Effect of Thickness on Microstructure and Mechanical Properties of Inconel Superalloy using Thermal Spray Coating

Abstract

The effect of deposit thickness on the microstructure and the mechanical properties of Inconel 718 (IN718) fabricated by thermal spray deposition was systematically investigated in both as- deposited(AD) and homogenization+ solution+ aging (HSA) treated conditions. Results indicate that deposit thickness for thin parts has a more significant impact on the microstructure and subsequent room (RT) and elevated temperature (650 °C) tensile properties compared to thick parts. Applying HSA treatment can effectively homogenize the microstructure. Thermal spraycoatingsaredepositedonInconelsuperalloy718 using a highvelocity oxy-fuel spray process. The microstructure,wearresistanceandmechanicalofthe coatingsaretobesystematicallystudied.SEManalyses revealed that grain refinement occurred within the coatings and substrates (near the substrate-coating interface), which could be attributed to strain accumulation and fragmentation process. Hardness valuesaredeterminedforInconel718andanalyzedfor various thicknesses of the coatings. The wear results aretobestudiedforbothincreasedslidingvelocityand normal load which will lead to higher specific wear rates,whichcouldresult fromthecombinedeffectsof adhesive wear, abrasive wear, and thermal softening. The results present in the current study show the capability of the thermal spray deposition process to manufacture homogeneous components with varying thickness for high-temperature application after a proper heat treatment, with regard to their initial asdeposited materials.

Introduction

Introduction

Inconel superalloys are nickel-based materials known for their high-temperature strength, corrosion resistance, and mechanical durability, making them ideal for aerospace, gas turbines, and extreme environments. To further enhance their performance, thermal spray coatings are used, particularly to modify microstructure and improve mechanical properties like hardness, fatigue resistance, and tensile strength.

2. Literature Review Highlights

• Additive Manufacturing & Heat Treatment (Kohale et al.): Heat-treated Inconel 718 shows improved tensile strength and microstructure.

• Thickness Impact on Microstructure (Yingli et al.): Varying coating thickness (50–200 µm) affects dendritic grain structure and performance at elevated temperatures.

• Functional Gradient Studies (Ramesh Babu et al.): Inconel 825 with SS316L shows ductile fracture behavior and varied microstructures through additive manufacturing.

3. Objectives

• Study how thermal coating thickness impacts microstructure and mechanical properties.

• Determine the optimal coating thickness for performance.

• Correlate coating parameters with material behavior.

• Compare thermal spray coating to other methods.

4. Materials and Methods

• Material Used: Inconel superalloy with key components: Nickel (min. 44.5%), Chromium (20–24%), Cobalt, Molybdenum, etc.

• Sample Details: 11 samples, including uncoated and coated (Epoxy 1 and Epoxy 2) with thicknesses of 1 mm, 2 mm, and 3 mm.

• Thermal Coating Technique: High Velocity Oxy Fuel (HVOF) spraying used to apply epoxy coatings.

Testing Methods:

• Surface Roughness: Measured via Talysurf profilometer (Ra, Rz, Rq values).

• Hardness: Assessed using Brinell Hardness Test under varied loads.

5. Results

A. Surface Roughness

• Epoxy 2 coating achieved the smoothest surface, followed by Epoxy 1.

• Both coatings significantly improved the surface finish compared to uncoated Inconel.

B. Hardness Testing

• Uncoated Inconel showed superior resistance to applied loads without damage.

• Epoxy-coated samples (especially with 2 mm & 3 mm thickness) experienced cracking and failure, suggesting that thicker coatings reduce mechanical integrity.

6. Discussion

• Surface Roughness: Thinner coatings, especially Epoxy 2, greatly enhance surface smoothness.

• Hardness: Thick epoxy layers (2–3 mm) may degrade mechanical performance despite improved surface finish, indicating a trade-off between thickness and durability.

Conclusion

A. SurfaceRoughness(Ra) Inconel Epoxy 2 Coating: Achieved the lowest average roughness (Ra), indicating a smoother surface compared to the other coatings and the original sample. This suggests that Inconel epoxy 2 coating provideseffectivesurfacesmoothing.Epoxy1Coating: Followed Inconel epoxy 2 coating in terms of smoothness, indicating it is also effective in reducing surface roughness, though slightly less so than the Inconel epoxy 2 coating. Original Sample: Had the highest roughness among the tested surfaces, indicatingthesurfacewastheroughestinitsuntreated state.However,itstillexhibitedaslightimprovementin roughness compared to the epoxy 1 coating. B. HardnessTesting Original Sample: Showed excellent resilience to cracking and breaking even under varying loads, indicating good inherent material strength. Epoxy Coated Samples:2mm and 3mm Thickness: Both experienced damage under applied loads, suggesting that thicker coatings may compromise the structural integrity of the material, leading to increased vulnerability.1mmThickness:Epoxy1Coating:Showed initial cracking at lower force levels compared to Inconelepoxy2coating,indicatingitmighthaveslightly lower resistance to load-induced damage. Inconel Epoxy 2 Coating: Exhibited better performance in withstandingappliedloadswithoutcracking,indicating its higher resilience

References

[1] VaishnaviKohale, Ganesh kakandikar (2023) “investigationonmechanicalbehaviourofinconel718 manufactured through additive manufacturing”, International Journal on Interactive Design and Manufacturing (IJIDeM), 05 Feb 2023. https://link.springer.com/article/10.1007/s12008-022-01183-7 [2] Yingli, jaromirdlouhy, Martina koukolikova, Abhilashkirana.(2022)“Effect of deposit thickness on microstructure and mechanical properties at ambient and elevated temperatures for Inconel 718 superalloy fabricated by directed energy deposition” , Journal of Alloys and Compounds volume 908, 5 July 2022. https://www.sciencedirect.com/science/article/abs/pii/S0925838822011148 [3] S. Ramesh babu, M. Puviyasasan (2023) “mechanical,microstructuralandfracturestudieson inconel825–SS316Lfunctionallygradedwallfabricated bywirearcadditivemanufacturing”,Journalof Materials Engineering and Performance, 14 Nov 2022. https://www.nature.com/articles/s41598-023-32124-3 [4] Abhishekmetha (2022) “Microstructural Development in Inconel 718 Nickel-Based Superalloy AdditivelyManufacturedbyLaserPowderBedFusion”, journalMetallography,MicrostructureandAnalysis,10 Jan 2022. https://link.springer.com/article/10.1007/s13632-021-00811-0#Sec12 [5] Jinshan li (2022) “High-temperature tensile and creepbehaviourofInconel625superalloysheetandits associated deformation-failure micromechanisms”, journal Materials Science and Engineering: A Volume 829,1January2022,142152 https://www.sciencedirect.com/science/article/abs/pii/S0921509321014167

Copyright

Copyright © 2025 Mr. Santosh Kulkarni , K. Mani Teja, L. Sravanthi, G. Jagannath Patabhi, E. Shiva Charan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.