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The Science and Technology of Materials in Automotive Engines Part 3

Tham khảo tài liệu 'the science and technology of materials in automotive engines part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 38 Science and technology of materials in automotive engines 2.32 Microstructure of hyper-eutectic Al-Si. The Si particles work in the same way as the SiC particles in Ni-SiC composite plating Fig. 2.31 . The Si dispersion in the casting must be carefully controlled from a tribological viewpoint. The hydrodynamic lubrication is greatest32 at an appropriate height of the exposed Si. A special finishing to expose the Si is required for the running surface. To expose Si particles the bore surface is chemically etched or mechanically finished after fine boring. Figure 2.33 is a scanning electron micrograph of the bore surface showing primary phase Si particles of about 50 ựm. Figure 2.34 is an atomic force micrograph clearly showing exposed Si particles after finishing. The slightly depressed aluminum matrix between the Si particles helps retain oil. Figure 2.35 shows the three steps of the mechanical honing process.33 The first prehoning stage a adjusts the dimensional accuracy of the bore shape. Secondly the basic honing b removes the broken Si. Finally the finish-honing c recesses the aluminum matrix. The final surface roughness value measures around 1 to 3 ựm in Rz value. Chemical etching is also used to expose Si particles. Since the counter piston consists of a similar high-Si aluminum alloy both running surfaces34 become a combination of aluminum alloy. To avoid seizure the piston surface is covered by plating such as Fe Sn or Cr Sn the Sn layer being on the outside . The machining of the hard A390 alloy is not so easy. However like a cast iron block the fact that the block material forms the bore wall is attractive. As a result production volumes are increasing. The cylinder 39 20 gm 2.33 Bore surface of an A390 alloy block. Si particles stand out from the matrix like stepping-stones. 2.34 Exposed Si crystals under atomic force microscopy. 2.6.6 Cast-in composite Honda first installed this composite in its Prelude model North America specification in 199 035 9