【作者】 孔旭；

【导师】 卢柯； 王镇波；

【作者基本信息】 中国科学技术大学 ， 材料学， 2023， 博士

【摘要】 奥氏体不锈钢(如AISI 304和316L)具有诸多优异的性能,如高耐腐蚀性和良好的焊接性能等,广泛应用于石油化工、厨具和核电等领域。然而,奥氏体不锈钢硬度较低,耐磨性较差,很大程度上限制了其在工业中的应用。细化晶粒和表面沉积硬质涂层有望提高其硬度和耐磨性。然而,奥氏体不锈钢的摩擦磨损性能受多方面因素的影响,如晶粒尺寸、相组成和塑性等,相关机理仍有待进一步研究。而硬质涂层与奥氏体不锈钢基体物理化学性质差异较大,在沉积及后续的使用过程中,涂层与基体的界面处可能会产生高度的应力集中,导致过早失效。本论文通过表面机械滚压处理(Surface mechanical rolling treatment,SMRT),成功制备出微观结构由表及里由纳米结构逐渐转变为粗晶结构的梯度纳米结构(Gradientnanostructure,GNS)304不锈钢板材。进而,选取有代表性层深处的样品,研究了初始微观结构及硬度等对其摩擦磨损性能的影响。随后,采用商用的电弧离子镀技术在GNS304不锈钢表面沉积TiN涂层,系统研究了 GNS表层对304不锈钢上TiN涂层结合性能和摩擦磨损行为的影响。主要研究结果如下:1、SMRT304不锈钢样品中,晶粒尺寸、奥氏体及形变诱发马氏体相的含量、以及显微硬度等随距表面深度的变化而逐渐变化。厚度为～1.9mm的平板样品双面经SMRT加工后,最表层组织的平均晶粒尺寸～36nm、主要由α’-马氏体组成,芯部组织主要为ε-马氏体板条细分的奥氏体晶粒。最表层及芯部位置的显微硬度分别为～5.1 GPa和～3.2 GPa,均显著高于原始粗晶样品的显微硬度(～2.1 GPa)。2、干摩擦条件下,相比于粗晶样品,SMRT样品中最表层样品的耐磨性有所提升,而芯部组织样品耐磨性提高幅度更大。分析表明,最表层纳米晶在摩擦过程中容易生成脆性摩擦层,可能导致耐磨性提升幅度的降低;而芯部组织样品优异的耐磨性主要源于其摩擦亚表层可在较大深度范围内协调摩擦导致的剪切变形。3、采用电弧离子镀技术在SMRT样品表面沉积出厚～6.8 μm的TiN涂层。微观结构研究表明,涂层样品从表面到内部依次为TiN外层、Ti中间层、非晶相和化合物界面层、以及304不锈钢基体。离子镀过程中,GNS304不锈钢表层的微观结构和硬度基本保持稳定。4、划痕结合力测量实验表明,SMRT镀膜(C-SMRT)样品上涂层从基体剥落的临界载荷比CG镀膜(C-CG)样品上的提高了～47%。结合机理研究表明,C-SMRT样品上涂层与基体之间具有更高的界面强度,这主要得益于其中界面化合物的形成受到抑制以及Ti元素向GNS基体中扩散能力的提高。此外,C-SMRT样品中较高的基体硬度、划痕尖端与涂层之间较低的摩擦系数以及涂层内部较低的内应力,也将导致其涂层剥落的临界载荷提高。5、干摩擦条件下,C-SMRT样品的耐磨性始终优于C-CG样品,且提高幅度随载荷的增大逐渐增大。低载荷(50N以下)时,两者摩擦系数相近;高载荷时,C-SMRT样品摩擦系数相对较低。C-SMRT样品的磨损形式以磨粒磨损为主,而C-CG样品的磨损形式包括磨粒磨损和塑性变形。6、分析表明,C-SMRT样品优异的耐磨性与GNS基体较高的硬度以及涂层/基体界面结合强度的提高有关。基体硬度对耐磨性的贡献主要体现在抵抗摩擦过程中塑性变形能力的提高上,而对涂层本身的磨损没有影响。高载荷下,C-SMRT样品摩擦系数较低主要是因为GNS基体表层较高的硬度有利于减小摩擦系数的犁耕分量。更多还原

【Abstract】 Austenitic stainless steels,such as AISI 304 and 316L,are widely used in petrochemistry,kitchenware,nuclear power plants,and other fields,mostly due to their outstanding corrosion resistance,welding properties,etc.But the relatively lower hardness and wear resistance of these materials also have limited its further applications.So,methods such as refining the grain size and depositing hard coatings are developed to improve the hardness and wear resistance.However,the friction and wear properties are affected by multiple factors in austenitic stainless steels,such as the grain size,phase composition,hardness,plasticity,etc.Further studies are needed to reveal the underlying mechanisms.Moreover,large differences in physical and chemical properties exist between hard coating and austenitic stainless steel substrate,so that a high stress concentration might be induced at the interface,leading to the premature failure.In this paper,plate surface mechanical rolling treatment(SMRT)was applied on 304 austenitic stainless steel plates to construct gradient nanostructure(GNS)layers,with the microstructure gradually changing from nanostructure at the surface to coarse grained structure in the core.Furthermore,effects of the initial microstructure and hardness on the friction and wear properties were studied on representative samples with different surface layers removed.Subsequently,a TiN coating was deposited on the GNS substrate by using a commercial arc ion plating approach.And effects of GNS surface layer on the bonding property and tribological behavior of the TiN coating were studied.The main results are as follows:1.The mean grain size,the content of austenitic and deformation-induced martensitic phases,and the microhardness gradually change with the depth from the surface in the SMRT 304 stainless steel sample.The plate sample of～1.9 mm in thickness was processed from both sides by SMRT.After SMRT process,the mean grain size in the topmost surface,with mostly α’-martensite,is～36 nm.And microstructure is composed of austenitic grains refined by ε-martensite in the core position.The microhardness values at the surface and the core are～5.1 GPa and～3.2 GPa,respectively.And both values are higher than that of the original coarse grain(CG)sample(～2.1 GPa).2.Dry sliding friction experiments showed that the wear resistance of topmost surface layer of the SMRT sample was better than that of CG sample.And the sample taken at the core position of the SMRT plate showed the best wear resistance.It was analyzed that a brittle friction layer was easy to form on the topmost nanograin layer,possibly leading into a decrease in wear resistance.In the sample taken at the core position of the SMRT plate,the outstanding wear resistance is mainly due to the accommodated friction-induced shear deformation in the subsurface layer over a large depth range.3.A TiN coating of～6.8 μm in thickness was deposited on the SMRT sample by arc ion plating technique.The typical microstructure contains a TiN outer layer,a Ti intermediate layer,an interfacial layer of amorphous and compounds,and the 304 stainless steel substrate from the surface to the interior in a coated sample.The microstructure and microhardness of the GNS substrate kept stable during the ion plating process.4.Scratch tests demonstrated that the critical load of coating detachment was significantly enhanced in the coated SMRT(C-SMRT)sample,～47%higher than that in the coated coarse-grained(C-CG)sample.Analyses of the underlying bonding mechanisms revealed that a higher interfacial strength was achieved in the C-SMRT sample,mostly due to the depressed formation of compounds in the interfacial layer and the accelerated diffusion of Ti into the GNS substrate.In addition,such factors as a higher substrate hardness,a lower coefficient of friction(COF)between the scratch tip and the coating,and a lower internal stress in the coating might also contribute to the enhanced bonding property in the C-SMRT sample.5.Dry sliding friction experiments demonstrated that the wear resistance of CSMRT sample was superior to that of C-CG sample,and the degree of improvement gradually increased with an increasing load.The COFs of both samples were similar under a low load(below 50 N),while the COF of the C-SMRT sample was lower than that of the C-CG sample under a high load.The wear mechanism of C-SMRT samples was mainly abrasive wear,whereas that of C-CG samples involved both abrasive wear and plastic deformation.6.The excellent wear resistance of the C-SMRT sample might be attributed to the higher surface hardness of the GNS substrate and the enhanced interfacial bonding strength between the coating and the substrate.The contribution of the hardness of the GNS substrate to wear resistance is mainly related with the improved resistance to plastic deformation,whereas shows no direct effect on the wear loss of the coating.Moreover,the higher hardness of the GNS substrate helps to reduce the plowing part under a high load,resulting in the lower COF in the C-SMRT sample.更多还原