Renzhong Huang,Wenhua Ma and Hirotaka Fukanuma
Plasma Giken Co.,Toshima,Tokyo,Japan
Cold spraying is a new emerging coating technology and has been widely used to produce coatings of various materials.It has been widely accepted that particle velocity prior to impact is one of the most important parameters for the cold spray Process,and bonding occurs when the impact velocities of the particles exceed a critical value.As we know,particle velocity is influenced by many parameters,such as nozzle design,particle size,particle morphology,working gas pressure and temperature.In this work,three types of commercial stainless steel powders with different sizes or morphologies were employed to prepare coatings.Their respective particle velocities were also measured.Thanks to the non-clogging nozzle developed in PLASMA GIKEN CO.,LTD.,the particle velocities can be adjusted by changing the working gas pressures and temperatures to values as high as 4 MPa and 1000℃.The in-flight particle velocity was monitored via the DPV-2000 system.The results show that the particle velocity was influenced by the working gas pressure,temperature,particle diameter and morphology.Much denser coatings can be obtained with higher particle velocities,and consequently higher micro-hardness values of the coatings can also be achieved.
Stainless steel 316L is an austenitic chromium-nickel stainless steel containing molybdenum.The existence of some additional elements in the iron increases general corrosion resistance,improves resistance to pitting from chloride ion solutions,and provides increased strength at elevated temperatures.Therefore,it was widely applied in the paper,textile and photographic industries etc(Ref 1-2).To prepare coatings of stainless steel,the traditional thermal spraying technologies,such as arc spray,flame spray and HVOF(high velocity oxy-fuel spray),can be employed(Ref 3).The process always used to prepare dense and low oxidated stainless steel coatings is HVOF method at present(Ref 4-5).However,oxidation cannot be avoided owing to the higher
particle temperature,which passes the melting point during the processing of the coatings.
As a emerging spray coating technology,cold spray was first developed in the mid 1980's at the Institute of Theoretical and Applied Mechanics in Russia(Ref 6).In the cold spraying process,the deposition of particles takes place through intensive plastic deformation upon impact in solid state at a temperature well below the melting point of the spray materials(Ref 7).As a result,spray particles experience little oxidation or decomposition during cold spray processing(Ref 8-9).
Since cold spray technology appeared,some papers concerning the properities of stainless steel coatings have been published(Ref 10-13).Fukanuma and Sun et al.disscussed the influences of the particle velocities and morphologies on the properties of the coatings.But the working gas temperature was limited to lower than 400℃(Ref 10-11).Yang et al.investigated effects of the transverse velocity on the density of the coatings,and the working gas was improved to 500℃ in that study(Ref 12).Lorenzana et al.improved the working gas to 800℃ to prepare stainless steel coatings.Howerer,the results showed that it is still difficult to obtain a dense coating under such conditions(Ref 13).
In the present study,three types of commercial stainless steel powder with different sizes or morphologies were utilized to prepare coatings and measure particle velocities.Thanks to the development of the cold spray equiment,the particle velocities can be adjusted by the change of working gas pressure and temperature as high as 4 MPa and 1000℃ so as to obtain dense coatings.The in-flight particle velocities was measured by DPV-2000 system.The microstructure of stainless steel coatings was characterized by OM(optical microscope)and scanning electron microscope(SEM),and the micro-hardness was also measured.The effects of particle size and morphology on particle velocity were discussed,and the relationships between particle velocities and the properties of coatings were also investigated.
Three types of commercially available stainless steel 316L powder were used as feedstock in the current experiment.The powder size distributions were characterized by a laser diffraction particle size analyzer(LMS-2000,Seishin Trading Co.,Ltd.Kobe,Japan).The morphologies and diameter distributions of the three types of powder are presented in Fig.1.Two types of sperical powder,MicroMelt,with nominal size ranges of 5***22 and 16***45μm were provided by Carpenter Technology Corporation(Wyomissing,PA USA).And a multiangular power,FE-101,with nominal size ranges 16***45μm was provided by Praxair Surface Technology Inc.The diameter distribution profiles show that the volumetric average diameter of the two types of MicroMelt powder is 18 and 28μm respectively with relatively sharp distributions,and 37μm for the FE-101 powder with a relatively wider distribution.The chenical compositions of the stainless steel excluding iron are shown in Table 1.It shows that the chemical compositions of the two kinds of stainless steel powder,Micromelt and FE-101,are almost the same,although some of the components of FE-101 powder are not given in Table 1.The particle hardness was measured with a micro-Vickers hardness tester.Ten particles for every powder were tested,and the averages and standard diviations of the measured micro-hardness are shown in Table 2.It can be seen that the hardness of MicroMelt powder was harder than that of FE-101 powder.
A commercial cold spray system,model number PCS-1000 designed by PLASMA GIKEN CO.LTD.,was employed in the current experiments.A converging-diverging(De-Laval)nozzle was configured in the cold spray system.The nozzle is cooled by chilled water in order to alleviate nozzle clogging and improve the reliability of this system.As a gas pressure controlled system,in the cold spray system the gas flow rate is adjusted by the change of gas pressure.The details of the cold spray conditions in this study are shown in Table 3.At the same time of the preparing of coatings,the deposition efficiency corresponding to the spray conditions was measured via the measurement of the powder consumption and the mass increase of the original test pieces.
The in-flight particle velocity was measured at the center of the flow,using a DPV-2000 system(Tecnar Automation Ltd.,St-Bruno,Qu *** bec,Canada)under the conditions of preparing coatings as shown in Table 3.The substrate was removed during the particle velocity measurement process.For cold spray process,the radiation intensity emitted from the in-flight particles is too weak to be detected by the optical sensor because of the low temperature of the particles.Therefore,a high-power diode laser system,CPS-2000,is equipped in the DPV-2000 system to beam the in-flight particles.By detecting the monochromatic light scattered by the particles,the velocities of particle can be measured by the DPV-2000 system.In this study,the velocity measurements were made at a position in the center of the gas flow 20mm away from the spraying gun exit.
Cross-sections of the coatings were prepared vertically to their surfaces by conventional mechanical polishing method using SiC sand papers and diamond suspensions.Then,a relatively new cross-section polishing method(CP)using a broad ion beam was utlized to minimize the artifacts such as exaggerated densification and/or reduced porosity which can be indused by the plastic deformation of sprayed metallic powder particles during sample preparation.The broad ion beam milling was conducted in a commercially available apparatus called a cross section polisher(SM-09020CP,JEOL,Japan).The acceleration voltage and milling speed were 4 kV and 50μm/h,respectively,under the chamber pressure of 2 *** 10-3 Pa.The mirror-polished milled vertical cross-sections of the coatings with the mechanical polishing method were examined by an optical microscope(Keyence,VHX-900),and the broad ion milled vertical cross-sections of the coatings with the CP method were examined by an emission scanning electron microscope(JSM-5200LV,JEOL,Japan).The micro-hardness of coatings were tested using micro-hardness tester,FM-700(Future-Tech.Crop),under the load of 300g and the load-time of 10 seconds.
Depending on the detection of scattering light in the experiment,the DPV-2000 system cannot always measure particle diameter correctly(Ref 14).Therefore,the number distributions of the particle velocities can be obtained instead of volume ones.The number distributions of particle velocity measured by DPV-2000 are shown in Fig.2.It can be seen that the particle velocities increased with the increase of working gas temperature or pressure no matter which powder was used.The profiles of the particle velocity distributions for all the spraying conditions approximate to the gaussian distribution with the range from 400 to 1100m/s even though the typical value of the distributions is somewhat different.
From the velocity distribution profiles mentioned above,the number-averaged velocities of the particles can be derived.Figure 3 shows the number-averaged particle velocities measured by DPV-2000 system.It is obvious that the particle velocity increased with the increase of gas temperature and pressure for all the three types of powder.Comparing the particle velocities between the two types of MicroMet powder,it can be seen that the fine powder can be accelerated to a higher velocity as opposed to the coarse powder.Therefore,the particle velocities of MicroMelt 5-22μm powder are faster than those of MicroMelt 16-45μm powder.Considering the influences of the powder morphologies,the particle velocities of FE-101 16-45μm powder are faster than those of MicroMelt 16-45μm powder,and even the particle velocities of MicroMelt 5-22μm powder.One of the reasons to yield the higher particle velocities of the FE-101 powder is the multangular shape of the powder,resulting in a larger drag coefficient of the particles in the gas flow.Another reason is its wider distributions of the particle diameter.Even though the volumetric average diameter of the FE-101 powder is 37μm,the larrgest value among the three types of powder,there are the many fine particles included in the powder because the volume of the fine particles is much smaller than the that of the coares particles.Consequently,the actual number average diameter must be much smaller than the volumetric average one.On the other hand,the DPV-2000 system can only measure the number distribution of in-flight particles for cold spray.Therefore,higher measured particle velocities of FE-101 powder were obtained.
The deposition efficiency of stainless steel coatings is shown in Fig.4.It can be seen that the spraying deposition efficiency increased with the increase of the gas temperature or pressure.It seems that higher particle velocity causes higher deposition efficiency.Therefore,the deposition efficiency of MicroMelt 5-22μm powder is higher than that of MicroMelt 16-45μm powder,and the deposition efficiency of FE-101 16-45μm powder is also higher than that of MicroMelt 16-45μm powder.However,the difference of the deposition efficency among the three types of powder became insignificant at the spray conditions of higher particle velocity because the deposition efficiency almost reached 100% for all of the three types of powder.
Figure 5 shows the comparison of the coating microstructure prepared by two different polishing methods.It can be seen that both dense and porous coatings can be correctly examined by the conventional mechanical polishing method compared with the ion beam milling method if suitable polishing parameters are applied in the conventional machanical polishing method.
With the suitable polishing parameters in the mechanical polishing porcessing,the prepared microstructures of the stainless steel coatings at different gas temperature are shown in Fig.6.It seems that the increase of gas temperature generated much denser coatings owing to the increase of particle velocity and temperature.The coatings of MicroMelt MicroMelt 5-22μm powder are denser than the ones of MicroMelt 16-45μm powder owing to the higher particle velocity.And for the same reason,the coatings of FE-101 16-45μm powder are denser than the ones of MicroMelt MicroMelt 16-45μm powder as shown in Fig.6.
Figure 7 shows the microstructures of the stainless steel coatings prepared at different gas pressures.It can be seen that the coatings became denser with the increase of gas pressure due to the increase of particle velocity.Even though it is difficult to prepare a dense stainless steel coating with the cold spray process,relatively dense coatings were obtained at a the higher gas pressure of 4 MPa and temperature of 1000℃ if the fine powder was utlized as shown in Fig.7(b).
In order to understand the densification of the coatings,the splats of MicroMelt 16-45μm powder on stainless steel substrate were observed.Figure 8 shows the splats prapared at two representative spraying conditions,taking consideration of the level of particle velocity.It reveals that the particle sprayed at the gas pressure of 4 MPa is much flatter than the one sprayed at the gas pressure of 2 MPa.The particle velocity difference between the two spraying condition is about 100m/s accoring to the results in Fig.2.It seems that the particle with higher velocity experienced much more intensive plastic deformation.The intensive deformation combined with the thermal melting or softening effects leads to adiabatic shear instability in the interficial(particle/substrate)region and the solid material jetting generates(Ref 15,16).As a result,denser coatings with stronger adhensive and cohesive strength were obtained based on the piling effect for the particles with higher velocity.
Figure 9 shows micro-hardness of the stainless steel coatings prepared by cold spray process.It can be seen that the micro-hardness of the all the coatings prepared by the three types of stainless steel powder increased with the increase of gas temperature or pressure.Compared with the increase of the deposition efficiency in Fig.4,it seems that the micro-hardness is more sensitive to the particle velocity.Even though the deposition efficiency increased with the particle velocity to some degree,the deposition efficiency would not increase owing to fact that the value was close 100% when the particle velocity was high enough.However,the densification of the coatings continuously increased with the increase of particle velocity,and consequently the micro-hardness increased as well.Excluding the lower particle velocity zone at the gas temperature of 600℃ where the prepared coatings are so porous that it is difficult to measure the micro-hardness correctly,it seems that the densification of coatings determines the micro-hardness.As a result,the micro-hardness of MicroMelt 5-22μm coatings is higher than that of MicroMelt 16-45μm coatings,and the micro-hardness of FE-101 16-45μm coatings is also higher than that of MicroMelt 16-45μm coatings.Even though the hardness of FE-101 powder is far lower than the MicroMelt powder,the hardness of the prepared coatings between the MicroMelt and FE-101 are very similar notwithstanding the hardness of the feedstock.The hardness increased from the feedstock powder to the coatings owing to the work-hardening effect during plastic deformation.Consequently,the hardness of coatings is mainly determined by the densification of the coatings instead of the hardness of the feedstock powder.
In this study,three types of stainless steel powders with different sizes and morphologies were used to prepare coatings using the cold spray process under different working gas temperatures and pressures.The experimental results showed that,the in-flight particle velocity increased with the increase of working gas temperature and pressure.The fine particles obtained a higher particle velocity than the coarse ones,and the multangular particles also obtained a higher particle velocity than the spherical ones.The deposition efficiency all of the three types of powder is strongly influenced by the particle velocity.Higher particle velocity generated higher deposition efficiency.Thanks to the high temperatures achieved by using the cold spray system enployed in this work,the deposition efficiency is up to almost 100% for all three types of powders under the optimal spraying conditions.As a result,the cold spray system can effectively decrease the production cost compared to traditional thermal spray methods.Higher particle velocity can provide denser coatings,resulting in higher micro-hardness values of the coatings.All of the obtained properties of coatings are related to the intensive plastic deformation of the particles at higher velocities.