| In order to improve the performance of machinable ceramics, we treated LaPO4/Al2O3 composite ceramic with structure design, made LaPO4 evenly distributed in the ceramic matrix to cut down its reunion. This could reduce the amount LaPO4, greatly increase the poor interface between LaPO4/Al2O3. So give it a good processing performance, while maintaining good mechanical properties.First, we used the non-homogeneous nucleation synthesized coated LaPO4/Al2O3 powder. To detect the phase composition of powder by XRD, there wereα-Al2O3 and amorphous LaPO4 existed in powder without calcined. The calcined powder is obvious monoclinic LaPO4 andα-Al2O3; this fully proved that the amorphous LaPO4 existed before burning. This would only produce a very shallow crack layerIn the case of certain content LaPO4, the coated LaPO4/Al2O3 composite ceramic in relative density, fracture toughness have been improved than those using traditional methods, while the hardness and bending strength decreased, but the processability index has also been greatly enhanced. We can clearly see the LaPO4 is layered structure by fracture surface analysis materials. When the rupture occurred, the grains were lamellar or step-like detachment. The fracture surface showed that the alumina grains were pulled out. The traces showed that the fracture form is intergranular fracture. In the LaPO4/Al2O3 composite ceramic materials processing, this layered structure LaPO4 cleavage occurred along the lamellar direction. Rather than the deep layer left by transgranular fracture cracks. The coated LaPO4/Al2O3 ceramic composite materials improved the processing, while leaving less processing damage.We have studied crack propagation behavior of coated LaPO4/Al2O3 composite ceramic. The slope of composite ceramic was-0.26 between strength and indentation load, indicating it have crack resistance curve effect. Kr increase from 4.153MPa·m1/2 to 4.281 MPa·m1/2, and slight decreased in the latter part. When the material cracks propagated, the two planes would have a relative displacement, Cracks compressed between the uneven grains, they produced a self-locking, and enhanced resistance to crack propagation. When the crack extended to the alumina grains, the extension path would be obvious to change, resulting in bridging. The two crack surfaces would be restrained by the alumina grains, thereby increased the material crack propagation resistance, improved material properties. As the interface effect or the impact of internal stress generated by heat mismatch, in particular the interactions of heterogeneity of internal stress and interfacial crack, when in the main crack tip generated microcracks, microcracks perpendicular with the principal stress axes, and then microcracks may be connected between one another, crack deflection occurred. This will increase the crack propagation path; consumed a large amount of crack propagation.We established the mechanical model of indentation tests; according to the mechanical model we could get the stress field at any point. Meanwhile we can analysis the formation of indentation cracks of ceramic materials. According to the formula we got the critical pressure of LaPO4/Al2O3 composite ceramic materials prepared by the conventional method was 1.3 N, the critical pressure of ones prepared by the coating method was 3.2 N; the initial crack length of LaPO4/Al2O3 composite ceramic material prepared by the coating method was 3.46μm, the initial crack length of ones prepared by the conventional method was 2.14μm.We analyzed toughening mechanism of ZTA ceramics and LaPO4/Al2O3 composite ceramic materials. ZTA ceramics was phase transformation toughening. The toughening mechanism of LaPO4/Al2O3 composite ceramic materials:under the pressure, materials would produce a large number of microcracks at the interface, and absorbed a large amount of elastic strain energy. Meanwhile during the crack expansion process, under the tensile stress, extension to form larger microcracks would confluence with the main crack, leaded the main crack propagation path was distorted and bifurcation, and made the main crack propagation path was more tortuous. This could absorb more elastic strain energy, thus greatly improved the fracture toughness. |
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