The Effect of TiSi Target Magnetic Field Intensity on the Residual Stress of TiAlSiN Hard Coating Deposited by Arc Ion Plating
Reference: CHEN Y F, GUO Z M, HUI Y, et al. The effect of TiSi target magnetic field intensity on the residual stress of TiAlSiN hard coating deposited by arc ion plating[J]. Vacuum and Cryogenics, 2024, 30 (1): 57−63.
Current research indicates that, compared to TiN coatings, TiAlN coatings exhibit higher hardness and cutting lifetime , yet their durability remains limited at elevated cutting speed and temperatures. The addition of elements such as carbon (C), silicon (Si), and boron (B) can enhance high-temperature performance. For TiAlSiN coatings, their nano-composite structure provides excellent wear resistance and oxidation resistance, though residual stresses limit practical applications. While internal stress studies predominantly focus on TiN coatings, TiAlSiN coatings receive relatively less attention. Therefore, exploring the mechanisms governing internal stress in TiAlSiN coatings holds significant importance.
Method:
Sample Preparation: The TiAlSiN coating was fabricated with arc ion plating(AIP) using the MA800 plus composite coating system developed by Guangdong Huasheng Nanotechnology Co., Ltd. This equipment features four arc source modules, each containing two units with a maximum output current of 400 A per unit. The experimental setup utilized two Ti50Al50 targets and one Ti85Si15 target, all maintaining 99.99% purity. The Ti50Al50 target was equipped with an MAG3 magnetic field configuration, while the Ti85Si15 target received both MAG3 and MAG6 configurations. The MAG3 magnetron consists of permanent magnets and electromagnetic coils, whereas the MAG6 magnetron only contains an electromagnetic coil. The base pressure of the vacuum chamber is about 0.5 mPa, with the Ti50Al50 target operating at 150 A current and a sample bias ranging from −40 to −60 V. Nitrogen gas was introduced to maintain a vacuum pressure of 2.3–4.6 Pa during the deposition of the transition layer. Subsequently, a Ti85Si15 target was activated to deposit the TiAlSiN functional layer, achieving a total coating thickness of 2.5 μm ± 0.3 μm.
Characteristic: The surface morphology of coated samples was analyzed using a Zeiss Sigma 300 field emission scanning electron microscope. EDS elemental surface scanning and compositional analysis were employed to measure atomic percentages of Ti, Al, Si, and N. Cross-sectional thickness measurements were conducted using ball pit and intersection electron microscopy techniques. The adhesion strength of coatings under varying magnetic fields was measured with an Anton Paar Revetest® high-load scratch tester, where the loading force increased from 1 N to 120 N at a rate of 238 N/min, with a scratch length of 3 mm and a scratching speed of 6 mm/min. Three scratches were made in the central region of each sample, with the minimum value recorded as the coating adhesion strength. Internal stress testing was performed using a SuProFST1000 film stress gauge based on the substrate bending method principle, employing Stony's equation for calculation.
Cutting Test: The WNMG080408 coated cemented carbide cutting insert was subjected to a cutting test using 316L stainless steel (hardness ≤200 HV) as the workpiece. The parameters were set at cutting speed Vc = 200 m/min, feed rate Fn = 0.2 mm/r, and depth Ap = 1.5 mm, with water-cooled emulsion cooling. Blade failure was determined by either post-cut face wear width VB ≥300 μm or blade fracture. A Kansai VHX-7000 series digital microscope system was employed to photograph and record wear patterns on both the front and rear cutting faces every 3 minutes.
Results and analysis
Figure 1: Magnetic field distribution of MAG3(a) and MAG6(b) and arc spot motion of Ti85Si15 target surface under MAG3(c) and MAG6(d)
Magnetic Field and Arc Spot Analysis: The magnetic field distribution was modeled using COMSOL, as shown in Figures 1(a) and 1(b), which reveals that the MAG3 target exhibits vertical magnetic fields at its edge and center, with parallel fields between these regions. The MAG6 coil maintains the same current intensity of 0.5 A as the MAG3, producing a central vertical magnetic field and peripheral parallel fields. Notably, MAG3 generates stronger magnetic fields than MAG6. Under MAG3's influence, arc spots predominantly form between the target's edge and center regions. In contrast, MAG6's weaker magnetic field causes more random arc spot movement. At a bias voltage of 50 V, the partial currents under MAG3 and MAG6 reach approximately 4.5A and 5.5A respectively, indicating faster deposition rates at lower magnetic fields.
Figure 2: The surface morphology and cross-section morphology of sample M3-1 and M6-1
Surface and cross-sectional morphology analysis: Samples prepared using MAG3 magnetic field exhibited fewer surface particles in M3-1, while those from MAG6 magnetic field showed more abundant and larger-sized particles in M6-1, as shown in Figure 2. The thickness of M6-1 was approximately 2.8 μm, compared to 2.2 μm for M3-1, attributed to the higher bias current and faster deposition rate of M6-1. Elemental analysis revealed that M3-1 contained atomic percentages of 49.14%,32.68%,16.81%, and 1.37% for N,Al,Ti and Si respectively, whereas M6-1 showed 49.17%,33.52%,15.15%, and 2.16% respectively. This indicates that the arc target surface magnetic field does not affect the atomic percentage of N elements in the coating, but primarily influences the ion percentage of target material ionization, thereby affecting element aggregation and nucleation processes as well as the final atomic percentages of different elements.
Figure 3: XRD patterns and partial enlarged details of M3-1 and M6-1
Coating structure analysis: XRD patterns reveal strong diffraction peaks at the (111), (200), and (220) crystal planes of M3-1 and M6-1. The cubic aluminum nitride (c-AlN) phase on the (200) plane indicates higher hardness and better wear resistance in the coating. M6-1 exhibits stronger diffraction peaks and narrower half-widths at both (111) and (200) planes compared to M3-1, suggesting superior crystallinity and finer grain size in M6-1. This indicates that M6-1 contains more silicon (Si) and has smaller crystal grains than M3-1.
Mechanical Properties and Elemental Analysis: The adhesion strength of samples M3-1 and M6-1 were 104.2 N and 100.6 N respectively, with hardness values of 40.6 GPa and 35.9 GPa. Both samples exhibited comparable adhesion strength exceeding 90 N, indicating excellent adhesion performance. The coating's internal stress and hardness decreased significantly with reduced magnetic field strength on the Ti85Si15 target surface: internal stress decreased from −5.84 GPa to −3.16 GPa, while hardness dropped from 40.6 GPa to 35.9 GPa. Although M6-1 contains higher silicon content than M3-1, M3-1 demonstrated superior hardness. Overall, M6-1 showed 11% lower hardness than M3-1 but approximately 50% reduced internal stress. The H3/E2 ratio and MDP parameters indicated that M3-1 exhibited better toughness compared to M6-1.
Figure 4: The flank wear width of M3-2 and M6-2
Cutting Test Analysis: Two coating blade samples (M3-2 and M6-2) underwent two cutting tests each. The results showed that at 24 minutes, the wear band width on the blade surface of M3-2 group reached 950 μm with chipping, while at 27 minutes, the wear band width of M6-2 group remained below 300 μm. Comprehensive analysis indicated that the coating sample M6-2 prepared using MAG6 exhibited lower internal stress and demonstrated superior wear resistance compared to the coating sample M3-2 prepared with MAG3.
Overall conclusion
Research findings indicate that two TiAlSiN coated samples were fabricated by modifying the magnetic field configuration of Ti85Si15 targets. When the magnetic field was changed from MAG3 to MAG6 (with the intensity decreasing from 1 mT to 0.5 mT), the internal stress of the TiAlSiN coating decreased by 46% from −5.84 GPa to −3.16 GPa. The study demonstrates that coatings prepared under low magnetic fields maintained nearly unchanged bonding strength and hardness, while achieving a 30% improvement in cutting life.
Explanation and Analysis: The low magnetic field strength reduces the ionization rate of materials evaporated from the target, thereby decreasing internal stress in the coating and significantly extending the tool's cutting life. Additionally, although hardness is reduced, the coating's toughness and wear resistance are enhanced, enabling the coated tool to demonstrate superior performance during high-speed machining and dry cutting operations.
Important: This study provides valuable experimental data for improving the performance of TiAlSiN coating, especially by adjusting the magnetic field strength in the magnetron sputtering coating technology to optimize the internal stress of the coating, which provides a new idea and technical support for the application of high-performance coating in the tool industry.