Repeated Laser Shock Adhesion Test for evaluating adhesion durability and its improvement of Ni-P coating

Hard coatings with a thickness of a few microns are widely applied to improve the performance of tools, for example: for cutting, forming and casting applications, and mechanical components. Among them, Ni-P plate coating is widely used for structural materials and plays a critical role in preventing mechanical failure of the substrate. It is well known that Ni-P material possesses high hardness, high strength and other superior mechanical properties, providing excellent performance for metallic ductile substrate/components. It has already been established characterization of the contact fracture and wear properties of Ni-P coatings with different microstructure by post-heating, so that they improve the wear resistance. However, the effect on adhesion strength has not been studied so far. The hindrance to this is the fact that it is challenging to evaluate adhesion strength quantitatively using conventional mechanical tests. In essence, quantitative mechanistic study for adhesion is needed to guide the improvement of wear resistance and mechanical performance of coatings.

Generally, for the integrity of coating materials, it is essential to quantitatively evaluate the adhesion strength and adhesion durability of hard coatings. Unfortunately, previous studies have only conducted general testing with contact methods, which may make quantitative evaluation difficult for good reproducibility. This is due to the fact that the testing result is affected by contact between the materials and/or equipment. On this account, researchers from Chuo University in Japan: Kohei Kanamori (graduate student), Yoshikatsu Kimoto (graduate student), Shuto Toriumi (graduate student) and led by Professor Akio Yonezu, proposed to evaluate the adhesion strength and durability of Ni-P coatings in a non-contact manner using the Laser Shock Adhesion Test (LaSAT). Their work is currently published in the research journal, Surface & Coatings Technology. This technique of LaSAT is very unique, and laser shock wave is employed to induce adhesive fracture with a non-contact manner. In addition, repetitive LaSAT provides useful information on adhesion durability, which is difficult to evaluate using general testing methods.

To begin with, a hard coating of electroless Ni-P deposited on carbon steel substrate (SKD11) was prepared, followed by the in-depth assessment of the effect of post heat treatment on adhesion. The researchers then calculated the interfacial tensile stress from the numerical simulation of wave propagation by using FEM. Lastly, the effect of post-heat treatment on adhesion was investigated using LaSAT, uniaxial tension, and microscopic observation using SEM-EDX.

The research team reported that credit to post heat treatment, the hardness of coating, adhesion strength and durability were improved. Especially, the repetitive LaSAT revealed that adhesion durability improved by twice or more. Further, observation of the delaminated surface of the Ni-P coating using SEM-EDX revealed the presence or absence of Fe-C compounds on the Ni-P surface.

In summary, the study used LaSAT to quantitatively evaluate the adhesion strength and adhesion durability of the hard coating in a non-contact manner. It was found that due to the heat treatment, Fe-C compounds on the substrate were deprived by the coating delamination. Overall, the team concluded that repeated LaSAT was useful for evaluating the adhesion strength of the electroless Ni-P coating deposited on the carbon steel substrate. In a statement to Advances in Engineering, Professor Akio Yonezu said the study provide important information on the effects of heat treatment on coating adhesion toward the development of better coating process, as well as repetitive LaSAT is extremely useful for evaluating coating life quantitatively. This is the first challenge and a pioneer work of films/coatings integrity.