J.A. Williams

Analytical models of scratch hardness

Abstract The ability of one material to abrade or scratch another has a long history as the basis of a scale of hardness and is clearly related to the now more common measure of hardness as the resistance of a material to indentation. However, there is no simple relation between the values of hardness from these two tests. Producing an abraded groove in the surface of a specimen involves large plastic strains, and a number of analytical models have been proposed for this process based on such techniques as slip line field theory and the theorems of plastic load bounding. While there is reasonable agreement between the predicted and measured values of the loads, these methods are much less satisfactory in providing estimates of the patterns of deformation or strain around the groove and these limitations may be associated with some of the idealizations relatively simple models make about material behaviour — such as isotropy and a neglect of elastic effects. The scratch testing of brittle solids can also involve plastic behaviour facilitated by the high local hydrostatic compressive stresses generated close to the tip of a sharp indenter. Scratch hardness is also finding applications as a test for the integrity of coated substrates, although the modes of failure here may be more complex than those observed in homogeneous materials.

Characterization of Receptors for Cholecystokinin and Related Peptides in Mouse Cerebral Cortex

Abstract: The characteristics of cholecystokinin (CCK) binding to its receptors in a particulate membrane fraction of mouse cerebral cortex were studied by employing biologically active radioiodinated CCK prepared by conjugation with 125I‐Bolton‐Hunter (125I‐BH) reagent. At 24°C binding was rapid, reversible, and linearly related to protein content. Binding was maximal at acidic pH (6.5) and reduced by the presence of monovalent cations. Under physiological conditions (pH 7.4, 118 mM‐NaC1, 4.7 mM‐KCl) Scatchard plots of CCK binding were linear with a KD value of 1.27 nM and binding capacity of 115 fmol/mg protein. Optimal binding required the presence of both Mg2+ and EGTA, and was inhibited by the addition of micromolar concentrations of Cu2+ (ID50= 30 μM). The cortical receptor recognized all major forms of CCK, with an order of potency of cholecystokinin octapeptide (CCK8) > CCK > cholecystokinin tetrapeptide (CCK4). Desulfated cholecystokinin octapeptide (dCCK8) had a 10‐fold lower affhity than CCK8. Dibutyryl cyclic GMP, a potent competitive inhibitor of CCK binding to receptors in pancreas, was not a specific inhibitor of CCK binding to brain receptors. These present results support the concept that CCK may function as a regulatory peptide in brain, and that the cortical CCK receptor is different from the receptors mediating the peripheral action of CCK.

Diamond and diamond-like carbon MEMS

Diamond and diamond-like carbon (DLC) thin films possess a number of unique and attractive material properties that are unattainable from Si and other materials. These include high values of Youngs modulus, hardness, tensile strength and high thermal conductivity, low thermal expansion coefficient combined with low coefficients of friction and good wear resistance. As a consequence, they are finding increasing applications in micro-electro-mechanical systems (MEMS). This paper reviews these distinctive material properties from an engineering design point of view and highlights the applications of diamond and DLC materials in various MEMS devices. Applications of diamond and DLC films in MEMS are in two categories: surface coatings and structural materials. Thin diamond and DLC layers have been used as coatings mainly to improve the wear and friction of micro-components and to reduce stiction between microstructures and their substrates. The high values of the elastic modulus of diamond and DLC have been exploited in the design of high frequency resonators and comb-drives for communication and sensing applications. Chemically modified surfaces and structures of diamond and DLC films have both been utilized as sensor materials for sensing traces of gases, to detect bio-molecules for biological research and disease diagnosis.

The prediction of friction and wear when a soft surface slides against a harder rough surface

The friction and wear caused by a single hard asperity in repeated sliding contact have been studied in our earlier publications. This paper outlines a method of combining all the contacts that occur between a soft surface and the asperities on a randomly rough hard surface and leads to predictions of the overall coefficient of friction and the wear rate of the soft surface. The hard asperities are assumed to be each capped with a spherical top of constant radius and their peak height distribution is taken to be Gaussian. Higher asperities in sliding contact may cut or plough the soft surface while lower asperities contact the soft surface elastically. Combining all the individual asperity contacts gives quantitative predictions of both friction and wear. Comparison between these and experimental results from the literature provides encouraging agreement.

Tribology and MEMS

Micro-electro-mechanical system, MEMS, is a rapidly growing interdisciplinary technology dealing with the design and manufacture of miniaturized machines with the major dimensions at the scale of tens, to perhaps hundreds, of micrometres. Because they depend on the cube of a representative dimension, component masses and inertias rapidly become small as size decreases whereas surface and tribological effects, which often depend on area, become increasingly important. Although our explanations of macroscopic tribological phenomena often involve individual events occurring at the micro-scale, when the overall component size is itself miniaturized it may be necessary to re-evaluate some conventional tribological solutions. While the absolute loads are small in such micro-devices, the tribological requirements, especially in terms of longevity—which may be limited by wear rather than friction—are particularly demanding and will require imaginative and novel solutions.

Evidence That Cholecystokinin Interacts with Specific Receptors and Regulates Insulin Release in Isolated Rat Islets of Langerhans

To determine the nature of the pancreatic islet cell cholecystokinin (CCK) receptor, we studied CCK receptor binding and biologic activity in isolated rat pancreatic islets. Binding of 70 pM 125I-CCK to collagenase-prepared isolated rat pancreatic islets at 24°C was one-half maximal after 5 min and maximal at 60 min. At 60 min, specific binding was 12% of total radioactivity per 100 μg islet protein; nonspecific binding (in the presence of 1 μM CCK 8) was less than 2% of total radioactivity. Unlabeled CCK 33 inhibited labeled hormone binding one-half maximally at 2 nM; Scatchard analysis showed one binding site (Kd, 2.3 ± 0.4 nM; Bmax, 8.1 pmol/mg protein). The agonist selectivity of this binding site was: CCK 8 = CCK 33>desulfated-CCK 8>CCK 4. Two CCK antagonists were studied; N-carbobenzoxy-L-tryptophan was more potent than dibutyryl-cGMP. When the effect of CCK on insulin release from the islets was studied, the order of potency of CCK agonists and antagonists on insulin secretion was the same as the order of their ability to inhibit 125I-CCK binding. The effect of CCK on insulin secretion was dependent on the glucose concentration in the media. CCK had no effect at 5.6 mM glucose and was fully effective at 11.0 mM glucose. These data, therefore, indicate that: (1) specific binding sites for CCK are present in rat pancreatic beta, cells; and (2) CCK acts in concert with glucose to stimulate insulin secretion.

The use of a shear instability criterion to predict local necking in sheet metal deformation

A shear instability criterion can provide a consistent approach to the onset of local necking in sheet metal forming under biaxial stretching. The neck is anticipated to initiate in the direction of pure shear when the shear stress attains some critical value. The yield function proposed by Hill is employed and the material is assumed to display only normal anisotropy. Empirically it is found that the additional material parameter required by this yield function is simply related to R, the coefficient of normal anisotropy, for a number of materials. This allows the limit strain to be predicted in terms of three well established plastic properties, viz. work hardening coefficient, coefficient of normal anisotropy and initial pre-strain. The influence of these on the limit strain curve is analysed and the coefficient of work hardening shown to play the most important role. Data available in the literature are employed in a comparison of the present theory with that due to Marciniak. In general the predicted limit strains are in reasonable agreement with the trend of experimental results for a wide range of materials. In the case of isotropic materials with work hardening coefficients in the range 0·2-0·6 predictions from the present theory are almost identical with those from that presented by Storen and Rice. The theory presented here exhibits a good correlation with experimental limit strains for materials with high work hardening coefficients, of approx. 0·4 or more. Generally, for low work hardening materials, with coefficients of 0·25 or less, the shear instability theory tends to an underestimate of limit strains and a Marciniak type of analysis may be more appropriate. However, bearing in mind the scatter of the experimental data the present theory constitutes a safe lower bound on limit strains and, in addition, has the advantage of simplicity in the mathematical calculation required.

Mechanisms of Abrasive Wear in Lubricated Contacts

Abstract Despite the fact that many tribological components are designed to operate with a comparatively thick film of lubricant, the bearing surfaces often still deteriorate with time as hard particulate contaminants are swept through the bearing gap. These may arise from the external environment or, perhaps, be wear debris from other pairs of surfaces lubricated by the same fluid. In order to investigate this phenomenon experimentally it is necessary to develop a predictable hydrodynamic film between the test surfaces which can be contaminated by small volumes of carefully graded abrasive particles. Here we describe the use of a foil bearing to generate such films between 10 and 50 μm thick to which contaminants such as powdered quartz or finely divided diamond can be added. As the ratio of the characteristic particle size to film thickness is varied not only do the wear rates of the solid surfaces change but examination of the wear tracks suggests that very different mechanisms of material loss come into operation. When the size ratio is low the worn surface of the cylinder is covered by what appears to be small erosion pits; these display virtually no alignment in the direction of relative sliding and it seems that the particles tumble and roll freely through the gap. Above some critical value of the ratio the appearance of the surface changes dramatically to a grooved or micro-machined surface with all the grooves aligned in the sliding direction. A relatively simple theoretical model is developed, based on what happens to a typical particle, which goes some way to explaining these observations. As well as being consistent with the observed transition from “tumbling” to “grooving”, the model can also explain why increasing the hardness differential between the hard and the soft surfaces does not always lead to a reduction in damage to the harder member of the pair.

The role of lubricants in machining

Abstract Friction between the rake face of a tool and the freshly formed chip surface plays a vital role in influencing the ease of the metal cutting process. The existence of clean surfaces and high hydrostatic stresses favours the formation of strong adhesive friction junctions; the extent of these can be limited by the provision of a suitable lubricant. Experiments using a novel planing machine, which enables the orthogonal cutting operation to be carried out in carefully controlled atmospheres, are described; it is suggested that processes of molecular transport within a network of interfacial capillaries are dominant in controlling the effectiveness of cutting fluids provided they are capable of reaction with, or adsorption by, the metal surfaces.

Localization of Saturable CCK Binding Sites in Rat Pancreatic Islets by Light and Electron Microscope Autoradiography

Cholecystokinin (CCK) is a known stimulus for the release of insulin and other islet hormones. To localize islet cell CCK binding sites, we measured the uptake of 125I-CCK by the isolated, perfused rat pancreas. Light microscope autoradiographs revealed uptake of label over both the endocrine islets of Langerhans and the exocrine acini. This uptake of 125I-CCK was saturable, as it decreased markedly when a large excess of unlabeled CCK8 was included in the perfusion solution. To define which cells in the islets bound CCK, electron microscope autoradiographs were prepared. The majority of silver grains in islets were localized over beta cells (69%), although saturable uptake was also observed over alpha (12%) and other islet cells. When grain densities were analyzed (grains/μm2), the highest density was observed over islet blood vessel cells. In contrast to islet blood vessels, there was no localization of 125I-CCK over acinar blood vessels. This study supports the concept, therefore, that there is a direct regulation of islet endocrine cells by CCK, and also raises the possibility that CCK influences islet hormone release via an indirect effect on the islet vascular endothelium.