Preparation And Characterization Of TiN/SiN_x Nanolayered Films And Their Fracture Behavior

High-speed cutting and dry machining significantly promote the progress in modem processing technology.Coating technology is one of its curial parts,to which more and more attentions have been paid for many years.Particularly,most of the related researches are focused on the improving of coating hardness and oxidation-resistance,which are two most important criteria for their performance under extreme conditions.The development of superhard coatings,defined by hardness values above 40 GPa,has been accelerated significantly during the last 15 years because of the interest in scientific and industrial applications.These superhard coatings possess an unusual combination of mechanical and chemical properties,such as high fracture toughness,high oxidation resistance and high thermal as well as chemical stability.Owing to its extreme hardness,high thermal and chemical stability,and low electrical resistance,titanium nitride(TiN)has been widely applied as a coating material,ranging from hard and protective coating on mechanical tools,decorative coatings,to the diffusion barrier in microelectronic industry.Many studies have been performed to improve the mechanical properties of TiN thin film through the growth of nanocomposites and multilayers.It is found that multilayers can combine the properties of the constituent materials and have improved properties when compared to the individual single layer film,and multilayers with optimized interface areas seem to be most promising with respect to an optimum hardness-to-toughness ratio.Multilayer coatings are useful as model systems since they allow the design and study of interfaces not readily accessible in three-dimensional nanocomposite.Xu et al.reported that the superhardness effect of a-Si₃N₄/nc-TiN nanostructured multilayers had a relationship with the layer thickness ratio and there existed a maximum hardness at a modulation period of 10 nm and layer thickness ratio of 3(Si₃N₄ to TiN).S(o|¨)derberg et al.found that SiN_x, which is amorphous in one single layer film,can exist as a crystalline structure in TiN/SiN_x multilayers when the thickness of the SiN_x layers is less than a critical thickness.The results of phase transformation for SiN_x from amorphous to crystalline offered an explanation for the superhardening effect determined by nanoindentation experiments.Chen et al.investigated the thermal stability of TiN/SiN_x multilayer coatings and found that the thermal stability depended on the SiN_x layer thickness.At SiN_x layer thickness of 0.8 nm or higher the layer structure of TiN/SiN_x multilayer coatings was maintained after 1 h of annealing at 1000℃and the hardness was as high as 37 GPa.Despite of the accelerated development in the preparation and characterization of TiN/SiN_x superhard coatings during the last decade,there is still a gap between the knowledge of preparation techniques as well as experimental parameters and structures.With extensive data obtained for the other systems,research interest is in understanding the mechanism of the hardness enhancement.Although it is not yet completely understood on the atomic scale,it seems that a large number of interfaces or laminated structures play important roles in the improvement of tribological properties.To have a future understanding of the hardness enhancement for the TiN/SiN_x films,it is of interest to systemically study the relationship between the deposition parameters and the microstructure,interface structure and mechanical properties of the multilayer films.Therefore,in this thesis,the influence of deposition parameters such as pressure,substrate bias,gas flow ratios and temperature on the mechanical properties,interfacial structure and microstructure of the multilayers is studied.Hardness enhancement for multilayer coatings is always influenced by the modulation period and internal stress.In this thesis,a series of TiN/SiN_x multilayer films with different modulation periods were deposited.The influence of modulation period on the structures(interfacial structure and texture)and mechanical properties(internal stress,hardness and elastic modulus)is studied.In addition,the material's microstructure can be designed during post-deposition annealing treatment.Studying the structures and mechanical properties of the multilayers after annealing treatment is significant for understanding the Ti-Si-N superhard nanocomposite system.Therefore,the thermal stability of the multilayers is also investigated by means of annealing at a series of temperatures from 700 to 1100℃in vacuum.Thin film adhesion is a very important issue for protective-film-coated cutting tools.Most adhesion tests for thin films empirically infer the adhesive strength by subjecting the specimen to some external load and measuring the critical value at which it fails.Indentation tests can be used to delaminate films from the substrates, if the films are weakly bonded to the.substrates,and thus to measure the thin film interracial strength.Basically,the cone and the wedge are the two most popular indenter geometries for measuring brittle thin film adhesion.Nanoindentation technology is normally used for measuring mechanical properties such as elastic modulus and hardness for thin film,but it is rarely used for evaluating adhesive strength in such small load and displacement scale.TiN/SiN_x multilayer coatings synthesized by various technologies show improved mechanical properties than their single component,and most researches focus on improving their mechanical properties such as hardness and elastic modulus.However,the fracture behavior, especially the interracial fracture,is rarely investigated.In this thesis,we investigate the interracial fractures for TiN/SiN_x nano-multilayer coatings by nanoindentation experiments.The main results of this thesis are summarized as follows:1.The effect of deposition pressureIn TiN/SiN_x multilayers deposited at different deposition pressure,it is found that the constituent TiN and SiN_x layers are polycrystalline fcc and amorphous, respectively.There are two main peaks(111)and(200)from TiN layers.However, the relative intensity of the TiN(200)peak decreases and the TiN(111)peak increases gradually as the deposition pressure increases from 0.4 Pa to 2.0 Pa, which is attributed to the competitive growth under low ion energy condition.The layer structure becomes more apparent and the interface width decreases monotonously with a decrease in deposition pressure.The surface morphology shows that the surface roughness increases with increasing deposition pressure, which is consistent with the XRR results.The hardness and elastic modulus increase with decreasing deposition pressure,which results from increased particles energy that making the multilayers denser and interfaces sharper with decreasing deposition pressure.2.The effect of substrate biasThe substrate bias voltage has a significant influence on the structures and mechanical properties of the multilayer films.The interfaces of the multilayers are not apparent at floating voltage,but they become sharper when the absolute value of substrate bias increases to 80,whereas they become almost diffused again when the substrate bias is -200 V.The results show that only moderate negative substrate bias is suitable to obtain sharp interfaces and substrate biases above the critical value would result in interface mixing.The variation of hardness and the interface width with the substrate bias show the same tendency.The maximum hardness occurs for the film deposited at the -80 V with the sharpest interface and the strongest(200)preferred orientation.3.The effect of substrate temperatureThe XRR patterns for TiN/SiN_x multilayer films deposited at different substrate temperatures indicate that the substrate temperature has little influence on the layer structure.As the substrate temperature increases from RT(room temperature)to 400℃,only strong(200)preferred orientation for TiN layers is observed.The hardness as well as elastic modulus is almost constant as the substrate temperature increases from RT to 400℃.4.The effect of gas flow ratiosThe N₂/Ar gas flow rate ratio has a significant influence on the structures of TiN/SiN_x film,in which the layer structure becomes not clear and the interface width increases monotonously with increasing the N₂/Ar gas flow rate ratio.The texture for TiN layer in the obtained multilayer film evolves from(200)to(111) with an increase in N₂/Ar flow rate ratio,and the film with a TiN(111)texture is harder than that with a TiN(200)texture.The hardness enhancement for the TiN/SiN_x multilayers results from two factors:(1)The preferred orientation for TiN layers;(2)Interfacial structure in the multilayers.All multilayer films exhibit nano-scale fracture characteristics,and the fracture characteristics do not depend on the hardness of multilayer film.5.The effect of SiN_x layer thickness,modulation period and thermal stability for the multilayer filmsWhen the thickness of TiN in the modulation period is fixed at 15.86 nm and the thickness of SiN_x increases from 0.49 to 9.05 nm,the texture of TiN changes from (111)to(200).The interfaces for the multilayers are not clear when the thickness of SiN_x is 0.49 nm,but it becomes more apparent when the thickness of SiN_x increases to 9.05 nm.The hardness and elastic modulus decrease as the thickness of SiN_x increases.When the modulation period is low(below 5.03 nm)the preferred orientation for TiN is(111),while,when modulation period exceeds 10.05 nm the preferred orientation turns to(200).There is a maximum value of 38.3 GPa at modulation period of 5.03 nm and it would decrease with the increase or decrease in modulation period.Stress measurements shows that SiN_x layers in the multilayer stack helps to relax the internal stress of TiN monolayer film,and hence the internal stress decrease with increasing modulation period.The thermal stability for TiN/SiN_x multilayer films depends on their modulation period,and multilayers with large a modulation period has a relative high thermal stability.The hardness enhancement for the as-deposited multilayers,compared to the monolayer TiN and SiN_x films,is attributed to two factors:the internal stress and sharp interfacial structure in the multilayer stack.The internal stress can be annealed off at a relative low temperature but sharp interfacial structure would maintain until interdiffusion between constituent layers occurs.6.Fracture behavior for the multilayer films and monolayer filmFracture behavior for TiN/SiN_x multilayer films with two different modulation periods,deposited on Si(111)substrate,is studied by nanoindentation experiments. The topographies of the indentations are studied by SEM images combined with in-situ AFM images.It is found that,during nanoindentation experiments,under the condition that the displacement limit mode is used and a strain rate is kept at 0.05/s,an interfacial fracture between the coating and substrate takes place as the maximum indenter displacement into the coating is 2500 nm,and the corresponding unloading segment in the load-displacement curve shows an obvious discontinuity due to the rebound of the detached film during unloading. Furthermore,it is found that the interfacial fracture toughness for the multilayer coatings is associated with the preferred orientation for the TiN layer and the interfaces between TiN and SiN_x layers in the multilayer stack.The TiN/SiN_x multilayer coating on Si(111)with a TiN(111)preferred orientation exhibits a higher interfacial fracture toughness and hardness comparing to that with a TIN(200)preferred orientation.The varying indenter strain rate shows no measurable influence on the fracture behaviors of the multilayers.