# Degradation of Hydraulic System due to Wear Particles or Medium Test Dust
## 1 Introduction  
Hydraulic system life refers to the period of time during which a machine can continuously perform its intended function under specified temperatures, pressures, and flow rates. Overall efficiency and service life are mainly influenced by factors such as hydraulic oil quality, temperature, and oil cleanliness. More than 70% of failures are due to the contaminant in hydraulic oil, with 60-70% of total failures due to solid particles [1]. The level of cleanliness of hydraulic fluids is one of the most important indicators for predicting possible failures of hydraulic components and consequently of the whole system [2; 3; 4]. There are several international standards for measuring the cleanliness of fluids, such as ISO 4406 [5] and SAE AS 4059 [6]. Using a fluid sample, the particle concentration and size distribution are evaluated according to the standard [5]. Thus, in the hydraulic system, there are not only wear particles but also particles that enter the system as contaminants from the environment or are present in the system from the beginning. The size and composition of the particles have a decisive influence on the system performance, especially if the gap between two mutually moving surfaces is similar in size to the particles themselves [7]. Particles entering such contacts cause wear [8] through mechanisms such as three-body abrasion [9] and erosion [10]. As a result of wear on the sliding surfaces, wear particles are generated and move through the system with the fluid [11]. The number of particles in the fluid is controlled by appropriate filtration [12]. Cleanliness stabilization is achieved when the number of particles in the hydraulic oil does not increase nor decrease but is constant. The increase in particles is due to wear of hydraulic components, built-in contaminants from manufacturing [2], and the decrease in particles is due to filtering of the oil [13]. When an equilibrium is reached between an input and output of the particles, the cleanliness stabilizes. To improve the quality of hydraulic filters and to calibrate the particle counters, test dust was introduced [14]. According to ISO 12103-1, A3 MTD, test dust has a well-defined particle size distribution and concentration [13]. Medium test dust (MTD) is used for testingvarious types of hardware devices. Since test dust is harder than typical wear particles [15], its use for acceleration testing has been explored [16]. Therefore, it is used to accelerate the wear of various hydraulic components [17; 18; 19; 20]. The increase in wear leads to an increase in leakage flow in hydraulic components and, moreover, to a decrease in the volumetric efficiency of hydraulic components [21]. Gear pumps are representative of hydraulic components, which have already been studied by [PERSON] et al. [22], [PERSON] [23], and [PERSON] [20] using test dust. They studied the chemical composition, size distribution, and concentration of test dust on the wear of the pump and consequently the loss of an effective flow rate. Studies on various valves, e.g., ball-seat valves [24] and directional valves [25], showed that performance decreased with increasing internal leakage. It was found that most of the system performance degradation occurs in the phase with improper filtration [25]. [PERSON] et al. [26] showed that the wear of the basic moving elements of the hydraulic motor leads to an increase in leakage. Corrosion wear can also enhance leakage. The corrosion of surfaces is a chemical process that converts metal surface atoms into a more chemically stable oxide. Pieces of debris are newly formed chemical compounds, usually agglomerated or sometimes mixed with fragments of surface materials [27]. Particles damage each component in the hydraulic system, so wear occurs. The wear of the sealing surfaces in the hydraulic system is noticeable by leakage and consequently by a loss in volumetric efficiency of the whole system.