Silicon Nitride Ceramics With High Hardness And Toughness Based On Microstructure Tailoring

Silicon nitride(Si3N4)has a distinctive combination of material properties such as high hardness,flexture strength and fracture toughness,inherent phase stability,good thermomechanical and tribological properties.Which make it an attractive choice for wear-resistant technological and engineering applications,such as bearings,and metal cutting and shaping tools.While,prepare the Si3N4 ceramics with high hardness and toughness is the key bottleneck of high reliability batch manufacturing of Si3N4 ceramics.The objective of this thesis is to design and prepare Si3N4 ceramics with high hardness and toughness based on the microstructure taloring.Research results are as followsGraded Si3N4 ceramics with sandwich-like microstructure were fabricated by two-step hot pressing(HP+HP)with the same homogenous starting composition(Si3N4+Al2O3+Y2O3).After HP+HP,the resulting ceramics exhibited a 2-zoned microstructure,differentiated by the phase assemblage of Si3N4 and grain size.The outer layers exhibited bimodal and fine-grained microstructure were well bonded to the inner layers with bimodal and coarse-grained microstructure.The hardness values of outer layers were higher than the inner layers,the toughness and strength values of the outer layers were lower than the inner layers.When the second-step HP temperature was 1600?,the microcantilever bending strength of the outer and inner layers were same(5.72 GPa),and with extremely low interfacial stress.These results showed that graded Si3N4 ceramics with hard surface(18.1 GPa)and tough core(5.5 MPa-m1/2)were achieved by two-step hot pressing procedureIn order to further improve the hardness ofthe outer layer and the toughness of the inner layer,the second-step HP was changed to spark plasma sintering(SPS)and?-SbNa seeds were introduced to the inner layer.Thus,the graded Si3Na ceramics with hard surface and tough core were prepared by HP+SPS process.After HP+SPS,main ?-Si3Na phase were detected in the outer layers,and only ?-Si3Na phase were observed in the inner layers.The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution.The hardness and wear resistance of outer layers were higher than the inner layers,the toughness of the outer layers were lower than the inner layers.When the SPS dwelling time was 3.5 min the microcantilever bending strength of the outer was higher than the inner layer were about 7.08 GPa and 4.31 GPa,respectively,and with low interfacial residual stress.These results showed the graded Si3N4 ceramics with hard surface(21.2 GPa),tough core(5.9 MPa·m1/2)and lower interfacial residual stress were achieved by the combination of HP and SPS process.Considering the higher cost and complex process of the graded Si3N4 ceramics by two-step sintering,self-synthesized ZrB2 powders was introduced to investigate the effects of the content(<10 vol.%)and impurities of ZrB2 on the phase evolution and microstructure tailoring of low temperature hot press Si3N4 ceramics.Results showed that:compared with homogeneous S13N4 ceramics,the addition of 2.5 vol.%ZrB2 could promote the phase transformation of a-to ?-Si3N4,the formation of bimodal microstructure,and increase the toughness and strength.On the other hand,the addition of ZrO2 inhabited the transformation trend of ? to ?-Si3N4 phase.B2O3 have no influence on the transformation of to ?-Si3N4 phase.The addition of boron could promote the phase transformation,but with lower density and properties.While,with the introduction of 2.5 vol.%ZrB2-B binary additives,toughness and strength of Si3N4 ceramics increased significantly from 5.2 MPa·m1/2 and 384 MPa to 7.2 MPa-m1/2 and 675 MPa,respectively,even with a sacrifice of hardness to 21.3 GPa.Due to ZrB2 is calssified to the IVB transition metal diborides,it is necessary to investigate the effect of other transition metal diborides(i.e.,MB2:TiB2,ZrB2,HfB2,TaB2,NbB2,CrB2,and MoB2)with 2.5 vol.%on the microstructure and mechanical properties of Si3N4 ceramics prepared by hot press at low temperaature.Results showed that:compared with the homogeneous Si3N4 sample,the addition of one of theses transition metal diborides could promote the transformation of ?-to ?-Si3N4 phase and formation of bimodal microstructure due to the formation of Si-Mg-O-N-Yb-M-B glass phase.The Si3N4 ceramic sample with HfB2 exhibited the lowest content of ?-Si3N4 phase,,the best ?-Si3N4 grain morphology and thus mechanical properties(hardness:20.3 GPa,toughness:8.2 MPa·m12,and flexural strength:780 MPa),while MoB2 exhibited the lowest transformation of ?-to/3-Si3N4 phase,the lowest density and mechanical properties.The differences in the ?-to ?-Si3N4 phase transformation of these seven transition metal borides were ascribed to the differences in melting point,ionic iadius,and the adsorption of O,and thus resulting in the difference in viscosity of the liquid phaseThe graded Si3N4 ceramics fabricated in-house by HP-HP and HP-SPS,and high-performance of Si3N4 ceramics with 2.5 vol.%ZrB2 were prepared as cutting tool inserts.The cutting performance of these Si3N4 ceramics was then evaluated against a commercial Si3N4 ceramic tool insert.Results showed that:the cutting life of in-house Si3S4 ceramics tools shows excellent wear resistance than commercial Si3N4 ceramic tool.The graded in-house Si3N4 cutting tool prepared by HP+SPS had the longest cutting life(575 m),and commercial cutting tool had the shortest cutting life(189 m)...