Selecting the wrong feedstock powder for plasma spray operations doesn’t just degrade coating performance — it drives rework rates above 15%, compromises bond strength below the 20 MPa threshold required for aerospace and turbine applications, and introduces porosity levels that accelerate corrosion in aggressive service environments. 茶色の溶融アルミナ (BFA) has become a preferred feedstock precisely because its chemistry, 形態学, and thermal stability can be engineered to meet plasma spray’s demanding process window. Understanding what separates a functional BFA grade from a liability-grade powder is the first step toward reproducible coating quality.
Why Brown Fused Alumina Suits the Plasma Spray Process
Plasma spray generates a plasma jet exceeding 10,000 °C at the torch core, where feedstock particles must melt completely within milliseconds, then solidify into dense, well-adhered splats on the substrate. BFA’s melting point of approximately 2,050 °C and high purity — typically 95–97% Al₂O₃ — make it thermally compatible with these conditions without the premature vaporization losses seen in lower-density oxides.
The material’s dominant α-Al₂O₃ (コランダム) phase contributes directly to coating hardness values in the range of 1,500–1,800 HV, making deposited layers highly resistant to abrasive and erosive wear. Because BFA is produced by fusion in arc furnaces rather than sintering, it delivers consistent phase purity lot-to-lot — a requirement for any thermal spray specification that calls out coating microhardness as a hold point. You can review the full material profile on our 褐色電融アルミナ product page.
Critical Powder Specifications That Govern Coating Outcome
Not every BFA grade is suitable for plasma spray. The particle size distribution, 形態学, flowability, and bulk chemistry each influence how the powder feeds through the injector, how uniformly it melts in the plasma plume, and what microstructure forms on impact.
| パラメータ | Recommended Range for Plasma Spray | Effect if Out of Spec |
|---|---|---|
| 粒子サイズ (D50) | 15–45 µm | Coarser particles under-melt; finer particles over-melt and vaporize |
| Al₂O₃純度 | ≥ 95.5 重量% | Fe₂O₃ and TiO₂ impurities promote spinel phases, reducing hardness |
| かさ密度 | 1.60–1.90 g/cm³ | Low density causes feed-rate instability and coating thickness variation |
| Flowability (Hall flow) | ≤ 35 s/50 g | Poor flow yields pulsed injection, producing lamellar porosity defects |
| Moisture content | < 0.2 重量% | Steam generation in the plasma causes splatter and delamination risk |
Morphology and Surface Texture: Angular vs. Spheroidized Grades
Standard crushed-and-classified BFA produces angular particles with irregular facets. This morphology is acceptable for many industrial plasma spray applications — pump sleeves, textile machinery guides, and wear-resistant overlays — where maximum packing density and mechanical interlocking are valued. Angular particles also present a higher specific surface area, which slightly improves heat absorption during plume transit.
For tighter porosity control in high-specification coatings (aerospace thermal barriers, medical implant surfaces), plasma-spheroidized or fused-and-crushed powders with a more equiaxed shape are preferred. Spheroidized BFA achieves Hall flow values below 25 s/50 g and produces more uniform splat morphology. The trade-off is cost: spheroidization adds 40–70% to feedstock price, so specifiers should benchmark the application requirement before upgrading the grade.
Comparing Brown Fused Alumina Against Alternative Plasma Spray Feedstocks
Engineers occasionally consider white fused alumina (WFA) or tabular alumina as alternatives. While WFA offers lower Fe₂O₃ content (typically below 0.1 重量%), its higher cost is only justified when coating color or extreme chemical purity is a customer deliverable — for example, in food-contact or semiconductor equipment applications. For a broader perspective on white alumina’s use cases, the article on What Is White Corundum Used For outlines where the grade differences matter most.
Tabular alumina, 対照的に, is sintered rather than fused, yielding a more porous internal grain structure. Under plasma spray conditions this translates to inconsistent melt behavior and elevated porosity in the deposited coating — a liability in any application requiring hermeticity. BFA’s fused, dense grain provides the most cost-effective combination of thermal stability, 硬度, and feed consistency for the majority of industrial plasma spray work.
表面処理: Grit Blasting BFA Before Spray Deposition
Before feedstock powder is introduced to the plasma torch, the substrate must be grit blasted to a surface roughness of Ra 4–10 µm to develop mechanical adhesion for the coating. BFA is also widely used as the blast media for this step. Using the same material family for both preparation and deposition eliminates cross-contamination risk from embedded foreign-oxide particles — a real concern when steel grit or silica sand is used on surfaces that will receive oxide ceramic coatings.
Recommended blast media for plasma spray substrate preparation includes:
- BFA grit 16–36 mesh for aggressive anchor profile generation on steel and titanium substrates
- BFA grit 46–60 mesh for aluminum and softer alloys where excessive surface damage must be avoided
- Recycled BFA (up to 3–5 passes) for high-volume production lines where media cost is a driver, provided contamination monitoring is in place
- Virgin BFA for aerospace and medical applications where substrate cleanliness is a certification requirement
Sourcing and Quality Verification for Plasma Spray Grades
Plasma spray coating shops operating to NADCAP, AS9100, or ISO 9001 quality systems typically require material test reports (MTRs) certifying chemistry, particle size distribution (PSD), and phase composition for each lot. When sourcing BFA, request X-ray diffraction (XRD) data confirming α-Al₂O₃ phase dominance and laser diffraction PSD reports showing D10, D50, and D90 values. A wide D90/D10 ratio above 4.0 is a warning sign for inconsistent melting behavior in the plasma plume.
HSA supplies 褐色電融アルミナ in both standard crushed grades and tight-distribution plasma spray grades, with full lot-level documentation. Given that high-performance ceramic coatings are also used in kiln and refractory environments, related feedstock considerations — such as those discussed for 日本製耐火物窯屋根煉瓦用アルミナ中空球 — demonstrate the breadth of alumina material engineering required across thermal applications. Procurement teams should confirm minimum order quantities, lead times, and certificate of conformance (CoC) availability before finalizing supplier qualification.
よくある質問
Q: What particle size range of brown fused alumina is recommended for plasma spray feedstock?
あ: For most industrial plasma spray applications, a D50 particle size of 15–45 µm is standard. Particles coarser than 60 µm risk incomplete melting in a typical 35–45 kW plasma torch, resulting in unmelted inclusions and reduced coating density. Finer fractions below 10 µm can vaporize or over-oxidize, increasing γ-Al₂O₃ phase content and reducing hardness. Tight PSD with a D90/D10 ratio below 3.5 is preferred for specification-controlled coating work.
Q: What minimum Al₂O₃ purity is required in brown fused alumina for high-performance plasma spray coatings?
あ: A minimum of 95.5 wt% Al₂O₃ is the standard threshold for industrial plasma spray grades. For aerospace or medical applications requiring NADCAP process compliance, suppliers typically provide BFA with ≥ 96.5 wt% Al₂O₃ and Fe₂O₃ content below 0.3 重量%. Elevated TiO₂ (その上 3 重量%) promotes hercynite and spinel phase formation during rapid solidification, degrading coating microhardness below the 1,400 HV target common in wear-resistant applications.
Q: How does brown fused alumina compare to white fused alumina as a plasma spray feedstock?
あ: 茶色の溶融アルミナ (BFA) contains 2–4 wt% TiO₂ which slightly lowers melting point and improves toughness of the deposited coating — beneficial for thermal cycling applications. 白色溶融アルミナ (WFA) offers higher purity (≥ 99% Al₂O₃) and is used where coating discoloration, 耐薬品性, or semiconductor cleanliness standards apply. For general industrial wear coatings, BFA provides a 30–50% cost advantage over WFA with equivalent hardness performance at equivalent PSD.
Q: Can the same brown fused alumina grade be used for both substrate blasting and plasma spray feedstock?
あ: Different size fractions are needed. Substrate blasting typically uses BFA grit 16–36 mesh (500–1,180 µm) to produce Ra 4–10 µm anchor profiles. Plasma spray feedstock requires 15–45 µm powder, several orders of magnitude finer. でも, using BFA for both steps eliminates the risk of embedding incompatible foreign oxides — such as iron from steel grit — which can nucleate coating delamination at the bond coat interface.
Q: What documentation should buyers request when sourcing brown fused alumina for certified plasma spray processes?
あ: Buyers operating under NADCAP, AS9100, or ISO 9001 quality systems should request: (1) a full chemistry certificate with XRF analysis confirming Al₂O₃, TiO₂, Fe₂O₃, SiO₂, and Na₂O values; (2) laser diffraction PSD report with D10, D50, D90, and span values; (3) XRD phase analysis confirming α-Al₂O₃ dominance; (4) bulk and tap density data; と (5) a certificate of conformance (CoC) traceable to the production lot. Reputable suppliers can provide all five documents per shipment.
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