Preparation of Gradient Carbide Coating Substrate (2)

Gradient cemented carbide substrate coating

In order to improve the cutting performance of cemented carbide, more than 80% of cemented carbide tools in industrialized countries are surface coated. For decades, double-coated, triple-coated and multi-coated composite blades have been developed at home and abroad, and some coatings even reach the level of tens of layers and hundreds of layers.

Carbide coating technology can be divided into chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology.

1. Selection of coating materials

Tool wear mechanism studies have shown that at high speed cutting, the tip temperature can reach up to 900 ° C. At this time, the wear of the tool is not only mechanical wear, but also bond wear, diffusion wear and oxidative wear. Therefore, the cutting process can be regarded as a physicochemical change process of a microdomain. The choice of coating material has a great influence on whether the coating can perform its intended function on the tool.

Titanium carbide is a high hardness wear-resistant compound with good anti-friction and wear properties; titanium nitride has a slightly lower hardness, but has higher chemical stability and can greatly reduce friction between the tool and the workpiece being machined. coefficient. From the perspective of coating process, both are ideal coating materials, but regardless of the titanium or titanium nitride, a single coating is difficult to meet the comprehensive requirements of high-speed cutting for tool coating.

Titanium carbonitride (TiCN) is a single TiC lattice in which a nitrogen atom (N) occupies the position of the original carbon atom (C) in the lattice to form a composite compound. The ratio of carbon to nitrogen atoms in TiCxNy is compared. The ideal mode is TiC0.5N0.5 and TiC0.3N0.7. Since TiCN has the comprehensive properties of TiC and TiN, its hardness is higher than that of TiC and TiN, so it is an ideal tool coating material.

No material can be compared with alumina (Al2O3) in terms of oxidation and wear resistance. However, due to the difference in physical and chemical properties between alumina and matrix alloys, a single alumina coating cannot produce an ideal coating tool. The emergence of multi-coating and related technologies enables the coating to improve the bonding strength with the substrate while at the same time having the comprehensive properties of a variety of materials.

So far, the coating of cemented carbide inserts can be roughly divided into four series: TiC/TiN, TiC/TiCN/TiN, TiC/Al2O3 and TiC/Al2O3/TiN. The first two are suitable for general semi-finishing and precision cutting, and the latter two are suitable for high speed and heavy duty cutting.

2. Chemical vapor deposition (CVD) technology

Chemical vapor deposition (CVD) is an important technological breakthrough in the field of cemented carbide. It uses one or several compounds containing a coating element or elemental gas in the gas phase of the reaction chamber in which the substrate is placed or on the surface of the substrate. The chemical reaction forms a coating. The common CVD technique is to use C/N-containing organic compound acetonitrile (CH3CN) as the main reaction gas, and decompose and chemically react with TiCl4, H2 and N2 at 700-900 °C to form TiCN. The coating effectively improves the surface hardness and wear resistance of the cemented carbide product, prolongs the service life of the cemented carbide product, reduces the loss, and improves the machining efficiency.

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