The Tribological Properties of Titanium Carbonitride TiCN Coating Lubricated with Non-Toxic Cutting Fluid

This paper presents the results of tribological studies of TiCN coatings formed on the HS6-5-2C steel by plasma-assisted chemical vapour deposition (PVD). The coating thickness was measured with the optical microscope. The surface texture analysis was performed before and after the tribological tests with a Talysurf CCI Lite optical profiler. The tribological properties were investigated using a T-01M tribotester for a ball-on-disc configuration in the sliding contact. The tests were carried out under lubricated friction conditions using lubricants. Cutting fluids and coatings have contributed to the reduction of resistance movement. The non-toxic cutting fluid has reached comparable coefficient of friction with a coolant containing mineral oil. Introduction Technological developments have created the need for coatings that meet demanding applications of advanced materials. Single layer and multilayer coatings, widely used especially on cutting tools [13], increase the quality of machined parts and enable high speed cutting [1]. Titanium carbonitride (TiCN) coating is characterized by high hardness and low friction, can be applied by physical deposition (PVD) at temperatures ranging from 300 to 450°C [3], exhibits high melting temperature [4], anti-wear behaviour, chemical stability and corrosion resistance [5, 6]. The effect of tools with coatings on the quality of surface treated and durability of the tool were examined [7, 8] to find that coated tools had lower wear than conventional high-speed steel tools. Two studies [9-11] describe experiments that involved machining with biodegradable, non-toxic coolants based on plant oils and oil-containing cutting fluids, in which the roughness of work materials and tool wear were compared. The results showed that the properties of plant oil coolants were comparable to or better than the properties of coolants containing mineral oil. To reduce tool wear and improve the surface quality of a workpiece, cutting tool lubricants are used during machining for cooling, lubricating, transporting chips from the machining zone, reducing the coefficient of friction, and temporary protection against corrosion [12-14]. Lubricants are applied in over 80% of metalworking operations [15]. Widely used mineral oil-based coolants are usually toxic. Plant oils have become a renewable, hazard-free and easily biodegradable alternative, characterized by good lubricity, low volatility, higher fire resistance and good thermal stability. Currently, plant oil based metal working fluids are the area of extensive research [16-18]. Materials and methods The tests were performed on a TiCN coating deposited on the HS6-5-2C high-speed tool steel by the PVD. The surface and cross-sections of the TiCN coating as well as its chemical analysis in the Terotechnology 2017 Materials Research Forum LLC Materials Research Proceedings 5 (2018) 47-53 doi: http://dx.doi. org/10.21741/9781945291814-9 48 micro-regions were carried out using a Jeol JSM 7100F field emission scanning electron microscope with an EDS analyser. A Talysurf CCI Lite interferometer was used to analyse the geometric structure of the specimen and counter-specimen surfaces before and after the tribological tests [16]. The tribological tests were carried out on a T-01M ball (100Cr6 steel, Ø = 10 mm)-on-disk (HS65-2C steel without coating or with the TiCN coating, Ø = 38 mm) testing machine in accordance with the requirements of ASTM G 99. Friction tests were performed at a load of P = 50 N, a sliding speed of v = 0.1 m / s, along the friction path s = 1000 m, at the constant relative humidity of 40 ± 5% and ambient temperature T0 = 23 ± 1 ̊C under technically dry friction, lubricated friction with a mineral oil-based coolant and with a non-toxic coolant, consisting of alkanolamine borate, a biodegradable polymer zinc aspartate and DEMI demineralised water. Test results Figure 1a shows an image of the TiCN coating surface morphology. Figures 1b and 1c show the results of elemental analysis in the micro-regions of the coating. b) Content, [% weight] C N Ti Fe Total 20,78 31,37 47,46 0,39 100,00 a) c) Content, [% weight] C N O Si Ti Total 12,95 38,13 3,40 0,12 45,90 100,00 Fig. 1. SEM: a) view of the disk area and the EDX radiation spectrum with the result of the chemical analysis for the TiCN coating: b) at point 1, c) at point 2. The elemental analysis in the micro-region of the TiCN coating (Fig. 1b) showed the presence of titanium, nitrogen and carbon as components of the coating and iron from the substrate. In the image of the surface morphology (Fig. 1a), droplet shaped particles of various sizes were observed. They were most likely formed during the process of coating deposition as a result of titanium droplets splashing on the surface of the substrate. The analysis of the chemical composition showed that in addition to elements such as titanium, nitrogen and carbon, oxygen and silicon were also present (Fig. 1c). In the following testing stage, metallographic specimens were made, cross-sections of the coating were observed and its thickness of approx. 2 μm, was measured. Terotechnology 2017 Materials Research Forum LLC Materials Research Proceedings 5 (2018) 47-53 doi: http://dx.doi. org/10.21741/9781945291814-9 49 Figure 2 shows the force-penetration depth changes and the view of the impression in the TiCN coating on the HS6-5-2C steel after the nanohardness measurement. The nanohardness results are summarized in Table 1. Fig. 2. Force penetration depth changes in the TiCN coating and the microhardness impression. Table 1. Results of nanohardness measurements for TiCN coating deposited on the HS6-5-2C steel. Parameter HIT, [GPa] EIT, [GPa] E*, [GPa] HVIT, [HV] CIT, [%] Fmax, [mN] hmax, [mm] S, [mN/mm] Ap, [mm] Welst, [nJ] Wplast, [nJ] Wtotal, [nJ] TiCN 38,18 311,57 342,38 3 535,70 0,36 49,99 0,32 352 1,31 4 1 5 The analysis of the loading-unloading curve indicates elastic properties of the TiCN coating, as can also be deduced from the values of the elastic modulus EIT = 311.57 GPa and the reduced elastic modulus E* = 342.38 GPa. Hardness measured by instrumental indentation according to the Oliver and Pharr method equals 38.18 GPa and the Vickers hardness equals 3,535.70 HV. The TiCN coating is characterized by a small creep module CIT 0.36%, a small contact area Ap 1.31 mm2, plasticity 1 nJ and 4 times higher elasticity 4 nJ. Figure 3 shows the coefficients of friction recorded during technically dry friction and lubricated friction with a mineral oil-based coolant and coolants with zinc aspartate for the HS6-5-2C steel 100Cr6 steel, and HS6-5-2C steel with the TiCN coating 100Cr6 steel pairs. Fig. 3. Mean coefficients of friction for the friction pairs tested under technically dry friction and with the use of coolants. 2.05