ICMCTF2018 Session TS1: Thermal and Kinetic Spray Deposition

In accordance with the key objectives -higher efficiencies and lower emissions- components of modern turbomachinery systems, i.e. stationary and rotating blades, are subject to very high demands, which need to be fulfilled among others with innovative coating solutions. Protective coatings are, therefore, the state of art to ensure functionality of turbomachinery components with prolonged life times.

In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray seems to be an ideal process for repair applications. In this study, Inconel 718 powder particles were cold-sprayed on Inconel 718 substrates by using nitrogen gas for an application as a repair tool for aero engine components. First, velocities of the cold sprayed particles have been determined as a function of process conditions and particle size. Critical velocities have been determined considering the deposition efficiencies.

Furthermore, the magnitude of the residual stress and its distribution through the thickness of the cold-sprayed coatings were measured by using the hole-drilling and the bending methods. Mainly compressive residual stresses were observed in cold-sprayed Inconel 718 coatings. Accumulation of residual stresses in the coatings is highly affected by peening during deposition and it decreases with increase in thickness. It has been observed that the bond--strengths of cold-sprayed Inconel 718 coatings are highly influenced by coating thickness and residual stress states of the coating/substrate system. A detailed discussion will be given.

In addition, also further results on cold spraying different Ni base superalloys on CMSX 4 type substrates will be presented and discussed. Especially the influence of substrate temperature will be highlighted.

With the emerging prevalence of 3D printed polymer materials for rapid prototyping, there is an increasing demand for a similar ease and quickness in producing metallic and multi-material components. One such approach is through the metallization of printed polymer parts using thermal spray, where thick deposits can be quickly and economically deposited. However, technical challenges include management of residual stress of the deposit, assessing the adhesion strength of sprayed metal onto polymers, and potential thermal degradation of polymer substrates from molten droplet impingement during spraying.

This presentation will discuss the methodologies used to accomplish successful metallization of polymer substrates (e.g., ABS, HDPE) including in-situ measurements of substrate deflection for calculation of residual stress, mechanisms and quantification of adhesion between polymers and sprayed metal, and optimization of spray processing and material layering.

The aerosol deposition (AD) method is a thick-film deposition process that uses ~500 nm particle size oxide powders to produce 95% dense films, up to several hundred micrometers thick with nanometer sized grains. The deposition can be performed on a variety of substrates because bonding and densification between the film/substrate interface are thought to be facilitated by local temperature rise, high pressure, particle fracture, and chemical bonding during deposition, which leads to the nano-grained microstructures created at room temperature. In this work, we present film characterization results of depositing dielectric and ferroelectric materials using aerosol deposition. Film properties will be compared when adjusting process parameters such as varying flow gas rate, gas type, and substrate material. Further characterization will be performed by changing annealing conditions, and electrode application.

Metal matrix composites (MMCs) provide a significant advantage for their tribological properties compared to pure metals. There is a long history of metal-ceramic composites for enhancement of load carrying capacity and solid lubricating composites for friction reduction. Both type of composites will generate ‘tribofilms’ that help to reduce wear and control friction. One may also engineer MMCs with both hard phase and solid lubricants.

There are many methods of manufacture for MMCs in bulk form and a few options for manufacturing them as coatings. Cold spray is one such coating deposition technique that has received increased attention in recent years. Researchers have developed a wide range of MMC coatings by cold spray, typically basing materials selection on metal-ceramic or metal-solid lubricant MMCs made by traditional methods. Recent work from our group has included cold spray coatings of Al-Al₂O₃, Ti-TiC, Cu-MoS₂ and Ni-WC.

Devices utilizing magnetic materials such as frequency selective limiters, circulators, inductors, and filters are critical components in many of today’s electronics [1]. The need for ferromagnetic materials in these devices poses many difficulties for minimizing device size, weight, and cost. One issue that hampers integration of ferromagnetic materials is the high-melting temperature of the ferrite compared with the low-melting temperature component structure [2]. Furthermore, the need for low-loss and narrow bandwidth operation adds another significant barrier to the advancement of integration of ferromagnetic materials into these device structures.

The high-frequency operation regime and strong uniaxial anisotropy of barium hexaferrite (BaFe₁₂O₁₉, BaM) makes this material particularly interesting to utilize as an oriented film for microwave components. In this study, we characterize BaM films deposited onto sapphire substrates by a room-temperature thick-film growth technique called aerosol deposition.

We perfomed alternating gradient magnetometry depth studies on a series of as-deposited films that show a variation in magnetization with depth. Cross-sectional SEM images indicate laterally uniform film density. Electron dispersive spectroscopy of the interfacial region suggest significant Al₂O₃ mixing into the film volume. Fe XPS spectra indicate a change in peak weighting as a function of thickness, possibly indicative of modified structure or oxygen incorporation due to Al incorporation.

To explore the possibility of magnetically orienting the films we deposited additional films in the presence of a 4 kOe static magnetic field. We report VSM, FMR, and XRD results of these films as-deposited and after post-deposition sintering at temperatures from 700C to 1000C.

The films deposited in the field show an increased saturation magnetization and remanence compared to the films deposited with no applied field. XRD results of all of the films in this study suggest good crystallinity. Rietveld refinement of the data also suggests that the films deposited in the field presence have a smaller unit cell volume compared to films deposited without the applied field. Post-growth sintering increases the crystallite size from about 10 nm to 25 nm. Annealing improves the overall properties of the films further increasing the magentic orientation and saturation.

[1] Adams, J., Davis, L., Dionne, G., Schloemann, E., and Stitzer, S., IEEE Transactions on Microwave Theory and Technology, 50 (2002), No. 3, pp.721.

[2] Johnson, S., Newman, H., Glaser, E., Cheng, S.-F., Tadjer, M., Kub, F., and Eddy, C., IEEE Trans. on Magnetics, 51, (2015), No. 5, pp. 2200206.

The hot section parts in aviation engines and stationary gas turbines are exposed to extreme environments, where high temperatures and eroding atmospheres lead to oxidation, corrosion and fatigue damage of the inserted parts. As the manufacture of the nickel based superalloys for those high temperature applications is expensive, the repair of worn or damaged parts is from an economic viewpoint desirable, however, usually difficult due to the poor weldability of these alloys.

The coating technology Cold Gas Spray has been tested to repair such worn parts and reduce the maintenance cost. In this process heated and pressurized gas is expanded through a Laval-Nozzle. This leads to a high-speed gas jet and accelerates the powder to supersonic velocities. The operating temperature is relatively low compared to the melting point of the used alloys. The particles hit the substrate, deform and are bonded to the substrate by mechanical clamping and formation of intermixing zones. An oxide free and dense coating is formed. With an increasing coating thickness the stored elastic energy in the coating increases and can lead the delamination of the coating at a critical thickness.

In this study the alloys CMSX-4, Rene 80, Inconel 625, Inconel 713 and Inconel 738 are deposited on single crystalline substrates that are similar to CMSX-4 and polycrystalline Inconel 738 substrates. All used powders are spherical and have a diameter between 5-45 µm. To show different residual stresses that evolve during the coating process curvature measurements were performed on round Inconel 738 samples. Each powder shows different adhesion to the substrate. To characterize the differences adhesion tests were performed.

To reduce residual stresses and increase the critical thickness of the coating a heated stage is tested to investigate the influence of a heated substrate. Additional heat treatments were performed to investigate the change of porosity and microstructure within the coating and at the interface between the substrate and the coating. The evolving microstructures were examined using scanning electron microscopy (SEM) and Electron Backscatter diffraction (EBSD).

The properties of most engineering materials depend on the characteristics of internal microstructures and defects. In metals, these features can include grains in the polycrystalline aggregate, impurities, multiple phases, and in the case of thermally sprayed coatings, significant levels of porosity. The microscopic details of the interactions between these internal defects, and the propagation of applied loads through the body, act in concert to dictate macro-observable properties like strength, conductivity, spall, etc. In order to achieve a comprehensive understanding and control of a material’s high-rate properties, the relevant structure-properties relationships must be understood. In this work, we used Sandia’s Alegra finite element software [1] to simulate the high-rate loading of metal coatings manufactured by thermal plasma spraying. These simulations include a direct representation of the microstructural details of the material, such that internal features like second phases and pores are represented and meshed explicitly as individual entities in the computational domain. We will discuss the dependence of the high-rate mechanical properties of these materials on microstructural characteristics such as the shapes, sizes, and volume fractions of the second phases and pores. We will also examine the effects of pore collapse on high-rate response, and how the details of the microstructural representation affect the microscopic material response to the applied load. In particular, we will discuss the effects of using “stairstep” (on a cubic finite-element “grid”) versus comformal (smooth) interfaces created via Sandia's SCULPT capability in CUBIT [3].

1. <http://www.cs.sandia.gov/ALEGRA/Alegra_Home.html>

2. <http://www.sandia.gov/mst/pdf/LENS.pdf>

3. <https://cubit.sandia.gov/>

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk. Though this process has been used for two decades the precise mechanisms of bonding and densification is still debatable. Therefore, we have used a combination of computational fluid dynamics (CFD) and finite element (FE) modelling and high speed videography to help understand the flight of the particles as well as the interaction of the particle and the substrate. The CFD model results show the flight of particles from the nozzle to just before the substrate. At the point just before impact, the velocity and direction is collected and used to inform the FE model of the flight of the particle. Initially the FE model is run with the parameters obtained from the CFD model but the work is extended to include variations in particle size, velocity, and material to show their effects on the deposition process. The model is developed as full 3-D implementation with symmetric boundary conditions applied when/where appropriate. The particle and substrate materials are independent and each is varied between Al₂O₃, SiC, and float glass. Each material is simulated using a Johnson-Cook constitutive model, which includes plasticity, damage initiation and evolution, and failure. The mechanical properties of each material are taken from bulk properties. The particle velocity is varied from 100 m/s to 300 m/s with sizes between 0.5 μm and 1.5 μm. The bounds were informed by the results of the CFD analysis. High-speed videography will help validate this model and to better understand the process as a whole.