A mixture of 30 vol.% SiC and 70 vol.%Ni-based alloy was clad onto steel substrate using laser multi-track overlapping process. Microstructure and dry sliding wear behavior of the overlapped cladding were studied. SiC particles completely dissociated during the processing. The microstructure of the cladding layer differs widely throughout the cladding layer, typical microstructure is composed of net-like dendrite M23(C,B)6, cellular-dendrite Ni31Si12, -Ni+Ni3B interdendritic lamellar eutectic, with very small amount of spherical graphite in the upper part of the cladding layer. Interestingly, significant amounts of net-like M23(C,B)6 carboboride dendrites as wear resistant skeleton were formed and uniformly dispersed in the cladding layer. Meanwhile, small graphite spheres scattered in the upper part of the cladding layer can also give contribution towards reducing friction as a self-lubricant. The novel microstructure, therefore, is beneficial for wear resistance. Friction and wear tests without lubricant show that the friction coefficients of the cladding layer is less than that of hardened steel, but the sliding distance characteristics of the friction coefficients of the cladding layer are in good agreement with that of the steel. There exists an approximately linear relationship between wear weights and sliding distances, and wear weight increases with an increase of sliding speed at the same sliding distance. Wear rate slightly increases with an increase of sliding distance, and the wear rate of the cladding layer is about one order less than that of the hardened steel.

Optimization of process variables for laser cladding of Ni-based alloys was performed using the pre-placed powder method. The microstructure of laser clad, under optimal processing conditions, and furnace melted, under near equilibrium conditions, Ni-based alloys has been comparatively investigated by X-ray diffraction XRD., scanning electron microscopy (SEM). and transmission electron microscopy (TEM). techniques. Comparison of the microstructures of the laser-clad and furnace-melted alloys revealed a remarkable difference. The microstructure of the laser cladding is complex, composed of blocky CrB type chromium carbon borides, orthorhombic structured Cr7C3 type dendritic carbides, cellular-dendritic v-Ni solid solution, different interdendritic eutectics and amorphous phases along grain boundaries. The interdendritic eutecti cs, either v-NiqM C or M23C or v-NiqNi3B Ni2B., can form depending on the local composition. v-NiqNi3B stable solidification and v-NiqNi2B metastable solidification exist simultaneously because of the non-equilibrium rapid solidification involved during the laser cladding. In contrast, the microstructure of furnace-melted Ni-based alloy under near equilibrium solidification is composed of hexagonal structured Cr7C3 type carbides with hexagonal prism morphologies, near-equiaxed n-Ni solid solution dispersed with fine Ni3Si precipitates, n-NiqNi B near lamellar eutectic and near-spherical Ni B compound. v-Ni is the main microstructural constituent in both the laser clad and furnace melted alloys. From the grain size, it was evident that the former is one to two orders finer than the latter.

A continuous-wave CO2 laser was used to clad a TiC particle-reinforced Ni-Cr-B-Si-C composite coating on to an AISI 1045 steel substrate. The microstructure of the clad was investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that partial dissolution of the TiC occurred on melting, and epitaxial growth of the remaining particles takes place by the precipitation of TiC upon cooling. This epitaxial TiC layer has an additional alloying effect by using elements such as chromium and silicon from the matrix. The pushing of TiC particles (TiCp) by the solidification front causes them to be unequally spaced. Besides the TiCp, the microstructure of the coating is a mixture of feather-like colonies and a small amount of (c-Ni+M23C6) eutectic. The feather-like colonies consist of CrB laths distributed in a c-Ni solid solution and grow radially from the TiC particles because of the large difference between the thermal properties of TiC and the Ni-Cr-B-Si-C matrix alloy.

30 vol.% WCp reinforced Ni-Cr-B-Si-C composite coatings were deposited on AISI1045 steel by laser cladding. The typical microstructures and the phases present in the coatings were investigated using scanning and transmission electron microscopies. It was found that WC particulates partially dissolved on the surface of the particulates in the melt pool during the laser cladding. Upon cooling, the bar-like a-W2C, blocky b-W2C and quadrilateral g1M6-C carbides were formed from the laser-generated melt pool, and the final eutectic reaction resulted in the formation of c-Ni+Ni3B lamellar eutectic. The rapidly solidified microstructure of the laser melt is composed of c-Ni+Ni3B lamellar eutectic in which bar-like a-W2C, blocky b-W2C and quadrilateral g1-M6C carbides are distributed. The coating thus consists not only of thermodynamically stable, but also of metastable phases. The presence of the latter can be attributed to the nonequilibrium solidification arisen from the high cooling rates involved during the processing.

In order to improve the strength and toughness of TiC materials, especially the elevated temperature strength so as to extend the applications in elevated temperature environment, TiC composites containing 20 vol.% short carbon fiber (Cf/TiC) were produced by hot pressing. The short carbon fibers were uniformly distributed in the TiC matrix by wet ball-milling mixing. The hot-pressing technique for Cf/TiC composites was optimized with respect to hot-pressing temperature and pressure, and the most suitable technique is vacuum sintering at 2100℃ under 30 MPa for 1 h. With carbon fiber addition, not only the room temperature strength and fracture toughness of TiC are remarkably increased, but the elevated temperature strength is increased as well. The flexural strength of the Cf/TiC composite is 593 MPa at room temperature and 439 MPa at 1400℃. The strengthening effect of the carbon fibers is estimated using a slightly modified role-of-mixture formulation, and the toughening effect is also calculated based on the toughening mechanisms of crack bridging, fiber pullout and crack deflection. The calculated results of the strength and fracture toughness of the composite agree well with the experimental results.