How to measure the coating adhesion of coating equipment?

Coating adhesion refers to the mutual adhesion between the coating and the substrate surface to be coated, that is, the difficulty of stripping the coating from the substrate. The harder it is to peel, the better the adhesion of the coating. In industry, there are three main methods to measure the coating adhesion of coating equipment, namely, scratch method, indentation method and ball mark method.

1. Scratch method

Scratch method is a standard test method to "quantitatively" measure the bonding strength and failure form of coating film substrate. A diamond spherical needle is used to continuously scratch on the coating surface while gradually increasing the load on the needle. The load applied at the moment when the coating is completely scratched or the coating is obviously peeled off is the bonding force of the coating, expressed in Newton N.

American ASTM C1624-05 standard is applicable to the adhesion test of hard ceramic coatings (Vickers hardness is higher than 5GPa, thickness ≤ 30um) deposited on ceramic or metal substrates at room temperature. The simple expression is to mark the points of LC1, LC2 or even LC3 by observing the scratch image under the microscope, and calculate the load applied at the location where these points occur.

Generally, LC1 means that cracks begin to appear on the scratch edge, indicating that the coating is beginning to fail, and LC2 means that the scratch begins to peel off, indicating that the coating has completely failed. Generally, LC2 is used to define the adhesion of the coating. At present, this standard is mostly used internationally.

The national standard JB/T8554-1997 is a scratch test method used to measure the film substrate bonding force in China. It is used to determine the coating failure point, namely the coating bonding force, by detecting the acoustic signal at the moment when the coating is scratched through. Because the sound is very small, it is difficult for human hearing to sense it accurately. Generally, a sensor capable of receiving weak sound is installed between the tested piece and the needle, and then the signal is amplified. Generally, the load corresponding to a peak value of the sound spectrum on the sound signal curve is used as the value of the membrane substrate bonding force.

Because the acoustic signal is greatly affected by the environment, many organizations still use the optical microscope to observe the scratch track and determine the LC2 value to determine the adhesion of the film substrate. Scratchers are generally expensive, and domestic scratch testers are generally more than 100000 yuan, so small enterprises rarely buy them. Large enterprises, schools or research institutes generally buy scratch testers.

2. Indentation method

Indentation test is a "qualitative" method to measure the film substrate bonding force. Use the diamond indenter of the Rockwell hardness tester to make a dent on the designated position of the sample. The sample substrate at the dent will generate corresponding stress between the substrate and the coating due to deformation. This stress may cause the coating around the dent to fall off. Different test methods shall be adopted for the matrix of different materials.

For steel matrix, HRC method is adopted, and the test compressive load is 150Kgf; For matrix with high hardness, such as cemented carbide, HRA method is adopted, and the test pressure load is 60Kgf. Observe the morphology of the pits under the microscope and compare with the standard to determine whether the adhesion of the coating is qualified. HF1-HF4 is qualified, and HF5-HF6 is unqualified. Most coating companies use this method to test the film substrate adhesion.

3. Ball mark method

The ball mark method is also a "qualitative" method to measure the film substrate bonding force. It is to grind a steel ball with a certain diameter on the surface of the Chuyuan layer, wear the coating through, and leave a circular (when the coating surface is flat) or elliptical (when the coating surface is cylindrical) dent on the coating and substrate.

The grinding time shall be well controlled, and the depth of the pit must exceed the thickness of the coating. In addition, because the coating is very hard, it is difficult for the steel ball to wear through the coating directly, so when grinding, a little diamond powder (particle size below 7um) should be coated on the grinding area of the steel ball, and alcohol should be sprayed to dilute the diamond powder and lubricate it during grinding.

When the pit is magnified by 50-100 times under the microscope, a decomposition line will be seen between the coating and the substrate. Observe the shape of this boundary to judge whether the adhesion between the coating and the substrate is good or not. If the boundary line is very clear and neat, the coating adhesion is good. If the boundary line has dog teeth staggered or faults, the coating adhesion is poor.