The metal forming industry employs forming tools such as punches and dies to deform plastically the metal to bring it into a desired shape. A force higher than the yield strength of the workpiece is applied in the normal direction. When the punch surface comes in contact with the workpiece, friction forces can induce wear at the edges of the punch and sides of the punch head but also on the inner lateral surfaces of the die. As a result, the tool lifetime decreases. To avoid that, the surface of the tool is protected by applying hard coatings.
The process temperature in PVD is low
Two methods are used to deposit hard coatings. These are chemical vapor deposition (CVD) and physical vapor deposition (PVD). In both processes, a vapor of the coating material is created and transported to the substrate where it condenses and adheres by adsorption (PVD) or chemical reaction (CVD).
In the case of CVD, the vapor of the coating material is created by chemical mean through reaction between gas species in a vacuum chamber. For the reaction to take place, a temperature of 600°C-1500°C is needed. CVD is suitable to deposit thick coatings because of the high adhesion strength due to chemical bonding with the substrate. The process has also a high deposition rate.
In PVD, the vapor of the coating material is created by physical mean through heating (thermal evaporation) or energetic particle bombardment (sputtering). The advantage of the PVD process is that heating is not necessarily needed. The temperature in PVD-sputtering without intentional heating is low (200°C). Using PVD, there is the possibility to deposit alloys with the same composition as in the source coating material.
Magnetron sputtering uses permanent magnets behind the target to produce a magnetic field perpendicular to the electric field. This configuration increases the electron density at the target vicinity and thereby the ionization probability. Therefore, lower pressure and lower voltage are needed in the process and the deposition rate is increased as well.
The density of the hard coatings deposited with magnetron sputtering can be enhanced by optimizing the pressure, target to substrate distance, the magnetic field strength and unbalancing degree, the power, and the substrate bias. This property results in coatings with higher hardness. The hard coatings include carbides, nitrides and borides that are deposited by sputtering a metal target in a reactive atmosphere.
The thickness of these coatings produced with PVD can be up to 10 µm with a deposition rate that can be 7 µm/h and can vary depending on the power, target to substrate distance, pressure, magnetic field configuration, and substrate bias.
High Power Impulse Magnetron Sputtering (HiPIMS)
In order to enhance further the coating density and properties, a variant of PVD magnetron sputtering was introduced in 1999 which is high power impulse magnetron sputtering (HiPIMS). In this coating method, a power is applied to the target in pulses with low duty cycle which increases the electron density during the on-time while the same average power, as in magnetron sputtering, is applied. This property increases the ionization probability of the target species and thereby the density of the coating.
The pulsing configuration (pulse on-time, and frequency) influence the density. The deposition rate in HiPIMS is lower than that of conventional magnetron sputtering but can be improved mostly through optimized pulsing and magnetic field configuration.
The HiPIMS process combines the benefits of magnetron sputtering and cathodic arc evaporation as the ionization probability of the target species is enhanced and coatings with lower defect density are obtained.
HiPIMS reduces poisoning of the target --> higher deposition rates
There is an important aspect to consider when conducting the magnetron sputtering in the reactive mode. This is the poisoning of the target which reduces significantly the deposition rate. This problem is reduced with the HiPIMS process to a degree that depends on the process parameters.
Backward cold forging