A study of adhesion and friction in intimate metal-metal contacts.

摘要

A novel method has been developed to study adhesion and its relationship to friction by establishing and perturbing the contact between two clean metal surfaces. An orthogonal cutting configuration is used to develop intimate contact between two nascent metal surfaces, free of contaminants and oxide layers. At the same time, the contact conditions are such that the real area of contact is equal to the apparent or geometrical area of contact. The unique nature of this contact has been exploited to study intimate metal-metal contacts by perturbing the contact in a controlled manner using low amplitude modulation superimposed in directions normal or tangential to the contact. The modulation allows the contact to be altered in a repeatable manner. Under certain conditions, using simultaneous measurements of the forces and the contact area, the contact between the tool and chip can be broken to study adhesion at the junction. The contacts between three different tools (steel, carbide, and TiN coated tools) and three different workpieces (aluminum 6061, commercially pure lead, and copper 110) have been investigated. Results have shown that the adhesive junction formed between the tool and the chip is influenced by the properties of the workpiece material and the tool. The strength of the junction, as well as the manner in which separation of the two materials occurs, varies with workpiece material. When the strength of the adhesive junction is greater than that of the chip, localized failure occurs within the chip and deposits form on the tool. When the adhesive junction is weaker than the chip material, separation has been shown to occur at the interface. Failure in the chip material does not occur in sliding contact due to the shear strength of the adhesive bond being less than the chip strength. However, when a tensile force is applied to the same junction, the chip material may fracture before the adhesive junction. The tool material also influences the strength of the junction when separation occurs at the interface. The TiN coated tool reduces the strength of the adhesive bond between the tool and chip by almost 20%, however, when using a carbide tool, the strength of the bond increases by almost 50%. Experiments with lubricants have shown that lubrication is effective in eliminating adhesion between the tool and chip, but the lubricant loses its effectiveness over time and adhesion occurs. In addition, the effects of modulation on the mechanics of cutting have been studied. The results have shown that modulation in the cutting direction or normal to the cutting direction alters the contact between the tool and the chip significantly. Both types of modulation lead to a reduction in the forces along the rake face of the tool. The friction force was seen to decrease by as much as 75% and the normal force was seen to decrease by up to 50%. In addition, a reduction in the amount of deformation the chip undergoes in its formation has been observed (up to 50%). The results confirm that the condition at the interface between the tool and the chip is a factor that influences the chip formation mechanism. The results from this work are used to explain results seen in prior work in which modulation is applied to cutting processes. It is believed that further development of this method to study adhesion will lead to a better understanding of the junctions formed in metal-metal contacts as well as the interactions occurring at the chip-tool interface.