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Laser cladding technology was utilized to construct protective coatings on the surfaces of service materials and to study the mechanical and fretting wear properties of the coatings. The coating mainly consists of diffuse Cr 2 AlC phase, interdendritic Cr 7 C 3 phase and dendritic Cr-based solid solution. The hardness of the coating was enhanced to 690 HV, which is 3.1 times higher than the hardness of the substrate. The change in H3/Er2 values of the coating and the substrate were calculated by nanoindentation to be 0.04 and 0.024, respectively, and the anti-fretting wear properties of the coating was excellent. The statistical distribution of the Schmidt factor of the coatings is between 0.41 and 0.5, which indicates that the coatings have a high probability of initiating slip and high plastic deformation capacity. For those grains with large Schmidt factors, dislocations can be easily activated in response to applied stresses. Strength is the first element of wear resistance, and with sufficiently high strength, the plasticity enhancement can be very effective in improving the fretting damage resistance. After the fretting wear test in liquid LBE, the Cr-Al-C composite coating has a huge increase in wear resistance compared to the substrate. The wear resistance of the coating was 33 times that of the 316 stainless steel substrate at a test temperature of 450 °C [ABSTRACT FROM AUTHOR]

To simulate the variational working conditions of the titanium fastener in the aero-engine, variable cycle fretting wear tests were carried out for normal loading (80/40 N), frequency (100/30 Hz), displacement amplitude (80/40 µm) and operating temperature (500/350 °C) in a self-lubricating composite coating on TC21 titanium alloy substrate, and the fretting wear behavior and wear mechanism were thoroughly investigated. The results shows that the coefficient of friction (COF) and the wear volume present a significant difference under variable cycle fretting conditions. The specimen exhibits a relatively stable COF under variable operating temperature. Whereas for the specimen under variable displacement amplitude presents a lowest wear volume. The adhesive wear is mainly found in the specimens under variable normal loading and frequency. Whereas for the specimens under variable displacement amplitude and operating temperature, abrasive wear and oxidation wear is mainly dominated, respectively. The dissipated energy calculation results demonstrate that the variable displacement amplitude and operating temperature have greater impact on the fretting wear behavior. The microstructure analysis in the cross-section of specimens reveals that the sub-surface is composed of amorphous-nano crystalline layer, plastic deformation layer and the origin coating. Additionally, the wear mechanism of the specimens under variable fretting conditions were deeply revealed. This work can provide essential information on fretting behavior and mechanism under the variational working conditions. • The fretting wear behavior and mechanism of a composite coating were investigated under variable cycle fretting conditions. • The variation of displacement amplitude has a significant effect on fretting wear. • The amorphous-nano crystalline layer provides a protective role and suppresses wear effectively. • This work can provide essential information on fretting behavior and mechanism under the variational working conditions. [ABSTRACT FROM AUTHOR]

Fretting wear is one of the main factors restricting the fastener service reliability for the high-temperature titanium alloys. Enhancing the high-temperature fretting wear resistance of titanium alloys with laser cladding coatings is an effective method. Thus, in this work, a self-lubricating composite coating was designed and fabricated on the surface of TC21 titanium alloy with laser cladding. Special attentions were made to the effect of the fractions of the hard and self-lubricating phases on high-temperature fretting resistance. In addition, the contribution of laser cladding power was also concerned. The primary phases of the self-lubricating composite coatings are α-Co, CaF 2 , WC, TiC and Cr 23 C 6 , and the reasons for the formation of TiC and Cr 23 C 6 are investigated from a thermodynamic point. The prepared composite coatings are dense and uniform. The crystallographic characteristics and the volume fractions of the hard phases (WC, TiC and Cr 23 C 6) and self-lubricating phase (CaF 2) were revealed, and the effect of grain size on the coating properties was analyzed. The hardness of the coatings were all maintained around 700 HV 0.2 , which was significantly higher than that of the substrate. The composite coating significantly improves the friction and wear resistance at 500 ℃. The hard phase was also found to be the most resistant to friction and wear. The frictional performance of the coating gets better when the hard phase and the self-lubricating phase increase. However, the hard phase contributes more to the high-temperature friction and wear resistance than the self-lubricating phase and the wear volume is 1/40 that of substrate. For the laser cladding power, the higher power can lead to a higher fraction of CaF 2 but a lower fraction of hard phase, causing a decline in COF but an increase in wear volume. The wear mechanisms of the coating and the substrate were analyzed, the subsurface stress distribution state of the wear scars were further analyzed. The current results present new insights on the anti-wear design strategy to the composite coatings consist of hard and self-lubricating phase. [Display omitted] • The high-temperature fretting wear volume is reduced about 40 times. • The strategy to the addition of the hard and lubricating phase were revealed. • The hard phase has a greater impact on the high-temperature fretting wear. • This work guide for the protection of the titanium alloys. [ABSTRACT FROM AUTHOR]

• The Co-based alloy/WC/CaF 2 composite coating is fabricated by laser cladding to improve the fretting wear resistance. • The surface microhardness of the composite coating can reach to 600 HV. • The fretting wear resistance are significantly improved, which can lead to the decrease of the COF and the wear loss. • This work guide for the protection of heat transfer tube in nuclear power plants. The coating fabricated by laser cladding on the surface has been an effective way to strengthen and enhance the wear resistance of alloys. However, the traditional Co-based alloy coatings rarely pay attention to the addition of self-lubrication constituents to form the composite coating. In addition, the effects of the Co-based composite coating on fretting wear resistance were few addressed. In this study, a Co-based alloy/WC/CaF 2 composite coating was fabricated by laser cladding on the surface of 316 stainless steel to improve the surface hardness and self-lubrication performance. The microstructure, phase constituents and fretting wear resistance at both room temperature (RT) and 500 °C were investigated. Results confirm that the composite coating is consisting of α-Co, Cr 23 C 6 and WC phases. And the F element is effectively dissolved into the α-Co phase. The fabricated composite coating is dense and uniform. The microhardness of the composite coating can reach to 600 HV 0.2 due to the presence of WC and Cr 23 C 6 phase. In addition, the grain refinement is another reason resulting in the higher microhardness. Fretting tests demonstrate that the fretting wear resistance at both RT and 500 °C are significantly improved by the composite coating, which can lead to the decrease of the COF and the wear loss. The lower COF can be ascribed to the dissolved F element in α-Co phase, especially at high fretting temperature. Additionally, the composite coating can effectively prevent the oxidation during fretting both at RT and 500 °C. This work provides an excellent method for the metallic materials to improve fretting wear resistance at both RT and high temperature. [ABSTRACT FROM AUTHOR]