Cold sprayed Aluminum-Quasicrystal Composite Coating: Bonding Mechanism Evaluation by SEM and TEM

Cold spraying (CS) is an emerging solid-state coating technology that has demonstrated significant industrial application potential. A wide variety of materials, such as metals and alloys, polymers, and ceramics can be effectively deposited by CS technology [1]. In addition, CS offers a promising route of composite coating formation to combine properties of several constituents and fabricate multifunctional layers, which could be challenging to obtain from single material coatings [2]. In principle, accelerating solid feedstock particles via pressurized and preheated gas, and subsequent particle impact at high speed onto a substrate lead to a coating build-up from deformed particles. Impacted particles undergo high strain rate deformation, which stimulates microscopic phenomena such as grain refinement, strain accommodation, and phase transformation [1]. In this regard, microstructural characterization is a crucial element in the development and optimization of novel coatings by revealing microscopic details.
In our previous works [3,4], CS was successfully employed to produce dense and well-integrated aluminum-quasicrystal (Al-QC) composite coatings with superior tribological properties and increased hydrophobicity compared to Al-based coating and bulk metallurgical counterparts. As a continuation, this work aims to investigate microstructural features of CS Al-QC composite coatings with a focus on probing the bonding state and particle-particle interface. Composite coatings were sprayed with optimized parameters using a high-pressure CS system with pressurized N2 that propels Al-QC feedstock blend toward the substrate. The process settings were also modified to accelerate a limited number of particles and obtain impact of several particles on the substrates (so-called wipe test) to simplify the deposition process. A SEM was utilized to evaluate the microstructure of the specimens. For an in-depth understanding of Al-QC interface, a (S)TEM equipped with EDS system was used to characterize a lamella extracted from the region of interest by FIB from polished surface of composite coating.
Examination of the Al-based substrate surfaces after the wipe test revealed various features as marked in Figure 1; Evidently, Al particles impact-induced plastic deformation could occur easier, acting as binder phase that retains the harder and more brittle QC particles inside the composite structure and provides a soft bed for the landing of subsequent QC phases. Al particles bonding to each other could be found randomly on the surface, whereas QC-QC bonding was not observed. Figure 2a depicts the SEM (secondary electrons) image from the region of interest for the TEM lamella prepared by FIB. TEM bright field image Figure 2b exhibits continuity and integrity alongside Al and QC interface. In the Al side, fine, elongated grains were formed due to heavy deformation near QC particle. Tracking the microstructure from the interface toward the interior region of Al particle is associated with a size gradient from fine elongated grains towards larger equiaxed grains. Furthermore, the EDS line scan in Figure 2c confirms the presence of a concentration gradient of core QC elements at the 30 nm-thick interlayers with Al particle, implying intermixing at the interface. The observed microstructural features can endow a strong bonding between dissimilar constituents in the composite coating structure. The findings regarding the bonding state can potentially justify the enhanced mechanical and tribological properties of composite coatings, and extended retention of the reinforcing phase in the structure under load, as observed in our previous works [3,4].

Figure 1. Surface features observed after wipe test by cold spraying of Al-QC. i) craters generated at rebounded particle, ii) flattening of Al particles with random signs of viscous flow of plastically deformed particles, iii) intruded hard QC particles imposing severe deformation on substrate, iv) cracked/fragmented QCs due to their natural brittleness and v) highly deformed Al particles upon impact to/by harder QC particle.

Figure 2. (S)TEM sample preparation and observed microstructure of CS Al-QC coating; a) location of TEM sample taken in FIB-SEM, b) brightfield TEM image of the marked region in (a) depicting QC, Al and their interface, c) STEM image from the interface of QC and Al and the corresponding EDS line analysis (yellow line).
References:
[1] H. Assadi, H. Kreye, F. Gärtner, T. Klassen, Cold spraying – A materials perspective, Acta Materialia. 116 (2016) 382–407. https://doi.org/10.1016/j.actamat.2016.06.034.
[2] L. He, M. Hassani, A Review of the Mechanical and Tribological Behavior of Cold Spray Metal Matrix Composites, J Therm Spray Tech. 29 (2020) 1565–1608. https://doi.org/10.1007/s11666-020-01091-w.
[3] R. Jafari, J. Kiilakoski, M. Honkanen, M. Vippola, H. Koivuluoto, Wetting Behavior and Functionality Restoration of Cold-Sprayed Aluminum-Quasicrystalline Composite Coatings, Journal of Thermal Spray Technology. (2023). https://doi.org/10.1007/s11666-022-01522-w.
[4] R. Jafari, J. Cizek, F. Lucak, L. Cvrcek, B. Matej, J. Walter, J. Kiilakoski, M. Honkanen, M. Vippola, H. Koivuluoto, Tribological assessment of cold sprayed aluminum-quasicrystal composite coatings, in: ITSC 2023, ASM International, Quebec, 2023.