## 3 Results and Discussion
### Correlations Between Coating Parameters and Mechanical Properties  
Figure 4 illustrates the relationship between the Cr-to-Ti ratio (denoted as Cr/Ti) and the hardness of all the samples. When the current of Cr increases from 80 to 175 A, Cr/Ti also increases from 0.428 to 2.571. The Cr/Ti ratio was determined to be the lowest (0.428, as shown in Table 4) for sample T1, while it was found to be the highest (2.571, as shown in Table 4) for sample T6. On the other hand, there is an opposite trend observed in the hardness of the coating surfaces. The hardness value exhibits variation between 31 and 26 GPa, depending on the chromium current employed. The literature reports the hardness values of binary CrN and TiN coatings to be 18 and 24 GPa, respectively [35; 36]. In this case, the highest level of hardness (30.9 GPa, as indicated in Table 4) observed in sample T1 can be attributed to the existence of multiple phases, as identified by the XRD spectra displayed in Figure 1b. The XRD analysis reveals the initial presence of a single-phase  
Figure 3: (**a**) Working principle of a Calo tester. (**b**) Image of a Calo tester. (**c,d**) Procedure for thickness measurement of the film using a Calo tester. (**e**) Image of TiCrN coating thickness measurements using the Calotte grinding measurement tool. (**f**) Effect of substrate current on coating thickness at fixed coating time and constant Ti current.  
material consisting of titanium nitride (TiN) with Cr substitution. Subsequently, a mixed phase of TiN and chromium nitride (CrN) is observed. Then, in the case of a higher current of Cr, Ti is introduced as a substitution in CrN. The higher concentration of the CrN phase in sample T6 is responsible for the observed lowest hardness value (26 GPa, as shown in Table 4). The reported values of these coatings lie somewhere between the hardness of TiN and CrN, as reported in the literature [37; 38]. The XRD spectra, which shows intense peaks corresponding to CrN (see Figure 1b), clearly demonstrates this.  
The modulus of elasticity and plasticity index of the samples are displayed in Figure 5. The elastic modulus of sample T1 was determined to be 495 GPa, which represents the highest value observed among all the samples. Conversely, the lowest value of 442 GPa was found in sample T6. The high value of the modulus of elasticity in T1 can be attributed to its higher hardness (30.9 GPa, as shown in Figure 4) in comparison to the other samples. The plasticity index, calculated by dividing the square of hardness by the cubic power of the modulus of elasticity, was observed to be the lowest in sample T6. The reason for this is because the T1 sample has the lowest values of hardness and modulus of elasticity.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Cr/Ti Ratio**} & **Hardness** & **Modulus** & **Plasticity Index** \\  & & **(H)** & **(E)** & **(GPa)** \\  & & **(GPa)** & **(GPa)** & **(H\({}^{3}\)/E\({}^{2}\))** \\ \hline T1 & 0.428 & 30.9 \(\pm\) 5.5 & 495 \(\pm\) 15.5 & 0.1216 \\ T2 & 0.587 & 29.9 \(\pm\) 2.7 & 470 \(\pm\) 17.3 & 0.1222 \\ T3 & 0.754 & 30.6 \(\pm\) 6.5 & 487 \(\pm\) 22 & 0.1256 \\ T4 & 0.923 & 29.7 \(\pm\) 2.1 & 457 \(\pm\) 20.9 & 0.1292 \\ T5 & 1.083 & 29.7 \(\pm\) 2.2 & 446 \(\pm\) 17.7 & 0.1357 \\ T6 & 2.571 & 26.0 \(\pm\) 4.0 & 442 \(\pm\) 22.5 & 0.0899 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Hardness and elastic modulus at different Cr/Ti ratios.  
Figure 4: Cr/Ti ratio and hardness of samples.