# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 2 Materials and Experimental Set-Up
### Experimental Set-Up  
The experimental set-up used in the tribological tests is shown in Figure 1. These tests, also called pi-on-bar tests, were conducted on a SOMAB T500 CNC lathe machine. A pin was mounted on the tribometer, and this tribometer was mounted on a piezoelectric dynamometer Kistler 9121 to measure the \(F_{x}\), \(F_{2}\) and \(F_{y}\) forces (see Figure 1) acting in the pin. Then, a dynamometer was mounted in a VDI attachment connected to the turret of the CNC machine. The force signals were acquired by a NI cDAQ-9174 card at a sampling frequency of 2 kHz. Two cooling/lubrication strategies were used: LN\({}_{2}\) and traditional metal working fluid (MWF) consisting of an oil-water mixture, also called flood. This MWF was used since it is the reference MWF used in the industry. The mixture was prepared by diluting 7% of mineral oil from Blaser, reference B-Cool 755, in 93% of water (in volume). This lubricant is qualified for machining heat-resistant alloys such as titanium and nickel-based alloys. The LN\({}_{2}\) tank was placed on a balance to estimate the flow rate based on the weight variation. The tank was connected to an insulated flexible pipe to deliver the LN\({}_{2}\) to the nozzle. The pressure of LN\({}_{2}\) in the tank was 4 bar and the estimated flow rate was 2 L/min. Axial force generated by pressurized CO\({}_{2}\) gas inside a cylinder was used to generate the contact pressure between the pin and the bar. This precludes the low temperatures of the LN\({}_{2}\) affecting the forces signals; the dynamometer was covered in an insulation material. The nozzle was placed behind the pin in the direction of sliding, in order to reproduce the cooling/lubricant conditions applied in machining. The details of the LN\({}_{2}\) nozzle diameter and position are shown in Figure 2. They were optimized by [PERSON] [44], and correspond to the following: nozzle outlet diameter equal to 3 mm; distance between the nozzle and the pin equal to 2 mm; distance from nozzle to workpiece equal to 2 mm; the angle between the pin axes and the workpiece equal to 75\({}^{\circ}\). The Ti-6 Al-4V bar was pre-machined to generate a surface roughness similar to that of the chip surface in contact with the tool, which was considered to be equal to 0.1 \(\upmu\)m.  
The apparent friction coefficient was determined using Equation (1):  
\[\mu_{app}=\frac{F_{t}}{F_{n}} \tag{1}\]  
where \(\mu_{app}\) is the apparent friction coefficient, \(F_{t}\) is tangential force, and \(F_{n}\) is the normal force.  
The tangential force \(F_{t}\) is equal to \(\sqrt{F_{z}^{2}+F_{y}^{2}}\), and the normal force \(F_{n}\) is equal to \(F_{x}\). The term \"apparent friction coefficient\" used here differs from \"interfacial friction coefficient\", which includes the adhesion at the pin-work material interface. [PERSON] and [PERSON] [46] proposed a simple decomposition for the apparent friction coefficient, shown by Equation (2):  
\[\mu_{app}=\mu_{loc}+\mu_{plast} \tag{2}\]  
where \(\mu_{loc}\) and \(\mu_{plast}\) are the local adhesive and macroscopic parts, respectively.  
Temperature was measured using a thermocouple type K inserted in a drilled hole from the bottom of the cylindrical body of the pin. The distance between the thermocouple and the contact surface between the workpiece and the pin was 1.53 mm [45].  
The adhesion volume of the Ti-6 Al-4V material to the pins was observed using optical microscope Keyence VHV 1000, then quantified using a dynamic focusing microscope Alicona InfiniteFocus [45].