| Great attentions have been paid to ceramics toughening due to the fact that the brittleness is the fatal shortcoming of structural ceramics. However, accurate and reliable assessment of fracture toughness is the foundation of research on improvement of fracture toughness for ceramic materials. So far no one method is able to test the facture toughness of structural ceramics precisely,reliably and quickly. It becomes a prime obstacle for selection of candidate material for engineering application, the study of fracture mechanism, and the development of structural ceramics with high toughness. In this dissertation, femtosecond laser micro-processing technique is innovatively applied in the assessment of fracture toughness for structural ceramics. An ultra-sharp V-notch with notch root radius smaller than 0.5 μm is cut by femtosecond lasers on representative ceramic sample. The effects of the width and depth of the V-notch, grain size, and the content of zirconia addition in ZTA on the fracture toughness are studied.In this dissertation, the femtosecond lasers with a wavelength, pluse width and repetition rate of 800 nm, 130 fs and 1 k Hz respectively, were used to cut V-notch with notch root radius smaller than 0.5 μm on the ceramic sample of silicon nitride, alumina, zirconia and zirconia toughen alumina(ZTA), when the laser power was controlled in the ranging of 50~100 m W and scan speed was kept at 100 μm/s. The V-notch can penetrate the test bar and the radius on both sides is equal. A thin melting layer with the thickness of 200-300 nm on V-notch surface was etched by femtosecond lasers. However, it had little effect on the testing result. This new SEVNB method can assess fracture toughness of structural ceramics precisely, reliably, quickly and reproducibly.In order to study the effect of notch radius on fracture toughness of silicon nitride, the silicon nitride samples with different notch radius were cut by femtosecond lasers, razor blades and diamond wheels, respctively. The testing results show that the fracture toughness of silicon nitride increase with the notch tip radius increasing. For testing samples with V-notch(ρ<0.5 μm), the KIc has no relationship with the U-groove width. And the KIc remains highly stable at 5.1 MPa?m1/2 with deviation smaller than 0.1 MPa?m1/2. The results of the finite element simulation and load-de?ection diagrams indicate that the tip of sharp V-notch offers a crack source for stable crack growth, which is the key to determine reliable and repeatable fracture toughness. The KIc value of silicon nitride measured by this method is its actual fracture toughness.The KIc of Y-TZP ceramics with V-notch(ρ<0.5 μm) is 4.42±0.12 MPa?m1/2 tested by SEVNB method. Whereas, the KIc of Y-TZP sample is 13.05±1.06 MPa?m1/2for ρ=100 μm tested by SENB method. The KIc of Y-TZP ceramics with U-groove width of 200 μm is about 3 times larger than that with V-notch(r<0.5 mm). Because the tip of sharp V-notch offers crack source, and the crack propagates stably before fracture. However, the root of U-groove does not offer a crack source. Crack formation, crack growth and fracture are happening at the same moment, when the flexure load increases big enough. In addition, the transformation zone R is minimal for 3Y-TZP sample with ultra sharp V-notch under the applied laod. The transformation of zirconia has little effect on fracture toughness in the zone. The fracture toughness measured by this method is its intrinsic value. The transformation zone R is large for 3Y-TZP sample with U-groove and result in increase the load for inducing crack.In order to eliminate the the effect of phase transformation, the alumina samples with average grain size of 0.8 μm and 3 μm are obtained after isostatic pressing and sintering. The effect of alumina grain size for mechanical behavior is studied. The experimental result shows that grain refining has little effect on fracture toughness of alumina, but it is helpful to improve bending strength.Finally, ZTA samples with zirconia content of 5 wt%, 10 wt%, 15 wt%, 20 wt% and 30 wt%, respectively were prepared to study the effect of zirconia content on the fracture toughness of ZTA compostes. The KIc values of ZTA samples increase at first and then decrease, as zirconia content increasing. The KIc of the ZTA sample reaches the maximum when the zirconia content is 15 wt%. Because in the ZTA sample with zirconia 15 wt% addition, the alumina grain size is relatively small, and the zirconia grains disperse homogeneously in matrix. Besides, the fracture toughness measured by SEVNB of the ZTA samples with 15 wt%,20 wt% and 30 wt% zirconia addition are larger than that of zirconia sample. Because the zirconia transformation zone R is minimal for the samples with ultra sharp V-notch under the applied laod, the transformation of zirconia has little effect on fracture toughness in the zone. However, the mutiphase microstucture of ZTA could improve fracture toughness.In this dissertation, an ultra-sharp V-notch on ceramic sample can be achived cut by femtosecond lasers. The fracture toughness of structural ceramics with the sharp V-notch is measured quickly, precisely and reliably. This modified SEVNB method would become a new standard for fracture toughness assessment of ceramic materials. It is of great significance for selection of candidate material for engineering application, the study of fracture mechanism, and the development of structural ceramics with high toughness. |
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