SDC99钢盐浴TD法制备VC覆层形成机理的研究

STUDY ON GROWTH MECHANISM OF SALT BATH VANADIZING COATING BY TD PROCESS ON SDC99 STEEL

对冷作模具钢SDC99盐浴渗V中VC覆层形成机理进行了研究.采用高分辨SEM和金相显微镜观察了渗V试样的横截面形貌及VC覆层微观形貌,并利用EDS对表面元素进行了能谱分析,利用XRD对盐浴试样表层进行了物相结构分析;采用原位纳米力学测试系统TriboIndenter对渗V试样截面硬度梯度进行了测试.结果表明:盐浴温度越高,在相同浸渗时间下形成的覆层越厚;VC覆层硬度约为22GPa,远高于基体硬度7.0GPa,覆层与基体之间硬度以较缓速度下降,表明界面处基体组织能对VC覆层起到良好的支撑作用.覆层生长的过程为:初始晶粒形核、长大,并有亚微晶粒在其表面生长;受钢中C活度的影响,覆层晶粒长成等轴晶,越靠近基体与覆层的界面,晶粒尺寸越小.在覆层晶粒生长初始阶段,SDC99钢上晶粒生长择优取向从(200)晶面转变为(111)晶面,随着时间的延长,覆层晶粒生长无明显择优取向,生长为等轴晶.

The SDC99 steel is a new kinds of cold work die steel with high strength, high wear resistant and toughness. However, in order to extend its service life, the surface modification technology is required. Many surface treatments can be used to improve the wear resistant of cold work die steel. Among these methods, transition metal nitrides and carbides （such as VC） fabricated by TD （thermal diffusion carbide coating process） have been commonly used in tribological applications to enhance the life of mechanical components because they possess high hardness, excellent wear resistance, low coefficient of friction and good corrosion resistance. During the TD process, materials are immersed in to a molten borax bath that contains the relevant carbide/nitride forming element such as V, Nb, Ta, Cr, Mo and W, and the coating is formed by the combination of metal elements as mention above with the carbon/nitrogen diffused from the substrate. In order to investigate the growth mechanism of VC coating on SDC99 steel, salt bath vanadizing by TD process were performed by molten borax bath containing Na2B4OT（75.6%）, BAC12（8.4%）, V2O5（10%） and AI powders（6%）. The experiments were carried at 850, 900, 950, 1000 and 1050 for various time. The microstructure and morphology of VC coating layer were observed using SEM and metallographic measurements, and the elemental analysis on the surface were carried out with EDS. The crystalline structures of samples were characterized with XRD. The hardness gradient of nanoindentation test was performed on Triboindenter in-situ nanomechanical test system.The results indicated that at the same immersion time, the higher the temperature of salt bath was, the thicker of prepared coating was. The VC coating layer possesses much greater hardness （about 22 GPa） than that of the substrate （about 7.0 GPa）. The hardness between VC coating layer and substrate decreases slowly, indicating that the microstructure of substrate adjacent to the interface can provide effective supporting for VC coating layer. The growth mechanism was the nucleation and growth of initial grains, and the formation of sub-micron sized grains on them. Moreover, the crystalline grain of coating grew into equiaxed grains, due to the influence of carbon activity in substrate, and the grain size decreased with the decrement of its distance to the substrate. In addition, at the initial growth stage of crystalline grain of coating, the growth preferred orientation of crystalline grains on SDC99 steel transformed from crystal plane （200） to （111）. However, with the increment of time, the growth preferred orientation of crystalline grains was unsignificance and they grew into enuiaxed grains.