Standardization of Metallographic Practices: Thermally Sprayed Coatings for State-of-the-Art and Common Coating Applications

Microstructural analysis plays a vital role in the formation of thermally-sprayed coatings. Laboratory personnel are able to examine the coating features in a confident and cost-effective manner, due to the advances made in consumable technology and equipment. Microstructural analysis has been crucial in the creation of new thermally-sprayed coatings for many high-tech and low-tech applications, including automotive, aerospace, bio-technology, electronic, petroleum and others. The coatings consist of a blend of materials with varying degrees of general microstructural properties and hardness. Accurate analysis of microstructure is essential to enable the creation of new and complex coatings that help in accurate microstructural analysis. The existing metallographic approaches are unable to generate accurate results in a consistent manner. The integrity and flatness of the coating structure can be maintained with a resin-bonded diamond surface that reduces the occurrence of damage when grinding various coating types.

Microstructural examination is the typical approach to determine the quality of thermally-sprayed coating. Precise evaluation depends on the metallographic preparation that is utilized to expose the microstructure. Before metallographic preparation is carried out, the sectioning and mounting approaches have to be analyzed [1]. Selection criteria for the sectioning and mounting method has been determined through testing performed by users of thermally sprayed coatings, as well as the Thermal Spray Society (TSS) Recommended Practices for Metallography committee [2]. Incorrect mounting of coatings can directly influence their properties, resulting in improper evaluation.

To study the performance of resin bonded diamond grinding discs (DGD), 13 coatings were selected covering an array of qualities, thicknesses, and coating properties (Figure 1). Semi-automatic equipment was used to prepare the coatings simultaneously. A preparation procedure was drawn up using the DGD, and was carried out several times to establish the consistency of the surfaces. Fully- and semi-automatic equipment generates high quality results in a consistent manner during the metallographic preparation of thermally-sprayed coatings [3].

Coating process and materials selected for the study. – First image, on first page

Figure 1. Coating process and materials selected for the study. – First image, on first page

The study demonstrated that the DGD surfaces generate good consistent results with no damage to the coating, irrespective of the coatings being made of soft or hard metals, composites or ceramic. The metallographic process allows the observer to properly view the microstructure, free of artifacts, if good sectioning and mounting approaches are implemented.

Coating Materials

Metallographic Preparation

Metallographic preparation was carried out by using an 8in. (250 mm) diameter EcoMet® semi-automatic grinder-polisher with an AutoMet® power head, to provide consistent loads when the specimens are rotating at 60 rpm with or against to base platen rotation. Automation enables simultaneous and controlled preparation of several specimens, [4] and promotes consistency in the finish with the precise control of platen, and time, pressure and head speed. Test results, including inclusive content or levels of porosity, can be compared for many years.

The preparation sequences began with a 45 µm DGD to ensure sufficient removal of material, approx. 1 mm. Grinding was carried out by utilizing 9 µm and 6 µm DGD to remove any deformation caused by the earlier planar grinding. 3 µm diamond suspension on a TexMet®, a napless cloth, was used for the final two steps. Colloidal silica (0.02 µm) suspension on a urethane foam cloth such as ChemoMet® was used for the final step. Figures 2-14 show the image results after the final polishing step.

Two Wire Arc Al-Bronze, 20x.

Figure 2. Two Wire Arc Al-Bronze, 20x.

Figure 3. Two Wire Arc Stainless Steel, 20x.

Electric Arc Spray NiCrAl, 20x.

Figure 4. Electric Arc Spray NiCrAl, 20x.

Electric Arc Spray 420 Stainless Steel, 20x.

Figure 5. Electric Arc Spray 420 Stainless Steel, 20x.

Top) Plasma Spray Al2O3 Coating, 20x. Bottom) Plasma Spray Al2O3 Substrate, 20x.

Figure 6. Top) Plasma Spray Al2O3 Coating, 20x. Bottom) Plasma Spray Al2O3 Substrate, 20x.

Plasma Spray Cast Iron, 20x.

Figure 7. Plasma Spray Cast Iron, 20x.

Plasma Spray T 800, 20x.

Figure 8. Plasma Spray T 800, 20x.

Top) Plasma Spray 8% Yttria Stabilized Zirconia Coating, 20x. Bottom) Plasma Spray 8% Yttria Stabilized Zirconia Substrate, 20x.

Figure 9. Top) Plasma Spray 8% Yttria Stabilized Zirconia Coating, 20x. Bottom) Plasma Spray 8% Yttria Stabilized Zirconia Substrate, 20x.

Plasma Spray Yttria Stabilized Zirconia 1494, 20x.

Figure 10. Plasma Spray Yttria Stabilized Zirconia 1494, 20x.

Plasma Spray Chrome Carbide, 20x.

Figure 11. Plasma Spray Chrome Carbide, 20x.

HVOF Aluminum 13 Titanium, 20x.

Figure 12. HVOF Aluminum 13 Titanium, 20x.

HVOF WC-Co, 20x.

Figure 13. HVOF WC-Co, 20x.

Figure 14. HVOF CrC-NiCr, 20x.

Results and Discussion

The resulting images displayed flat surfaces devoid of scratches, low coating defects, and well-defined substrate-coating interfaces. The results were almost the same when the preparation was carried out five more times by utilizing the same parameters and surfaces.

Conclusion

DGD consistently generates flat surfaces on different thermally-sprayed coatings, and can be effectively used for a prolonged period of time. Initial data shows that Apex® DGD can be utilized to grind many hundreds of specimens, saving costs for laboratories which usually make use of silicon carbide abrasives. The damage is less when diamond is imbedded into a resin binder, compared to when a metal-bonded diamond disc is used. Metal-bonded diamond discs enable higher material removal, but generate deeper surface damage and friable coating damage.

This study demonstrates that Apex® DGD is a cost-effective process for metallographic preparation of thermally-sprayed coatings shown in this report and others with identical features, with no unacceptable damage caused to coatings.

References

1. Blann, G.A., “The Effects of Thermosetting and Castable Encapsulation Methods on the Metallographic Preparation of Ceramic Thermally Sprayed Coatings,” Journal of Thermal Spray

Technology, 3 (1994) pp.263-269.

2. ASM International, Thermal Spray Society, TSS Accepted Practices for Metallography of Thermal Spray Coatings, Accepted Practices Document – NiCrAl/Bentonite Abradable coatings, Aug. 2007.Pp.4

3. Sauer, J.P., “Metallographic Preparation of Thermal Sprayed Coatings: Coating Sensitivity and the Effect of Polishing Intangibles”, Proceedings from the 9th National Thermal Spray Conference, October 1996, Cincinnati, Ohio, USA pp.777-783

4. Geary, A., “Metallographic Evaluation of Thermally Sprayed Coatings”, Technical Meeting of the 24th Annual Convention: International Metallographic Society, July 1991. Monterey, California, USA, Materials Characterization, pp. 637-650.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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