The curve of the friction coefficient of the composite coating of laser cladding with different components with friction time is shown infigure 1. It is clearly obvious that the friction coefficient of composite coating K1 without the BN lubricating phase has an obviously increases compared with the matrix, which is mainly related to the content of TiN reinforcing phases and NiTi intermetallic compound in the composite coating. During the dry sliding friction process, the friction pair and the reinforcing phase in the composite coating cause frictional wear of the double abrasive. As the friction process progresses, part of the reinforcing phase of embedded in the coating is peeled off. The friction process is changed from catamaran friction wear to trisomic friction wear, which leads to the increase of friction coefficient of composite coating. h-BN is similar to graphite and has good lubricity during dry sliding friction. With the increase of mass fraction of BN lubricating phase, friction coefficient of composite coating decreases. The decrease in friction coefficient is mainly due to the uniform distribution of BN on the surface of the coating, and the hexagonal structure is prone to shear during the friction process. When the BN mass fraction is 15%, the friction coefficient of the composite coating is significantly reduced and stabilized at about 0.35. Combined with the XRD and microhardness analysis of the coating, as the BN mass fraction increases, the content of the lubricating phase BN and NiTi intermetallic compound in the coating is higher, which leads to the greatly improves the toughness and self-lubricating property of the composite coating. However, the hardness of the coating is reduced due to the high content of lubricating phase.
Figure 1.Friction coefficient curve of composite coating.
Figure 2 shows the relative wear volume and wear amount of laser cladding metal-ceramic composite coating. It can be seen that the wear volume and wear amount of the composite coating are obviously reduced compared with the matrix. Combined with SEM and microhardness analysis, the content of TiN reinforcing phase and BN are Evenly distributed in composite coating K2, which improves the hardness of the coating. These reinforcing phases play a pinning effect and embedded on the metal surface to reduce the contact between the friction pair and the metal during the friction process. The BN lubricating phase and the NiTi intermetallic compound effectively improve the toughness of the coating and act as a lubricant. Therefore, the coating has excellent wear resistance. With the improvement of BN mass fraction, the content of TiN in the coating decreased, lead to the coating hardness decreased. During the dry sliding friction process, the coating is easily deformed under the shear stress of the frictional force, causing the hard phase to fall off and reducing the wear resistance of the coating.
Figure 2. Relative wear volume and wear amount of laser cladding self-lubricating composite coating.
Figure 3 is a surface friction and wear morphology of a wear-resistant composite coating. As can be seen from thefigure, a large number of pear groove and exfoliation pits appear on the surface of the matrix under the action of a hard friction pair(WC ceramic ball). This is due to the surface of the cladding layer under the normal stress of friction micro-bulge result a cutting effect on the coating during the friction process, resulting in a slight plastic deformation, and the wear debris and part of the hardness phase fall off resulting in abrasive wear during the friction process. It can be seen fromfigure3(b)that when the BN mass fraction is 5%, the pear groove of the surface of the composite coating is narrower and shallower than the other ones, and the surface peeling pit is less. This is because a large number of TiN hard phase particles are generated under the action of laser and evenly distribute in the coating to improve the hardness of the coating, hindered the friction pair produces continuous plastic shear to coating. And with the solid lubrication of the BN lubricating phase during the friction process, the wear resistance of the coating is obviously improved.
Figure 3.Surface friction and wear morphology of composite coating:(a)Matrix wear morphology;(b)5% content of BN wear morphology.
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