Adhesion testing of thermal sprayed coatings
The Test Method
The ASTM C633 testing standard was written to allow for coating adhesion in a tensile testing machine.
- A 25mm diameter steel cylinder end is sprayed with the required coating.
- Another cylinder with the same diameter is glued on top of this.
- After positioning this assembly into the tensile testing machine they were pulled apart.
- Maximum pull stress is recorded.
Complicated universal link joints were required in order to achieve a pull force which was normal to the coating surface.
The method works but is associated with a number of drawbacks:
- To spray-coat a cylinder end is not necessarily a good representation of spraying another surface;
- If a coating adhesion on a particular substrate is to be investigated special test cylinders of the same material need to be made;
- The test procedure is slow and cumbersome;
- Not everybody has access to a tensile testing machine, and they can be very expensive to purchase
The PAT adhesion tester has addressed all these problems.
With PAT there is a choice. If required:
- You can test with Ø25mm cylinders strictly in accordance with ASTM C633
- You can test with Ø25mm cylinders on test panels instead of on another cylinder
- You can test with Ø25mm cylinders directly on the actual component (if space allows)
- You can test with many other cylinder sizes in accordance with ASTM C633
- You can test with many other cylinder sizes on test panels instead of on another cylinder
- You can test with many other cylinder sizes directly on the actual component (if space allows)
ASTM C633 testing standard is often specified as a default standard. Expensive equipment is then required to carry out this inflexible test method. Questions like: Why does the test cylinder have to be one inch diameter? What about the tensile increase rate? These parameters were originally set because of the limitations of tensile testing machines. They were big machines with a measurably moving traverse (to measure elongation). The strain in a thin coating cannot be sensibly measured so tensile rate in millimeters per minute is not relevant.
How bad is it? An example:
A brittle coating of 10µm (1/100 mm) thickness is tested.
A conventional tensile tester with cross-head travel between 13µm and 21µm per second (as specified by the ASTM standard) is used.
Assume the strain in the coating is 1% of its thickness at maximum stress (= 0.1µm).
In other words, (ignoring the strain in the glue layer) the duration of the actual test period (from zero N/mm2 to fracture) is only between 1/130 and 1/210 seconds.
Assuming a fracture value of 20 MPa (2900 psi) the stress increase during testing with conventional tensile testers is more than 2600 MPa (377,000 psi) per second!
If the coating is only 1µm thick the testing speed is reduced to between 1/1300 and 1/2100 seconds.
With The ASTM standard the rate of stress increase in the coating will depend on the thickness of the coating, which is another weakness of this standard: The stress increase should be the same regardless of coating thickness!
So instead of controlling the distance the the dolly is traveling during pull testing the machine should be able to control the materials pull stress increase (measured in MPa or psi per second).
Ultimately, the important thing is to consistently and accurately measure the true mechanical strength of a coating or a coating/substrate interface, so why not simplify matters? —
Two cylinders with Ø25mm/one inch diameter, one of which is coated with the sample coating, are glued together end-to-end and then pulled apart in order to measure the pull stress (measured in Newton/mm2 or psi) required to fracture the coating.
The ASTM-C633 tensile testing jig is used with the PAT range of pull-off testers to perform pull testing in accordance with ASTM-C633.