简介概要

Mechanism of Zr in in situ-synthesized particle reinforced composite coatings by laser cladding

来源期刊：Rare Metals2017年第12期

论文作者：Yao-Wei Yong Wei Fu Qi-Lin Deng Jian-Guo Yang

文章页码：934 - 941

摘要：By adding mixture of ZrO₂ and carbon, a Zrenhanced composite coating was produced onto an AISI1045 substrate by laser cladding. The microstructure and phase formation, microhardness and wear resistance of the composite coating were studied. The experimental results indicate that the composite coating with metallurgical bonding to substrate consists of y-Ni, massive ceramic particles of ZrC,NiZr₂, Ni₇ Zr₂,（Fe,Ni）₂₃C₆ and Fe₃ C. The in situ-synthesized ZrC particles are uniformly dispersed in composite coating, which refines the microstructure of composite coating. With different Zr02 and carbon additions, the properties are improved differently. Finally, the fine in situ ZrC particles improve the microhardness of composite coating to HV_0.2 650, which is nearly 2.7 times that of Ni25 coating. Also, the composite coating has an advantage in wear resistance; it offers better wear resistance when more mixture of ZrO₂ and carbon was added in nickel alloys.

稀有金属(英文版) 2017,36(12),934-941

Mechanism of Zr in in situ-synthesized particle reinforced composite coatings by laser cladding

Yao-Wei Yong Wei Fu Qi-Lin Deng Jian-Guo Yang

School of Mechanical Engineering, Shanghai Jiao Tong University

School of Mechanical Engineering, Ningxia University

State Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology

收稿日期：17 August 2016

基金：financially supported by the National Natural Science Foundation of China (No.51275303);

Mechanism of Zr in in situ-synthesized particle reinforced composite coatings by laser cladding

Yao-Wei Yong Wei Fu Qi-Lin Deng Jian-Guo Yang

School of Mechanical Engineering, Shanghai Jiao Tong University

School of Mechanical Engineering, Ningxia University

State Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology

Abstract：

By adding mixture of ZrO₂ and carbon, a Zrenhanced composite coating was produced onto an AISI1045 substrate by laser cladding. The microstructure and phase formation, microhardness and wear resistance of the composite coating were studied. The experimental results indicate that the composite coating with metallurgical bonding to substrate consists of y-Ni, massive ceramic particles of ZrC,NiZr₂, Ni₇ Zr₂,(Fe,Ni)₂₃C₆ and Fe₃ C. The in situ-synthesized ZrC particles are uniformly dispersed in composite coating, which refines the microstructure of composite coating. With different Zr02 and carbon additions, the properties are improved differently. Finally, the fine in situ ZrC particles improve the microhardness of composite coating to HV_0.2 650, which is nearly 2.7 times that of Ni25 coating. Also, the composite coating has an advantage in wear resistance; it offers better wear resistance when more mixture of ZrO₂ and carbon was added in nickel alloys.

Keyword：

In situ-synthesis; Zirconium carbide(ZrC); Composite coating; Laser cladding; Nickel-based alloy;

Author： Qi-Lin Deng e-mail:dengqilin@sjtu.edu.cn;

Received： 17 August 2016

1 Introduction

Many investigations have proved that laser cladding is one of the most promising processes for producing the coating of metal matrix composites on different bulk substrates.As a matter of fact,it has impressive advantages over other techniques such as chemical and physical vapor deposition or self-propagating high-temperature synthesis and so on because of its localized energy input as well as controllable and precise laser beam size [ 1] .Moreover,laser cladding could fabricate a coating with sound metallurgical bonding between coating layer and superficial layer of substrate without side effect on the bulk of substrate.

A widespread attention has been paid to the process called coating of ceramic metal matrix composites(CMMCs) due to that it improves the wear and corrosion resistance of products.In terms of ceramic-reinforcing phases,it can be categorized into two groups:ex situ and in situ.The former is to add foreign ceramic particles such as WC [ 2] and TiC [ 3] directly into the cladding materials,and the latter is known as in situ thermal synthesis method through in situ formation reactions during laser cladding [ 4] .With in situ synthesis method,the ceramic particles are synthesized in a metal matrix by chemical reactions between elements or between element and compounds during fabrication process [ 5] .Compared with ex situ method,the reinforcements produced by in situ synthesis method are much more compatible with the matrix.Furthermore,the in situ-synthesized reinforcements are stable thermodynamically and distributed evenly in the matrix [ 6, 7] .Using laser cladding,the CMMCs can be fabricated onto various bulk substrates such as Fe-based,Ni-based,etc.,and the reinforcements phase also varies from carbides,borides,nitrides or their mixtures [ 1] .As for reinforcements,the refractory element seems to be the ideal material for enhancing the coating such as ZrC,VC,TaC [ 8] ,TiC [ 9, 10] NbC [ 11] ,etc.

Among the refractory carbide reinforcements,ultrahightemperature ceramic of zirconium carbide (ZrC) has attracted considerable attention because of its characteristics and excellent properties,such as high melting point(～3420℃) and ultrahigh microhardness (～HV 2550).Moreover,ZrC combines good chemical resistance with outstanding mechanical properties [ 12, 13] .In addition,as a ceramic member,ZrC and its composites are expected to function as a promising material for wear resistance and high-temperature structures under extremely severe working conditions,rocket engines and hypersonic spacecraft for example [ 14] .However,mass production of ZrC is very difficult.Technically,ZrC is synthesized by carbothermal reduction reaction between ZrO₂ and carbon at elevated temperature [ 15, 16] ,mechanical alloying [ 17] and chemical vapor deposition (CVD) [ 18] .While the formation of ZrC is under quite high temperature [ 19, 20, 21] ,the high energy input by laser prompts the formation.Zhang et al. [ 1] formed ZrC coating by laser single cladding and overlapping cladding on steel,respectively.They found that when ZrC_0.7 distributed in the coating,the hardness of coating was enhanced significantly.Jayaseelan et al. [ 19] heated ZrC sample by laser and investigated the microstructure,and they announced that the layer was characterized by dendritic and ZrC+C eutectic phase.

The aim of this work is to fabricate an in situ ZrC reinforcement which embedded in Ni-based alloy coating.This paper covers the microstructure of the ceramic metal matrix composite coatings as well as the distribution and morphologies of ZrC phase in the coating.Furthermore,the microhardness and wear resistance were also revealed and compared for different proportions of ZrC in nickel matrix.

2 Experimental

2.1 Material and specimen preparation

The specimens of AISI 1045 carbon steel with dimension of 50 mm×50 mm×20 mm were selected as the substrates in experiments.The grooves with dimension of40 mm×50 mm×1 mm were milled by CNC mill machine onto the substrate surface so as to ensure the preplaced powder 1 mm of thickness.The surface of grooves was polished with SiC sandpaper grit 400-1500,dried and ultimately cleaned with acetone before laser cladding.The powder mixture for alloy coating is composed of commercial self-fluxing Ni-based alloy (Ni25,Shanghai Stellite Co.,Ltd,China) and (ZrO₂+C),which is mixed 1:3 in mole ratio according to the reaction equation[ZrO₂+3C=ZrC+2CO(g)↑],the commercial powder of ZrO₂ and graphite have purity of 99.5%and 99.8%,respectively (Aladdin Industrial Corporation,China).Particle size of Ni25 varies from 50 to 150μm,while that of graphite is≤30μm and that of ZrO₂ is≤50μm.The substrate and the Ni25 alloy are shown in Table 1.The different percentages of (ZrO₂+C) doping were mixed into Ni-based powder.The composite powder was preplaced into the grooves by an organic binder.Then,the preplaced powder was dried in an ambient atmosphere.

2.2 Laser cladding

A 3-kW continuous-wave CO₂ laser was used to fabricate the composite coatings.The processing parameters were determined as follows:input power was 2.2 kW and the scanning speed was 120 mm·min^-1.The size of the laser beam was about 4 mm×6 mm,and the overlapping ratio was set to 50%.All experiments were carried out under the protection of high-purity argon gas atmosphere.

2.3 Metallographic examination

The transverse cross sections of the coating samples were treated using standard mechanical polishing procedures after laser cladding.The microstructure of coatings was etched using the Aqua-regia for metallographic examination.Scanning electron microscope (SEM,JSM-6610) was used to study the microstructure characteristics.Energydispersive spectroscopy (EDS,Aztec X-Max 20) was used to study the chemical composition of coatings and phases.Phase constituents of coatings were analyzed by a Bruker D8 Advance X-ray diffractometer (XRD,Cu Kαradiation withλ=0.154056 nm,40 kV and 40 mA) with a scan rate of 6(°)·min^-1.A MVC-1000A micro-Vickers microhardness machine was used to obtain the microhardness profile across the coating cross section under a load of 200 g and a loading time of 15 s.

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Table 1 Chemical composition of nickel-based alloy powder (wt%)

The wear tests were carried out by MMUD-5B pin-disk sliding wear tester in room temperature.The pin was cut from the laser coating and shaped to a cylinder with a dimension ofΦ4 mm×15 mm.The surface of coatings was polished until the surface roughness reached Ra0.8μm.In the test,the pins were pushed onto the stationary counterpart,which is made of GCr15 bearing steel with a hardness of～HRC 60,and the surface roughness of counterpart was R_a 0.8μm as well.The pin rotates at constant speed of 300 r·min^-1,with 35 mm of rotation diameter,and the loads in test were set to 100 N.Wear tests were repeated five times continuously and each of them lasted 5 min.Thus,the mass loss of each sample was measured five times using a high-precision electronic balance (Model:G&G JJ224BC,made in China) with accuracy of 0.1 mg,and the average mass loss was calculated to evaluate the wear characteristics.Before and after wear test,the sample and counterpart were ultrasonic ally cleaned in acetone.To further investigate the wear surface morphology,SEM was adopted to assess the worn surface of laser cladding coating and analyze the wear mechanisms.

3 Results and discussion

3.1 Microstructure

As observed from Fig.1,all Ni25+(ZrO₂+C) ceramic composite coatings are free from cracks and pores despite of different proportions of (ZrO₂+C).It also can be found that the white particles are uniformly distributed in the matrix.Figure 2 shows SEM images of Ni25+5 wt%(ZrO₂+C) ceramic composite coating.Figure 2a shows that there exists an excellent metallurgical bonding with about 20μm in thickness between the coating and the substrate.This is caused by the formation of planar growth at the bottom of molten pool due to the extremely high ratio of temperature gradient (G) to solidification velocity(V).Moreover,a few ultra-fine white particles with size of1-3μm distribute in the plane crystal,as shown in Fig.2c marked by the white frame.However,above the planar,the dendrites grow up with non-oriented direction regardless of the large ratio of G/V.The growth direction of dendrites is influenced by both preferential direction and heat flow.Preferential growth direction has larger effects than heat flow.In the growing process,the preferential growth direction mostly parallel to temperature gradient direction is the dominant direction for growth of dendrite.Therefore,the dendrite growth direction tends to be perpendicular to the surface.However,the influence of preferential direction has another effect of inclining the structure of dendrites.So,the dendrites appear to have an angle rather than be completely perpendicular to the bottom of molten pool.

Fig.1 SEM image of Ni25+(ZrO₂+C) composite coating

Figure 2b,c shows a homogeneous microstructure embedded many white particles,and the eutectic phases can be observed in the middle area and upper area.According to the theory of microstructure in rapid solidification [ 22] ,the grain size tends to be smaller as the velocity of solidification increases.The process of laser cladding is a process of rapid solidification;therefore,the structure of laser coating tends to be fine.In the late stage of solidification,the degree of super-cooling increases and thus the dendritic grain size in upper area is evidently smaller than that in bottom area.In upper area of the coating,the primary dendrite arm spacing is about 10μm,while it is about 5μm in bottom of the coating,which contributes to the higher velocity of solidification at the upper area than that at the bottom area.

The white particles show the orientation of tetragonum and triangle morphologies dominant.And its size is in the range of 1-3μm,as shown in Fig.3.

3.2 XRD phase analysis

Figure 4 shows typical XRD patterns of Ni25 coating and Ni25+5wt%(ZrO₂+C) composite coating by laser cladding.It reveals that the diffraction peaks of possible phases are close,so some phases are difficult to identify for composite coating according to the reflection in XRD patterns.The main reason is that the planes of lattice corresponded to diffraction peaks of possible phases in the coatings are close to each other.The other reason is the oversaturation,and thus distortion of the lattice caused by non-equilibrium solidification involved in the laser cladding [ 1, 23] .The results indexed by JCPDS show that main phases in composite coating areγ-Ni,ZrC,NiZr₂,Ni₇Zr₂,(Fe,Ni)23C₆ and Fe₃C.In contrast,the Ni25 coating mainly includes the austenitic y-Ni and carbide of Fe₂₃C₆.But,the quantity of each phase cannot be estimated exactly,and the formation of phases is understood roughly by the observation of micros truc ture and analysis of the chemical reaction in process.The formations depend on many factors such as the nucleation and its growth,the thermodynamical reaction,etc.The addition of (ZrO₂+C) will prompt the formation of ZrC,which can enhance the hardness of coating as a ceramic enforcement.By comparing XRD patterns of Ni25 coating and Ni25+5 wt%(ZrO₂+C) composite coating,it can be found that the diffraction peak of Ni25 coating close to 2θ=44.166°and2θ=75.601°is higher than that of Ni25+5 wt%(ZrO₂+C) composite coating,which means that Fe₂₃C₆in Ni25 coating diffuses and forms new carbides like (Fe,Ni)₂₃C₆.

Fig.2 SEM images of Ni25+5 wt%(ZrO₂+C) composite coating:a bottom area,b upper area and c middle area

Fig.3 Typical SEM morphology of ZrC particles

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Table 2 EDS analyses of phases marked in Fig.2 of Ni25+5wt%(ZrO₂+C) coating (wt%)

As shown in Fig.2,the ceramic-reinforced composite coating mainly includes the white particles (marked as SO and S1),bulk eutectic (S2),cellar dendrite phase (S3) and needle-like phase (S4).The compositions of these phases are listed in Table 2.By combing XRD patterns (Fig.4)with EDS analysis results (Table 2),it can be concluded that the white particles are ZrC,marked as SO and S1.It is known that the laser cladding is a rapid cooling process;thus,it is likely to happen that the reaction ZrO₂+3C=ZrC+2CO(g)↑does not react sufficiently in a very short time.Therefore,there is a little oxygen existing in SO.The content of nickel in S3 is extremely higher than that of Fe.It is suggested that the cellar dendrite is the primary austeniteγ-Ni solid solution.Observed from the needle-like phase S4 and its EDS result,S4 is rich in Ni,Fe with little Zr and C.Such phase should be (Fe,Ni)₂₃C₆ based on XRD analysis.In addition,a small amount of Zr is likely to nucleate Ni₇Zr₂ phase.By analyzing EDS and SEM results,it is inferred that the phase marked as S2 is probably eutectic ofγ-Ni+ZrC and NiZr₂,since the content of Ni is about 33 wt%in group of Ni and Zr.As a result,NiZr₂ is apt to form.So,it can be concluded that the composite coating layer is mainly composed of ZrC,austeniteγ-Ni,interdendritic eutecticγ-Ni+ZrC and NiZr₂.It also can be found that the white ZrC particles are mainly dispersed inγ-Ni,which may be caused by the trapping effect during the forward moving of solid and liquid interface.

Fig.4 XRD patterns of a Ni25 coating and b Ni25+5 wt%(ZrO₂+C) composite coating

3.3 Formation mechanism of composite coatings

During laser cladding process,the powder mixture melts and forms the molten pool under high irradiation of laser input.The affinity of transition metals elements for carbon is the highest for valence number of 4,but higher valence than 4 will lead to a decrease of affinity to carbon.Thus,Zr should have the highest affinity for C [ 24] .Besides the factor of affinity,the thermodynamic effect on formation should be considered as well.

Figure 5 reveals the Gibbs free energy versus temperature of ZrC formation by the reaction of ZrO₂+3C=ZrC+2CO(g)↑under standard conditions.When the temperature reaches about 1680℃,the Gibbs energy of ZrC formation becomes negative,which means that the reaction would happen spontaneously with elevated temperature.As a result,the reaction happens and therefore generates ZrC.Owing to the high melting point of ZrC,it will nucleate firstly from the alloy solution in molten pool as the primary phase freely grows up by the bonding of surrounding atoms of carbon and zirconium.Therefore,the carbide of (Fe,Ni)23C₆ subsequently disperses among the solution and grows into needle-like shape due to the shortage of carbon after formation of ZrC.With the temperature further decreasing,ZrC particles become the heterogeneous nucleation sites for eutectics and finally are trapped by the eutectics.

Fig.5 Gibbs free energy (ΔG) versus temperature for ZrC formation

When theγ-Ni dendrite grows up,its tips will be obstructed when reaching ZrC particles.Consequently,the dendrites have to bypass those particles to continue the growth,and therefore the dendrites split and form the arms [ 25, 26] .As a result,the fine microstructure of coating will be achieved.Furthermore,the formation of (Fe,Ni)23C₆also prompts the structure to be refined.

The compounds of Ni-Zr have several types according to the Ni-Zr binary phase diagram [ 27] ,while the values of Gibbs energy for ZrC are about 4-5 times lower than those of other compounds such as NiZr,NiZr₂,Ni₇Zr₂,etc.,and NiZr does not seems to exist during ZrC-Ni interaction due to much stable ZrC bonding [ 28] .

From the binary phase diagram of Ni-Zr [ 27] ,the intermediate phases melt congruently,such as NiZr₂ and Ni₇Zr₂.As mentioned above,when the temperature reaches above 1680℃,ZrC is generated.But Ni₇Zr₂ and NiZr₂phases form at 1440 and 1120℃,respectively.For this point,ZrC is precipitated as the sites for the later formation of Ni₇Zr₂ or NiZr₂.Furthermore,zirconium carbide generally has a Zr/C ratio varied from 0.55 to 0.99 [ 13] ;thus,it can be described as ZrC_x,which will cause C to diffuse into Ni matrix and to make Ni/Zr ratio change.As a result,according to analysis of the ternary diagram of C-Zr-Ni [ 28] ,the different compounds of NiZr₂ and Ni₇Zr₂ are likely to form with suitable ratio among Ni,Zr and C and suitable temperature.

3.4 Microhardness

Figure 6 illustrates the profile of microhardness of Ni25coating and Ni25 composite coating with 5 wt%,7 wt%and 10 wt%(ZrO₂+C) addition.The composite coatings show the similar three-stepwise microhardness distribution:coating,heat affected zone (HAZ) and substrate.It also exhibits that the microhardness of Zr-enhanced coating increases as more (ZrO₂+C) were added.The(ZrO₂+C) doping will disperse ZrC into the microstructure with a hardness of around HV 2500.So,the microhardness of composite coatings should normally increase when more (ZrO₂+C) were added.The highest hardness of Zr-enforced coating can reach about HV_0.2650,which is about 2.7 times that of Ni25 coating.While the hardness of Ni25+5 wt%(ZrO₂+C) composite coating is about HV_0.2 350 on average,and that of Ni25+7 wt%(ZrO₂+C) composite coating is HV_0.2480.The first reason,as mentioned above,is the dispersion of ZrC by in situ reaction of ZrO₂ and carbon,and then,the hardness is enhanced.And the second is that the increase in hardness of composite coatings may be ascribed to the grain refinement strengthening because of Zr introduction [ 29] .Therefore,the dendritic grain size in upper area is evidently smaller than that in bottom area.According to the H-P equation [ 30] ,increasing Zr proportion will reduce the grain size of Ni-Zr micros truc ture,leading to higher hardness.

Fig.6 Hardness distribution for different coatings

Fig.7 Wear resistance of coatings versus sliding time

Nevertheless,from Fig.6,it seems strange that the hardness of composite coatings varies enormously along the direction of thickness except for Ni25 coating.This phenomenon can be explained that the presence of eutectic in bottom is more than that of upper of coating.Meanwhile,the martensitic transformation would happen in HAZ.Therefore,the hardness of HAZ is higher than that of the substrate.But the continuous overlapping decreases the hardness of HAZ.Finally,the hardness of HAZ is about HV_0.2 300.

3.5 Wear resistance

Figure 7 illustrates the average mass loss of composite coatings and Ni25 coating under dry sliding wear conditions.The average mass loss of coating under a load of100 N and rotating speed of 300 r·min^-1 was weighted every 5 min with total sliding time of 30 min.Observed from the results,the average mass losses for all coating are approximately linear with sliding time.Moreover,the mass loss of composite coatings decreases in the dry sliding wear test when more (ZrO₂+C) were added into the coating.

Fig.8 SEM images of worn surface of clad coating:a Ni25+5 wt%(ZrO₂+C),b Ni25+7 wt%(ZrO₂+C),c Ni25+10 wt%(ZrO₂+C),and d Ni25

Wear is a typical phenomenon for two moving surfaces in contact,it depends on the material properties of the moving elements,surface chemistry,applied load,etc.In these works,the applied load and rotating speed are constant for all experiments.Therefore,it can be concluded that the change in wear resistance can attribute to the change in microstructure of Ni25 coating and Ni25+(ZrO₂+C) composite coatings.The main phase constituents of Ni25 coating include Fe₂₃C₆ andγ-Ni,while the Ni25+(ZrO₂+C) composite coatings contain not onlyγ-Ni and (Fe,Ni)23C₆,but also the phase of ZrC,NiZr₂,Ni₇Zr₂,and Fe₃C.Among these phases,γ-Ni has fcc structure and thus has good plasticity,which will lead to plastic deformation.The hardness of M₂₃C₆ (HV 1200-HV1600) offers only little resistance to the passage of counterpart,while ceramic particles of ZrC (～HV 2550) will improve the wear resistance to the counterpart evidently.Evidently,the mass of synthesized ZrC increases as more(ZrO₂+C) were added into the coating.Therefore,the stronger wear resistance could be accomplished by composite coatings.As a result,the wear loss should decrease with the further addition of (ZrO₂+C).

The worn surface of Ni25 coating and Ni25+(ZrO₂+C) composite coatings is shown in Fig.8.After wear testing,the worn surface of Ni25 coating (Fig.8d)shows plastic deformation and many craters,which illustrates the typical characteristic of adhesive wear.This phenomenon ascribes to the good plasticity and low hardness of Ni25 coating.

The unevenly distributed debris and white scrap marks are shown in Ni25+5 wt%(ZrO₂+C) and Ni25+7wt%(ZrO₂+C) composite coatings.Moreover,the bumpy stuffs stick to the worn surface of Ni25+5 wt%(ZrO₂+C) composite coating,while Ni25+10 wt%(ZrO₂+C) composite coating shows typical abrasive wear characterized by parallel plowing grooves onto the worn surface.As mentioned above,the hardness can be enhanced by adding more (ZrO₂+C).Thus,different contents of (ZrO₂+C) lead to different hardness values of the surface.These differences in hardness shifts adhesive wear to abrasive wear gradually.The reason is that the enhanced hardness decreases the plasticity of composite coatings,so the surface cannot stick to the counterpart easily under the external loading.Moreover,the harder particles such as ZrC particles will contact and plow grooves onto the surface of counterpart.

4 Conclusion

In this work,the effect of Zr in reinforcing microstructure of Ni25 coating was studied,through adding the mixture of ZrO₂ and C into Ni25 powder,and the ceramic-reinforced composite coatings were produced by in situ synthesis laser cladding on ferrous substrate.And the composite coating with excellent metallurgical bonding to substrate consists ofγ-(Ni,Fe),ZrC,NiZr₂,Ni₇Zr₂,(Fe,Ni)23C₆ and Fe₃C.Furthermore,the hardness of composite coating can be enhanced by addition of mixture of ZrO₂ and C.When more (ZrO₂+C) is added in the range of 5 wt%-10 wt%,the composite coatings become harder.The hardness of the Ni25+10 wt%(ZrO₂+C) composite coating can be about HV_0.2 650,which increases by about 27%compared to that of Ni25 coating.As a ceramic enforcement,ZrC provides good wear resistance to counterpart.The mass losses of composite coatings decrease as the addition amount of mixture ZrO₂ and C increases.