The CVD method is a coating method inducing a chemical reaction on the base metal surface of cemented carbide. Layers are deposited onto the surface in a reactor at 900°C ~ 1100°C (1650°F-2010°F) with gas reaction at atmospheric pressure.
Since the development of TiC coating in 1969, coated grades with a single layer of TiN, TiCN, or Al2O3 have been put to practical use one after another. In the late 1970s, coated grades with double layers and then multi-layers were developed. Presently multi-layer CVD coated grades are the main trend.
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Substrates coated using CVD
Substrates specifically for machining steel
When machining steel, high cutting forces and long periods of contact with the chip leads to high cutting edge temperatures. To combat this and prevent crater wear, the substrate needs to contain TiC, therefore it must be a mixed cemented carbide type (WC-TiC-Co).
In addition, the substrate needs to have a tough surface layer. This layer is an area that is enriched in Co and is employed to prevent cracks that develop in the hard coating layer from damaging the substrate.
Substrates specifically for machining cast iron
In comparison with steel, the chips developed when machining cast iron are shorter in length and the chip contact length is reduced. Therefore when machining cast irons the cutting forces tend to be located close to the cutting edge.
Additionally when machining cast irons the C within the cast iron chemically reacts with the Co in the substrate and diffusion wear can easily occur. In order to prevent this the substrate needs to have a low Co content. As mentioned previously, the cutting forces are localised at the cutting edge and this leads to vibration when machining, meaning that high fracture and wear resistance is required. Therefore the substrate for machining cast irons needs to be a straight cemented carbide type (WC-Co).
This is a process in which a reaction gas is fed into a high temperature environment chamber. A chemical reaction takes place and the result of this chemical reaction is that a thin film of hardened particles is left on the surface of the carbide substrate.
Advantages and disadvantages with CVD method
To be able to select a tool grade for a certain machining application it is necessary to understand the differences and features of both the CVD and PVD coating methods.
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Coating material and machining performance
Tool material properties and machining properties are closely related. However, the actual machining performance is determined based on a number of material properties. In other words, if a tool material consists of a material that theoretically has excellent properties it does not mean the tool material will provide high machining properties.
For example, flank wear and crater wear resistance of coated grades depends on the properties of the material that make up the coating layer, and the thickness of the coating layer. Whereas the plastic deformation and fracture resistance are dependant on the properties of the substrate.
The table and the graph below shows the material properties of hard particles that make up the coating layer.
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Hardness (Flank wear resistance)
Flank wear caused mechanically by rubbing is mostly influenced by the hardness of the tool material. In the coating, the TiC layer is the hardest and offers high flank wear resistance. Additionally, by applying a thicker coating layer it is possible to slow down the development of wear and in turn extend the tool life. However, there is one draw back, as the coating layer becomes thicker the grain structure becomes rougher and this can result in micro-chips forming, leading to brittle wear*. To overcome this problem, the use of titanium carbonitride (TiCN), a tougher coating material, has become widely used.
Free energy formation (Crater wear resistance)
During machining, the chips developed rub / slide across the surface of the rake face subjecting it to severe high pressure and temperature condition. Under these conditions a chemical reaction can occur between the workpiece and the coating making it necessary for a coating layer to have thermal stability even under high temperature conditions. The term that defines thermal stability is Free Energy Formation. As the minus value becomes larger it offers greater stability. Aluminium oxide (Al203) is the most stable.
Thermal conductivity (Plastic deformation resistance,
thermal crack resistance) / Thermal impact coefficient
Materials that have a low thermal coefficient cannot radiate heat effectively and causes heat to gather around the cutting edge. When this occurs it can result in the cutting edge becoming soft, thus leading to plastic deformation. The thermal impact coefficient can be found using the formula below. As shown in the table, TiC has a much lower thermal impact coefficient than WC.
Oxidation property (Notch wear resistance)
Notch wear is a form of wear that appears at the cutting edge boundary. Notch wear also occurs at the same point when machining surface scale or on materials that easily work harden. Additionally, if the tool is coated then there is a possibility that the coating layer will suffer from chipping and spalling. When this occurs the TiC layer will be exposed. The TiC layer has little oxidation resistance and notch wear will rapidly progress. As notch wear develops the surface finish of the workpiece will deteriorate.
Solubility in Fe (Welding resistance)
As much as 7％ of WC dissolves in iron at 1,250°C, while less than 0.5％ of TiC dissolves in iron at the same temperature. This shows that TiC is less likely to react with workpiece materials at high cutting edge temperatures.
Features of Al2O3
An AI2O3 layer coated using the CVD method has the following features.
Even under high temperature conditions the hardness is maintained, thus providing superior wear resistance.
The graph below shows the wear resistance properties of three test pieces (TiC, TiCN and AI2O3 coated inserts) when machining carbon steel. It can be seen when the cutting speed is 400m/min the wear resistance of the AI2O3 coated insert is better than that of the TiC and TiCN coated inserts.
Protects the substrate at high temperature.
The graph below shows that at low temperatures there is little difference in the thermal properties of the three test pieces. However as the temperature increases it can be seen that AI2O3 prevents heat from being transferred when compared to the TiC and TiCN. This means that during machining, the heat generated is not transferred to the substrate and prevents crater wear and plastic deformation of a cemented carbide substrate.
Special carbide substrate
Cross section of the micro-structure