ASTM C633 Standard Test Method for Adhesion or Cohesion Strength of Thermal Spray Coatings

Get Certified ASTM C633 Thermal Spray Coating Testing

Testing to ASTM C633 assists coating manufacturers, coating suppliers, engineers, and quality-control laboratories in assessing the bond strength and integrity of the thermal spray coating. The technician prepares the coated fixture, bonds it to a loading fixture, and applies a tensile load perpendicular to the coating surface. The test enables laboratories to compare similar thermal spray coatings and to test coating application procedures. ASTM states that the method is not to be used as an intrinsic strength value for use in determining performance under particular service stresses.

• Adhesion Strength: Determines the bonding strength between the coating and substrate.
• Cohesion Strength: Evaluates the internal strength of the thermal spray coating.
• Thermal Spray Processes: Covers combustion flame, plasma arc, two-wire arc, HVOF, and detonation processes.
• Feedstock Forms: Applies to thermal spray feedstock supplied as wire, rod, or powder.
• Coating Thickness: Applies to coatings that can be applied at thicknesses greater than 0.015 in. (0.38 mm) under the method’s current limitation.

What are the uses of ASTM C633 Testing?

ASTM C633 gives quantitative data on adhesion or cohesion strength for thermal spray coatings. The results can be used for coating quality assessment, to compare similar coating systems, and to assist in process development by manufacturers and engineers. The test helps to:

• Evaluate coating-to-substrate bonding strength.
• Determine coating cohesion strength.
• Support thermal spray quality control.
• Support acceptance testing.
• Compare similar thermal spray coating materials.
• Help qualify thermal spray equipment and procedures.
• Support development of coatings with improved adhesion and integrity.

Which Materials Can Be Tested According to ASTM C633?

ASTM C633 covers appropriate thermal spray coatings and their compatible substrates. The standard contains metallic and ceramic coating materials, and coatings made of different spray materials are permitted. This test applies to:

• Metallic Coatings: Tests thermal-sprayed metal coatings.
• Ceramic Coatings: Includes ceramic materials such as metal oxides and carbides.
• Multilayer Coatings: Evaluates systems containing different sprayed materials, such as an oxide layer over a sprayed metal bonding layer.
• Metal Substrates: Commonly used as substrates for thermal spray coatings.
• Ceramic Substrates: Suitable ceramic substrates, including oxide or graphite materials, may also be used.

Why is ASTM C633 Testing Important?

Thermal spray coatings need to have good adhesion and internal structural integrity for proper service performance. ASTM C633 is a standardized tensile test to determine these properties in a controlled laboratory. The results are used to help manufacturers determine coating or processing issues and to compare similar coating systems. They also provide assistance and guidance for quality control, acceptance testing, and development of better thermal spray processes.

ASTM C633 Testing Process and Data Collection

Technicians prepare the thermal spray-coated fixture and bond the loading fixture to the coating surface. They mount the assembly in the tensile testing machine and apply a controlled tensile load perpendicular to the coating. The laboratory records the applied load, coating information, test conditions, and failure location. Technicians use the measured load and applicable test area to determine the adhesion or cohesion strength.

Common Challenges and Troubleshooting

Improper surface preparation, adhesive penetration, weak bonding, misalignment, and inconsistent loading can affect ASTM C633 results. The standard specifically limits the method to coatings greater than 0.015 in. (0.38 mm) because bonding agents can penetrate thermal spray coatings and influence the results. Technicians should prepare the bonding surfaces carefully and align the fixtures correctly. They should also use suitable bonding materials and follow the required coating thickness and test conditions.

• ASTM C633 provides adhesion or cohesion strength results based on tensile loading normal to the coating surface.
• Laboratories report strength in psi or MPa, as applicable.
• The report specifies whether the failure was at the coating-substrate interface or in the coating itself. 
• The report details the coating material, the substrate, the coating thickness, bonding conditions, loading information, and the relevant test conditions. 
• Laboratories can achieve reliable and repeatable results with suitable fixture preparation, adhesive bonding, tensile loading, alignment and calibration.

Link to ASTM C633