Results and discussion
The synthesis of 1,3-diazole derivatives was performed both under ultrasound (US) irradiation and under conventional thermal heating (TH). N-alkylation of the imidazole under US gives slightly higher yields. In addition, the amount of solvent needed was about two times less compared with conventional TH, whereas the reaction times reduce from hours or even days to minutes. Taking into consideration these advantages the N-alkylation methods could be considered environmentally friendly. Overall, the use of US proved to be more efficient than TH, the efficiency of the former in the N-alkylation reaction of 1,3-diazole being assigned to cavitation effects. The different behavior of imidazole and benzimidazole in the N1-alkylation reactions under US irradiation should relate to the different acidity of the hydrogen haspin inhibitor from the 1-st nitrogen in the two heterocycles and also to their different solubilities. Some of the melting points of the synthesized imidazolium salts being lower than 30°C encourage us to claim that these compounds could be considered ionic liquids.
Cavitation erosion is a process that affects a significant number of components used in hydraulic equipments, like pumps, ship propellers or turbines for hydroelectric power plants . When subjected to cavitation erosion, materials behave differently, depending on their composition, structure, surface treatments etc . During the cavitation erosion of metals and alloys, the work-hardening process due to the bubble collapse in the superficial layer is associated with a surface hardness increase . For ductile materials it was also observed that the erosion rate is scaled by the ratio of the thickness of the hardened layers to the covering time, but also depending on the flow aggressiveness . Franc and Michel  also pointed out that fatigue mechanisms have to be expected due to the repetitive nature of the process, involving high strain rates and short impact duration.
In order to asses one material’s resistance to such conditions, specific tests are standardized, aiming to simulate the cavitation erosion process in accelerated conditions performed in a laboratory. However, there is significant difference compared to real cavitation phenomena that occur in components of hydraulic machines and there are concerns on accepting the accelerated erosion tests versus the full scale erosion. Choi et al.  studied the influence of different erosion intensities and testing methods and concluded that the relative ranking of erosion resistance of some materials depends on the cavitation intensity. According to Chahine et al. , the ultrasonic method leads to the formation of a cavitation bubble cloud, always at the same location, with bubbles of nearly uniform size and form obtained at a fixed frequency, compared to the real cases where a distribution of nuclei size exists as well as various exciting frequencies. They also emphasized that the standardized test does not allow a full characterization of the behavior in real conditions due to the absence of a real liquid flow or the interaction of bubble nuclei with turbulent vortex filaments.
Compared to real cavitation erosion that occurs after a long duration of exposure, the standardized accelerated tests provide however relevant laboratory results that can be used to compare materials tested under similar conditions. The equipment for this purpose leads an intensive erosion process in a controllable and reproducible manner, by generating bubbles clouds that erode the surface of a sample made out of the tested material. Such equipments can be used to assess the resistance to cavitation erosion of a material in terms of the erosion rate, thus allowing materials to be classified based on this property. Ultrasonic equipments have been developed for the purpose of evaluating the cavitation erosion process, according to ASTM G32-09 standards [7,8]. They have the advantage of using simple equipments, with easily controllable parameters, that generate longitudinal vibrations, amplified and transmitted into the liquid as ultrasonic waves. The bubbles that form during these vibrations implode at the surface of the samples leading to a cumulative effect that has a destructive effect on the surface with an energy that depends on the parameters used in the process and the characteristics of the ultrasonic probe. The cavitation erosion pits depend on the material’s particularities with the predominant failure related to the fatigue process . Although the ultrasonic cavitation erosion experiments can be performed in various fluids more intense cavitation occurs for higher surface tension of the fluid. The increase of the viscosity is expected to lead to a reduced erosion of the surface due to the decrease in the rate of growth and collapse of the bubbles .