Investigation On Mechanical Behavior And Thermal Stability Of TaN And TaN/AlN Thin Solid Films

TaN is a kind of material with good mechanical properties,high thermal stability and good oxidation resistance.However,TaN also has a lot of complicated structures,which will lead to the diversity of film performance.So in order to study the relationship between properties and structures of the films,we preparing a series of films with different structures by adopting the method of magnetron sputtering.Al N coating is also a very outstanding material that own well oxidation resistance and very stable structure（stable structure as hexagonal phase）.When the Al N combine with the Na Cl type transition metal nitrides（TMN） to form a multilayer film,will form a metastable cubic structure which was studied a lot.Alternately depositing the amorphous structure and crystalline structure as a thin solid film,will firstly epitaxial and then back into amorphous.The presence of the amorphous structure blocking the epitaxial growth,in consequence broken the crystallization integrity.Nowadays, most of the researches on this multilayer structure are about the hardness enhancement and the principle of this phenomenon. While we consider the influence of amorphous layer on film oxidation resistance, therefore, we prepared TaN/Al N multilayer films with different Al N layer thickness. We applied to the analysis of the characterization methods include: XRR, XRD, HRTEM, SEM, nano indentation, friction and wear,vacuum annealing and annealing in the air, etc.Display the results of the experiment below:（1） Prepared TaN thin film with 11 different structures by using nitrogen as the reaction gas and changing the ratio of Ar-N₂ flow. Their phase structures have undergone TaN0.1- Ta2N- ε-TaN-c-TaN-Ta3N5 and the mixture of two phases. TaN film hardness value increases at first then a little bit decreases with the changing of nitrogen flow.The maximum hardness is 34.043±1.931 GPa in TaNx- 5 sample, whose structure is c- TaN and ε-TaN mixture. Moreover, hardness of TaNx- 2 to TaNx- 5are also more than 30 GPa. High hardness is the result of the phase structure and the joint action of internal stress. The TaN0.1 friction and wear behavior is much lowerthan other samples, the average friction coefficient is 0.51926 is too much higher than other samples（0.3876,0.3381 and 0.3374）.Only TaN0.1 can be clearly observed the dent, other sample cross section curve almost no dent and grinding crack width of TaN0.1 is twice as many as other samples. This phenomenon is owing to the sample hardness, modulus, oxidation resistance combined action, and when these factors are the same, the friction coefficient of sample seriously affect the value of the wear rate.TaN film’s starting oxidation temperature is from 700 ℃ to 750 ℃, 800 ℃, 850 ℃,with the increase of nitrogen flow, oxidation resistance of films are improved. After annealing, part of the film appeared the phenomenon of phase transition, which is caused by the generation of oxide lattice mismatch, the volume expansion and stress accumulation.（2） Choosing the same experimental parameters of c- TaN（Ar/N₂ = 60/20） from TaN experiment with changing the Al N layer thickness to preparae five TaN/Al N multilayer films, that which modulation period is 4 nm / 0.4 nm, 4 nm / 0.8 nm, 4 nm/ 1.6 nm, 4 nm / 3.2 nm, 4 nm / 4 nm,respectively. Multilayer film in 4 nm / 0.4 nm has c- TaN（111） and（200） texture, then increasing the Al N layer thickness, the multilayer film change into c- TaN（111） structure. Observed from HRTEM images, 4nm / 0.4 nm film completely coherent, lattice structure throughout the entire film,while 4 nm /1.6 nm film appeared obvious amorphous layer, the crystallization of film integrity was damaged. The measured thickness of the amorphous layer is about0.89 nm, indicating that the amorphous Al N is epitaxial on TaN film as long as its thickness below the critical thickness（0.89 nm）. The multilayer film hardness is higher than that of mixed law calculated value, meanwhile decreasing accompany with increasing the Al N layer thickness,this is the influence of film stress and the combined action of the presence of the amorphous layer. After vacuum annealing,TaN/Al N multilayer film was founded that there is a diffusion phenomena at the film interface during high temperature conditions.A 4 nm / 0.4 nm multilayer film, will lost modulation structure after 700 ℃ annealing, while the other four multilayers well maintained the modulated structure.Hardness of multilayer show rapidly increasesthen slightly decreases after annealing, multilayer films perform good high temperature thermal stability. Finally, we conducted a preliminary exploration about the oxidation resistance of TaN/Al N multilayer, initial oxidation temperature of the multilayer films increasing with the increase of the Al N layer thickness.Oxidation resistance has a great relationship with the thermal insulation holding time, 30 min insulation, leads to the initial oxidation temperature fell sharply.