Surface energy measurements from Griffith cracks in boron fibres

Journal of Materials Science -     

The initial stages of growth and the origin of proximate voids in boron fibres

The formation of so-called proximate voids (i.e. voids near the core-mantle interface) in boron fibres has been studied experimentally. The mantle growth was interrupted in very early stages of nucleation and growth of boron nodules on the tungsten wire substrate. Scanning electron microscope investigations of surface topography and cross-sections revealed a morphology of preferential nucleation of boron nodules along axial die-mark ridges on the tungsten wire surface, where tight parallel axial rows of growing nodules are formed which, particularly around deep wire surface grooves, may hedge in axially extended voids of a morphology earlier found to be typical of proximate voids. The physical conditions of this mechanism are studied with respect to geometry, surface energy and diffusion parameters.     

A specimen preparation technique for transmission electron microscopy of surface layers

A TEM specimen preparation method is described, with the aid of which electron transparent foils can be obtained across the external surface of a specimen. After careful pre-treatment, steel specimens have been electrolytically coated with nickel. Conventional thinning in a plane cutting the substrate-coating interface, gave thin foils displaying the internal structure as a function of depth under the initial free surface. The method has also been applied to minute metal particles, of dimensions too small to allow manipulating and foil preparation by conventional methods. Image examples are shown, and the applicability of the method is discussed.     

Performance and failure of high speed steel drills related to wear

Three high speed steel grades, representing low, medium and high contents of alloying elements, were investigated in a comparative drill performance test using two different work materials. The results are discussed with reference to the observed wear mechanisms. Since drill performance is usually expressed as number of holes to failure, relationships between gradual wear and final failure of the drills are emphasized.The performance tests resulted in chisel edge, crater, flank and margin wear. The corresponding wear mechanisms were studied with the aid of scanning electron and light optical microscopy. The most important mechanisms are (1) abrasive wear for drilling in a plain carbon steel and (2) adhesive wear for drilling in a quenched and tempered steel.     

Radial cracks in boron fibres

The morphology of so-called radial cracks in boron fibres is studied in transverse and axial cross-sections, as well as in fracture surfaces. A formation mechanism is proposed in terms of the transverse residual stress pattern. Small pre-existing voids, (so-called proximate voids) are found to act as stress concentrators which contribute to the opening up of radial cracks in quantitative agreement with the Griffith criterion for brittle fracture.     

A method of using the incident beam tilt as a eucentric goniometer in transmission electron microscopy

The principal difficulties in constructing and operating a eucentric specimen tilting goniometer in a transmission electron microscope are discussed, together with the goniometric function of the incident beam tilt. The latter function is found easy to operate in a eucentric manner. The imaging beam then will have a non-axial path, which will increase particularly the field chromatic aberration. Earlier, however, a technique for the compensation of the chromatic aberration during displaced aperture dark field image formation has been developed. In combination with this technique, it proved possible to use the ordinary incident beam tilt as a eucentric goniometer. Image sequences were obtained, with accurately varied diffraction conditions. The tilt angles and the direction of the tilt axis can be very accurately determined from the displacements of the diffraction pattern.     

Adhesive mechanisms in the wear of some tool steels

The adhesive wear properties of a number of tool steels have been investigated, with special emphasis on the formation and action of prows. The test pieces were worn by dry sliding in a pin-on-ring machine. A special specimen preparation technique made it possible to study the internal structure of wear fragments and prows as a function of depth under the external surface. It was found that prows form in successive steps. The initial step has been reported earlier for pure metals and low carbon steel.A detailed explanation of material deterioration in adhesive wear has been worked out. The dominating mechanism was found to be an abrasive dead zone action, rather than the shearing off or destruction of prows.Investigations by X-ray and electron microscope techniques revealed extensive transformations between the austenite and martensite phases, an extremely fine grain size and a redistribution of carbide and oxide particles during the formation of prows. Correspondingly high microhardness values, up to 1600 HV_0.15N, were measured in the transformed material. Distortion of the retained austenite lattice contributes to the high hardness.     

Force and Energy Measurements During Controlled Grooving—A Basic Study of Abrasive Wear

Basic studies of abrasive wear have been performed by controlled grooving in a modified impact tester equipped with a cemented carbide tip. Specimen holders were constructed to permit normal and tangential force measurements during grooving and to enable quick-stop tests. The grooving energy is read directly from the standard pendulum meter or integrated from tangential force curves.

A series of metals were studied by single-tip grooving and the grooving energy was plotted versus weight loss W within a large W interval. Mettallographic studies reveal characteristic friction layers in the groove bottom and walls and also show that the development of these layers is governed by the mechanisms of chip formation.

A particular purpose of this work is to find relations between internal structure and microhardness profiles on one hand and grooving forces/energy and wear resistance on the other. There are indications that the specific grooving energy e = E/W can be used to predict abrasive wear resistance under widely varying conditions from “mild” to “severe” wear.     

Influence of Normal Force and Frequency in Fretting©

The contact conditions for fretting between spherical caps are reviewed. In particular, the relations between critical displacement and tangential force amplitudes for incipient slip on one hand, and normal force and frequency on the other, are analyzed. Experiments are carried, out with a testing apparatus, specially designed for fretting under controlled conditions within wide intervals of testing parameter values. An austenitic stainless steel and a Cu-3%Si alloy are investigated.

It is confirmed that the critical tangential force is proportional to the normal force, and that the critical displacement is proportional to the 213 power of the normal force for two different frequencies, 300 and 800 Hz. The observed amplitude values are significantly higher than the theoretical values for the higher frequency. The frequency dependence is discussed.