Strength and fracture origins of a feldspathic porcelain.
OBJECTIVE
To identify the strength limiting flaws in in vitro test specimens of a fine-grained feldspathic dental porcelain.


METHODS
Four-point flexural strengths were measured for 26 test specimens. The fracture origin site of every test specimen was studied using stereoptical and scanning electron microscopy. A fractographically labeled Weibull strength distribution graph was prepared.


RESULTS
The complex microstructure of the feldspathic dental porcelain included a variety of feldspars, tridymite, and a feldspathoid as well as pores/bubbles and residual glass. The relatively high flexural strength is due in part to the fine grain size. Fractography revealed five flaw types that controlled strength: baseline microstructural flaws, pores/bubbles, side wall grinding damage, corner machining damage, and inclusions. The baseline microstructural flaws probably were clusters of particular crystalline phases.


CONCLUSIONS
Each flaw type probably has a different severity and size distribution, and hence has a different strength distribution. The Weibull strength distribution graph blended the strength distributions of the five flaw types and the apparent good fit of the combined data to a unimodal strength distribution was misleading. Polishing failed to eliminate deeper transverse grinding cracks and corner damage from earlier preparation steps in many of the test pieces. Bend bars should be prepared carefully with longitudinal surface grinding whenever possible and edge chamfers should be carefully applied. If the grinding and preparation flaws were eliminated, the Weibull modulus for this feldspathic porcelain would be greater than 30. Pores/bubbles sometimes controlled strength, but only if they touched each other or an exposed surface. Isolated interior bubble/pores were harmless.
American Dental Association Foundation, Paffenbarger Research Center, National Institute of Standards and Technology, Gaithersburg, MD, USA. george.quinn@nist.gov
George D
GD
Quinn
Kathleen
K
Hoffman
Janet B
JB
Quinn
D000535:Aluminum / Q000737:chemistry; D017607:Aluminum Compounds / Q000737:chemistry; D000538:Aluminum Silicates / Q000737:chemistry*; D003460:Crystallization; D003775:Dental Polishing / Q000379:methods; D003776:Dental Porcelain / Q000737:chemistry*; D003799:Dental Stress Analysis; D005898:Glass / Q000737:chemistry; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D010316:Particle Size; D018583:Pliability; D016062:Porosity; D011188:Potassium / Q000737:chemistry; D017680:Potassium Compounds / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry; D012825:Silicon / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D012964:Sodium / Q000737:chemistry; D017670:Sodium Compounds / Q000737:chemistry; D013052:Spectrometry, X-Ray Emission; D013314:Stress, Mechanical; D013499:Surface Properties; D014961:X-Ray Diffraction
D017607:Aluminum Compounds; D000538:Aluminum Silicates; D017680:Potassium Compounds; D017640:Silicates; D017670:Sodium Compounds; C545846:sanidine; D003776:Dental Porcelain; C016447:feldspar; C024205:nepheline; D012822:Silicon Dioxide; C016024:orthoclase; D012964:Sodium; D000535:Aluminum; D011188:Potassium; D012825:Silicon