Direct Inkjet Printing of Digitally Designed 2D TiN Patterns

Abstract

TiN is a non-oxidic ceramic widely employed as a hard coating material for cutting tools due to its high thermal and chemical stability. Among all 2D coating techniques, Inkjet printing (IJP) is one of the most promising for the fabrication of layers with customized designs. However, despite its advantages, this process has not been used so far with this material. In this work, we prepared TiN suspensions for their implementation in IJP with a nozzle of 70 μm. A complete study of the ink properties was performed to formulate a suitable ink with a high level of dispersion and to evaluate the jetting during the printing process. Moreover, after a sintering process at 1100 °C under vacuum, a complete hardness analysis of the coated discs was performed, resulting in values ranging from ~4 to 7 GPa, depending on the grid design.

1. Introduction

TiN is a refractory compound that presents a number of characteristics, such as its excellent mechanical properties, that make it an interesting material for use as a coating in different applications such as protective coatings [1], catalysis, medical implants [2,3,4], and superconductors [5]. TiN is also used as a hard coating material for cutting tools [6], as it has a high hardness and resistance to oxidation and it is a common component in special refractories [7,8] and cermets [9] due to its high thermal shock resistance and chemical durability.

Several physical and chemical methods, including evaporation, ion plating, chemical vapor deposition (CVD), sputtering, and electrophoretic deposition (EPD), can deposit thin TiN films. Mendoza et al. [10,11], Kavanlouel et al. [12] and Gonzalez et al. [13] used EPD to develop TiN coatings. Morton et al. used laser-beam to form microdetritic surface coatings of TiN [14]. Moreover, a laser combined with a spray method has been employed by Pang et al. to get hybrid deposition of TiN/TiC-Ni/Mo for solar applications [15]. CVD and cathodic arc evaporation (CAE) are other methods employed by Shen et al. and Meindlhumer et al. [16], respectively, for the fabrication of TiN layers to improve the mechanical behavior of the coated materials.

In the last decade, there has been a growing interest in using additive manufacturing (AM) techniques for the fabrication of ceramic materials [17,18,19]. These highly customizable technologies enable the fabrication of hierarchically structured bodies with complex shapes allowing the control over the architecture of the sample and microstructural features, such us total porosity and pore size distribution. Among all the rapid prototyping technologies, Inkjet printing (IJP) provides excellent possibilities for new design styles, short production runs, sustainable printing environments, quick response time, and customization [20,21,22,23,24]. Thus, IJP is an interesting option, enabling the extension of the range of properties, and therefore possible applications, of TiN coatings.

IJP is a deposition technique of fluids or complex fluid materials such as inks. This technology is based on the drop deposition by gravity of the material through the nozzle, and the consolidation of the printed pattern is done by solvent evaporation [25]. These drops are created by a pulse, which could be generated thermally or by the action of a piezoelectric material. IJP is widely used in the industry for its versatility and low cost of implementation to obtain laminated materials and cover surfaces with a two-dimensional pattern. This technique has been used in a wide range of applications such as printing text or graphics, solar cells, dense coatings of hard materials, and in the functionalization of surfaces from sol-gel. Moreover, it is one of the most precise direct-write techniques because of its availability and versatility. It is a quick processing technique and an inexpensive ecological tool which offers a low temperature process for microfabrication of layers and coatings in 2D.

In this rapid prototyping technique, two-dimensional coatings are built drop-by-drop via the ejection of the ink through the action of a piezoelectric contraction produced by an electric pulse (drop-on-demand). IJP is made up of nozzles that are able to control every droplet jetting. Depending on the nozzle, the shape and volume of the drop change. Besides, another of the most important factors affecting the quality of the designed patterns and the performance of the printing process is the use of an ink with the right rheological properties. To avoid droplet deformation, requirements such as wettability, contact angle, and surface tension on the substrates must be adjusted [18]. Inks are generally prepared in different ways, such as suspending nanoparticles in a carrier solvent. One of the strategies for accomplishing a good quality ceramic-ink is also employing a cosolvent (diethylene glycol is used in this work) [26,27,28]. The choice of the solvent and co-solvent is related with the printability of the ink that can be determined by the dimensionless parameter Z, which is the inverse of the Ohnesorge number (Oh) and relates the Reynolds number (Re) and the Weber number (We), and could be expressed as in equation 1 [29,30].

$$Z=O{h}^{-1}=\frac{Re}{{\left(We\right)}^{1/2}}= \frac{{\left(\gamma \rho a\right)}^{1/2}}{\eta }$$

(1)

where γ is the surface tension (N/m), ρ is the density (Kg/m³), a is the inner diameter of the nozzle (m), and η is the apparent viscosity (Pa·s). Thus, surface tension, viscosity, and density can be regarded as the key physical parameters to optimize the quality of jetting and deposition. An ink is considered printable when the Z parameter ranges between 1 to 14 [31,32,33,34].

According to our best knowledge, no work dealing with IJP of TiN has been reported so far. The objective of the present work focuses on the validation of IJP technology for the fabrication of 2D TiN patterns with high design resolution. These digitally designed TiN patterns will improve the resistance to aggressive environments and high temperatures of the coated substrates.

2. Materials and Methods

2.1. Materials

TiN used for the formulation of the inks was a commercial powder from Hefei Kaier Nanometer Energy and Technology (Hefei, China). These powders were characterized based on their specific surface area (SS), density (ρ), and particle size. The result of this characterization is summarized in

Table 1. Morphological characterization of the TiN powder.

SS (m2/g)	ρ (g/cm3)	Particle Size (nm)
44.42 ± 0.01	3.3 ± 0.1	29.1 ± 0.9

.

The suspension was prepared by dispersing the TiN powder in isopropanol with a solid content of 2 wt% using polyethyleneimine (PEI, Sigma Aldrich, Darmstadt, Germany) as dispersant (1.5 wt% of PEI) following the procedure described by Mendoza et al. [35], reaching a Zeta potential value of +70 mV. However, to improve the printing behavior of the ink, a dilution with diethylene glycol (DEG, Sigma Aldrich, Darmstadt, Germany) was carried out. The DEG is a common cosolvent used for ink formulation due to its specific physical properties (density, viscosity, and surface tension) [36,37,38,39].

The substrates used in this work were unpolished Ti discs 16 mm diameter and 3 mm height, obtained from the metallurgical processing of the powders and sintered at 1250 °C.

2.2. Characterization of the Ink Properties

The inks were characterized based on their physical properties. The zeta potential and size of the TiN particles in the dispersion media was measured with a Zetasizer Nano ZS (Malvern, UK). The viscosities were examined using a Haake Mars rheometer (Thermo Scientific, Waltham, MA, United States) with a double-cone plate fix of 60 mm of diameter and angle of 2° (DC60/2°) at 25 °C. The densities were determined using a micropipette and an analytical balance with a precision of 0.1 mg. Both the surface tension and contact angle were measured under ambient conditions using, a Drop Shape Analyzer—DSA30 Tensiometer (Krüss GmbH Wissenschaftliche Laborgeräte, Hamburg, Germany) in pendant drop configuration system.

2.3. Inkjet Printing

The IJP system used in this work is an XCEL system (Aurel automation, Modigliana FC, Italy) with an inkjet head MD-K-140 (Microdrop technologies, Norderstedt, Germany), which presents an inner nozzle diameter of 70 μm and allows to heat the nozzle until 100 °C. The conditions for the drop formation were settled using a printing station included in the same printing system featuring, a live camera with a stroboscopic light-emitting diode (LED) which allows the study of drop formation (Figure 1a). The drops were optimized to obtain a homogenous pattern (Figure 1b) by adjusting the voltage (230 V), frequency (1036 Hz), and the pulse of the piezoelectric (135 us). Negative air pressure (−20 mBar) at the nozzle was established to avoid dripping. Moreover, constant pass conditions with a separation of 0.01 mm between drops, and 30 °C of nozzle temperature, were selected. Figure 1a shows the spherical drops after optimizing the printing parameters. Low operating pressures provide uniform jetting without satellite drops. In Figure 1b, different defects were obtained due to the fact that the oscillation frequency and pulse were not adjusted, producing ink smudges (first, third, and fourth lines) and discontinuity of thickness (second line). Finally, a well-defined line—bottom line—was created by adjusting the frequency, the pulse voltage, and the speed movement of the printer head.

2.4. Characterization of the Samples

The printed samples were sintered at 1100 °C for one hour under vacuum (10–5 atm). The microstructures of the samples were examined using a XL-30 scanning electron microscope (Philips, Eindhoven, the Netherlands). The hardness of the sintered parts was measured using a nanoindenter (Zwick/Roell, Zhu2.5, Ulm, Germany) with a Berkovich diamond tip of 100 nm radius, recording the Young’s modulus and hardness as a function of the penetration depth. The analysis was performed by making nine 3 × 3 matrixes with 0.7 mm between them to perform indentations inside the printed lines. During the indentation test the maximum load was 500 mN and the depth limit was 2.4 μm.

3. Results and Discussion

3.1. TiN Inks Characterization

The starting TiN ink was prepared using isopropanol as solvent, and its characteristics are summarized in

Table 2. Characterization of the inks.

Solvent	Zeta Potential (mV)	Particle Size (nm)	Density (g/cm3)	Surface Tension (25 °C) (mN/m)	Viscosity (mPa·s)	Z
iPrOH	+70	29 ± 3	0.79	1.75 ± 0.01	3.1	3.91
iPrOH:DEG (3:1)	+70	29 ± 3	0.88	1.94 ± 0.03	3.3	4.00

, including the calculated Z parameter. This ink presents a zeta potential of +70 mV due to the PEI adsorption, thus the dispersion and stabilization of the particles against aggregation is manly done by electrical repulsion but also steric interactions, obtaining a stable and well dispersed suspension that will not flocculate even at the isoelectric point. Basically, the negatively charged surface of the TiN nanoparticles adsorbs the positively charged amine groups of the PEI, until fully covering the particle surface. The density, surface tension and viscosity of the ink were used for Z parameter calculation, resulting in a value of 3.91, inside of the printable range [29,39]. As can be observed in Figure 2a the printed patterns were poorly defined due to the spreading of the ink on the surface. Therefore, it was necessary to modify the characteristics of the suspension by diluting with DEG in a suspension:DEG ratio of 3:1. In this case the suspension was suitable for printing, obtaining a solid line as shown in Figure 2b. Characteristics of initial and employed inks are summarized in Table 2.

3.2. Characterization of the Printing

New diluted TiN ink was employed for printing on the Ti substrates. Two grids with different pattern were printed using the conditions described above, the distances between lines being 1 mm² (Figure 3a) and 0.5 mm² (Figure 3b). The total surface area coated by these grids were 16% and 31% respectively. In order to confirm the high precision of the printed patterns, electron microscopy images were taken. Figure 3c,d show the SEM micrographs of 1 mm² and 0.5 mm² grid, respectively.

Next, the printed samples were thermal threated at 1100 °C for 1 h in vacuum to sinter the TiN patterns. Figure 4 shows the surface microstructure of the 0.5 mm grid after sintering. The TiN coating partially diffuses into the sample, with only a few particles remaining on the surface, making it difficult to visualize the pattern after sintering. Similar diffusion was obtained for 1 mm grid sample.

After the thermal treatment, the mechanical properties of the patterns were characterized.

Table 3. Surface and mechanical properties of the uncoated Ti substrate and coated with both patterns.

	Contact Angle (Water 25 °C)	Roughness (μm)		Mechanical Properties
		Ra	Rq	Hardness (GPa)	Young’s Modulus (GPa)
Ti substrate	81.3° ± 0.1	1.19	2.3	3 ± 0.7	110 ± 20
TiN 1.0 mm grid	85° ± 2	1.02	1.98	4 ± 0.6	100 ± 10
TiN 0.5 mm grid	89° ± 9	1.22	2.28	5 ± 1	140 ± 20

summarizes the result of this characterization, including the uncoated Ti substrate.

Contact angle increasing with the TiN amount from the Ti substrate (81.3°) to the 0.5 mm grid (89°) is related to the nonpolar nature of the TiN. On the other hand, the roughness of the samples was studied based on the roughness average (Ra) and the root mean square roughness (Rq). Both the Ra and Rq show similar values for the uncoated Ti and the 0.5 mm grid but decrease for the 1 mm grid, which indicates that the 1 mm grid decreases the roughness of the sample while the 0.5 mm does not modify the roughness of the Ti substrate because it covers more surface. Table 3 shows the average values of both hardness and Young’s Modulus of the samples measured by microindentation. The average hardness increases with the TiN coating while the young modulus does not follow this trend. However, as samples have not been fully coated, there are also uncoated and coated areas. Thus, in order to understand the contribution of the printed patterns to the mechanical properties of the samples, the dispersion of the hardness values obtained was studied. Figure 5 shows the distribution of the hardness (black circles) and Young’s Modulus (white triangles) as a function of the indentation depth for the sample coated with the 1 mm grid (Figure 5a) and the 0.5 mm grid (Figure 5b).

In view of the data collected, both grids showed a similar distribution of hardness and Young’s modulus points, the 0.5 grid (Figure 5b) being slightly higher. In both graphs, the distribution of points shows clear differences in the mechanical compression from uncoated areas (3.0–3.5 GPa) to coated ones (5.0–5.8 GPa). In addition, the micrograph in Figure 4 shows areas where after sintering the coating disappears after sintering, this is because part of the TiN enters into solid solution with the Ti of the substrate, which also causes a hardness gradient and in the point spread in the plots of Figure 5 two distinct zones cannot be easily detected. However, the mechanical hardness properties of both coatings were estimated, resulting in a hardness value of 4 GPa and Young’s Modulus of 120 GPa for the 1 mm grid and a hardness value 5 GPa and Young’s Modulus of 160 GPa for the 0.5 mm. This increment in the mechanical properties between both grids has been attributed to the fact that there is a higher amount of coated surface in the 0.5 mm grid. The larger the coated surface, the higher the hardness values obtained.

4. Conclusions

IJP technique allows us to design complex 2D patterns coatings with high reproducibility. The printability features depend on the machine and nozzle. Setting up the suspension according to the requirement of the printing head is necessary to get homogeneous coatings. Two grid designs were characterized, 1 mm² and 0.5 mm² grid, with a coated surface area of 16% and 31%, respectively. The TiN coating diffuses into the sintered Ti disc, blurring the drawn pattern on the surface. However, the superficial properties of the sample (wetting angle, roughness, etc.) changed depending on the printed design. In the sequential microindentation tests of both pattern designs, 1 mm² and 0.5 mm², the average hardness of the sample increased by 36% and 78%, respectively. Moreover, in a more detailed hardness study of the surface, three different regions with diverse hardness values were obtained: uncoated regions, the naked Ti surface between the TiN patterns, showed values around ≈3 GPa. The coated region with TiN lines showed values higher than 5 GPa, and the intermediate zones, where the diffusion of the pattern took place, presented hardness values from 3 to 5 GPa.