Plasma Yttria-Stabilized Zirconia (8YSZ) Coatings
A Ceramic Built to Keep Heat Out
Yttria-stabilized zirconia, or YSZ, is the workhorse ceramic for thermal barrier coatings. It conducts heat poorly, stands up to thermal cycling, and holds its structure at temperatures that would destroy most metals.
The version on this page is 8YSZ. Zirconium oxide with 8 percent yttria by weight. That ratio is the industry standard for plasma sprayed thermal barriers.
A&A Coatings applies 8YSZ with the plasma spray process. This page covers what it is, what it does, and what to check before you spec it.
Need a quote? Call 888-725-0150 or send us a request. Our team replies the same business day.
Why Yttria
Pure zirconia has a problem. As it heats and cools, it changes crystal structure. Each change comes with a volume shift. Those shifts crack the coating from the inside.
Yttria fixes that. It holds the material in a stable phase so the volume change does not happen. Eight percent by weight is the standard ratio. Lower ratios do not stabilize as well. Higher ratios reduce the coating’s toughness.
The yttria also creates oxygen vacancies in the crystal lattice. Those vacancies scatter phonons, which is the technical way to say they break up heat flow at the atomic level. The result is lower thermal conductivity.
Thermal Conductivity
Air plasma sprayed 8YSZ typically lands between 0.8 and 1.5 W/m·K at high temperature. The exact value depends on the coating structure, the porosity, and the spray parameters. Those are not small variables. Two coatings with the same chemistry can measure differently if the microstructure differs.
The low conductivity comes from two things. First, the material itself conducts heat poorly. Second, the plasma sprayed structure has pores and micro-cracks that interrupt heat flow. If you densify the coating, conductivity climbs and the barrier effect weakens.
If you need a conductivity figure for a spec, tell us the part and the service conditions. We test per job and provide the number with the finished coating.
Service Temperature
The practical ceiling for 8YSZ sits around 1200°C. Past that line, two things happen.
First, the phase structure starts to change. Yttria-stabilized zirconia is stable at high temperature, but not indefinitely. Above the limit, the structure shifts, and the coating loses the properties that made it useful.
Second, the coating sinters. Pores close up. Thermal conductivity rises. Strain tolerance drops. A coating that was flexible becomes brittle.
None of that happens all at once. The coating degrades over time at temperature. The rate depends on the actual service conditions, the cycle profile, and the fuel. If your part runs near or above 1200°C, tell us up front and we will give you a realistic service estimate.
Pairing With a Bond Coat
An 8YSZ topcoat does not go directly onto the metal. It needs a bond coat first.
On an 8YSZ system, the bond coat is usually MCrAlY. The metal in the mix is nickel or cobalt. The chromium and aluminum form the oxide that anchors the ceramic.
The bond coat does two jobs. It gives the ceramic a surface it can grip. And it forms a protective oxide layer called the thermally grown oxide, or TGO.
The TGO is mostly aluminum oxide. It is dense, it sticks to the bond coat, and it blocks oxygen from reaching the metal underneath. That is what keeps the substrate from oxidizing away.
Bond coat thickness for 8YSZ systems usually runs in the 100 to 150 micron range. The exact number depends on the substrate and the service conditions. We set it per job.
Some systems add an aluminizing step on top of the MCrAlY. That adds a thin layer of aluminum that forms a more stable TGO. The result is longer coating life in high temperature service.
Typical Thickness
Topcoat thickness for plasma sprayed 8YSZ usually runs 200 to 500 microns. For gas turbine blades, 250 microns is a common target.
Thicker is not automatically better. A thicker ceramic layer stores more elastic energy. When a crack forms at the interface, that energy releases and the coating spalls faster. Thicker coatings often fail sooner under thermal cycling than thinner ones.
The right thickness is a balance between insulation and strain tolerance. Tell us the operating temperature and the cycle profile, and we will recommend a range for your part.
Substrates
8YSZ works on high temperature alloys when the surface is prepped correctly. The usual substrates are:
- Nickel superalloys
- Cobalt superalloys
- Stainless steels
Nickel and cobalt superalloys are the standard for turbine and engine service. Stainless steels show up in lower temperature industrial applications where the coating still helps.
Prep is not optional. The surface has to be clean and free of oxides. We use a grit blast to strip the surface and roughen it. That roughness is what gives the bond coat something to grip. Without it, the coating system fails at the interface.
How 8YSZ Fails
Failure usually starts at the interface between the bond coat and the ceramic.
TGO growth. The bond coat oxidizes. The TGO thickens. Stress builds at the interface. Cracks form and spread. The ceramic lifts off.
CMAS attack. Sand, ash, and dust melt on the hot coating surface and soak into the pores. On cooling, the infiltrated layer cracks and takes the ceramic with it. This is a particular problem in dusty environments.
Sintering. At high temperature, the pores close up. Conductivity rises. Strain tolerance drops. The coating becomes brittle and cracks under cycling.
Thermal cycling. Every heat and cool cycle stresses the interface. Over thousands of cycles, cracks grow and the coating spalls.
Each of these is covered in more detail on our thermal barrier coatings guide, along with inspection and repair options.
Where 8YSZ Is Used
Gas turbines. Turbine blades, vanes, combustor liners, and transition pieces. 8YSZ is the standard topcoat for thermal barrier systems in both aero and industrial turbines.
Aeroengines. High-pressure turbine blades run at the highest temperatures in the engine. 8YSZ works with internal cooling and film cooling to keep the metal below its limit.
Diesel engines. Piston crowns and exhaust valves. The coating lowers surface temperature, which lets the engine run higher combustion temperatures without damaging the metal.
Industrial components. Nozzle liners, exhaust valves, and other high temperature parts benefit from the same thermal protection.
Finishing
8YSZ is usually left as-sprayed for thermal barrier service. The rough surface actually helps with thermal performance because it breaks up heat flow.
If your application needs a smoother surface, we can grind it. Diamond wheels are the standard tool. Wet grinding is preferred over dry because it keeps the surface cool and reduces the risk of micro cracking.
Tell us the finish requirement at quote time and we will match it.
Quality and Specifications
A&A Coatings holds AS9100 and ISO 9001 certifications and meets NADCAP requirements for coatings. Our in-house lab checks thickness, bond strength, hardness, and microstructure before parts ship. Customer specs welcome, with certification paperwork supplied on request.
Hardness and bond strength for 8YSZ depend on the substrate and the spray parameters. We test per job and can share the results on request.
Related Pages
For a broader look at thermal barrier systems, including bond coat selection, failure modes, and inspection, see our Thermal Barrier Coatings guide.
For the three-part zirconia, titania, yttria composite, which trades some thermal performance for better wear resistance, see Plasma Zirconia, Titania, Yttria Composite.
Start Your Quote
Call 888-725-0150 or send your part details through our quote form. Tell us the operating temperature, the cycle profile, and what the part does. We will recommend a coating system and tell you what it costs.
Same business day reply.
