3.1. Microstructure and Mechanical Properties
The surface morphology of the coating obtained by SEM is shown in
Figure 2a. There are microscopic defects such as white particles and pits on the surface of the coating; this is because, in the process of ion plating deposition, droplets splashed and adhered to the surface of the coating. The defects will seriously affect the surface quality of the coating, and the surface defects can be reduced by adjusting process parameters [
26,
27,
28]. The elements of the AlTiZrN coating were scanned by EDS, and the distribution of each element is shown in
Figure 2b. It can be seen from the figure that the distributions of the Al and Zr element are relatively uniform, while that of the Ti and N elements are relatively dispersed, which are related to the ratio of each component of the target and N
2 flow rate.
Figure 3 shows the XRD pattern of the AlTiZrN coating. The AlTiZrN coating shows a face-centered cubic (fcc) structure of TiN, with four diffraction peaks at 35.669°, 39.167°, 73.147°, and 75.530° corresponding to (111), (200), (311), and (222) respectively, and the diffraction peak at (111) is narrow and high. The XRD pattern was first fitted by Jade software, and then data analysis was used to obtain the full width at half maximum and lattice size of four diffraction peaks of 35.669°, 39.167°, 73.147°, and 75.530°. The specific values are shown in
Table 5. When the diffraction peak is (111), the lattice size reaches 687 Å, and when the diffraction peak is (220), the lattice size is 158 Å, which indicates that grain refinement occurs at this time.
Uncoated tools and AlTiZrN-coated tools are shown in
Figure 4a, and the colour of the AlTiZrN coating is golden yellow. The cross-sectional morphology of the AlTiZrN coating and substrate is shown in
Figure 4b. The measured thickness of the coating was about 1.792 μm, and the coating was dense, well-bonded with the substrate, and had no obvious gap. There were some pits on the top of the coating caused by the uneven cutting of the sample. The chemical composition of the deposited coating was analyzed by the EDS software of the SEM. The results are shown in
Table 6.
The average hardness of the AlTiZrN coating measured by the Micro-Vickers hardness tester is 3887 HV
0.05. The hardness of the traditional AlTiN coating is 2800–3300 HV [
29,
30,
31]. Doping Zr can improve the hardness of the AlTiN coating by at least 17.8%. Some studies had shown that when the percentage content of the Al, Ti, and Zr was close to 1:1:1, the possibility of the extreme hardness of the coating was greater [
32,
33].
For coated tools, the bonding strength between the coating and substrate will affect the cutting performance and life of the tools to a certain extent. Due to the complex shape of the tool, it is impossible to accurately measure the bonding strength of the coating. Therefore, this paper chose to use a Rockwell hardness tester to test the bonding strength of the coating on the surface of YG8 samples.
Figure 5a shows the standard diagram for judging the bonding strength between the PVD coating and substrate. Under the HF1~HF4 standards, slight cracks were generated around the indentation opening, which is the qualified bonding strength. Under the HF5 and HF6 standards, the coating near the indentation opening was peeling, and the strength was unqualified. The indentation morphology of the AlTiZrN coating on the surface of the cemented carbide was observed by an optical microscope, as shown in
Figure 5b. There are only cracks around the indentation, and the coating did not peel. By comparing the number and shape of cracks, it can be judged that the bonding strength between AlTiZrN coating and cemented-carbide substrate meets the HF3 standard.
Coefficient of friction (COF) is another index to measure the mechanical properties of the coatings. For fine machining tools, the COF of the surface is the main parameter that reflects the cutting performance. The smaller the COF, the smaller the machining deformation of the tool, and the better the cutting ability when the cutting force is reduced. The COF curve of the AlTiZrN coating measured by the friction and wear tester is shown in
Figure 6. The COF curve of the AlTiZrN coating fluctuates in the range of 0.2~0.4. After calculation, the average value of COF is 0.32. In [
14], COF values of 0.65 were reported for AlTiN-TiSiN coating deposited on the nitrided tool steels. In [
34], COF values between 0.62 and 0.68 were reported for AlCrN coatings deposited on HS6-5-2 steel substrates, and in [
35], values of around 0.5–0.75 were reported for nanocomposite TiAlN coatings deposited on different steels. Therefore, the AlTiZrN coating prepared in this paper had a better COF.
3.2. Cutting Performance
The flank wear curves of the uncoated and AlTiZrN-coated cemented-carbide tools at different cutting speeds are shown in
Figure 7. At the cutting speed of 85 mm/min, the wear of the uncoated and coated tools can be divided into three stages. Within 0–9 cm, AlTiZrN-coated tools are in the initial wear stage, and the wear amount is small. With the increase of cutting length, the wear amount of the flank increases steadily, and the tool wears sharply after cutting 33 cm. When the cutting length is 36 cm, the wear width of the tool flank exceeds 0.1 mm, and the tool reaches the failure state. Through the curve, the cutting length of the tool can be calculated to be about 35.5 cm. However, the uncoated tool showed sharp wear after cutting 21 cm and finally reached the failure state when the cutting distance was 29.4 cm. When the cutting speed was 105 or 125 mm/min, the flank wear did not increase slowly, and the wear curve of the uncoated tool was steeper than that of the coated tool. Finally, the cutting length of the uncoated and coated cemented-carbide tools was 20.5 and 25.1 cm, respectively, at the speed of 105 mm/min; At the speed of 125 mm/min, the cutting lengths of the uncoated and AlTiZrN-coated cemented-carbide tools were 14.5 and 19.6 cm, respectively. Comparing the flank wear curve, it can be seen that the wear stage of the AlTiZrN-coated cemented-carbide tools is more obvious and stable under three different cutting speeds, which proves that AlTiZrN-coated cemented-carbide tools have less wear in cutting than uncoated tools.
Figure 8 shows the cutting length of the uncoated tool and AlTiZrN-coated tool at different cutting speeds. With the increase in cutting speed, the lives of both types of tools will be greatly shortened. At the cutting speeds of 85, 105, and 125 mm/min, the lives of the AlTiZrN-coated tools are increased by 20.7%, 22.4%, and 35.2%, respectively. The data shows that under the condition of high-speed cutting, the coated tools had more advantages than the uncoated tools.
3.3. Wear Mechanism
Figure 9 shows the relationship between flank wear and cutting length of an uncoated tool and an AlTiZrN-coated tool at different wear stages at a cutting speed of 125 mm/min. In the early stage of the cutting process, the wear of the two kinds of tools was relatively uniform, which was simple flank wear, and the uncoated tool wore down at a greater rate at the same cutting distance. With the increase in cutting distance, the cutting edge of the tool gradually dulled. When the cutting distance was 9 cm, the micro-edge collapse of the two kinds of tools occurred, which led to a decline in cutting performance [
36].
Because the uncoated tool had no coating protection, it led to large-area wear on the flank, which accelerated the failure of the tool. When the cutting distance was increased to 15 cm, the uncoated tool reached the failure standard, and a large number of cutting edges collapsed. The small pits and grooves on the flank of the AlTiZrN-coated tool are due to the existence of small particles with high hardness in the workpiece material. During the process of cutting, the small particles rub against the rotating tool, and this will lead to defects on the flank. When the cutting distance reached 21 cm, due to periodic impact and alternating thermal stress, the AlTiZrN coating peeled off, and obvious cracks and grooves appeared on the flank. By analyzing the wear morphology of the flank, it can be seen that the wear resistance of the AlTiZrN-coated tools is better than that of the uncoated tools.
In order to study the wear failure form of the flank of the AlTiZrN-coated tool, the coated tools with the cutting length of 9 and 15 cm, respectively at the speed of 125 mm/min are selected. The cemented-carbide tool was cut by a wire-cutting machine, and a small part was intercepted for EDS analysis.
Figure 10 shows the results of EDS. In the middle stage of tool wear, the content of the Al, Ti, Zr, and N in the surface layer of coated tool flank was lower than that of the coating, and slight friction and wear occurred. At the same time, a small amount of Fe and O elements appear. Because the coating and cemented-carbide did not contain Fe and O elements, it indicates that a small amount of high-chromium cast iron adhered to the back surface of the tool at this time, and an oxidation reaction occurred. At this time, the wear forms are slight bonding wear and oxidation wear. In the later stage of cutting, a large number of Fe and O elements, as well as an appropriate amount of Cr, a small amount of Ni, and trace Si elements begin to appear on the flank of the coated tool. The constituent elements of the AlTiZrN coating are greatly reduced, the cutting part of the tool contacts and rubs against the chip, and some hard spots of the workpiece material will be taken away by the tool. At this time, the wear mechanisms of the flank of the tool are serious oxidation wear and bonding wear. Therefore, the wear mechanism of the AlTiZrN coating in high-speed cutting includes friction wear, oxidation wear, and bonding wear, among which the oxidation wear and bonding wear are the main wear forms. The wear mechanisms of the AlTiZrN-coated tools are similar to that of the AlTiN- and TiAlSiN-coated tools in reference [
31].
3.4. Chip Formation
Under different cutting conditions, the shapes of chips produced by cutting KmTBCr12 are also different.
Figure 11 shows the chips produced by the uncoated tools and AlTiZrN-coated tools when the cutting length was 9 cm at the speed of 125 mm/min. The chips produced by the two kinds of tools are curled, but the chips produced by coated tools are generally shorter, which indicates that the coated tools have a better chip-breaking performance than the uncoated tools, and the coated tools are more suitable for cutting.
Figure 11c,d shows the machining path of the tool on the workpiece surface. Combined with the wear diagram of the flank in
Figure 9, it can be seen that the wear of the coated tool was relatively slight, the scratches were more uniform and smooth, and the surface quality of the workpiece was good. The cutting edge of the uncoated tools was seriously worn, the scratches were more rough and uneven, and the surface of the workpiece was rough. Therefore, it can be judged that under the same conditions, the coated tool has better chip-breaking performance and good machining surface quality.
Figure 12 shows the chip and machining path generated by the AlTiZrN-coated tool when the cutting length was 9 cm at different cutting speeds. It can be seen from the figure that when the cutting speed was low, the generated chips were bent and long; the scratches formed by tool machining were not uniform, and the surface was rough. With the increase in cutting speed, the chips gradually became smaller, and the scratches tended to be uniform and smooth, indicating that the chip-breaking performance of the tool improved at this time. Therefore, under the relatively high-speed cutting conditions, the chip-breaking performance of the AlTiZrN coating was also better.