Ploughing Mechanism Research Articles

Combustion‐thermal‐sprayed recycled poly(ethylene terephthalate)

AbstractThermal‐sprayed polymer coatings have been used as protection against corrosion and wear. In this study, poly(ethylene terephthalate) (PET) powder, which was obtained from postconsumer beverage bottles, was deposited on 1020 steel by low‐velocity flame‐spray technology. The chemical and structural changes in PET due to the thermal‐spray processing were investigated with Fourier transform infrared spectroscopy and X‐ray diffraction. Changes in tribological behavior were examined by pin‐on‐disk testing and three‐dimensional profilometry. The results show that coatings had the same functional groups as the PET beverage bottles. However, the degree of crystallinity was modified. These changes were rationalized in terms of possible structural modifications of the PET polymer. The study showed evidence that the pin‐on‐disk wear developed by an abrasion process through a ploughing mechanism, although a fatigue mechanism could not be disregarded. A low friction coefficient between PET and steel was confirmed. In the as‐sprayed condition, the PET coatings showed higher friction, likely because of a higher coarse debris production rate during the pin‐on‐disk testing. Heat treating the as‐sprayed coating to increase the amorphous PET content improved the sliding behavior by increasing wear resistance. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3159–3166, 2004

Possible erosion mechanisms in snow avalanches

AbstractSnow erosion and entrainment processes in avalanches are classified according to their mechanisms, the flow regimes in which they occur, and their spatial position within the avalanche. Simple, but process-specific, models are proposed for erosion by impacts, abrasion, plowing and blasting. On the basis of order-of-magnitude estimates, the first three mechanisms are clearly expected to be important. The fourth mechanism stipulates that the compaction of the snow cover ahead of the avalanche leads to the flow of escaping air just in front of the avalanche that may disrupt the snow cover and support formation of a saltation layer. The effects of this hypothetical mechanism resemble those of the plowing mechanism. All mechanisms depend strongly on the snow properties, but, with plausible parameter values, erosion rates at or above the experimentally found rates are obtained. The entrainment rate of an avalanche is most often limited by the shear stress needed to accelerate the eroded snow to avalanche speed.

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5–85nm sputtered on Si(100) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200nm and contact (normal) loads between 10 and 1200μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.

Estimation of in-process cutting constants in ball-end milling

Two methods are presented for the estimation of tangential, radial and axial cutting coefficients for the shearing and ploughing mechanisms from a single set of cutting forces in ball-end milling. These estimation methods are based upon the invertibility of the analytical milling force model, which considers both the shearing and the ploughing mechanisms by incorporating their respective cutting constants in the local force model. The periodic milling forces are established as the convolution integral of the differential local cutting forces and their Fourier coefficients are derived and expressed in a matrix expression as a linear function of the unknown cutting constants in terms of cutting conditions and cutter geometry. This linear expression thus leads to a systematic formulation of the estimation methods allowing the six unknown cutting constants to be determined from the measured milling forces. The first method uses the first harmonic forces as the source signal while the second method extracts the six cutting constants from the average force as well as the first harmonics. Limitations of both estimation methods are discussed. The consistency and accuracy of the estimated cutting constants are confirmed by the experimental results.

An analytical force model with shearing and ploughing mechanisms for end milling

An analytical force model with both shearing and ploughing mechanisms is established for the end milling processes. The elemental forces are defined as the linear combination of shearing and ploughing forces in six cutting constants. The analytical model for the total milling forces in the angular and frequency domain are derived by convolution approach and Fourier transform respectively and are expressed as the superposition of the shearing force component and ploughing force component. This dual-mechanism model is analyzed and discussed in the frequency domain and compared with the lumped shear model. An expression is derived for identifying the cutting constants of the dual-mechanism model from the average milling forces. Explicit inclusion of ploughing force in the model is shown to result in better predictive accuracy and yields a linear force model with constant cutting coefficients. Experiments verify the accuracy and the frequency analysis of the dual-mechanism model and show that cutting constants for the dual-mechanism model are fairly independent of chip thickness.

A New Model and Analysis of Orthogonal Machining With an Edge-Radiused Tool

A new machining process model that explicitly includes the effects of the edge hone is presented. A force balance is conducted on the lower boundary of the deformation zone leading to a machining force model. The machining force components are an explicit function of the edge radius and shear angle. An increase in edge radius leads to not only increased ploughing forces but also an increase in the chip formation forces due to an average rake angle effect. Previous attempts at assessing the ploughing components as the force intercept at zero uncut chip thickness, which attribute to the ploughing mechanism all the changes in forces that occur with changes in edge radius, are seen to be erroneous in view of this model. Calculation of shear stress on the lower boundary of the deformation zone using the new machining force model indicates that the apparent size effect when cutting with edge radiused tools is due to deformation below the tool (ploughing) and a larger chip formation component due to a lower shear angle. Increases in specific energy and shear stress are also due to shear strain and strain rate increases. A consistent material behavior model that does not vary with process input conditions like uncut chip thickness, rake angle and edge radius can be developed based on the new model. [S1087-1357(00)01302-2]

Scratch test of soda-lime glass

A series of single point scratch tests have been performed on soda-lime glass slides using spherical and conical diamond indenters. The conical indenters had different apex angles, 2 θ. The force ratio (horizontal to normal) is independent of normal load or driving speed, but increases linearly with cot θ, indicating a plowing mechanism in which the scratch hardness (normal load divided by one half of the vertically projected contact area) is equal to the plowing stress (horizontal force divided by the horizontally projected contact area). As the driving speed increases, the width of the scratch decreases so both the scratch hardness and the plowing stress increase proportionally while the crack density decreases. For spherical indenters, the friction appears to be caused by adhesion rather than plowing. In a liquid environment (water) the horizontal force increases and stick–slip appears. The rate of crack formation seems to follow first order kinetics in which the probability of cracking within an interacting distance is proportional to the fraction of the uncracked portion of the scratch. The damage mechanism changes from deformation to cracking with the increase of scratch depth.

Strain Hardening of Fcc Metal Surfaces Induced by Microploughing

Microploughing experiments were used as a method for better understanding the ploughing mechanism in gold and iridium single crystals. The plough depths ranged from 20 nm in iridium to 1600 nm in gold. Yield stress profiles and TEM analyses indicate that both materials strain harden even when very small volumes of material are involved. Strain hardening theory, as applied to bulk material, is useful in analyzing the results.

A Dual-Mechanism Approach to the Prediction of Machining Forces, Part 1: Model Development

A cutting-process model addressing the chip removal and edge ploughing mechanisms separately yet simultaneously is presented. The model is developed such that it is readily applicable in an industrial setting, its coefficients have physical meaning, and it can be calibrated with a concise quantity of orthogonal cutting data. The total cutting and thrust forces are each the summation of its individual components acting on the rake face and clearance face. These components are calculated using the rake and effective clearance angles from the normal and friction forces acting on each of these tool surfaces. These normal and friction forces are calculated by the chip removal and edge ploughing portions of the model, respectively, using four empirical coefficients. To calculate the clearance face forces, the interference volume is required, the calculation of which is based on a geometrical representation of the clearance face interference region. This representation is characterized in part by the depth of tool penetration, which is influenced by thermal energy generation and is therefore determined using a fifth empirical model.

Tribological properties of thin silver films sputter-deposited on superalloy substrates

Thin silver films have been deposited on superalloy (alacrite) substrates with an average roughness of 0.2–0.3 micrometer by radio frequency magnetron sputtering of a silver target in argon atmosphere. The silver films, of 0.4–20 μm thickness, exhibited a crystallographic structure preferentially oriented in the (111) direction, with grain sizes of 150 nm. The compressive residual stresses in these films were as low as -0.05 GPa. Argon-free silver films deposited on silicon substrates at the floating potential possessed an electrical resistivity similar to the bulk-resistivity value. The tribological tests were carried out with an alumina ball-on-disk apparatus operating under loads of 9.8, 2.9 and 1 N at room temperature and 500 °C with a sliding velocity of 0.02 m s −1 for 5000 to 90 000 cycles, i.e. for an average sliding distance varying from 385 m to 7 km. After an initial stage of about 1000 cycles, the friction coefficient of silver-coated alacrite disks was essentially independent of the number of cycles. The average friction coefficient values were in the range 0.14 to 0.42, depending upon film thickness and test temperature. After 5000 cycles under loads of 2.9 and 1 N, the bottom of the wear tracks exhibited a periodic sequence of bumps and hollows. The periodicity of these undulations was dependent on the load and test temperature. The formation of these undulations probably resulted from the adhesive friction mechanism of silver films. Under a load of 9.8 N, the bottom of the wear tracks was flat and the wear of silver films was caused by a ploughing mechanism. The variation of the average friction coefficient of silver-coated alacrite disks with film thickness at room temperature was interpreted on the basis of a mechanism of friction proposed for soft metal films deposited on steel substrates when sliding against hard antagonists.

The unlubricated reciprocating sliding wear of a martensitic stainless steel in air and CO 2 between 20 and 300 °C

A limited programme of work has been undertaken to investigate the self-wear behaviour of a high strength martensitic stainless steel. This material is used as boiler tube supports in advanced gas-cooled reactors. Some tests were carried out in air at room temperature to permit comparison with data sets for other materials but most of the tests were in CO 2 in the temperature range 20–300 °C. Wear rates were measured by specimen weight loss and by continuous collection and intermittent weighing of wear debris. At room temperature, both in air and CO 2, specific wear rates were initially high, i.e. of the order of 10 −12 m 3 N −1 m −1. The severe wear rate however decreased with increasing temperature in CO 2 reaching a value of about 1 × 10 −13 m 3 N −1 m −1 at 300°C. Wear transitions were observed at all temperatures except 200 °C and resulted in a decrease in wear rate by between a factor of two and two orders of magnitude. The extent of the decrease in rate, however, showed no consistent trend with either temperature or load. Associated with the wear transition was a change in friction behaviour from widely fluctuating during the severe wear stage to smoother sliding in the post-transition stage. The duration of the severe wear stage at 20°C in CO 2 appeared to be proportional to load. A correlation was found between the initial wear kinetics and the mode of wear surface deformation. The highest wear rates occurred when the surface contact interactions resulted in gall formation; galling being defined as the displacement of material from one part of a surface to another region by a ploughing mechanism. The severe wear rate progressively decreased as the surface deformation mechanism changed to one of transfer from one surface to the other with the subsequent formation of features called prows. There was clear metallographic evidence that wear debris was generated by breakdown of these surface features.

The unlubricated wear of sintered iron

The unlubricated wear of sintered pure iron has been investigated under different sliding conditions using a dynamic wear rig operating in the scanning electron microscope and a simple pin-on-disk apparatus operating in air. Delamination and ploughing mechanisms were found to be responsible for wear during sliding under the different conditions. Using normalized variables, the measured wear rates agreed well with the predictions of Archard's law and correlated with wear data from other sources. The effect of porosity in the iron has also been studied. Open pores on sliding surfaces were found to be important in generating and trapping wear debris. When the trapping mechanism was bypassed by cleaning, or rendered ineffective by pore closure, the wear behaviour of high porosity sintered iron approached that of the lower porosity specimens.

Wear of boundary-lubricated metal surfaces

In the past, the friction and wear of boundary-lubricated metallic surfaces were attributed to adhesion and shearing of lubricants. However, examination of the friction and wear of pure metals lubricated with mineral oil indicates that while the friction coefficient was typical of the values obtained in boundary-lubricated sliding, the predominant wear mechanism was an abrasive-type mechanism. Scanning electron microscopy and surface profilometry revealed many ploughing grooves on the surfaces. These results indicate that the ploughing mechanism may be the predominant factor in controlling friction and wear of boundary-lubricated surfaces. On the basis of slip line field analysis and surface topography statistics an approximate expression for the wear coefficient was derived. It was found that the wear coefficient depends on the sharpness of the surface asperities (or the entrapped wear debris), the interfacial “friction” and the extent of the plastically deformed zone below the surface.

The influence of surface roughness on abrasive wear

Analyses are given of the mechanism of friction and abrasive wear and of the effect of surface roughness on them. Theoretical expressions are derived for ploughing, adhesion and the total friction coefficient of hard conical asperities ploughing a soft metal surface, with the assumption that the asperities of the hard metal are cones with randomly distributed slopes, the mean value of which varies with surface roughness. Simple expressions for the abrasive wear rate and the mean wear particle size are also derived on the basis of a ploughing mechanism of the hard conical asperities on the soft metal surface. A comparison of calculated values based on these theories with experimental data of single-pass wear tests for various soft metals such as copper, cadmium, lead and zinc sliding on low carbon steel plates shows good agreement. The effects of surface roughness on the tangential forces under unlubricated and lubricated conditions as well as the mean wear particle size are theoretically discussed and the theoretical results are compared with experimental data.

Paper 16: Solid Friction and Boundary Lubrication

During the last decade considerable progress has been made in our understanding of the contact between solid surfaces. Earlier work emphasized the plastic deformation and more recent work the elastic deformation of the surface asperities. However, when sliding occurs combined normal and tangential stresses in the contact zone greatly enhance the plastic régime. Broadly speaking, recent work supports the earlier view that friction is attributable to adhesion at the points of real contact and to the ploughing or tearing of one surface by asperities on the other. In high vacuum the surface films normally present can be removed either by heating or by repeated rubbing. As a result the adhesion, friction and wear can rise to very high values. However, even under very clean conditions some metallic pairs appear to show relatively weak adhesion. With non-metals similar processes occur but with brittle solids they are complicated by surface and sub-surface cracking. Here crystal orientation can be very important. With polymeric solids, where strength properties are rate and temperature dependent, marked variations in friction with speed and temperature are often observed. The more rigid the polymer the smaller the effect. In the field as a whole little work has been carried out on the interaction between the adhesion and the ploughing mechanisms. In the field of lubrication it is now apparent that a considerable amount of what was formerly termed boundary lubrication was probably elastohydrodynamic. However, recent studies show that elastohydrodynamic films can operate under more severe conditions if small quantities of surfactants are added. These do not affect the viscous properties of the lubricant but form protective surface films of molecular dimensions. During the last decade little work has been carried out on the formation, structure and strength properties of these boundary films. It remains an area of great importance.