Evaluation Of Fracture Toughness Of Ceramic Matrix Composites By SEVNB Method Based On Femtosecond Laser

SEVNB method based on femtosecond laser has been proved to be able to measure the fracture toughness of ceramic materials accurately and rapidly.However,for ceramic matrix composites that have complex,diverse and heterogeneous structure,this method has hardly been studied and applied.At present,SENB method is still the most commonly used test method for ceramic matrix composites.But the test value of this method may be larger than the actual value due to inactivation of notch root.In order to study whether SEVNB method based on the femtosecond laser is superior to SENB method in the field of ceramic matrix composites,three types of ceramic matrix composites composed in meso scale,micro scale and macro scale respectively have been researched in this dissertation.These ceramic matrix composites are whisker toughened ceramic matrix composites,three-dimensional continuous fiber toughened ceramic matrix composites,and ductile interlayer laminated composites.Evaluation of three-dimensional continuous fiber toughened ceramic matrix composites:In this experiment,the composites for research were C_f/Si C prepared by PIP method,in which the preform was prepared by 3K plain laminated suture,including C_f/Si C with interphase(Y-C_f/Si C)and C_f/Si C without interphase(N-C_f/Si C).To study the effect of notch tip radius,femtosecond laser and different thicknesses of the diamond wheel were used to process Y-C_f/Si C and N-C_f/Si C samples with different notch tip radii.The results show that the notch tip radius has little effect on the test values of fracture toughness for these two kinds of C_f/Si C,and SENB method is sufficient to measure the fracture toughness of this type of composites accurately.In addition,the test values are discrete caused by intrinsic defects and fiber distribution of samples.Further,the influence of the V-notch depth,the total notch depth and the loading rate of the testing machine on the test values of fracture toughness for these two kinds of C_f/Si C was researched.The results show that,V-notch depth has little effect on the test values of fracture toughness for these two kinds of C_f/Si C;the test values of fracture toughness of C_f/Si C generally increases at first and then decreases with the increase of a/W,and the test values of fracture toughness for Y-C_f/Si C composites can be accurately measured when the a/W range is 0.28?0.66;the significant increase of loading rate from 0.05 mm/min to 5 mm/min has little effect on the test values of fracture toughness test values for C_f/Si C composites.Evaluation of whisker toughening ceramic matrix composites:In this experiment,Si C_w/Al₂O₃ samples with different whisker contents were prepared by hot pressing.In order to research the effect of notch tip radius,Si C_w/Al₂O₃ with whisker content of 20 vol%was used as the research object.Femtosecond laser,razor blade and different thicknesses of the diamond wheel were used to process Si C_w/Al₂O₃ samples with different notch tip radii.It was found that the test values of fracture toughness increased with the increase of the notch tip radius.The results of research on laser processing rate show that the V-notch processed by femtosecond laser with the processing power of 100 m W and the processing rate of 100?m/s is sharp enough,and the test values of fracture toughness are accurate and reliable.In order to study the effect of whisker content,the fracture toughness of Si C_w/Al₂O₃ with different whisker content was measured.It was found that with the increase of Si C_w content,the test values of SEVNB method based on femtosecond laser had little change when the whisker content increased from 5 vol%to 40 vol%.This is probably because the whisker agglomeration was intensified with the increase of whisker content,and the toughening effects were almost equal to the performance degradation effect.But the test values of SENB method showed a trend of increasing after decreases first,and reach the peak value(6.08 MPa·m^1/2)when the whisker content is 20 vol%.This is probably because there are some"weak micro zone"in the sample,which is easy to initiate a crack earlier under an external force,and it is different to samples with different whisker contents,as a result,the samples are affected differently by the inactivation of notch root.Evaluation of ductile laminated ceramic composites:In this experiment,Al₂O₃/steel wire laminated ceramic composites and Al₂O₃/Al foil laminated ceramic composites were prepared by mould pressing method.In order to study the effect of notch tip position on fracture toughness of Al₂O₃/steel wire laminated ceramic composites,samples with different notch tip position were prepared.The test results of fracture toughness show that the test values of fracture toughness are different at different notch tip position.In detail,the test values of fracture toughness of this composite at the fourth steel interlayer is 7.01 MPa·m^1/2,which is higher than 5.51 MPa·m^1/2 at the third interlayer,and slightly lower than the 6.88 MPa·m^1/2between the third and fourth interlayers.In order to study the effect of the notch tip radius on fracture toughness of the above two kinds of composites,samples with different notch tip radii were prepared.The experimental results show that the notch tip radius has little effect on the test values of fracture toughness of Al₂O₃/steel mesh laminated ceramic composites with strong interlaminar bonding,but has great effect on the test values of fracture toughness of Al₂O₃/Al foil laminated ceramic composites with weak interlaminar bonding.In this dissertation,SEVNB method based on femtosecond laser was used to research the fracture toughness of three types of ceramic matrix composites.The results show that this method has no obvious advantages over SENB method for three-dimensional continuous fiber toughened ceramic matrix composites and ductile laminated ceramic composites with strong interlaminar bonding,the fracture toughness of these materials can be measured accurately by SENB method;but for whisker toughened ceramic matrix composites and ductile laminated ceramic matrix composites with weak interlaminar bonding SEVNB method has higher accuracy than SENB method.