## 1 Introduction  
Ionic liquids as high-tech technical fluids have been in practical use in various fields of technology for several years. Ionic liquids are applied in the field of Technology as additives, solvents, electrolytes, catalysts, reagents, lubricants and heat transfer and storage fluids, etc. In addition, they are also used in the areas of heating, ventilation and air conditioning (HVAC), as a sealing fluid, as a cutting fluid designed specifically for metalworking processes, e.g., for cutting and drilling, and as operating fluids in different technical systems, they are referred to by a unified term: ionic engineering fluid. Therefore, their applications cover many diverse technical fields, and even megatrends like mobility, health and the green economy.  
Due to their excellent lubricating and other physicochemical properties, they have also made their way into the field of Hydraulic Drive Technology. In most of the mentioned cases of using ionic liquids, they only need to provide one, or maybe two or three good physicochemical properties. In the case of use as a hydraulic fluid, an ionic liquid must have several excellent properties at the same time, of ten or more. Therefore, the development of ionic liquids appropriate for hydraulic systems took more than a decade. The development included the identification of material properties important for their use as a hydraulic fluid, extensive pre-selection processes and the laboratory testing of suitable ionic liquids, as well as long-term testing using real hydraulic components under real operating conditions [1]. Now, we can talk about a completely new type of hydraulic fluid, ionic hydraulic fluid--HIL.  
Ionic hydraulic fluids are (for) now being considered alternative hydraulic fluids. This is similar to the use of water, or water-based fluid, as a hydraulic fluid, except that, in this case, it is necessary to compensate for or adapt to the shortcomings of the use of water by using special materials or coatings [2; 3; 4]. Today, ionic hydraulic fluids are suitable for use mainly in hydraulic applications in areas where classic hydraulic fluids cannot solve the problem satisfactorily, or the challenge of the application, or cannot solve it at all. Compared to classic hydraulic fluids, ionic hydraulic fluids have excellent individual physicochemical properties, and, due to their properties, cover the field of both highly additive mineral hydraulic oils and non-flammable and more environmentally friendly classic hydraulic fluids simultaneously.  
Before a new type of hydraulic fluid is used in a hydraulic system, it is absolutely necessary to check the impact of the new fluid on individual components and their materials. In particular. the compatibility of the fluid with all the materials incorporated in the component is definitely an extremely important aspect. This concerns the compatibility of metallic materials (e.g., valve housings, pumps, filters and cylinders, as well as hydraulic pipes and fittings), as well as non-metallic materials such as flexible pipes, the corrosion protection of component surfaces (including the painting of the tank, especially the interior), and, of course, all the seals. This is particularly important, as seals are a key element that ensures flawless operation of the system, where we use a whole range of different materials for different seals.