Mechanical Behavior of Spray-Coated Metallic Laminates
Thermal spray (TS) coatings have been extensively utilized for various surface modifications such as enhancing wear/erosion resistance and thermal protection. In the present study, a new function of TS material is explored by studying its load-carrying capability. Due to the inherent microstructures containing voids and interfaces, it has been presumed TS materials were not suitable to bear loads. However, the recent advances in TS technology to manufacture near fully dense TS coatings have expanded their potential applications. In the current experiments, TS nickel coatings are deposited onto metallic substrates, and their mechanical behaviors are closely examined. Based on the measured data, the estimated elastic modulus of TS Ni is about 130 GPa (35% less than bulk value), and the maximum tensile strength is about 500 MPa (comparable to bulk value). It was found that such a high value is attainable because the coating is deposited onto a substrate, enabling a load-transfer mechanism and preventing coating failure at a much lower stress level. Three distinct deformation stages are identified to describe this behavior. Such a clarification is critical for enabling TS process to restore structural parts as well as to additively manufacture load-bearing components.
Keywordsfinite element modeling HVOF mechanical behavior repair
Most of the modern thermal-spray (TS) applications seek to enhance surface functionality of structural substrate materials (Ref 1). However, the roots of the technology lie in machine component repair, reclamation, and restoration. From its inception in the early 1900s to the 1960s, worn-out engineering components such as shafts, hydraulic cylinders, paper mill rolls, etc., have been restored using TS overlays followed by secondary finishing operations (Ref 2, 3). Given the presence of porosity and defects inherent in these spray-assembled materials, the regenerated surfaces were not considered to have the same mechanical integrity as bulk materials and treated as such in applications.
The recent decades have seen the introduction of advanced high-velocity TS processes such as high-velocity oxy-fuel (HVOF), high-velocity air-fuel (HVAF), and even non-thermal cold-spray-based solid-state consolidation (Ref 4-8). In all the above processes, the very high particle kinetic energies allow for the synthesis of deposits with near-full density. This enhancement in density with concomitant improvements in properties and performance has led to expanded applications of TS coatings to meet stringent performance requirements. For instance, these advanced coatings offer excellent wear and corrosion resistances and even improved functionalities such as near-bulk electrical properties (Ref 6, 9-11).
Increasingly, there is significant increase of interest in considering TS for not just a protective barrier coating but enabling duality of function (structure and surface) through synergistic benefits arising from the structurally integrated coatings (Ref 12). Several examples embody this development, including landing gears of large airplanes (through replacement of electroplated Chromium), hydraulic cylinders in earth moving machinery, and even repair and reclamation of superalloy gas turbine engine components. More recently, directed TS has been considered as potential repair solutions for selective locations of infrastructures experiencing severe corrosion and material loss (Ref 13). These new opportunities have pushed the requirement landscape of thermal sprayed layers from just surface modification to one requiring system level functionalities. Today, advancements in TS technology may allow for their applications in true structural restoration and even additive manufacturing via layered spray-based assembly.
There has been a significant amount of past research studies producing near-bulk materials using a spray-forming process, generally using low-pressure plasma (Ref 14-17). Microstructural characteristics and defects were correlated to mechanical properties of the spray-formed materials, often with post-processing heat treatments for improved performance (Ref 17). Materials formed ranged from ceramics and layered composites (Ref 16) to superalloy compositions for gas turbine applications (Ref 14). Mechanical testing results of these spray-formed materials were assessed on free-standing coatings, using substrates only for the collection of the deposited materials and their removal either mechanically or through chemical dissolution.
Mechanical behavior/strength of the spray-assembled materials.
Adhesion with the parent metal and bonding at the restored/substrate interface.
Changes if any to the characteristics of the remaining original structure, after having been subjected to impacting particles (peening) and thermal effects during processing.
Fatigue and cyclic load performance of spray-coated metallic composites.
Corrosion response of the reclaimed system and potential galvanic issues.
In addition to microstructural integrity, residual stresses play an integral role in defining the properties and performance of a TS coating, including hardness, reclaiming strength, and wear and corrosion behaviors (Ref 18). The evolution of these stresses are complex, arising from impact, thermal mismatch, and phase changes. In processes involving melting and solidification, large tensile quenching stresses arise due to constrained shrinkage and cooling of impacting droplets (Ref 19). Thermal mismatch stresses are imposed due to expansion mismatch between the depositing material and substrate. In high-velocity spray processes, additional stress variants arise due to solid-state peening, which results in local compressive stresses. All of the above stress variants and their intensities are affected by material, process condition, and parameters. Impact and peening processes may offer the benefit of compaction of deposited layers, enhancing the characteristics of these coatings. Much progress has been achieved in understanding formation and quantification of residual stresses including process parameter effects to control such stresses (Ref 20). The above attributes of substrate-deposit interaction, microstructural integrity, and state of stress in deposited material all affect the effective properties of the coating, and ultimately that of the substrate-coating system. The latter is of significance in structurally integrated coating’s major role of which requires load-bearing applications (Ref 12).
The present study seeks to address these questions through a novel analysis of the mechanical behavior of coating-substrate specimens. In these tests, HVOF thermal-sprayed metallic (nickel) coatings were deposited onto steel tensile specimens under various processing parameters. The resultant coating-substrate laminates were tested under uniaxial tension to examine the overall stress-strain responses. With various test results, the tensile behaviors of sprayed material are analyzed, and their contributions to the load-carrying capacities are estimated. These results are necessary to quantify and confirm possible enhancement in the residual strength of structural materials.
Process and Coating Characterization
Spray conditions for the JP-5220 used to deposit nickel coatings
Its processibility under TS conditions. Nickel and nickel alloys experience limited in-flight oxidation compared to ferrous alloys during TS processing.
Relatively small thermal expansion mismatch with steel and ferrous alloys.
Excellent ductility, enabling densification and peening during high-velocity TS processing.
Reasonable corrosion resistance.
Inflight particle properties (average temperature and velocity) were measured using Accuraspray G3™ (Tecnar Automation LTEE, St-Bruno, QC, Canada), while coating-formation dynamics and residual stresses were obtained from in situ beam curvature monitoring (Ref 21-23). Here, the slope of the curvature change during deposition is related to stress development within coating and consequently the deposit cohesion. In the case of HVOF spraying of metals, this feature is generally negative, indicative of compressive stress state due to impact peening. Details of these procedures are available in ref (Ref 24-26). Cross-sectional metallographic analysis of the samples was also conducted with backscattered scanning electron microscopy. Vickers Hardness was measured using a 300-g load for 15 s on the polished top surface of coupons sprayed alongside the beam curvature monitoring, as well as the substrate material.
Tensile Sample Preparation, Testing, and Analysis
Average thickness, mm
A, B, C
B (0.08, 0.35, 0.70 mm)
Heat treated (300 °C)
Heat treated (500 °C)
Heat treated (300 °C)
To ensure good bonding during spraying, the tensile substrates were grit blasted using 24-mesh alumina grit using a suction blaster set to 80 psi on both sides, followed by air, blowing away excess grit. They were then placed in an acetone ultrasonic bath for cleaning and for removing any embedded grit or grit dust. For coating application, the substrates were mounted on a disk and spun on a carousel with the torch rastering past the necked region with shielding to reduce overspray on the gripping portion of the dogbones. After coating one side, the specimens were flipped and coated to equal thickness on the other side. Air cooling was directed at the samples during spraying, which maintained the substrate temperature to be below 300 °C (monitored with infrared thermal camera). This was the basis for future heat treatment of the steel substrate prior to spraying.
All the tensile tests were conducted with a servo hydraulic tensile testing machine with static 200-kN load cell. The deflection within the dogbone was measured with a clip-on extensometer (with resolution of ~1 µm) with the clips placed 25 mm apart. The mechanical arm pull rate was set at 2 mm/min, producing a strain rate of 0.033/min between the two grips, 60 mm apart. In all cases, at least three samples from each condition were tested for repeatability. The averaged results are meant to show typical tensile data for each sample set.
Nickel Deposit Condition Characterization
The hardness and residual stress for these coatings are shown in Fig. 2(b). All the coatings have significantly higher hardness than that of steel. The higher hardness of coatings A and B than coating C can be explained from the high densities of these coatings as evident from the SEM micrographs shown in Fig. 2(c). Coatings A and B are almost fully dense at >99% of bulk Ni, whereas Coating C has significant visible porosity and is considered a poorer-quality coating. It is estimated that 95 MPa compressive stress is present in coating B, with lesser amounts being present in coatings A and C.
The repeatability of multiple coated specimens was very good, as their measured stresses were within ±4 MPa for any given strain. The only major variability occurred in the failure above 15% strain.
Due to it being the best-performing coating in terms of hardness, compressive residual stress, and composite stress-strain behavior, coating B is used for additional mechanical characterization.
Coating Thickness Effect
These measured results raise an important question. In elastic plastic bimetallic specimens, if response of each phase is independent of the other, the effective behavior should still show the nonlinearity near the lower yield strain of the two metals. Here the nonlinear behavior of steel begins at about 0.1% strain but the average stress-strain curve of coated specimen remains linear well past this strain. The only way to explain this phenomenon is that the spraying Ni alters the stress state of steel substrate. This aspect is closely investigated in the following sections.
Coating on Stress-Relieved Substrates
Ni coating at condition B was then deposited on the “heat-treated” steel substrate. The stress-strain behaviors of coated and steel-only specimens are shown in Fig. 5. Here the stress-strain relation of Ni coating on the as-received steel is also included. It essentially overlaps that of Ni coating on heat-treated steel. These results suggest that heat input from spray essentially causes the similar effects on the rolled steel as the heat treatment at 300 °C with the differences in substrate heating time from the furnace or spray process not producing significant differences in the sprayed specimen tensile behavior.
Estimated Stress-Strain Relation of Ni Coating
The large increase in the yield strength of Ni-coated specimen can be explained by the raise in the yield strength of the heat-treated steel. However, there is a large discrepancy between the post-yielding behavior between the steel and the Ni-coated specimens. Based on the results shown in Fig. 5, the difference in behavior between coated and non-coated stress-relieved specimens were analyzed to estimate the property of Ni coating. First, the elastic modulus of the annealed 1008 steel was extracted from the stress-strain relations and determined to be E = 204 GPa. Then the tangent modulus of Ni-coated specimen between 0.02% < ε < 0.07% was measured and found to have an effective modulus of 196 ± 3 GPa. It is assumed that the strain in this range to be nearly linear elastic although it exhibited some variations. From the thickness ratio of coating and substrate, the effective/average tangent modulus of the Ni coating was then determined to be 131 ± 20 GPa. The relatively large error bound arises from the fact that the coating is only about 9% of the total specimen thickness and also the variability in the measured tangent elastic modulus of Ni-coated specimen. The latter observation suggests that the inhomogeneous microstructure may cause TS Ni coat to exhibit some degree of non-linearity. Then, the entire stress-strain of Ni coat is essentially estimated from subtracting the steel-only result from the composite Ni-coated-steel result (after adjusting for thicknesses). In the procedure, a residual stress of −95 MPa in Ni coat (Fig. 2b) was also taken into account to obtain the Ni coat’s property, although residual stress could vary based on spray orientation as well as other factors. Thus, this may not represent a perfect match between the residual stresses of the sprayed tensile specimens and curvature measurement samples.
Nickel Coating on Copper Substrate
The extracted stress-strain relation of Ni coat indicated the maximum stress of about 530 MPa at 0.46% strain, which then starts to lose its stiffness. Note without adjusting for the residual stress, the peak stress would be 540 MPa. However, because of Cu having a lower modulus and being more compliant than steel, the Ni coat carries a larger fraction of load here. Hence, the estimated stress-strain relation of TS Ni prior to reaching the maximum tensile strength should be somewhat more accurate. Although the general trend of post-peak behavior is similar to the one estimated with 1008 steel substrate, this result exhibits oscillations. This is not experimental noise, but likely to be from unstable damage and local microcracking taking place in the Ni coat. Compared with the stiffer steel substrate, the compliant Cu substrate amplifies the unstable events. More detailed discussion is given on post-peak behavior in the section 4.
Nickel Coating on 1018 Steel
The stress-strain relation of Ni coat was extracted similarly with the estimated residual stress of −13 MPa. It is noted that this residual stress value is different from the value measured via bilayer curvature in section 3.1, although it is not surprising given that a flat beam versus rotating carousel substrate mounting and rastering pattern differences would produce a different magnitude of residual stress. The peak stress reaches about 535 MPa which is consistent with the 1008 steel and Cu substrate tests. However, due to the higher yield and post-yield stress of 1018 steel, the difference between the steel-only and Ni-coated specimens is very small, as shown in the Figure. Thus, the estimated property is less accurate than those obtained earlier. In fact, a small noise in the measured data translates to larger error and oscillation as shown in the Figure. The estimated post-yielding behavior of this Ni coat is somewhat different from the previous tests. It still exhibits softening but at a lower rate. Nevertheless, the general trend is still consistent with the other cases, and this is described more in detailed in the section 4.
Repairing of “damaged” Substrates
In order to analyze the state of stress more closely, a 3D finite element model of the grooved and coated specimen was constructed. The mesh for nickel-steel specimen contains ~30,000 eight-noded linear brick elements. In this initial study, the steel and nickel were modeled as elastic-perfectly plastic materials. The experimentally determined Young’s moduli of 204 and 131 GPa and yield stresses of 230 and 500 MPa for the steel and Nickel, respectively, were used for the model. The softening observed for post-yielding of Ni coat (e.g., Fig. 9) was not considered. Incorporation of such a behavior would require a damage model and will be left for a future study.
The present study investigated a potential role of TS metallic coatings as load-supporting components. The application of dense TS coating onto steel leads to enhanced strength of the existing structures due to the added metal as well as through process induced changes to the system, as in the case of the as-received 1008 steel. To further interpret the results, it is of importance to identify the respective contributions of substrate (subjected to TS): the coating material and any synergistic effects of the two.
The question of how the surface preparation and coating application affect the parent material’s stress strain response requires critical examination. As shown in Fig. 5, the stress-strain response between the coated structures made with the stress-relieved and the as-received 1008 steels is nearly the same, although the response of the stress-relieved steel is different from that of the as-received steel. It is likely that the as-received substrates contained residual stress from manufacturing and machining, indicated by the lack of the upper and lower yield points seen in the heat-treated specimen. Nickel particle impact and latent heat during deposition, as well as heat from the torch may have contributed to alterations to the substrate steel. Thermal camera images indicate substrate temperatures to remain below 300 °C (with spray time under 10 min), which indicates full annealing accompanied with microstructural changes is not likely to occur, although some stress relief is possible. A mixture of residual stress in the as-received specimens, grit-blasting, and accumulation and relaxation during deposition from particle impact and cooling, respectively, may also induce the phenomenon of static strain aging in the substrate steel, although no change in yield point was observed in samples that were grit blasted and heated by the torch plume (no introduction of powder into the plume).
Based on several tests of repeat samples, the modulus of TS Ni coating B was estimated to 130 GPa, while its maximum strength was estimated to be about 500 MPa, which is greater than many of structural metals. The reduction of 35% in the Young’s modulus as compare to bulk Nickel is due to presence of weak interfaces (i.e., boundaries between splats) within TS Ni coating. Since the coating is 99% + dense, the modulus reduction due to porosity should be small. On the other hand, the maximum strength is comparable to the strength of bulk Ni. There are two scenarios for this high stress. The rapidly solidified metal is known to comprise of fine grains, which are known to provide higher hardness and consequently higher yield strength (Ref 30). In addition, the HVOF sprayed Ni could have finely dispersed oxides which can contribute to dispersion strengthening, although no oxide phases were detectable through x-ray diffraction measurements of Coating B. However, in this particular case, the explanation of high tensile strength lies on the “load-transfer” mechanisms of coating-substrate structure. As damage initiates at some voids and/or weak interface, their progression is prevented by the local load transfer through the substrate.
The present study elucidated the mechanical behavior of TS nickel coating and its capacity to carry mechanical load. Such an understanding is needed for establishing potential applications of TS metals to be used as load-bearing components. TS Ni’s deformation behavior can be described in three stages. The key mechanism is the load transfer which occurs when it is deposited on to sufficiently ductile substrate. With a non-brittle substrate, the TS coating is prevented from brittle failure at much lower loads, as it would do if it were the sole component of the specimen. With the substrate, the maximum strength of TS Ni reaches near the strength of bulk Ni. It is expected that other near-fully dense metallic coatings have similar deformation stages as introduced here. Although only tensile behaviors were examined here, the compression behavior of TS coating is expected to be different, especially at large strains. The softening behavior may not occur under compression. Since many of engineering structures are subjected to compression (e.g., bridges, buildings), investigation of TS coatings under compression is clearly needed and will be studied in the near future.
This research was supported through funding by the Innovations Deserving Exploratory Analysis (IDEA) program of the Transportation Research Board of the National Academy of Sciences, managed by Dr. I. Jawed (NCHRP IDEA 155). The authors also acknowledge support from the Stony Brook’s Industrial Consortium for Thermal Spray Technology.
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