Inﬂuence of the Substrate on the Formation Peer Reviewed ...

Influence of the Substrate on the Formationof Metallic Glass Coatings by Cold Gas

SprayingJohn Henao, Amadeu Concustell, Sergi Dosta, Nuria Cinca, Irene G. Cano, and Josep M. Guilemany

(Submitted February 9, 2016; in revised form April 9, 2016)

Cold gas spray technology has been used to build up coatings of Fe-base metallic glass onto differentmetallic substrates. In this work, the effect of the substrate properties on the viscoplastic response ofmetallic glass particles during their impact has been studied. Thick coatings with high deposition effi-ciencies have been built-up in conditions of homogeneous flow on substrates such as Mild Steel AISI1040, Stainless Steel 316L, Inconel 625, Aluminum 7075-T6, and Copper (99.9%). Properties of thesubstrate have been identified to play an important role in the viscoplastic response of the metallic glassparticles at impact. Depending on the process gas conditions, the impact morphologies show not onlyinhomogeneous deformation but also homogeneous plastic flow despite the high strain rates, 108 to109 s21, involved in the technique. Interestingly, homogenous deformation of metallic glass particles ispromoted depending on the hardness and the thermal diffusivity of the substrate and it is not exclusivelya function of the kinetic energy and the temperature of the particle at impact. Coating formation isdiscussed in terms of fundamentals of dynamics of undercooled liquids, viscoplastic flow mechanisms ofmetallic glasses, and substrate properties. The findings presented in this work have been used to build upa detailed scheme of the deposition mechanism of metallic glass coatings by the cold gas sprayingtechnology.

Keywords coating, cold gas spray, deposition mechanisms,metallic glass, substrate effect

1. Introduction

Metallic glasses have garnered extensive interest inrecent years due to their remarkable mechanical proper-ties, excellent corrosion resistance, magnetic behavior,and unique processing capabilities, attributable to the lackof grain boundaries and crystal defects (Ref 1-3). Metallicglasses are used in a variety of applications such as in cellphone cases, golf clubs, watch and ornamental parts, aswell as sensors, actuators, electromagnetic casting, andmagnetic recording heads. Since the discovery of metallicglasses, different families have been developed based onthe combination of more than 30 elements includingtransition metals, rare earth, and alkali elements (Ref 4-7).Among the different families of metallic glasses, Fe-basemetallic glasses have attracted attention due to their rel-atively low-cost, good glass forming ability, high corrosionresistance, high compressive strength, excellent soft mag-netic properties, and catalytic properties (Ref 2, 8, 9).

The amorphous structure of metallic glasses providesthem high strength, hardness, large elastic strains andenhanced corrosion and magnetic properties. However,the lack of a long-range ordered structure leads to acomplex mechanism of plastic deformation including ab-sence of strain hardening. Inhomogeneous deformation,i.e., localized plastic flow, is promoted at high strain ratesand low temperatures, while homogeneous flow takesplace at low strain rates and high temperatures (Ref 10).As a result, metallic glasses present limited plasticity atroom temperature restricting their commercial prolifera-tion. Different techniques have been proposed to over-come this drawback like promoting multiple shear bandformation or homogeneous flow, and/or hindering shearband propagation. The fundamentals of these techniquesconsist in controlling sample size, composition of the glass,and the presence of a second crystalline phase (Ref 11). Atsmall scales, metallic glasses exhibit enhanced plasticityand toughness, and they can be thermoplastically pro-cessed like polymer materials. The basis of forming ametallic glass coating from small-scale metallic glass par-ticles is their enhanced processability at temperaturesabove the glass transition (Ref 10, 12). This allows toproduce larger and complex parts, always keeping themechanical and functional properties offered by metallicglasses.

Cold gas spray (CGS) coatings are mainly built-up onthe basis of kinetic energy. The CGS system consists of agas stream which is conducted through a convergingdiverging nozzle (De-Laval type nozzle) (Ref 13, 14). Thegas is used to accelerate and transport particles up to the

John Henao, Amadeu Concustell, Sergi Dosta, Nuria Cinca,Irene G. Cano, and Josep M. Guilemany, Thermal Spray Centre(CPT), Universitat de Barcelona, C/ Martı Franques 1,08028 Barcelona, Spain. Contact e-mail: [email protected].

JTTEE5 25:992–1008

DOI: 10.1007/s11666-016-0419-3

1059-9630/$19.00 � ASM International

992—Volume 25(5) June 2016 Journal of Thermal Spray Technology

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substrate, developing velocities in the range from 300 to1200 m/s. The gas (typically N2 or He) can be preheatedand its pressure can be controlled in order to modifyparticle temperature and velocity at impact. Impact con-ditions consist in extreme strain rates, 108 to 109 s�1 andhigh cooling rates, 105 to 106 K/s (Ref 15, 17). The particletemperature usually remains below the melting point ofthe material; for this reason CGS is known as a solid stateprocess. Therefore, the coating is built-up by the plasticdeformation of the impacting particles (Ref 12-19). Forpolycrystalline materials, the most important criterion fordeposition is the so-called critical velocity (Ref 12, 19).The critical velocity is a parameter used to control and topredict the optimum deposition conditions of a givenmaterial, i.e., the range of proper temperatures andvelocities of particles before impact. The deposition effi-ciency tendency follows a like-sigmoidal shape in functionof the impact velocity of the process where high deposi-tion efficiencies are obtained just above the criticalvelocity. The concept of critical velocity rises from ballisticexpression of hydrodynamic penetration of particles atimpact, combined with the Johnson-Cook model forsoftening of crystalline materials, which is often used formodeling and simulation of deforming metals (Ref 12, 19).In this manner, the conversion of kinetic energy, at con-ditions predicted by the critical velocity, results into plasticdeformation and subsequent heat generation, localized atthe impact region, promoting bonding at the interfaceparticle/substrate (Ref 19-21).

On the other hand, metallic glasses, due to theiramorphous nature, do not present strain-hardening anddefects such as dislocations involved in the plasticity ofcrystalline materials (Ref 9, 22). In this manner, the con-cept based on the critical velocity results inappropriatesince the softening of amorphous metals above the glasstransition differs vastly from the Johnson-Cook model(Ref 28). For this reason, a good understanding of themechanisms involving deformation in metallic glasses isrequired to develop any estimation of the optimum con-ditions for metallic glass deposition in CGS. Plasticity ofmetallic glasses is based in two different deformationmodes: inhomogeneous flow (localized deformation bymeans of shear band formation) and homogeneous flow(non-localized deformation uniformly distributed). Thetype of flow is exclusively dependent on temperature andstrain rate (Ref 9, 10, 22, 23). In this sense, previousstudies, related with the production of metallic glasscoatings by CGS (Ref 24-26), have been carried out underthe concept of thermal softening between the glass tran-sition temperature (Tg) and the crystallization tempera-ture (Tx). Metallic glasses can deform homogenouslybetween Tg and Tx and therefore, bonding is achievedmore easily. However, preheating particles above Tg is notthe unique requirement to deposit metallic glasses sincetheir mechanical response is also dependent on the strainrates experienced at impact. At high strain rates and hightemperatures, inhomogeneous flow can occur and poordeposition of metallic glass particles obtained (Ref 9, 22).In fact, it is well known that shear bands, i.e., inhomoge-neous flow, are not only promoted at high strain rates and

low temperatures, but they are also formed at tempera-tures around and above Tg if the strain rate is high enough(Ref 27). Recently, based on the equations governing thedynamics of undercooled liquids, the conditions forbuilding up metallic glass coatings by CGS were redefinedin terms of the Reynolds number (Re) of the metallic glassin the supercooled liquid state. The Re relates the inertialforces to the viscous forces acting throughout the metallicglass particle according to the following equation (Ref 28).

Re ¼ qv0d0

lðEq 1Þ

where q, v0, d0, and l are the liquid density, the impactingparticle velocity, the particle diameter, and the liquidviscosity, respectively. When the Re is too small becauseof the large viscosity of the liquid, large elastic deforma-tions are produced during impact leading either torebounding or inhomogeneous deformation (shear bandformation). This behavior is typically seen in non-Newtonian liquids undergoing yielding (Ref 27). Theseare conditions where metallic glass coatings are not able togrow-up. On the other hand, when the Re overtakes acritical value, namely Recrit, shear banding and elasticdeformation are avoided, homogenous deformation ispromoted, and high deposition efficiencies can be ex-pected. The advantage of including the undercooled liquidproperties using the Re of metallic glasses lies in that theanalysis can be extrapolated to different metallic glasssystems. Interestingly, the formation of metallic glasscoatings by CGS can be predicted by means of a windowof deposition based on the Re, which is used to determinethe ideal temperature and velocity range at whichimpacting particles are able to flow and building-up ametallic glass coating. The window of deposition ofmetallic glasses differs from the window of deposition ofcrystalline metallic coatings due to its amorphous natureitself in which the change of viscosity with temperature isexponential (Ref 28). In addition to homogeneous defor-mation, another condition to obtain metallic glass coatingsis the suppression of the crystallization phenomena duringthe spraying process. Crystallization can hinder thedeformation of particles at impact since mechanisminvolving viscous flow begins to be restricted by the atomicplanes in the new crystal structure. Consequently, depo-sition efficiency deteriorates when crystallization occurs inmetallic glasses (Ref 28, 29). In previous studies, thecrystallization of the metallic glass particles could be to-tally avoided due to the high heating rate (around1 9 106 K/s), the high cooling rate (around 1 9 105 K/s),and due to the control of the maximum temperatureachieved by the particles during the CGS process (Ref 28).According to the classic theory of nucleation and growth(Ref 30), the crystallization is determined by the energybarrier for nucleation, the driving force for the liquid-crystal transformation, and the viscosity of the metallicglass. In overall, if the heating and cooling rates are fastenough to avoid the onset of crystallization of the metallicglass, the amorphous phase can be maintained duringspraying process and in the final coatings. Therefore, the

Journal of Thermal Spray Technology Volume 25(5) June 2016—993

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spraying conditions for deposition of metallic glass parti-cles need a balance between high energetic conditions, inorder to induce a Re above Recrit, and the kinetics ofcrystallization of the metallic glass.

Deposition of metallic glasses should be tackled notonly from the above mentioned factors, but it must also beconsidered the contribution of the substrate material. It isknown that impact-induced particle deformation dependson the mechanical properties of the substrate such as itselastic modulus and hardness (Ref 31). Previous CGSstudies using crystalline particles suggest that particlesundergo different strain rate conditions depending on thesubstrate hardness. The impact of a high velocity particleis classified according to the substrate/particle hardness asfollows: similar hardness, softer substrate or harder sub-strate material. For example, a softer substrate leads tovery low particle deformation and embedment. By con-trast, a harder substrate promotes flattening andmicrostructural changes such as recrystallization (Ref 21,32, 33). In the case of crystalline metals, the characteristicsof the substrate deformation affect bonding (Ref 31, 34-37). Usually, bonding is promoted by shear instabilities atthe particle/substrate interface (localized deformation andheat generation) (Ref 20, 34). When the deformation ofthe substrate is low compared to the deformation of theparticle, the shear instabilities are not formed at theinteracting surfaces. Meanwhile, a larger deformation ofthe substrate can induce jet-type flow formation at theinteracting surfaces (Ref 31, 34-38). Due to deformation,the flow of both materials promotes intimate contactwhich can result in metallurgical bonding (because flow ofmaterials and heating favor atomic diffusion) and/or agood mechanical interlocking (flow favors anchoring) (Ref31, 39). Just under conditions promoting shear instabili-ties, high deposition efficiencies are obtained. Up to date,the influence of the substrate on the deposition of metallicglasses by CGS has been barely tackled. Early studiessuggest shear instabilities as the main mechanism actingon bonding and deposition of metallic glasses by CGS(Ref 40, 41); however, this argument has been based onmodels from crystalline metals, which are not relevant formetallic glasses. Regarding this fact, a complete studyabout bonding mechanisms and about the interaction be-tween the metallic glass particle and the substrate stillneeds to be explored.

The thermal properties of the substrate material arealso considered important for bonding of crystalline par-ticles by CGS. Using numerical simulation, it has beendemonstrated that thermal conduction might affect thedeformation of both the particle and the substrate to someextent and hence the critical velocity in crystalline metalcoatings (Ref 33). However, there is a sort of consensussuggesting that the heating induced by impacts of sprayedparticles does not affect too much the particle deformationbehavior and the impact process could be assumed to beadiabatic because the distances involved in the process aretoo small in comparison with the rate of heat dissipation(Ref 20, 21). That assumption could be reasonable forsolid crystalline metals without induced softening, how-ever for metals with a certain degree of softening and

clearly for metallic glasses, the thermal interaction andrate of cooling might represent important changes in thebehavior of these materials during deformation. In theparticular case of metallic glasses, both mechanical andthermal properties of the substrate material might lead tochanges in the Recrit, and therefore the deposition effi-ciency could vary as substrate properties are changed.

This work explores the substrate influence on thedeposition efficiency of Fe-base metallic glass coatings byCGS, and it shows a way to predict the building up con-ditions of coatings of any metallic glass by CGS regardingthe intrinsic mechanical and thermal properties of bothsubstrate and metallic glass particle. The effects of themechanical properties of the substrate material on thedeformation of metallic glass particles at impact have beenstudied based on ballistic expressions for plastic work, thelaw of conservation of energy, and considerations for aliquid in the undercooled state. Furthermore, experimentsand numerical simulations have been carried out with aimsof studying the thermal conduction interaction betweenparticle and substrate at impact.

2. Experimental Procedure

Commercially available spherical Fe-base metallic glasspowder of composition Fe72,8Si11,5Cr2,2B10,7C2,9 at.%(Kuamet 62B, Epson Atmix Corp, Japan) was sieved intoa size distribution from 20 to 40 lm. A laser diffractionequipment (LS 13 320 Laser Diffraction, Beckman Coul-ter, Inc., 250 S. Kraemer Blvd, Brea, CA) was used toverify the particle size distribution after sieving. X-raydiffraction (XRD) with a Cu-Ka radiation was employedto check the amorphous nature of the feedstock powder.Mild Steel AISI 1040, Stainless Steel 316 L, Aluminum-7075-T6, commercially pure Copper (99.9%) and Inconel-625 plates with a flat geometry of 100 9 50 9 5 mm wereused as substrate material. Plates were prepared bygrinding using 240 grit SiC paper prior to spraying. Thedetailed process parameters used in this work for obtain-ing the Fe-base MG coatings are shown in Table 1. Twocommercial CGS Systems: Kinetics� 4000/17 kW (ImpactInnovations, Ampfing, Germany) with a maximum oper-ating pressure of 40 bar and Impact 5/11 (Impact Inno-vations, Ampfing, Germany) with a maximum operatingpressure of 50 bar were employed to deposit coatingsusing Nitrogen as carrier gas. A nozzle of type D24 WC-Co and a pre-chamber extension of 43 mm were used. Thetraverse speed of the spraying gun was set as 250 mm/s,while the powder feeding rate was 42 g/min during allexperiments. The deposition efficiency was calculatedmeasuring the final coating mass after spraying divided bythe total sprayed powder mass onto the sample withoutoverspray.

The Tg, Tx and the crystallization enthalpy (DHx) of theinitial powder and coatings were determined by differen-tial scanning calorimetry (DSC) at a heating rate of0.167 K/s in a Mettler Toledo DSC1. Cross sections of theas-sprayed coatings were prepared for a metallographic


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study according to the ASTM E3-01 standard. Cross sec-tion porosity of the MG coatings was measured accordingto ASTM E2109-01 standard and using the ImageJ soft-ware (Ref 42). Microstructure of the as-sprayed coatingsand single impacts were studied using scanning electronmicroscopy (SEM). At least 100 single impacts werestudied by sample, and measurements of the mean pene-tration depth (dexp) and the mean aspect ratio were per-formed for each particle/substrate interface. The aspectratio was calculated as the ratio of the total height of theparticle post impact, Ly, and the particle width post im-pact, Lx, i.e., Aspect ratio = Ly/Lx, in this way Aspectratio = 1 is considered as no deformation.

With the aim of understanding the influence of thesubstrate material on the deformation and deposition ofthe metallic glass particles at impact, a one-dimensionaland isentropic model for the gas flow was used in order toestimate the particle velocity and temperature just at themoment of impact. The model includes equationsexpressing the conservation of mass, momentum, and en-ergy. Details of the process used to determine bothvelocity and temperature of the particle from the initialgas temperature and gas pressure by CGS are reported indetail in previous works (Ref 28, 39, 43). Particle tem-perature was employed to estimate the metallic glass vis-cosity according to the Vogel Fulcher Tamman (VFT)equation. VFT equation is commonly used to describe theequilibrium viscosity of metallic glasses over 15 orders ofmagnitude, i.e., from the melting point to the glass tran-sition temperature. Details of the viscosity estimation forthe present metallic glass composition are explained in(Ref 28). The particle velocity and viscosity were used tocalculate the Re of the Fe-base metallic glass particles atimpact following Eq 1 for the different spraying condi-tions studied in this work. Finally, the study about thethermal interaction particle/substrate was carried out by

means of the commercial software COMSOL multiphysics3.5. The particle was supposed to impact in a directionnormal to the substrate surface and the meshing wasconducted using a 2D lagrange quadratic element. Vickershardness measurements were performed in a micro-Vickers indenter (Matsuzawa indenter MXT CX-1, Mat-suzawa Co., Ltd, Akita-shi, Akita, Japan). At least, 10indentations were collected on the polished surface ofeach substrate specimen setting a load of 500 gF and dwelltime of 10 s. Nanoindentation technique was employed toevaluate the hardness (H) and Young Modulus (E) formetallic glass particles using a Berkovich indenter at 5 mNload. Table 2 shows the material properties used in thepresent work (Ref 44).

3. Results

Figure 1(a) shows the particle morphology of the initialFe-base metallic glass powder, and Fig. 1(b) the particlesize distribution after sieving. The powder presents aspherical smooth surface typical from gas-atomized pow-ders. The sieved Fe-base metallic glass powder has amicrometric particle size, with a portion of the powderbetween 25 and 40 lm corresponding to a 72% in volume,according to Fig. 1(b). A 12% in volume is below 25 lmand a 16% in volume of the powder is above 40 lm. TheXRD analysis of the feedstock powder, Fig. 1(c), shows abroad halo typical in amorphous materials, while DSCmeasurements in Fig. 1(d) reveal an endothermic heat fluxcorresponding to the glass transition at 476 �C (Tg) and anexothermic peak due to crystallization of the supercooledliquid at 561 �C (Tx). DSC analysis, Fig. 2(a), also revealsthat Fe base metallic glass coatings are totally amorphouswhen gas conditions are set from Re1 (750 �C 40 bar) toRe6 (800 �C 45 bar). However, crystallization takes place

Table 2 Thermo-mechanical properties of the studied substrates (Ref 44)

Property/material Fe-base metallic glass 1040-steel 316L-steel Inconel-625 Aluminum-7075-T6 Copper (99.9%)

Density, kg/m3 7000 7845 7990 8440 2810 7940Thermal conductivity, W/(mÆK) 10 50.7 16.3 9.8 130 360Specific heat, J/(gÆK) 785 486 500 410 960 385Yield stress, MPa ÆÆÆ 370 290 460 375 333Elastic modulus, GPa 146* 200 193 208 71.7 110Hardness, Hv 1100*(10.79 GPa) 178*(1.75 GPa) 175*(1.72 GPa) 330*(3.24 GPa) 155*(1.52 GPa) 100*(0.98 GPa)

* Measured in the present work

Table 1 CGS process parameters for the different experiments carried out in this work

Feedstock SubstrateProcess gaspressure, bar

Process gastemperature, �C

Stand-offdistance, mm

Fe-base metallic glass powder 1040-steel316L-steelAluminum-7075-T6Inconel-625Copper (99.9%)

40 750 5040 750 3040 800 5040 800 3040 800 1045 800 1050 800 10


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when the gas pressure is increased to 50 bar at Re7, wherethe crystallization peak clearly changes. Figure 2(b) showsthe crystallization enthalpy measured by DSC for the Fe-base MG coatings onto the different substrates at (800 �C45 bar) Re6 and (800 �C 50 bar) Re7, respectively. Theseresults show that crystallization occurs in the metallic glasscoatings at the highest pressure (50 bar) independently ofthe substrate used. In fact, early studies (Ref 28, 29) pointout that crystallization of Fe-base metallic glass particlestakes place during the particle flight according to theJohnson-Mehl-Avrami-Kolmogorov theory. In the presentwork, crystallization also takes place in-flight due to thehigh temperatures experienced by the particles inside thenozzle. Using the same procedure as (Ref 28, 45), thecrystallized volume fraction (Vcrys) of each specimen hasbeen estimated by normalizing the difference of crystal-lization enthalpy of the initial amorphous powder and thespecimen by the enthalpy of crystallization of the amor-phous powder. The Vcrys of the Fe-base metallic glasscoatings sprayed at 50 bar is around 12%, as determinedfrom the data in Fig. 2(a) and (b).

Table 3 shows the Re of the Fe-base metallic glassparticle at the moment of impact for the different sprayingconditions used in this work. According to the LS mea-surements, Fig. 1(b), estimations of Re have been con-ducted using a mean particle diameter of 30 lm. Note thatthe Re increases with gas temperature and gas pressuresince both temperature and pressure promote variations inparticle temperature, i.e., viscosity, and particle velocity.Figure 3 shows the deposition efficiency of the Fe-basemetallic glass coatings onto the different substrates versusthe Re. In all the cases, the deposition efficiency initiallyincreases with increasing Re; however, three different

behaviors are found depending on the substrates used.The first group of materials, the 1040-steel and 316L-steel,displays a sharp rise in deposition efficiency withincreasing the Re, followed by a plateau of a maximumachievable deposition efficiency and finally, a decrease inthe deposition efficiency when the Re becomes too high.The second group consists of the Aluminum-7075-T6, thebehavior is similar to the previous case, but the plateau isnot present. Finally, coatings sprayed onto Copper (99.9%) and Inconel-625 alloys show a monotonical increasein the deposition efficiency until reaching a maximum atthe largest Re studied.

Cross section of the sprayed coatings onto 1040-steelfor conditions corresponding from Re1 to Re7 has beeninvestigated by SEM, Fig. 4, in order to reveal the dif-ferences in particle deformation. When the Re is too low,metallic glass particles deform in homogeneously, asshown in Fig. 4(a) and (b). Metallic glass particles presentsoftening and their flattening becomes more severe as theRe increases, Fig. 4(c) and (d). Particles present lateralviscous flow in the impact zone promoted by the highstrain rates and low viscosity of the metallic glass at im-pact, see Fig. 4(e) and (f). When the viscosity reaches verylow values, i.e., a large Re, metallic glass particles stilldeform homogenously; however, the formation of verythin sections by viscous deformation and the rapid coolinglead to their fracture by the new incoming particles, asshown in Fig. 4(g).

Cross section of the coatings obtained onto the differ-ent substrates has been also studied. Since the differenceof deposition efficiency is especially notorious at low vis-cosities, cross section micrograph has been taken forcoatings at Re5 conditions. Figure 5 shows the Fe base

Fig. 1 Characteristics of the Fe-base metallic glass powder, (a) Surface morphology, (b) particle size distribution, the vertical lines showthe range between +25-40 microns, (c) XRD pattern and (d) DSC scan of the initial powder


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metallic glass particles after impacting onto the surface ofthe different metallic substrates at Re5 conditions. Parti-cles impacting onto Copper (99.9%) and Aluminum-7075-T6 undergo very low deformation and the penetrationdepth is in the order of the particle diameter. Particlesimpacting onto 1040-steel, 316L-steel and Inconel-625show homogeneous deformation and they present lowpenetration depth.

Figure 6 shows the porosity of the Fe base metallicglass coatings according to the different studied metallicsubstrates. Porosity of the Fe base metallic glass coatingsdecreases as the Re increases up to Re6 for all cases, but itis interesting to note that it is higher in the case of coatingsonto Aluminum-7075-T6 and Copper (99.9%). Whenmetallic glass coatings start to grow-up, the particle/par-

ticle impact generates a compact structure in those caseswhere the particle/substrate impact promotes homogenousdeformation. This is observed in metallic glass coatingssprayed onto 1040-steel, 316L-steel, and Inconel-625. Inthe case of the metallic glass coatings obtained ontoCopper (99.9%) and Aluminum-7075-T6, where the par-ticle/substrate impact promotes inhomogeneous defor-mation, a porous structure is produced during theformation of the metallic glass coatings and therefore theyhave a higher porosity for identical spraying conditions.At Re7, the porosity of the metallic glass coatings is thehighest independently of the substrate. Interestingly, theincrease of porosity after Re5 matches with the results inFig. 4(g). At this condition, small debris appears in thecoatings as a consequence of the fracture of the jetsformed in the metallic glass particles after impact con-tributing to the increase of the total porosity.

4. Discussion

4.1 Impact Conditions in Metallic Glasses

As shown in Fig. 3, small Re leads to low depositionefficiency because inhomogeneous deformation is pro-moted, see Fig. 4(a). Under these conditions, the viscosityof the metallic glass particles is high and they are not ableto deform plastically during impact. Thus, metallic glassparticles either bounce-off or break by means of shearband formation. However, the mechanism of deformationduring impact changes at higher Re. As concluded inprevious work (Ref 28), the deposition efficiency is supe-rior to 50% just reached a given Re, namely Recrit. Abovethis Recrit, the viscosity of the metallic glass is low enoughto allow homogeneous deformation, coating growth, andgood particle adhesion. The results in Fig. 3, 4 and 6confirm this behavior. Note that the Recrit is differentdepending on the substrate material. Figure 3 allowsmeasuring the Recrit for coatings sprayed onto 1040-steel,316L-steel, Inconel-625, Aluminum 7075-T6, and Copper(99.9%), and the results are 0.005, 0.007, 0.0131, 0.0356,and 0.054, respectively. The variation in the Recrit shouldbe related to the influence of both the mechanical prop-erties and the thermal properties of the substrate material.A hard substrate leads to high strain rates, and thus, itcould change the deformation mechanism of the metallicglass particle, while a material with high thermal diffu-sivity could quickly decrease the particle temperature andthus it could also affect its deformation mode as well.

In the case of Fe-base metallic glass coatings sprayedonto 1040-steel and 316L-steel, see Fig. 3, the plot depo-sition efficiency versus Re follows a similar tendency thanthe plot ratio of bonds versus impact velocity in crystallinemetal coatings deposited by CGS. In crystalline metalcoatings, the deposition efficiency increases, reaches amaximum, presents a plateau, and finally decreases. Suchdrop in efficiency of crystalline metals is related to anincrease in the rebound energy because of the high impactenergy. This leads to the particle eroding the substratematerial instead of building-up a coating (Ref 46). A

Fig. 2 (a) Crystallization enthalpy of the feedstock powder, andthe coatings deposited onto 1040-steel at different Re, i.e.,spraying conditions shown in Table 3. (b) Crystallization en-thalpy of the Fe-base metallic glass coatings at Re6 and at Re7deposited onto the different studied substrates


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similar behavior has been observed for different metallicglass compositions (Fe-base, Ni-base and Cu-base)sprayed by CGS onto steel substrates. High energeticconditions (temperatures above the glass transition) led toimproved deposition efficiency, although it was limited byerosion and/or excessive softening (Ref 26, 47, 48). The Febase metallic glass composition studied in this workclearly shows a tendency to deform homogeneously whenthe Re increases, see Fig. 4, even if the deposition effi-ciency decreases after a maximum value or increasesmonotonically as the sprayed coatings onto 1040-steel and316L-steel or Inconel-625 and Copper (99.9%), respec-tively. SEM micrographs in Fig. 4 also show particlesexperiencing fracture after impact at the largest Re, de-spite of homogenous deformation. The high porosity fordeposited coatings at Re7 condition is related to thebreakup of the jets formed in the metallic glass particlesafter impact or to the excessive spreading of the particleitself due to its low viscosity, see Fig. 6. It is well knownthat splashing phenomenon can occur in liquids impactingwith large Re. When a liquid drop undergoes splashing,the drop breaks into many droplets since either the shockenergy is able to overcome the cohesion energy of thesurface or the inertial viscous dissipation is not enough todissipate the kinetic energy (Ref 49). In fact, previous

studies suggest that metallic glass particles can fractureand bounce-off if they soften excessively (Ref 50). Underthat condition, most of the deformation is concentrated inthe metallic glass particle side and therefore the influenceof the substrate on the coating formation is depleted. Inthis manner, the deposition efficiency decreases in the bestof the cases (1040-steel, 316L-steel, Aluminum-7075-T6),and it increases in the others (Inconel-625, Copper(99.9%)) leading to the same deposition efficiency valueindependently of the substrate used. For coatings de-posited onto Inconel-625 and Copper (99.9%), the specialmechanical and thermal properties of these substratematerials can be associated to the different behaviors ofthe deposition efficiency with respect to the other sub-strates, as discussed in the following section.

Particles of different sizes reach the surface at differenttemperatures and velocities; therefore, it is important toconsider the Re for different particle sizes. Figure 7 showsthe Re in terms of the spraying conditions and the particlesize. One can note that the Re increases as gas pressureand gas temperature are increased in all cases. Smallerparticles (25 and 30 lm) present smaller Re than thebiggest ones in all conditions. As mentioned in the resultssection, the sprayed Fe-base metallic glass powder con-sisted of 72% in volume of particles in the range of 25 to40 lm with a mean particle size of 30 lm. Taking intoaccount this particle size distribution, metallic glass par-ticles below 35 lm impact onto the substrate with a Resmaller than Recrit at 750 �C 40 bar, as represented by thedashed line at the bottom of Fig. 7 (deposited onto 1040-steel). Under this condition, most of the metallic glassparticles deform inhomogeneously and the contribution tothe deposition efficiency is mainly due to the biggestparticles (>35 lm). At 800 �C and 40 bar, all particlesimpact at a Re above the Recrit, the smaller metallic glassparticles present enough softening and contribute to im-prove the deposition efficiency as observed in Fig. 3 and 4.Increasing gas pressure (from 40 bar to 50 bar), themetallic glass particles deform excessively leading tosubsequent fracture (Re7), Fig. 4(g). According to thementioned above, Re7 is taken as a reference for excessivesoftening. Therefore, the deposition efficiency, Fig. 3, be-gins to decrease when the majority of particles within therange used overcome Re7 (at 800 �C 45 bar particles>35 lm) because the bigger particles may splash orbounce-off at impact. Nevertheless, generalized splashingand fracture are only shown in the coating when most ofthe particles impact at a Re above Re7. Consequently, the

Table 3 Re number and impact conditions calculated at the different studied spraying conditions

Gas pressure, bar Gas temperature, �C Stand-off distance, mm Vimp, m/s Timp, �C Re number(dimensionless)

40 750 50 595 527 Re1=5.0E-540 750 30 532 549 Re2=8.0E-440 800 50 607 570 Re3=1.28 E-240 800 30 546 592 Re4=8.20 E-240 800 10 516 628 Re5=1.20045 800 10 535 636 Re6=2.31950 800 10 572 644 Re7=4.006

Fig. 3 Deposition efficiency in function of the Reynolds num-ber for the different studied substrates. The continuous guide linefollows the behavior presenting a maximum of deposition effi-ciency, whilst the discontinuous guide line shows the monotonicalincrease of deposition efficiency in Copper (99.9%) and Inconel-625


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deposition decreases and is independent of the impactsurface. On the other hand, Recrit is affected by theinteraction particle/substrate because the transition frominhomogeneous flow to homogeneous flow depends on thesubstrate used, as mentioned above. Therefore, the con-tribution of different particle sizes to coating build-up alsodepends on the substrate used. Further details aboutsubstrate/particle interaction will be discussed in section4.2 and 4.3. In any case, the above description shows thatthe Re of metallic glass particles at impact change simi-larly in the range of particle used. Therefore, a narrowdistribution of particles is preferred to perform a goodcontrol of the process based on the Re since this ensuresthat most of the particles are impacting under similarconditions.

Thus, it can be concluded that conditions for obtainingmetallic glass coatings are associated to those promotingviscous flow without excessive softening at impact. The Reis useful to control the range of conditions in whichmetallic glass coatings can be obtained. As in the case ofcrystalline materials, a narrow particle size distributionensures that most of the metallic glass particles softensimultaneously. In this way, metallic glass particles over aRecrit can easily flow after impact promoting metallic glasscoating formation. The experiments carried out in thiswork show a Remax ~ Re7 that leads to excessive softening.It is worth mentioning that it cannot be taken as a generalvalue since jet formation and splashing are phenomenadepending not only in the Re but also on the impact

velocity. Finally, it is worth mentioning that Recrit is af-fected by the properties of the substrate material leadingto different deposition efficiencies and different metallicglass coating microstructures.

4.2 Particle Viscoplastic Deformation Dependenceon the Mechanical Properties of the SubstrateMaterial

The Re corresponding to the different impact condi-tions expresses the state of the particle prior to impact, butas seen in the results section, the final deformation statealso depends on the substrate material. Figure 8(a) showsthe aspect ratio of the particles, in contact to the substrate,in function of the Re. In all cases, the deformation im-proves increasing Re, which is represented by the decreaseof aspect ratio. The increment of Re is associated to thesoftening of the particles at impact, and therefore they canflow easily at impact, and achieve higher degree ofdeformation. This result has also been observed in previ-ous studies (Ref 26, 28, 29, 47, 48, 50) spraying metallicglass particles onto steel substrate, the improved defor-mation led to better deposition efficiency. In addition tothe above discussed, the present result also shows that theharder the substrate material, for instance Inconel 625(open squares in Fig. 8a), the smaller the aspect ratio atthe same Re, i.e., different degrees of particle deformationat the same initial impact conditions. However, if onecompares 1040-steel and Aluminum-7075-T6 alloys, which

Fig. 4 Cross section of the Fe-base metallic glass coatings deposited onto 1040-steel using the spraying conditions of Table 3 for: (a) Re1,(b) Re2, (c) Re3, (d) Re4, (e) Re5, (f) Re6 and (g) Re7. The inset in the figure shows a high magnification view of the particles in the coating


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are materials with a similar hardness, 178 and 155 Hv,respectively, it would be expected a very similar aspectratio distribution. However, according to Fig. 5, Alu-minum-7075-T6 alloy presents very low resistance topenetration compared to 1040-steel, i.e., metallic glassparticles are embedded into the Aluminum-7075-T6 sub-strate, while particles deform more and penetrate less inthe 1040-steel. This difference rises from the strain hard-ening and strength coefficient of Aluminum-7075-T6 withrespect to the 1040-steel substrate under compression,since both parameters determine the flow stress behaviorof metals. Under compression, Aluminum-7075 presents alower strain hardening and strength coefficient (i.e., easierdislocation cross slip) and the substrate flows easily underthe application of similar stresses in comparison to thesteel substrate (Ref 51, 52). Moreover, SEM images,Fig. 5, show very different characteristics of particledeformation upon impact, which also suggests a veryimportant effect of the particle/substrate thermal interac-tion, as discussed in section 4.3.

With the aim of studying in more detail the parti-cle/substrate mechanical interaction at impact, a simple

model based on the experimental penetration depth (dexp)and the aspect ratio of the metallic glass particles has beendeveloped to estimate the viscous work spent during im-pact to deform the metallic glass particles. According tothe law of conservation of energy, an energy balance be-tween the kinetic, the viscous dissipation and the surfaceenergy has been conducted (Ref 21, 53):

EKo þ Eso ¼ Esf þWsubstrate þ ER þWviscous;

Eeff þ Eso ¼ Esf þWviscous

ðEq 2Þ

where EKo represents the kinetic energy calculated fromthe impact velocity and the mean particle size of themetallic glass particles at the different conditions in thiswork [EKo = ½m vo

2) see impact velocity in Table 3. Eso isdefined as the surface energy of the particle before impact(Eso = pD2r; where r is the surface tension for the Fe-base metallic glass (Ref 54)). Esf corresponds to the sur-face energy after impact at its maximum splat diameterDmax (Esf = p/4 9 D2r (1 � Cosha)) for ha =90º (Ref 53).Wsubstrate is defined as the energy dissipated by the fric-tional and the plastic work during the substrate deforma-

Fig. 5 Impact morphology of the Fe-base metallic glass particles onto: (a) 1040-steel, (b) 316L-steel, (c) Inconel-625, (d) Aluminum-7075-T6, and (e) Copper (99.9%) at Re5 (=1.2), 800 �C, 40 bar, 10 mm


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tion and ER is the recoverable strain energy by both thesubstrate and the particle. Wviscous represents the energyavailable to deform the metallic glass particle in thesupercooled liquid state upon impact. For simplicity, theterm Eeff, namely effective kinetic energy, was introducedin Eq 3. Eeff is defined as the difference between EKo andWsubstrate.

In order to evaluate the plastic work carried out on asubstrate by a hard particle impacting its surface,Wsubstrate, a model developed by Chen and Hutchinson(Ref 55) has been used. Their work was based on metallicparticles impacting metallic substrates at high velocity(300-400 m/s) and high strain rates (104 s�1). These arevery similar conditions compared to the CGS process.They used finite element analysis to determine the resid-ual stresses and geometric stress concentration on the

substrate material. The most important non-dimensionalparameters governing impact indents were identified, sig-nificantly reducing the set of material independentparameters. In (Ref 55), the parameter X is defined as thekinetic energy of the impacting particle (EK) normalizedby the yield stress of the substrate material (ry) given inTable 2.

X ¼ EK

ryD3¼ p

12

qp

ry

� �

v2 ðEq 3Þ

Interestingly, Chen and Hutchinson found that the pene-tration depth of a particle into a substrate (d) normalizedby its initial diameter (D) can be expressed in terms of Xas long as the combination of deformation and densityratio between particle and substrate material are highenough (Ref 55):

d

D¼ 0:27

ffiffiffiffi

Xp

ð0:1þ 0:70ffiffiffiffi

Xp

Þ ðEq 4Þ

Equation 4 includes the pile-up effect that typically ap-pears in ductile materials and is independent of the elas-ticity of the substrate. In the present work, Eq 4 wassolved introducing D, namely 30 lm, to find out the the-oretical value of X (Xtheo) from Eq 3 for conditions cor-responding from Re1 to Re5. The estimated value of d foreach Re and substrate material was called the theoreticalimpact depth (dtheo). In this sense, it was supposed thatEq 4 is valid only in the case that dexp £ dtheo since not allthe kinetic energy is consumed at impact as plastic workon the substrate side. On the other hand, Eq 4 was alsosolved using dexp in order to find out the experimentalvalues of X (Xexp). The ratio Xexp/Xtheo versus the ratiodexp /dtheo has been plotted in Fig. 8(b). Most of the con-ditions studied in this work fulfill the condition dexp £

dtheo, i.e., points to the left of the vertical dashed line inFig. 8(b). Two conditions corresponding to Re5 in metallicglass coatings deposited onto Aluminum-7075-T6 andCopper (99.9%) present a dexp > dtheo due to a largerexperimental impact depth, i.e., points to the right of thevertical dashed line in Fig. 8(b). Interestingly, Xexp/Xtheo < 1 suggests that part of the initial kinetic energyhas been spent in viscoplastic deformation of the metallicglass particle.

In order to solve Eq 2, Xexp values have been intro-duced in Eq 3 and Eeff calculated using Ek=Eeff. For thetwo experiments with dexp > dtheo, Eeff has been calcu-lated assuming that a 95% of the initial kinetic energy wasspent as plastic work. This assumption has been takenconsidering the almost unaffected particle spherical shapeupon impact.

Figure 8(c) shows Wviscous in function of the experi-mental aspect ratio of the metallic glass particles onto thedifferent metallic substrates for conditions from Re1 toRe5. One can note that harder substrates lead to largerWviscous. For instance, Inconel 625 has values of Wviscous

around 20 9 10�6 J, whileWviscous is close to 0 for metallicglass particles impacting onto Copper (99.9%). Interest-ingly, in Fig. 8(c), the aspect ratio is smaller as the Wviscous

takes larger values describing a linear behavior in each

Fig. 6 Porosity of the Fe-base metallic glass coatings accordingto the substrate used and the Re

Fig. 7 Re number for different particle sizes: (s: 25microns;d: 30microns; h: 35 microns; n: 40 microns) as a function of the tem-perature and pressure of the carrier gas, and the stand-off distance.The dashed line at bottom represents the Recrit measured experi-mentally on 1040-steel, while the dashed line at the top shows Re7


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single Re, for example at Re5, the aspect ratio is 0.63 athigh Wviscous values and 0.93 at small Wviscous values.Previous studies using crystalline metals deposited byCGS suggest that the energy dissipated in plastic defor-mation is mostly spent in the softer part of impact (Ref 21,31, 36), just as happened in this work using a metallicglass. Therefore, Fig. 8(c) reveals quite different viscousdissipation energies, and different deformation behaviorsin the case of metallic glass particles depending on themechanical properties of the substrate, which is in agree-ment with the dissimilar aspect ratio observed in Fig. 8(a).This suggests that a substrate with a large hardness pro-

motes extremely high strain rates, similar to crystallinematerials (Ref 21, 31), and therefore the energy spent inviscous dissipation is larger than in softer substrates.

According to the definition of viscous work (Ref 56),the viscous work is expressed in terms of the metallic glassviscosity (g), the shear strain rate ( _cÞ and the finite strain(e), see Eq 5. In 2-dimensions, the e can be expressed interms of the aspect ratio (AR) giving a measure of theoverall intensity of distortion of a circle of area (A), whichrepresents the metallic glass particle of mass (m) anddensity (q), becoming an ellipse upon impact (Ref 57).

d Wviscousð Þ¼ s �dAð Þ �v¼ shear stress times impact velocity

Wviscous ¼Z

g � _c2 � e �mq

� �

dA

Wviscous ¼ g: _c2� �

:

m

q:A:ð1�ARÞ

ðEq 5ÞEquation 5 describes qualitatively the experimentalbehavior of the metallic glass particles shown in Fig. 8(c).The variation of the slope for the same Re and differentsubstrates in Fig. 8(c) is related to the viscosity and theshear strain rate undergone by the metallic glass particles.For the same initial Re, i.e., same viscosity and velocity, alarger shear strain rate is achieved with increasing sub-strate hardness leading to higher Wviscous and lower aspectratio. When Re is small, for instance Re1, the viscositybecomes too large and strain rates extreme. Under thiscondition, the slope increases and the energy spent toreach certain degree of distortion becomes large. At largerRe, i.e., Re5, the viscosity decreases various orders ofmagnitude, and therefore the viscous energy necessary todeform the MG particle for a given aspect ratio is smaller.

In overall, the results of this section show that metallicglass particles impacting at any Re onto soft metallicsubstrates present almost no deformation after impact.This behavior is very similar in crystalline particlesimpacting onto softer substrates (Ref 21, 25, 31, 36). Inboth cases, the plastic deformation is concentrated in thesoft substrate leading to excessive plastic flow of thesubstrate and embedment of the particle. Increased sub-strate hardness allows the activation of viscous flow in themetallic glass particles and plastic deformation is possibleat the different studied Re, Fig. 3 and 8. A similar result isreported for crystalline materials, where particlesimpacting onto hard substrates show higher deformation(Ref 21, 25, 35). However, the deformation mechanismsleading to this result are completely different dependingon the nature of the sprayed particles. Particles of crys-talline materials impacting hard substrates experiencesoftening due to the higher induced shear strain rates andtemperatures that allow easier dislocation movement, i.e.,easier plastic deformation. According to Fig. 8 and Eq 5,metallic glass particles impacting onto hard substrates alsodeform more extensively due to the induced higher shearstrain rates and temperature at the particles but in thiscase, the higher softening is due to the shear-thinningexperienced by the undercooled liquid. On the other hand,if the substrate is sufficiently hard, the rebound energy

Fig. 8 (a) Aspect ratio (dimensionless) vs. Re; (b) Ratio ofdimensionless impact energy vs. ratio of penetration depth, and(c) viscous energy vs. aspect ratio


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developed by the metallic glass particle is sufficiently highto bounce back at impact, especially in the case of stiffermaterials. The energy of the process associated to theparticle rebounding, namely the rebound energy (ER), isdefined in function of the elastic properties of the sub-strate material and the velocity of the particle at impact(Ref 46). For example, in this work, the impact velocityand temperature of the particle have been exactly thesame independently on the substrate used, Table 3;therefore the difference of behavior exclusively comesfrom the substrates properties. Interestingly, the deposi-tion efficiency onto the hardest substrate, Inconel-625, israther low despite the large particle deformation, largeWviscous and large Eeff, in contradiction to the high depo-sition efficiency obtained onto 1040-steel and 316L-steelwith lower Wviscous. The high elastic limit, i.e., high hard-ness, and high elastic modulus of Inconel-625, see Table 2,makes ER large in this case, leading to a higher probabilityof particle rebounding onto this substrate and therefore alower deposition efficiency, as observed in the presentwork.

4.3 Effect of the Thermal Properties of theSubstrate

Typically, bonding in CGS metallic coatings is studiedunder the assumption of an adiabatic process (Ref 12, 20).This assumption is in many cases true due to the fact thatthe rate of heat dissipation in the distances associated tothe process does not affect the conditions of the plasticdeformation. However, it has been demonstrated thatunder some particular situations, such as particle thermalactivation, the thermal interaction substrate/particle be-comes crucial changing the distribution of temperature inthe contact area and all over the particle (Ref 21, 33). Thistype of differences between the adiabatic and thermalanalysis becomes really important when the materials in-volved are very sensitive to temperature, where any vari-ation in temperature might represent the change ofdeformation behavior and/or phase transformations. Atleast for metallic glasses, an adiabatic study cannot sup-port and explain all the results related to the process and itbecomes crucial considering the heat transfer due to theviscous nature and its exponential dependence on tem-perature. Upon this point, calculations have been carriedout taking into account the metallic glass particle/sub-strate thermal conduction in the early stages of impact.

In CGS, when particles are propelled and impact ontothe surface of a substrate, there is a time in the range of1 9 10�8 to 5 9 10�8 s in which particle deformation doesnot take place (Ref 20). During this time of impact, anelastic collision takes place, and it is followed by an elasticunloading whereas the shape of the particle is partiallyrecovered. In this range of time, a temporal change intemperature in the particle/substrate contact area is given,and it could be different depending on the thermal prop-erties of the substrate material. The temperature changemight go on as soon as the particle deformation occurs andthe contact surface becomes larger. Although the heattransfer in the case of a metallic glass particle impacting

on a substrate is a dynamic process, the temporal changein temperature in the early stages could be considered asthe precursor of very different subsequent metallic glassdeformation modes and substrate plastic deformation. It isworth mentioning that the Fe-base metallic glass used inthis investigation is characterized to be a fragile liquid(Ref 28). This means that the viscosity of the metallic glassparticle could change from the order of 102 to 1012 Pa swith just a change in temperature of 50 �C. A rapid heatextraction from the particle due to a high thermal diffu-sion coefficient of the substrate material could lead to acompletely different deformation behavior, i.e., as a brittlesolid instead of a liquid. Calculations were carried out forsingle impact of a metallic glass particle for times in theorder of 10�8 s and for the different metallic substratesstudied in this work. It was assumed, based on previousinvestigations (Ref 31, 33), particle/substrate perfect con-tact. This assumption is supported by the fact that CGSparticles impact occurs at very high velocity developingvery high contact pressures.

Figure 9(a) shows the viscosity change due to temper-ature evolution in function of the radius of a metallic glassparticle after 10�8 s of the impact onto the different sub-strates studied in this work for Re5 conditions. The vis-cosity from the surface until 250 nm inside the metallicglass particle increases from the initial viscosity of 102 upto 1012 Pa s when the impact takes place onto Copper andAluminum-7075-T6. Furthermore, the viscosity is still ashigh as 105-106 Pa Æs at 350 nm in these two substrates.This big increase in viscosity means that the impact sur-face changes from a supercooled liquid to a brittle andextremely hard solid, i.e., a metallic glass. Further particledeformation is hindered due to this hard surface layer inthe case of high heat conductivity substrates. On the otherhand, substrate materials with much lower heat conduc-tivity present a much more moderate increase of viscosityat 250 nm from the particle surface and at 350 nm theviscosity is lower than 104 Pa s for the 1040-steel, 316L-steel and Inconel-625 allowing the deformation of theparticle.

According to the present calculations, in agreementwith the experimental results, it is clear that differences inparticle deformation and penetration rise not only fromthe mechanical properties of the substrate, but also fromits thermal properties. The largest heat transfer rate at thesubstrate impact surface of Copper and Al alloys has beenalready proved in previous works using crystalline parti-cles (Ref 32). In the present case, when a metallic glassparticle is sprayed onto materials with high thermal con-ductivity such as Copper (99.9%) and Aluminum-7075-T6,the metallic glass particle itself has almost no deformationdue to the high heat flux from the particle to the materialwhich leads to a high increase of viscosity at the metallicglass particle surface. This hard surface allows the particleto penetrate much more into the substrate. Thus, com-pared to 1040-steel, 316L-steel and Inconel-625 substrates,the metallic glass particle penetrates more deeply and itpresents almost no deformation on Copper (99.9%) andAluminum-7075-T6. Of course, at the beginning of theplastic deformation factors as frictional heating and the


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strain rate effect on the viscosity could deplete the in-crease of the viscosity. However, the strain rate andheating are often localized and the fact of increasing theparticle/substrate contact area could result in a mono-tonical increase in viscosity (when the substrate possessesa high thermal diffusivity) even after the initial impactstage. Previous works using crystalline materials sprayedby cold gas showed similarly improved deposition whensubstrate presents low heat extraction (Ref 32, 38, 58).Nevertheless, unlike crystalline materials, metallic glassparticles are more sensitive to thermal properties of thesubstrate because viscosity depends exponentially ontemperature.

The influence of the thermal properties of the substratematerials on the particles deposition has also been takeninto account. The formation of the metallic glass coatingsconsists in simultaneous and successive impacts onto asubstrate. Once a first set of particles impact the substrate,another set of particles are deposited onto the first onesand the thermal field can be rapidly perturbed in the im-pact location. Figure 9(b) shows the experimental depo-sition efficiency versus the thermal diffusivity of thesubstrate materials after one and three passes of thespraying gun. Interestingly, the deposition efficiency in thehigh thermal diffusive materials decreases between thefirst and the third pass. This suggests that the sub-strate/coating temperature during the first pass of thespraying gun and during the second pass and third pass canbe really different depending on the thermal properties ofthe substrate. In this case, if the first layer is cooled by aparticular substrate material faster than another, thecharacteristics of particle deformation in the subsequentlayers will definitely be affected. The substrates withhigher thermal diffusivity, i.e., Copper (99.9%) and Alu-minum-7075-T6, cool the first metallic glass layer morequickly than the rest of the substrates. Therefore, themetallic glass particles forming the second and third layerimpact onto a cooler and very hard metallic glass surfacepromoting high strain rates, high rebounding energies andhigher surface viscosity. In these conditions, a decrease indeposition efficiency should be expected in agreement tothe results displayed in Fig. 9(b).

4.4 Mechanisms of Metallic Glass ParticleDeposition

The deposition of metallic glass particles onto verydifferent substrates has revealed the mechanisms ofmetallic glass particle deformation and the necessaryconditions for metallic glass coating build-up. In this sec-tion, the similarities and differences between the mecha-nisms of particle deposition of a crystalline material and ametallic glass are discussed and clearly illustrated.

A metallic glass particle deformed in the homogeneousflow regime onto 1040-steel is observed in Fig. 10(a) forthe experimental conditions of Re3 in this work. Note thatintimate contact is promoted upon impact of the metallicglass particle. A clear limit between the substrate and themetallic glass particle is difficult to observe. The substrateis severely deformed due to the high stress level experi-

enced at impact and as consequence a jet is formed. InFig. 10(b), a higher magnification view of the metallicglass particle in homogeneous regime after etching for 5min with Nital is presented. Note the area limited by thedashed lines, where the metallic glass seems to haveexperienced extreme viscous flow and a structure like aliquid. As mentioned previously, non-Newtonian regime ispromoted in metallic glasses deforming in homogenousflow, depending on the combination of strain rate andtemperature. Considering the high strain rates concen-trated in the impact zone, Fig. 10(b) suggests that metallicglass particles impacting under this condition show non-Newtonian behavior around the surface of impact. This istypically seen in viscous liquids with shear thinning phe-nomena, and it has been already reported in metallicglasses in the supercooled liquid state (Ref 59). Undersuch conditions, the metallic glass viscosity drops expo-nentially and the liquid can flow easily. In the case of ametallic glass particle in the CGS process deforming in

Fig. 9 (a) Gradient of viscosity in the surface of a Fe-basemetallic glass particle during the temporal impact in a time in theorder of 1 9 10�8 s obtained by finite element simulation. (b)Experimental deposition efficiency for 1 layer and 3 layers of Fe-base metallic glass coatings versus the thermal diffusivity of thedifferent substrates


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homogenous flow, Fig. 10(b), the shear thinning phe-nomena are given due to the stress localization at thesurface of impact, promoting an increase in the vis-coplastic deformation, an intimate particle/substrate con-tact, and high deposition efficiency as mentionedpreviously. If this condition is not fulfilled, it could giverise to two possible situations: one case associated to verysmall Re, and another associated to very large Re. Fig-ure 10(c) shows a metallic glass particle sprayed at con-ditions of Re1 (a small Re number in this work). Under thiscondition, metallic glass particle deforms inhomogenouslypromoting shear bands and cracks. Since the deformationis localized through the shear bands, then the impactsurface is not able to flow, and therefore bonding is notachieved. Figure 10(d) shows a particle sprayed at condi-tions of a large Re (Re7). Under this condition, themetallic glass particle deforms in the homogenous flowregime and presents excessive shear thinning, i.e., lowviscosities, leading to the formation of jets at the edge ofthe particle. This type of deformation is also suitable for agood bonding because a large substrate/particle contactarea is promoted. However, as mentioned previously, theparticle/particle impact under conditions of a very largeRe leads to fracture of the metallic glass particles. Thus,coating growth is affected and highly porous coatingsobtained, as shown in Fig. 6.

In this manner, according to Fig. 10 and following thediscussed above, good particle bonding and high deposi-tion efficiency are promoted under homogeneous defor-

mation around the surface of impact for metallic glassesimpacting in the supercooled liquid state. When theseconditions are not satisfied, either by a poor or excessivesoftening of the metallic glass particle, the characteristicsof the impact and coating formation are changed, leadingto poor deposition efficiency and a porous microstructureas a result of particle rebounding or particle fracture.

A description of the deposition mechanisms of metallicglasses by CGS is presented in Fig. 11 as a flowchart concerning both a crystalline metal and a metallicglass. Although the conditions promoting metallic glassdeposition have shown similarities with the conditionsneeded to obtain bonding of crystalline metals, themechanisms involved in the deformation process anddeposition for metallic glasses have a different physicalexplanation once compared with those already known forcrystalline metals. In a crystalline metal, the bonding isgiven when the particle, overcoming a critical velocity,impacts onto the substrate and it experiences highlylocalized plastic deformation. In crystalline metals, plas-ticity is mainly governed by the movement of dislocations.For example, the breeding and interaction of dislocationsis responsible for the work hardening. During the particleimpact and due to the high strain rates experienced by theparticles, the stress flow varies monotonically with theplastic strain while subsequent dislocation movements areoccurring. The plastic strain energy is dissipated as heatand it promotes softening of the material. The thermalsoftening can lead to processes of dynamic recovery,

Fig. 10 Different deformation states for the Fe-base metallic glass particles onto 1040-steel substrate at: (a) Re5, (b) Re5 etched withNital for 5 min, (c) Re1 and (d) Re7


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recrystallization, and even shear banding (Ref 31, 60). Thetype of process following the thermal softening is exclu-sively a function of the properties of the particle materialand its interaction with the substrate. Typically, shearbands can appear in the impact surface of the crystallinemetallic particle in materials with a low thermal conduc-tivity. In any case, the plastic flow becomes localized at theparticle/substrate impact surface as a consequence of thethermal softening and stress concentration. This meansthat the thermal softening dominates over the workhardening upon impact. The shear localization causes theflow of interfacial region consisting in a highly deformedmaterial; it promotes intimate contact particle/substrateand therefore particle bonding (Ref 20, 21, 39).

On the other hand, a metallic glass presents twodeformation modes: homogenous deformation and inho-mogeneous deformation according to the deformationmaps given by Spaepen (Ref 61). The present and ourprevious studies (Ref 28, 29) demonstrate that deforma-tion in conditions of homogenous flow is essential becauseit promotes viscous dissipation at the interfacial parti-cle/substrate area and a high particle/substrate contactarea. This means that the total energy involved in the CGSprocess must be high enough in order to activate viscousflow in the metallic glass particles at impact and subse-quent shear-thinning. This condition can be easilyachieved by controlling the Re of the metallic glass par-ticle. However, the thermo-mechanical properties of thesubstrate play an important role to activate viscous flow inthe impacting particles. A hard substrate favors particledeformation and shear thinning because of the high strainrates experienced by the particles. At the same time, avery hard substrate also promotes large ER and thenumber of rebounding particles becomes more importantleading to a deposition efficiency decrease. In addition, arequirement for a good bonding is the substrate defor-mation. A very hard substrate will need higher impactenergies in order to reach a good mechanical anchoring, asshown in Fig. 3 for Inconel-625. In this type of substrate,when viscous flow is thermally activated in the metallic

glass particle (sufficiently large Re), deposition efficiencyis still low compared to the coatings sprayed onto 1040-steel or 316L-steel substrates since the ER is still highenough, producing either a poor mechanical anchoring ora subsequent particle erosion in the particle/particle im-pact, or directly particle rebounding. On the other hand,substrates with high thermal diffusivity require metallicglass particles in conditions of a large Re in order to keepthe viscous flow activated during enough time to produceparticle deformation, as suggested by Fig. 9. This is clearlyshown in Fig. 3 for metallic glass coatings onto Copper(99.9%) and Aluminum-7075-T6, which have a largerRecrit compared to the other substrates.

5. Conclusions

The effect of the substrate properties on the charac-teristics and deposition efficiency of the Fe-base metallicglass coatings produced by CGS were studied at differentprocessing conditions. The experimental data were com-bined with different physical concepts related to the im-pact dynamic and the mechanics of deformation ofmetallic glasses in the supercooled liquid state. The com-parison between calculations and experimental data haveshown that mechanical and thermal properties of thesubstrate material have an important effect in the char-acteristics of the metallic glass particle deformation, in thebonding and in the microstructure of the metallic glasscoatings by CGS. A summary of conclusions in this studyis as follows:

1. The mechanical properties of the metallic substrateinfluence the strain rate conditions and the rebound-ing energy of the process. A hard substrate promoteshigh deformation rates and larger deformation of theparticles, while soft substrates absorb most of the ki-netic energy during impact leading to poor particledeformation. Therefore, hard substrates promotehigher densification of metallic glass coatings. It is alsoworth mentioning that if the substrate is very hard andpresents a large elastic limit, the rebound energy ofthe deformed particles can be sufficiently high tobounce back after impact, and the efficiency of theprocess could be deteriorated.

2. Thermal properties of the metallic substrate influencethe rate of heat dissipation out of the metallic glassparticle during impact, and thus it affects the defor-mation of the metallic glass. A high thermal diffusivityleads to a rapid surface cooling, and therefore metallicglass particles get temperatures below its glass tran-sition point promoting inhomogeneous deformation.This finding proves that adiabatic analysis commonlyused for crystalline metallic coatings should be avoi-ded for metallic glasses.

3. It is confirmed that the deposition efficiency ofmetallic glass coatings build-up by CGS scales with theReynolds number of the impacting particles. Inter-estingly, the analysis carried out gives further ele-

Fig. 11 Flow chart of the deposition mechanisms for a crys-talline metal and a metallic glass


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ments with respect to the limits of the Re for maxi-mum deposition efficiency. Maximum deposition effi-ciency of metallic glass particles is expected just aboveRecrit. In the present work, the Recrit was estimated forFe-base metallic glass coatings sprayed onto 1040-steel, 316L-steel, Inconel-625, Aluminum 7075-T6,and Copper (99.9%), as 0.005, 0.007, 0.0131, 0.0356,and 0.054, respectively. Spraying conditions leading tomuch larger Re than Recrit can promote excessivemetallic glass softening and a decrease in the deposi-tion efficiency. On the other hand, low Re produceslow deposition efficiency because deformation islocalized into thin shear bands, generally leading toparticle fracture and/or particle rebounding. More-over, this study shows that a narrow range of particlesizes is needed to better control the impact conditionssince Re depends on particle size. In overall, the Reremains a good criterion for describing the state of themetallic glass particles at impact and for predictingparticle adhesion because it takes into account boththe inertial and viscous forces acting on the particle.

4. This investigation demonstrates that the mechanismsinvolved in the metallic glass deposition are differentto the mechanisms usually accepted for crystallinemetals. Homogeneous flow and shear thinning are themain mechanisms of deformation involved in thedeposition of metallic glass coatings. These mecha-nisms reflect the amorphous nature of metallic glassesand require thermal activation which is controlled bythe Re. Interestingly, shear thinning is an essentialstep to achieve coating formation with high depositionefficiency.

Acknowledgments

The authors would like to thank IMPACT INNOVA-TIONS GmbH for their collaboration in coating deposi-tion, EPSON-ATMIX for providing the powder used inthis work, and the Generalitat de Catalunya through 2014SGR 1558 for the financial support. The authors alsoacknowledge the CPT grant for supporting ‘‘John Henao’’in his PhD program at the Universitat de Barcelona.

References

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2. J.Q. Wang, Y.H. Liu, M.W. Chen, G.Q. Xie, D.V. Louzguine-Luzgin, A. Inoue, and J.H. Perepezko, Rapid Degradation ofAzo Dye by Fe-Based Metallic Glass Powder, Adv. Funct.Mater., 2012, 22, p 2567-2570

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