Study On Preparation, Structure And Properties Of SAZ Nano-composite Ceramics By In-situ Controlled Crystallizing From The SAZ Amorphous Bulk

As one of promising high-temperature structural materials, SiO₂-Al₂O₃-ZrO₂(SAZ) composite ceramic with mullite as their one of the main phases is well-known for its excellent chemical stability, good high-temperature strength, high creep resistance, low thermal conductivity and low thermal expansion co-efficient. However, the lower intrinsical toughness of mullite restricts its use. In general, ZrO₂ can improve its room-temperature mechanical properties with unique characteristics at high temperature, Which promotes the wide spread applications of SAZ composite ceramics. However, zirconia-mullite composite ceramics are difficult to sinter to high density by conventional route because it normally requires high sintering temperature which will result in the grain abnormal growth. In addition, the source of toughening agents is limited and they are usually expensive. In this paper, SAZ nano-composite ceramics with dense and homogenous microstructure were fabricated at relatively low temperature (1100-1200℃) by in-situ controlled crystallizing from SAZ amorphous bulk. This method can overcome the negative effects due to abnormal growth of nano-grains at high sintering temperature, and can control the microstructure effectively. The preparation and crystallization behaviors of SAZ amorphous bulk were investigated by DSC, XRD, SEM, TEM, EDS and IR techniques. Some important relationships between compositions, processing, structure and properties were studied. The effects of the additives on the structure and mechanical properties of SAZ nano-composite ceramics prepared by this method were analyzed. Several important conclusions can be summarized as follows:1 It is difficult to obtain homogeneous amorphous bulk from the composition zone near the eutectic point in the system of SiO₂-Al₂O₃-ZrO₂ due to thire strong ability of crystallization, although the melting temperatures are low. The transparent amorphous bulks are obtained in the zones with rich silicon. The dopant of TiO₂ shows a positive effect on the formation of SAZ amorphous bulk, lowering the high-temperature meltage viscosity. Al₂O₃ and ZrO₂ have great effects on the formation of amorphous bulk and thire optimum contents are 35-40 wt% and 10-20 wt%, respectively. The more homogeneous sol was obtained by raising the melting temperature or prolonging the melting time. In addition, it is important to control the cooling speed. By careful selection of composition and additives, the homogeneous SAZ sol was prepared at 1650-1700℃and transparent amorphous bulk was obtained after the controlled cooling speed quenching.2 There are two crystallization reactions during the heat treatment for SAZ amorphous bulk. The first crystallization reaction occurs in 930-1050℃. The tetragonal zirconia is formed. The second crystallization reaction occurs in 1100-1200℃. The main crystalline phases of mullite and cristobalite are produced. The amorphous bulks undergo structural changes by heat treatment at different temperature. Phase segregation occurs at about 900℃, resulting in the formation of Si-rich and Al, Zr-rich regions. The t-ZrO₂ is crystallized from the Al, Zr-rich region at 920-950℃followed by poorly defined Al-Si spinel. With the increase of temperature, mullite forms by reaction between Al-Si spinel and amorphous silica, and at the same time, cristobalite is formed from the excessive amorphous silica. The crystallization process can be characterized as follows:3 With the help of thermal analysis, crystallization kinetics parameters were calculated. It has been found that the crystallization activation energy of the SAZ amorphous is low. The crystallization activation energy of t-ZrO₂ for one typical sample TZ2 is 518-538kJ·mol^-1 and that of mullite is 522-545kJ·mol^-1. The crystallization activation energy of t-ZrO₂ for another typical sample Z1 is 556-578kJ·mol^-1 and that of mullite is 497-522kJ·mol^-1.4 The composite additives of ZrO₂ and TiO₂ are useful to improve the crystallization behaviors of SAZ amorphous bulk. With the increase of TiO₂, the crystallization temperature of t-ZrO₂ is decreased and there is no evident effect on that of mullite formation. With the increase of ZrO₂, the crystallization temperatures of them are decreased together. When the ratio of TiO₂/ZrO₂ is excess 1/2, the unfavorable phases such as cordierite and ZrTiO₄ are formed easily at low crystallization temperature with the increase of TiO₂. When the ratio of TiO₂/ZrO₂ is less than 1/2, the unfavorable phases such as cordierite and ZrTiO₄ are not easy to form with the increase of ZrO₂. But the unfavorable phase of cordierite is formed easily with the increase of ZrO₂ in the samples doped with ZrO₂ only.5 The comprehensive mechanical properties and structure of T, TZ and Z nano-composite ceramics are affected greatly by the heat treatment processing. Nucleating temperature and time and crystallization temperature and time are interrelated. The optimal heat treatment processing of Z1 and TZ1 samples was determined by means of DSC analysis and property measurement evaluation. The fracture toughness and flexural strength of Z1 sample obtained under the optimal heat treatment processing (nucleated at 950℃for 2.0 h and crystallized 1150℃for 2.0 h) are 5.13 MPa·m^1/2, 520MPa, respectively. The fracture toughness of the typical sample TZ1 obtained under the optimal heat treatment processing (nucleated at 920℃for 2.0 h and crystallized 1150℃for 4.0 h) is 7.48 MPa·m^1/2. The promoted properties are attributed to the improved microstructure. With the research of the effects of thermal treatment processing on the microstructure of the samples, it shows that the heat treatment temperature and time have an important effect on the size and shape of grains, especially the nucleating and crystallization temperature. Too high or too low nucleating temperature is not advantageous to nucleate. The sample nucleated at 920-950℃followed by heated at 1100- 1150℃shows very dense and homogenous microstructure with the size of ball-like grains about 20-50 run. With the increase of heat treatment temperature up to 1300℃, the grains grow quickly and some grow into platelike grains with the size about 5μm. In addition, the phases and bulk density are effected by crystallization temperature. The higher of crystallization temperature, the less of t-ZrO₂ and the more of m-ZrO₂, and the lower of bulk density. Optimizing the two-step heat treatment is crucial for the microstructure control of SAZ nano-composite ceramic.6 With the research of the effects of the additives of TiO₂ and ZrO₂ on the properties of T, TZ and Z nano-composite ceramics, it has been found that the fracture toughness of SAZ nano-composite ceramics are improved a lot by the composite additives and the optimal content of TiO₂ additive is no more than 5 wt%. With the increase of TiO₂ in the T samples, the microhardness is increased then decreased, but the fracture toughness declined. With the increase of ZrO₂ in the TZ samples, the fracture toughness is increased while the change of microhardness is related to crystallizing temperature. Under 1200℃, the microhardness is increased, but above 1200℃, it is decreased.7 With the research of the effects of the additives of Y₂O₃ and La₂O₃ on the crystallization behaviors of SAZ amorphous bulk, it has been found that the addition of Y₂O₃ and La₂O₃ has some beneficial effects on the formation of SAZ amorphous bulk by lowering the melting temperature, but has no effects on the main phases. Y₂O₃ makes zirconia more stable by the formation of solid solution. With the increase of Y₂O₃, the fracture toughness and flexural strength of ZY samples are firstly raised, then retained and dropped finally. The comprehensive mechanical properties of the sample doped with 1.0 wt% Y₂O₃ are optimal, its fracture toughness and flexural strength were 4.5MPa·m^1/2 and 460Mpa respectively. Its strengthening and toughening mechanism is the transformation from t-ZrO₂ to m-ZrO₂. The fracture toughness and flexural strength of ZYL samples firstly increases and then decreases with the content of La₂O₃. The sample with 0.6 wt% La₂O₃ has high fracture toughness 4.74 MPa·m^1/2 and the sample with 1.2 wt% La₂O₃ has high flexural strength 514 MPa. In the ZYL samples, in addition to the contribution of bridge join of large particles, the boundary strengthening is also one of the main strengthening and toughening mechanism since La₂O₃ are mailly distributed in glass phase.