4.3. Mechanical properties and adhesion of oxide films
The deviation in indentation diameter under the same impact conditions depended upon both the scattering of impact velocity and film thickness, but the diameter of the detached area related well to the indentation diameter shown in Fig. 9. The ratios of b to a were regarded to be constant depending upon the environments. The detaching of the films seemed to be associated with the surface strain, as explained in Section 4.2. Fig. 13 shows the relations between a relative distribution of lip height hD−1 and relative distance dD−1 caused by the impact of glass beads of 2 and 5 mm in diameter. D is a diameter of a glass bead. d at the maximum value of hD−1 shows the indentation diameter, and the periphery of the indentation is in the outer region from the maximum hD−1. The indentation ratio aD−1 is approximately determined by the impact velocity, independent of particle size [13]. It can be found that the relative distribution of a lip surface (hD−1 versus dD−1 curve in the outer region) depends upon aD−1. If the maximum hD−1 of ca. 4 × 10−3 corresponds to 30% of surface strain, which is roughly obtained as the nearly same indentation ratio in the previous paper [14], the surface strain at given ba−1 is roughly estimated from the constant value of ba−1 in Fig. 9, and aD−1 and dD−1 (b is equal to d) in Fig. 13. The surface strain obtained by the two ways of the different aD−1 or impact velocity was consistent with each other. The surface strain is regarded as the shearing strain γf on the fracture of the films and listed in Table 2.
The relations between the fracture volume Vf and fracture energy If derived in Section 4.2 are shown in Fig. 14, and are generally good despite the scattering of data which is probably caused by irregular strength of the multi-layer of the oxide film (see Fig. 7). The slopes were different depending upon the type of the corrosive environments and showed the fracture pressure Pf. σf modified by Eq. (7), Ef and τf obtained from Eqs. (3) and (4) are also listed in Table 2. It was found that Ef, σf and τf took lower values with the stronger corrosiveness. These values in the 10 vol.% HCl environment was extremely small and reflected the porous oxide films and the low apparent density. The adequacy of the values of these mechanical properties was sustained by the fact that the contact pressure on the mild steel with the oxide films was slightly higher than that without the oxide films. The σf value of 530 MPa in Table 2 was comparable to a mechanical property of mild steel. These results, therefore, propose ones of mechanical properties under high temperature corrosive environments, obtained by the particle impact method.
On the other hand, considerations on adhesion of the films have been argued among many researchers [15] but adhesion obtained by a scratch test is often affected by parameters such as an indenter, a load and film thickness. In the case of this study, we considered that the adhesion is defined by surface stress σa generated in the metal surface,which detaches the oxide film with the surface strain γf obtained from Fig. 13. The stress–strain curve on mild steel is generally expressed as
σa is also listed in Table 2 and is larger in the more corrosive environment in contrast with the other mechanical properties. In fact, the shearing strain on the metal surface when the films were detached was the largest in the most corrosive environment of 10 vol.% HCl.