Accepted Practice for Testing Bond Strength of Thermal Spray Coatings

The ASM Thermal Spray Society (TSS) Accepted Practices Committee prepared this document to address critical aspects in testing and to provide step-by-step guidance for obtaining representative and reproducible test results based on ASTM C633 and other applicable industry standards.

Sufficient adhesion bond between the coating and the surface of a coated part/material (often called the substrate) is critical for the coated part functionality. The coating integrity and durability also directly depends on the cohesion bond strength—bonding of coating internal elements such as the internal layers and individual lamellas.

ASTM C633, “Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings,”^[1] is the baseline and mandatory process to follow for tensile testing of bond strength of thermal spray coatings. The test is required as a condition of approval for new coatings and their suppliers, and it serves as the core qualification test for coatings for aviation, oil/gas, automotive, power, marine, and many other industries. This practice document clarifies details of ASTM C633 requirements and gives examples of the best practice confirmed by hundreds of tests performed world-wide, adopted by numerous industrial standards, and requested to comply with international technical standardization and certification organizations such ISO, AS, SAE, and Nadcap.

The ASTM C633 test applies tensile stresses to a coated system consisting of a coated sample (bond cap/substrate) glued to another cylindrical sample (mating cap) as shown in Fig. 1. As an alternative, the coating could be applied on a cylindrical “button” that is glued between two cylindrical samples as shown in Fig. 2. The load is applied in the direction perpendicular to the interface between the coating and the substrate. The level of tensile load is gradually increased from “0” to the load that results in the sample failure—the coating pulls out from the substrate or fractured in two pieces inside the coating. The coated surface is always a flat circle with a diameter 1 in. + 0/-0.005 in., and the sample dimensions are standardized by ASTM C633. The bond strength is calculated as the load at sample failure divided on the coating failed area.

1. Inspect coating quality on bond caps. (Coating chipping, cracks, delamination, separation, overspray, are not allowed.)

2. Prepare mating caps—grit blast their flat surface. (Do not grit blast coating surface.)

3. Apply a layer of glue on the coating surface.

4. Place the coated samples with applied glue layer into V groove of curing fixture. (See example of V groove gravity fixture in Fig. 3.)

5. Add mating cap in the fixture with the grit blasted surface facing the glue layer on coated sample.

6. Apply compression pressure to the assembled samples.

   Note:Fig. 3 does not show the additional weight on top of the assembled sample to keep them in compression during glue curing (see the details below in “Requirements”).

7. Place fixture with assembled and compressed samples in the preheated oven.

8. Keep the samples under pressure in heated oven until the glue cures, cool samples, and release the pressure.

9. Carefully remove excess glue from each assembly with grinding. Recommended grinding parameters:

   • Abrasive size for grinding paper or wheel: not coarser than 120 mesh

   • Grinding direction: parallel to the glued surface

   • Interface damage/removing of the sample material(s) are not allowed.

10. Place assembled samples in grips of tensile machine, and gradually apply tensile load, record the failure load.

11. Measure diameter (D) of sample failed face, and calculate the failure area as:

    $3.14×D2/4$

12. Bond Strength = Load of Failure / Failure Area.

Coating thickness of 0.01 in. (0.25 mm) is recommended if the coating has a porosity below 2%. If the coating has high porosity, increase the coating thickness to 0.015 in. (0.38 mm) and above as recommended by ASTM C633 to prevent glue penetration through the coating to the substrate, but do not exceed the thickness required for the coated part application. Thicker coating has a lower bond strength because the residual stress accumulation that can lead to failure is not a representative result of the test.

The coating thickness shall not vary across the surface by more than 0.001 in. (0.025 mm). The coating surface may be finished by grinding or machining when the thickness variation is excessive. Other treatment such as grit blasting should not be used to level the coating thickness. Coating tapering increases the sample misalignment that results in the failure at lower load.

Polyamide-epoxy FM 1000 adhesive film is recommended (and is mandatory in many industrial specifications) as a bonding glue for the coating tensile test. The advantage of using FM 1000 is that it enables testing of high porosity coatings (such as abradable and ceramic coatings) without glue penetration to the substrate. The disadvantage is that it loses strength if stored at temperatures of 85 °F (29 °C) and above. However, the adhesive can perform well for years if stored in a refrigerator.

Recommended cycle for FM 1000 firm: Heat assembled samples in the oven to 340 ± 10 °F (170 ± 6 °C), then cure for 90 min ± 10 min at the temperatures at bond line of 340 ± 10 °F (170 ± 6 °C). Curing conditions may vary based on the manufacturer recommendations and for different types of glue. Control the curing temperature with a calibrated thermocouple touching (or bonding to) the assembled samples. A class 5 furnace (per AMS2750) with temperature uniformity in the working zone ±250 °F is sufficient.

Figure 3 shows a V-grooved gravity fixture. This design works with FM 1000 adhesive film. To keep the assembled samples in compression during the glue curing, one solid steel cylinder (1 in. diameter and 2 in. long) must be placed on the top of the two assembled samples. The fixture can be made from steel or aluminum alloy, and it should support samples at a position of 30 degrees to vertical. The horizontal groove in the middle of the fixture shown in Fig. 3 protects it from contact with excessive glue during the curing process.

Protect samples from contamination. Do not touch the coated surface. Keep coated samples in clean envelopes (plastic envelopes are recommended). Use filtered compressed air when removing grit or use liquid degreaser to dry the sample surfaces. Protect samples from excessive humidity.

Sample misalignment always leads to low accuracy test results. Verify the alignment of samples in the fixture before and after curing. Remove excessive glue from the sample sides. Use a tensile machine that has universal joints to ensure the self-alignment of the samples when the tensile load is applied.

The recommended crosshead speed for the tensile load is 0.03–0.05 in./minute.

It is recommended to test five (5) samples and to identify the coating bond strength as an average of the five results. For well-established dense coatings with high adhesion and cohesion strength (such as cermets) the number of samples can be reduced to three (3) for 1 data point.

Always test at least one uncoated reference sample with each set of curing samples to verify/confirm that the glue itself has sufficient bond strength (10,000 psi as a minimum for FM 1000).

Determine, and keep records for all test conditions and file test reports. The bond test report should include (at minimum):

• The glue strength

• The coating bond strength

• The percentage of each mode of coating failure (see Fig. 2)

• The date the test was conducted

• The name and signature of the test operator

• Reference(s) to applicable standard(s)

• Specifications and local instructions documenting the test conditions and requirements

This Accepted Practice is intended to be used as a baseline, but it does not replace local test or laboratory instructions. Additional requirements may apply based on the available equipment, testing materials, customer requirements, and other criteria. Acceptance testing should always be performed in accordance with the latest revision of ASTM C633.

This document was prepared in 2013 by the ASM Thermal Spray Society Accepted Practices Committee, which was then chaired by Dr. Tetyana Shmyreva of Rolls-Royce and comprised of an international team (25 total members) of scientists and engineers from academia and industry. Notable organizations represented included Rolls-Royce Corp. (US), Swinburne University (Australia), Sulzer Metco, and GKN Aerospace (EU). The charter of the Accepted Practices Committee is to provide the thermal spray industry with technical guidance for common evaluation techniques.

A version of this document was published as “Accepted Practice to Test Bond Strength of Thermal Spray Coatings,” Journal of Thermal Spray Technology, Vol 22, 2013, p 1263–1266, https://doi.org/10.1007/s11666-013-0033-6.