Additives The Saz Ceramics Crystallization Behavior, Morphology And Performance

The SAZ composite ceramics, which have unique high temperature property, have been one of the hottest issues in the research of novel ceramic materials. In the present study, a new method, high temperature sol-gel and in-situ controlled crystallization, was used to prepare SAZ composite ceramics with ultra fine grains. First, the homogeneous sol was obtained at 1650-1700℃and additions. Second, gel was attained by controlling cooling processing. Finally, the ceramics samples were obtained by a two-step heat treatment.The effects of MgO, CaO, and TiO2 additions on the crystallization behaviors of Si-Al-Zr-O system amorphous bulk were investigated by DSC, XRD, SEM, TEM, EDS, Raman and IR techniques. According to the results, the homogeneous and dense Si-Al-Zr-O composite ceramics with ultra fine grains were prepared by the the two-step heat treatment schedule. In the paper, the effection of the armorphous on the structure and properties of SAZ ceramics prepared by sintering method were also studied. Several important conclusions can be summarized as follows:1. During the heating process of the amorphous, Zr4+ firstly precipitates in form of t-ZrO2, Al3+ firstly formes Al-Si spinel phase. As further increase in temperature, Al-Si spinel phase firstly increases then disappeares; when the temperature rises to 1100～1150℃, Raman peaks of m-ZrO2 appears, while the cristobalite is formed, the main phase are t-ZrO2 and mullite; a part of t-ZrO2 phase transforms into m-ZrO2, while the surplus amorphous silica formes cristobalite phase. The main two reactions during crystallization process of SAZ amorphous are:during 930-1050℃crystallization reaction t-ZrO2 precipitates; during 1100-1200℃crystallization reaction mullite and cristobalite phases precipitate.2. The phases, microstructure and mechanical properties of samples were effectively affected by CaO and MgO additions. For the only MgO-doped sample, the main crystalline phases were identified as t-ZrO2 and mullite with a small amount of cristobalite (SiO2), forsterite (Mg2Si04) and m-ZrO2. When 1wt% CaO replaced the same content of MgO, the main crystalline phases did not change, but Mg2Si04 disappeared and cordierite (2MgO·2Al2O3·5SiO2) existed. Meanwhile, the content of cristobalite was increased obviously and mullite was decreased due to the formation of cordierite. With the increase of CaO content (1-3wt%), more cristobalite formed, less cordierite precipitated, and no new phase for CaO additives occured. The only MgO-doped specimen has homogenous structure with fine, equiaxed grains of< 100nm, but the anisotropic growth of mullites occurred in CaO, MgO-doped samples. As CaO doping concentration increased, the aspect ratio of mullite grains was increased, which implied that the addition CaO played a major role in the anisotropic growth of mullite. The possible reason for this is that CaO additions promote cristobalite forming which results in the formation of the Al-rich zone. Compared with the only MgO-doped sample, the doping of CaO and MgO exhibited a strong increasing in the KIC of the doped samples. It is most likely related to the anisotropic growth of grains.3. The amount of TiO2 additives (1 wt%-7 wt%) decreases the formation temperature of t-ZrO2 and mullite, promotes the growth of mullite and ZrO2 grains, while has little effect on the orders of precipitation. The results show that when the addition of TiO2 is less than 5 wt%, most of Ti4+dissolves in the t-ZrO2, a small amount of Ti4+are soubled into mullite grain; and When TiO2 additives are more than 5 wt%, the excessive Ti4+ formates a new phase, ZrTiO4. Meanwhile, With TiO2 additives increase, a small amount of cordierite phase is formed, while cristobalite phase decreases. The hardness increases with the content of TiO2, while the fracture toughness presentes a slightly increase firstly and then the fracture toughness decreases with the TiO2 addition higher than 3 wt%.4. With the increasing incorporation of amorphous, the main phase of SiO2 disappears, while ZrSiO4 phase increases significantly, which indicates adding the amorphous promotes the reaction between ZrO2 and SiO2 or promotes ZrO2 solute into SiO2. By adding more amorphous, crystallization process becomes more complete; the matrix phases are mainly mullite and zirconium silicate phase, while zirconia is the most important precipitation. The density grows fastest when the percentage of amorphous content is 0 wt%-10 Wt%; porosity shows a obvious growth trend when the amorphous content is 0 wt%-8 wt%, with the increase of amorphous content, porsity maintains the relatively stable change; shrinkage decline is most apparent when amorphous content is the 0 wt%-10 wt%. Hardness growth trend is most apparent when the amorphous content is 0 wt%-20 wt%.The fracture toughness presentes a relatively large increase with the amorphous content growth. Combination of all those tests, when the amorphous content was 20 wt% better propersity and simpler procedure can be achieved at the same time.