Cold Spray Additive Manufacturing Research Articles

A meshless computational framework for studying cold spray additive manufacturing including large numbers of powder particles with diverse characteristics

Cold spray (CS) has emerged as an appealing additive manufacturing (AM) technique for producing or repairing individual components or entire structures. Compared to fusion-based AM technologies, cold spray additive manufacturing (CSAM) offers distinct advantages in the fabrication of components, while avoiding some melting/solidification-related issues such as phase transformation and oxidation. It involves intricate processes that pose significant challenges for numerical modeling, particularly when simulating the entire process at a large scale. The smoothed particle hydrodynamics (SPH) method is highly suitable for handling large material deformations due to its Lagrangian and meshless nature. In this work, we develop an enhanced SPH method to conduct large-scale simulations of CSAM with different powder sizes, morphologies, and distributions. A modified material model has been incorporated to accurately capture the strain-rate hardening effects during the plastic stage. The computational scale is greatly improved by using a Message Passing Interface (MPI) based framework, enabling the simulation of approximately ten million SPH particles. To the authors’ knowledge, this study marks the first attempt to numerically reproduce the entire process of CSAM with real powder sizes and distributions. Experimental data measured for a wide range of powder velocities are used to validate the simulation results and assess the prediction accuracy. Subsequently, we comparatively study the bonding mechanisms of powders with the same or different sizes, while also identifying a four-stage coating process. The effects of powder morphology on the bonding process are thoroughly investigated. A large-scale CSAM process is finally reproduced to demonstrate the capability of the present meshless scheme, and mechanisms of pore formation are analyzed, providing valuable insights for practical engineering applications.

Mechanical and microstructural behavior of Inconel 625 deposits by high-pressure cold spray and laser assisted cold spray

Thick deposits of Inconel 625 (IN625) by high-pressure cold spray (HPCS) have been described as cold spray additive manufacturing (CSAM). However, given the solid solution and numerous precipitation phases of the IN625 superalloy, these deposits can exhibit low ductility and high residual stresses due to severe deformation of the particles upon impact against the substrate. To counteract this phenomenon, laser-assisted cold spray (LACS) was used here to soften the material using two laser surface temperatures (650 and 900 °C), and to elucidate their effect on the microstructure and mechanical properties, specifically elastic modulus, hardness, and stress relaxation behavior of the deposits. For comparison, IN625 was also deposited by HPCS. The microstructure of the deposits was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties were determined by nanoindentation. The HPCS and LACS 650 °C deposits showed an interdendritic network of intermetallics, a feature also present in the powder feedstock material. However, the deposit produced with LACS at 900 °C showed a microstructure suggesting a dissolution of this network, a critical surface laser temperature for inducing an evolution on the microstructure in LACS processes. The EBSD maps show strong grain deformation in the material deposited using HPCS, whereas the sample made with LACS at 900 °C showed significant recrystallization and grain growth (3.5 μm in average). Mechanical properties, including stress relaxation phenomena, were measured using nanoindentation. The hardness of the LACS 900 °C sample was found to be 7.2 GPa, which is a decrease from the 8.9 GPa found in the HPCS sample and 9.1 GPa found in the LACS 650 sample. This decrease in hardness can be attributed to the recrystallized microstructure due to the localized laser heating. In contrast, the elastic modulus remained practically the same for all three samples, around 265 GPa.

Strain rate-dependent behavior of cold-sprayed additively manufactured Al–Al[formula omitted]O[formula omitted] composites: Micromechanical modeling and experimentation

Metal matrix composites (MMCs) fabricated by cold spray additive manufacturing (CSAM) are increasingly gaining attention as structural materials due to their rapid production and scalability. Herein, the failure behavior of CSAM Al–Al2O3 composites under quasi-static and dynamic compression was studied by an experimentally informed/validated 3D microstructure-based finite element (FE) model. The debonding mechanism was found to grow at a higher rate consequently dampening the particle cracking mechanism when the strain rate rises to dynamic regimes. The stress-bearing capacity of the particles plays a key role in enhancing the flow stress and elongation at failure of the CSAM composite under high strain rates due to the lower propensity of particle cracking. Eventually, the model was exercised to study the microscale failure progression in the material under elevated temperatures. For the first time in the literature, this study informs on the correlation between the microscale failure mechanisms and the mechanical performance of CSAM MMCs at the macro scale across strain rates and temperatures whose outcomes are applicable to the design of next-generation materials with a tailored performance.

Cold spray deposition of high density polyethylene composite powders

In this paper, we investigate the use of the cold spray additive manufacturing technique to deposit micron-sized composite particles of high-density polyethylene (HDPE) compounded with either micron or nanometer sized particles of silicon dioxide (SiO2) or copper (Cu). In this study, the mass fraction of the SiO2 and copper particles added into the final composite powders was varied between 0.5 % and 9.1 % and the deposition efficiency of each composite powder was measured. Deposition of the composite powders onto an HDPE substrate was successful even at the highest particle loadings tested. In each case, an optimal mass fraction of silicon dioxide or copper particles was observed with a maximum value of deposition efficiency larger than that of the pure HDPE. For the silicon dioxide composite powders, the deposition efficiency nearly doubled compared to the pure HDPE powders. The improvement of deposition efficiency at moderate particle loadings is likely due to the increase in the impact kinetic energy of the particles resulting from the increase in the density of the composite powders. While the decrease in deposition efficiency at high particle loadings is likely the result of the increased coverage of the silicon dioxide and copper particles on the surface of the HDPE powders limiting the interaction between the HDPE of the composite powder with the HDPE substrate. SEM images show that it is possible to create a smooth, dense, and uniform thin-film polymer composite coating using cold spray.

Additive manufacturing of Invar 36 alloy

Invar 36 alloy, renowned for its ultra-low coefficient of thermal expansion (CTE), is a functional Fe–Ni alloy that finds wide applications in aerospace and precision instruments. Additive manufacturing (AM), as a rapid digital manufacturing process, can efficiently and flexibly fabricate Invar 36 alloy parts with complex geometric and exceptional properties. Considering the advantages of AM and the increasing interest and demand for Invar 36 alloy parts fabrication by AM in recent years, it is imperative to provide a comprehensive summary of the current research status and technological advancements pertaining to AM Invar 36 alloy. Firstly, an overview of the AM processes currently employed for fabricating Invar 36 alloy is provided, including laser powder bed fusion, selective laser sintering, directed energy deposition, cold spray additive manufacturing, wire arc additive manufacturing, and binder jetting. Additionally, the microstructure, mechanical properties, and CTE of Invar 36 alloy manufactured by AM are summarized. Even eliminating the post heat treatment, the as-printed Invar 36 alloy can achieve excellent low CTE and mechanical properties, comparable to or exceeding traditional manufacturing processes. Moreover, the advantages and research progress of AM Invar 36 alloy composites are introduced. Enhancing mechanical properties while reducing CTE is a common challenge for both AM Invar 36 alloy and Invar composites. Finally, the technical gaps, research trends, and potential applications of AM Invar 36 alloy are summarized and prospected.

Effects of a hybrid post-treatment on microstructure and mechanical properties of cold sprayed AA7050 deposits

Cold spray additive manufacturing (CSAM) technology is a promising and efficient method for preparing high-strength aluminum alloy components. In this work, AA7050 deposits were prepared by high-pressure cold spraying, and a hybrid post-treatment (i.e., annealing + hot compression + solid solution and aging) was proposed to improve the mechanical properties of the deposits. The results of the study indicated that the annealing alone was insufficient to eliminate all defects within the AA7050 deposits. After annealing, the porosity of the material decreased from 2.34% to 1.62%. The subsequent hot compression and aging treatment promoted the precipitation of strengthening phases and homogenized the microstructure, reducing the porosity of the AA7050 deposits to 0.18%. As a result, the mechanical properties of the materials further improved. The ultimate tensile strength of the CSAMed AA7050 after aging treatment reached 557.7 MPa, which was 337.4% and 149.3% higher than that of the as-sprayed and annealed materials, respectively. Furthermore, fracture surfaces analysis indicated that the fracture mechanism of the specimens changed from brittle to ductile mode after the hybrid post-treatment.

Mixed-Material Feedstocks for Cold Spray Additive Manufacturing of Metal–Polymer Composites

High-performance polymers such as poly(ether ether ketone) (PEEK) are appealing as composite components for a wide variety of industrial and medical applications due to their excellent thermomechanical properties. However, conventional PEEK metallization methods can often lead to poor quality control, low deposition rate, and high cost. Cold spray is a promising potential alternative to produce polymer–metal composites rapidly and inexpensively due to its relatively mild operating conditions and high throughput. In this study, we investigated the deposition characteristics of metal–polymer composite feedstock, composed of PEEK powder and copper flake in varying ratios, onto a PEEK substrate. Copper-PEEK powder blends were prepared by both hand-mixing and cryogenic milling (cryomilling), which predominantly creates composite particles with micron-scale copper domains coating PEEK particle surfaces. This process non-monotonically affects the relative dominance and length scales of the multiple contributing deposition mechanisms present in mixed-material cold spray. In low-pressure cold spray, deposits showed significant changes in deposition efficiency and composition as a result of milling, with improvements in these characteristics most dramatic at lower Cu fractions. Deposits of a cryomilled blend of nominally 30 vol.% copper in PEEK exhibited minimal porosity under scanning electron microscopy, complete retention of powder composition, and the highest deposition efficiency among all samples tested. Notably, neither neat PEEK nor neat Cu meaningfully deposited at the same mild conditions as this 30 vol.% Cu blend, indicating a synergistic departure from linear mixing rules driven by the relative balance of local deposition interactions (e.g., hard–soft, soft–soft, etc.). Intentional powder and process design toward optimizing this balance may facilitate cold spray metallization applications.

Prediction of Geometry-Induced Porosity in Cold Spray Additive Manufacturing of Leading Edges

Prediction of Geometry-Induced Porosity in Cold Spray Additive Manufacturing of Leading Edges

Data-Driven Overlapping-Track Profile Modeling in Cold Spray Additive Manufacturing

Cold spray additive manufacturing is an emerging solid-state deposition process that enables large-scale components to be manufactured at high-production rates. Control over geometry is important for reducing the development and growth of defects during the 3D build process and improving the final dimensional accuracy and quality of components. To this end, a machine learning approach has recently gained interest in modeling additively manufactured geometry; however, such a data-driven modeling framework lacks the explicit consideration of a depositing surface and domain knowledge in cold spray additive manufacturing. Therefore, this study presents surface-aware data-driven modeling of an overlapping-track profile using a Gaussian Process Regression model. The proposed Gaussian Process modeling framework explicitly incorporated two relevant geometric features (i.e., surface type and polar length from the nozzle exit to the surface) and a widely adopted Gaussian superposing model as prior domain knowledge in the form of an explicit mean function. It was shown that the proposed model could provide better predictive performance than the Gaussian superposing model alone and the purely data-driven Gaussian Process model, providing consistent overlapping-track profile predictions at all overlapping ratios. By combining accurate prediction of track geometry with toolpath planning, it is anticipated that improved geometric control and product quality can be achieved in cold spray additive manufacturing.

Tuning the mechanical properties of CrCoFeMnNi high entropy alloy via cold spray additive manufacturing associated with heat treatment

The CoCrFeMnNi high entropy alloy (HEA) is the most studied HEA because of its excellent combination of strength and ductility. However, despite the advantage of CoCrFeMnNi HEA, its application is limited owing to its low yield strength. In this study, an emerging cold spray additive manufacturing technique was applied to fabricate a bulk CrCoFeMnNi high entropy alloy to improve the yield strength of CrCoFeMnNi. The paper presented an experimental investigation of the microstructure and mechanical properties of the as-sprayed and heat-treated CrCoFeMnNi high entropy alloy samples under compressive loading conditions at different strain rates. The microstructural analysis of the as-sprayed samples exhibited undeformed particles, numerous refined grains, and an interparticle incomplete interface. Post-heat treatment was applied to tune inter-particle bonding and ductility. Quasi-static compression tests (at 0.001 s−1) and dynamic impact tests (up to 3900 s−1) were carried out on the as-sprayed and heat-treated samples along the printing direction (vertical samples) and perpendicular to the printing direction (horizontal samples). The extraordinary mechanical properties of the as-sprayed vertical samples reached a yield strength of ∼ 1036.6 MPa and an ultimate compressive strength of ∼ 1117.8 MPa at a strain deformation of 10.1%. The as-sprayed horizontal samples had a yield strength of ∼ 1033.1 MPa and an ultimate compressive strength of ∼ 1102 MPa at a strain of 5.6%. Thus, the as-sprayed (vertical and horizontal) samples showed mechanical anisotropic properties. Further, the as-sprayed samples showed brittle behavior. However, as-sprayed samples exhibit high ductility, up to 70%, and high plastic deformation after heat treatment. The deformation mechanism of as-sprayed and heat-treated samples is dominated by deformation twinning. This work showed excellent yield and ultimate compressive strength compared to other CrCoFeMnNi high entropy alloy manufacturing methods.

Importance of feedstock powder selection for mechanical properties improvement of cold spray additively manufactured Ti6Al4V deposits

CSAM (cold spray additive manufacturing) of Ti6Al4V is a challenging task and high-quality deposits conforming to the AM application standards have not been developed so far. In our study, two distinct feedstock Ti6Al4V powders with different morphology and microstructure, spherical and crystalline, were used and their influence on the deposits was investigated in terms of microstructure as well as tensile properties. The results indicate the mechanical strength and ductility of the as-deposited samples to be in the range of 8–30 % compared to wrought Ti6Al4V and highlight a significant anisotropy in different in-plane directions. The post-treatments of the deposits from the spherical, plasma atomized powder effectively reduced the porosity and triggered microstructural homogenization and recrystallization, leading to a significant increase in the yield and tensile strengths, reaching 892 MPa and 954 MPa, respectively, while achieving an enormous enhancement in the elongation to 21.6 % at the same time. This was in a striking contrast to the deposits from the crystalline powder: despite the yield and tensile strength increase to 853 MPa and 1058 MPa, respectively, the elongation remained virtually zero, highlighting the importance of the feedstock powder selection in cold spray additive manufacturing of Ti6Al4V.

Effect of strain rate on compressive behavior and deformation mechanism of CoCrNi medium entropy alloy fabricated via cold spray additive manufacturing

The equiatomic CoCrNi medium entropy alloy (MEA) has shown better mechanical properties than many MEAs and high entropy alloys (HEAs) at room temperatures. However, the mechanical behaviors and deformation mechanisms of CoCrNi MEAs have yet to be studied, especially at different strain rates. In this study, cold spray additive manufacturing (CSAM) was applied to fabricate freestanding CoCrNi deposits. The investigation of compressive behaviors and deformation mechanisms were studied at a low-strain rate of 0.001 s−1 and at a high strain rate ranging from 2300 s−1to 4200 s−1. The as-fabricated sample exhibited refined grains, unfused powders, and large porosity, resulting in limited metallurgical bonding within the bulk CoCrNi deposit and deformed particle interface. Therefore, post-heat treatment at 1350 °C was applied to improve the interparticle bonding and strengthen the as-fabricated CoCrNi deposits. As expected, the annealed deposit significantly improved the ultimate compressive strength (UCS) and strain elongation of the as-fabricated deposits from ∼90 MPa and ∼5% to ∼1400 MPa, and ∼70% at a strain rate of 0.001 s−1. Besides, the annealed samples showed improvement twofold in UCS and ductility of the as-fabricated deposits from ∼67 MPa and ∼8.6% to ∼1266 MPa and ∼34% at a strain rate of 4200 s−1 and 3800 s−1 respectively. The TEM image of the annealed CoCrNi sample showed deformation twining, nano twins, planar slips, and microbands, which are the dominant deformation mechanisms that help to promote the plasticity and the work hardening behavior of this alloy. This work validates the compressive behavior and deformation mechanism of cold-sprayed CoCrNi MEA associated with post-heat treatment at different strain rates.

Examining the contribution of tamping effect on inter-splat bonding during cold spray

In Cold Spray Additive Manufacturing (CSAM), the 'tamping effect'—resulting from continuous impact of incoming particles enhances the mechanical properties of deposited layers. While the impact of tamping on porosity is well-studied, its influence on inter-splat bonding remains unexplored. In this study a multiparticle simulation based on Finite Element Analysis (FEA) is employed, featuring 120 particles distributed across 12 layers to model the deposition process in cold spray. A methodology is developed to discern the effect of tamping, enabling the estimation of particle boundary temperatures under conditions with and without tamping. Energy analysis and inter-splat boundary temperature variations obtained from simulations are examined to understand the contribution of tamping to bonding. Our findings reveal a significant increase in inter-splat boundary temperatures, attributed to energy transfer from overlying layers, positively impacting bonding. The proposed hypothesis is substantiated through mechanical testing and the assessment of inter-splat bonding-related properties, including hardness, tensile strength, scratch resistance, and corrosion resistance, as a function of deposit thickness. CSAM IN718 and Cu deposits with a thickness of 4 mm are used as subjects for this study.

Microstructure and tribology of cold spray additively manufactured multimodal Ni-WC metal matrix composites

Cold spray produces thick and dense deposits that can be applied in additive manufacturing or as a repair to recover damaged components. One potential materials system is WC-reinforced Ni composites, which are known for a combination of high hardness and toughness. WC particle size and distribution are some of the most important variables that impact the mechanical properties and wear resistance. In this study, thick multimodal Ni-24 vol%WC composite coatings were additively manufactured using the cold spray technique. Spherical Ni powder and two types of WC particles, i.e., cast and agglomerated, were used as feedstock materials. The microstructure, porosity, WC distribution, and microhardness of coatings were investigated in comparison with cold-sprayed Ni and Ni-agglomerated WC coatings with similar WC fill ratios. Dry sliding wear of multimodal Ni-WC coatings was studied with a sliding speed of 3 mm/s under normal loads of 5 and 12 N and was then compared to Ni and composite coatings made with only agglomerated WC. Multimodal Ni-WC coatings with an altered distribution of WC particles offered an improvement in friction and wear as compared to Ni-agglomerated WC coatings. Characterization of worn surfaces revealed the formation of tribofilms following compaction of wear debris. A more stable tribofilm that developed to a higher coverage was identified for coatings with multimodal WC particles at both tested loads. However, the protective role of tribofilms was reduced at a higher load, caused by subsurface crack propagation. Subsurface microstructure of worn surfaces that are induced by sliding wear was studied, and a correlation between the mechanical properties and microstructure and wear mechanism of coatings was made. Overall, our findings highlight the potential of cold spray additive manufacturing in producing multimodal Ni-WC composite coatings with enhanced wear resistance.

Spray Trajectory Planning for Complex Structural Components in Robotized Cold Spray Additive Manufacturing

Spray Trajectory Planning for Complex Structural Components in Robotized Cold Spray Additive Manufacturing

An application of cold spray additive manufacturing for thermoelectric generator thermal interfacing

The influence of thermal interface resistance is an important design problem for the optimisation of heat transfer systems. For thermoelectric generators (TEG), it can limit the ability to convert thermal energy directly to electrical energy. This paper describes an experimental and numerical study of the influence of thermal interface resistance on TEG module performance. A test apparatus was constructed to evaluate TEGs under varying temperature and heat source/sink conditions. The effect of heat source/sink interface finish grades, corresponding to medium quality (N8-N6) and very fine (N5-N4) surface finishes, at varying clamp forces, is evaluated. Then the application of cold spray additive manufactured copper, as a novel method of TEG module thermal interfacing is explored. Experimental data is accompanied with numerical analysis using TEG ideal equations and the effective material properties approach with the incorporation of temperature dependency and fabrication quality factors to improve modelling accuracy. A system of simultaneous heat flow equations is then established to numerically solve for the presence of thermal interface resistance. The results show that high clamp forces (3600 N) and very fine finish grades were sufficient to minimise thermal interface resistance to the range 0–5.98 ×10−5m2KW−1, representing a loss of 0–9.9% in maximal achievable power. For the cold spray thermally interfaced TEG module, resistance estimates were lower and in the range 0–5.4 ×10−6m2KW−1, corresponding to a 0–1.0% loss in power.

A continuous toolpath strategy from offset contours for robotic additive manufacturing

Toolpath planning is an essential component of robotic additive manufacturing. An efficient toolpath strategy allows parts to be made that are geometrically accurate, free of defects, have good mechanical properties and have low residual stress. Toolpaths for cold spray additive manufacturing have some technical constraints that need to be considered compared to their counterpart designed for conventional 3D printing machines. This study presents an automated toolpath planning method based on offset contours. The generated toolpath is globally continuous, layer-wise setting, making it suitable for robotic cold spray additive manufacturing. The toolpath algorithm was tested on a variety of geometries to demonstrate its robustness. One model was selected for printing using a commercial high-pressure cold spray system. The experimental results show that our method is applicable to cold spray robotic additive manufacturing for near-net shape construction. The method is particularly good for web-rib structures.

Unraveling microforging principle during in situ shot-peening-assisted cold spray additive manufacturing aluminum alloy through a multi-physics framework

Cold spray (CS) is a highly potential solid-state additive manufacturing (AM) technique. In situ shot-peening-assisted CSAM was proposed to additively manufacture fully dense deposits using cost-effective and renewable nitrogen gas. The role of in situ shot-peening particles is critical but remains unclear. Here, the process was quantitatively modeled to visualize the dynamic deformation, energy conversion, as well as cell/sub-grain size and microhardness evolutions, compared to those during the conventional CSAM process, identifying the key role of in situ shot-peening particles in the AA6061 extreme deformation and microstructure characteristics during in situ shot-peening-assisted CSAM. High-fidelity modeling was verified fully by comparing the experimental and model-reproduced deformation profiles, cell/sub-grain size distributions, and increases in microhardness. The results show that the kinetic energy of in situ shot-peening particles was 470 times higher and dissipated mainly through AA6061 plastic deformation (86.36% of total energy), leading to significant enhancement of microhardness and tensile strength. Moreover, the mixing ratio of large-size SS410 particles required to create a fully dense deposit was evaluated from an energy perspective, in good agreement with the experiment. This study elucidates the microforging principle during in situ shot-peening-assisted CSAM, providing scientific guidelines for high-quality and low-cost CSAM of high-strength aluminum alloys.

Microstructure and mechanical properties of cold spray additive manufactured and post heat treated high-entropy alloys with mixed CoCrFeNi and Ti powders

Solid-state cold spray additive manufacturing has the potential to fabricate high-strength bulk high-entropy alloys (HEAs). Through systematic experiments, this study examined the microstructure and mechanical properties of cold spray fabricated bulk HEAs with mixed CoCrFeNi and Ti powders and explored the effects of Ti content in 4 and 7 at% and post-heat treatment at 800 °C/6 h and 1000 °C/6 h. It is found that the bulk HEAs are highly densified and remain nearly defect-free after the post-heat treatment. The addition of Ti serves as an mixing powder effect to further densify the alloys. Moreover, the addition of Ti and post-heat treatment promote the formation of fine precipitates, including Fe2Ti, Co2Ti, CrFe, CoTi2, and Ni3Ti, which contributes to the strengthening of the alloys. Furthermore, post-heat treatment improves the cohesion of the alloys and yields fine and relatively uniform grains by stimulating static recovery and recrystallization. These synergistic effects improve the hardness, elastic modulus, strength, and ductility properties. The average hardness and elastic modulus of the bulk alloys are as high as 377.6 HV and 94.8 GPa, respectively; the best ultimate tensile strength and elongation to failure are 626.7 MPa and 22.1 %, respectively. Synchrotron X-ray diffraction validates the complex precipitates formed and confirms that there is no phase change during tension. The study offers insights into the microstructure and mechanical properties of cold spray additive manufactured HEAs and the effects of mixed powders and post-heat treatment on tuning microstructure and properties.

Cold spray forming: a novel approach in cold spray additive manufacturing of complex parts using 3D-printed polymer molds

AbstractThe solid-state additive manufacturing (AM) process cold spraying (CS) offers advantageous properties such as melt-free near-net-shape part fabrication and high deposition rates. Compared to other metal-based AM processes such as the powder bed fusion of metals (PBF-LB/M) or directed energy deposition (DED) processes such as laser metal deposition (DED-LB), CS features lower part resolution. One solution to increase the achievable level of detail is spraying onto removable molds. No study exists that investigates the general feasibility and manufacturing boundaries, from which design guidelines could be derived. In this paper, the applicability of material extruded and thermally bonded polymer (MEX-TRB/P) shapes, which is especially suitable for flexible low-cost production of small batches, as molds for cold spray additive manufacturing (CSAM) is investigated. For this purpose, material extruded thermoplastics are examined regarding their suitability for the CS process. Furthermore, geometrical and thus constructive restrictions of this new approach “Cold Spray Forming” (CSF) are analyzed using an industry-relevant use case. It was shown that the feasibility of this approach could be determined by the material value hardness of the sprayed polymer substrates.