Properties and research progress of cold-sprayed aluminum matrix composite coatings: a concise review

ABSTRACT

Cold spray is a rapidly developing solid-state deposition technology in recent years. It has the characteristics of low spraying temperature and high velocity of spraying particles, hence showing a good prospect for preparing metal matrix composite (MMC) coatings and bulk materials. In particular, aluminum matrix composites have received great attention in electronics, aerospace and automobile industries due to their excellent physical and mechanical properties. In this paper, the research progress of cold-sprayed aluminum matrix composite coatings and materials was systematically introduced. Firstly, state of the art on cold sprayed coatings and materials with different reinforcing phases was presented. Then, research progress on different post-spray treatments, to improve the properties of as-sprayed deposit, was briefly discussed. Finally, the application fields and problems of cold-sprayed aluminum matrix composite coatings and materials were summarized.

KEYWORDS:

• Cold spray
• metal matrix composite
• aluminum and aluminum alloys

Introduction

Metal matrix composite (MMC) is relatively a new class of material prepared by adding a reinforcing phase into the metal matrix. The metal matrix mainly endows the material with macro morphology and basic physical properties, while the reinforcing phase offers special properties and functions, thus providing strong designability. As a new structural-functional-integrated material, MMC has the advantages of high specific strength, high specific stiffness, heat resistance, wear resistance and low coefficient of thermal expansion. It has a wide application prospect in the fields of national defense, military industry, electronics, aerospace, aviation and so on [1]. At present, the main methods for preparing metal matrix composites include stir casting, liquid infiltration, powder metallurgy, spray deposition and so on [2]. However, these methods generally have a complex process, long production cycle and high cost. Moreover, the process temperature is high enough to instigate harmful interfacial reactions between metal and reinforcement, which are not desirable in MMC containing heat-sensitive materials (such as nanocrystalline and amorphous phases).

Recently, the rapidly developing cold spraying technology has shown a certain application prospect in the preparation of metal matrix composite coatings and materials. In the process of cold spraying, the high-pressure gas (generally N₂, He or air) is divided into main gas and auxiliary gas. The main gas is heated to the spraying temperature by the heater, and the auxiliary gas carries the spraying powder into the main gas after passing through the powder feeder. The powder and main gas are mixed in front of the de-Laval nozzle and form a two-phase (gas–solid) supersonic flow through the nozzle, and then the powder impacts the substrate at a very high speed to produce strong plastic deformation. Consequentially, deposition of the powder particles is achieved (Figure 1) [3]. Compared with other traditional composite preparation technologies, the process of preparing composite materials by cold spraying is simple, the production cycle is short, the material composition is easy to control, and its low-temperature characteristics can avoid the occurrence of harmful interface reactions in other preparation processes [4].

Figure 1. The schematic representation of cold spraying.

Based on the above advantages, the research on the preparation and performance of metal matrix composite coatings and materials has become a hotspot in the field of cold spraying. Every year, a lot of literature related to Cold Sprayed Metal Matrix Composite Coatings and materials is published, covering the preparation of composite powder, the microstructure of prepared materials, wear resistance, corrosion resistance, and mechanical properties. In addition, the influence of post-treatment on the microstructure and properties of prepared materials is also researched. The metallic matrix of composite materials is mainly composed of aluminum, copper, nickel, and titanium as the matrix phase [5,6]. The types of reinforcing phases mainly include oxides, carbides, nitrides, sulfides, diamonds, carbon nanotubes (CNTs), amorphous and intermetallic compounds [7,8]. The application fields of cold spraying include the preparation of functional coatings, additive manufacturing, and repairing engineering parts and components.

Aluminum and aluminum alloys have the advantages of lightweight, good ductility and high corrosion resistance. Therefore, these are the most widely studied materials by cold spraying. Compared with other metal matrix composite coatings and materials, cold-sprayed aluminum matrix composite coatings and materials have the most varieties containing different reinforcing phases such as Al₂O₃, SiC, TiN, B₄C, WS₂, intermetallic compounds, etc. [8]. A large number of research results show that the addition of a hard reinforcing phase can significantly reduce the porosity of aluminum-based composite coatings and materials, improve their adhesion and hardness, and then improve the corrosion resistance and wear resistance of composite coating materials. However, the type, content, particle size, morphology, etc. of the reinforcement phase have a significant influence on the microstructure and properties of the aluminum-based composite coating and material. This paper will systematically introduce the latest research progress of cold sprayed particle reinforced aluminum matrix composite coatings (materials).

Aluminum matrix composite coating (materials)

Al₂O₃ reinforced aluminum matrix composite coatings (materials)

The Al₂O₃ is the most commonly used reinforcement phase for cold-sprayed aluminum-based composite coatings and materials. Lee et al. [9] first prepared Al/Al₂O₃ composite coatings using agglomerated spherical Al₂O₃ and molten method to prepare irregular Al₂O₃ powder as the reinforcing phase. The results show that the impact of irregular Al₂O₃ on the substrate surface is remarkable, and the adhesion of the prepared composite coating is higher. Spencer et al. [10] prepared Al/Al₂O₃ and 6061Al/Al₂O₃ composite coatings, with different Al₂O₃ content, on the surface of AZ91E magnesium alloy using cold spraying. They found that although the content of Al₂O₃ in the coating was significantly lower than that of the original powder, the addition of Al₂O₃ could improve the adhesion of the coating, and the composite coating had excellent corrosion resistance and wear resistance. Tao et al. [11] prepared Al/Al₂O₃ coatings on the surface of AZ91D magnesium alloy using cold spraying, and found that the addition of Al₂O₃ can significantly reduce the porosity of the coating and increase its adhesion and hardness. Shockley et al. [12] studied the wear resistance of cold-sprayed Al/Al₂O₃ composite coatings using in situ friction and wear experiments and found that with the increase of Al₂O₃ content in the coating, the plastic flow and pore formation at the Al/Al₂O₃ interface were inhibited, thereby improving the wear resistance of the coating. Subsequently, they [13] studied the influence of Al₂O₃ morphology on the coating formation process as well as friction and wear performance of the coating. They revealed that the deposition efficiency of spherical Al₂O₃ is lower than that of irregularly shaped Al₂O₃, and the hardness of the coating is only related to the content of Al₂O₃ retained in the coating, which is shown in Figure 2. When the Al₂O₃ content inside the coating exceeds a certain critical value, a well-combined hard oxide film is generated on the worn surface during the friction and wear process. Consequentially, the wear resistance of the coating is improved. In contrast, the critical content of spherical Al₂O₃ is lower than that of irregular Al₂O₃. Wang et al. [14,15] systematically studied the effects of Al₂O₃ morphology and content on the surface morphology, microstructure, hardness, friction, wear and tensile properties of AlSi₈Cu₃/Al₂O₃ composite materials and found that only a small amount of spherical Al₂O₃ (named ‘S’ in Figures 3–4), which mainly provides compaction of the pre-deposited layer, is deposited inside the material. However, the compactness and the tensile properties of the prepared coating were significantly improved. Irregular Al₂O₃ (named ‘I’ in Figure 4) is easy to deposit inside the material, and it greatly improves the hardness and wear resistance of the prepared material. Besides, the retention mechanisms of the two kinds of Al₂O₃ were studied in detail, which is shown in Figures 3 and 4. Hang et al. [16] employed the electron backscattered diffraction (EBSD) method to reveal that the compaction effect of Al₂O₃ leads to more severe deformation of Al particles, thereby improving the compactness of the coating. As a result, the hardness, modulus of elasticity and wear resistance of cold-sprayed Al/Al₂O₃ composite coating were significantly higher than those of ordinary pure Al blocks. However, the micromechanical properties of the composite coating were found to be anisotropic. Gong et al. [17] studied the corrosion resistance of cold-sprayed Al/Al₂O₃ composite coating under-insulated conditions, and found that the addition of Al₂O₃ improves the hardness and compactness of the coating. Moreover, they revealed that the addition of Al₂O₃ does not affect the corrosion properties of the coating; however, the degradation rate of the coating is significantly accelerated with an increase in the Cl-concentration. Heimann et al. [18] prepared Al/Al₂O₃ composite coating on the surface of 7075 aluminum alloy using cold spraying and found that it has a very low thermal conductivity and solar absorption rate as well as high infrared emissivity and oxidation stability. Therefore, the prepared coating has great application prospects for the protection of space materials in low earth orbit. Fernandez et al. [19,20] systematically studied the effects of Al₂O₃ content and morphology on the deposition behavior of cold-sprayed Al/Al₂O₃ composite coatings. The results showed that the retention rate of spherical Al₂O₃ in composite coatings was significantly reduced compared with that of irregular Al₂O₃. With the increase in Al₂O₃ content in the feedstock powder, the overall deposition efficiency of spherical Al₂O₃ composite powder showed a downward trend, while the overall deposition efficiency of irregular Al₂O₃ composite powder initially increased and then decreased. The addition of Al₂O₃ can play an important role in roughening the matrix in the pre-deposited layer and removing the oxide film, thus resulting in improved deposition efficiency and adhesion of Al.

Figure 2. Recovery of Al₂O₃ (left) and deposition efficiency (right) of spray Al/Al₂O₃ coatings [13].

Figure 3. Schematic diagrams of deposition behavior for spherical and irregular alumina coatings [14].

Figure 4. Representative cross-sectional SEM micrographs of different samples at different magnifications: (a, b) pure A380 alloy coating; (c, d) (S) composite coating; (e, f) (I) composite coating [15].

Sic reinforced aluminum-based composite coating(materials)

The SiC is also a commonly used reinforcing phase for cold-sprayed aluminum-based composite coatings (materials). Jeong et al. [21] prepared the Al/SiC composite coating by cold spraying and greatly improved the high-cycle fatigue properties of the cast A356 aluminum alloy. Jodoin et al. [22] prepared Al-12Si/SiC composite coating by cold spraying. Their results show that, depending on the initial SiC volume fraction of the blend, between 50% and 33% of the SiC in the feedstock powder was retained in the coatings, which is due to the different deposition efficiency of these two powders. Lee et al. [23] studied the effect of SiC size on cold-sprayed Al/SiC composite coatings and found that the size of the impact crater formed by large-sized SiC particle at the coating/matrix interface was greater than that of the one formed by small-sized SiC. Li et al. [24,25] studied the effects of spray gas temperature and SiC content on the microstructure and properties of cold-sprayed Al5056/SiC composite coatings and found that spraying temperature only improves the deposition efficiency, but it does not affect the content of SiC in the coating. With the increase in SiC content in the coating, the hardness of the coating increases, and the volume of wear is significantly reduced. Yu et al. [26] studied the influence of SiC size on the deposition behavior of cold-sprayed Al5056/SiC composite coatings, and found that as the average size of SiC increases from 2 to 67 μm, although the speed of the particles decreases, but the kinetic energy of the particles is significantly improved. The kinetic energy (E_k) of the SiC particle was obtained via the kinetic equation as follows: $E_k={\displaystyle \frac{1}{2}m{v}^2={\displaystyle \frac{1}{2}\rho \left[{\displaystyle \frac{4}{3}\pi {\left({\displaystyle \frac{D}{2}}\right)}^3}\right]v_p^3}}$Here, ρ is the density, D is the equivalent particle diameter and v_p is the particle velocity. From this formula, and their tests of particle velocities as a function of particle diameters, it can be calculated that the kinetic energy gained by a particle decreases as its size increases. Further, the content of retained SiC, the hardness, and adhesion strength of the composite coating are gradually increased. The relevant results are shown in Figure 5. Normand et al. [27,28] also prepared Al5056/SiC composite coatings using cold spraying and studied the effects of SiC content and size on the

Figure 5. The variation of SiC content in the coating, porosity, particle velocity, kinetic energy, microhardness and cohesion strength of the coating with the size of SiC particles [26].

microstructure and corrosion behavior of composite coatings. The results showed that the addition of SiC particles reduces the porosity and improves the surface roughness and interface morphology of the coating. The Al5056/SiC composite coating exhibited better corrosion resistance compared with that of the Al5056 coating. When the size of SiC particles was about 20 μm, the composite coating exhibited higher spray efficiency and corrosion resistance. Wang et al. [29] prepared Al/SiC composite materials with a thickness of 5 mm by cold spraying. They systematically studied the effects of SiC content and heat treatment on the mechanical properties of the prepared composite materials. According to Figure 6, the increase in SiC content, the flexural strength and hardness of the material generally increase, and a suitable heat treatment will promote the fusion of particle interfaces, thereby significantly improving the bending strength of the composite materials. Kumar et al. [30] found that with the increase in SiC content, the hardness, mechanical strength, modulus of elasticity and wear resistance of Al/SiC composite coatings are improved, and heat treatment can further improve the wear resistance of the composite coatings, the relevant results are shown in the Figure 7. Kayali et al. [31] prepared a 7075Al/SiC composite coating using cold spraying and revealed that the addition of ceramic particles improves the compactness, hardness and wear resistance of the coating. With the increase in ceramic particles content in the coating, the hardness of the coating increases, but it has little effect on the wear resistance of the coating. Compared with the 7075Al coating, the addition of ceramic particles increases the corrosion current density of 7075Al/SiC composite coating.

Figure 6. Typical load-deflection curves of as-sprayed and heat-treatment at 200°C, 300°C, 400°C and 500°C: (a) deposit 20 (b) deposit 30 (c) deposit 40 (Defined by weight fraction of SiC in powder) [29].

Figure 7. Some relevant results of cold spray Al/SiC coatings (a) Hardness of the composite coatings as a function of load, (b) hardness of the Al-SiC composite coatings in as-sprayed and heat treatment conditions, (c) stress–strain data from micro tensile test, (d) Ultimate Tensile Strength as a function of SiC content, (e) elastic modulus of the coatings and (f) strength coefficient of the coatings [30].

B₄c reinforced aluminum-based composite coating (materials)

Recently, Al/B₄C composites have been widely studied as a common wear-resistant material due to the very high hardness of B₄C particles. Moreover, a high neutron absorption cross-section of ¹⁰B (∼3845 barns) makes B₄C/Al composites an attractive thermal neutron absorbing material [32].

Yandouzi et al. [33] compared the microstructure and mechanical properties of the composite coating prepared by ball milling and mechanical mixing of Al-12Si/B₄C powders using cold gas dynamic spray (CGDS) and pulsed gas dynamic spray (PGDS) processes, respectively. They found that B₄C particles are evenly distributed in the Al-12Si matrix phase after ball milling by these two methods. Further, the content of B₄C retained in the composite coating was higher, while the hardness and wear resistance were better. This conclusion is consistent with the experimental results obtained by Chesnokov et al. [34,35] that is, the use of ball-milled Al – B₄C composite powder in the cold spray process may increase the deposition efficiency and the weight fraction of B₄C in the coating. Besides, the B₄C particles were broken in the ball milling process and the size decreased obviously, which is shown in Figure 8. Huang et al. [36] prepared ultra-think (5 mm) Al + 30wt%B₄C coatings on 5083 aluminum alloy by cold spray method. Results indicated that the B₄C particles, retained in the coatings, were much lower than that in the feedstock powder. Further, the results showed that the quality of the prepared Al/B₄C composite coatings is very good with the bond strength reaching 26.5 MPa, and the average porosity reducing to 1.5%. The wear resistance of the Al/30% B₄C coating was also greatly improved compared with that of the 5083 Aluminum alloy. Although the cold spray Al/B₄C composite coatings exhibited a higher corrosion rate than 5083 aluminum alloy in the neutral salt spray test, it still has an excellent anticorrosion performance. Due to the large thermal neutron absorption cross-section of ¹⁰B (∼3845 barns) in natural B₄C, Tariq et al. [32] believed that Al/B₄C composite material, prepared by cold spraying method, could be used as an effective strategy for repairing unserviceable neutron shielding products. They prepared 6 mm thick neutron shielding B₄C/Al composite coating by cold spray and performed a series of post-treatments including heat treatment, hot rolling, etc. to improve the mechanical properties of the as-sprayed composites [32,37]. The results showed that the strength and elongation of the composites were greatly improved from 37 to 185 MPa and 0.3% to 6.2%, respectively. To improve the neutron absorption capacity of Al/B₄C composite materials, Zhao et al. [38] studied the effects of Al and B₄C particle size on the retention of B₄C particles in cold-sprayed Al/B₄C composites. They found that the cold sprayed composites, prepared by mixing small-sized Al particles and optimum-sized B₄C particles (～15μm), result in higher retention of B₄C (∼30 vol.%) in the coating. The influence of two sizes of Al powder on the deposition of B₄C is shown in Figure 9. Besides, the B₄C is widely used as wear-resistant material due to its high hardness. As reported by Shikalov et al. [39], with the addition of B₄C, the friction coefficient of the Aluminum coating is reduced, and the wear resistance is significantly improved.

Figure 8. SEM-images of the coating cross sections obtained from mixtures:(a) V-shaped mixer and (b) Planetary mill [35].

Figure 9. Schematic diagrams showing deposition behavior of cold sprayed MMCs coatings with large (a–c) and small (d–f) sized Al particles [38].

Miscellaneous particles reinforced aluminum-based composite coating (materials)

In addition to above-mentioned aluminum-based composite coating (materials), the relevant literature also highlights the research on the microstructure and properties of particle-enhanced Al-based composite coatings (materials) such as TiN, diamond, WS₂ and intermetallic compounds. Li et al. [40–42] studied the microstructure and frictional properties of cold-sprayed Al5056/TiN and Al2319/TiN composite coatings and found that the content of TiN in The Al5056/TiN composite coating is consistent with that in the original powder, while the content of TiN in the Al2319/TiN composite coating (38.7 vol.%) is higher than that in the original powder (32.7 vol.%). With the increase in TiN content in the coating, the hardness and wear resistance of the two composite coatings are significantly improved. Moreover, the wear resistance of the cold sprayed coatings prepared by using ball-milled Al5056/TiN composite powder was further improved.

Heat treatment significantly reduces the hardness of composite coatings by eliminating cold working/work hardening effects of cold spraying process [44]. Osswald et al. [43] prepared nano-diamond-reinforced Al-based composite coatings (ND–Al MMC) using high-energy ball milling and cold spraying, respectively. As shown in Figure 10, the hardness of the composite increases with the proportion of nano-diamond, and the highest hardness and elastic modulus values for the composite coatings reached 3.27 and 98.3 GPa. Kwon et al. [45] prepared diamond-reinforced Al matrix composites using low-pressure cold spraying. Compared with cold-sprayed pure aluminum, the hardness and thermal conductivity of composite materials were improved. Bu et al. [46] studied the microstructure, mechanical properties and corrosion resistance of the Mg₁₇Al₁₂/Al composite coating. The results showed that although the Mg₁₇Al₁₂ intermetallic compound particles retain only a small part (less than 10 vol.%), the addition of hard Mg₁₇Al₁₂ particles can significantly reduce the porosity of the coating, improve the hardness and bond strength of the coating. Further, the composite coating can significantly improve the corrosion resistance of AZ91D magnesium alloy. Agarwal et al. [7] prepared an aluminum-based composite coating, reinforced with 0.5 and 1 wt.% of CNTs, by cold spraying. They found that due to the impact and shear between the aluminum-silicon eutectic particles and the aluminum matrix shown in Figure 11, the length of the CNTs deposited inside the coating is shortened, and the elastic modulus of the CNTs enriched area in the coating is greatly improved. Ng et al. [47] prepared Ti₆Al₄V/Al composite coatings (with different components) by cold spray as an attractive coating material for application in the artificial hip joints. The wear behavior of the prepared coatings was assessed by the scratch test, and the results revealed that the friction coefficient of Ti₆Al₄V/Al coatings could be greatly reduced by the reduction of Al content, especially for applied loading forces less than 20 N. For hip implants, the contact force is expected to be less than 4 MPa; therefore, the prepared coating basically meets the requirements for their use on the artificial hip joints.

Figure 10. (a) Hardness of ND–Al MMC with different ND content milled for 10 h using ball powder ratio of 10:1, 20:1, and 30:1. (b) Linear relationship between hardness of ND–Al MMC, produced using a ball powder ratio of 10:1, and ND content for different milling times [43].

Figure 11. Transmission electron microscope images showing (a) tip of CNT broken due to impact, and (b) tip of a CNT broken due to shearing. Schematic showing the two mechanisms for fracture of CNTs during cold spraying, namely by (c) impact and (d) shear [7].

Concluding remarks

This article discusses various types of cold sprayed Al-based MMC, which are widely used as aerospace, transportation, nuclear and biomedical materials, were discussed. Based on these references, the following important conclusions were drawn:

1. The addition of ceramic reinforcement, such as SiC, Al₂O₃ and B₄C, etc. can improve the hardness and wear resistance of MMC due to the adequate tamping and pinning effects.

2. Some reinforcement, such as CNTs and some irregular ceramic particles can enhance the interparticle bonding of the metallic matrix. As a consequence, the bonding strength and the tensile strength of the composite are significantly enhanced.

3. The particle size, morphology and ratio of the reinforcement greatly influence the deposition efficiency, porosity and retention rate of the reinforcement in the coating, which is directly related to the performance of the composite coating. Therefore, the selection of appropriate materials, spraying parameters and post-treatment process are the key to improving the performance of the coating.

As a solid-state process, cold spray offers a unique space in the areas of composite fabrication and additive manufacturing of structural materials. However, the poor plasticity of cold sprayed materials still needs to be improved. With the recent proliferation of cold-sprayed materials in structural applications, advanced characterization techniques, the improvement of this problem is still worth looking forward to.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by IMR Innovation Fund [grant number 2021-PY06] from the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS).