Proper surface treatment of small circular tools can increase tool life, reduce cycle time and improve surface quality. However, the correct choice of tool coating depending on the processing needs can be a confusing and laborious task. Each coating has advantages and disadvantages in the cutting process. If an improper coating is selected, it may lead to lower tool life than uncoated tools, and sometimes even lead to more problems than before coating. .
A wide variety of tool coatings are available, including PVD coatings, CVD coatings, and composite coatings with alternating PVD and CVD coatings, which are readily available from tool manufacturers or coating suppliers. Floor. This article will introduce some of the common properties of tool coatings as well as some common PVD and CVD coating options. Each of the properties of the coating plays an important role in determining which coating is most beneficial for cutting.
1. Characteristics of the coating
(1) The high surface hardness brought by the hardness coating is one of the best ways to improve the tool life. In general, the higher the hardness of the material or surface, the longer the life of the tool. Titanium Nitride (TiCN) coatings have higher hardness than Titanium Nitride (TiN) coatings. Due to the increased carbon content, the hardness of the TiCN coating is increased by 33%, and the hardness varies from about Hv 3000 to 4000 (depending on the manufacturer). The use of CVD diamond coatings with surface hardness up to Hv9000 is relatively mature on tools, and the lifetime of CVD diamond coated tools is 10 to 20 times higher than that of PVD coated tools. The high hardness and cutting speed of diamond coatings can be 2 to 3 times better than uncoated tools, making it a good choice for non-ferrous materials.
(2) Abrasion resistance Wear resistance refers to the ability of the coating to resist abrasion. Although the hardness of some workpiece materials may not be too high, the elements added during the production process and the processes used may cause the cutting edges of the tool to crack or blunt.
(3) Surface Lubricity High friction coefficient increases the heat of cutting, resulting in shortened or even lost coating life. Lowering the coefficient of friction can greatly extend tool life. A smooth, smooth or textured coating surface helps reduce heat of cut because the smooth surface allows the chips to slide away from the rake face quickly and reduce heat generation. Compared to uncoated tools, coated tools with better surface lubricity can be machined at higher cutting speeds to further avoid high temperature welding with workpiece materials.
(4) Oxidation temperature The oxidation temperature refers to the temperature at which the coating starts to decompose. The higher the oxidation temperature value, the more advantageous the cutting process under high temperature conditions. Although the TiAlN coating may have a lower room temperature hardness than the TiCN coating, it has proven to be much more effective than TiCN in high temperature processing. The reason why the TiAlN coating retains its hardness at high temperatures is the formation of a layer of alumina between the tool and the chip, which transfers heat from the tool to the workpiece or chip. Carbide tools typically have higher cutting speeds than high-speed steel tools, making TiAlN the preferred coating for carbide tools. Carbide drills and end mills typically use this PVD TiAlN coating.
(5) The anti-adhesion of the anti-adhesive coating prevents or reduces the chemical reaction between the tool and the material being processed, and prevents the workpiece material from depositing on the tool. When machining non-ferrous metals (such as aluminum, brass, etc.), built-in chips (BUE) often occur on the tool, causing tool chipping or workpiece size. Once the material being processed begins to adhere to the tool, the adhesion will continue to expand. For example, when an aluminum workpiece is machined by a forming tap, the aluminum adhered to the tap increases after each hole is machined, and finally the diameter of the tap becomes too large, causing the workpiece size to be out of service. Coatings with good anti-blocking properties work well even in applications where the performance of the coolant is poor or the concentration is insufficient.
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