H.A. Abdel-Aal

On the interdependence between kinetics of friction-released thermal energy and the transition in wear mechanisms during sliding of metallic pairs

Abstract Sliding of complying solids is often associated with the release of thermal energy. This energy accumulates within the mechanically affected zone (MAZ) of the rubbing pair. The accumulation of thermal energy within the MAZ tends to maximize the potential energy at the interface. Now, since a maximized potential energy renders the sliding system unstable, one (or both) materials will respond in a manner that consumes (dissipates) part or all of the accumulated energy in order to re-establish system stability or at least equilibrium. The material response may be in many forms: oxidation, crack initiation, wear debris generation, transition in wear mechanism, etc. As such, one may consider that these processes are intrinsic responses by the material to dissipate energy. Moreover, many of these responses are triggered at different stages of rubbing according to the balance between the rate of external thermal energy release (which is a factor of the nominal operation parameters) and the rate of thermal energy accumulation—RTEA (which is mainly a function of thermal transport properties of the rubbing pair). An interesting feature of this view is that the later quantity—RTEA—is directly related to the ability of the particular solid to dissipate thermal loads. This quantity, which is termed here as the heat dissipation capacity (HDC), is directly related to the state of blockage of energy dissipation paths within the rubbing solid. The objective of this paper is therefore to study the relation between the change in the HDC of a sliding solid and the transition in the mechanism of wear. It is shown that there exists an inverse correlation between the change in the HDC and the transition in the mechanism of wear. Moreover, it is also shown that a so-called ratio of residual heat (RRH, representing the ratio between the actual thermal load and the part of that load that is not dissipated by the solid) is a significant parameter that influences the magnitude and mechanism of wear. The findings are applied to explain the wear behavior of two tribo systems: a titanium (Ti–6Al–4V) sliding on itself and sliding on a steel (AISI M2) counterpart.

A remark on the flash temperature theory

The classical solution of the temperature development in a sliding solid pair treats the thermal conductivity as a non-variant quantity that is not affected by temperature elevation. This may introduce an erroneous estimate of the temperature distribution in the contacting pair. This error may be intensified in the presence of considerable temperature gradients. In this note, we apply the well known Kirchoff transformation to correct the temperature estimates in a practical case of sliding friction. Thus, accounting for the point wise variation of the thermal conductivity in the contacting pair. We compare temperature estimates, obtained by the classical solution, to those obtained by the variable conductivity solution. The results indicate that the behavior of the thermal conductivity with temperature is influential to the magnitude of the peak flash temperatures developed at the tip of the contact asperities.

On friction-induced temperatures of rubbing metallic pairs with temperature-dependent thermal properties

Abstract This paper investigates the temperature rises for dry sliding systems when the variation in the thermal conductivity with temperature is taken into account. For the purpose of the analysis, it has been assumed that the thermal conductivity of the rubbing materials vary linearly with temperature. Accordingly, materials are classified into three categories based on that variation: materials for which the conductivity drop with temperature elevation (class a): materials for which the conductivity increases with temperature elevation (class b): and materials for which the conductivity-temperature curve has an inflation point (class c). The variable conductivity temperatures are obtained by applying the so called ‘Kirchoff transformation’ to the fundamental solution of the heat equation. The results indicate that the behavior of the conductivity with temperature is significantly influential to the magnitude of the temperatures reached by the rubbing pair. For a variety of sliding pairs analyzed in this work, significant variation between the constant and the variable conductivity predictions were found. For example, the temperature rise for a mild steel (AISI 1020) rubbing pair, sliding at 6 m/s and 30 N nominal load, predicted by the variable conductivity solution is about 30% higher than that predicted using a constant conductivity solution. It is also shown that the estimates of the heat conducted through the surface may be in error (by about 30–40%) if based on a constant conductivity solution. Such behavior has direct effects on the thickness of the thermally affected subsurface layer (the so-called thermal skin), and the thermal distortion of the contact interface. The error introduced in the estimates of the temperature rises for class c materials is shown to be proportional to the ratio between the inflation to the melting temperatures of the moving solid.

The correlation between thermal property variation and high temperature wear transition of rubbing metals

This paper reports the findings of a preliminary investigation of the correlation between the thermal properties of a rubbing material and its wear behavior at elevated temperatures. It is conjectured that, the potential of a material to accommodate a sudden change in its thermal state upon contact is an important factor that influences wear resistance of the material. Accordingly, metals which intrinsically maintain a smooth transition between two thermal states are more likely to display a favorable wear behavior at higher temperatures. Thus for such metals, the nature of transition is conducive to protective layer formation. The results indicate that for all alloys, an equilibrium between the conductive and the diffusive effects is maintained throughout the temperature intervals for which a favorable wear behavior was experimentally reported. Moreover, the bounds of such intervals mark the respective reported wear regime transition temperatures.

Efficiency of thermal energy dissipation in dry rubbing

The changes in friction and wear, especially during the run-in period, are strongly correlated to the blockage of energy dissipation paths within the sliding materials. As such, the preservation of the tribological integrity of a rubbing material depends, mainly, on the efficiency of dissipation of the friction induced thermal energy (FITE). This paper therefore, studies the mechanistic evolution of FITE and the process of its dissipation during sliding. The study focuses on the intrinsic aspects of the dissipation process. In particular, the role of the mechano-physical properties (conductivity, diffusivity, hardness, etc.) and their respective variation with temperature is studied comprehensively. This is achieved by studying the behavior of two contrasting rubbing pairs: mild steel (AISI 1020) sliding on itself and, a stainless steel (AISI 304HN) sliding on a mild steel counterpart. The results indicate that the external sliding parameters (nominal load and speed) are of limited influence on the dissipation process. The major influence, however, is assumed by the change in the thermal properties and more dominantly, by the rate of degradation of the conductivity in the contacting layers of the rubbing pair. These results support the view that wear generation is, in essence, an intrinsic energy dissipation mechanism the aim of which is to consume part of the FITE released at the surface.

Estimation of Temperature Distribution in Silicon During Micro Laser Assisted Machining

Silicon is machined using a diamond tool and the process is assisted with an IR Laser for the purpose of heating and thermal softening the work piece material. The laser beam passes through the tool and into the work piece, where the material is both thermally heated (by the laser) and mechanically deformed (by the tool). The laser is used to increase the work piece temperature (up to the softening temperature of silicon, about 500–800°C [10]), while the tool deforms and cuts the heated and softened silicon in the ductile regime, without producing cracks. This hybrid laser assisted machining process results in a smooth plastically deformed surface and extends the life of the diamond tool when cutting a hard and abrasive material, e.g. silicon. Scratch tests were done using the micro laser assisted machining method with diamond tools, which demonstrated enhancement in the depth of cut from 60 nm to 120 nm with (a 2x increase in depth of cut, at a constant load) while the cutting speed varied from 0.305 mm/sec to 0.002 mm/s. An analytical and numerical method was used to estimate the temperature rise in the vicinity of the diamond tool due to laser irradiation and absorption by the silicon work piece. It is assumed that the layer of silicon that absorbs the heat from the laser radiation is silicon II. Silicon II is a metallic phase of silicon, commonly referred to as the beta-tin structure, formed by a high pressure phase transformation (HPPT). In this context, the analytical and numerical models are solved using the heat conduction equation for semi-infinite solid over time with a Gaussian laser beam intensity distribution. The temperature rise for different cases (laser intensity, depth of cut, cutting speed, etc.) was modeled using point, and plane heat source method with Gaussian intensity distribution. These results are discussed in detail to estimate the temperature distribution while machining.Copyright

A note on the intrinsic thermal response of metallic pairs in dry sliding friction

Abstract This paper develops a general expression for the specific rate of heat dissipation ( SRHD ) - ability of a material to dissipate an applied thermal load per unit surface temperature rise- of metals. The expression incorporates the variation in each of the thermal diffusivity and the thermal conductivity of the material with temperature elevation. It is shown that the general expression for SRHD is a super position of three functions which represent the competing effects of the room temperature thermal properties and their respective variation with temperature. Such expression is believed to facilitate parametric studies which investigate the effect of heat accumulation on the transition of wear mechanisms.

Error bounds of variable conductivity temperature estimates in frictionally heated contacts

Abstract We analyze the effect of using a linear conductivity-temperature model to solve the nonlinear heat equation for two rubbing solids. We divide errors into two groups, sampling errors and linearization errors. We relate the first to the choice of a reference conductivity in the linear model. Whereas, we relate the second to the modeling of the conductivity values when the k-Θ curve of the material includes an inflation point (classC materials). The results suggest that the temperature estimates are sensitive to the choice of the reference conductivity, when the variation in the conductivity of the material with temperature is considerable (e.g., for Sapphire). We show that the errors for class c materials are proportional to the ratio of the inflation, to the melting temperatures of the sliding solid.

On the development of surface temperatures in precision single-point diamond abrasion of semiconductors

Abstract This paper presents an analytical, variable-conductivity, thermal model for single-point diamond precision machining of semi conductors. The model is used to calculate the temperature development during the simulated machining of silicon and germanium wafers. The heat generated at a tool-work piece contact is taken as the product of the coefficient of friction, the relative sliding speed, and the contact stress. Whereas, heat partition between the diamond abrasive and the work piece is obtained through a modified Jaeger-Blok analysis that incorporates the coupling of the thermal properties of the tool-work piece system. The results indicate that: for the ranges of nominal loads, cutting speeds and, feeds encountered, the work piece surface temperature rise is well below the thermal softening temperatures for these materials. This, has direct implications to the ductile removal mechanisms. The low values calculated for the work surface are shown to be consistent with several calculations based on the analysis of conventional machining.

On the bulk temperatures of dry rubbing metallic solid pairs

Abstract The calculation of temperature fields that develop in rubbing metallic pairs is customary performed under the assumption of a steady state and; that heat is released at the interface of the rubbing pair. These assumptions have several drawbacks as they neglect the role of the subsurface structural deformations which gives rise to internal heat generation. This paper introduces a solution for the transient temperature fields developed within two rubbing metallic solids with internal heat generation that varies exponentially with the distance from the nominal contact surface. The results indicate that in contrast to the customary models, the maximum temperature appears at some distance in the subsurface region. A result that is supported by the experimental observations of different authors.