Magnesium alloy surface treated film's performance test

The surface treatment films on magnesium alloy (such as aluminum magnesium alloy, boric magnesium alloy, silver magnesium alloy, etc.) materials and workpiece can be divided into two categories: one is the film with thickness less than 30μm, such as chemical conversion film, anodic oxidation film, vapor deposition film, etc. The other is the thickness of up to 100μm thick film such as spray film. The performance of the film can be measured according to the thickness of the film and its formation mode. Detection content: appearance, film thickness, corrosion resistance, hardness, wear resistance, and adhesion between film and substrate.

Appearance inspection

The general magnesium alloy workpiece after surface treatment can be observed by natural light or fluorescent lamp color and uniformity of the film layer. For anodic oxidation film or micro-arc oxidation film, the pore size, color, spot and peeling of the oxide layer should be observed, and for the gas deposition film, the ripple, color difference and coverage state should be detected.

Film thickness determination

Non-destructive method can be used to determine the thickness of the film, but also damage method, the former includes mass method, magnetic method, eddy current method, β -ray reflection method, X-ray fluorescence method, double beam microscopy method, mechanical measuring method, etc. The latter include metallographic microscopy, dissolution method, liquid flow method, drop method, Coulomb method, profilometer method, interference microscopy, glow discharge spectrum and so on. Because the measurement methods are based on their respective mechanisms and different mathematical models, the test results will be different. It is important to specify the measurement method so that the test results are meaningful. At present, there are many thickness measurement methods, such as thickness meter measurement, metallographic detection and glow discharge spectroscopy.

Hardness measurement

The instrument usually used is microhardness tester. Under certain load, the diamond indenter is pressed slowly into the surface layer within tens of seconds, forming diamond indentation on the surface, measuring the diagonal length of the indentation, and then calculating the surface Vickers hardness according to the formula. Load was selected according to film thickness and estimated hardness before measurement. High load, the hardness error will be relatively small, but high load may press through the film, sometimes also cause the film rupture.If the thickness of the film is needed to understand the hardness distribution along the thickness direction, you can first prepare metallographic samples along the interface, and then test the hardness according to the way shown on the right.

Microhardness tester can not accurately determine the hardness of vapor deposition film or chemical conversion film, but can be measured by nano hardness tester. This durometer has a small load of nanoscale and can be used to measure the hardness of nanoscale films.

Coating adhesion and bonding strength determination

Bonding force or bonding strength, also known as adhesion, is the force required to separate the coating, coating, oxide film or other surface treatment film per unit area on the substrate. Due to the difficulty of actual detection, although there is no lack of quantitative and semi-quantitative methods, most of them are qualitative lattice method, tension method, friction method, stripping method, deformation method, heating method, etc. Film thickness, properties, substrate state and other factors should be considered to determine which method to use.

The micro arc oxidation film of magnesium alloy is hard and thin, so the scratch method can be used. The scratch method is based on the indenter made of material whose hardness is greater than that of the film layer, such as the diamond indenter, with a certain speed across the film layer, while increasing the vertical pressure on the indenter (step or continuous) until the film layer is separated from the matrix. At this time, the load is the critical load, and then the bond strength of the film is calculated. Acoustic emission technology or friction measurement method can be used to determine whether the film is separated from the matrix.

Other coatings may be tested for hardness by indentation. The operation is simple, but not precise. Although the tensile method is not difficult, it needs to prepare special samples. Laser stripping method is accurate and destroys the film surface, but the equipment is noble and the operation is complex. For the thick coating on the surface, repeated heating method, namely thermal vibration method, can also be used to determine the hardness.