| The current research status of Ni-B, Ni-W, Ni-W-B composite coatings at home and in abroad is discussed in this thesis, and the author puts forward a new method to obtain RE-Ni-W-B multifunctional composite coatings by means of pulse electrodeposition. Furthermore, some grains of PTFE, BN, MoS2, SiC, B4C, and A12O3 are added into the RE-Ni-W-B system to reduce its internal stress, eliminate its surface cracks and enhance its hardness, wear-resistance and the resistance to oxidation at high temperature.The theories of alloy co-deposition and composite electrodeposition and the mechanism of pulse electrodeposition of RE-Ni-W-B coatings are expounded. By use of E-pH patterns of W-H2O, B-H2O and Ni-B-H2O, the possibility of electrodeposition of RE-Ni-W-B composite coatings is discussed from the standpoint of thermodynamics.Through analysis of hardness and thickness of the composite coatings, the optimal process conditions of DC electrodeposition of RE-Ni-W-B composite coatings are screened out by orthogonal experiment. But there are many cracks on the surface of RE-Ni-W-B composite coatings owing to high internal stress.After determining the optimal process conditions of DC electrodeposition of RE-Ni-W-B composite coatings, the composite coatings are also obtained by means of pulse electrodeposition so as to decrease the internal stress, eliminate the cracks and enhance the hardness and wear-resistance of the coatings. The best pulse frequency and duty cycle are obtained, but the surface cracks of composite coatings are still obvious.The effects of wear-resistant grains such as SiC, B4C, AI2O3 and antifriction grains such as PTFE, BN, MoS2 on the properties of pulse electrodeposited RE-Ni-W-B multifunctional composite coatings are studied. Experimental results indicate that the sizes of other crystalline grains decrease during the co-deposition of these grains, and the higher the contents of these grains in the bath are, the smaller the sizes of other crystalline grains. The surface cracks of the RE-Ni-W-B composite coatings are eliminated and the smoothness is improved owing to the uses of these grains.The composition analysis shows that pulse electrodeposition can increase W and B contents in the RE-Ni-W-B composite coatings, compared with DC electrodeposition. All ofthese solids such as SiC, B4C, AI2O3, PTFE, BN, MoS2 can deposit into the RE-Ni-W-B composite coatings to a different degree, but this will affect W and B contents in the coatings.The X-ray diffraction results of RE-Ni-W-B-B4C-MoS2and RE-Ni-W-B-PTFE-B4C composite coatings show that (1) As to the RE-Ni-W-B-B4C-MoS2 composite coating, the coating is mainly amorphous and partially crystallized as-deposited, and the coating is been crystallized after heat treatment at 500. (2) As to the RE-Ni-W-B-PTFE-B4C composite coating, the coating is between amorphous and micro-crystalline as-deposited, and the coating changes into micro-crystalline state after heat treatment at 200; the diffraction peak of PTFE decreases after heat treatment at 400; PTFE begins to decompose and the composite coating is crystallized after heat treatment at 600. CeO2 and B4C in the RE-Ni-W-B-B4C-MoS2and RE-Ni-W-B-PTFE-B4C composite coatings are very stable and do not decompose after heat treatment at 800 .Through analysis and comparison of the hardness, wear rate and the resistance to oxidation at high temperature of the composite coatings, the effects of three different grains SiC, B4C, AI2O3 on the properties of pulse electrodeposited RE-Ni-W-B-PFTE, RE-Ni-W-B-BN and RE-Ni-W-B-MoS2 multifunctional composite coatings are studied. The results indicate that the properties of the multifunctional composite coatings including SiC, B4C, AI2O3 grains are much better than those of the coatings without these grains, the hardness improves and the wear rate decreases obviously. The results also indicate that the best self-lubricating material is PTFE in the RE-Ni-W-B composite coatings obtained by pulse electrodeposition. |
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