Ya-Wen Li

Effect of processing on the morphology of α-sialon grains

Abstract In order to explore the effect of processing on the formation of elongated α-sialon grains, a mixture of α-Si 3 N 4 and β-Si 3 N 4 (5–40 wt.%) powders combined with variant firing cycles was used for preparing an α-sialon doped with Dy 2 O 3 . The results showed that the morphology of α-sialon grains was more closely relevant to the firing conditions than to the composition of the starting powder. The specimen sintered by gas pressure sintering (GPS) or through heat treatment at 1800°C or1900°C (for hot-pressed (HP) specimen) possessed a bimodal microstructure. The mechanism of forming elongated α-sialon grains is discussed in the present work.

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 formation and microstructural evolution of Ca α-sialon using different Si3N4 starting powders

Abstract Silicon nitride has two polymorphous structures, α-Si 3 N 4 and β-Si 3 N 4 . In this study three different Si 3 N 4 starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca 1.8 Si 6.6 Al 5.4 O 1.8 N 14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si 3 N 4 resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si 3 N 4 remained until 1800° in samples prepared with ∼100%β-Si 3 N 4 powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si 3 N 4 starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.

Effect of additives on microstructure of Ca α-sialon

Abstract Three kinds of additives were added into Ca α-sialon, (Ca 0.6 Si 10.2 Al 1.8 O 0.6 N 15.4 ). It was found that the additives resulted in more of the liquid phase during sintering, which promoted the completion of the α→α′ transformation and the development of the elongated morphology characteristic of α-sialon grains. Comparison was made concerning the effect of the amount and type of additives on the microstructural development of Ca α-sialon. Phase and property analysis was also carried out on the hot-pressed samples.

Effect of amount and atomic ratio of dual modifiers Ca and Mg on phase formation and mechanical properties of Ca,Mg-α-Sialons

Multi-cation α-Sialons ceramics containing calcium and magnesium for the compositions ((Ca,Mg)xSi12 − 3xAl3xOxN16 − x), where x equals 0.6 and 1.0 with the different atomic ratios of Ca to Mg, were fabricated by hot-pressing. An exploration for Ca,Mg-α-Sialon involving phase compositions, cell dimensions, microstructure and mechanical properties were carried out in the present work.

Formation behavior, microstructure and mechanical properties of multi-cation α-sialons containing calcium and neodymium

Abstract Dual modifying cation (Ca+Nd) α-sialons were hot pressed for the x=1.0 composition ((Ca,Nd)xSi12-3nAl3nOnN16-n) with mixtures of CaO:Nd2O3 in the molar ratios of 0:1, 0.3:0.7, 0.5:0.5, 0.7:0.3 and 1:0. Experimental results showed that except for pure Nd- and Ca-α-silaon compositions, the main crystalline phase in samples hot-pressed at 1750°C for 1 h was α-sialon together with a small amount of melilite solid solution and AlN polytypoid. The higher CaO:Nd2O3 ratio in the starting composition, the higher α-sialon content and the lower melilite solid solution content in the final phase assembly. The α-sialon lattice also expanded with increasing substitution of Ca for Nd. Elongated grain morphology was observed in (Ca+Nd) α-sialon. EDS analysis revealed higher solubility of Ca than Nd in the α-sialon lattice and lower solubility of Ca than Nd in the intergranular phase. The hardness increased, ranging from 15.87 to 18.79 MPa, when the amount of α-sialon phase in the material became higher. The fracture toughness varied from 4.2 to 5.1 MP am1/2 for (Ca+Nd) α-sialon compositions, in which the composition with CaO:Nd2O3=0.5:0.5 possessed the highest value.

Phase formation and microstructural evolution of R-α′-AlN-polytypoid ceramics (R=Y, Y+Sm and Sm)

Abstract In the present work, Y, Sm and (Y+Sm) were separately used to explore the effects of rare-earth elements on the reaction sequence and microstructural evolution in α-sialon-AlN-polytypoid composite ceramics. The results showed that the reaction sequences in the composite ceramics are affected by rare earth elements. With the existence of AlN-polytypoid α-sialon develops into elongated morphology. The Sm–α-sialon composition exhibits the feature of fine grains with high aspect ratio and Y-composition possesses rather coarse morphology with low aspect ratio. In the composite ceramics, AlN-polytypoid was not distributed homogeneously around the α-sialon grains, especially in the Y-composition. The variation in microstructure with rare-earth elements in the Sialon materials was discussed based on their reaction sequences.

Anisotropic grain growth of R-α-sialon (R = Nd and Er)

Two R-α-sialon (R0.6Si9.3Al2.7O0.9N15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the α → α′ phase transformation. Elongated α-sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R-α-sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R-α-sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.

Effect of processing on toughness of Ca α-sialon ceramics

The influence of processing on microstructural development of Ca α-sialon composition (CaxSi12−3xAl3xOxN16−x) with x = 1.8 was studied by dwelling at different intermediate temperatures before reaching the final sintering temperature. The microstructural observation results have showed the different aspect ratios of elongated grains obtained by the various processing conditions, reflecting the effect of the number of nuclei of α-sialon on morphology of grains during sintering. Improved toughness was achieved by applying low temperature dwelling for Ca α-sialon compositions with low x values. The toughness showed an increase of 33% and 16% for x = 0.6 and 1.0 compositions respectively with middle dwelling processing at 1350°C for 3 h before reaching 1750°C for 1 h by hot-pressing.

TEM observation on nucleation of Ca α-Sialon

Ca α-Sialon compacts pressurelessly-sintered to the intermediate temperature (1 450°C) were investigated with TEM for an overall composition Ca1.8Si6.6Al5.4O1.8N14.2. It was found that in most cases, the newly-formed α-Sialon grains had no epitaxial orientation relationship with the Si3N4 particles; only occasionally heteroepitaxial nucleation of α-Sialon on α-Si3N4 was detected. Further EDAX analysis revealed a much higher Ca concentration in the non-epitaxially nucleated α-Sialon than in the heteroepitaxially nucleated α-Sialon. Thus a possible correlation between the concentration of metal cations in the α-Sialon structure and the nucleation mechanism has been proposed, i.e. α-Sialon compositions with higher Ca concentration show a stronger trend of non-epitaxial nucleation.