Research Of Cr-V Series Self-lubricating Hard Coatings

"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".