Cutting tool coatings improve the friction and wear properties of cutting tools, which is why they are essential in cutting operations. For many years, surface processing technology providers have been developing customized coating solutions to improve cutting tool wear resistance, machining efficiency and service life. The unique challenge comes from the attention and optimization of four elements: (i) pre- and post-coating processing of cutting tool surfaces; (ii) coating materials; (iii) coating structures; and (iv) integrated processing technology for coated cutting tools.
Cutting tool wear sources
During the cutting process, some wear mechanisms occur in the contact zone between the cutting tool and the workpiece material. For example, bonded wear between the chip and the cutting surface, abrasive wear of the tool by hard points in the workpiece material, and wear caused by frictional chemical reactions (chemical reactions of the material caused by mechanical action and high temperatures). Since these frictional stresses reduce the cutting force of the cutting tool and shorten the tool life, they mainly affect the machining efficiency of the cutting tool.
The surface coating reduces the effect of friction, while the cutting tool base material supports the coating and absorbs mechanical stress. The improved performance of the friction system can save material and reduce energy consumption in addition to increasing productivity.
The role of coating in reducing processing costs
Cutting tool life is an important cost factor in the production cycle. Among other things, cutting tool life can be defined as the time of a machine can be machined without interruption before maintenance is required. The longer the cutting tool life, the lower the costs due to production interruptions and the less maintenance work the machine has to do.
Even at very high cutting temperatures, the use life of the cutting tool can be extended with coating, thus significantly reducing machining costs. In addition, cutting tool coating can reduce the need for lubricating fluids. Not only reduces material costs, but also helps to protect the environment.
Effect of pre- and post-coating processing on productivity
In modern cutting operations, cutting tools need to bear high pressures (>2 GPa), high temperatures and constant cycles of thermal stress. Before and after the coating of the cutting tool, it must be treated with the appropriate process.
Before cutting tool coating, various pretreatment methods can be used to prepare for the subsequent coating process, while significantly improving the adhesion of the coating. By working in conjunction with the coating, the preparation of the tool cutting edge can also increase cutting speed and feed rate, and extend cutting tool life.
The coating post-processing (edge preparation, surface processing and structuring) also plays a determinant role in the optimization of the cutting tool, in particular to prevent possible early wear by the formation of chip (bonding of workpiece material to the cutting edge of the tool).
Coating considerations and selection
The requirements for coating performance can be very different. Under machining conditions where the cutting edge temperature is high, the heat-resistant wear characteristics of the coating become extremely important. It is expected that modern coatings should also have the following characteristics: excellent high-temperature performance, oxidation resistance, high hardness (even at high temperatures), and microscopic toughness (plasticity) through the design of nanostructured layers.
For efficient cutting tools, optimized coating adhesion and a reasonable distribution of residual stresses are two decisive factors. Firstly, the interaction between the substrate material and the coating material needs to be considered. Secondly, there should be as little affinity as possible between the coating material and the material to be processed. The possibility of adhesion between the coating and the workpiece can be significantly reduced by using an appropriate tool geometry and polishing the coating.
Aluminum-based coatings (e.g. AlTiN) are commonly used as cutting tool coatings in the cutting industry. Under the action of high cutting temperatures, these aluminum-based coatings can form a thin and dense layer of aluminum oxide that continually renews itself during machining, protects the coating and the substrate material beneath it from oxidative attack.
The hardness and oxidation resistance performance of a coating can be adjusted by changing the aluminum content and the coating structure. For example, by increasing the aluminum content, using nano-structures or micro-alloying (i.e., alloying with low content elements), the oxidation resistance of the coating can be improved.
In addition to the chemical composition of the coating material, changes in the coating structure can significantly affect the performance of the coating. The different cutting tool performance depends on the distribution of the various elements in the coating micro-structure.
Nowadays, several single coating layer with different chemical compositions can be combined into a composite coating layer to obtain the desired performance. This trend will continue to develop in the future – especially through new coating systems and coating processes, such as the HI3 (High Ionization Triple) arc evaporation and sputtering hybrid coating technology that combines three highly ionized coating processes into one.
As an all-round coating, titanium-silicon based (TiSi) coatings offer excellent machinability. These coatings can be used for processing both high hardness steels with different carbide contents (core hardness up to HRC 65) and medium hardness steels (core hardness HRC 40). The design of the coating structure can be adapted accordingly to the different machining applications. As a result, titanium silicone-based coated cutting tools can be used for cutting and processing a wide range of workpiece materials from high-alloyed, low-alloyed steels to hardened steels and titanium alloys. High finish cutting tests on flat workpieces (hardness HRC 44) have shown that coated cutting tools can increase its life by almost two times and reduce surface roughness by about 10 times.
The titanium-silicon based coating minimizes subsequent surface polishing. Such coatings will be expected to be used in processing with high cutting speeds, high edge temperatures and high metal removal rates.
For some other PVD coatings (especially micro-alloyed coatings), coating companies are also working closely with processors to research and develop various optimized surface processing solutions. Therefore, significant improvements in machining efficiency, cutting tool use, machining quality, and the interaction between material, coating and machining are possible, and practically applicable. By working with a professional coating partner, users can increase the utilization efficiency of their tools throughout their life cycle.
Post time: Nov-07-2022