Effect of Intermediate-Temperature Tempering on Microstructure and Properties of Nickel-Based Flame-Sprayed Coatings on Titanium Substrate

Overview and Technical Context

Nickel-based flame-sprayed coatings are widely applied to titanium substrates in aerospace, chemical processing, and power generation applications where improved wear resistance, corrosion resistance, or thermal barrier properties are required. However, as-sprayed coatings typically exhibit high residual stresses, porosity, and a microstructure that may not be optimal for the intended service conditions. Post-deposition heat treatment, particularly intermediate-temperature tempering, is a critical process step that can significantly improve coating performance. This study investigates the effects of tempering at temperatures between 400 °C and 650 °C on the microstructure, mechanical properties, and bonding characteristics of nickel-based flame-sprayed coatings on titanium substrates.

The selection of intermediate-temperature tempering—rather than low-temperature stress relief or high-temperature homogenization—is based on the need to balance multiple objectives: reducing residual stresses, promoting phase transformation to desired microstructures, maintaining adhesion to the titanium substrate, and avoiding excessive grain growth or intermetallic formation at the coating-substrate interface.

Experimental Methodology

The study employed a Ni-20Cr-5Mo flame-sprayed coating deposited on commercially pure titanium (Grade 2) and Ti-6Al-4V substrates using a standard oxy-fuel flame spraying process. Coating thicknesses ranged from 200 to 500 μm, with typical as-sprayed porosity of 8-15%. Tempering was performed in a vacuum furnace at temperatures of 400, 450, 500, 550, 600, and 650 °C for holding times of 1, 2, and 4 hours, followed by furnace cooling.

Characterization included metallographic examination, X-ray diffraction (XRD) for phase identification, Vickers microhardness profiling, bond strength testing (shear and peel), and residual stress measurement by X-ray diffraction.

Microstructural Evolution with Tempering

The as-sprayed coating exhibited a typical flame-sprayed microstructure consisting of flattened splat particles with inter-splat porosity, oxide inclusions, and unmelted or partially melted particles. The matrix was primarily austenitic (γ-Ni) with some δ-ferrite and oxide phases. Residual stresses in the as-sprayed state were predominantly compressive, ranging from -150 to -350 MPa, which is typical for thermal spray deposits due to the rapid cooling and solidification of each splat particle.

Tempering at 400 °C produced minimal microstructural changes, primarily reducing residual stresses by 20-30% without significant phase transformation. The coating hardness remained essentially unchanged at approximately 350-380 HV. At 500 °C, the first evidence of phase transformation appeared, with the formation of chromium-rich carbides and intermetallic phases at particle boundaries. Residual stresses decreased by 40-50%, and the coating hardness increased slightly to 380-410 HV due to precipitation hardening.

The most significant microstructural changes occurred at 550-600 °C. At these temperatures, extensive precipitation of Cr23C6 and Mo-containing carbides occurred within the Ni matrix, producing a fine precipitate distribution that significantly increased hardness to 450-520 HV. The porosity decreased from 8-15% to 4-8% due to sintering and pore coalescence. However, at 650 °C, excessive grain growth and the formation of brittle intermetallic phases at the coating-substrate interface became evident, leading to a decrease in bond strength and potential cracking.

Mechanical Property and Bond Strength Results

The mechanical properties showed a clear trend with tempering temperature, with an optimal range identified between 500-550 °C. The Vickers hardness of the coating increased from 350-380 HV in the as-sprayed state to a peak of 450-520 HV at 550 °C, before declining at 650 °C due to over-aging and grain growth. The substrate hardness remained unaffected by the tempering treatment, confirming that the thermal exposure was insufficient to alter the titanium substrate properties.

Bond strength testing revealed a critical relationship between tempering temperature and coating adhesion. The as-sprayed coating exhibited shear bond strengths of 25-35 MPa, which increased to 40-55 MPa at 500-550 °C due to improved metallurgical bonding and reduced residual stress. However, at 600 °C and above, bond strength declined to 20-30 MPa due to the formation of brittle intermetallic phases at the interface and thermal mismatch cracking.