Study On The Failure Mechanism Of Tungsten Carbide Coating Under The Combined Action Of Temperature And Cyclic Stress

Two kinds of thickness of WC-17co coating (0.15mm and0.3mm) on718nickel-basealloy are prepared by high velocity oxy-fuel spraying(HVOF) technology, and a systematicstudy of the microstructure, mechanical properties and low cycle fatigue performance ofWC-17Co coating sample respectively in three different environment (room temperature,150℃6h and300℃6h) is made. SEM/BSE technologies are adopted to make acomparative analysis of micro-structure morphology of coating surface and cross-section, andmake a comparative analysis of phase and element content of the coating using X-ray andEDS technology. The three-point bending test is used to measure the Young`s modulus (E) ofthe coating, and the indentation method is used to test its hardness and fracture toughness,while using the split method, the residual stress of coated specimens are tested along thedirection of the coating thickness. Finally, make a test and comparative analysis ofenvironment low cycle bending fatigue performance of nickel-based alloy/WC coatingspecimen, as the fatigue fracture and failure mechanisms is analyzed. Main conclusions:1) The microhardness of the untreated coating is1436HV, Young’s modulus is303.76Gpa, and fracture toughness is13.80MPa m1/2; after the heating treatment of150℃6h, the WC-17Co coating’s microhardness is1423HV, Young’s modulus is291.35Gpa, andfracture toughness is13.80MPa m1/2; after the heating treatment of300℃6h, the coating’smicrohardness is1392HV, Young’s modulus is271.55Gpa, and fracture toughness is13.80MPa m1/2。2) WC-17Co coating sample residual stress test results show that: there is compressiveresidual stress in WC-17Co coating, and it distributes in gradient along the direction of thecoating thickness. The compressive stress arrives at the maximum value of161.7MPa for theuntreated thin coating (0.15mm) sample at the thickness of0.09mm, and the average residualstress in the coating is66.1Mpa. The average residual stress of the coating which is under theheating treatment of150℃6h is34.1MPa, decreased by48.4%; the average residual stressof the coating which is under the heating treatment of300℃6h is13.0MPa, decreased by80.3%. The compressive stress arrives at the maximum value of329.8MPa for the untreatedthick coating (0.30mm) sample at the thickness of0.24mm, and the average residual stress in the coating is134.7MPa; the average residual stress of the coating which is under the heatingtreatment of150℃6h is62.5MPa, decreased by53.6%; the average residual stress of thecoating which is under the heating treatment of300℃6h is39.7MPa, decreased by70.5%.3) Fatigue test results show that: thick coating(0.3mm) has a better fatigue property thanthin coating (0.15mm) at home-temperature small bending amplitude(A=6mm); fatigueperformance significantly reduces at home-temperature large bending amplitude(A=8mm); inisothermal environment of150°C and300°C, the fatigue properties of nickel-based alloy/WCcoating system become obviously worse; whether large or small bending amplitude, fractureanalysis shows that: the fatigue source region of a thin coating (0.15mm) appears mainly inthe defects between coating and substrate interfaces and expand to both sides, which lead tothe failure of nickel-based alloy/WC coating specimen; while that of the thick coating (0.3mm)mainly forms at the surface of the coating, expand along the thickness till to the failure.