PVD Coating Coating Adhesion Inspection

- Jan 12, 2019-

PVD coating coating adhesion inspection

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Coating adhesion refers to the ability of coating and coated substrate surface adhesion, that is, the degree of difficulty in stripping the coating from the substrate. The harder it is to peel, the better the coating adhesion. In general, if the boundary between the coating and the substrate is not obvious, it indicates that the coating has a very good binding force. By observing the interface icon of the coating substrate under scanning electron microscope, it can be seen that the more fuzzy the interface of the coating substrate is, the better the binding force of the coating is proved by the field measurement.


The testing methods of coating adhesion include bending test method, hot and cold cycling test method, adhesion stripping test method, adhesion stretching test method, scratch test method, indentation test method and ball mark test method. For PVD coating, the last three methods are commonly used, as described below:


1.Scratch test method

Three needles with diamond spheres are used to row continuously on the surface of the coating at a certain speed and gradually increase the load on the needle. The load at the moment when the coating is completely penetrated is the binding force of the coating.The coating will make a different "sound" at the moment it is cut through. Of course, the sound is very small, and it is difficult for the human auditory sense to accurately sense.To improve the accuracy of the measurement, a sensor that can receive weak sound is installed between the measured part and the needle, and then the signal is amplified.With this sensor, the sound of the needle during the whole process of stroking can be recorded, and the load and sound signal intensity curve can be drawn automatically.The load corresponding to the first spectral peak on the curve is the critical value of the coating's adhesion to the substrate.


2. The instrumented indentation test(IIT)

Indentation test is carried out on rockwell hardness tester. Different test methods are used for different materials. For steel matrix, HRC method was adopted, and its test pressure load was 15okgf. For the matrix with high hardness, such as hard alloy, HRA method is adopted, and its test pressure load is 60kgf.


When testing the rockwell hardness tester of the conical head (cone Angle is 120, spire radius of 0.2 mm), to a certain test pressure for coating the surface under test, keep constant time (hardness tester program automatically set, about 6 s) after the unloading pressure, pressure head at the specimen surface pressure will be a net conical depression (indentation), viewed from the front, indentation is a circle.


After the workpiece or test piece (with the coated workpiece in the same batch of secondary coating) is measured by the rockwell hardness tester, the adhesion of the coating is checked under the projector, as shown in the judgment reference diagram of indentation edge crack, the serial number HF0 ~ HF6 is from good to bad.


The degree of coating adhesion can be determined by magnifying and observing the coating condition of trace edge with the projector and comparing with the crack criterion. This coating adhesion measurement method is widely used in manufacturing enterprises.


3. Ball mark test method


The indentation test method is to use steel balls of a certain diameter to grind the coating on the coating surface and wear the coating through the substrate, leaving a circular (when the coating surface is flat) or oval (when the coating surface is cylindrical) pit.Grinding time to master, the depth of the pit must exceed the thickness of the coating.In addition, because the coating is very hard, it is difficult to directly wear the coating through the steel ball, so when grinding, first in the ball grinding area coated with some diamond powder (particle size below 7um), and spray alcohol to dilute the gold and copper powder and grinding lubrication.When the pits are enlarged by 50 ~ 100 times under the microscope, a boundary line can be seen between the coating and the substrate, and the shape of the boundary line can be observed to determine the adhesion between the coating and the substrate."If the boundary is very clear and neat, then the coating adhesion is not good;If this line of demarcation is staggered or faulted.The coating has good adhesion.