Nanoindentation testing of nanoparticle reinforced PEO coating

Overview of activities
The mechanical response of the PEO coatings was investigated in this phase of the research. Nano mechanical characterization of the coatings was performed on a Bruker Hysitron TI 980 Triboindenter. The nanoindenter tip was made of diamond in Berkovich type geometry with half-angle 65.27° and included angle 142.30°. The Young’s modulus and Poisson’s ratio for the diamond provide by the manufacturer as 1140 GPa and 0.07, respectively. The equipment has active feedback load and displacement control. For all the specimens, a peak load of 150 mN was used to create the indentation, and then load was released while recording the displacement, which was recorded in nm (10^-9 m). At least 7 indentations were made on each specimen.

 

Results
The deformation in indentation follows elastic-plastic mode, where a part of the deformation is elastic in nature. A typical load-displacement curve obtained in a nano-indentation experiment is shown in Figure 1. The loading and unloading curves do not follow the same path due to the plastic deformation occurring in the sample. The maximum deformation is denoted by h_max and final displacement of the indenter after elastic recovery is denoted by h_f.

Figure 1. Schematic load-displacement graph during nanoindentation.

Figure 2 shows the relationship between the dimensionless parameters h_f/h_c and E^*/H_c for the coatings. The parameter h_f/h_c is a measure of the recovery of the material being tested with an inverse relationship. So, higher values of h_f/h_c point to less elastic and more plastic behavior; for instance, a value h_f/h_c= 0 means perfectly elastic and h_f/h_c= 1 means perfectly plastic and 0 – 1 means elastic-plastic behavior. The parameter E^*/H_c is the ratio between the modulus and hardness of the material. For all specimens tested, there is an increasing trend in h_f/h_c with E^*/H_c , which is consistent with results from numerical simulations of nanoindentation. The range of values of E^*/H_c and these results show that the tests reflect local mechanical response rather than bulk. It can be inferred from the plots that higher processing time produces coatings with higher values of the two dimensionless parameters h_f/h_c and E^*/H_c , which are indications of low elastic recovery and high stiffness materials. It is interesting to note that the slope of line fitting the data points is lowest for the coatings produced at 10 minutes.

Figure 2. Plots showing E^*/H_c vs h_f/h_c relationship for (a-e) PEO@TiN and (f-j) PEO@SiC coatings

It is important to understand the relationship between coating stiffness (E^*) and hardness (H_c). While the E^*/H_c parameter is useful in understanding the effect of depth of deformation on mechanical properties, both values can be high or low simultaneously, causing the parameter to be effectively the same. The plots of E^* vs H_c are shown in Figure 3. There is a clear increasing trend in E^* with H_c for all specimens. This shows that hardness is directly proportional to the contact stiffness of the phase being indented. The outliers in the Figure 9a could be a due to a TiN nanoparticle aligned such that the deformation takes place in the TiN(1 1 1) preferred direction.

Figure 3. Young’s Modulus, E^* vs contact hardness, H_c plot for (a) PEO@TiN and (b) PEO@SiC coatings.

The calculated mean and standard deviations are listed in Table 1. The hardness value of TiN incorporated PEO coating was 1.90±1.02 GPa, which was close to similar coating on MA8 alloy (2.2±0.3 GPa @ 1 g/L NP). The fact that the average values are so close means that it reflects the bulk properties and the standard deviation accounts for the local behavior. This value reported in the [29] was obtained by microhardness testing, however, and reflects properties of a larger scale than in this study. This is why the values of standard deviation listed in Table 1 are so high. Similarly, the modulus in the other study is 62±7 GPa, whereas in this study it is 55.2±20.9 GPa.

Table 1. Mean value and standard deviation of h_c and E^* for the coatings.

Conclusion
Nanoindentation testing reveals that PEO@TiN coatings have higher average modulus and hardness than the PEO@SiC coatings. The high variance in data is due to the difference in local properties and bulk properties of the coating. There is a clear increasing trend in the dimensionless parameter h_f/h_c and E^*/H_c.

CFLA 1.1.1.1/19/A/148

Being implemented with the financial support of the ERDF.