HVOF sprayed Al–Cu–Cr quasicrystalline coatings from coarse feedstock powders

HVOF sprayed Al –Cu –Cr quasicrystalline coatings from coarse feedstock powdersYingqing Fu ⁎,Tianxiang Peng,Deming Yang,Chengqi Sun,Yuzhen Chen,Yang GaoDepartment of Materials Science and Engineering,Dalian Maritime University,Dalian 116026,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 25October 2013Accepted in revised form 5May 2014Available online 10May 2014Keywords:HVOF thermal spray QuasicrystalParticle breakage MicrostructureAl –Cu –Cr quasicrystalline (QC)coatings were prepared from coarse feedstock powders by a high velocity oxy-fuel (HVOF)system DJ2700.The contrast experiments were performed by low power atmospheric plasma spray (LPAPS)using the same Al 65Cu 20Cr 15QC powders,and by HVOF spraying Cu180powders with the size of 44–61μm.The phase composition,microstructure and microhardness properties of the QC coatings were investigated.XRD results showed that the feedstock and coatings contained a predominant phase,icosahedral quasicrystal (IQC)I-Al 65Cu 24Cr 11,and three minor crystalline phases:α-Al 69Cu 18Cr 13,θ-Al 13Cr 2(i.e.Al 83Cu 4Cr 13)and ε-Al 2Cu 3.A qualitative analysis on the XRD patterns indicated that,a HVOF-sprayed coating contained more IQC and fewer crystalline phases in the one deposited by LPAPS.Moreover,the higher the input heat energy,the fewer IQC and the more crystalline phase the coating contained,for not only HVOF but also LPAPS.As con firmed by experimental results,when the HVOF-sprayed QC particle impacted onto the substrate surface,the unmelted solid part of particle broke up,and the previously deposited coating portion was deformed and densi fied and even cracked by impingement of the in-flight particles with high velocity;which mini fied splats and densi fied the as-sprayed coating.Thus,the HVOF-sprayed coatings from such coarse (61–74μm)QC powders had much smaller splats and much denser microstructures with lower porosity and higher microhardness,compared with those deposited by LPAPS using the same feedstock,although the preferential particle size of feedstock powders for HVOF spraying is conventionally 5–45μm.Furthermore,based on the contrast experimental results,the necessary and suf ficient conditions for occurrence of the particle impact breakage behavior in the thermal spray process are:(1)the high brittleness of feedstock and (2)the high velocity and low melting degree of spray particles,respectively.©2014Elsevier B.V.All rights reserved.1.IntroductionSince being firstly reported in 1984[1,2],quasicrystalline (QC)materials have become the subject of intense study due to their excep-tional structure and properties [3–9].After 30years of quasicrystal studies,the focus of research is currently shifting closer to the reality;much interest is nowadays concentrated on finding practical production techniques and applications for these materials.The established technology of aluminum fabrication makes the Al-based QC alloys more attractive than many other quasicrystalline alloys [10].However,because quasicrystals are brittle in bulk at ambient temperature,most proposed applications employ QC films/coatings [4–6].Depositing QC coatings onto metallic substrates allows the advantages of QC alloys,such as their surface properties,to be emphasized,while their dis-advantages,such as room-temperature brittleness,can be compensated for by the substrate materials.Various techniques are available for QC coating production,such as magnetron sputtering [11],simultaneous vapor deposition [12],laserablation [13],ion-beam mixing [14],ion implantation [15],and thermal spraying [16–21].Compared with the other techniques,thermal spraying is more versatile in industrialized QC coating production,with a more extensive adaptability;and the coating fabricated by these techniques is more robust [22]and wear-resistant than the one by the others.Moreover,a commercial application of QC coating has emerged,and the product is a cookware surface coating named Cybernox®,deposited by thermal spraying techniques [20].Thermal spraying techniques are divided,according to the way the energy or heat is provided to melt the material or give it enough plasticity,to allow the formation of the coatings [23].They usually include flame spraying,high velocity oxy-fuel (HVOF)spraying,deto-nation spraying,wire arc spraying,plasma spraying and so on.Widely used for many industrial applications,HVOF spraying is probably the best among the thermal spray techniques for some speci fic needs [24].It has been well understood that the highest particle velocity can be achieved by spray particles with heat source of the highest velocity;and the highest particle temperature is associated with heat source of the highest temperature [25,26].Because the maximum temperature of plasma jet reaches over 10,000K [27],spray particles can be heated to a high temperature by plasma jet with a medium velocity.AlthoughSurface &Coatings Technology 252(2014)29–34⁎Corresponding author.Tel.:+8641184723586;fax:+8641184729611.E-mail addresses:yingqing_fu@ ,fuyingqing@ (Y.Fu)./10.1016/j.surfcoat.2014.05.0030257-8972/©2014Elsevier B.V.All rightsreserved.Contents lists available at ScienceDirectSurface &Coatings Technologyj ou r n a l h o m e p a ge :w ww.e l s e v i e r.c o m /l oc a t e /s u r f c oa tHVOF is characterized by high velocity and low flame temperature [27],the flame temperature is signi ficantly in fluenced by the type of fuel gas and flame conditions.As shown in [28],the HVOF flame temperature using propane as fuel could reach to over 3100K,and the spray particle velocity in it ranged from 300to 1200m/s [29].HVOF-sprayed coatings are commonly thick and dense with less oxidation [25,26],and typically with reduced changes in phase composition,compared with atmospheric plasma-sprayed ones [30].Due to the above-mentioned features,HVOF spraying is advantageous for the fabrication of QC coatings,and develop-ing HVOF-sprayed QC coatings for industrial applications may be inter-esting and valuable.It can be recognized that the heating and accelerating rates of spray particles increase with decreasing particle size [31].It has also been found that the oxygen content in HVOF-sprayed alloy coatings increases in an exponential fashion with the decrease of spray particle size,and the oxidation of spray alloy particles becomes remarkably severe when the particle size is smaller than 45μm [32].Thus,in the current study,the Al 65Cu 20Cr 15QC powders with a size distribution ranged from 61to 74μm,were chosen as feedstock,although the preferentialparticle size of feedstock powders for HVOF spraying is conventionally 5–45μm [26].Such coarse powders,which reduced the vaporization of aluminum and oxidation of spray particles to facilitate the formation of Al-based QC phases,were HVOF and low power atmospheric plasma [33–37]sprayed respectively.Another finer feedstock,the Cu180powder with the size of 44–61μm,was HVOF-sprayed using identical process parameters.Based on the contrast experiments,the breakage behavior of HVOF-sprayed QC particles after impacting onto the AISI 1045steel substrate was examined.The in fluences of spray particle impact breakage on the coating microstructure,the necessary and suf ficient conditions for its occurrence in the thermal spray process,were clari fied respectively in this paper (which is extremely different from the previous paper [36]).Furthermore,no study on the impact breakage of HVOF-sprayed QC particles has been reported.2.Experimental details2.1.Spraying materials and processScanning electron microscope (SEM)micrographs of the feedstock powders are shown in Fig.1.The spherical QC powders (Fig.1a),pro-duced by gas atomization of a liquid melt with a nominal composition of Al 65Cu 20Cr 15[36–38],had the size ranged from 61to 74μm.Another water atomized feedstock,Cu180powders (Fig.1b)for the contrast experiment with the size of 44–61μm,had a nominal composition of Cu –5wt.%Ni –10wt.%Al.Both the feedstocks were thermally sprayed onto the previously grit-blasted AISI 1045steel substrates respectively,with an oxygen –propane HVOF torch DJ2700(Diamond Jet 2700-hybrid,Sulzer Metco,Westbury,NY,USA),and the HVOF spray para-meters are presented in Table 1.The low power atmospheric plasma spray (LPAPS)parameters of the same Al 65Cu 20Cr 15QC powders have been reported in a previous paper [36].The reaction equation of the complete combustion of propane is as follows:C 3H 8þ5O 2¼4H 2O þ3CO 2:ð1ÞAccording to this equation,the stoichiometric propane –oxygen ratio is 1/5=0.2,where the flame enthalpy is maximum.Two spray condi-tions,reducing (R)and oxidizing (O),were used.Reducing condition was propane rich and oxidizing was oxygen rich.Since 20%oxygen is present in the high-pressure air used to transport the feedstock pow-ders,the fuel-oxygen ratios of the reducing and oxidizing flame were set at 0.204and 0.196respectively,as listed in Table 1.The thickness of the as-sprayed coatings ranged from 200to 900μposition and structure examinationX-ray diffraction (XRD)was done using an X-ray diffractometer (Rigaku D/MAX-Ultima+/PC)with a Cu anode (λ=0.15406nm).A Philips XL30scanning electron microscope (SEM)was used to charac-terize the topographic morphologies of the feedstock and coatings,and the Al and O contents in the coatings were estimated by its energy dispersive X-ray spectroscopy (EDS).The coating cross sections were observed and photographed by an Olympus GX51F computerized light microscope,whose Olycia M3software was used to evaluate the coating porosity.A MH-6hardness tester was used to measuretheFig.1.SEM micrographs of Al 65Cu 20Cr 15QC (a)and Cu180(b)feedstock powders.Table 1High velocity oxy-fuel spray parameters.Coating code Powder Particle size (μm)Feed rate (g/min)Compressed air flow (L/min)Propane flow (L/min)Oxygen flow (L/min)Fuel –oxygen ratio Spray distance (mm)HF1(R)Al 65Cu 20Cr 1561–7410256532080.204180HF2(O)Al 65Cu 20Cr 1561–7410262582430.196180HF3(O)Cu18044–6132262582430.19618030Y.Fu et al./Surface &Coatings Technology 252(2014)29–34microhardness at a load of100g and a dwell time of5s,and Vickers in-dentation marks were performed on10different locations for each sample.3.Results and discussion3.1.Phase compositionThere are two types of QC phases in Al–Cu–Cr system,icosahedral quasicrystal(IQC)and decagonal quasicrystal(DQC).IQC forms in rapidly solidified alloys with a nominal composition of Al65Cu20Cr15 [38–40].Simple cubic Al65Cu24Cr11IQC and face-centered cubic Al69 Cu21Cr10IQC were observed in[38].DQCs with two different perio-dicities along the tenfold axis,1.24and3.72nm were found in rapidly solidified Al67Cu18Cr15[41].The DQC composition in[42,43]varies in the range of71–73at.%Al,11–12at.%Cu and15–18at.%Cr,and differs from that in Al70.5Cu18Cr11.5[44],Al71.4Cu19.2Cr9.4[45]and Al78Cu10Cr12 [38].Friedel[46]pointed out that the QC and approximant phases are Hume-Rothery phases,so they arise preferentially in the characteristic ranges of the valence concentration e/a,known as the Hume-Rothery rule[44–46].Although the DQCs mentioned above form over a wide compositional range,they exist on or near to a line at constant valence concentration e/a=2.09,called the“decagonal QC line”in the Al–Cu–Cr composition–structure diagram[38,39].Similarly,Al–Cu–Cr IQCs are formed by conforming to the“icosahedral QC line”with e/a= 1.86[38,39].As mentioned in Section2.1,the spraying feedstock was fabricated by gas atomization,and had a nominal composition of Al65Cu20Cr15 [36–38],which determined that the QC phase in the feedstock and thermal sprayed coatings should be Al65Cu24Cr11IQC,instead of a DQC.Besides this main phase I-Al65Cu24Cr11(I),three minor phases are also found in the phase assemblage.As the approximants of Al65-Cu24Cr11IQC,α-Al69Cu18Cr13[38]andε-Al2Cu3[47]are1/1-type and B2-based respectively.θ-Al13Cr2(i.e.Al83Cu4Cr13)has a monoclinic structure with lattice parameter a=2.52nm,b=0.76nm,c= 1.09nm,andβ=128.7°[38,39].Imitating the previous paper[36],XRD peak intensity ratios(PIRs)of α/I,ε/I andθ/I[48,49],as shown in Table2and Fig.2were calculated, based upon a qualitative analysis of XRD patterns of the Al65Cu20Cr15 QC feedstock and as-sprayed coatings.And the value of PIR had been confirmed to roughly equal the volume fraction of each crystalline phase in the sample[48,49].In the present work,the formation of IQC follows two mechanisms expatiated in[36],which are unnecessary to repeat pared with the LPAPS jet[36],the HVOFflame has a relatively low tempera-ture[27],which decreases the vaporization of aluminum,and facilitates the formation of IQC rather than any crystalline phase(α,θorε). Furthermore,the feedstock particles accelerated to a much higher speed[27]by the HVOFflame in the nozzle inside,resides a much briefer time in the nozzle outside to expose in the oxidizing atmosphere, so the oxidation of particles decreases.Consequently,the HVOF-sprayed coatings contained more IQC and fewer crystalline phases(α,θandε), compared with the ones deposited by LPAPS(except PS1),as shown in Table2and Fig.2.Even though the same thermal spray technique (HVOF or LPAPS)was used,the energy increase of the heat source caused a rapid growth of heat transfer to in-flight particles,promoted the vaporization of aluminum and oxidation of particles(Fig.3), reduced the formation of IQC,and made cooling rate of droplets or splats so“slow”that some crystalline phases were prone to form. So the higher the input energy was,the fewer the IQC and the more crystalline phase(α,θorε)the coating contained.3.2.MicrostructureThe coating cross-sectional SEM micrographs are shown in Fig.4. PS3and PS4comprised layers of deformed particles,in which porosities, cracks and remains of semi-molten particles were observed and regarded as typical features of plasma sprayed coatings,as shown in Fig.4(a)and(b).The microstructure characteristics of the coating deposited by LPAPS and its changes with increasing heat source energy have been discussed in the previous paper[36].As shown in Fig.4(c) and(d),porosities,microcracks and partially-melted particles were also detected in the HVOF-sprayed coatings,but these microstructures were much smaller-sized than those of the coatings deposited by LPAPS[36].Generally,the density of thermal spray coating depends not only on the state parameters of the feedstock particles,including structure,size, velocity and temperature before impinging on a substrate,but also on the subsequentflattening and solidification of liquid or partially lique-fied droplets after collision onto a substrate;and ultimately affects the coating microstructures and properties[50,51].In the present experiments,because the feedstock QC powders for HVOF and LPAPS were the same,the structure and size of the feedstock particles could be regarded as identical,and their influences could be neglected.The microstructural difference of the as-sprayed coatings should be determined by the velocity and temperature of in-flight par-ticles,as well as theflattening and solidification of liquid or partially liq-uefied droplets after impact onto the pared with the Al65Cu20Cr15QC particles in LPAPS process[36],those in HVOF spraying had a much higher average velocity and a much lower average temper-ature[27],leading to insufficient melting of the particles,as confirmed by the SEM micrographs of the coating surface morphologies shown in Fig.5.The molten fraction of spray material plays an important roleTable2XRD peak intensity ratio ofα/I,ε/I andθ/I in the Al65Cu20Cr15QC feedstock and as-sprayed coatings[36].Coating code PS1PS2PS3PS4PS5HF1HF2Feedstock Thermal spray technique LPAPS LPAPS LPAPS LPAPS LPAPS HVOF HVOFθ/I peak intensity ratio(%)49.556.561.766.873.547.348.846.8α/I peak intensity ratio(%)25.128.731.034.638.520.522.618.7ε/I peak intensity ratio(%)13.114.816.016.918.313.914.612.8Fig.2.XRD peak intensity ratio ofα/I,ε/I andθ/I in the feedstock and as-sprayed coatings.31Y.Fu et al./Surface&Coatings Technology252(2014)29–34on producing a high density coating [52],so the low melting degree of HVOF-sprayed Al 65Cu 20Cr 15QC particles facilitates a low coating densi-ty.However,a higher coating density than that of LPAPS could be ob-tained using the same QC powders (Fig.4),because the in-flight velocity of Al 65Cu 20Cr 15QC particle reached to 400–700m/s by the DJ2700HVOF system,in comparison to 200–400m/s by LPAPS [36],which were estimated from [27,52].In the present cases,pores in the coatings were originated not only from insuf ficient filling of droplet ma-terials into the cavities of previously deposited coating surface,but also from insuf ficient densi fication through the deformation of unmelted particle fractions [52].High particle velocities of HVOF bene fited the de-crease of apparent pores of the resultant coating [53],overwhelmed the disadvantage of low melting degree and induced a high coating density.In addition,when HVOF-sprayed Al 65Cu 20Cr 15QC particles impacted onto the substrate or previously deposited coating surface,the unmelted particle part broke up due to the inherent brittleness ofquasicrystals;and the previously deposited portion was deformed,den-si fied and even cracked by high-velocity-particle impingements.The particle breakages and deformations of particles and the previously deposited coating upon high-velocity-particle impacts,as shown in Fig.5(a),mini fied splats,facilitated suf ficient filling of droplet materials into the surface cavities and densi fied the coating.3.3.Particle impact breakage behaviorThe impact breakage behavior of HVOF-sprayed Al 65Cu 20Cr 15QC particles signi ficantly affects the microstructure and properties of the resultant coating.It is valuable and essential to clarify the occurrence conditions for this behavior.Hence,the contrast experiments were performed by LPAPS [36]with the same feedstock,and by HVOF spraying Cu180powders.Three typical coatings —HF2,HF3(as shown in Table 1)and PS4(as shown in Table 2and [36])were chosen for comparison.Their SEM surface morphologies and cross-sectional light microscopy images are shown in Fig.5and Fig.6,respectively,and their cross sections were etched with 10wt.%NaOH –water solution after polishing treatment,to make the splats and microstructure in them more visible.Under otherwise identical conditions,the feedstock for HF3has the smaller-sized powders,much higher thermal conductivity and tough-ness than that for HF2,so the microstructure of HF3conventionally may be finer and denser than that of HF2.On the contrary,HF2presents a much finer microstructure with much smaller splats compared with HF3,as shown in Fig.6(a)and (b).Why?It positively attributes to the particle breakages (“A ”in Fig.5(a))and deformations of particles (“B ”in Fig.5(a))and the previously-deposited coating portion (“C ”in Fig.5(a))upon high-velocity-particle impacts.This particle breakage phenomenon can be clearly con firmed in Fig.5(a),but nearly impercep-tible in Fig.5(b).Based on the results mentioned above,the high brittle-ness of feedstock powders (e.g.quasicrystal)should be the necessary condition for occurrence of the particle impact breakage in thermal spray process.In other words,no high-brittleness feedstock,no visible particle breakage in thermalspraying.Fig.3.Effect of aluminum and oxygen mean content in as-sprayed coatings on the loss of IQC (based on EDSanalyses).Fig.4.Cross-sectional SEM micrographs of as-sprayed Al –Cu –Cr QC coatings:(a)PS3,(b)PS4,(c)HF1,and (d)HF2.32Y.Fu et al./Surface &Coatings Technology 252(2014)29–34HF2also gives a much finer microstructure than PS4,as shown in Figs.4(b)and (d)and 6(a)and (c),although they were fabricated with the same feedstock.The finer microstructure of HF2directly relates to the characteristics of HVOF-sprayed particles.As mentioned above,HVOF-sprayed QC particles had a much higher average velocity and melting degree than those of LPAPS.Moreover,Al 65Cu 20Cr 15QC pow-ders have such coarse particles with a very low thermal conductivity (≤1/5that of AISI 304stainless steel)[39],which can aggravate insuf fi-cient melting.Therefore,most in-flight QC particles (N 80%)[54]in HVOF were the solid –liquid two-phase particles similar to those report-ed by Li et al.[51,52,55,56],where unmelted cores and completely melted surface layers were at solid and liquid states,respectively.How-ever,most QC particles of LPAPS [36]melted more suf ficiently than those in HVOF,and some small-sized particles even completely melted into single-phase liquid droplets.The great difference of particle melting degree can be con firmed in Fig.5(a)and (c),but the particle-breakage phenomenon mentioned above cannot be perceptible in Fig.5(c),which indicates that the high melting degree and low velocity of in-flight particles of LPAPS signi ficantly suppressed the particle breakage.Hence,the suf ficient condition for occurrence of thermal spray particle impact breakage may be the high velocity and low melting degree of spray particles (e.g.HVOF-sprayed low thermal conductivity powders).3.4.Porosity and microhardnessThe porosity and Vickers microhardness (HV 0.1)of as-sprayed QC coatings are shown in Table 3.In order to decrease the negative effects of porosity on the accuracy of hardness examination,the microhardness value in Table 3is an average of 10different locations on each sample.Gas porosity and shrinkage porosity,the 2main types of porosities in thermal sprayed coatings,as well as their formation mechanism,had been discussed before [36,57,58],and should not be repeated here.Sig-ni ficant phase assemblage and atomic volume differences between QC and crystalline phases facilitate the formation of porosity [59,60].There-fore,the QC coatings in the present study may have higherporosities,Fig.5.SEM micrographs of coating surface morphologies:(a)HF2,(b)HF3,and (c)PS4.Fig.6.Cross-sectional light microscopy images of coatings:(a)HF2,(b)HF3,and (c)PS4.33Y.Fu et al./Surface &Coatings Technology 252(2014)29–34than the Al–Cu–Cr crystalline coatings thermally sprayed by the same technique,despite having similar elemental compositions.The HVOF-sprayed QC coatings had a much lower porosity compared with those by LPAPS[36],which related to thefiner and denser coating microstruc-tures resulted from the particle impact breakage mentioned above.The rise in HVOF energy promoted the oxidation of particles and vaporiza-tion of aluminum,as illustrated in Fig.3,thus inducing the pores.How-ever,the growth of particle velocity facilitated the particle impact breakage,minified splats,increased splatflattening ratio and adhesion among splats to suppress interlamellar shrinkage porosity formation, and consequently,diminished the coating porosity.Under the present experimental conditions,the latter effect may overwhelm the former, thereby causing the coating porosity to decrease with increasing the HVOF energy,so HF2had a lower porosity than HF1.The coating micro-hardness depended on the coating phase composition and microstruc-ture,and increased with the coating porosity decrease in the present study,which agreed well with the experimental results of E.Fleury et al.[61].As a result,HF2had a higher microhardness than HF1,though HF1contained more IQC that contributes to hardness(Fig.3).4.ConclusionsAl–Cu–Cr QC coatings were prepared by a HVOF DJ2700system.The influences of heat source energy and particle impact breakage on the phase constituent,microstructure and hardness properties of the coat-ings were investigated.The results are summarized as follows:pared with the one deposited by LPAPS using the same Al65Cu20Cr15QC feedstock,(1)the HVOF-sprayed coating contained more IQC,due to the less oxidation and vaporization of the feedstock;and(2)presented a lower porosity and higher hardness with a much denser microstructure,which resulted from the particle impact breakage behavior.2.With increasing the HVOF energy,(1)the volume fraction of suchcrystalline phases asε,αandθin the coating increased,but that of the IQC decreased,because the higher energy reduced the cooling rate and increased the particle oxidation and vaporization of alumi-num;(2)the coating porosity decreased while its microhardness increased due to the increase of the splatflattening ratio and the adhesion among minified splats.3.The particle impact breakage behavior can minify splats,deform anddensify the coating.The necessary and sufficient conditions for its occurrence in the thermal spray process are:(1)the high brittleness of feedstock and(2)the high velocity and low melting degree of spray particles,respectively.AcknowledgementThe 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34Y.Fu et al./Surface&Coatings Technology252(2014)29–34。