When the thermal sprayed coating is quite thin, for example, coating thickness is less than 0.15mm, the Vicker and Knoop hardness testing for metal coating and other relevant coating could be refered to (see GB 9790-88).
When the thermal sprayed coating thickness is more than 0.15mm, the surperficial Rockwell scale N and T should be applied according to different thickness of the coatings. The specimen hardness value is between 20HRC to 65HRC, apply scale N, while specimen hardness value is between 99HRB to 32 HRB, apply scale T.
1. The Smallest Thermal Sprayed Coating Thickness of Scale N and T
According to the difference of specimen hardness value, the smallest thermal sprayed coating thickness of scale N and T is shown in Table 1 and Table 2.
Table1. The Smallest Thermal Sprayed Coating Thickness of Rockwell Scale N Testing.
Table2. The Smallest Thermal Sprayed Coating Thickness of Rockwell Scale T Testing.
The specimen surface should be fine shined to make it not distorted after effect of temperature and producing process.
The recommended total test force holding time is: For specimen which will not be distorted with time going after loading test force, keep no less than 2 seconds. For specimen which will be distorted with time going after loading test force, keep 6 to 8 seconds. For specimen which will be obviously distorted with time going after loading test force, keep 20 to 25 seconds. The distance between two indentation center or distance between indentation center and specimen edge should be no less than 3mm.
The test resolution is 0.5. Hardness values of 5 continuous points should be tested for each specimen. Besides, the arithmetic mean value of the 5 points should be given, as well as the minimum value and maximum value.