Study On Forming Mechanism Of Self-lubricating Composite Coating In HVOF Spraying Process And Its Characteristics

Reducing the weight of automobiles is a remarkable tendency in the development of modern automobile industry. The warm forming technology of aluminum alloy sheet with high specific strength plays a more and more important role in application of light weight productions, and lubrication in aluminum sheet forming is one of the key techniques. Nowadays, Brush application of lubricating grease on mould surface is a common method to provide lubricant, which leads to low producing efficiency, serious pollution, and instability of product quality, etc. Therefore, to avoid a series of negative effect come with lubrication in warm forming of aluminum alloy sheet, the research of a new kind of alternative lubricant material is a valuable direction with good prospect.In this paper, a Ni-based solid self-lubricating coating with low friction coefficient and good wear-resistant was prepared on a medium steel substrate by using of high velocity oxygen fuel (HVOF) technology. After optimizing the components and process of the mixture consisted of ultra-micro lubricant powders and Ni45 or Ni60A self-fluxing alloy powder, Ni-based composite agglomerating powders for HVOF spraying were prepared through high energy ball milling, spray dried agglomerate, and vacuum microwave sinter. The melting-deforming behaviors of composite particles in HVOF spraying, forming mechanism of coating, and coating microstructure, tribological properties of coatings were also discussed, which provides a stable foundation for further application of the novel Ni-based composite coating with low friction coefficient and good wear- resistant.A cooling- device was designed to collect melted particles in HVOF spraying process for holding the melting microstructure, and the melting status, impact deformation morphology of particles were observed. Also the melting-flatten behavior of Ni-based self-lubricating composite particles, and compacting mechanism of coatings were discussed. The results showed that composite particles flattened easily at the distance of 150～250mm before the spraying gun exit, and composite coating deposited on substrate was mainly composed of melted flat particles, partially melted distorted particles, and some flatten particles with a hollow core.An orthogonal experiment was designed for Ni60A-5wt. %CaF₂-10wt. %WS₂ composite agglomerated powders, the important influencing factor on the porosity of coating was in the sequence of kerosene flow, powder feeding ratio and spraying distance. And the greatest influencing factor on bonding strength of coating was in the sequence of kerosene flow, spraying distance and powder feeding ratio. The results indicated that Ni-based self-lubricating composite particles could be in-situ deposited on the substrate, and Ni-based composite coating has very dense microstructure, low porosity, and high bonding strength.The bonding strength of composite coatings was deeply influenced by the characteristics of original powders and HVOF spraying parameters. Such as the bonding strength of Ni-based hBN coatings was commonly lower than that of Ni-based CaF₂-WS₂ coatings. And for Ni60A based CaF₂-WS₂ composite powders, the bonding strength of the coatings increased with kerosene flow, and the maximum bonding strength was 52.9MPa. Fracture analysis of the composite coatings indicated that the fracture mechanism was mainly cleavage crack or quasi-cleavage crack.Friction wear testing was carried on Ni-based composite coatings, the results showed that the tribological properties of Ni60A-based hBN coatings were excellent than that of Ni45-based hBN coatings, but the friction coefficient was not lower to 0.4, and the abrasion mechanism of which was spalling. In addition, the friction coefficient of Ni-based CaF₂-WS₂ coatings was lower. At ambient temperature, the friction coefficient of Ni45-5wt. %CaF₂-10wt. %WS₂ and Ni45-10wt. %CaF₂-5wt. %WS₂ coating was in the range of 0.46-0.48, 0.32-0.39, respectively. At 400℃, the friction coefficient of Ni45-10wt. %CaF₂-5wt. %WS₂ coating was 0.32-0.38. As for Ni60A-based CaF₂-WS₂ coatings, the minimum friction coefficient of which was 0.22-028 at room temperature, and 0.37-0.4 at 400℃, respectively. The abrasion mechanism of Ni-based CaF₂-WS₂ coatings were mainly spalling at room temperature and coaction's of spalling, sticking or grain abrasion at 400℃.In this work, Ni-based self-lubricating composite coatings prepared by using of HVOF spraying technology have good high temperature friction wear properties, which is a promising process. It is a valuable trial and discussion on the incorporation of novel materials and new process, and manifests a certain theory and good application values.