Fabrication And Characterization Of Micro-nano Al₂O₃/LaPO₄ Composites With Coating Structure

In order to improve the machinability of ceramic materials, one method is to introduce another weak interphase or layered structure materials into matrix to facilitate crack deflection during machining. Lanthanum phosphate (LaPO4) is a suitable and effective oxide interphase. Incorporating LaPO4 into Al2O3 matrix, the LaPO4/Al2O3 interface is weak enough to make the composite easily machined. The composites can be machined using conventional metal-working methods and have been widely used owning to their chemical stability,high melting point,excellent oxidation resistance and weak interfaces bonding.The traditional way to make the LaPO4/Al2O3 composite is to mix the two powders directly. In this case large amount of LaPO4 is needed, leading to serious degradation of the composite strength. In order to avoid such disadvantage, powder coating method is introduced to minimize this degradation. This method could make weak interfacial bond at the LaPO4/Al2O3 interface. The requested LaPO4 amount is therefore decreased to some extent. Among powder surface coating methods, the heterogeneous nucleation process has been widely applied recently due to its uniformity of the coating thickness by controlling the concentration of coating materials between the critical values of homogeneous and heterogeneous nucleation. The preparation of LaPO4-coatedα-Al2O3 powders through heterogeneous precipitation method is not reported up to now in the literature.The LaPO4-coatedα-Al2O3 powders are synthesized by incorporating LaPO4 ontoα-Al2O3 particle surface using the heterogeneous precipitation method and hydro-thermal method. The coated powders are characterized through XRD,TEM,SEM,XPS and Zeta potential measurements.The LaPO4 powders are synthesized via chemical precipitation method and hydro-thermal method. The LaPO4-coatedα-Al2O3 powders are successfully synthesized through heterogeneous precipitation method and hydro-thermal method. We arranged a series of parallel experiments through changing the pH value,concentration and temperature. The coated powders are characterized using XRD,TEM,SEM and Zeta potential measurements. According to the XRD result, the coated powder consists of onlyα-Al2O3 and monoclinic LaPO4. The TEM examinations show that under alkaline condition a homogeneous coating with thickness of ~10 nm is obtained on theα-Al2O3 powder surface. The Zeta potential measurement shows that the coated powders have a pH value of 7.85 at IEP, similar to that of LaPO4 (pH value of 8.21). This is in good agreement with the TEM results, indicating a successful coating process in our experiment. The best coating condition is 20℃,alkaline suspension,0.01M。According to the best coating process, The LaPO4/Al2O3 ceramics with 20% LaPO4 are synthesized.With the same content of LaPO4, the LaPO4/Al2O3 ceramics which made by coating method have excellent properties.The relative density,fracture toughness,bending strength of the LaPO4/Al2O3 ceramics are higher than those synthesized in traditional way,as well as the mechinablity.The structure of the material decides its property,the right structure design of the LaPO4/Al2O3 ceramics leads to the good properties.