The surface finish of cylinder bores in an automotive engine plays a significant role in lubrication and friction reduction. The traditional honing leaving cross hatch marks with peaks and valleys served well for many years.  Improved surface finish through plateau honing with fewer peaks has been in use in many engines.  The recent trend is thermal spray coating on aluminum blocks eliminating cast iron liners to save weight.  There are several variants of the coating depending on the process used; atmospheric plasma spray, twin wire arc spray, electric wire arc spray, plasma transferred wire arc, etc. Most of these coatings have \<2% porosity and iron oxide (wuestite) on the honed surface which helps retaining engine oil but not necessarily help in reducing friction.  This paper will briefly describe development of high porosity thermal spray coatings deposited by plasma transfer wire arc process initially on free standing liners with the focus on friction reduction.  The coating is mechanically bonded to the parent aluminum surface by mechanical roughening followed by coating deposition and honing. The initial work was focused on free-standing aluminum liners to understand critical coating deposition parameters and their levels through a Design of Experiments (DoE). This was followed by developing the honing process to expose pores on the liner surface and identifying the key process parameters. The mechanism of pore generation was identified.  A lot of efforts were made in quantifying and characterizing pores in the absence of any standard method. The team established detailed characterization method for defining percent porosity on the surface, pore size, volume, and distribution using 2D stylus profilometry, 2D (Keyence), and 3D optical microscopy techniques, and analytical model. In addition, detail surface roughness characterization was also performed.  The microstructure of the coating was characterized in addition to hardness and thermal conductivity.  The process of coating deposition and honing was applied to cylinder bores of in-line four cylinder engine blocks.  Friction evaluation was conducted on laboratory bench test rig where a section of production and other coated rings reciprocated against a liner section.   Friction evaluation was also conducted under motoring conditions on coated engine blocks as well as on engines built with coated blocks with speed ranging from 700 RPM to 5000 RPM and at engine oil temperature ranging from 35C to 120C. The friction reduction potential will be presented on liner sections through laboratory bench tests and engine blocks with motored friction tests. It was observed that friction decreased with increasing porosity and also the selection of appropriate ring face coating can provide additional friction benefit.  


High Porosity Thermal Spray Coating: Development, Characterization, and Friction Evaluation

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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