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Combinatorial sputter deposition of CrMnFeCoNi high entropy alloy thin films on agitated particles

A method for combinatorial sputter deposition of thin films on microparticles is presented. The method is developed for a laboratory-scale magnetron sputter system and uses a piezoelectric actuator to agitate the microparticles through oscillation. Custom-made components enable to agitate up to nine separate batches of particles simultaneously. Due to the agitation, the whole surface of the particles can be exposed to the sputter flux and thus can be completely covered with a thin film. By sputtering a CrMnFeCoNi high entropy alloy target, separate batches of polystyrene microspheres (500 um monodisperse diameter), Fe alloy particles (300 um mean size) and NaCl salt particles (350 um mean size) were simultaneously coated with a homogeneous thin film. In contrast, a CrMnFeCoNi thin film that was deposited on agglomerating Al particles (5 um mean size) only partially covers the surface of the particles. By co-sputtering a CrMn, an FeCo and a Ni target, nine separate batches of Al particles (25 um mean size) were coated with a CrMnFeCoNi thin film with a composition gradient. These depositions demonstrate the ability to coat different types of particles with uniform films (from elemental to multinary compositions) and to deposit films with composition gradients on uniform particles.


Introduction
Powders that consist of microparticles are used in various industries and current fields of research such as foods, pharmaceuticals, cosmetics, powder metallurgy, additive manufacturing, lacquers and catalysis [1][2][3]. Many powder properties such as flowability [3], electrocatalytic activity [4,5], compactibility [9] and oxidation resistance [6] depend completely or to a great extent on the surface properties of the particles. Since many materials with desirable surface properties are valuable, core-shell particles made of relatively cheap core particles and functional coatings are of high scientific and technological interest. Among different methods to produce core-shell particles, several studies demonstrate sputter deposition as an effective means to coat particles to alter the properties of powders. For example, stainless steel coatings on WC particles used in additive manufacturing improve the flowability and the oxidation resistance of the WC powder [3,6]. Ni thin films deposited on quasi-crystalline Al-Cu-Fe microparticles act as binder to enable cold-pressing the powder into dense macrocomposite cylinders, which is otherwise accompanied by crumbling or cleavage [9]. Several studies demonstrate the fabrication of cost-efficient catalytic powders by depositing catalytic thin films on microparticles, e.g. Pt coatings on polymer and Al2O3 particles, which show the same electrocatalytic properties as bulk Pt [4,5].
To modify powder properties effectively, it is important that the coatings cover the whole surface of the individual particles. To achieve this with sputtering, the particles must be agitated during the deposition process. Due to the line-of-sight characteristic of sputtering, not-agitated particles are mainly coated on the exposed top surfaces whereas the bottoms are shadowed and remain uncoated. Prior literature comprises different agitation techniques that can be classified according to their basic principles into four groups: rotating cylinders [1][2][3][4][5][6], rotating tilted bowls [7,8], vibrating bowls [10][11][12][13][14] and dusty plasmas [9,15]. These agitation techniques are schematically depicted in Fig. 1. They were successfully used to homogeneously coat particles (2 … 500 µm size) with metallic and metal oxide thin films. In some cases, difficulties with stuck particles made it necessary to modify the technique. For example, Abe et al. modified a rotating cylinder to a hexagonal barrel, which improved the agitation of the Al2O3 particles (20 µm size) [1,4]. In [7], a concussion mechanism was added to a rotating tilted bowl to loosen stuck glass microspheres. The technique from Fernandes et al. is based on a rotating cylinder that simultaneously vibrates [16].
We introduce a method for combinatorial sputter deposition of thin films on agitated particles. This allows to deposit a homogeneous thin film in one process on different kinds of particles.

Development of the sputter method
Inspired by the agitation technique depicted in Fig. 1c, a device referred to as oscillation plate was developed. Fig. 2a  constant composition on the particles. However, a thickness gradient of the deposited thin film must be considered, because the sputter cathodes are not centered above the oscillation plate and the latter cannot be rotated due to its cabling. Co-sputtering of different target materials from two or three cathodes results in a thin film with a composition gradient across the particles.

Experimental procedure
To test the developed method, the oscillation plate was used to agitate different types of microparticles in three deposition processes. The high entropy alloy CrMnFeCoNi was chosen as coating material, as it shows interesting structural [18] and electrocatalytic [19] properties.
It has been investigated in thin film form on flat substrates [20,21] and also on nanotip arrays to study its phase stability [22,23]. The coated particles were investigated with scanning electron microscopy (SEM, Jeol JSM-7200F). The reported average chemical compositions (in at. %) of the deposited films refer to the average film composition of ten coated particles of the same sample and were measured by energy dispersive X-ray spectroscopy (EDX).
During the first deposition process, a CrMnFeCoNi high entropy alloy target was used and a thin film was deposited on Al particles (5 µm mean size). The sputter target was manufactured at Ruhr University Bochum and its chemical composition was determined by EDX as Cr20.6Mn20.4Fe20.1Co19.8Ni19.1. An AC electrical signal with a square waveform and a peak-topeak amplitude VP-P of 20 V was applied on the actuator. 20 V is the maximum VP-P of the function generator and was chosen because it caused the most visible movement in the particles.
The frequency of the driving signal was set to a linear frequency sweep in the range of 0.1 … a) b) c) During the second deposition process, the same CrMnFeCoNi high entropy alloy target as in the first process was used and a thin film was deposited on three different powder substrates: polystyrene microspheres (500 µm monodisperse diameter), Fe alloy particles (300 µm mean size) and NaCl salt particles (350 µm mean size), which were placed in separate pits of the oscillation plate. The driving signal applied on the actuator was the same as in the first process, except for VP-P, which was reduced to 7 V. At VP-P > 7 V, the larger travel range of the actuator caused the particles to fall off the oscillation plate, whereas at VP-P < 7 V, they moved visibly less. The sputter duration was 50 min and the sputter power was 30 W DC.
The third process was a co-deposition of Cr50Mn50 (99.8 % purity, EvoChem), Fe50Co50 (99.95 % purity, Robeko) and Ni (99.99 % purity, EvoChem). Nine batches of Al particles (25 µm mean size) were used as substrates (Fig. 2b). For a clear identification of the particle batches, they are numbered with respect to the sputter set up as depicted in Fig. 9a. The driving signal applied on the actuator was the same as in the first deposition process. The sputter duration was 45 min. The sputter powers were 25 W DC (CrMn), 30 W DC (FeCo) and 23 W DC (Ni).

Results and discussion
The first deposition process resulted in an uneven distribution of the sputtered elements on the Al particles (5 µm mean size), as shown in Fig. 3. This can be explained by the agglomerating characteristic of these Al particles. When the particles were agitated at ambient conditions, there was little movement in the agglomerates. At the sputter pressure, the movement was reduced and after a few minutes of sputtering, the agglomerates stuck to the oscillation plate and did not move at all. Consequently, only the particle surfaces exposed to the sputter flux got coated. A similar observation was reported by Schmid et al. [7]. In their case, glass microspheres (2 … 30 µm diameter) trickled freely in a rotating tilted bowl at ambient conditions, but stuck to the bowl at the sputter pressure of 1.5 Pa. The authors suggest that at ambient conditions, the air humidity leads to thin water layers that lubricate the particles to move freely, whereas at sputter conditions there are no such layers. They also report that the deposited metal and metal oxide coatings further promote the sticking. Sticking was also reported for glass microspheres (100 d) e) f) µm and 230 µm diameters) agitated in vibrating bowls [11,12]. In all cases, the stuck particles could be separated by introducing O2 into the sputter chamber, which leads to oxide layers on the already deposited material [11,12].
During the second deposition process, the oscillation plate agitated the spherically shaped polystyrene and Fe alloy particles as well as the cubic NaCl particles to jump up and down.

Conclusion and outlook
The presented sputter method was demonstrated to be a promising approach to rapidly produce samples of coated particles with different combinations of core and coating materials. Different kinds of particles were coated with a CrMnFeCoNi thin film, and Al particles were coated with a composition gradient CrMnFeCoNi thin film. Due to the nature of particles of being separate individuals, a composition gradient film on particles inherently has discrete compositions (one composition for each particle coating), whereas films on flat continuous substrates can have continuous composition gradients. To increase the efficiency of the presented sputter method, a scaled-up version of the oscillation plate to accommodate a larger number of particle samples can easily be designed and adapted to any sputter chamber. To increase the efficiency of the sample characterization, high-throughput methods suitable for the fabricated particle samples are recommended to be developed. For the homogeneity of the films, thorough agitation of the particles is necessary. As observed for the agglomerating 5 µm Al particles, insufficient agitation leads to uneven coatings. The oscillation frequency and the travel range are important parameters for the particle agitation, which are controlled by the electrical driving signal.
However, to homogeneously coat particles that tend to agglomerate, the method has to be modified. For example, agglomerates may be separated by introducing O2 into the sputter chamber or by using an electrical driving signal with a larger VP-P to cause the actuator to oscillate with a larger travel range.