Macrohardness and Microhardness of Thermal Spray Coatings

       The hardness of the coating is a very important mechanical performance index of the coating, which is related to the wear resistance, strength and service life of the coating and other functions. Most of the coating materials that make up the coating are multi-alloy or high-alloy materials and composite materials. There are compound phases and hard point dispersion phases in the coating structure, so the detection of coating hardness can be divided into macro-hardness and micro-hardness.



The macroscopic hardness of the coating refers to the average hardness value measured by using a general Brinell hardness or Rockwell hardness tester with a large-scale (macro) indentation on the entire surface of the coating as the measurement object. Since the coating material is different from the base material, defects such as pores and oxides existing in the coating will have a certain impact on the measured macroscopic hardness value. The microhardness of the coating refers to the microhardness tester, taking the particles in the coating as the measuring object, and the measured hardness value reflects the hardness of the coating particles.



The macrohardness and microhardness of a coating are inherently different. The macrohardness of the coating reflects the average hardness of the coating surface, while the microhardness of the coating reflects the hardness of the particles in the coating.



The macrohardness and microhardness of the coating are also different in numerical value. For example, the hardness value of the particles constituting the high carbon steel coating is 67HRC if converted to a microhardness meter, while the macrohardness (average hardness) value of the coating is 38 ~ 40HRC. In addition, particles of the compound intercalated between the coatings may give higher values when measuring notch hardness.



The above two aspects determine the different adaptation conditions for the hardness of the two coatings. Generally speaking, for thin coatings with a thickness of less than tens of microns, in order to eliminate the influence of the matrix material on the hardness of the coating and the limitation of the indentation size of the coating thickness (the coating is too thin, it is easy to reflect the hardness of the matrix material to the measurement The results come in), the microhardness test can be selected. Conversely, for thick coatings (thickness greater than tens of microns), the macro hardness can be selected.