TABLE II. Calculated residual stresses in Si3 Na based laminates Thickness of layers (um) Composition Si3N4 Si3N4 with Tin (MPa) (MPa) 4. Results and discussion Si3 Na/Sin 188246.5 4.1. Mechanical proper t o wt oTiN Mechanical properties such as the strength, Young Si3N4/2(Si3N4-245 279.5 151 modulus, and fracture toughness of the laminates are presented in Table Ill. The parameters of the tested 765515.5 50 wt oTiN laminates, such as composition and layer thickness are Si3 NaTiN 2467 1078 given in Table Il. Besides these four designs, one more design of Si3 N4/Si3 N4 laminate was used as a base for comparison. The laminates of this design were pre pared in the same way as the others, however, all layers were of the same composition. Therefore, no residual done. For rolling, a crude rubber(4 wt %)was added stresses can appear during cooling. It is worth noting to the mixture of powders as a plasticizer through a that both the Young's modulus and fracture toughness 39 solution in petrol. The powders were then dried up of these Sis N/Sis N4 laminates were measured to be on o a 2 wt% residual amount of petrol in the mixture. the same level as standard Si N4 ceramics prepared by After sieving powders with a 500 um sieve, granulated e standard powder route, which includes no rolling powders were dried up to the 0.5 wt. residual petrol. The strength of the Si3 N4/Si3 N4 laminate was less than A roll mill with 40 mm rolls was used for rolling. The that of the standard Si3 N4 ceramics with values of 507.6 velocity of rolling was 1.5 m/min. Working pressure +3.2 and 750+ 20.7 MPa, respectively. As one can varied from 10 MPa for a 64% relative density of tapes see from Table Ill, while the strength of Si3 N4/Si3N to 100 MPa for a 74% relative density. The thickness of 20wt. TiN laminates are approximately on the same tapes was either 0. 40.5 mm or 0.8-1.0 mm, the width level as the Sis N/Si N4 laminates, further increase of was 60-65 mm. Samples of ceramics were prepared by the TiN content to 50 and 100% results in a significant the hot pressing of tapes stacked together. Each layer decrease of both strength and Young's modulus. The contained one or a few tapes. Graphite dies were used measured fracture toughness of the Si3N4/TiN lami for hot pressing, and the hot pressing was performed nates also showed a decrease similar to strength and at a temperature of 1820C, with a dwelling time of Young's modulus values 20 min and a pressure of 30 MPa [20]. During hot The Si3 N4/Si3N4-20 wt TiN laminates showed an pressing,the shrinkage of layers occurred 3 times such increase in apparent fracture toughness. This increase that after rolling, the thickness of the individual tape can be explained by the introduction of the residual bulk was 450 um, while after hot pressing it decreased to 150 um. After hot pressing, the thickness of the Si3N compressive stresses in Si3N4 layers In the case where the thicknesses of the Si3 N4 and the si3 N4-20 wt TIN layers was in the range of 150-300 um, and the thick ness of the Si3N4 layers with TiN additive varied fr layers were similar, the calculated residual compressive stress was about 1 88 MPa and the residual tensile stress 200to500m. about 246.5 MPa The measured value of the apparent Fracture toughness was also measured by Single- fracture toughness was 7.41+ 1.79 MPa m/2.There Edge-V-Notched-Beam (SEVNB) method [21]. 4 was a further increase in KIe(8.5+0.01 MPa m/)for point bending strength of the machined specimens was the laminates with 20 wt%tin when the relative thick determined using a jig with an inner span of 20 mm and ness of the SiN4- 20 wt %TiN layers was increased an outer span of 40 mm. The notch tip was located in a second Si3 N, layer in the case of layered composites. reason for this is a significant increase of the residual Strength and Youngs modulus were also calculated at room temperature by measuring the deflection of the residual stress in the Si3N4-20 wt TiN layers(Ta- samples during 4-point bending tests according to the ble ID). However, an increase of TiN content to 50 wt% standard. The bending strength calculation was based resulted in a significant increase of the residual tensile on a monolithic sample analysis. Optical and scanning stress in the laminates. The calculated tensile stress val electron microscopy was used for a microstructure in- vestigation equipped with a Leica microscope, an XYZ mapping different layer compositon proper A Renishaw 1000 Raman microspectrometer was TABLE III. Mechanical properties of SigNa based laminates with stage and 514.5 nm argon ion laser. The laser generated Composition ar(Mpa) E(GPa) KIc(MPa/2) 12.5 mW of power. a plasma filter was used to remove plasma lines from the spectra taken. The laser spot wa SiN4/Si3N4507.6±32 306.6 5.54±0.01 about 1-2 um for the 100 x objective lens used during S seIN the measurements. Autofocusing was used to collect SisN/(SigNa- 450.4+82.9 8.5±0.01 the Raman spectra because it maintains a good focus 20 wt %TIN) on the sample during line mapping experiments. The SiaNa/Si3N4- 157.9+ 14.9 297.7 system was set up to take spectra from all points along 50 wt%TiN a single line of interest on the surface. Before the si3N4 SinA/TiN 140.8±1091574 3.97±0.52 5445T A B L E I I . Calculated residual stresses in Si3N4 based laminates Thickness of layers (µm) Composition Si3N4 Si3N4 with TiN σcom. (MPa) σtens. (MPa) Si3N4/Si3N4— 20 wt.%TiN 250 210 188 246.5 Si3N4/2(Si3N4— 20 wt.%TiN) 245 530 279.5 151 Si3N4/Si3N4— 50 wt.%TiN 200 330 765 515.5 Si3N4/TiN 200 400 2467 1078 done. For rolling, a crude rubber (4 wt.%) was added to the mixture of powders as a plasticizer through a 3% solution in petrol. The powders were then dried up to a 2 wt.% residual amount of petrol in the mixture. After sieving powders with a 500 µm sieve, granulated powders were dried up to the 0.5 wt.% residual petrol. A roll mill with 40 mm rolls was used for rolling. The velocity of rolling was 1.5 m/min. Working pressure varied from 10 MPa for a 64% relative density of tapes to 100 MPa for a 74% relative density. The thickness of tapes was either 0.4–0.5 mm or 0.8–1.0 mm, the width was 60–65 mm. Samples of ceramics were prepared by the hot pressing of tapes stacked together. Each layer contained one or a few tapes. Graphite dies were used for hot pressing, and the hot pressing was performed at a temperature of 1820 ◦C, with a dwelling time of 20 min and a pressure of 30 MPa [20]. During hot pressing, the shrinkage of layers occurred 3 times such that after rolling, the thickness of the individual tape was 450 µm, while after hot pressing it decreased to 150 µm. After hot pressing, the thickness of the Si3N4 layers was in the range of 150–300 µm, and the thick￾ness of the Si3N4 layers with TiN additive varied from 200 to 500 µm. Fracture toughness was also measured by Single￾Edge-V-Notched-Beam (SEVNB) method [21]. 4- point bending strength of the machined specimens was determined using a jig with an inner span of 20 mm and an outer span of 40 mm. The notch tip was located in a second Si3N4 layer in the case of layered composites. Strength and Young’s modulus were also calculated at room temperature by measuring the deflection of samples during 4-point bending tests according to the standard. The bending strength calculation was based on a monolithic sample analysis. Optical and scanning electron microscopy was used for a microstructure in￾vestigation. A Renishaw 1000 Raman microspectrometer was equipped with a Leica microscope, an XYZ mapping stage and 514.5 nm argon ion laser. The laser generated 12.5 mW of power. A plasma filter was used to remove plasma lines from the spectra taken. The laser spot was about 1–2 µm for the 100 × objective lens used during the measurements. Autofocusing was used to collect the Raman spectra because it maintains a good focus on the sample during line mapping experiments. The system was set up to take spectra from all points along a single line of interest on the surface. Before the Si3N4 measurements, the spectrometer was calibrated using a standard Si wafer band with position at 520.3 cm−1. 4. Results and discussion 4.1. Mechanical properties Mechanical properties such as the strength, Young’s modulus, and fracture toughness of the laminates are presented in Table III. The parameters of the tested laminates, such as composition and layer thickness are given in Table II. Besides these four designs, one more design of Si3N4/Si3N4 laminate was used as a base for comparison. The laminates of this design were pre￾pared in the same way as the others, however, all layers were of the same composition. Therefore, no residual stresses can appear during cooling. It is worth noting that both the Young’s modulus and fracture toughness of these Si3N4/Si3N4 laminates were measured to be on the same level as standard Si3N4 ceramics prepared by the standard powder route, which includes no rolling. The strength of the Si3N4/Si3N4 laminate was less than that of the standard Si3N4 ceramics with values of 507.6 ± 3.2 and 750 ± 20.7 MPa, respectively. As one can see from Table III, while the strength of Si3N4/Si3N4- 20wt.% TiN laminates are approximately on the same level as the Si3N4/Si3N4 laminates, further increase of the TiN content to 50 and 100% results in a significant decrease of both strength and Young’s modulus. The measured fracture toughness of the Si3N4/TiN lami￾nates also showed a decrease similar to strength and Young’s modulus values. The Si3N4/Si3N4-20 wt.% TiN laminates showed an increase in apparent fracture toughness. This increase can be explained by the introduction of the residual bulk compressive stresses in Si3N4 layers. In the case where the thicknesses of the Si3N4 and the Si3N4-20 wt.% TiN layers were similar, the calculated residual compressive stress was about 188 MPa and the residual tensile stress about 246.5 MPa. The measured value of the apparent fracture toughness was 7.41 ± 1.79 MPa m1/2. There was a further increase in K1c (8.5 ± 0.01 MPa m1/2) for the laminates with 20 wt.%TiN when the relative thick￾ness of the Si3N4-20 wt.%TiN layers was increased compared to the thickness of pure Si3N4 layers. The reason for this is a significant increase of the residual compressive stress, and at the same time, a decrease of the residual stress in the Si3N4-20 wt.% TiN layers (Ta￾ble II). However, an increase of TiN content to 50 wt.% resulted in a significant increase of the residual tensile stress in the laminates. The calculated tensile stress val￾T A B L E I I I . Mechanical properties of Si3N4 based laminates with different layer compositions Composition σf (Mpa) E (GPa) KIC (MPa. m1/2) Si3N4/Si3N4 507.6 ± 3.2 306.6 5.54 ± 0.01 Si3N4/Si3N4- 20 wt.%TiN 356.2 ± 76.4 312.9 7.41 ± 1.79 Si3N4/2(Si3N4- 20 wt.%TiN) 450.4 ± 82.9 – 8.5 ± 0.01 Si3N4/Si3N4- 50 wt.%TiN 157.9 ± 14.9 297.7 – Si3N4/TiN 140.8 ± 10.9 157.4 3.97 ± 0.52 5445