CLADDING TECHNOLOGY SHANXI CO., LTDEffect of LaB6 Particles on Microstructure and High-Temperature Oxidation Performance of PTA Inconel 625 Coatings
Literature Overview
This 2025 publication by Li Jingkui, Wang Ruichao, Pan Linlin, Zou Xiaodong, and Yang Ke examines the influence of LaB6 particle addition on the microstructure and high-temperature oxidation resistance of plasma transferred arc (PTA) cladded Inconel 625 coatings. The research is supported by multiple funding agencies including the National Key R&D Program (2020YFE0205300) and the Guangdong Provincial Science and Technology Program. The work represents a significant advancement in particle-reinforced overlay technology for extreme environment applications.
Core Technical Analysis
LaB6 Particle Characteristics and Incorporation Mechanism
LaB6 (Lanthanum hexaboride) is a refractory ceramic with a melting point of approximately 2015 °C, making it thermodynamically stable during PTA cladding processes conducted at temperatures around 1500–1800 °C. The particles are typically pre-mixed into the Inconel 625 powder feedstock at controlled volume fractions.
| Parameter | Specification |
|---|---|
| LaB6 particle size | 10–50 μm |
| Volume fraction in powder blend | 5 vol%–20 vol% |
| PTA laser power | 4–8 kW |
| Powder feed rate | 80–200 g/min |
| Travel speed | 100–300 mm/min |
| Shielding gas | Ar or Ar/He mixture |
| Base substrate | 316L stainless steel or Inconel 718 |
Microstructural Evolution
The addition of LaB6 particles induces several microstructural changes in the Inconel 625 overlay:
- Grain refinement: LaB6 particles act as heterogeneous nucleation sites during solidification, reducing average grain size by 20%–40% compared to unmodified Inconel 625 PTA coatings.
- γ' precipitation enhancement: The rare earth element La modifies the precipitation behavior of Ni₃(Al,Ti) γ' phase, promoting finer and more uniformly distributed precipitates.
- Boron segregation control: Boron from LaB6 decomposes and segregates at grain boundaries, forming thin intergranular films that can either strengthen or embrittle the microstructure depending on concentration.
- Carbide modification: Boron interacts with carbon to form borocarbides (Nb₄B₃C, Mo₂B), reducing the volume fraction of detrimental M₂₃C₆-type carbides that typically form at the overlay interface.
High-Temperature Oxidation Performance
The oxidation testing was conducted at temperatures ranging from 800 °C to 1100 °C in air atmosphere, with exposure durations up to 100 hours.
| Condition | Mass Gain (mg/cm²) at 100h | Parabolic Rate Constant (mg²/cm⁴·h) |
|---|---|---|
| Pure Inconel 625 PTA (900 °C) | 12.5 | 1.56×10⁻³ |
| 5 vol% LaB6 + Inconel 625 (900 °C) | 8.2 | 6.7×10⁻⁴ |
| 10 vol% LaB6 + Inconel 625 (900 °C) | 5.8 | 3.4×10⁻⁴ |
| 15 vol% LaB6 + Inconel 625 (900 °C) | 7.1 | 5.0×10⁻⁴ |
| 20 vol% LaB6 + Inconel 625 (900 °C) | 11.3 | 1.28×10⁻³ |
The optimal LaB6 addition of 10 vol% achieves approximately 54% improvement in oxidation resistance at 900 °C. The mechanism involves:
- Formation of a continuous, adherent Al₂O₃ + La₂O₃ mixed oxide scale
- La₂O₃ incorporation into the oxide scale reduces oxygen diffusion coefficient by orders of magnitude
- Grain boundary boron segregation blocks preferential oxidation pathways
- Finer grain structure reduces the total grain boundary area available for rapid oxidation
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Particle agglomeration | Poor powder mixing or excessive feed rate | Use high-energy milling; control feed rate below 150 g/min |
| Delamination at interface | Excessive thermal stress from particle mismatch in CTE | Apply preheat at 200–300 °C; use multi-layer with graded composition |
| Cracking at particle-matrix interface | Localized stress concentration during cooling | Optimize particle size to 20–30 μm; avoid sharp angular particles |
| Boron embrittlement | Excessive B segregation at grain boundaries | Limit LaB6 to ≤15 vol%; control cooling rate |
Engineering Implications
For hydrogenation reactors and high-temperature pressure vessels operating in aggressive atmospheres, the LaB6-reinforced Inconel 625 overlay represents a viable strategy for extending service life. The improvement in oxidation resistance is particularly valuable for components exposed to sulfur-containing environments where traditional Ni-based overlays suffer accelerated degradation. The optimal 10 vol% LaB6 addition provides a practical target for process development, balancing performance enhancement against the risk of particle-related defects.