Thermal spraying is widely used in aero-engine manufacturing to apply sealing coatings and enhance performance.
As critical engine components, the casing parts use plasma-sprayed nickel-chromium coatings to improve performance. The TC25 titanium-alloy substrate, large coating area, and thin-walled design make spraying and machining difficult.
Coating application often causes localized or widespread detachment, and stripping and recoating do not resolve the issue. To improve coating yield and ensure delivery and operational requirements, …
This study examines coating detachment causes and validates measures to address nickel-chromium issues on thin-walled titanium-alloy components.
Current Status
To better understand the persistent coating-detachment issues on thin-walled titanium-alloy casings, it is necessary to first review the current coating practices and the characteristics of the applied process.
A clear understanding of the coating workflow, critical parameters, and influencing factors provides the foundation for identifying the root causes of failures and developing targeted improvement measures.
Therefore, the following section outlines the plasma-sprayed nickel-chromium coating process used on these components.
Overview of the Plasma Sprayed Nickel-Chromium Coating Process
Thermal-sprayed coatings offer superior properties, but mishandling key factors can cause failure.
A specific engine casing requires plasma-sprayed nickel-chromium coatings on three surfaces (Figure 1), with thickness based on machining. The coating sequence is:
Part preparation → Degreasing and cleaning → Masking and protection → Sandblasting for surface roughening → Coating application → Post-spray treatment → Coating machining → Inspection.
Issues with Plasma Sprayed Nickel-Chromium Coatings
Statistics show nickel-chromium coatings on a certain engine casing rarely pass the first attempt (<10%), typically requiring 2–4 passes.
In production, the nickel-chromium coating on this engine casing exhibits flaking, large-area peeling, and detachment, including post-machining.
These large, thin-walled titanium casings (Φ>500 mm, ~3 mm) risk deformation and scrap if re-sprayed after machining.
Improving first-pass yield of the nickel-chromium coating ensures quality and cuts coating and machining costs.
Analysis of Factors Affecting
Understanding the underlying mechanisms that drive coating detachment requires a detailed examination of the factors influencing coating integrity during plasma spraying.
Although the process steps are well established, the interaction between thermal, mechanical, and material-related variables becomes particularly critical for large, thin-walled titanium-alloy components.
These factors collectively determine coating adhesion, residual stress distribution, and overall stability.
The following sections analyze the key influences on coating performance, beginning with the effects of thermal expansion mismatch between the TC25 substrate and the nickel-chromium coating.
Impact of Thermal Expansion on Coating Quality
During the spraying process, the coating simultaneously experiences tensile and compressive stresses. Compressive stress promotes coating adhesion, whereas tensile stress hinders it.
Plasma spraying of thin-walled titanium parts should preserve compressive stresses and minimize tensile stress effects on the coating.
Plasma spraying creates thermal disparity between layers, with later layers cooling at ~10⁵ °C/s under tensile stress; titanium substrates must remain below 120 °C.
The TC25 titanium substrate’s high thermal expansion requires considering sub-100 °C effects for coating–substrate matching.
TC25 expands 18 times slower than nickel-chromium (8.1×10⁻⁶ vs. 14.5×10⁻⁵/°C). Stabilizing them aligns contraction, reduces tensile stress, and maximizes bond strength (Figure 2).
Reducing temperature differences and using intermittent, faster spraying improves contraction matching and bond strength.
Influence of Coating Thickness on Coating Quality
Layered coating (Figure 3) raises heat and stress; differing expansion causes tensile stress as the substrate cools, risking coating detachment.
For this large, thin-walled titanium casing, plasma spraying must consider turning, focusing on two main points:
First, the large diameter requires generous coating allowance for turning, increasing the chance of achieving the final diameter in one pass.
Second, the thin walls are prone to deformation, making post-deformation turning difficult. A thicker coating provides greater compensation for deformation.
From the above analysis, it is evident that thicker coatings are more advantageous for subsequent turning operations.
However, thicker coatings raise tensile stress, risking detachment; beyond a certain thickness, spraying can cause the coating to peel during application.
Influence of Coating Structure on Coating Quality
The bond between the coating and substrate involves metallurgical-chemical bonding, physical bonding, and mechanical bonding.
Metallurgical bonding is strongest, but sprayed coatings rely mainly on mechanical bonding.
A Ni/Al interlayer forms nickel aluminide, boosting droplet temperature and creating a dense, low-porosity metallurgical bond.
Without a substrate layer, adhesion drops as coating thickens; with a Ni/Al layer, adhesion remains strong.
Analysis and testing show a Ni/Al transition layer between titanium and nickel-chromium coating improves affinity and overall adhesion strength.
Table 1 shows that adding a substrate layer significantly improves nickel-chromium coating bond strength.
Influence of Substrate Surface Condition on Coating Quality
Thermally sprayed coatings bond mainly mechanically; increasing surface area strengthens this bond.
Substrates are cleaned and sandblasted before coating; surface roughening maximizes bonding area without affecting part performance.
Early flat substrates yielded poor adhesion. Adding grooves increased roughness and contact area, significantly improving coating quality.
Process Optimization Verification
Key factors affecting nickel-chromium coating quality—expansion, thickness, structure, and substrate condition—guide spraying and machining controls.
Currently, the nickel-chromium coating exhibits strong adhesion to the substrate, achieving a 100% first-pass pass rate after mechanical turning.
Statistics show the coating now passes inspection in a single spray and turning, with no rework needed.
Conclusion
This study examined how substrate, coating, and spraying control affect nickel-chromium quality, focusing on expansion, thickness, structure, and substrate condition.
Experiments confirmed the critical factors, achieving 100% first-pass coating success with no detachment. Key measures are:
- Increase the gun travel speed.
- Adopt the gap spraying method and reduce the coating thickness.
- Add a nickel-aluminum transition layer as the base layer.
- Groove the substrate in the spraying area to increase the bonding surface area between the coating and the substrate.