Crack healing in alumina bioceramics

Microscopic cracks can occur at the surface of oxide ceramic restorations as a result of the manufacturing process and mainly due to the final mechanical preparation in the dental laboratory. A method is presented to heal up such microscopic cracks by a glass infiltration process.

Bar specimens made of high purity bio-alumina were manufactured. On two batches of specimens microscopic cracks were induced using the Vickers indentation technique. The small microscopic cracks at the tip of the resulting half-penny-shape cracks were extended by the bridge loading method. The indentation pattern of the specimens of one batch was subsequently glass-infiltrated. The surface layers of the specimens with the Vickers indentation were removed by grinding as far as only the extended microscopic cracks (with and without glass) remained at the surface. The strengths of untreated, micro-damaged, and micro-damaged and glass-infiltrated specimens were determined. The microstructure of the fracture surfaces was analyzed using SEM.

The characteristic strength of the specimens decreased from sigma(0)=378 to 196 MPa and the Weibull modulus from m=13.7 to 2.3 due to the micro-damaging. The strength and the scatter-in-strength were significantly improved by the glass infiltration process. The strength of the "healed" specimens (sigma(0)=434 MPa, m=17.3) was even better than that of the untreated samples.

Microscopic cracks that can occur at the surface of dental restorations made of alumina like abutments or cores of crowns and bridges during the manufacturing and preparation process could reliably be healed by a glass infiltration process.