Technical Data - Metal Matrix

Thermal Properties

Heat Capacity

Given two equal masses of aluminum and cast iron; aluminum is able to absorb twice as much heat energy as cast iron for an equal rise in temperature. This allows an aluminum brake disc to be half the weight of the cast iron disc and still have the same thermal storage capacity.

Thermal Conductivity

Aluminum has a thermal conductivity that is up to 4 times greater than cast iron. This has multiple benefits in braking.

Increased Cooling Rate

Wheel Acts as a Heat Sink

Reduction in Thermal Stress

The Alumatrix brake disc is able to quickly spread heat evenly throughout the disc. This helps to reduce the thermal stress generated between the hat and ring portion of a fixed brake disc. When a brake disc goes through a high-power stop, the ring portion heats very rapidly causing thermal expansion. In the case of a cast iron fixed disc, the ring is not able to expand because it is restrained by the hat section, which is much cooler due to the low thermal conductivity. The large temperature gradient between the ring and the hat generates thermal stresses in the brake disc. With aluminum’s high thermal conductivity, the heat quickly flows out of the ring and into the hat section, reducing the thermal stress generated in a high-powered stop.

Brake Dynamometer Testing

Metal Matrix has conducted and continues to conduct significant on-vehicle and dynamometer testing to ensure the performance, durability and longevity of the Alumatrix brake disc. Testing has been conducted in-house and also at third party testing facilities. The Alumatrix brake disc has been extensively tested on a modified AK Master friction test, as well as the SAE J2928 Thermal Cracking test. Through these tests, we have been able to verify that our brake discs are durable and provide reliable friction. If you are interested in the performance details, you can download test reports below.

Prototype Disc and Pads

Disc

A prototype brake disc has been developed based on the geometry of the rear disc found on the 2013-present Ford Fusion. This solid cast iron disc was used as the bench mark for testing. The MMC prototype was manufactured to the identical dimensions of the cast iron brake disc.

With the geometry of the cast iron and Alumatrix disc being identical, the volumes are also identical. Aluminum has a density 1/3 of cast iron, so with equal volumes, a weight savings of greater than 50% is achieved. This therefore reduces the heat capacity ratio, giving the Alumatrix Brake a heat capacity ratio of less than one. In order the match the heat capacity ratio’s the geometry of the Alumatrix rotor would have to change to increase the volume and weight. An equal heat capacity ratio would provide a weight savings of about 50% on a rear disc.

Protype Disc Dimensions
(revolved section view)

Protype Specifications

Pads

Standard off the shelf NAO pads were used against the cast iron disc while an organic pad, developed by a collaborating partner was used against the Alumatrix disc.

Test Inertia

Rotors were tested with a dyno inertia of 40 kg∙m²

Thermal Cracking Test

Friction vs. Temperature

Brake Emissions

Temperature vs. Initial Speed

Thermal Cracking Test

Alumatrix Brake Disc - 1000 Thermal Cracking Stops

Procedure

The SAE J2928 Thermal Cracking test is a very intense braking test designed to test the disc’s ability to handle thermal stresses during high power braking events. This test runs 150 cycles (300 stops) from 160 kph (100 mph) down to 0.5 kph (0.3 mph) at a constant deceleration rate of 5.0 m/s². The first stop is initiated when the brake disc reaches 100°C (212°F). At the completion of the first stop, the disc then cools for 70 seconds while it spins back up to 160 kph (100 mph). After 70 seconds, the second stop is initiated. The disc then cools down to 100°C (212°F) and the cycle is repeated.

Multiple Alumatrix brake discs have run through 300 or more of these high power stops with 40 kg∙m² of inertia and show no signs of cracking. Maximum operating temperatures reached near 400°C (752°F), which will be considered the maximum temperature threshold. The figure to the right shows an Alumatrix Brake that has done over 1000 stops.

Cast iron discs crack due to their inability to conduct heat away, but with aluminum, the difficulty becomes going over 400°C (752°F). At this point, the aluminum will soften at the disc/pad interface causing smearing and destruction of the transfer layer.

Friction vs. Temperature

Friction vs. Alumatrix Brake Disc Temperature

A specific pad formulation has been developed to work with the MMC braking surface. The plot below shows how the friction coefficient performs with increasing rotor temperatures. It can be seen that as the temperature increase above 300°C (572°F) the friction begins to fade below 0.3. This happens because the resin that holds the pads together begins to sublime, changing from a solid to a gas. The gas is then trapped between the rotor and pad interface acting like an air hockey table. Depending on the pad composition, similar circumstances can be observed with the cast iron friction couple.

Brake Emissions

Most automobile emissions are associated with internal combustion engine exhaust, but there are additional non-exhaust emissions that need to be considered. One of the core contributors to non-exhaust emissions is brake dust, which is released into the environment as brake pads are worn away. Since brakes are needed regardless of a vehicle’s powertrain, brake emissions will remain a concern even as environmentally friendly electric vehicles become more popular.

Pad Wear of Alumatrix Brake Friction Couple Compared To Grey Cast Iron

The SAE Thermal Cracking test (300 stops from 160 kph (100 mph) down to 0.5 kph (0.3 mph) was used to study the wear performance of Alumatrix brake disc and pads compared to grey cast iron. The pads were measured throughout the test to see how they wore in thickness and weight.

The Alumatrix Brake friction couple showed much less wear, with the cast iron losing 3.5 times as much thickness and 2.4 times as much pad weight. This means that not only does the Alumatrix Brake friction couple produce less brake emissions over a set number of stops, it offers longer pad life and lower brake pad costs over the lifetime of a vehicle.

Temperature vs. Initial Speed

Maximum Brake Temperature Reached for a Given Initial Speed

Tests were performed by stopping the Alumatrix brake disc at a constant deceleration from varying initial speeds to measure the maximum temperature that the disc would reach. The same test was performed on a cast iron disc for comparison. The results can be seen in the adjacent figure.

This test demonstrated the effects of the lower heat capacity of Alumatrix brake discs. For a given initial starting speed, and therefore kinetic energy, the disc reached a higher temperature during the braking event. For only having 76% the heat capacity of the cast iron, the maximum temperature seen on the Alumatrix Brake was not much higher. This shows how aluminum's high thermal conductivity can quickly wick the heat away from the friction surface. By changing the disc geometry, the heat capacity can be increased to equal that of the cast iron while still offering a significant weight savings.