Study On The Synthesis And Reaction Mechanism Of Cr-Al-C Ternary Ceramics

The ternary layered ceramic Cr₂AlC,as a member of the MAX phase compound,is widely concerned by researchers because of its excellent properties of metal and ceramics.At present,the traditional methods usually required high synthesis temperature or long time to synthesis high-purity Cr₂AlC.These problems gravely restrict the wide application of Cr₂AlC ceramic.In order to rapid synthesis high-purity Cr₂AlC at low temperature,the Cr₂AlC was synthesized by a new method-microwave hybrid heating,and its synthetic process,reaction mechanism and property were studied in this work.The research mainly focused on the Cr₂AlC by microwave hybrid heating using Cr,Al,C and Cr,Al,Cr₃C₂ as the raw materials,and compared with the pressureless sintering.This paper discussed the ratio of raw materials and process parameters（such as microwave power,synthesis temperature,holding time,et al）on its content and microstructure,and the phase evolution rule and the reaction mechanism was also studied.Meanwhile,the nanometer Cr₂AlC powders were prepared by ball milling-ultrasonic using the preparation of Cr₂AlC as raw material.And its composition,specific surface area and particle size were analyzed.In addition,the tribological properties of as-received and nanometer Cr₂AlC powders were discussed.Cr₂AlC was synthesized by microwave mixing and heating with graphite as a carbon source.Under the protection of Ar atmosphere,the molar ratio of n（Cr）:n（Al）:n（C）=2:1:1,and the temperature was kept at 1100°C for 5min,Cr₂AlC with a small amount of Cr₇C₃ was synthesized.Using Cr₃C₂ as carbon source,the mole ratio was n（Cr）:n（Al）:n（Cr₃C₂）=1:2:1,sintering temperature was at 1050℃for 3min,single-phase Cr₂AlC was synthesized at Ar atmosphere.The result showed that high-purity Cr₂AlC could be synthesized at lower temperature in the research was attributed to the introduction of Cr₃C₂ as carbon source instead of C（graphite）.For Cr/Al/C system,Cr₇C₃ phase was preferentially formed compared with Cr₂AlC phase and the solubility of C in Al is much lower than that in Cr so that the impurity Cr₇C₃ was retained in the final products.Using Cr₃C₂ as carbon source instead of C,Cr₃C₂ could provide the Cr and C at nearly equal molar lever,Cr₃C₂ and molten Al could react together to form the Cr₂AlC phase.In addition,compared to pressure less sintering,the microwave hybrid heating temperature of Cr₂Al C was decreased by 300℃,and the dwell time was reduced by 10h.Using Cr,Al,Cr₃C₂ powders as raw materials,Cr₂AlC was synthesized by pressure less sintering.In the temperature range of 700-1000℃,the molten Al reacted with Cr,Cr₃C₂ to form Cr₂Al,Cr₅Al₈,Cr₂AlC.At this time,a small amount of Al₄C₃ was also found in the sample.When the temperature up to 1100℃,Cr₂Al,Cr₅Al₈ reacted with Cr,Cr₃C₂ to form Cr₂AlC.With the rise of the sintering temperature,a small amount of new impurity phase Cr₇C₃ was produced in the sample,which was caused by Cr₂AlC decomposition at high temperature.Using Cr,Al,Cr₃C₂ powders as raw materials,Cr₂AlC was synthesized by microwave hybrid heating.At below 900℃,Cr,Al reacted with Cr₃C₂ to produce Cr-Al intermetallic compound（Cr₂Al,Cr₅Al₈）and Cr₂AlC.At above 900℃,Cr-Al intermetallic compound reacted with Cr₃C₂,Cr to form Cr₂AlC.Based on the XRD results,it is concluded that a large amount of Cr₂AlC phase was detected in the sample.When the sample was sintered by pressure less sintering,the sample is heated by heating element,the temperature was transferred from outside to inside which was easy to produce the temperature gradient.However,the microwave hybrid heating is heated by internal friction vibration,so that the whole material is heated to avoid the heat transfer from the outside to produce the temperature gradient,which can rapid synthesis Cr₂AlC at low temperature.Nanometer Cr₂AlC powder has been successfully prepared by ball milling and ultrasonic stripping using as-received Cr₂AlC powder as raw material,and the average particle size was458nm.The specific surface area of nanometer Cr₂AlC was 29.89m²/g calculated by BET formula,which was about 7 times of the as-received Cr₂AlC powder.At room temperature,800r/min,under a load of 300 N,the base oil containing 2wt%Cr₂AlC sample presented excellent tribology performance.This is because during the friction,Cr₂AlC powder can form a layer of lubricating film on the surface of the friction pair to reduce the wear and tear.When the addition of Cr₂AlC is 2wt%,the friction coefficient and the friction width increase with the increase of loading force,which may be caused by the failure of the lubricating film due to the large loading force.Under the same conditions,the nanometer Cr₂AlC showed a better performance of reducing wear.Compared with the as-received powder,the friction coefficient and the grinding width decreased by 70.65%and 69.00%,respectively.