Effect of cooling rate and quenching temperature on fracture of chevron-notched dicor glass-ceramic

Effects from cooling rate and quenching temperature on load to fracture and, where applicable, on fracture toughness of Dicor glass were determined with the short-rod method. As-cast and properly cerammed Dicor were heated at 600, 700, 800, and 1000 degrees C for 5 minutes prior to ambient air-quench or furnace cooling. Air-quenching included cooling on hot ceramic tray, on cold metal block, and with forced air. The extent of crystallization of as-cast Dicor was determined using differential thermal analysis (DTA), and the fracture surfaces of short-rod samples and polished sections were characterized using scanning electron microscopy, and image analysis. Increased cooling rates and higher quenching temperatures increased fracture loads by up to three times with forced air compared to furnace cooling. The plane-strain fracture toughness of as-cast and properly cerammed Dicor were respectively, 0.95 and 1.20 MPa . m(1/2), which were significantly (different p less than or equal to 0.05); crack propagation occurred by jumping and by smooth growth advance, respectively. Load-mouth opening displacement plots showed evidence for micro-crack and deflection toughening mechanisms. The fracture plane was the chevron plane for furnace cooled samples, while for air-quench, for the most part, the fracture planes were inclined by up to 90 degrees. This was attributed to the inclusion of mixed-mode stress states in air-tempered samples. An inner core and an outer shell of material contained residual tensile and compressive stresses, respectively. Fractographic differences existed between plane-strain and mixed-mode fractures.