D.S. Yan

Characteristics of Ca-α-sialon : Phase formation, microstructure and mechanical properties

Abstract Ca-α-Sialon (Ca x Si 12 -( m+n ) Al m+n O n N 16-n ,) ceramics with extensive compositions on the line of Si 3 N 4 –CaO:3AlN (where m =2 n ) ranged from x =0·3 up to x =2·0 were fabricated by hot-pressing. An exploration for Ca-α-Sialon involving reaction sequences, phase compositions, cell dimensions, microstructure and mechanical properties was carried out in the present work.

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m−2 for unreinforced Si3N4, to 4770 J m−2; its fracture toughness,Klc, increased from 3.7 MN m−3/2 for unreinforced material, to 15.6 MN m−3/2. The strength remains about the same as unreinforced Si3N4 and the thermal expansion coefficient is only 2.51×10−6 ° C−1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.

Effect of agglomerates in ZrO2 powder compacts on microstructural development

Ultrafine zirconia powders were prepared by a coprecipitation and spray-drying method. Agglomerates may be fragmented or present in green bodies after compaction. The effect of agglomerates on sintering and microstructural development was studied and it was found that the agglomerate content in compacts was a major factor affecting the microstructure development and the sintered densities. The interaction between agglomerates themselves, and between agglomerates and the primary particle matrix is discussed. It is argued that the hard agglomerates in the powder from the water-washed coprecipitates are formed by oxobridging between non-bridging hydroxyl groups present in the zirconium hydroxide structures due to the effect of hydrogen bonding in the aqueous system. The substitution of organic -OR groups for the non-bridging hydroxyl groups removes this hydrogen-bonding effect between the zirconium hydroxide units and thus eliminates the cause of agglomeration.

Preparation of nanosized titania powder via the controlled hydrolysis of titanium alkoxide

By controlling the hydrolysis of titanium butoxide, followed with or without an ethanol washing process, the preparation of nanoscale titania powder was studied in detail. The characteristics of different powders produced by the direct precipitation (without an ethanol wash) and ethanol wash processes were studied by X-ray diffraction, transmission electron microscopy, BET, thermogravimetry and differential thermal analysis techniques. By comparison, it was found that both the direct precipitation and ethanol wash methods can obtain slightly agglomerated nanoscale titania powders of less than 15 nm, but the ethanol wash can further reduce the agglomeration. The particle sizes of titania powders can be modified while still retaining the anatase structure.

Mechanical properties of AlN-polytypoids: 15R, 12H and 21R

Abstract Mechanical properties of three types of AlN-polytypoids with Ln2O3(Ln=Nd,Sm) and Ln2O3:Al2O3=1:1, respectively, as sintering additives were studied. The results showed that the hardness increased with the AlN-polytypoid compositions shifting towards AlN-rich side, that was 21R having the highest hardness (12.7–14.4 GPa) and 15R the lowest values(11.5–12.8 GPa). The fracture toughness and flexural strength had no such close relationships to the type of AlN-polytypoids, which varied in a range of 3.8–4.8 MPa·m1/2 and 313–366 MPa (at room temperature), respectively. However, for the compositions with suitable amount of additives, the flexural strength greatly increased with increasing temperatures, especially for 15R and 12H. This characteristic was discussed on the basis of the fracture morphology observed by SEM.

Preparation and characterization of nanoscale Y-TZP powder by heterogeneous azeotropic distillation

A heterogeneous azeotropic distillation process was effectively used to dehydrate hydrous zirconia and therefore prevent the formation of hard agglomerates in the preparation of nanoscale zirconia powder. The mechanism of azeotropic distillation to prevent hard agglomerate from forming was studied by investigating the interaction of hydrous zirconia with n-butanol. The prepared powder was sintered to 99.5% of theoretical density by slow-sintering (average grain size of ∼200 nm), 97.5% by fast-sintering (average grain size of ∼120 nm) at 1250°C.

Formation behavior and microstructure of multi-cation α-sialons containing neodymium and ytterbium or yttrium

Abstract Multi-cation α-sialons containing neodymium and ytterbium, as well as neodymium and yttrium, were prepared by hot pressing at the temperatures ranging from 1550 to 1750°C for 2 h with the compositions (Nd 0.18 Yb 0.18 /Nd 0.18 Y 0.18 )Si 10.36 Al 1.62 O 0.54 N 15.46 . The densification process, reaction sequence, cell dimensions and microstructure were studied in comparison with the corresponding single cation α-sialon. Experimental results have shown that the samples hot-pressed at 1750°C mainly consist of α-sialon phase, whose content (around 95 wt%) is higher than that of counterpart single rare earth doped α-sialon, especially much higher than the one of Nd-α-sialon. On the other hand, the multi-cation α-sialons compositions are beneficial to lower the eutectic temperature in the systems, thus promoting the dissolution of intergranular crystalline melilite phase, and facilitating the precipitation of α-sialon phase. TEM and EDS revealed that the amount of Yb 3+ or Y 3+ , which possess relatively smaller ionic radii than Nd 3+ , is higher than that of the Nd 3+ in α-sialon grains and more Nd 3+ are remained in the grain boundary phase. Small amount of elongated α-sialon grains were observed by TEM and the preferred orientation also occurred under hot-pressing for such a low x value composition ( x =0.36) used in this study.

Preparation of ultrafine zirconia powder by emulsion method

Abstract Ultrafine 14 nm 3 mol% Y 2 O 3 stabilized tetragonal ZrO 2 powder was prepared by emulsion processing. The obtained particles were weakly agglomerated into polyhedral or spherical shaped powders. The powder characteristics were investigated by using X-ray diffraction (XRD), X-ray fluorescence (XRF), inductively coupled plasma spectroscopy (ICP), nitrogen adsorption (BET model) and thermal analysis (DTA-TG). The experiments indicated that the heterogenous distillation used in the process was an effective method to remove residual water in the gelprecipitate and reduce the formation of strong agglomerates. The powder presented good formability and sinterability.

Effect of dual elements (Ca, Mg) and (Ca, La) on cell dimensions of multi-cation α-sialons

Abstract The lattice parameters of multi-cation α-sialons containing Ca and Mg have been studied for the compositions M x Si 12–3 n Al 3– n O n N 16– n (M=0.5Ca+0.5Mg) with x ranging from 0.3 to 1.4 by using a Guinier–Hagg camera film data. The results indicated that the cell dimensions of Ca, Mg-α-sialon were obviously smaller than that of Ca-α-sialon with the same compositions. The EDAX analysis showed that the contents of Ca in the grains were much higher than Mg and the average value of Ca contents could be close to the one in nominal composition, implying that Ca was easily “pushed” towards α-sialon structure in multi-cation α-sialons case. Based on these results, α-sialon ceramics using 70%Ca+30%La as additives were prepared. The lattice parameters of the α-sialons, in which 30%Ca were replaced by La as additives, were close to the single-cation Ca-α-sialon under the same compositions, indicating that most of Ca were incorporated in α-sialon because La cannot be absorbed to α-sialon structure. The advantage of using additives Ca combined with La to form α-sialon was discussed in the paper. Consequently, one composition of α-sialon with Ca and La as additives possessing hardness H v10 and fracture toughness K 1c to be 18.5 GPa and 5.5 MPa m 1/2 , respectively, was obtained.

Phase relationships in Si3N4-AIN-MxOy systems and their implications for sialon fabrication

Phase relationships in Si3N4-AIN-MxOy systems involving α′-sialon, where M represents lithium, magnesium, calcium, yttrium, neodymium, samarium, gadolinium, dysprosium, erbium and ytterbium are outlined. Their implications for the formation and fabrication of single-phase α′-sialon and two-phase β:α′ sialon ceramics are discussed.