| Titanium and its alloys are widely used in aerospace,marine,chemical and biomedical fields due to their low density,high specific strength,good corrosion resistance and oxidation resistance.However,low hardness,high friction coefficient and poor wear resistance limit their applications,especially to the fields where specific surface performances are require.Titanium carbide has high hardness(28-35 GPa),high melting point(3067 ?),low friction coefficient,excellent wear resistance and similar density to titanium(4.93 g/cm3),which can be used as a coating material to extend the service life of titanium alloys.Therefore,in the paper,gray cast iron is used as the solid carbon source and commercial pure titanium as the substrate.The TiC-Fe gradient coating was prepared on the surface of pure titanium by a simple two-step heat-treatment process(1150 ? × 5 min + 1000 ? ×10 h).The microstructure,phase distribution and morphology of the TiC particles of the TiC-Fe gradient coating were characterized by X-ray diffractomer(XRD),scanning electron microscopy(SEM),electron backscattered diffraction(EBSD),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS),and the formation mechanism was analyzed in detail;the hardness,elastic modulus,fracture toughness,coating/substrate interface bonding stre strength and wear resistance of the coating were evaluated by nanoindentation testing,scratch testing,and friction-wear testing,and the toughening mechanism and wear mechanism were also analyzed.The experimental results showed that:(1)The microstructure of the TiC-Fe gradient coating exhibits a gradient variation,namely,from the surface of the coating to the titanium substrate,the volume fraction of the TiC particles gradually increases,and the volume fraction of the a-Fe phase gradually decreases.The thickness of the coating is approximately 30.4 ?m,and the phase composition of the coating is mainly TiC phase and a small amount of a-Fe phase distributed along the grain boundary of TiC Moreover,the coating is completely dense,free of cracks and voids,and the interface between the coating and the substrate exhibits a good metallurgical bond.During the first step of the heat treatment,the formation mechanism of TiC particles is mainly the nucleation-growth of TiC grains;during the second step of the heat treatment,the formation mechanism of TiC particles is the diffusion-type solid phase transition formed by the diffusion of carbon atoms into the titanium lattice.The formation mechanism of TiC-Fe gradient coating is the change of TiC nucleation density caused by carbon concentration gradient.(2)From the surface of the coating to the substrate,as the thickness of the coating increases,the hardness increases from 19.1 GPa to 31.7 GPa,and the elastic modulus increases from 368.1 GPa to 464.4 GPa.Moreover,the fracture toughness values of the columnar crystal zone,the equiaxed crystal zone and the large particle zone of the TiC-Fe gradient coating are(3.5 ± 0.15)MPa·m1/2,(1.9 ± 0.14)MPa·m1/2 and(3.1 ± 0.21)MPa·m1/2,respectively.In addition,the coating exhibites excellent coating/substrate adhesion strength.Due to the in situ inward growth of the TiC-Fe gradient coating and the gradient distribution of high volume fraction of the hard-phase TiC(>90%)and a small amount of high toughness a-Fe phase in the coating,the coating exhibits high hardness,high elastic modulus,high toughness and excellent interface bonding strength.(3)At wear loads of 10 N and 20 N,the wear rate of the iron-rich zone and iron-poor zone of the TiC-Fe gradient coating decreases with increasing TiC particle volume fraction.When the wear load is 20 N,the wear rate of the iron-rich zone is about 14.3%of the pure titanium substrate,and the wear rate of the iron-poor zone is about 5.6%of the pure titanium substrate.For the iron-rich zone,the distribution of a small amount of high-toughness a-Fe phase along the grain boundary of TiC particles significantly improves the toughness of the coating.Therefore,only a small amount of TiC particles are broken and peeled off during high load wear,the wear mechanism of the iron-rich zone is mainly abrasive wear and slight plastic deformation;For the iron-poor zone,the dense bulk TiC particles in the high load wear process are partially cracked under the action of positive pressure and friction,and the abrasive chips generated by the abrasive parts are adhered to the wear surface.Thus,the wear mechanisms of the iron-poor zone are mainly microcracks,micro-ploughing and adhesive wear. |
PDF Full Text Request