Formation Mechanism And Mechanical Properties Of In Situ Fabricated Nbc Layers On Gray Cast Iron

Niobium carbide(NbC)reinforced layers on gray cast iron(Fe)substrates are fabricated by in situ techniques.The microstructure,chemistry composition,and phase distribution of the reinforcing layers are characterized.Based on reaction thermodynamic analysis,a carbon atom diffusion model is established to study the growth kinetics,microstructure evolution and formation mechanism of the layers.The fracture behavior of reinforcing layers are assessed by indentation fracture testing,then the fracture toughness and residual stress are calculated.Adhesion strength and failure behavior of the interface of NbC/Fe are explored by scratch experiment.The main results are as follows:(1)Niobium carbide(NbC)reinforcing layer is produced by niobium plate and gray cast iron at 1150? for 5 mins,followed by a heat treatment at 990?,1010? and 1030? for 5,10,15,and 20 mins.Results show that the coating consists of NbC and a-Fe phases.The thicknesses of NbC reinforcing layer ranges from 12.51±1.4 to 29.17±2.0?m depending on the heat treatment temperature and time.The NbC volume fraction is almost constant about 95%along the depth towards the matrix,and the average particulate diameter of NbC gradually increases from 80nm to 600nm.In addition,the growth kinetics of NbC reinforcing layers are estimated.A diffusion model based on Fick's laws is used to explore the relationships between the growth kinetics of NbC layer with C concentration distribution in the layer and heat treatment parameters during heat treatment.It is indicated that the experimental results of the kinetics of the niobium carbide layer are in good agreement with those estimated using diffusion model.The diffusion activation energy of C in the NbC layer is 98.59 kJ/mol.(2)The values of hardness for the surface and cross-section of NbC reinforcing layers are as high as 19.70GPa and 21.78GPa.Their elastic modulus are 418.85GPa and 474.05GPa,respectively.Nanoindentation apparatus was applied at this stage.According to the profiles of the indentation,crack size and related equations,the types of crack on the surface of reinforcing layers are evaluated as palmqvist crack.The fracture toughness of the surface of NbC layer is 9.41-10.71MPa·m1/2 according to the selected formula,and the residual stress measured by XRD is-404.71±23.64MPa.In addition,the crack of the cross-section of reinforcing layers is half-Penny crack.The fracture toughness and residual stress of the cross-section of NbC layer are 6.41±0.82MPa·m1/2 and-96.32±1.2MPa.A analytical model is established as a research method in this stage.Meanwhile,it is found that the fracture toughness of NbC layer is higher than that of other materials because of fine grain strengthening,ductile phase toughening and residual stress toughening.(3)Moreover,adhesion strength and failure behavior of the interface of NbC/Fe are explored using scratch experiment under a gradually increasing load of 0-100N.Investigation results show that the adhesion strength at NbC layer/Fe interface is 63N.The cracks propagate along the grain boundaries,and the failure of the reinforcing layers is caused by the interaction of the cracks in the bottom and edge of scratch grooves.