Preparing high performance TiCN coatings by HIPIMS

Abstract: The high hardness and low friction coefficient of TiCN coating is prepared by HiPIMS technology, which greatly improves the service life of tap coating. The influence of different C contents on coating hardness, friction coefficient and cutting life is verified by experiments.

Key words: HiPIMS; TiCN coating; microstructure; mechanical properties; frictional wear; tap

1. Introduction

TiCN coatings, combining amorphous carbon and nanocrystalline structures, exhibit superior hardness, enhanced toughness, and lower friction coefficients compared to TiC and TiN1. These properties make them widely used in tapping and drilling, particularly suitable for processing non-ferrous metals and alloys2. HiPIMS primarily employs pulsed DC discharge mode. During discharge, high trigger voltage induces peak power density and current density, causing a sharp increase in plasma electron density. The enhanced electron density increases the probability of sputtered atoms colliding with electrons, achieving high ionization rates (20%-100%)³. The high plasma density relies on peak power density and low duty cycle. During pulse switching, plasma particles collide and exchange charges without sputtering the target material, reducing thermal accumulation and preventing ceramic targets from cracking due to overheating4. To some extent, HiPIMS technology integrates the advantages of DC magnetron and cathode arc methods, achieving high ionization rates while avoiding metal particle generation. High ionization rates and current density effectively improve coating deposition quality, reduce surface roughness, and enhance film adhesion and density5. Additionally, high ionization rates enhance particle diffraction properties, effectively addressing thickness non-uniformity issues on complex workpiece surfaces. In this paper, a series of TiCN coatings were prepared on the surface of hard alloy by HiPIMS technology to investigate the influence of power variation on the content of C, microstructure and mechanical properties of the coating, and thread cutting tests were carried out to investigate the actual cutting performance of TiCN coating.

2 Trial 

2.1 Coating preparation

The preparation of TICN coating film is conducted on the the HA800 composite coating machine developed by Guangdong Huasheng Nano Technology Co.LTD(hereinafter referred to as "the Company") ,which contains four arc source units, one DCMS power supply, and one HiPIMS power supply, and is equipped with pulse bias power supply, closed-loop temperature control system, coil current regulation, disk arc magnetic field, and sputtering magnetic field. The target materials used are Ti (99.9%) and Ti50C50 (99.9%), with a polished substrate of 17mm×17mm×5mm hard alloy as the base material. Before preparation, the substrate undergoes cleaning and drying treatment using the Company's mature cleaning line. Prior to coating deposition, the substrate is heated to 450℃ and subjected to H₂ heating and Ar glow discharge for further sputtering cleaning. Subsequently, the bias voltage is reduced to-60V, and high-purity Ar and N₂are introduced in specific proportions to achieve the chamber pressure of 0.6Pa . The duty cycle of HIPIMS is controlled at about 5%, and the peak current was controlled at 200-300A. TiCN coatings with different C contents are are by adjusting the power of the Ti target, and TiN coatings are prepared under the same conditions as a reference. Specific deposition parameters and coating information are shown in Table 1.

Table 1 Coating deposition parameters

2.2 Coating structure analysis and mechanical property testing

The phase structure of the coating is analyzed using the BrukerD8 Advance X-ray diffractometer with Cu-Kα radiation in θ/2θ mode, operating at a step size of 0.02° and scanning range from 20° to 80°. The surface and cross-sectional morphology are observed using Zeiss Sigma 300 field emission scanning electron microscope, complemented by energy dispersive spectroscopy (EDS) for coating composition analysis. The hardness and elastic modulus of the coating are measured using a nanoindentation tester (Anton Paar NHT3) with a maximum load of 10mN and a holding time of 10 seconds. To minimize measurement errors, 16 indentations were pressed into each sample. After removing abnormal data, the average value is taken as the final result. The friction coefficient of the coating under room temperature was measured using a tribological testing instrument (MS-T3001) with a rotating ball of φ3mm stainless steel, operating at 200 rpm with a 2mm rotational radius and 2N load for 20 minutes.

2.3 Tap cutting test

The tap used in the test is M6×1 spiral pointed tap made of Suzhou YongPu, and the cutting material is 55C steel plate (200mm × 200mm × 15mm). The material hardness ranges from 210 to 230 HBW. The coated tap cutting tests are conducted on a Nokin-V855G machine, with processing parameters set at linear speed of 25 m/min, backcutting depth of 25 mm, and coolant application using emulsion. During machining, tools are mounted on Spike tool holders from Germany's Pro-micron GmbH, connected to a computer system for real-time measurement and output of torque variations during tap threading and thread formation. Kistner equipment is employed to photograph the wear surfaces after cutting.

3. Results

3.1 Crystal structure of TiCN coating

As shown in Figure 1, both TiN(T0) and TiCN(T1-T4) exhibit a face-centered cubic (FCC) structure. With the decrease in Ti target power, the carbon content gradually increases, causing a small-angle tilt in the 111 orientation (the growth direction code) of TiCN. According to Bragg's formula, the interplanar spacing progressively increases with rising carbon content. This phenomenon primarily occurs because carbon atoms replace nitrogen atoms in the TiN lattice, forming a substitutional solid solution. Since carbon atoms have a larger atomic radius than nitrogen atoms, this 

substitution results in angular distortion.The lattice distortion observed leads to increased interplanar spacing. As carbon content increases, more nitrogen atoms are replaced by carbon, causing continuous lattice distortion and gradual enlargement of the 111 orientation's small-angle shift. Notably, the 200 (structure growth direction code) orientation remains unchanged despite increasing carbon content, indicating that this peak primarily originates from the bottom AIP layer. When carbon atoms reach 32% or higher, the 111 peak exhibits significant broadening. This occurs because after carbon saturation and solid solution formation, excess carbon atoms predominantly exist as amorphous lubrication phases such as sp² (graphite-like) and sp³ (diamond-like) structures. These amorphous phases predominantly form at grain boundaries, inhibiting nanocrystalline growth and leading to continuous reduction of grain size. Consequently, X-ray diffraction (XRD) measurements reveal progressively broadened and weakened peaks.

3.2 Cross-sectional morphology of TiCN coating

The cross-section of TiN (T0) and TiCN (T1,.T3, T4) with different C contents of the coating in SEM is shown in Figure 2. Prepared under the same conditions, the TiN coating shows an obvious columnar crystal growth mode (shown in Figure 2a),and the columnar crystals exhibit slight tilting. Kuratani et al. Have reported that the tilt of columnar crystal structures is primarily related to the energy of bombardment particles. When ion energy is sufficiently high, it minimizes tilting or even achieves a completely non-tilting state. As shown in Figures 2b,2c, and 2d, when the C atom content reaches 10%, the cross-sectional columnar crystals become finer with enhanced coating density. However, when the C content exceeds 32%, columnar crystals completely disappear, and the crystalline grains further refine with improved density. This phenomenon further demonstrates that increased C content leads to an increase in amorphous phases, which inhibits nanocrystal growth, reduces grain size, and causes excessive C to exist in amorphous form, ultimately resulting in the disappearance of columnar crystals.

Figure 1. XRD spectra of TiCN coatings prepared at different powers

Figure 2. Cross-sectional morphology of TiCN coatings prepared at different powers under SEM

3.3 Mechanical properties of TiCN coating

The hardness of TiCN is significantly higher than that of TiN, and with the increase of C content, the hardness increases first and then decreases. When the proportion of C atoms reaches 10% (shown in position T1 in Figure 3), the hardness reaches the maximum value 40.7GPa.    This is primarily due to the substitution of N atoms in TiN by C atoms forming solid solutions, which enhances hardness through solid solution strengthening. As the solubility increases, the film's hardness continuously rises. The amorphous C concentrated at grain boundaries hinders nanocrystalline growth, resulting in fine-grained strengthening and ultimately a sharp increase in hardness. When Ti target power is further reduced, excess C atoms induce an amorphous phase transition in the coating, leading to a hardness decline as shown in Figure 3. This phenomenon aligns with the broadening of the 111 peak observed in XRD. Both H³/E² values of TICN are significantly higher than those of TiN, indicating enhanced plastic deformation resistance. This improvement mainly stems from stronger C-N and C-C bonds formed in the coating. Additionally, H³/E² serves as a key parameter for evaluating wear resistance, with higher values reflecting greater wear durability.

Figure 3. Hardness of TiCN coatings prepared at different powers (black line) and H³/E² (red line)

3.4 Friction and wear properties of TiCN coating

The friction coefficient of TiCN (shown in T1~T4 in Figure 4) is significantly lower than that of TiN (shown in T0 in Figure 4), mainly because there is an amorphous lubricating phase in the TiCN coating, which has a certain wear-reducing effect on the coating. With the increase of C and content, the friction coefficient of the coating decreases first and then increases. For high-C-content TiCN coatings, the amorphous phase primarily exists in sp² and sp³ structures. In sp² structures, C atoms facilitate the formation of transfer films during friction wear, providing solid lubrication effects on material surfaces to reduce interfacial friction and consequently lower surface friction coefficients. The C atoms in sp³ structures enhance coating hardness. When C atoms account for 45%, the friction coefficient shows a certain increase, mainly due to excess C atoms existing in amorphous form, which leads to reduced hardness. The introduction of C atoms significantly improves the coating's friction-wear performance and enhances its oxidation resistance. This phenomenon primarily results from reduced heat accumulation caused by decreased friction coefficients. Simultaneously, the addition of C atoms makes the coating more dense and eliminates columnar crystal structures.

Figure 4. Friction coefficients of TiCN coatings prepared at different powers

3.5 Tap cutting

Under identical machining conditions, the introduction of C atoms significantly reduces torque during cutting, primarily due to decreased friction coefficients. When machining 200 holes, the wear width of TiCN (T3) was notably smaller than that of TiN (TO), further demonstrating that TiCN exhibits superior frictional wear performance compared to TiN under equivalent conditions. This also indicates that enhancing coating hardness and reducing friction coefficients contribute to improved tap cutting performance, as shown in Figure 5.

a) Peak tap torque at 198-200 holes

b) The wear of flank face of the tap at the 200th hole

Figure 5 Torque and wear of tap under the same machining conditions

4. Conclusion

1) TiCN prepared by HiPIMS mainly presents the 111 orientation. With the increase of C content, the orientation gradually shifts to a small Angle, the peak appears wider and weaker, the grain size becomes smaller, and the columnar crystal of the coating disappears, and the coating becomes more dense.

2) The hardness of the TiCN coating first increases and then decreases with increasing C content, reaching its maximum value 40.7GPa when the C atom proportion reaches 10%. The friction coefficient shows a trend of decreasing initially and then increasing with rising C content. At 32% C atom proportion, the average friction coefficient during stable wear phase reaches its minimum at approximately 0.17.

3) The introduction of C atom significantly reduces the torsion force during tapping and significantly improves the cutting performance of tap.

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