Do not look for maximising returns in debt funds. They are for safety, security and stability: the 3 S in your portfolio. The sub classes in debt funds called overnight and liquid funds are for Liquidity.
Having said that, many debt funds (gilt class) have given above 10% returns because of the way interest rates have moved recently. But this is not gurantee-able.
Invest in a mix of debt funds with the help of a SEBI Registered Investment Advisor.