Effect of LaB6 Particles on Microstructure and High-Temperature Oxidation Resistance of PTA Inconel 625 Overlay Coating

Literature Overview

This study investigates the influence of lanthanum hexaboride (LaB6) ceramic particles on the microstructural evolution and high-temperature oxidation behavior of Inconel 625 overlay coatings produced by plasma transferred arc (PTA) cladding. LaB6 is a high-temperature refractory ceramic with excellent thermal stability, high melting point (approximately 2000 °C), and strong bonding capability with nickel-based superalloys. The research introduces LaB6 particles into the PTA feedstock powder at varying volume fractions and evaluates the resulting coating microstructure through metallographic examination, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Oxidation tests were conducted at elevated temperatures in air to assess the protective capability of the reinforced coating compared to the as-cladded baseline Inconel 625.

Core Technical Findings

The addition of LaB6 particles fundamentally alters the solidification microstructure of the PTA overlay. In the baseline coating without reinforcement, the microstructure consists of equiaxed dendrites with interdendritic precipitation of the delta phase (Ni3Nb) and gamma prime (γ′) strengthening phases. When LaB6 particles are introduced, they act as heterogeneous nucleation sites, effectively refining the dendrite arm spacing and suppressing the growth of the brittle delta phase. The LaB6 particles distribute along the dendrite boundaries and at the fusion line, creating a composite-like microstructure that enhances both mechanical integrity and oxidation resistance.

From an oxidation resistance perspective, the LaB6-reinforced coatings exhibit significantly reduced mass gain during prolonged exposure at temperatures between 800 °C and 1100 °C. The mechanism is attributed to the formation of a more compact and adherent oxide scale, enriched in lanthanum-containing oxide phases (such as La2O3 and LaAlO3) that plug grain boundary diffusion pathways and reduce oxygen ingress. This is consistent with the well-documented rare earth modification effect in superalloy systems, where trivalent rare earth ions segregate to oxide-metal interfaces and inhibit cation outward diffusion.

Engineering Practice Implications

For engineers designing high-temperature components requiring corrosion and oxidation resistance, this study provides a practical pathway to enhance PTA overlay performance without changing the base alloy composition. The key takeaway is that LaB6 particle size, distribution uniformity, and volume fraction must be carefully controlled during powder preparation. Over-concentration of particles can lead to agglomeration, porosity, and reduced bond strength at the fusion interface. In pressure vessel applications where overlay layers serve as corrosion-resistant linings for hydrogenation reactors or heat exchangers operating above 600 °C, the LaB6-reinforced PTA approach offers a promising route to extend service life and reduce maintenance intervals. However, the cost of LaB6 powder and the additional powder blending steps must be weighed against the performance gains, particularly for large-area cladding operations.

Study Insights and Reflections

The most intellectually stimulating aspect of this work is the demonstration that a simple ceramic particulate addition can simultaneously address multiple performance requirements—microstructural refinement, delta phase suppression, and oxidation resistance improvement—in a single process step. This multi-functional approach is highly attractive from an engineering economics standpoint. Nevertheless, further investigation into the long-term thermal cycling stability of the LaB6/Inconel 625 composite coating is warranted, as thermal fatigue cracking at the particle-matrix interface could become a limiting factor under cyclic thermal loading conditions typical of pressure vessel startup and shutdown sequences.