2023 Volume 64 Issue 6 Pages 1210-1216
The cold spray technique is expected to effectively form a metallic coating with fine crystal grains originating from the microstructure of the original powder. We previously reported that a pure iron coating with fine crystal grains can be formed by the cold spray technique by using mechanically-milled pure iron powder. In this study, the tensile strength of the pure iron coating was investigated. The as-sprayed coating showed significantly low Young’s modulus, tensile strength, and ductility owing to the low cohesion strength between particles. For tensile strength improvement, the coating was subjected to spark plasma sintering (SPS) treatment. As a result, the Young’s modulus was considerably improved by the SPS treatments at 740 and 786°C; moreover, the tensile strength of the SPS-treated coating was approximately four times higher than that of the bulk material. In contrast, the ductility was not improved by the SPS treatment. The low ductility was likely attributed to the presence of Fe oxides at the particle–particle interface.
This Paper was Originally Published in Japanese in J. Japan Thermal Spray Society 59 (2022) 84–90.
Alloys containing rare metals and strategic materials are indispensable for piping and turbine components in power plants owing to their high-strength and high corrosion resistance. Structural materials that do not contain rare metals or strategic materials are desired for the effective utilization of resources, avoidance of price fluctuation risk, and improved recyclability. Grain refinement is one of the most effective methods of strengthening metallic materials without using additive elements. Particularly, metallic materials with submicron grain sizes have received significant attention because of their high strength and excellent properties, such as superplasticity and high corrosion resistance.1)
Thermo-mechanically controlled process (TMCP), equal channel angular pressing (ECAP), accumulative roll bonding (ARB), and mechanical milling (hereinafter called “MM”) have been proposed as methods for grain refinement.2–5) These methods introduce large strain into the material and refine crystal grains through continuous dynamic recrystallization and are classified as plastic forming. Among these methods, the MM method can conveniently produce metallic powders with fine crystal structures. However, the processing of the powder using the MM method is time-consuming and costly, thereby making it difficult to apply to large structural materials and limiting its applications.
The cold spray (hereinafter called “CS”) method is a coating technique based on the impact and deposition of powder particles with several tens of micrometers. The particles are accelerated up to supersonic velocities using high-pressure working gases such as air or nitrogen.6–9) It is a low-temperature process where the crystal growth can be suppressed by the heat input. In addition, crystal grains in the particles become finer owing to the high-speed deformation.10,11) Therefore, thick metallic coatings with a fine crystal structure can be fabricated over a large area in a short time via the CS method using the powder that has been finely crystallized by the MM method. In addition, the CS method is expected to be applicable for coating the necessary areas of large structural components.
Several studies on microcrystalline coatings using the combination of MM and CS methods have been conducted worldwide.12–14) Ajdelsztajn et al. investigated the microstructure and hardness of an aluminum alloy A5083 coating, deposited by the CS method using an MM-treated powder in a liquid nitrogen environment,15) and reported that the hardness of the A5083 coating was significantly higher than that of the cold-rolled material coating. Kumar et al. deposited Ni–20Cr powder, which was microcrystallized by the MM method, on carbon steel plates for pressure vessels using the CS method.16,17) The Ni–20Cr coatings with fine crystals showed oxidation and corrosion resistance superior to the base material and coatings with coarse crystals.
Our research group has been investigating the CS coatings using pure iron powder, finely crystallized using the MM method, to replace conventional stainless steels and high-chromium steels. We evaluated the microstructure, hardness, and deposition efficiency of the pure iron coatings in our previous study18) and demonstrated that high-hardness coatings can be formed. Herein, the tensile strength properties of the pure iron coating were evaluated. In addition, post-treatment methods to improve the tensile properties were explored.
The pure iron powder (C < 0.007 mass%, D50 = 26.8 µm, ITOH KIKOH Co. Ltd., Japan) was mechanically milled using the planetary ball mill machine (P-7 Classic line, Fritsch Co. Ltd., Germany). The powder and SUJ2 steel balls of 8 mm diameter were encapsulated in a silicon nitride pod of 45 ml volume and treated at 1400 rpm for 20 h under atmospheric conditions. In our previous study,18) the average crystal grain size was reduced to about 100 nm using this MM treatment. However, the thick coatings were difficult to be fabricated because of the low deposition efficiency of the MM-treated powder. The deposition efficiency can be improved by mixing the MM-treated powder with the as-received powder. Herein, a mixture of MM-treated powder and 20 mass% as-received powder was prepared. The as-received powder, which means non-MM-treated powder, was also prepared. These powders are hereafter referred to as MM20 and NonMM, respectively. The powders were deposited on a 40 mm × 40 mm × 10 mm substrate of structural steel (SS400) using a CS system (PCS-100, Plasma Giken Co. Ltd., Japan). Nitrogen was used as the working gas at pressure and temperature of 5 MPa and 1000°C, respectively. A target coating thickness was set to 1.5 mm. The nozzle traverse speed, stand-off distance, and pitch were set to 300 mm/s, 30 mm, and 3 mm, respectively. In addition, coatings with only the MM-treated powder, without any addition of the as-received powder, were prepared as specimens for heat treatment, which will be described in section 3.2.
After deposition, the cross-section of each coating was observed using a scanning electron microscope (SEM, SU-70, Hitachi High-Tech Corporation, Japan). An electron backscatter diffraction (EBSD) analyzer (OIM detector: AMETEK, Inc., USA, OIM Data Collection: TSL solutions, Japan), installed in the SEM, was used to determine the grain size. The specimen surface was mirror-polished and treated using an ion milling system (IM4000, Hitachi High-Tech Corporation, Japan). The EBSD analysis was performed at low and high magnifications with step sizes of 200 and 15 nm, respectively.
To evaluate the tensile strength properties of the coatings, the test specimens, with the shapes shown in Fig. 1, were cut in the in-plane direction of each coating using a wire electric discharge machining. The parallel section of the specimen was perpendicular to the traverse pattern. A high-temperature tensile observation device (CATY-T3H, Yonekura MFG. Co. Ltd., Japan) was used for the tensile tests. The test was conducted under displacement control, with a crosshead displacement speed of 0.45 mm/min. The load was measured using a load cell (MRDT-2kN, Showa Sokki Corporation, Japan), and the strain was measured by a strain gauge (Kyowa Electronic Instruments Co. Ltd., Japan) attached to the parallel part of the specimen. The tensile tests were performed twice for each coating.
Schematic of the tensile test specimen [unit: mm].
Figure 2 shows the backscattered electron images of the cross-sections of each as-sprayed coating. A dense coating is seen in the NonMM, although particle–particle interfaces and pores are observed in the coating. In contrast, visible gaps were observed at the particle–particle interfaces in the MM20. This indicates that the particles are not sufficiently bound to each other.
Cross-sectional SEM image of each as-sprayed coating; (a) NonMM and (b) MM20.
Figure 3 shows the inverse pole figure (IPF) images of each coating obtained by EBSD analysis at low magnification (LM) and high magnification (HM). The difference in color contrast indicates the difference in the crystal orientation. The crystals are not regularly polygonal, instead, they are highly distorted in the NonMM sample (Figs. 3(a) and (b)). Furthermore, the crystal orientation is observed to change continuously within the crystal grains, indicating that large plastic strain is induced in the crystal grains. In addition, fine crystal grains are observed in a few areas in the vicinity of the particle–particle interface. These results suggest a significant plastic deformation of the particles during the impact. Some of the grains with large plastic strains are refined by recrystallization.
IPF image of each as-sprayed coating obtained through EBSD analysis; (a) NonMM (LM), (b) NonMM (HM), (c) MM20 (LM) and (d) MM20 (HM).
In contrast, the MM20 comprises coarse grains with continuously changing crystal orientation and fine grains. The coarse and fine grains originate from the as-received powder and MM-treated powder, respectively. The HM analysis (Fig. 3(d)) demonstrates that some of the grains are refined to less than 100 nm, i.e., nanocrystals.
The nominal stress–strain curve, obtained from the tensile test, is shown in Fig. 4 for each coating. Table 1 lists the Young’s modulus and tensile strength, calculated from the curve. The plastic deformation is barely observed, and the coating is brittlely fractured in the NonMM specimen (Fig. 4). The Young’s modulus is approximately 192 GPa, which is relatively close to that of a typical pure iron bulk material. The tensile strength is approximately 150 MPa, which is lower than that of the bulk material. Similarly, the coating was observed to be brittlely fractured with almost no plastic deformation in the MM20 like that in the NonMM. However, the Young’s modulus and tensile strength were significantly lower than those of NonMM. Moreover, a significant fluctuation was observed in the results of each specimen. This is attributed to the insufficient bonding between particles in the MM20 (Fig. 2). In other words, the Young’s modulus is speculated to reduce because of the slippage between particles under tensile loading, and the low tensile strength is caused by the lack of bonded areas. Therefore, we attempted to improve the tensile strength properties by post-treatment in the next section.
Stress–strain curve of each as-sprayed coating obtained via the tensile test.
We performed cold rolling as a post-treatment to improve interparticle bonding while suppressing grain coarsening. A four-stage cold rolling mill (120DX270W-300DX250W, Nippon Cross Rolling Co., Japan) was used. The specimen thickness was reduced to 4 mm via milling the base material side of the specimen because the mill is capable of rolling specimens with a thickness of 5 mm or less. The rolling ratio was controlled by gradually narrowing the gap between the rolls to investigate the effect of rolling ratio on the coating.
Figure 5 demonstrates the appearance of the MM20 specimen after cold rolling at a rolling ratio of 10%. The coating is completely delaminated from the substrate, and the edges are chipped (Fig. 5). During rolling, shear stress acts near the specimen surface owing to the friction between the roll and the specimen.19) Therefore, the shear stress is speculated to exceed the adhesion strength between the coating and the substrate, resulting in delamination. This indicates that the tensile strength properties of coatings with low adhesion strength are difficult to improve by cold rolling.
Appearance of the MM20 specimen after 10% cold rolling.
Our previous study20) reported that post-heat treatment using spark plasma sintering (hereinafter called “SPS”) on a pure copper coating, deposited using the CS method, can improve interparticle bonding while suppressing crystal grain coarsening. Therefore, we attempted to improve the tensile strength properties of the MM20 coating by applying SPS treatment.
First, conventional heat treatment was performed using an electric furnace, and the effect of heat treatment temperature on grain size was investigated. The effect of heat treatment temperature on grain size cannot be evaluated quantitatively in the MM20, because the coating contains coarse grains originated from the as-received powder (Fig. 3). Therefore, a coating made of only MM-treated powder, without adding as-received powder, was subjected to the heat treatment in an electric furnace (KDF-S70, DENKEN-HIGHDENTAL Co. Ltd.). The holding temperatures were set to 755, 805, and 855°C. The holding time was 5 min, which was set relatively short to match the SPS treatment (described later in this section). The heating rate was 10°C/min, and the specimen was air-cooled in the furnace after the holding time. After heat treatment, the EBSD analysis was performed on the coating cross-section to investigate the grain size.
Figure 6 shows the IPF images of the MM coating after the conventional heat treatment at different temperatures. The coarsening of the crystal grains increases with increasing heat treatment temperature. However, Fig. 6(c) shows that not all the grains coarsen uniformly, but some fine grains remain. Figure 7 shows the results of a quantitative evaluation of the crystal grain size distribution based on the results of this analysis. Fine crystals with an average grain size of less than 1 µm are observed at 755°C. However, the average grain size exceeds 1 µm at 805°C. Furthermore, the ratio of crystal grains with a size of 1 µm or less is significantly reduced at 855°C. Therefore, heat treatment should be performed at temperatures below 800°C to suppress crystal grain coarsening and maintain fine crystals of 1 µm or less as much as possible.
IPF image of the MM coating after the conventional heat treatment at various temperatures; (a) 755°C, (b) 805°C and (c) 855°C.
Crystal grain size distribution in MM coating after conventional heat treatment at various temperatures.
Based on the heat treatment results, the MM20 coating was further subjected to SPS treatment at temperatures below 800°C. A discharge plasma sintering apparatus (SPS-1030, Sumiseki Materials Co., Ltd.) was used for SPS treatment. The holding temperatures were set to 700, 740, and 786°C. The holding time was fixed at 5 min. The heating rate was 30°C/min, and the specimen was air-cooled in the furnace after the holding time. Hereafter, the specimens are denoted as SPS700, SPS740, and SPS786, corresponding to their respective holding temperatures. A compressive load was applied to the specimen during the holding time to induce a compressive stress of 50 MPa in the direction perpendicular to the coating surface. After SPS treatment, the SEM and EBSD analyses were performed on the cross-section of each coating. In addition, tensile test specimens with the shape, shown in Fig. 1, were cut from each coating by wire electric discharge machining and subjected to tensile tests. The test conditions were the same as those described above.
Figure 8 shows the cross-sectional backscattered electron images of each MM20 coating after the SPS treatment at different temperatures. Comparing with the result of the as-sprayed coating (Fig. 2(b)), visible interparticle interfaces and pores are significantly reduced at all treatment temperatures. Figure 9 shows the IPF images of each coating cross-section. In the region originated from the as-received powder, recrystallization occurs and the crystal grains, with a size of several micrometers to several ten micrometers, are observed. In contrast, the crystal grains become coarser in the region, originated from the MM-treated powder, with increasing SPS treatment temperature. However, most of the crystal grain sizes are observed below 1 µm, even in the SPS786.
Cross-sectional SEM image of each MM20 coating after the SPS treatment at various temperatures; (a) 700°C, (b) 740°C and (c) 786°C.
IPF image of each MM20 coating after the SPS treatment at various temperatures; (a) 700°C (LM), (b) 700°C (HM), (c) 740°C (LM), (d) 740°C (HM), (e) 786°C (LM) and (f) 786°C (HM).
Figure 10 shows the stress–strain curve obtained via the tensile test of the SPS-treated coatings at each temperature. The Young’s moduli and tensile strengths, obtained from these results, are listed in Table 2. As shown in Fig. 10, the tensile strength is improved significantly compared to that of the as-sprayed coating (Fig. 4), although brittle fracture behavior is observed in all specimens. The Young’s modulus and tensile strength of the SPS700 are lower than those of the other specimens, treated at a higher temperature. Moreover, the fluctuation in the results of the SPS700 is more significant. The SPS740 exhibits the highest tensile strength of approximately 835 MPa, which is ∼12 times and ∼4 times the tensile strength of the as-sprayed coating of the MM20 and general pure iron bulk material, respectively. In addition, the Young’s modulus is improved up to a value close to that of the bulk material. In contrast, the SPS786 exhibits the same Young’s modulus as the SPS740, and lower tensile strength than that of the SPS740. Although the 0.2% proof stress could not be quantitatively evaluated from this result, Fig. 10 confirmed the reduction in the yield stress.
Stress–strain curve of each MM20 coating after the SPS treatment obtained via the tensile test.
Figure 11 shows the SEM images of the fracture surface of each specimen after the tensile test. The as-sprayed coating is brittlely fractured at the particle–particle interface. The formation of minute dimples is observed at a few places on the fracture surface of the SPS700. In contrast, the minute dimples are formed nearly all over the fracture surface in the SPS740 and SPS786, indicating the ductile fracture surface. These observations reveal that the interparticle bonding is improved significantly in the SPS740 and SPS786 via the SPS treatment. In the SPS treatment, elemental diffusion is accelerated by the electric current and mechanical pressure, which are named as electromigration and stress migration, respectively. Particularly at the particle–particle interface, the current density increases in the bonded region because the current does not flow in the unbonded region. Consequently, electromigration occurs remarkably in the vicinity of the bonding region, and diffusion is speculated to be promoted to improve the bonding between particles, even at relatively low temperatures.20)
SEM image of the fractured surface of each coating after the tensile test; (a) as-sprayed MM20, (b) SPS700, (c) SPS740 and (d) SPS786.
Thus, the high tensile strength observed regardless of treatment temperature is considered the result of grain refinement strengthening because of the improved interparticle bonding by the SPS treatment. The fluctuations in the tensile strength and low Young’s modulus of the SPS700 indicates that the bonding at the particle–particle interface is not completely improved under this SPS treatment condition. In addition, the decrease in the yield stress and tensile strength in the SPS786 is because of the grain coarsening (Fig. 9). However, the fracture strain of the SPS786 is like that of the SPS740, and no improvement in ductility is observed.
The coarsening of grains, decrease in yield stress, and formation of dimples on the fracture surface of the SPS786 suggest an improved ductility. Energy dispersive X-ray spectroscopy (EDX) analysis was performed on the cross-section of the SPS786 to identify the reason for no improvement in ductility. Figure 12 shows the cross-sectional SEM image of the SPS786 and elemental mapping images of Fe and O. Dark gray dot-shaped areas are observed in the SEM image at the particle–particle interface. The elemental mapping image revealed that these dots are iron oxides rather than pores. This indicates an abundance of iron oxides at the particle–particle interfaces which cause precipitation strengthening and inhibit the movement of dislocations, thereby preventing the improvement in ductility. These iron oxides are speculated to originate because the MM treatment was carried out in an atmospheric environment and the particle surface was oxidized during the CS treatment. We will work on improving the ductility by optimizing treatment conditions in our future studies.
Cross-sectional SEM image and element mapping profile of SPS786 obtained by through EDX analysis.
Herein, pure iron powder, finely crystallized via the MM method, was deposited on structural steel (SS400) using the CS method, and the tensile strength properties of the fabricated coating were evaluated. In addition, improvements in the tensile strength properties by post-treatment were investigated. The conclusions are summarized as follows:
This study was supported by a research grant from the Amada Foundation in 2018 (AF-2018038). We would like to express our gratitude to Mr. A. Hashimoto for conducting the EBSD analysis.
