Preparation Of Ni-TiC Powder And Study For The Performance Of Lasser Cladding Coating

Titanium carbide(TiC)has extremely high hardness,high melting point,high modulus and relatively low density,thus it has been widely used as the reinforcing phase for metal matrix composites or metal surface composite coatings to improve their mechanical properties and wear properties.In these metal matrix composites or coatings,TiC reinforced nickel matrix composites have excellent shear strength and compressive resistance because of their combine properties of metal(ductility and toughness)and ceramic(high strength and elastic modulus).However,traditional methods for preparing nickel-based composites,including mechanical mixing and in-situ synthesis,cannot avoid the disadvantages of uneven TiC distribution and low TiC content.Therefore,the Ni-TiC composite coating prepared by these methods have low hardness and uneven hardness value,and there are soft spots in the coating.In addition,existing literatures mainly focused on the wear resistance of Ni-TiC composite coatings,while the corrosion resistance of coatings was rarely studied.In many application conditions,the composite coating not only needs excellent wear resistance,but also needs good corrosion resistance.Therefore,in the present thesis,a new Ni-TiC composite powder with a higher TiC content up to 50 wt.%was developed.Meanwhile,the Ni-TiC composite coating was prepared by advanced laser cladding technology to study the wear resistance of the composite coating.In the final part of thesis,the corrosion resistance of the composite coating can be further improved by adding Cr elements with different contents.The main research contents and achievements of this thesis are as follows.(1)Ni-TiC powder was prepared by Hot Water-High Pressure H₂ process.By orthogonal experiment,the optimal process parameters are got based on cladding speed.The cladding temperature is 150?,the density of(NH₄)₂SO₄ is 180 g/L,the density of ammonia water is 40 g/L,the pressure of H₂ is 5 MPa.The mechanism of Ni cladding TiC is that,in the beginning of reaction,Ni element deposited on the TiC surface steps which were very chemical active,deformed new cores.These new cores which also were very active,accelerated new Ni atom deposition.These can cause Ni cores growing up and finally cladding the whole TiC particle surface.(2)Laser cladding process was applied to produce almost fully dense nickel-titanium carbide composite coatings on a 45 carbon steel with varied titanium carbide content.In the as prepared Ni-30TiC and Ni-50TiC composite coating,titanium carbide particles with a bimodal microstructure could be homogeneously distributed in the nickel based matrix.Some other metal elements such as iron and titanium were diffused into nickel matrix leading to a solid solution strengthening effect.Due to the presence of the harder nickel-titanium carbide composite coating on the 45 carbon steel,the surface hardness and wear resistance were significantly improved.The determined Vickers hardness for the Ni-50TiC composite coating was in the(5.51?6.82)GPa range.The coefficient of friction and volume wear rate of Ni-50TiC composite coating was reduced down to 0.41 and 9.3×10^-6 mm³/N·m,respectively,which was due to presence of harder nickel-titanium carbide composite coating and formation of varied soft and lubricant metal oxides such as titanium oxides,iron oxides and nickel oxides etc.(3)In both Ni-50TiC-10Cr and Ni-50TiC-20Cr coating,TiC particles distributed in Ni-based matrix very homogeneous,similar to the Ni-TiC coating.Ni-Cr solid solution was formed during laser cladding,thus caused solid solution strengthening.Cr element didn't affect the microstructure of Ni-TiC coating and distribution of TiC in the composite coating.But Cr could restrain the Fe diffusion into the Ni matrix.The measured Vicker's hardness of Ni-50TiC-10Cr coating and Ni-50TiC-20Cr coating was in the range of 5.7-8.2 GPa and 5.2-7.4 GPa,respectively.The friction coefficient of Ni-50TiC-10Cr was 0.40 while the friction coefficient of Ni-50TiC-20Cr increased slightly up to 0.45.Similar to the friction coefficient,the wear resistance of Ni-50TiC-10Cr and Ni-50TiC-20Cr decreased slightly compared to Ni-50TiC.(4)The presence of Cr element and high TiC content also improved corrosion resistance of the as-prepared composition coating.With higher chemical potential,TiC had a function which is like passive film on the coating surface.All these four coatings EIS showed the typical characters of capacitive reactance.With the increasing of TiC and Cr content,the electrochemical transferring resistance Rct increased.In the Ni-TiC-Cr coatings,Cr could form a dense and stable passive film,leading to the excellent corrosion resistance of composition coating.The results of immersion corrosion testing showed that,Ni-30TiC and Ni-50TiC coatings appeared a typical localized corrosion behavior.And Ni-TiC-Cr coating appeared a general corrosion behavior.With the TiC content increasing,the number of corrosion pitting in the Ni-TiC decreased.And the passive character on the coating surface was enhanced obviously with the Cr content increasing.During the period of corrosion testing,corrosion almost didn't happen on the Ni-50TiC-20Cr coating surface which shows the superior corrosion resistance.