Coating Microhardness (HV) Testing

       The microhardness method is to use a special diamond indenter on the microhardness tester to press into the surface of the sample coating under a certain static load to obtain the corresponding square pyramid indentation, and then use the hardness tester to measure the microhardness. The eyepiece magnifies the indentation by a certain magnification, and then measures the diagonal length of the indentation, and checks the microhardness of the coating by looking up the table.

    This method is applicable to a wide range of hardness and sample types, and can be used for general spray and spray melt layers.

    The commonly used loads for microhardness tests are 0.25N, 0.5N, 1N, 2N, 3N, 5N and 10N. For special and fine specimens, it is necessary to select smaller loads, such as 0.1 and 0.05N. Some special ultra-micro hardness tester, its load can reach 0.01N.

    The load selected for the test depends on the thickness and hardness range of the test piece. A larger load should be used as far as possible within the allowable range of the thickness of the specimen, in order to obtain a larger size indentation, thereby reducing the relative error of the measurement.

However, it is not advisable to use an excessive load for the hard material, so as not to damage the indenter. However, it is also not suitable to use loads with a diagonal of the obtained indentation of less than 10 μm. The selection of the test load is based on the hardness range of the material. Usually, the load size can be estimated by the following formula:

   

    where m------load mass (g);

         HV-------Estimated coating hardness value (N/mm2);

         δ-----(μm);

     As the load decreases, the ratio of the diagonal length of the indentation obtained to the load is not constant.

     When determining the hardness value of the material, try to make the diagonal of the obtained indentation larger than 10 μm within the allowable range of the thickness of the specimen.

     Microhardness test sample requirements:

     The surface of the sample should be smooth, clean, flat and free of oil stains. The surface of the sample should be ground and polished, and its surface roughness Ra should not be higher than 12 μm. The coating should have a corresponding thickness. After the test, the test piece should not have any trace of deformation, and its thickness is generally not less than 10 times the depth of the indentation. For extra-small specimens, they should be embedded in plastic or other softer materials, but the embedded material should have a certain strength so that it does not shift during the test.

     The test piece should have enough test surface, and its area diameter should not be less than 5 times the indentation diagonal.

     Microhardness test load application and unloading:

     The load should be applied smoothly and slowly, without any vibration and shock, and the loading speed should not be too slow, especially when the holding time is short. Generally, the loading speed should not be greater than 50μm/s, and the loading speed is mostly in the range of 15μm/s~70μm/s.

     The load holding time also has an impact on the test results. The longer the holding time, the more sufficient the deformation of the material and the lower the hardness value. At the same time, if the holding time is too long, it is easily disturbed by external conditions and also affects the test results. Usually about 30S is used to complete the loading.

     When the load is unloaded, collision and vibration are forbidden, and it should be unloaded smoothly and evenly. Generally, the unloading speed is faster than the loading speed.

     The microhardness test should pay attention to the following problems:

     (1) The test should be carried out in a clean environment with room temperature of 20°C±1°C, dry and free of corrosive gas, with no vibration source nearby, and the hardness tester should be installed on a solid foundation and adjusted to a level.

     (2) The magnification of the microscope is generally not less than 400 times. The division value should not be greater than 0.5μm, and the reading accuracy should not be lower than ±0.2s.

     (3) The hardness tester can only be used after passing the inspection.

     (4) At least 5 points shall be measured on the same test piece, and the average value of the 5 points shall be taken as the tested hardness value.

     (5) The distance between the center of the indentation and the edge of the specimen and the center of two adjacent indentations shall not be less than 2.5 times the length of the diagonal of the indentation. For non-ferrous metals, the distance shall not be less than 5 times the diagonal length of the indentation.