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In-situ observation of strain and cracking in coated laminates by digital image correlation
Accepted Manuscript In-situ observation of strain and cracking in coated laminates by digital image correlation
Gregory Smith, Olivia Higgins, Sanjay Sampath PII: DOI: Reference:
S0257-8972(17)30866-6 doi: 10.1016/j.surfcoat.2017.08.057 SCT 22618
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
3 July 2017 21 August 2017 24 August 2017
Please cite this article as: Gregory Smith, Olivia Higgins, Sanjay Sampath , In-situ observation of strain and cracking in coated laminates by digital image correlation, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.08.057
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ACCEPTED MANUSCRIPT In-situ observation of strain and cracking in coated laminates by digital image correlation Gregory Smitha, Olivia Higginsa, and Sanjay Sampatha a
Center for Thermal Spray Research, Dept. of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794
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Abstract
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Overlay coatings are widely used in engineering components to impart a range of surface
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functionalities including thermal, wear, and corrosion protection, as well as material reclamation. In most surface engineering applications, the coating’s role is restricted to the surface, with
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limited integration with the underlying substrate. However, the situation is changing: there is an
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emerging need for so-called structurally integrated coatings, where the coating and substrate are intimately bonded, resulting in a coupled system. Of interest are the emerging applications of
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thermal spray and cold spray overlay coatings applied on loaded engineering components such as
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landing gear, heavy machinery hydraulics, and steel infrastructure. These coatings, even metals or cermets, respond in a brittle manner associated with their layered processing and ultra-fine
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grain structures resulting from rapid quenching. As such, it is of importance to understand their
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coupled mechanical response, especially stress-strain behavior and strain to fracture. In this study, an approach involving strain monitoring of coated steel via digital image
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correlation has been developed. Both elastic response and strain beyond the yield point of the system are assessed to examine load transfer between the coating and substrate and onset of cracking. Three different coating materials (Ni, WC-CoCr and Al2O3) were deposited to near full density via high velocity thermal spray. The results point to a powerful new approach for understanding mechanical behavior of heterogeneous composites using advanced imaging techniques.
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ACCEPTED MANUSCRIPT Keywords: Overlay coatings, thermal spray, digital image correlation, tensile stresses, crack propagation
1. Introduction Overlay coatings continue to gain attention as they not only provide surface enhancement
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to structural metals but also provide a cost effective and sustainable utilization of advanced
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materials [1-3]. A wide range of applications exist for overlay coatings, encompassing metallics,
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ceramics, cermets, and composite coatings for addressing wear, corrosion, thermal barrier,
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electrical insulators, and structural repair [3-6]. Processes involved in depositing overlay coatings include electroplating, thermal spraying, laser cladding, and vapor deposition. Among
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these methods, thermal spraying encompasses the greatest diversity in materials, thickness, and applications.
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In the past, coatings were added as an after-thought to provide surface functionality to a
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priori designed structural component [1, 6]. In that sense, coatings had limited impact on the
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structural attributes of the underlying structural material. Current and future coating applications require more exacting specifications including the need for prime-reliant coatings, where-in the
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performance of the coating is tied to both system functionality and component durability [3, 4]. In this emerging scenario, understanding and defining coating-substrate system response in the
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operational environment becomes critical. Characterizing physical, mechanical, and functional properties of coatings is challenging due to dimensional constraints, anisotropy, and substrate attachment. This has led to use of small volume and local characterization techniques such as nano and micro-indentation or measurements on free-standing films and coatings [7, 8]. Coating-substrate interfacial adhesion is assessed through indirect techniques such as bond-pull or bend testing [9-12]. Although these
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ACCEPTED MANUSCRIPT techniques have been very useful, they often do not provide full field description of the properties, especially when the substrate is constrained and adhesion is considered. Uniaxial tensile testing of spray coated laminates has been used successfully to extract mechanical system properties of laminate structures, and is an important indicator of coating adhesion, strength and
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compliance, as shown in Vackel, et al. and other ongoing work [13]. This methodology is useful
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in ascertaining critical system properties; however, it has focused predominately on composite
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strength, adhesion, and failure mechanisms. Strain evaluation is more limited in this context. The strain and fracture response of overlay coatings are comparable to the bulk behavior
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of hard and brittle materials (cermets and ceramics), with even metallic overlay coatings
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displaying brittle responses due to process induced defects and residual stresses [14-17]. Measurement of residual stress can be readily performed on these coated systems and has been
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shown to have significant impact on mechanical and fatigue behavior in these systems, however
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also introduces additional complexities [18-22]. The paucity of such information in the literature is in part due to difficulty in characterization of strains and fracture initiation in these constrained
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systems. This paper seeks to exploit digital image correlation (DIC) techniques to characterize
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strain and cracking in coated metal systems [23-28]. The goal here is to understand not only the response of the coating but to also couple coating and substrate deformation.
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DIC strain measurements were conducted on spray coated steel substrates subjected to uniaxial tensile testing. The principle materials for the study were formed by supersonic combustion thermal spray technique (referred to as high velocity oxy-fuel or HVOF), yielding near full density coatings with high adhesion strength. Within these structures, defects such as oxides and interfaces were present, along with nano-crystalline grains associated with rapid quenching. As such even these metallic coatings showed little or no ductility. Coating failure (via
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ACCEPTED MANUSCRIPT cracking) in these systems can be detected with the DIC technique, and presents a methodology to better understand strain to failure in these systems, as well as provide a basis for a technique that can be suitable for a range of component geometries that are inaccessible with traditional
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extensometers.
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ACCEPTED MANUSCRIPT 2. Material and methods Tensile testing with an Instron uniaxial tensile machine (Instron, Norwood, MA) was used to evaluate 1008 low carbon steel, "dog-bone" shaped specimens (150 mm x 25 mm x 3 mm, with approx. 65 mm x 12.5 mm gauge). As-received specimens underwent a stress relief
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treatment at 450o C with a 1 hour hold in air to normalize internal stress states. Surfaces were
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cleaned with isopropanol and grit blasted at 3.5 bar, as per typical pre-spray preparation.
materials and processing parameters specified in Table 1.
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Coatings were fabricated at the Center for Thermal Spray Research (CTSR) Stony Brook using
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Coatings were deposited uniformly on both sides of the specimens by HVOF technique
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over the substrate gauge length, while leaving the grip areas uncoated. Coating both sides ensured balanced stresses on the substrate and allowed for even loading during testing. In
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addition to fully coated specimens, one set was sprayed using overlay masking which allowed
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for a thin strip of nickel to be deposited along the longitudinal surface of the specimen. Figure 1 shows the cross-section microstructures of each of the three coatings deposited with nominally
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(>98%) dense structure, imaged via scanning electron microscope (TM3000, Hitachi, Japan).
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Uniaxial tensile tests were performed with a 100-kN load cell (Instron 2518, Norwood, MA) and monitored with an externally attached, clip-on extensometer (Instron 2620, Norwood,
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MA) with a 50 mm span (L), placed directly onto the coated region within the specimen gauge. Displacement controlled loading was performed at 0.5 mm/min cross head speed, with the tensile load applied on the uncoated ends of the specimens. Testing setup and typical result can be seen illustrated in Figure 2.
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ACCEPTED MANUSCRIPT 2.1 Image recording and analysis Simultaneously during uniaxial tensile loading, a digital camera (PixelLink, Ottawa, ON) was held approximately 150 mm from the front face of the specimen and set to record images at 1 frame/second. Image acquisition was synchronized with the start of the loading, allowing
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image reconciliation with position and load. Tensile loading resulted in elongation of the
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substrate-coating system and eventual cracking of the coating, which always occurred after
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substrate yielding. Post elongation, load-displacement values were converted to engineering stress and strain values. Cracking and specific features captured were subsequently linked to
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calculated stress and extensometer derived strain by cross-referencing with the image
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timestamps.
Applied tensile loads were actuated by mechanical grips on the uncoated ends of each
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specimen. With this loading configuration, the only load to which the coating is exposed is via
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shear forces acting on the coating-substrate interface. So long as the integrity of this mechanical bonding is upheld, the applied tensile loads will be translated to the coating. Constant strain can
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be assumed throughout both the substrate and coating, however as will be shown later, the
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breakdown of this interaction and development of coating cracks will influence the loading and observed behavior.
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2.2 Strain tracking using digital image correlation Images collected during the tensile test were processed with DIC techniques to measure specific or localized strains. Here, the primary use of the DIC was for linear strain tracking over a series of images using a virtual linear extensometer, rather than full-field strain mapping commonly found in commercial software. Reflected and scattered light from the coated surface roughness peaks provided disparate points that could be followed by an image tracking program
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ACCEPTED MANUSCRIPT (as an alternative to the more traditional paint speckling methods). Recorded images were fed into an open source MatlabTM program developed by Eberl, et al [29]. Details about program operation, control, and successful implementation in measurement of mechanical and thermal properties have been discussed in prior literature [23-28]. The operation can be broadly described
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as using an agglomerated average of specific points on the observed surface, digitally tracked
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throughout a sequential image stack. The program provides for various patterns of point
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overlays, as well as operations to manage data with respect to image orientation. Here, 30 to 90 points were tracked on the specimen (coating) surface. The region of interest (ROI) was set to 8
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mm x 5.25 mm, inside the extensometer span for the full coated samples, and 4.5 mm x 1.8 mm
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on the strip sample for more localized measurement. Crack spacing is on the order of 0.5 mm to 1 mm, so multiple cracks are found within the set ROI.
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In the cases described here, the DIC tool was used to extract strain evolution with time of
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uncoated and coated substrates. However, because the visual references on the coating surface show localized deviations (i.e. cracking) with increasing strains, the localized average linear
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distance across the cracked area will deviate from the global strain rate measured by the
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extensometer. If there is localized movement or cracking occurring within the ROI, some of the individual tracked points will experience localized increases changes in strain. When these
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movement changes are averaged across all the points, the result is a small shift or jump in the DIC strain, as compared to the extensometer strain. This discrepancy between the extensometer strain and DIC strain corresponds well with a localized movement or cracking events. Additionally, these changes locally can cause a shift in the slope of the strain versus time response. In the case of significant cracking, some of these tracked points become "lost", in that they can no longer be tracked. This can result in a more pronounced residual change to the
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ACCEPTED MANUSCRIPT average measurement, especially at higher strains, where more deviations and lost points have accumulated. Although "strain" is used as the primary nomenclature to maintain continuity, effective strain or apparent strain would be a more correct definition to define the behavior and
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measurement after cracking. Elaboration on the method and analysis process will be discussed in
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conjunction with the results of specific experiments.
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ACCEPTED MANUSCRIPT 3. Results 3.1 Overview of the tensile test Baseline testing was performed using only extensometer and load cell feedback. Converting the output load-displacement values into engineering stress versus strain relations,
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i.e. normalizing for initial cross-sectional area and displacement versus starting position, can
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give the composite strength of the coating-substrate system. Figure 3 shows both uncoated and
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nickel coated specimens, with a notable strength increase seen in the nickel coating over the uncoated steel specimen. This is similar to the measurement approach and results seen in recent
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work from Vackel, et al [13]. Figure 3a shows an increase in yield strength of the coated
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laminate, which can be as much as 30 – 50 MPa higher at the yield point than the uncoated beam. As the prior work discusses, this synergistic benefit is highly subject to material and
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deposition parameters. The given stress in all the data presented is for the overall substrate-
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coating system (substrate only in the cases of the uncoated steel). In reality, the stress within the coating will be different from that in the substrate, due to the difference in elastic moduli
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between coating and substrate. This was extensively discussed previously, where an approach
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was proposed to extrapolate the stress in the coating and the strength contribution of this load bearing component [13]. However, to maintain clarity the stress values presented here are the
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overall load measurement normalized by the cross-sectional area, directly measurable with only the tensile machine.
Figure 3b illustrates the evolution of strain with time (as a function of pull rate, 0.5 mm/min). Image acquisition is undertaken at a constant frame rate throughout test progression, with the resulting strain versus time relationship straightforward in linking image data and the coating-substrate behavior as the test progresses. Single test results are shown; however, tests
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ACCEPTED MANUSCRIPT were typically repeated three times. Nominal variation and spread in data encompasses a combination of processing variation and data acquisition noise; however, trends are consistent in repeated specimens. 3.2 Intial benchmarking with DIC technique on uncoated steel
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Benchmarking and verification of the DIC technique was initially undertaken on
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uncoated steel substrates. Shown in Figure 4 is the correlation results of two repeated uncoated
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steel substrates. Figure 4a shows the stress versus strain relationship, while 4b shows strain evolution versus time. Strain was measured on the front face of the uncoated steel using the DIC
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analysis technique and by standard clip-on extensometer. Both measurement techniques should
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yield identical strain evolutions, driven by the constant machine loading rate. In the data shown, this relationship seems reasonable, especially at lower strains. Uncertainty (for this and other
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results) grows as the test progresses, as defects or jumps in the point tracking are compounded,
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and makes point tracking more unstable. In subsequently presented results, the clip-on extensometer data will continue to be shown for validity and to ensure the image techniques are
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reporting reasonable data.
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As additional confirmation of the DIC technique, a static, no load test was performed. In this static test, 40 seconds’ worth of images were recorded of a specimen held in the load frame
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with no applied load. These were analyzed with the DIC method, with measured strain evolution presented in Figure 5. After adjusting the axis scale to match Figure 4, measurement noise from the DIC technique is shown to be negligible. The noise is consistent in both "x" and "y" and negligible in comparison with measured strain during specimen testing, and is likely due to machine vibrations or inconsistencies in specimen illumination.
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ACCEPTED MANUSCRIPT 3.3 Nickel coated substrates In the uncoated substrates shown previously, the measured DIC strain correlated well with the extensometer strain. As mentioned before however, a discrepancy or divergence between the DIC strain and extensometer strain indicates local movement or cracking. In the
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coated systems, deviation of the strains correlates with the beginning of observable cracking on
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the coated substrates. In all cases presented here the coating undergoes failure - cracking and
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eventual delamination - well before the steel substrates fail. During testing the loading rates and test times are sufficient to pass through elastic deformation and into plastic deformation of the
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steel resulting in brittle (or near brittle) failure of the coating.
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The divergence between the two strain measurements of the coated material can be seen in the case of a fully coated beam, shown in Figure 6 Figure 6a shows the stress versus strain
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relationship, while 6b shows strain evolution versus time. The timing of the divergence between
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the digitally tracked strain and extensometer strain corresponds with the visualization of cracks on the coating surface, shown in 6c. Initial cracking begins approximately 50 seconds into the
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test, and then accelerates around 60 seconds. By 75 seconds, significant cracking is observed.
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Divergence and apparent slope changes in the DIC tracked images around 50 seconds may correspond to internal cracking or disruption at the substrate-coating interface, with full
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observable cracking and divergence around 60 seconds. Specific localized strains at these regions will cause DIC variance from extensometer data. To capture both the behavior of the substrate and coating together, a partially covered strip of coating was applied to a steel specimen and observed under loading, with results shown in Figure 7. This configuration allowed for direct observation and use of the DIC technique on both the substrate and coating within a single frame. Extensometer data specific to this test is not
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ACCEPTED MANUSCRIPT shown. For these specimens, the extensometer would observe the overall beam trend and cannot differentiate between strain evolution of the coating and that of the substrate. Instead, two thin lines are shown from previously tested, fully coated substrates, representing extensometer data from other repeated tests.
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The DIC program allows for specific selection of a ROI . Images were processed twice
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with the DIC software, once on a ROI on the coating and then again on a ROI on the uncoated
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substrate. The plotted DIC results in 7a indicate two distinct regions of behavior in the coating, which are marked on the figure insert. In Region 1, the coating and substrate undergo an
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identical evolution of strain with time, indicated by their overlapping curves. The beginning of
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region 2 however, reveals the beginning of surface cracking. The DIC strain curves diverge from one another as the apparent strain changes between the coating and substrate. This remains
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relatively constant as cracks appear with consistent separation and at greater frequency (7b),
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accounting for an apparent increase in localized strain in the coating. Due to the likelihood of cracks developing internally prior to their manifestation on the surface, the jump between Region
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1 and 2 could be explained by the internal occurance of such cracking (cohesive failure) or
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delamination. Images with cracks are shown in 7c, and crack locations are marked with boxes. To verify that the coating and substrate were both subjected to the same global strain
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during elastic loading, manual digital strain measurements were made on the nickel strip specimen. Three points on both the substrate and coating strip were individually selected for monitoring, and tracked manually, during progressive loading. Average position pathways of each point are shown in the first image of Figure 8, with dashed line illustrating linear distance from the image edge. The subsequent images only show the tracked points, with distance
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ACCEPTED MANUSCRIPT measured from the edge of the frame due to relatively large overall movement that made tracking between two points difficult. The distance each point traveled over the full duration of the tensile test (275 seconds) was averaged for the coating and substrate points separately. The first image corresponds to the
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beginning of the test (0 applied strain). The middle image was taken immediately before cracks
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were detected, and the third after crack saturation and test completion. Average movement on the
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coating surface was 0.55 ± 0.08 mm compared to 0.48 ± 0.02 mm on the uncoated surface,
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showing no discernable difference within the standard deviation of the three measurements. Once cracking began, the average distance moved was 1.27 ± 0.1 mm for the coated and 1.27 ±
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0.09 mm between 80 and 275 seconds, again undifferentiable within the standard deviation. This
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indicates that overall strain rate was consistent between the coating and substrate. Increased distances measured on the coated section due to additional displacement from crack opening
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matched overall displacement measured in the substrate, indicating congruence in overall linear
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extension.
3.4 Cermet and ceramic coated substrates
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Figure 9 shows the strain evolution with detectable buildup of cracking in the WC-CoCr
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coating, and shows similar general behavior to the other tested materials. Like the nickel coated substrates, there is a divergence in the DIC data which correlates to the start of visible cracking in the related images. In the WC-CoCr coatings detectible cracking occurred at 46 seconds, corresponding to the divergence seen in the processed DIC data. Compared to the nickel coating, the cracks appear at a greater frequency in the WC-CoCr (3.6 cracks/mm versus 1.1 cracks/mm in nickel), which is likely due to the higher adhesion strength (due to peening of the large carbides) and lower fracture toughness of WC-CoCr. This results in a well bonded system, with a
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ACCEPTED MANUSCRIPT propensity for the coating to carry more load, but which undergoes cracking when the stress in the coating overcomes its fracture strength. Because of these conflicting actions, micromechanical behavior can be highly localized and may account for the difference in the direction of the DIC tracked strains, when compared to the nickel coating.
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In Figure 10 the strain evolution for the Al2O3 coating is shown. In this case, cracking in
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the coating occurs within the first 10 seconds of the test. The rapid divergence from the
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extensometer data in the highly brittle Al2O3 coating may indicate near-immediate initial cracking and even delamination from the substrate. The initial jump in this data correlates again
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to the detectable start of cracking. However, the DIC response is much noisier, potentially a
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function of the highly brittle nature and large, rapid crack development in the system.
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ACCEPTED MANUSCRIPT 4. Discussion 4.1 Strain to failure of varied coating materials Metallic and cermet coatings can lose much of their ductility due to oxidization, ultra-fine grain structures associated with rapid quenching, interfaces within and between coating splats,
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and buildup of internal stresses resulting from the deposition process [5, 15, 16]. Thus, the
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coatings tend to fail in a brittle manner in the form of cracking during elongation, a failure mode
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which has been well studied [14, 15, 30-32]. This result is captured in Figure 11 and discussed
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below.
The stress and strain plot (11a) of the different coating laminates shows the overall
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strength contribution of the coating in relation to the uncoated 1008 steel, with the nickel showing the highest yield point. At higher strains, well past coating failure, these stress values
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merge, indicating that even after cracking there is residual contribution of the remaining attached
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coating ligaments. The Al2O3 coating however, does not contribute to any strength enhancement
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and the thickness contribution to the cross-sectional area deflates the composite strength for the entirety of the test. This is likely due to insufficient bond strength between the substrate and
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coating. Unlike the nickel and WC-CoCr coating, there is no ability for the Al2O3 coating to contribute because it has no remaining connection to the underlying substrate, even while the
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stress value accounts for the additional thickness of the coating. From within the highlighted region in 11a, the stress and strain values at the onset of the initial cracking are shown in 11b and indicate an expected material response with regards to the inherent brittleness of the laminates. The nickel, which has already been shown to have the highest contributed strength, also can delay onset of cracking to higher strains. Using the data presented in Figure 11, it is feasible to establish design guidelines for coatings, potentially
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ACCEPTED MANUSCRIPT providing a framework for incorporating quantitative parameters for design considerations, such as strain to failure. Furthermore, the presented technique can be used to screen and optimize different coating types, thicknesses, and process induced variants (microstructure and stress), enabling enhanced input into structurally integrated coating design in a variety of applications.
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4.2 Delamination versus coating fracture
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Some uncertainty exists in the standard tensile technique at several different levels.
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Potential slippage of the beam in the grips and attachment of the extensometer are two reasons
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why the DIC technique can offer a more rigorous assessment of these composite coated systems. However, the materials behavior picked up in the DIC measurement is subjective to how that
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material fails. In these coated systems, there is an inherent competition between adhesive failure/delamination and cracking in the coating, which has been explored in detail elsewhere.
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This is evidenced by the differences in the direction of the DIC strain between the nickel and the
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WC-CoCr coatings. However, in both cases the deviation was discernable and cracking was
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detected.
As the material strains, some degree of shear force is applied across the interface [14, 30-
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32]. If adhesion is maintained, the coating laminate will crack and fail. However, if these shear forces overcome the adhesion strength, the coating will instead delaminate. In the case of
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delamination there will be no apparent cracking in the DIC images. However, movement and apparent changes in strain can still be measured because of the now "floating" coating layer. This introduces a potential discrepancy because these two events are likely to overlap and occur because of the statistical distribution of fracture strengths and adhesion strengths over the range of the material system. This was potentially noted in the Al2O3 coatings, where both coating
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ACCEPTED MANUSCRIPT fracture and delamination were indistinguishable, both occurring simultaneously shortly after the start of the test. This general relationship needs further examination in future studies. In general, the DIC method presents a greater ability to measure and collect repeatable data over a more traditional extensometer setup, especially for these composite systems. Offering
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high data fidelity is critical if such a technique is expanded for more general usage. Especially
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with the potential ability to operate as a non-contact measurement on varied geometries, there is
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potential for the technique to be applied in a range of coating measurement applications, both at academic and industrial levels.
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To our knowledge, the application of DIC in this context of structurally integrated
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coating systems is shown here for the first time. As such, it requires continued development of the technique and interpretation of the data. Although only three materials are shown here, many
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more have been tested, resulting in a range of supplementary data and enhanced confidence in
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the approach. Data not presented here includes variation of materials, surface roughness of substrates prior to deposition (to affect adhesion), thicknesses, and processing variants resulting
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in a range of coating mechanical properties, including residual stress. Much of the data has
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shown the potential and efficacy for the proposed approach but has suggested interlinking between coating adhesion, cracking/fracture, and residual stress dependence. Decoupling these
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influences is a target of future work including:
Supplementary use of acoustic emission for confirmatory results of crack onset
Compression testing of coated systems.
Deliberate modification of processing to affect bonding strength and character
Determination of residual stress influence on strain to failure
Evaluation of geometry effects to determine detection of strains on non-uniform coatings
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Role of coating thickness and stiffness on cracking response
5. Conclusion Understanding coupled mechanical behavior of overlay coated metals is an ongoing concern in the emerging class of structurally integrated coatings. In this paper, tensile testing of
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thermal spray coated materials in conjunction with digital image correlation was used to
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understand strain and fracture response of coatings under substrate constraint. Specifically, a DIC tool was used to observe the onset of coating cracking behavior, determine variable coating
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strain evolution, and quantifiably measure strain to failure of three discrete types of coating materials (metal, cermet, and ceramic) deposited on steel. This evaluation builds upon a
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traditional uniaxial tensile test and isolates the strain response of the coating and the substrate to
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identify cracking phenomena and critical stresses for the onset of these events. Although typical
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operation of these coating systems is well below the yield strength of the parent metal, insight gained into how coated systems can support enhanced load bearing capability and the nature of
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strain and fracture will enhance integration of such coating into structural design. The technique points to significant advantages of using DIC over conventional
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extensometer based observations. Any application where mechanical loading of the component is
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expected, but simple strain gauges or extensometers are not feasible due to geometric constrains could present an opportunity for such DIC analysis. With appropriate further development, DIC can also be used to examine thermo-mechanical loading from thermal expansion mismatch, which can contribute to coating failure in specific service. It is envisioned that the DIC technique can become a valuable tool in enhanced design integration and expand utilization of thermostructural overlay coatings in industrial machinery.
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ACCEPTED MANUSCRIPT Acknowledgements The authors would like thank the Industrial Consortium for Thermal Spray Technology at Stony Brook University in part for their support of the Center's research activities. 6. References
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[1] H. Herman, S. Sampath, R. McCune, Thermal spray: Current status and future trends, MRS Bull. 25 (2000) 17-25.
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[3] A. Vackel, G. Dwivedi, S. Sampath, Structurally Integrated, Damage-Tolerant, Thermal Spray Coatings, JOM 67 (2015) 1540-1553.
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[13] A. Vackel, T. Nakamura, S. Sampath, Mechanical Behavior of Spay-Coated Metallic Laminates, J. Therm. Spray. Technol. 25 (2016). [14] Z.B. Chen, Z.G. Wang, S.J. Zhu, Tensile fracture behavior of thermal barrier coatings on superalloy, Surf Coat Technol. 205 (2011) 3931-3938.
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[15] M. Gui, R. Eybel, B. Asselin, F. Monerie-Moulin, Cracking and Spalling Behavior of HVOF Thermally Sprayed WC-Co-Cr Coating in Bend and Axial Fatigue Tests, J. Mater. Eng. Perfor. 24 (2015) 1347-1356.
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[16] T. Varis, T. Suhonen, A. Ghabchi, A. Valarezo, S. Sampath, X. Liu, S.-P. Hannula, Formation Mechanisms, Structure, and Properties of HVOF-Sprayed WC-CoCr Coatings: An Approach Toward Process Maps, J. Therm. Spray. Technol. 23 (2014) 1009-1018.
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[17] S. Kuroda, Y. Tashiro, H. Yumoto, S. Taira, H. Fukanuma, S. Tobe, Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel, J. Therm. Spray. Technol. 10 (2001) 367-374.
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[18] A. Vackel, S. Sampath. Fatigue behavior of thermal sprayed WC-CoCr- steel systems: Role of process and deposition parameters, Surf Coat Technol. 315 (2017) 408-416.
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[19] J. Matejicek, S. Sampath. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis, Acta Mater. 51 (2003) 863-872.
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[20] S. Sampath, X.Y. Jiang, J. Matejicek, L. Prchlik, A. Kulkarni, A. Vaidya, Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats, Mater. Sci. Eng., A 364 (2004) 216-231.
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[21] J. Matějíček, S. Sampath, T. Gnäupel-Herold, H.J. Prask, Residual stress in sprayed Ni+5%Al coatings determined by neutron diffraction, Appl. Phys. A 74 (2002) s1692-s1694.
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[22] X.C. Zhang, M. Watanabe, S. Kuroda, Effects of processing conditions on the mechanical properties and deformation behaviors of plasma-sprayed thermal barrier coatings: Evaluation of residual stresses and mechanical properties of thermal barrier coatings on the basis of in situ curvature measurement under a wide range of spray parameters, Acta Mater. 61 (2013) 10371047. [23] B. Pan, K.M. Qian, H.M. Xie, A. Asundi, Two-dimensional digital image correlation for inplane displacement and strain measurement: a review, Meas. Sci. Technol. 20 (2009). [24] C. Eberl, G. D., K.J. Hemker, Mechanical Characterization of Coatings Using Microbeam Bending and Digital Image Correlation Techniques, Exp. Mech. 50 (2010) 85-97.
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ACCEPTED MANUSCRIPT [25] C. Eberl, D.S. Gianola, X. Wang, M.Y. He, A.G. Evans, K.J. Hemker, A method for in situ measurement of the elastic behavior of a columnar thermal barrier coating, Acta Mater. 59 (2011) 3612-3620. [26] A. Pandey, A. Shyam, T.R. Watkins, E. Lara-Curzio, R.J. Stafford, K.J. Hemker, The Uniaxial Tensile Response of Porous and Microcracked Ceramic Materials, J. Am. Cer. Soc. 97 (2014) 899-906.
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[27] R.J. Thompson, K.J. Hemker, Thermal Expansion Measurements on Coating Materials by Digital Image Correlation, SEM Annual Conference & Exposition on Experimental and Applied Mechanics. (2007)
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[28] M.D. Novak, F.W. Zok, High-temperature materials testing with full-field strain measurement: Experimental design and practice, Rev. Sci. Instrum. 82 (2011) 115101.
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[29] C. Eberl, R. Thompson, D. Gianola, K. Hemker. Digital Image Correlation and Tracking, MATLAB Central File Exchange (2006).
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[30] B.F. Chen, J. Hwang, G.P. Yu, J.H. Huang, In situ observation of the cracking behavior of TiN coating on 304 stainless steel subjected to tensile strain, Thin Solid Films 352 (1999) 173178.
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[31] L. Qian, S. Zhu, Y. Kagawa, T. Kubo, Tensile damage evolution behavior in plasmasprayed thermal barrier coating system, Surf Coat Technol. 173 (2003) 178-184.
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Figure Captions
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[32] M. Zhou, W.B. Yao, X.S. Yang, Z.B. Peng, K.K. Li, C.Y. Dai, W.G. Mao, Y.C. Zhou, C. Lu, "In-situ and real-time tests on the damage evolution and fracture of thermal barrier coatings under tension: A coupled acoustic emission and digital image correlation method", Surf Coat Technol. 240 (2014) 40-47.
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Table 1. List of spray processes and parameters. Figure 1. Scanning electron micrographs of the three coatings, measured at 500x magnification, showing nickel (a), WC-CoCr (b), and Al2O3 (c). Figure 2. Typical engineering stress versus strain curve from uniaxial tensile test, resolved from load cell data and extensometer attached to specimen face. Schematic shows position of camera and extensometer on coating face. The coating is shown as the darker region over the substrate gauge. 22
ACCEPTED MANUSCRIPT Figure 3. Tensile tests showing increase in composite strength of the coated nickel structure, over an uncoated specimen (a) and strain evolution versus time (b). Figure 4. DIC measured strain compared to extensometer based strain measurement using only reflected light from the roughness of the grit-blasted steel surface, for verification on a uncoated
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system. Typical stress versus strain curve with insert shows the magnified yield point (a) and the
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strain evolution versus time (b) with a digital grid overlay.
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Figure 5. Static observation of coated beam to evaluate measurement noise and drift due to vibration or image processing at a similar scale with regards to other figures. Insert is zoomed in
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to show fluctuations at magnified strains.
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Figure 6. Stress (a) and strain (b) buildup between the DIC and extensometer tracking. Cracking can be visually seen in (c), and starts to occur between 50 and 75 seconds in the coating, which
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can be back correlated to strain and stress values. The coating surface was polished to allow for
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clearer crack observation.
Figure 7. Stress (a) and strain (b) buildup on the fine strip coating. Strain buildup was measured
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using DIC on the coating strip and the uncoated beam, as shown in regions marked in insert.
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Note differences in strain and time scale between plots (a) and (b), with corresponding images with cracks shown below (c).
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Figure 8. Linear displacement distances (shown with dashed line in the first image (a)) made by manually tracking specific points, both on and off coating strip at three different time intervals, corresponding to start of loading, immediately before cracking, and after crack saturation. Displacement measurement was made between the top of the image frame and marked point, due to typically large displacements over the duration of the test.
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ACCEPTED MANUSCRIPT Figure 9. Strain evolution in WC-CoCr coating, with indicated divergence between DIC and extensometer at approximately 46 seconds. Figure 10. Strain evolution in Al2O3 coating, with indicated divergence between DIC and extensometer at approximately 8 seconds.
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Figure 11. Overlaid stress and strain curves of the three different coating systems and uncoated
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steel (a). The detected onset of cracking at specific stress and strain (b) is shown from the
Tables
Torch
Feedstock
Thickness per side
O2
Kerosene/ Hydrogen
SD
Nickel
JP-5220 (Praxair Surface Technologies, Indianapolis, IN)
Ni-914-3 (Praxair)
150 µm
940 SLPM
25.4 L/hr.
406 mm
WC10Co4Cr
JP-5220 (Praxair)
150 µm
940 SLPM
24.6 L/hr.
330 mm
Al2O3
HV-2000 (Thermach, Appleton, WI.)
150 µm
283 SLPM
708 SLPM
152 mm
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Table 1. List of spray processes and parameters.
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marked region of the stress versus strain curve.
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WOKA 3652 (Oerlikon Metco, Westbury, NY)
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Al-1110-HP (Praxair)
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Graphical abstract
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Highlights
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A digital image technique for monitoring strain in coated laminates is discussed The technique can identify the onset of surface cracking during uniaxial loading Coatings undergo brittle fracture after yielding of the steel substrates Different coating systems show variable strain to failures, critical for design
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Gregory Smith, Olivia Higgins, Sanjay Sampath PII: DOI: Reference:
S0257-8972(17)30866-6 doi: 10.1016/j.surfcoat.2017.08.057 SCT 22618
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
3 July 2017 21 August 2017 24 August 2017
Please cite this article as: Gregory Smith, Olivia Higgins, Sanjay Sampath , In-situ observation of strain and cracking in coated laminates by digital image correlation, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.08.057
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Manuscript cover page
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ACCEPTED MANUSCRIPT In-situ observation of strain and cracking in coated laminates by digital image correlation Gregory Smitha, Olivia Higginsa, and Sanjay Sampatha a
Center for Thermal Spray Research, Dept. of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794
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Abstract
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Overlay coatings are widely used in engineering components to impart a range of surface
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functionalities including thermal, wear, and corrosion protection, as well as material reclamation. In most surface engineering applications, the coating’s role is restricted to the surface, with
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limited integration with the underlying substrate. However, the situation is changing: there is an
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emerging need for so-called structurally integrated coatings, where the coating and substrate are intimately bonded, resulting in a coupled system. Of interest are the emerging applications of
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thermal spray and cold spray overlay coatings applied on loaded engineering components such as
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landing gear, heavy machinery hydraulics, and steel infrastructure. These coatings, even metals or cermets, respond in a brittle manner associated with their layered processing and ultra-fine
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grain structures resulting from rapid quenching. As such, it is of importance to understand their
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coupled mechanical response, especially stress-strain behavior and strain to fracture. In this study, an approach involving strain monitoring of coated steel via digital image
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correlation has been developed. Both elastic response and strain beyond the yield point of the system are assessed to examine load transfer between the coating and substrate and onset of cracking. Three different coating materials (Ni, WC-CoCr and Al2O3) were deposited to near full density via high velocity thermal spray. The results point to a powerful new approach for understanding mechanical behavior of heterogeneous composites using advanced imaging techniques.
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ACCEPTED MANUSCRIPT Keywords: Overlay coatings, thermal spray, digital image correlation, tensile stresses, crack propagation
1. Introduction Overlay coatings continue to gain attention as they not only provide surface enhancement
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to structural metals but also provide a cost effective and sustainable utilization of advanced
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materials [1-3]. A wide range of applications exist for overlay coatings, encompassing metallics,
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ceramics, cermets, and composite coatings for addressing wear, corrosion, thermal barrier,
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electrical insulators, and structural repair [3-6]. Processes involved in depositing overlay coatings include electroplating, thermal spraying, laser cladding, and vapor deposition. Among
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these methods, thermal spraying encompasses the greatest diversity in materials, thickness, and applications.
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In the past, coatings were added as an after-thought to provide surface functionality to a
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priori designed structural component [1, 6]. In that sense, coatings had limited impact on the
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structural attributes of the underlying structural material. Current and future coating applications require more exacting specifications including the need for prime-reliant coatings, where-in the
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performance of the coating is tied to both system functionality and component durability [3, 4]. In this emerging scenario, understanding and defining coating-substrate system response in the
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operational environment becomes critical. Characterizing physical, mechanical, and functional properties of coatings is challenging due to dimensional constraints, anisotropy, and substrate attachment. This has led to use of small volume and local characterization techniques such as nano and micro-indentation or measurements on free-standing films and coatings [7, 8]. Coating-substrate interfacial adhesion is assessed through indirect techniques such as bond-pull or bend testing [9-12]. Although these
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ACCEPTED MANUSCRIPT techniques have been very useful, they often do not provide full field description of the properties, especially when the substrate is constrained and adhesion is considered. Uniaxial tensile testing of spray coated laminates has been used successfully to extract mechanical system properties of laminate structures, and is an important indicator of coating adhesion, strength and
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compliance, as shown in Vackel, et al. and other ongoing work [13]. This methodology is useful
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in ascertaining critical system properties; however, it has focused predominately on composite
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strength, adhesion, and failure mechanisms. Strain evaluation is more limited in this context. The strain and fracture response of overlay coatings are comparable to the bulk behavior
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of hard and brittle materials (cermets and ceramics), with even metallic overlay coatings
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displaying brittle responses due to process induced defects and residual stresses [14-17]. Measurement of residual stress can be readily performed on these coated systems and has been
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shown to have significant impact on mechanical and fatigue behavior in these systems, however
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also introduces additional complexities [18-22]. The paucity of such information in the literature is in part due to difficulty in characterization of strains and fracture initiation in these constrained
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systems. This paper seeks to exploit digital image correlation (DIC) techniques to characterize
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strain and cracking in coated metal systems [23-28]. The goal here is to understand not only the response of the coating but to also couple coating and substrate deformation.
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DIC strain measurements were conducted on spray coated steel substrates subjected to uniaxial tensile testing. The principle materials for the study were formed by supersonic combustion thermal spray technique (referred to as high velocity oxy-fuel or HVOF), yielding near full density coatings with high adhesion strength. Within these structures, defects such as oxides and interfaces were present, along with nano-crystalline grains associated with rapid quenching. As such even these metallic coatings showed little or no ductility. Coating failure (via
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ACCEPTED MANUSCRIPT cracking) in these systems can be detected with the DIC technique, and presents a methodology to better understand strain to failure in these systems, as well as provide a basis for a technique that can be suitable for a range of component geometries that are inaccessible with traditional
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extensometers.
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ACCEPTED MANUSCRIPT 2. Material and methods Tensile testing with an Instron uniaxial tensile machine (Instron, Norwood, MA) was used to evaluate 1008 low carbon steel, "dog-bone" shaped specimens (150 mm x 25 mm x 3 mm, with approx. 65 mm x 12.5 mm gauge). As-received specimens underwent a stress relief
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treatment at 450o C with a 1 hour hold in air to normalize internal stress states. Surfaces were
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cleaned with isopropanol and grit blasted at 3.5 bar, as per typical pre-spray preparation.
materials and processing parameters specified in Table 1.
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Coatings were fabricated at the Center for Thermal Spray Research (CTSR) Stony Brook using
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Coatings were deposited uniformly on both sides of the specimens by HVOF technique
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over the substrate gauge length, while leaving the grip areas uncoated. Coating both sides ensured balanced stresses on the substrate and allowed for even loading during testing. In
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addition to fully coated specimens, one set was sprayed using overlay masking which allowed
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for a thin strip of nickel to be deposited along the longitudinal surface of the specimen. Figure 1 shows the cross-section microstructures of each of the three coatings deposited with nominally
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(>98%) dense structure, imaged via scanning electron microscope (TM3000, Hitachi, Japan).
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Uniaxial tensile tests were performed with a 100-kN load cell (Instron 2518, Norwood, MA) and monitored with an externally attached, clip-on extensometer (Instron 2620, Norwood,
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MA) with a 50 mm span (L), placed directly onto the coated region within the specimen gauge. Displacement controlled loading was performed at 0.5 mm/min cross head speed, with the tensile load applied on the uncoated ends of the specimens. Testing setup and typical result can be seen illustrated in Figure 2.
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ACCEPTED MANUSCRIPT 2.1 Image recording and analysis Simultaneously during uniaxial tensile loading, a digital camera (PixelLink, Ottawa, ON) was held approximately 150 mm from the front face of the specimen and set to record images at 1 frame/second. Image acquisition was synchronized with the start of the loading, allowing
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image reconciliation with position and load. Tensile loading resulted in elongation of the
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substrate-coating system and eventual cracking of the coating, which always occurred after
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substrate yielding. Post elongation, load-displacement values were converted to engineering stress and strain values. Cracking and specific features captured were subsequently linked to
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calculated stress and extensometer derived strain by cross-referencing with the image
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timestamps.
Applied tensile loads were actuated by mechanical grips on the uncoated ends of each
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specimen. With this loading configuration, the only load to which the coating is exposed is via
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shear forces acting on the coating-substrate interface. So long as the integrity of this mechanical bonding is upheld, the applied tensile loads will be translated to the coating. Constant strain can
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be assumed throughout both the substrate and coating, however as will be shown later, the
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breakdown of this interaction and development of coating cracks will influence the loading and observed behavior.
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2.2 Strain tracking using digital image correlation Images collected during the tensile test were processed with DIC techniques to measure specific or localized strains. Here, the primary use of the DIC was for linear strain tracking over a series of images using a virtual linear extensometer, rather than full-field strain mapping commonly found in commercial software. Reflected and scattered light from the coated surface roughness peaks provided disparate points that could be followed by an image tracking program
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ACCEPTED MANUSCRIPT (as an alternative to the more traditional paint speckling methods). Recorded images were fed into an open source MatlabTM program developed by Eberl, et al [29]. Details about program operation, control, and successful implementation in measurement of mechanical and thermal properties have been discussed in prior literature [23-28]. The operation can be broadly described
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as using an agglomerated average of specific points on the observed surface, digitally tracked
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throughout a sequential image stack. The program provides for various patterns of point
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overlays, as well as operations to manage data with respect to image orientation. Here, 30 to 90 points were tracked on the specimen (coating) surface. The region of interest (ROI) was set to 8
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mm x 5.25 mm, inside the extensometer span for the full coated samples, and 4.5 mm x 1.8 mm
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on the strip sample for more localized measurement. Crack spacing is on the order of 0.5 mm to 1 mm, so multiple cracks are found within the set ROI.
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In the cases described here, the DIC tool was used to extract strain evolution with time of
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uncoated and coated substrates. However, because the visual references on the coating surface show localized deviations (i.e. cracking) with increasing strains, the localized average linear
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distance across the cracked area will deviate from the global strain rate measured by the
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extensometer. If there is localized movement or cracking occurring within the ROI, some of the individual tracked points will experience localized increases changes in strain. When these
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movement changes are averaged across all the points, the result is a small shift or jump in the DIC strain, as compared to the extensometer strain. This discrepancy between the extensometer strain and DIC strain corresponds well with a localized movement or cracking events. Additionally, these changes locally can cause a shift in the slope of the strain versus time response. In the case of significant cracking, some of these tracked points become "lost", in that they can no longer be tracked. This can result in a more pronounced residual change to the
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ACCEPTED MANUSCRIPT average measurement, especially at higher strains, where more deviations and lost points have accumulated. Although "strain" is used as the primary nomenclature to maintain continuity, effective strain or apparent strain would be a more correct definition to define the behavior and
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measurement after cracking. Elaboration on the method and analysis process will be discussed in
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conjunction with the results of specific experiments.
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ACCEPTED MANUSCRIPT 3. Results 3.1 Overview of the tensile test Baseline testing was performed using only extensometer and load cell feedback. Converting the output load-displacement values into engineering stress versus strain relations,
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i.e. normalizing for initial cross-sectional area and displacement versus starting position, can
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give the composite strength of the coating-substrate system. Figure 3 shows both uncoated and
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nickel coated specimens, with a notable strength increase seen in the nickel coating over the uncoated steel specimen. This is similar to the measurement approach and results seen in recent
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work from Vackel, et al [13]. Figure 3a shows an increase in yield strength of the coated
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laminate, which can be as much as 30 – 50 MPa higher at the yield point than the uncoated beam. As the prior work discusses, this synergistic benefit is highly subject to material and
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deposition parameters. The given stress in all the data presented is for the overall substrate-
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coating system (substrate only in the cases of the uncoated steel). In reality, the stress within the coating will be different from that in the substrate, due to the difference in elastic moduli
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between coating and substrate. This was extensively discussed previously, where an approach
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was proposed to extrapolate the stress in the coating and the strength contribution of this load bearing component [13]. However, to maintain clarity the stress values presented here are the
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overall load measurement normalized by the cross-sectional area, directly measurable with only the tensile machine.
Figure 3b illustrates the evolution of strain with time (as a function of pull rate, 0.5 mm/min). Image acquisition is undertaken at a constant frame rate throughout test progression, with the resulting strain versus time relationship straightforward in linking image data and the coating-substrate behavior as the test progresses. Single test results are shown; however, tests
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ACCEPTED MANUSCRIPT were typically repeated three times. Nominal variation and spread in data encompasses a combination of processing variation and data acquisition noise; however, trends are consistent in repeated specimens. 3.2 Intial benchmarking with DIC technique on uncoated steel
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Benchmarking and verification of the DIC technique was initially undertaken on
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uncoated steel substrates. Shown in Figure 4 is the correlation results of two repeated uncoated
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steel substrates. Figure 4a shows the stress versus strain relationship, while 4b shows strain evolution versus time. Strain was measured on the front face of the uncoated steel using the DIC
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analysis technique and by standard clip-on extensometer. Both measurement techniques should
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yield identical strain evolutions, driven by the constant machine loading rate. In the data shown, this relationship seems reasonable, especially at lower strains. Uncertainty (for this and other
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results) grows as the test progresses, as defects or jumps in the point tracking are compounded,
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and makes point tracking more unstable. In subsequently presented results, the clip-on extensometer data will continue to be shown for validity and to ensure the image techniques are
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reporting reasonable data.
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As additional confirmation of the DIC technique, a static, no load test was performed. In this static test, 40 seconds’ worth of images were recorded of a specimen held in the load frame
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with no applied load. These were analyzed with the DIC method, with measured strain evolution presented in Figure 5. After adjusting the axis scale to match Figure 4, measurement noise from the DIC technique is shown to be negligible. The noise is consistent in both "x" and "y" and negligible in comparison with measured strain during specimen testing, and is likely due to machine vibrations or inconsistencies in specimen illumination.
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ACCEPTED MANUSCRIPT 3.3 Nickel coated substrates In the uncoated substrates shown previously, the measured DIC strain correlated well with the extensometer strain. As mentioned before however, a discrepancy or divergence between the DIC strain and extensometer strain indicates local movement or cracking. In the
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coated systems, deviation of the strains correlates with the beginning of observable cracking on
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the coated substrates. In all cases presented here the coating undergoes failure - cracking and
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eventual delamination - well before the steel substrates fail. During testing the loading rates and test times are sufficient to pass through elastic deformation and into plastic deformation of the
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steel resulting in brittle (or near brittle) failure of the coating.
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The divergence between the two strain measurements of the coated material can be seen in the case of a fully coated beam, shown in Figure 6 Figure 6a shows the stress versus strain
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relationship, while 6b shows strain evolution versus time. The timing of the divergence between
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the digitally tracked strain and extensometer strain corresponds with the visualization of cracks on the coating surface, shown in 6c. Initial cracking begins approximately 50 seconds into the
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test, and then accelerates around 60 seconds. By 75 seconds, significant cracking is observed.
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Divergence and apparent slope changes in the DIC tracked images around 50 seconds may correspond to internal cracking or disruption at the substrate-coating interface, with full
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observable cracking and divergence around 60 seconds. Specific localized strains at these regions will cause DIC variance from extensometer data. To capture both the behavior of the substrate and coating together, a partially covered strip of coating was applied to a steel specimen and observed under loading, with results shown in Figure 7. This configuration allowed for direct observation and use of the DIC technique on both the substrate and coating within a single frame. Extensometer data specific to this test is not
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ACCEPTED MANUSCRIPT shown. For these specimens, the extensometer would observe the overall beam trend and cannot differentiate between strain evolution of the coating and that of the substrate. Instead, two thin lines are shown from previously tested, fully coated substrates, representing extensometer data from other repeated tests.
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The DIC program allows for specific selection of a ROI . Images were processed twice
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with the DIC software, once on a ROI on the coating and then again on a ROI on the uncoated
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substrate. The plotted DIC results in 7a indicate two distinct regions of behavior in the coating, which are marked on the figure insert. In Region 1, the coating and substrate undergo an
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identical evolution of strain with time, indicated by their overlapping curves. The beginning of
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region 2 however, reveals the beginning of surface cracking. The DIC strain curves diverge from one another as the apparent strain changes between the coating and substrate. This remains
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relatively constant as cracks appear with consistent separation and at greater frequency (7b),
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accounting for an apparent increase in localized strain in the coating. Due to the likelihood of cracks developing internally prior to their manifestation on the surface, the jump between Region
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1 and 2 could be explained by the internal occurance of such cracking (cohesive failure) or
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delamination. Images with cracks are shown in 7c, and crack locations are marked with boxes. To verify that the coating and substrate were both subjected to the same global strain
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during elastic loading, manual digital strain measurements were made on the nickel strip specimen. Three points on both the substrate and coating strip were individually selected for monitoring, and tracked manually, during progressive loading. Average position pathways of each point are shown in the first image of Figure 8, with dashed line illustrating linear distance from the image edge. The subsequent images only show the tracked points, with distance
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ACCEPTED MANUSCRIPT measured from the edge of the frame due to relatively large overall movement that made tracking between two points difficult. The distance each point traveled over the full duration of the tensile test (275 seconds) was averaged for the coating and substrate points separately. The first image corresponds to the
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beginning of the test (0 applied strain). The middle image was taken immediately before cracks
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were detected, and the third after crack saturation and test completion. Average movement on the
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coating surface was 0.55 ± 0.08 mm compared to 0.48 ± 0.02 mm on the uncoated surface,
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showing no discernable difference within the standard deviation of the three measurements. Once cracking began, the average distance moved was 1.27 ± 0.1 mm for the coated and 1.27 ±
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0.09 mm between 80 and 275 seconds, again undifferentiable within the standard deviation. This
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indicates that overall strain rate was consistent between the coating and substrate. Increased distances measured on the coated section due to additional displacement from crack opening
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matched overall displacement measured in the substrate, indicating congruence in overall linear
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extension.
3.4 Cermet and ceramic coated substrates
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Figure 9 shows the strain evolution with detectable buildup of cracking in the WC-CoCr
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coating, and shows similar general behavior to the other tested materials. Like the nickel coated substrates, there is a divergence in the DIC data which correlates to the start of visible cracking in the related images. In the WC-CoCr coatings detectible cracking occurred at 46 seconds, corresponding to the divergence seen in the processed DIC data. Compared to the nickel coating, the cracks appear at a greater frequency in the WC-CoCr (3.6 cracks/mm versus 1.1 cracks/mm in nickel), which is likely due to the higher adhesion strength (due to peening of the large carbides) and lower fracture toughness of WC-CoCr. This results in a well bonded system, with a
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ACCEPTED MANUSCRIPT propensity for the coating to carry more load, but which undergoes cracking when the stress in the coating overcomes its fracture strength. Because of these conflicting actions, micromechanical behavior can be highly localized and may account for the difference in the direction of the DIC tracked strains, when compared to the nickel coating.
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In Figure 10 the strain evolution for the Al2O3 coating is shown. In this case, cracking in
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the coating occurs within the first 10 seconds of the test. The rapid divergence from the
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extensometer data in the highly brittle Al2O3 coating may indicate near-immediate initial cracking and even delamination from the substrate. The initial jump in this data correlates again
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to the detectable start of cracking. However, the DIC response is much noisier, potentially a
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function of the highly brittle nature and large, rapid crack development in the system.
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ACCEPTED MANUSCRIPT 4. Discussion 4.1 Strain to failure of varied coating materials Metallic and cermet coatings can lose much of their ductility due to oxidization, ultra-fine grain structures associated with rapid quenching, interfaces within and between coating splats,
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and buildup of internal stresses resulting from the deposition process [5, 15, 16]. Thus, the
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coatings tend to fail in a brittle manner in the form of cracking during elongation, a failure mode
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which has been well studied [14, 15, 30-32]. This result is captured in Figure 11 and discussed
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below.
The stress and strain plot (11a) of the different coating laminates shows the overall
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strength contribution of the coating in relation to the uncoated 1008 steel, with the nickel showing the highest yield point. At higher strains, well past coating failure, these stress values
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merge, indicating that even after cracking there is residual contribution of the remaining attached
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coating ligaments. The Al2O3 coating however, does not contribute to any strength enhancement
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and the thickness contribution to the cross-sectional area deflates the composite strength for the entirety of the test. This is likely due to insufficient bond strength between the substrate and
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coating. Unlike the nickel and WC-CoCr coating, there is no ability for the Al2O3 coating to contribute because it has no remaining connection to the underlying substrate, even while the
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stress value accounts for the additional thickness of the coating. From within the highlighted region in 11a, the stress and strain values at the onset of the initial cracking are shown in 11b and indicate an expected material response with regards to the inherent brittleness of the laminates. The nickel, which has already been shown to have the highest contributed strength, also can delay onset of cracking to higher strains. Using the data presented in Figure 11, it is feasible to establish design guidelines for coatings, potentially
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ACCEPTED MANUSCRIPT providing a framework for incorporating quantitative parameters for design considerations, such as strain to failure. Furthermore, the presented technique can be used to screen and optimize different coating types, thicknesses, and process induced variants (microstructure and stress), enabling enhanced input into structurally integrated coating design in a variety of applications.
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4.2 Delamination versus coating fracture
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Some uncertainty exists in the standard tensile technique at several different levels.
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Potential slippage of the beam in the grips and attachment of the extensometer are two reasons
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why the DIC technique can offer a more rigorous assessment of these composite coated systems. However, the materials behavior picked up in the DIC measurement is subjective to how that
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material fails. In these coated systems, there is an inherent competition between adhesive failure/delamination and cracking in the coating, which has been explored in detail elsewhere.
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This is evidenced by the differences in the direction of the DIC strain between the nickel and the
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WC-CoCr coatings. However, in both cases the deviation was discernable and cracking was
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As the material strains, some degree of shear force is applied across the interface [14, 30-
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32]. If adhesion is maintained, the coating laminate will crack and fail. However, if these shear forces overcome the adhesion strength, the coating will instead delaminate. In the case of
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delamination there will be no apparent cracking in the DIC images. However, movement and apparent changes in strain can still be measured because of the now "floating" coating layer. This introduces a potential discrepancy because these two events are likely to overlap and occur because of the statistical distribution of fracture strengths and adhesion strengths over the range of the material system. This was potentially noted in the Al2O3 coatings, where both coating
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ACCEPTED MANUSCRIPT fracture and delamination were indistinguishable, both occurring simultaneously shortly after the start of the test. This general relationship needs further examination in future studies. In general, the DIC method presents a greater ability to measure and collect repeatable data over a more traditional extensometer setup, especially for these composite systems. Offering
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high data fidelity is critical if such a technique is expanded for more general usage. Especially
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with the potential ability to operate as a non-contact measurement on varied geometries, there is
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potential for the technique to be applied in a range of coating measurement applications, both at academic and industrial levels.
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To our knowledge, the application of DIC in this context of structurally integrated
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coating systems is shown here for the first time. As such, it requires continued development of the technique and interpretation of the data. Although only three materials are shown here, many
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more have been tested, resulting in a range of supplementary data and enhanced confidence in
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the approach. Data not presented here includes variation of materials, surface roughness of substrates prior to deposition (to affect adhesion), thicknesses, and processing variants resulting
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in a range of coating mechanical properties, including residual stress. Much of the data has
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shown the potential and efficacy for the proposed approach but has suggested interlinking between coating adhesion, cracking/fracture, and residual stress dependence. Decoupling these
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influences is a target of future work including:
Supplementary use of acoustic emission for confirmatory results of crack onset
Compression testing of coated systems.
Deliberate modification of processing to affect bonding strength and character
Determination of residual stress influence on strain to failure
Evaluation of geometry effects to determine detection of strains on non-uniform coatings
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Role of coating thickness and stiffness on cracking response
5. Conclusion Understanding coupled mechanical behavior of overlay coated metals is an ongoing concern in the emerging class of structurally integrated coatings. In this paper, tensile testing of
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thermal spray coated materials in conjunction with digital image correlation was used to
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understand strain and fracture response of coatings under substrate constraint. Specifically, a DIC tool was used to observe the onset of coating cracking behavior, determine variable coating
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strain evolution, and quantifiably measure strain to failure of three discrete types of coating materials (metal, cermet, and ceramic) deposited on steel. This evaluation builds upon a
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traditional uniaxial tensile test and isolates the strain response of the coating and the substrate to
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identify cracking phenomena and critical stresses for the onset of these events. Although typical
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operation of these coating systems is well below the yield strength of the parent metal, insight gained into how coated systems can support enhanced load bearing capability and the nature of
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strain and fracture will enhance integration of such coating into structural design. The technique points to significant advantages of using DIC over conventional
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extensometer based observations. Any application where mechanical loading of the component is
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expected, but simple strain gauges or extensometers are not feasible due to geometric constrains could present an opportunity for such DIC analysis. With appropriate further development, DIC can also be used to examine thermo-mechanical loading from thermal expansion mismatch, which can contribute to coating failure in specific service. It is envisioned that the DIC technique can become a valuable tool in enhanced design integration and expand utilization of thermostructural overlay coatings in industrial machinery.
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ACCEPTED MANUSCRIPT Acknowledgements The authors would like thank the Industrial Consortium for Thermal Spray Technology at Stony Brook University in part for their support of the Center's research activities. 6. References
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[1] H. Herman, S. Sampath, R. McCune, Thermal spray: Current status and future trends, MRS Bull. 25 (2000) 17-25.
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[2] S. Sampath, G. Dwivedi, A. Valarezo, B. Choi, Partnership for accelerated insertion of new technology: case study for thermal spray technology, IMMI. 2 (2013) 1.
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[3] A. Vackel, G. Dwivedi, S. Sampath, Structurally Integrated, Damage-Tolerant, Thermal Spray Coatings, JOM 67 (2015) 1540-1553.
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[4] M.B. Beardsley, J.L. Sebright, Structurally Integrated Coatings for Wear and Corrosion. Caterpillar Inc., Peoria, IL, Report No. DOE/GO14037 (2008) 1-129.
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[5] A. Valarezo, G. Bolelli, W.B. Choi, S. Sampath, V. Cannillo, L. Lusvarghi, R. Rosa, Damage tolerant functionally graded WC-Co/Stainless Steel HVOF coatings, Surf Coat Technol. 205 (2010) 2197-2208.
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[7] S.-H. Leigh, C.-K. Lin, C.C. Berndt, Elastic Response of Thermal Spray Deposits under Indentation Tests, J. Am. Cer. Soc. 80 (1997) 2093-2099.
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[8] W.B. Choi, L. Li, V. Luzin, R. Neiser, T. Gnaupel-Herold, H.J. Prask, S. Sampath, A. Gouldstone, Integrated characterization of cold sprayed aluminum coatings, Acta Mater. 55 (2007) 857-866. [9] C.K. Lin, C.C. Berndt, Measurement and analysis of adhesion strength for thermally sprayed coatings, J. Therm. Spray. Technol. 3 (1994) 75-104. [10] C.C. Berndt, C.K. Lin, Measurement of adhesion for thermally sprayed materials, J. Adhes. Sci. Technol. 7 (1993) 1235-1264. [11] H.-J. Kim, Y.-G. Kweon, Elastic modulus of plasma-sprayed coatings determined by indentation and bend tests, Thin Solid Films 342 (1999) 201-206. [12] G. Marot, J. Lesage, P. Démarécaux, M. Hadad, S. Siegmann, M.H. Staia, Interfacial indentation and shear tests to determine the adhesion of thermal spray coatings, Surf Coat Technol. 201 (2006) 2080-2085. 20
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[13] A. Vackel, T. Nakamura, S. Sampath, Mechanical Behavior of Spay-Coated Metallic Laminates, J. Therm. Spray. Technol. 25 (2016). [14] Z.B. Chen, Z.G. Wang, S.J. Zhu, Tensile fracture behavior of thermal barrier coatings on superalloy, Surf Coat Technol. 205 (2011) 3931-3938.
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[15] M. Gui, R. Eybel, B. Asselin, F. Monerie-Moulin, Cracking and Spalling Behavior of HVOF Thermally Sprayed WC-Co-Cr Coating in Bend and Axial Fatigue Tests, J. Mater. Eng. Perfor. 24 (2015) 1347-1356.
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[16] T. Varis, T. Suhonen, A. Ghabchi, A. Valarezo, S. Sampath, X. Liu, S.-P. Hannula, Formation Mechanisms, Structure, and Properties of HVOF-Sprayed WC-CoCr Coatings: An Approach Toward Process Maps, J. Therm. Spray. Technol. 23 (2014) 1009-1018.
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[17] S. Kuroda, Y. Tashiro, H. Yumoto, S. Taira, H. Fukanuma, S. Tobe, Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel, J. Therm. Spray. Technol. 10 (2001) 367-374.
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[18] A. Vackel, S. Sampath. Fatigue behavior of thermal sprayed WC-CoCr- steel systems: Role of process and deposition parameters, Surf Coat Technol. 315 (2017) 408-416.
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[19] J. Matejicek, S. Sampath. In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings: Part 1: apparatus and analysis, Acta Mater. 51 (2003) 863-872.
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[20] S. Sampath, X.Y. Jiang, J. Matejicek, L. Prchlik, A. Kulkarni, A. Vaidya, Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats, Mater. Sci. Eng., A 364 (2004) 216-231.
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[21] J. Matějíček, S. Sampath, T. Gnäupel-Herold, H.J. Prask, Residual stress in sprayed Ni+5%Al coatings determined by neutron diffraction, Appl. Phys. A 74 (2002) s1692-s1694.
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[22] X.C. Zhang, M. Watanabe, S. Kuroda, Effects of processing conditions on the mechanical properties and deformation behaviors of plasma-sprayed thermal barrier coatings: Evaluation of residual stresses and mechanical properties of thermal barrier coatings on the basis of in situ curvature measurement under a wide range of spray parameters, Acta Mater. 61 (2013) 10371047. [23] B. Pan, K.M. Qian, H.M. Xie, A. Asundi, Two-dimensional digital image correlation for inplane displacement and strain measurement: a review, Meas. Sci. Technol. 20 (2009). [24] C. Eberl, G. D., K.J. Hemker, Mechanical Characterization of Coatings Using Microbeam Bending and Digital Image Correlation Techniques, Exp. Mech. 50 (2010) 85-97.
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ACCEPTED MANUSCRIPT [25] C. Eberl, D.S. Gianola, X. Wang, M.Y. He, A.G. Evans, K.J. Hemker, A method for in situ measurement of the elastic behavior of a columnar thermal barrier coating, Acta Mater. 59 (2011) 3612-3620. [26] A. Pandey, A. Shyam, T.R. Watkins, E. Lara-Curzio, R.J. Stafford, K.J. Hemker, The Uniaxial Tensile Response of Porous and Microcracked Ceramic Materials, J. Am. Cer. Soc. 97 (2014) 899-906.
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[27] R.J. Thompson, K.J. Hemker, Thermal Expansion Measurements on Coating Materials by Digital Image Correlation, SEM Annual Conference & Exposition on Experimental and Applied Mechanics. (2007)
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[28] M.D. Novak, F.W. Zok, High-temperature materials testing with full-field strain measurement: Experimental design and practice, Rev. Sci. Instrum. 82 (2011) 115101.
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[29] C. Eberl, R. Thompson, D. Gianola, K. Hemker. Digital Image Correlation and Tracking, MATLAB Central File Exchange (2006).
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[30] B.F. Chen, J. Hwang, G.P. Yu, J.H. Huang, In situ observation of the cracking behavior of TiN coating on 304 stainless steel subjected to tensile strain, Thin Solid Films 352 (1999) 173178.
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[31] L. Qian, S. Zhu, Y. Kagawa, T. Kubo, Tensile damage evolution behavior in plasmasprayed thermal barrier coating system, Surf Coat Technol. 173 (2003) 178-184.
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Figure Captions
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[32] M. Zhou, W.B. Yao, X.S. Yang, Z.B. Peng, K.K. Li, C.Y. Dai, W.G. Mao, Y.C. Zhou, C. Lu, "In-situ and real-time tests on the damage evolution and fracture of thermal barrier coatings under tension: A coupled acoustic emission and digital image correlation method", Surf Coat Technol. 240 (2014) 40-47.
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Table 1. List of spray processes and parameters. Figure 1. Scanning electron micrographs of the three coatings, measured at 500x magnification, showing nickel (a), WC-CoCr (b), and Al2O3 (c). Figure 2. Typical engineering stress versus strain curve from uniaxial tensile test, resolved from load cell data and extensometer attached to specimen face. Schematic shows position of camera and extensometer on coating face. The coating is shown as the darker region over the substrate gauge. 22
ACCEPTED MANUSCRIPT Figure 3. Tensile tests showing increase in composite strength of the coated nickel structure, over an uncoated specimen (a) and strain evolution versus time (b). Figure 4. DIC measured strain compared to extensometer based strain measurement using only reflected light from the roughness of the grit-blasted steel surface, for verification on a uncoated
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system. Typical stress versus strain curve with insert shows the magnified yield point (a) and the
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strain evolution versus time (b) with a digital grid overlay.
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Figure 5. Static observation of coated beam to evaluate measurement noise and drift due to vibration or image processing at a similar scale with regards to other figures. Insert is zoomed in
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to show fluctuations at magnified strains.
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Figure 6. Stress (a) and strain (b) buildup between the DIC and extensometer tracking. Cracking can be visually seen in (c), and starts to occur between 50 and 75 seconds in the coating, which
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can be back correlated to strain and stress values. The coating surface was polished to allow for
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clearer crack observation.
Figure 7. Stress (a) and strain (b) buildup on the fine strip coating. Strain buildup was measured
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using DIC on the coating strip and the uncoated beam, as shown in regions marked in insert.
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Note differences in strain and time scale between plots (a) and (b), with corresponding images with cracks shown below (c).
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Figure 8. Linear displacement distances (shown with dashed line in the first image (a)) made by manually tracking specific points, both on and off coating strip at three different time intervals, corresponding to start of loading, immediately before cracking, and after crack saturation. Displacement measurement was made between the top of the image frame and marked point, due to typically large displacements over the duration of the test.
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ACCEPTED MANUSCRIPT Figure 9. Strain evolution in WC-CoCr coating, with indicated divergence between DIC and extensometer at approximately 46 seconds. Figure 10. Strain evolution in Al2O3 coating, with indicated divergence between DIC and extensometer at approximately 8 seconds.
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Figure 11. Overlaid stress and strain curves of the three different coating systems and uncoated
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steel (a). The detected onset of cracking at specific stress and strain (b) is shown from the
Tables
Torch
Feedstock
Thickness per side
O2
Kerosene/ Hydrogen
SD
Nickel
JP-5220 (Praxair Surface Technologies, Indianapolis, IN)
Ni-914-3 (Praxair)
150 µm
940 SLPM
25.4 L/hr.
406 mm
WC10Co4Cr
JP-5220 (Praxair)
150 µm
940 SLPM
24.6 L/hr.
330 mm
Al2O3
HV-2000 (Thermach, Appleton, WI.)
150 µm
283 SLPM
708 SLPM
152 mm
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Table 1. List of spray processes and parameters.
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marked region of the stress versus strain curve.
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WOKA 3652 (Oerlikon Metco, Westbury, NY)
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Al-1110-HP (Praxair)
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Highlights
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A digital image technique for monitoring strain in coated laminates is discussed The technique can identify the onset of surface cracking during uniaxial loading Coatings undergo brittle fracture after yielding of the steel substrates Different coating systems show variable strain to failures, critical for design
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