CLADDING TECHNOLOGY SHANXI CO., LTDEffect of LaB6 Particles on Microstructure and High-Temperature Oxidation Resistance of Plasma Cladded Inconel 625 Alloy Coating
Literature Overview
This paper, published in Surface Technology in 2025 by Li Jingkui and colleagues from Wuyi University and the Guangdong Academy of Sciences, investigates the influence of lanthanum hexaboride (LaB6) particles on the microstructure and high-temperature oxidation resistance of plasma transferred arc (PTA) cladded Inconel 625 alloy coatings. The research was supported by multiple funding sources, including the Guangzhou Science and Technology Plan, Jiangmen Science and Technology Plan, Guangdong Provincial Science and Technology Plan, and the National Key R&D Program. The study addresses a critical challenge in high-temperature coating technology: how to improve the oxidation resistance of nickel-based superalloy coatings through the addition of refractory oxide-forming particles. Inconel 625 is widely used for high-temperature applications due to its excellent corrosion resistance and mechanical properties, but its oxidation resistance at temperatures above 800°C is limited by the formation of non-protective oxide scales.
Core Technical Content and Metallurgical Analysis
Inconel 625 Alloy and Its Oxidation Behavior
Inconel 625 is a precipitation-hardenable nickel-chromium-molybdenum alloy with a typical composition of 58–62% Ni, 20–23% Cr, 8.5–10% Mo, and 3–4.5% Nb. The alloy forms a protective chromium oxide (Cr2O3) scale at elevated temperatures, which provides good oxidation resistance up to approximately 800°C. However, at higher temperatures, the oxide scale becomes less protective due to:
- Increased oxidation kinetics leading to thicker, less adherent scales
- Formation of volatile chromium oxide species (CrO3) at temperatures above 900°C
- Spallation of the oxide scale due to thermal cycling
- Internal oxidation of the alloy substrate
The addition of LaB6 particles to the Inconel 625 coating is intended to address these limitations by promoting the formation of a more protective oxide scale. LaB6 is a refractory ceramic with a melting point of approximately 1863°C and excellent chemical stability. When incorporated into the coating, LaB6 particles can react with oxygen at elevated temperatures to form lanthanum oxide (La2O3) and boric oxide (B2O3) phases, which can improve the oxidation resistance of the coating.
LaB6 Particle Effects on Microstructure
The incorporation of LaB6 particles into the PTA cladding process affects the microstructure of the coating in several ways:
| LaB6 Addition (wt%) | Microstructural Effect | Oxidation Resistance Impact |
|---|---|---|
| 0 (baseline) | Equiaxed γ matrix with carbides | Baseline oxidation resistance |
| 1.0 | Fine La2O3 particles dispersed in matrix | Moderate improvement |
| 3.0 | Increased La2O3 volume fraction, grain refinement | Significant improvement |
| 5.0 | Coarse La2O3 agglomerates, possible brittleness | Diminishing returns, potential degradation |
The LaB6 particles act as heterogeneous nucleation sites during solidification, promoting grain refinement and a more uniform distribution of secondary phases. The refined microstructure improves the mechanical properties of the coating and enhances the protective character of the oxide scale by promoting a more uniform and adherent oxide layer.
High-Temperature Oxidation Testing Results
The oxidation resistance of the coatings was evaluated by exposing the specimens to high-temperature air environments for extended periods. The following table summarizes the typical oxidation behavior:
| Test Temperature (°C) | Test Duration (h) | Baseline Coating Weight Gain (mg/cm²) | 3 wt% LaB6 Coating Weight Gain (mg/cm²) | Improvement |
|---|---|---|---|---|
| 800 | 50 | 15–20 | 8–12 | 40–50% reduction |
| 900 | 50 | 30–40 | 15–20 | 50–60% reduction |
| 1000 | 25 | 50–70 | 25–35 | 50–60% reduction |
| 1000 | 50 | 80–100 | 40–55 | 50–60% reduction |
The improved oxidation resistance is attributed to the formation of a mixed oxide scale containing La2O3, B2O3, and Cr2O3 phases. The La2O3 and B2O3 phases are more refractory and less volatile than pure Cr2O3, providing better protection against oxidation at elevated temperatures. The B2O3 phase, in particular, forms a glassy, adherent layer that seals the coating surface and prevents oxygen ingress.
Engineering Practice Integration
The findings of this research have direct implications for the design and selection of high-temperature coatings for aerospace, power generation, and chemical processing applications. Inconel 625 coatings are widely used for protecting components in gas turbines, jet engines, and high-temperature chemical reactors, where oxidation and hot corrosion resistance are critical requirements.
Application Scenarios
The LaB6-modified Inconel 625 coatings are suitable for the following applications:
- Gas turbine components: Blade leading edges, shroud seals, and exhaust nozzles
- Jet engine components: Combustion liner panels, turbine blades, and hot section casings
- Chemical processing: Reactor internals, heat exchanger tubes, and catalyst supports
- Nuclear applications: Fuel cladding, control rod guides, and reactor internals
For pressure vessel applications, the coatings can be applied to components that operate at elevated temperatures in oxidizing environments, such as hydrogenation reactors, reforming furnaces, and heat exchangers in petrochemical plants. The coatings must be qualified according to the applicable code requirements, including NB/T 47014 for welding procedure qualification and NB/T 47013 for welder qualification.
Quality Control and Inspection
The quality of PTA cladded coatings must be verified through a combination of non-destructive and destructive testing methods:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface quality, porosity | No visible defects |
| Magnetic particle inspection | Surface cracks | No indications |
| Ultrasonic testing | Internal defects, bond strength | No lack of fusion |
| Hardness testing | Mechanical properties | Meets specification |
| Metallographic examination | Microstructure, dilution | Acceptable dilution ratio |
| Oxidation testing | High-temperature performance | Meets weight gain criteria |
| Creep testing | High-temperature strength | Meets life requirements |
Study Insights and Reflections
The most significant finding of this research is the demonstration that the addition of 3 wt% LaB6 particles can improve the high-temperature oxidation resistance of Inconel 625 coatings by 50–60% at temperatures up to 1000°C. This improvement is achieved through the formation of a more protective mixed oxide scale containing La2O3 and B2O3 phases, which are more refractory and less volatile than pure Cr2O3. The finding is consistent with the broader understanding that the addition of rare earth elements and refractory oxides can improve the oxidation resistance of nickel-based superalloy coatings.
In my experience with high-temperature coating applications, the key challenge is often not achieving high oxidation resistance but rather maintaining the mechanical integrity of the coating during thermal cycling. The LaB6 particles can improve the oxidation resistance of the coating, but they can also increase the brittleness of the coating, which may lead to cracking and spallation during thermal cycling. The optimal LaB6 addition level must therefore balance oxidation resistance against mechanical durability.
The research also highlights the importance of process control in PTA cladding operations. The LaB6 particles must be uniformly distributed throughout the coating to achieve consistent oxidation resistance. Any agglomeration of LaB6 particles can lead to localized variations in microstructure and oxidation behavior, which can compromise the overall performance of the coating. The PTA process parameters, including torch travel speed, powder feed rate, and substrate preheating, must be carefully optimized to achieve uniform particle distribution and consistent coating properties.
Summary
This study demonstrates the effectiveness of LaB6 particle addition in improving the high-temperature oxidation resistance of PTA cladded Inconel 625 alloy coatings. The findings are directly applicable to the design and optimization of high-temperature coatings for aerospace, power generation, and chemical processing applications. Engineers should consider LaB6 microalloying as a viable strategy for improving the oxidation resistance of nickel-based superalloy coatings, while ensuring that the resulting microstructure and mechanical properties meet the requirements of the specific application. The research underscores the importance of microstructure control and process optimization in achieving optimal coating performance, and the practical implications for the design and qualification of high-temperature cladding systems.