S Li et al. Materials Letters 57(2003)1670-1674 1671 2. Experimental methods 3. Results and discussion 2. 1. Raw materials and fabrication 3.1. Determination of the thermal shock resistance Si3N4(Founder High-Tech ceramic, China) pow- ders with 8 wt. Y2O3 (purity >99.9%), 4 wt% A thermal shock weakens the fracture strength of Al2O3(99.9%)were ball milled with 20 wt. Sic the material significantly, because of the cracks whisker (TWS-400, Tokai Carbon Japan)in ethanol formed by the thermal stress [10]. The presence of for 24 h to achieve a homogenous mixture. The the thermal stress is the primary reason for decrease in mixture was mixed with organic binders and then the strength. The tensile stress on the surface by the produced green filaments using an extrusion pr thermal shock is given b ess. The green filaments were subsequently coated (1) dewaxing, the green body was hot pressed in a where o is the tensile stress on the surface by the graphite resistance furnace under N2 at 1820Cfor thermal shock, a is the thermal expansion coefficient, 1.5 h and under pressure 22 MPa. a detailed E is the elastic modulus, p is the poisson ratio andAT description of the fabrication process can be found equals To-T which is the temperature difference in Ref. 9]. The SiC whisker reinforced Si3N4 In general. the gth of the ceramics remains ceramic was fabricated by hot pressing in the same constant until the temperature difference reaches a critical value(ATs). Therefore, AT is often used to characterize the thermal shock behavior of ceramics 2.2. Thermal shock test The fracture initiation and crack propagation resist ance are two design principles used to express thermal Thermal shock experiments were performed by shock resistance For fracture initiation resistance. the measuring the retained bending strength after capacity is described by [ll], quenching specimens from successively higher tem 4 3 X 36 mm'rectangular bars, then polished with R=oB(I-D) peratures in water. The test specimens were cut into (2) diamond pastes down to 3.5 um. After that, the where aB is the room-temperature bending strength and maintained at that temperature for 10 min to under tensile stress eliminate any temperature gradient within them. The Higher R represents greater resistance to the ini- samples dropped parallel to their long axes into the tiation of fracture during rapid quenching and during water. and the time is 0.3 s from the furnace to the a steep temperatur quench bath. For each condition, seven specimens gradient. According to this equation, to obtain an were tested. The retained strength of the thermally improved thermal shock resistance, it is necessary to shocked composites was measured at room temper have higher strength but lower Poisson ratio, thermal ature and a crosshead speed of 0.5 mm/min by three- expansion coefficient, and elastic modulus point bending using an Instron universal testing After cracks are initiated. the resistance of crack machine. The work of fracture (woF) of the propagation is very important. Such a parameter, Si3N4/Bn fibrous monolithic ceramic and the si defined by Hasselman [12], is shown below four-point bending test with lower and upper spans R=-WoE whisker reinforced Si3 N4 ceramic was measured in a G(1-) of 30 and 10 mm, respectively. The bending tests were performed at a crosshead speed of 0.05 mm/ where wo is the work of fracture. The resistance to min using specimens of nominal dimensions 4 x3x crack propagation therefore increases as the fracture 36 mm energy, elastic modulus, and Poisson ratio increase2. Experimental methods 2.1. Raw materials and fabrication Si3N4 (Founder High-Tech ceramic, China) pow￾ders with 8 wt.% Y2O3 (purity >99.9%), 4 wt.% Al2O3 (>99.9%) were ball milled with 20 wt.% SiC whisker (TWS-400, Tokai Carbon Japan) in ethanol for 24 h to achieve a homogenous mixture. The mixture was mixed with organic binders and then produced green filaments using an extrusion proc￾ess. The green filaments were subsequently coated with slurry of 25 wt.% BN and 75 wt.% Al2O3, dried and parallel packed into a graphite die. After dewaxing, the green body was hot pressed in a graphite resistance furnace under N2 at 1820 jC for 1.5 h and under pressure 22 MPa. A detailed description of the fabrication process can be found in Ref. [9]. The SiC whisker reinforced Si3N4 ceramic was fabricated by hot pressing in the same condition. 2.2. Thermal shock test Thermal shock experiments were performed by measuring the retained bending strength after quenching specimens from successively higher tem￾peratures in water. The test specimens were cut into 4
3
36 mm3 rectangular bars, then polished with diamond pastes down to 3.5 Am. After that, the specimens were heated up to the testing temperature and maintained at that temperature for 10 min to eliminate any temperature gradient within them. The samples dropped parallel to their long axes into the water, and the time is 0.3 s from the furnace to the quench bath. For each condition, seven specimens were tested. The retained strength of the thermally shocked composites was measured at room temper￾ature and a crosshead speed of 0.5 mm/min by three￾point bending using an Instron universal testing machine. The work of fracture (WOF) of the Si3N4/BN fibrous monolithic ceramic and the SiC whisker reinforced Si3N4 ceramic was measured in a four-point bending test with lower and upper spans of 30 and 10 mm, respectively. The bending tests were performed at a crosshead speed of 0.05 mm/ min using specimens of nominal dimensions 4
3
36 mm3 . 3. Results and discussion 3.1. Determination of the thermal shock resistance parameters A thermal shock weakens the fracture strength of the material significantly, because of the cracks formed by the thermal stress [10]. The presence of the thermal stress is the primary reason for decrease in the strength. The tensile stress on the surface by the thermal shock is given by rf ¼ aE 1 t ðT0 TÞ ð1Þ where rf is the tensile stress on the surface by the thermal shock, a is the thermal expansion coefficient, E is the elastic modulus, t is the Poisson ratio, and DT equals T0 T which is the temperature difference. In general, the strength of the ceramics remains constant until the temperature difference reaches a critical value (DTc). Therefore, DTc is often used to characterize the thermal shock behavior of ceramics. The fracture initiation and crack propagation resist￾ance are two design principles used to express thermal shock resistance. For fracture initiation resistance, the capacity is described by [11], RI ¼ rBð1 tÞ aE ð2Þ where rB is the room-temperature bending strength under tensile stress. Higher RI represents greater resistance to the ini￾tiation of fracture during rapid quenching and during steady-state heat flow down a steep temperature gradient. According to this equation, to obtain an improved thermal shock resistance, it is necessary to have higher strength but lower Poisson ratio, thermal expansion coefficient, and elastic modulus. After cracks are initiated, the resistance of crack propagation is very important. Such a parameter, as defined by Hasselman [12], is shown below: RII ¼ w0E r2 Bð1 tÞ ð3Þ where w0 is the work of fracture. The resistance to crack propagation therefore increases as the fracture energy, elastic modulus, and Poisson ratio increase S. Li et al. / Materials Letters 57 (2003) 1670–1674 1671