PVD technologies, including vacuum arc evaporation and DC-magnetron sputtering, have been utilized in industrial settings since the early 1980s for depositing protective coatings. These coatings encompass a range of materials such as metal nitrides, carbonitrides, oxides, oxynitrides, and DLC, serving diverse applications such as cutting and forming tools, automotive components, and decoration. Vacuum arc evaporation generates a highly energized and ionized particle flux toward the substrate, while “classical” gas-ion-dominated direct current magnetron sputtering (DCMS) has limitations in generating ionized and energetic species of the sputtered target material. The development of High-Power Impulse Magnetron Sputtering (HiPIMS) has exhibited significant potential in addressing DCMS's limitations by enabling the production of highly energetic particles. This innovation, with its industrial applicability for protective coatings, was introduced around 2010. This paper aims to provide an industrial perspective on HiPIMS, serving as a guide for scientists and engineers in comprehending and implementing HiPIMS solutions. It covers historical context and fundamental characteristics. Basic features as well as state-of-the-art configurations of PVD systems are also described. Graphical representations of experimental results illustrate HiPIMS features, including operational modes, deposition rate effects, thickness uniformity, and sustainability, particularly in terms of energy efficiency. The discussion focuses on the application prospects, advantages, and constraints of industrially applied HiPIMS protective coatings, emphasizing cutting and forming tools, within the context of the findings presented.
Most of the industrial applied protective coatings used in tribological applications (such as tools, components, decoration, and corrosion reduction) are deposited through PVD processes that utilize cold plasma states, specifically arc evaporation and magnetron sputtering processes.1,2 For a detailed explanation of physical vapor deposition with plasma assistance, you can refer to a recent review.3 One crucial aspect of PVD processes is the energy flux of the species that impact the substrate, including both layer-forming species and process gas species. This energy flux plays a significant role in tailoring coating properties depending on the process parameters.4
Classical magnetron sputtering solutions, based on direct current magnetron sputtering (DCMS) and the pulsed mode (P-DCMS), are dominated by gas ions. Gas-ion-dominated sputtering means that layer-forming species are generated from gas-ion impacts on the target, especially in non-reactive processes. When a bias is applied to the substrates, coating growth is primarily modified by the impact of accelerated gas ions.5
In most cases, argon (Ar) is used as the sputtering gas. However, when depositing hard coatings, appropriate gases such as nitrogen (N2), oxygen (O2), hydrocarbons (CxHy), and gas mixtures are added. Magnetron sputtering technology has been available for mass production of tribologically stressed parts for more than 30 years,6–8 whereas arc evaporation for hard coatings, including hard hydrogen-free carbon coatings, began approximately 50 years ago.9,10 Vacuum arc discharge is characterized by high ionization of the evaporated cathode material and high process stability on an industrial scale.11
Figure 1 provides an overview of the main operation modes of DCMS at the target. The discharges occur at different power levels, with the maximum, or at least temporary, applied power being categorized as low-power density (Lpd) and high-power density (Hpd). Both power-density modes can be achieved with discharge parameters (U, I) in DCMS processes that may or may not vary over time or with pulsed discharge parameters (P-Lpd or P-Hpd). It is important to note that all these modes are part of the DCMS family. For instance, we refer to DCMS running with constant discharge parameters at low-power densities as Lpd DCMS or simply classical DCMS, and if it is pulsed, we call it P-DCMS (P-Lpd DCMS).
Gas-ion-dominated processes in low-power-density mode imply that most of the target sputtering results from gas-ion impacts, and coating growth is primarily modified by these gas-ion impacts. In contrast, target-material-ion-dominated processes running in high-power-density modes indicate that, at least temporarily, most of the target sputtering results from metal-ion impacts, and the coating growth is mainly modified by target-material-ion impacts. If the high-power-density mode is continuously applied over a period, it can lead to self-sputtering, which essentially means gasless sputtering or SS-DCMS. This mode can be categorized as Hpd-DCMS and can be initiated for various target materials.
HiPIMS, an abbreviation for High-Power Impulse-DCMS, represents a pulsed high-power-density process (P-Hpd-DCMS). HiPIMS operates between the two modes, transitioning from Lpd to Hpd-DCMS, even during a single pulse. Additionally, the ionization of gas and target material strongly depends on the specific target material and the pulse parameters, such as current density at the target. A previous terminology for HiPIMS was HPPMS, which stood for high-power P-DCMS (P-Hpd-DCMS).12
Industrial magnetron designs for circular, rectangular, or cylindrical targets enable sputter processes with sputter power densities of several 10 W/cm2, especially with metallic targets like Cr and AlTi.13,14 Often, the limit of power density is around 30 W/cm2. P-DCMS processes have been optimized with high duty cycles to achieve the highest deposition rates while minimizing arcing at the targets.3,15 Some characteristics of traditional DCMS and P-DCMS are illustrated in Fig. 1. Both discharges typically operate at relatively low-power densities at the target. The development of P-DCMS processes, which apply high-power densities at the target during pulses, originated from research conducted in the 1980s in the Soviet Union.16–21 It was subsequently patented in Russia in the early 1990s.22 A notable paper on HiPIMS (High-Power Impulse Magnetron Sputtering) in the Western world was published in 1999.23
The operational range of magnetically enhanced glow discharge for sputtering processes has been expanded through the development of suitable pulsed power supplies and tailored experimental setups and processes that enable high-power densities at the target area. The higher ionization of the sputtering plasma (with an ion density ni ≤ 5 × 1019 m−3) was measured during high-current pulses.16 Similar values (ranging from 1018 to 1019 m−3) were observed by several other research groups.17,24,25
It is important to note that alongside pulsed processes, investigations of non-pulsed processes at high-power densities were also conducted in the early 1990s. These studies demonstrated that by surpassing a certain threshold of power density in DCMS, a self-sputtering process of the target material occurs in addition to gas sputtering.24,26–29 This gave rise to a gasless process known as self-sustained magnetron sputtering, SS-DCMS. It was soon discovered that self-sputtering also plays a significant role in HiPIMS processes.25,30–32
The term HiPIMS, referring to the pulsed high-power-density sputtering process that falls between P-DCMS and SS-DCMS and is characterized by significant ionization of the sputtered species, was called HiPIMS in the scientific community around 2000. Research showed that HiPIMS offers greater flexibility in tailoring the microstructure and topography of various coatings, whether they are non-reactive (e.g., metallic Cr) or reactive (e.g., AlTiN-based) coatings, compared to conventional low-power-density DCMS and P-DCMS.33–35
However, it was also observed that the material to be sputtered significantly influences the ionization degree of the HiPIMS plasma. For instance, carbon exhibits an extremely low ionization degree even at high-peak power densities.12,36,37 It is worth mentioning that HiPIMS discharge not only has the potential to modify coatings but also to perform ion cleaning processes38 and nitriding processes.39 Additionally, the high-density plasma stimulates plasma-assisted chemical vapor deposition (PA-CVD) processes in the form of plasma decomposition, as demonstrated in acetylene deposition for a-C:H.40,41 This paper summarizes and discusses the features of industrial HiPIMS solutions and their potential applications in tribology, especially in cases where limitations of the performance of coatings deposited by arc or low-power-density DCMS have been identified.
II. INDUSTRIAL VIEW OF HiPIMS FEATURES
A. Basics of HiPIMS physics for engineering processes
First and foremost, it should be noted that both the discharge parameters (voltage and current) determine the plasma condition, and the film-forming species change within one pulse over the pulse duration depending on the pulse shape and length.42 Time- and space-resolved optical emission spectroscopy (OES) has identified up to four different phases for a triangular current pulse shape: (1) the ignition phase, (2) the high-current metal-dominated phase, (3) the transient phase, and (4) the low-current gas-dominated phase.43 It is widely accepted that for describing the strength of a specific HiPIMS process, only the peak values of the pulse densities are provided, as discussed earlier. However, it should be noted that this description has limitations when comparing processes with different pulse shapes, such as rectangular pulses. Figure 2 schematically illustrates the power densities of HiPIMS operations and the possible duty cycle to prevent thermal overload of the magnetron, depending on the magnetron design (including cooling, target type, and magnets). The duty cycle must be reduced as peak power increases.
The two different published peak power densities, either peak power per erosion track area30,44 or peak power per whole target size,45 are based on different calculations. From a plasma physics perspective, the erosion track calculation is more relevant. However, for practical purposes and in comparison to standard DCMS operation, the whole target area calculation provides the thermal overload limit in kW for a specific magnetron design and target material.
In conclusion, the reported density data must be critically reviewed because they can vary by a factor of 2–4 depending on the race track area at the whole target size. The authors suggest using the calculation method based on the whole target area for industrial applications. It should be noted that in addition to critically assessing the reported calculated densities (power and current), the dependence of discharge parameters on the magnetic field of the magnetron assembly itself should be reviewed, along with considering the erosion depth of the target.46 It is also crucial to acknowledge that the target material itself plays a role.47 For example, research has shown that almost no self-sputtering effect can be achieved for carbon,36 whereas a strong HiPIMS self-sputtering effect is measurable for materials like tungsten (W) and titanium (Ti).30,48
For industrial applications, it seems important to classify HiPIMS operation modes based on their distance from the low–power-density regime of DCMS to tailor coating properties and growth rates. The following classifications have been suggested: low-energy HiPIMS and moderate-energy HiPIMS.49 However, the transition between P-DCMS in the low-power-density regime and low-energy HiPIMS regime is not sharply defined and only partially investigated.
We believe that a high-energy HiPIMS region should be considered. Based on our industrial experience, the practical peak power density is typically limited to around 1 kW/cm2. If we calculate this using the entire target size, for example, with dimensions of length 50 cm and width 10 cm, and assuming a typical peak voltage near 1000 V, we reach a peak current density of 2 A/cm2. As shown in Fig. 3, a significant ionized fraction of the target material flux, depending on the target material, is achieved below 1 kW/cm2 for various metals but not for carbon.50 It should be noted that these values were calculated for the racetrack area. If the total target area is considered, lower peak power-density values, roughly a factor of 2 or even lower, should be used. For example, titanium (Ti) exhibits a significant ionized fraction at around 0.3 kW/cm2 on the racetrack but approximately 0.15 kW/cm2 when the total target area is considered.
The classification of HiPIMS operations based on peak power density guides technological development groups. However, a more physical approach involves using peak current density for basic plasma investigations and modeling because the burning voltage differs on different target material surfaces. Assuming a discharge voltage of 1000 V for simplicity and considering the entire target size for the ranges shown in Fig. 2, we roughly define the peak current density (jpcd) ranges for HiPIMS operations as follows:
- Low-energy HiPIMS: LE-HiPIMS: 0.1 < jpcd ≤ 0.3 A/cm2.
- Moderate-energy HiPIMS: ME-HiPIMS: 0.3 < jpcd ≤ 0.9 A/cm2.
- High-energy HiPIMS: HE-HiPIMS: 0.9 < jpcd (typical industrial applied maximum values are about 2 A/cm2).
It has been demonstrated that the HiPIMS effect, including ionization of the target material and self-sputtering, begins for aluminum (Al) targets at approximately 0.3 A/cm2 when calculated using the racetrack area or about 0.15 A/cm2 when considering the whole target area. The corresponding peak power densities (at 400 V) are 0.12 and 0.06 kW/cm2, respectively.30,44
It is well-established that the deposition flux of ions and neutrals toward the substrate significantly influences coating growth. While challenging to measure, calculations have been performed for various target materials. Figure 4 illustrates calculated values of the ionized flux fraction based on the ionization region model (IRM) for copper (Cu), titanium (Ti), tungsten (W), and carbon (C).3,36,51,52
Calculations have shown a strong dependence on both the material itself and the peak current density. As mentioned earlier, the ion flux fraction for carbon is relatively low, while for W, Ti, and Cu, values in the range of 30%–35% were calculated for a peak current density in the range of 0.7–1 A/cm2. It should be noted that the pulse mode (fixed voltage or fixed current) affects the ion flux fraction.53 Additionally, the flux fraction is influenced by pulse duration, magnetic field, and distance from the target track.25,54
Furthermore, it is important to emphasize that only a fraction of the sputtered target material, leaving the ionization region as ions and neutrals, is directed toward the substrate. Some of the flux goes to the chamber walls. Ions and neutrals have different specific spreading characteristics. Experimental evidence has shown that when the magnet configuration is altered at a constant discharge voltage, and consequently the discharge current, the flux parameter for neutral atoms remains relatively constant, while transport parameters for ions vary significantly.55
The classification of HiPIMS operation modes into three energy ranges, as shown in Fig. 1, provides a general approximation of the trend. However, each target material and each reactive gas process exhibit different characteristics in terms of current evolution and ionization degree for given pulse parameters.
As an example, Fig. 5 presents OES measurements for different parameters of reactive deposition of Al67Ti33 N. The target had a composition of Al67:Ti33 at. %. It is important to note that for this and all subsequent coatings described below, the metallic content of the coatings is assumed to be nearly the same as in the target, and the stoichiometry of the coating is provided. These coatings are simply named based on the target content in at. %, such as Al67Ti33 N. When values are not given, as in Chap. 4, it is considered proprietary knowledge.
The ionization of metals Ti and Ar is shown, with the threshold observed in the region of a peak power density/peak current density ranging from 0.11 to 0.16 W/cm2 and 0.26–0.36 A/cm2, respectively. Variations in pulse forms, using constant toff and varying the ton time for power control, are noticeable for the three different operation regimes: LE-HiPIMS (0.26 A/cm2), ME-HiPIMS (0.36; 0.53; 0.79 A/cm2), and HE-HiPIMS (2.04 A/cm2). It should be noted that the variation in pulse form is influenced not only by plasma effects but also by power supply characteristics, especially the current drop during long pulse times.
Reactive HiPIMS processes are commonly employed for depositing various wear protection coatings, especially for tools. These processes involve three fundamental mechanisms during reactive deposition processes: self-sputtering of the target material, sputtering by noble gas ions, and sputtering by reactive gas ions. These mechanisms are illustrated in Fig. 6 using industrial AlTiN coatings as an example,33 which provides a simplified schematic of fluxes occurring in a reactive deposition process. Additionally, secondary electrons are emitted during the process, as described by the parameter (Yse).31 The sputtering and secondary electron emission yields depend on the ion type and the chemical composition of the target material. Factors such as the probability of ions returning to the target (β) and the probability of ionization (α) vary among different ions. Furthermore, the sputtering yields of gases (Yng, Yrg) and the bonding of reactive gases to solids (δrg) also play essential roles. These mechanisms are interrelated; for example, when nitrogen is added to deposit AlTiN coating, a portion of the nitrogen chemically bonds to the AlTi target, while another part forms the coating. Noble gases like argon are almost not consumed.
The key to achieving a HiPIMS process is ensuring that the discharge conditions lead to sufficient self-sputtering of the selected target material (Yss) in correlation with a specific electron emission (Ysetm). Achieving a satisfactory ionization of the sputtered material, a high value for (αtm), and, if possible, a low probability of the sputtered target species returning to the target, often referred to as back-attraction (βtm), are all necessary to achieve the desired coating modifications while maintaining industrially acceptable growth rates. Achieving this balance requires optimized gas management tailored to the target material and pulse modes.
In general, the HiPIMS process is a pulsed high-current sputtering process that involves a degree of self-sputtering of target ions. To reach the critical threshold required to initiate self-sputtering, the current density at the target or power density within the pulse needs to be approximately one order of magnitude higher than for low-power-density (conventional) DCMS and P-DCMS.
B. Pulse forms for HiPIMS discharges
Pulse control can be achieved through the management of average power, voltage, or current settings by selecting the ton and toff times. The achievable discharge parameters are constrained by power supplies and the magnetron/target design. The pulse patterns required to achieve HiPIMS effects encompass a wide range of shapes, as schematically shown in Fig. 7. ton times for the pulses span several orders of magnitude, ranging from about 101 to 103 μs. The shortest ton times are often referred to as ultra-short pulses and have values smaller than 10 μs.56,57 A rough classification of pulse length includes the following categories:
The basic pulse forms are triangular and rectangular pulses. Well-defined triangular pulses, as shown in Fig. 7(a), are typical for short and ultra-short pulses. Long rectangular single pulses with lengths of up to 5 ms have been reported,59 as seen in Fig. 7(b). Triangular (and rectangular) pulses can be combined within one pulse group, as shown in Fig. 7(c).60,61 These groups are also utilized in what is known as DOMS or deep oscillation magnetron sputtering.62–65, Figure 7(d) illustrates a stepwise process of a pulse group with a low current for pre-ionization of the discharge, followed by the HiPIMS pulse.66 Both the pre-ionization step and the subsequent HiPIMS pulse can consist of pulse groups, also referred to as MPPMS (modulated pulsed magnetron sputtering).35, Figure 7(e) shows a mode that has been more extensively investigated recently, where a positive reverse voltage with a lower value than the negative sputtering voltage is applied.67–73 Finally, Fig. 7(f) demonstrates a bipolar operation using one pulser for two magnetrons in a switching mode.74
It should be noted that in most practical discharges, more complex pulse patterns are often observed, depending on both the power supply and the pulse parameters (ton, toff). For instance, as shown in Fig. 7, not only can the peak current change with ton time at a given toff time, but the pulse shape can also vary. A clear picture of the advantages and disadvantages of various pulse forms in industrial coating applications does not exist. It largely depends on specific applications, including the process and the coating. Various research and development teams, commercial power supply manufacturers, and PVD-system manufacturers offer different solutions. Key criteria for selecting the power supply and a dedicated pulse mode during processing for industrial systems include the availability of a pulser, process stability, and the cost of pulsers.
C. Influences of HiPIMS plasma flux at coating growth
The growth of the coating is strongly influenced by the number and type of impinging species, which include neutrals, energized neutrals, and ions, in addition to the thermal conditions of the growing film.4,5 These species carry their kinetic energy and enthalpy of vaporization. Furthermore, ions from the gas and sputtered material are accelerated toward the negatively biased growing surface. Biasing, in the form of a self-bias or applied bias voltage, allows for tuning the interaction between the growing film and ions. Physical properties of the coating, such as composition, density, texture, structure, and stress, can be adjusted by controlling species with the HiPIMS parameters and biasing. Technical properties of the coating, such as hardness, wear resistance, friction, and corrosion protection, are determined by the resulting physical properties. The composition of the HiPIMS plasma changes during a single pulse,3,42,52,75,76 providing the possibility to select a portion of the pulse to accelerate specific ions.77,78 Moreover, ensuring uniformity in both coating thickness and coating properties on complex substrates is crucial for various applications.
HiPIMS holds significant potential for enhancing uniformity, especially in applications like trench filling.79 Research has demonstrated notable differences in the uniformity of titanium coatings deposited on the inner (concave) and outer (convex) surfaces of bowl-shaped workpieces when using both DCMS and HiPIMS. Despite HiPIMS exhibiting a lower deposition rate, it offers advantages such as slightly improved thickness uniformity and a substantial enhancement in coating property uniformity along with denser (harder) coating morphology.80–82 It is essential to note that the highly ionized plasma can introduce edge effects on complex substrate geometries, arising from both gas ions and ionized target materials. For example, variations in chemical composition have been reported for AlTiSiN coatings.83,84 Selected results illustrating the influence of pulse parameters on coating growth and uniformity for industrially applied coating types are discussed in Chap. 3.
D. Deposition rate and energy consumption
Industrial PVD technologies for mass production always strive to optimize deposition rates while considering sustainability factors like energy consumption. Magnetrons often operate at their highest thermal power density limits to achieve high deposition rates and shorter cycle times. Prolonging the coating step not only increases the energy consumption of the sources but also affects the entire system, including pumps and cooling systems. Especially with the rising concern for energy consumption and greenhouse gas emissions, it is essential to consider these factors, at least until we transition to using purely green energy sources.
Deposition rates are often expressed in μm/h, while the power applied to the magnetrons is measured in kW. Calculating the ratio of the deposition rate to the consumed power of one source provides insight into the necessary optimization goal: μm h−1 kW−1. The question then arises regarding the expected values for various PVD deposition technologies. Several authors have explored the phenomenon of lower deposition rates in HiPIMS compared to DCMS.4,85–88 Figure 8 illustrates the ratio between the deposition rates of DCMS and HiPIMS for non-reactive deposition of various metals, depending on the peak current density, calculated as the ratio between peak current and the total target area.87 These selected elements are commonly used in hard coatings, either as primary or doping elements. The deposition rate for Ti and Cr decreases nearly with jpcd−0.5 in the range of jpdc 0.1–3 A/cm2. A similar qualitative drop in the deposition rate with increasing current density, within the range of 0.26–2 A/cm2, is seen for reactive HiPIMS in Fig. 9. A reduction in the deposition rate is also observed for LE-HiPIMS with current densities up to 0.3 A/cm2. The rate drop increases for ME-HiPIMS up to 0.9 A/cm2. For most materials, there is only a slight additional drop in the HE-HiPIMS range up to 2 A/cm2.
It has been shown that the reduced deposition rate of HiPIMS compared to DCMS for a constant target power can be attributed to various factors. The primary effects identified include the return effect of ionized sputtered species and the yield effect, as shown in Fig. 10. Yield effects represent the higher required discharge voltage of HiPIMS. In simpler terms, a certain percentage of the discharge energy is consumed to generate and ionize the target species flowing toward the substrates. Other minor effects include the ion-species effect, the magnetic field, and the film effect.89 To illustrate the industrial experience, Fig. 10 includes the total rate drop measured for AlTiN coatings, which dropped to 30% of the DCMS rate. A unique scenario was observed for reactive processes involving oxygen. In such cases, the HiPIMS process can achieve the same or even higher deposition rates than DCMS, especially in oxide deposition cases at the transition to the poisoning range.33,90,91 This can be explained by reaching a stoichiometric coating composition at a working point with a less poisoned target state.92 The mechanism behind this effect involves reduced target oxidation due to the high erosion rate of the target material during a single pulse, combined with gas rarefaction effects in front of the target. This effect results in a suppression of the hysteresis effect at a given pumping speed.
Figure 11 provides a comparison of deposition rates and power consumption for AlTiN deposition processes using DCMS, HiPIMS, and arc evaporation with an Al67:Ti33 at. % target. Two flanges were equipped with face-to-face arranged magnetrons, and one flange featured two circular arc evaporators. Both setups achieved the same coating height. Consequently, it can be concluded that HiPIMS processes consume the highest energy per coating volume when compared to DCMS at low-power density. The energy consumption of a HiPIMS process was measured to be a factor of 10 higher than that of the arc evaporation process. To mitigate the decrease in the deposition rate (increased energy consumption per coating thickness) observed in pure HiPIMS processes, various approaches have been suggested:
- Variation of pulse parameters.88,94
- Variation of pulse forms.46
- Variation of magnetic field strength and setup.53,85,95–97
- Variation of working gas pressure, e.g., Ar.34
- Application of a positive reverse pulse acting as an ion extraction pulse to the target.68,70
- Addition of a focusing coil in front of the target, resulting in at least a locally increased rate.98
However, these methods do not eliminate the physical limitations of the rate phenomenon. Achieving a balance between the two goals of HiPIMS applications—obtaining a high ionized flux fraction of the sputtered material to tailor coating properties (e.g., stress and structure) and achieving a high deposition rate—is a challenging task.3,94 Often, process parameters resulting in a lower deposition rate need to be selected to achieve the best coating properties, as demonstrated for AlCrN coatings.99
E. Hybrid processes
Hybrid processes, which involve the combination of various deposition or treatment technologies, are under investigation and being implemented to achieve higher deposition rates, unique coating characteristics, and tailored surface solutions. Below, we provide a brief overview of some of the most industrially relevant hybrid processes.
1. Combination of HiPIMS plus DCMS or P-DCMS deposition
One significant development involves the combination of HiPIMS with DCMS or P-DCMS in hybrid processes aimed at mitigating the decrease in deposition rates. Specially designed hardware has been developed for this purpose.18,100,101 The simplest and widely adopted hybrid setup connects at least one magnetron to a DC power supply for low-power-density operations, while at least one other is connected to a HiPIMS power supply.100,102–106 Another approach is to use HiPIMS pulses in conjunction with DCMS or P-DCMS processes on the same magnetron, thereby modifying the discharge process characteristics.21,107,108 For example, one can improve the deposition rate by interspersing P-DCMS pulses between HiPIMS pulses.100,109,110
2. Combination of HiPIMS plus arc evaporation deposition
Another exciting development in hybrid processes combines arc evaporation with HiPIMS, both of which are high-ionization deposition processes, opening up new possibilities for tailoring coating architectures (see Fig. 12). This hybrid approach allows for the creation of advanced multilayer, nano-multilayer, and nanocomposite coatings.93,111,112 Arc evaporation is primarily limited to specific cathode material properties, typically metal alloys. In contrast, HiPIMS can atomize and ionize materials that are difficult or impossible to evaporate using a cathodic arc, such as Si and B. In the hybrid process, materials from both arc evaporation and HiPIMS are utilized, combining direct arc evaporation with constant current and pulsed HiPIMS processes. This not only offers control over coating composition, architecture, and morphology but also modifies the plasma discharge conditions. Initial solutions for tool coatings have been successfully implemented in industrial settings.113
3. HiPIMS mode plus arc mode
Typically, the generation of an arc discharge at the target surface during HiPIMS processes is suppressed to the best extent possible. However, for hard carbon coatings, a mixed sputtering/arc mode has been found to offer certain advantages. Short-lived cathode spots are generated in the magnetic racetrack, producing a significant number of carbon ions necessary for a high C–C sp3 content.114,115
III. INDUSTRIAL HARDWARE AND PROCESS STEPS
In Secs. III A–III C, we will briefly describe selected hardware components used in HiPIMS processes to highlight their critical features for understanding industrial systems. We will discuss two types of industrial magnetron solutions: rectangular magnetrons operated with triangle-like current pulses and circular magnetrons operated with rectangular current pulses. We will also introduce the basic system setup for a pure HiPIMS configuration.
A. Type of magnetrons and magnetic fields
HiPIMS processes have been investigated using three primary magnetron configurations: circular, rectangular, and even rotating cylindrical.116,117 Circular and rectangular configurations are dominant in industrial applications for protective coatings. Rectangular magnetrons are equipped with stationary magnetic arrays, while specific circular magnetrons are configured with rotating magnetic arrays.79,118,119 The setup of the magnetic field significantly influences discharge characteristics,118 plasma generation,120 growth rates,85 and coating properties.121 Different magnetic field arrays with various balancing grades (BM to UBM) are utilized, with the maximal magnetic field strength at the target typically ranging from 25 to 50 mT. Generally, a lower magnetic field strength tends to result in higher growth rates.85,121 However, in industrial applications, the choice of the magnetic field must strike a balance between achieving the optimum growth rate and ensuring desirable coating properties.
Figure 13 illustrates flange-mounted magnetron targets used in industrial coating systems. In Fig. 13(a), an industrial rectangular long magnetron, specifically engineered for HiPIMS, is shown. The magnetic field is configured in the UBM arrangement, and its strength is adjustable by varying the distance between the target and the magnetic array, with a maximal field strength of 50 mT. Distance regulation is used to compensate for erosion effects or alter discharge parameters.122 Figure 13(b) shows an array of three circular magnetrons mounted on a flange of the coating chamber. Both systems are equipped with movable shutters to cover the magnetron targets and radiant heaters.
B. HiPIMS power supplies
For HiPIMS discharges, power supplies must deliver sufficient power densities (current densities) to the dedicated magnetron source area. Several manufacturers offer complete industrial-quality units. Additionally, in-house developed solutions are installed by PVD-system manufacturers. For example, power supplies for constant current pulsing (referred to as the S3p process) have been developed.59
Pulses are generated by pulse units based on MOSFET or IGBT electronics and capacitor banks.18,58 Smaller magnetrons, such as 1″ targets with a maximum continuous power of 50 W/cm2 and an isolation voltage of 1000 V, require only a standard pulsed power supply with a maximal peak power of about 10 kW. However, for larger magnetron areas typically installed in industrial coating systems, a sufficiently large capacitor bank is needed to supply the high-peak currents required (several 100–1000 A). This capacitor is charged by a sufficiently large primary DC power supply of several 10 kW. HiPIMS power supplies are mostly designed with maximum discharge voltages ranging from 1000 to 1200 V. Achievable peak powers can reach several MW and peak currents can go up to 2000 A.18,123 It is worth noting that the selection of on-time and off-time to achieve the maximum peak current strongly depends on the amount of stored charge (As) in the capacitor and the plasma impedance. Recently, HiPIMS sources enabling a definite positive reverse pulse have been developed and implemented in industrial applications.67,69–71State-of-the art power supplies offer three different modes for controlling the input power into the discharge:46
- fixed pulse power control,
- fixed voltage control, and
- fixed current control.
It is important to note that the pulse forms and discharge parameters measurable with an oscilloscope are influenced by the power supply characteristics (both hardware and software), the wiring between the power supply, the target material, magnetic field strength, and the gases used during the discharge. In general, the pulse forms for fixed power control vary with the pulse on-time for a fixed off-time in power control, as shown in Fig. 5. Longer on-times result in a lower peak current, and at the longest times, a current decrease may occur. This dynamic variation during one pulse can be mitigated by employing constant current pulsing.
It is worth mentioning that recent investigations have shown that current-controlled pulses result in higher stability of the growth rate against magnetic field changes caused by erosion effects in the racetrack.46,53
Due to the high-power density delivered to the target surface (racetrack), micro-arcing processes at the target can cause serious damage.58 Additionally, micro-particles are emitted, making arc management in HiPIMS processes a crucial aspect to generate defect-free coatings. To prevent the incorporation of micro-particles, filtering methods similar to those used in arc processes have been suggested.124 Modern power supplies incorporate flexible arc suppression solutions, which are more or less efficient at quenching arcs within a few microseconds. A special arc suppression method involves igniting a positive reversal voltage between negative pulses.18,125
C. Coating system setup and process
Figure 14 shows the setup of an industrial HiPIMS system for tribological coatings equipped with two face-to-face mounted HiPIMS sources. These magnetron assemblies have a UBM magnetic field, resulting in a closed field configuration. Typically, each magnetron assembly has its power supply, but in specific configurations, bipolar pulsing can be achieved with only one power supply.93 A movable shutter is often positioned in front of the magnetrons, and the installed radiant heater provides sufficient power to heat the substrate to temperatures ranging from 200 to 600 °C. It is worth noting that during the HiPIMS discharge, the bias power supply experiences a high-current load. Standard bias power supplies used for arc or DC-magnetron processes may become overloaded, leading to the use of adapted power supplies.18 Some power supplies designed for HiPIMS magnetron assemblies are also used as bias sources.126 Synchronizing the HiPIMS magnetrons can be used to avoid an overload of the bias. Furthermore, bias synchronizations are a tool to optimize coating properties. In such cases, the bias is only active during a selected time within the pulse, utilizing only the metal-ion-rich portion of each HiPIMS pulse, as seen in processes combining HiPIMS and DCMS.77,78,106,127–129
Coating systems equipped with HiPIMS sources often use various ion cleaning methods after pumping and heating steps to remove contaminants and impurities from the surface before coating deposition. Additional plasma sources are frequently installed to generate gas ions, although ion cleaning is physically possible with the HiPIMS discharge itself. Ion cleaning typically employs inert gas ions such as Ar or ion mixtures containing hydrogen, e.g., Ar + H2. For example, Oerlikon Balzers utilize two different gas-ion etching solutions: side-wall etching, based on an arrangement of plasma sources along the chamber wall, and arc-enhanced glow discharge (AEGD).11,113 However, ion cleaning can also be performed using the HiPIMS plasma itself. In this case, both noble gas ions and target ions are accelerated toward the surface with a sufficiently high bias voltage (approximately 1000 V).38,130–132 Additionally, implantation of target atoms has been demonstrated, resulting in substantial modifications of surface composition and microstructure.133 HiPIMS, when used as a conditioning process for the interface, may improve not only coating adhesion but also the performance of coated components.30,34,134–136 Industrial coating systems are available in various sizes, ranging from smaller systems with a coating volume of approximately 0.05 m3 (e.g., diameter 40 cm and height 40 cm) suitable for coating specific tools or components or for rapidly introducing new product lines. Standard systems designed for tool coatings typically offer a coating volume of at least 0.15 m3 (e.g., diameter 70 cm and height 40 cm). A unique example of the industrial implementation of HiPIMS processes involves a large coating system for broaches, with lengths of up to 2.2 m. This system is equipped with HiPIMS sources using Cr targets for both ion cleaning and the coating process, in combination with DCMS.137
Process control in HiPIMS processes demands various instrumentation, particularly during the system and/or coating development phases. Pulse waveforms and bias parameters are documented through oscilloscopic measurements, as exemplified in Fig. 5. Plasma characteristics are often optimized using advanced optical emission spectroscopy (OES) measurement systems, both in terms of hardware and software.138,139 Research has demonstrated that combining OES data with other process parameters can establish a robust method for ensuring process repeatability.140 Additional tools such as Langmuir probes, gridded energy analyzers, absorption spectroscopy, and mass spectrometry are also employed to characterize plasma generation and plasma expansion toward the substrates in a three-dimensional scanning mode.25 Furthermore, coating systems increasingly integrate mass spectrometers for residual gas analysis, primarily to optimize the ion cleaning process.
IV. HiPIMS PROCESS FEATURES AND COATING PROPERTIES FOR TRIBOLOGICAL APPLICATIONS
Specifications for tribological applications are becoming progressively complex. There is a growing demand for the development of hard coatings that exhibit high strength, high toughness, low friction, and exceptional thermal and chemical stability to withstand the generation of frictional heat under high-speed sliding and high surface pressure. This necessitates the development of new thin film materials and advanced deposition process technology, such as HiPIMS.
A. Coating types and influences at properties
To achieve seemingly contradictory physical and mechanical properties, such as the coexistence of high hardness and high toughness, a new film structure design is required that goes beyond the conventional concept of a monolayer film structure. Common film structures include gradient films, multilayer films, nanolayer films, and nanocomposite films. In recent years, these film structures have transitioned from the R&D phase to industrialization and have found applications in various industrial products. To create these novel tribological coatings, precise control of the deposited particles at the atomic and molecular levels is essential. In this context, the high ionization of film-forming species in HiPIMS discharge significantly enhances the controllability of the kinetic behavior of incident particles during film growth. As summarized in Table I, industrially available hard coatings deposited by the HiPIMS process exhibit a wide range of selectivity in film hardness and stress management, allowing flexibility to meet the requirements of various tribological applications.
|Coating types .||Cathode/target content (at. %) .||Hardness (GPa) .||Elastic modulus (GPa) .||Compressive stress (GPa) .||Start of oxidation (°C) .||Coating temperature (°C) .|
|Coating types .||Cathode/target content (at. %) .||Hardness (GPa) .||Elastic modulus (GPa) .||Compressive stress (GPa) .||Start of oxidation (°C) .||Coating temperature (°C) .|
1. Process characteristics on example for nitrides
The ability to tailor coating properties is achieved through the process controllability offered by “pulsed” plasma, which introduces a “time axis” to the conventional input parameters. Notably, the peak discharge power and peak current applied to sputtering targets can be freely adjusted by varying the pulse on and off times, i.e., the duty ratio and pulse frequency, as shown in Fig. 5. As previously discussed, since the ionization and kinetic energy distribution of sputtered particles are strongly influenced by the applied peak current, controlling the peak current in HiPIMS discharge plays a critical role in governing film growth. For instance, consider the deposition of AlTiN using a target composition of Al67:Ti33 at. %. Figure 15 presents a grazing incident x-ray diffractogram of AlTiN films grown at different peak current densities, as shown in Fig. 5. Current densities were calculated based on the entire target area. Films produced at lower discharge current densities (jpcd = 0.26 and 0.36 A/cm2) consist of a mixture of the NaCl-cubic phase and wurtzite-AlN phase. In contrast, films grown at higher jpcd > 0.53 A/cm2 exhibit a single phase with a cubic structure, despite the relatively high Al concentration.
The variation in phase composition with increasing jpcd significantly affects mechanical properties, as illustrated in Fig. 16, which depicts nanoindentation hardness HIT and elastic modulus EIT as functions of jpcd. HIT values increase significantly from jpcd = 0.36–0.53 A/cm2, coinciding with the observed phase shift at the same jpcd, as seen above in GIXRD analysis. HIT reaches a maximum value of approximately 40 GPa at jpcd = 0.78 A/cm2 and then slightly decreases at a higher jpcd = 2.04 A/cm2. While jpcd control in this series of depositions was achieved by varying pulse duration under constant time-averaged power, recent advancements in HiPIMS power supply technology enable the realization of very long pulse durations of up to ∼100 ms, while maintaining a high-peak discharge current density. Figure 17 provides an example of target voltage and current waveforms in an argon atmosphere. A relatively high-peak current density of ∼3.5 A/cm2 (calculated based on the racetrack area) on the target was successfully maintained for a very long pulse duration of 5 ms. This long-duration pulse, combined with continuous high-power density throughout its duration, expands the process window for controlling film growth. As shown in Fig. 18, the grain size of Ti50Al50 N (target Al50:Ti50 at. %) coatings roughly doubles with an increase in constant pulse power density from 1.0 to 2.0 kW/cm2. Additionally, an increase in pulse power density leads to a transition from a crystal texture with a strong (111) orientation to a texture with a partially (200) oriented microstructure, which is a typical evolution of the microstructure in polycrystalline NaCl-structured materials under the influence of high-flux ion incidence during film growth.5
The high flux of ionized particles with extended irradiation during film growth also enhances film adhesion, as demonstrated in Fig. 19. This figure highlights the significant improvement in critical load during scratch tests with a pulse duration of 2 ms compared to shorter pulse lengths. Three different pulse durations were evaluated: 50 μs, reflecting the state-of-the-art HiPIMS; a longer pulse length of 300 μs; and a pulse length of 2 ms. In all cases, the power density was held constant at 2.0 kW/cm2. For TiAlN coatings deposited with a 50 μs pulse duration, cohesive failure occurred at a scratch load of 66 N. An improvement in coating adhesion, up to 88 N, was observed for coatings deposited with a 300 μs pulse duration. Coatings deposited with a 2 ms pulse duration exhibited no cohesive failure, even at loads of up to 100 N, which was the maximum load used in these experiments. Furthermore, the impact on the reactive process was studied under this long pulse operation at a high pulse power density of 2 kW/cm2. Not only do coating properties vary with pulse length, but the target reaction is also influenced by it. Figure 20 illustrates the variation in nitrogen partial pressure with nitrogen flow for a reactive process of TiN coatings. The hysteresis of the reactive process is significantly affected by the applied pulse duration, with almost no hysteresis observed at a very long pulse duration of 10 ms. This extends the transition region where the film can be grown at a high deposition rate.
2. Oxygen containing coatings
Pure oxide coatings, particularly Al2O3 in the corundum structure, and also Cr2O3,141 are of special interest for protective coatings. However, generating the corundum structure through PVD processes has proven to be challenging.142 Studies have shown that AlCrO coatings in the gamma phase can be produced via HiPIMS + arc discharge.72,112 An even more intriguing approach involves the deposition of MeNO coatings. These coatings derive their unique properties from the addition of oxygen to MeN, resulting in properties that fall between those of metal nitrides, MeN, and insulating oxides, MeOx.142 By adjusting the oxygen/nitrogen ratio, it becomes possible to tailor physical, mechanical, and tribological characteristics. Nevertheless, oxide-containing coatings deposited by HiPIMS are still in the developmental phase at an industrial scale.
3. DLC coatings
Various DLC coatings, including hydrogen-free and hydrogen-containing types, have been industrially deposited by magnetron sputtering for numerous tribological applications for several decades.10,143 Hydrogen-free carbon coatings deposited by HiPIMS were first described in 2003.12 Hydrogen-free coatings can achieve a maximum hardness of approximately 20–40 GPa when deposited by HiPIMS.63,64 Several attempts to optimize properties toward ta-C, deposited by arc evaporation, have been published, involving variations in pulse characteristics, inert gas composition, magnetic field strength, and bias.37,63,64 Additionally, it has been demonstrated that coating properties can be tailored in terms of sp3 content and stress by applying a positive pulse.73 However, plasma modeling has revealed that the ionization of carbon remains relatively low, typically below 5%.36 Even the addition of Ne resulted in a-C coating-type growth.144 In general, the ionization of carbon is insufficient to form the necessary CC–sp3 bonds for ta-C. To address this limitation, hybrid processes combining HiPIMS discharge with a short-duration arc process were investigated (see Chap. 1.5.2). However, HiPIMS-deposited a-C coatings exhibit potential for industrial applications due to their low defect density and dedicated tribological properties. Additionally, hydrogen-containing (a-C:H) coatings have been deposited by introducing CH4 or C2H2.40,41,145,146 These additions were shown to increase the deposition rate, albeit with a reduction in hardness as the hydrogen content increased.
Furthermore, a-C:H:Me coatings, such as a-C:H:W, a-C:H:Ta, a-C:H:Al, a-C:H:Nb, and a-C:H:Ti, were investigated by HiPIMS sputtering of WC, Ta, or Al, respectively.147–152 Multilayers with the Me/a-C architecture were also deposited, such as those used to develop coatings for thermal management, featuring Cu/C coatings.153
B. Thickness distribution
Homogeneous thickness and uniform distribution of coating properties are preferred for many applications. One such application requires conformability to complex shapes, particularly at cutting edges. The major coating process for such an application is arc evaporation; however, achieving homogeneous coatings can be challenging due to the complex geometry of cutting tools. HiPIMS demonstrates better thickness and coating property distribution homogeneity.154 Figure 21 shows SEM micrographs of the cross sections of coatings deposited using arc evaporation and HiPIMS on 1 mm diameter endmills with Al60Cr40 N coatings on cemented-carbide substrates. A power density of 1.0 kW cm−2 was applied to Al60:Cr40 at. % targets. On the flank face, parallel to the targets, the deposition rate was approximately 5.6 μm/h by arc evaporation and 1.9 μm/h by HiPIMS. On the flank face not facing the target due to complex geometry, the deposition rate was approximately 1.6 μm/h by arc evaporation and about 1.7 μm/h by HiPIMS. During cutting, chips form over the rake face, resulting in the adhesion of workpiece materials and abrasion of the rake face. A more homogeneous thickness distribution of HiPIMS coatings prolongs tool life in this tooling application, compared to arc coatings.
C. Defect densities
Defect-minimized coatings offer advantages in several tribological systems. These advantages address not only the roughening effect caused by micro-particles (droplets) but also growth defects resulting from dust.143 Direct arc evaporation, a major coating process for cutting tools, is characterized by significant incorporation of micro-particles into the coating. While it is challenging to completely eliminate micro-particles in industrial HiPIMS processes, achieving a significantly lower level of micro-particle content is possible. However, both technologies are susceptible to growth defects due to dust in the working area and chamber. In general, growth defects often have adverse effects on cutting performance.155
To illustrate the importance of defects, their influence on surface reactions in coating and diffusion processes during cutting is highlighted by TEM. Figure 22 presents the bright-field TEM image of the cross section of a coating after cutting with stainless steel (SUS304, DINX5CrNi18-10), deposited using arc evaporation on a cemented-carbide indexable tool. The coating comprises an Al50Ti50 N base coating with a functional Al60Cr40O top coating. Micro-particles were observed in the Al60Cr40O coating, and a significant layer of adhesive workpiece material was found on the surface of the Al60Cr40O coating. Figure 22(b) shows the dark-field TEM image of the area marked in Fig. 22(a). The dark-field TEM and EDS analyses (Table II) revealed that defects originated from micro-particles in the coating. Furthermore, the diffusion of the workpiece material was observed along these defects, as indicated in measuring point 3 of Table II. This ongoing diffusion effect can be significantly reduced by defect-reduced coatings deposited by HiPIMS or other PVD-compatible processes, such as filtered arc or low-voltage arc.
|Element .||O (net counts) .||Al (net counts) .||Cr (net counts) .||Mn (net counts) .||Fe (net counts) .||Ni (net counts) .|
|Point 1||123||…||9 023||450||32 932||3556|
|Point 2||2131||…||14 119||6771||180||…|
|Point 3||2709||7265||8 494||294||4 559||621|
|Point 4||3096||9111||5 363||…||…||…|
|Element .||O (net counts) .||Al (net counts) .||Cr (net counts) .||Mn (net counts) .||Fe (net counts) .||Ni (net counts) .|
|Point 1||123||…||9 023||450||32 932||3556|
|Point 2||2131||…||14 119||6771||180||…|
|Point 3||2709||7265||8 494||294||4 559||621|
|Point 4||3096||9111||5 363||…||…||…|
D. Surface preparation issues
The demand for small cutting tools used in machining small precision molds has increased due to the growing miniaturization of electronic components and the adoption of advanced safety features in cars and automated driving technology. Arc evaporation is the primary method for producing these small cutting tools, and post-treatments are often applied to enhance their performance. One such post-treatment involves combining hard abrasive particles, like diamonds, with elastic materials such as rubber and ejecting them from a nozzle with compressed air. These particles, typically sized between 0.1 and 2 mm, move at high speeds across the surface of the coated part, polishing it through frictional forces.156 However, the particle size is often too large to effectively polish complex-shaped tools with diameters of 1 mm or less, and it may lead to tool breakage due to the impact of the particles.
Figure 23 shows SEM micrographs of the cutting edge of 1 mm diameter ball-nose endmills coated with Al60Cr40 N after milling hardened stainless steel (SUS420 J, hardness 52 HRC). Figure 23(a) depicts a tool with an arc-deposited coating, while Fig. 23(b) shows one with HiPIMS coating. Both coating types underwent post-treatment before cutting. The tool coated by arc evaporation exhibits areas of polished micro-particles on the flank face, with numerous micro-particles visible on the rake face. In contrast, the HiPIMS-coated tool presents a smooth surface with minimal damage near the cutting edge, particularly in the direction of the rake face. This comparison clearly highlights the advantage of the HiPIMS process for small-diameter tools that may not be amenable to effective post-treatment, especially on the rake face.
V. SELECTED APPLICATIONS OF HiPIMS COATINGS
Currently, HiPIMS technology is primarily applied in niche applications. This is because the significant benefits it offers to coated parts’ performance must outweigh the associated costs. Consequently, HiPIMS is particularly used in applications where a low density of surface defects or a very high precision of coated surfaces is desired. It is worth noting that almost all standard coatings, including DLC coatings, deposited by arc or DCMS, are also available through HiPIMS. However, HiPIMS coatings themselves can be optimized in the coating architecture for specific applications, as demonstrated in Fig. 24. The AlTiN coating architecture was tailored for micro-tools using a multilayer generated through different pulse parameters. Chipping was observed for the monolayer, whereas the multilayer prevented it. One reason for this may be the significant reduction in stress within the coating.
HiPIMS coatings are now used as wear- and friction-reducing coatings in a wide range of industrial applications, including various tools and components. Additionally, functional decorative and medical applications are increasingly becoming the focus of research and development. It should be noted that HiPIMS coatings are sometimes applied merely because parts have been successfully qualified using HiPIMS without exploring alternative solutions.
One example demonstrates the performance of micro-drills coated with AlCrN during deep hole drilling of stainless steel. In this application, the main challenge is the workpiece material adhering to the tool surface, leading to difficult chip evacuation and, consequently, tool breakage. Coatings produced by arc evaporation tend to have a significant number of surface defects, resulting in coated tools that exhibit spontaneous tool breakage, low performance, and high variation in tool life. Applying HiPIMS coatings with very low defect density significantly improves tool life, ensuring stable and safe chip evacuation, reproducible results, and increased tool life, as shown in Fig. 25.
Another critical application where tool surface roughness is of utmost importance is threading. During threading, the tool surface comes into tight contact with the workpiece material, resulting in extremely high friction. In particular, during thread forming, the friction between the coated tool surface and the workpiece material significantly affects tool performance. An application example demonstrates thread forming with tools coated using arc evaporation and HiPIMS. Tools with coatings produced by arc evaporation underwent post-treatment to reduce surface roughness. Torque, which reflects the friction between the tool surface and the workpiece material during thread formation, plays a key role. Higher torque values typically indicate higher tool surface roughness or tool wear. To prevent tool breakage, lower torque values are preferred. Positive torque values are measured during thread formation, while negative values are measured during the tool's reverse motion to remove it from the threaded hole. As Fig. 26 illustrates, tools coated with HiPIMS exhibit lower torque values. Tools coated with arc evaporation followed by post-treatment exhibit significantly higher torque values compared to those coated with HiPIMS. Tools coated with HiPIMS display a longer tool life and a more stable manufacturing process.
For many forming tool applications, there is mixed friction contact between the tool surface and the surface of the manufactured part. This frictional contact generates high temperatures and mechanical loads in the contact area. In many cases, the tool's lifespan is limited by the buildup of workpiece material on the tool surface, resulting in deterioration of the surface quality of the produced parts. Additionally, galling and abrasive wear are often limiting factors of a tool’s lifespan. Due to these wear mechanisms, achieving surface quality with no defects is crucial for extending tool service life and improving the quality of the produced parts. Figure 27 presents results for cold extrusion of screw head punches. During this application, the buildup on the tool surface leads to adhesive wear and damage to the punch. HiPIMS-deposited AlCrN exhibits significantly better performance compared to AlCrN coatings deposited by arc evaporation.
B. Components: Evaluation of the market potential of HiPIMS
In general, HiPIMS technology has potential in the same fields where arc and traditional DCMS are applied, provided that it offers an added value. Research activities are ongoing in all fields, including automotive,102 medical,157–159 general machine building, decoration,160 and hydrogen-based technologies such as fuel cells and electrolyzers.161,162 The added value can stem from the coating properties themselves, the homogeneity of coating thickness and properties, or the elimination of post-treatment efforts. Additionally, the direct metallization of plastics holds promise, particularly regarding adhesive strength to plastics.163 However, as aforementioned, HiPIMS solutions are still characterized by high investments and relatively high process costs due to low growth rates. Furthermore, there is limited service volume availability in the market. The growth potential for various applications depends on the achievable superior quality of respective goods.
Several industrial implementations are already in place, with components coated in both mass production and small-batch production. For instance, TiN coatings deposited by HiPIMS are used for high-speed rotating axles (several 10 000 rpm) of blowers to prevent wear during the starting and landing phases. Figure 28 shows an application of hydrogen-free DLC coating (a-C) deposited by HiPIMS with constant current pulsing (S3p process). It is used for wear protection in the open tribosystem yarn-steel dent. Another application, shown in Fig. 28(b), is the wear protection of a valve stator in the closed tribosystem steel stator-polymer rotor of an HPLC (high-performance liquid chromatography) device.
HiPIMS coatings are also applied for high-end deco, e.g., for i-phone covers.160
VI. CHALLENGES AND OUTLOOK
The potential of HiPIMS technology to produce superior film properties has been demonstrated. However, the industrial breakthrough for HiPIMS technology is still in its early stages. Figure 29 illustrates the technological/industrial performance parameter of HiPIMS technology over time. After nearly 30 years of scientific and industrial development, it has reached a level between 50% and 75%, roughly 66%. Nevertheless, there is a high potential for further development in the coming decades.
Foundational knowledge is sufficient to implement HiPIMS technology on an industrial scale. The operating ranges for different pulse shapes are defined, including both extremely short pulses lasting a few microseconds and rectangular pulses in the millisecond range, which are already in industrial use. Highly ionized plasma states have been investigated for processes to deposit metals, nitrides, carbides, borides, oxides, and their mixtures. Recent research has shown the potential of positive reverse pulses to impact coating growth positively. Calculations based on the ionization region model (IRM) support these experimental results. It appears that process modeling's potential needs to be more fully harnessed for industrial processes. It has been experimentally and theoretically demonstrated that the achievable coating growth rates are lower than those of DCMS and arc evaporation.
The industrial-scale use of HiPIMS processes demands productive systems that deliver highly reproducible coating results. The most crucial prerequisite for industrial implementation is the availability of stable and cost-effective power supplies for HiPIMS. HiPIMS power supply technology has reached an industrial level, meeting demands for delivering high-power pulses with stable voltage and current across various pulse lengths, forms, and frequencies. These power supplies are equipped with units to suppress micro-arcing, although reducing micro-arcing to the level of DCMS processes remains challenging. Industrial users are actively seeking solutions to enhance micro-arc management.
Industrialized coating systems are designed as pure HiPIMS systems, and also as hybrid systems that combine DCMS with HiPIMS or arc with HiPIMS. Specially designed magnetron assemblies, tailored in size, shape, and magnetic field setup, are available for HiPIMS applications. Solutions for synchronizing HiPIMS pulses with the bias voltage show significant potential for customizing coating properties. A unique aspect of HiPIMS is its ability to function as a metal-ion cleaning method. However, many industrial systems employ alternative ion cleaning solutions based on gas-ion cleaning, such as AEGD.113
The number of applications using HiPIMS coatings for protective purposes in tools, components, and decorative coatings will continue to rise, particularly when the added value can be achieved. This has been demonstrated, especially for micro-tools, small punching tools, and more. Virtually, all traditional types of hard coatings, including DLC coatings, have been at least partially implemented in an industrial context. Unique application fields may emerge, particularly involving non-conductive substrates and low coating temperatures, such as glass and plastics. HiPIMS has the potential to create advanced coating architectures. However, HiPIMS solutions will not replace other technologies like arc or DCMS, but rather complement them by providing an added value.
Key criteria for choosing between HiPIMS and direct arc (where the cathode faces the substrate holder directly) for a specific application are summarized in Fig. 30. The color-coded bars roughly indicate the application potential of both deposition techniques. HiPIMS applications will gain traction in the market when low defect densities and low roughness for small parts (such as micro-drills and molds) are essential. The HiPIMS process technology holds a promising future if new high-performance coatings become available that cannot be deposited using arc methods. Additionally, HiPIMS's coating homogeneity can offer advantages. However, direct arc deposition has its own merits, including cost-efficiency and lower specific energy consumption per unit of coating volume, as well as the capability to deposit hard ta-C coatings.
Considerations need to be addressed from two angles. First, the added value in specific applications must be substantial enough to offset the higher energy consumption associated with the HiPIMS coating process compared to classical DCMS and arc methods. A particular advantage can be achieved if HiPIMS can produce excellent coatings at lower temperatures than those required by traditional DCMS or arc processes. Development in the coming decades will focus on refining fundamental knowledge, enhancing hardware and coating systems, and implementing tailored solutions for specific applications.
The authors express their gratitude for the input and fruitful discussions with all their colleagues at Oerlikon Balzers, with special thanks to Florian Rovere. They would also like to acknowledge the contributions of Yoshikazu Teranishi at Tokyo Metropolitan Industrial Technology Research Institute, Shuji Takahashi (currently at Nissan Automotives), and Hidetoshi Komiya at Tokyo Metropolitan University for their assistance in TiAlN depositions. Additionally, Professor Tomomi Shiratori at Toyama University and Dr. Yohei Suzuki at Komatsu Seiki, Co. Ltd., contributed to the evaluation of micro-piercing punches. The authors extend their appreciation to Kazuyuki Kobota and Rick Welty for their support and constructive discussions.
Conflict of Interest
The authors have no conflicts to disclose.
Joerg Vetter: Formal analysis (lead); Writing – original draft (lead); Writing – review & editing (lead). Tetsuhide Shimizu: Data curation (lead); Investigation (lead); Writing – review & editing (supporting). Denis Kurapov: Data curation (lead); Investigation (lead); Writing – review & editing (supporting). Tomoya Sasaki: Data curation (lead); Investigation (lead); Writing – review & editing (supporting). Juergen Mueller: Data curation (supporting); Investigation (supporting). Dominic Stangier: Writing – review & editing (supporting). Markus Esselbach: Conceptualization (supporting).
The data that support the findings of this study are available from the corresponding author upon reasonable request.