Inconel alloys are oxidation- and corrosion-resistant materials well suited for service in environments subject to extreme pressure and heat. Specifically, Inconel625 is a Ni–Cr–Mo alloy that was developed for high-temperature strength. Its highly alloyed composition provides a good level of general corrosion resistance in a wide range of oxidizing and non-oxidizing environments. Some of its typical applications are in chemical processing, aerospace and marine engineering, pollution-control equipment, and nuclear reactors [1
Inconel625 and 718 coatings obtained by conventional thermal spray processes such as high-velocity oxy fuel (HVOF) have been extensively used in the power industry to improve the corrosion and wear resistance of metallic surfaces [4
]. Most of the studies relate the corrosion activity to the presence of more or less oxidation and porosity content; for example, according to Planche et al., electrochemical activation time increases with the oxygen/fuel ratio used in the combustion, leading to a higher coating density due to a more difficult electrolyte penetration into the coating [6
Low thermal, high kinetic energy-based processes such as cold gas spray (CGS) offer promising techniques for the avoidance of porosity and oxidation problems [7
]. These techniques initially proved to be well suited to addressing the plastic deformation of ductile raw materials with relatively low melting points, high densities, low mechanical strength, and low heat capacities, such as Zn and Cu. The subsequent development of gun systems, however, has made the challenge of effectively treating higher strength materials such as Inconel more achievable [10
Our previous studies [11
] investigated the mechanical and microstructural properties as well as the fatigue behavior of Inconel625 cold-gas-sprayed coatings, using different starting particle dimensions (−45 + 15 μm, spherical shape) under different process conditions. Many authors [13
] have used different particle shapes of feedstock powder to demonstrate that changing gas pressure and temperature greatly influences the particle velocity, producing denser and less porous coatings and spherical and smaller particles, giving better results. The results have shown that the best combination of low porosity and high flattening ratio for Inconel [13
] was achieved using Kinetics 4000 equipment (Impact Innovations GmbH, Rattenkirchen, Germany) with a gas temperature of 800 °C at 4 MPa pressure. Moreover, the coatings produced with larger dimensions of starting particles showed lower porosity levels than the finer particles. Monotonic bend tests were performed on different V-notched (30°, 60°, and 90°) substrates coated under the above-mentioned process conditions; it was found, during cyclic tests, that the crack growth rate increases with decreasing V-notch aperture and increasing maximum bending load. Increasing the process conditions up to 1000 °C and 5 MPa, using PSC100 equipment (Plasma Giken Co. Ltd., Saitama, Japan) [12
], showed lower levels of porosity, higher hardness values, and improved corrosion properties. The microstructurally and mechanically improved features led to improved fatigue properties under crack initiation and growth tests in bending.
Recent results from other authors also studying the microstructural features and mechanical and corrosion properties of cold-gas-sprayed Inconel625 coatings demonstrate the growing interest in this topic. The chronology starts in 2017, when Chaudhuri et al. [16
] illustrated the microstructural evolution of Inconel625 coatings cold-sprayed onto a medium carbon steel substrate, and observed a significant strain accumulation in the coating due to severe deformation of the particles. Moreover, the substrate region close to the coating–substrate interface showed a heavy grain refinement, as a result of the severe deformation of the substrate by particle impact, followed by thermally activated dynamic recrystallization. In the same year, the high-temperature corrosion of Inconel625 cold-gas-sprayed coatings was evaluated by Fantozzi et al. [17
], who subjected the coatings to chlorine-induced active oxidation. This was done because Inconel625 coatings are applied as protective coatings in many industrial fields where high-temperature corrosion resistance is required. The cold gas spray coatings were sprayed onto stainless steel substrates, using two different gas processes (N2
and He) and two different gas-atomized powders (fine and coarse). All the coatings performed well, preventing corrosion of the substrate and acting as a barrier against the corrosive environment, notwithstanding the conclusion that combining the finest particles with the use of He in the gas process seems to have better results.
In a very recent study, Azarmi et al. [18
] investigated the elastic properties of Inconel625 powders that were cold gas sprayed onto aluminum substrate. They concluded that the major microstructural feature affecting the Young’s modulus of cold-gas-sprayed coating is the dislocation density at the grain boundaries. This result is a consequence of the stress/strain test that indicates a reduction of the in-plane Young’s modulus of the CGS-deposited coating of about 30%, as compared to the bulk material. To better understand this reduction, they performed transmission electron microscopy (TEM) experiments, which confirmed the occurrence of a very high dislocation density at the grains and grain boundaries in CGS-deposited coatings.
Given the parallel growth of CGS technology, not only as a coating process but also as an additive manufacturing technique, Sun et al. [19
] used it to cold gas spray Inconel625 in a process to repair metal components. The resulting low porosity levels and high hardness values demonstrated the good quality of the cold-gas-sprayed Inconel625 coatings and showed that cold gas spray is a promising additive manufacturing technique for repair applications of Ni-based superalloy parts [20
As previously mentioned, Inconel718 is also an important alloy used in the aerospace sector and its spraying feasibility is worth considering. Levasseur et al. [21
] demonstrated that a cold gas spray process can be used advantageously to produce high density Inconel718 coatings, but post-deposition sintering was necessary because of a lack of inter-particle bonding. Singh et al. [22
] investigated the influence of coating thickness on residual stress and the adhesion strength of Inconel718 coatings sprayed onto Inconel718 substrate. Luo et al. [23
] used an in situ micro-forging technique to reduce the porosity of Inconel718 coatings deposited onto 316 L substrate. To introduce the micro-forging effect during spray deposition, 410 martensitic stainless steel powders with a spherical morphology were blended into the Inconel718 powders. The authors concluded that porosity present in the Inconel718 deposit was gradually reduced and the inter-particle bonding improved with the enhanced in situ micro-forging effect [24
Complementing the above-mentioned works, Ni-based superalloy coatings reinforced with alumina have also been observed to improve wear resistance properties. For example, Ni20Cr was successfully deposited by spraying it in a blend with ceramic alumina particles, leading to many practical advantages [25
]. Ceramic particles have been added by other authors to produce improved coatings [26
] and were observed to function in more ways than just reinforcing, such as (i) preventing nozzle clogging, (ii) activating the sprayed surface, and (iii) helping in the compaction of the structure [25
Following our previous studies, we wanted to go one step further by producing alumina-reinforced Inconel625 cold-gas-sprayed coatings with different Al2O3-to-Inconel625 ratios and evaluating the main mechanical, tribological, corrosion, and oxidation properties of the optimized deposits.