**2.1. Test material measurements**

Coating survival and RCF test life is dependent on coating and substrate material properties. For example, a thin film deposited onto an M50 Steel substrate is likely to spall if the applied contact stress during the RCF test exceeds the ultimate strength of the substrate. When considering the ultimate strength and hardness of rolling bearing elements it is best to directly measure surface hardness and then approximate the ultimate strength. Substrate geometry influences surface hardness and it is to be expected that ball, rod, and race geometries will influence the hardness measurement as well. Table 2 presents hardness and material property data for the test elements presented in Figure 2b. Table 3 contains measured surface roughness data. When collecting hardness and surface roughness data, it is recommended to carryout repeated measurements over multiple samples. For example, the data presented in Tables 2 and 3 were repeated three times per sample on five samples and the average was taken based on 15 measurements of each test element.


**Table 2.** Hardness and material property data for RCF test elements in Figure 2b.


**Table 3.** Average surface roughness Ra data in microns for RCF test elements.
