that collect in a static flow area around the tool in certain drilling applications in the particular embodiment depicted the mud ringing occurs on a radially outward facing surface of the tool body as well as on the outwardly facing surface of the blade i e the side of the blade facing the borehole wall this surface of the tool is sometimes referred to as the mud ringing zone generally the surface of the tool that includes the mud ringing zone is at an outermost radial extent of the tool body however it will be understood that the invention is not limited in this regard as the location of mud ringing can vary based on the particular tool type and the drilling application exemplary embodiments of the present invention are intended to improve the hardness and therefore the wear and abrasion resistance of vulnerable downhole tool surfaces e g a mud ringing zone or a contact zone which routinely contacts the borehole wall exemplary method embodiments in accordance with the present invention include at least one step in which a vulnerable tool surface is friction stirred so as to increase the hardness of the surface exemplary tool embodiments in accordance with the present invention include at least one hardened outer surface having a friction stir zone certain embodiments of the invention are described below with respect to an underreamer e g as depicted on fig2 it will be understood that the invention is explicitly not limited to such underreamer embodiments the invention is potentially applicable to substantially any downhole tool having an outer surface that is vulnerable to abrasion and wear for example methods in accordance with the present invention may be advantageously applied to.