Z. Eliezer

Influence of high velocities and high current densities on the friction and wear behavior of copper-graphite brushes

Abstract The friction and wear behavior of Morganite CM1S powder metallurgy copper-graphite brushes at sliding velocities up to 160 m s−1 and current densities up to 870 A cm−2 is presented. The brushes had a cross-sectional area of 1.2 cm2 and the loads employed ranged from 9 to 45 N. The wear rates as a function of velocity and the voltage drop per brush as a function of sliding velocity, brush pressure and current were determined. The wear rates under a current of 600 A are of the same order as those obtained under no current conditions. The minimum difference was obtained at a sliding velocity of 100 m s−1 (4.1 × 10−4 cm km−1 with current compared with 3.4 × 10−4 cm km−1 without current). The wear rate exhibited by the positive brush was lower than that of the negative brush at any sliding velocity. At constant current and sliding velocity the contact voltage drop decreases with increasing brush load. The voltage drop exhibited by the positive brush is always lower than that of the negative brush. The contact voltage drop varies little with sliding speed when the current and the brush load are kept constant. At constant brush load and constant sliding speed the voltage drop increases monotonically when the current is increased. It has been determined that local rotor waviness, even of small amplitude, can produce sufficient brush bouncing to cause excessive sparking which results in pronounced damage to the brushes and rotor surface.

Wear mechanism in composites: a qualitative model

Abstract A qualitative model of wear in composite materials is presented. The model is based on the possibility of formation of loose wear particles as a result of debonding at the fiber/matrix interface. The experimental results lend support to the suggested wear mechanism.

Residual stresses and sliding wear

Abstract The residual stresses that develop during the wear of AISI-SAE 1018 and 4340 steels have been examined. The entire three-dimensional stress tensor was obtained. A normal stress perpendicular to the surface, predicted by theory, has been found, but its magnitude is too small to affect the wear rate. There are also significant shear stresses. The wear process rapidly alters any initial stress distribution produced by heat treatment or peening to such a degree that the wear rate is not affected by these stresses, unless they are initially larger than those that can be produced by the wear parameters.

High speed tribological properties of graphite fiber/ Cu-Sn matrix composites

Abstract High speed dry friction experiments of graphite fiber/Cu-Sn matrix composites against steel were conducted at sliding velocities up to 235 m s−1. The composite samples were prepared by the method of liquid metal infiltration. It has been determined that the friction coefficient and the wear rate depend on the amount of tin in the matrix, orientation of fibers relative to the sliding surface, the sliding velocity, the graphite grain size and the degree of liquid metal infiltration within the fibers. The increase in tin content leads to a decrease in both friction and wear due to an increase in matrix hardness. Specimens tested with the fibers oriented perpendicular to the sliding surface exhibit better frictional behavior than those with fibers parallel to the sliding surface. Both friction coefficient and wear rate reach a minimum value at a velocity between about 120 and 180 m s−1. Large graphite grain size and poor liquid metal infiltration within the fibers have a detrimental effect on wear.

Friction and wear characteristics of powder metallurgy copper-graphite brushes at high sliding speeds

Abstract Friction experiments using several commercial powder metallurgy copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities ranging from 20 to 235 m s−1. The measured wear rates ranged from a minimum of 4.3 × 10−5 cm km−1 at a sliding velocity of 100 m s−1 for a brush with high graphite content to a maximum of 8.4 × 10−3 cm km−1 at a sliding velocity of 230 m s−1 for a brush with high metal content. The coefficients of friction ranged from a minimum of about 0.08 to a maximum of about 0.47 and were greatly affected by the presence of oxide layers at the sliding interface. Almost all the brushes exhibited some degree of edge breaking. The velocity at which edge breaking occurred was dependent on the powder grain size. Brushes with a large grain size seem to exhibit edge breaking at a lower speed than brushes with a fine grain size. High interface temperatures which occur at high sliding speeds result in melting of the lead-tin binders used in most powder copper-graphite brushes.

Friction and wear properties of two types of copper — Graphite brushes under severe sliding conditions

Abstract High speed dry friction experiments using two copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities up to 230 m s −1 and at current densities up to 526 A cm −2 . One brush was a commercial powder metallurgy (PM) specimen and the other was a graphite fiber-Cu/Sn matrix composite material. The composite brush was prepared by a proprietary process of liquid-metal infiltration and was run with the graphite fibers perpendicular to the rotor surface. The coefficient of friction was determined as a function of velocity, the wear rates were determined as a function of velocity and the voltage drop was determined as a function of velocity and current. The results show that the coefficient of friction exhibited by the PM brush is lower than that of the composite material at any velocity tested. The wear rates without current are much higher for the PM than for the composite brush, but they are of the same order when a current of 600 A is passed through. The voltage drop at the brush-rotor interface shows a similar variation with velocity for the two brushes, but the variation of the voltage drop as a function of current is different for the two specimens. The voltage drop increases almost linearly with increasing current for the PM brush. For the composite brush it exhibits a sharp increase up to about 50 A and then varies very little up to the maximum current of about 600 A. The damage done to the rotor surface in the case of heavy sparking is more pronounced with the PM brush than with the composite brush. It appears that the difference between the high temperature mechanical properties of the two types of brushes is responsible for their different behavior under severe sliding conditions.

Friction and wear properties of an epoxy-steel system

Abstract The friction and wear properties of a cured epoxy resin pin sliding against a steel disc were examined. It was found that the initial (single traversal) coefficient of friction is relatively low (about 0.25) and temperature dependent, while the steady state friction coefficient is relatively high (about 0.8) and temperature independent. It has been determined that the steady state value reflects the friction of iron oxide sliding on itself rather than the epoxy-steel friction. Optical microscopy observations and preliminary electron spectroscopy for chemical analysis experiments suggest that, as in the case of metallic friction, the wear particles form by oxidation of the steel fragments transferred by adhesion to the epoxy surface rather than by direct oxidation of the steel counterpart.

Tribological behavior of metal matrix composites

Abstract The wear and friction behavior of continuous graphite fiber reinforced metal matrix composites was investigated. Composite materials were tested against 4620 steel at 54 m s −1 at room temperature in air without lubricant. The graphite fibers studied included rayon-, pitch- and polyacrilonitrile (PAN)-based fibers. Both high modulus and high strength PAN-based fibers were examined. The fibers were incorporated into copper- and silver-based alloys by means of a liquid metal infiltration technique. The results of this study indicate that the type of graphite fiber in the composite is the most significant factor in the wear and friction behavior of metal matrix composites. In some high modulus fiber tin-bronze composites the fiber fraction influences the wear rate but not the coefficient of friction. Neither the matrix alloy nor the composite tensile strength per se correlate with the friction and wear properties; however, there are specific trends for the various matrix alloys.

Solid lubricant coatings using electromagnetic railgun acceleration

Obtention de revetements de lubrifiants solides avec un accelerateur a rails electromagnetique

The effect of sliding time and speed on the wear of composite materials

Abstract The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s −1 . The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.