New understanding of hardening mechanism of TiN/SiN x -based nanocomposite films
© Li et al.; licensee Springer. 2013
Received: 21 August 2013
Accepted: 1 October 2013
Published: 17 October 2013
In order to clarify the controversies of hardening mechanism for TiN/SiN x -based nanocomposite films, the microstructure and hardness for TiN/SiN x and TiAlN/SiN x nanocomposite films with different Si content were studied. With the increase of Si content, the crystallization degree for two series of films firstly increases and then decreases. The microstructural observations suggest that when SiN x interfacial phase reaches to a proper thickness, it can be crystallized between adjacent TiN or TiAlN nanocrystallites, which can coordinate misorientations between nanocrystallites and grow coherently with them, resulting in blocking of the dislocation motions and hardening of the film. The microstructure of TiN/SiN x -based nanocomposite film can be characterized as the nanocomposite structure with TiN-based nanocrystallites surrounded by crystallized SiN x interfacial phase, which can be denoted by nc-TiN/c-SiN x model ('c’ before SiN x means crystallized) and well explain the coexistence between nanocomposite structure and columnar growth structure within the TiN/SiN x -based film.
KeywordsTiN/SiN x film Nanocomposite Hardening mechanism Microstructure
As superhard (hardness H ≥ 40 GPa) film material, nanocomposite films have been widely investigated in the past decades for use as wear-resistant coatings on tools and mechanical components [1, 2]. Among them, the pseudobinary TiN/SiN x is a representative film due to strong surface segregation of the constituent phases (TiN and SiN x have essentially no solid solubility). Especially, since hardness as high as 80 to 105 GPa was reported by Veprek et al. in 2000 , it has attracted much attention from the scientific community. So far the nanostructure and hardening mechanism have been widely explained by nc-TiN/a-SiN x model proposed by Veprek et al. in 1995 , in which equiaxed TiN nanocrystallites (nc-TiN) were embedded in an amorphous SiN x (a-SiN x ) matrix.
However, this model is in dispute due to the lack of direct experimental evidence, which mainly reflects in two aspects. On one hand, whether TiN crystals are transformed from columnar crystals into equiaxed nanocrystallites is disputed, since there was no direct cross-sectional transmission electron microscopy (TEM) observation for the isotropic nature of the TiN grain. On the other hand, whether SiN x phase exists as amorphous state is also disputed, since Veprek et al.  suggested SiN x was amorphous because no obvious SiN x Bragg reflections in X-ray diffraction (XRD) patterns were found, which lacked direct observational evidence so far. Later, based on their high-resolution TEM (HRTEM) observations, Kong et al.  reported that TiN were columnar nanocrystals, rather than equiaxed nanocrystals, separated by crystallized SiN x interfacial phases. Hultman et al.  suggested that SiN x interfacial phase could be crystalline located around TiN nanocrystals according to their ab initio calculations. However, they did not give direct experimental evidence. In addition, the cross-sectional TEM published by Zhang et al.  and Kauffmann et al.  showed that even with increased Si content up to 12 at.%, the TiN/SiN x nanocomposite films still had a columnar morphology, which increases the uncertainty of the existing model and hardening mechanism of TiN/SiN x film.
To clarify these controversies about hardening mechanism, TiN/SiN x and TiAlN/SiN x nanocomposite films with different Si content were synthesized since the hardness of TiN/SiN x -based nanocomposite films was highly sensitive to the thickness of SiN x interfacial phase [3, 4]. The relationship between microstructure and hardness for two series of films would be studied. Special attention would be paid to the morphology and structure of constituent phases in two films.
The TiN/SiN x and TiAlN/SiN x nanocomposite films were fabricated on the silicon substrates by reactive magnetron sputtering system. The TiN/SiN x and TiAlN/SiN x nanocomposite films were sputtered from TiSi and TiAlSi compound targets (99.99%), respectively, with 75 mm in diameter by RF mode and the power was set at 350 W. The TiSi and TiAlSi compound targets with different Si content were prepared by cutting the Ti (at.%, 99.99%), TiAl (Ti at.%/Al at.% = 70%:30%) and Si targets (at.%, 99.99%), respectively, into 25 pieces and then replacing different pieces of Ti and TiAl with same piece of Si. Adopting this method, TiSi and TiAlSi targets with different Si/Ti (or Si/Ti0.7Al0.3) volume or area ratios, including 1:24, 2:23, 3:22, 4:21, and 5:20 were prepared. The base pressure was pumped down to 5.0 × 10-4 Pa before deposition. The Ar and N2 flow rates were 38 and 5 sccm, respectively. The working pressure was 0.4 Pa and substrate was heated up to at 300°C during deposition. To improve the homogeneity of films, the substrate was rotated at a speed of 10 rpm. The thickness of all the TiN/SiN x and TiAlN/SiN x nanocomposite films was about 2 μm.
The microstructures of TiN/SiN x and TiAlN/SiN x nanocomposite films were characterized by XRD using a Rigaku D/MAX 2550 VB/PC (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation and field emission HRTEM using a Philips CM200-FEG (Philips, Amsterdam, Netherlands). The preparation procedures of cross-section specimen for HRTEM observation are as follows: The films with substrate were cut into two pieces and adhered face to face, which subsequently cut at the joint position to make a slice. The slices were thinned by mechanical polishing followed by argon ion milling. The hardness was measured by a MTS G200 nanoindenter (Agilent Technologies, Santa Clara, CA, USA) using the Oliver and Pharr method . The measurements were performed using a Berkovich diamond tip at a load of 5 mN with the strain rate at 0.05/s. The indentation depth was less than one-tenth of the film thickness to minimize the effect of substrate on the measurements. Each hardness value was an average of at least 16 measurements.
Results and discussion
The influence of Si content on crystallinity throws doubt upon the nc-TiN/a-SiN x model proposed by Veprek [3, 4]. If SiN x phase exists as amorphous state, the increase of Si/Ti ratio from 1:24 to 5:20 (SiN x fraction accordingly rises from 4 to 20 at.%) only leads to thickening of amorphous SiN x interface, which cannot improve the crystallization degree of film, but lowers it due to the increasing impeditive effect on TiN growth. In addition, as amorphous SiN x interfacial phase thickens, TiN and TiAlN phases cannot only present (200) orientation, but may also grow along other directions owing to the randomicity of crystallite growth . Therefore, whether SiN x interfacial phase is amorphous deserves to be further deliberated.
In fact, the effect of Si content on crystallinity of TiN/SiN x and TiAlN/SiN x films brings into our mind the influence of amorphous modulation layer thickness on the crystallization degree of nanomultilayered films, such as TiN/SiC , TiAlN/SiO2, and CrAlN/SiN x . In these nanomultilayered film systems, with the increase of amorphous layer thickness, the crystallization degree of films firstly increases and then decreases, which can be attributed to two facts. On one hand, the initial increase of amorphous layer thickness could not only crystallize the amorphous layer and grew epitaxially with crystal layer, but also the newly deposited crystal layer could grow epitaxially on crystallized amorphous layer, leading to the 'mutual promotion effect’ of growth in nanomultilayers and improvement of crystallization integrity. The thicker the crystallized amorphous layer thickness is, the higher the crystallization degree of the nanomultilayered film. On the other hand, with further increase of amorphous layer thickness, the amorphous layers cannot keep the crystallization state and change back into the amorphous state, which destructs epitaxial growth structure and decreases the crystallization integrity of the nanomultilayer. Therefore, it can be deduced that with increase of Si content, SiN x interfacial phase may undergo the crystallized process, helping to improve the crystallization degree and maintain (200) preferred orientation of TiN/SiN x and TiAlN/SiN x films. Nevertheless, this deduction needs to be verified from HRTEM observations.
From Figure 2a, it can also be seen that many small equiaxed nanocrystallites exist within the TiN/SiN x film in the cross-sectional morphology. In the medium-magnification image of Figure 2b, it is obvious that many equiaxed nanocrystallites with dark contrast are surrounded by the interfacial phase with bright contrast. According to the nanocomposite structure of TiN/SiN x film, it is not difficult to conclude that the equiaxed nanocrystallites with dark contrast and interfacial phase with bright contrast correspond to TiN and SiN x phases, respectively, indicating that the film constituted of equiaxed TiN nanocrystallites encapsulated with SiN x interfacial phase, rather than columnar-like TiN nanocrystals proposed by Kong et al. . The average size of TiN crystallite is about 6 to 10 nm, with average SiN x interfacial thickness of 0.5 to 0.7 nm. In the high-magnification image of Figure 2d, the TiN crystallites basically grow along with the same direction and present the epitaxial growth structure. The SiN x interfacial phase is observed to exist as crystallized state, rather than amorphous state, such as the E zone between A and C crystals, F zone between A and B crystals, H zone between B and D crystals, and G zone between C and D crystals, which verifies the validity of above deduction. It is worth noting that there are also some amorphous areas present in Figure 2d. Actually, these regions are not composed of real amorphous phase. When we slightly tilted the specimen, the regions that appeared to be amorphous could change into crystallized structure, which suggested that there existed the misorientation difference between different regions and that the 'amorphous’ regions are not really composed of amorphous phase, but crystallized phase. Therefore, it is reasonably believed that there exists the same 'crystallized effect’ of nanomultilayered films in nanocomposite films, namely, when Si content increases to an appropriate value, that is, SiN x interfacial phase reaches to a proper thickness, the SiN x interfacial phase can be crystallized under the template effect of adjacent TiN crystallites, which can coordinate the misorientations between TiN crystallites and grow coherently with them. In high magnification of TiAlN/SiN x film, it can also be observed that the lattice fringes continuously go across adjacent TiAlN crystallites through SiN x interfaces, suggesting that SiN x phase has been crystallized between adjacent TiAlN crystallites and grows coherently with them (Figure 2e). Comparatively, the SiN x interfacial thickness of TiAlN/SiN x film is smaller (about 0.3 to 0.5 nm) based on Figure 2e than that (about 0.5 to 0.7 nm) of TiN/SiN x film in Figure 2d, which is agreement with the fact that the Si/Ti0.7Al0.3 ratio of 3:22 in TiAlN/SiN x film is lower than Si/Ti ratio of 4:21 in TiN/SiN x film.
It is not difficult to find that the variation of hardness with increase of Si content is in accord with crystallization degree. According to the hardening mechanism proposed in nc-TiN/a-SiN x model [3, 4, 14], the TiN crystallite size is too small for dislocation activities, and the film can only deform by grain boundary sliding (i.e., by moving single undeformed TiN nanocrystallites against each other). However, based on this mechanism, TiN nanocrystallites that slide along grain boundary must cause the coordinate movement of adjacent nanocrystallites, such as crystallite rotation and shift , and leave trace in the sliding boundary, which both lack direct experimental evidence from the existing literatures. In addition, the dependence of hardness on Si content should not have related to crystallization degree.
Nevertheless, with further increase of Si content, the SiNx interfacial phase thickens and cannot maintain the crystallized state between adjacent TiN nanocrystallites, resulting in the transformation back into the amorphous state and breakage of epitaxial growth structure. Accordingly, the blocking effects on the dislocation motions decrease. Despite that the amorphous phase can also act as an obstacle for dislocation movement, its impeding effect on the dislocation motion is much smaller than that of coherent interface. Therefore, the hardness of the film decreases. It is worth noting that the Si/Ti ratio at which film presents the highest crystallinity and hardness for TiAlN/SiN x film is 3:22, lower than that of 4:22 for TiN/SiN x film. That is to say, the maximal crystallized SiN x interfacial thickness maintained by TiAlN is smaller than that by TiN, which can be attributed to the misfit difference between TiN/SiN x and TiAlN/SiN x . The lattice parameter of TiN decreases with the addition of Al , resulting in the increase of misfit between TiAlN and SiN x , which reduces the epitaxial breakdown thickness of SiN x and might also be the reason for lower maximal hardness for TiAlN/SiN x film relative to TiN/SiN x film.
In summary, in order to clarify the controversies of hardening mechanism for TiN/SiN x -based nanocomposite films, the microstructure and hardness for TiN/SiN x and TiAlN/SiN x nanocomposite films with different Si content were studied. With the increase of Si content, the crystallization degree for two series of films firstly increases and then decreases. The microstructural observations suggest that when SiN x interfacial phase reaches to a proper thickness, it can be crystallized between adjacent TiN or TiAlN nanocrystallites, which can coordinate misorientations between nanocrystallites and grow coherently with them, resulting in blocking of the dislocation motions and hardening of the film. The microstructure of TiN/SiN x -based nanocomposite film can be characterized as the nanocomposite structure with TiN-based nanocrystallites surrounded by crystallized SiN x interfacial phase, which can be denoted by nc-TiN/c-SiN x model ('c’ before SiN x means crystallized) that can well explain the coexistence between the nanocomposite structure and columnar growth structure within the TiN/SiN x -based film.
The present work was financially supported by the National Natural Science Foundation of China under grant no. 51101101, 'Shanghai Municipal Natural Science Foundation’ under grant no. 11ZR1424600 sponsored by Shanghai Municipal Science and Technology Commission, 'Innovation Program of Shanghai Municipal Education Commission’ under grant no. 12YZ104, and 'Shanghai Leading Academic Discipline Project’ under grant no. J50503 sponsored by Shanghai Municipal Education Commission.
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