Composite materials reinforced (filled) with ceramic particles/fibers (e.g., alumina) are extensively used in the applications in which tribological properties (wear rate, coefficient of friction) are important.
Wear resistance of Metals and particularly Polymers is much lower. However it may be considerably increased in composite materials with alumina particle reinforced phase.
Alumina particles dispersed throughout a relatively soft matrix (metal or polymer) not only increase the material hardness and strength but also improve its tribological behavior.
Wear resistance is not only tribological property enhanced in alumina particle reinforced composites.
In some cases coefficient of friction of the composite is lower than that of matrix material.
Additionally hard alumina particles in a metal matrix composite decrease the probability of seizure between the sliding counterparts due to the abrasive effect of the particles polishing the surface of the counterpart.
The methods of fabrication of metal matrix composites reinforced with alumina:
Stir Casting. In this method alumina particles or short fibers are mixed with a molten matrix metal by means of mechanical stirring.
Squeeze Casting Infiltration. The method is using a movable mold part (ram) for applying pressure on the molten metal and forcing it to penetrate into a performed dispersed alumina phase, placed into the lower fixed mold part.
Sintering of a powder of a matrix metal mixed with alumina in form of particles or short fibers.
Different metals are used as matrices for alumina reinforced metal matrix composites:
Aluminum alloys have been widely used as matrix materials for fabrication of metal matrix composites reinforced with alumina particles or fibers.
Engine pistons, engine blocks and other automotive and aircraft parts operating under severe friction conditions are fabricated from alumina reinforced aluminum matrix composites.
Alumina particles dispersed in an aluminum alloy increase its wear resistance more effectively than hard silicon (Si) particles in eutectic and hypereutectic Al-Si alloys .
The disadvantages of alumina reinforced aluminum matrix composites are relatively high coefficient of friction and high wear rate of the counterpart.
Addition of dispersed particles of a solid lubricant (e.g., graphite together with the alumina particles/fibers (heterophase composite) allows to decrease the coefficient of friction and the wear rate of the both sliding parts.
The coefficient of friction of the aluminum alloy (Al12SiCuNiMg) matrix heterophase composite containing 25-wt% of particulate Al2O3 with the particle size of 50 μm (~0.002”) and 5-wt% of glass carbon particles is lower by 20% than that of the composite reinforced with 30% of alumina only .
Copper matrix composites are widely used in sliding electrical contacts of welding electrodes, railway overhead current systems, transfer switches, homopolar machines and other electrical applications.
Alumina particle reinforced copper matrix composites have an increased wear resistance and better refractory properties than non-reinforced copper.
However it was shown in  that the wear rate decreases firstly and then increases with the increase of addition of Al2O3 in the range (wt.%) from 1 to 5, exhibiting a minimum at 2% addition of Al2O3 and a maximum relative wear-ability of 3.13 times as much as that of copper. Copper-alumina composite was prepared in this work by the method of coprecipitation using NH4HCO3 as precipitation and CuSO4+NH4Al(SO4)2 as maternal solution.
Zinc alloy matrix composites reinforced with alumina particles have been studied since eighties.
The results of tribological investigations of composites with the matrix made of the ZA-27 (Zn68,Al28.5Cu2.5) alloy reinforced with Al2O3 particles of sizes 12 and 250 μm (~0.0005” and 0.01”) in quantities of 3.5 and 10 mass % are presented in . The highest wear resistance in the tests without lubrication was exhibited by the composite material reinforced by the Al2O3 particles of size 250 μm (~0.01”) in the amount of 5 mass %.
The best tribological properties are provided by the polymeric solid lubricant Polytetrafluoroethylene (PTFE) with the coefficient of friction varying within the range 0.02 – 0.1.
However polymers are very soft and therefore their wear rate is high. The wear resistance may be considerably improved by addition of hard ceramic particles/fibers (e.g., alumina). Unfortunately the reinforced phase micrometer-sized particles and fibers dispersed in a polymer matrix cause increased wear of the counterface and lead to higher coefficient of friction due to rough mating surface and unstable transfer film.
Undesirable abrasive effect may be eliminated if nanosized hard alumina particles are used as the filler (reinforcing phase) dispersed in a polymer matrix.
Friction and wear behavior of PTFE filled with alumina nanoparticles was studied in . A solid lubricant composite material was made by compression molding blended PTFE and 40 nm alumina particles. The wear resistance increased monotonically with increasing filler concentration from 0 to 20 mass %. The composite has wear resistance over 600x higher than the unfilled samples. Only slight increase of the coefficient of friction was observed.
Effect of the nanoparticles shape on the wear resistance of alumina-PTFE nanocomposites was investigated in . It was found that the inclusion of irregular shaped filler particles considerably reduced the wear of PTFE, but also led to increased friction. The wear resistance of PTFE was increased 3000x with 1 wt.% filler.
Comparative investigation of the tribological properties of micrometer- and nanometer-Al2O3-Particle-Filled poly(phthalazine ether sulfone ketone) (PPESK) composites  proved that the lowest wear rate was obtained for the composite filled with 1 vol %-nanometer alumina particles.
Effect of lubrication on the tribological properties of nano-alumina reinforced Polyoxymethylene (POM) composites was studied in . It was concluded that alumina nanoparticles were more effective in enhancing tribological properties of the POM composites under oil lubrication conditions.
Shekhar Nath and Bikramjit Basu  proposed High Density Polyethylene (HDPE) filled with particles of alumina and hydroxyapatite as biocompatible composite for bone replacement (e.g., Total Hip Joint Prosthesis). The properties of the proposed biocomposite were compared to those of pure HDPE. The composite demonstrated higher wear resistance, hardness and modulus of elasticity (stiffness).
Polymer matrix composites filled with hard alumina particles are also used as coatings applied over the engine bearing surface. Polyamide-imide, Epoxy or Phenolics may be used as the matrix material. Solid lubricants such as Molybdenum disulfide, Graphite or Polytetrafluoroethylene (PTFE) are also added to the polymer matrix for reducing the coefficient of friction. 2-5% of alumina particles (or other hard particles like Silicon carbide, Silicon nitride or Silica) enhance the wear resistance of the coating.
Source : SubsTech