Interpolated Selective Area Mechanical Roughening for Thermally Sprayed Engine Bores

Thermally sprayed engine bores require surface preparation prior to coating to ensure adequate adhesion. Mechanical roughening methods produce repeatable surfaces with high adhesion strength and are attractive for high volume production. The currently available mechanical roughening methods are finish boring based processes which require diameter-specific tooling and significant clearance at the bottom of the bore for tool overtravel and retraction. This paper describes a new mechanical roughening method based on circular interpolation. This method uses two tools: a peripheral milling tool, which cuts a series of concentric grooves in the bore wall through interpolation, and a second rotary tool which deforms the grooves to produce an undercut. This method produces equivalent or higher bond strength than current surface preparation methods, and does not require diameter-specific tooling or bottom clearance for tool retraction. In contrast to boring-based methods, it also permits roughening and coating only a portion of the bore (e. g. the ring travel portion), reducing steel wire and energy consumption and enabling the elimination of a bimetallic cut on the joint face of the block. Following a review of existing mechanical roughening methods, the interpolation method is described, including finite element analysis, adhesion testing to optimize profile dimensions, and preproduction testing. Finally, low volume production experience for a V8 engine and the application of the method to brake rotor mechanical roughening are discussed.