Surface and elemental alterations of dental alloys induced by electro discharge machining (EDM)

Zinelis, Spiros

doi:10.1016/j.dental.2006.03.021

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Volume 23, Issue 5 , Pages 601-607, May 2007

Surface and elemental alterations of dental alloys induced by electro discharge machining (EDM)

Department of Biomaterials, School of Dentistry, University of Athens, Thivon 2, Goudi 115 27, Athens, Greece

Received 18 October 2005; received in revised form 1 March 2006; accepted 9 March 2006.

Abstract

Objectives

To evaluate the surface and elemental alterations induced by electro discharge machining (EDM) on the surface of dental cast alloys used for the fabrication of implant retained meso- and super-structures.

Methods

A completed cast model of an arch that received dental implants was used for the preparation of six wax patterns which were divided into three groups (Au, Co and Ti). The wax patterns of the Au and Co groups were invested with conventional phosphate-bonded silica-based investment material and the Ti group with magnesia-based investment material. The investment rings of the Au and Co groups were cast with an Au–Ag alloy (Stabilor G) and a Co–Cr base alloy (Okta C), respectively, while the investment rings of group Ti were cast with cp Ti (Biotan). One casting of each group was subjected to electro discharge machining (EDM); the other was conventionally ground and polished. The surface morphology and the elemental compositions of conventionally and EDM-finished surfaces were studied by SEM/X-ray EDS analysis. Six spectra were collected from each surface employing the area scan mode and the mean value of each element between conventionally and EDM-finished surfaces was statistically analyzed by t-test (a=0.05). Then the specimens of each group were cut perpendicular to their longitudinal axis and after metallographic grinding and polishing the cross-sections studied under the SEM.

Results

The EDM surfaces showed a significant increase in C due to the decomposition of the dielectric fluid during spark erosion. Moreover, a significant Cu uptake was noted on these surfaces from the decomposition of the Cu electrodes used for EDM. Cross-sectional analysis showed that all alloys developed a superficial zone (recast layer) varying from 2μm for Au–Ag to 10μm for Co–Cr alloy.