Wetting of Cu-SiC Composite Material Modified by Nanosecond Laser Radiation and Liquid Spreading over It

Abstract

In the framework of this work, the surface properties of Cu-SiC composite material were studied when spreading micro- and nanoliter liquids. The Cu-SiC samples with a SiC content of 5 to 20 wt.% were fabricated by spark plasma sintering at temperatures from 700 to 850 °C. The Cu-SiC surfaces were processed by two different methods: using abrasive materials and nanosecond laser radiation. Surface analysis was performed by scanning electron microscopy, profilometry, energy dispersive spectroscopy and Vickers methods. The surface properties (wetting and dynamic characteristics of spreading) were studied using a shadow optical technique when interacting the Cu-SiC surfaces with water (up to 10 μL). It was proved that the recorded deterioration of the wettability properties of Cu-SiC surfaces processed by abrasive materials with an increase in their sintering temperature and the reason for the spontaneous hydrophobization of the Cu-SiC composite materials modified by nanosecond laser radiation, are due to the adsorption of airborne hydrocarbon contaminants, similar to the known wetting inversion of metal surfaces. It was established that the wetting properties of materials prior to modification by laser radiation do not affect the intensity, duration of stages, and steady-state values of contact angles upon wetting inversion of Cu-SiC composite materials. It was also found that the processing of Cu-SiC surfaces by laser radiation makes it possible to change the dynamic characteristics of the liquid spreading (at a flow rate of 5 μL/min, the liquid front speed is more than three times, and the dynamic contact angles are in the range of 30°).

1. Introduction

Copper reinforced with silicon carbide is widely used in various industries because of its thermophysical (low thermal expansion coefficient and high thermal conductivity) and mechanical properties (high specific strength and wear resistance) [1]. The combination of such unique properties of Cu and SiC determines the application of Cu-SiC composite material in the production of electronic circuits, power break electric contact, high-power electrical network switches and vacuum arc-quenching chambers [2,3]. The most promising direction of the use of Cu-SiC in practice is the high-tech sector of the micro- and nano-industry. However, the widespread use of Cu-SiC as a structural material in the production of microfluidic devices, which are used for separating and mixing micro- and nanoliter liquids, systems for ensuring the temperature regime in microelectronic devices, and 5G communication systems is unfeasible without studying its surface properties and developing ways of controlling small and ultra-small coolant flows on its surface.

The processing of technological surfaces with nanosecond laser radiation is a relatively new, science-based, technically simple and non-contact method for changing the surface properties of materials. It is applicable to parts of complex geometric shapes on an industrial scale [4]. Laser processing of materials allows changing the electrical properties, e.g., imparting conductive properties to a non-conductive material (diamond) [5], as well as modifying thermal properties, e.g., increasing thermal insulation performance of a coating [6]. In addition, it allows creating a given type of texture, roughness and radically changing the elemental composition of the near-surface layer [7]. This makes it possible to control (intensify or suppress) heat transfer, mass transfer, and chemical reaction occurring on technological surfaces modified by laser radiation. In this case, a key factor is surface wetting, which can be changed by laser processing of surfaces to extreme states (superhydrophilicity/superhydrophobicity) [8]. Nowadays, it is well known that it is possible to significantly increase the resistance to corrosion [9], biofouling [10], organic contaminations [11], abrasive and cavitation wear [12], icing [13], and change the reflective ability of electromagnetic radiation [14] by processing surfaces with laser radiation.

By setting the laser radiation parameters when processing the surface of a material, taking into account its optical and thermal properties, it is possible to form texture elements of various geometric shapes and sizes (from micro- to nano-scale) and supplement the main elements of texture with smaller-scale aggregates. Based on analysis of well-known works, it has been found that using laser radiation of different durations, including pico- [15], femto- [16,17] and nanosecond [18,19], when processing the surface of the same material, in particular steel, it is possible to achieve similar specified extreme wettability properties. Immediately after laser processing, metal surfaces are superhydrophilic [20]. Wetting improvement after processing is explained by the oxide film formation [21]. Oxides are polar compounds; the surface energy of metals and their alloys increases after laser processing, therefore, water droplet spreads into a thin film on such surfaces. The inversion of wetting from superhydrophilicity to hydrophobicity of surfaces modified by laser radiation happens during storage under atmospheric conditions. Such changing of the surface wetting properties of metals is known as spontaneous hydrophobization [22]. The deterioration of wetting properties over time is due to the fact that hydrocarbon compounds from the environment are adsorbed on the surface [22,23]. These are non-polar compounds, so the surface energy of a metal or alloy textured by laser radiation decreases and, as a result, the wetting properties deteriorate (the contact angle increases). After laser texturing, wetting properties become steady, as a rule, after 10 to 120 days [16,24,25,26,27,28]. The scale of the formed textures depends on the duration of the laser radiation. Femto- and pico-second radiation, compared to nanosecond radiation, is characterized by a smaller depth of thermal diffusion into the material and a smaller heat-affected zone [4].

There are studies of ceramic materials after texturing their surfaces with laser radiation. In [29], a change in the wetting properties of SiC ceramic materials was recorded after laser processing similar to metals: superhydrophilicity showed immediately after processing, followed by an increase in the angle with time up to 127°. Another result was obtained when forming a micro-square-convexed texture on SiC [30]. Here, the surface immediately after texturing showed hydrophobicity with a static contact angle (SCA) of 119°.

To the best of our knowledge, no studies of the near-surface properties of composite matrix materials (combination of ceramic and metal) after laser texturing have been conducted. In addition, the mechanism of liquid spreading over surfaces modified by laser radiation has not been studied at a level sufficient to predict the main spreading characteristics. In the literature, there are models to describe the spreading of liquid droplets and the liquid meniscus movement, such as the molecular-kinetic model of Blake and Haynes [31], the hydrodynamic model of Cox and Voinov [32] and the combined model of Petrov and Shikhmurzaev [33,34], as well as recently developed approaches by McHale, Newton and Shirtcliffe [35], and Johansson and Hess [36]. However, experimental studies of spreading over textured surfaces by laser radiation indicate a complex nature of spreading, which depends on the texture, wetting, flow velocity, and liquid properties [20,37,38,39]

The presented work is aimed at studying the effect of processing using abrasive materials and nanosecond laser radiation on the surface properties of Cu-SiC composite material, as well as evaluating the possibility of controlling the dynamic characteristics of micro- and nanoliter liquid spreading over Cu-SiC surfaces.

2. Experimental Procedure

2.1. Materials and Processing Methods

The samples of the Cu-SiC composite material were made from commercial copper powders (nanopowder, 98%, 6.8 m²/g, OCHV, Moscow, Russia) and silicon carbide (abrasive micropowder F1200, 96%, LitPromAbrasive, Moscow, Russia) by spark plasma sintering. The powders were mixed in a ratio of 5, 10, 15, 20 wt.% SiC in isopropyl alcohol using an ultrasonic bath in order to achieve a uniform distribution of the components. Further, the resulting dispersed mixtures were placed in Petri dishes and dried for 120 h under laboratory conditions (at a temperature of 22–24 °C, relative humidity of 45–55%, atmospheric pressure). The portions of prepared mixtures weighing 4 g were used to fabricate bulk composite samples according to a well-tested procedure [40] by spark plasma sintering on an SPS 10-4 unit (GT Advances Technologies, Hudson, NH, USA). Simultaneous pressing and sintering were conducted in vacuum in graphite molds using graphite punches with a diameter of 12.7 mm at a pressure of 60 MPa. The initial powder sample was heated to a temperature of T_SPS = 700–850 °C at a rate of 100 °C/min with a unipolar current up to 5 kA. After holding at T_SPS for 10 min, the current supply was stopped and the samples were cooled to a temperature of 25 °C under a pressure of 60 MPa. The duration of the manufacturing process of a bulk ceramic sample did not exceed 30 min. The component composition, sintering conditions and characteristics of the fabricated bulk Cu-SiC samples are presented in

Table 1. Composition, sintering temperature and hardness of final bulk Cu-SiC samples.

Sample No.	Precursor		TSPS, °C	Hardness, HV
	Cu, wt.%	SiC, wt.%
1	100	0	850	90 ± 2
2	90	10	800	124 ± 3
3	90	10	750	110 ± 3
4	90	10	700	107 ± 3
5	90	10	850	140 ± 3
6	95	5	850	138 ± 3
7	85	15	850	137 ± 3
8	80	20	850	141 ± 3
9 *	0	100	1600	-

* The values of SiC parameters were taken from [41].

.

The reference sample No. 1 (copper) was sintered at a temperature of 850 °C. This was chosen based on specially conducted experiments, which showed that at a temperature below 850 °C, the density of the sintered sample significantly decreases. However, at temperatures above 850 °C, uncontrolled melting processes are realized.

Using a Forcipol 1 V grinding and polishing machine (Metkon, Bursa, Turkey), the surface of the fabricated samples was mechanically processed by abrasive paper containing silicon carbide particles in order of decreasing grain size 320, 600, 800 and 1200 mesh. The modification procedure by laser radiation was conducted on preliminary polished Cu-SiC surfaces under laboratory conditions (ambient air, atmospheric pressure, temperature 20–22 °C, relative humidity 45–55%), using an ytterbium pulsed fiber laser (IPG Photonics, Moscow, Russia) as part of the TurboMarker-V50A4 RA system (Laser Center, Saint-Petersburg, Russia) with a wavelength of 1064 nm. An anisotropic texture with a multimodal developed roughness was designed according to a well-tested graphic-analytical technique [41] based on known geometrical shape and dimensions of the texture element (ablation crater). The selected type of texture makes it possible to change the wettability properties to extreme states (superhydrophilicity/superhydrophobicity) [4]. It also makes it possible to intensify the chemical reaction [42], heat and mass transfer [7] and phase transformations [43]. Special experiments were conducted to determine the geometric shape and dimensions of texture elements formed under the action of a single laser pulse on polished Cu-SiC surfaces. Based on an analysis of ablation craters (Figure 1), the following parameters of laser radiation were taken to form an anisotropic texture with a multimodal developed roughness: output power N = 50 W, pulse duration τ = 120 ns and repetition rate f = 20 kHz. A pulse energy of 0.92 mJ in the TEM₀₀ mode was focused on the surfaces into a spot 40 μm in diameter. Under such parameters of laser radiation, the diameter of the ablation crater on the fabricated Cu-SiC surfaces is 62 µm ± 3 µm. The error in determining the diameter of the ablation crater did not exceed 4%.

In addition to parameters of laser radiation, the beam speed along the surface (V) and the number of beam passes per 1 mm (n) were determined to obtain a given texture. These parameters are controlled using a two-axis galvanometric scanner based on the Cambridge Technology VM2500+ drive included in the laser system. The laser beam travel speed V = 118 m/s and the number of beam passes n = 170 mm⁻¹ were determined from the conditions of 90.5% overlap of ablation craters:

$$V=d\cdot f\cdot x,$$

$$n=\frac{1}{d\cdot x},$$

where d is the ablation crater diameter, µm; f is the repetition rate of laser radiation, Hz; x is the relative distance between the centers of ablation craters determined as $x=1-\%\mathrm{of}\mathrm{overlap}/100\%$. Assuming a 90.5% overlap, the relative distance between the centers of ablation craters is x = 0.095.

Reference sample No. 1 (copper) was not subjected to the laser modification procedure due to the fact that copper has high light reflectivity (more than 95%) and low absorption (less than 5%) of infrared laser radiation (1.064 µm).

After laser modification, the Ci-SiC samples were cleaned in an ultrasonic bath for 10 minutes successively in isopropyl alcohol and distilled water. The Cu-SiC samples were stored under laboratory conditions.

2.2. Microtexture Analysis and Chemical Composition Study

Using a Tescan Vega 3 SBU scanning electron microscope 150 (Tescan, Brno, Czech Republic) with an OXFORD X-Max 50 energy dispersive adapter (Oxford Instruments, High Wycombe, UK), operated at 10–20 kV accelerating voltage, specimen current of 3–12 nA, and a spot diameter 152 of ~2 µm, the microtexture and elemental composition of the samples were analyzed. A Micro Measure 3D station profilometer (STIL, Aix-en-Provence, France) was used to study surface roughness according to the well-tested procedure [44] based on the three-dimensional roughness parameters: Sa (average roughness); Sz (maximum height of asperities); Sdr (increment of the surface area). These parameters were found to have a dominant influence on the liquid spreading [20].

Hardness was measured using a Pruftechnik KB 30 S microhardness tester (Pruftechnik, Munich, Germany) according to the Vickers method with an indentation time of 15 s and a load of 200 g.

2.3. Wetting and Spreading Experiments

Figure 2 presents a model of an experimental setup for determining wetting properties and studying the liquid movement over the surfaces of composite materials.

The light beam from the source MI-150 (Edmund Optics, Barrington, IL, USA) 1 passes through the fiber optic cable glass fiber cable BX4 (Dolan-Jenner, Boxborough, USA) 2, entering the telecentric tube 52 mm telecentric background illuminator 62-760 (Edmund Optics, Barrington, IL, USA) 3, where it is converted to plane-parallel light. Then it illuminates a droplet 4 sitting on the surface of a sample 5. The shadow image of the droplet was obtained by a high-speed camera Fast Video 500 M (Fast Video, Moscow, Russia) 6. The geometrical dimensions of the droplet were determined from shadow images using the LB-ADSA [45] and tangential 1 [46] goniometry methods. When determining wetting properties (static contact angles), a 5-μL droplet of distilled water was dosed onto the surface with a Lenpipet Stepper single-channel electronic dispenser (ThermoScientific, Russia) 7. In order to evaluate the wetting inversion (spontaneous hydrophobization), static contact angles were measured on laser-modified Cu-SiC surfaces at least once every 13 days until the wetting properties stabilized. Static contact angles were determined as the arithmetic mean of the values obtained from five measurements in randomly selected areas. The error in determining the contact angles did not exceed 5%.

Liquid spreading with a volume not exceeding 10 μL over the Cu-SiC surfaces consisted in determining the dynamic contact angles (DCA), the speed of the liquid front, and the contact angle hysteresis (CAH). In the experiments, a liquid supply system was used, consisting of a Cole Parmer 8 high-precision syringe pump (Cole Parmer, Vernon Hills, IL, USA), a tube 9, and a hollow tip 10. The pump operated in sequentially changing modes of supplying and pumping out liquid. When distilled water was added to a droplet formed on the surface under study, an advancing contact line was formed; when the liquid was pumped out, a receding line was formed. During the measurement of dynamic contact angles, the volume of liquid delivered by the pump (10 μL) remained constant. Two flow rates have been selected. The flow rate of 5 µL/min was chosen based on recommendations [47]: quasi-static advancing and receding angles should be measured at very low fluid flow rates/contact line speeds to avoid dynamic effects. The value of 100 µL/min was chosen to compare the obtained values of CAH and DCA.

Spreading, from the moment the liquid was formed on the surface to the complete pumping out of the liquid, was recorded by a high-speed video camera. After the experiments, the video files were storyboarded into separate photographic images using Fast Video Lab software (Fast Video, Russia). The moment of the droplet formation was taken as the beginning of the experiment. The resulting shadow images were processed by the LB-ADSA [45] and tangential 1 [46] goniometry methods. Liquid spreading over laser-modified Cu-SiC surfaces was studied during the period of wetting inversion from hydrophobic to wetting stabilization. For 19 days after laser processing, an unstable formation of a droplet in the form of a spherical segment was recorded on hydrophilic modified Cu-SiC surfaces, which did not allow one to determine the spreading characteristics with sufficient accuracy. The stabilization of wetting properties was detected according to the recommendations [37], which show that the contact angle hysteresis should be constant over time when the wetting properties are stabilized. Each experiment was repeated 3–5 times according to a well-tested procedure [20] under identical conditions. The contact angle hysteresis was determined by experimentally obtained advancing θ_0A and receding θ_0R quasi-static contact angles measured at zero liquid front speed [47]. It is known [48] that the SCA should be in the range between θ_0A and θ_0R. The error in determining the DCAs, CAH, and the liquid front speed did not exceed 5%.

3. Results and Discussion

3.1. Analysis of Surface Microtexture and Roughness

Table 2. SEM images of surface and elemental composition of the near-surface layer of Cu-SiC material.

Mechanical Processing					Laser Processing
Sample No. 1
					This sample was not processed by laser radiation due to its high light reflectivity (more than 95%) and low absorption (less than 5%) of infrared laser radiation (1.064 µm)
Region	wt.%				-
	C	O	Si	Cu
1	19.6	1.8	0.0	78.6
2	21.5	1.5	0.0	77.0
3	21.8	1.6	0.0	76.6
Sample No. 2
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
4	37.4	0.0	57.7	4.9	25	8.8	11.4	6.3	73.5
5	4.0	2.0	0.8	93.2	26	8.6	11.2	6.3	73.9
6	22.2	2.2	15.6	60.0	27	8.7	11.0	6.4	73.9
Sample No. 3
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
7	36.2	0.0	58.7	5.1	28	8.1	11.3	5.6	75.0
8	5.2	1.3	0.6	92.9	29	8.2	11.0	6.0	74.8
9	22.4	1.7	16.9	59.0	30	8.1	11.9	5.0	75.0
Sample No. 4
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
10	34.8	0.0	59.4	5.8	31	9.8	8.2	4.4	77.6
11	4.6	0.8	0.8	93.8	32	9.0	9.5	4.8	76.7
12	21.9	1.3	17.0	59.8	33	9.1	9.1	4.9	76.9
Sample No. 5
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
13	34.5	0.0	59.8	5.7	34	13.7	10.5	5.8	70.0
14	4.2	0.8	0.8	94.2	35	12.2	12.1	5.2	70.5
15	22.5	1.2	16.2	60.1	36	13.1	10.1	5.1	71.7
Sample No. 6
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
16	34.9	0.0	59.1	6.0	37	11.1	10.3	9.1	69.5
17	2.1	1.5	0.6	95.8	38	13.2	9.2	7.2	70.4
18	17.5	1.8	12.1	68.6	39	12.7	10.4	7.7	69.2
Sample No. 7
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
19	33.9	0.0	60.3	5.8	40	6.0	12.1	8.2	73.7
20	6.3	0.4	0.9	92.4	41	8.2	10.1	7.3	74.4
21	21.7	0.9	18.5	58.9	42	8.0	10.5	8.8	72.7
Sample No. 8
Region	wt.%				Region	wt.%
	C	O	Si	Cu		C	O	Si	Cu
22	35.0	0.0	61.8	3.2	43	7.9	9.2	9.1	73.8
23	7.5	0.6	1.4	90.5	44	10.3	9.3	7.9	72.5
24	15.2	1.3	26.9	56.6	45	8.1	9.8	8.6	73.5

presents SEM images of the surfaces of composite materials processed by abrasive paper 1200 mesh and laser radiation. In addition, the results of EDX analysis of these surfaces are summarized in Table 2. It should be noted that energy dispersive spectroscopy determines the elemental composition not on the surface, but the near-surface layer at a thickness of up to several tens of micrometers. The content of light elements (carbon, oxygen) was detected with a fairly large error [48]. However, all the studied samples were modified by laser radiation and studied by the energy dispersive spectroscopy under identical conditions, which makes it possible to compare the elemental composition of samples No. 2–8 qualitatively. In addition, it is known that with similar characteristics of laser radiation, the layer is modified at a thickness of up to hundreds of micrometers [4], which is much greater than the thickness of the elemental composition measurements by EDX.

3.1.1. Analysis of Cu-SiC Surfaces Processed by Abrasive Materials

Based on the analysis of SEM images (Table 2), it can be seen that mechanical processing forms chaotically arranged microchannels (grooves) on the copper surface (sample No. 1). The following values of roughness parameters were obtained for Sample No. 1: Sa = 0.3 µm, Sz = 3.4 µm; Sdr = 1.9%. The obtained values agree well with previously determined parameters for M1 copper processed by abrasive paper with 1200 mesh (Cu 99.9, the remainder Fe, Ni, S, As, Pb, Zn, O, Sb, Bi, Sn) [44]. Table 2 shows C and O in the surface layer of sample No. 1, in addition to the dominant element Cu (76–78 wt.%). The presence of oxygen (1–2 wt.%) is due to the oxidation process, which is realized during machining with abrasive materials [49]. The presence of carbon (19–22 wt.%), is due to the uncontrolled adsorption of non-polar molecules of saturated hydrocarbons from the environment [50] under the action of polarization of inactive hydrocarbon molecules by the electric field of the metal surface [51].

The SEM images show that samples No. 2–8 processed by abrasive materials (Table 2) have no scratches in comparison with sample No. 1. The absence of defects had the effect of reducing the roughness of samples No. 2–8 compared to sample No. 1. The surfaces of samples No. 2–8 were polished mechanically under identical conditions; the difference in the initial component composition (SiC content varied from 5 to 20 wt.%) had no significant effect on the formation of texture (roughness). Therefore, the roughness of samples No. 2–8 is almost identical: Sa = 0.1–0.2 µm, Sz = 1.8–2.4 µm; Sdr = 1.3–1.4%. The difference in the values of the roughness parameters is comparable with the measurement error. The absence of defects in the form scratches on samples No. 2–8 and the decrease in roughness compared to sample No. 1 is explained by the higher hardness of samples No. 2–8 (Table 1). The wear mechanism and the effect of material hardness on texture formation under the conditions of metal and ceramic surface treatment with abrasive materials have been studied in [41,44].

Local sites predominantly consisting of Cu and SiC were observed on the surfaces of composite samples No. 2–8 and processed mechanically. The sites designated as 5, 8, 11, 14, 17, 20, and 23 in Table 2 consist mainly of copper (90–96 wt.%) with an insignificant content of C (2–8 wt.%), Si (up to 2 wt.%), and O (up to 2 wt.%). In addition, the local sites 4, 7, 10, 13, 16, 19, and 22 (Table 2) consist mainly of Si (57–62.0 wt.%) and C (34–37 wt.%) with low copper content up to 6 wt.%. It should be noted that the oxygen content was not recorded in these local sites. Despite the fact that the energy dispersive spectroscopy is rather imprecise for determining the content of C and O, it can be concluded at a qualitative level that the elemental composition of the surfaces of Cu-SiC composite materials No. 2–8 is inhomogeneous after mechanical processing. For this reason, the practical application of composite materials after mechanical processing as a structural material in the production of microfluidic devices, systems for separating and mixing micro- and nanoliter liquids, is associated with a significant disadvantage. The disadvantage is that the functioning of microfluidic devices, systems for separating and mixing micro- and nanoliter liquids, is based on the controlling the liquid movement (e.g., three-phase contact line speed, dynamic contact angles) [52]. It is extremely difficult, and sometimes impossible, to control such characteristics over a surface with a heterogeneous elemental composition. The wetting properties depend on the elemental composition, and the wetting determines the three-phase contact line (TPCL) movement and the dynamic contact angles [53,54]. On the surface of mechanically processed Cu-SiC composite materials, local sites containing mainly either Si and C or Cu will randomly replace each other along the trajectory of TPCL movement. The surfaces of SiC and Cu have different wetting properties. It is concluded that under conditions of liquid spreading over mechanically processed Cu-SiC surfaces with a heterogeneous elemental composition, it is impossible to control the spreading characteristics.

3.1.2. Analysis of Cu-SiC Surfaces Processed by Laser Radiation

The SEM images of laser-processed samples No. 2–8 show that a multimodal texture with developed roughness was formed on the surfaces due to melting, ablation, and crystallization of the material. The roughness parameters of the surfaces of samples No. 2–8 are Sa = 4.1–4.3 µm, Sz = 26.4–26.8 µm, Sdr = 19.0–20.0%. As a result of laser processing, the elemental composition of the near-surface layer of all studied samples became homogeneous (Table 2). It should be added that no obvious changes were recorded in the content of Si and C on the surfaces containing SiC precursors from 5 to 20 wt.% in the initial composition of the Cu-SiC bulk product. Therefore, laser modification of Cu-SiC composite materials makes it possible to form both a developed and multilevel roughness on the surface, as well as a near-surface layer with a uniform elemental composition.

3.2. Analysis of Wetting Properties

3.2.1. Wetting Analysis of Cu-SiC Surfaces Processed by Abrasive Materials

In the studies conducted during the manufacture of the bulk Cu-SiC composite materials, two main factors changed (Table 1) with other conditions being equal: (1) sintering temperature (from 700 °C to 850 °C) with the composition of the materials unchanged; (2) the content of SiC (from 5 to 20 wt.%) at a constant sintering temperature of 850 °C. Figure 3 presents water static contact angles on the surfaces of Cu-SiC grouped according to the above variation factors.

It can be seen from Figure 3a that samples sintered at higher temperatures demonstrate higher static contact angles. The wetting properties are known to depend on the texture (roughness) and elemental composition of the surface [55]. According to the results of the texture (roughness) analysis conducted in Section 3.1, the values of the roughness parameters of samples No. 2–8 processed by abrasive materials were almost equal to each other. Therefore, the roughness does not significantly contribute to the difference in wetting properties of samples No. 2–8 (Figure 3a,b). In order to estimate wetting properties, in addition to the static contact angle, it is necessary to analyze the contact angle hysteresis or the roll-off angle [56]. In this study, the value of obtained contact angle hysteresis is presented in Figure 3 as the confidence interval for static contact angle. It can be seen from Figure 3a that the difference in the values of the static contact angles of samples No. 2–5 are close to the values of the contact angle hysteresis, which is more than 10°. This indicates that the deterioration of wetting properties (an increase in the contact angle) recorded on surfaces fabricated at high sintering temperatures is insignificant. For example, the minimum difference between the largest value of the static contact angle on sample No. 5 and the smallest value on sample No. 4 (Figure 3a), taking into account the hysteresis, is:

Δθ_min = (θ_{No. 5} − H_{No. 5}/2) − (θ_{No. 4} +H_{No. 4}/2) =

     = (88.9 − 14.4/2) − (75.2 + 11.2/2) = 0.9°

where θ_{No. 4}, θ_{No. 5} are static contact angles on the surfaces of samples No. 4 and 5°; and H_{No. 4}, H_{No. 5} are contact angle hysteresis on the surfaces of samples No. 4 and 5°.

As the surface roughness of samples No. 2–8 are almost identical, the contact angle hysteresis of more than 10° on the Cu-SiC surfaces processed by the abrasive materials is connected with the chemical heterogeneity (chaotically arranged local sites of Si and C or Cu). Under the experimental conditions, the recorded effect of the sintering temperature of composite materials on the degradation of their wetting properties (Figure 3a), as well as in the case of hysteresis, could be mainly due to the difference in the elemental composition of surfaces No. 2–5. The higher the sintering temperature of composite materials, the more intense the surface oxidation. With oxidation, the surface energy increases, so the wetting properties should improve (the contact angle decreases at higher sintering temperatures). However, in the conducted experiments, the surface wetting properties deteriorate with an increase in the sintering temperature. After sintering and mechanical processing with abrasive materials, the samples were stored under ambient conditions. Therefore, the recorded degradation of wetting properties can be explained by the adsorption of airborne hydrocarbon contaminants, in comparison with the spontaneous hydrophobization of surfaces modified by laser radiation [4]. The high sintering temperature of composite materials contributes to the subsequent intensification of the adsorption and deposition of hydrocarbon contaminants. To prove it, special experiments were conducted. Samples No. 2–5 were placed in methyl alcohol, which is a solvent for organic contaminants, for 1 h. The samples were then dried under laboratory conditions for 3 h. After cleaning, the static contact angles on the Cu-SiC surfaces decreased to θ_{No. 4} ≈ 73°, θ_{No. 3} ≈ 78°, θ_{No. 2} ≈ 80°, θ_{No. 5} ≈ 84°.The contact angle decreased more on surfaces produced at higher sintering temperatures. The contact angle hysteresis also slightly decreased by 2–3%. The decrease in the contact angle and hysteresis is due to the removal of hydrocarbon contaminants.

It can be seen from Figure 3b that there is no clear relationship between the surface wetting properties and the change in the SiC content in the Cu-SiC composition (samples No. 5–8) at the sintering stage. It should be noted that some of the conclusions made for the group of samples No. 2–5 (Figure 3a) are also valid for the group of samples No. 5–8 (Figure 3b): the difference in the wetting properties of surfaces No. 5–8 and contact angle hysteresis (more than 10°) are mainly due to the inhomogeneous elemental composition of the surfaces.

It should be noted that the difference in the contact angles of Cu-SiC composite materials in Figure 3b, taking into account the hysteresis, is more than 18°:

Δθ_min = (θ_{No. 5} − Δθ_{No. 5}/2) − (θ_{No. 8} + Δθ_{No. 8}/2) =

= (88.9 − 14.4/2) − (56.4 + 13.2/2) = 18.7°.

In addition, Figure 3b presents the SCA and the CAH measured on copper sample No. 1 (without SiC) and SiC No. 9 (without copper) surfaces from [41]. It can be seen that the change in the content of SiC from 5 to 20 wt.% in the composition of the initial powder makes it possible to obtain a Cu-SiC composite material with a wide range of wetting properties (from 57 to 90 degrees). Moreover, the wetting properties of Cu-SiC composite materials differ significantly from the properties of materials made from Cu (No. 1) and SiC (No. 9) components. In the same way as the group of samples No. 5–8 (Figure 3a), samples No. 2–5 (Figure 3b) were cleaned in a methanol medium. After cleaning, the wetting properties of samples No. 2–5 improved. The surfaces exhibited angles: θ_{No. 5} ≈ 84°, θ_{No. 6} ≈ 60°, θ_{No. 7} ≈ 71°, θ_{No. 8} ≈ 54°. The contact angle hysteresis also decreased by 2–3%. This is due to the removal of adsorbed hydrocarbons. After removal of the hydrocarbons, the difference in wetting properties of samples No. 2–5 was preserved and exceeded the contact angle hysteresis value. Most likely, the implicit relationship between the surface wetting properties and the change in the SiC content in the composition at the sintering stage is due to the following reasons: (1) uneven distribution of SiC grains in the bulk and over the surface of the Cu-SiC composite material; (2) uncontrolled processes of formation of the Cu melt during sintering of the bulk product, as a result of which SiC grains are immersed in the copper melt at the sintering stage.

3.2.2. Wetting Analysis of Cu-SiC Surfaces Processed by Laser Radiation

Figure 4 shows the wetting inversion over time of Cu-SiC surfaces modified by laser radiation.

Immediately after laser modification, samples No. 2–8 demonstrated superhydrophilic properties; water droplets spread into a thin film. When Cu-SiC samples were stored under laboratory conditions, the wetting properties deteriorated (contact angle increased). After 19 days from the moment of surface modification, samples No. 4–6 had hydrophobic properties (θ > 90°); samples No. 2 and No. 3—after 26 days; sample No. 8—42 days; and No. 7—60 days.

Stabilization of the wetting properties of samples No. 2–6 was registered after 53 days, No. 7 and 8 after 73 days. After stabilization, the contact angle hysteresis did not exceed 10°, after which it was constant over time. This is in good agreement with the recommendations [37] for determining the stabilization time of surface properties during “spontaneous hydrophobization” of metal surfaces. It should be noted that among the studied group of samples modified by laser radiation, the surfaces of samples No. 2–6 have the worst wetting properties (θ ≈ 135–150°), and the surface of sample No. 7 (θ ≈ 98°) is the best under conditions of contact angle stabilization (Figure 4). Based on the analysis of the typical time intervals of the wetting inversion (achievement of superhydrophilicity, hydrophobicity, stabilization of wetting properties), it was concluded that the inversion mechanism and, accordingly, the reason for the “spontaneous hydrophobization” of the surfaces of Cu-SiC composite materials modified by nanosecond laser radiation, are similar to the studied mechanism and cause of wetting inversion of metal surfaces [16,22,23,24,25,26,27,28]. Consequently, the wetting inversion of the surfaces of Cu-SiC composite materials modified by nanosecond laser radiation is due to the adsorption of hydrocarbon compounds from the environment. It was also concluded that SiC precursors with a content of 5 to 20 wt.% in the composition of the initial powder, as well as the sintering temperature (from 700 to 850 °C) in the manufacture of bulk material, do not affect the values of the steady-state contact angles due to wetting inversion of the surfaces of this material modified by laser radiation.

The change in the static contact angle under conditions of the wetting inversion of Cu-SiC composite materials during their storage in laboratory conditions can be described by an exponential dependence, similar to metals [16]:

θ = θ_steady·(1 − e^−t/a),

where θ_steady is the steady-state contact angle on the surface modified by laser radiation, °; a is a constant characterizing the time during which the contact angle exceeds 50% of the steady-state value, days; t—stabilization time of wetting properties, days.

The values of the fitting coefficients and the times of wetting stabilization are summarized in

Table 3. Fitting coefficients and times of wetting stabilization of Cu-SiC samples modified by laser radiation.

Sample	No. 2	No. 3	No. 4	No. 5	No. 6	No. 7	No. 8
θsteady, °	144.2	135.8	149.5	143.4	138.7	98.2	120.9
a, day	21.4	16.6	15.9	17.1	15.5	21.8	21.8
t, day	53	53	53	53	53	73	73

. The fitting curves and experimental data show good agreement (Figure 4).

An analysis based on a comparison of wetting properties (static contact angles) of Cu-SiC samples after abrasive processing (Figure 3) and after modification by laser radiation at the stage of achieving wetting stabilization (Figure 4) allowed the following conclusions to be drawn. Under conditions of spontaneous hydrophobization of Cu-SiC composite materials modified by laser radiation, the dynamics of wetting inversion (change in contact angles over time), the values of contact angles established during stabilization as well as the duration of time required to stabilize the wetting properties, do not depend on the wetting properties of Cu-SiC composite materials before modification. The results and conclusions obtained are important for understanding the mechanism of spontaneous hydrophobization and for identifying the main factors affecting the intensity of the wetting inversion of materials modified by laser radiation. It can be concluded that the wetting properties of materials prior to modification by laser radiation do not affect the intensity, duration of stages, and steady-state values of the contact angles upon the wetting inversion.

3.3. Analysis of Droplet Spreading over Cu-SiC Surfaces

3.3.1. Analysis of Droplet Spreading over Cu-SiC Surfaces Processed by Abrasive Materials

Figure 5 presents the change in the speed of the liquid front (three-phase contact line) in the time that a 10-µL water droplet spreads over the surfaces of Cu-SiC composite materials processed by abrasive materials. Positive values in Figure 5 correspond to the advancing liquid front (supplying liquid by a pump), negative values—receding liquid front (pumping out liquid by a pump). At the initial moment of time (0–2 s), it is technically impossible to record the values of the liquid front speed, so the dependencies do not start from zero (along the X-axis).

It can be seen from Figure 5 that the liquid front speed at the initial moment of spreading increases. After a slight increase, the speed decreases monotonically. The operating time of the pump under supplying conditions at a flow rate of 5 µL/min was 120 s and at 100 µL/min, 5 s. The liquid supplying mode of the pump was replaced by the liquid pumping out mode. When the pump stops in the supplying mode, the liquid front continues to move due to the action of inertia forces. Next, the pinning of the three-phase contact line occurs. It should be noted that during pinning, the pump is already operating in the liquid pumping out mode. The mechanism of the three-phase contact line pinning with a description of the forces acting on the droplet, as well as the causes due to roughness (asperities are energy barriers to the liquid front movement) are well studied in [20]. When the balance of forces is disturbed, the liquid front begins to move toward the previously wetted surface (the speeds are shown as conventionally negative in Figure 5) and the droplet contact diameter and height decrease. Figure 5 shows that the speeds of advancing liquid front (positive values) decrease on the samples in the order of increasing static contact angles: No. 8—No. 6—No. 7—No. 4—No. 3—No. 2—No. 5. It was concluded that the more hydrophilic the surface, the higher the speed of the liquid front. Based on the analysis of the liquid front speeds in the pumping out mode (negative values) (Figure 5), it was concluded that the surface wetting properties do not affect the liquid front speed over the pre-wetted surface.

Figure 6 shows the time dependences of the dynamic contact angles obtained for water droplet spreading over the surfaces of Cu-SiC composite materials processed by abrasive materials.

The advancing DCAs (I, Figure 6) registered when liquid supplied by the pump remains constant in time when the front of a liquid droplet moves over a previously unwetted surface. When switching the operating mode of the pump to pumping out liquid from a droplet formed on the surface (II, Figure 6), the receding DCAs decrease rapidly until the droplet is completely removed from the surface (for a flow rate of 5 μL/min). At a flow rate of 100 µL/min, the receding DCAs at Stage II remain constant for some time (2–3 s), despite a decrease in the droplet volume. Further DCAs also decrease.

From Figure 7, it can be seen that SCAs lie in the range of θ_0A–θ_0R. As noted earlier in Section 3.2, the contact angle hysteresis on the Cu-SiC surfaces is mainly due to the non-uniformity of the elemental composition (Table 2). As can be seen from Figure 7, the difference between CAH measured at 5 µL/min and 100 µL/min is not significant. However, according to the recommendation [47], the CAH values are obtained at very low fluid flow rates, therefore, measurements at 5 µL/min are taken as CAH values. Thus, the CAHs on Cu-SiC surfaces processed by abrasive materials are 10.1–14.9°.

3.3.2. Analysis of Droplet Spreading over Cu-SiC Surfaces Processed by Laser Radiation

The experimentally obtained change in the liquid front speed in time during the advancing and receding movement of water droplet contact line over the surfaces of Cu-SiC composite materials modified by laser radiation when the surfaces demonstrate hydrophobic properties are shown in Figure 8, and when the surface properties are stabilized—in Figure 9.

It was found (Figure 8a) that the liquid front speed on the surfaces of samples No. 2–6 is significantly lower than that recorded on samples No. 7 and No. 8. It was concluded that at a low liquid flow rate (no more than 5 μL/min), the liquid front speed on the hydrophobic modified surfaces of Cu-SiC composite materials depends on the wetting properties. The worse the wetting properties, the lower the movement speed is.

It was found (Figure 9a) that the liquid front speed when the surface wetting properties are stabilized decreases compared to that measured before stabilization (Figure 8a), which is also associated with the deterioration of their wetting properties. It should be noted that the sequence of speed change from sample to sample remained the same, as in the conditions when the wetting properties continued to change (Figure 8a). At a flow rate of 100 µL/min (Figure 9b), the speeds change significantly. On the samples showing hydrophobic (θ_{No. 2} = 144.2°; θ_{No. 3} = 135.8°; θ_{No. 5} = 143.4°; θ_{No. 6} = 138.7°) and close to superhydrophobic properties (θ_{No. 4} = 149.5°), the speed increased significantly (the maximum increase by 37% on sample No. 2) compared to that obtained before stabilization of the wetting properties (Figure 8b). According to the analysis of the liquid front speed values (Figure 8 and Figure 9) and dynamic contact angles (Figure 10 and Figure 11), it was found that at low liquid flow rates (5 μL/min), the degree of hydrophobization does not affect the liquid front movement. This is due to very low speeds at which the liquid freely penetrates into micro- and nano-sized cavities, displacing the air located there. Therefore, despite the hydrophobic and close to superhydrophobic properties of the samples, the dynamic contact angles recorded on their surfaces (No. 2–6) do not exceed 110° (Figure 10a) and 120° (Figure 11a) before and after stabilization of the SCAs, respectively.

At high liquid flow rates (100 μL/min), the flow rate is significant, the liquid does not have time to fill the cavities, so the liquid front moves along the “air cushion” formed in the irregularities. This explains the increase in the liquid movement speed after stabilization of the wetting properties, as well as the fact that on surfaces with worse wetting properties after stabilization (samples No. 2–6), the speeds are much higher than on surfaces of samples No. 7 and No. 8 (Figure 9b). It is concluded that the surface areas containing an “air cushion” in the irregularities of the surface texture reduce the resistance to the liquid front movement. In addition, in this case (100 μL/min) the DCAs are significantly higher than the DCAs at 5 μL/min.

Another distinctive feature of the dynamic properties of a liquid on the surfaces of Cu-SiC composite materials processed by abrasive materials and laser radiation is the nature of the change in dynamic contact angles for advancing and receding liquid front. In contrast to constant DCAs in the stage of liquid supplying by a pump (I, Figure 6), DCAs increase significantly on surfaces modified by laser radiation in Stage I (Figure 10 and Figure 11). At the stage of pumping out the liquid from the droplet formed on the surface, the DCAs sharply decrease until the droplet completely disappears both on surfaces processed by abrasive materials (II, Figure 6), and on surfaces modified by laser radiation (II, Figure 10 and Figure 11).

The values of the quasi-static advancing and receding DCAs and the CAH determined by them for the Cu-SiC samples modified by laser radiation are shown in Figure 12.

It was found that under the conditions of SCA growth, the CAH is greater (Figure 12a,b) on laser-modified samples than on mechanically processed Cu-SiC samples (Figure 7). After wetting stabilization, the CAH of the laser-modified Cu-SiC surfaces has decreased and does not exceed 7°. It is concluded that laser modification of the nanosecond duration of the Cu-SiC composite materials due to the developed multimodal roughness makes it possible to change the wetting properties in a wide range from the extreme state (superhydrophilicity) to the hydrophobic state. The mechanism of wetting inversion is similar to the inversion mechanism that occurs on the surfaces of metals modified by laser radiation. After processing Cu-SiC surfaces with laser radiation, the elemental composition of the surfaces becomes uniform. This eliminates one of the main disadvantages of Cu-SiC surfaces processed by abrasive materials. This, in turn, makes it possible to change the dynamic characteristics of the droplet spreading (at a flow rate of 5 μL/min, the liquid front speed is more than three times (Figure 8 and Figure 9), and the dynamic contact angles are in the range of 30° (Figure 10 and Figure 11)). These advantages and the small contact angle hysteresis make Cu-SiC suitable for use as a structural material for microfluidic devices, systems for separating and mixing micro- and nanoliter liquids. To develop practical recommendations for the development of research topics, it is planned to study the dynamic characteristics of known industrial methods of hydrophobization of Cu-SiC based on the chemisorption of a hydrophobic agent [4] which is devoid of the spontaneous hydrophobization disadvantages.

4. Conclusions

1. The surfaces of Cu-SiC composite materials containing up to 20 wt.% of SiC and mechanically processed were found to have a significantly heterogeneous elemental composition. This causes the variability of the surface properties and a significant contact angle hysteresis (more than 10°). The use of such materials in practice as a structural material in the production of microfluidic devices, systems for separating and mixing micro- and nanoliter liquids is associated with a significant disadvantage. It is extremely difficult, and sometimes impossible, to control the dynamic characteristics of the nano- and micro-liter liquid spreading over such surfaces.

2. It was shown that laser modification of Cu-SiC composite materials makes it possible to form a developed, multilevel roughness on the surface, as well as to form a near-surface layer with a uniform elemental composition.

3. It was proven that the recorded deterioration of the wettability properties of Cu-SiC surfaces after abrasive processing with an increase in their sintering temperature, as well as the inversion mechanism and the reason for the spontaneous hydrophobization of the Cu-SiC composite materials modified by nanosecond laser radiation, are due to the adsorption of airborne hydrocarbon contaminants, similar to the known wetting inversion of metal surfaces.

4. It was established that the wetting properties of materials prior to modification by laser radiation do not affect the intensity, duration of stages, and steady-state values of contact angles upon wetting inversion of Cu-SiC composite materials.

5. The processing of Cu-SiC surfaces by laser radiation makes it possible, at a given flow rate of 5 μL/min, to change the dynamic characteristics of the micro-droplet spreading: the liquid front speed of more than three times, advancing dynamic contact angles in the range of 30°. These advantages and the small contact angle hysteresis (less than 10° under the wetting stabilization conditions) make Cu-SiC suitable for use as a structural material for microfluidic devices, systems for separating and mixing micro- and nanoliter liquids.