Effect Of Carbon Source On TiC Composite Coating Prepared By Laser Composite Sol-gel Method

As one of the main failure forms of the workpiece in industry,wear has brought huge losses to industrial production.TiC has excellent physical and chemical properties such as low density,high strength,high elastic modulus,excellent oxidation resistance,wear resistance and corrosion resistance.The addition of TiC strengthening phase to the metal matrix can significantly improve the performance of the substrate to meet the requirement in extreme conditions.Laser composite sol-gel technology,as a new ceramic coating preparation method,combining with the advantages of laser cladding and sol-gel technology,can prepare sub-micron structured powder,synthesize reinforced particles in situ on the substrate surface.The resulting coating is metallurgically bonded to the substrate,with no defects,uniformly distributed fine particles,improved hardness and large thickness.In this paper,TiO2 and C mixed powder was prepared by sol-gel method.The composite powder was preliminarily placed on a 45 steel substrate,and the preset layer was irradiated with a continuous diode laser to finally obtain a TiC enhanced coating.And then the type and proportion of carbon sources were changed,with carbon nanotubes(CNTs)and nano-graphite replaceing part of the micro-graphite to obtain different coatings.The microstructure and phase composition of the coatings were analyzed by optical microscopy,scanning electron microscopy(SEM)and X-ray diffraction(XRD).The hardness and wear resistance properties of the cladding layer were tested by micro-hardness tester and friction and wear tester.Finally,the formation mechanism of TiC reinforced coating was analyzed.The results show that the processing parameters have influence on the microstructure and properties of the coating.During the process of coating preparation with micro-graphite as carbon source,the increase of the laser power will lead to increasing the hardness.However,when the laser power is higher than 1200 W,the content of TiC particles in the coating is very small.The increase of the scanning speed will lead to reducing the hardness,meanwhile TiC particles remain less.The experiments without shielding gas will cause more defects in coating,but the particles are distributed uniformly and cladded well after adding the protective gas.The optimum processing parameters for the coating were 1000 W of laser power,2 mm/s of laser scanning speed and 10 L/min of protective gas flow rate.The addition of nano-carbon materials will have a greater impact on the cladding.When CNTs were added,the CNTs have higher reactivity and larger specific surface area,which results TiC particles larger and agglomerated.The addition of excess CNTs will lead to powder burning.Finally,20 wt.%CNTs can get the optimal coating.When adding nano-graphite,the properties of nano-graphite are similar to that of micro-graphite,so the basic properties of the powder are not changed except increasing the specific surface area.Finally,the fine TiC particles uniformly distributed in the coating.But the addition of excess nano-graphite will also lead to powder burning.Finally,60 wt.%nano-graphite can get the best coating.The average hardness of the micro-graphite coating,20 wt.%CNTs coating and 60 wt.%nano-graphite coating are 700 HV0.2,800 HV0.2 and 950 HV0.2,respectively.The wear resistance of the micro-graphite coating is up to 4 times that of 45 steel subsrate,the 20 wt.%CNTs coating is up to 8 times of substrate,and the 60 wt.%nano-graphite coating is up to 12 times of substrate.The main wear mechanisms of the three types of coatings are abrasive wear.After thermodynamic analysis of the mechanism of in situ synthesized TiC,it is concluded that solid-solid reaction mechanism is viable during the carbothermal reduction.The effect of different carbon sources on the growth mechanism of TiC particles was analyzed.It is found that nano-graphite can produce more fine TiC particles.The strengthening mechanism of TiC particles was analyzed.It can be resulted that TiC particles can make grains finer,and the particles in the coating act as the inner crystal distribution.In summary,the preparation of TiC enhanced coatings by laser composite sol-gel method was studied in this paper.The exploration of the powder preparation,the optimization of the laser processing parameters,the addition of different carbon sources and the analysis of strengthening mechanism were carried out.The effect of nano-carbon materials on the preparation of TiC coating by laser composite sol-gel method was studied.The laser composite sol-gel technology with high performances is expected to be applied in industrial production.