Application of Silicon Carbide Wear-Resistant Coating in Bushings

Core TBC Technology for Stop Valve Coatings

Thermal barrier coatings on valve parts must provide adhesion, erosion resistance, and thermal insulation while preserving dimensional fit and sealing integrity.

• YSZ-based ceramic topcoat for insulation and thermal fatigue resistance

• MCrAlY bond coat layer for oxidation protection and TBC anchoring

• Plasma spraying under inert atmosphere for porosity control

• Coating thicknesses from 80–250 μm depending on application and exposure

All processes follow AMS 2437, ISO 14923, and NADCAP coating guidelines.

Materials Commonly Coated in Stop Valve Assemblies

These substrates benefit from ceramic coatings that reduce surface oxidation and thermal gradients during cyclic operation.

Case Study: YSZ Coated Inconel 625 Valve Plug for Steam Turbine

Project Background

A customer required a plasma-applied YSZ coating on an Inconel 625 valve plug used in a high-pressure steam turbine operating at 920°C. Coating target thickness was 150 μm with Ra ≤ 5 μm surface finish. MCrAlY bond coat was applied first, followed by YSZ top layer.

Typical Coated Valve Components and Applications

Coatings are designed for thermal insulation, sealing surface preservation, and dimensional stability over long service intervals.

Challenges in Thermal Barrier Coating Stop Valve Components

1. Bond coat adhesion failure occurs after 500 thermal cycles when substrate surface roughness exceeds Ra 6.0 μm before coating.

2. Erosion from steam particulates leads to YSZ topcoat thinning beyond 25% within 1000 hours in 30 m/s flow systems.

3. Cracking in topcoat layers originates at sharp corners with radii under 1.0 mm under thermal shock above 950°C.

4. Delamination risk increases when thermal expansion mismatch exceeds 15 × 10⁻⁶/K between substrate and ceramic layers.

5. Surface finish requirements below Ra 5 μm are critical for sealing zones and must be maintained post-coating.

Plasma TBC Solutions for Stop Valve Assemblies

1. LPPS plasma spraying achieves porosity under 5% and bond strength exceeding 30 MPa for high-cycle thermal environments.

2. Masking accuracy ±0.1 mm protects sealing faces during spray process, ensuring no ceramic on machined surfaces.

3. YSZ with 8 wt.% Y₂O₃ stabilizer maintains tetragonal phase stability up to 1200°C under repeated thermal cycling conditions.

4. Pre-spray HIP treatment at 1030°C removes internal porosity before ceramic layer application for better adhesion.

5. CMM verification ensures all critical coated dimensions remain within ±0.01 mm post-coating.

Results and Verification

Manufacturing Methods

Substrates were CNC machined from forged or cast Inconel and Hastelloy. Bond coats were plasma-applied followed by ceramic topcoats under controlled temperature and atmosphere.

Precision Finishing

Post-spray surface finish was honed to Ra 4.8 μm. Critical dimensions were re-checked by CMM and hand-lapped where sealing integrity was required.

Post-Processing

Parts received heat treatment after TBC application for bond stabilization. Final passivation was performed to eliminate residual contaminants.

Inspection

X-ray testing validated coating adhesion and layer thickness. SEM confirmed no delamination or cracks. All coatings met customer specs for adhesion (≥30 MPa) and thermal resistance.

FAQs

1. What ceramic compositions are used for valve TBCs?

2. How do you ensure adhesion of coatings on curved valve surfaces?

3. What is the typical coating thickness for turbine stop valves?

4. Can sealing surfaces be coated or must they be masked?

5. Are TBCs applicable to stainless steel valve bodies?