Prepartion Process, Reaction Mechanism And Properties Of Multiphase Ceramic Coatings Made By Solid State Reaction

The ultra-fine powders of Al-TiO₂-B₂O₃ system were prepared by ball milling.The multiphase ceramic coating of Al₂O₃-TiB₂ on the surface of Q235 and AZ31B bases were made by solid-phase reaction method.The reaction systems of coatings were designed and properties of the coating were predicted through the Empirical Electronic Theory(EET) in solid and molecules.Behaviors of mechanochemistry and thermal chemistry of Al-TiO₂-B₂O₃ system were investigated by scanning electron microscopy(SEM), differential thermal analysis(DTA),X-rays diffraction(XRD) and laser granularity distributing apparatus.The forming mechanisms of coatings were studied through kinetics and thermodynamics.The phase composition and the characteristic of interface between coating and base were analyzed.The properties of coatings,such as hardness,bonding strength,thermal shock,wear resistance,corrosion resistance,etc.,were test too.The calculation of EET showed that the multiphase ceramic coating of Al₂O₃-TiB₂ on the bases of Q235 and AZ31B are scientific and reasonable.A higher interface bonding can be obtained between the coating and the base,the strength and toughen of coating itself could be adjustable for the base.The calculation of thermodynamic showed that the reaction of Al-TiO₂-B₂O₃ system will happen in practice and the ultimate products are Al₂O₃ and TiB₂.The composite powders of Al-TiO₂-B₂O₃ were refined enough by ball milling and the granularity appeared in normal distribution.The medium particle diameter of powders with 4.35μm in diameter added by process control agent(PCA) could be obtained after 20h ball milling and the grain size was in nanometer degree.The shapes of particles were close to spherical and a few of new phases,such as Al₂O₃,TiB₂,etc., appeared.The system could be activated obviously by the mechanochemistry reaction. The solid reaction would happen slowly at 400℃in powders with 20h ball milling and severely at 670℃.The final products are mostly Al₂O₃ and TiB₂ after heat treatment at 700℃and completely Al₂O₃ and TiB₂ at 1000℃.The kinetics analysis showed that the activation energy E_a in Al-TiO₂-B₂O₃ system decreased with the increased ball milling time and the kinetics mechanism belonged to the reaction-diffusion model.The comprehensive properties of Al₂O₃-TiB₂ multiphase ceramic coatings on the surface of Q235 bases prepared with coated solid-state reaction method improved with the increasing of thermal curing temperature.The properties increased firstly with the ratio of aggregates and binders and then decreased.The comprehensive properties of coatings were best when the ratio of aggregates and binders is 1.5:1. The mode of interface bonding was mainly in chemical,metallurgical and diffusional bonding.The highest microhardness of coating was HV_0.11000 and the bonding strength of coatings on Q235 bases was 13.4MPa;the thermal shock times of them was 45 and the wear-resistance improved 3.21 times than the bases. The wear-resistance of coating improved 4.00 times than the bases after lubricated and 3.38 times after sealed.The corrosion resistance was best after thermal curing at 700℃.The average corrosion ratio was 1.06 g·m^-2·h^-1 in aqueous solution with 15wt.%H₂SO₄.The corrosion resistance improved 19.52 times than Q235 bases and 36.48 times after sealed.The Al₂O₃-TiB₂ multiphase ceramic coatings on the surface of AZ31B bases were prepared with coated solid-state reaction method at 400℃thermal curing temperature.The comprehensive properties increased firstly with the ratio of aggregates and binders and the added amount of Al₂O₃ in nanometer and then decreased.When the ratio of aggregates and binders is 1.35:1 and the added amount of Al₂O₃ in nanometer is 50%,the comprehensive properties of coatings were best.The mode of interface bonding was mainly in chemical,metallurgical and diffusional bonding.The bonding strength of coatings on AZ31B bases was 11.28MPa and the thermal shock times of them was 45.The highest microhardness of coating was HV_0.1750.The abrasive wear-resistance and the adhesive wear-resistance improved 6.27,6.41 times than the bases,respectively.The corrosion resistance was superior to the bases.The average corrosion ratio was 0.2254 g·m^-2·h^-1 in aqueous solution with 3.5wt.%acetic acid.The corrosion resistance improved 40.12 times than AZ31B bases.The average corrosion ratio was 0.1488 g·m^-2·h^-1 and the corrosion resistance improved 60.79 times after sealed.The comprehensive properties of Al₂O₃-TiB₂ multiphase ceramic coatings on the surface of AZ31B bases prepared with thermal spraying were superior to that prepared with coated solid-state reaction method.The mainly mode of interface bonding included mechanical,chemical and metallurgical bonding.The thermal shock times was 49.The highest microhardness is HV_0.11200 and it was HV_0.11400 after remelted;the dry adhesive wear-resistance and the oil adhesive wear-resistance improved 10.07,11.48 times than the AZ31B bases,respectively; the abrasive wear-resistance improved 9.3 times than the AZ31B bases,which were superior to the common ceramic coating made by thermal spraying method. The remelted treatment could further improve the wear-resistance of coatings.The average corrosion ratio was 26.10909 g·m^-2·h^-1 in aqueous solution with 5wt.% acetic acid.The corrosion resistance improved 42.25 times than AZ31B bases.The average corrosion ratio was 15.96028 g·m^-2·h^-1 and the corrosion resistance improved 67.48 times after sealed.A better oxidation resistance appeared in the coatings too.