The best coating for high speed dry cutting is Nitrogen Aluminum Titanium
A big reason why cutting fluids are often no longer necessary today is because of coatings. They mitigate temperature shocks by inhibiting heat transfer from the cutting zone to the insert (tool). The coating acts like a thermal barrier because it has a much lower thermal conductivity than the tool base and workpiece material. As a result, these tools absorb less heat and can withstand higher cutting temperatures. Whether turning or milling, coated tools allow for more efficient cutting parameters without reducing tool life.
The coating thickness is between 2 and 18 microns, and it plays an important role in tool performance. Thinner coatings withstand temperature changes better during impact cutting than thicker coatings because thinner coatings have less stress and are less prone to cracking. On rapid cooling and heating, thick coatings tend to shatter like a glass that heats and cools very quickly. Dry cutting with thinly coated inserts can extend tool life by up to 40%, which is why physical coatings are often used to coat round tools and milling inserts. PVD coatings tend to be applied thinner than chemical coatings and bond more strongly to the contour. In addition, PVD coatings can be deposited on cemented carbide at much lower temperatures, so they are more used for very sharp edges and large positive rake milling and turning tools.
Although the coating material is titanium nitride, it accounts for 80% of all coated tools. However, in the case of high-speed dry cutting, the best PVD coating is titanium-aluminum-nitride (TiAlN), which outperforms titanium nitride by a factor of four in high-temperature continuous cutting, such as in high-speed turning. TiAlN coating also outperforms other coatings for tools under higher thermal stress conditions. Such as dry milling and deep hole drilling of small diameter holes where cutting fluids are difficult to reach
TiAlN is harder than TiN at cutting temperatures and is thermally stable. PVD coatings take advantage of its resistance to chemical wear. It has a hardness of up to 3500 degrees Vickers and its operating temperature is up to 1470°F. Materials scientists speculate that these properties can be attributed to amorphous aluminum oxide films, which form at the chip/tool interface when some of the aluminum in the coating surface oxidizes at high temperatures.
Ultra-thin multilayer PVD coatings were deliberately selected for this study, and the deposition process produces coatings consisting of hundreds of layers, each just a few nanometers thick. The deposition of general PVD coatings is only a few micrometer-thick coatings.
Although PVD coating has many advantages, CVD coating is still more popular for machining most ferrous metals. In the CVD process, the higher deposition temperature helps to improve the bonding strength and allows a higher cobalt content in the matrix, so that the toughness of the cutting edge is good and the ability to resist plastic deformation is improved. Due to the CVD coating ratio
CVD is the process of depositing a useful layer of aluminum oxide on the tool, the most heat and oxidation resistant coating known. Alumina is a poor conductor, it isolates the tool from the heat generated by cutting deformation, and promotes heat flow into the chip. This is an excellent CVD coating material mainly for carbide turning tools used in dry cutting. It also protects the substrate during high-speed cutting and is the best anti-abrasive and crater wear coating.
Coated inserts have longer tool life and are more stable in dry milling than wet milling. Higher cutting speeds will further increase the cutting temperature. For example, dry machining cast iron at a cutting speed of 14,000 rpm and 1,575 inches/min can heat the cutting zone in front of the tool to 600° to 700°C. The metal removal rate is similar to milling aluminum, while the resulting temperatures are higher on cast iron than on conventional tools.
Selection of cermets, ceramics, CBN, PCD
Higher cutting speeds require more wear-resistant tool materials and higher thermal hardness. Cermets, cubic boron nitride, and two ceramics suitable for fine-processing needs—alumina and silicon nitride (the modern term "ceramic" includes both aluminum oxide and silicon nitride, as opposed to just referring to aluminum oxide in the past.), they applications are becoming more and more popular. Polycrystalline diamond is another tool material used in dry cutting situations. In all of these materials, they have higher red hardness and wear resistance, the trade-off is greater brittleness.