The hardness of the coating is inseparable from the properties of the coating material, but the hardness of the coating is different from the hardness of the sprayed material. Even for the same sprayed material, the hardness of the coating is usually different, so in the evaluation and inspection Coating hardness should take into account the characteristics of the coating.
1. Particularity of coating structure
The structure of the coating is very complicated. Different coatings, even the same coating material and different spraying conditions, the structure of the coating, that is, the size and structure of the particles that make up the coating, the number and size of pores, oxidation The content of compounds such as substances are not the same. Moreover, due to the inclusion of pores and oxide slag in the coating, the structure of the coating is heterogeneous, resulting in the non-uniformity of the hardness of the coating.
The microstructure of the coating can be directly observed through metallographic examination. A qualified coating microstructure should be a layered structure of deformed particles, and a certain proportion of undeformed spherical particles should be controlled. At the same time, the total microscopic distance between oxide slag inclusions and pores on the interface between the coating and the substrate should generally not exceed 15%. There should be no large holes and slag inclusions on the interface.
2. Coating hardness and spraying conditions
Spraying conditions include working parameters of spraying equipment, spraying distance, thermal properties of sprayed parts, temperature during spraying, properties of sprayed materials, conveying speed of sprayed materials, moving speed of Spray Gun or sprayed parts, etc. Different spraying conditions result in different coating hardness.
As shown in Figure 11-6-2, when the conveying speed, oxygen pressure, compressed air pressure and other spraying conditions of the spraying material are changed respectively, the four kinds of material coatings are made by the fusible line gas spraying (oxygen & hydrogen combustion flame) method hardness. The hardness of the coating will increase no matter which one of the spraying material delivery speed, oxygen pressure, and air pressure is increased. Especially when the oxygen pressure and air pressure are increased, the oxide content in the coating will increase, and increasing the air pressure can increase the impact speed of the sprayed particles on the surface of the substrate, so it can be used to make a dense coating.
Figure 11-6-2 Relationship between fusible line gas spraying conditions and coating hardness
The high-frequency induction spraying method will also cause the hardness of the coating to vary greatly due to changes in the spraying conditions. Figure 11-6-3 shows the relationship between coating hardness and spraying distance for carbon steel when the air pressure is 3.9×10 5 Pa . When the spraying distance is about 100mm, the coating hardness can reach the maximum value of HB400.
Figure 11-6-4 shows the influence of air pressure on the hardness of carbon steel coating (spraying distance is 100mm). It can be seen from the figure that as the air pressure increases, the hardness of the coating increases. When the air pressure is 4.9×10 5 Pa , the hardness of the coating is HB420.
Also shown in Figure 11-6-5 is the relationship between the hardness of the carbon steel coating and the carbon content in the carbon steel material ( when the air pressure is 3.9×10 5 Pa, the spraying distance is 100mm). It can be seen from the figure that as the carbon content increases, the hardness of the coating also increases. The hardness of carbon steel (0.65% carbon) coating is HB450.
3. Macrohardness and microhardness of the coating
Due to the different methods of measuring and testing coating hardness, the meaning of coating hardness is also different in nature. For example, use the same indentation method for hardness measurement, you can use Brinell or Rockwell Hardness Tester, these two kinds of measurement are based on the indentation in a large range as the measurement object, and the macroscopic hardness value is measured, because The hardness of the coating is non-uniform due to the influence of factors such as pores and oxides, which affects the measured macroscopic hardness. The use of a microHardness Tester can measure the hardness value of the coating particles, that is, the microhardness of the coating.
The meaning of macrohardness and microhardness of a coating is essentially different. Of course, the numerical values are also different. For example, the hardness value of the particles constituting the high-carbon steel coating is HRC67 if converted according to the microhardness meter, while the average hardness of the coating itself is HRC38-40. In addition, compound particles interspersed between coating particles may give higher values when measuring the indent hardness. For coatings with a small thickness, usually less than tens of microns, in order to eliminate the influence of the base material on the hardness of the coating and the limitation of the thickness of the coating on the size of the indentation, the microhardness method is generally used.
