Synthesis of Yttria-Stabilized Zirconia Nano Powders for Plasma Sprayed Nano Coatings

Plasma sprayed Yttria Stabilized Zirconia (YSZ) coatings, with few microns sized microstructure/grain morphology has beenwell researched, reported and established as an industrial Thermal Barrier Coatings (TBC) material/system. However, nano structured YSZ coatings possess improved characteristics when compared with their micron sized counterparts. However, due to their nano sizes, light weight, and low density, plasma spray coating process of nano powders suffers from flowability issues due to lack of nano powder inertia/momentum, leading to poor deposition/ uneven coating thickness. In this research work, nano structured YSZ coatings were synthesized by using an Atmospheric Spray Coating (APS) facility. Nano powders of YSZ were used as the starting materials to prepare micron sized plasma sprayable powders. 80μm thick NiCrAlY bond coat (commercial)
and 200μm thick YSZ top coat with nano microstructure (lab synthesized) were built on steel substrates. The starting nano crystalline (YSZ) powders, measuring 30-70 nanometers (nm) were synthesized in the laboratory via chemical method (sol-gel) by employing zirconium oxy chloride hexa-hydrate and yttrium nitrate as precursors, citric acid as chelating agent and ethylene glycol for the diversification reaction followed by calcination @ 1000ºC. They were then re-constituted into micron sized (53- 106 μm) plasma sprayable powders by agglomerating with polyvinyl alcohol (PVA) binders. The nano crystallite morphology of powders and coatings were analyzed by Scanning Electron Microscope (SEM), chemical composition by Energy Dispersive spectroscopy (EDS) and crystal structural phase by X-ray diffraction (XRD). The influence of calcination temperature of 1150ºC
on nano crystallite morphology was also studied.

1. Background
Nanotechnology refers to any manipulation of matter in the dimensions of 10-9 meters. The first time the conceptwas addressed was in the famous lecture by Dr Richard Feynman “There’s plenty of Room at the Bottom”, but it was only after a decade later the term “nanotechnology” was coined by Dr Norio Tgunichi. Nanotechnology [1] has seen tremendous growth from then to the present age. In the aerospace and automotive world, Yttria Partially Stabilized Zirconia based Atmospheric Plasma Sprayed (APS) or Electron Beam Physical Vapor deposited (EBPVD) coating is a well-known and tested coating system, highly suitable as thermal barrier coatings (TBCs) and is one of the prime players in the industry. The role for this coating is to protect mechanical components from high temperature in service. The structural properties and the atomic nature of stabilized (with yttrium oxide) zirconium oxide make it the most advanced one for the present day’s research on TBCs. Thermal barrier coatings (TBCs) are currently widely used as defending coatings on hot section modules in advanced gas turbine engines to resist increased inlet temperatures and thus refining engine performance[2]. The property and performance delivered by YSZ thermal barrier coatings with nanostructured grain morphology
have been reported to be superior when compared to their micron seized counterpart.The possibilities for the development of yttria-partially-stabilized zirconia (YSZ) coatings have been explored and achieved by using YSZ
nano powders [3]. The nano powders were used as ceramic (Top coat) material to be converted into nano-grained microstructure. In this research work, an attempt is made to develop nano sized plasma spray coatings via in-house synthesis of nano powders followed by plasma spray coating on suitable metal substrate. Thus, this work involves the synthesis of nano sized YPSZ powders in the laboratory, analysis using materials analysis instruments such as Scanning Electron Microscope – energy dispersive X-ray spectroscope (SEM –EDS), Xray diffractometer (XRD), reconstitution of the nano sized powders into micron sized plasma sprayable powders for enhancing flowability and deposition characteristics in plasma, and finally plasma spray coating process via the free flowing powder on bond coated (NiCrAlY) substrate (Steel).
1.1. Literature
Nanomaterials are any material whose at least one dimension is in the range of 1nm to 100nm. Nanomaterial’s are a material sciences oriented approach to nanotechnology and are of much academic and industrial interest
because at this scale, unique properties emerge. The unique properties of nano materials find wide application in the fields of electronics, mechanics, optics, steel making industries and aero-space. In general, materials are classified into nano-materials if they obey the particle size range, structures and properties range. Nanomaterials can be nanoscale in a dimensional range as 1- D, 2-D and 3-D (eg: surface films strands or
fibers, precipitates, colloids). They may also exist in single, fused, agglomerated or aggregated form. Advancements in the biomedical, mechanical, electronics and other domains that govern the socio-economic
developments today are greatly dependent on nano metal oxides. The unexplored chemical and physical properties shown by these materials have attracted interests and are an important catalyst to today’s breakthroughs in research and development. It is extremely difficult to obtain nano metal oxides in a large scale using any chemical process and is one of the major roadblocks in the research and development of nano metal oxides. When compared to emerging trends in materials, Nanomaterials and coatings possess the properties resulting from reducing microstructural features. Even at ambient temperature the mechanical properties in ceramics may be consummate with small defect size and better grain boundary stress relaxation. Diffusivity is prominently better, related with a greater volume of grain boundaries. The nanostructures in TBCs are known to result in ample growth in thermal barrier effect and efficiency. The stable microstructure strength of the nano-coating is also an important constraint to be considered in nano coating research. Nano-sized grains and pores can be formed by atmospheric plasma spray (APS) and electron beam physical vapor deposition (EB-PVD).
In general many factors affect or influence a developmental process of nano coatings like the materials obtainability, materials and process development, critical control parameters, determination of materials and
deposition parameters for interface control in multilayered structures, choice of materials to develop thermal stability etc. play critical roles. Some of the barriers to the developmental progress are consistent and reproducible properties data, thermal stability in the service temperature over a long period of time.

1.2. Plasma Sprayed nano Coatings
Thermal Spraying [4] includes a number of coating techniques in which molten or semi-molten fineceramic or/and metallic particles are sprayed on specially prepared substrate’s surfaces for coating development. The feed materials can be ceramic, cermet, metal or even plastic in form of powder, wire or rod. Melting of feed materials is achieved through gas combustion, electric arc or plasma. Nano ceramic coatings may be prepared via thermal spray technology and specifically by employing atmospheric plasma spray (APS) [5] technique. In thermal spray coatings process spray powder/particles are accelerated and then melted before impacting onto the prepared substrate where they compress and form flattened lamellae, called splats, from the layering of which results the coating. Coatings are used in an extensive range of applications including automobile and boiler components, aircraft engines, orthopaedics [6] and dental applications etc. However, observations of coatings after service indicate a coating’s failure is most commonly associated with its poor mechanical properties and micro- or crystal structural changes.
1.3 Nano Powders
Nanomaterials show excellent mechanical properties compared with conventional materials because ~50% vol. of their atoms are concentrated around grain boundaries. A novel method to produce nanomaterials is the sol-gel process; [7] a chemical mixture of oxides including hydrolyzable alkoxides that undergo an evolution of sol-gel. This transition represents a linking of nanometer-size units into an oxide network of infinite molecular weight. Nano powders, due to their low density and small size are very low in flowability. This makes it extremely difficult to perform the coating process. To overcome this problem the nano powders are reconstituted into
agglomerates using organic binders such as Poly Vinyl Alcohol (PVA). The proportions of PVA added also plays a key role in the flowability of powders as they are a reducing agent. Redox reactions results in critical changes to the nano metallic oxides and hence must be cautiously dealt with.

1.3. Challenges in nano coating development

Primarily, there are two modes for synthesis of nano powders and they are (a) Bottom-up (building from atoms) and (b) Top-down (reducing from larger particles) approaches. Bottom-Up processes mainly deal with the chemical process and are normally found to produce much finer particles than the top-down approach. Top-down approaches follow the physical processes like milling, ablation, ionized sieving etc. The bottom up process is associated with an impediment and that is poor yield of the synthesized powders.
The top down process normally gives much higher yield. Although nano-structured ceramic coatings possess mechanical properties including adhesion, stiffness, ductility, and toughness when compared with their micron sized counterpart coatings, difficulty lies in the fact related with the complex nature of synthesis where for most industrial applications these nano-sized powders need to be densified without the loss of their nanostructure to obtain bulk products. Each and every technique has its own advantages and disadvantages. The biggest challenges in thermal spraying nano-materials are
i. to make the nano sized feed powder through the atmospheric plasma flame and
ii. to hold the nanostructure of the feedstock (agglomerated to microns) to deposit.
If the feed powder does not flow easily through the plasma flame, isolated patches of non-uniform deposition are found on the substrate.
During spraying the nano-structured powder particles are completely molten; the normal behavior of the spray particles, such as solidification, nucleation and growth will take place. Such processes will destroy the nanostructured features of the feedstock. Partially melting of powder particles is necessary to achieve the physical conditions for cohesion and adhesion.

1.5. Scope of the present investigation
Keeping the above background in view, the aim of the present research work was to fabricate ceramic nanocoatings using 6-8% Y2O3 stabilized ZrO2 nano sized plasma sprayable powders synthesized in the laboratory, the focus being on preparing plasma sprayable powders, comprised of agglomerates of the nano powders. The developed plasma spray powders were deposited (200μm thick) on bond coated (80 μm NiCrAlY) steel substrate (40mm x 35mm x 3mm thick). The method used for synthesis of nano-powders here is a sol-gel derived chemical process [8]. Sol-gel process is one of the most common processes used to synthesize nano powders along with the likes of reduction, pyrolysis, solvo-thermal etc.

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Post time: May-27-2020
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