Fabrication of in-situ synthesized ceramic reinforced Ni-based alloy composite coatings by reactive braze coating processing

Pei Xinjun,Liu Wenbin,Cheng ge,Pan Xinyu,Li Aina,Li haishen

Yangjiang Polytechnic,Yangjiang 529566,China

Abstract In-situ synthesized ceramic such as TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating was fabricated on the surface of mild steel substrate by reactive braze coating processing with colloidal graphite,Cr,Ni,ferro-boron,Si and titanium powders as the raw materials at low temperature of 1 000 ℃,and a new kind of coating materials was developed.By means of SEM,EDS,XRD and surface hardness tester,the microstructures,phases,hardness and wear-resistance of the coating were analyzed,respectively.The results revealed that the coating was mainly composed of the ceramic in-situ synthesized reinforcement phases of TiC,Cr7C3 and Cr5B3 and the binder phases in-situ synthesized of Ni31Si12 and(Ni,Fe)solid solution;The ceramic reinforcement phases of TiC,Cr7C3 and Cr5B3 were randomly distributed in the binder phases of Ni31Si12 and(Ni,Fe)solid solution;The coating had about 15vol% pores and can possibly be applied as a self-lubrication coating;The coating and the substrate were integrated together by metallurgical bonding;The coating had a hardness up to 91-94HR15N.

Key words reactive braze coating processing,composite coating,Ni-based alloy,ceramic,self-lubrication coating

0 Introduction

Ceramic such as TiC with a high hardness(3 000HV)and a low density(4.93 g/cm3),make it a potential material for high wear-resistant coatings.Monolithic TiC,however,is brittle at ambient temperatures,and metals such as Ni,Co and Fe have been incorporated as a ductile second phase to improve its fracture toughness.Nickel is the most commonly used metallic binder phase in TiC reinforced composite coating,which is mainly due to the low wetting angle,30°under vacuum(1.33×10-3Pa)at 1 450 ℃[1],that liquid Ni forms with solid TiC.Addition of molybdenum to nickel reduces the wetting angle with TiC to zero,and this leads to TiC reinforced composite coatings with good wearresistant properties[1].Compared with Ni,Ni-based alloy has higher hardness.Using Ni-based alloy as binder phase,it will certainly improve the hardness of TiC reinforced composite coatings.Compared to TiC,Cr3C2 and Cr5B3 have lower hardness,while-ever adding Cr3C2 into the raw materials of the coatings can decrease the temperature of the fabricating coating[2].Flame spray,plasma spray,laser cladding and braze coating are the common production techniques used in the fabrication of wear-resistant coatings[3-10].An alternative processing technique for the production of ceramic reinforced metallic binder phase composite coatings is reactive braze coating processing[2],and by which a high density(TiC+Cr3C2)-Fe composite coatings with in-situ synthesized TiC has been successfully fabricated with carbon,Cr3C2,Fe,ferrochromium and titanium powders as the raw materials at 1 300 ℃ on a mild steel surface[2].

In this study,the possibility of using reactive braze coating processing is investigated for the fabrication of ceramic in-situ synthesized such as TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating,which contain 10%(volume ratio)TiC.Ni-based alloy has lower melting point(≥1 000 ℃),but if only using it as the raw material to prepare coatings,the temperature of the fabrication should be increased by 1.15-1.3 times of its melting point to improve the infiltration of liquid Ni-based alloy.The TiC,Cr7C3 and Cr5B3 in-situ decrease the temperature of the fabrication of this coating,and utilizing reactive braze coating processing TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating was prepared at 1 000 ℃ on a mild steel surface with colloidal graphite,Cr,Ni,ferro-boron,Si and titanium powders as the raw materials.And the microstructure,phases,hardness and wear resistance of the obtained composite coatings were analyzed by electron scanning microscopy(SEM),energy dispersive spectrum(EDS),X-ray diffraction(XRD),surface Rockwell hardness tester and wear tester(SRV),respectively.The TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating is a new coating system,if it can be fabricated by reactive braze coating processing means that the scope of the coating materials will be broadened.

1 Experimental procedure

The substrate was a mild steel machined into rectangular plates with size of 55 mm×30 mm×3 mm.The coating surface was ground with sand paper to get rid of oxide,and then rinsed with ethanol.The raw materials used in the experiments were colloidal graphite(99.5wt%,700 mesh),Cr(99wt%,300 mesh),Ni(99.5wt%,700 mesh),ferroboron(B18.64wt%,300 mesh),Si(99.5wt%,300 mesh)and titanium(99.7wt%,300 mesh)powders.The raw materials were weighed according to an appropriate ratio.To prepare some slurry,the raw materials weighed were mixed and milled in ethanol medium about 24 h at the rotational speed of 120 r/min with the ratio of steel ball weight to the mixture weight as four.The slurry was uniformly coated on one of the substrate surfaces to make a pre-layered coating whose thickness was 0.4-1 mm(seen in Fig.1).The assembled samples(as shown in Fig.1)were put into an oven to be dried below 200 ℃.

Fig.1 Schematic of an assembled specimen’s cross-section before braze coating processing

Reactive braze coating processing was carried out in a resistance heating vacuum furnace of the molybdenum lath with an effective chamber size of?150×200 mm.Reactive braze coating processing can be summarized as follows.Firstly,the assembled specimens(the dried samples below 200 ℃)were placed in the chamber;Secondly,the chamber was evacuated to the vacuum level of 4.0×10-3Pa with a diffusion pump followed by heating with pre-determined parameters;Finally,the coated specimens were cooled down to room temperature in the chamber.The predetermined heating parameters was:heating specimens from room temperature(25 ℃)to 1 000 ℃ in 60 min and holding 30 min at 1 000 ℃,then cooled to room temperature.

The cross-sections of the obtained ceramic reinforced Ni-based alloy composite coatings were prepared for metallographic examination by standard polishing techniques.Microstructural studies including electron scanning microscopy(SEM)(LEO-1450),energy dispersive spectrum(EDS)and X-ray diffraction(XRD)techniques were used in order to identify the microstructures and phases that were present in the ceramic reinforced Ni-based alloy composite coatings.SEM was performed on the cross-section of the ceramic reinforced Ni-based alloy composite coatings.The phases present in the ceramic reinforced Ni-based alloy composite coatings were determined using XRD(with Cu Kα).EDS was used to analyze chemical composition of the ceramic reinforced Ni-based alloy composite coatings.Surface macro-hardness measurements were carried out using a surface Rockwell hardness tester with a load of 147.1 N.

2 Results and discussed

Fig.2 is the over-view of the cross-section of the ceramic reinforced Ni based alloy composite coating,and Fig.3 shows backscattered electron micrograph and second electron image of the typical microstructures of the coating,respectively.From Figs.2 and 3 it can be seen that there are many pores which do not evenly distribute in the coating.According to the amount of the pores the coating is divided into three layers,namely the top layer,the mid layer and the bottom layer near to the substrate.In the top layer and the bottom layer the amount of pores are smaller than that in the mid layer.By estimating,the mount of pores is about 15%volume in the coating.The coating with many pores may be applied as a self-lubrication coating.

Fig.2 Over-view of the cross-section of the ceramic reinforced Ni based composite coating(a)Overview of the cross-section(b)Layers in the cross-section

Fig.3 Typical structure of the ceramic reinforced Ni based composite coating(a)Backscattered electron micrograph(b)Second electron image

Although the coating can be divided into three layers,Except F zone in the mid layer of the coating,the microstructures in these layers are the same(seen in Fig.4).The whole coating is mainly sorted into five zones of A,B,C,D and E by different colors of the zones.The quantity of A and E zones is the most.While B,C and D zones are evenly distributed in A and E zones.Thus,it can be said that the zones of A and E are the binder phase of the coating while B,C and D zones are reinforced phases.B zone takes on different shape,and it is an aggregate of some small grey particles(seen in Fig.4b).Fig.4h shows X-ray line scanning of B zone.Because of the drift of the information,the result of X-ray line scanning is not corresponding to the actual position.Nevertheless,from the result of X-ray line scanning it can be found that the distribution of elements is not uniform in B zone.In B zone,the fuscous zone has the higher concentration of Ti,the white zone has more Si,Fe and Ni,and the change of the concentration of Cr is relatively small.D zone looks like catenulate aggregate.C zone appears hexangular and takes on oval and polygonal.F zone becomes rod.Exception of the above zones in the coating,there still presents a G zone that adjoins the substrate and lies in the interface.The zones of A,C,D,E,F and G were analyzed by the EDS.The results of EDS(in Table 1)showed that A and E zones were composed of Ni,Si,Cr and Ti,and the concentration of Ni was in large amount while-ever the concentration of Ti was very low;C,D and F zones mainly consisted of Cr,however the concentration of Cr was different in them.G zone was made up of Fe in great extent(seen in Fig.5).

Table 1 EDS analysis results of A,C,D,E,F and G zones(at%)

Fig.4 SEM micrographs of the three layer of the coating(a)Top layer(b)Magnification of B zone(c)Mid layer(d)Magnification of(Ⅰ)zone(e)Bottom layer(f)Magnification of D zone in bottom layer(g)Magnification of B zone in bottom layer(h)X-ray line scanning of B zone in top layer

Fig.5 SEM micrographs of the coating’s interface with Cr,Ni,Fe,Si and Ti Kα X ray line scanning(a)Interface(b)Cr Kα X ray line scanning(c)Ni Kα X ray line scanning(d)Fe Kα X ray line scanning(e)Si Kα X ray line scanning(f)Ti Kα X ray line scanning

Table 2 phases possibly present in the zones of A,B,C,D,E,F and G

Because there are several elements in the raw materials,at the elevated temperature these elements will react to each other and as a result many compounds will occur in the coating.These compounds(phases)which will be most possibly synthesized at elevated temperature in zones of A,B,C,D,E,F and G are shown in Table 2.Owing to the overlap of the diffraction peaks of the different phases,some diffraction peaks are enhanced and/or some diffraction peaks are covered up,it is difficult to identify the phases in the coating merely by the result of XRD.In order to ascertain the phases in the coating,the elements of the various zones were analyzed by means of EDS.Every compound has its own constitutes and the elements composing of the compound come from different raw materials during the fabrication of coatings by the reactive braze coating processing.For example,to synthesize Cr5B3,it needs the elements of Cr and B,and the element of B can only be obtained from ferroboron added in the raw materials of the coatings.As a result,the EDS analysis results of the zone containing Cr5B3 show high concentration of Fe.Considering the ratio of the raw materials of the coatings,combining the results of XRD and EDS,the phases present in every zone are ascertained.Namely,the phases in the zones of A,B,C,D,E,F and G are Ni31Si12,TiC,Cr7C3,Cr5B3,Ni31Si12+(Ni,Fe)solid solution,(Cr,Fe)3C7 and Fe3C,respectively.All of the phases in the coating are in-situ synthesized.

From SEM micrographs(seen in fig.4)it can be found that the TiC has different morphology.The reason why the TiC appears different morphology mainly consists in the powder morphology of Ti particle in the raw materials.Because the Ti particle has enough toughness so that it cannot fragmentate and its form changed from granular to rod-like during ball milling the mixture of the raw materials.When Ti reacts with C diffused across the near zone into the Ti particle to form TiC at elevated temperature,due to C diffusing into Ti particles needing time,with the uneven distribution of C in the Ti particle the concentration of TiC is not uniform in Ti particles.Where there is more concentration of C,there is more concentration of TiC in Ti particles.Meantime,Ti particle distributes randomly in the mixture of the raw materials,the resulted TiC has random orientations.In the SEM micrographs of the cross-section TiC takes on different morphology.

For the same reasons as the orientation of the TiC,the morphology of hexangular Cr7C3 takes on different shapes such as oval and polygonal.Compared to TiC and Cr7C3,the morphology of Cr5B3 which looks like catenulate aggregate is simple.

There is an interface between the coating and the substrate(shown in Fig.5).The interface consists of A,E and G zones and in which the elements of Cr,Ni,Fe,Si and Ti become graded distribution.The fact that the interface is present means that the coating and the substrate are integrated together by a metallurgical bonding.

The distribution of Ti,Ni,Fe,Cr and Si in the interface is very helpful to decrease the stress between the coating and its substrate.Because the coating has higher hardness,while the mild steel substrate has lower hardness.If there is no interface between the coating and the substrate,due to unmatched hardness of the coating and the substrate there is inevitably higher stress between them,and the coating only combines with the substrate by mechanical bonding.At the same time due to the gradient distribution of Ti,Ni,Fe,Cr and Si in the interface between the coating and the substrate,the hardness of the interface is gradient so that the stress between the coating and the substrate will be weakened in great extent.

The hardness of the composite coating was tested by a surface Rockwell hardness tester.It is showed that the hardness is in the range of 91-94HR15N.

3 Conclusions

(1)By means of reactive braze coating processing a new coating system,the ceramic such as TiC,Cr7C3 and Cr5B3 reinforced Ni-based alloy composite coating was prepared on an surface of a mild steel substrate at low temperature of 1000 ℃.The coating and the substrate were integrated together by metallurgical bonding.

(2)The microstructures of the coating were mainly composed of TiC,Cr7C3,Cr5B3,Ni31Si12 and(Ni,Fe)solid solution.All of the phases in the coating are in-situ synthesized.The ceramic reinforcement phases of TiC,Cr7C3 and Cr5B3 are randomly distributed in the binder phases of Ni31Si12 and(Ni,Fe)solid solution.

(3)The volume of the pores is about 15% in the coating and can be used as self-lubrication coating.

(4)The hardness of the coating is in the range of 91-94HR15N.

Acknowledgment

This work was supported by Yangjiang science and technology project(0202010067),and the authors gratefully acknowledge Yangjiang polytechnic for providing financial assistance for this study.