## 4 Discussion
### Hardness Recovery  
The hardness variation in the He-ion-irradiated specimen saturated for a nuclear energy deposition of 1.8 \(\times\) 10\({}^{20}\) keV\({}_{\rm nucl.}\)cm\({}^{-3}\). It should be indicated that this finding is in good agreement with ion irradiation-induced hardness reduction [4; 5; 7; 8; 13; 14; 15]. Moreover, an obvious recovery was observed in the hardness variations of Ar-ion-irradiated specimens. Figure 8 illustrates that when the electronic energy deposition in electron irradiated glass exceeded 1 \(\times\) 10\({}^{22}\) keV\({}_{\rm elec.}\)/cm\({}^{3}\), the corresponding hardness decreased significantly. [PERSON] et al. showed that Au ion irradiation slightly affects the hardness variations. Moreover, it was found that the variations of the start point are similar to that of the electron irradiated glass [16]. Consequently, the hardness recovery may be attributed to the effects of electronic energy deposition. Accordingly, it was inferred that for the hardness recovery of ion-irradiated borosilicate glass, the deposited electronic energy threshold is 1 \(\times\) 10\({}^{22}\) keV\({}_{\rm elec.}\)/cm\({}^{3}\). Since the hardness evolution induced by electronic energy deposition changes as a function of the chemical compositions, such a threshold depends on the chemical composition of the specimen. Reported results for different glass compositions showed that the mechanical characteristics of the specimen change differently with ionizing radiations [5; 13; 15; 22; 23].  
For electronic energy deposition, the displacement damage caused by ion irradiation has the self-restoration phenomenon [13; 24]. In this regard, [PERSON] et al. observed the hardness recovery in the He ion irradiated with gold pre-damaged glasses [21]. Moreover, it was found that the nuclear energy deposition leads to displacement damage in the glass after ion irradiation. However, considering the effect of electronic energy deposition, the microdamage in the glass can be recovered partially. Moreover, for single ion-implanted borosilicate glass, the electronic energy loss for the Ar ion was greater than that of He ion. As a result, the corresponding self-restoration effect is more obvious while depositing the same level of electronic energy.  
Figure 6 presents different degrees of He ion-induced recovery in the hardness of the Ar pre-irradiated glass (I-IV samples). It was observed that both the maximum and intermediate damage levels recovered to the saturation damage level of the He ion irradiation, while there was no such a recovery effect in the hardness of sample I. Accordingly, it was inferred that there is a damage threshold for the hardness recovery of ion-irradiated borosilicate glass. When the damage level exceeds the threshold, the effect of hardness recovery caused by electronic energy deposition is more pronounced. [PERSON] et al. observed the recovery phenomenon induced by He-ion-irradiated and Au pre-irradiated amorphous silica when the pre-existing damage level exceeded the threshold. It was found that, when the pre-existing damage level is lower than this threshold, the superposition effect appears in the damage [25]. This is the main reason for the hardness saturation after He ion irradiation and the absence of recovery. One explanation that supports the recovery effect induced by ion irradiation in borosilicate glass is the low-temperature non-melting thermal spike originating from the electron energy loss. In this regard, an alpha-induced thermal spike has been proposed to improve the nanoligament connectivity and hardness of nanoporous silica [26]. However, the thermal peak scheme has not yet been applied to borosilicate glass. Consequently, the mechanism of the hardness recovery and the damage threshold of recovery induced by ion-irradiated borosilicate glass has not been clarified yet.