Microstructuring of steel surfaces via cold spraying with 316L particles for studying the particle-wall collision behavior

Surface & Coatings Technology 379 (2019) 125054

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Microstructuring of steel surfaces via cold spraying with 316L particles for studying the particle-wall collision behavior

T

Paul Breuningera,∗, Fabian Krulla, Katharina Huttenlochnerb, Christine Müller-Renob, Christiane Zieglerb, Rolf Merzc, Michael Kopnarskic, Sergiy Antonyuka a

Technische Universität Kaiserslautern, Institute of Particle Process Engineering, Gottlieb-Daimler Straße 44, 67663, Kaiserslautern, Germany Technische Universität Kaiserslautern, Department of Physics and Research Center OPTIMAS, Erwin Schrödinger Straße 56, 67663, Kaiserslautern, Germany c IFOS GmbH, Institute for Surface and Thin Film Analysis GmbH, Trippstaedter Straße 120, 67663, Kaiserslautern, Germany b

A R T I C LE I N FO

A B S T R A C T

Keywords: Microstructuring Cold spray Stainless steel particles Laval nozzle CFD Restitution coefficient

The surface morphology can have a significant influence on the contact behavior during particle collisions with surfaces. This work focuses on the design of surfaces to study the dynamic interactions between particles and component surfaces. Within this study two different methods with particle-substrate collisions were used. A cold spray method utilised for surface coating was adapted to generate different nub structures by applying single spherical stainless steel particles (316L) in the size range of 1–8 μm on stainless steel substrates (316Ti). EulerLagrange Computational Fluid Dynamics simulations of the spray process were performed to obtain impact velocities and temperatures of the particles. In cold spray experiments, different nub concentrations on the surface were generated by varying the spraying time. Measurements with Scanning Force Microscopy show a significant increase of the roughness with increasing spraying time. Collisions of polystyrene particles on the produced surfaces were observed by two high-speed cameras. A strong effect of the surface roughness on the coefficient of restitution during the particle collisions were obtained. These results can be used to create surfaces with a certain energy dissipation in future studies.

1. Introduction The surface of any component determines its interactions with the immediate surroundings. The knowledge of the relationship between the surface roughness and the component properties can be a crucial part of process optimization. It is known that the microstructure of surfaces determines friction [1,2] and wetting [3] behavior. In Ref. [4], the cultivation of the bacteria Lactobacillus delbrueckii lactis and the effectiveness of lactic acid production was related to the microstructure of the used surface. In the field of bulk solids handling, the surface microstructure plays also an important role. The understanding of the particle dynamics in unit operations for particulate processes in chemical industry as fluidized beds, pneumatic transport, mixers, mills and others are based on the knowledge about the physical properties and the mechanical contact behavior of particles including collision between the particles and particles with wall surfaces of apparatuses and instruments [5]. In the last decades, the increasing computer capacity has made it possible to model even large apparatuses. Precise experimental investigations of the particle-wall contact behaviour are necessary for model ∗

development. An established way to investigate these properties is to determine the restitution coefficient by particle-wall collision experiments realised as a free-fall test [5]. The restitution coefficient (COR) is a very important parameter to describe the energy dissipation and damping force during particle-wall interactions. The COR is an essential parameter in many contact models [6–11], which are used for the simulation of interactions between particulate materials and walls of apparatuses or machines in different solids processes [6,9–12]. Although there are many studies on different materials, only few papers [13,14] deal with the influence of the micromorphology of the substrate. For systematic investigations, there is a need for surfaces with a defined surface microstructure. In this work a cold spray method was used to create such surfaces with statistically distributed asperities in different concentrations. For this purpose, an own constructed cold gas setup was used. Particles are dispersed in a heated gas stream, accelerated to supersonic velocities and sprayed onto a substrate surface, which is moved perpendicular to the spraying jet. During the impact, the high kinetic energy is transformed into heat and plastic deformation of both the particle and the substrate and a strong bonding of particle can be created.

Corresponding author. E-mail address: [email protected] (P. Breuninger).

https://doi.org/10.1016/j.surfcoat.2019.125054 Received 21 October 2018; Received in revised form 19 September 2019; Accepted 6 October 2019 Available online 08 October 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 379 (2019) 125054

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velocity for a given powder was studied in the work of Alkimov et al. for a wedge shaped nozzle [26]. The optimal nozzle geometry depends on the particle size. Li et al. examined the optimal nozzle exit diameter for a given 316L stainless steel powder with a mean size of 9.9 μm by CFD simulations which were validated with experiments [27]. The authors obtained a nozzle expansion ratio of ε = 4,93. In a similar study Buhl et al. developed a nozzle geometry, based on CFD simulations of the gas flow inside of the Laval nozzle taking the wall friction into account, which accelerates titanium particles with the sizes of 1 and 10 μm [28]. Dykhuizen et al. investigated the particle impact on the surface by means of numerical and experimental methods. It was stated that neither particle nor substrate needs to be melted to obtain high bond strengths [29]. Stoltenhoff et al. described the spraying process via simulations and made recommendations on process optimizations for a given powder [30]. Assadi et al. determined that local temperatures in the contact zone can even reach the melting temperature of the used materials due to strong plastic deformation. Based on empirical studies Assadi et al. developed a model of the critical bonding velocity vcrit [31], which the particles need to exceed to be successfully fixed. This model was improved in the work of Schmidt et al. [15], resulting in:

Nowadays, the cold spray process is well established for surface coating of various metal substrates. Usually the process is carried out at pressures up to 5 MPa and temperatures up to 1100 °C for utilizing particles in a size range of 5–45 μm [15,16]. The latest developments prove that it is also possible to use cold spray for additive manufacturing and high strength materials [17–19]. A successful deposition of cold sprayed particles for a given material is dependent on the impact velocity and the impact temperature. Schmidt et al. developed an empirical model that predicts a successful particle bonding in a narrow velocity range between a lower (critical velocity) and an upper limit (erosion velocity) [15]. Outside of those limits, erosion of the surface occurs. The highest bonding strength is achieved between those limits at a certain temperature level. The most dominant bonding mechanism was determined as an adiabatic shear instability occurring in the contact zone at impact. Beside this, there are further mechanisms, which enhance a stable bonding, such as metalmetal diffusion, interlocking or van der Waals forces. A detailed description of the bonding mechanisms can be found in Refs. [16,17,20]. For the generation of surface asperities with defined shape and size, the particle plastic deformation during the impact becomes an important parameter. The deformation depends on the material of both contact partners, the size of the particles and the collision scenario (direction, velocity and impact angle). Assadi et al. related the actual impact velocity to the critical velocity and defined the dimensionless vimpact parameter η = v , a value that can predict the bonding strength and crit the actual flattening ratio [20]. For the application of hemispherical asperities, process parameters close to η = 1 are suitable, otherwise the particle flattens too strongly and loses its original shape. The more the impact velocity exceeds the lower limit for bonding (critical velocity), the deformation of the particle and the penetration depth become stronger. The temperature is another important parameter, since material behavior also changes with the temperature. To adapt the cold spray process for surface microstructuring, the relationships between the process parameters and the impact scenario (velocity, temperature and angle) are needed. However, due to the small sizes and the high velocities of particles, the phenomena are difficult to obtain experimentally. Therefore, only few works are available in the literature. In the recent work of Hassani-Gangaraj et al. [21] the recording of a particle impact experimentally by high-speed cameras are presented. A more common strategy to obtain unknown information about the impact velocity and temperature is the simulation by different numerical methods [22]. There are already various articles and book chapters on the numerical modelling of the cold gas process [15,23], the results of which can be used as a basis for the present study. However, since most studies deal with layers of larger particles, a detailed simulation of the carrier gas flow and particle movement through the nozzle is needed to obtain the particle motion and temperatures during the process. The own constructed cold spray setup was applied for the creation of variable asperity concentrations on technical surfaces. To study the influence of the produced microstructure on the various interactions of the surfaces with their surroundings the collision behavior with solid polystyrene (PS) particles of the size 665 μm was studied by a single particle impact test using a free-fall setup, which was developed in our previous work [24].

vcrit =

F1 ⋅

Timp − Tref ⎞ 4 + F2⋅c p⋅(Tmel − Timp) ⋅σTS ⋅ ⎛1 − Tmel − Tref ⎠ ρ ⎝ ⎜

⎟

(1)

where cP is the specific heat capacity, ρ is the particle density, σTS is the tensile strength and F1 and F2 are empirical coefficients. Temperatures include the melting temperature Tmel , the impact temperature Timp and the reference temperature Tref = 20°C . The first term of the equation describes the plastic deformation according to the Johnson-Cook model. The second term takes the heat dissipation due to the increase of the temperature in the contact zone into account. The authors of [15] also took the particle size in a theoretical approach into account. With decreasing particle size strain rate hardening effects increase and smaller particles have a lower adeabaticity due to thermal conductivity. These has been related to an empirical equation of the critical velocity −0.14 v 316L crit = 950⋅d particle ,

(2)

which though contain neither material properties nor process parameters. However, they also determined a minimum diameter for adeabatic straining of 316L particles significantly below 1 μm. Above this diameter, the thermal diffusion is slow enough that local shear instability of a spherical particle can occur. A successful fixation can also take place with very small particle sizes. 2.2. The coefficient of restitution (COR) for measuring the energy dissipation of a surface in contact with a polystyrene (PS) particle The influence of the produced surface microstructure on the contact behavior and adhesion properties of granules during a collision was evaluated by the coefficient of restitution (COR). The COR is defined as the kinetic energy ratio of the translational particle movement after (Ekin, rebound ) and before (Ekin, impact ) particle collision with the surface and can be given as:

COR = 2. Theory

Ekin,rebound = Ekin,impact

1−

Ediss v = rebound Ekin,impact vimpact

(3)

where Ediss describes the energy dissipation during a collision, vimpact and vrebound are the velocities of the particle before and after impact, respectively. Thus, the COR is defined in a range from 0 to 1, where 0 means that the total impact energy is dissipated and no rebound of particle occurs. On the other hand, at a coefficient of 1 no energy is lost, which correspond to the ideal elastic impact behavior. The determination of the COR can be achieved with different measurement methods [5]. In the present study a setup of two highspeed cameras, perpendicular to each other, is used to determine the

2.1. Surface modification by cold spray deposition For a stable particle-substrate bonding, a high acceleration of the particles to sufficient velocities is an essential step, which is achieved by using a Laval nozzle. For process optimization, the geometry of Laval nozzles was studied by various authors. Dykhuizen and Smith studied the gas dynamics and developed equations for optimal particle acceleration to layout spraying nozzles [25]. The maximum possible particle 2

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Fig. 1. SEM image of 316L particles (a); number based cumulative particle size distribution (b).

particle velocity before and after the collision (see chapter 3.5). A similar experimental method was applied in Refs.: [32–35]. Most studies described in the literature focus on the effect of material behavior of contact partners (from elastic to plastic) but neglect the effect of the micromorphology of surfaces at the particle-wall contact assuming a perfectly smooth surface. In our previous work [24] a novel method was introduced to study the relationship between the surface microstructure and the collision behavior of fine particles. 3. Materials and methods 3.1. Materials The samples of the substrates made from 316Ti stainless steel were prepared with the size of 10 × 20 mm2. They were polished in three steps with diamond paste from coarse to fine in the grain size of 6 μm, 3 μm and 1 μm. The particle system used was 316L particles produced by Goodfellow (Goodfellow, Hamburg, Germany). Scanning Electron Microscopy (SEM) images (Fig. 1a), obtained by a Phenom G2 pro (Thermo Fischer Schientific, Eindhoven, The Netherlands) show that the particles have a spherical shape. The particle size distribution shown in Fig. 1b, was determined by a laser diffraction spectroscopy (Horiba LA-950, Retsch, Haan, Germany). The particles had a mean diameter of 3.1 μm, while all present sizes are in the range from 1 to 8 μm. The particle density of 7700 ± 5.39 kg/m³ was measured with a helium pycnometer (Heliumpyknometer, 3P-Instruments (Quantachrome), Odelzhausen, Germany).

Fig. 2. Experimental cold spray setup.

but also higher temperatures. However, the nozzle was extended, which leads to a decrease of the impact temperature and an increase of the final velocity of the particles. The higher the process parameters pressure and temperature are chosen, the more the particles in the stagnation area are decelerated again. In commercial cold spray devices the window of deposition starts at particle sizes above 5 μm [23,36]. The present cold gas setup utilizes lower pressures and a nozzle particularly designed for the acceleration of fine particles. Therefore, the particles are not slowed down so much and the limit shifts slightly to smaller particle diameters. The distance between the nozzle and the substrate is 5 mm. Two linear engines move the substrate perpendicular to the jet stream and meander over the substrate surface with a speed of 10 mm/s. After sample manufacturing an ultrasonic cleaning procedure in distilled water was conducted to remove loosely and weakly bonded particles. For 10 min, an ultrasonic sonotrode with a diameter of 13 mm in a distance of 3 mm above the sample applied a specific intensity of 1.8 W/cm2.

3.2. Cold spray setup The experimental cold spray setup used for morphological microstructuring of steel surfaces is shown in Fig. 2. Nitrogen flow (pressurized to 0.9 MPa; concentration of other gases as oxygen or argon below 200 ppm) is divided into two different streams. The first one is pre-heated in a tube furnace, the second stream is loaded with the steel powder by a powder disperser (Topas SAG 409, Topas, Germany), adapted to work under pressure conditions. The powder feeder is equipped with a transportation ring, which transports the powder into the gas stream. In this study, the dosage rate was set constant at 1.06 g/ h. Both gas streams are mixed again in the mixing/heating chamber and are heated up to temperatures of 500 °C. Within the Laval nozzle, the generated aerosol is accelerated to supersonic velocities with a Mach number up to 2.4. The nozzle has a length of 29.5 mm. The outlet has a diameter de of 1.5 mm, and the diameter of the minimum orifice d∗ is 0.8 mm. The nozzle geometry was adapted to the acceleration of small particles. The performed simulations and experiments described in a previous work [28] showed a good usability of the nozzle for particle sizes between 1 and 10 μm. The geometry has an expansion ratio of

3.3. Characterization of the substrates The cold sprayed samples were cleaned with acetone to remove loose particles from the surface. The isopropanol was used to remove residuals of acetone from the surface. The SEM images of samples were obtained with a Phenom G2 pro. The surface roughness was determined by scanning force microscopy (SFM), with a Molecular Force Probe MFP3D (Asylum Research, USA). For the measurements, a cantilever (OMCL AC240TS-R3, Olympus, Tokio, Japan) with a resonance frequency of 70 kHz, a stiffdF N ness of dx = 1.7  m and a tip radius of 7 nm was used. The cantilever scans the surface with a scanning rate of 50 Hz and determines the relative height by the deflection of the cantilever. For each sample,

d2

ε = ∗e 2 = 3.58, which is slightly lower in contrast to common cold (d ) spray setups. A low expansion ratio results in lower impact velocities 3

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three representative measurements with a scan size of 50 × 50 μm2 were carried out. The analysis was conducted with the software MountainsMap of the company Digital Surf (MontainsMap, DigitalSurf, Besançon, France). The height retrace was tilt corrected and analysed according surface parameters. The roughness parameters for these measurements were determined: Sa is the arithmetic height, Sq the root mean square of the absolute height. Sz is defined as the peak to valley of a surface. The area weighted material proportion Smr indicates the ratio of the area of the material 1 μm below the peak in relation to the total area and is therefore a measure of the surface concentration of the asperites. The area weighted density of peaks Spd , determined by Wolf pruning of 5%, respectively is also a scale for the surface concentration of peaks. The particle deformation and bonding quality were observed with a cross-section cut by an ALTURA 875 (Thermo Fischer Science (FEI) DualBeam™ focused ion beam (FIB), Eindhoven, The Netherlands). The FIB used a 30 keV gallium ion beam for milling. A platinum cap layer was deposited on the surface to protect the original sample topography and to avoid rounding of the edges. 3.4. CFD setup A steady and axis-symmetric simulation of the flow field in the Laval nozzle (described in chapter 3.2) and the adjacent flow areas on the substrate was established with Computational Fluid Dynamics (CFD). The substrate is firmly clamped and connected to a cold wall on the side. The k-ω SST turbulence model [37] was used, due to its robust computation of flows near walls (y+ < 1) and in open jets. An inlet temperature of 400 °C and 500 °C was applied, which corresponds to a volume flow of 9.04 L/min and 9.65 L/min, respectively. The ambient pressure of 0.112 MPa is used as outlet boundary condition. The simulation was performed with the software ANSYS Fluent 17.1. The compressible gas density was achieved by the ideal gas law, while the temperature dependence of the fluid viscosity was obtained by the model of Sutherland [38]. The substrate was calculated as a steel wall with a density of 8030 kg/m3, a thermal conductivity of 16.27 W/mK and a heat capacity of 502.48 J/kgK. Particle motion in the gas flow was calculated with the Euler-Lagrange approach in a one-way coupling. The drag force was approximated by the model of Morsi and Alexander [39]. The initial particle velocity is set to zero, which underestimates the initial velocity. However, it was checked that the neglect of the initial particle velocity of some meters per second and gravity has no influence on the calculated impact velocity, the acceleration due to the strong gas expansion was too dominant. In contrast, the particle density has a strong effect on the obtained impact velocity. Therefore, if possible, it should be measured, because the particle density can differ from the solid density due to pores occurring for example during particle production. The particle density was set to 7700 kg/m3 according to the measurement described in chapter 3.1. The specific heat capacity of the particles was set constant to 502.48 J/kgK. The initial particle temperature was chosen according to the initial temperatures of the fluid, which was simulated at 400 °C and 500 °C.

Fig. 3. Sketch of the free fall test setup.

vacuum pump or opening a valve with compressed air to overcome the adhesion force in the particle-needle contact. The experiments were performed in ambient air. The impact velocity of the particle could be adjusted by the drop height in the range of 0.1–5 m/s. The particle impact was recorded with two high-speed cameras (OS7, IDT) at with 10'000 frames per second. The cameras were arranged at an angle of 90° to obtain three-dimensional particle trajectory and velocity before and after the collision. The captured impacts were analysed using Image Processing Toolbox of MATLAB. The impact velocity was set to 0.55 m/ s, which corresponds to a drop height of 2 cm. The experiment was repeated 50 times with each surface to measure the distribution of the energy dissipation depending on the surface micromorphology.

4. Results 4.1. Experimental manufacturing of particle structured surfaces Experiments were conducted with the setup described in chapter 3.2 by the two initial temperatures 400 °C and 500 °C. The sample was moved perpendicularly to the nozzle with a speed of 10 mm/s. When the edge of the sample was reached the focus point was shifted 80 μm to the side, before the sample was moved back. In comparison to commercial cold gas systems, both the traverse speed and the scanning line distance are rather small values. This is due to the relatively small dimensions of the nozzle and the lower particle concentration in contrast to commercial systems. However, reproducible results have been achieved with these settings. The width of the spraying zone is about 1 mm, thus each point of the surface is passed several times. Two representative SEM images of the manufactured and by ultrasonic treatment cleaned samples are shown in Fig. 4. Significantly more particles were deposited on the substrate at the initial temperature of 500 °C. At 400 °C initial temperature, between the few bonded particles many craters resulting from particle rebounds can be seen. However, it is striking that a bonding of some significantly large particles occur on the surface. To manufacture samples with varying surface coverage by particles, the gauge distance between two scanning lines was halved to 40 μm, resulting in the double of absolute spraying time. In Fig. 5 representative SEM images of a polished surface and two cold sprayed substrates are displayed. The sample with the scanning line distance of

3.5. Determination of the specific energy dissipation of the produced surfaces To determine the influence of the produced surfaces on the contact behavior the coefficient of restitution (COR), described in chapter 2.2, was measured with the free-fall setup displayed in Fig. 3. A single soft spherical polystyrene particle (produced by Microparticles GmbH, Berlin, Germany) with a diameter of 665 ± 20.5 μm and a density of 1050 kg/m³ was firstly fixed at a certain height above the surface with a vacuum nozzle, which consists of a dosage needle and a vacuum pump. The vacuum nozzle was held perpendicular to the surfaces to perform normal particle impact. The particle was released by switching off the 4

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Table 1 Surface roughness. Sample

Sa Sq Sz Spd Smr

nm nm μm 1000/mm2 %

Polished

l = 80 μm

l = 40 μm

2.8 ± 0.5 8.6 ± 3.0 0.3 ± 0.06 19.9 ± 9.3 100.0 ± 0.0

166.7 ± 9.7 225.0 ± 16.4 1.7 ± 0.1 47.3 ± 2.1 57.1 ± 21.0

372.7 ± 13.0 461.7 ± 18.7 3.2 ± 0.2 65.6 ± 3.1 14.1 ± 5.5

Fig. 4. SEM images of steel surfaces cold sprayed with steel particles at the initial gas temperature of 400 °C (left) and 500 °C (right) at 0.8 MPa and a line speed of 10 mm/s with scanning line distance l=80 μm.

Fig. 6. Cut through the bonded particles on the surface performed by a focused ion beam.

that the number of peaks also increases significantly. While the value for the polished surface of almost 20 results from the small absolute dimensions, a clear increase of the peak number can be seen with increasing surface concentration. The material proportion Smr decreases in the opposite direction to the surface concentration according to the expectation that more peaks mean a smaller number. The shape of the particles was recorded by SEM images in a side view of the cross-section through two particles (Fig. 6) produced by a focused ion beam. The platinum cap layer can be seen as white bar covering the particles. The SEM image of the cross-section cut demonstrates a homogeneous deformation of the originally spherical particles, resulting in a regular nub. Although some slight cracks appeared, under the particles no significant cavities or voids are present. A strong flattening of the particles and only a slight plastic deformation of the substrate can be observed, which is caused by a significantly higher material stiffness due to the lower substrate temperature at particle impact. The deposited particle is divided into two spherical calottes. With the measured width and height of the calottes, the initial particle sizes are recalculated as 2.1 μm for the larger particle and 1.3 μm for the smaller particle.

Fig. 5. SEM images of experimental samples with different particle loadings on the surface.

l = 80 μm show a low particle loading of the surface where the particles are mostly separated, while almost the whole surface of the second sample is covered with particles. By doubling the spraying period, twice as many particles on the surface are expected. In addition to that, an interlocking of the particles may occur. Many deformed areas on the particle surface can be observed caused by particle-particle impacts. A quantitative statement about the deposition efficiency cannot be made at present state. The samples were weighed before and after the structuring process to determine a volume-based deposition efficiency. However, the increase in mass was within the error tolerance of the balance. A number-based deposition efficiency cannot be determined from SEM images since not every particle impact can be clearly detected at present state. Any error here leads to an overestimation of the deposition efficiency. However, obviously more particles and with a larger size are deposited at the sample with the longer spraying time, which is caused by local heating of the substrate. The higher surface temperature results in a lower yield strength and a higher ductility of the surface. A gradually increase of the deposition efficiency with the increase of the spraying time has already been observed for AISI 304 substrates in Ref. [40].

4.3. Simulation of the spraying process with Computational Fluid Dynamics In Fig. 7 the calculated profiles of the gas pressure, temperature and velocity for the initial temperature of 500 °C are plotted in an axissymmetric view of the nozzle. The flow in the nozzle accelerates quickly and cools down due to the gas expansion. The gas flow stagnates in a region of the jet focus point, where the pressure rises and the gas temperature increases to a maximum of 407 °C. In the stagnation area, the velocity of the gas flow decelerates strongly and is deflected to the sides. At the boundary inner layer of the laval nozzle the temperature can increase up to 50 °C above the initial temperature due to wall friction. Since this effect does not affect the particle properties, the scale was limited to the initial temperature of 500 °C for a better understanding. The particle velocity for four particles from the inlet of the nozzle to the impact point is plotted in Fig. 8 for the particle size 1 μm and 3 μm. The smaller particles accelerate quickly, almost reaching the gas velocity inside the nozzle, but they decelerate above the surface due to stagnation. The increase of the size from 1 to 3 μm (Fig. 8 b) results in the decrease of impact velocity by approximately 25% because of their

4.2. Characterization of the produced surfaces The measured surface roughness values Sa , Sq , Sz as well as the area weighted number of peaks Spd and the area weighted material fraction Smr according DIN EN ISO 25178-1&2 are displayed in Table 1. The values Sa , Sq , Sz increase with increasing particle concentration on the surface. Two phenomena cause this increase. Firstly, on the sample with the scanning line distance l = 40 μm the deposited particles are slightly larger than on the sample with l = 80 μm. Secondly, also particle-particle bonding occurred with a higher possibility at the sample with l = 40 μm, resulting in aggregated structures. The Spd parameter show 5

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Fig. 7. Results of the CFD simulation for the half of the axisymmetric nozzle and the substrate: geometry (a), profiles of pressure (b), velocity (c) and temperature (d) for an initial temperature of 500 °C and an inlet pressure of 0.9 MPa.

Fig. 8. Evolution of particle velocity inside the Laval nozzle and open jet depending on the particle size: 1 μm (a) and 3 μm (b) particles.

Fig. 9. Averaged particle impact and critical impact velocities (a); averaged particle impact temperatures (b); Comparison of different drag models (c) for a process pressure of 0.9 MP and a working distance of 5 mm.

higher momentum. The values of impact velocities of particles with the size of 3 μm show a high dispersion. The particles starting at the outer positions in the nozzle inlet cross the rotational axis and spread. For particles with the size of 1 μm (Fig. 8 a), the particles are focused within the nozzle to a small focus point with an impact velocity of about 800 m/s for all particles trajectories. The impact velocities are almost independent of their initial starting position, the four considered particles show a deviation of impact velocities of ± 4% (Fig. 8 a). This indicates a separation effect of the particles within the Laval nozzle. Smaller particles have a small momentum and follow the stream lines of the gas, consequently the particle jet widens within the divergent part of the nozzle. Larger particle sizes cross the symmetry axis shortly behind the throat of the nozzle. This also causes the particle jet to widen. Both effects result in a higher dispersion of impact velocity values

dependent on different initial position of the particles in the nozzle. The particle size was varied in a wide range from 0.3 μm to 50 μm. A normal impact with the substrate was observed for all considered particle sizes, except dP = 0.3 μm. A generated particle in the 2D model represents particles in a circle with increasing radius from the rotational axis to the nozzle wall. Consequently, a particle close to the nozzle wall represents more particles as a particle modelled on the symmetry axis. The obtained velocities were weighted by their initial starting position, assuming a homogeneously distributed aerosol. The averaged and weighted impact velocity for each particle size is plotted in Fig. 9a. The vertical deviation bars show the range of the minimal and maximum impact velocities for each particle size obtained by varying initial position in the radial direction in the nozzle inlet. The particles with a size slightly below 1 μm have the highest impact

6

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velocities. As was shown above (Fig. 8 a), at this size, the variation of particle velocities is very small, which means that all particles are accelerated similarly. The particle impact temperature shows a contrary trend, as plotted in Fig. 9b. In the nozzle a strong expansion occurs, which is attended by a loss of the gas and particle temperatures. In the stagnation region, above the substrate, the gas temperature recovers almost to the initial starting temperature. At an initial temperature of 400 °C and 500 °C the gas reaches 100 μm above the surface 396.64 °C and 496.53 °C, respectively. The temperature of fine particles below 1 μm increases strongly in this region. Due to the thermal momentum, larger particles react slower to the heat loss inside the nozzle and the heat gain due to stagnation. Consequently, there is a minimum in the particle impact temperature, which occurs at particles, slightly larger than the particles, with the highest impact velocity. This can explain the deposition of larger particles at the lower initial gas temperature in Fig. 4, the impact temperatures of particles > 5 μm are significantly higher than the smaller particles of around 1 μm. A wide range of different drag models can be found in literature, although a complete comparison is still missing [22]. To prove the validity of the used model of Morsi and Alexander [34], which describes the drag of a spherical particle assuming an incompressible flow, the results were compared with those obtained by the high-Mach number drag model [3], which considers the compressibility of the fluid at high Mach numbers (Fig. 9c). For particles up to a size of 5 μm the deviation between both models is below 1%. At particle sizes above 5 μm, the high-Mach number model predicts slightly higher impact velocities. Thus, the drag model of Morsi and Alexander is sufficient to calculate the particle impact velocity in the present study. The obtained impact velocity and temperature were compared to the analytical model of the critical bonding velocity described in chapter 2.1. The unknown parameters were taken from the CFD simulation. The surface temperature of the substrate in the area where particle impact occurs was averaged at 340 °C (400 °C inlet temperatures) and 407 °C (500 °C), respectively. The particular surface temperature was averaged with each particle impact temperature to obtain an equivalent impact temperature depending on the particle size. These temperature values were used to calculate the critical bonding velocity with Eq. (1) depending on the particle size. The specific heat was calculated with the correlation cP = 462 ± 0.134⋅( θ+ 273°C) , with the actual temperature θ in °C given by Antony et al. for flat surfaces [41]. The empirical factor F1 was set to 1.2, F2 was chosen as 0.3 as suggested in Ref. [15]. All values are summarised in Table 2. The calculated values of the critical velocity of all considered particle sizes vary in the range of 623 to 652 m/s for an inlet temperature of 500 °C (Fig. 9a). The critical velocity for an inlet temperature of 400 °C is only slightly smaller than the maximum achieved impact velocities. The model predicts only the deposition of the smallest particles of the used material (Fig. 1). A comparison of the calculated critical impact velocities with the obtained particle impact velocities reveals that the process window for sufficient particle bonding on the surface gets larger with increasing process temperatures, which is caused by both the decrease of the critical velocity and the increase of particle impact velocities. According to the CFD simulations, both particles observed in the FIB cut (Fig. 6) exceeded the critical velocity for bonding. The SEM image shows a slightly deeper penetration of the smaller particle, which corresponds to

Fig. 10. Influence of the micromorphology of the stainless steel surface on the coefficient of restitution by impact of the polystyrene spherical particles (a) and on the impact and rebound angles (b).

the results of the CFD simulation. The smaller particle has a lower impact temperature resulting in a higher stiffness and additionally a particle impact velocity up to 75 m/s higher than the one of the larger particle. 4.4. Influence of surface produced micromorphology on the particle collision behavior The effect of the micromorphology of the surfaces on the particle collision behavior was estimated by free fall experiments as described in chapter 2.2. The measured coefficients of restitution of spherical polystyrene particles were compared for the surfaces cold sprayed by different concentrations and initial polished surface in Fig. 10a. The measured COR of 0.98 ± 0.01 for particle impact on the polished surface showed a dominant elastic collision behavior. The surface microstructure produced by cold spray increased the energy dissipation. In comparison with the polished surface, the COR decreased to 0.70 ± 0.04 at low concentration of particles on the surface and to 0.6 ± 0.06 at high particle concentration. This additional energy dissipation can be explained by the multiple contacts between the polystyrene particle and the surface asperities. With increasing concentration of cold sprayed particles fixed on the surface the number of asperities in the contact area with the colliding polystyrene particle increases, which leads to the additional dissipation of the kinetic collision energy. During impact, an elastic deformation of the particle occurs at the contact surface. Within this contact surface there are many small asperities which dissipate the impact energy. The contact behaviour depends on the number, size and especially on the material of the utilised particles. The use of other materials used in conventional cold spray applications is possible in future studies. The energy dissipation Ediss by collision with each surface given in Table 3 was calculated with Eq. (3) for the given impact velocity, particle size and particle density. Additionally, the influence of the microstructure on the energy dissipation was provided by comparison of the dissipated energies of the microstructured surfaces with the polished surface. The energy dissipation increased with the particle concentration on the surfaces by a factor of 11.0, respectively 21.1. Moreover, the rebound angle of the particle from three different surfaces was investigated. Fig. 10b shows the mean impact and rebound angle with all three surfaces. The particle-wall impact is close to a normal impact. The small deviation of the impact angle from zero can be explained by a small deviation of the particle trajectory from vertical Table 3 Energy dissipation of the different surfaces (Definition of the indices: i = structured and pol = polished).

Table 2 Parameters for the critical bonding velocity model. Parameter Unit

Value

Surface

Tsubs

Tmel

ρ

σTS

F1

F2

cP

°C

°C

kg/m3

MPa

-

-

J/( kg K)

7700

485

340/407

1370

1.2

0.3

565

7

Ediss

Ediss,i /Ediss,pol

J

-

Polished

8.65⋅10−10

1

l = 80 μm

9.53⋅10−9

11.0

l = 40 μm

1.82⋅10−8

21.1

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to misinterpretations of the local deformation. Nevertheless, the section shows that a microstructure of regular asperities can be produced by this method. The shape of the final asperities depends, among other reasons, on the original particle shape. In order to produce a surface of homogeneous quality, however, a precise analysis of the process parameters is necessary.

during the release. The van der Waals forces between the particle and nozzle tip were overcome with compressed air, resulting in a minor deviation of an ideal vertical falling. A larger rebound angle on the microtextured surfaces was observed. The small difference of the mean rebound angle between the polished and low particle concentration surfaces results from the distance between the deposited stainless steel particles on the surface (Fig. 5). Therefore, particles collide more frequently with the smooth part of the surface, which led to a normal particle impact and small rebound angles. With increasing surface particle concentration the rebound angle increases. Beside this, a higher deviation occurred due to a larger concentration of asperities on the surface. Most of the particles impact on these asperities. Therefore, considering the contact geometry on the microscale the normal impact can be assumed only for nearly ideal smooth surfaces of contact partners.

6. Conclusion In this work, the cold spray process was adapted for morphological microstructuring of 316Ti surfaces with single spherical 316L particles with the aim of manufacturing of surfaces with a certain microstructure in the range of a few microns. 316L particles with the size in the range of 1–8 μm were successfully deposited at an initial gas temperature of 500 °C in different particle concentrations on 316Ti steel surfaces. The model of the critical bonding velocity with η-values slightly above 1 can be used for the manufacturing of a certain microstructure in the size range of a few micrometers. For the generation of a microstructure detailed information about the impact conditions of each single particle are necessary. The performed CFD simulations determine the impact velocity and impact temperature of the steel particles in a wide size range from 0.3 to 50 μm. By using the particle properties of a real powder the influence of the particle size and the initial particle position could be determined. A variation of the particle size and the initial particle position showed a significant change on the impact conditions. The obtained process window reacts very sensitive on the particle size and expands with increasing initial gas temperature. Small uncertainties exist due to the transient process. However, the presented simulative tools can be used well for qualitative statements. Existing models need to be extended to the processing of fine powders. The manufactured surfaces could be well used for the determination of the dynamic interactions with polystyrene (PS) particles of size 665 μm. The increase of the energy dissipation during particle collision corresponds with increasing surface roughness and concentration of asperities. The dissipated energy correlates to an increased rebound angle and reduced coefficient of restitution. The surfaces produced proved to be suitable for the impact tests. It could be shown that a simple but targeted change of the surface morphology is possible by the cold spray process. This can be used to generate microstructures on technical steel surfaces with a specific energy dissipation. The presented method can be extended to other metallic materials in different concentrations. An exact knowledge of the impact condition of the particles is necessary to consider also the plastic deformation of the particles during the impact as a structure-forming process. The process can also be combined with post thermal treatment to increase the adhesion of the particles and sinter them with the base substrate. The cold gas process is then only used to create the surface topography.

5. Discussion Regarding CFD modeling, there are a few points that can be discussed. The authors assume, that the calculated substrate temperature overestimates the real existing substrate temperatures, since of the steady model. The heat loss through the clamping connection and the energy input through the jet form an equilibrium. The substrate does not have large dimensions and does not move far, thus the substrate heats up strongly. However, the surface temperature determined by the simulation results in a theoretical maximum value of the surface temperature, which is never quite reached in practice. A transient simulation, which takes this effect into account, is much more complex. A heating of the substrate, however, leads to a decrease of the critical velocity due to a higher ductility of the substrate at higher temperatures, which was also observed experimentally. On the substrate manufactured with the small scanning line distance l = 40 μm, larger and more particles as expected were deposited, which is also explained by the increased process time. A local warming of the surface decreases the critical velocity, which magnifies the window of deposition. This corresponds to the findings of other groups, who suggest preheating of the substrate [40,42,43]. All these findings indicate that local temperature changes have a sensitive influence on the size of the fixed asperities. A crucial aspect is the calculated particle velocities using Lagrangian method. This method is well suited to make qualitative statements and to weight influencing factors as in our previous study [44]. However, the absolute values should be used carefully. In Ref. [15] the authors determined the critical velocity of 316L particles with the size of 25 μm between 700 and 750 m/s. The velocities of particles obtained by simulation with the Lagrangian method for most studied particle sizes are in a range, which is clearly below the model predictions. However, the impact temperatures of 20 °C assumed in Ref. [15] are well below the prevailing process temperatures. Higher temperatures will result in softer material behaviour and a decrease of the bonding velocity. The own determination of the critical velocity, which account for the actual impact temperature, is exceeded by particle sizes smaller than 2.5 μm. Taking equation (2) into account; the critical velocity exceeds the impact velocity for all particle sizes significantly. Although this model bases on experimental data, it does not depend on any material parameters or process parameter. However, Schmidt et al. predicted a minimum particle diameter of a few hundred nanometres for 316L steel [15], which fits the observations from this study. In order to integrate all occurring phenomena into a valid model, further studies are necessary. The performed FIB cut through deposited particles indicates a strong but regular deformation of the particles during the impact. Due to the strong deformation during the impact, a half-shell structure can develop. However, the images only show a small section of the surface. Furthermore, although the exact center of both particles was targeted during the cut, a slight deviation from this line of intersection can lead

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