D465 car brake pads with different content of artificial graphite were prepared using dry powder metallurgy mixing and mould pressing. Through the LINK inertia bench test，the effects of artificial graphite on the friction material performance were studied. The results show that the artificial graphite has a little influence on physical and mechanical properties of friction materials; With the increase of artificial graphite content，the friction coefficient of friction material shows a trend of gradually decreasing，the wear quantity presents the tendency of decreasing firstly then increasing; the effect of artificial graphite on noise occurrence of friction materials has the same trend as the wear quantity. Combining and comparing with the physical properties，chemical properties，mechanical properties，friction coefficient and wear data，when the artificial graphite content is controlled in 8%，friction and wear properties of friction materials are optimal.
Artificial graphite, also known as Synthetic Graphite (SG), is mainly produced by calcining high-quality petroleum coke at a high temperature of 2500 ~ 3000 °C. Compared with natural graphite, it has high purity, less ash, complete crystal form and uniform particle size. Artificial graphite can form a friction film with other components in the friction material. The proper amount can improve the lubrication performance, reduce the abrasion and ease the friction. At the same time, its high temperature performance, high thermal conductivity, low thermal expansion coefficient, stable chemical properties, can effectively reduce vibration and noise, has been widely used in brake materials for many years. However, when the temperature is higher than 500 °C, the graphite will rapidly oxidize, making the surface of the friction material loose and the wear is intensified. Therefore, in the air environment, artificial graphite is suitable for use at low and medium temperatures, and when it exceeds 500 °C, the lubricating performance is lowered.
Artificial graphite has good chemical stability. At the working temperature of the friction material, artificial graphite has almost no chemical reaction with other friction materials. In high temperature sintered friction materials, artificial graphite will not react with special friction materials such as brass and copper in the formulation, artificial graphite will not melt, soften or become sticky. Therefore, it does not form a bond or gel on the friction surface; the graphite at normal temperature is relatively stable in chemical properties, and is insoluble in dilute acid and alkali.
In this experiment, the effect of artificial graphite was studied by a single variable method: five formulations gradually increased the content of artificial graphite, and the components of the friction material included a resin matrix, a fiber reinforcement, a friction modifier, and a filler.
Performance Testing
According to the test method of the apparent porosity of the automotive brake lining of QCT583-1999, the porosity of the sample opening is measured by the oil immersion method. The hardness of the test piece was measured on a HR-150A Rockwell hardness tester, and the indenter was a steel ball having a diameter of 12.7 mm, and a force of 980 N was applied.
According to the ISO 6310 road vehicle brake lining compression strain test method, the Model 1620 compression performance tester of American LINK Company was used to test the deformation of the brake pad sample at 7 Pa and 16 Pa under load at room temperature.In this way, the compressibility of the friction material is evaluated.
According to GB / T 26739-2011 road vehicle brake lining material internal shear strength test method, the internal shear strength of the friction material was tested on the RP300 shear strength tester.
According to the JC / T685-1998 friction material density test method, the density of the brake pad samples was measured by the Archimedes principle drainage weighing method.
The pH of the friction material is measured using a pH meter in accordance with the JASO C458-1986 pH test procedure for brake linings, gaskets and clutch discs for automobiles.
The friction and wear performance test was carried out on the US LINK 3000 inertial bench test machine according to the SAE J2522 standard. The brake pad samples were mounted on the quarter brake assembly and the hydraulic brake method was used to test the friction material samples’ changes in friction performance at different speeds, different brake pressures, and different brake temperatures in different brakes. The surface morphology of the friction block samples after the test was analyzed by a Japanese JSM-6490LV scanning electron microscope.
The friction material brake noise test is tested on the US LINK 3900 NVH inertia test bench according to SAE J2521 standard. The test results of brake speed, brake pressure, brake temperature and braking direction are tested for the occurrence of brake noise.
2 Results and analysis
2. 1 Effect of artificial graphite on physical and chemical properties of friction materials
The physical and chemical properties of different artificial graphite samples show that the porosity of the material increases with the increase of artificial graphite content; the density and pH of the material do not change with the addition of artificial graphite.
In the process of increasing the content of artificial graphite from 4% to 8%, the increase of porosity is small; and when the content of artificial graphite is increased from 8% to 12%, the effect of artificial graphite on the formation of pores is enhanced, and the porosity is increased. obvious. By observing the SEM morphology of the artificial graphite, the increase of the porosity is due to the porous structure of the artificial graphite, and various mixed powders cannot effectively fill the internal pores of the artificial graphite during the hot press forming process. This phenomenon becomes more pronounced as the amount of artificial graphite increases. The larger the porosity of the friction material, the smaller the mass per unit volume and the lower the density. Therefore, as the content of artificial graphite increases, the density decreases. In the five basic formulations, the variable artificial graphite and filler barium sulfate are neutral, so the content of artificial graphite in this series of formulations has little effect on the pH value.
2. 2 Effect of artificial graphite on mechanical properties of friction materials
The mechanical properties of samples with different artificial graphite content show that the hardness of the sample and the shear strength at room temperature are decreasing, and the compressive amount at room temperature is increasing. These changes are mainly due to the soft and loose porous of artificial graphite, and the phase with resin. Poor capacitance, low interface bonding strength, and reduced material mechanics.
2. 3 Effect of artificial graphite on the wear properties of friction materials
2. 3. 1 Effect of artificial graphite on friction coefficient
The SAE J2522 standard was used to investigate the variation of friction coefficient of friction materials under different braking speeds, different braking pressures and different braking temperatures. Due to the large amount of experimental data, this paper selects the nominal friction coefficient and the lowest friction coefficient to discussion.
The nominal friction coefficient characterizes the overall friction coefficient level of the friction material, and the lowest friction coefficient represents the material’s resistance to thermal decay. With the increase of artificial graphite content, the nominal friction coefficient of the material first decreases and then increases, and the lowest friction coefficient remains basically unchanged. This trend is determined by the lubricating properties and chemical stability of artificial graphite.
With the increase of the content of artificial graphite, on the one hand, the residual substances generated by the oxidative decomposition of organic matter at high temperature during working will combine with graphite to form a structurally stable lubricating film, and the lubricating effect is gradually increased, so the friction coefficient is lowered; As the porosity of the material increases gradually and the structural strength gradually decreases, the shear resistance of the material gradually decreases, which also leads to a decrease in the friction coefficient. By observing the SEM morphology of the sample with 4%, 8%, and 12% artificial graphite after performing the SAE J2522 test standard, it can be seen that the graphite content is low, the lubricating film is thin and incomplete, and thus the lubrication effect is low and the friction coefficient is high. . The amount of artificial graphite added is moderate, the friction film formed by the material is complete in structure, the lubrication effect is good, and the friction film formed by friction is dense and complete, and thus the friction coefficient is lowered. When the artificial graphite is further increased to 12%, the nominal friction coefficient does not decrease due to the poor oxidation resistance of the graphite. When the temperature exceeds 500 °C, the excess graphite is exposed to the air and is rapidly oxidized, destroying the friction membrane, the lubrication is reduced, and the friction coefficient is slightly increased.
2. 3. 2 Effect of artificial graphite on the wear properties of friction materials
The thickness and weight of the friction material before and after the SAE J2522 test standard were measured and recorded. The difference in thickness and weight before and after the test was the amount of wear during the test.
As the content of artificial graphite increases, the thickness wear and mass wear of the friction material first decrease and then increase. The wear amount is the lowest when the artificial graphite content is 8%. This is because as the artificial graphite increases, the friction film of the material gradually becomes complete, and the friction coefficient is gradually reduced to reduce the wear amount. However, when the graphite is further increased, since the excess graphite is exposed to the air, it reacts with oxygen at a high temperature. The friction film is broken and the coefficient of friction is also increased, thereby increasing the amount of wear.
2. 4 Effect of artificial graphite on the noise of friction materials
The SAE J2521 noise test standard was carried out to investigate the occurrence of frictional noise of friction materials under different test conditions such as braking speed, braking temperature, brake pressure and braking direction. Brake sounds with a sound frequency range of 2 000 Hz to 17 000 Hz and a decibel value of 70 dB or more are recorded as brake noise.
From the frequency of occurrence of braking noise and the distribution of decibel values of friction materials, the frequency of braking noise of friction materials with different artificial graphite content is mainly distributed at 8 000 Hz, 12 500 Hz, 15000 Hz, and the decibel value is up to 110 dB. The noise generated is mainly caused by drag and deceleration braking. As the content of artificial graphite increases, the frequency of braking noise decreases first and then increases. The noise frequency distribution shifts from low frequency (800 Hz) to high frequency (15 000 Hz), and the decibel value decreases first and then increases. When the artificial graphite content is 8%, the braking noise occurrence probability and the noise intensity are the lowest, and the comprehensive score is the highest. When it is increased to 12%, there is a high frequency and high decibel noise (ie screaming). It is known from the frictional noise occurrence probability and the scoring value of the friction material with different artificial graphite content that as the graphite content increases, the probability of occurrence of noise decreases first and then increases. When the graphite content is 8%, the probability of occurrence is the lowest; the overall score is reversed.
The artificial graphite content is in the range of 4% – 8%. When the artificial graphite content is 8%, the noise incidence rate is the lowest and the score value is the highest. On the one hand, the artificial graphite itself is soft and porous, and the amount of compression increases with the artificial graphite content. Correspondingly, it is because the increase of the compression amount may compensate for the influence of the DTV of the brake disc on the brake jitter to some extent, thereby reducing the noise occurrence rate; on the other hand, when the artificial graphite content is 8%, the friction material’s surface forms a dense and structurally complete friction film. The characteristics of the transfer film are the key factors affecting the braking noise. The smooth and stable transfer film can play a lubricating role, so that the friction is stable and the noise generation rate is the lowest. Since the artificial graphite itself has poor compatibility with the resin, when the artificial graphite content is further increased, the resin cannot satisfy the requirement of sufficient bonding between the components, and the artificial graphite is peeled off from the friction material matrix, and is more easily oxidized and decomposed in the high temperature process. Therefore, a large number of pits or scratches are formed on the surface of the friction material substrate, and the magnitude of the braking torque is proportional to the contact area of the brake disk, the pressing pressure, the friction coefficient of the brake disk material, and the equivalent working radius, so It can be seen that the pit on the surface of the friction material causes the friction to vibrate, so that the friction material noise frequency and the decibel value increase.
3 Conclusion
1) As the artificial graphite content increases from 4% to 12%, the porosity of the friction material increases, the density decreases, and the pH increases.
2) The effect of increased artificial graphite content on the mechanical properties of friction materials is as follows: Rockwell hardness decreases, material compressibility increases, and internal shear strength decreases.
3) Artificial graphite is beneficial to the stability of the friction coefficient of the friction material. As its content increases, the amount of wear decreases first and then increases. At around 8%, the amount of wear is lowest.
4) Artificial graphite is beneficial to suppress the generation of noise. As the content increases, the frequency of noise decreases first and then increases. At 8%, the noise performed best.
Author: Liu Bowei ，Liu Shenfei ，Yang Yang
