【作者】 仇越秀；

【导师】 李波； 张善勇；

【作者基本信息】 钢铁研究总院 ， 材料物理与化学， 2013， 博士

【摘要】 “绿色加工技术”具有高效、节能、低碳、环保的特点,是未来的发展方向；但对加工工具及其涂层材料提出了苛刻的要求。过渡金属氮化物涂层,如：TiN, TiAIN和CrN,摩擦系数较高；传统的固体润滑剂涂层,如：类金刚石涂层,MoS2和h-BN,在湿润或高温等条件下易于氧化失效,均难以满足绿色加工技术的需要。针对现有涂层材料在绿色加工的高速摩擦、高温、无润滑剂条件下易于氧化、磨损而失效的问题,提出在硬质涂层中加入自润滑成分,降低涂层的摩擦系数,减小摩擦磨损,提高其使用寿命。通过纳米微结构设计,将Cr基耐磨层和V基自润滑层复合制备成摩擦学硬质涂层,系统研究涂层的微观结构与力学性能、氧化行为和自润滑等性能之间的关系,探索纳米结构自润滑摩擦学涂层的润滑机理。本论文采用磁控溅射技术制备了VN涂层,CrN/VN、CrAlN/VN多层膜结构涂层和CrAlSiVN纳米复合膜结构涂层,采用场发式电子探针(FE-EPMA)、掠入射X射线衍射分析仪(GIXRD)、场发式扫描电镜(FE-SEM)、透射电子显微镜(TEM)、原子力显微镜(AFM)和X射线光电子能谱仪(XPS)表征涂层的微观结构和化学组成；利用纳米压痕仪表征涂层的纳米硬度和弹性模量；利用Ball-on-disc磨耗实验仪表征涂层的摩擦性能。论文的主要研究结论如下：采用磁控溅射制备VN涂层,研究氮气分压和偏压对涂层性能影响,结果表明：在无偏压条件下,随着氮气分压从0.007Pa增加到0.29Pa,VN涂层晶化完全,结构变得更加致密,硬度也随之增加,最高达22.9GPa。在氮气分压为0.18Pa条件下,随着直流偏压从OV增加到-150V,VN涂层内的残余压应力增加到-1.69GPa,与此同时,涂层变得更加致密；当偏压进一步增加到-200V时,涂层晶粒长大,内部的残余应力略有减小。VN涂层的硬度随着偏压的增加而增大,最大到24.5GPa。采用磁控溅射技术,通过多层膜结构设计将CrN和VN结合在一起,制备了调制周期厚度(Λ)在7nm到27nm范围内的CrN/VN涂层。和CrN单层涂层相比,涂层的硬度从16.7GPa提高到25.2GPa,而与WC硬质合金的摩擦系数从0.40降低到0.21。CrN/VN涂层硬度的提高归因于CrN与VN之间的模量差以及热膨胀系数的差异。V在摩擦磨损过程中能够被氧化形成具有润滑作用的钒氧化物从而能降低涂层体系的摩擦系数。调制周期厚度7～27nm对CrN/VN涂层的硬度未见显著影响。由于多层膜结构的存在,CrN/VN涂层的磨耗率比单层膜结构涂层有所增加。将Al加入CrN层后,CrAIN层和VN层存在1%晶格错配度；常温下,由于形成了钒氧化物润滑层,CrAlN/VN涂层同样具有较低的摩擦系数,与WC的摩擦系数随着VN含量的增加而降低,最低达0.26。和CrN/VN涂层相比,CrAlN/VN涂层的硬度有了显著提高。固定CrAIN/VN的调制比为1：2时,调制周期在3nm到30nm范围内变化,当调制周期为20nm, CrAlN/VN涂层具有最高的硬度为32.4GPa以及弹性模量为375GPa,与WC合金的磨耗率最低为1.1×10-7mm3/Nm。当固定CrAlN/VN周期厚度为20nm时,随着CrAlN/VN调制比增大或减小,伴随着CrAIN或VN子层厚度的减薄,涂层的硬度有增强的趋势；当CrAlN/VN调制比为1：2时,涂层具有最高的硬度32.4GPa。CrAlN/VN涂层的高温氧化及摩擦学行为研究结果表明,CrAlN/VN涂层(CrAlN/VN=1:2(at.%), A=20nm)的硬度随着温度升高而下降,当温度升到700℃时,涂层的硬度仅为1.3GPa,基本失去硬质涂层的功效。随着温度升高CrAlN/VN涂层与氧化铝的摩擦系数上升,到400℃时最高达0.72；随着温度进一步升高到550℃,涂层表面形成V2O5,涂层的摩擦系数降低到0.56；当温度升高到700℃时,由于氧化物熔化,体系摩擦系数降低到了0.42。CrAlN/VN涂层仅适用于常温条件下的机械加工,不能在高温条件下应用。在CrAlVN涂层中加入Si能够显著提高涂层的力学性能,当Si含量为5.4at.%时,涂层具有最高的硬度为38.7GPa,弹性模量347.2GPa,及抗塑性形变系数0.8GPa。CrAlSiVN涂层和WC合金的摩擦系数在0.48-0.60之间,Si的含量变化对其没有显著影响；CrAlSiVN涂层的磨耗率随着涂层抗塑性形变性的提升而降低。钒在CrAlSiN涂层中能够起到晶粒细化的作用；CrAlSiN涂层的机械强度随着钒的加入而降低,当涂层中钒的含量由0增加到25.8at.%,涂层的硬度由34.8GPa降低到29.4GPa。当CrAlSiN涂层中加入少量钒时,未能起到润滑作用,由于涂层强度降低,摩擦系数反而增大,磨耗率也增大；当钒的含量达到25.8at.%时,起到润滑作用,涂层的摩擦系数降低到0.39,磨耗率也呈现下降趋势。CrAlSiVN涂层的高温氧化行为研究结果表明,CrAlSiVN涂层的硬度随着温度升高而下降；但是和CrAlN/VN涂层相比,CrAlSiVN涂层的抗氧化性得到了显著提高,当温度升到800℃时,涂层的硬度降低到约11.0GPa,仍然具有硬质涂层的效果。CrAlSiVN涂层能够抵抗800℃的热处理,可以应用于800℃左右的绿色机械加工条件。通过纳米多层膜结构和纳米复合膜结构将Cr、V氮化物进行复合,可以获得既硬又润滑的CrN/VN, CrAIN/VN和CrAlSiVN摩擦学硬质涂层。由于在摩擦磨损过程中被氧化形成了VOx化合物,Cr-V系列涂层表现出优良的摩擦学性能,能够满足绿色加工条件的需求。更多还原

【Abstract】 "Green machining technology" is environmentally friendly, cost effective, efficiency, and becomes more and more popular. But it also brings strict requirements for cutting tool’s coatings. Transition metal nitride coatings like TiN, TiAIN and CrN perform poorly due to high coefficient of friction; conventional solid lubricating coatings like diamond like carbon, MoS2, and h-BN fail due to oxidization under extreme conditions like ambient moisture and elevated temperatures; thus they are not suitable for green machining. Combining lubricant with hard coatings is an effective way out which can decrease the coefficient of friction of coatings as well as the wear and improve the life.Vanadium based coatings are easily oxidized to form Magneli-phase vanadium oxides and becomes ’lubricious’. Chromium based coating is hard, thus, the combination of Chromium and Vanadium based hard coatings render both hardness and lubricious properties attractive in green machining.This study focuses on preparation and properties of Cr and V based tribological coatings. The effects of nano-strucutre on the coating’s mechanical and tribological properties will be studied, and the lubricant mechanism of self-adaptive Cr-V series coatings will be explored, either. VN, CrN/VN, CrAlN/VN multilayer coatings, and CrAlSiVN nano-composite coatings are deposited through pulsed magnetron sputtering in this study. X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and electron probe micro-analyzer are employed to characterize the microstructures and chemistry. Nanoindentation and ball-on-disc wear test are used in mechanical and tribological studies.The main results of this study are as follows:In reactive magnetron sputtering of VN coatings, increasing of the nitrogen partial pressure (from0.007Pa to0.29Pa) brings about fine crystalized denser coatings with higher hardness (22.9GPa). At nitrogen partial pressure of0.18Pa, increasing of substrate bias voltage generates more residual stress in the coating (-1.69GPa at-150V), and further increasing the bias voltage to-200V slightly releases the residual stress and at the same time further enhances the density of the coating and the hardness to24.5GPa.CrN and VN are combined together through multilayer structure and prepared by magnetron sputtering. The period thickness of CrN/VN varies from7nm to27nm. Compared with CrN coating, multilayering with VN gives a hardness improvement from16.7GPa to25.2GPa, and the coefficient of friction (COF) with tungsten carbide alloy decreases from0.40to0.21. The hardness enhancement of CrN/VN coatings contributes to the difference of elastic modulus and thermal expansion coefficient between CrN and VN. The decrease of COF is attributed to the formation of vanadium oxides in the tribo process. There is no significant relationship between hardness and the CrN/VN bilayer thickness (from7to27nm). After doping aluminum into CrN generates about1%lattice mismatch in between the CrAIN and VN sublayers for magnetron sputtered CrAlN/VN multilayer coatings. The multilayer coatings are found self-lubricating with a coefficient of friction of0.26at room temperature because of formation of VOx. At period thickness of20nm, the coating exhibit nanoindentaiton hardness of32.4GPa, elastic modulus of375GPa and low wear rate of1.1×10-7mm3/Nm against cemented tungsten carbide. When period thickness is set as20nm, the multilayer coating with higher or lower CrAlN/VN layer thickness ratio, as well as lower sublayer thickness, has higher hardness. The coefficient of friction (COF) of the multilayer coating decreases with VN content increasing. And at CrAlN/VN layer thickness ratio of1:2, the multilayer coating obtains the best hardness of32.4GPa and the lowest COF of0.26with WC alloy. Because of oxidization, the hardness of CrAlN/VN coating (CrAlN/VN=1:2(at.%), A=20nm) decreases greatly after annealing. After annealed at700℃for2h, the hardness is only1.3GPa, and it can not act as hard coating anymore. The COF of the coating with alumina increases with test temperature increasing. It is0.72for400℃. When test temperature is550℃, V2O5forms on the surface of the coating, and the COF drops to0.56, and when test temperature further increasing to700℃, because the melting of vanadium oxides, COF further drops to0.42. So, CrAlN/VN is only suit for green machining at room temperature.The mechanical properties of CrAlVN coatings can be significantly improved after doped with Si. Si content of about5.4at.%provides the best hardness of38.7GPa, the elastic modulus of347.2GPa, as well as plastic deformation resistance (H3/E*2) of0.8GPa. The COF of CrAlSiVN coatings with WC alloy is from0.48to0.60. There is no relationship between COF and the content of Si. The wear rate of CrAlSiVN coatings decreases with plastic deformation resistance (H3/E*2) increasing. The crystalline in CrAlSiN coatings can be refined after doped with vanadium. The mechanical properties of CrAlSiN coatings decrease after doped vanadium. When the content of vanadium increasing from0to25.8at.%, the hardness of the coatings decrease from34.8GPa to29.4GPa. Small addition of vanadium has no lubricant effects, on the contrary, the COF and the wear rate increases due to the low coating strength. When the vanadium content is about25.8at.%, the coating presents lubricant effect, the COF of the coating with WC alloy drops to0.39and the wear rate drops, either. The hardness of CrAlSiVN coatings decreases with test temperature increasing. But compared with CrAlN/VN multilayer coatings, the oxidation resistance of CrAlSiVN coatings increases greatly; after annealed at800℃for2h, the hardness of the coating remains11.0GPa, which can still act as hard coatings, which means CrAlSiVN coatings can be applied at800℃green machining.Chromium and vanadium nitrides are combined together through nano-multilayer and nano-composited structure to get CrN/VN, CrAlN/VN and CrAlSiVN coatings render both hardness and lubricious properties. The Cr-V series nitride coatings are found of good tribological properties attributing to the formation of vanadium oxides in the tribo process which can be applied to "Green machining".更多还原