Effect of TiC Content and TaC Addition in Substrates on Properties and Wear Behavior of TiAlN-Coated Tools

: The present paper reports a new way to improve the wear resistance of coated carbide tools by increases in TiC content and the addition of TaC in substrates. The results suggest that the average grain size of the substrate increased with the increases in TiC (0–14 wt.%) content, and the hardness of the TiAlN coating deposited on the substrate exhibits a similar trend. In addition, the adhesion strength of the TiAlN-coated carbide increases with increasing TiC content, which can be attributed the formation of the (Ti,W)C phase and the similar hardness of the substrate and coating. The addition of TaC into the substrates inhibits the grain growth and thereby causes the hardness and adhesion strength of the TiAlN coatings to improve from 24.6 GPa and 16.7 N to 30.1 GPa and 17.3 N, respectively. In turning tests, the TiAlN coating deposited on the substrates with the TaC addition achieved the best wear resistance in turning stainless steel because it possessed the highest substrate and coating hardness and sufﬁcient adhesion strength. However, the TiAlN coating deposited on the substrates with a higher TiC content shows the better wear resistance in turning titanium (TC4), which can be attributed to it having the highest adhesion strength.


Introduction
Coated carbide tools have been developed for high-efficiency and high-precision machining applications owing to their combined properties of a hard coating and cemented carbide substrate [1].However, they are still subject to high cutting temperatures and severe adhesive wear when machining hard-to-cut materials [2].To meet the increasing demands of advanced machining techniques, further optimization of coated tools is required.Studies show that the performance of coated tools is closely correlated with the mechanical and thermal properties of the coating.Previous research indicated that the mechanical and thermal properties of the coating can be improved via composition diversification and multilayer structure technology [3][4][5][6].An alternative method is improving the properties of the cemented carbide substrate.As reported by previous researchers, the hardness, adhesion strength, and wear resistance of coated tools can be affected by the composition, mechanical properties, and surface properties of the substrate [7][8][9].Among them, substrates with an ultrafine grain size can accelerate nucleation, refine the grain size of the coating, and, hence, improve the hardness of the coating and the adhesion strength between the coating and substrate [7].Sprute et al. [9] found that the adhesion strength of the TiAlN-based coated tool is improved by increasing the hardness and residual stress of the substrate.The coating often possesses a higher hardness than the substrate.Therefore, increasing the hardness of the substrate is one route to improving the adhesive strength between the coating and the substrate by increasing the cooperative deformation ability of the system [10].Moreover, the adhesion strength can also be improved by reducing the lattice mismatch between coating and substrate, which is more favorable in forming a coherent interface between the coating and substrate [11].Although the performance of the coating is important, it is not fully representative of the properties of a coated tool when the substrate performance is significantly different [12,13].For instance, coating failure is significantly affected by the substrate material's properties, which is vital for the wear resistance of coated tools [13,14].In addition, one of the notable advantages of deposited coatings is their high hardness, which can remarkably improve the wear resistance of substrates [15,16].Sveen [14] found that the Vickers hardness of a coating substrate system is close to that of the substrate with a loading of 0.5 kg, which can be attributed to the higher hardness of the substrate exerting a strong supporting effect on the coating.Actually, the cutting edge and face of coated tools sustain a large mechanical pressure in the cutting process [17][18][19], so that the wear resistances of the coated tools also depend on the properties of the substrate.
The incorporation of TiC into cemented carbide substrates is a promising method to improve the wear resistance of WC-Co cemented carbide [20].The incorporation of TaC into WC-TiC-Co cemented carbide is widely accepted to decrease the grain size and increase the hot strength and oxidation resistance, while the hardness, heat resistance, and wear resistance can be improved by modulating the TiC content [21,22], which is conducive to the cutting performances.Xian et al. found that the adhesion strength of TiAlN-coated cemented carbides can be improve by the addition of TiC owing to the formation of fcc-(Ti,W)C, which is beneficial for the epitaxial growth between coating and substrates [11].However, the average grain size of the WC-TiC-Co cemented carbide decreased with the increases in TiC content, which is bad for the mechanical properties of the deposited coating [7].However, the effects of TiC content and TaC addition in the substrate on the mechanical properties and wear resistance of TiAlN-coated carbides has not been reported.
In this work, the TiAlN coating is deposited by cathodic arc evaporation on WC-TiC-Co cemented carbide with different TiC and TaC contents.The microstructure and mechanical properties of these cemented carbides, especially the effects of the TiC content and TaC addition on the adhesion strength and wear resistance of the coated tools, were systematically investigated.

Materials and Methods
Herein, WT1, WT2, WT3 and WT4, the four cemented carbide materials (Zhuzhou Cemented Carbide Cutting Tools Co. Ltd., Zhuzhou, China) representing different TaC and TiC contents, were used as substrates (in Table 1), which were prepared by the powder metallurgy method.Before mounting into the vacuum chamber, all the substrates were first polished using a 1500# diamond grinding disc and then sandblasted for 4 min.The TiAlN coatings were deposited using a cathodic arc evaporation system from powder metallurgy Ti0.5Al0.5 targets.The deposition process was conducted under N2 atmosphere at ~2.2 Pa and a deposition temperature of 500 • C, with a target current of 100 A and DC bias of −80 V for the substrates.The substrates were cleaned before deposition using our previously reported cleaning parameters [23].Table 1 shows the chemical compositions and mechanical and physical properties of the four cemented carbide substrates.Scanning electron microscopy (SEM; FEI Quanta 250 FEG, FEI, Hillsboro, OR, USA) was used to identify the cross-sectional morphologies and thickness of the coated cemented carbides.The structural characterization was performed using X-ray diffraction (XRD; Rigaku D/max2550pc, Rigaku, Tokyo, Japan) equipped with Cu Kα radiation.The nanohardness and elastic modulus of the TiAlN-coated cemented carbides were measured using a nanoindenter (CSM) with a Berkovich diamond tip under a loading force of 10 mN according to the Oliver and Pharr method.The adhesion strength of the film with the substrate was examined by Rockwell indentation with a load of 100 kg.The adhesion and abrasion resistance of the TiAlN-coated cemented carbides was assessed using a scratch tester (MST) with an indenter tip with a radius of 50 µm.The applied load was increased from 0 to 30 N at a loading speed of 30 N/min.
Generally, the traditional ball-disk friction experiments with low load are used to evaluate the wear resistance of coatings.In addition, the friction experiment under normal temperature cannot fully characterize the friction environment of the tool-chip interface under cutting conditions [24,25].Here, the wear resistance of the TiAlN-coated carbide tools was assessed using an LBR-370 CNC lathe.The cutting parameters are shown in Table 2.The wear mechanism was elucidated by investigating the worn tools using SEM and energy dispersive X-ray spectroscopy (EDS).

Microstructure
Figure 1 shows the backscattered electron (BSE) images of the WT1, WT2, WT3, WT4 cemented carbides.It can be seen that the cemented carbides have a typical multi-phase structure.WT1 (in Figure 1a) is composed of a white WC hard phase and black Co bonded phase, in which there are a few large particles and many small particles in the WC hard phase.Large particles of WC are formed by the dissolution-precipitation of small particles during sintering [26].It can be observed that there are some grey phases in Figure 1a, which originate from the addition of other carbides during the preparation process.With the addition of TiC, a large amount of gray (Ti,W)C hard phase appears in WT2 (in Figure 1b) and WT3 (in Figure 1c).Owing to the abundance and close contact of the (Ti,W)C phase, the number of small WC particles decreases substantially, resulting in an increase in the average grain size of the hard phase.Additionally, the abundance and grain size of the (Ti,W)C phase increases with the increase in TiC content.Moreover, the number of large-grain-size WC decreased with the addition of TaC in WC-TiC-Co cemented carbides (in Figure 1d), which can be attributed to the TaC hindering the WC grains' growth [21].As shown in Figure 1, it can be concluded that the average grain size for the four cemented carbides is ordered as follows: WT3 > WT2 > WT1 > WT4.
The XRD patterns of the four coated tools are shown in Figure 2. All samples showed fcc-TiAlN(111), TiAlN(200) and TiAlN(220) diffraction peaks.The TiAlN(200) diffraction peaks had higher intensity than the TiAlN(111) ones, which can be explained by the high surface diffusion capacity of the ions by cathodic arc evaporation [27].TiAlN/WT1 (in Figure 2a) mainly comprised WC, TiAlN and Co. Fcc-(Ti,W)C was formed when TiC was added to the substrate to give TiAlN/WT2 (in Figure 2b), which can be attributed to a sintering process in which WC dissolves into the TiC lattice [28].The increase in the TiC content in TiAlN/WT3 (in Figure 2c) resulted in a notable increase in the intensity of the (Ti,W)C peaks, owing to the formation of a large amount of the (Ti,W)C phase.According to Figure 2d, the addition of TaC induces a shift of the peak position to lower 2θ angles, which can be attributed to the larger TaC dissolved into the (Ti,W)C crystal structure.The XRD patterns of the four coated tools are shown in Figure 2. All samples showed fcc-TiAlN(111), TiAlN(200) and TiAlN(220) diffraction peaks.The TiAlN(200) diffraction peaks had higher intensity than the TiAlN(111) ones, which can be explained by the high surface diffusion capacity of the ions by cathodic arc evaporation [27].TiAlN/WT1 (in Figure 2a) mainly comprised WC, TiAlN and Co. Fcc-(Ti,W)C was formed when TiC was added to the substrate to give TiAlN/WT2 (in Figure 2b), which can be attributed to a sintering process in which WC dissolves into the TiC lattice [28].The increase in the TiC content in TiAlN/WT3 (in Figure 2c) resulted in a notable increase in the intensity of the (Ti,W)C peaks, owing to the formation of a large amount of the (Ti,W)C phase.According to Figure 2d

Mechanical Properties
The hardness of the coatings deposited on the different cemented carbide substrates was determined by nanoindentation.Due to the large differences in the hardness of the four substrates (in Table 1), the interference of the substrate on the hardness of the coating was excluded during testing.Generally, when the depth of the indentation is higher than 1/10 of the thickness of the coating, the test results show some contribution from

Mechanical Properties
The hardness of the coatings deposited on the different cemented carbide substrates was determined by nanoindentation.Due to the large differences in the hardness of the four substrates (in Table 1), the interference of the substrate on the hardness of the coating was excluded during testing.Generally, when the depth of the indentation is higher than 1/10 of the thickness of the coating, the test results show some contribution from the substrate 10.According to our previous research [29], the indentation depth was ~200 nm during the nanoindentation test under a loading force of 10 mN.Table 3 shows that the hardness of TiAlN/WT1, TiAlN/WT2, TiAlN/WT3 and TiAlN/WT4 is 27.9, 24.6, 22.9 and 30.1 GPa, respectively.Table 3 also lists the elastic modulus values of the TiAlN coating.It can be found that the hardness and elastic modulus of the TiAlN coating decreases with the increase in TiC content in WC-Co cemented carbides substrates, which can be attributed to the average grain sizes of the substrate decreasing with the increases in TiC content in substrate.In addition, the TiAlN/WT4 coated cemented carbides had the highest hardness, owing to the smallest average grain size of the substrate (in Figure 1).Figure 4 shows the results of the Rockwell indentation test with a load of 100 kg.It can be found that there are a lot of small radial cracks and spalling around the indentation shape of the TiAlN/WT1 coated cemented carbides.The radical cracks are caused by the tensile stresses, which come from collaborative deformation between substrate and coating under the indenter.As the tensile stress reaches a certain degree, the coating begins to peel off [13].With the increases in TiC content and TaC addition in the substrates, the spalling around the indentation disappears, as shown in Figure 4b-d.According to the standard in the German engineer's Manual (VDI3198), it can be concluded that the adhesion strength of TiAlN coated cemented carbides increases with the addition of TiC and TaC into the substrates.Figure 4 shows the results of the Rockwell indentation test with a load of 100 kg.It can be found that there are a lot of small radial cracks and spalling around the indentation shape of the TiAlN/WT1 coated cemented carbides.The radical cracks are caused by the tensile stresses, which come from collaborative deformation between substrate and coating under the indenter.As the tensile stress reaches a certain degree, the coating begins to peel off [13].With the increases in TiC content and TaC addition in the substrates, the spalling around the indentation disappears, as shown in Figure 4b-d.According to the standard in the German engineer's Manual (VDI3198), it can be concluded that the adhesion strength of TiAlN coated cemented carbides increases with the addition of TiC and TaC into the substrates.In order to give a more sophisticated understanding of the influence of the TiC and TaC content in substrates, the adhesion strength of the hard coating-substrate system was tested by scratch tests.During the scratching process, the coating and substrate are stressed by a diamond indenter that applies shear stress and squeezes the carbide material.With an increase in the normal load, the plastic deformation in the coated carbide surface increased, resulting in the formation of a scratch furrow behind the indenter.The coating at the edge of the scratch was squeezed by the sliding indenter and subjected to severe shear stress [13].The coating fractures when the stress exceeded the critical value (Lc1), and the crack proceeded to rapidly expand under the compressive stress of the indenter.The acoustic emission (AE) and friction coefficient (µ) begin to fluctuate significantly, and the coating spalling localizes at the rims of the scratch (in Figure 5).With the increase in normal load (>Lc2), the plastic deformation of the coating surface further increases, and the coating is completely peeled off.The fluctuation in AE is not significantly different from that at Lc1, but the friction coefficient begins to stabilize (Figure 5).Therefore, there are some limitations in using AE to evaluate the adhesion strength between coating and substrate, and the scratch failure morphology and friction coefficient should be combined to comprehensively evaluate the adhesive strength of coatings.
cantly, and the coating spalling localizes at the rims of the scratch (in Figure 5).With the increase in normal load (>Lc2), the plastic deformation of the coating surface further increases, and the coating is completely peeled off.The fluctuation in AE is not significantly different from that at Lc1, but the friction coefficient begins to stabilize (Figure 5).Therefore, there are some limitations in using AE to evaluate the adhesion strength between coating and substrate, and the scratch failure morphology and friction coefficient should be combined to comprehensively evaluate the adhesive strength of coatings.On account of the variation in the microstructure and properties of the substrates, the adhesive strengths of the coatings are diverse, as presented in Table 3 and Figure 5.The values of Lc1 and Lc2 for TiAlN/WT1, TiAlN/WT2, TiAlN/WT3 and TiAlN/WT4 are 10.1, 13.1, 13.8 and 13.2 N, and 14.2, 16.7, 18.3 and 17.3 N, respectively.The introduction of TiC increases the average grain size of the hard phase, which is not conducive to the adhesive strength of the coating.However, the results indicate that the adhesive strength of the TiAlN-coated cemented carbides increases with an increase in the TiC content in the substrate, which can be attributed to the change in the structure and properties of the substrate.Firstly, compared to TiAlN/WT1 and TiAlN/WT2, TiAlN/WT3 possessed the lowest hardness and elastic modulus mismatch between the coating and substrates, which allows for the coating and substrate to cooperatively deform [9].Sec- On account of the variation in the microstructure and properties of the substrates, the adhesive strengths of the coatings are diverse, as presented in Table 3 and Figure 5.The values of Lc1 and Lc2 for TiAlN/WT1, TiAlN/WT2, TiAlN/WT3 and TiAlN/WT4 are 10.1, 13.1, 13.8 and 13.2 N, and 14.2, 16.7, 18.3 and 17.3 N, respectively.The introduction of TiC increases the average grain size of the hard phase, which is not conducive to the adhesive strength of the coating.However, the results indicate that the adhesive strength of the TiAlN-coated cemented carbides increases with an increase in the TiC content in the substrate, which can be attributed to the change in the structure and properties of the substrate.Firstly, compared to TiAlN/WT1 and TiAlN/WT2, TiAlN/WT3 possessed the lowest hardness and elastic modulus mismatch between the coating and substrates, which allows for the coating and substrate to cooperatively deform [9].Secondly, the FCC-(Ti,W)C formed after the introduction of TiC has a face-centered cubic structure, which is more similar to the structure of the TiAlN coating [11].Thus, it is more favorable to form a coherent interface between the coating and substrate, and the adhesive strength is improved.The TiAlN/WT4 with a value of 17.3 N possessed a higher adhesion strength than that of TiAlN/WT2 after the addition of TaC into the substrates.Compared to (Ti,Ta,W)C, (Ti,W)C had a lower lattice constant (4.31 Å), which is close to that of the TiAlN coating (4.18 Å) 11 and beneficial for forming a coherent interface during deposition.However, the addition of TaC into the substrate leads to an increase in adhesion strength rather than weakening, owning to grain refinement [29][30][31][32].

Wear Behavior
To evaluate the effects of different TiC and TaC contents in the substrate on the wear resistance of the coated carbide tools, cutting experiments with stainless steel and titanium alloy were carried out.According to the tool wear morphology of the four coated tools (in Figure 6), after 2 min of dry cutting stainless steel, there is a notable mechanical furrow on the four tool surfaces, which is typical of abrasive wear morphology [33,34].Positions A and B (Figures 6a and 7b,c) near the cutting edge of the coated tool did not possess the coating material elements, such as Ti and Al, indicating that the coating materials here are completely polished.Moreover, a large amount of Fe, Cr and other elements is found in this area, indicating that serious adhesive wear has occurred.Point C (in Figure 6a) contains a large amount of Ti, Al and N species (in Figure 7c), indicating that the coating still exists.In addition, a large amount of O species is detected in the cutting wear area of the coated tool, which indicates that severe oxidative wear occurs in the cutting process [35].This can be attributed to the high cutting heat, which is not easy to discharge under high-speed dry cutting conditions.The main wear modes of the four coated carbide tools in cutting stainless steel are adhesive, abrasive and oxidative wear.Based on the wear morphology of the four coated tools, it is clear that the descending order of the flank wear values for all the coated tools is TiAlN/WT1, TiAlN/WT2, TiAlN/WT3 and TiAlN/WT4 coated carbide tools, with the same cutting time during turning stainless steels.Due to the highest hardness of the substrates and coating, and the sufficient adhesion strength between coating and substrates, the TiAlN/TW4 coated carbide tool possessed the best wear resistance [36].The wear resistance of TiAlN/TW3 is close to that of TiAlN/TW4.The poorest wear resistance of TiAlN/WT1 is mainly related to it having the weakest adhesion and substrate hardness.According to the tool wear morphology of the four coated tools (in Figure 8), after 2 min of dry cutting titanium alloy, the cutting wear area is mainly composed of three different contrast areas.EDS reveals a large amount of V (Figure 9a) species in the gray area (Figure 8a) near the cutting edge, which is the main component of TC4 titanium alloy, indicating that serious adhesive wear has occurred 8. Chipping is formed after the adhesion flakes off in the cutting process [37].However, a large number of elements such as  According to the tool wear morphology of the four coated tools (in Figure 8), after 2 min of dry cutting titanium alloy, the cutting wear area is mainly composed of three different contrast areas.EDS reveals a large amount of V (Figure 9a) species in the gray area (Figure 8a) near the cutting edge, which is the main component of TC4 titanium alloy, indicating that serious adhesive wear has occurred 8. Chipping is formed after the adhesion flakes off in the cutting process [37].However, a large number of elements such as According to the tool wear morphology of the four coated tools (in Figure 8), after 2 min of dry cutting titanium alloy, the cutting wear area is mainly composed of three different contrast areas.EDS reveals a large amount of V (Figure 9a) species in the gray area (Figure 8a) near the cutting edge, which is the main component of TC4 titanium alloy, indicating that serious adhesive wear has occurred 8. Chipping is formed after the adhesion flakes off in the cutting process [37].However, a large number of elements such as W and Co appear in the white area (Figures 8a and 9b), indicating that the TiAlN coating there has been completely polished.However, point C in Figure 8a contains a large amount of Ti, Al and N species (Figure 9c), indicating that the coating there still exists.Moreover, it can be found that a large number of O species are detected in the wear area, which indicates that serious oxidative wear occurs in the cutting process.It follows from the above that the wear modes of the four coated tools in cutting titanium is adhesive and oxidative wear.It can be found that the TiAlN/TW3 coated carbide tool shows the least damage compared with the others, indicating that the TiAlN/TW3 coated carbide tool possesses the best wear resistance, which can be attributed to it having the highest adhesive strength between the coating and substrate 8.The wear resistance of TiAlN/TW4 is close to that of TiAlN/TW3, while TiAlN/WT1 possesses the worst wear resistance.

Conclusions
In this work, The TiAlN coatings were deposited on four different substrates, WT1, WT2, WT3 and WT4, representing different TaC and TiC contents.With the increases in TiC content (0-14 wt.%) in substrates, the hardness of the TiAlN coating decreases and the adhesion strength increases.The higher adhesion strength can be attributed to the

Conclusions
In this work, The TiAlN coatings were deposited on four different substrates, WT1, WT2, WT3 and WT4, representing different TaC and TiC contents.With the increases in TiC content (0-14 wt.%) in substrates, the hardness of the TiAlN coating decreases and the adhesion strength increases.The higher adhesion strength can be attributed to the

Conclusions
In this work, The TiAlN coatings were deposited on four different substrates, WT1, WT2, WT3 and WT4, representing different TaC and TiC contents.With the increases in TiC content (0-14 wt.%) in substrates, the hardness of the TiAlN coating decreases and the adhesion strength increases.The higher adhesion strength can be attributed to the fcc-(Ti,W) C formed and decreases to the hardness difference between TiAlN and WC-Co cemented carbide.The addition of TaC into the substrates inhibits the grain growth and thereby causes the hardness and adhesion strength of the TiAlN coatings to improve from 24.6 GPa and 16.7 N to 30.1 GPa and 17.3 N, respectively.In particular, the cutting test indicates that the main wear mechanism of a coated tool turning stainless steel is abrasive, adhesive, and oxidation wear, while that of a coated tool turning TC4 is adhesive and oxidation wear.The higher TiC content in the substrate improved the TC4 turning performance of TiAlN-coated tools due to the high adhesion strength between coating and substrates, while the TaC addition improved the stainless steel turning performance of the TiAlN-coated tools, owing to the high hardness of substrates and coating and the sufficient adhesion strength.
, the addition of TaC induces a shift of the peak position to lower 2θ angles, which can be attributed to the larger TaC dissolved into the (Ti,W)C crystal structure.The cross-sectional structure of the four coated cemented carbides is shown in Figure 3.The four coatings are compact and tightly bonded to the cemented carbides.The coating thickness of the four coated tools is approximately 3 μm, indicating the deposition rate of TiAlN coating did not change with the change in the cemented carbide substrates' composition.

Figure 2 .
Figure 2. XRD patterns of (a) TiAlN/WT1, (b) TiAlN/WT2, (c) TiAlN/WT3 and (d) TiAlN/WT4 coated cemented carbides.The cross-sectional structure of the four coated cemented carbides is shown in Figure3.The four coatings are compact and tightly bonded to the cemented carbides.The coating thickness of the four coated tools is approximately 3 µm, indicating the deposition rate of TiAlN coating did not change with the change in the cemented carbide substrates' composition.

Figure 7 .
Figure 7.The EDS results of TiAlN coated cemented carbides after stainless steel turning: (a) Point A, (b) Point B and (c) Point C.

Figure 7 .
Figure 7.The EDS results of TiAlN coated cemented carbides after stainless steel turning: (a) Point A, (b) Point B and (c) Point C.

Figure 7 .
Figure 7.The EDS results of TiAlN coated cemented carbides after stainless steel turning: (a) Point A, (b) Point B and (c) Point C.

Figure 9 .
Figure 9.The EDS results of TiAlN-coated cemented carbides after TC4 turning: (a) Point A, (b) Point B and (c) Point C.

Figure 9 .
Figure 9.The EDS results of TiAlN-coated cemented carbides after TC4 turning: (a) Point A, (b) Point B and (c) Point C.

Figure 9 .
Figure 9.The EDS results of TiAlN-coated cemented carbides after TC4 turning: (a) Point A, (b) Point B and (c) Point C.

Table 2 .
Cutting parameters of test.