Operating parameters for suspension and solution plasma-spray coatings
Introduction
Nanostructured materials offer significant improvement in engineering properties due to the reduction of grain sizes by a factor of almost two orders of magnitude over conventional coatings [1]. Since the beginning of the mid-1990s, works have started to use the spray technology for the deposition of finely or nano-structured coatings [2], [3]. To produce finely structured coatings by thermal spray techniques, four routes have been suggested:
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to spray complex alloys containing multiple elements, such as for example the SAM2X5 (Fe–Cr–Mo–W–Mn–B–C–Si) [4] which exhibits a glass forming capability when cooled-down. After deposition, upon subsequent heating in the 500–750 °C range, the metallic glass precursor transforms into multiple crystalline phases and the resulting structures are nanoscaled [5];
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to spray conventional micrometric particles (in the 30–90 μm range) made of agglomerated nanoparticles. Nevertheless, the operating parameter window for which particles are only partially melted (i.e., the molten part acts as a “cement” bonding unmolten nanograins) is narrow (see the review of Lima and Marple [6]);
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to spray sub-micron-sized or nano-sized particles via a suspension (suspension thermal spraying, STS). To circumvent the too high gas flow rates to spray particles below 5 μm [7], a liquid carrier is used instead of a gas one [8], [9], [10]. Once it has been fragmented and vaporized by the plasma flow or the HVOF flame, particles contained in the droplets are heated, accelerated and sprayed onto the substrate;
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spray solutions of final material precursor (solution precursor thermal spraying, SPTS). As with suspension, the liquid undergoes rapid fragmentation and evaporation once injected in the SPTS jet. This is followed by precipitation or gelation, pyrolysis and melting to result finally in the impact of molten liquid droplets with average diameters ranging from 0.1 to a few micrometers [11], [12], [13].
In this paper, only the two last routes; i.e., suspension plasma spraying (SPS) and solution precursor plasma spraying (SPPS) will be developed. Only d.c. plasma jets will be considered to process feedstock. Compared to conventional coatings, those manufactured from solutions or suspensions exhibit quite interesting features such as the absence of lamella boundaries and cracks and porous microstructures with nanometer-sized grains. However, liquid injection within plasma jets is quite different from particle injection by a carrier gas in particular the fragmentation and vaporization mechanisms taking place, as well as the manner liquid droplets or liquid jets are produced. Indeed, mechanisms encountered in SPS or SPPS are by far more sensitive to arc root fluctuations than those encountered with conventional spraying. In the following will be successively presented:
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the liquid injection with the parameters quantifying fragmentation and vaporization,
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the way coatings are produced considering SPS and SPPS.
Section snippets
Atomization
Four different atomizer technologies were used for example by Jordan et al. [14]:
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home made capillary atomizer (proprietary system),
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fan nozzle: a narrow angle atomizing fan nozzle (mechanical secondary atomization),
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air cap transverse air blast atomizing nozzle, typified as air blast atomizer,
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nebulizer (transverse jet secondary atomization).
Fig. 1 displays droplet size distributions measured at the systems exit implementing phase Doppler anemometry (PDA) for the different considered technologies.
Plasma–liquid interaction
According to the complexity of involved phenomena, a quasi-stationary plasma flow will be considered at first to depict them before considering a fluctuating flow. This is the case of an Ar–He plasma gas mixture for which voltage fluctuations can be limited to ΔV/Vm = 0.3 (Table 2) (corresponding to the takeover plasma torch operating mode) when the torch is operated under an arc current intensity of 700 A (and an anode internal diameter of 6 mm). Fluctuating plasma jet will be characteristic of
General remark
Both for SPS and SPPS, three types of particles can be collected on the substrate [26]:
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those which have been well treated (i.e., fully molten) and which form lamellae upon impact and spreading,
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those which have traveled in the jet fringes and are either only pyrolyzed (for the solution) or in a powdered state (for the suspension when fragmentation starts in the jet fringes resulting in poorly treated droplets),
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those which have been ejected from the plasma jet warm core by thermophoresis force
Conclusions
Since about a decade, the interest to manufacture on large surfaces “thick” finely structured or nano-structured layers has been increasingly growing. If nano-structured architectures can be manufactured by gas condensation routes (CVD, PE-CVD, PVD, EB-PVD, etc.), their thicknesses can hardly be higher than a few micrometers. On the contrary, plasma-spray coating thicknesses between 50 and a few millimeters are easily achieved but with no nano-structured architectures after particle melting.
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2020, Ceramics InternationalCitation Excerpt :In SPS deposition technique, fine particles (typically 100 nm to 5 μm) are carried within a liquid medium (organic solvent or water) in the form of a suspension to be injected into the plasma. Liquid injection makes the plasma spray process much more complex than the conventional APS [19]. One of the major challenges of SPS is preparation of industrially scalable suspensions which must be well-dispersed and stable without particle agglomeration or sedimentation.