Synthesis and Sintering Kinetics of Ultrafine Cr2AlC MAX Phase Powders

Yembadi, Rajkumar (2017) Synthesis and Sintering Kinetics of Ultrafine Cr2AlC MAX Phase Powders. PhD thesis, Indian Insitute of Technology Hyderabad.

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Abstract

The MAX phase is a group of layered ternary compounds, which has combined properties of metals and ceramics. Like metals they are ductile, readily machinable, relatively soft, good thermal and electrical conductive and has good damage tolerance. Li ke ceramics they have good high temperature strength, high elastic modulus and good oxidation and corrosion resistance. Cr 2 AlC is one of the most promising candidate in the MAX phase group. Due to its unique combination of properties, it has potential appl ications for diverse field of applications, especially as high temperature materials and protective coatings, etc. Since last two decades, a lot of research has been carried out on synthesis, sintering and property evolution, but commercial exploitation of these materials is still limited. Recent reports on MAX phases revealed the importance of powder production and development of sintering methods. Producing Cr 2 AlC powders with high purity, has been a major challenge; whereas reaction kinetics and thermody namics have been rarely explored. Moreover, understanding the sintering mechanism in multi - element system is difficult, whereas identification of a particular element which would be controlling the sintering is still a major challenge. The present investig ation has been initiated with three major objectives: i) to synthesize the high purity ultrafine Cr 2 AlC powders through mechanically activated synthesis route, ii) to study the reaction kinetics and thermodynamic aspects of Cr 2 AlC formation, and iii) to de termine the sintering mechanisms of Cr 2 AlC powders. Powder synthesis was done in three stages: firstly, CrC x (x = 0.5) powder was prepared from Cr and C (graphite) at 1100 ºC. CrC x powder consists mainly two types of carbides, Cr 7 C 3 and Cr 3 C 2 phases in eq uimolar ratio, as represented by a chemical reaction: 10Cr + 5C  Cr 7 C 3 + Cr 3 C 2 . Secondly, these synthesized CrC x powders were milled for 15 and 30 h using high energy planetary ball mill, with the aim of particle size refining and increasing the chemical reactivity of the powder. Finally, these mechanically activated carbide powders were reacted with pure Al powders at various temperatures. The formation reaction can be represented as: Cr 7 C 3 + Cr 3 C 2 + 5Al  5Cr 2 AlC. Milling energies (during milling of chro mium carbide powders) were estimated for 15 h and 30 h duration as 1.6 kJ/g and 3.2 kJ/g respectively. Effect of mechanical activation (i.e., milling durations) of chromium carbide powders on the reaction with pure Al powder was clearly noticed. Nearly pur e Cr 2 AlC powders with xii submicron particles (below 400 nm) were produced at 800  C, using 15 h milled carbides. Little excess amount (about 0.4 mol) of Al was required to achieve very high purity Cr 2 AlC phase. DSC technique was employed to assess the thermod ynamic factors for Cr 2 AlC formation. Cr 2 AlC formation reaction was exothermic in nature, which occurred as soon as aluminium started melting. Changes in enthalpy of reaction was found from - 53 kJ/mol (for 15 h milled) to - 66 kJ/mol (for 30 h milled). The G ibb’s free energy of reaction was found to become more negative with increasing milling time; it changed from - 107 kJ/mol to - 139 kJ/mol, when milling was increased from 15 h to 30 h. This has enabled the conversion at relatively lower temperature of about 700 °C. Reaction kinetics was analysed through the activation energies of 366 - 369 kJ/mol and 315 - 316 kJ/mol for Cr 2 AlC formation from 15 h and 30 h milled carbides respectively. The Cr 2 AlC powder produced at 800 ºC using 15 h milled reactants, was subjec ted to further sintering studies. Green compacts (of about 61% relative green density) were sintered using vertical dilatometer system , where axial shrinkage data were recorded as a function of temperature and time. Relative density was found to be increas ed to about 74% after sintering at 1300 ºC for one hour. Imaging through SEM showed the formation of stepped structures or layered structures, typical characteristics of MAX phases. Further, the observed shrinkage was analyzed in two different stages, firs t for non - isothermal shrinkage and second for isothermal shrinkage. Using the non - isothermal shrinkage, activation energy of sintering was derived for possible volume diffusion or grain boundary diffusion mechanism through the Young and Cutler’s model. Act ivation energy of 352 kJ/mol obtained for volume diffusion mechanism indicates possibility of operating volume diffusion mechanism during non - isothermal heating. When isothermal shrinkage data were analysed through conventional Arrhenius relation, it yield ed the apparent activation energy of sintering to be 472 kJ/mol. It appears that measurement of activation energy alone, does not give clear understanding about the controlling mechanism. Therefore attempts have been made to evaluate complete diffusivity p arameters, i.e. frequency factor of diffusion coefficient along with its activation energy, for both volume diffusion and grain boundary diffusion mechanisms using existing models of Johnson (1969) and Ashby (1974). The derived frequency factors with respe ctive activation energies were compared with the diffusivity parameters available in the literature for constituent elements, i.e. Cr, Al, and C, as well as compounds and alloys containing some xiii of these elements. Such comparison revealed that current resul ts are in close proximity with the diffusivity parameters reported for Cr in bulk and pure polycrystalline metal and in multi - element alloys. Thus it could be concluded that diffusion of Cr through volume diffusion mechanism controls the sintering of Cr 2 Al C powder. This thesis has been organized into five chapters and a brief overview has been given below: The first chapter mainly contains two sections. The first section briefly presents the general introduction of MAX phase and their important mechanical p roperties. A detailed literature review on synthesis techniques of general MAX phases and particularly Cr 2 AlC powder production have been presented, and its sintering methods have been discussed. In the next section, the fundamentals of sintering, and vari ous sintering mechanisms and models during isothermal and non - isothermal sintering have been discussed. At the end of this chapter, the short comings of the existing literature and specific objectives of the present work have been discussed. Second Chapter deals with the experimental procedure including mechanical milling, powder synthesis, powder compaction, sintering using dilatometer and characterisation of the powders and sintered samples using various characterisation tools like, Differential Scanning calorimeter, X - ray diffraction (XRD) and Scanning electron microscope, etc. Third Chapter describe the results about powder synthesis and characterization . The effect of various parameters on Cr 2 AlC conversion; such as composition, mechanical milling of re actant , especially CrC x and reaction temperature have been presented in detail. Estimation of milling energies and its correlation with Cr 2 AlC conversion, such as Enthalpy and Gibb’s free energies have been discussed. Fourth Chapter comprises of the observ ations of the dilatometer sintering plots and analysis of the shrinkage measurements. In addition, the phase analysis and microstructures of the sintered samples have been presented. It also presents the analysis of the observed sintering data during non - i sothermal and isothermal sintering through the employment of various sintering models. Diffusion coefficients and activation energies derived from the shrinkage data have been shown. The final section of the fourth chapter attempts to analyse the outcome o f sintering data . Mainly the results predicted through the sintering models for various sintering mechanisms have been explained and compared with existing data to reveal the sintering mechanism and to identify the major diffusin g species. Last Chapter (fi fth chapter) summaries , all the major results about powder synthesis, thermodynamics of phase formation, powder sintering and sintering mechanisms.

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