HUANG Xiaochen,YAN Yu,DING Yunfei,GE Jinlong,

LI Zongqun1,2,LI Tong1,2,ZHOU Zijue4

(1.School of Material and Chemical Engineering,Bengbu University,Anhui 233030,China;2.Engineering Technology Research Center of Silicon-based Materials,Anhui 233030,China;3.Jiangsu Academy of Marine Resources Development (Lianyungang),Jiangsu Ocean University,Lianyungang 222005,China;4.School of Materials Science and Engineering,Hefei University of Technology,Anhui 230009,China)

【Abstract】The influence of Ti3 SiC2 content on the arc erosion behavior of Ag-Ti3 SiC2 contact material was comprehensively investigated.After 10 k V arc erosion,the Ag-Ti3 SiC2 composite decomposed and oxidized by the formation of AgO,TiO2, and SiO 2.The morphologies of protrusions,pores,peelings,and folds formed on the cathode.With an increasing Ti3 SiC2 content,the electric arc was more dispersed on the eroded surface of Ag-Ti3 SiC2 composites.With 30 vol.%of Ti3 SiC2 addition to the composite,the gridlock effect of Ti3 SiC2 could lock the molten Ag tightly,which effectively reduced the splashing phenomenon of Ag matrix and the erosion effect of arc erosion on the contact material.

【Key words】 Ag-Ti3 SiC2;Arc erosion;Gridlock effect;Oxidation

1 Introduction

Relays have been widely used in aerospace and national defense equipment systems such as satellite,rocket,and accelerator engineering due to their functions in signal transmission,circuit conversion and long-distance regulation.It is common to see the erosion effects of electric arc on the contact material of a relay,which greatly shortens the lifespan and causes substantial economic loss to a country.Compared with other commonly used metals,silver(Ag)has better electro-conductivity and thermal conductivity.In addition,a high thermal conductivity consequently promotes the heat transfer of the arc or Joule heat source to the surrounding environment,shortens the arc combustion time,and reduces arc erosion.However,the arc concentrates on the surface of pure silver,which might lead to a large number of materials splashing.The strength of Ag will decrease seriously.Besides,Ag has a relatively lower hardness and inferior wear resistance,which cannot meet the requirements of the mechanical strength for a good relay.Therefore,the addition of Ag to a composite as the contact material can reduce contact resistance and the local temperature rise caused by the electric arc.The addition of a reinforced phase in the silver matrix can promote the dispersion of arc and weaken the arc erosion of silver,contributing to the better performance of the contact materials.

Through changing the preparation process and adding additives,the arc erosion abilities of traditional contact materials have been improved.It was found that the splashing phenomenon caused by arc erosion of AgZn O as a contact material prepared by co-precipitation method was less than that by ammonification method[1].Compared with internal oxidation and chemical co-precipitation,AgSnO2prepared by powder metallurgy had less arc erosion and shorter arc burning time[2].The conductivity of AgSnO2with (La,Bi)codoped was further enhanced[3].A further study found that the addition of CuO particles and its irregular distribution in the matrix significantly reduces the arc erosion of AgSn O2material[4].This could be resolved by the applications of Bi2O3,Y or NiO as the additives for AgSn O2.And thus,the electrical contact properties of AgSn O2material can be improved significantly[5-7].Those findings reveal that the addition of Ni improved the arc erosion resistance and dielectric strength of Ag TiB2and CuCr materials,respectively[8].Cu W alloys have been widely used in electrical and electronic fields because of its good combination of intrinsic properties of a hard W phase with a soft Cu phase.With the addition of graphite to Cu W contact,the arc originally occurred in the direction of Cu specific crystal transferred to graphite,which greatly improved the arc erosion resistance of Cu W alloy[9].A recent study investigated copper matrix composites reinforced with carbon nanotubes (CNTs)and TiB2micro-particles.It was found that CNTs addition to TiB2/Cu can effectively promote the dispersion of electric arc and greatly reduce the arc energy of TiB2/Cu materials[10].The increased W content can reduce the welding force of Al2O3-Cu(Cr,W)material and shorten the arc burning time[11].Additives make the composition of electrical contact materials more complex,and the preparation process is more challenging to control stably.Therefore,it is urgent to develop a new type of electrical contact materials without additives.

The MAX phases are a group of layered ternary compounds in which M is a transition metal element,A is theⅢorⅣmain group element,and X is C or N element.The typical representative of the MAX phase is Ti3SiC2material[12-14].MAX phase Ti3SiC2material contains strong covalent bonds,ionic bonds and weak van der Waals forces,which endow Ti3SiC2with metal and ceramic properties.It has low resistivity (0.227μΩ· m)and high thermal conductivity(37 W·m-1·K-1).The linear expansion coefficient of Ti3SiC2is 9.1×10-6K-1[15],and the melting point of Ti3SiC2can reach about 3 000℃.Ti3SiC2material has excellent high-temperature oxidation resistance.After 20 hours of high temperature oxidation,the weight only increases by 3.5×10-2kg/m2.Ti3SiC2has excellent mechanical properties in Young’s modulus(322 GPa)and compressive strength(1 050 MPa).Because of the above behavior in electrical,thermal and mechanical properties,Ti3SiC2material has attracted more attention in the past decade.Sun et al.proved that Ag-Ti3SiC2composite had good wettability,and Ag and Ti3SiC2materials remained stable under high temperatures,did not chemical reactions occur,which proved that Ag-Ti3SiC2composite was suitable for contact material[16].Our previous studies proved that Ti3SiC2material had the function of dispersing electric arc on the Ag matrix,effectively avoiding the concentrated erosion of the Ag matrix by the electric arc.However,the principles of dispersed arc action and arc erosion mechanism of Ti3SiC2reinforced phase are still not well understood.Thus,this paper concentrated on the influence of Ti3SiC2content on the arc erosion behavior of Ag-Ti3SiC2contact material.Microstructure and compositions of eroded Ag-Ti3SiC2were characterized systematically.With the content of Ti3SiC2addition reached or exceeded 30%,the gridlock effect of Ti3SiC2was exhibited,which locked the Ag protrusions tightly due to the good wettability between Ag and Ti3SiC2material,leading to the reduction of the splashing of the silver matrix.

2 Material and methods

2.1 Fabrication of Ag-Ti3 SiC2 composites

Ti3SiC2powders(-300 mesh,＞98%purity)were fabricated by molten salt shielded synthesis[17].Ag powders and Ti3SiC2powders with a constant volume ratio of 60∶40,70∶30,80∶20 and 90∶10 were mechanically mixed for 2 h,respectively.Then,the mixtures were transferred into stainless steel molds(Φ15 mm).The pressure with 700 MPa was applied to the mold for 90 s.Each mixture was turned into a planchet.Then the planchets with different Ti3SiC2additions were heated to 600℃in a tube furnace with a constant Ar atmosphere for 1 h.The mixtures were placed in a furnace for an adequate time until room temperature.After that,Ag-Ti3SiC2specimens were obtained for the following experiment.All the specimens were polished with abrasive papers,and the specimens were thoroughly cleaned with alcoholic solution,acetone,and deionized water.The phase compositions of the fabricated Ag-Ti3SiC2were characterized using X-ray diffraction(XRD,Smart LabSE)with Cu Kαradiation at 40 k V and 50 m A.

2.2 Microstructure

The microstructure of the Ag-Ti3SiC2with various Ti3SiC2additions was investigated using scanning electron microscopy (FE-SEM,JEOL)coupled with energy-dispersive X-ray spectrometry(EDX)and an element-mapping technique.XRD was used to characterize the phase constituents of Ag-Ti3SiC2before and after arc erosion.All samples for micro-structure observation were cleaned to remove any contamination.

2.3 Arc erosion of Ag-Ti3 SiC2

A self-made arc generation device was utilized for the arc erosion of Ag-Ti3SiC2[18].Ag-Ti3SiC2contact materials with various Ti3SiC2additions(10,20,30,and 40 vol.%)were selected as the cathodes,and a tungsten rod with a tip was the anode.To avoid the influence of the last arc erosion,the anode was renewed after the completion of each test.After the application of the load voltage of 10 k V on the electrodes,the cathode and anode moved closer at 2 mm/minute until the occurrence of an electric arc.The arc lasted about 35 ms.The morphological characteristics were reconstructed by three-dimensional laser scanning confocal microscopy(3D LSCM,VK-X1000)in the mode of laser color observation.The field-emission scanning electron microscopy (FE-SEM,JEOL)was carried out to display the phase and element distribution of the eroded Ag-Ti3SiC2composites.The eroded products of the Ag-Ti3SiC2composite were analyzed by a Raman spectrometry (LabRAM HR,HORIBA JOBIN YVON),which was performed over a wavenumber range of 100-2 000 cm-1with an Nd∶YAG laser.

3 Results and Discussions

3.1 Microstructure

Fig.1 shows the XRD patterns of the Ag-Ti3SiC2with various Ti3SiC2additions.The peaks of the phases,including Ag (JCPDF No.87-0597)and Ti3SiC2(JCPDF No.89-8255),were identified.The peaks of Ti3SiC2intensified with increasing Ti3SiC2content from 10%vol.to 40%vol..

Fig.1 XRD patterns of Ag-Ti3 SiC2 with various volume percentages of(a)10 vol.%, (b)20 vol.%, (c)30 vol.%and(d)40 vol.%Ti3 SiC2

The microstructures of Ag-Ti3SiC2with various Ti3SiC2additions are shown in Fig.2.It can be seen that with the increment of Ti3SiC2from 10 to 40 vol.%,the dark gray phase increased dramatically.To further investigate the phase distribution of Ag-Ti3SiC2,EDX mappings of the specimens were further conducted using FE-SEM.Fig.3 shows the distribution of elements in the matrix.It can be seen that gray phase and dark phase were identified in Fig.3(a),and they were further illustrated in Fig.3(c)and Fig.3(d).The gray and dark phases were the elements of Ti and Si,respectively.Those findings were coincident with the XRD patterns in Fig.1,which suggested that Ti3SiC2particles distributed in the Ag matrix uniformly without aggregations,indicating the stable preparation process and suitable Ag-Ti3SiC2composite for arc erosion.

Fig.2 SEM images of Ag-Ti3 SiC2 composites with various volume percentages of(a)10 vol.%, (b)20 vol.%, (c)30 vol.%and(d)40 vol.%Ti3 SiC2

Fig.3 (a)SEM images of Ag-10 vol.%Ti3 SiC2 composite,mappings of(b)Ag(c)Ti and(d)Si

3.2 Gridlock effect of Ti3 SiC2

To investigate the effects of Ti3SiC2content on the behavior of Ag-Ti3SiC2composites,the eroded morphologies of Ag-Ti3SiC2against the volume of Ti3SiC2were characterized and the results were shown in Fig.4.As can be seen,Ag-Ti3SiC2with different Ti3SiC2additions suffered various attacks from arc erosion,and the electric arc was more dispersed on the surface of Ag-Ti3SiC2composite.The phenomenon proved that Ti3SiC2has arc dispersing effect,which effectively avoid the concentrated erosion of Ag matrix.As can be seen from Fig.4(a),a large site of the Ag-Ti3SiC2was attacked on account of the arc erosion,and a layer of eroded sites was observed and splashed off the surface of the Ag-Ti3SiC2composite with 10%Ti3SiC2.With the increasing Ti3SiC2addition,Ag/Ag oxides protrusions formed on the surface of Ag-30 vol.%Ti3SiC2composite,as shown in Fig.4(c),accompanied by Ti3SiC2and its oxide particles.The enlarged image inside the yellow rectangle in Fig.4(c)was shown in Fig.6(a)and Fig.6(b),it can be seen that the Ag-Ti3SiC2composite has been melted by the arc erosion.After its solidification,refined protrusions formed on the surface of Ag-Ti3SiC2composite.In order to identify the composition of the protrusion and the flat place,the microstructure and the phase were further investigated using SEM and EDX.As shown in Fig.6(c),the EDX on the selected region indicated that the protrusion mainly contained Ag and O.While,Fig.6(d)showed that the flat place mainly contained Ti,O,and Si.Those findings presented that both Ag matrix and Ti3SiC2were oxidized.

Fig.4 SEM images of eroded Ag-Ti3 SiC2 composites with the volume percentage of(a)10 vol.%, (b)20 vol.%, (c)30 vol.%and(d)40 vol.%Ti3 SiC2

Fig.6 (a)Enlarged image inside the yellow rectangle in Fig.4(c); (b)enlarged image inside the yellow rectangle in Fig.6(a); (c)and(d)EDX results of blue rectangles in Fig.6(b)

In order to better explain the dispersing function of Ti3SiC2,the morphological characteristics were reconstructed by three-dimensional laser scanning confocal microscopy in the mode of laser color observation.The results are shown in Fig.5.Compared with Fig.4,the images in Fig.5 showed the color change of Ag-Ti3SiC2composite after arc erosion more clearly.The surface conditions of Ag-10 vol.%Ti3SiC2and Ag-20 vol.%Ti3SiC2composites were compared in Fig.5(a)and 5(b).Due to the function of arc erosion,a layer of material on the surface has been stripped and leaked out of the fresh surface in Fig.5(a),shown by the white circles.There was no such phenomenon in Fig.5(b).The content of Ti3SiC2in the composite in Fig.5(b)was higher than that in Fig.5(a),and Ti3SiC2particles played the role of consuming arc energy and dispersing arc,therefore,there was no material peeling phenomenon in Fig.5(b).When the content of Ti3SiC2material increased to 30%and 40%,the arc jumped from the arc starting position to the side,and finally a larger erosion image formed in Fig.5(c)and(d)than that in Fig.5(a)and(b).In summary,Ti3SiC2can disperse the arc.

Fig.5 Microscopic images of eroded Ag-Ti3 SiC2 composite with the volume percentage of(a)10 vol.%, (b)20 vol.%,(c)30 vol.%and(d)40 vol.%Ti3 SiC2

The above findings presented that with the increasing Ti3SiC2content,Ag-Ti3SiC2exhibited different behavior after arc corrosion application.This could be explained by the schematic model as shown in Fig.7.It is known that the melting point of Ag is 961.78℃,which is far lower than that of Ti3SiC2(＞3 000℃).After applying load voltage of 10 k V on the cathode (Ag-Ti3SiC2)and anode(Tungsten rod),high energy and high heat arc made Ag matrix melt first.Due to the surface tension and arc plasma force[19-20],Ag formed the shape of protrusion,as shown in Fig.7(a).When the cathode contained 10%or 20%of Ti3SiC2,some Ag protrusions splashed off the surface,while the others remained on the eroded surface,as shown in Fig.4(a)and 4(b).Sun et al.found that the contact angle of molten Ag/Ti3SiC2was 14°,which represented the good wettability between Ag/Ti3SiC2[16].When Ti3SiC2content exceeded 30%,Ti3SiC2formed a grid-like structure.Before the molten Ag splashed off the material surface,the excellent wettability made Ti3SiC2lock the Ag protrusions tightly,as shown in Fig.7(b).After the arc erosion was extinguished,Ag protrusions solidified rapidly,leading to the morphologies as shown in Fig.4(c),4(d)and Fig.6(a).Thus,Ti3SiC2effectively reduced the splashing phenomenon of the Ag matrix and the erosion effect of arc erosion on the contact material.

Fig.7 Schematic illustration for the morphology formation mechanism of eroded(a)Ag-10 vol.%Ti3 SiC2, (b)Ag-30 vol.%Ti3 SiC2 composite

3.3 Microstructure of eroded Ag-Ti3 SiC2 composite

It is well known that the microstructure of the eroded surface of Ag-Ti3SiC2composites varied with the increasing Ti3SiC2content(as shown in Fig.4 and Fig.6).Fig.8 showed the surface morphologies of Ag-Ti3SiC2after arc erosion.It can be seen from Fig.8(a)to Fig.8(d)that protrusions,pores,peelings and folds form on the eroded Ag-Ti3SiC2composite.The regions around the pores were rich in Ag and O,as shown in Fig.8(c).In similar,the fold also mainly contained Ag and O in Fig.8(f).In order to further confirm the products of the Ag-Ti3SiC2composite after arc erosion,Raman spectrometry was conducted on the Ag-Ti3SiC2composite.Through analyzing the shift value of Raman peaks,the specific substance can be identified precisely.The laser of the Raman spectrometry acts at the yellow cross,shown in the insets of Fig.9.In Fig.9(a),the laser served at the flat place,similar to the place in Fig.6(b)and Fig.6(d).The peaks of 793 and 1 035 cm-1belonged to SiO2(R061064,R070565),and the peak of 327 cm-1belonged to TiO2(R050363).The results were consistent with the EDX result in Fig.6(d).In Fig.9(b),the pattern obtained at the fold morphology was similar to that of the morphology in Fig.8(e).The shift of 993 cm-1arose from Ag O[21]while the peaks at 1 312 and 1 586 cm-1arose from carbides[21],which was agreed with EDX result in Fig.8(e)and Fig.8(f).The Raman shifts in Fig.9 further certificated that Ag-Ti3SiC2decomposed and oxidized into AgO,TiO2and SiO2.

Fig.8 Microstructures of erode Ag-Ti3 SiC2(a)protrusions, (b)pores, (d)peelings, (e)folds;EDS results of(c)spectrum 1 and(f)spectrum 2

Fig.9 Raman shifts of eroded Ag-Ti3 SiC2 composite(a)TiO2 and SiO2 and(b)Ag O

4 Conclusion

The influence of Ti3SiC2content on the arc erosion behavior of Ag-Ti3SiC2contact materials was comprehensively investigated.After the application of the load voltage of 10 k V on Ag-Ti3SiC2composite,the composite decomposed and oxidized into AgO,TiO2and SiO2.Consequently,protrusions,pores,peelings,and folds formed on the eroded surface.Due to the effect of the dispersed arc of Ti3SiC2,the eroded regions by the arc were more scattered with the increasing Ti3SiC2content.When the Ti3SiC2content was less than 30%,some molten Ag particles splashed off the cathode surface,while some Ag particles formed protrusions and remained on the surface dispersedly.When Ti3SiC2content reached or exceeded 30%,due to the excellent wettability between Ag and Ti3SiC2material,Ti3SiC2had the gridlock effect,which could lock the Ag protrusions tightly.Ti3SiC2can effectively reduce the splashing phenomenon of Ag matrix and the erosion effect of arc erosion on the contact material.