Microstructuring of titanium surfaces with plasma-modified titanium particles by cold spraying

Particuology 44 (2019) 90–104

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Microstructuring of titanium surfaces with plasma-modified titanium particles by cold spraying P. Breuninger a,∗ , F. Krull a , S. Buhl a , A. Binder b , R. Merz c , M. Kopnarski c , B. Sachweh b , S. Antonyuk a a

Technische Universität Kaiserslautern, Institute of Particle Process Engineering, Kaiserslautern, Germany BASF SE, Ludwigshafen, Germany c IFOS GmbH, Institute for Surface and Thin Film Analysis GmbH, Kaiserslautern, Germany b

a r t i c l e

i n f o

Article history: Received 28 March 2018 Received in revised form 9 August 2018 Accepted 14 August 2018 Available online 18 February 2019 Keywords: Chemical vapor deposition Cold spray Surface modification Computational fluid dynamics

a b s t r a c t Although the deposition mechanisms of the cold spray process are well studied, few reports regarding the use of surface-modified particles exist. Herein, titanium particles 3–39 ␮m in size and with an angular shape were modified in a plasma-enhanced chemical vapor deposition process in Ar, Ar-C2 H2 , and N2 plasmas. After Ar-C2 H2 and N2 treatments, the respective presence of TiC and TiN on the particle surface was confirmed via transmission electron microscopy and energy-dispersive X-ray, X-ray photoelectron, and Raman spectroscopies. The powders were deposited on titanium substrates by cold spray experiments, where unmodified particles up to 10 ␮m in size exhibited a successful surface bonding. This finding was described by an existing analytical model, whose parameters were achieved by computational fluid dynamics simulations taking the particle shape factor into account. A good deposition of plasma-modified particles up to 30 ␮m in size was experimentally observed, exhibiting an upper size limit larger than that predicted by the model. Higher surface roughness values were found for plasmamodified particles, as determined by 3D scanning electron microscopy. The water contact angle indicated that argon treatment influenced the wettability. Tribological tests showed a decrease of the initial friction coefficient from 0.53 to 0.47 by microstructuring. © 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Introduction Targeted alteration of a surface topography is a current subject of research where, in particular, the cold spray method is effective as a surface coating process that can enhance various component properties such as friction, wear, and corrosion behavior (Moridi, Hassani-Gangaraj, Guagliano, & Dao, 2013). Since its invention in the late 1980s (Alkhimov, Papyrin, Kosarev, Nesterovich, & Shushpanov, 1994) the cold spray method has undergone continual optimization for specific applications and is an economically competitive method widely used in the industry today. In the cold spray process, particles are dispersed in a gas stream that is heated up and accelerated to supersonic velocities, which is usually achieved by dispersing the particles into a high pressure (up to 4 MPa) gas stream that is subsequently expanded via a Laval nozzle. The particles are affixed to the surface during impact by their high kinetic

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

energy, resulting in plastic deformation of the particles and the substrate and a local warming of the contact area during the impact (Papyrin, Kosarev, Klinkov, Alkhimov, & Fomin, 2007). In contrast to other thermal spraying methods, the temperatures used in the cold spray method are relatively low. Therefore, the cold spray process can be applied to a variety of applications and materials, such as thermally sensitive particles. For example, titanium dioxide particles have been applied in the anatase phase to create active photocatalytic surfaces (Yang, Li, Han, Li, & Ohmori, 2008). The conditions whereby particles are successfully bonded to a substrate surface have been investigated by Dykhuizen et al. (1999), whose experiments and 2D finite element method (FEM) simulations showed that bonding takes place at temperatures below the melting point of the particles and substrate. Further, they observed that material is pressed out at the particle–substrate interface to form a so-called material-jet, which can be considered as an indicator for successful bonding of the particle to the surface. Grujicic, Zhao, DeRosset, and Helfritch (2004) have discussed several bonding mechanisms present during cold spraying. They studied the deposition of different particle/substrate materials by

https://doi.org/10.1016/j.partic.2018.08.002 1674-2001/© 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustrations of different cold spray technologies.

FEM simulations and reported that adiabatic shear instability is a dominant bonding mechanism. A quantification of the conditions under which bonding takes place has been reported by Schmidt et al. (2009). Using thermal softening data and the findings obtained in the field of ballistics, they developed an empirical model for a critical impact velocity that must be exceeded to provide enough energy for a sufficient bonding. However, the exact mechanisms of the particle bonding have not yet been conclusively clarified and depend strongly on the powder properties and the process conditions present during particle impact. The nozzle design is a critical aspect of the process, as has been shown in Alkhimov, Kosarev, and Klinkov (2001). Dykhuizen and Smith (1998) have performed a fundamental study on an optimal particle acceleration in Laval nozzles, which established the basis for industrial production. The nozzle geometry can be optimized for certain applications where, for example, Yin, Zhang, Guo, Liao, and Wang (2013) determined the optimal nozzle length for the maximum acceleration of copper particles in the size range of 10–20 ␮m. Using geometric variations, Suo et al. (2013) have conducted a numerical study on the particle distribution in cold spraying. In our previous work, a method has been shown for adapting the nozzle geometry for various materials in a single nozzle. This optimization was performed by numerical simulations of the gas flow and particle movement, which was validated in cold spray experiments (Buhl, Breuninger, & Antonyuk, 2017). There is extensive literature concerning the reduction of friction by cold spraying. Ji et al. (2013) have applied a mixture of tungsten carbide and cobalt particles to a steel surface, wherein the soft cobalt was used as a kind of binder to fix the very hard tungsten carbide particles to the surface. It was found that layers created by cold spraying exhibit better wear resistance than similar layers created by high-velocity oxygen fuel spraying. In another work (Spencer, Fabijanic, & Zhang, 2009), a similar process was used to produce a layer from a mixture of Al2 O3 and Al particles on a magnesium substrate, resulting in a better wear resistance and a lower coefficient of friction than layers made exclusively from aluminum particles. Further, cold-sprayed layers comprising mixtures of even harder diamond and nickel particles on steel surfaces that were simultaneously heated up by a laser beam exhibited significantly lower friction coefficients than layers that were fixed by a laser treatment of the same particles (Yao, Yang, Li, & Li, 2015). Finally, Buhl et al. (2015) have investigated the friction properties of case-hardened steel chain pins microstructured by cold-sprayed TiO2 particles. Most studies on the cold spray process focus on the creation of dense coating layers from untreated particle materials on the components. In our previous studies (Buhl et al., 2015, 2017; Schmidt et al., 2017), we suggested that the generation of a certain surface

micromorphology was possible by applying a single layer of particles. The purpose of the present work is to investigate a new variant of this process in which a monolayer of particles with a modified particle surface is fixed to a surface for the purpose of generating a certain microstructure with a variable surface chemistry (Fig. 1). The usage of surface-modified particles may change the conditions whereupon particle bonding occurs in the cold spray deposition. Various possibilities exist for the surface modification of particles, such as a fluidized bed coating or chemical reactions in the gas or liquid phase. One method for the surface modification of particles is chemical vapor deposition (CVD), which is a chemical process to produce high-purity, high-performance solid materials that include coatings by the decomposition of suitable precursor gases or vapors, and can be used for microfabrication processes to deposit materials in various forms (Choy, 2003). In recent years, CVD has been increasingly investigated for particle coating and structuring (Binder, Heel, & Kasper, 2007; Binder & Seipenbusch, 2011; Vahlas, Caussat, Serp, & Angelopoulos, 2006). The CVD process involves chemical reactions of gaseous reactants on or near a heated substrate surface and, in the case of particle coating, a good gas–solid contact, an intensive solids mixing and, in particular, a sufficient accessibility of the single particle surface area are required for a homogeneous coating of all particles. One of the most promising reactor types for this task is a fluidized bed reactor (FBR), which provides very good mixing, heat exchange, and mass transport. Normally, thermal energy is applied to initiate the chemical reaction in the CVD process, and a suitable alternative energy source is the use of a plasma. In such a plasma-enhanced CVD (PECVD) process the vapor reactants are ionized and dissociated by electron impact, and hence generate chemically active ions and radicals that undergo heterogeneous chemical reactions at or near a substrate surface and deposit the solid material (Mueller et al., 2015). For the coating of particles, a PE-CVD process can be conducted in an FBR to combine the advantages of both processes (Flamant, 1994; Sathiyamoorthy, 2010; Von Rohr & Borer, 2007). In a plasma process, it is possible to modify particle surfaces without changing the chemical or physical bulk properties of the material, and especially when the process is running at low temperatures. For example, Shin and Goodwin (1994) described the coating of silica particles with carbon in a microwave-frequency PE-CVD FBR without changing the bulk properties of the silica particles. The PE-CVD process can also be used to deposit carbides, nitrides or carbonitrides on metal surfaces (Sachdev & Scheid, 2001), which can be especially interesting for tribological applications owing to their hard surfaces.

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The bonding models for the cold spray process presented in the literature are based on empirical studies and cannot describe possible additional effects arising from a surface layer on the particles or a change of their mechanical properties during CVD treatment. Therefore, the aim of the presented work is to prove the usability of particles modified by CVD for the cold spray process. Herein, titanium particles were processed in a PE-CVD FBR with argon-acetylene and nitrogen plasmas to create layers of titanium carbide and titanium nitride. Treatment in argon plasma under similar conditions was performed as a reference. The untreated and plasma-modified particles were then deposited on titanium substrates and their deposition efficiency was compared using scanning electron microscope (SEM) images. Changes in the friction and the wetting behaviors were determined with a tribometer and contact angle measurements.

Materials and methods Substrate and particles Titanium (grade 2) samples were used as a substrate for the cold spray experiments, where the substrate sample had an area of 30 × 30 mm2 and a thickness of 2 mm. The samples were polished in three steps from course to fine with Joke Magic Diamond paste with grain sizes 6, 3, and 1 ␮m, sequentially. They were subsequently cleaned with isopropanol in an ultrasonic bath for 10 min. Residues of isopropanol were then removed in two sequential heptane beakers with increasing purity. The surface of a polished and an unpolished sample is shown in Fig. 2. The roughness of the samples was measured with a triboindenter (TI Premier, Hysitron, USA) using a diamond Berkovich tip with a tip radius of 190 nm. To obtain the surface roughness, 25 squares with an edge length of 50 ␮m were measured in an even distribution above the sample. Data analysis was conducted with Gwyddion 2.5 (Released 2018-02-02). While the arithmetical mean deviation of the roughness profile of the unpolished sample was Ra = 203 ± 0.04 nm, the polished sample had a roughness of Ra = 144 ± 0.01 nm.

Fig. 2. Scanning electron microscope image of the titanium surface before (left) and after (right) the polishing procedure.

Titanium particles (Alfa Aesar by Fisher Scientific, Karlsruhe, Germany; −325 mesh, 99.5% metals basis) were used for CVD plasma modification and cold spray experiments. The Ti particles were square-edged or angular in shape, which was caused by the mineral production and purification processes. The particle size was measured with a laser diffraction spectrometer (LA-950V2, Horiba, Japan) and was in a range from 3.4 to 39 ␮m (mean volume-based size d50,3 = 14.9 ± 0.03 ␮m and number-based size d50,0 = 8.5 ± 0.05 ␮m). The particle size distribution was related to equivalent spheres. The obtained results were nearly identical for the two used dispersion liquids (i.e., ethanol and water), which excludes any major influence of the liquid phase. The analysis was repeated eight times to determine uncertainties in the measurements. Setup for the plasma modification of titanium particles The principal setup of the PE-CVD FBR is shown in Fig. 3. The reactor was a customized inductively-coupled radio-frequency (RF)-PE CVD FBR on a lab scale for powder treatment in controlled gas atmospheres (Mueller et al., 2015). The reactor consisted of a water-cooled glass reactor with a glass frit serving as sample holder and gas distribution plate. This setup allowed high flexibil-

Fig. 3. Experimental setup for powder modification in a fluidized bed PE-CVD reactor.

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Fig. 4. Experimental setup for cold spray experiments.

ity regarding reaction time and energy input as well as a statistical treatment of the individual powder particles within a sample. An RF generator (PFG 500, Huettinger, Germany) was used to produce the inductively-coupled RF plasma (5 kW, 13.56 MHz). For the experiments herein, the titanium powder described in Section “Substrate and particles” was used. Per experiment, a powder mass of 40 g was filled into the reactor and dried overnight in the evacuated reactor at a base pressure of ∼10−2 mbar. In the coating experiments, the total gas flow was kept constant at 80 sccm (standard cubic centimeter per minute), which was sufficient for a good fluidization of the particles. The gas composition and the reaction time were varied; and the argon, nitrogen, and argonacetylene were used at different mixing ratios (all gases with purity >99.999%). Downstream of the reactor, a rotary vane pump provided a typical working pressure between 1 to 5 mbar for all CVD experiments. The modified particles were characterized using Xray photoelectron spectroscopy (XPS) and SEM and transmission electron microscope (TEM) image analysis, as well as Raman spectroscopy.

the particles at a low concentration and to avoid the formation of a solid covering layer on the surface. The flow of nitrogen, used as the carrier gas, was compressed to 9 barabs and split up into two separated streams. The first stream was preheated in a tube oven to 300 ◦ C, while the second stream was loaded with titanium particles using a powder disperser. Both streams were mixed again in a heating/mixing chamber, where the mixture was heated to 500 ◦ C. Through the Laval nozzle, the aerosol was accelerated to supersonic velocities and sprayed onto the substrate. The substrate was kept a distance of 4 mm from the nozzle outlet and was moved in a meandering way by two magnetic linear drives. The speed of the substrate was 100 mm/s and the distance between spraying lines was 10 ␮m, which led to an overall spray velocity of about 0.5 cm2 /min. The bond durability of the deposited particles was evaluated using the resistance of the bonds to powerful ultrasonic forces. The ultrasonic treatment was performed by a sonotrode with a 10 mm diameter placed at a distance of 3 mm to the substrate inside a beaker filled with demineralized water. The samples were treated for 10 min with an overall specific ultrasonic power of 4.3 W/cm2 .

Setup for the cold spray experiment

Results and discussion

The microstructuring of the titanium substrate surfaces with untreated and with plasma-modified particles was performed with the cold spray setup shown in Fig. 4. A Laval nozzle was used to achieve high gas velocities up to a Mach number of 2.4 at the outlet of the nozzle. The nozzle possessed an orifice 0.8 mm in diameter and a total length of 30 mm, where the geometry was optimized for efficient particle acceleration of the titanium particles as described in Buhl et al. (2017). The sample substrate was moved perpendicular to the fixed Laval nozzle during the experiment, where the sample velocity and particle volume flow were adjusted to apply

Characterization of the powders Table 1 gives an overview over the conditions used for the coating experiments. Three different titanium powder samples were treated by the CVD process for these cold spray experiments. To investigate the influence of the high temperatures of plasma and the fluidized bed process on the titanium particle size distribution, herein we used a pure argon (Ar) plasma because argon gas is inert at the plasma condition. For the surface modification of the titanium particles, a pure nitrogen (N2 ) plasma and an argon-

Table 1 Experimental conditions of the chemical vapor deposition experiments. Modification

Ar plasma N2 plasma Ar-C2 H2 plasma

Gas composition (sccm) Ar

C2 H2

N2

80 – 50

– – 30

– 80 –

Reaction time (min)

Generator power (kW)

15 30 15

1 1,5 1

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Fig. 5. SEM images of the processed titanium powders, showing the initial Ti powder (a), and the Ti powder modified with Ar (b), Ar-C2 H2 (c), and N2 (d) plasmas.

processing in the fluidized bed. This lack of the finest fraction can also be seen in the SEM images (Fig. 5). The nitrogen plasma-treated powder sample, which has the longest reaction time of 30 min, exhibits the largest particles with d10,3 = 13.54 ␮m and almost no particles below 8 ␮m. This can be explained by the fact that smaller particles are more likely to exceed the elutriation velocity in the fluidized bed, and thus larger particles are more likely to remain inside the bed. In the case of the nitrogen plasma, a larger fraction of small particles was elutriated out owing to the longer process time. Particle losses in the argon and argon-acetylene plasmas were comparable, which corresponds to the observation that the fine fraction of particles are elutriated with proceeding process time. The slightly higher entrainment in the argon-acetylene plasma can be explained by the slightly higher gas temperature in the plasma zone, and subsequent higher local gas velocities. For TEM imaging, powder samples were dispersed in ethanol and applied on an ultra-thin carbon TEM grid. The samples were imaged by a TEM (Tecnai G2-F20ST, FEI, Hillsboro, USA) operated at 200 keV under bright-field conditions. Energy dispersive X-ray spectroscopy (EDX) was applied to determine chemical compositions at specific locations in the sample using an EDX detection system with an energy resolution of 131 eV at MnK˛ (EDAX, Mahwah, USA). The images and spectroscopy data were evaluated using the Olympus (Tokyo, Japan) iTEM 5.2 (Build 3554) and FEI TIA 4.1.202 software packages. Fig. 7 shows TEM images of the unmodified (i.e., initial) and nitrogen- and argonacetylene-plasma-modified titanium particles. The unmodified particles exhibit no visible coating layers. The EDX analysis qualitatively shows that the surfaces of the particles are partially oxidized, with decreasing oxygen concentration in the particle core direction. More quantitative statements cannot be given from the EDX analysis because of the overlapping peaks of the oxygen K˛ peak (0.525 keV) and the titanium L˛ peak (0.452 keV). The natural amorphous surface layer cannot be avoided when the sample is exposed to ambient air (e.g., during filling or removing from the reactor or by TEM sample preparation). Surface modification of the titanium particles treated in nitrogen plasma could not be proven by TEM. This is because the nitride layer was too thin to distinguish differences in the crystal lattices and because small elements like nitrogen or carbon could not be clearly detected in the EDX spectra owing to the large particle scale of the titanium particles and the peaks overlapping the titanium L˛ peak (nitrogen K˛ = 0.392 keV, carbon

Fig. 6. Particle size distribution of the titanium particles before and after plasma processing in the PE-CVD process, as measured by laser diffraction.

acetylene (Ar-C2 H2 ) plasma were applied. During the experiments, the samples processed in the nitrogen plasma exhibited particle agglomeration whose large lumps were no longer fluidized. To overcome this issue, titanium particles 33 wt% larger (Alfa Aesar −100 mesh, 99.5% metals basis) were mixed with the smaller titanium particles as a fluidization agent. The larger particles exhibited a good fluidization behavior and broke up the agglomerates of the small titanium particles, resulting in a homogenous fluidization during the entire process time. After the process, the larger titanium particles were removed from the sample by sieving and were not considered during further processing. Before sampling, the powders were thoroughly mixed to avoid segregation. Fig. 5 shows SEM images of the initial and plasma-modified powders, which shows that the angular shape of the particles was retained after the plasma process. The particle size distributions of the plasma-modified powders were measured with laser diffraction spectrometry (LA-950V2, Horiba) and compared to those of the initial powder (Fig. 6). All measurements were repeated eight times. The particle size was quantified using characteristic values of the particle size distribution, where the volume-based size d10,3 , which corresponds to 10% of particle mass, is shown in Table 2. It is evident that a significant fraction of the small particles was lost during the

Table 2 Characteristic values of volume-based (Index 3) and number-based (Index 0) particle size distributions of the powders.

d10,3 d10,0 d50,3 d50,0 d90,3 d90,0

(␮m) (␮m) (␮m) (␮m) (␮m) (␮m)

Initial powder

Plasma modified powder

Before CVD

CVD with Ar

8.8 4.4 14.9 8.5 23.0 15.2

± ± ± ± ± ±

0.00 0.10 0.03 0.05 0.01 0.00

10.6 7.1 16.5 11.7 24.1 18.2

± ± ± ± ± ±

0.00 0.00 0.01 0.0 0.01 0.04

CVD with Ar-C2 H2 11.7 8.3 17.1 13.0 24.4 19.3

± ± ± ± ± ±

0.00 0.01 0.02 0.01 0.02 0.01

CVD with N2 13.5 9.5 18.6 15.4 24.4 21.0

± ± ± ± ± ±

0.01 0.04 0.04 0.02 0.06 0.02

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Fig. 7. Transmission electron microscope images of the Ti particles (a) unmodified and after modification using (b) N2 and (c, d) Ar-C2 H2 plasma.

Fig. 8. X-ray photoelectron spectral measurements of the unmodified and modified titanium particles.

K˛ = 0.277 keV). The argon-acetylene plasma processing, however, was found to result in an amorphous surface layer on the titanium particles. Again, TEM analysis was challenging owing to the large particle size, but image analysis revealed a thin but irregular amorphous layer attributed to carbon but cannot be clearly proven by EDX. Therefore, the surface composition was measured by XPS (Phi Versa Probe 5000, Ulvac-Phi, Japan) using monochromatic Al K˛ radiation (49 W). Detailed information about the analytical procedure can be found in Weber et al. (2017). The results of the XPS measurements are shown in Fig. 8. The native amorphous oxidized layer of the unmodified titanium particles can be clearly seen by the high surface oxygen content (approx.

53.8 at%). The carbon content (∼22 at%) is most likely owing to absorbed hydrocarbons from the environment. The nitrogen content is negligible. The reference sample, which was exposed to a pure argon plasma, exhibits a nearly identical surface composition of these elements to that of the unmodified sample, indicating that no contaminants were introduced by the CVD process. The successful surface modification after nitrogen plasma treatment can be seen by the increased nitrogen content of about 21.7 at%, where the oxygen and carbon content is still present. While the oxygen surface concentration decreased significantly to about 33.5 at%, the carbon content remained nearly constant. This indicates a replacement of the amorphous oxygen layer by nitrified surface groups, while the carbon content can still be attributed to absorbed environmental hydrocarbons. Detailed analysis of the Ti 2p and N 1s spectra confirms the presence of titanium nitride (TiN, 454.5 eV), titanium oxynitride (TiON, 455.8 eV), and titanium oxide (TiO2 , 458.7 eV). A quantitative analysis is difficult, however, and strongly depends on the fitting parameter. Qualitatively, the main contents comprise titanium nitride and titanium oxide. Therefore, the surface is not a pure titanium nitride, but comprises oxynitride and partially unmodified TiO2 as well. The argon-acetylene plasmaprocessed particles exhibit a high carbon surface content of about 65.5 at%, while the oxygen content decreased to 14.8 at%. Detailed analysis of the Ti 2p and C 1s peaks indicate a primary contribution from titanium carbide (454.6 and 281.7 eV) and a mixture of sp2 (284.2 eV) and sp3 (284.8 eV) hybridized carbon species. Carbon oxide species such as C–O, C O, or COOR groups are negligibly observed. This indicates that, at the present conditions, the argonacetylene plasma process results in a mixture of carbide formation

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Fig. 9. X-ray diffraction patterns of the initial and modified titanium particles.

as well as carbon coating, whereas the main content is owing to the carbon coating. Raman spectra were recorded using a micro Raman spectrometer (NTEGRA, NT-MDT, Russia) operated in the backscattering mode. Measurements were performed with a laser excitation wavelength of 514 nm and an optical objective of 50×. The laser intensity at the sample surface was maintained below 1 mW to avoid laser-induced altering of the sample. Powder X-ray diffraction measurements (XRD; D8, Bruker, USA) were also performed. Fig. 9 shows the XRD patterns of the initial and plasma-modified titanium particles. Without exception, the patterns of the initial particles and the argon plasma-processed particles exhibit peaks attributed to hexagonal titanium. These results were confirmed by the Raman spectra of both samples (Fig. 10). No Raman peaks were

observed in the spectrum of the initial titanium sample because it is metallic in nature and not Raman active. Furthermore, the spectrum of the argon plasma-treated particles exhibits no peaks, indicating that the surface was not chemically changed during the plasma process. In addition to hexagonal titanium peaks, the XRD pattern of the nitrogen plasma-modified particles exhibits tetragonal titanium nitride peaks, indicating a modification of the particle surface without changing the titanium bulk character of the particles (Fig. 9). In addition, very small peaks of titanium oxynitride can be observed. The formation of oxynitride can be the result of nitration of the amorphous oxidized titanium surface or of incorporation of oxygen from minor leaks in the reactor or gas line. However, the oxynitride content is negligible. The Raman spectrum of the nitrogen plasma-processed sample in Fig. 10 indicates the formation of titanium nitride by the presence of three broad bands at wavelengths around 201–230, 330, and 550 cm−1 . These bands are known to belong to the acoustic mode band (wavelength region between 180 and 360 cm−1 ) and to the optical mode band (between 500 and 600 cm−1 ) of titanium nitride (Kataria et al., 2012; Chen, Liang, Tse, Chen, & Duh, 1994). In the case of the argon-acetylene plasma-treated sample, the XRD pattern exhibits peaks attributed to cubic titanium carbide in addition to the dominating peaks of hexagonal titanium. Again, this indicates a modification of the particle surface without changing the titanium particle bulk (Fig. 9). The Raman spectrum of the argon-acetylene-processed sample is shown in Fig. 10. Stoichiometric titanium carbide is not Raman active, but any nonstoichiometry or defects will produce a Raman signal. The peaks in the wavenumber region between 200 and 800 cm−1 are attributed to the presence of a hypo-stoichiometric or defective Ti carbide phase (Lohse, Calka, & Wexler, 2005; Klein, Holy, & Williams, 1978). This under-stoichiometric carbide will most likely be formed on the boundary layer between the surface Ti carbide and the titanium bulk. Furthermore, peaks of a pure carbon phase are visible at a wavelength of 1350 cm−1 , representing the D band, and 1584 cm−1 , representing the G band (Tuinstra & Koenig, 1970). The presence of

Fig. 10. Raman spectral measurements of the unmodified and modified titanium particles.

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Fig. 11. Scanning electron microscope images of the various cold sprayed titanium particles: (a) unmodified particles and particles processed in (b) Ar, (c) Ar-C2 H2 (d) and N2 plasma.

pure carbon peaks clearly proves a carbon coating of the titanium particle simultaneous with the carbide formation.

Cold spray experiments with initial and plasma-modified titanium particles The initial titanium particles and the samples modified in the CVD process were deposited on titanium substrates using the cold spray method. All cold spray experiments were conducted with the same process parameters described in Section “Setup for the cold spray experiment”. Fig. 11 shows SEM images of the titanium substrates after cold spraying of four different titanium particle samples: initial and post-processing in argon, nitrogen, and argonacetylene plasmas. An obvious difference is observed between the substrate surfaces cold sprayed with the initial particles and with particles modified in the CVD process. The deposited initial particles are evenly distributed on the surface and range in size up to 10 ␮m, which corresponds to 8.8% mass fraction of the sprayed powder. It is striking that larger particles are not deposited on the substrate. In contrast, the surfaces after microstructuring with particles processed in the FBR show a large number of particles with sizes up to 40 ␮m. As discussed in Section “Setup for the plasma modification of titanium particles”, a significant fraction of the fine particles present in the initial titanium powder is lost after plasma processing in the fluidized bed process because of elutriation. With the plasma-modified samples, therefore, particles are deposited on the surface that are significantly larger than

the unmodified sample. Nevertheless, fine particles can also be observed in the SEM images of the modified particle samples, covering the entire surface. Notably, particles larger than 10 ␮m occurred also in the initial powder but were not observed in the deposited sample. Because the cold spray process conditions were the same for the unmodified and modified titanium particles, it must be concluded that the deposition mechanisms during collision between these two sample types were different. Consequently, the reason must be found in the surface change during the plasma process. The XRD and XPS measurements show that, beside the tetragonal bulk material, the surface chemistry was only changed after nitrogen and argon-acetylene plasma processing, and stayed constant in the argon plasma (compare Figs. 8 and 9). Because the size range of the deposited particles was about the same for all three plasma-modified samples, the kind of plasma treatment should play a secondary role in the deposition mechanisms. However, being treated in a plasma could change the mechanical properties of the particles in a way that enhances the deposition of larger particles, and it is possible that the high process temperatures change the mechanical properties of the surfaces, such as hardness. Fig. 12 shows SEM images of a sample before and after ultrasonic treatment with the method described in Section “Setup for the cold spray experiment”. Only a few particles were detached by the procedure, while the majority of the deposited particles remained bonded on the entire substrate. This result was observed in the samples with surface-modified particles,

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Fig. 12. Scanning electron microscope images of a section of cold sprayed titanium substrate (with unmodified titanium particles) before (left) and after (center) ultrasonic treatment. Plastic deformation can be observed at the impacted edge of a particle (lower right).

regardless of the particle size. It was seen that, for a stable bonding, it was sufficient for a square-edged region of the particle to be penetrated into the surface (lower right image, Fig. 12). CFD simulation of the cold spray process To achieve a deeper understanding of the impact behavior of the titanium particles, we carried out a simulation of the gas flow and the particle motion from the nozzle inlet to the impact point on the substrate surface using computational fluid dynamics (CFD; ANSYS Fluent Version 17.1). A rotationally symmetric model of the Laval nozzle and the adjacent areas was generated, as shown in Fig. 13(a), with a stuctured grid with 50000 cells for the flow region and 6500 cells for the substrate. The flow field of the simulation was compared with that of a simulation with 4 000 000 grid cells, whereby no significant deviations were found. The flow field inside the nozzle and in the jet was estimated by a density-based, stationary CFD simulation assuming a steady and axisymmetric flow. The k–ω shear stress transport (SST) model was used because of its capability of describing flows close to walls and within open jets. Previous works have assumed the gas viscosity to be constant, but a significant temperature decrease is present owing to the strong gas expansion in the Laval nozzle. Therefore, this dependency should be taken into account. According to the review of Yin, Meyer, Li, Liao, and Lupoi (2016) discussing different models, there is still deliberation regarding the correct modeling, although the model of Sutherland (1893) has commonly been used to account for the temperature dependence of the viscosity. Herein, the substrate was modeled as titanium with a density of 4858 kg/m3 , a heat capacity of 544.25 J/(kg K) and a thermal conductivity of 7.44 W/(m K). The fluid-side heat transfer coefficient and the temperature gradient in the substrate were determined by solving the energy equations. A nozzle inlet pressure of 0.9 MPa was set as the boundary condition and the outlet pressure was 0.1 MPa, all in absolute values. The initial temperature of the gas was 773 K, according to the conducted experiments. The calculation resulted in a gas mass flow rate of 2.3 kg/h, where the mean velocity of the fluid at the inlet was 3.2 m/s. The gas velocity and temperature fields obtained by CFD simulations of the gas are given in Fig. 13(b) and (c). The scale was the fluid flow, which was initially slow, was highly accelerated inside the nozzle, while the gas temperature in the nozzle decreased by 300–400 ◦ C from the initial temperature of 500 ◦ C. The gas jet was focused on the flat surface of the substrate, where a stagnation region was generated in which the gas decelerated quickly and was heated to a temperature almost the same as the initial temperature. Thus, a surface temperature Tsubs = 450 ◦ C was obtained at the axis of the nozzle jet.

The particle velocities were calculated with an equation of motion, such that mp ·

៝p du = F៝D , dt

(1)

៝ p and mp are the velocity vector and mass of the particle, where u respectively; and the drag force FD is calculated using the approach proposed by Haider and Levenspiel (1989) that considers the shape factor of the particles according to the Wadell factor  Wa (Wadell, 1934). This shape factor (Eq. (2)) is defined as the surface of a sphere, As , with the same volume as a real particle divided by the real particle surface area, AP , given as Wa =

As . AP

(2)

The effect that gas flow compressibility has on the drag at high Mach numbers was not considered in the present study. However, the particle impact velocity was slightly underestimated by neglecting this effect, resulting in a conservative consideration for the impact velocity. A parametric study was conducted to evaluate the influence that the particle size distribution and the initial position of the particles in the nozzle inlet cross-section has on the impact velocities, where the initial velocity of the particles was zero. Fig. 14 shows the evolution of the velocity of a flow particle from the nozzle inlet starting at the rotational axis to the collision point on the substrate. The simulation was performed for six different particle sizes. Small spherical particles ( Wa = 1) 0.5 ␮m in size accelerate to the gas velocity very quickly, but they decelerate as soon as they come into the stagnation area above the substrate. Although particles of this size were not present in the applied titanium powder, this size was considered in the simulation to describe the effect of a small particle size in general. Owing to their greater inertia, larger spherical particles were accelerated to smaller velocities, but their deceleration in the stagnation area was not significant. All considered particles exhibited a collision angle that was normal to the wall surface. Further, the particle temperature followed the change of the gas temperature. The specific heat capacity of particles was assumed to be isotropic, and the heating or cooling of the particle depends on the actual heat transfer coefficient, which is a function of the actual particle velocity. Specifically, smaller particles experience a more rapid temperature change according to the larger heat transfer coefficient. Small particles 0.5–1 ␮m in size can rapidly lose heat, but they experience heating right before impact. The temperature of large particles 50 ␮m in size remains almost constant. Simulations using particles with a smaller shape factor (i.e.,  Wa = 0.65) exhibited greater particle velocities than those of spheres. Specifically, 10 ␮m particles with a shape factor of 0.65

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Fig. 13. Numerical results of the velocity and the temperature profile during the cold spray process.

Fig. 14. Particle velocity (top row) and temperature (bottom row) along the rotational axis of the nozzle and spray obtained by CFD simulation for titanium particles with the shape factor of 1 (left column) and 0.65 (right column).

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Fig. 15. Impact velocities (top) and temperatures (bottom) of titanium particles with varying shape factors obtained by computational fluid dynamics simulations of the spray process.

reached impact velocities that were a factor of 1.5 greater than spherical particles. Further, it was found that smaller equivalent diameters reduced the effect of the shape on the velocity. In addition, the temperature of the non-spherical particles was slightly greater over the complete distance up to impact than that of the spherical particles. Taking the initial particle position into account, a parametric study was conducted to estimate the impact velocities for a wide range of particle sizes. In Fig. 15, the calculated impact velocity and the corresponding particle impact temperature are plotted, where the deviation bars indicate the spread of the impact velocity according to the initial position in the nozzle inlet. The obtained impact velocities were compared with the critical impact velocities resulting from different analytical models found in the literature (Fig. 15). Grujicic et al. (2004) conducted a finite element study to localize adiabatic shear instabilities during particle impact for a titanium–titanium pair, and obtained the critical velocity of 657 m/s. Schmidt, Gärtner, Assadi, and Kreye (2006) estimated a semi-empirical equation for the critical velocity vcrit , given as

vcrit =

     F1 · 4 · TS · 1 − Ti −Tref  Tmel −Tref 

+ F2 · cp · (Tmel − Ti ),

(3)

where Ti is the impact temperature;  is the particle density; cp is the heat conductivity;  TS and Tmel describe the averaged tensile strength and melting temperature, respectively, of both the substrate and the particle material; and F1 and F2 are empirical

Table 3 Constants and variables used for the model of critical impact velocity. F1 F2 Tref (K) Tmel (K) TP (K) Tsubs (K) cp (J/(kg K))  TS (MPa)  (kg/m3 )

1.2 0.3 293 1943 700 723 523 340 4506

factors. For 25 ␮m-diameter titanium particles impacted on a titanium substrate, Schmidt et al. (2006) calculated the critical bonding velocity of 750 m/s with a range up to 810 m/s owing to uncertainties. Herein, the unknown parameter of impact temperature Ti was obtained by the CFD simulations. The calculated mean particle impact temperature TP = 427 ◦ C was averaged with the surface temperature Tsubs = 450 ◦ C to Ti = 438 ◦ C. This value was used in the model and results in a critical particle impact velocity of 680 m/s. All used values are given in Table 3. The obtained values of the critical bonding velocity show a wide deviation between the existing models. Nevertheless, for all of the models herein, bonding is predicted only for 1 ␮m-diameter particles. For larger particle sizes up to 5 ␮m, the performed CFD simulations and the model of Grujicic predict a successful bonding for particles with small shape factors ( Wa = 0.5), which agrees with the experiments conducted with the initial titanium powder (Fig. 12). The experimentally-obtained maximum size of the deposited particles was about 10 ␮m, where the small deviation

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Fig. 16. 3D scanning electron microscope images of the surfaces microstructured with (a) initial particles, and particles modified with (b) Ar, (c) Ar-C2 H2 , and (d) N2 plasmas.

from the model relates to the assumption of an ideal spherical particle shape. This assumption makes it impossible for the models to exactly describe the real stresses in the edge-flat contacts between the used angular shaped titanium particles and the surface. The SEM images in Fig. 12 show that the particles often collide with a sharp edge impacting the surface. The small radius of the curvature in the contact may result in increased contact stresses compared to a spherical particle. This leads to a larger plastic deformation, and thus larger particles can be fixed successfully to the surface. The good bonding of the plasma-treated particles cannot be explained with any of the used models. Nevertheless, a bonding of particles with the sizes up to 39 ␮m was experimentally observed (Fig. 11), which was most likely caused by an additional bonding mechanism governed by the particle surface properties.

actual surface area was determined by transferring the obtained 3D images into triangles and calculating their area. The magnification factor r in Table 4 denotes the ratio of the real surface area to a flat area. The determined roughness values demonstrated a large deviation between the surfaces microstructured with the initial titanium powder and those with the plasma-modified particles. The mean value of the maximum height RZ for the surface microstructured with the initial powder was about 14 ␮m, while the plasmamodified particle-microstructured surfaces were in the range RZ = 30–35 ␮m. These dimensions agree well with the obtained 2D SEM images (Fig. 11). Small variations of the roughness values were found between the surfaces microstructured with the various plasma-modified particles, but the higher standard deviation made comparison of these surfaces impossible.

Measurement of the surface roughness Wetting behavior of microstructured titanium samples The surface roughness of the cold sprayed surfaces could not be determined with the triboindenter, as was used in Section “Substrate and particles”, because the piezo element that controlled the height shift had a maximum amplitude of 6 ␮m that was smaller than the largest deposited particles. The surface roughness was therefore estimated by the “shape from shading” method (Zhang, Tsai, Cryer, & Shah, 1999), where a scanning electron microscope (SEM; 2Gpro, Phenom, Netherlands) was applied for this measurement. In a 3D roughness mode, the backscatter electron detector of the SEM capture images from different angles to observe the “shadows”, from which a 3D reconstruction of the surface was performed. In this mode, the SEM had a constant image resolution of 512 × 512 pixels. Nine representative measurements of the surface were carried out, and one image for each microstructured surface is shown in Fig. 16. A representative surface area of 100 × 100 ␮m2 was chosen to obtain variations in the surface roughness, where the obtained height profiles were analyzed in MATLAB line by line. The obtained roughness data concerning the RA (i.e., mean value of the arithmetic average of each of 512 lines), the RRMS (i.e., mean value of the root mean square of each line), and the RZ (i.e., total height between the five lowest and the five highest peak values) are given in Table 4. The Table 4 Determined surface roughness of the microstructured with titanium particles. Initial

Plasma modified in CVD process Ar

RA (␮m) RRMS (␮m) RZ (␮m) r (␮m)

0.95 1.38 13.91 3.44

± ± ± ±

0.10 0.11 1.41 0.24

4.38 5.63 33.66 12.47

Ar-C2 H2 ± ± ± ±

1.82 2.21 8.21 4.71

3.33 4.88 34.84 12.38

± ± ± ±

N2 0.67 1.09 6.12 7.72

2.65 3.62 29.95 7.70

± ± ± ±

1.17 1.66 12.65 3.06

The static water contact angles were determined with the sessile drop method using a contact angle measuring system (Contact Angle Measurement System G2, Krüss GmbH, Hamburg, Germany). A constant drop size was obtained by adjusted by the system automatic dosing unit. Further, the measurements were repeated three times on each sample and each measurement repetition was performed in a fresh area region of the sample. Herein, double-distilled water was used as the test fluid for the contact angle measurements, while the environmental conditions of humidity and temperature were recorded and found to remain constant during the measurement series. A tangent fitting method was applied to analyze the camera images of the sessile drop and determine the contact angles, and the results are shown in Fig. 17. As a reference, a polished titanium sample treated with the cold spray process with no particles was used to obtain the wetting properties of the substrate surface after gas treatment. The microstructured surface with the initial titanium particles exhibited an increase of the contact angle from 64.06◦ ± 0.72◦ to 75.57◦ ± 2.9◦ . A comparison of the experimental results with Wenzel’s law (Wenzel, 1936), which describes the wetting behavior depending on the surface roughness, exhibited no sufficient agreement because the model predicts an increase in hydrophilicity with increasing roughness r (Table 4). The particle modifications exhibited different effects on the wetting behavior. The substrate surfaces microstructured with argon-acetylene- or nitrogen-plasma-modified particles exhibit a higher contact angle (∼80◦ ) than those microstructured with the initial particles, wherein the nitrogen-plasma-modified particlemicrostructured surface exhibits a slightly higher contact angle. Interestingly, the deposition of particles treated in argon plasma had a significantly larger effect on the contact angle, reducing it to 42◦ . This effect corresponds to the findings of Doundoulakis (1987)

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Fig. 17. Wetting behavior of the microstructured surfaces (The images show the wetting of surfaces measured before storage).

Fig. 18. Coefficient of friction between titanium substrates with various surface structures and a titanium ball with a linear reciprocating tribometer, using a moving average of ten data points (a); SEM images of the contact area (b).

and Duske et al. (2012), who observed a significant decrease of the contact angle of water on argon-plasma-treated flat titanium surfaces, most likely owing to a complete removal of minor surface contaminants. To determine the effect of storage time on these results, the measurements were repeated after six months of storage at ambient conditions in a closed and dark sample container. Each surface was measured 10 times with 6 ␮L droplets and the contact angle was measured on both sides of the droplet. The storage time showed a significant change only for the argon-modified particlemicrostructured samples (Fig. 17). Friction behavior of microstructured titanium samples To characterize the tribological properties of the microstructured titanium samples, the coefficients of friction were determined by a linear reciprocating tribometer (Tribometer, Anton Paar Ltd., Austria), which was a pin-on-disk tribometer equipped with linear reciprocating module and a 6-mm-diameter titanium ball as the counterpart. The titanium substrates with and without coldsprayed particles were compared, and the test was performed in reciprocating mode with a constant normal force of 2 N, a maximum

velocity of 1 cm/s between the turning points, and an amplitude of 2 cm. The results are shown in Fig. 18. The coefficient of friction of the polished titanium substrate without particles starts at about 0.73 and decreases to about 0.53 after 550 cycles, which corresponds to a distance of 22 m. However, no constant coefficient of friction value was reached during this number of cycles. The large fluctuations result from surface fractures that occur at the beginning of the tribological exposure. In comparison to the polished titanium substrates, all surfaces microstructured with particles exhibit a lower coefficient of friction at the beginning of the experiments of about 0.56. The measured curves all exhibit a similar behavior, where the coefficient of friction of all of the studied samples with particles decreased with the distance and reached a constant value of about 0.47 after 200 cycles, which corresponds to a displacement of 8 m. A dependence of the friction behavior upon the particle size of the fixed particles or upon the surface modification of the particles could not be observed. The SEM images of the sample with the unmodified particles (Fig. 18 right) show that the particles in the contact area were partially sheared off and partially crushed owing to the high normal stress. This was also previously observed in Buhl et al. (2015). Fluctuations of the coefficient of friction value

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of the polished substrate indicate stick-slip effects owing to strong adhesion between the substrate and the friction body. In the case of the surfaces structured with rougher particles, the direct contact between the counterparts was owing to the roughness asperities. Furthermore, the debris material produced owing to wear also acts as asperities, and the ploughing of the asperities and debris material will dominate the friction and wear. After a number of cycles, the coefficients of friction are equivalent with those of the samples with the plasma-modified particles. The initial reduction of the friction is likely owing to the interaction of various influencing variables such as surface roughness, particle material, and particle abrasion that serves as a solid lubricant.

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cation of the particles. Compared to a polished reference sample, the measured contact angle increased after cold-spraying of particles that were unmodified (17%) and those that were modified in argon-acetylene (22%) and nitrogen (30%) plasmas. The sample with argon-modified particles showed a 35% lower contact angle. The present study shows the possibility of applying CVD surfacemodified particles on surfaces via the cold-spray process, and can be extended in the future to create surfaces with specific properties. For further investigations of the wetting and tribological behavior, a separation of the individual phenomena of surface chemistry and microtopography may be performed. Acknowledgements

Conclusions Titanium powder with particles in the size range of 3–39 ␮m was modified in Ar, N2 , and Ar-C2 H2 plasmas using a customized inductively-coupled RF-PE-CVD-FBR reactor. The XPS, Raman, and XRD measurements demonstrated the formation of titanium nitride (in case of the N2 plasma) and titanium carbide as well as deposited carbon films (in case of the Ar-C2 H2 plasma) on the particles following plasma modification. Layers of these groups could not be identified clearly on TEM images, though deposited amorphous carbon films could be detected. With EDX analysis, the layers were too thin to observe differences in the crystal lattices, which supports the spectroscopy results showing that only the particle surface was modified instead of a major formation of the core–shell structure. The plasma-treated and untreated particles were successfully fixed on titanium surfaces using the cold spray technique, where the particle–substrate bonding quality was verified by highintensity ultrasonic sample treatments. The plasma-modification of the particles led to an increase of the deposition efficiency of larger particles for all of the plasmas herein. Though unmodified particles only in the size range of 3–10 ␮m were fixed on the surface, after CVD treatment a sufficient bonding of particle diameters up to 39 ␮m was achieved under otherwise identical process conditions. To further elucidate this effect, the unknown substrate temperature at impact was estimated by CFD simulations of the cold spray process, while the particle acceleration and temperature were calculated by solving the equation of motion and the energy equation for different particle diameters. The particle size showed a strong influence on the particle impact velocity and the impact temperature, where fine particles about 1–2 ␮m in diameter impacted with the highest velocities (of 790 m/s) and the lowest temperatures (about 182 ◦ C). The particle shape factor was varied in the used drag model, and revealed that the impact velocity and impact temperature increased with increasing deviation from a spherical shape. The obtained particle impact velocities of the untreated titanium particles were in good agreement with the different analytical models for critical impact velocity, which predicts a successful particle–substrate bonding, and with the experimental results. The small differences existing between the experiment and simulation can be explained by the severe deviation of the particle shape from spherical. The experimental results show that only a small region of an angular particle, typically a sharp edge, was penetrated into the substrate and resulted in a sufficient bonding. In the present study, additional bonding mechanisms were not considered and may be investigated in future studies. The coefficient of friction was significantly smaller for all coldsprayed substrates than that of the polished surface without particles. The measured coefficient of friction vs. sliding distance curves were approximately the same for all microstructured surfaces. Thus, the surface microstructure created by the cold-sprayed particles played a more important role than the chemical modifi-

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