Nannaji Saka

Abrasive wear mechanisms and the grit size effect

Abstract The classical cutting model for abrasive wear deviates from reality in that it cannot predict the effect of grit size on wear and that the observed wear rates are at least an order of magnitude smaller than the predicted values. Through experimental and theoretical work it is shown in this paper that the grit size effect is a consequence of the transition from a cutting mechanism to a sliding wear mechanism. The wear coefficient is shown to be less than that predicted by a cutting model owing to the plastic deformation of the surface being worn.

Boundary Lubrication Studies on Undulated Titanium Surfaces

It is customary in tribological research, and especially in practice, to employ smooth surfaces, for it is generally believed that smooth surfaces should have low sliding friction. However the wear particles produced during sliding get trapped at the interfaces, and plowing by the wear particles becomes the principal mechanism of friction. Through experimental and theoretical investigations, it is shown that the plowing friction component can be substantially reduced by means of surface undulations. Steel-titanium and titanium-titanium pairs were tested to investigate the role of surface undulations in boundary lubricated sliding. Compared with the experimental results of flat titanium surfaces, friction coefficient and wear rate were considerably reduced with the undulated titanium surfaces, especially when lubricants considered ineffective for titanium were used. In addition, the effects of pad width and cavity volume fraction of the undulated surface were also investigated. A plowing model proposed for...

Wear of metals at high sliding speeds

Abstract High speed sliding wear of AISI 1020 steel, AISI 304 stainless steel and commercially pure titanium (75A) was studied using a pin-on-ring geometry. All the tests were carried out in air without any lubricant. The sliding speed was 0.5–10.0 m s−1 and the normal force was 49.0 N (5 kgf). The friction coefficient of all the materials tested decreased with the sliding speed; this appears to be a consequence of oxide formation. The wear rate of 304 stainless steel increased monotonically with speed, whereas the wear rate of 1020 steel and titanium first decreased and then increased and again decreased, with a maximum occurring at about 5 m s−1. The complex variation of the wear rate as a function of speed is explained in terms of the dependence of the friction coefficient, hardness and toughness of the materials on temperature. Microscope examinations of the wear track, the sub-surface of worn specimens and the wear particles indicate that the wear mode was predominantly by subsurface deformation, crack nucleation and growth processes, i.e. the delamination process, similar to the low speed sliding wear of metals. Oxidative and adhesion theories proposed in the past to explain the high speed sliding wear of metals are found to be incompatible with the experimental observations.

Finite element analysis of an elastic-plastic two-layer half-space: normal contact

Abstract A two-dimensional finite element stress and strain analysis of the sliding contact of a two-layer elastic-plastic half-space for various friction coefficients was conducted. First the contact pressure of the two-layer half-space under normal indentation was determined. Then the normal and tangential loadings were applied proportionally and incrementally on the surface. It was found that surface deformation, location of initial yielding, stresses and strains along the interfaces between layers strongly depend on the friction coefficient. When the friction coefficient is small (less than 0.3), yielding initiates in the subsurface region near the leading edge of the contact, provided that the normal load is large enough. When the friction coefficient is large (greater than 0.3), yielding initiates on the surface at the trailing edge of the contact. The surface strains, especially the shear strains, are large owing to the unconstrained deformation. The magnitudes of shear stresses and strains along both interfaces are significantly large for high friction. On the basis of the analysis, the implications for interface failures are qualitatively addressed.

Wear of two-phase metals

Abstract The effect of the second-phase particle structure on the wear properties of two-phase metals was investigated by using precipitation-hardened copperchromium alloys (Cu-0.58 at.% Cr and Cu-0.81 at.% Cr) aged for different periods of time at 500 °C. The hardness of these materials was found to increase initially with the aging time and then to decrease; the maximum value was reached after about 100 min of aging. Metallographic examination of worn specimens indicated that the wear process proceeded by subsurface deformation, crack nucleation and crack propagation, i.e . by delamination. The friction coefficient was found to be constant for both alloys and for all aging times. In the early stages of precipitation both the wear rate and the wear coefficient decreased. As the aging continued the wear rate and the wear coefficient increased even though the hardness also increased. The wear coefficient remained constant for the overaged alloys. The decrease in both the wear rate and wear coefficient in the early stages of precipitation is due to the fact that particles are small and coherent and require a large amount of subsurface deformation for crack nucleation. With further aging, the particles grow and become incoherent, increasing the wear rate due to easier crack nucleation. For the case of the overaged alloys, the wear coefficient tends to become constant because the wear process is controlled by the crack propagation rate which is found to be independent of the aging time for both alloys.

The role of tribology in electrical contact phenomena

Abstract The mechanism of electrical contact resistance between lightly loaded sliding surfaces was investigated. It was found that the increase in contact resistance of non-noble or base metal contacts, such as Sn-Pb, is due to the oxidation of metallic wear debris that gets entrapped at sliding contacts. It was hypothesized that when the wear debris is continuously removed from the sliding surfaces even non-noble or base metals ( e.g. copper, nickel and Sn-Pb) exhibit low contact resistance similarly to the noble metals. Experimental work on a modulated contact surface of a base metal contact has shown that the electrical contact resistance was low because the wear debris was effectively trapped, thus confirming the validity of the hypothesis. The implication of these findings for the usage of base metals in electrical contacts is discussed.

Evolution of Copper-Oxide Damascene Structures in Chemical Mechanical Polishing

Nonplanarity arising from the chemical mechanical polishing of Cu-oxide damascene structures results in the exposure field (die-size) being partially out of focus in subsequent lithography process. Thus the corresponding mechanisms of within-die polishing must be determined and the within-die nonplanarity due to polishing needs to be minimized to increase the process yield In this paper, contact mechanics models were developed to explain the role of pattern geometry on the variation of material removal rate. The effects of Cu linewidth, area fraction, and the elastic properties of the polishing pad on pad displacement into low features were examined to focus on the mechanical aspects of the process. The pressure distribution On the high features was determined and the rate of pattern planarization was quantified. Experiments on patterned Cu wafers were conducted to verify the model. Based on these results. The planarization and polishing behavior and the within-die nonplanarity due to the variation of pattern geometry were discussed.

Negative thermal expansion ceramics (a review)

Abstract The literature was searched to identify ceramics which were reported to have negative coefficients of thermal expansion (CTEs). Even though the CTE is an inherent thermophysical property of a material, the observed CTEs were found to depend on the sample preparation techniques, and the experimental data showed hysteresis and scatter. Microcracking, grain boundary separation, plastic deformation and sluggish phase transformations have been suggested to be responsible for the observed hysteresis. Many published results therefore are not reproducible and are even suspect. It is suggested accordingly that care should be exercised in using the published data.

Boundary Lubrication of Undulated Metal Surfaces at Elevated Temperatures

In the boundary lubricated sliding of metals, lubricant molecules desorb from metal surfaces as the interfacial temperature exceeds the transition temperature. As a consequence, numerous metallic contacts will be established, leading to adhesion and wear particle formation. The wear particles so formed plow the sliding surfaces, resulting in high friction and severe wear. In this paper, it is shown that friction can be reduced at elevated temperatures even with additive-free lubricants by using undulated surfaces. Flat and undulated OFHC copper surfaces were tested with various lubricants at different temperatures. Experimental results and theoretical analysis show that undulated surfaces minimize the plowing component of friction due to wear debris, thereby keeping the friction coefficient at a low value after the transition. Presented at the 43rd Annual Meeting in Cleveland, Ohio May 9–12, 1988

Friction and wear of fiber-reinforced metal—matrix composites

Abstract The friction and wear behavior of three uniaxial metal-matrix composites (graphite/Al, stainless steel/Al and Al 2 O 3 /Al-Li) were investigated. Unidirectional sliding tests were conducted in the three principal orientations (longitudinal, transverse, and normal). The experimental results suggest that graphite/Al is a low-friction, low-wear composite. In contrast, stainless steel/Al is a high-friction, high-wear and highly anisotropic composite. Scanning electron microscopic observations of the worn surfaces revealed that in all three composites fiber pull-out was the predominant mechanism of wear in transverse sliding, and is the principal reason for wear anisotropy.