Modeling of Constriction Resistance in Coated Joints

Abstract

Metallic coatings applied to surfaces in contact have been shown to be effective at reducing thermal contact resistance. Contact resistance is primarily caused by the constriction of heat flo w as it passes through individual contact spots. Most analyses of coated constrictions have been limited to plane contacts of a semi-infinite cylinder, while an actual constriction terminates in a shape like the frustum of a cone. A numerical model has been developed to determine the constriction resistance of such a coated asperity. The gap between the cone and contact surface is considered to either be evacuated or filled with a gas, and the temperature jump phenomenon is included in the gas-gap model. The effects of radiation heat transfer are also included. The results indicate that an optimum coating thickness for minimizing constriction resistance exists in all cases. Most gases are found to reduce the coating effectiveness very slightly, especially compared to the effect of radiation. The effect of radiation on the model is shown to be highly dependent on the joint temperature, substrate and coating thermal conductivities, and constriction ratio. Contrary to current belief, radiation is shown to be important even for temperatures below 300°C when either the substrate conductivity or the constriction ratio is very low.

An experimentally validated thermo-mechanical model for the prediction of thermal contact conductance

“…This correlating equation is valid for the following ranges of parameter values: 0.01 < a/b < 0.1; 5.83  10 -5 < k gas /k sub < 1.61  10 -3 ; and 0.0175 <  < 0.628. Details of the constriction resistance analysis are available in [18,20].…”

“…This geometry best represents individual deforming asperities [18]. A steady-state heat transfer analysis was performed with a finite-volume approach using the commercial CFD software FLUENT [19].…”

Analysis and Prediction of Constriction Resistance Between Coated Surfaces

“…Adiabatic conditions are prescribed along the axis of symmetry as well as the outside edge of the cylinder. The top surface is held at a constant temperature of 400 K, while the bottom surface is at 300 K. In addition, radiation through the gas gap is permitted, and accounted for as described by Olsen et al 6 For including the radiation contribution, an emissivity value of ε = 0.05 was used for the electroplated metal surfaces. All other participating surfaces in the domain were assumed to have an emissivity of 1 to model them as reradiating surfaces.…”

“…In addition, when the number of grids was more than tripled, the heat flow rates changed by less than 1%, confirming grid independence of the results. The length of the cylinder chosen was shown to satisfy the semi-infinite cylinder approximation by noting that the heat flux across the top surface was spatially uniform 6 . Black et al 7 performed a number of other checks on a similar but uncoated geometry to justify the modeling method.…”

“…Olsen et al 6 proposed that an asperity on a real surface is best modeled as a cylinder terminating in the frustum of a cone. Using a finite difference conduction analysis of this geometry, the effects of contact radius, contact angle, and substrate conductivity on constriction resistance were explored.…”