Research On Thermal Fatigue Properties Of Laser Textured Mo Coatings On Cast Iron Surface

In order to solve the friction damage problem of piston ring-cylinder liner of diesel engine,self-lubricating Mo coating is usually prepared on the surface of piston ring by plasma spraying.However,thermal fatigue problems such as coating cracking,fault and peeling failure still appear under alternating load and alternating high and low temperature;Laser surface texture technology can effectively strengthen the mechanical bonding between coating and substrate,therefore,this study mainly combines laser surface groove texture with plasma spraying Mo coating effectively,and studies the influence of micro-texture morphology on thermal fatigue resistance of Mo coating by means of experimental exploration and theoretical analysis,and elucidates the thermal fatigue behavior mechanism of interfacial grooves textured Mo coating.It is of great significance to the stable service of engine ring-sleeve system.The main study contents are as follows:Two kinds of micro-texture morphology-linear groove and circular pit-were designed.The interfacial grit-blasting pretreatment was used as the experimental control group.Mo coating was prepared on the substrate surface by atmospheric plasma spraying,and the coating showed a typical layered structure.Scanning electron microscopy(SEM),XRD and microhardness analyzer were used to characterize the coating morphology and other microscopic characteristics,and the cross section porosity of groove texture coating is only 9.1%.The coating shows good compaction.And the mechanical bonding between the textured matrix and the spraying coating is the main part.In view of the theoretical analysis of thermal matching between coatings and substrates,the effects of texture unit shape and morphology parameters on thermal fatigue resistance of Mo coatings were discussed.Due to the smaller diameter to depth ratio and higher surface roughness,the coating of linear groove texture has higher cohesive strength and toughness,which can hinder the growth of thermal fatigue cracks in the groove position.The depth of the groove texture is proportional to the thermal fatigue resistance of the coating.The coating sample with laser scanning for 5 times shows good thermal fatigue resistance in the thermal cycle experiment.The distribution spacing of grooves has a direct effect on the area ratio of the texture and the interface bonding mode.When the distribution spacing of grooves is 200 μm,the coating shows better thermal fatigue resistance.The coating mainly falls off between the layers close to the bonding surface,indicating that the mechanical bonding strength of the texture to the coating is greater than the cohesion strength of the spray coating.Based on the synergistic effect of thermal mismatch stress at the coat-matrix interface and high temperature oxidation of Mo coating on thermal fatigue performance of coatings,the thermal fatigue behavior of grooved textured coatings under different temperature gradients was compared and analyzed.With the increase of temperature gradient,the oxidation phenomenon of Mo coating intensifies when the alternating thermal stress is concentrated.When the upper limit temperature reaches 500℃,the high temperature oxidation and crack growth promote each other,which aggravates the failure of Mo coating.Crack nucleation is more likely to occur at the pore defects and interlayer bonding sites of the coating,and the cracks propagate zigzag under the synergistic action of thermal shock and oxidation until multiple main cracks overlap and fall off and fail.Cracks in different degrees and forms are blocked,passivated,bifurcated,deflected or even terminated near the grooves,which reduces the crack growth rate and effectively prolong the thermal fatigue life of the coating improves the thermal fatigue resistance of the coating.