The emergence of vacuum coating technology has been relatively recent. In the international arena, the CVD (chemical vapor deposition) technology was applied to hard alloy cutting tools in the 1960s. However, during its early development, this technology faced many obstacles. It needed to operate in a high-temperature environment (with a process temperature above 1000ºC) and had a limited variety of coatings, which significantly limited its development potential.
By the end of the 1970s, PVD (physical vapor deposition) technology emerged, opening up a promising new space in the field of vacuum coating. In just a few decades after that, PVD coating technology developed rapidly.
Nowadays, new technologies such as PCVD (physical chemical vapor deposition) and MT-CVD (medium-temperature chemical vapor deposition) have emerged in the vacuum coating technology field. Various coating equipment and processes have been springing up continuously, presenting a prosperous and diverse scene.
In the future, the development trends of tool coating technology will include the following points:
(I) Diversification and complexity of coating components
a. The first generation of PVD coatings mainly consisted of TiN. On this basis, various single metal coatings such as TiC, TiCN, ZrN, CrN, WC were developed successively. With the further development of PVD deposition technology, aluminum was added to the coatings, and multicomponent metal alloy coatings such as TiAIN and TiAICN emerged. Their wear resistance and red hardness have significantly improved compared to single-metal coatings, enabling them to be used at higher cutting speeds, for example, up to 150m/min in rolling cuts.
b. Later, it became a trend to deposit multiple different types of coatings on the tool layer by layer to leverage the advantages of each coating. For example, TiN + TiCN + TiN, TiN + TiALN, TiAIN + WC/C, etc. combinations were used.
c. In recent years, PVD coating technology has taken another significant step forward. Several coating companies abroad have successfully developed pulse coating technology and begun to apply it. For instance, the P3E (Pulse Enhanced Electron Emission) technology from Balzers in Switzerland and the H.I.P_ (High Ion Pulse) technology from Cemecon in Germany. These two new technologies both utilize pulsed electrons to activate the arc evaporation of the target material. Due to this process operating in an oxygen atmosphere, theoretically, this technology can deposit any metal oxide (such as Al2O3, ZrO2, Cr2O3, Ta2O5, etc.) and its compound coatings. Currently, the Al2O3 coating has entered the practical trial stage, and it is believed that it will be widely applied in the near future.
(II) The application development of coatings becomes more targeted
To meet different application requirements, the development and design of coatings have become increasingly targeted. According to the characteristics and requirements of different application fields such as drilling, milling, dry rolling cutting, stamping, and deep drawing, coatings with relative advantages in these aspects have been developed. Through continuous efforts and experiments, success has been achieved in certain fields, such as the application of TiX (Al:Ti = 2:1) coatings in milling, AICrN coatings applied to high-speed dry rolling cutting, CrN + TISIN composite coatings applied in drilling, and TIN + TCX composite coatings applied in deep drawing molds. Their lifetimes are significantly better than those of other coatings. In addition, various targeted coatings with functions such as corrosion resistance (Crx coatings), "self-lubrication (WC/C coatings), processing of soft materials (MoS2 coatings), and processing of hard materials (CBN, Diamond coatings)" have already been widely applied. Although these coatings have been very successful in their respective fields, with the continuous development of PVD coating technology, new more targeted coatings will be continuously developed to replace these existing coatings.
(III) The deposition particles of coatings tend to be nanometerized
With the development of nanotechnology and the advancement of coating technology, nanometer-coated cutting tools have attracted great attention from researchers and PVD coating service companies. The nanometerization of coating deposition particles can enhance the bonding strength between the coating and the substrate as well as among different layers, and can also reduce the surface roughness of the coating. Currently, the deposition particles of most coatings are still relatively large. Although there are some coatings called nano-level, large particles can still be found on the final surface of the coating, and the coating surface is still relatively rough. Reducing the size of coating deposition particles while maintaining process stability to avoid the appearance of large abnormal particles will become an important direction for the development of coatings, especially in mirror surface applications. Although some companies have developed mirror surface coatings, their quality and stability are poor, and the process is also relatively complex. In the future coating research and development, the nanometerization of coating particles and the nanometerization of interlayer thickness of coatings will be the main development directions, which is of great significance for improving the comprehensive performance of coatings and reducing interlayer stress, and will further improve the smoothness of the mirror surface, thereby further expanding the application of coatings in the precision forming industry.
(IV) The process temperature of coatings is getting lower
From the deposition temperature of about 1000℃ for general CVD coatings to about 500℃ for PVD and PECVD coatings, the deposition temperature of coatings has decreased, thus expanding the application range of coatings. However, a deposition temperature of about 500℃ still has adverse effects on the coating workpiece, such as causing deformation of the workpiece and a decrease in the hardness of the substrate. Therefore, special requirements for the preheating of the coating workpiece need to be proposed, such as the back heating temperature of the workpiece not being lower than the coating temperature. Coatings with lower temperatures, such as those with a coating temperature below 200℃, will eliminate these limitations, allowing for a wider range of materials for coating applications, more flexible selection of preheating, and more feasible comprehensive application of different surface modification technologies. At the same time, the application of low-temperature coatings will reduce the energy consumption of coating equipment, having a certain environmental protection effect in energy conservation. Moreover, the reduction of coating temperature allows for shorter heating and cooling times, shortening the delivery cycle of coatings and improving efficiency. Therefore, low-temperature coatings will greatly promote the application and popularization of coatings, and will become an important direction for the development of PVD coatings.
