The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings

STATEMENT OF PROBLEM: Different types of investment materials affect the formation of a surface contamination zone within commercially pure titanium (cpTi) castings. This contamination zone may possibly alter the mechanical properties of cast titanium, which may be problematic for castings used in the fabrication of removable and fixed prostheses. PURPOSE: The purpose of this study was to evaluate the effect of different types of investments on the extent of contamination zone and the modulus of elasticity, yield strength, elongation, and hardness of cpTi castings. MATERIAL AND METHODS: Forty wax patterns were fabricated according to ISO 9693 for tensile testing. The patterns were divided into 2 groups of 20 patterns each, invested, and cast in pairs using cpTi. The first group (P) was invested with a phosphate-bonded silica-based investment material (Ticoat S+L), and the second group (M), with a magnesia-alumina investment material (Rematitan Ultra). Investment materials were examined by x-ray diffraction analysis (XRD). One specimen from each group was sectioned and prepared for metallographic observation. The extent of the contamination zone was determined by scanning electron microscopy, using back-scattering electron imaging and energy dispersive spectroscopy analysis, as well as microhardness testing. The tensile strength of the specimens was determined in a universal testing machine. From the derived tensile curves, the modulus of elasticity, yield strength, and percentage elongation were calculated and statistically evaluated among the groups using the Student t test (alpha=.05). Three fractured specimens from each group were examined by scanning electron microscopy to determine the mode of fracture. RESULTS: XRD analysis showed that silica and magnesia were the dominant phases of Ticoat S+L and Rematitan Ultra, respectively. The contamination zone was found to extend 50 to 80 mum for the P specimens and 15 to 20 mum for the M specimens. No significance difference was found for the modulus of elasticity (P=85 +/- 11 GPa, M=79 +/- 13 GPa), whereas significant differences were found for the yield strength (P=462 +/- 48 MPa, M=321 +/- 54 MPa; P<.001) and percentage elongation (P=12 +/- 2, M=21 +/- 7; P=.002) between the groups tested. The fracture mode was brittle externally and ductile internally for both groups. CONCLUSIONS: According to the results of this study, the extent of the contamination zone as well as the yield strength and percentage elongation of the cpTi castings were significantly affected by the type of the investment material.