Thickness Effect of (0001) Textured Al₂O₃ Layer on the Optical and Mechanical Properties of MgAl₂O₄/Al₂O₃ Laminated Transparent Composite

The effect of the thickness of (0001) textured Al₂O₃ layer on the optical and mechanical properties is examined in the MgAl₂O₄/Al₂O₃ transparent laminated composite. The textured Al₂O₃ layers with various thicknesses are formed on the MgAl₂O₄ polycrystals using a magnetic field alignment technique followed by one step spark‐plasma‐sintering (SPS) processing. Depending on the thickness of the textured Al₂O₃ layer, the transmittance efficiency of the composite sensitively changes and attains the higher in‐line transmittance of T_in,0.64 ≈ 54% at a visible range of λ = 0.64 μm for the thin Al₂O₃ layer with a thickness of ≈400 μm. In addition to the transparency, the hardness H_v measured on the textured Al₂O₃ layer surface also changes sensitively with the thickness and can attain H_v = 30.4 GPa, which is much higher than 25.5 GPa of monolithic Al₂O₃ polycrystals. The higher hardness can be ascribed to residual stresses formed in the textured Al₂O₃ layer. These results indicate that by controlling both the thickness and the texture structure of the Al₂O₃ layer, the MgAl₂O₄/Al₂O₃ composite simultaneously achieves excellent optical and mechanical properties.