Plasma Giken Co.,Ltd.,Saitama,Japan
Cold spray is a new emerging coating technology in which particles in a solid state are deposited via plastic impact on a substrate at a high velocity and a temperature that is much lower than the melting point of the starting powder.Compared to the conventional thermal spray processes,dense coatings without any degradation can be obtained by cold spray process with high deposition efficiency.CoNiCrAIY coatings are widely used for land-based gas turbines to resist high-temperature oxidation and hot corrosion.Owing to the high cost of the low-pressure plasma spray(LPPS)or some degradation in the hyper-velocity oxy-fuel(HVOF)spray process,cold spray process is a prospective candidate for coating preparation.
In the current study,CoNiCrAIY coatings were prepared by cold spray and LPPS processes,and a comparison of the coating's properties between the LPPS and cold spray process was carried out.The spray conditions of cold spray were optimized by the measurements of deposition efficiency and the observations of microstructure.
In recent years global warming as a result of carbon dioxide emissions has become a problem.As a result there is a need to increase the operational efficiency of gas turbines in coal power stations in order to control carbon dioxide emissions.Improving the efficiency of gas turbines calls for elevated gas temperatures at the turbine inlet,causing the turbine blades to be exposed to high temperatures.A thermal barrier coating is therefore applied to protect the turbine blades.Standard thermal barrier coatings involve the application of an MCrAIY bond coat to a substrate of nickel-based superalloy,followed by a top coat of yttria-stabilized zirconia(YSZ).The bond coat is commonly applied with low pressure plasma spray(LPPS),however the procedure is extremely costly and therefore another method is in demand(Ref 1-3).
This demand has recently called attention to cold spray as a method of application for MCrAIY bond coats(Ref 4***6).Cold spray is a comparatively new technology in which particles are heated below their melting point and accelerated by a high velocity inert gas flow to create a coating upon impact with a substrate.The fact that the sprayed particles are heated below their melting point allows for numerous special characteristics of cold spray,namely that the oxide content of the coating is low,thermal impact on the substrate is low,and high deposition efficiency is achievable.The prime factor in building up layers of a cold spray coating is particle velocity,which must pass a specific velocity(the critical velocity)in order to from a coating.As a high temperature material,MCrAIY has a high critical velocity,and consequently is considered to be comparatively difficult to cold spray,but recent advances in cold spray equipment have increased the operating temperature and pressure,leading to increases in particle velocity,which have made it possible to create cold sprayed MCrAIY coatings with high density and high deposition efficiency.
In this paper CoNiCrAIY is investigated as a cold spray coating material.CoNiCrAIY was sprayed at multiple conditions while measuring particle velocity and deposition efficiency,after which cross-section analysis and hardness testing was carried out to determine the impact of varying conditions on the resulting coating.Also,the cold spray results are compared with LPPS coating results to evaluate the utility of cold spray as a coating method.
This research uses CoNiCrAIY powder from Sulzer Metco with the brand name AMDRY9951.It is a spherical powder with a composition as follows:Co-32Ni-21Cr-8A10.5Y.Powder size distribution was measured with the laser diffraction particle size analyzer(Seishin Trading Co.,Ltd.Kobe,Japan).The measured powder distribution and photographs taken by SEM are shown in Fig.1.
Plasma Giken's own LPPS and cold spray systems were used to create the coatings.LPPS spray conditions are shown in Table 1,and cold spray conditions are shown in Table 2.Cold spray was conducted with two types of gas,namely helium(He)and nitrogen *** at 600℃,800℃,and 1000℃.Gas pressure was set at 2MPa for helium and 3MPa or 4MPa for ***,for a total of 9 different operating parameters at which the coatings were created.
The oxide levels within the LPPS and cold spray coatings were measured using inert gas fusion - non-dispersive infrared absorption method(EMGA650,HORIBA,Ltd,Kyoto,Japan).
The in-flight particle velocities of coatings prepared with cold spray were measured at each set of spray conditions.Measurement was conducted with DPV-2000,(Tecnar
Automation Ltd.,St-Bruno,Qu***bec,Canada)focused on the center of the spray at a distance of 20mm from the nozzle exit where it picks up particle velocity and particle diameter.The results were then compared with calculated results from a one dimensional simulation model.
Cross section of the LPPS and cold spray coatings were analyzed by optical microscope,and the porosity was measured by image analysis.Vickers Hardness of the coating was acquired by measuring with a hardness tester at 10 locations in the coating and then taking the average.The load was applied at 300g for 10 seconds.
Figure 2 displays the measured and simulated particle velocities at each set of spray conditions.Particle velocity measurements comprise of the velocity and diameter of 10,000 particles at each set of spray conditions.The measured average particle diameter was 25μm,so the simulation was carried out using 25μm as the particle diameter.Looking at the particle velocity results with *** as the propellant gas,it is apparent that higher gas temperature and pressure lead to higher particle velocity.It is also clear that high temperature leads to higher particle velocity with He as the propellant gas. A comparison of the simulated and measured particle velocity values reveals that at each set of conditions the simulated particle velocity is higher than the measured particle velocity.This can be considered to be the result of the variety of large and small particles have higher velocity.The specifications of the measurement equipment are set such that particles traveling above 1500m/s are considered to be traveling at 1500m/s,and taking account of this can explain why the measured value is lower than the actual average particle velocity.This phenomenon in turn demonstrates that simulation is an effective method for determining particle velocity with cold spray.
Oxide content results are shown in Table 3.A comparison of the results from LPPS and cold spray shows that,although the difference is slight,coatings formed with cold spray have lower oxide levels.Comparing oxide levels of coatings formed at varying cold spray conditions,we find the highest oxide content in coatings sprayed at high temperature using helium.Figure 2 shows the measured and simulated particle velocities at each set of spray parameters.Looking at this diagram we can see that using helium at high temperature achieves the highest particle velocity.It is reasonable to consider that the oxide levels are highest in the coating sprayed with helium at 1000℃ because of oxidation occurring at the interface of each pass,which is the result of the heat energy transmitted to the substrate upon impact of the high velocity particles.
Table 4 shows the deposition efficiency results of cold spray coatings measured at each set of spray conditions.Beginning with *** coatings,we can see that deposition efficiency is high when the coating is sprayed at higher temperature and pressure.It is clear that deposition efficiency increases greatly with the use of helium.This can be considered as a result of the high particle velocity achieved with helium,which results in a large number of sprayed particles reaching critical velocity.
Results of the cross section analysis are shown in Fig.3,while Fig.4 shows the hardness results.Looking at the cross section results we can see that within the LPPS coating the pores are uniformly dispersed,and the coating surface is smooth.In the cold spray *** coating,however,the pores are scattered,and the surface is rougher than that of the LPPS coating.This can be
considered to be the result of spraying particles below their melting point in the cold spray process as opposed to the melted particles that are sprayed with LPPS.Accordingly,the sizes of the solid state particles influence the dispersion of the pores,and the size and shape of the particles strongly influence surface roughness.In the cold spray He coating,surface of the coatings are as smooth as the LPPS coatings.This may be due to the fact that a large amount of plastic deformation of the particle increases with higher particle velocity.Taking a look at the differences in cross sections of the cold spray coatings at various spray conditions,we find first that with the *** coatings,higher temperature and pressure lead to denser coatings with higher deposition efficiency,but the porosity is higher than that of the LPPS coating.Looking at the He coating cross section results,we see that although it is difficult to confirm the presence of variations that are a result of raising the chamber gas temperature,the coatings are far denser than those created with ***,and the deposition efficiency is also high.It was not possible to identify a difference in porosity upon comparison with LPPS coatings.In this way the quality of cold spray coatings varies greatly depending on the spray conditions,and it can be thought that the faster the particles are traveling when they impact with the substrate,the better the quality of the coating.
Comparing the hardness results of the LPPS coatings with cold spray *** coatings,we find that the hardness of LPPS coatings is more uniform than that of cold spray coatings.This can be considered as a result of the differences in hardness between the vicinity of pores and non-porous parts of the coating.Helium sprayed coatings displayed less variation in hardness and a overall higher level of hardness than *** coatings.This can be considered to be the result of the higher density in the coating,and a work hardening effect achieved through higher particle velocity.
The effects of spray conditions on cold spray CoNiCrAIY coatings were investigated.First,it became apparent that particle velocity is a critical factor in the formation of coatings with cold spray.It became clear that using He as the propellant gas raised the particle velocity,which improved the coating quality and the deposition efficiency.In comparison with LPPS coatings,it was found that *** coatings do not perform as well,but it proved impossible to detect clear differences in oxide content and porosity between He cold spray and LPPS coatings.As such,cold spray process could be an effective method for deposition of CoNiCrAIY as a bond coat.Currently it is necessary to spray with He gas,but if improvements in equipment allow for higher density and deposition efficiency with *** gas,further cost reduction will become possible.