steel and exotic alloys. Still, for farriers, the additional cost of TiN coated tools over uncoated or oxide coated tools is money well spent as the coating can significantly increase the life of the tools. In our own tests we found that TiN coated tools lasted over twice as long as oxide coated tools.
Our recommended coating for taps for farriers is Titanium Carbonitride (TiCN). This coating is close in price to TiN but is harder (3000 vickers/87 Hrc vs 2300 vickers/81 Hrc) and has a lower coefficient of friction (.45 vs .65) than TiN. For farriers tapping by hand or using impact guns, that lower coefficient of friction is critical to being able to quickly get shoes tapped and minimize broken taps. The hardness means you get more for your money and have to stock fewer taps.
Two other common coatings used in industrial applications are TiAlN and AlTiN. Both are great coatings for tooling used in CNC milling machines and we use a lot of AlTiN coated tooling in the shop but the only advantage of that coating is heat resistance and its a lot more expensive. Its worth the cost when machining 4140 at 5000 rpm but for a tap in an impact gun, there's not nearly enough heat involved to make a difference, let alone justify the cost. In talking with application engineers, it was suggested that the higher heat is actually necessary for the AlTiN to get to a coefficent of friction comparable to TiCN and that it'd be more susceptible to chipping in an impact gun.
At the top of the line of cutting tool coatings is the advanced proprietary nano coatings (nACo) like used on our high performance carbide drill bits. These coatings are generally only found on carbide cutting tools. They are significantly harder, smoother, and more heat resistant than other coatings to the degree that they can be used in steel horseshoes without cutting lubrication, saving money and mess. They are more expensive but the incredible tool life achieved by nACo coated carbide tooling and elimination of lubrication significantly cuts overall costs and hassle making it a much cheaper option for farriers that drill and tap regularly.
Regardless of your choice in cutting tools, high quality coatings are a cost effective way to increase the life and performance of the cutting tools you carry in your shoeing rig.