| Wear is one of the main forms of metallic material failure. The wear not only causes huge economic losses, but also brings significant security risk. Gray iron is the most common cast iron and the most widely used as a cast material due to its low cost,outstanding castability and excellent machinability. The wear resistance of gray iron,however, is quite poor. So, improving the wear resistance of gray cast iron has a significant economic and social benefits. Enhancing the wear resistance of metal surface is an efficient, simple and economical method to improve wear resistance of the metal and prolong the service time of metal parts. Both surface alloying technique and ceramic particle-reinforcement technique are effective means of improving the wear resistance of metallic materials.In this work, to enhance the bonding of alumina toughed zirconia(ATZ) ceramic and gray iron, the Ni layer was prepared onto ATZ ceramic particle surface by sintering. A porous preform, consisting of high-carbon ferrochrome powders(HCFPs), borax, binder and ATZ ceramic particles, was first produced and then the ATZ ceramic particle reinforced and chromium surface alloyed gray iron matrix composite was successfully fabricated by the pressureless infiltration. The effect of alumina contents on the properties of ATZ ceramic was systematic studied. The microstructure, wear behavior and wear mechanism of the composite were investigated by optical microscopy,scanning electron microscopy(SEM), X-ray diffraction(XRD), energy dispersive spectrograph(EDS), etc. The main results are as follows:(a) With less than 20 vol% alumina, hardness and relative density vary slightly and flexural strength of ATZ ceramic increase dramatically. For alumina content over 20vol%, hardness and relative density reduce severely and flexural strength varies slightly.Compared with zircona ceramic, the hardness and relative density of ATZ ceramic with 50 vol% alumina decrease by about 29.4% and 5.8%, respectively. But the flexural strength of ATZ ceramic with 20 vol% alumina increases by ~15.8%. With increasingalumina content from 0 to 50 vol%, the fracture toughness of the ATZ ceramic increases by approximately 32.7%.(b) The decrease in the hardness of ATZ ceramic with increasing alumina content is consistent with the relative density decrease. This is believed to be due to the reason that alumina, as a rigid particle, can inhibit the densification behavior of ATZ ceramic. The strengthening is mainly attributed to the dispersion strengthening effect of alumina particles. The balance between the harmful effect of porosity and the profitable dispersion strengthening effect should determine the flexural strength. The increase in fracture toughness should be due not only to the crack deflection by dispersed alumina particles and the increase in transgranular fracture of ATZ ceramic, but also by the phase transformation of t-ZrO2 accelerated by the residual stress from the coefficient of thermal expansion(CTE) mismatch between zircona and alumina. For alumina content over 10 vol%, the increase in fracture toughness is mainly attributed to the transformation toughening of t-Zr O2.(c) Considering the Vickers hardness, flexural strength and fracture toughness of ATZ ceramic, the optimum content of alumina is 20 vol%. The density, fracture toughness,flexural strength and Vickers hardness of ATZ ceramic are 5.55-5.61 g/cm3, 5.9-6.2MPa?m1/2, 600-670 MPa and 1250-1280 HV30, respectively.(d) ATZ ceramic particle reinforced and chromium surface alloyed gray iron matrix composite was fabricated by pressureless infiltration. ATZ ceramic particles uniformly distribute in gray iron matrix and the shape of the ATZ ceramic particles distribution is alike to that of the porous preform, indicating that the strength of the porous preform is enough to bear the impact of high-temperature and high-speed liquid metal. The addition of high-carbon ferrochromium powders(HCFPs) leads to the formation of white iron during solidification. The phase constitutions of the composite consist of ATZ ceramic particles, ?-Fe, ?-Fe, graphite, ledeburite, pearlite, M3 C and M7C3.(f) The wear volume loss rates of ATZ ceramic particle reinforced and chromium surface alloyed gray iron matrix composite decreases first then tends to be stable. Thewear resistance of the composite is approximately 2.7 times higher than that of gray iron matrix. In addition, rigid ATZ ceramic particles and alloy carbides also improve the wear resistance of the composite. With the increase of wear time, the iron matrix subsides, the ATZ ceramic particles and alloy carbides protrude, and rigid ATZ ceramic particles and alloy carbides gradually bear main the wear force of the abrasive particles.So, the mechanism of wear resistance increase is a combination of the increase in surface hardness of iron matrix and the protecting effect of ATZ ceramic particle and alloy carbides on iron matrix. |
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