In-situ Preparation And Mechanicial Properties Of WC Reinforced Iron-matrix Surface Composites

Tungsten carbide(WC)is commonly used as a reinforced phase for preparation of the metal matrix composites,which can pro-long service life of the material because of its high melting point(3410?),high hardness(17.8 GPa),excellent wear resistance,and well corrosion resistance.The development of iron(Fe)-matrix surface composites is attracting considerable attention because of its good mechanical properties and the low cost.In the present work,the tungsten carbide-reinforced iron-matrix surface composites,of which the raw materials were high purity tungsten plate and gray cast iron(HT300),were produced by in-situ process.The microstructure and phase composition of the composites were characterized by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron backscattered diffraction(EBSD)and X-Ray diffractomer(XRD);the growth kinetics of the tungsten carbide layer were analyzed by classical kinetics theory;the microhardness,fracture toughness and wear properties of the composites were studied by Vickers hardness tester,nano indenter and friction-wear tester.The experimental results showed that:(1)The thermodynamically favored products of the Fe-W-C ternary system were WC,W2C and Fe3C,and the reaction Gibbs free energy of them decreased with the increase of temperature.According to the value of the ?rG,the stability of carbides can be placed in the order:W2C>WC>Fe3C.(2)At the eutectic temperature for different heat treatment times,the WC layer was formed between the tunsgten plate and the matrix,and the fishbone-like Fe3W3C phase appeared in the matrix.With the heat treatment time prolonged,the thickness of the WC layer gradually increased.Moreover,the WC layer came to smooth from the initial cell interface.The density decreased,the WC grains were dispersed by Fe.In addition,the activation energy of pure WC layer was estimated to be 184.06 kJ·mol-1 on the basis of the classical kinetics theory by the temperature(1085?,1100?,1125?)and heat treatment time(15min,45min,75min,105min)as the variables.(3)By EBSD analysis,the phase between the tungsten plate and the WC layer was determined,which is Fe6W6C.The crystallization process of WC grains was grown in the form of lamellae,{0001} crystal plane as the basal plane was the most stable and finally {1010}crystal plane submerged,showing {0110} crystal plane only.Therefore,the triangular columnar WC grains were observed.The growth process of WC layer can be divided into four stages:Fe-W-C ternary micro-region formed because of the dissolution and diffusion of tungsten atoms and carbon atoms from the tungsten plate and the matrix,respectively;the formation of Fe6W6C interstitial compound,WC phase and Fe3W3C interstitial compound;the WC layer growth,while the Fe3W3C phase reduction;the gradient of WC layer formed.(4)The microhardness of the WC layer presented a gradient distribution from the surface to the iron matrix,and the maximum microhardness value arrived at the WC layer which was 2169 HV0.1.The fracture toughness(KIC)of the surface WC layer was about 5.21 MPa·m1/2 by the indentation method.Under a 10 N load,the friction coefficient of the surface WC layer and the close to the matrix WC layer was 0.311 and 0.485,respectively.It could be seen that only a slight wear scratches on the WC layer,and there were no WC particles peeling,which showed excellentinterface bonding strength;the wear scratches of the WC layer close to the matrix became deeper and the density increased,but the WC particles were not pulled out of the matrix,indicating that the WC particles were well bonded to the matrix.The main wear mechanism was adhesive and abrasive.