Assuming zero taxes, the effect of increasing leverage in the capital structure of a firm...

The value of a business derives itself from the value of its cash flows, and not its capital structure. However, the availability of tax deductions on debt allow increasing the value of the business to equity holders by using the tax shield.

In the absence of taxes, there is no such advantage. Even if debt is nominally priced lower than the cost of equity, substituting debt for equity makes the remaining equity more risky and increasing the cost of equity to offset any advantage gained from the lower cost of debt. Changing the capital structure does not change the value of the firm, and this in essence is the conclusion of the Modigliani-Miller theorem.