| Recently developed Mg-Al transparent ceramics which not only has good transmittances from ultraviolet to infrared band, but has excellently chemical stability and mechanical properties, has been widely used in the fields of civil and military. For instance, it has been used as dome of guided missile, bulletproof windows, high-voltage light tube, and the faceplate of back projection color TV, etc. During its applications, transparent ceramics was found that it has the weakness of brittleness which also belongs to other ceramics. However, it is not suitable to toughen transparent ceramics using traditional toughening techniques which has been proved to sacrifice transparence of ceramics. This paper presents the in-situ toughening technique which is used to toughen transparent ceramics taking the advantage of segregated Al2O3 in main grain boundary of non-stoichiometric ceramics. The non-stoichiometric ceramics was obtained through changing the molar ratio of Al2O3 and MgO in the lattices of spinel. This technique gives a thinking direction of toughening transparent ceramics.In this work, MgO·nAl2O3 powders with n=1, 1.3, 1.5 and 1.8 are obtained by calcining NH4Al(SO4)2·12H2O and MgSO4·7H2O at high temperature through changing the molar ratios of the raw materials. Then transparent Mg-Al spinel ceramics with different stoichiometries are prepared by two stages sintering techniques: vacuum sintering (temperature 1550℃, holding time 2h, vacuum degree 1× 10-3Pa) and post hot isostatic pressing (temperature 1700℃, pressure 200MPa, holding time 2h), using previously obtained powders. Using X-Ray diffraction, Scanning Electrical Microscopy (SEM) analysis and Transmission ElectricalMicroscopy(TEM) methods, compositional phases of powders and ceramics, figurations and sizes of grains. Morphologies of grain boundaries and fracture face are being tested. At the same time, micro-hardness, fracture toughness, tree-points flexural strengths, compressive strengths and optical transmittances of ceramics are also tested.The XRD and TEM results of powders show that, Mg-Al spinel powders with different-stoichiometries have structures of single spinel phase. The powders whose grain sizes are around 20 nanometers all have good homogeneities and dispersal characters. The XRD results of ceramics show that transparent Mg-Al spinel ceramics with non-stoichiometries have segregating Al2O3 phase besides spinel phase. And the diffractive intensities of segregating phase increase with increasing stoichiometrics.SEM results show that, the sizes of Mg-Al spinel ceramics are from around 40μm to 70μm. Grain boundaries of spinel phase gradually become deflected with increasing stoichiometry n. The fracture behavior of stoichiometric ceramic is mainly transgranular and its fractural figuration looks like rock sugar. The fracture behaviors of non-stoichiometric ceramics consist of transgranular and intergranular models. And its crack propagations are more deflected. The toughening mechanisms of non-stoichiometric ceramics are as following: thermal expansion mismatching, cracking deflection and dispersively toughening of second phase. The effects of many toughening mechanisms lead strengths and toughnesses of non-stoichiometric ceramics to be higher than that of stoichiometric ceramic.Analysis results of ceramics properties show that, the transmittances of ceramics with n=1.3 and 1.5 in all experimented wavelength of non-stoichiometric ceramics are similar to that of stoichiometric ceramic, which is around 50%. However, MgO·nAl2O3 ceramic with n=1.8 has lower transmittance compared to other ceramics because of scattering caused by Al2O3. Transmittances of transparent ceramics with different n appear good consistency. Compared to stoichiometric ceramic, ceramic with n=1.3 has the highest flexural strength of 223 MPa. With increasing n, flexural strengths of non-stoichiometric ceramics are decreasing.Strengths and toughness of non-stoichiometric ceramics are enhanced. Compressive strengths of ceramics with different n are in the trend of increasing followed by decreasing, the highest value is 1400MPa of n=1.5. However, hardness and fracture toughness of ceramics with various n increase continuously. The highest value of hardness and toughness of n=1.8 is 1100Kg/mm2 and 2.54MPa1/2 respectively, which is enhanced by around 20% and 14% respectively. All this results have not been found to report yet.
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