Selective laser-assisted deposition of silver nanoparticles on a Mg-2wt.%Ca substrate

The raw material was an Mg-2wt.%Ca alloy prepared by gravity casting in a steel mold at CanmetMATERIALS laboratories. The composition of the as-cast ingot is reported in table 1. The as-cast material was extruded at 400 °C in a plate shape die (extrusion ratio equals approximately to 5:1). This thermomechanical treatment results in a relatively homogeneous microstructure consisting of magnesium grains with an approximate average size of 8 µm and Mg₂Ca second phase particles with an average size ranging between 1 and 14 µm [29]. The substrate was cut from the extruded plate material with the following dimensions: 5 mm width, 5 mm length and 1.5 mm thick. The substrate was polished with abrasive papers up to 4000 grit. Samples were cleaned in ultrasonic bath with ethanol for 1 min prior to the silver nanoparticle deposition. A minimum of three samples were used to validate the optimized parameters for the LAMM.

The deposition process was conducted with a mesoscale maskless method using an aerosol-jet printing machine manufactured by OPTOMEC Inc. The ink used for the deposition process was obtained from Cabot Corporation (MA, USA). This material was a suspension of silver nanoparticles (45–55 wt.% and average size  <60 nm) in ethylene glycol. The ink was diluted with 1 ml of the silver suspension in 3 ml of distilled water. Micro-droplets of ink were created inside a vial by an ultrasonic bath and then deposited onto the substrate which was attached to a motorized stage. A deposit was then created by controlling the stage motion in the x and y directions. The trajectory program was made with the 2D application of the AutoCAD software. In order to improve the wettability during this deposition step, the substrate sample was continuously heated at 80 °C using a hot plate attached to the stage [27].

Once the deposit was achieved, the whole sample was heat treated using a laser beam, which was scanned on its surface. A continuous erbium fibre laser from IPG Photonics, characterized by a TEM₀₀ transverse mode with a wavelength of 1550 nm, was used. The laser power was set to 8 W, a value in the middle of the working range, and in the present optical set-up the beam diameter was 85 µm at the focused point.

Vertical scanning interferometry (VSI) of a white light (WYKO NT 1100 optical profiling system, Veeco) was used to characterize the deposit topography. A LEO 1530 scanning electron microscope (SEM) equipped with a field emission gun, was used to investigate the effect of the laser treatment on the microstructure of the deposit, and the deposit-substrate interface. The SEM samples were prepared by cross sectional cutting of samples using a focused ion beam (FIB) machining technique. The FIB machine (ZEISS NVision 40), was also used to prepare the samples used for transmission electron microscopy (TEM). A carbon layer with an approximate thickness of 400 nm was deposited to protect the surface against damage during FIB machining.

Conventional TEM imaging (bright field and dark field), in the STEM mode, and chemical analysis by energy-dispersive x-ray spectroscopy (EDS) were carried out using a JEOL 2100F microscope. The microscope was fitted with a specific device, so-called ASTAR, for automatic mapping of the alloy phases and grain orientation [29]. The operation of the ASTAR device is based on the scanning of the TEM sample by a narrow parallel electron beam and a synchronized acquisition of the local diffraction pattern. An automatic indexation of the diffraction pattern dataset allowed us then to build a map of the distribution of phases and the grain orientation in the substrate.

The deposition process is done in two steps: first, the deposition step, which controls the thickness and width, as well as the geometry of the deposit, and second, laser scanning, which aims to improve the deposit cohesion through a controlled sintering process. In order to obtain a cross-line deposit, the stage motion is controlled by a trajectory program.

3.1.1. Deposition step.

The deposition process aims to obtain well-defined micron scale topography with continuous dense lines of silver nanoparticles and minimal overspray. To achieve this goal, deposition trials are conducted by varying the processing parameters and examining the resultant deposits through optical micrography, followed by SEM analysis. As the exemplar micrograph in figure 1 illustrates, non-optimal process parameters result in obtaining non-uniform deposited structures with extended overspray.

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The process-characterization trials lead to determining the parameters for obtaining the highest quality of deposition (i.e. well defined geometry and minimal overspray).

Figure 2(a) shows the SEM image of the deposited surface that is obtained by using the optimized LAMM parameters, as reported in table 2. The deposited lines in figure 2(a) are well defined with 20 µm width and 40 µm separating distance, as specified in the trajectory program. As the micrograph shows, the amount of overspray is very limited. The profilometry map shown in figure 2(b), the deposit forms an even and almost periodic pattern. A profilometry mapping at higher magnification is performed to evaluate more precisely the thickness of the deposited lines, figure 2(c). According to the color scale used for the z dimension, the deposit thickness varies between approximately 100 nm to 1 µm along a single line.

Table 2. Optimized LAMM process parameters.

Parameter	Value
Atomizer gas flow rate (cm3 · min−1)	20
Sheath gas flow rate (cm3 · min−1)	48
Ultrasonic atomizer voltage (V)	50
Deposition velocity (m · s−1)	0.7
Deposition tip diameter (µm)	150
Hot plate temperature (°C)	80

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3.1.2. Laser heat treatment.

The deposition step is followed by a laser heat treatment, which is applied to the entire surface. The goal of this process is to obtain an early stage of sintering (i.e. necking) of the nanoparticles, so that the nano-scale topography of the sintered deposit is preserved. A systematic sequence of altering the laser scanning speed and characterizing the surface by SEM is conducted to achieve the desired sintering characteristics.

As illustrated by figure 3(a), the laser treatment using 8 W power and a scanning speed of 0.1 m · s⁻¹, results in an advanced state of sintering with particles completely bonded together. Using the same laser power but changing the scanning speed to either 0.2 or 0.3 mm · s⁻¹, figures 3(b) and (c), leads to an early stage of sintering where necking between particles has begun but the surface topography has still nano-scale characteristics. By further increasing the velocity to 0.4 mm · s⁻¹, figure 3(d), the degree of necking between nanoparticles does not seem to be highly impacted. Thus, a laser power of 8 W and speed of 0.25 mm · s⁻¹ has been chosen in order to obtain desirable sintering condition.

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A sample processed with the parameters listed in table 2 and a laser speed of 0.25 mm · s⁻¹, is characterized to examine the impact of the laser processing on the microstructure of the substrate. A cross section is obtained by FIB cutting, figure 4(a). The microstructure of the silver deposit layer and the substrate are shown in figures 4(b) and (c).

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Figures 3(c) and 4(b) show that the silver layer has achieved relatively homogeneous necking between particles across the layer. We note that, though it has been possible to cut a FIB section, the interface between the silver deposit and the substrate shows however some de-cohesion. The microstructure of the substrate consists of magnesium grains and Mg₂Ca particles. The grain size varies from less than 1 µm in a thin (~1 µm) layer below the surface (referred to as sublayer) to several microns well below the surface. Mg₂Ca particles also vary in size with micro-size particles distributed in the substrate below the sublayer. These microstructural features suggest that the bulk substrate retains the structure of the as-extruded material [29], while the sublayer is affected by the surface deposition process.

To better understand the effect of the deposition process on the sublayer, a cross section, using FIB cutting, is prepared from a location of the sample free of deposition (the location lies in between to deposit lines), figure 5(a). The TEM analysis, as shown in figure 5(b), shows that the sublayer of small grain also exists in this region. A crystallographic mapping at the vicinity of the surface (as specified in figure 5(b)), using ASTAR and the classical crystallographic orientation code, is displayed in figure 5(c). The ASTAR study clearly indicates that there is a 1 µm thick sublayer formed by small grains ranging in size from about 100 nm to 500 nm, with the smallest grains located the nearest to the surface. These findings indicate that the formation of the sublayer is independent of the silver deposition process. The ASTAR mapping also shows that the sublayer grains are not elongated, i.e. have no preferential orientation and show no sign of a high dislocation density or twins. The grain boundaries appear with well-defined edges and large misorientation angles. Associated with the TEM study, EDS is used to obtain the compositional profile along the cross-section. The results show no evidence for the diffusion of silver in the substrate.

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A 3D thermal model has been developed using COMSOL Multiphysics. It aims to simulate the thermal effect of a laser source scanning a substrate which is held at a constant temperature. This is achieved by the numerical modelling of the local temperature as a function of time and depth in the materials (i.e. deposit and substrate). Figure 6 shows the front view of the domain of the silver deposition, which is 200 µm long, 20 µm wide and 1 µm thick.

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The following equation is considered for governing heat transfer in the solid:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle \rho {{C}_{p}}\frac{\partial T}{\partial t}+\nabla \cdot \left( -k\nabla T \right)=Q\nonumber \end{align} \tag{ 1 }$$

where ρ, C_p, k and Q are, respectively, the density, the specific heat capacity, the thermal conductivity and the power generation per unit volume. The heat transfer module and the boundary conditions used in Jabari et al [30] were adapted to solve the heat transfer equation.

In the present study, the effect of temperature on properties is considered negligible. Therefore, three different boundary conditions were implemented for this semi-infinite domain: the convective heat transfer, the moving power input and the constant temperature of the bottom of the substrate.

The convective heat transfer is applied to all surfaces except the bottom facet. In these cases, the boundary condition is given by:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle -k\nabla T{{n}_{|\Omega }}=\left\{\begin{array}{@{}llcccccccccccccccccc@{}} Q-{{h}_{S}}\left( {{T}_{ext}}-T \right) & {\rm if}\,\Omega \notin \left[ {\rm laser}\,{\rm beam}\,{\rm location} \right] \nonumber \\ {{h}_{S}}\left( {{T}_{ext}}-T \right) & {\rm if}\,\Omega \notin \left[ {\rm laser}\,{\rm beam}\,{\rm location} \right] \nonumber \end{array} \right.\nonumber \end{align} \tag{ 2 }$$

where n is the normal vector to the surface boundary, Q is the heat flux from the laser beam and h_S is the convective heat transfer coefficient associated to the corresponding surface. At the laser beam location, the moving power input is then applied as a boundary condition. Thus, the moving power input is given by:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle Q={{\beta }_{M}}I\left(x,y,t \right)\nonumber \end{align} \tag{ 3 }$$

where ${{\beta }_{M}}$ is the absorption coefficient of the material (substrate or silver deposition). The laser intensity distribution, characterized by a TEM₀₀ transverse mode, is given by:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle I\left(x,y,t \right)=\frac{2{{P}_{l}}{{M}^{2}}}{\pi r_{l}^{2}}\exp \left(\frac{-2{{r}^{2}}{{M}^{2}}}{r_{l}^{2}} \right)\nonumber \end{align} \tag{ 4 }$$

where P_l, M², r_l and r are respectively the laser power, the beam factor quality, the spot size at the process zone and the distance from the center of the spot size. As the laser beam is moved on the substrate surface at a defined velocity, v_l, the distance from the center of the sport size can be written as follows:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle r=\sqrt{{{\left(x-{{v}_{l}}t-{{x}_{0}} \right)}^{2}}+{{\left(y-{{y}_{0}} \right)}^{2}}}\nonumber \end{align} \tag{ 5 }$$

where $\left({{x}_{0}},{{y}_{0}} \right)$ is the Cartesian coordinates of the center of laser beam at t  =  0 and v_l is the velocity of the laser beam.

Following Foroozmehr et al [27], the silver deposit is described as a porous material made of packed silver nanoparticles with a layer thickness of 1 µm. According to Dullien et al [31], the porosity for a random packing of spheres is approximately 37.5%. Thus, the density of the silver deposition is considered equal to 6563 kg · m⁻³. For simplification, the porosities in the deposition are considered spherical. According to Cernuschi et al [32], using this assumption, the thermal conductivity can be calculated as follows:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle {{k}_{p}}={{k}_{s}}{{\left(1-\varphi \right)}^{3/2}}\nonumber \end{align} \tag{ 6 }$$

where k_s and φ are, respectively, the thermal conductivity of silver and the ratio of porosities.

In addition, due to the small size of the silver nanoparticles (≈60 nm), the roughness of the deposition has an impact on its absorbance [33]. According to Bennett et al [34], the effective absorbance due to the roughness of the material can be determined using the following expression:

$$\begin{align} \newcommand{\e}{{\rm e}} \displaystyle {{A}_{r}}=1-\left(1-A \right){{e}^{-{{\left(\frac{4\pi \delta }{\lambda } \right)}^{2}}}}\nonumber \end{align} \tag{ 7 }$$

where A is the absorbance of the material, δ is the root mean square (RMS) roughness and λ is the wavelength of the incident beam. Considering the surface as an array of spheres, δ is assumed to be equal to 60 nm. Concerning the substrate absorbance, the measured RMS roughness for polished samples is approximately 0.2 µm which is of the same order as the laser beam wavelength. Thus, it was assumed that the impact of the roughness of the substrate on its absorbance is negligible.

Finally, by using the previous assumptions and literature [35–37], we obtain the parameters summarized in table 3.

Figure 7(a) displays the cross section of the temperature maps at the center of the laser beam focused on the sample surface in two cases: without deposition, figure 7(a), and with a silver deposition on the substrate, figure 7(b).

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According to the simulation, a thin layer substrate is significantly thermally affected by the laser treatment: 180 °C is reached at the first micrometer below the surface. The temperature rapidly decreases with the thickness of the substrate. Also we can conclude that the silver deposition has a negligible impact on the temperatures in the substrate.