Supersaturated Layer Research Articles

Wide stacking fault of aluminum for multilayered TiB2/Al–Ni composite by roll bonding process

Abstract In this paper, a multilayered TiB 2 /Al–Ni composite was successfully processed via roll bonding process using TiB 2 particles reinforced aluminum matrix composite (TiB 2 /Al) foils and Ni foils as starting materials. There existed a supersaturated solid solute layer (denoted by Al (Ni) layer) near the interface between TiB 2 /Al–Ni layers. Wide stacking faults, up to 20 nm in width, were observed in the Al (Ni) layer. High-resolution electron microscope was used to identify the wide stacking fault. The broadening of stacking fault is attributed to addition of Ni and the local high stress during the rolling process. The presence of stacking fault increases the hardness of interface, and accommodated stress concentration during rolling between TiB 2 /Al–Ni layers to increase deformability of the TiB 2 /Al–Ni composite.

Sunshine on a Cloudy Forecast

The observation of supersaturated layers in Mars' atmosphere has implications for modeling the dynamics of its climate and water cycle.

Cirrus clouds triggered by radiation, a multiscale phenomenon

Abstract. In this study, the influence of radiative cooling and small eddies on cirrus formation is investigated. For this purpose the non-hydrostatic, anelastic model EULAG is used with a recently developed and validated ice microphysics scheme (Spichtinger and Gierens, 2009a). Additionally, we implemented a fast radiative transfer code (Fu et al., 1998). Using idealized profiles with high ice supersaturations up to 144% and weakly stable stratifications with Brunt-Vaisala frequencies down to 0.0018 s−1 within a supersaturated layer, the influence of radiation on the formation of cirrus clouds is remarkable. Due to the radiative cooling at the top of the ice supersaturated layer with cooling rates down to −3.5 K/d, the stability inside the ice supersaturated layer decreases with time. During destabilization, small eddies induced by Gaussian temperature fluctuations start to grow and trigger first nucleation. These first nucleation events then induce the growth of convective cells due to the radiative destabilization. The effects of increasing the local relative humidity by cooling due to radiation and adiabatic lifting lead to the formation of a cirrus cloud with IWC up to 33 mg/m3 and mean optical depths up to 0.36. In a more stable environment, radiative cooling is not strong enough to destabilize the supersaturated layer within 8 h; no nucleation occurs in this case. Overall triggering of cirrus clouds via radiation works only if the supersaturated layer is destabilized by radiative cooling such that small eddies can grow in amplitude and finally initialize ice nucleation. Both processes on different scales, small-scale eddies and large-scale radiative cooling are necessary.

Numerical simulations of contrail-to-cirrus transition – Part 2: Impact of initial ice crystal number, radiation, stratification, secondary nucleation and layer depth

Abstract. Simulations of contrail-to-cirrus transition were performed with an LES model. In Part 1 the impact of relative humidity, temperature and vertical wind shear was explored in a detailed parametric study. Here, we study atmospheric parameters like stratification and depth of the supersaturated layer and processes which may affect the contrail evolution. We consider contrails in various radiation scenarios herein defined by the season, time of day and the presence of lower-level cloudiness which controls the radiance incident on the contrail layer. Under suitable conditions, controlled by the radiation scenario and stratification, radiative heating lifts the contrail-cirrus and prolongs its lifetime. The potential of contrail-driven secondary nucleation is investigated. We consider homogeneous nucleation and heterogeneous nucleation of preactivated soot cores released from sublimated contrail ice crystals. In our model the contrail dynamics triggered by radiative heating does not suffice to force homogeneous freezing of ambient liquid aerosol particles. Furthermore, our model results suggest that heterogeneous nucleation of preactivated soot cores is unimportant. Contrail evolution is not controlled by the depth of the supersaturated layer as long as it exceeds roughly 500 m. Deep fallstreaks however need thicker layers. A variation of the initial ice crystal number is effective during the whole evolution of a contrail. A cut of the soot particle emission by two orders of magnitude can reduce the contrail timescale by one hour and the optical thickness by a factor of 5. Hence future engines with lower soot particle emissions could potentially lead to a reduction of the climate impact of aviation.

Effect of valence electron structure on temper process and hardness of the supersaturated carburized layer

By measuring the hardness of carburized layer of a new type supersaturated carburizing steel (35Cr3SiMnMoV) at different temper temperature for 2 h, the relationship curve between the carburized layer hardness and the temper temperature is established. The result indicates that the hardness goes down firstly, then up and down, just like a wave consistent with the temperature increase. A secondary hardening peak appears at 570 °C or so. Based on Empirical Election Theory (EET) of Solids and Molecules, the valence electron structures (VESs) containing α-Fe-C, α-Fe-C-Me segregation structure units and carbide are calculated. The laws of temper process and hardness change with the temper temperature are explained, and the fact that reconstruction of θ-Fe3C is prior to that of special carbide at high tempering is analyzed with the phase structure formation factor, S, being taken into consideration. Therefore, the laws of temper process and hardness change of supersaturated carburized layer at different temper temperature can be traced back to valence electron structure (VES) level of alloy phase.

Modeling solute clustering in the diffusion layer around a growing crystal

The mechanism of crystal growth from solution is often thought to consist of a mass transfer diffusion step followed by a surface reaction step. Solute molecules might form clusters in the diffusion step before incorporating into the crystal lattice. A model is proposed in this work to simulate the evolution of the cluster size distribution due to the simultaneous aggregation and breakage of solute molecules in the diffusion layer around a growing crystal in the stirred solution. The crystallization of KAl(SO(4))(2)12H(2)O from aqueous solution is studied to illustrate the effect of supersaturation and diffusion layer thickness on the number-average degree of clustering and the size distribution of solute clusters in the diffusion layer.

Formation of large (≃100 μm) ice crystals near the tropical tropopause

Abstract. Recent high-altitude aircraft measurements with in situ imaging instruments indicated the presence of relatively large (≃100 μm length), thin (aspect ratios of ≃6:1 or larger) hexagonal plate ice crystals near the tropical tropopause in very low concentrations (<0.01 L−1). These crystals were not produced by deep convection or aggregation. We use simple growth-sedimentation calculations as well as detailed cloud simulations to evaluate the conditions required to grow the large crystals. Uncertainties in crystal aspect ratio leave a range of possibilities, which could be constrained by knowledge of the water vapor concentration in the air where the crystal growth occurred. Unfortunately, water vapor measurements made in the cloud formation region near the tropopause with different instruments ranged from <2 ppmv to ≃3.5 ppmv. The higher water vapor concentrations correspond to very large ice supersaturations (relative humidities with respect to ice of about 200%). If the aspect ratios of the hexagonal plate crystals are as small as the image analysis suggests (6:1, see companion paper (Lawson et al., 2008)) then growth of the large crystals before they sediment out of the supersaturated layer would only be possible if the water vapor concentration were on the high end of the range indicated by the different measurements (>3 ppmv). On the other hand, if the crystal aspect ratios are quite a bit larger (≃10:1), then H2O concentrations toward the low end of the measurement range (≃2–2.5 ppmv) would suffice to grow the large crystals. Gravity-wave driven temperature and vertical wind perturbations only slightly modify the H2O concentrations needed to grow the crystals. We find that it would not be possible to grow the large crystals with water concentrations less than 2 ppmv, even with assumptions of a very high aspect ratio of 15 and steady upward motion of 2 cm s−1 to loft the crystals in the tropopause region. These calculations would seem to imply that the measurements indicating water vapor concentrations less than 2 ppmv are implausible, but we cannot rule out the possibility that higher humidity prevailed upstream of the aircraft measurements and the air was dehydrated by the cloud formation. Simulations of the cloud formation with a detailed model indicate that homogeneous freezing should generate ice concentrations larger than the observed concencentrations (20 L−1), and even concentrations as low as 20 L−1 should have depleted the vapor in excess of saturation and prevented growth of large crystals. It seems likely that the large crystals resulted from ice nucleation on effective heterogeneous nuclei at low ice supersaturations. Improvements in our understanding of detailed cloud microphysical processes require resolution of the water vapor measurement discrepancies in these very cold, dry regions of the atmosphere.

Impact of ice supersaturated regions and thin cirrus on radiation in the midlatitudes

In this study we investigate the radiative impact of ice supersaturated regions (ISSRs, i.e., cloud free air masses in the upper troposphere that are supersaturated with respect to ice) and thin cirrus. For this purpose we use corrected radiosonde data obtained from routine measurements over the meteorological observatory in Lindenberg, Germany. The radiative effect of the measured ice supersaturation is determined. By constructing an idealized profile from the measurement data the radiative properties of ISSRs and thin cirrus containing ice supersaturation were studied. The impact of ISSRs on the surface forcing is negligible but locally, within the vertical profile, changes in the heating rates up to 1 K d−1 for typical values of 130% relative humidity with respect to ice compared to the saturated profiles are found. This is also important for the local dynamics within the supersaturated layers. The outgoing longwave radiation due to the enhanced water vapor content inside ISSRs decreases up to 0.8 W m−2. The radiative impact of thin cirrus is much stronger. Thin cirrus influence the surface budget, the top of the atmosphere radiation and the vertical profile of the heating rates. Changes in the outgoing longwave radiation and in the reflected shortwave flux at top of the atmosphere up to 64 W m−2 and 79 W m−2, respectively, are possible. Changes in the surface flux (downward) up to 89 W m−2 are found. The maximal heating rate differences between thin cirrus and ISSR amount to 15 K d−1. The radiative impact of thin cirrus clouds depends strongly on cloud ice content and the size of the ice crystals. Additionally, the radiative impact of cirrus formed by parameterizations not allowing ice supersaturations in large‐scale models is estimated. The errors due to artificially formed cirrus are quite large; differences up to 3 K d−1 in the vertical heating profiles and up to 38 and 40 W m−2 in the outgoing longwave radiation and the surface flux, respectively, are found. Thus we recommend using physically based parameterizations in GCMs which allow ice supersaturation.

Evaluation of the use of radiosonde humidity data to predict the occurrence of persistent contrails

AbstractRegular visual observations of persistent contrails over Reading, UK, have been used to evaluate radiosonde measurements of temperature and humidity defining cold ice‐supersaturated atmospheric regions which are assumed to be a necessary condition for persistent condensation trails (contrails) to form. Results show a good correlation between observations and predictions using data from Larkhill, 63 km from Reading. A statistical analysis of this result and the forecasts using data from four additional UK radiosonde stations are presented. The horizontal extent of supersaturated layers could be inferred from this to be several hundred kilometres. The necessity of bias corrections to radiosonde humidity measurements is discussed and an analysis of measured ice‐supersaturated atmospheric layers in the troposphere is presented. It is found that ice supersaturation is more likely to occur in winter than in summer, with frequencies of 17.3% and 9.4%, respectively, which is mostly due to the layers being thicker in winter than in summer. The most probable height for them to occur is about 10 km. Copyright © 2007 Royal Meteorological Society

Displacements of Sb atoms in supersaturated Sb-doped Si layer formed by molecular beam epitaxy growth

Rutherford backscattering channeling measurements with angular scans have been carried out on ⟨100⟩ Sb-doped Si superlattice structures formed by molecular beam epitaxy growth. Three different crystallographic axes ⟨100⟩, ⟨111⟩, and ⟨110⟩ have been explored. We have observed Sb displacements from their substitutional sites with displacement amplitudes of 0.22 Å in as-grown samples. The displacements increase with increasing post-growth annealing temperatures, associated with formation of nanometer size Sb precipitates.

Mesoscale modelling of water vapour in the tropical UTLS: two case studies from the HIBISCUS campaign

Abstract. In this study, we evaluate the ability of the BRAMS (Brazilian Regional Atmospheric Modeling System) mesoscale model compared to ECMWF global analysis to simulate the observed vertical variations of water vapour in the tropical upper troposphere and lower stratosphere (UTLS). The observations are balloon-borne measurements of water vapour mixing ratio and temperature from micro-SDLA (Tunable Diode Laser Spectrometer) instrument. Data from two balloon flights performed during the 2004 HIBISCUS field campaign are used to compare with the mesoscale simulations and to the ECMWF analysis. The observations exhibit fine scale vertical structures of water vapour of a few hundred meters height. The ECMWF vertical resolution (~1 km) is too coarse to capture these vertical structures in the UTLS. With a vertical resolution similar to ECMWF, the mesoscale model performs better than ECMWF analysis for water vapour in the upper troposphere and similarly or slightly worse for temperature. The BRAMS model with 250 m vertical resolution is able to capture more of the observed fine scale vertical variations of water vapour compared to runs with a coarser vertical resolution. This is mainly related to: (i) the enhanced vertical resolution in the UTLS and (ii) to the more detailed microphysical parameterization providing ice supersaturations as in the observations. In near saturated or supersaturated layers, the mesoscale model predicted relative humidity with respect to ice saturation is close to observations provided that the temperature profile is realistic. For temperature, the ECMWF analysis gives good results partly attributed to data assimilation. The analysis of the mesoscale model results showed that the vertical variations of the water vapour profile depends on the dynamics in unsaturated layer while the microphysical processes play a major role in saturated/supersaturated layers. In the lower stratosphere, the ECMWF model and the BRAMS model give very similar water vapour profiles that are significantly drier than micro-SDLA measurements. This similarity comes from the fact that BRAMS is initialised using ECMWF analysis and that no mesoscale process acts in the stratosphere leading to no modification of the BRAMS results with respect to ECMWF analysis.

Corrosion Properties of Supersaturated Magnesium Alloy Systems

The range of applications for magnesium alloys is still limited due to their relatively poor corrosion behavior. In recent years, various new magnesium alloys were developed, some of them with improved corrosion properties, thus opening new fields of application. However, the number of alloying elements for the use in conventional cast processes is limited due to their interaction with liquid magnesium, other alloying elements or large differences in the melting temperatures. The possibilities for grain refinement by post-processing are also restricted. PVD techniques can help to produce supersaturated precipitation free and microcrystalline magnesium layers. Using ion beam and magnetron sputtering, binary or ternary Mg-Al, Mg-Ti and Mg-Sn alloy systems as well as standard alloys (AM50, AZ91 and AE42) were deposited on silicon and on magnesium substrates. The effect of the microstructure on the corrosion properties was studied by comparing as cast material and PVD coatings using potentiodynamic polarization, linear polarization resistance, and electrochemical impedance techniques.

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H 2SO 4 + 2 ppm F − simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm 2 at the compaction force of 150 N cm −2.

Structural stability and hardness of carburized surfaces of 316 stainless steel after welding and after neutron irradiation

Surface hardening treatments offer promise of mitigating the threat of liquid cavitation pitting erosion at the interior surfaces of the austenitic 316 stainless steel vessel that will hold the liquid mercury target of the Spallation Neutron Source. One treatment is a commercial carburization process in which carbon is impregnated at low temperature at concentrations up to 6 wt% in supersaturated solid solution to depths of about 33 μm. The surface hardness of 316L steel is raised from 150 to 200HV 0.05 (micro-Vickers hardness number at a 50 g load) to 1000–1200HV 0.05. It is shown that during subsequent electron beam welding the supersaturated carburized layer in the heat affected zone decomposes to a tiered microstructure of carbide phases in austenite. The hardness of this complex decomposition microstructure is in the range 530–1200HV 0.05, depending on the exposure temperature, the local carbon level, and the size of the carbide particles. To test whether the carburized solid solution layer would break down under atomic displacements from proton and neutron irradiation in service, specimens of annealed and 20% cold-rolled 316LN steel were neutron irradiated to 1 dpa at 60–100 °C. No softening of the layer was detected. Rather, the hardness of the layers was increased by 2–12%, compared to increases of 81% and 43% for the annealed and 20% cold rolled substrate materials, respectively. Optical microscopy examinations of the surfaces of the as-carburized-and-irradiated specimens revealed no sign of decomposition attributable to irradiation.

Relationships between radiosonde and RUC‐2 meteorological conditions and cloud occurrence determined from ARM data

Relationships between modeled and measured meteorological state parameters and cloudy and cloud‐free conditions are examined using data taken over the ARM (Atmospheric Radiation Measurement) Southern Great Plains Central Facility between 1 March 2000 and 28 February 2001. Cloud vertical layering was determined from the Active Remotely Sensed Cloud Location product based on the ARM active sensor measurements. Both temperature and relative humidity (RH) observations from balloon‐borne Vaisala RS80‐15LH radiosonde (SONDE) and the Rapid Update Cycle (RUC) 40‐km resolution model are highly correlated, but the SONDE RHs generally exceed those from RUC. Inside cloudy layers, the RH from SONDE is 2–14% higher than the RH from RUC at all pressure levels. Although the layer mean RH within clouds is much greater than the layer mean RH outside clouds or in clear skies, RH thresholds chosen as a function of temperature can more accurately diagnose cloud occurrence for either data set than a fixed RH threshold. For overcast clouds (cloud amount greater than or equal to 90%), it was found that the 50% probability RH threshold for diagnosing a cloud, within a given upper tropospheric layer, is roughly 90% for the SONDE and 80% for RUC data. For partial cloud cover (cloud amount is less than 90%), the SONDE RH thresholds are close to those for RUC at a given probability in upper tropospheric layers. Cloud probability was found to be only minimally dependent on vertical velocity. In the upper troposphere, SONDE ice‐supersaturated air occurred in 8 and 35% of the clear and cloudy layers, respectively. The RH was distributed exponentially in the ice supersaturated layers as found in previous studies. The occurrence of high‐altitude, ice‐supersaturated layers in the RUC data was roughly half of that in the SONDE data. Optimal thresholds were derived as functions of temperature to define the best RH thresholds for accurately determining the mean cloud cover. For warm clouds the typical SONDE threshold exceeds 87%, while the RH thresholds for cold clouds are typically less than 80% and greater than 90% with respect to liquid and ice water, respectively. Preliminary comparisons with satellite data suggest that the relationships between cloudiness and RH and T determined here could be useful for improving the characterization of cloud vertical structure from satellite data by providing information about low‐level clouds that were obscured by high‐level clouds viewed by the satellite. The results have potential for improving computations of atmospheric heating rate profiles and estimates of aircraft icing conditions. Similar analyses are recommended for later versions of the RUC analyses and forecasts.

Formation of ice supersaturation by mesoscale gravity waves

Abstract. We investigate the formation and evolution of an ice-supersaturated region (ISSR) that was detected by means of an operational radiosonde sounding launched from the meteorological station of Lindenberg on 21 March 2000, 00:00 UTC. The supersaturated layer was situated below the local tropopause, between 320 and 408 hPa altitude. Our investigation uses satellite imagery from METEOSAT and the Advanced Very High Resolution Radiometer (AVHRR) and analyses of the European Centre for Medium-Range Weather Forecasts (ECMWF). Mesoscale simulations reveal that the ISSR was formed by a temporary vertical uplift of upper tropospheric air parcels by 20 to 40 hPa in 1 to 2 h. This resulted in a significant local increase of the specific humidity by the moisture transport from below. The ascent was triggered by the superposition of two internal gravity waves, a mountain wave induced by flow past the Erzgebirge and Riesengebirge south of Lindenberg, and an inertial gravity wave excited by the anti-cyclonically curved jet stream over the Baltic Sea. The wave-induced ISSR was rather thick with a depth of about 2 km. The wave-induced upward motion causing the supersaturation also triggered the formation of a cirrus cloud. METEOSAT imagery shows that the cirrus cloud got optically thick within two hours. During this period another longer lasting thin but extended cirrus existed just beneath the tropopause. The wave-induced ISSR disappeared after about half a day in accordance with the decaying wave activity.

Discontinuous tracks in relaxed SiGe alloy layers: Formation and thermal evolution

Strain-relaxed epitaxial Si 1− x Ge x alloy layers partially doped with arsenic were irradiated with U 238 ions of energy 0.8–2.7 GeV in the electronic stopping regime. Using transmission electron microscopy, clear evidence is found that in some of the Si 1− x Ge x alloy layers latent tracks are created. Depending on the composition of the Si 1− x Ge x alloy and on the doping level of As, the tracks exhibit a more or less discontinuous character. Larger GeAs precipitations are observed in supersaturated alloy layers. We also find indications that the velocity of the projectile ions plays an important role. Additional thermal treatment results in gradual annealing of track-related defects at low temperature (200–400 °C) and in the formation of GeAs precipitates at high (850 °C) temperature.

Formation of a tropopause cirrus layer observed over Florida during CRYSTAL‐FACE

On 13 July 2002 a widespread, subvisible tropopause cirrus layer occurred over the Florida region. This cloud was observed in great detail with the NASA Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE) instrumentation, including in situ measurements with the WB‐57 aircraft. In this paper, we use the 13 July cloud as a case study to evaluate the physical processes controlling the formation and evolution of tropopause cirrus layers. Microphysics measurements indicate that ice crystal diameters in the cloud layer ranged from about 7 to 50 μm, and the peak number mode was about 10–25 μm. In situ water vapor and temperature measurements in the cloud indicated supersaturation with respect to ice throughout, with ice saturation ratios as large as 1.8. Even when the ice surface area density was as high as about 500 μm2 cm−3, ice supersaturations of 20–30% were observed. Trajectory analysis shows that the air sampled near the tropopause on this day generally came from the north and cooled considerably during the previous few days. Examination of infrared satellite imagery along air parcel back trajectories from the WB‐57 flight track indicates that the tropopause cloud layer formation was, in general, not simply left over ice from recently generated anvil cirrus. Simulations of cloud formation using time‐height curtains of temperature along the trajectory paths show that the cloud could have formed in situ near the tropopause as the air was advected into the south Florida region and cooled to unusually low temperatures. If we assume a high threshold for ice nucleation via homogeneous freezing of aqueous sulfate aerosols, the model reproduces the observed cloud structure, ice crystal size distributions, and ice supersaturation statistics. Inclusion of observed gravity wave temperature perturbations in the simulations is essential to reproduce the observed cloud properties. Without waves, crystal number densities are too low, crystal sizes are too large, and the crystals fall out too fast, leaving very little cloud persisting at the end of the simulations. In the cloud simulations, coincidence of high supersaturations and high surface areas can be produced by either recent nucleation or sedimentation of crystals into supersaturated layers. The agreement between model results and observed supersaturations is improved somewhat if we assume that the steady state relative humidity within cirrus at T < 200 K is enhanced by about 30%. The WB‐57 measurements and the model results suggest that the cloud layer irreversibly dehydrated air near the tropopause.

PULSED ELECTROCHEMICAL MACHINING OF IRON IN NaNO3: FUNDAMENTALS AND NEW ASPECTS

ABSTRACT The interface of ARMCO iron during electrochemical machining (ECM) was investigated by microelectrochemical techniques. The results lead to a duplex layer structure model, a solid oxide film, and a supersaturated viscous layer. The consequences on pulsed ECM, especially on the important parameters such as pulse/pause ratio, potential, current density of oxygen evolution, Fe 2+ and Fe 3+ formation, pH, efficiency, temperature, thicknesses of solid, and supersaturated layers are discussed.

Comments on “Thermophysical-property-based selection of tool protective coatings for dry machining of steels” by Wit Grezesik et al., Journal of Manufacturing Science and Engineering., Nov. 2003, Vol. 125, pp. 689–695

Contributed by the Manufacturing Engineering Division for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received January 26, 2004; revised July 20, 2004. Associate Editor: M. A. Davies.Thermal matching of coating-tool materials to optimize dry machining of steels is an interesting idea. Grezesik et al. find that the thermal properties of the materials involved, namely, the thermal diffusivity and the thermal conductivity, control the thermal and tribological behavior outputs of the machining process. Such a finding has been detailed in the discussor’s own work when studying dry sliding of metallic contacts 12, however, the physical interpretation of the contact parameter b in the manuscript is not apparent. The parameter b is originally given by: b=KρC1/2 where K is the thermal conductivity, ρ is the mass density, and C is the specific heat. In this form, the contact coefficient b represents the interaction between the injection of frictional heat normal to the surface of contact and the thermal storage of the bulk material. Now recasting the contact coefficient to the form b=KD−1/2 is perhaps more revealing of its physical significance. Note that in this form, b represents the interaction between the speed of heat injection through the surface and the speed by which the heat spreads through a given area. Now, the diffusivity will attempt to homogenize the distribution of heat throughout a given area of the material, thus attempting a uniform distribution of temperature. The conductivity, on the other hand, will attempt to inject heat normal to the contact surface. Thus, the parameter b represents a balance between heat injection and heat distribution, that is, b is a measure of heat intensity at the contact spot. Perhaps more indicative of the physical role of the parameter b is the term “coefficient of heat penetration” 3 or thermal effusivity, which indicates another physical attribute of transient thermal transport between sliding contacts. The effusivity plays a detrimental role in the evolution of heat transfer in sliding as it represents the effects of thermal inertia on a medium. In two contacting materials, the material with higher effusivity will resist the penetration of the thermal flux. Whereas the material with lower effusivity will allow deeper penetration. As such, the ratio of the effusivity of two conducting materials (such as the coating and the substrate) will influence the thickness of the thermal layer and, thereby, the dissipation of the heat flux generated in machining. Moreover, the rate of change in the effusivity will affect the rate of change in the depth of thermal penetration and, thereby, the growth of the temperature gradient. The ultimate effect will be the creation of a thermally supersaturated layer 4 that controls the quality of the machined surface and tool wear. While the generation of heat flux in machining is not directly linked to the variation in the thermal conductivity, conduction of heat away from the surface is linked to that variation. The variation of the thermal conductivity, along with the evolution of the temperature gradient, will affect the partition of the generated heat among the tool-coating assembly, the work piece, and the chip. That is, the thermal loads on each of the previous components will follow the change in the conductivity. An increase in the conductivity will lead to a corresponding increase in the heat conducted to either the work piece or the tool. If such an increase is associated with a decrease in thermal diffusion, the intensity of heat will increase locally. This will reduce the material flow stress and, therefore, frictional stress. The notion that the contact length can be effectively used for the control of heat flux and temperature at the interface was based on the temperature dependence of the thermal number Pec. It seems that the authors have avoided the physical interpretation of that number. Figure 5 of Grezesik’s paper shows a delay in reaching the maximum temperature at higher thermal numbers. This is due to the dominance of convection, which helps cooling the interface. Thus Fig. 5 suggests that the rate of temperature rise and not the temperature (and thereby thermal property variation) may be controlled by controlling the thermal number. However, this may prove difficult to achieve, as the thermal number is not a process parameter.