Date of Award

9-13-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Cylinder bores, Engine valves, Lightweight alloys, Surface coatings, Surface roughness, Tribology

Supervisor

Alpas, A.

Rights

info:eu-repo/semantics/openAccess

Abstract

The objective of this study is to develop new tribological methods for reducing friction and wear of critical internal combustion (IC) engine components, namely engine valves and cylinder bores, either using lightweight materials or advanced coatings. Increasing the components’ durability by improving their wear resistance and optimizing vehicles’ fuel economy by controlling the friction are among this study’s principal goals. Most lightweight materials used in IC engines, Al and Ti alloys, suffer from higher wear rates than conventional cast irons (CI) and steels. Ti alloy valves are proposed to replace steel valves, but their high temperature wear resistance needs to be enhanced. Thus the oxide formation on Ti alloys and its behaviour during high temperature sliding were studied for the development of robust valve contact surfaces. Similarly, Al engine blocks are used to supersede CI blocks, but the wear resistance of cylinder bores needs to be improved. Accordingly, the oxidational wear on thermal spray (TS) coated Al bores and its effect on roughness and lubricated friction were studied. Diamond-like carbon (DLC) coatings as adhesion mitigating counterfaces for Al and Ti alloys exhibit low friction properties, thus emerge as promising coatings for both valve seat inserts and piston rings. Thermal oxidation (TO) treatment performed on Ti alloys generated a TiO2 layer with an oxygen diffusion zone underneath. Compared to untreated alloys, lower sliding wear losses were achieved on TO-Ti alloys at 350 °C (intake valve working temperature) and 550 °C (exhaust valve working temperature). The propensity of crack growth in oxide layers was also reduced as determined by high temperature impact tests conducted using a self-built wear and impact tribometer. The high temperature friction and wear behaviour of W-DLC coating deposited by physical vapour deposition were studied. Low coefficient of friction (COF) values and wear rates were maintained up to 500 °C. The transfer layers consisting of amorphous carbon and monoclinic γ-WO3, as determined by transmission electron microscopy, were responsible for the low friction and wear. Requirements of cylinder bores differ from those of engine valves, as they operate under transitions of lubrication regimes. Thus Stribeck curves were constructed to screen suitable piston ring coatings and bore coatings with proper surface honing to reduce COF. TS coatings, deposited by a plasma transferred wire arc method, could improve the durability of Al bores, but their friction properties remain to be studied especially when oxidational wear occurs. Reciprocating tests were performed with in-situ Raman spectroscopy using base oil with and without zinc dialkyldithiophosphate (ZDDP). The ZDDP containing oil refrained oxidational wear and delayed the transition from the boundary to the mixed lubrication regime. This observation emphasized the importance of using smooth bore surfaces to reduce COF. The friction behaviour of smooth TS coatings were studied with reference to smooth and rough CI liners. TS coatings reduced COF mainly in the mixed lubrication regime due to the improved oil retention capability. DLC coated rings further reduced the boundary lubricated COF, as the formation of carbon transfer layers was favored when interfacial contact was more prominent. From an engineering point of view, this study showed the importance of understanding the microstructural aspects of sliding damage process in designing new surface engineering methods. These methods include generating preferential oxides to reduce high temperature friction and wear for lightweight engine valves; and developing durable and energy-efficient surfaces using the synergy of TS coatings, surface honing, lubricant additives and piston ring coatings for lightweight cylinder bores.

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