Study on Wear-resistant and Antifriction Ni-based Composite Coating by Plasma Cladding

Author:Zhang Xin Jie

Supervisor:cui hong zhi


Degree Year:2018





As one of the most commonly encountered failure modes(fracture,wear,corrosion)of engineering component in metallurgy,mining,transportation,cement and other industries,wear causes great security risk and economic loss.Cladding coatings have been widely used in industries because it can modify the chemical and physical properties of material surface,then the wear-resistance can be improved.The main technologies are plasma cladding and laser cladding,the plasma cladding has became an important method to develop the technology of maintenance and reproduction,and extend the service time of mechanical products with cheap equipment and low maintenance.Ni is good matrix material for composite coating because of similar thermal expansion coefficient with steel and good ductility,toughness and corrosion resistance.TiB2,TiC ceramics can significantly improve the wear resistance of composite coatings with the advantages of high hardness,strength and good chemical stability.In this paper,TiB2+TiC reinforced Ni-based wear-resistant coatings were fabricated by plasma cladding technology on Q235 low carbon steel.The phase constitution,evolution of microstructure in the coatings were analyzed.The effects of ceramic content,cladding speeds and wear conditions on the wear resistance were studied.and effects of ceramic content,cladding speed and wear conditions on wear-resistance of coatings were studied.Based on this,Ti2SC/TiB2+TiC/Ni self-lubricating coatings with Ti2SC as antifriction component were prepared,and the microstructure and tribological properties were researched.The two-step method of sintering-plasma remelting was used to prepare coatings with high content ceramics,and the morphology and size of ceramic phase were controlled reasonably.The microstructure and wear resistance of coatings with high content ceramics were investigated.TiB2-TiC/Ni composite coatings with different content of ceramics were prepared by plasma cladding.The coatings mainly consists of TiB2、TiC,and γ-Ni.TiB2 with rectangular or hexagonal shape and TiC with irregular blocky shape generated in-situ,and distributed uniformly in the coatings.The content and size of TiB2 and TiC increased as increasing content of Ti+B4C.Thermal field models under different plasma cladding speeds were established.When the cladding speed decreased from 380 mm/min to 152 mm/min,cooling speed slowed down,the driving force of nucleation decreased,Additionally,the dilution rate increased with deeper penetration.Therefore,the number of strengthening phases decreased,and the size increased in coatings,TiB2 easily grew along close-packed {0001} planes in two dimensions as sheets.As temperature field distribution cloud maps of molten pool showed,the coating could form a close metallurgical bond with the substrate when the coating cladding speed was less than 228 mm/min,the results of the simulation are in agreement with the experimental results.When the content of Ti+B4C increased from 0 to 50%(wt.%),the content of TiB2 and TiC increased,the wear mechanism change as follows:abrasive+adhesive wear→micro-cutting→micro-cutting+reinforced phase peel out.The wear resistance of coating with content of 40%Ti+B4C is the best.The results of wear test at different cladding speeds show that,the wear resistance of coating with 228 mm/min cladding speed is better.With higher cladding speeds,the coatings has higher brittleness.The reinforced particles abnormally grow,and easily cause stress concentration at lower cladding speeds.Therefore,the coating occur delamination wear resulting from the greater brittleness and stress concentration with the cladding speeds of 380 mm/min and 152 mm/min.Effect of wear load(30 N,60 N and 120 N)and counter grinding pairs(Al2O3 ceramic ball,stainless steel ball)on wear process and wear mechanism of coatings were studied.When the Al2O3 ceramic balls were the counter grinding pairs,the wear mode is micro-cutting at 30 N load.The structure of compaction layer appear at the 60 N load,and the wear mechanism transform into adhesive wean When the load increased to 120 N,the wear mechanisms are oxidation and delamination wear.When used the stainless-steel grinding balls,shear damage occur in the grinding balls,and stainless-steel material transfer partially to the surface of the coating because the hardness of coating is greater than the hardness of the counter grinding stainless-steel balls.The stainless-steel balls exhibit a greater adhesive effect,and the adhesive wear is aggravated with the load increased.Ti2SC/TiB2+TiC/Ni self-lubricating composite coatings were produced by plasma cladding using Ti,B4C,Ni and different content of MoS2.With the increase of MoS2 addition,the size of antifriction phase(Ti2SC)increased.Microstructure continuity of coating is destroyed by long needle Ti2SC when content of MoS2 is 15%.The results show that,when the wear speed is 10 mm/s,the 10%MoS2 content coating has the lowest friction coefficient and wear rate,and the partial extraction of Ti2SC is attached to the ceramic particles to form the excellent antifriction structure.When the wear rate speed to 50 mm/s,the addition of MoS2 causes the coating to form the structure of the compacted layer of the transfer material during the wear process.The friction coefficient and wear volume decline with the increase of MoS2.Energy principle of anti-friction reducing wear rate mechanism was analyzed through calculating energy consumption in the wear process.At wear speeds of 10 mm/s and 50 mm/s,the energy consumption partially act on the“pull-out”mechanism and forming of compaction layer,resulting in the decrease of damage consumption of the coatings,thus reducing the wear damage.The size and distribution of ceramic particles in the coating can be reasonably controlled by using spark plasma sintering and plasma remelting composite technology when the content of ceramic phase is increased.First,TiB2-TiC/N1 composites were fabricated by spark plasma sintering,the results show that the mainly phases of sintered samples are γ-Ni,TiBZ,TiC.With the increase of Ni content,the size of TiB2 and TiC ceramics decrease gradually,and the denser composites are obtained.When Ni content is low(20%and 30%),severe delaminate occurs.Micro-cutting wear appears with lower and smoother COF with the increase of Ni content(40%).However,when the Ni content continues to increase(50%),the adhesive wear results in poorer wear resistance.The TiB2-TiC/Ni composite coatings were prepared by sintering-plasma remelting technique on the basis of studying sintering TiB2-TiC/N1.The sintering coatings was remelted by plasma with different currents.With increase of plasma currents,the bonds between coatings and substrates gradually change from mechanical bonds to metallurgical bonds.When the remelting currents were 60 and 75 A,the wear mechanism is abrasive wear,and the wear-out of the coatings are slight.When the remelting current rose to 90 A,TiB2 particles grew into long strips.Defect in TiB2 increase,the wear mechanism of the coating changed to delamination with peel out of TiB2,resulting in increase of wear volume.It was found that coatings prepared by the sintering-plasma remelting process exhibited better wear resistance than the plasma cladding coating.