Functionally graded ceramics by a new in situ processing route: Residual stress and wear resistance

A new in situ method of producing ceramic functionally graded materials (FGMs) with continuously changing microstructure by pressureless sintering in combination with chemical reactions and evaporation is reported. Solid solutions of Fe2O3 in Al2O3 were first sintered in air and then reduction aged in 4 vol% H2/N2. Metallic Fe formed near the surface and was able to evaporate, producing a surface layer with graded composition. The thermal expansion mismatch led to compressive residual stresses in the surface region of ∼500 MPa and the graded structure avoided delamination. The abrasive wear resistance of the FGM ceramics was improved by a factor of 5 times compared with monolithic alumina. The improvement was attributed to the suppression of crack propagation normal to the surface by the compressive surface stresses.