Coating properties

Each characteristic of the coating plays an important role in determining which coating is most beneficial to the cutting process.This article will describe some common properties of cutting tool coatings to help us choose the appropriate coating for the cuting tool.

1. Hardness

The high surface hardness of the coating is one of the best ways to increase tool life. In general, the harder the material or surface, the longer the cutting tool life. Titanium nitride (TiCN) coating has higher hardness than titanium nitride (TiN) coating. Due to the increased carbon content, the hardness of the TiCN coating is increased by 33%, which ranges from Hv3000 to 4000 (depending on the manufacturer). CVD diamond coating with surface hardness up to Hv9000 has been widely used in tools. Compared with PVD coated tools, the life of CVD diamond coated tools is 10 ~ 20 times longer. The high hardness and cutting speed of diamond coatings are two to three times higher than that of uncoated tools, making them a good choice for non-ferrous materials.

2. Wear resistance

Wear resistance refers to the ability of a coating to resist wear. Although some workpiece materials themselves may not be too hard, the addition of elements and processes during production may cause the cutting edge of the cutting tool to collapse or blunt.

3. surface lubricity

High friction coefficient will increase the cutting heat, resulting in shorter coating life and even failure. Reducing the friction coefficient can greatly prolong the cutting tool life. A smooth or well-textured coated surface helps to reduce cutting heat because the smooth surface allows chips to quickly slide away from the front cutter face, reducing heat generation. Coated tools with a better lubricated surface can also be machined at higher cutting speeds than uncoated tools, further avoiding high-temperature fusion welding with the workpiece material.

4. Oxidation temperature

Oxidation temperature refers to the temperature at which the coating begins to decompose. The higher the oxidation temperature is, the better the machining is under the high temperature condition. Although TiAlN coatings may be less hard at room temperature than TiCN coatings, they have proved to be much more effective in high-temperature processing than TiCN coatings. The reason the TiAlN coating retains its hardness at high temperatures is that a layer of alumina is formed between the tool and the chip. The alumina layer transfers heat from the cutting tool to the workpiece or the chip. Carbide tools typically have higher cutting speeds than HSS tools, which makes TiAlN the coating of choice for carbide tools. Carbide bits and end mills often use this PVD TiAlN coating.

5. anti-bonding

The anti-bonding properties of the coating prevent or mitigate chemical reactions between the cutting tool and the material being processed and prevent the workpiece material from being deposited on the tool. In the processing of non-ferrous metals (such as aluminum, brass, etc.), the tool will often produce chip nodules (BUE), resulting in tool collapse or workpiece size out of tolerance. Once the processed material begins to adhere to the tool, the adhesion will continue to expand. For example, when aluminum workpiece is processed with molded taps, the aluminum attached to the tap will increase after each hole is processed, so that the diameter of the tap will eventually become too large and the workpiece will be discarded out of tolerance. A coating with good adhesion resistance can work well even in processing situations where coolant properties are poor or concentration is insufficient.