Microstructure and Properties of TiN/TiCN/Al₂O₃/TiN Coating Enhanced by High-Current Pulsed Electron Beam

Abstract

In this work, a TiN/TiCN/Al₂O₃/TiN coating deposited onto cemented carbide matrix by chemical vapor deposition was irradiated by high-current pulsed electron beam (HCPEB). The influence of pulse times on the phase composition, microstructure, and mechanical properties of the coating investigated. The results showed that no new phase was produced, the grain size of the coating surface was refined, the surface became flat, and the surface roughness decreased after HCPEB treatment. The TiN/TiCN/Al₂O₃/TiN coating presented a smooth surface with good mechanical performance after HCPEB. A maximum hardness was obtained after 15 pulses, and the 15-pulse irradiated coating showed better wear resistance. The improvement in the coating’s performance after irradiation was mainly attributed to the formation of grain refinement and crystal defects, as well as the change of stress field inside the coating. The objective of this study was to evaluate the potential of HCPEB modification in the preparation of high-performance coating by analyzing the microstructure and property of coating under different pulses.

1. Introduction

WC-Co cemented carbide holds a significant position in the modern manufacturing and processing field due to its characteristics of high hardness, strength, and excellent wear resistance [1,2,3]. However, the rapid development of the modern manufacturing industry has higher and higher requirements for the service performance and production efficiency of cemented carbide tools; under high-speed cutting conditions, the interaction between the tool and the cutting workpiece is enhanced, resulting in serious wear and deformation, reducing the performance and life of the tool. Therefore, it is challenging to fulfill the requirements of modern manufacturing and processing.

In view of the above problems, coating one or more layers of high hardness and good wear resistance on the cemented carbide matrix has become an effective solution [4,5,6,7]. TiN is a very hard and wear-resistant material that is widely used in various industries on account of its high hardness and chemical stability. The thermal expansion coefficient of TiN is similar to that of high-speed steel, and the thermal stress generated during the cutting process is small, so it is the earliest coating material used on the surface of high-speed steel tools [8,9]. In addition, TiN also has a wide range of applications in molds, medical devices, auto parts, and decorative coatings [10,11,12]. However, TiN is prone to oxidation during processing, limiting its application in high-temperature working conditions, for which ternary coating begins to appear [13,14]. TiCN, developed from TiC and TiN, combines the advantages of both and has better wear resistance. TiCN prepared by medium temperature chemical vapor deposition (MT-CVD) not only has less cracks and high toughness, but also has improved surface finish, and is widely used in tribological research and processing of stainless steel and ductile iron [15,16]. In addition, TiCN coating is used in protective films for automotive parts and biomedical instruments due to its excellent oxidation resistance, high hardness, and good wear resistance [17,18,19]. Al₂O₃ is widely used in the field of coating materials because of its excellent thermal stability and can still have excellent performance at high temperatures [20,21]. It has also been reported that, compared to single-layer coating, multi-layer structured coating exhibits superior mechanical and frictional performance [22,23].

However, the coating prepared by CVD usually limits the performance of the coating tool because of the large surface roughness, large residual stress, cracks, and other problems [24,25,26], so it is necessary to eliminate these unfavorable factors through post-treatment technology. In recent years, high-current pulsed electron beam (HCPEB) treatment, as a new surface treatment technology, can induce surface melting followed by rapid curing, resulting in a significant change in structure, consequently affecting its mechanical, and other, properties [27,28]. HCPEB treatment can induce crater eruptions of materials with irregular composition and structure, purify or homogenize the melted surface, and also form a remelt protective layer [29]. The rapid heating and cooling of HCPEB make the surface of the material obtain the self-quenching effect, so as to obtain the ultra-fine grain [30]. In addition, HCPEB treatment can induce the stress field inside the material, and at the same time, strong plastic deformation occurs under the action of thermodynamic coupling. This non-equilibrium process can change the growth orientation of the grain, and then affect its texture transformation [31]. Guan et al. [32] explored the impact of HCPEB treatment on TiAlN coating, and the results showed that the grain refinement, transition zone formation, and residual stress regulation of TiAlN coating after HCPEB irradiation were the reasons for improving its mechanical and tribological properties. Guo et al. [33] studied the effect of HCPEB modification on the structure and tribology of cold-sprayed Al coating. The results showed that electron beam irradiation not only eliminated cracks and defects on the surface of the coating, but also formed a remelted layer with higher hardness and compressive stress values, thus enhanced wear resistance. Many researchers have suggested that HCPEB irradiation can effectively improve the mechanical properties of various coatings. However, the research on the treatment of multi-layer coating by HCPEB is not complete at present, and HCPEB has gaps in the field of cemented carbide. Therefore, the surface treatment of coating by HCPEB to improve their structure and properties needs further research.

In this work, the HCPEB surface modification of the TiN/TiCN/Al₂O₃/TiN coating prepared by chemical vapor deposition was investigated. The structure transformation of the TiN/TiCN/Al₂O₃/TiN coating before and after HCPEB treatment was characterized in detail, and the mechanism for the improvement of the coating properties was explored. This work will provide a possibility to improve the coating preparation technology and provide a new technology path and application prospect for the coating industry.

2. Materials and Methods

2.1. Preparation of Substrate and Coating

WC-Co cemented carbide was prepared by powder metallurgy as the base material of cutting tools. Firstly, the WC powder (94 wt.%) and Co powder (6 wt.%) were fully mixed with the forming agent. Subsequently, alcohol was utilized for ball milling to ensure effective amalgamation between the powder particles. The resultant dry mixture was then molded into a block body and subsequently sintered to form the cemented carbide matrix. Figure 1 shows the flow chart of cemented carbide matrix preparation. The cemented carbide matrix was cut and polished into blocks measuring 12 mm × 12 mm × 4.8 mm. Subsequently, the sample was subjected to edge passivation treatment to alleviate stress concentration, followed by ultrasonic cleaning in acetone and alcohol solutions. Finally, the samples were thoroughly dried in an oven in preparation for the coating process.

A TiN/TiCN/Al₂O₃/TiN coating was deposited on cemented carbide using CVD equipment with the model BernexTM BPXpro from Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Hunan, China.

The deposition of TiN coating was carried out under conditions of 900 °C temperature and 200 kPa pressure, and utilized N₂ and TiCl₄ as the N source and Ti source, respectively. The gas ratio of N₂ to TiCl₄ was maintained at 1:30. The chemical reaction is:

$$2{\mathrm{TiCl}}_4(\mathrm{g})+{\mathrm{N}}_2(\mathrm{g})+4{\mathrm{H}}_2(\mathrm{g})\stackrel{950 °\mathrm{C}, 200\mathrm{kPa}}{\to }2\mathrm{TiN}(\mathrm{s})+8\mathrm{HCl}(\mathrm{g})$$

(1)

The deposition of TiCN coating was carried out under conditions of 885 °C temperature and 60 kPa pressure. The process involved using CH₃CN as the source for C and N, and TiCl₄ as the source for Ti. The gas ratio of TiCl₄ to CH₃CN was maintained at 2:1. The chemical reaction is:

$${\mathrm{TiCl}}_4(\mathrm{g})+1/3{\mathrm{CH}}_3\mathrm{CN}(\mathrm{g})+3/2{\mathrm{H}}_2(\mathrm{g})\stackrel{885 °\mathrm{C}, 60\mathrm{kPa}}{\to }{\mathrm{TiC}}_{2/3}{\mathrm{N}}_{1/3}(\mathrm{s})+4\mathrm{HCl}(\mathrm{g})$$

(2)

The deposition of Al₂O₃ coating was carried out under conditions of 1000 °C temperature and 65 kPa pressure. In this process, the reaction between CO₂ and H₂ produced gaseous H₂O, which subsequently reacted with AlCl₃ to form Al₂O₃. H₂S was added as a catalyst to enhance the deposition rate during the deposition process. The gas ratio AlCl₃ to CO₂ to H₂S was maintained at 5:18:1. The chemical reaction is:

$$2{\mathrm{AlCl}}_3(\mathrm{g})+3{\mathrm{CO}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\stackrel{1000 °\mathrm{C}, 65\mathrm{kPa}}{\to }{\mathrm{Al}}_2{\mathrm{Cl}}_3(\mathrm{s})+3\mathrm{CO}(\mathrm{g})+6\mathrm{HCl}(\mathrm{g})$$

(3)

Finally, a TiN (~0.5 μm) + TiCN (~10 μm) + Al₂O₃ (~5 μm) + TiN (~1 μm) multilayer coating was deposited on the YG6 cemented carbide.

2.2. HCPEB Treatment

High-current pulsed electron beam (HCPEB) treatment (“Hope-I” type) was conducted on TiN/TiCN/Al₂O₃/TiN coating. The treatment conditions were set as follows: accelerating voltage of 27 keV, energy density of 5 J/cm², background vacuum of 5.5 × 10⁻³ Pa, pulse duration of 1.5 μs with a 10-s pulse interval. The diameter of the beam spot was maintained at 45 mm to ensure that the sample was completely irradiated, and the number of pulses were 5, 10, 15, and 20. The schematic diagram of the HCPEB system is depicted in Figure 2.

2.3. Characterization

X-ray diffraction (XRD) with CuKα radiation was used to characterize the surface phase composition of the coating. The microstructure before and after HCPEB was observed with a Nova/Nano-450 Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy EDS) from School of Materials, Jiangsu University, China. Surface roughness measurement was conducted by an Olympus LEXT OLS4100-type three-dimensional laser scanning microscope (3D-LSM).

The parameter of microhardness measurement was a working load of 500 g for 15 s, averaging ten measurements for accuracy. The friction and wear experiment were performed with SFT-2M equipment (from School of Materials, Jiangsu University, China), using a Si₃N₄ steel ball at 230 mm/min for 30 min under a load of 10 N and averaging three measurements for accuracy. The wear rate of the coating was calculated, and the image of the worn surface was investigated with SEM.

3. Results and Discussion

3.1. Physical Phase Analysis

The XRD patterns of samples before and after HCPEB are displayed in Figure 3. It is obvious that the original coating contains Al₂O₃, TiCN, and TiN phases, no new phase is observed after HCPEB irradiation, and the peak of TiN phase is reduced. The reason for this is that after HCPEB irradiation, the coating surface temperature increases and melts, and the stress field inside the coatings changes and causes eruption, resulting in a decrease in the peak value. At this time, the main working coatings become Al₂O₃ and TiCN coatings, so the peaks of Al₂O₃ (0 1 2) and TiCN (2 2 0) are amplified. The intensity of the two peaks increases, and the peak position shifts to a higher angle in different degrees after HCPEB, with the most noticeable shift occurring after 5 and 15 pulses. According to the Bragg equation, residual compressive stress exists in the coatings under irradiation [34]. Compared with 5 and 15 pulses, the peak position of 10 and 20 pulses shifts to a lower angle, indicating that the internal stress of the coating became the residual tensile stress at this time. The reason for this phenomenon is that HCPEB irradiation forms impact pressure stress on the surface of the coating, making its crystal lattice shrink [35,36]; at the same time, due to the introduction of dynamic temperature field, thermal stress is generated inside the coating, leading to crystal lattice expansion [37]. The interaction between the two causes different degrees of lattice distortion in the coatings, resulting in different stress states inside the coatings. In addition, the broadening of Al₂O₃ (0 1 2) and TiCN (2 2 0) peaks may be attributed to the formation of grain refinement and crystal defects caused by HCPEB. This will also have a certain impact on its performance.

3.2. Microstructure Analysis

Figure 4 shows the surface morphology of the coating before and after HCPEB treatment. It can be seen from Figure 4a,b that the surface of the original coating is composed of coarse particles with uneven surface distribution. In addition, there are some network microcracks on the surface of the coating, which has a high roughness. After 5 pulses of irradiation, the surface is relatively flat and has no granular feeling, and a relatively small melt pit has been generated (Figure 4c), indicating that the surface coating is melted by electron beam irradiation and then forms into a relatively flat surface after cooling and solidification. A large number of previous studies [38,39] have shown that melt pits are typical features of the surface of materials irradiated by HCPEB. When a high-energy electron beam bombards the surface of materials, a certain region of the subsurface of the material preferentially melts due to the deposition of energy. This results in a rapid expansion of material volume in this area, leading to its eruption from the surface. Subsequently, during the ultra-rapid cooling process, the material solidifies rapidly, forming a melt pit. A bulge-like structure is also found in Figure 4d. Under 10 pulses, a large number of crater phenomena appear, distributed along the direction of cracks, and the size of the crater also increases, indicating that the energy of the electron beam is concentrated and the crater eruption is intense due to poor thermal conductivity between the coatings, leaving a large-sized crater. In addition, the bulge-like morphology of the coating surface still exists and increases. Then, with the increase of pulse times, the number of melt pits gradually decreases, the width of cracks are sutured, and some bulge-like structure appears on the surface. After magnification (Figure 4h,j), it is found that these bulge-like structures are composed of fine nanoscale grains, indicating that HCPEB can refine the coating grains. The grain refinement phenomenon increases with the increase of pulse times, which also verifies the broadening of XRD diffraction peaks. The phenomenon of grain refinement occurs due to the rapid injection of high energy during HCPEB irradiation, causing ultra-rapid heating and melting, followed by a rapid solidification and cooling of the surface [40,41]. In addition, the microcracks on the surface are gradually patched after HCPEB, but still exist. Based on this result, the appropriate amount of HCPEB irradiation can cause the surface crack area to melt and fill the microcracks, and subsequent solidification can fill the microcracks and effectively bond the surface microcracks.

Figure 5 shows the 3D image and surface roughness curve (Sa) of the coating before and after HCPEB. Figure 5a shows that the surface roughness of the original coating is the highest, and the surface structure has many raised small particles. As shown in Figure 5b, after 5 pulses of HCPEB irradiation, large peaks and craters have formed on the surface of the coating. After 10 pulses of irradiation (Figure 5c), the number of mountain peaks and craters decreases significantly, but the size of pits increases, resulting in an overall flattening of the coating. After 15 pulses of irradiation (Figure 5d), the number of peaks further decrease, while the bulges increase, contributing to a relatively smoother surface. However, large-sized pears and bulges become obvious after 20 pulses. HCPEB irradiation eliminates most of the sharp protrusions and pits; the topography of small pits and peaks are smoothed gradually with the increase of irradiation times and the surface of the coating becomes smoother. This is basically consistent with the analysis results in Figure 4. Figure 5f shows the surface roughness (Sa): the roughness of the initial coating is 0.214 μm. After 5, 10, 15, and 20 pulses of irradiation, the surface roughness becomes 0.177, 0.185, 0.135, and 0.141 μm, respectively. After HCPEB irradiation, the roughness decreases significantly, with the lowest being at 15 pulses. This means that when the TiN/TiCN/Al₂O₃/TiN coating is irradiated by HCPEB, the surface becomes smooth and flat, which can effectively reduce its roughness.

Figure 6 shows the cross-section morphology and line-scanning analysis of TiN/TiCN/Al₂O₃/TiN coating before and after HCPEB irradiation. From Figure 6a,b, it can be seen that the TiCN (~11 μm) + Al₂O₃ (~6 μm) + TiN (~1 μm) coating was actually prepared, but the bottom coating TiN was too thin and was not be significantly observed. From the cross-section, the prepared coating is relatively dense, but there are microcracks in the vertical direction and some pores are found in the TiCN coating. The boundary between TiCN and Al₂O₃ curves is obvious from Figure 6b, which may be caused by the mismatch between the thermal expansion coefficients of the two. After 5 pulses, the surface TiN coating melts and disappears, and the outermost coating becomes Al₂O₃ when the thickness is reduced. There are a lot of pores and voids in TiCN and Al₂O₃ coatings, and there are still microcracks. This is because the surface temperature increases during HCPEB irradiation, which causes the reaction between TiN, Al₂O₃, and TiCN coatings. In the process of repeated heating, melting, expansion eruption, and cooling solidification, the interface between coatings produces mismatched thermal stress [9], which results in a large number of pores and voids. The line scan data (Figure 6d) shows that the TiCN and Al₂O₃ curves are tighter than the original coating. The boundary between the coatings becomes blurred and the coating becomes denser because the temperature gradient caused by HCPEB irradiation promotes the diffusion of elements between the coatings. After 10 pulses, the thickness of the Al₂O₃ coating is further reduced, and no microcracks are observed inside, but the number of pores further increases. After 15 pulses, the pores in the coating are reduced and the coating is relatively smooth and dense, but the pores increase again after 20 pulses. This may be because the effect of HCPEB reaches a critical point at 15 pulses, and subsequent irradiation causes the stress field to be too large, with adverse effects.

3.3. Mechanical Properties

The microhardness of TiN/TiCN/Al₂O₃/TiN coating before and after HCPEB is shown in Figure 7. The microhardness of the original coating is 2092.10 HV. With the increase of irradiation times, the microhardness of the coatings gradually increases to 2212.58 HV, 2307.49 HV, 2425.68 HV, and 2210.13 HV, respectively. It can be seen that the surface microhardness of the coating first increases and then decreases after HCPEB treatment, reaching a peak at 15 pulses. This is because HCPEB irradiation can form nanoscale grains on the surface of the coating (Figure 4), increase the yield strength of the coating, and then increase the hardness value, that is, fine crystal strengthening. Secondly, the HCPEB treatment induces plastic deformation inside the coating, forming deformation structures such as slip bands. These crystal defects lead to the increase of dislocations in the coating, and thus improve the deformation resistance of the coating, that is, the dislocation strengthening. Finally, the change of stress state inside the coating has a positive effect on the increase in hardness. Anthony et al. [42] found that coating with compressive stress could enhance fatigue life, inhibit crack propagation, and improve the stress resistance of a material. Figure 3 shows that after HCPEB irradiation, residual compressive stress is introduced inside the coating, which can inhibit the expansion of cracks on the coating surface and thus improve its hardness.

Figure 8 shows the friction coefficient and wear rate of TiN/TiCN/Al₂O₃/TiN before and after HCPEB. The original coating has a higher wear rate of 0.411, while the friction coefficients after HCPEB irradiation are reduced to 0.308, 0.292, 0.183, and 0.367, respectively. The friction coefficient of the coating after 15 pulses is the lowest, and the wear rate of the coating after 15 pulses is also the lowest. The significant improvement of friction and wear properties can be attributed to the increase of hardness, the increase of plastic deformation resistance, and the decrease of surface roughness. As a result, HCPEB treatment effectively enhances the friction and wear properties of the coating, with the 15-pulse coating showing the most superior properties.

In order to better explain the wear mechanism, the wear image of the TiN/TiCN/Al₂O₃/TiN coating before and after HCPEB is shown in Figure 9. The significant grooves can be clearly seen in the original coating; in addition, there are some debris and cracks attached to the surface, the surface of the coating is slightly damaged. At this time, there is mainly abrasive wear. The furrow morphology on the surface is greatly reduced, and debris and dust are attached to a large area of the surface after 5 pulses (Figure 9b), involving adhesive wear and slight abrasive wear. As the pulse times increases, the furrow morphology disappears, and there is a substantial accumulation of debris and abrasion particles (Figure 9c–e), indicating that the mechanism involves mainly adhesive wear. Among them, the wear surface at 15 pulses is relatively smooth, and only some debris are attached to it. This stratification phenomenon is also found in the wear surface at 20 pulses. This observation is consistent with the mechanism analysis results presented earlier for Figure 7 and Figure 8 above. In addition, the improvement of wear performance is also attributed to the role of the surface Al₂O₃ coating as a protective film. In summary, HCPEB can effectively improve the wear performance of TiN/TiCN/Al₂O₃/TiN coating.

4. Conclusions

In this work, TiN/TiCN/Al₂O₃/TiN coating deposited onto cemented carbide matrix by chemical vapor deposition was irradiated by high-current pulsed electron beam (HCPEB). The influence of irradiation pulses on the phase composition, microstructure, microhardness, and frictional resistance of these coatings was investigated. The main conclusions are as follows:

(1)

After HCPEB treatment, the diffraction peak of the coating is widened and strengthened due to the formation of grain refinement and crystal defects. In addition, the stress state inside the coating also changes, and the diffraction peak shifts to a higher angle in different degrees compared with the original coating.

(2)

After HCPEB treatment, the surface of the coating has a typical melt-pit appearance, and the nanoscale grain appears. In addition, the surface roughness of the coating decreases after irradiation, reaching its minimum at 15 pulses. The thickness of the coating decreases with the increase of pulse time.

(3)

HCPEB treatment can effectively improve the surface microhardness of TiN/TiCN/Al₂O₃/TiN coating. The microhardness of the coating increases first and then decreases with the number of pulses, reaching a peak at 15 pulses.

(4)

HCPEB treatment can improve the friction and wear properties of TiN/TiCN/Al₂O₃/TiN coating. The wear mechanism of the original coating is mainly abrasive wear, the friction coefficient decreases after HCPEB irradiation, and the wear morphology is mainly adhesive wear. Among them, the coating wear performance after 15 pulses is the best.

(5)

It is proved that HCPEB treatment can change the internal structure and improve the hardness and wear resistance of the coating. This work presents the possibility of a new technology for the preparation of high-performance multilayer coatings, and also provides a new technological innovation and application prospect for the coating industry.