Design Of Strengthening-toughening And Investigation On Tribological Properties For Transition Metal Carbides Nitrides-based Films

The transition metal carbides（nitrides）（TMC（N））films are generally used for protecting surfaces from scratch,wear and corrosion,due to their high hardness,high melting point and high chemical inertness.As a result of these excellent properties,they have shown great potential in precision machinery,bio-implantation,transportation,cutting tools,aerospace industry,and geological exploration.However,with the rapid industrial technologies growth,there is growing demand for films with superior hardness.In this context,strengthening of TMC（N）-based films can broaden their application field and scope,and thus become a hot research topic.Hardness and toughness are generally conflicting properties,where the enhancement of the hardness is achieved usually at the expense of toughness.Obviously,for the TMC（N）films,simultaneously improving hardness and toughness remains a great challenge.While improving the hardness and toughness of TMC（N）films,a low coefficient of friction（CoF）and wear rate are required to ensure their durability and service life.Thus,reducing CoF and improving tribological properties of the films are also important evaluation standards for designing and selecting strengthening-toughening strategies.Based on the above analysis,we adopt the following three different strategies to realize the strengthening-toughening design of TMC（N）-based films and observe their effect on tribological properties:（1）Ag alloying to modulate the electronic structure for optimizing the hardness,toughness,friction and wear behavior;（2）inserting soft ductile Ag layers to construct nano-multilayer films,and exploring the influence of the ductile layer size effect on the strengthening-toughening behavior and tribological properties;（3）incorporating high-melting point tough Ta layer to form ordered encapsulated nanocomposites,and facilitating an improvement in the comprehensive performance of the films.1.Although soft noble metals（Au,Ag,Cu,etc.）typically only exist in the form of nanoparticles in the TMC（N）grain boundary,we found that Ag with a high valence electron concentration（VEC）at very low content can alloy with TMN,and dissolve into TMN sublattice.Furthermore,incorporating only a few solute Ag atoms can achieve significant improvement in the combined properties of the films,including hardness,toughness,friction,wear-resistance,hydrophobic and bacteriostatic properties.The specific experimental scheme:involves the introduction of few solute Ag into NbN sublattice by co-sputtering to obtain Nb-Ag-N solid solution films.Then the effect of solute Ag content on the strengthening-toughening behavior and tribological properties is investigated.Results show that the incorporation of few solute Ag atoms（¹.5 at.%）not only improves the hardness,toughness and wear-resistance of the film,but also significantly reduces CoF.Theoretical calculations reveal that the enhanced hardness is mainly due to the hybridization between Ag 5s,4d and N p electron orbitals;whereas,improved toughness is mostly attributed to the appearance of the Ag e_g state.Besides a decrease in wear rate due to high hardness and toughness,it is also confirmed that the main reason for lower CoF is ascribed to the fact that solute Ag atoms can promote the formation of the self-lubricating AgNbO₃ phase by activating the tribochemical reaction.In addition,solute Ag atoms can effectively tailor the chemical bond states on the film surface and accelerate the formation of the hydrophobic group of Ag₂O,thereby resulting in high hydrophobicity.Finally,we verified the universality of the scheme by demonstrating that the solid solution Hf-Ag-N and Ta-Ag-N films induced by few solute Ag atoms exhibit excellent strengthening-toughening behavior and tribological properties.Especially,Hf-Ag-N films induced by few solute Ag atoms（¹.48 at.%）can catalyze the base oil（PAO 10）at the sliding interface.As a result,the base oil could self-assemble in situ to form a self-lubricating onion-like-carbon（OLCs）tribofilm,which significantly improves their lubricating properties.Interestingly,the Ta-Ag-N films activated by few solute Ag atoms show a significant inhibitory effect on E.coli,thereby establishing excellent antibacterial activity.2.Based on studies relating to TM-Ag-N solid solution films,we introduced soft ductile metal Ag layer to build Ag/TMC（N）nano-multilayer films for exploring the size effect of Ag nanolayer on strengthening-toughening behavior and tribochemical reactions.When the ductile metal Ag layer is very thin,the discontinuous nano-multilayer structure can significantly enhance hardness and toughness,reducing CoF and wear rate of the films;in the case of a thicker Ag layer,the formation of a continuous Ag layer leads to a significant decrease in the hardness,toughness and wear-resistance.Therefore,the incorporation of the Ag nanolayer to construct a discontinuous nano-multilayer structure may be an effective strategy to achieve films with high strength and toughness along with improved tribological performance.The specific experimental procedure is as follows:we prepared the Ag/TaC nano-multilayers with various Ag thickness(l_Ag)ranging from 2 nm to 14 nm by magnetron sputtering,and investigated l _Ag-dominated growth,tribochemistry and strengthening-toughening behavior.The results show that the formation of discontinuous nano-multilayer structure induced by solid-state dewetting achieved a super hardness,high toughness and excellent friction and wear-resistance.Among them,the rapidly drop in CoF⁰.227 is mainly due to the synergistic effect of lubricious Ag nanoclusters and silver tantalate（AgTaO₃）phase.However,for l_Ag37nm,continuous soft Ag layers would easy slip,thereby leading to dramatically decrease in the hardness and toughness as well as wear-resistance.3.In addition to the nano-multilayer structure,the formation of core-shell-like structure encapsulated by tough metal layer is also an effective way to achieve the strengthening-toughening for the TMC（N）-based films.However,there is still no effective preparation method to obtain the ordered structure.Specifically,obtaining the composite structure consisting of TMC（N）and“high melting point TM”（miscible system）is a great challenge.We propose a new strategy to overcome this by activating solid-state dewetting during magnetron sputtering layered deposition to accomplish the self-assembly of ordered TaC@Ta core-shell-like nanocomposite film.This process yields thin Ta-shells（¹.5 nm）coherently grown into pseudocrystal c-Ta under the template effect of TaC-cores.The TaC@Ta film induced by the special core-shell-like structure achieves excellent strengthening-toughening behavior and tribological properties.Apparently,solid-state dewetting may provide a new route to build ordered TMC（N）@TM core-shell-like nanocomposite structure,with optimized properties for the TMC（N）-based film.Specific conclusions are as follows:the TaC@Ta core-shell-like structure film(l_Ta¹.5 nm)can simultaneously achieve super hardness and high toughness,together with drastically enhanced wear and corrosion resistance.A thorough analysis indicates that the hardness enhancement is mainly dominated by the Orowan strengthening mechanism.Enhanced toughness is attributed to the indenter-induced phase transformation from the pseudocrystal（c-Ta）to body-centered cubic（a-Ta）.The dropping in CoF is ascribed to thin pseudocrystal Ta layer（¹.5 nm）in the TaC@Ta core-shell-like structure,which can induce worn surface spontaneous oxidation to form lubricious Magnéli phase?TaO_x.Furthermore,superior corrosion-resistance is also obtained for the TaC@Ta nanocomposite film,since the ordered TaC@Ta core-shell-like structure can help to suppress the columnar growth and hinder the diffusion of corrosive solution.The proposed three strategies can improve the comprehensive properties of TMC（N）-based films,thereby providing a new thinking for the design of strengthening-toughening and in-depth investigation of friction and wear behavior.As a result,these films can be used in cutting-edge applications across several fields like mechanical,biology and aerospace engineering along with military applications.