Research on Process and Mechanisms of Active Diffusion Bonding of ZrC_x Ceramics

Author:Pan Rui

Supervisor:he peng lin tie song


Degree Year:2018





Transition Metal Carbides(TMCs),such as ZrCx,TiCx,TaCx,VCx,etc.,are also known as interstitial carbides.These carbides have a combination of remarkable properties,such as a high melting point(>3000℃),high hardness,excellent wear and corrosion resistances and good electrical conductivity.Such superior materials’ features make these ceramics to be the prime candidates for various ultra-high-temperature applications,such as the next-generation rocket engines,sharp leading edges of future-generations of reentry vehicles,gas-cooled fast nuclear reactors,to mention just a few.Stringent high temperature working environments of TMCs require material’s joints to have as high quality as possible.The requirement is not only to obtain a joint with as good mechanical properties as possible,but an ability to sustain a highly enhanced temperature resistance as well.In the present study,to meet the above requirements,a novel joining method of active diffusion bonding for ZrCx ceramics using transition metals as the interlayer is promoted.The method is based on the presence of carbon vacancies in ceramics substrates and the mutual solubility between TMCs.Homogeneous joints can be formed at the mating parts’ interface by using this method.The joints formed possess low residual stress,good high temperature resistance and high reliability.All these features have been overcoming the shortcomings of traditional bonding,such as high residual stress,often poor high temperature resistance and unacceptably low strength.To verify the feasibility of the proposed joining method for ZrCx ceramics,a single layer of Ti was initially considered as the interlayer to active diffusion bond ZrCx ceramics(x=1,0.85,0.7 and 0.55).The effect of joining parameters and carbon vacancy population on the microstructures and mechanical properties of the joints was studied,the diffusion kinetics of the interfacial metal element was analyzed,and the formation process of homogeneous joints was revealed.It has been experimentally established that,by using 10μm thick Ti as the interlayer,the non-stoichiometric ZrC0.7,with a large population of carbon vacancies,can achieve a virtually homogeneous joint domain at 1300℃ after 1h.However,ZrC,with little population of carbon vacancies,would not form a homogeneous joint domain even if the joint would have been exposed to 1400℃ for 8h.Based on this benchmark study,it has been established that the formation of homogeneous joints in ZrC0.7can be attributed to the promoting effect of carbon vacancies on the diffusion of interfacial elements and the mutual solubility between ZrCx and TiCx.To understand the promoting mechanism of the carbon vacancy,first principle calculations were performed to analyze the activation energy of different vacancy combinations in ZrCx ceramics and to evaluate the effect of carbon vacancy content on the miscibility gap of ZrCx-TiCx pseudobinary phase diagram.It has been shown that the increase of carbon vacancy content decreases the activation energy of VZr-(VC)n(n=1,2,3,…6)vacancy cluster,thus promoting the diffusion of interfacial elements in ZrCx ceramics.This is why the homogeneous joints can be formed from the point of view of the process kinetics.It has been demonstrated that,in the ZrCx-TiCx pseudobinary phase diagram,with the increase of carbon vacancy content,the size and the shape of the miscibility gap of ZrCx and TiCx changes and shrinks,thus promoting the solubility between ZrCx and TiCx.This is why the homogeneous joints are formed from the point of view of thermodynamics.The analysis of mechanical properties of ZrC0.7joints reveals that the homogenization of the joint domain greatly improves both the room temperature and the high temperature(measured at 1000℃)three-point bending strength.The homogeneous joint strengths can become comparable to that of the base ceramics.To solve the problem of ZrC(with small population of carbon vacancies)not forming homogeneous joints when using single Ti interlayer,other transition metals(Zr,Ta,Hf and Nb)have been considered to be utilized as the interlayers to facilitate an active diffusion bonding of ZrC0.85ceramics.It is worth noting that this ceramic has the highest melting point among all the ZrCx ceramics.The investigation has shown that both the increase of temperature and time would promote the homogenization of the joint and do result in forming homogeneous joints,but under different process conditions,depending on the material of the interlayer.When using Zr as the interlayer,it is easy to form Kirkendall voids within the homogeneous joints.When using Ta or Hf as the interlayer,the homogeneous joint domains can be formed only under pretty high temperature,e.g.1600℃ and 1700℃.Only by using Nb as the interlayer,ZrC0.85ceramics can form homogeneous joints with good qualities.This indicates a need to modify the process by decreasing the technological parameter for the formation of homogeneous joints to take place.In addition,by keeping the thickness of different transition metals constant,the activity,the ease of implementation and the technological parameter controllability of different transition metals to form homogeneous joint with ZrC0.85ceramics were mutually compared.The results show that the sequence of activity from better to worse is Ti>Zr≈Nb≈Hf>Ta>V;the sequence of ease from better to worse is Zr>Ti>Nb>Hf>Ta;the sequence of bonding temperature controllability from better to worse is Ta>Ti>Nb>Zr>Hf.Finally,to further decrease the bonding parameter set conditions for forming homogeneous joints of ZrCx ceramics,a novel design of using multiple interlayers,e.g.,Ti/M/Ti(M=Zr,Ta,Hf and Nb,respectively)has been proposed.The effect of bonding parameters on the microstructures and mechanical properties of the formed joints has been studied.It is established that,by using multiple Ti/M/Ti interlayers,when compared with using only a corresponding single M interlayer,a thicker Ti/M/Ti results in a lower bonding temperature,a shorter holding time and a thicker homogeneous joint.This verifies the feasibility of proposing the active diffusion bonding using multiple Ti/M/Ti as the interlayers.The mechanism behind this phenomenon is attributed to the fact that a layer M would form the(Ti,M)solid solution with Ti before the reaction with base ceramics takes place.This prevents a heavy formation of carbides which is in turn difficult to get dissolved into the base ceramics at the interface.In addition,the study of mechanical properties of different ZrC0.85joints facilitated by using multiple Ti/M/Ti as the interlayer also demonstrates,as expected,that the homogenization of the joint domain greatly enhances the strength and the high temperature resistance of the joints(comparable to that of the base ceramics).