The bonding force of the coating refers to the adhesion of the coating to the surface of the substrate, that is, the difficulty of stripping the coating from the substrate. The harder it is to stripping, the better the coating adhesion. In general, if the boundary between the coating and the substrate is not obvious, it shows that the adhesion of the coating is very good.
The detection methods of coating adhesion include bending test, adhesion stripping test, adhesive tension test, scratch test, indentation test and ball mark test. For PVD coating, the latter three methods are usually used.
1. Scratch test method
Using three needles filled with diamond balls to continuously row at a certain speed on the coating surface, at the same time, the load on the needle is gradually increased, and the load that the coating is fully worn at that moment is the binding force of the coating. In order to improve the accuracy of the measurement, a sensor that can receive a weak sound is installed between the measured piece and the needle, and then the signal is amplified. Through the sensor, the sound of the needle during the entire stroke can be recorded, and the applied load and the intensity curve of the sound signal are automatically drawn. The load corresponding to the peak of the first sound spectrum appearing on the curve is the critical value of the binding force of the coating to the substrate.
2. Indentation test method
The indentation test is carried out on the Rockwell hardness tester, and the different testing methods are used for the substrate of different materials. For steel substrates, the HRC method is used and the test pressure load is 15 okgf, and for high hardness substrates, such as hard alloy, the HRA method is used and the test pressure load is 60kgf.
When testing, the cone shaped indenter of the Rockwell hardness tester (Cone angle 120, tip radius 0.2mm) is pressed into the surface of the tested coating under certain test pressure. After a certain time (Hardness program is automatically set, approximately 6s), the pressure is removed, and the pressure head will squeeze out a web of conical concave on the surface of the sample. It is observed from the front side and the indentation is a circle.
The workpiece or test piece is measured by the Rockwell hardness tester and is placed under the projector to check the viscosity of the coating. The condition of the coating on the edge of the trace is amplified by a projector, which is compared with the criterion of the crack determination, thus determining the grade of the adhesion force of the coating. This coating adhesion measurement method is currently used in more production companies.
3. Ball mark test method
The ball mark test is to grind the coating on the coating surface with a certain diameter of the steel ball, leaving a circle on the coating and the substrate, or the ellipse (the surface of the coating is cylindrical). The time of grinding must be mastered, 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 the coating directly. Therefore, when grinding, first apply some diamond powder (grain size below 7um) in the grinding area of the ball, and spray alcohol to dilute the diamond powder. Enlarging the pit by 50~100 times under the microscope, we will see a boundary between the coating and the substrate, and observe the shape of the boundary line to judge the bonding force between the coating and the substrate. If the line is very clear and neat, the coating binding force is not good, and if this line of demarcation is staggered or a fault occurs, the coating has good adhesion.