YSZ Powder for Thermal Barrier Coatings: Why '8YSZ' Names Two Different Materials

The most dangerous word on a thermal barrier coating purchase order is "8YSZ". In thermal spray practice it means yttria (Y₂O₃) at 8 wt%, which is about 4.5 mol%. In solid oxide fuel cell practice the same name means 8 mol%, which works out arithmetically to roughly 13.7 wt%. Two materials with different phase content, different function, and about 1.7 times the difference in yttria loading circulate under one label. The practical conclusion first: write "Y₂O₃ 7.0–8.0 wt%" on the specification, not "8YSZ", and check the unit on the incoming certificate of analysis before the drum reaches the spray booth.

The problem: one name, three notations

Any YSZ composition written with the number "7" or "8" can mean one of three things, depending on which industry's convention the writer was following.

The third row is the nastiest. Papers that count yttrium oxide as YO₁.₅ rather than Y₂O₃ write the standard TBC composition as "7.8 mol%" — a number that collides by coincidence with the "7 to 8" of the weight-percent convention. Seeing "about 8" tells you nothing on its own about whether it is wt%, mol% Y₂O₃, or mol% YO₁.₅.

The gap is not a performance difference. It is a difference in material. What a TBC needs is the metastable t′ (tetragonal prime) phase that forms in the partially stabilized 7–8 wt% window, and that phase is the source of the coating's toughness and phase stability. A fully stabilized cubic composition with far more yttria exists to conduct oxygen ions, which is a different job entirely — the composition trade from the electrolyte side is covered in our YSZ vs ScSZ comparison. One frequent source of confusion is worth clearing here: "7YSZ" and "8YSZ" in TBC usage are not the two materials in question. One major supplier's 8% YSZ datasheet specifies Y₂O₃ at 7.0–8.0 wt%; another supplier labels its 7YSZ line "93-7", a nominal 7 wt% yttria — the same TBC material, differently rounded.

Why the notations diverged, and what a datasheet actually specifies

Thermal spray powder datasheets settled on weight percent; electrolyte and ionic conduction literature settled on mole percent. Neither industry is confused internally, so neither had reason to change, and the ambiguity survives at the boundary — at powder suppliers who sell into both markets, and at buyers who research a specification by searching for it. Below are the axes a real TBC powder specification is built from.

Composition: why 7–8 wt% specifically

Below full cubic stabilization, this composition forms the metastable t′ phase that carries high-temperature toughness. The resulting coating gives an effective thermal conductivity on the order of 1 W/m·K, and its thermal expansion coefficient of about 11×10⁻⁶/K sits unusually close, for a ceramic, to the roughly 14×10⁻⁶/K of a nickel-base superalloy substrate; the residual mismatch is absorbed by coating porosity of around 10%. The ceiling comes from the same composition. Above roughly 1200 °C in long exposure, detrimental decomposition of the metastable t′ phase and accelerated sintering compound each other, and 1200 °C is the figure usually quoted as the long-term service limit for this family. One reading note for COAs: the "ZrO₂ balance" line on these datasheets includes HfO₂ up to 2.5%, because hafnia occurs naturally with zirconia and is conventionally not treated as an impurity.

Impurity grades: a 14-fold spread inside one product family

Within a single supplier's 8% YSZ datasheet, impurity limits are not one set of numbers but four. The figures below are from the Oerlikon Metco HOSP 8% YSZ datasheet (wt%, maximum). The tier names in the left column are ours, added for readability — the datasheet distinguishes the tiers by product designation, and the row-to-product mapping did not survive our extraction of the source PDF, so we do not assign specific products to specific tiers here.

On SiO₂ alone the spread from top tier to general purpose is fourteenfold. The rationale for the structure is sintering resistance: the fewer low-melting impurities present, the better the coating holds up. The datasheet states that high purity material improves coating performance "even at temperatures significantly above 1200 °C", and, for its premium products, attributes extended coating life to high sintering resistance and low shrinkage. Burner-rig thermal cycling work points the same way, treating low-SiO₂ high-purity YSZ as an initial prerequisite for minimizing sintering and loss of compliance; in that study, coatings from agglomerated and sintered powder measured 0.21 wt% SiO₂, about twice the level of the HOSP-derived coatings.

Monoclinic phase content has to be read differently. The limit is real as a specification item — the datasheet caps it at 6–10% depending on grade — but the same burner-rig study reports that a low monoclinic content in the feedstock showed no positive effect on TBC cycle life. Verify compliance with whatever limit the specification carries; the evidence for paying a premium to go lower than the limit is not there.

Particle size and manufacturing route: the powder pre-determines the microstructure

Powders for atmospheric plasma spray (APS) come from three manufacturing routes, each aimed at a different coating structure.

The general rules: higher porosity lowers thermal conductivity and improves thermal shock resistance, lower porosity improves erosion resistance; finer powder produces a denser coating, coarser powder a more porous one. Datasheet practice is to state D10/D50/D90 from laser diffraction, referenced to ASTM B822. Apparent density and Hall flow, where they appear, are normally referenced to ASTM B212 and B213.

All of the above concerns APS powder. Other processes do not take powder at all in the same form: EB-PVD evaporates an ingot, and suspension plasma spray (SPS) feeds a suspension of submicron powder that is not flowable enough to feed dry. If the coating process changes, the physical form of the feedstock changes with it, and a powder specification does not transfer.

Specifications: OEM documents govern the powder

On the coating process side there is an SAE AMS standard, and for plasma spray deposition it is AMS 2437. It is worth naming the error this replaces: AMS 2447 is sometimes cited in a TBC context, but that standard covers the high velocity oxygen/fuel (HVOF) process and does not apply to a plasma-sprayed YSZ top coat. For the powder itself, public standards are not what governs — engine OEM specifications are. Examples visible in one supplier's published approval table include Pratt & Whitney PWA 1375, GE Aviation A50TF278 Class A through D, GE A50A557 and A50A558, Honeywell EMS 57750, MTU MTS series, and Rolls-Royce RRMS 40042. The specification texts themselves are mostly not public, which is why "approved to which OEM specification" functions in practice as the quality signal.

Supply and demand pulling in opposite directions

The supplier landscape for TBC-grade YSZ powder is established and largely Western: Oerlikon Metco (the Metco 204 family), Höganäs (Amperit), Saint-Gobain (APS powders plus EB-PVD ingots), and Treibacher Industrie among them. The pressure is upstream of them. China's April 2025 MOFCOM Announcement No. 18 placed yttrium, along with six other medium and heavy rare earths, under export licensing, and the effect is measurable: yttrium exports from China to the United States ran to 17 tonnes across April–December 2025, against 333 tonnes in the eight months before the controls, and February 2026 came in at 20 tonnes against more than 66 tonnes in January 2025. Manufacturers of thermal coating materials for aero engines have warned of shortage and rationing. Prices outside China rose by a large multiple while domestic Chinese prices stayed low — sources differ widely on the size of that multiple, so we do not anchor on a specific number. The structure of the licensing regime and what happens at its next decision point are covered in our yttrium export control analysis. On the zirconia side, market research puts China at roughly 90% of zirconium chemical production; that is a single-source estimate and we treat it as indicative only.

Demand points the other way. GE Vernova's gas turbine backlog plus slot reservations reached 116 GW in Q2 2026 and Siemens Energy has reported record gas services backlog; global aviation MRO demand grew to about $136 billion in 2025, with the engine share of the commercial aftermarket forecast to keep rising as the average fleet age climbs. TBC powder is consumed both in new engines and turbines and in recoating during overhaul, so both indicators point the same direction — though we have found no public basis for converting either into tonnes of powder, and we do not attempt it. In Korea, Doosan Enerbility has described securing coating technology as part of its H-class gas turbine localization, and Sungil Turbine in Busan performs thermal barrier coating and refurbishment of gas turbine hot section components. Both accounts rest on company press material, and we cite them only as confirmation that TBC demand and coating capability exist domestically.

What to put in the purchase specification

At minimum, we suggest the specification document state the following.

• Composition with the unit attached: "Y₂O₃ 7.0–8.0 wt%", never "8YSZ" alone. Recording that ZrO₂ balance includes HfO₂ (up to 2.5% on the Metco datasheet cited above) heads off an argument over the COA later.
• Impurity limits: at least a SiO₂ maximum matched to the application. For hot-section, long-life duty the case for a low-SiO₂ grade is well founded in sintering resistance.
• Monoclinic maximum: request it within the conventional 6–10% band; the case for paying to go below the limit is weak (see above).
• Particle size: D10/D50/D90 per ASTM B822, worked backwards from the coating you want, dense or porous.
• Manufacturing route: F&C, A&S, or HOSP, stated explicitly. Identical composition and size distribution from different routes give different coatings.
• Governing specification: where an applicable OEM specification exists, pinning the order to that number is the strongest single line in the document.

The trap this article opens with is caught at incoming inspection, in the COA comparison. Check the unit on the composition line first. If Y₂O₃ reads 13–14 wt%, an 8 mol% material has arrived — electrolyte grade, not coating grade. If the COA is written in mol%, then roughly 4.5 mol% is the TBC figure and 8 mol% is not. The general protocol for lot-level measurement verification, cross-checking morphology, composition, particle size, and crystal phase, is set out in our SEM/EDS powder verification method.

Frequently asked questions

What happens if SOFC-grade 8YSZ (8 mol%) is used for a TBC?

It is a specification violation and, more to the point, the wrong material. The metastable t′ phase that gives a TBC its properties forms in the partially stabilized 7–8 wt% window, while 8 mol% (roughly 13–14 wt%) is a fully stabilized cubic composition. We have not found published coating-life data for that specific misapplication, but the compositions differ on their face, which is why the unit check at incoming inspection is the control that matters.

Is 7YSZ different from 8YSZ?

Not in the sense buyers usually fear. Within TBC practice both labels refer to the same partially stabilized material — one supplier's "8% YSZ" datasheet specifies Y₂O₃ 7.0–8.0 wt%, and another's "7YSZ" portfolio is designated "93-7", a nominal 7 wt%. The distinction that actually changes the material is wt% versus mol%, not 7 versus 8.

Isn't lower monoclinic content always better?

It is a genuine specification item, capped at 6–10% depending on grade. But burner-rig testing has reported that low monoclinic content in the feedstock showed no positive effect on thermal cycle life, so the causal link is contested. Verify compliance with the limit; justifying a grade premium on this line alone is hard.

Should we simply always buy the highest purity tier?

No. The four tiers exist because applications differ. For hot-section parts and long-life duty above 1200 °C the argument for a low-SiO₂ high-purity grade is clear on sintering resistance, but not every location on the machine runs those conditions. The datasheet's own guidance is to choose the product that meets the required customer material specification — the criterion is the specification, not the tier.

References (public sources)

• Oerlikon Metco, datasheet DSMTS-0001.8, 8% yttria stabilized zirconia (HOSP) — composition, the four impurity tiers, monoclinic limits, ASTM B822 particle sizing.
• Höganäs, Amperit thermal barrier coating technical bulletin — the three manufacturing routes and the OEM approval table (no publication date shown; current version as accessed August 2026).
• Vaßen et al., Journal of the American Ceramic Society, 2021 — YSZ thermal conductivity, the 1200 °C limit, and an example of YO₁.₅ mole fraction notation.
• Journal of the European Ceramic Society, 2022 — burner-rig thermal cycling of 8YSZ coatings: the effect of SiO₂ and the reported absence of a monoclinic-content effect.
• Materials (open access), 2022 — the "8 wt.% (~4.5 mol%) Y₂O₃" formulation.
• MDPI Coatings, 2021 — review of the t′ phase and partially stabilized compositions.
• Heliyon, 2023 (open access review) — YSZ thermal expansion of about 11×10⁻⁶/K against roughly 14×10⁻⁶/K for a nickel-base superalloy, and coating porosity of around 10% absorbing the mismatch.
• SAE AMS 2437 (plasma spray deposition) and AMS 2447 (HVOF) — process standards, cited to distinguish their scopes.
• ASTM B212, B213, B822 — apparent density, Hall flow, and light-scattering particle size distribution.
• Holland & Knight commentary, April 2025 — China MOFCOM and General Administration of Customs Announcement No. 18, licensing of seven medium and heavy rare earths.
• CSIS, 2026 — yttrium export statistics one year into the controls and the reported effect on coating material producers.
• Market research (fused zirconia market report), 2026 — the estimate that China accounts for roughly 90% of zirconium chemical production; single-source, treated as indicative only.
• Turbomachinery Magazine and GE Vernova quarterly disclosure, 2026 — gas turbine backlog; and Turbomachinery Magazine coverage of Siemens Energy's record gas services backlog, 2026.
• Oliver Wyman Global Fleet & MRO Forecast, February 2026 — aviation MRO market size and engine share.
• Doosan Enerbility newsroom material and Korean trade press on Sungil Turbine — company-sourced, cited as such.
• Byun Eung-seon, "Thermal Barrier Coatings for Gas Turbine Engine Components", Prospectives of Industrial Chemistry, 2014 — Korean-language technical review.

Figures above reflect public sources as of the time of writing; the yttrium export statistics and the price situation in particular can change quickly.

Nami Tech Solutions (NTS) does not hold TBC-grade YSZ powder as standing inventory. We work project by project instead — fixing the specification in one document down to the composition unit and the impurity limits, and reading the mill COA against that document line by line, so that a material sharing the name does not reach the line in place of the material that was ordered.