Tensile Fracture Test Analysis And High Temperature In-Situ Performance Characterization Of Nickel-Graphite And Aluminum-Silicon Abradable Coatings

The abradable coating is widely used in the aero-engine airway clearance material,mainly to improve engine efficiency,reduce fuel consumption,and protect the role of the blade tip.However,long-term service in harsh and complex environment makes the coating strength and wearability inevitably deteriorate.Therefore,it is important to study the mechanical properties and wearability of the coating after long-term service to improve flight safety and engine efficiency.In this thesis,the evolution of mechanical properties of sealant coatings with heat treatment time and the mechanism of action are investigated for the first time after long time heat treatment.The main contents and conclusions of this thesis are as follows.(1)In this work,nickel-graphite and aluminum-silicon sealant coatings were firstly heat-treated at 200 ℃,300 ℃ and 400 ℃ for 400 h,800 h,1200 h,1600 h and2000 h,respectively;then the tensile strength of the coatings before and after the heat treatment was tested and analyzed based on the digital image correlation method.The results showed that the strength of the coatings under all conditions except for the nickel-graphite coating heat-treated at 200 ℃ and the aluminum-silicon coating heattreated at 300 ℃ showed a great degree of deterioration after 2000 h of heat treatment.Since the grain growth of the Nickel-graphite coating at 200 ℃ was slow,the grain growth of a certain size at the later stage of the heat treatment could hinder the propagation of the shear band,which led to the increase of the strength of the Nickelgraphite coating after 1200 h of heat treatment at 200 ℃.The ultimate tensile strengths of the Nickel-graphite and Aluminum-silicon after heat treatment at 200 ℃,300 ℃,and400 ℃ for 2000 h were 173.39 MPa,145.78 MPa,50.80 MPa,and 269.83 MPa,373.62 MPa,315.51 MPa,respectively.These parameters can directly reflect the mechanical strength of the coatings for reasonable maintenance of the coatings.(2)High temperature and ultra-high speed bench testers were used to analyze the frictional wear of Nickel-graphite and Aluminum-silicon at room temperature,100 ℃,200 ℃,300 ℃,and 400 ℃,respectively,to investigate the wear mechanisms of the coatings at different temperatures.The results showed that Nickel-graphite has a larger porosity and thus its wearability is better than that of Aluminum-silicon coatings.Due to the increase in temperature,the nickel-graphite and aluminum-silicon coatings softened,hardness and tensile strength decreased,and thermal stress increased,leading to a change in the main wear mechanism of the coatings from abrasive and adhesive wear to fatigue wear.(3)The tensile strengths of Ni-graphite and Al-Si coatings were tested at different temperatures using high temperature in-situ tensile tests.The results showed that the strength of Ni-graphite coating reached a minimum value of 29.41 MPa at 300 ℃;the strength of Al-Si coating reached a minimum value of 65.45 MPa at 400 ℃.The reasons can be attributed to the softening of the coating by heat and the increase of defects within the coating due to the differences in the thermophysical properties between the substrate,the bonding layer,the coating and the internal phases of the coating.As well as the growth of silicon grains,which produce cuttings on the coating.