Date of Award

Spring 2016

Degree Type

Thesis

Degree Name

MS Metallurgy

Department

Metallurgical Engineering

First Advisor

Alan Meier

Second Advisor

KV Sudhakar

Third Advisor

Bruce Madigan

Fourth Advisor

Gagan Saini

Abstract

Diamond tools are increasingly gaining importance as cutting and drilling materials for a wide variety of industrial applications. Polycrystalline diamond (PCD) is the main ultrahard material commercially used in the oil and gas drilling industry. In this study, a reactive brazing process was developed to join polycrystalline diamond (PCD) to WC-13 wt% Co, to form the cutter for fixed-cutter drill bit applications.

Most nonmetals including polycrystalline diamond are not wet by and cannot easily be joined with conventional brazing alloys due to their chemical stability. The experimental approach was first to analyze the effect of adding an active metal (Ti, Zr, or V) to copper, silver, or a silver-copper eutectic alloy on the wettability of diamond and WC-Co substrates. Sessile drop tests were utilized to compare wettability between the liquid braze alloy and the substrate. The addition of Ti, Zr, and V decreased the apparent contact angle, which improved both the wetting and bonding behavior between braze alloy and diamond substrate. For all three alloy systems evaluated, all three base alloys (Cu, Ag, and Ag-Cu) with active metal additions (Ti, Zr, or V) exhibited good wettability on diamond and WC-Co substrates.

Microstructural analysis of the diamond and WC-Co sessile drop samples was performed via scanning electron microscopy (SEM) to characterize the interfacial layers formed. Two different types of reactions were observed between the braze alloys and the WC-Co substrates: reduction and dissolution reactions. For the diamond sessile drop samples, only intermetallic solidification products were observed at the interface for the Ag-Cu eutectic based alloys with additions of 2 and 5 wt% Ti. SEM/EDS analysis revealed that the chemical changes at the interface between the braze alloy and diamond substrate were in agreement with the intermetallic solidification products predicted from the phase diagrams. Based on the Gibbs energies of formation for carbides, it is predicted that the formation of TiC is thermodynamically favored at the interface. However, no TiC reaction product was identified within the resolution of SEM/EDS analysis possibly because the TiC reaction layer is too thin.

Based on the results of the wetting studies, an effort was made to optimize the shear strength of diamond brazed to WC-Co. This phase study was focused on the relationship between the braze alloy composition, the braze layer thickness, the brazing thermal cycle, the braze microstructures and the resulting joint mechanical properties. The average shear strength for Ag-2 wt% Ti alloy was approximately constant in the braze thickness range of 0.1 to 0.2 mm. It was observed that the brazed samples failed in the silver braze layer. More visible cracking and larger cracks were observed on the surface region of diamond substrates of the joint thickness of 0.2 mm for the Ag-Cu-2 wt% Ti alloys. It is possible that thermal stresses generated from coefficient of thermal expansion (CTE) mismatch resulted in the formation of interfacial cracks. The Ag-Cu eutectic alloy with addition of a 2 wt% Ti has the highest average shear strength of 95 MPa when the hold time is 30 minutes and the cooling rate is 5 °C/min.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Metallurgical Engineering

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