Microstructure And Wear Property Of Graphite,Al₂O₃ And Carbon Fiber Reinforced Magnesium Matrix Composite

As the lightest metal structural materials magnesium alloy has low density and high performance in specific strength and stiffness. It is widely used in automobile, aerospace and electric area. But its low strength as increasing the temperature and low resistance in corrosion and wear property limits its application area. One method to solve problem is to make magnesium matrix composite.Using fibers,particles and whisker as reinforcement fabricating magnesium matrix composite can increase its wear property. Usually the researches force on hard particle like SiC or TiC particles reinforced composites, after adding these reinforcement can increase the strength of the matrix, moreover, increase its wear resistance. But the investigation on lubricating effect during friction and wear process is less, especially the effect of solid lubricant which can decrease friction coefficient and wear loss on friction pair and the form mechanism of lubricating film. This article designs self-lubricating magnesium matrix composite. The composites reinforced by graphite particles and Al2O3 short fibers can be fabricated by combining perform with squeeze-infiltration technique. The effect of reinforcements on microstructure,microhardness,tensile and wear properties of the composites along with wear mechanism is investigated. The research gives the proof of the development and application of self-lubricating magnesium matrix composite.For graphite/Al2O3/AZ91D-Cex composite, investigates different Ce content, different graphite content and particle size on microstructure and properties of the composite. The results show that:(1) For Gr/Al2O3/AZ91D-Cex composite with different Ce content, the composite can be fabricated by combining perform with squeeze-infiltration technique. The graphite particle and Al2O3 short fibers combine closely with the matrix and no obviously defect can be found. For the reason of difference in element electronegativity, Ce forms Al11Ce3 with Al. Adding Ce in the matrix can affect the grain size by affecting the nucleation rate of the composite, refine the dendrite and form rare earth phase. The grain size of the composite decreases as increasing Ce content, but microhardness and wear resistance increase as increasing Ce content, and the composite with 1.0% Ce has the best wear resistance. When the wear test load changes the wear loss of the composite increases as increases load, at low load the mechanism is mainly abrasive wear and oxidation wear; at high load it changes to delamination wear. When velocity changes at 0.1m/s the composites have similar wear loss and the wear mechanism is mainly oxidation wear, but as increasing the velocity it changes to abrasive wear along with oxidation wear, at 2.0m/s the wear mechanism of all the composites transform to delaminaiton wear.(2) For Gr/Al2O3/AZ91D-0.8Ce composite with different graphite content, when the graphite content increases from 5% to 10% the grain size changes less, when the graphite content come up to 15% the grain size becomes less. The graphite particle affects the grain size of the composite by increasing nucleation rate and embarrassing grain size from growing up, so the composite with more graphite content has little grain size. The microhardness decreases as increasing the graphite content, and the wear resistance increases as increasing graphite content, the composite with 20% graphite has the best wear resistance. Analysis the wear loss under different wear test load, at 20N the worn surface has oxidative film mixed with graphite on it which can work as lubricate and decrease wear loss; but as increasing the load the worn surface can form oxidative film and the wear mechanism is abrasive wear and oxidation wear; at high load some broken Al2O3 short fibers are found in the subsurface which decreases the strength of the matrix, so the graphite is easier been peeled of from the worn surface and can't still work as lubricant, the wear loss increases sharply and the wear mechanism changes to delamination wear.(3) For Gr/Al2O3/AZ91D-0.8Ce composite with different graphite particle size, as decreasing the graphite particle size the grains become uniform but the grains size changes less and the effect of graphite particle size affect the process of nucleation and growth less. The microhardness decreases as increasing the graphite particle size, but the wear resistance increases as increasing the graphite particle size, the composite with graphite particle size of 240μm has the best wear resistance. When the wear test load changes, the composite with graphite particle size of 240μm has bare graphite on the worn surface which increases the lubricating effect of the friction pair, so its wear loss increases slowly as increasing the load. At low load the wear mechanism is abrasive wear and oxidation wear; at high load except the composite with graphite particle size of 240μm is still abrasive wear and oxidation wear, the wear mechanism of others change to delamination wear.(4) For CF/Al2O3/AZ91D-Cex composite, the effect of Ce on microstructure,microhardness,tensile and wear properties of the composite are investigated. The results show that: When the Ce content increases from 0.0% to 1.5%, the grain size of the composite decreases at first then increases, and the composite with 1.0% Ce has the less grain size. After adding Ce some rod liked Al11Ce3 are found in the matrix, but when the Ce content increases to 1.5% the shape of rare earth phase changes to block. The microhardness,tensile strength and wear resistance of the composite increase at first then increase as increasing Ce content, the properties of the composite with 1.0% Ce is better than the others. Analysis the fracture surface of the composite, when carbon fibers are pull out some groove are left on the fracture, there are rare earth in the groove which decreases the combination between carbon fibers and matrix. Because the dislocation pile-up can form tiny hole and decrease the tensile strength of the composite. Although adding Ce can increase the toughness of the matrix, the fracture mechanism is still brittle fracture.When the wear test load changes the wear loss increases as increasing load, and the composite with 1.0% Ce has the best wear resistance but the composite without Ce is the lowest. Choosing these two composite investigate the wear property under different test load and sliding velocity. At low load the oxide film mixed with C element appears on the worn surface of the composite with 1.0% Ce, analysis the formation mechanism of the oxidative film; at high load the worn surface has melting phenomenon along with the wear mechanism of delaminaiton wear. Investigate the effect of test load and velocity on the subsurface of wear specimen and analysis its wear mechanism, the results shows that after adding Ce the changes between mild wear to severe wear is delayed. Finally the effect of properties is compared between graphite and carbon fibers reinforced composites.