Resin Bonded Diamond Surface Enhance Microstructural Analysis of Thermally Sprayed Coatings

Microstructural analysis continues to play a significant role in the development of thermally sprayed coatings. The improvements in equipment and consumable technology for examining coatings have enabled laboratory personnel to accurately observe the coating characteristics with great confidence and in a cost-effective manner. Microstructural analysis is considered to be important in the development of new thermally sprayed coatings used for automotive, aerospace, electronic, petroleum, bio-technology and other low and high tech applications.

The coatings that are being developed comprise of a combination of materials that differ in general microstructural characteristics and hardness. Accurate microstructural analysis is considered to be more important than ever because these new, complex coatings cannot be developed successfully without control of the microstructure. Established metallographic techniques have not always been able to produce accurate results. A resin bonded diamond surface was developed which brings down the amount of damage created during the lapping and grinding of a variety of coating types while maintaining the flatness and integrity of the overall coating microstructure.

Introduction

Microstructural evaluation is considered to be a common approach for determining the quality of a thermally sprayed coating. Accuracy of the evaluation depends on the metallographic preparation used to expose the microstructure. It is essential to carefully consider the sectioning and mounting techniques before metallographic preparation.[1] Testing by users of thermally sprayed coatings as well as the TSS Recommended Practices for Metallography committee helps in establishing the selection criteria. The characteristics of the coating causing improper evaluation will be directly affected by improper mounting of a few thermally sprayed coatings.

Ten coatings representing a range of coating properties, thicknesses and qualities were selected for this study of the performance of diamond/resin disks. All coatings were prepared on EcoMet® 3000/AutoMet® 2000 equipment in a simultaneous manner. A preparation sequence was established using Apex® DGD Disks and the process was repeated for several times in order to establish the consistency of the surfaces. The best, consistent results are produced by EcoMet 3000/AutoMet 2000 and BuehlerVanguard® 2000 equipment when metallographically preparing thermally sprayed coatings.[2]

The study revealed that whether the coatings are ceramic, hard or soft metallic or composite in nature, the resin/diamond surfaces produce excellent consistent results without damaging the coating. The metallographic process will enable the observer to view a coating microstructure that is accurate in appearance if good sectioning and mounting techniques are followed. The SEM and light microscopy photographs in Figure 1 and 2 show the surfaces used for the grinding and lapping of the coatings.

Apex DGD Surface.

Figure 1. Apex DGD Surface.

Apex DGD Cross Section.

Figure 2. Apex DGD Cross Section.

Coating Materials

Thermal spray equipment Manufacturers provided 10 diverse thermally sprayed coatings. The coatings provided were Electric Arc Spray NiCrAl and 420 Stainless Steel, HVOF WC/CO, Plasma Sprayed NiAl, Chrome Oxide, Copper, Two Wire Arc Stainless Steel and Aluminum Bronze, Chrome Carbide and 8% Yttria Stabilized Zirconia. Spray parameters and coating thickness and substrate material differed with each coating type.

Metallographic Preparation

The metallographic preparation was carried out on the 8 inch, EcoMet 3000/AutoMet 2000 Grinder-Polisher, which is available with a power head that provided consistent applied loads while rotating the specimens against or with the rotation of the base platen during each step. This allowed for controlled, simultaneous preparation of the specimens.[3]

All the preparation sequences were started with a 45 µm Apex disk in order to guarantee satisfactory material removal (1-2 mm). Two addition grinding steps were performed using a 9 µm and 6 µm Apex disk respectively which removed the earlier deformation from the initial planar step. The final two steps involved the use of 3 µm MetaDi® Supreme Diamond Suspension on a TexMet ® 1000 Polishing Pad and MasterMet® (0.03 µm) Colloidal Silica Suspension on a ChemoMet ® Cloth. The surface characteristics after the 45 µm, 9 µm and 3 µm resin/diamond disk procedures are shown by Figures 3, 4 and 5. Images were captured after the final polishing step revealing the results of this process and are shown in Figures 6-15.

45 micron DGD.

Figure 3. 45 micron DGD.

9 micron DGD.

Figure 4. 9 micron DGD.

3 micron Napless Cloth.

Figure 5. 3 micron Napless Cloth.

8% YSZ.

Figure 6. 8% YSZ.

EAS NiCrAl.

Figure 7. EAS NiCrAl.

2 Wire Arc Al-Bronze.

Figure 8. 2 Wire Arc Al-Bronze.

CrO.

Figure 9. CrO.

EAS 420 Stainless Steel.

Figure 10. EAS 420 Stainless Steel.

Plasma NiAl.

Figure 11. Plasma NiAl.

Chrome Carbide.

Figure 12. Chrome Carbide.

8% YSZ

Figure 6. 8% YSZ.

EAS NiCrAl.

Figure 7. EAS NiCrAl.

2 Wire Arc Al-Bronze.

Figure 8. 2 Wire Arc Al-Bronze.

Results and Discussion

The images captured for documentation revealed flat, scratch-free surfaces, minimal coating defects and well defined coating/substrate interfaces. Similar results were obtained when the preparation procedure was repeated five times using the same parameters and surfaces.

Conclusions

Apex DGD Disks will constantly produce flat surfaces on a wide range of thermally sprayed coatings and can be used for an extended period of time before losing their effectiveness. Initial observations show that these surfaces can be used to grind hundreds of specimens, which provide a big cost savings for laboratories that regularly use silicon carbide abrasive papers. The damage produced during the grinding operation is much less than a metal bonded diamond disk because the diamond is imbedded in a resin binder.

Metal bonded diamond disks produce deeper surface damage and provide better material removal, but tend to develop more damage on friable coatings. This study concludes that Apex DGD Disks can provide a cost effective method for metallographically preparing the thermally sprayed coating shown in this report and others with similar characteristics without the need for producing unacceptable damage to the coatings while reducing the preparation cost.

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. 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
  3. 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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