Property Characterization And Technical Study On Combustion Synthesis Of AlN-Based Conductive Ceramics Under High Nitrogen Pressure

The conductive ceramics is a kind of high-performance functional material, which combines the electrical properties of metal and structural properties. Therefore it has many advantages over common metal in certain physicochemical properties. In this paper, based on the reaction Al+N2 AlN, with the proportional additive of TiB₂ or TiN conductive phase, AlN-based conductive ceramic has been fabricated by the technology of combustion synthesis under the 100Mpa nitrogen atmosphere. Through adjusting the ratio of conductive phase and insulating phase, high temperature conductive ceramic with different conductivity can be obtained. Because each composition in this new composite ceramic has a high melting point, this sort of conductive ceramic system has some special features like good high temperature properties, excellent corrosion resistance and anti-wear property.Firstly, much more focuses have been paid on the combustion synthetic mechanism, conductive property and anti-oxidation property of AlN-TiB₂ composite conductive ceramics. Some researches show that: with the increase of the content of TiB₂, material properties just like relative density, Rockwell hardness, flexural strength and fracture toughness have the similar tendency of first increment and later decrement. And when the TiB₂ content is up to 45%, all mechanical properties have reached the peak value. This regularity can be explained by the densification mechanism. In the combustion synthesis reaction region, the melting Al will overflow on the surface of TiB₂ and strong liquid sintering will happen. At the same time densification can realize under the action of external pressure. Also with the increase of volume content of TiB₂ conductive phase, the electrical resistivity of samples decreases. When the volume content of TiB₂ is between 30% and 60%, the AlN-TiB₂ system has a good conductive property and the conductivity is around 1800¹50μ?·cm.Secondly, the method of in situ synthesis is used to introduce the BN phase (B4C+Si BN+SiC). Meanwhile in order to reduce the complication of reaction TiB₂+N2 TiN+BN, TiN is used to replace the TiB₂ and a new composite ceramic AlN-TiN system is obtained. The results of thermodynamic analysis show that when the reaction occurs under high pressure (100Mpa) and low combustion temperature, TiN is a steady phase. While under high combustion temperature, the reaction of TiN+B4C TiC+TiB₂+N2 will happen. In fact the XRD results also validate the above analysis. And the influence of TiN content on the mechanical property and electrical resistivity of samples is similar to the AlN-TiB₂ ceramic system. With the increase of BN content, the relative density and strength of sample decrease, however the thermal shock resistance has been greatly improved. When the BN content is 30% or 40%, the residual strength of sample change little with the increase of heat shock temperature difference and it shows an excellent property of thermal shock resistance.Finally, SiC powders are introduced into the AlN-TiB₂ system to getting AlN-SiC-TiB₂ composite ceramic system and the solution strengthening effect of AlN-SiC to the sample property is studied. Research results show that AlN-SiC in the sample is fully solid solution under the high temperature during the combustion process. With the increase of SiC content, on the one hand, the solid solution effect increase, while on the other hand due to the decrease of Al content, the densification procedure is weakened. So with these cons and pros, the relative density and flexural strength both have a regularity of first increase and later decrease. Especially, when the SiC content is 35%, all the properties reach the peak value.Heat treatments are carried to improve the properties such as mechanical property and microstructure of AlN-SiC-TiB₂ sample after 600-1200℃for 1-10h. Obviously, with the temperature increasing and the prolongation of holding time, the mechanical properties have been greatly improved. AlN-SiC-TiB₂ composite ceramic has been fabricated by the optimum technology, whose relative density is 95%, flexural strength is high to 517.8MPa, fracture toughness is 5.79MPa?m1/2 and the Rockwell hardness is 94.1. Another studying in XRD and SEM show that: there is obvious phase separation is observed and this phenomena is becoming more and more intensified with the increase of heat treatment temperature and holding time. More importantly, the TEM results indicate that there is a lot of nano-scale TiB₂ crystal precipitated phase in the interface of AlN and TiB₂ particles.Accordingly, three different kinds of toughening and reinforcing mechanism of AlN-SiC-TiB₂ system have been discussed. The first one is that heat treatment technology has reduced the thermal stress in the combustion samples, the second one is after heat treatment the rich SiC from the phase separation of solid solution phase will have a function of particles dispersion strengthening. The third one is the uniform distribution of nano-scale TiB₂ which is precipitated between the AlN and TiB₂ particles phase interface, so a kind of composition gradient effect is formed between two phases to highlight the fracture toughness of samples.In addtion, the oxidation behavior under the constant temperature of AlN-TiB₂ and AlN-SiC-TiB₂ composite ceramics is studied. The oxidation experiment results show that: the relationship between oxidation weight gain and oxidation time satisfies the parabolic law. The oxidation rate and the scale thickness of AlN-SiC- TiB₂ system increase at a slow rate with the increase of oxidation temperature, and its oxidation resistance have advantage over the AlN-TiB₂ system. The low temperature oxidation products of both system are mainly rutile structure TiO2. In addition the formation of Al4B2O9,Al18B4O33 makes the AlN-TiB₂ ceramic has a good property of self-sealing and the high temperature oxide of AlN-TiB₂ is Al2TiO5. The high temperature oxidation products of AlN-SiC-TiB₂ system are Al6Si2O13 with long rods, which have specific properties such as high strength, low modulus and low thermal expansion coefficient. Therefore both composite ceramics have good anti-oxidation property.