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A running-in and starved lubrication experiment is designed to investigate the heavy-duty scuffing behavior of piston ring coatings against cast iron (Fe) cylinder liner using the piston ring reciprocating liner test rig. The scuffing resistance of the piston ring with the chromium-based ceramic composite coating (CKS), and that with the thermally sprayed nickel-chromium-molybdenum coating (NCM) is compared at different nominal pressures (40~100 MPa) and temperatures (180~250 °C). With the failure time as a criterion, the rank order is as follows: NCM/Fe > CKS/Fe. Before the scoring occurs at the interface of the piston ring and cylinder liner (PRCL), the cast iron liner enters into a “polish wear” stage, and iron-based adhesive materials begin to form on the piston ring surface. With the macroscopic adhesion formation, the plastic shearing cycle causes surface damages mainly due to abrasive effects for the CKS/Fe pairs and adhesive effects for the NCM/Fe pairs.

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In order to investigate the friction and wear behavior between the nodular cast iron cylinder liner (Fe) and CuSn coated piston ring under heavy-duty conditions, piston rings with chromium(Cr) coating and CuSn-Cr coating were tested using the piston ring reciprocating liner test rig at the simulated working conditions of 56 MPa, 200 r/min, 190 • C. Compared with the Cr/Fe pair, the CuSn coating consumption of the CuSn-Cr/Fe pair made friction coefficient and cylinder wear loss decrease by 2.8% and 51.5%, respectively. Different size Sn patches worn from the CuSn coated piston ring were embedded into the cylinder liner surface based on the surface topography. This process was shown to reduce the surface roughness of a cylinder liner and form flatter plateau structures. Chemical elements analysis indicated that plateau structures on the cylinder liner surface matched with CuSn-Cr coated ring are helpful to promote the tribo-chemical reaction and generate the reactive products to protect the mutually contacted asperities.Although surface texturing on the cylinder liner or piston ring is an effective method to improve the wear behavior [7-9], various coatings prepared on the piston rings are also important means to improve heavy-duty friction and wear performance [10]. The CrN coating made by the physical vapor deposition or magnetron sputtering method has shown good high temperature resistance to adhesion [11][12][13][14]. In order to improve the thermal stability, oil compatibility, and internal stress of ceramic coatings, nanocomposite coatings (like TiN, TiAlN, TiSiN and TiSiCN) were developed to minimize the frictional losses and wear of piston rings in an automotive engine [15,16]. Shen et al. compared chromium-based ceramic composite (CKS) and nickel-chromium-molybdenum (NCM) coated rings sliding against cast iron liner and demonstrated that the anti-scuffing behavior of NCM is better than that of the CKS with the failure time as a criterion [17]. Wan et al. indicated that the presence of amorphous graphite-like carbon not only combated the scuffing damage and running instability effectively for conventional chromium-based coatings, but also improved the reliability and robustness of the piston rings [18]. Based on the increasing scuffing resistance experiments, diamond-like carbon (DLC) coated piston ring can protect the cast iron liner from scuffing up to 600 N normal load [19]. Its lower friction coefficient and wear loss were attributed to the formation of a mixed tribolayer [19][20][21]. But the existence of residual stress in the preparation process of DLC coating would cause the coating to crack. If stress concentration exerted on the contact interface was too high, it would lead to the coating wear or peeling [22].Compared with hard coatings, soft coatings have also attracted widespread attention for weakening the friction surface damage. Hamilton et al. investigated the relationship between macroscopic wear and the temperature of the MoS 2 coating [23]. Meng et al. indicated that the stabilized fricti...

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