Study On The Toughness And Fracture Damage Mechanism Of CrN-Pt Coatings

To improve the service life of electrical connectors in the marine environment,CrN coatings doped with Pt elements were used for surface protection.Previous studies found that the coating has excellent electrical conductivity and thermal stability.In this project,CrN coatings doped with different contents of Pt elements were prepared on316L stainless steel substrate to reveal the influence law of Pt element content on the strength and toughness of CrN coatings and to analyze the damage mechanism when cracks are generated.In this thesis,CrN coatings(Pt content of 0),CrN-Pt(Pt content of 5.47 at.%)coatings and CrN-3Pt(Pt content of 15.00 at.%)coatings were prepared on 316L stain-less steel substrate using plasma-enhanced magnetron sputtering technology for elec-trical connectors operating in marine environment.The microstructure morphology of each coating was observed by SEM,and the hardnesses of 17.86 GPa,21.57 GPa,and20.66 GPa and the moduli of elasticity of 247.7 GPa,284.6 GPa,and 261.5 GPa for the three coatings were obtained by nanoindentation test,respectively.H/E and H~3/E*~2were introduced to evaluate the coating strength and toughness,and the H/E of the three coatings were 0.072,0.076,and 0.079,and the H~3/E*~2 were 21.07,32.30,and 30.89,respectively.The scratch test and finite element simulation results showed that all three coatings produced composite cracks,and the CrN coating had the least damage and the smallest three-way stress gradient;the CrN-Pt coating had increased damage and the three-way stress gradient;the CrN-3Pt coating had the heaviest damage and the largest three-way stress gradient.The simulated vacuum annealing processes at 400°C,600°C and 800°C revealed that the coating sprouted type I cracks with the largest positive stress gradient in the X-direction(S11)for the CrN coating and the smallest S11 stress gradient for the CrN-Pt coating.At the end of annealing at 400°C and 600°C,the CrN coating had the worst strong toughness and the CrN-3Pt coating had the best strong toughness;at the end of annealing at 800°C,the CrN coating had the worst strong toughness and the CrN-Pt coating had the best strong toughness.The following conclusions are obtained in this study:Pt elements can improve the denseness of CrN coatings and reduce the roughness of the coatings,and the more Pt elements are doped,the better the denseness of the coatings;Pt elements can improve the hardness,elastic modulus,yield strength and toughness of CrN coatings,and CrN-Pt coatings have the highest hardness,elastic modulus and toughness,and CrN-3Pt coatings have the best yield strength.The doping of Pt elements in CrN coatings dis-rupts the co-grid growth of the coating,reduces the bond strength between the coating and the substrate,and the coating damage increases with the increase of Pt element content.The coating fracture damage mechanism is that the positive stresses in the X-direction(S11)and Y-direction(S22)induce cracks along the X-direction on the surface of the CrN coating,and S11 induces cracks along the Y-direction on the surface of the CrN-Pt and CrN-3Pt coatings;S22 dominates the main crack extension along the Y-direction;S11 and S22 together induce the main crack extension along the bonding surface.The different thermal expansion coefficients of the coating and the substrate during annealing lead to the first sprouting of type I cracks at the interface,and S11promotes the expansion of cracks to the coating surface.Doping the CrN coating with Pt elements for annealing can significantly inhibit the emergence and expansion of cracks and promote the alloying reaction of the coating.At the same time,the annealing of CrN coating with Pt element can effectively improve the toughness of the coating.The results of this thesis provide a more comprehensive theoretical support for the study of the toughness properties and fracture damage mechanism of CrN-Pt coatings.