Characteristics and Cutting Performance of CVD Al₂O₃ Multilayer Coatings Deposited on Tungsten Carbide Cutting Inserts in Turning of 24CrMoV5-1 Steel

Abstract

In this work, TiN/Ti(C,N)/Al₂O₃ multilayer coatings were deposited using an industrial-scale thermal CVD system. Two polymorphs of Al₂O₃, the stable α- and the metastable κ-Al₂O₃, were obtained by the deposition of specific bonding layers at the Al₂O₃/Ti(C,N) interface. The comparable hardness and elastic moduli of α- and κ-Al₂O₃ multilayer coatings were measured. The tribological behavior of Al₂O₃ multilayer coatings was studied at room temperature using 24CrMoV5-1 balls; friction coefficients were comparable for both α- and κ-Al₂O₃ multilayer coatings. As a result of the relatively high hardness of coatings and the generation of abrasive wear particles, larger wear tracks were observed on balls. In Rockwell C tests, good adhesion at Al₂O₃/Ti(C,N)-based layer’s interface was reported in κ-Al₂O₃ multilayer coatings, which could be attributed to the deposition of κ-bonding layers consisting of needle-shaped grains. The cutting performances in the turning-roughing of 24CrMoV5-1 steel under different parameters—cutting speed, feed, and depth of cut—were investigated. Herein, κ-Al₂O₃ multilayer coatings showed the longest tool life, double of that of a commercial CVD Al₂O₃ multilayer coating. The results obtained could enrich the existing database for the development of prediction models of tool wear and machined surface quality and help improve tool performance for the machining of 24CrMoV5-1 steel.

1. Introduction

Metal cutting is a manufacturing process achieved through different operations, mainly categorized as turning, milling, and drilling [1]. During metal cutting, high forces act at contact zones between the tool and workpiece. The chip formation involves the plastic deformation at shear zones, generating contact stress and heat in cutting tools, which are subjected to abrasive and adhesive wear. When using workpiece materials with relatively high thermal conductivities, the generated heat is transported away with chip flow over the tool rake face [2,3,4,5]. In the case of continuous cylindrical turning, the workpiece rotates around its center axis, while a sharp-edged tool is set to a certain depth of cut for performing a facing operation towards the workpiece rotational center [6]. In contact zones, high temperatures are reached, where thick coatings (≈5–20 µm) with low thermal conductivities are accepted as favorable since they provide a thermal barrier to the substrate, and the generated heat is deflected into the chip, preventing the plastic deformation of the underlying substrate [1,7].

CVD α- and κ-Al₂O₃ coatings were found to be efficient to protect cutting tools from crater wear on the rake face, due to their low adhesion tendency in contact with metals and excellent chemical stability at high temperatures [8]. α- and κ-Al₂O₃ differ in their crystal structure, grain size and thermal conductivity; the low thermal conductivity of κ- Al₂O₃, approx. three times lower than α-Al₂O₃, suggests that this metastable phase of alumina could be an excellent thermal barrier for cutting tools [9]. However, κ-Al₂O₃ can transform irreversibly to α-Al₂O₃ at high temperatures, resulting in a volume contraction of approx. 8% due to the heat generated during thermal CVD process and metal cutting [10]. It was also reported that the mechanical properties of CVD α-Al₂O₃ coatings could be enhanced by choosing an optimized texture for the α-Al₂O₃ phase [11]. Consequently, CVD Al₂O₃ coatings have continuously been optimized with respect to the phase and film texture control.

In previous studies, CVD TiN/Ti(C,N)/Al₂O₃ multilayer coatings were extensively studied [12,13]. It was shown that the growth of α- and κ-Al₂O₃ on Ti(C,N) could be controlled with the deposition of bonding layers at the Al₂O₃/Ti(C,N)-based layer’s interface [12]. The epitaxial growth of α-Al₂O₃ on rutile as well as that of κ-Al₂O₃ on needle-like Ti(C,N) were reported [12,13]. It is essential to achieve the phase control to compare the performance of CVD α- and κ-Al₂O₃ multilayer coatings in the present work.

In this work, 24CrMoV5-1 was selected as the workpiece material, which is widely used to produce molds for light metals and plastics. Based on a previous study, tungsten carbide tools with coatings of hard refractory materials such as Ti(C,N) and Al₂O₃ were used for turning operations of 24CrMoV5-1 steel herein [14]. The aim of the present work is to study mechanical and tribologicial properties of CVD α- and κ-Al₂O₃ multilayer coatings and to investigate the effects of operational conditions on tool wear. The cutting performances of coated tungsten carbide inserts were characterized and compared to that of a commercial CVD Al₂O₃ multilayer coating.

2. Materials and Methods

2.1. Sample Preparation

Four CVD Al₂O₃ multilayer coatings were deposited, corresponding to the following samples: A, B, A1 and B1. All coatings were deposited using an industrial-scale thermal CVD system (Ionbond BernexTM BPXpro 325S, IHI group, Olten, Switzerland). The process was described in a previous study, principally composed of four systems: reactor, AlCl₃ generator, by-products treatment system, gaseous and liquid precursors (TiCl₄ and CH₃CN) supply systems [15]. As shown in Figure 1, the multilayers in samples A and B were deposited on 30 mm diameter cemented carbide substrates (WC-Co 6 wt.%) to study the mechanical and tribological properties of CVD Al₂O₃ coatings. All substrates were mirror-polished (1μm diamond slurry) and cleaned using ethanol.

This coating architecture is described as: TiN buffer layer/TiN gradient layer (Grad 1)/MT-Ti(C,N); gradient layer (Grad 2)/MT-Ti(C,N)/MT-Ti(C,N); gradient layer (Grad 3)/HT-Ti(C,N); gradient layer (Grad 4)/HT-Ti(C,N); and α- or κ-bonding layer/Al₂O₃. For clarity, the TiN buffer layer acts as a diffusion barrier of Co from the substrate, Ti(C,N) layers provide high hardness and good wear resistance; bonding layers help to control the growth of α- and κ-Al₂O₃ on Ti(C,N); and the Al₂O₃ top layer can improve the oxidation resistance of coatings. Herein, α-Al₂O₃ was obtained in samples A and A1, whereas κ-Al₂O₃ was obtained in samples B and B1. Importantly, multilayers with a 0.8 µm thick TiN top layer were deposited in samples A1 and B1 on tungsten carbide inserts for the turning of 24CrMoV5-1 steel. TiN top layers can facilitate the wear detection. TiN top layers in samples A1 and B1 were deposited at 1005 °C and 70 mbar from a TiCl₄-H₂-N₂ gas mixture (H₂: balance, N₂: 39.4 vol.%, TiCl₄: 1.70 vol.%, the total flow rate: 34.0 L.min⁻¹). Due to an increasing deposition temperature in the CVD process, MT (medium-temperature) and HT (high-temperature) Ti(C,N) layers, as well as different gradient layers, were deposited in order to enhance the process continuity and the coating adhesion. The detailed deposition parameters are given in

Table 1. The deposition parameters of different layers in samples A, B, A1 and B1.

Parameters	TiN	MT-Ti(C,N)	HT-Ti(C,N)	α-Bonding Layer	κ-Bonding Layer	Al2O3
Temperature (°C)	900	880	1005	1005	1005	1005
Pressure (mbar)	70	70	70	70	70	70
Deposition time (min)	40	60	15	15	15	120
Total flow rate (L.min−1)	33.8	26.1	32.5	26.0	32.5	25.0
H2 (vol.%)	balance
N2 (vol.%)	39.6	20.3	18.4	-	9.2	-
TiCl4 (vol.%)	1.3	2.4	1.4	-	1.2	-
CH3CN (vol.%)	-	0.8	-	-	-	-
CH4 (vol.%)	-	-	3.4	-	2.2	-
CO2 (vol.%)	-	-	-	3.8	-	4.0
CO (vol.%)	-	-	-	-	0.9	-
AlCl3 (vol.%)	-	-	-	-	0.4	1.6
HCl (vol.%)	-	-	-	-	-	2.0
H2S (vol.%)	-	-	-	-	-	0.3

and

Table 2. The deposition parameters of different gradient layers.

Parameters	Grad 1	Grad 2	Grad 3	Grad 4
Temperature (°C)	900 → 890	890 → 880	880 → 890	890 → 1005
Pressure (mbar)	70	70	70	70
Deposition time (min)	10	15	15	60
Total flow rate (L.min−1)	33.9	26.1	26.1	26.2
H2 (vol.%)	balance
N2 (vol.%)	39.5	20.3	20.3	16.8
TiCl4 (vol.%)	1.6	2.2	2.3	1.6
CH3CN (vol.%)	-	0.8	0.8	-
CH4 (vol.%)	-	-	-	5.3

.

2.2. Characterization

X-ray diffraction (XRD) studies were carried out using a Bruker D8 Advance diffractometer (Bruker, Billerica, MA, USA) operating at 40 kV/40 mA with Kα radiation of Cu (λ Kα₁ = 1.54059 Å and λ Kα₂ = 1.54443 Å). Phase identifications were conducted in Bragg–Brentano geometry with a step size of 0.02° for a 2θ-range from 20° to 60°. Peak positions were determined by fitting the diffraction peaks with the Pseudo–Voigt function, and the software MAUD (version 2.92) was used. The instrumental contribution was determined using the standard material^® 640d.

Surface morphology was observed using a Zeiss Gemini 500 scanning electron microscope (SEM, Jena, Germany), and cross-sectional fractures were prepared by a FIB/SEM FEI Helios NanoLab 600i (FEI, Hillsboro, OR, USA) equipped with platinum gas injection system. The FIB column used a gallium liquid metal ion source operating at an accelerating voltage up to 30 kV. A Pt-layer was primarily deposited to protect the coating surface from ion beam damage. However, the film thickness is more than 11 µm, which exceeds the limit of the FIB/SEM used, and only the Al₂O₃ and Ti(C,N)-based layer’s interface was highlighted.

The average surface roughness Ra was evaluated by an AltiSurf© 500 optical profilometer (Altimet, Thonon-Les-Bains, Switzerland) with a scanning rate of 50 μm/s, using a measurement length of 5 mm on ten different positions at the coating surface. Surface profiles were characterized after ball-on-disc tests with a scanning rate of 50 μm/s (analyzed area: 2.5 × 2.5 mm²).

The hardness and modulus of samples A and B were evaluated by a Nano-indenter HYSITRON TI980 using a Berkovitch diamond tip with a maximum load of 11 mN. Loading and unloading rates were 2 mN/s with a residence time of 2 s at the maximum load. Prior to measurements, the coating surface was mirror-polished to eliminate asperities and to obtain an average roughness Ra less than 15 nm. Twenty-five indentations were performed for each sample and an average value was considered. The Oliver and Pharr method and the Poisson’s ratio of 0.24 for both α- and κ-Al₂O₃ were used for calculations [16,17]. Vickers hardness HV_0.05 values were measured using a Wilson^® TUKONTM 1202 (Buehler, Leinfelden-Echtergingen, Germany): ten indentations were performed for each sample, and an average value was considered [18].

A CSM tribometer was used for ball-on-disc tests. Samples A and B were tested using 6 mm diameter grade 24CrMoV5-1 balls (workpiece material for cutting tests) without lubricant. A normal force of 10 N (maximum of the tribometer used) was applied; the sliding distance was 1000 m with a speed of 10 cm/s and a rotational radius of 2.5 mm. Wear tracks on coating surfaces were observed by an optical microscope KEYENCE VH-Z250R (KEYENCE, Bois-Colombes, France). Rockwell C tests were carried out for samples A and B to investigate the film adhesion and its brittleness; a conical diamond indenter with a load of 150 kg was used [19].

Furthermore, Ø 200 × 300 mm annealed bars of 24CrMoV5-1 (Dijon, France: ANFOR NFA) with a hardness of 220 HB were selected as the workpiece material; its composition (C: 0.39 wt.%, Si: 1.03 wt.%, Mn: 0.33 wt.%, Cr: 5.07 wt.%, Ni: 0.16 wt.%, Mo: 1.23 wt.%, V: 0.39 wt.%, P: 0.02 wt.% and S: <0.001 wt.%) is given in a previous study [14]. The tool holder DCLNL 3232 P16 was used with uncoated (CNMX 16 06 12 E8 CH01) and coated (CNMX 16 06 E8 CFX 1025) inserts provided by Evatec Tools. According to the ISO standard 3002:1982, the tool cutting edge angle (k_r) is 95°, the tool cutting edge inclination (λ_s) is −6°, the orientation of the cutting face (γ_o) is −6° and the orientation of the flank (α_o) is 6°.

A spinner TC600 CNC lathe machine was used for the cylindrical turning of 24CrMoV5-1 steel; the experimental set-up is shown in Figure 2 [14]. A water–oil mixture (8.5% S-Aero Fluch soluble oil) was used as lubricant during turning. Cutting edge temperatures were measured by an Actarus^® System (CIRTES, Saint-Dié-des-Vosges, France); a temperature probe was 1 mm under the cutting edge. A triaxial piezoelectric dynamometer-Kistler 9257B and accelerometer-Kistle 8763B were placed to measure cutting forces and accelerations in feed, in tangential and radial directions with reference to tools [14,20]. Herein, cutting parameters, cutting forces, accelerations, and temperature are inputs of the ‘‘Machine Learning Model’’, which can provide an estimation of tool wear and machined surface roughness as outputs [14].

The COM (Couple Tool-Material) protocol defined on the standard NF E 66-520 was devoted to the determination of cutting parameters, for a precise association of tool, workpiece material and machining operation. This protocol provides the guidelines for choosing minimum values for cutting speed-Vc, as well as maximum and minimum values of depth of cut-a_p and feed-f. The determination of these values is based on cutting parameters and other process information (cutting forces, accelerations, cutting edge temperatures, roughness of the machined surface, tool wear, chip shape, etc.).

Herein, 39 experiments were carried out following the COM protocol using commercial coated inserts from Evatec Tools to choose the optimum cutting parameters. As can be seen in

Table 3. Experiments carried out for the determination of cutting parameters: cutting speed-Vc (m/min), feed-f (mm/rev) and depth of cut-a_p (mm).

Series	Experience Number	Vc (m/min)	f (mm/rev)	ap (mm)
(A)	1 to 3	150	0.3	2.5
	4 to 6	240	0.3	2.5
	7 to 9	340	0.3	2.5
	10 to 12	500	0.3	2.5
(B)	13 to 15	240	0.2	2.5
	16 to 18	240	0.3	2.5
	19 to 21	240	0.45	2.5
	22 to 24	240	0.6	2.5
(C)	25 to 27	240	0.35	1
	28 to 30	240	0.35	2.5
	30 to 32	240	0.35	3.5
	33 to 36	240	0.35	4.5
	37 to 39	240	0.35	5.5

, cutting speed-Vc was increased from 150 to 500 m/min in the series (A), feed-f was increased from 0.2 to 0.6 mm/rev in the series (B), and depth of cut-a_p was increased from 1 to 5.5 mm in the series (C). These parameters (Vc: 150–500 m/min, f: 0.2–0.6 mm/rev and a_p: 1–5.5 mm) were suggested by Evatec Tools for turning-roughing operations.

Each experiment repeated three times consists of performing a cylindrical turning operation for about 30 s with given cutting parameters, to obtain stable signals of temperature and a visible evolution of tool wear. Accelerations and cutting-edge temperatures were measured, and specific cutting force-kc (cutting force per unit area of cut) was calculated according to the standard NF ISO 3002-4. Tool flank wear-Vb was measured as the average width of the flank land by an optical microscope according to the ISO standard 3685:1993. At the end of each experiment, an average roughness Ra of the machined surface was measured by a portable roughness tester: Mitutoyo SJ 210 (Mitutoyo, Roissy, France). A new cutting edge was used for each experiment.

Based on the results obtained from 39 experiments and the “Machine Learning Model”, cutting parameters were determined: cutting speed—240 (m/min), feed—0.35 (mm/rev) and depth of cut—3.5 (mm). In these conditions, a low flank wear with a low machined surface roughness was obtained for turning of 24CrMoV5-1 steel using commercial coated cutting inserts (CNMX 16 06 E8 CFX 1025) from Evatec Tools (Evatec tools, Thionville, France). However, it was not possible to characterize this TiN/TiB₂/Al₂O₃ commercial coating in terms of coating thickness, composition or microstructure. Afterwards, tool-life tests were carried out according to the ISO standard 3685:1993 using these determined parameters. When tool flank wear-Vb reached 300 µm or edge breakage occurred, the cutting edge was considered as reaching the end of life. The results obtained are discussed in Section 3.5.

3. Results and Discussion

3.1. Structure, Morphology and Microstructure Analyses

The X-ray diffractograms of samples A, A1, B and B1 are presented in Figure 3 and Figure 4. It is shown that α-Al₂O₃ was obtained in samples A and A1, due to the deposition of the α-bonding layer that was obtained by oxidizing the uppermost part of the HT-Ti(C,N) layers. This oxidizing step resulted in the formation of rutile; the epitaxial growth of α-Al₂O₃ on rutile was evidenced [12]. On the contrary, κ-Al₂O₃ was obtained in samples B and B1, due to the deposition of κ-bonding layer composed of Ti(C,N) needle-shaped grains [12]. Compared with Figure 3, diffraction peaks belonging to TiN were pronounced in Figure 4, since 800 nm thick TiN top layers were deposited in both samples A1 and B1 to facilitate the wear detection after the manufacturing process.

Figure 5 shows surface morphologies and cross-sectional microstructures of samples A and B. Sample A consists of facetted grains, in agreement with earlier observations reported [12,21,22]. Previous studies found that κ- to α-Al₂O₃ phase transformation can occur during the CVD deposition process, and α-Al₂O₃ obtained through this phase transformation shows large equiaxed grains [21,23]. Herein, neither diffraction peaks originating from κ-Al₂O₃, nor large equiaxed grains was observed, and the phase transformation is ruled out. In contrast, sample B is composed of small, rounded grains, thus a similar morphology was reported [12]. Some cracks were observed due to thermal stresses generally found in CVD coatings. Cross-sectional fractures were prepared by FIB, the Al₂O₃ and Ti(C,N)-based layer’s interface was highlighted, and Ti(C,N) needle-shaped grains were grown to promote the nucleation of κ-Al₂O₃ in sample B.

3.2. Nano- and Micro-Indentation

The hardness of 28.0 ± 0.8 GPa and 25.6 ± 0.4 GPa was measured for samples A and B, in good agreement with earlier studies: polycrystalline α-Al₂O₃ exhibits slightly higher hardness than κ-Al₂O₃ [24,25,26,27]. Young’s moduli of 333 ± 6 GPa and 292 ± 6 GPa were measured for samples A and B. Accordingly, Ruppi et al. reported elastic moduli of 390 ± 44 GPa and 340 ± 15 GPa for CVD α- and κ-Al₂O₃ multilayer coatings [25]. However, Rebelo de Figueiredo et al. obtained lower moduli of 280 ± 8 GPa and 250 ± 6 GPa [24]. It is believed that the mechanical properties of CVD Al₂O₃ are strongly dependent on deposition conditions. Additionally, elastic properties of CVD Al₂O₃ coatings depend also on residual stress state [27,28]. High tensile residual stresses could provoke low moduli. Regarding CVD coatings, thermal stresses induced by dissimilar coefficients of thermal expansion play a crucial role, and the stress behavior of anisotropic α-Al₂O₃ is not independent from the in-plane orientation [27,29,30]. Such a conclusion requires further investigations.

The Vickers hardness of 27.3 ± 1.1 and 25.9 ± 1.0 GPa was measured for samples A and B, respectively. Comparable values of approx. 29.4 GPa were reported for both CVD α- and κ-Al₂O₃ multilayer coatings from the company Bernex. Since the applied indentation load was relatively small in both the present study and company measurement standard, no visible radial or lateral cracks were observed at the surroundings of the imprints by an optical microscope.

3.3. Ball-on-Disc Test

As shown in Figure 6, friction coefficients (µ) of 0.76 and 0.72 were determined for samples A and B, respectively. This could be attributed to relatively high surface roughness Ra, which is 125 nm and 108 nm for samples A and B. Accordingly, a slightly lower µ of sample B could be related to its smaller grain size [31,32]. As shown in Figure 7, wear tracks were observed on the surfaces of samples A and B, while larger tracks were found on 24CrMoV5-1 balls. It is speculated that the counterpart crushed coating surface asperities and abrasive wear particles were generated. Furthermore, the counterpart shows an adhesion tendency to the coating surface, leading to the stick-slip phenomenon and scattering friction forces [33]. Meanwhile, coatings exhibit much higher hardness levels than 24CrMoV5-1 steel, which could also be responsible for larger tracks on balls [34,35].

3.4. Rockwell C Test

As shown in Figure 8 and Figure 9, samples A and B were characterized by SEM after Rockwell C tests. In general, coatings under extreme stress conditions show two different aspects. On the one hand, the normal components of the stress tensor could be responsible for the brittle failures of coatings [36]. Normal stresses greater than the coating strength can provoke coherence release or chipping. In this case, the mechanical properties of coatings, such as the internal cohesion, play an important role. On the other hand, interfacial bonds between the coatings and the substrates—the so-called adhesion—are key. The release of interfacial bonds could be correlated to the shear stress components of stress tensors, which could result in micro- or macro-delamination.

As shown in Figure 8, extended delamination and radial cracks surrounding the imprint are observed. As can be seen in Figure 8a,b, film delamination with buckling at the center of the imprint is evidenced. In EDS analysis (1), an Al₂O₃ layer was detected. In EDS analyses (2) and (3), Ti(C,N)-based layers and a substrate were characterized. It was observed in EDS analyses (4) and (5) that Al₂O₃ layers pile up at the center of imprint, and Ti(C,N)-based layers were exposed. As shown in Figure 9, film delamination and cracks at the vicinity of imprint were observed, similar to sample A. On the contrary, no significant buckling at the center of the imprint could be observed in sample B, consistent with EDS analysis (4). EDS analysis (1) demonstrates that only an Al₂O₃ layer was detected, whereas Ti(C,N)-based layers and a cemented carbide substrate were characterized in EDS analyses (2) and (3).

In such case, it is difficult to conclude whether the fracture results from cohesive or adhesive failures, so the so-called mixed failure mode is considered. This kind of failure could be caused by a combination of normal and shear stresses. At least, the poor delamination at the center and small radial cracks at the surrounding of imprint indicate that sample B shows a better adhesion between the Al₂O₃ and based layers, compared with sample A. However, these results must be put into perspective with respect to the nature of the substrate. However, indentations were performed on these coatings deposited on cemented carbide substrates, whereas in conventional Rockwell C tests, a quenched and tempered high-speed steel (62 HRC) substrate is used.

3.5. Metal Cutting Test

Tool-life tests consist of executing cutting experiments with the same cutting edge until the end of life. Using the determined cutting parameters in Section 2.2, tool-life tests of 24CrMoV5-1 steel using sample A1 and B1 coated cutting inserts, commercial uncoated and coated cutting inserts were carried out according to the ISO standard 3685:1993. It is noticeable that cutting-edge radiuses are comparable before and after the deposition of CVD Al₂O₃ multilayer coatings (samples A1 and B1). Regarding commercial uncoated inserts, edge breakage occurred after the very first seconds. It is shown clearly that these uncoated inserts are not adaptable to the severe cutting conditions applied.

Tool-life plots are shown in Figure 10. Flank wear, cutting edge temperature, roughness of the machined surface, accelerations and cutting forces were measured every 30 s during the cutting tests, as displayed in Figure 11. In these conditions, sample B1 (the coated tungsten carbide cutting insert) exhibits the longest tool life of approx. 650 s, double that of commercial coated inserts. In this study, tool life was evaluated by flank wear-Vb; when flank wear reaches 300 µm or edge breakage occurs, the tool reaches the end of its life.

As can be seen in Figure 11, while cumulated machining time increased, cutting temperatures did not increase until the end of life. As aforementioned, the generated heat in the shear zone could be transported away with chip flow. In this respect, CVD Al₂O₃ is considered a good thermal barrier for preventing the plastic deformation of substrates [1,7,8]. Compared with α-Al₂O₃, κ-Al₂O₃ shows a much smaller thermal conductivity. This could be a plausible explanation for the lower cutting-edge temperatures measured during cutting tests using sample B1 coated cutting inserts [9]. Concerning the roughness of the machined surface, comparable results of approx. 3 µm were obtained at the end of tool life in all three tests.

Cutting forces in different directions were measured. Cutting forces in all directions for tool-life tests using commercial coated cutting inserts were always higher than those using sample A1 and B1 coated inserts, particularly F_t and F_f, indicating that more important powers were required on the spindle for turning using commercial coated inserts. Accelerations in all directions were measured and root mean square values were considered. Indeed, accelerations correlated to cutting vibrations could significantly influence tool service life and tool deflection, and the vibration of tool tip could seriously deteriorate the quality of the machined surface. In this study, smaller accelerations were measured during the test using sample B1 coated cutting inserts. In contrast, it is speculated that large grain size and the higher hardness and roughness of sample A1 generated important vibrations. This could be responsible for the significant chipping reported in Figure 12a.

As shown in Figure 12 and Figure 13, sample A1 and B1 coated inserts were characterized, but we could not characterize this commercial coating due to an industrial confidential issue. Important chipping on the flank face of sample A1 can be observed, showing that wear penetrated the α-Al₂O₃ layer, revealing Ti(C,N)-based layers, which is not the case for sample B1 except for a small area near the cutting edge. As sample B1 exhibits a good adhesion at the Al₂O₃/Ti(C,N)-based layer’s interface, this could be a plausible explanation for its good performance in these cutting tests.

Figure 12b,c shows SEM images of sample A1 corresponding to the red and blue squares indicated in Figure 12a. EDS analyses were conducted. The adhesion of the workpiece material to the tool surface and visible cracks were observed. However, the diffusion of cobalt from the substrate is not evident, but it could be possible in the zone where the adhesion of the workpiece material occurs. A similar phenomenon is found in sample B1, as shown in Figure 13b,c, but the diffusion of Co from tungsten carbide substrates into coatings needs further cross-sectional EDS analyses. Accordingly, the adhesion of the workpiece material on tools could generate progressive tool wear. It was suggested that diffusion of Fe and Cr from the workpiece material through grain boundaries into coatings accelerated the degradation of coatings [37]. Furthermore, Ti and Al appear in very limited zones, similar to sample B1. It is assumed that the delamination only occurred at the Al₂O₃/Ti(C,N)-based layer’s interface, but Ti(C,N)-based layers are adherent to cutting inserts.

Regarding sample B1, as shown in Figure 13a,d, the Built-Up-Edge (BUE) is observed, due to the adhesion of the workpiece material. Moreover, since the cutting-edge temperature was low (approx. 300 °C), only thermal cracks were observed, but there was no evidence for secondary cracks resulting from κ- to α-Al₂O₃ phase transformation.

4. Conclusions

In this work, four CVD Al₂O₃ multilayer coatings were deposited without (samples A and B) and with thin TiN top layers (samples A1 and B1). The morphology, microstructure, and mechanical and tribological properties of samples A and B were studied, while turning operations of 24CrMoV5-1 with uncoated and coated tools (commercial coating, samples A1 and B1) were carried out. The main results can be summarized as follows:

(1)

By means of nano-indentation, hardness levels of 28.0 ± 0.8 and 25.6 ± 0.4 Gpa and Young’s moduli of 333 ± 6 and 292 ± 6 GPa were measured for CVD α- and κ-Al₂O₃ coatings, respectively. Due to the anisotropic properties of α-Al₂O₃, the influence of film texture on its mechanical properties needs further investigations. Vickers hardness levels of 27.3 ± 1.1 and 25.9 ± 1.0 GPa were measured for CVD α- and κ-Al₂O₃ coatings, comparable with CVD α- and κ-Al₂O₃ coatings from the company Bernex.

(2)

In tribotests using 24CrMoV5-1 balls, friction coefficients of approx. 0.7 were measured for samples A and B, respectively. The effects of grain size, hardness and surface roughness were discussed. In Rockwell C tests, poor delamination but no evident bulking at the center of imprint indicate that sample B could be a good adherent coating.

(3)

Cutting parameters for tool-life tests of 24CrMoV5-1 were determined according to the COM protocol: cutting speed—240 (m/min), feed—0.35 (mm/rev) and depth of cut—3.5 (mm). In this study, commercial uncoated inserts were not adaptable. Sample B1 coated inserts exhibited the longest tool life of approx. 11 min, double of that of commercial coated inserts from Evatec Tools. Compared to sample A1, good adhesion between κ-Al₂O₃ and Ti(C,N)-based layers was evidenced in sample B1. The large grain size and high hardness and surface roughness of sample A1 could be responsible for important vibrations that could seriously deteriorate the tool service life.

The results obtained can enrich the existing database for the development of prediction models of tool wear and machined surface quality. They can also help in improving the tool performance for the turning-roughing of 24CrMoV5-1 steel, as the tool life has been dramatically improved compared with the existing commercial CVD Al₂O₃ multilayer coating. Further works will focus on the optimization of multilayer coating architecture and film texture to improve the adhesion and mechanical properties. The crater wear on the tool rake face also needs more investigations, to study the tool-chip interface.