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Plasma surface hardening process was performed to improve the performance of the AISI 1045 carbon steel. Experiments were carried out to characterize the hardening qualities. A predicting and optimizing model using genetic algorithm-back propagation neural network (GA-BP) was developed based on the experimental results. The non-linear relationship between properties of hardening layers and process parameters was established. The results show that the GA-BP predicting model is reliable since prediction results are in rather good agreement with measured results. The optimal properties of the hardened layer were deduced from GA. And through multi optimizations, the optimum comprehensive performances of the hardened layer were as follows; plasma arc current is 90 A, hardening speed is 2. 2 m/min, plasma gas flow rate is 6. 0 L/min and hardening distance is 4. 3 mm. It concludes that GA-BP mode developed in this study provides a promising method for plasma hardening parameters prediction and optimization.

CNTs-SiC/Al2O3-TiO2 coatings were prepared by micro-plasma spraying. Microstructure, high temperature oxidation resistance, complex permittivity and absorbing performance of CNTs-SiC/Al2O3-TiO2 coatings were studied. The coatings were density, low porosity. Little oxidation occurred during the spraying process. The phase of β-SiC and C were successfully retained during the plasma spraying process. The high temperature oxidation of CNTs-SiC/Al2O3-TiO2 composite coating is improved compared with that of CNTs. The initial weight loss temperature of CNTs-SiC/Al2O3-TiO2 composite coating is raised to 511°C. The weight loss of the sample heated from 35°C to 700°C is 0.71 wt%, while the weight loss of the sample held for 60 min at 700°C is 0.41wt%. The real parts of complex permittivity of the CNTs-SiC/Al2O3-TiO2 composites decrease from 17.3 to 10.3 with the increasing frequency. And with increasing frequency, the real parts of complex permittivity of the composites decrease from 6.3 to 2.9. This tendency is beneficial for the absorption of the microwave with wide frequency band. The CNTs-SiC/Al2O3-TiO2 coatings have ability of absorbing microwave. The microwave absorbing performance of the coating with thickness of 0.9 mm is pure. However, with the increase of the coating thickness, the microwave absorption properties were enhanced and the minimum reflection loss shifts to low frequency with the increase of the coating thickness. 1188 无 机 材 料 学 报 第 25卷

Ni-Cr coating and Ni-Cr/ZrO2 gradated coating were deposited on C45 carbon steel by high velocity arc spraying and micro-plasma spraying to solve the high temperature oxidation problem of medium carbon steel components. The oxidations of Ni-Cr coating, Ni-Cr/ZrO2 gradated coating and C45 carbon steel substrate were carried out for up to 108 hours in air atmosphere at 1100°C. The oxidation behaviors were investigated after detailed examinations by thermal gravimetric analysis (TGA), x-ray diffraction (XRD) and scanning electron microscopy (SEM). Ni-Cr coating and Ni-Cr/ZrO2 gradated coating show similar and low dynamics curve near to the logarithms function. Surface observations with SEM and XRD reveal that the oxidizing surface of the C45 carbon steel substrate is mainly a loose spherical structure consisted of mainly Fe2O3 and Fe3O4. The surface structures of Ni-Cr coating and Ni-Cr/ZrO2 gradated coating after 108 hours oxidization are rather denser than that of C45 carbon steel, which can effectively improve the properties of the oxidation resistance of C45 carbon steel substrate.

Samples of AISI 1045 carbon steel were surface hardened by micro plasma transferred arc (PTA) process. The hardened layer was characterized using both light optical and scanning electron microscopy and microhardness techniques. The tribological properties of the surface hardened layer and untreated substrate were investigated using a block-on-ring tribometer sliding against GCr15 steel under unlubricated condition. The worn surface morphologies and dominant wear mechanisms were identified using microscopy techniques. Results show that the surface hardened layer consists mainly of martensite and retained austensite with fine and dense structure, the microhardness of hardened layer increases from approximately HV 200 to HV 600. The wear volume loss of plasma hardened layer was 81.86×10-11m3 much better than that of untreated AISI 1045 carbon steel (743.44×10-11m3). Wear of untreated AISI 1045 carbon steel occurred by combined mechanisms of adhesion, abrasion and plastic deformation. While the worn surface of surface hardened layer is quite better with slight track and thin oxides on the worn surfaces. Plasma surface hardening has essentially changed the wear mechanism of the AISI 1045 carbon steel to slight abrasion and oxidation wear.

Nonastructured Al2O3-13wt%TiO2 (AT13) coatings were deposited by multi-function micro-plasma spray and Metco 9M plasma spray, respectively. Constituent phases and the microstructure of the powder particles and coatings prepared were examined with the aid of scanning electronic microscope (SEM) and X-ray diffraction (XRD). Mechanical properties including hardness and bonding strength were also evaluated by microhardness tester and electron tensile tester. Multi-function micro plasma sprayed nanostructured AT13 Coating is fully-melted, dense and uniform. However, AT13 Coating deposited by Metco 9M plasma spray is partial-melted. The microhardness of multi-functional micro plasma sprayed AT13 Coating is HV975.7~1441.7, much higher than that of Metco 9M plasma sprayed AT13 Coating (HV655.3~946.6). The bonding strength results present the same, increased from 19.8 MPa to 42.7 MPa.