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Thermal Transport Properties of Aluminum Metal Matrix Composites
for Brake Applications

  TPUC

  Diffraction User Center
  Residual Stress User Center
  Diff. and Therm. Prop. Group
  High Temperature Materials Laboratory
  Metals and Ceramics Division
  Oak Ridge National Laboratory
....



Users and Staff

    R. L. Hecht, Materials Science Department, Ford Research Laboratory
    R. B. Dinwiddie, W. D. Porter, H. Wang, ORNL

Background

Aluminum metal matrix composites (Al-MMC's) are candidate materials for automotive brake applications as a means of reducing vehicle weight and brake operating temperatures. One major limitation to widespread commercial use of Al-MMC brakes is their high cost, often more than double the piece cost of gray cast iron rotors. Another issue concerning the use of Al-MMC's for brake components is that current brake systems operate at temperatures higher than the melting temperature of Al alloys. Modeling of the thermal events is a key step in brake design, but this is especially important for the design of Al-MMC brakes. Material properties such as thermal diffusivity, thermal conductivity and heat capacity are inputs to most finite element and analytical models for brakes. In order to provide materials properties for modeling, the thermal transport properties of a series of current, commercially available Al-MMC alloys proposed for use in brake applications have been measured. The overall goal of this project is to determine the effects of particle reinforcement type, particle volume fraction and elevated temperature exposure on the thermal transport properties of Al-MMC's.


Optical Micrograph of material 3-SiC. Dark grey areas are SiC and the light grey areas are Si.


Optical Micrograph of material Al2O3-B.
Grey areas are agglomerates of small Al2O3 particles.

Experimental Details

  • Test specimens were all cut from Al-MMC brake components produced by two US suppliers.
  • Six Al-MMC's have been measured to date.
  • Optical microscopy, SEM, EDAX and image analysis were used to characterize the materials' microstructure at the Ford Research Laboratories.
  • Room and elevated temperature thermal diffusivity was measured on the HTML's Laser Flash Thermal Diffusivity System.
  • Specific heat was measured at HTML via differential scanning calorimetry (DSC).

Micrography Results

Al-MMC's Microstructural & Chemical Characteristics
Material
Label 		Particle Particle
Volume
% 		Particle Ave.
Size (µm) 		Matrix alloy composition
(approximate)
1-SiC SiC 18.2 6.9 Al 359: 9% Si, 0.6% Mg
2-SiC SiC 26.1 7.6 Al 359: 9% Si, 0.6% Mg
3-SiC SiC 32.5 13.5 Al 359: 9% Si, 0.6% Mg
4-SiC SiC 41.4 8.5 Al 359: 9% Si, 1.0% Mg
Al2O3-A Al2O3 40.7 8 x 3 Pure Al
Al2O3-B Al2O3 37.6 8 x 3 Pure Al
 

Thermophysical Property Results

Thermal diffusivity

  • Diffusivity decreases with increasing temperature for the range studied.
  • The greatest effect on diffusivity appears to be the difference between Al2O3 and SiC particle reinforcement.
  • Both the SiC and Al2O3 reinforced materials experienced approximately a 35% drop in diffusivity value from room temperature to 475oC. (In a similar temperature window, gray cast iron diffusivity drops by 50%.)


Heat capacity

  • Heat capacity increases as a function of temperature.
  • Up to about 300oC, the heat capacity of the Al-SiC materials is lower than that of pure Al because it is depressed by the presence of SiC particles and Si in the matrix alloy Al359.
  • The heat capacity of Al2O3 is slightly higher than that of SiC, hence the Al2O3 reinforced materials are closer to the Al values for the whole temperature range.

 

Thermal conductivity, W/m K

  • Thermal conductivity was calculated from K = a r Cp
  • Thermal conductivity decreases with increasing temperature. For all of the alloys, there is approximately a 15% drop in conductivity from Troom to 475oC.
  • For a similar temperature increase, gray cast iron experiences a 35% decrease in thermal conductivity.


Elevated temperature exposure

  • Solutionizing consisted of 16 hours at 538oC. Aging was done at 250oC.
  • Thermal diffusivity of all of the SiC Al-MMC's increases in response to aging. (The matrix alloy of the SiC reinforced materials is Al 359, a heat-treatable alloy.)
  • Thermal diffusivity of the Al2O3 reinforced materials shows little response to long term thermal exposure.
  • After 20 hours of aging, the thermal diffusivity has stabilized in the SiC reinforced materials.


Conclusions

  • Thermal transport in Al-MMC's is influenced by matrix alloy, particle reinforcement type and particle loading volume.
  • Diffusivity decreases with increasing temperature.
  • Elevated temperature aging at 250oC had little effect on the Al2O3 reinforced materials and a slight positive influence on the diffusivity of the SiC-Al 359 alloys.

 

Typical Room Temperature Thermal Transport Values
of Automaobile Brake Materials

Material

Thermal Diffusivity   (cm2/sec)

Thermal cponductivity (W/mK)
SiC Al-MMC 0.85 190
Al203 Al-MMC 0.42 110
Gray cast iron .017 57

 

Publications

"Predicted Influence of Materials’ Thermal Properties on Disc Brake Roughness Due to Thermoelastic Instability". Hecht, R. L., Dinwiddie, R. B., and Porter, W. D., to be published in Proc. of TMS Fall Meeting, 1999.

 

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Acknowledgments
URL: http://www.html.ornl.gov/tpuc/brakes.html