1. Introduction
Nowadays, a strong challenge towards machinability improvement noticed in production and manufacturing areas results in the widespread application of cutting tool materials coated by multi-functional layers which can withstand high contact pressure and high temperature. These efforts are predominantly addressed to a group of difficult-to-machine aerospace materials (superalloys) which are classified within the ISO S group of engineering materials. It is expected that they should not only retain greater hot hardness and excellent resistance to slow down diffusion including oxidation resistance in atmospheric air on account of penetration air to the cutting zone in dry machining [
1].
In particular, Ti
1-xAl
xN and Al
xTi
1-xN coatings with various stoichiometric relationships of Al/Ti (x = 0.5−0.7) have versatile applications in the restricted machining operations of Ni-and Ti-based heat resistant superalloys (HRSA) [
2,
3]. It is very important from the practical point of view that by varying the Al content in the TiAlN coating, its mechanical and tribological properties improve due to promoting the formation of an outer Al
2O
3 layer during machining operations. However, the higher Al content causes a lower oxidation rate [
4]. On the one hand, dual-phase and hcp (hexagonal close-packed)-structured coatings with high Al contents are more wear resistant in comparison to fcc (face-centered cubic) structured coatings [
1,
5,
6]. The crystal structure of TiAlN is fcc when the AlN content is less than 60 mol%, whereas hcp structure occurs if the AlN content is higher than 70 mol% [
5]. On the other hand, increasing the Al content above x = 0.60 results in a lower oxidation resistance of the AlTiN coating due to the precipitation of the wurtzite-type AlN [
6]. A potential effect is either increasing the rate of oxygen inward diffusion or decreasing the rate of Al diffusion into the surface.
It was documented, based on the pin-on-disc tests [
7], that for sliding materials with dominating severe adhesive wear, the protective function of the TiAlN coating is not sufficient. As a result, the research problem arises of how the formation of Al
2O
3 protective layer can be predicted for machining applications. At present, some oxidation and diffusion investigations are carried out [
3,
8]. They are carried out in air atmosphere or in vacuum at different times and oxidation temperatures. These conditions allow replicating crater or notch wear conditions, respectively. However, it is not clear whether the contact zones between the coating and the workpiece material are subjected to atmospheric oxygen or not. In particular, research on the oxidation behavior of cermets, uncoated cemented carbides and pCBN tools in coupling with Inconel 718, alloyed steel and hard steel and was performed in a controlled-environment chamber [
2,
3]. Hatt et al. [
9] proposed replicating the tool crater wear in machining of (α + β) titanium alloys using the diffusion couple method in vacuum. However, only crater wear at the rake face was taken into account. Moreover, the oxidation test on uncoated and TiAlN/TiN coated carbide inserts were reported in [
10,
11]. They revealed the evolution of the visible notch wear (VB
N) resulting from severe oxidation of the peripheral zone of the flank face.
Extended investigations of tool wear mechanisms when machining of Inconel 718 and Ti6Al4v alloys using a set of TiAlN coated inserts were carried out in other research studies [
11,
12,
13]. They confirmed the predominant notch wear mechanism on the flank face in dry machining of these superalloys. In this paper, the oxidation tests were carried out at four different temperatures: 700 °C, 800 °C, 900 °C and 1000 °C with regard to the connection between TiAlN-coating and the diffusion couple interface created by the Ti6Al4V and Inconel 718 (IN 718) and this coating. After the tests were finished, the obtained data were related to tool wear data under dry machining condition and different machining times. It was reasoned that oxidation effects documented by X-ray diffraction (XRD) and EDS analysis of the oxidized coating surfaces are similar for the static diffusion-couples and dynamic tool wear tests.
2. Experimental Details and Measurements
In this experimental study, two aerospace heat resistant alloys (HRSA) including Ti-6Al-4V alloy (Bimotech, Wroclaw, Poland) with typical two-phase α + β structure (hardness of 36 HRC) and nickel-based superalloy of Inconel (IN) 718 (Bimotech, Wroclaw, Poland) (brand name PWA 1469-4, hardness is 36 HRC) are used. Cutting tools coated with PVD (Physical Vapour Deposition) TiAlN (atomic ratio Ti:Al of 0.45:0.55) layer with coating thickness of about 3 μm were selected. They were a KC5010 rhombic shaped cutting inserts (Kennametal, Poznan, Poland). For machining trials, cutting speed vc = 200 m/min, constant depth of cut ap = 0.25 mm and feed rate of f = 0.1 mm/rev were selected.
Two short machining trials of 0.5 and 2 min were carried out for the two alloys in order to expose the oxidized Al
2O
3 layer on the rake and flank faces and to show the development of notch wear which is predominantly caused by ambient air penetrating from the periphery of the flank face [
1,
11].
In this experimental study, three different oxidation techniques were used: classical oxidation test in a free contact with air in the furnace chamber, static diffusion-couple test performed in the same chamber and dynamic diffusion-couple test under real machining conditions.
The oxidation experiments for the TiAlN-coated inserts were carried out in air at four different temperatures: 700 °C, 800 °C, 900 °C and 1000 °C and for three annealing times of 2.5, 10 and 30 min. The computer-aided acquisition system was used to verify the actual test temperature. In the second stage, the diffusion couples, consisting of TiAlN-coated inserts and special coupons (see
Figure 1) made of Inconel 718 and Ti6Al4V alloys were annealed at the constant temperature of 900 °C for 2.5, 10 and 30 min. In all tests, at least three samples were used and each oxidation test was repeated at least three times.
The oxidation effect was assessed by measurements of the mass before and after annealing using precision weighting with a resolution of ±0.1 mg [
14]. Consequently, the thickness (
tol) of the deposited Al
2O
3 layer was assessed from Equation (1) and mass of the deposited Al
2O
3 layer by area unit (mol) was designate:
where:
- -
Δmol = mass increment, mg/cm2
- -
ρ = density of Al2O3, 3986 kg/m3
- -
Au = unit area, cm2.
Furthermore, the coating adhesion was tested by a scratch test using a micro scratch tester (MST Instrument) (Anton Paar, Warsaw, Poland) equipped with a diamond cone-shaped indenter (Anton Paar, Warsaw, Poland) by Rockwell with a tip radius of 100 μm and concurrently the coating thickness was measured accurately.
The static diffusion couple method was applied in order to quantify the diffusion interactions between the TiAlN coating and both Ti6Al4V and Inconel 718 samples in the form of a thin disc of about 2 mm in thickness. The contact disc surfaces were polished to a mirror finish using ultra fine abrasives and ultrasonically cleaned in isopropanol. During the test, they were pressed by a mass of 15 kg to promote strong contact and intensify chemical reactivity at high temperatures.
After the test was finished, the coating and coupon’s surfaces were examined by scanning electron microscopy JEOL JSM 840A (JEOL Companies, Tokyo, Japan) and were also subjected to energy-dispersive spectroscopy X-ray microanalysis (EDS) (JEOL Companies, Tokyo, Japan) in terms of diffusion processes and oxidation effects. Phase identification of the oxide layers was performed by X-ray diffraction (XRD) (X’Pert PRO PANalytical) (Panalytical Companies, Malvern, United Kingdom).
Figure 1 presents the setup of diffusion couple test in which the commercial cutting tool insert (1) is coupled with a disc-shaped material (2) (
Figure 1a). As a result, the diffusion at the couple interface is developed correspondingly to the contact conditions created by the normal load of 150 N on the flank face (
Figure 1b). The chemical compositions of the contacted materials detected by EDS analysis are specified in
Table 1.