Nanocrystalline HardAmorphous Matrix Nanocomposites

nc-MenN/a-Si3N4 (Me = Ti, W, V) The synthesis and properties of Si3N4/TiN composite coatings have been the subject of scientific and technological interest. The first efforts to co-deposit silicon nitride and titanium nitride were undertaken in the 1980s [588-592]. The TiN/Si3N4 composite coatings were prepared by chemical vapor deposition (CVD) from SiCl4+TiCl4+NH4 +H2 [588-592] and TiCl4/SiH2Cl2/NH3 [593] at a low pressure of 1.33-10.7 kPa and a relatively high temperature of 1050-1450 °C, which is typical of conventional CVD processes. It has become advantageous to make use of the magnetron sputtering of TiSi targets in order to produce conductive diffusion barriers for copper metallization [594, 595]. The crystal structure of the Si3N4 matrix is varied from the amorphous to the a and fi type of Si3N4 [591, 596]. The average crystallite size of TiN in the amorphous Si3N4 matrix from the half-width of X-ray diffraction peaks is estimated to be about 3 nm [588, 591, 592]. It has been reported [588] that the solid solubility of TiN in Si3N4 is not significant at such elevated temperatures.

Recently, novel superhard nanocrystalline-amorphous composite coatings of nc-Me„N/a-Si3N4 type with hardness higher than 50 GPa, used as an alternative for diamond-like carbon, were developed by means of plasma-assisted chemical vapor deposition [163, 208, 577, 587, 597-604] and physical vapor deposition (PVD) [605-610]. As is known [277], the diamond-like carbon has:

• High solubility in ferrous materials

• Limits in high-temperature applications, for example, etching by oxygen at high temperatures

• Low threshold of temperature stability, that is, limited by 500 °C

• Low coefficient of thermal expansion, that is, noncom-patible with most substrates

As a further extension, the PVD technology was developed to overcome the highly corrosive and flammable precursors (i.e., TiCl4, SiCl4, SiH4, etc.) of CVD processing [605, 606].

The research and development strategy regarding the deposition of nc-MeBN/aSi3N4 has been extensively described [164, 577, 598, 599] and can be briefly summarized as follows:

• The components of the composite should themselves have high hardness.

• A ternary/quaternary system is used in order to form a nanocrystalline-amorphous composite with very marked interfaces.

• High adhesion of the nanocrystalline-amorphous interface should be realized.

• Relatively low deposition temperatures should lead to nanostructure formation, avoiding interdiffusion during deposition and a structure change in the steel substrates (550 °C).

Small nanocrystals (2-4 nm) of nitrides of refractory transition metals (Ti [605, 606], Zr, Hf, V, Nb, Ta, W [597]) have been embedded in a thin (0.4-0.6 nm) amorphous silicon nitride (a-Si3N4) matrix (Fig. 10).

The approach to deposition of nc-TiN/a-Si3N4 and nc-W2N/a-Si3N4 provides that these nanocomposites crystallize into two separate phases, which creates an n-Me„N/ a-Si3N4 nanocomposite mixture at a relatively low temperature of 500-550 °C from precursors of TiCl4+SiH4+N2+H2 [1, 577] and TiCl4+SiCl4+N2+H2 [611] by vacuum plasmaassisted chemical vapor deposition. The deposition temperatures are suitable for using steel substrates without any irreversible changes in their structure during deposition processing. The deposition rate used was 2.5-5 ¡xm/h.

In particular, as the a-Si3N4 content increases up to 16-23 mol%, the hardness reaches a maximum value of approximately 50 GPa, whereas the average size of the nanocrystals becomes minimal at about 4 nm [577]. For this composition, the nanocomposite system reaches a minimum free-energy level when the specific area of the nc-TiN/a-Si3N4 interface

Figure 10. Schematic representation of the nc-TiN/a-Si3N4 nanocomposite consisting of TiN nanocrystals embedded in an a-Si3N4 amorphous matrix. Reprinted with permission from [166], R. Hauert and J. Patscheider, Adv. Eng. Mater. 2, 247 (2000). © 2000, Wiley-VCH.

reaches maximum [208]. Further increasing the a-Si3N4 content leads to an increase in the average nanocrystalline size and a decrease in hardness.

Further vacuum annealing of nanocomposites suggests [612] that the increase in crystallite size during annealing over a temperature of T/Text > 0.5, where Text is the melting/decomposition temperature, is absent up to 1150 °C during 30 min annealing. For example, the 1150 °C is sufficiently higher than 0.5Text ~ 800 °C for TiN and Si3N4 forming the nc-TiN/a-Si3N4 component. It was concluded [233] that such high temperature stability is due to the absence of solubility of TiN in the interface nc-TiN/a-Si3N4.

A correlation between the average nanocrystallite size, the content of a-Si3N4, and the hardness of nc-TiN/a-Si3N4 was experimentally found. The dependence of the average nanocrystallite size on the content of a-Si3N4 reaches the minimum of the nanocrystal size. Therefore, the average nanocrystal size, which is about 9 nm, corresponds to 16-20 mol% of a-Si3N4, and the maximum hardness of the nc-TiN/a-Si3N4 nanocomposite was found to be about

50 GPa.

Recently, there have been reports on ultrahard nanocrys-talline composite coatings of nc-TiN/nc-TiSi2/a-Si3N4/a-TiSi2 composition [1, 612-614] having a microhardness of 80-105 GPa. The nc-TiN/nc-TiSi2/a-Si3N4/a-TiSi2 nanocrystalline multiphase composite coatings possess a high elastic recovery up to 90% [1]. Two kinds of ultrahard nanocomposite coatings, namely, nc-TiN/a-Si3N4/a-TiSi2 with about 5 at%

51 and nc-TiN/nc-TiSi2/a-TiSi2 with about 17 at% Si, were formed [1]. The nc-TiN/a-Si3N4/a-TiSi2 nanocrystalline composite coatings with a microhardness of 80 GPa consist of TiN nanocrystals with an average grain size of 10-11 nm surrounded by the a-Si3N4/a-TiSi2 amorphous matrix. At about 10 at% Si, formation of the TiSi2 nanocrystalline precipitates commences. The nc-TiN/nc-TiSi2/a-TiSi2 nanocrys-talline composite coatings with a microhardness of 90-105 GPa consist of TiN nanocrystals with an average grain size of 4-6 nm and TiSi2 nanocrystals with an average grain size of 3 nm. This nanocrystalline mixture is surrounded by the TiSi2 amorphous matrix.

A gas mixture of WF6+SiH4+N2+H2 has been used as a precursor for high-frequency (13 MHz) plasma chemical vapor deposition of nc-W2N/a-Si3N4 nanocrystalline composite coatings at approximately 100 Pa on Si wafer and stainless-steel substrates [597, 598]. Similar correlations were found for the nc-W2N/a-Si3N4 nanocomposite coatings (Fig. 11). A maximum microhardness of about 50 GPa is achieved when the average nanograin sizes are 3-4 nm and the optimal Si content is approximately 7 at%.

It has been concluded [587, 597] that:

• Nanocrystalline deposits are trying to maximize the specific surface of the interfaces and, therefore, decrease the Gibbs free energy of nanocomposite formation.

• An increase in the a-Si3N4 content above 20-30 mol% leads to a decrease in the hardness of the nanocomposite.

• Deposits gain thermodynamic stability when the total area of the nanocrystalline-amorphous interfaces is maximized and both the average size of the TiN nanocrystals and the average thickness of the a-Si3N4 streaks are minimized.

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