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Rensselaer Polytechnic instituteAutomotive Disc Brakes
Friction & Wear Mechanisms in Braking
Anthony Beeman
5/5/2015
Abstract:
The intent of this paper is to provide the reader with a fundamental introduction to the friction and contact between two rubbing surfaces, describe the specific contact situations found between a brake pad and disc, and to outline the friction and wear phenomena typical of brakes. This paper shall investigate the various types of brake pad materials from a historical prospective in order to see the progress made in brake pad design and to understand the current design limitations that will be a challenge for future brake pad innovation. Results from this paper conclude that the science of tribology has never been more interdisciplinary than that in the tribological system of automobile brakes. Wear behaviors of disc brakes is complex due to the variation in speeds, load, dissipation of energy, and temperature rise. In addition, several wear mechanisms such as abrasion, adhesion, surface fatigue, and tribochemical reactions simultaneously occur in a brake pad tribological system. Friction behaviors are also complex due to the friction coefficient variations that occur due to fade and recovery of the brake pad and rotor. The large number of varying parameters make it difficult to accurately model and predict the friction coefficients of automobile brake systems. Overcoming these design challenges is paramount for future progress and innovation of brake pad design.
Table of Contents
Abstract: 2
1. Purpose: 4
2. Theory and Methodology 4
2.1. Fundamentals of Friction: 4
2.2. Automotive Brake Systems: 5
2.3. Brake Pad Materials Categories: 6
3. Tribological Contact in Brakes 8
3.1. Microscopic Contact 8
3.2. Contact Pressure Effects 9
3.3. Thermal Loading Effects 10
3.4. Wear Mechanisms 11
3.5. Wear Model 12
4. Conclusion 12
5. References 14
1. Purpose:
The purpose of this paper is to investigate the tribological contact and friction phenomena in the brake pad and disc couple. The intent of this paper is to provide the reader with a fundamental introduction to the friction and contact between two rubbing surfaces, describe the specific contact situations found between a brake pad and disc, and to outline the friction and wear phenomena typical of brakes.
2. Theory and Methodology:
2.1. Fundamentals of Friction:
Friction is the resistance that is encountered when two solid surfaces slide or tend to slide over each other. Dry friction occurs when the two surfaces are free from a contaminating fluids. Figure 1 provides a free body diagram of a block on an incline.
Figure 1: Friction Free Body Diagram
The subject free body diagram illustrates that one body is being pressed against another body by a force P. The reaction force R has the same magnitude and line of action as the force P. The reaction force can be resolved into two components, a force normal to the surface, FN, and a force tangential to the surface, FT. From the above statement it follows that, for motion not to occur:
FT= FN tan (ao) = N fo
Where fo=tan (ao) is the static coefficient of friction and ao is the angle of friction at rest. Sliding motion occurs when the tangential force, FT, exceeds the value of N fo. During sliding motion, the tangential component of force P will need to overcome the friction force, F. The force F is commonly expressed as F=f N where f is the coefficient of sliding friction. Normally, the coefficients of sliding friction, f, are smaller than the coefficients of static friction. With small velocities of sliding and very clean surfaces, the two coefficients do not differ appreciably3.
2.2. Automotive Brake Systems:
A vehicle brake is used to slow down a vehicle by converting its kinetic energy into heat. Typically cars today utilize disc brakes which are comprised of the caliper assembly, disc brakes, and a rotor. Figure 2 illustrates an assembly view of an Automobile brake system. The caliper receives a hydraulic input pressure as a result of the driver pressing the brake pedal. The hydraulic pressure squeezes the disc pads closer together which results in the disc pads to come into contact with the rotors. As a result, the energy of motion is converted into heat because of the friction between the brake pad and the rotor.
Figure 2: Automotive Disc Brake System Assembly View
2.3. Brake Pad Materials Categories:
For many years up until the late 1970's asbestos was viewed as having the optimum performance for brake pads. However, due to the health hazards associated with asbestos today's brake pad materials fall into one of three categories; semi-metallic, low steel, and non-asbestos organics. Figure 3 provides a brief timeline of the type of brake pad material category used since 1970. It should be noted that the increases in copper use in brakes is the result of the increased market penetration of non-asbestos organics rather than an increase in the percentage of copper per pad. [7]
Figure 3: Disc Brake Pad Formulation Trend[7]
From 1976 to 1988 semi-metallic materials were predominantly used. Typical semi-metallic formulas contain steel fibers, porous iron powder, abrasives and graphite lubricants. Semi metallic brake pads have a low to medium coefficient of friction that ranges from 0.28-0.38. However, semi-metallic brake pads have a relatively high coefficient variation with increased temperature. Semi-metallic brake pads exhibit excellent fade resistant and wear properties at temperatures over 200C. However, for temperatures under 100C the semi-metallic brake pad exhibits poor wear characteristics.
From 1987 to 2004 low steel brake pad materials were utilized. Low steel materials are comprised of some steel fiber and iron powder, various abrasives and lubricants, some non-ferrous metals. Low steel brake pads have a higher coefficient of friction than semi metallic brake pads that ranges from 0.38-0.50. These brake pads are long lasting but require additional force to slow the vehicle down. As a result, they tend to be very loud and cause extreme rotor wear.
From 1988 to present non-asbestos organic materials have been utilized. Non-asbestos organics are made from a combination of synthetic substances bonded to copper flakes and filaments. Non-asbestos organics formulas contain no ferrous metal, contain nonferrous metals, various abrasives and lubricants, mineral fibers, and other reinforcements. Non-asbestos organic materials are often referred to as ceramics. Semi metallic brake pads have a low to medium coefficient of friction that ranges from 0.33-0.40 and exhibit excellent wear at temperatures under 200C. Non-asbestos organic materials provide a good compromise between the durability of the metal pads the grip and fade resistance of the synthetic material. The biggest drawback with non-asbestos materials is the fact that they do not dissipate heat well as a result of the copper flakes having a high thermal conductivity. The excess heat can result in the brake components to warp.
3. Tribological Contact in Brakes:
The tribological system between brake pads and rotor is very complex due to the variations between temperature, load, environment, and wear mechanisms.
3.1. Microscopic Contact:
Typically brake pads are composed of composite materials that use two or more materials that have very different material properties. As a result, the brake pads show a wide spectrum of wear resistance. Unevenly distributed wear and compaction of wear debris results in a the formulation of contact plateaus. Figures 4 and 5 illustrates a 3D profile of a single contact plateau on a pad (left) and disk surface (right).
Figure 4: Brake Pad Contact Plateau[4] / Figure 5: Rotor Contact Plateau[4]The contact plateaus can be classified as either primary or secondary plateaus. Figure 6 illustrates a composite brake pad cross section with primary and secondary plateaus. The primary plateaus form due to the low removal rate of the wear resistant materials, fibers, within the brake pad composite. Over time the primary plateaus begin to form gaps large enough for the wear debris from both disc and pad to deposit. As brake pressure is applied the debris is compacted and forms secondary plateaus. However, when brake pressure is reduced during sliding, large parts of the secondary plateaus will peel off in flakes.[2] As a result, the plateaus in the brake pad are easily observable with a microscope after a braking application.
Figure 6: Composite Brake Pad Primary and Secondary Plateau Cross Section[4]
3.2. Contact Pressure Effects:
Several experiments have been conducted that show the size of plateaus varies with brake pressure and temperature. It has been shown that the average size plateaus fall within the range of 50 to 100 μm and constitute 10-30% of the normal braking area.[3]. Figure 7 and 8 illustrates the contact plateaus after braking at low pressure/temperature and high pressure/temperature combinations respectively. As pressure and temperature increases the plateaus can grow up to a millimeter in size.
Figure 7: Contact plateaus on standard pad to Volvo 850 after braking(Low Brake Pressure and Temperature) [3] / Figure 8: Contact plateaus on standard pad to Volvo 850 after braking
(High Brake Pressure and Temperature) [3]
3.3. Thermal Loading Effects:
Brake fade is the gradual loss of braking power resulting from a decreased friction coefficient between the disc pad lining and rotor. The effects of brake fade further complicate the tribological contact system for brakes because as the temperature increases the friction coefficient will vary. Brake fade occurs most often during high performance driving or going down a long steep hill in which the sliding time between the brake pad and rotor is increased. Over time as the brake pads continue to contact the rotors, temperatures will increase and the coefficient of friction will decrease. Figure 9 illustrates the decreased friction coefficient over a 20 second time span for three common brake pad materials AP-0, AP-0.75 and AP-1.
Figure 9: Friction Coefficient as a Function of Sliding Time[6]
After cooling, faded brakes usually perform as well as before and there is no visible change to the brake pad. However, if the brakes have been at an elevated temperature for a prolonged period of time glazing can occur on the friction lining of the brake pad which reduces braking efficiency.
3.4. Wear Mechanisms:
Pressure, contacting temperature, and sliding velocity are the three critical factors that affect wear of break pad materials. The major wear mechanisms seen in a brake pad tribological system consist of abrasion, adhesion, surface fatigue, and tribochemical reactions.
As previously discussed, over time the primary plateaus begin to form gaps large enough for the wear debris from both disc and pad to deposit. As a result, three-body abrasionoccurs. The relatively hard contaminant debris will becomes imbedded in one metal surface and is squeezed between the two surfaces, which are in relative motion. This results in surface ploughing to occurs and parallel grooves in the direction of motion are created in the rotor. The subject grooves are illustrated in Figure 5.
Adhesion interaction between the moving rotor and fixed brake pad will result in adhesive wear at the brake pad surfaces. Adhesive wear is the transfer of material from one contacting surface to another. High loads, temperatures, and pressures cause the asperities on two contacting surfaces to spot-weld together then immediately tear apart. As a result the surface of the rotor is left rough due to the deformation of the metal.
Surface fatigue is a process by which the surface of a material is weakened by cyclic mechanical or temperature loading . As brake pressure is applied and temperature increases repeatedly fatigue wear is produced. As a result, wear particles are detached by cyclic crack growth of microcracks on the surface. The fatigue cracks start at the brake pad surface and spread to the subsurface regions. The cracks may connect to each other resulting in separation and delamination of the brake pad material.
Atop of the secondary plateaus a thin tribofilm of dense and sintered debris forms. The tribofilm acts as a case hardening for the secondary plateaus as it is much harder than the bulk material properties of the secondary plateaus. The tribofilm that is formed helps to stabilize the friction coefficient and reduce wear. However, the tribological sliding interaction between the brake pad and rotor leads to chemical reactions that increase wear rates with increased temperatures. As a result, this tribofilm will slowly wear away.
3.5. Wear Model:
When two solid bodies are rubbed together they experience material removal. The amount of material removal is a function of normal pressure, sliding distance, and specific wear coefficients. The wear equation for friction materials can be represented with Rhee's wear equation:
ΔW=kFavbtc
where F is the contact force, v is the sliding speed, t is the time, and k is the wear constant which is a function of the material and temperature. One negative aspect of Rhee's wear model is that a, b, and c are required to be obtained experimentally. As a result, detailed experimentations and correlations are required to obtain the subject wear constants for each brake pad's manufacture proprietary composition.
4. Conclusion:
The science of tribology has never been more interdisciplinary than that in the tribological system of automobile brakes. Wear behaviors of disc brakes is complex due to the variation in speeds, load, dissipation of energy, and temperature rise. In addition, several wear mechanisms such as abrasion, adhesion, surface fatigue, and tribochemical reactions simultaneously occur in a brake pad tribological system. As a result, brake pad wear is not an exact science, its study incorporates many scientific disciplines and principles whose complex interactions lead to considerable areas of uncertainty. Rhee's wear model provides an experimental wear equation for friction materials that has been shown to correlate well with brake pad wear data. However, experimentations and correlations are required to obtain wear constants for each brake pad's manufacture proprietary composition. As a result each unique brake pad proprietary composition and rotor combination will need unique experimentation to accurately correlate the wear rate of brake pads. Friction behaviors are also complex due to the friction coefficient variations that occur due to fade and recovery of the brake pad and rotor. As temperatures increase due to brake fade the coefficient of friction will decrease. The large number of varying parameters make it difficult to accurately model and predict the friction coefficients of automobile brake systems. Overcoming these design challenges is paramount for future progress and innovation of brake pad design.