Preparation And Application Of Textured Chromium Coatings With Promising Tribological Properties

The demand of high quality cold-rolled is increasing dramatically with the surge in the rapid development of automobile and home appliances industries.The roller,which is the key component of mill,plays a critical role in controlling the quality of rolled products.The existing surface texturing technologies,however,have the disadvantages of short service life,easy to produce roll marks and schedule free rolling,which produces severe effects on the quality of the rollers and the economic benefits of enterprises.The steel enterprises have urge to stay ahead in the competitiveness race only by reducing the cost,improving both the efficiency and product quality.A novel surface texturing technology based on the electrodeposition method was proposed to overcome the shortcoming of existing technologies.Textured surfaces with the high hardness,low friction and excellent wear resistance were fabricated by means of electrodeposition directly.In addition,this method has the little damage on the surface of roller during the repairing process.The mechanisms of bumps formation and growth were revealed.The textured surfaces exhibit the excellent tribological properies in both the abrasive wear and oil lubrication conditions.The following innovations have been achieved:1.The textured surfaces with the high hardness,wide surface roughness range and high peak counts were obtained by electrodeposition method.The bumps arranged in random array are distributed on the textured surfaces.The bumps with a variety of diameters and heights can be achieved by adjust the electrodepositing parameters.The Vickers hardness of the textured chromium coatings are as high as 750～1050;the surface roughness Ra range from0.47μm to 5.9μm and the peak density RPC ranges from 68 to 193/cm.2.The textured chromium coatings exhibit excellent tribological properties under both abrasive wear and oil lubrication conditions.Under abrasive wear conditions,the friction coefficient of the textured surface can be reduced by up to 40%compared with the untextured surfaces.The wear rate of untextured surface is 4.25×10^-5 mm³ N^-1 m^-1,and the wear rate of structured surface can be as low as 3.6×10^-6 mm³ N^-1 m^-1.The bumps covered on the textured surfaces can reduce the contact area,and the surrounding troughs can act as the reservoir for both abrasive particles and wear debris.The larger the volume of structured pits,the greater the wear resistance of the coating.Better wear resistance of coatings is achieved due to the greater volumes of troughs.3.In the case of lubrication conditions,the textured surfaces can not only as the reservoir for lubricating oil and wear debris which contribute to reduce the surface friction coefficient effectively,but also promote lubricate regimes transition from the boundary to mixed lubrication,leading to a increase in load capacity and a further reduction in the coefficient of friction.The effects are more pronounced under the conditions of high sliding speeds and low loads.4.The tribological performance of textured chromium coatings were further improved by ion implantation.Compared to the samples without ion implantation,the friction coefficient of the samples implanted by ion implantation decreased by 21.9%and the wear rate decreased by 46.1%.The improvement in tribological properties of ion implanted samples is attributed to the formation of hard Cr N and/or Cr₂N.The greater improvement is achieved in the samples with high dose of ion implantation.5.A improved method for quantitative analysis of wear properties of textured surface was developed.This method,relying on surface bearing ratio curve,can calculate the volume fraction of material wear accurately,so that the accurate measurement of the wear volume of textured surface can be realized.The surface texturing technology based on the electrodeposition provides a new idea for the fabrication of rollers with high wear resistance.The textured chromium surfaces could increase the transfer ratios of textured asperities and prolong the service life of rollers,which helps to improve production efficiency.In addition,the excellent tribological properties of the textured surfaces achieved under oil-lubricated conditions make it have a potential application in the cylinder-piston system.Textured chromium coatings have a positive effect on reducing frictional losses of mechanical parts,reducing fuel consumption and improving energy efficiency.