The overall goal of the proposed research is to determine the mechanisms responsible for the thermally induced changes in coefficient of thermal expansion that are experienced by dental porcelains and to use the understanding thus gained to develop porcelains more resistant to such changes. The development of such thermally stable porcelains would greatly reduce the tendency for cracking and checking of the dental porcelains during cooling of porcelain-fused-to-metal restorations, with consequent reductions in cost, inconvenience, and re-treatment trauma to the dental patient receiving this type of restoration. The first specific aim is to measure the effects of multiple firings, isothermal heat treatments, and different cooling rates on the coefficients of thermal expansion for a variety of dental porcelains. These measurements will be accomplished in a conventional differential dilatometer and in a laser dilatometer developed in the Dental Materials Laboratory at the Medical College of Georgia. The second specific aim is to determine the mechanisms responsible for porcelain expansion changes during various heat treatments. The methods which will be used to discriminate among the possible mechanisms are: quantitative x-ray diffractometry, high-temperature x-ray diffractometry, hot-stage scanning electron microscopy, and measurement of the mid-span deflection of bimaterial porcelain-metal strips in a high rate infrared bending beam viscometer developed at the Dental Materials Laboratory at the Medical College of Georgia. Possible mechanisms for thermal instability of dental porcelains involve the crystallization or dissolution of leucite, the conversion of leucite to sanidine, the retention of metastable cubic leucite upon cooling from high temperatures, the decoupling and recoupling of leucite particles in the glass matrix via microcracking and sintering, and trapping of various levels of excess volume in the glass matrix owing to different cooling rates. The development of methods for improving the thermal stability of porcelain frits is the third specific aim of the proposed work. The strategies for developing improved porcelain frits involve reduction or elimination of leucite coupling/decoupling, stabilization of the leucite fraction, avoidance of metastable cubic leucite retention upon cooling from the porcelain firing temperature, and minimization of the effects to structural relaxation of the glass matrix.

Agency
National Institute of Health (NIH)
Institute
National Institute of Dental & Craniofacial Research (NIDCR)
Type
Research Project (R01)
Project #
1R01DE007806-01
Application #
3221584
Study Section
Oral Biology and Medicine Study Section (OBM)
Project Start
1986-08-01
Project End
1989-07-31
Budget Start
1986-08-01
Budget End
1987-07-31
Support Year
1
Fiscal Year
1986
Total Cost
Indirect Cost
Name
Medical College of Georgia (MCG)
Department
Type
Schools of Dentistry/Oral Hygn
DUNS #
City
Augusta
State
GA
Country
United States
Zip Code
30912
Twiggs, S Warren; Mackert Jr, J Rodway; Oxford, Amalia L et al. (2005) Isothermal phase transformations of a dental porcelain. Dent Mater 21:580-5
Mackert Jr, J Rodway; Sheen, Geoffrey W; Williams, Amalia L et al. (2003) Effects of local cooling rate and processing variables on leucite in dental porcelain. Int J Prosthodont 16:647-52
Mackert Jr, J R; Twiggs, S W; Russell, C M et al. (2001) Evidence of a critical leucite particle size for microcracking in dental porcelains. J Dent Res 80:1574-9
Mackert Jr, J R; Williams, A L; Ergle, J W et al. (2000) Water-enhanced crystallization of leucite in dental porcelain. Dent Mater 16:426-31
Mackert Jr, J R; Twiggs, S W; Williams, A L (2000) High-temperature X-ray diffraction measurement of sanidine thermal expansion. J Dent Res 79:1590-5
Khajotia, S S; Mackert Jr, J R; Twiggs, S W et al. (1999) Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains. Dent Mater 15:390-6
Mackert Jr, J R; Russell, C M (1996) Leucite crystallization during processing of a heat-pressed dental ceramic. Int J Prosthodont 9:261-5
Mackert Jr, J R; Williams, A L (1996) Microcracks in dental porcelain and their behavior during multiple firing. J Dent Res 75:1484-90
Denry, I L; Mackert Jr, J R; Holloway, J A et al. (1996) Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain. J Dent Res 75:1928-35
Mackert Jr, J R; Khajotia, S S; Russell, C M et al. (1996) Potential interference of leucite crystallization during porcelain thermal expansion measurement. Dent Mater 12:8-12

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