The most common misconception when buying "round SiC powder" is that spherical means one and the same thing. In reality, SiC powders that look round divide into at least two categories, with entirely different manufacturing principles, prices, and applications. The bottom line: SiC powder should be graded into three types — crushed angular, spray-dried granule, and plasma-spheroidized — and the crux of supplier verification is not "is it spherical" but "is it a spray-dried granule or a plasma bead."
Three Grades of SiC Powder
| Grade | Process | Morphology | Rough price | Main use |
|---|---|---|---|---|
| ① Crushed (angular) | Milling + sieving | Sharp angular | ~$2–3/kg | Abrasives |
| ② Spray-dried granule | Fines + binder + sintering aids, spray-dried | Porous spherical (dimpled) | $5–15/kg | Press sintering |
| ③ Plasma spheroidized | Plasma rounding in a narrow thermal window | Dense true beads | Tens of $/kg+ | Thermal spray, 3D printing (AM) |
Of these, the grades ordinarily bought as feedstock for pressing and sintering now sit on two separate lines: the ready-to-press granules at 60–90 µm, listed with their bulk density on our spray-dried SiC granules page, and the submicron sintering fines — the W0.5 cut — on our green SiC micro powder page, where the polishing grades sit too and the size is stated by standard rather than a bare grit number, since JIS, FEPA and the Chinese W grade put similar digits on materially different powders.
① Crushed (Angular) Powder
The most basic form. It is made by milling SiC lumps and classifying by sieve. The morphology is sharp and angular, and it is used mainly as an abrasive. One misconception needs correcting here: angular does not mean low quality. Morphology (angular vs spherical) and purity (impurity content) are independent axes. Angular powder can be high purity. The real weakness of angular powder is not purity but flowability — angular particles interlock, flow poorly, and fail to fill a press die uniformly, creating density variation in the green body.
② Spray-Dried Granule (RTP, Ready-to-Press)
This is the grade used for press-sinter forming. A slurry of fine (sub-micron) SiC mixed with binder and sintering aids (carbon, B₄C, and similar) is spray-dried, and as the small droplets dry they form porous spherical granules. These granules look round, but they are not solid beads — they are porous agglomerates of fines held together.
The most recognisable mark of a spray-dried granule is a surface dimple: during droplet drying the surface collapses inward, leaving a donut-shaped depression. Read that test in one direction only. A dimple under SEM is strong evidence of spray drying, but its absence is not evidence against it — dimple formation depends on solids loading and drying rate, and a dense, fast-dried granule can come out smooth. In practice these granules are designed to crush during press forming, and the binder that was mixed in is removed by thermal decomposition during sintering. Detecting a carbon-rich composition or B₄C is a normal signal of a pressureless-sintering sintering-aid formulation.
③ Plasma-Spheroidized Powder
The most expensive grade, and the one whose mechanism is most often described wrongly. SiC has no stable liquid at atmospheric pressure — heated far enough it decomposes rather than melts, peritectically into a silicon-rich liquid and vapour plus free carbon, in the region of 2500–2800 °C. So plasma spheroidization is not "melt and resolidify." The route depends instead on a thin, transient silicon-rich layer at the particle surface that surface tension can pull round, which means it runs inside a narrow thermal window: too little energy and the particle stays angular, too much and it loses silicon to the gas phase. Some silicon loss and a shift in surface stoichiometry are therefore normal consequences of the route rather than defects — which makes surface composition (free carbon, free silicon, surface Si/C ratio) a fair thing to ask a supplier to report, particularly if the beads are headed for a coating or a build where surface chemistry drives adhesion. What the route does deliver is dense beads with almost no internal voids, unlike the porous granules. Their excellent flowability and packing density make them a feedstock for thermal spray coating and metal 3D printing (additive manufacturing, AM). The price climbs to tens of dollars per kilogram and up.
Why the Distinction Matters
Even among "round SiC powders," a mismatch in application causes a process to fail. Feeding plasma beads into a press-sinter line gives different forming and sintering behavior; conversely, a green press-forming granule fed to a thermal-spray or AM line is liable to break down in handling and injection instead of arriving intact. The disqualifying property there is crushability, not spray drying as such — agglomerated-and-sintered (A&S) thermal-spray powders are spray-dried and then sintered to strength, and A&S YSZ is a standard thermal-barrier-coating feedstock. So for a round powder headed to a spray booth, the question is not whether it was spray-dried but whether it was sintered to strength afterwards. Notably, some products sold under the name "granule" in the market are in fact closer to crushed angular powder, and they are hard to tell apart from photos or the naked eye alone. That identification problem is examined with a real case in Powder quality verification methodology.