# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 1 Introduction  
The contact between the tool and the chip/workpiece has a significant impact on tool life and the surface integrity. This contact strongly depends on several factors (contact conditions), including sliding speed, contact pressure, temperature, lubrication/cooling strategy, etc. [1, 2]. The friction coefficient is often used to model this contact to correlate the shear stress with the normal stress acting at the tool-chip and tool-workpiece interfaces. This coefficient should be determined experimentally through specially designed tribological tests able to reproduce the contact conditions observed in metal cutting [3]. This is essential for the accurate simulation of the metal cutting process. Especially for the simulations when coated tools and cooling conditions are applied, the knowledge about the tribological behavior at the workpiece/chip interface is limited. Unfortunately, classical methods for determining the friction coefficient between two instances based on pin-on-disc are not suitable to describe the tribological conditions in the metal cutting process. The standard pin-on-disc tribometer uses a pin to rub repeatedly over a rotating disc to determine the friction coefficient and the wear volume. However, the pins rub over the same surface of the disc at lower speed and contact pressure when compared to metal cutting process [4, 5, 6]. Two main approaches to investigate the tribological phenomena (including the determination of the friction coefficient) at the tool-chip interface can be found in the literature. One is using the cutting process itself [7, 8, 9, 10]; apparently it is the best way to provide the relevant friction conditions, but this approach is not able to extractquantitative results, because it is impossible to have first-hand information about the local contact pressure, sliding velocity or temperature. Even the approach based on analytical models linking the local shear strength, temperature or sliding velocity is not suitable, due to the limitations of these models, and to the large uncertainty in determining their parameters [11]. The second is based on specially designed tribometers able to reproduce the contact conditions in metal cutting [5; 12; 13]. A pin rubs on the refreshed surface of a workpiece in rotation, and the helical movement of the pin on the bar avoids the superposing of the scratch on the cylinder. So, the surface of the workpiece in contact with the pin is always different and never repeated where the contact area of the pin remains the same (as the cutting tool). Meanwhile, the workpiece needs to be refreshed each time. As mentioned by [PERSON] et al. [3], such tribometers are able to reproduce the contact pressure, temperature and velocity between the tool and the chip in metal cutting, thus contributing to a better understanding of the tribological phenomena in this process. The apparent friction coefficient is strongly correlated to the sliding speed and the contact pressure. Many researchers have shown that the friction coefficient drops when increasing the sliding speed [14; 15; 16; 17; 18]. [PERSON] et al. [3] conducted tribological tests on various work materials with a TiN-coated carbide tool; the friction coefficient dropped significantly with increasing sliding speeds. Similar results were obtained by other researchers for other materials and contact conditions [4; 11; 19; 20]. Besides this, the contact pressure between the pin and workpiece is also a crucial parameter that affects the friction coefficient. [PERSON] et al. [21] tested the friction coefficient using different normal forces between 5 N and 400 N, under dry and flood conditions. Using lower loads, the friction coefficient increases with the sliding velocity. However, using higher loads, the friction coefficient decreases when the sliding speed increases.