SPS Guide: Suspension Plasma Spraying & TBC Coatings
Release time:
2026-08-17
Author:
Zhenzhong Fused New Material
Article overview
This guide covers the core engineering concepts of Suspension Plasma Spraying (SPS) with structured technical definitions, a coating process comparison table, industry application contexts, and advanced feedstock material requirements. Use the table of contents to jump directly to the thermal spray context relevant to your advanced surface engineering needs.
Table of contents
- 1. What is Suspension Plasma Spraying (SPS)? (process definition)
- 2. SPS process fundamentals: how liquid feedstock feeding works
- 3. SPS vs. traditional thermal spray processes: technical comparison
- 4. Key material feedstocks: zirconia, rare-earth pyrochlores, and oxides
- 5.Major industrial applications: aerospace, energy, and semiconductor IT
- 6. Common misconceptions about SPS coating technology
- 7. How to determine if SPS is right for your component surface engineering
- 8. Frequently asked questions
WWhat is Suspension Plasma Spraying (SPS)? (process definition)
Suspension Plasma Spraying (SPS) is an advanced atmospheric thermal spray process in which submicron or nanometer-sized ceramic powders—dispersed in a liquid medium (ethanol, water, or organic solvent)—are directly injected into a high-temperature plasma jet. By atomizing and evaporation-vaporizing the liquid carrier, SPS enables the deposition of ultra-thin, columnar-structured, or highly dense nano-ceramic coatings that are impossible to produce using conventional powder-fed plasma spraying (APS).
If you landed on this page seeking high-performance thermal barrier coatings (TBCs) or specialized ceramic surface treatments, understanding SPS is critical. In modern high-temperature industrial manufacturing, thermal and environmental barrier coatings must withstand extreme thermal cycling, corrosion, and erosion. SPS has emerged as the premier technology to bridge the performance gap between traditional Atmospheric Plasma Spraying (APS) and costly Electron Beam Physical Vapor Deposition (EB-PVD).
Why SPS feedstock material quality matters
In traditional APS, feeding dry powders smaller than 10–15 μm causes severe nozzle clogging and fluidization instability due to high interparticle van der Waals forces. SPS solves this fundamental fluid dynamics limitation by suspending ultrafine submicron particles (such as 0.1–1.0 μm fused monoclinic zirconia or rare-earth oxides) in a liquid slurry. The quality, purity, and phase stability of these fused ceramic raw materials directly dictate the coating's phase composition, strain tolerance, and thermal insulation efficiency.
SPS process fundamentals: how liquid feedstock feeding works
In standard plasma spraying, dry powder particles are carried by an inert gas stream directly into the plasma plume. In Suspension Plasma Spraying, the mechanism relies on sophisticated liquid fluid dynamics and thermodynamic evaporation stages.
The 4-step droplet evolution in the plasma jet
Mechanical Atomization: The liquid suspension (slurry) is pressurized and atomized through a specialized liquid injector nozzle into fine droplets upon entering the high-velocity plasma jet.
Solvent Flash-Evaporation: As the droplets encounter plasma temperatures exceeding 8,000–10,000 K, the liquid carrier (ethanol or water) instantaneously vaporizes, releasing the agglomerated submicron ceramic particles.
Particle Heating & Melting: The liberated submicron/nano ceramic particles absorb heat from the plasma, fully or partially melting while accelerated toward the substrate.
Deposit & Microstructure Formation: Due to the small inertia of submicron droplets, the flow of plasma gas around surface topographies forces particles to deposit at specific angles, naturally building columnar microstructures similar to EB-PVD coatings.
According to technical research in thermal spray processing, the high momentum drag on small particles allows fine-tuning of the deposit architecture—ranging from segmented vertical cracks to fully dense nano-structured barriers—by adjusting the plasma enthalpy and suspension solid load (typically 10–30 wt%).
According to technical research in thermal spray processing, the high momentum drag on small particles allows fine-tuning of the deposit architecture—ranging from segmented vertical cracks to fully dense nano-structured barriers—by adjusting the plasma enthalpy and suspension solid load (typically 10–30 wt%).
SPS vs. traditional thermal spray processes: technical comparison
Choosing the right surface modification technology requires evaluating microstructure requirements, deposition rates, equipment setup, and raw material specifications. The comparison below highlights how SPS fits alongside conventional thermal spray methods:
| Feature / Attribute | Atmospheric Plasma Spray (APS) | Suspension Plasma Spray (SPS) | High-Velocity Oxy-Fuel (HVOF) | Electron Beam PVD (EB-PVD) |
|---|---|---|---|---|
| Feedstock State | Dry powder (15–100 μm) | Liquid suspension (0.1–2 μm) | Dry powder (15–45 μm) | Solid ingot (vaporized) |
| Microstructure | Lamellar (splats) with pores | Columnar or dense nano-structure | Dense, low porosity | Columnar strain-tolerant |
| Coating Thickness | 100–1000 μm | 20–200 μm | 50–300 μm | 50–150 μm |
| Thermal Conductivity | Moderate (~0.8–1.2 W/mK) | Extremely Low (~0.5–0.8 W/mK) | High (N/A for TBC) | Moderate (~1.5–2.0 W/mK) |
| Process Environment | Atmospheric air | Atmospheric air | High-pressure flame | High vacuum |
| Relative Equipment Cost | Moderate | Moderate to High | Moderate | Very High |
| Submicron Powder Handling | Impractical (clogging) | Excellent (via liquid carrier) | Poor | N/A |
Key material feedstocks: zirconia, rare-earth pyrochlores, and oxides
The performance of an SPS-deposited coating depends heavily on the chemical purity, phase stability, and particle size distribution of the starting material. Advanced fused electro-melting techniques provide the high-purity ceramic powders required for stable suspensions.
1. Zirconia-based thermal spray materials
Yttria-Stabilized Zirconia (YSZ): The gold standard for Thermal Barrier Coatings (TBCs). 7–8 wt% YSZ suspended in liquid carriers provides superior low thermal conductivity and exceptional phase resistance up to 1200°C.
Calcia & Magnesia Stabilized Zirconia (CaSZ / MgSZ): Fused calcia/magnesia stabilized zirconia powders offer high thermal expansion coefficients matching base metal alloys, reducing thermal mismatch stresses in industrial machinery and steel mill rolls.
Fused Monoclinic Zirconia: High-purity monoclinic ZrO₂ serves as the primary raw material for custom stabilizer formulations and specialized high-temperature wear coatings.
2. Next-generation rare-earth pyrochlores
Gadolinium Zirconate & Ytterbium-Co-Doped Zirconia: For ultra-high temperature gas turbines (>1300°C), traditional YSZ undergoes phase transformation and sintering. Fused gadolinium zirconate and complex multi-component zirconate powders deposited via SPS provide high resistance to CMAS (calcium-magnesium-alumina-silicate) molten salt attack.
3. Yttrium oxide & YAG powders
Yttrium Oxide & Yttrium Aluminum Garnet (YAG): High-purity spray granulation powders designed for semiconductor plasma etching chamber components, where high resistance to fluorocarbon plasma erosion is required.
Major industrial applications: aerospace, energy, and semiconductor IT
SPS technology is rapidly transitioning from laboratory R&D to full-scale commercial manufacturing across multiple demanding industrial sectors:
Aerospace gas turbines & jet engines
Modern aero-engines run at temperatures exceeding the melting point of nickel-based superalloys. SPS-sprayed columnar YSZ and rare-earth zirconate coatings act as thermal barriers on turbine blades, combustor liners, and vanes. The strain-tolerant columnar structure allows the coating to expand and contract under severe thermal shocks without spalling off.
Power generation & heavy-duty industrial gas turbines (IGT)
In stationary power generation, operating efficiency scales with combustion temperature. Industrial gas turbines utilize SPS thermal barrier coatings to increase firing temperatures while protecting underlying hot-section hardware from oxidation, hot corrosion, and thermal fatigue.
Semiconductor manufacturing equipment
In plasma-etching vacuum chambers, process components suffer from aggressive halogen plasma erosion. Submicron yttrium oxide and specialized yttrium aluminum garnet coatings sprayed via SPS produce dense, pore-free ceramic surfaces that minimize particle contamination on silicon wafers.
Automotive engine & exhaust systems
SPS coatings are increasingly evaluated for thermal insulation in internal combustion engine piston crowns and manifold channels to increase thermal efficiency, lower emissions, and prevent thermal heat transfer into structural engine blocks.
Common misconceptions about SPS coating technology
Despite its adoption in surface engineering, technical misunderstandings remain regarding the operation and performance of Suspension Plasma Spraying:
Misconception 1: SPS is just standard plasma spraying with wet powder
Fact: SPS requires fundamentally different fluid delivery systems, specialized liquid injection torch hardware, modified plasma gas enthalpy profiles (often utilizing higher helium/hydrogen ratios), and controlled solvent vaporization dynamics. Standard dry powder feeders cannot be retrofitted into SPS without dedicated liquid suspension control units.
Misconception 2: Liquid carriers lower plasma jet temperature and decrease coating quality
Fact: While liquid carrier evaporation does absorb heat energy from the plasma plume, proper plasma torch power tuning (high enthalpy) compensates for solvent cooling. The resulting atomization produces vastly finer molten droplets, enabling high-density or unique columnar microstructures that dry APS cannot achieve.
How to determine if SPS is right for your component surface engineering
Use this decision matrix to evaluate whether Suspension Plasma Spraying matches your surface coating requirements:
30-Second selection checklist
Choose SPS if: You require strain-tolerant columnar microstructures, ultra-thin high-density ceramic barriers (20–100 μm), or need to utilize submicron/nanometer precursor ceramic materials.
Choose APS if: You require thick thermal barriers (>300 μm) on broad surface areas where standard pore-and-splat lamellar microstructures offer acceptable performance at lower operational complexity.
Choose HVOF if: Your target application requires metallic coatings, cermets (WC-Co), or maximum bond strength with dense, wear-resistant mechanical properties at lower process temperatures.
Frequently asked questions
Q: What is the main difference between SPS and SPPS?
A: SPS (Suspension Plasma Spraying) uses a pre-synthesized submicron ceramic powder suspended mechanically in a liquid carrier (ethanol or water). SPPS (Solution Precursor Plasma Spraying) uses chemical precursor salts (like nitrates or acetates) dissolved in a solvent, which synthesize into ceramic particles in-flight via thermochemical reaction within the plasma torch.
Q: Why are YSZ materials preferred for SPS thermal barrier coatings?
A: Yttria-Stabilized Zirconia (specifically 7–8 wt% YSZ) exhibits a rare combination of ultra-low thermal conductivity, high coefficient of thermal expansion (matching metallic substrates), and superior phase stability under thermal cycling conditions up to 1200°C.
Q: How does liquid carrier selection affect the SPS coating process?
A: Ethanol carriers atomize into smaller droplets and evaporate faster due to lower heat of vaporization, promoting finer columnar microstructures. Water carriers offer lower raw material cost and safer operation but demand higher plasma torch power to overcome the heat required for water vaporization.
Q: What raw material parameters matter most for SPS zirconia suspensions?
A: Phase purity, controlled particle size distribution, low impurity levels (low silica/iron content), and optimal electro-kinetic surface chemistry for stable dispersion without premature agglomeration in slurry holding tanks.
Understanding Suspension Plasma Spraying is essential for modern surface engineers seeking high-performance thermal insulation, wear resistance, and plasma erosion protection. By choosing raw materials—such as high-purity fused zirconias, rare-earth pyrochlores, and YAG powders—manufacturers can tailor SPS coating microstructures to survive the world's most demanding thermal environments.
Understanding Suspension Plasma Spraying is essential for modern surface engineers seeking high-performance thermal insulation, wear resistance, and plasma erosion protection. By choosing raw materials—such as high-purity fused zirconias, rare-earth pyrochlores, and YAG powders—manufacturers can tailor SPS coating microstructures to survive the world's most demanding thermal environments.
Understanding Suspension Plasma Spraying is essential for modern surface engineers seeking high-performance thermal insulation, wear resistance, and plasma erosion protection. By choosing raw materials—such as high-purity fused zirconias, rare-earth pyrochlores, and YAG powders—manufacturers can tailor SPS coating microstructures to survive the world's most demanding thermal environments.
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