Mechanical properties of dental investment materials

Measurement of the elastic modulus (E) of investment materials has been dif ficult because of their low strength. However, these values are essential f or engineering simulation and there are many methods available to assess th e elasticity of materials. The present study compared two different methods with one of the methods being non-destructive in nature and can be used fo r specimens prepared for other tests. Two different types of investment mat erials were selected, gypsum-and phosphate-bonded. Method 1 is a traditiona l three-point bending test. Twelve rectangular bars with dimension of (70 x 9 x 3 mm) were prepared and placed on supports 56.8 mm apart. The test was conducted at a cross-head speed of 1 mm/min by use of a universal testing machine. The load applied to the test specimen and the corresponding deflec tion were measured until the specimen fractured. The E value was calculated from a linear part of the stress-strain plot. Method 2 is an ultra micro-i ndentation system to determine near surface properties of materials with na nometer resolution. The measurement procedure was programd such that the sp ecimens were indented with an initial contact force of 5 mN then followed b y a maximum force of 500 mN. Measurement consisted of 10 indentations condu cted with a spherical stainless steel indenter (R = 250 mu m) that were equ ally spaced (500 mu m). The E value rose asymptotically with depth of penet ration and would approach the three-point bending test value at approximate ly four time's maximum contact depth for both materials. Both methods are p ractical ways of measuring the E of investment materials. (C) 2000 Kluwer A cademic Publishers.