# Tribological Analysis of Several Coatings under Flood and Cryogenic Cooling Conditions
## 1 Introduction
Studies conducted on several metallic and non-metallic work materials using different gases and liquids as cooling/lubricant strategies have shown that the friction coefficient is the lowest when liquid nitrogen (LN\({}_{2}\)) is used, followed by gaseous nitrogen and finally air [22; 23; 24]. But there are also contradictory studies showing that the low temperature induced by LN\({}_{2}\) will increase friction coefficient and wear rate [25; 26]. It is also revealed that for some metallic materials, like aluminium, brass-lead pairs and self-mating niobium, the reduction in the friction coefficient and wear rate is significant [26]. [PERSON] et al. [23] found that the friction coefficient and wear rate decreased significantly in 8 K and 77 K He gas compared to in air at an ambient temperature (300 K) for polymer materials. In contrary, the modeling of the tribological behavior of titanium alloys [27] under cryogenic conditions using high loads and speeds shows that the friction coefficient of Ti\(\delta\)Al4V is higher than in air, as well as in the CFRP drilling procedure [28]. LN\({}_{2}\) and CO\({}_{2}\) are the two most frequently used very-low-temperature cooling methods in machining operations. [PERSON] and [PERSON] [29] reported that both LN\({}_{2}\) and CO\({}_{2}\) resulted in lower cutting temperatures compared to conventional flood MWF, but LN\({}_{2}\) (reduction of 47%) is more effective in reducing the temperature than CO\({}_{2}\) (reduction of 36%). [PERSON] [30] reported that LN\({}_{2}\) and CO\({}_{2}\) resulted in 240% and 150% longer tool lives compared to dry machining, respectively. [PERSON] et al. [31] reported that in turning Ti-6 Al-4V titanium alloys, the use of LN\({}_{2}\) and CO\({}_{2}\) significantly improves the tool life compared to conventional flood MWF; in particular, the LN\({}_{2}\) increases the tool life five-fold. LN\({}_{2}\) can extend the tool life in the machining of titanium alloys at higher cutting speeds [32; 33; 34; 35; 36; 37; 38]. A comparison of the main advantages is shown in Table 1.  
[PERSON] [40] reviewed various more environment-friendly cutting fluids, including cryogenic cooling, MQL, solid lubricants, etc. [PERSON] et al. [41] employed a combination of MQL + CO\({}_{2}\) in milling Inconel 718. Although conventional cutting fluids (mixtures oil-water) exhibited the best performance, from the ecological perspective MQL + CO\({}_{2}\) significantly reduces the amount of mineral oil by 90% in comparison with conventional cutting fluids. The objective of this work is to investigate the tribological behaviors of several new multilayer coatings specially developed for the cryogenic-assisted machining of Ti-6 Al-4V alloy. The coatings are CrN and AlCrN monolayers, as well as CrN/AlCrN-based multilayers with two different architectures. The choice of Cr-based coatings instead of the most common Ti-based coatings is justified by the higher corrosion resistance and, more importantly, good thermal properties of Cr-based ones. [PERSON] et al. [42] investigated the tribological and thermal functions of various cutting tools coated by TiAlN, TiN and TiAlN + MoS\({}_{2}\). The results show that these coatings do not have a significant influence on heat transmission on the substrate. [PERSON] et al. [43] reported that no contribution of the TiAlN coating was found to tool life in milling Ti-6 Al-4V under cryogenic cooling conditions. Firstly, the tribological tests were performed to analyze the capability of the new coatings for the selection of coatings and for further tool wear tests; besides this, they can also provide information relevant to simulation jobs. Secondly, works on tribological tests relevant to cryogenic-assisted machining are few, and those addressing coatings are even fewer; as such, it is necessary to perform full-scale experiments to fill up the blank page in this field, especially for the Ti-6 Al-4V alloy.