New PVD tool coating

- Mar 13, 2019-

New PVD tool coating

 

TiAlN and AlTiN are PVD tool coatings formed by depositing Al into TiN. Up to now, increasing the aluminum content in TiAlN and AlTiN coatings to enhance the high temperature resistance and hardness of tool coatings has been a major technical issue concerned by tool manufacturers and coating companies. Since 1995, the related vapor deposition process has been continuously studied and improved. By 2000, the ratio of aluminum to titanium in TiAlN and AlTiN coatings had increased from 1:2 to 3:2, that is, the aluminum content had increased from 33% to 60%.

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In order to further improve the aluminum content in the coating, the Liechtenstein based Balzers coating company developed a large number of research and development, invented the coating technology of replacing titanium element with chromium element, and introduced the single-layer AlCrN coating named "BalinitAlcorna" in 2004. AlCrN coating has a higher aluminum content than the general AlTiN coating and is suitable for a variety of high speed steel and carbide cutting tools including gear hobs, end mills and milling blades. In addition, it can be used for turning tools, but only in heat resistance and diffusion stability of excellent matrix materials, such as PCBN and Si3N4 ceramics. In the exhibition of EMOHannover2005, balchas company also launched the product named "BalinitHelica", which is a multi-layer AlCrN coating specially designed for drilling tools. This super-smooth coating can be applied to any hard alloy or high speed steel bit, significantly enhancing the wear resistance and shear strength of the bit, as well as improving the chip removal performance of the bit.

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When the section microstructure of BalinitAlcorna coating on cemented carbide matrix was observed by scanning electron microscope (SEM), it could be clearly seen that the thickness was 3 ~ 4 m and the coating was a single layer with continuous structure. In the SEM image of BalinitHelica coating, the multi-layer coating structure is clearly discernible. The total thickness of the Helica multilayer coating is approximately 4 m, while 1 ~ 2 m is more suitable for a small bit less than 1/8 "in diameter.

 

Although 100% Al2O3 (pure alumina) PVD coating provides the best thermal protection for cutting tools (AlN components in AlTiN and AlCrN coatings "convert" to Al2O3 during machining), this coating has limited applications. Although PVDAl2O3 coating technology has been patented, and has been used in the use of ac power (or pulse dc power supply) on the laboratory scale of small reaction furnace on the tool Al2O3PVD coating, but now in the commercial scale of large reaction furnace Al2O3PVD coating is still a lot of problems. This is because Al2O3, unlike other conductive coating materials, is an insulator. Therefore, the PVD coating process based on the plasma deposition principle requires external bias voltage during the coating deposition process, that is, there must be a certain potential difference between the coated tool and the plasma target source. In short, the insulating properties of alumina make the PVD process rather difficult to control. In addition, the economy of PVDAl2O3 coating is also poor.

 

Due to the limitation of coating structure stability, the aluminum content in AlTiN coating has actually reached the maximum value (about 65%). In TiN - based coatings, the crystal structure is transformed from cubic lattice to hexagonal lattice. However, in the CrN - based coating, the aluminum content can be further increased without changing the crystal structure of the AlCrN coating.

 

Like other coatings, the deformation resistance of AlCrN coatings also depends on the lattice shape of the coating material. The coating with cubic lattice can maintain high red hardness, that is, the coating can maintain high hardness when exposed to the cutting temperature of the tool/workpiece interface. Once the crystal structure of the coating is transformed into a hexagonal lattice, the hardness of the coating will be reduced due to the decrease of deformation resistance. TiAlN coating hardness at about 800 or have fallen sharply, AlTiN coating when temperature is less than 900 , the phenomenon of the hardness is lower; And at the time of the temperature of 1100 AlCrN coating can still maintain its hardness. Even under the high temperature of 1100 , also can protect the AlCrN coating matrix without tools. Balzers AlCrN series coating has a hardness level of 2800 ~ 3200HV at a load of 50g.

 

Because AlCrN coating high red hardness so that it can maintain a stable performance under extremely high thermal load, so AlCrN coating tool than other types of coating tool processing performance is better, especially in high-speed cutting, dry (or quasi-dry) cutting conditions more advantages. For example, the cutting experiments carried out by Balzers' cutting laboratories demonstrated that when the Ck45 steel workpiece was precisely milled at speeds of 200m/min and 400m/min respectively, the service life of the AlCrN coated milling cutter was much longer than that of the TiAlN coated milling cutter. However, there are exceptions, in the processing of high hardness (54HRC) workpiece materials (such as steel mold), the performance of AlTiN coating is better than the AlCrN coating (tool life increased 20% ~ 40%, while the tool price reduced by about 25%). However, when machining materials with low hardness (< 54HRC) (such as alloy steel and cast iron) and materials with high viscosity (such as low carbon steel and stainless steel), the cutting efficiency of AlCrN coating is significantly higher than that of AlTiN coating (the tool life can be increased by 25% ~ 100% under the same cutting speed and feed).

 

In addition, AlCrN coating is also recommended for machining workpiece materials (such as stainless steel) that produce long chips, because the long chips have more contact with the tool's front cutting surface during forming, and the increase in the contact degree between the tool and the workpiece will generate a lot of cutting heat. However, AlCrN coating has a small friction coefficient, which can reduce the adhesion between the cutting tool and the chip and reduce the cutting heat.

 

In addition, compared with tin-based coating, AlCrN coating can lead to changes in tool wear and chip forming mode during processing, that is, AlCrN coated tool rear surface wear (VB) is significantly less than TiAlN coated tool, and the width (KB) of wear groove is small, but the depth (KT) of wear groove is increased.

 

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