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Metallography

date2025-07-24tags:meche:

Introduction

Thermal spray coatings are used in many everyday applications, from coffee makers to cars to jet engines. Proper preparation and examination of these coatings are crucial to ensure their functionality and prevent failures.

Coating material (wire or powder) is melted by a high-temperature heat source. Molten particles are accelerated by a flame or plasma jet and projected onto the substrate. Particles mechanically bond to the surface, deform, and cool rapidly. Materials deposited include ceramics, carbides, and metal alloys. Coatings provide properties the substrate lacks, such as:

The first commercially viable thermal spray plant was established in the early 1900s. One of the first applications was a zinc coating for corrosion protection. Thermal spray coating technologies include:

Examples of applications:

Analysis

Coating properties must be analyzed for:

Materialographic analysis aids in developing new products and application methods.

Edge Rounding

Occurs due to incorrect mounting resin, gaps, or unmatched abrasion rates. Silicon carbide grinding can contribute to this.

Interface Delamination

Can be caused by incorrect sectioning or spray errors.

Porosity

Accurate assessment of porosity is important. Fluorescent dye in the mounting resin can help identify porosity. Smearing from silicon carbide grinding can mask porosity.

Preparation of Thermal Spray Coatings

Knowledge of coating and substrate materials is crucial. Different spray processes result in varying densities and structures. Expected porosity and oxide content can be estimated based on the process. HVAF: Dense coating Plasma spray: Up to 20% porosity Cutoff wheel selection depends on coating thickness: <20% of substrate thickness: Select based on substrate properties.

20% of substrate thickness: Select based on coating properties.

Sectioning

Fragile coatings may require epoxy mounting before cutting. Protective padding can be used to protect the coating during clamping. Adequate cooling water flow is essential. Delamination can occur if the substrate is not used as support during cutting. Mounting before sectioning can help determine if delamination existed before cutting.

Mounting

Hot mounting: Resin is cured under pressure and heat. Not recommended for brittle coatings. Cold mounting: Resin hardens through a chemical reaction. Preferred for brittle or coarse coatings. Mold release agents should be used to prevent sticking. Specimens should be cleaned and dried before mounting to improve adhesion. Porous samples should be placed in a vacuum chamber to remove entrapped water and alcohol. A good mount has minimal shrinkage, good adhesion, and a matching abrasion rate between resin and materials. Vacuum impregnation can be used to fill pores and cracks in the coating. Accessories like multi-clips can be used to secure specimens during mounting. Adequate spacing between specimens and between the specimen and the mount edge is important to prevent cracking. Proper mixing of resin and hardener is crucial.

Grinding and Polishing

Plane grinding: Removes large amounts of material and creates a flat surface. Avoid aggressive grinding that can cause damage. Fine grinding: Uses diamond abrasives to further refine the surface and remove scratches from plane grinding. Diamond polishing: Removes fine scratches and prepares the surface for final polishing. Oxide polishing: Achieves a sub-micron finish suitable for high-quality imaging.

Examples of Preparation Methods

Hard facing: Four-step process (plane grinding, fine grinding, diamond polishing, oxide polishing). Comparison of customer-prepared standard (nine steps) vs. updated method (four steps): The updated method resulted in better edge retention and reduced processing time while maintaining comparable porosity levels.

Potential Issues

Smearing Pullout False porosity levels

Examination

A properly prepared sample allows for the examination of: Coating thickness Porosity Oxides Cracks Unmelted particles Adhesion to the substrate Contamination at the interface

Resources

Struers website: Knowledge section, free preparation methods ASM Handbook Volume 9 Metallography.com: Ask the Expert forum Carpenter Technologies: Online guide for etching specialty alloys

Q&A

Sonification is generally not recommended for cleaning between polishing steps due to the risk of cavitation damage. Best practices for cleaning include using a diluted solution of dish soap and cotton balls, cleaning sample holders, and washing hands. Feed rate can be adjusted during cutting, but RPM should generally remain constant. Complementary rotation is recommended for grinding and polishing steps. Counter rotation can be used for oxide polishing. Coolant additives can help prevent burning and extend the life of cutoff wheels. Differentiating between oxides and porosity can be challenging. For ceramic materials, consult the Struers website or contact their lab for specific preparation methods. Backfilling voids in a mount is possible but not commonly practiced.

Conclusion

Proper preparation and examination of thermal spray coatings are essential for quality control, process monitoring, and failure analysis. By following the guidelines and utilizing the available resources, technicians and engineers can ensure accurate and reliable results.

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