Stress-strain and thermal expansion characteristics of a phosphate-bonded investment mould material for dental super plastic forming.
OBJECTIVE
The stress-strain and thermal expansion characteristics of a phosphate-bonded investment material were measured to determine its suitability as a die material for superplastic forming of dental appliances. Titanium alloy denture bases and implant superstructures have been fabricated successfully using this investment material, but a greater understanding of its properties was sought to optimize the forming technique before exploitation could proceed. The effect of a boron nitride powder on the properties of phosphate-bonded investment was also investigated.


METHODS
Three-point bend test specimens were cast in the phosphate-bonded investment material. Four groups of test samples were produced at specified water-to-powder ratios (w/p ratios). For one group 5 wt.% boron nitride powder was added to the investment powder before mixing with water. For stress-strain measurements specimens were heated to 920 degrees C and held for 30 min prior to the application of load. For thermal expansion measurements specimen length was measured for the entire heating and cooling cycle.


RESULTS
Curves of stress versus strain and thermal expansion were recorded for all four test groups. For increasing initial w/p ratio failure stress decreased with lowest failure stresses for specimens containing boron nitride. Strain to failure also decreased with increasing initial w/p ratio, except for the group containing boron nitride which showed relatively large elongations to fracture. Specimen surfaces with higher initial w/p ratios were observed to have larger surface porosity. Thermal expansion curves for three groups were very similar, but differed considerably from the group containing boron nitride.


CONCLUSIONS
The lowest initial w/p ratio should be used for highest hot strength. Hot deformation of phosphate-bonded investment dies for superplastic forming is an issue that requires further investigation because of potential die deformation and the resulting loss of fit between the superplastic superstructure and implant components. Boron nitride could be used in phosphate-bonded investment to decrease thermal expansion, but the consequences of reduced hot strength and increased strain to failure may make the modified material unsuitable as a die for superplastic forming.
Curtis
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RV
UMDS, Dental School, London, UK.
D000497:Alloys; D000704:Analysis of Variance; D001896:Boron Compounds / Q000737:chemistry; D003722:Dental Alloys / Q000737:chemistry*; D003734:Dental Casting Investment / Q000737:chemistry*; D003735:Dental Casting Technique; D003799:Dental Stress Analysis; D006358:Hot Temperature; D008422:Materials Testing; D003765:Models, Dental; D010710:Phosphates; D018583:Pliability; D016062:Porosity; D011208:Powders; D013718:Tensile Strength; D014025:Titanium / Q000737:chemistry; D014867:Water
D000497:Alloys; D001896:Boron Compounds; D003722:Dental Alloys; D003734:Dental Casting Investment; D010710:Phosphates; D011208:Powders; D014867:Water; C031462:titanium alloy (TiAl6V4); D014025:Titanium