Influence of Powder Characteristics on Hot-pressed Si 3 N 4 Ceramics

In this paper, the influence of the starting powder characteristics for five commercial Si3N4 powders on sintering behavior, microstructure and mechanical properties of hotpressed Si3N4 ceramics with and without the addition of La2O3-Yb2O3-MgO sintering additives was investigated. For the system without sintering additives, the high amount of the impurities in the starting powders could facilitate the densification process and promote β-Si3N4 grain growth. However, large whisker-like particles (α phase) present in the initial powders inhibited the sintering process, which led to a lower density of Si3N4 ceramics. On the other hand, when the sintering additives were introduced in the beginning stage of the powder processing step, the effects of impurities content and particle morphology in the initial powders on the densification and microstructure were not apparent. There was tendency that finer powder size resulted in finer microstructure. However, a high α-phase ratio in the initial powders could accelerate the abnormal grain growth and thus lead to better mechanical properties.


Introduction
Silicon nitride (Si 3 N 4 ) ceramic has been one of the most promising materials for applications as high-temperature structural components (e.g., engine hot-section parts components, cutting tools, and bearing balls etc.), which is due to its good mechanical properties at high temperatures, superior thermal shock resistance, wear and corrosion resistance.It is known that the powder characteristics of initial powders employed may have a significant influence on the densification behavior and microstructure of Si 3 N 4 ceramics.Mitomo etc. reported that the metallic impurities, morphology, and size distribution of the Si 3 N 4 particles had significant impact on the densification behavior of four kinds of powder using gas pressure sintering [1].Wötting etc also showed that the densification degree of Si 3 N 4 strongly depend on the particle size, oxygen and carbon content in pressure-less sintering [2].Homma etc. pointed out that the density and strength of HIP-ed Si 3 N 4 were strongly affected by the specific surface area, α/β ratio, and green density [3].On the other hand, Tanaka reported that a few hundred PPM impurities (Fe, Ca, Al) were sufficient to achieve full densification by HIP without sintering additives [4].Therefore, many studies investigated the effects of the sintering additives on the microstructure and mechanical properties of Si 3 N 4 [5][6][7][8].The general conclusion is that proper design of sintering additives lead to a significant increase in the mechanical properties of the Si 3 N 4 ceramics.
With the development of powder technology, the characteristics of the Si 3 N 4 powders such as particle size and morphology, impurities content, α/β ratio have progressively changed with time.Thus, the sintering behavior and mechanical properties varied among different commercial powders, which were synthesized by various methods (imides decomposition, direct nitridation, self-propagating and plasma chemical reaction).However, the relationship between the powder characteristics and microstructure development by hot pressing, so far is still not well understood.
In the present work, the densification behavior of five kinds of Si 3 N 4 powders with and without the addition of sintering additives were investigated via hot pressing.The differences in the densification, microstructure, and mechanical properties of the sintered samples were investigated and compared.The desirable properties of the starting powder were investigated with respect to high fracture toughness and strength.

Sample preparation
The five kinds of Si 3 N 4 powders were acquired from 5 different commercial manufacturers, and 4 kinds were prepared by different methods.Sample SN-A was manufactured using imides decomposition method, which was synthesized by silicon tetrachloride with ammonia (UBE E10, Japan).Sample SN-B was prepared by the direct nitridation of silicon (Starck M11, Germany).Sample SN-C was prepared by the nitridation of silicon (Ansaimei, China), but with sufficient nitridation additives, which resulted in high αratio.Sample SN-D was prepared by self-propagating high temperature synthesis (Tongli, China).Sample SN-E was prepared by plasma-induced chemical vapor deposition (Kaier, China).The characteristics of the five types of powders are listed in Tab.I.The particle size was measured by the Malvern 2000 (Malvern, Britain).The BET (specific surface area) was measured by the surface area and pore size distribution analyzer (SA3100, Beckman Coulter, USA).The α/β phase ratio was measured by X-ray diffraction (Bruker D8, Germany) and calculated according to the equation of the Gazzara [9].The impurities present were measured by the GDMS (VG9000, Thermo Elemental, UK).The morphology of the powders was shown in Fig. 1.It showed that the particle size of sample SN-A measured was not accurate, which due to agglomeration.The sample SN-A was finest, and the particle size was about 200 nm.It consists of spherical (α-phase) and some small rod-like (β-phase) particles.The sample SN-B also consists of small spherical particles and big irregular particles.Sample SN-C and sample SN-D were irregular in shape.The size distribution was much wider in sample SN-C, and it was coarser.The sample SN-E consists of spherical particles and whiskers (mostly αphase) with very high aspect ratio.The powder mixtures consisted of Si 3 N 4 powder with an additive composition of 4wt% La 2 O 3 and 4wt% Yb 2 O 3 (Fandecheng Corp., Beijing, China)] and 2wt% MgO (Hangzhou Wanjing Corp., China).In order to homogeneously mix the powders, ethanol was added to each powder mixture and the slurry was ball-milled for 24h using silicon nitride balls, and then poured through a 30-mesh sieve to separate milling media with the powderalcohol slurry in beakers.The slurry was ultrasonically dispersed for 5 min, and dried by rotary evaporation.After drying, the powder was manually ground and passed through a 100mesh sieve.A desired amount of the mixture was then placed in a graphite die and the uniaxial pressure of 10 MPa was applied via the graphite rams.The whole die assembly was then placed in a hot press furnace (VHP300/50-2200, Shenyang Weitai Corp., China), which was evacuated and back filled with nitrogen.Billets of Si 3 N 4 plus additive compositions were densified at 1800 o C for 1h under an applied uniaxial pressure of 30 MPa with a continuous flow of nitrogen through the chamber.The powders without additives were prepared and sintered via the same processing steps.

Characterization
The density of all samples was determined using the Archimedes principle.The morphologies were observed by scanning electron microscopy (SEM, Nova Nano430, Dutch) on the polished surfaces etched by 90%CF 4 +10%O 2 .The mechanical properties were measured using the Instron 5567 universal machine.Test specimens with dimensions of 3 mm x 4 mm x 45 mm were cut and machined from the hot-pressed disks.The flexure strength was measured by four-point bending method with inner and outer span of 20 mm and 40 mm, respectively under a cross-head speed of 0.5 mm/min at room temperature (ASTM-C1161-02).The fracture toughness was determined by single edge-notched beam (SENB, 3-point bending) method with an outer span of 40 mm at a cross-head speed of 0.05 mm/min at room temperature following ASTM standard of C1421.All notches were cut by the diamond disk with thickness of 150 μm.The width of the notch was 150±2 μm.The relative densities of sintered ceramics without and with additives are shown in Tab.II.Without sintering additives, the relative densities of SN-C and SN-D were higher than other samples without additives after hot pressing.The observed difference in the sintered densities might be attributed to impurities content, i.e., A1 2 O 3 , Fe 2 O 3 , CaO and particles morphology in the initial powders.Samples SN-C and SN-D contained high metallic impurities contents, which results in higher densification densities measured.The α-Si 3 N 4 grains would be dissolved into liquid SiO 2 and precipitates as elongated β-Si 3 N 4 grains through the dissolution and precipitation process during the sintering.The oxygen content (exist as SiO 2 or Si-O amorphous phase) was almost the same in all initial powders.However, it is known that the atomic diffusion was supposed to be much slower in highly pure silica than SiO 2 with metallic impurities.Therefore, those metallic impurities present in SiO 2 would lower the viscosity of the liquid phase formed and thus enhance the diffusion process and densification rate [10].However, sample SN-E with the highest amount of impurities exhibited the lowest density, which might be due to the presence of the high amount of whisker-like particles.The densification behavior would be impeded by the networks formed by those whisker-like particles, which decrease the green compact density and also particle rearrangement during the sintering process [11].The low densities of SN-A and SN-B were mainly due to the low metallic impurities.Previous studies showed that the oxygen content, particle size, and size distribution in the starting powders would be the key parameters which impact the densification process of Si 3 N 4 via gas pressure sintering [1].In the HIP-ed Si 3 N 4 without additives, the relative densities were strongly affected by the specific surface area, green density, and α/β ratio [3].However, in this study, the significance of the initial powder properties (oxygen content, α/β ratio, particle size, and distribution) was decreased during the hot-pressing without additives.The metal impurities and morphology of the powders have significant effect on the densification without the sintering additives.

Relative Density
The relative densities of Si 3 N 4 with La 2 O 3 -Yb 2 O 3 -MgO additives were very similar, and they are all above 95 % of theoretical density.That was because the sintering additives introduced could provide adequate amount of liquid phase for densification.When large amounts of additives were employed, the characteristics of the powders (oxygen content, impurities, α-ratio, particle size and distribution) became less significant.Sintering without additive: Fig. 2 shows that the microstructures of samples SN-A, SN-B, SN-C, SN-D and SN-E without sintering additives.All samples showed porous microstructure.The distance between the particles decrease and the neck size increased with shrinkage in sample SN-C and SN-D.The neck size increased with little shrinkage in sample SN-A, SN-B, and SN-E.The high amount of the metallic impurities in initial powder could result in the microstructural difference, which promoted the materials transport, facilitated the β-Si 3 N 4 grain growth, and led to the longer and fatter rod-like grains and low porosity.However, the sample SN-E also with highest metal impurities showed high porosity, and the whiskers-like particles were not detected after sintering.Obviously, the whiskers-like particles (most was α-phase) were not sTab.in high temperature and dissolved into the spherical particles (β-phase).The whiskers-like particles impeded the rearrangement and solution-reprecipitation stage during the liquid phase sintering and hindered the growth of grains [11].The microstructure of the β-Si 3 N 4 was influenced strongly by the starting powder characteristics during hot pressing without sintering additives.The high content of impurities can facilitate the growth of the grains extensively and decrease the porosity.The whiskerslike particles would dissolve into spherical particles and impeded the microstructure development.Sintering with additives: Fig. 3 shows the micrographs of samples with sintering additives.The elongated grains were apparent in SN-A and SN-C.The two samples showed a bimodal grain size distribution, which consisted of elongated grains, referred as reinforcing grains, in the fine grains matrix.The high α-phase content in the initial powders could enhance the formation of the elongated grains with high aspect ratio.[12] The samples SN-B and SN-D showed broad grain diameter distribution and coarser matrix.The influence of the particle size on the microstructure was not well studied in those previous studies.Lee etc. reported that abnormal grain growth was enhanced that resulted in significant bimodal distribution of grain size with decreasing powder size [13].However, Rhee etc. showed that the specimens prepared with coarse and fine initial α-Si 3 N 4 powders resulted in coarse and fine grained β-Si 3 N 4 ceramics, respectively [14].In this study, grain size of SN-E was finer than other samples, due to the finer matrix grains.It showed that there was tendency that finer powder size resulted in finer microstructure.The influence of the impurities on the microstructure was different between the sintering with additives and without additives.As one would anticipate that the impurities would decrease the melting temperature, and accelerate the densification and the materials transport, and thus promote β-Si 3 N 4 grain growth during the sintering.The sample SN-D and SN-E had high content metal impurities in the initial powders, but former was resulted in coarser β-Si 3 N 4 grains and latter was finer.So the influence of the metal impurities was not distinct.Comparing the morphology of the 5 starting powders, it was hard to confirm the effectiveness on the microstructure.Maybe, the whisker-like particles in SN-E would decrease the grain size, due to the low sintering ability.As result, the α/β phase ratio in the initial powder would be the main material parameter for bimodal distribution.And the starting powder size would be the main reason for grain size distribution.Then the influence of the impurities and the powder morphology was not apparent in this study.

Mechanical Properties
Tab. III Mechanical properties of the samples at room temperature.

Sample
Fracture Strength Fracture Toughness (MPa) (MPa*m The mechanical properties of SN-A, SN-B, SN-C, SN-D, and SN-E with additives are summarized in Tab.III.The samples SN-A and SN-C with distinctly bimodal microstructure have the combination of excellent bending strength (852 MPa and 805 MPa) and fracture toughness (11.84 MPa•m 1/2 and 8.83 MPa•m 1/2 ).The samples SN-B, SN-D, and SN-E had lower strength and toughness.Lange had reported that fracture toughness increased with the higher α-phase content in the initial powders [12].The high α-phase in the initial accelerated the abnormal grain growth resulting in the distinctly bimodal microstructure.The mechanical properties are thus strongly dependent upon the microstructure.The bimodal microstructure consisting of larger elongated grains dispersed in a finer matrix with a submicron grain size was one approach to achieve both high strength and toughness [15].The oxide impurities (A1 2 O 3 , Fe 2 O 3 and CaO) have no apparent effect on the room temperature mechanical properties of hot-pressed Si 3 N 4 [16].In this study, the high α-phase composition in the initial powders would be one of the key reasons for the high mechanical properties.

Conclusion
The influence of powder characteristics on sintering behavior, microstructure, and mechanical properties of hot-pressed β-Si 3 N 4 ceramics with and without the addition of La 2 O 3 -Yb 2 O 3 -MgO additives was investigated.During hot pressing, the effect of the powder characteristics was different between the sintering with additives and without additives.
Without the sintering additives, the higher amount of the impurities in the initial powders enhanced the densification behavior, and promoted the β-Si 3 N 4 grain growth.The whisker-like particles in the initial powders showed low thermal stability and decreased the density.The influence of particle size was not obvious on the densification and microstructure.
With the sintering additives, the effect of initial powders characteristics (impurities content, morphology of the particles) was not apparent on the densification behavior and resulting microstructure.And the starting powder particle size would be the main reason for β-Si 3 N 4 grain size.The high α/β phase ratio accelerated the anisotropic grain growth and thus increased the high mechanical properties.
Characteristics of starting Si 3 N 4 powders.