# Evaluation of Anti-Wear Properties of Metalworking Fluids Enhanced with Halloysite Nanotubes
## 1 Introduction  
Metalworking fluids (MWFs) are used in metal removal processes such as cutting, milling, and grinding to reduce friction between the tool and the workpiece, increase tool life by reducing wear, and improve surface finish [1; 2; 3; 4]. MWFs with nanoparticle additives have been widely investigated for the purpose of reducing the mechanical friction and wear between components obtaining very promising results [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. For example, the wear scar diameters (WSDs) of a syntheticfluid and base oil for machining applications were lowered by 10% and 86%, respectively, with 0.01 wt. % CuO nanoparticles [10]. Similarly, [PERSON] et al. [15] dispersed MoS\({}_{2}\), WS\({}_{2}\), and hexagonal boron nitride (hBN) nanoparticles in MWFs obtaining wear reductions of up to 66%. This was attributed to nanoparticles filling surface valleys and the shearing of trapped nanoparticles at the interface of contacting surfaces. Another study by [PERSON] et al. [12] found that nanoparticles also deagglomerate metallic transfer films further reducing friction and wear. [PERSON] et al. [16] dispersed CaCO\({}_{3}\) nanoparticles in a poly-alpha-olefin (PAO) base oil improving load-carrying capacity, and reducing friction and wear. Significant friction reductions in boundary and mixed lubrication regimes were obtained by [PERSON] et al. [17] through the formation of a protective film by dispersing MoS\({}_{2}\) nanotubes into PAO. Moreover, [PERSON] et al. [5] achieved reductions in friction and wear by CuO, TiO\({}_{2}\), and nano-diamond nanoparticles due to a rolling effect provided by their spherical shape. The addition of nanoparticles to MWFs has also demonstrated improvements in tool life and surface finish of products. Exfoliated graphene nanoparticles added to a vegetable oil reduced edge chipping of cutting tool edges in ball milling experiments [18]. A cutting fluid with dispersed CuO nanoparticles developed by [PERSON] et al. [11] was employed in a Computer Numerical Control (CNC) gear generator obtaining an improvement on the surface finish of workpieces of ~52%. Nanoparticles of SiO\({}_{2}\) blended into a mineral oil investigated for plain bearings by [PERSON] et al. [19; 20] reduced friction temperatures by acting as rolling and sliding particles between surfaces. Furthermore, a nanolubricant containing MoS\({}_{2}\) nanoparticles for end-milling of AL6061-T6 developed by [PERSON] et al. [21] improved the quality of the machined surfaces by the rolling, mending, and polishing tribological mechanisms provided by MoS\({}_{2}\) in the tool-workpiece interface. Halloysite nanotubes (HNTs), with a molecular formula of Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\cdot\)nH\({}_{2}\)O, are hollow clay multiwalled tubular nanoparticles with lengths of 50-5000 nm, diameters of 20-200 nm, and inner lumen of 10-70 nm that naturally form in the Earth [22; 23; 24]; however, their dimensions may vary depending on the deposit [25]. These nanoparticles have negatively charged SiO\({}_{2}\) groups on the outside of the layers and positively charged Al(OH)\({}_{3}\) on the inner lumen [26; 27; 28]. HNTs are non-toxic even at high concentrations, and are much less expensive compared to other tubular-like nanoparticles such as carbon nanotubes (CNTs) [22]. Due to these characteristics they have been studied for diverse applications, such as for preparing colloids with biopolymers providing tunable properties [29], for drug transport and delivery [28], as nanocontainers of additives for paper preservation [30], for obtaining inorganic reverse micelles for the entrapment of antibacterial agents [31], and for capturing hydrocarbon and aromatic oils [32]. Recently, our group demonstrated that HNTs dispersed in metal-forming lubricants tested under extreme pressure conditions are able to increase the load-carrying capacity, scuffing load, and seizure load (load for lubricant film breakdown) under contacts pressures of 3.5-3.9 GPa [33].