Results and Discussion

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" colspan=2 height=80 BGCOLOR="#000000" TEXT="#F7F7FF" bgproperties="fixed" background="03c00500dvldtc.gif"> <div> <FONT SIZE="5" COLOR="#FFCC66" FACE="Arial" ><B>Metal Matrix Composites (MMC)</B></FONT><B>     |     </B><A HREF="mmc.htm" TARGET="_top" TITLE="Metal Matrix Composites (MMC)"><U><B>home</B></U></A></div> <div> <A HREF="id2.htm" TARGET="_top" TITLE="Introduction"><U>Introduction</U></A>   |   <A HREF="id3.htm" TARGET="_top" TITLE="Experimental Procedure"><U>Experimental Procedure</U></A>   |   <B>Results and Discussion</B>   |   <A HREF="id6.htm" TARGET="_top" TITLE="Conclusions and Acknowledgement"><U>Conclusions and Acknowledgement</U></A></div> <div> </div> </td> </tr><tr> <td valign="top" width=210 BGCOLOR="#000000" TEXT="#F7F7FF" background="03dd2c00dvldtc.gif"> <div> <IMG SRC="052b1b10.gif" border=0 width="177" height="177" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <IMG SRC="054b0b00.gif" border=0 width="176" height="176" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <IMG SRC="066b0b00.gif" border=0 width="176" height="176" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <bR> </td> <td valign="top" height=400 BGCOLOR="#000000" TEXT="#F7F7FF"> <div> Results and Discussion</div> <div> <I><B>Density, porosity and microindentation hardness :</B></I><FONT SIZE="3" COLOR="#FFCC66" FACE="AdvTTf90d833a.I" ><I><B> </B></I></FONT></div> <div> The densities, percentage porosity and microindentation hardness data for each of the infiltrated MMCs are given in Table 2. It can be noted that all</div> <div> the composites are close to full density with minimal porosity measured. The density depends on the percentage of tungsten, the type of infiltrant and the type of secondary abrasive. Heavy infiltrant alloys, such as the 60Cu–40Ag, gives MMCs with higher densities.</div> <div> Similarly, the use of heavy secondary abrasives, even below the 2% by volume range, produces a MMC of higher density.</div> <div> The microindentation hardness measurements indicate that the properties were affected by the type of infiltrant and by the presence of secondary abrasive. W/Cu–Zn–Sn showed a higher hardness than the W/Cu–Ag alloy. This can be explained by the higher percentage of tungsten present in the W/Cu–Zn–Sn alloys (68% W compared to 62% for the Cu–Ag). Changes in microindentation hardness measurements within a single infiltrant alloy were not expected, because these measurements were located in areas away from the secondary abrasive. However, the presence of secondary abrasives seemed to harden the infiltrant alloy. This hardening behavior can only be explained by the partial dissolution of the secondary abrasives in the liquid metal (solid solution hardening).</div> <bR> <div> <IMG SRC="05771c50.gif" border=0 width="625" height="197" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> <I><B>Infiltrated microstructures :</B></I></div> <div> Typical microstructures of the matrix (avoiding the secondary abrasives, when possible) are shown in Fig. 3. The presence of bright tungsten equiaxial particles, having a wide range of particle size distribution, is noted. The matrix surrounding the tungsten particle corresponds to the infiltrated metal (Table 1). </div> <div> In the 56Cu–43Zn–1Sn MMC, the infiltrant is a single phase alloy (Fig. 3a), while in the 60Cu–40Ag, the infiltrant consists of a proeutectic copperrich</div> <div> phase (dark phase) and a silver-rich eutectic phase (light gray phase) (Fig. 3b). These results are in agreement with the phase diagrams obtained from each of the alloys. No reaction was detected between the infiltrant alloy and the tungsten particles, which is one of the requirements for successful infiltration. It was expected that the nickel used as infiltration aid in the Cu–Ag alloys would partially react with the tungsten particles. However, this effect was not corroborated in the observed microstructures. </div> <div> <IMG SRC="05ddc030.gif" border=0 width="476" height="259" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Examples of the microstructural features obtained by the addition of secondary abrasives in the infiltrant alloys are shown in Fig. 4. It can be noted that there is no interface between the matrix and the SiO2 secondary abrasive. This phase can be considered well adhered to the matrix in both alloys (Fig. 4a).</div> <div> <IMG SRC="05f07120.gif" border=0 width="519" height="530" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <bR> <div> <I><B>Volume loss</B></I></div> <div> The volume loss during abrasion of each of the composites is given in Table 3. By plotting microindentation hardness of the matrix against the volume</div> <div> loss, the wear behavior of the copper alloys can be established (Fig. 5). All the composites were abraded under identical conditions. Therefore, the results obtained can be compared. It can be seen that in a specific W–Cu alloy system (with either Cu–Zn–Sn or Cu–Ag), as the microindentation hardness of the matrix increases, the wear resistance of the composite also increases. The harder copper alloy (Cu–Zn–Sn) has a lower wear resistance than the softer copper alloy (Cu–Ag). These results emphasized that the wear behavior of the composite is not ruled by a single material</div> <div> property such as hardness. Instead, it is thought that the mechanism of abrasive wear depends on ductility, toughness, hardening coefficient and the strain rate and that, together with the hardness, are the controlling factors in the abrasive wear behavior of the material. The slope of the curves in Fig. 5 indicates the effect of secondary abrasives in each of the copper infiltrantalloys. It can be noted that the harder the matrix (W/ Cu–Zn–Sn), the more pronounced is the effect of the secondary abrasive.</div> <bR> <div> <IMG SRC="06f6eea0.gif" border=0 width="366" height="234" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> <IMG SRC="060edec0.gif" border=0 width="749" height="236" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> <I><B>Microstructural features of the abrasive wear surfaces </B></I></div> <div> A comparison of the different wear surfaces indicates that as delamination and microplowing decreases, the extent of microcracking increases. This can be explained by a loss in ductility in the matrix, which means that microplowing will require less plastic deformation to produce fracture (microcracks) and that the wear sheets formed by delamination should be progressively shorter. The ranking, in regard to appearance of  microplowing and visible delamination, is as follows: W/Cu–Zn–Sn + SiO2>W/Cu–Zn–Sn>W/Cu–Zn–Sn + SiC>W/Cu–Zn–Sn + WC, which is the same order exhibited by the volume loss.</div> <bR> <div> <IMG SRC="06e133b0.gif" border=0 width="275" height="315" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <bR> <div> The results obtained suggest that the use of a harder composite (W/Cu–Zn–Sn instead of W/Cu–Ag) does not result in a composite with an improved abrasion resistance. However, within a specific W/Cu alloy, solid solution hardening of the matrix was accompanied by an improved wear resistance. This obeys the empirical observation of higher hardness for better abrasion resistance. When two alloys are compared, the results indicate that the wear behavior is controlled by a synergistic effect of hardness and ductility.</div> <bR> <bR> <bR> <bR> <bR> </td> </tr></table></body>