Coating of the turning insert

In order to obtain the best cutting performance, the cutting insert must be surface-coated. The most commonly used cutting insert coating process is the chemical vapor deposition (CVD) process, a metal target is ionized by high current, and then deposited on the turning insert by evaporation and condensation. This process can be vividly compared to that when the temperature of the asphalt road becomes very low and the air is full of high concentration of water vapor, a thin layer of ice will form on the road. However, the difference is that although the temperature of the turning insert placed in the coating furnace is relatively low, the actual furnace temperature may exceed 480°C.

Another commonly used blade coating process is the physical vapor deposition (PVD) process. Compared with the CVD process, the use of PVD technology can deposit a thinner coating, so that the cutting edge can be sharper, and it can be more excellent when cutting difficult-to-machine materials (such as hardened steel, titanium alloy and heat-resistant super alloy) Cutting performance.

In a typical turning insert CVD coating process, the first layer of coating on the turning insert is titanium carbide nitride (TiCN). This coating material can provide excellent wear resistance, and also has the advantage of being easy to bond with the cemented carbide substrate. Usually, aluminum oxide (Al2O3) is used as the second coating. This coating has excellent thermal and chemical stability, and can protect the turning insert from the adverse effects of cutting high temperatures and the chemical composition in the coolant.

The thickness of TiCN and Al2O3 coatings mainly depends on the processing type of the turning insert. For example, when turning hard materials, the turning insert needs to be fully protected, so the thickness of each coating may need to reach 10μm. For the finishing of soft materials, it may be more appropriate to coat a 5μm thick TiCN layer and a 2μm thick Al2O3 layer.

After finishing the preparation of TiCN and Al2O3 coatings, the cutting insert is close to the finished product in terms of function. Unfortunately, the color of the Al2O3 coating is completely black, and it is difficult for users to distinguish which working surfaces of the turning insert have been used and whether the cutting edge has been worn. In order to solve this problem, most tool manufacturers will finally apply a layer of titanium nitride (TiN) coating on the turning insert. This bright golden coating has good visibility, and the user can easily evaluate the wear state of the cutting insert through the change of its color.

In the past, the completion of the TiN coating marked the completion of the manufacturing of the cutting insert. But in recent years, there is one last process that has become increasingly popular. In the CVD or PVD coating process, when the cutting insert cools, the degree of shrinkage of different coating materials varies. Therefore, stresses are generated in each layer of the coating, and microcracks appear. In order to eliminate these stresses and minimize micro-cracks, people have adopted an advanced technology of sandblasting the cutting insert with a mixture of alcohol, alumina and fine sand. After the sandblasting process is completed, the manufacturing of the cutting insert is complete.