铝热剂法原位合成农机刀具Al_2O_3-Ti(C,N)复合涂层组织结构及性能

Microstructure and properties of in-situ synthesis Al_2O_3-Ti(C,N) composite coatings of agricultural machinery tool by thermit process

为了提高旋耕刀、犁铧等农机触土刀具表面强度,以铝热剂的放热反应提供内在热源、等离子弧作为外在热源,采用反应等离子熔覆技术在Q235钢表面原位合成了Al_2O_3-Ti(C,N)复合材料涂层。利用扫描电镜、能谱仪、X射线衍射仪、显微硬度计、金相显微镜等对复合涂层的微观结构及强质硬化相的成分、组织及性能进行了分析。结果表明:涂层与基体呈冶金结合,涂层主要由网状、嵌套、球状等3种结构组成,硬质相Al_2O_3、Ti(C,N)与粘结相Fe-Ni之间相互包裹、互相嵌套,构形成空间网状骨架结构;涂层硬度最高可达HV_(0.5)2160,平均硬度HV_(0.5)1870,约为基体Q235钢的7.7倍;涂层摩擦系数约为0.372,其磨损量约为65Mn钢及Q235钢的1/7和1/17,与基体相比,复合涂层具有较高的硬度和较好的摩擦磨损性能,可以为农机材料表面强化提供参考。

In order to improve surface strength of agricultural machinery key parts such as rotary blade and plough under atmospheric conditions, the heat released from the exothermic reaction of termite as an internal heat source, and plasma arc column as an external heat source, the in-situ synthesis of Al_2O_3-Ti（C,N）（AT composite materials） composite coating was prepared on Q235 A steel specimen surface by reactive plasma cladding technology. Firstly, raw materials were the cheap thermite（iron oxide and aluminum powder）, titanium powder, graphite powder, Ti N powder and Ni60 A powder, which were mixed in a planetary-type ball mill DQM according to the mass fraction ratio of Fe_2O_3∶Al∶Ti∶C∶Ti N∶Ni60A=36.5∶13.5∶8.4∶2.1∶4.5∶35, milled for 6 hours, was made a mixed powder paste with the self-made glue which was the adhesive, and then was coated on the surface of pretreated Q235 A specimen and the thickness was 2 mm ±0.5 mm, reserving 2-3 mm arc end.Secondly, the sample was placed in the vacuum drying oven and were dried under 80 ℃ during 12 hours, then cooled the sample to room temperature. Using the plasma arc column of DML-300 plasma welding machine as an external heat source,condition of the arc could be ignited successfully, increased appropriately the working distance（about 8 to 10 mm） and reduce the cladding current（120 A）. The plasma arc single channel was used to scan the clad and reached thermite ignition temperature, then the thermite in the pre-coated raw material occurred chemical reaction to generate Al_2O_3 and Fe. After the first cladding operation was finished, it immediately reduced the operating distance and appropriately increased the cladding current, using the optimized process parameters： cladding current of 200 A, cladding speed of 2.5 mm/s, preset coating thickness of 2.0 mm, ion gas flow of 0.6 L/min, working distance of 4 mm. Then the in-situ synthesized Al_2O_3-Ti（C,N） composite coating was remelted and placed in Argon protection atmosphere cooling. The composition, microstructure,phase and performance of Al_2O_3-Ti（C,N） composite coating were analyzed by scanning electron microscope（SEM）, energy disperse spectrometer（EDS）, X-ray diffractometer（XRD）, micro hardness tester and friction testing machine. The results indicated that the new phase Al_2O_3 and Ti（C,N） were synthesized in-situ on Q235 A steel during the plasma cladding process and the composite coatings were made, which had a good metallurgical combination with Q235 A steel substrate. The coating was mainly composed of reticular structure, nested structure and spherical structure. The hard phase Al_2O_3 and Ti（C,N） and Fe-Ni binding phase were mutual inclusion between themselves body, nested within each other, constituting the spatial reticulate structure. The highest hardness value of the coatings was up to HV0.52160, and the average hardness was HV_（0.5）1870, and about 7.7 times of Q235 A steel substrate. The friction coefficient of AT composite coating was about 0.372 and the wear loss of the AT composite coating was about 1/7 of 65 Mn steel, and 1/17 of Q235 A substrate surface. Compared with the substrate materials, the AT composite coating had higher hardness and more excellent friction and wear properties,which provided an experimental and theoretical reference for the surface strengthening of agricultural machinery materials.