A.W. Ruff

Measurement of solid particle velocity in erosive wear

Abstract A method is described for determining the velocity of solid particles in a gas-particulate stream applied to erosion testing of materials. A simple mechanical configuration allows the measurement to be made under a wide range of equipment conditions. The time-of-flight of the particles is determined over a controlled path length between two rotating disks. Examples of measurements on several test apparatus are presented. The importance of nozzle design is discussed. A comparison between particle and gas stream velocity is presented.

Friction and wear properties of WC-Co and Mo-Mo2C based functionally graded materials

Abstract Wear resistant coatings based on functionally graded materials (FGMs) applied on industrial machinery components can reduce weight, increase adhesion strength, decrease internal stresses and improve the resistance against propagation of surface defects. Macroscale FGMs offer a new method of surface engineering to produce tailored tribological properties. In order to fully exploit the FGM concepts, an efficient fabrication with advanced process control assuring the stability of the resulting properties is desirable. The fundamental understanding of wear damage modes of each layer, as well as the development of predictive models for through-thickness behavior can increase the industrial applicability of graded coatings. In the present study, two industrially relevant FGM coatings were investigated: high-velocity-oxygen-fuel (HVOF) deposited WC-Co/stainless steel and plasma sprayed Mo-Mo2C/stainless steel FGMs. For both deposition processes, high degree of automation was achieved and linearly graded coatings were successfully prepared. Sliding friction and abrasive wear responses were evaluated through thickness and damage mechanisms controlling the coefficient of friction and wear rates were described. A correlation between composition, microstructure characteristics and damage modes was established. The enhancement of Mo-Mo2C coating properties through FGM approach with stainless steel was reported.

Transmission and scanning electron microscopy studies of deformation at erosion impact sites

Abstract Scanning and transmission electron microscopy methods have been employed to study topographic features and subsurface damage associated with erosive-particle impact craters in annealed 310 stainless steel surfaces. Angular Al 2 O 3 and spherical glass particles approximately 50 μm in diameter were projected at a velocity of 59 m s −1 to impact the surface at attack angles of 90° and 20°. Under these conditions, material was found to be displaced but not removed from the surface at isolated impact sites. A comparison was made with damage produced at diamond pyramid hardness indentations. Substantial differences were not observed. In general, a high dislocation density zone a few microns wide was found to surround both impact craters and hardness indentations. The width of this zone varied according to the size and shape of the crater and the direction of particle motion. Deformation twinning occurred at some impact sites. The plastic strain associated with impact craters in 310 stainless steel and copper was also determined by a method that is based on an analysis of selected-area electron channelling patterns.

Effect of layer spacing on wear of Ni/Cu multilayer alloys

Abstract Alloys consisting of multiple alternate layers of nickel and copper have been prepared by electrodeposition and studied in sliding wear. Most recently a multilayer alloy with a layer spacing of 3.8 nm was studied, to complement earlier studies of multilayer alloys with layer spacings of 10 and 100 nm. In order to determine wear characteristics, the alloys were tested in unlubricated sliding against type 52100 steel in a crossed-cylinder geometry at various loads. It was found that the most wear-resistant alloy was the one having the smallest layer spacing, 3.8 nm. Further, all the multilayer alloys showed less wear than either copper or nickel deposited from the same solution and tested under the same conditions. The multilayer structure in these alloys is thought to provide internal barriers to wear which occurred by plastic deformation processes. The data suggest that the individual layer spacing determines the level of stress necessary to initiate severe wear.

Sliding wear studies of ni-cu composition-modulated coatings on steel

Abstract Sliding wear measurements have been conducted on Ni-Cu composition-modulated coatings on steel. The electrodeposited coatings consisted of alternate layers of nickel and copper with equal layer spacings of either 10 or 100 nm. Pure nickel and pure copper coatings were also prepared in a similar manner and studied. The coated cylinders were slid against AISI type 52100 steel in a crossed cylinder geometry under clean conditions without lubrication at various loads and sliding distances. It was found that the most wearresistant coating was the composition-modulated alloy having the smallest layer spacing, 10 nm. This effect was most pronounced at low loads. Both composition-modulated coatings showed less wear than coatings of pure copper and nickel deposited from the same solution. The layer microstructure in these composition-modulated coatings may provide internal barriers to wear damage, thus leading to the increases in wear resistance. Results from examination of the worn specimens and collections of wear debris are also described.

Deformation studies at sliding wear tracks in iron

Abstract Determinations have been made of strains on the surface and subsurface of specimens of high purity iron after different distances of sliding wear. The method involved the measurement of loss of intensity (contrast) of particular electron channeling lines obtained from small selected areas near the wear track. A calibration specimen, deformed plastically to a range of strain values, was used to relate the channeling line contrast loss to plastic strain. Strain maps lateral to the wear track and below the original surface were obtained after removing controlled thicknesses of iron by electropolishing. In all cases the maximum strain was found at the track center location at the surface and the strains decreased steadily with depth below the track. With a 50 g load the strains vanished at about 40 μm depth. Significant strains were found to exist outside the wear track boundaries. There was no indication of a soft or less-hardened surface layer in any of the specimens studied.

Damage processes in ceramics resulting from diamond tool indentation and scratching in various environments

Abstract Studies have been carried out to determine the influence of different chemical environments and tool shapes on damage produced during diamond tool scratching and indenting of two advanced ceramics: chemical-vapor-deposited silicon carbide and a composite aluminum oxide-titanium carbide. A nanoindentation/scratching apparatus was used in a controlled-load mode for the studies. Two diamond tool shapes were used: a wedge with −45° rake and 0.5 mm radius curved edge, and a Vickers pyramid. The environments studied were: air, water, mineral oil, mineral oil + stearic acid and two commercial water-based fluids. It was of interest to identify damage mechanisms, critical loads for initiation of severe damage, specific scratching energies and the effect on damage of the different tool shapes. It was found that chemico-mechanical interactions during scratching in the different environments led to significant differences in such parameters as damage threshold load, maximum tool penetration depth and specific scratching energy. In some cases surface chemical films appeared to form and control the scratching process, especially at low loads. At higher loads significant chemical influences on the mechanical damage processes also were found. Damage morphology involved discrete surface cracking, spalling, grain pull-out and plastic deformation. In many cases considerable plastic response of the relatively brittle ceramics was observed. The proportions of cracking and plasticity varied substantially with tool shape, environment and material. Discontinuous surface cracking damage was observed in some cases, suggesting that a sub-surface process of damage or strain energy accumulation was involved.

Characterization of debris particles recovered from wearing systems

Abstract Studies have been conducted on wear debris collected from various systems involving both sliding and rolling contacts. Results of size, shape and chemical composition analyses of typical debris particles are presented. Different techniques for debris recovery and analysis are discussed. Some results are presented on electron transmission studies of individual debris particles. The effect of small particle size on X-ray emission microanalysis is also described.

Deposited thin-film wear sensors: materials and design

Abstract Vacuum deposited thin-film sensors have been developed for monitoring wear and temperature in a variety of bearing applications. The sensor typically consists of one or more metallic or insulator films. This lamination sputtered thin-film resistance elements and thermoelectric elements with appropriate insulators is deposited directly on the bearing surface. The thin-film package can be designed to have wear behavior similar to that of the bearing materials, and to have a small surface area that conforms to the bearing contour. The wear sensor monitors the regression of the surface, i.e. the loss of material due to wear, in the range 0.1–10 μm (or more) over the small area that the sensor occupies. The thermoelectric sensor monitors local temperature. The effect of the materials and fabrication on the properties of the sensors are described as they effect the electrical response and the wear characteristics. Desired properties needed by the conducting film component include adequate electrical conductivity, adhesion to the insulating layers, low ductility, adequate toughness, and hardness. The critical properties of the insulating film component were found to be adhesion, low conductivity, strength, and hardness. Test results were obtained for sliding ring and block tests in both air and oil-lubricated environments. For conducting films, aluminum, platinum, tantalum, and type K + and K − thermoelectric alloys were studied and are reported here. Copper and two metal silicides were also evaluated. For insulating films, results on aluminum oxide, tantalum oxide, and borosilicate glass are reported. The substrate block material was 304 stainless steel. It was found that the electrical performance of the conducting films in the sensor package can be described using a simple electrical resistance model.

Measurements of plastic strain in copper due to sliding wear

Abstract Wear experiments have been conducted to determine the plastic strains that are introduced in surface material near sliding wear tracks. Both oillubricated and dry-sliding experiments have been carried out at different sliding distances on surfaces of copper. The strain values were determined from selected-area electron channelling patterns obtained from regions as small as 10 μm in size and 0.05 μm deep around the wear track using a scanning electron microscope. A deformed calibration specimen was used to relate the electron channelling band contrast to the deformation strain. Strain maps were obtained on the wear surface lateral to the wear track and also below the surface using electropolishing metal removal techniques. Particular attention was given to the near-surface strain values. In all cases the maximum strain was found at the wear surface located at the track center and the strains decreased uniformly with depth. Significant large strains were also found outside the wear tracks. The results are compared with those previously reported for iron and with other relevant studies.