The damage forms of cutting inserts are mainly divided into wear and breakage (chipping). The hardness and thickness of the coating directly affect the wear resistance of the insert, while its toughness affects the insert's resistance to impact and breakage. During cutting, the formation of adherent layers on the insert surface and their forced removal during continuous machining can also lead to wear and breakage. The stability of the coating material in terms of heat resistance and chemical resistance, as well as its non-affinity with the workpiece material, helps to prevent adhesion and the corresponding damage. The thermal conductivity of the coating is also important; it reflects the coating's ability to dissipate heat generated during machining. A low thermal conductivity means poor heat dissipation, which keeps heat from entering the insert and instead transfers it to the chip for removal, thereby reducing thermal wear on the insert.
![CNMG carbide inserts]( "CNMG carbide inserts")
Coating a few micrometers of hard material on the surface of substrate materials such as tungsten carbide can endow cutting tools with the toughness of the substrate material while also incorporating the high hardness of the coated hard material. Generally, a coating thickness of no more than 0.2% of the insert thickness can significantly enhance the cutting performance of the insert and tool. Therefore, the application of coated inserts and tools is becoming increasingly widespread. Under different conditions, depositing different coatings can improve cutting efficiency and extend tool life. Nowadays, uncoated inserts and tools are only used in certain special anti-adhesion machining processes, non-ferrous metal machining, and low-cost applications.
Coatings are generally divided into two methods: Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD). The basic principle of CVD coating is to introduce compound-containing gases into a high-temperature furnace (900-1100°C) to produce a chemical reaction on the surface of the object being coated, thereby depositing a hard material. The principle of PVD coating is to use vacuum deposition techniques such as ion plating, sputtering, and ion mixing to deposit hard materials on the surface of the object at lower temperatures (100-700°C).
Due to the high processing temperature, CVD coatings can only be applied to substrates that are heat-resistant. CVD coatings have strong adhesion to the substrate and can form relatively thick layers. By changing the raw gas, multiple materials can be continuously coated in the same furnace. Therefore, CVD coatings are suitable for high-speed, high-feed, deep-cutting applications where a large volume of material needs to be removed in a short time. Generally, turning inserts mainly use the CVD method to deposit coatings to enhance their cutting performance. However, due to the difference in thermal expansion coefficients between the coating and the substrate material, tensile residual stresses can easily form, which is detrimental to the insert's resistance to breakage and fatigue.
Let's look at the advantages of PVD coatings. They can be applied at lower temperatures, which means lower heat resistance requirements for the substrate. The coating process does not reduce the strength and toughness of the cutting edge. The coating is thinner and produces compressive residual stress in the coating film, which improves resistance to breakage and fatigue. According to the ISO international standard, cutting tool materials are categorized as follows: P for steel, M for stainless steel, K for cast iron, N for non-ferrous metals, S for heat-resistant steel, and H for high-hardness steel. Each material is further divided into several categories based on performance and composition: 01, 10, 20, 30, and 40. The lower the number, the harder the material; the higher the number, the tougher it is. For example, cutting tool materials for steel are marked as P01, P10...P40, and the same applies to others. For different materials and cutting conditions, inserts made of materials corresponding to different codes should be used for machining. 
