Nanocrystalline SoftAmorphous Matrix Nanocomposites

nc-MeN/a-Ni (Me = Zr, Cr) Coatings of the Zr-Ni-N and Cr-Ni-N ternary systems [663, 665-669], deposited using direct-current magnetron sputtering of Zr-Ni (90/10 at%) and Cr-Ni alloy targets in Ar-N2 mixture onto steel substrates, correspond to a nanocomposite of nc-MeN/a-Ni type combining a hard nc-ZrN phase (nanograins of 10-50 nm in diameter) embedded in a soft Ni matrix. The nanocom-posite coatings displayed the following characteristics [665, 666]:

• Strong influence of the nanostructure and the Ni content in the nanocomposite coatings on their microhard-ness

• High hardness of up to 57 GPa for Zr-Ni-N and up to 37 GPa for Cr-Ni-N nanocomposite coatings

• The microhardness of the Zr-Ni-N and Cr-Ni-N nanocomposite coatings regulated by variation of partial pressure of nitrogen and negative bias voltage

• The nc-ZrN/a-Ni nanocomposite coating, possessing the highest microhardness, consisted of 4 at% Ni, 50 at% Zr, and 46 at% N.

The investigations demonstrated that the incorporation of nitrogen into the ZrNi deposit transforms the ZrNi alloy into a nc-ZrN/a-Ni nanocomposite. The microhardness of the nc-ZrN/a-Ni nanocomposite coatings reaches a maximum of about 55 GPa at a nitrogen partial pressure between 0.03 and 0.15 Pa depending on the substrate bias voltage

As stated [665], the influence of the substrate bias on the microhardness is very similar to the influence of the nitrogen partial pressure [665]. The nanocomposite coatings with a microhardness of 51.1 GPa prepared at a substrate bias of —75 V and with a microhardness of 54.6 GPa prepared at a substrate bias of —125 V differ in nanograin size. Therefore, the nanocomposite coatings with 51.1 GPa have an average nanograin size of 23 or 57 nm depending on the method of evaluation. The nanocomposite coatings with 54.6 GPa have an average nanograin size of 14 or 5 nm.

The nc-CrN/a-Ni nanocomposite coating possessing the highest microhardness of up to 37 GPa has been deposited from a Cr target fixed with a Ni ring at a partial nitrogen pressure of 0.08 Pa and a substrate temperature of 300 °C

[666]. Later, a microhardness of approximately 45 GPa for the Cr-Ni-N system was achieved [663].

Recently, the origins of the high hardness of ZrN/Ni and CrN/Ni nanocomposites was investigated [669]. It was indicated [669] that the hardness enhancement is due to a high biaxial compressive stress in the ZrN/Ni and CrN/Ni nanocomposites. No hardness enhancement due to the nanostructure formation could be attributed to the ZrN/Ni and CrN/Ni nanocomposites [669].

The attempt to obtain such hard nanocomposite coatings of TiN/Ni has not met with much success [668]. The maximum hardness measured in the TiN/Ni was 10.5 GPa only. It was suggested [668] that the hardness could be increased with decreasing Ni content.

nc-MeN/a-Cu (Me = Zr, Cr, Al, Al-Si) The nc-ZrN/a-Cu nanocomposite coatings were prepared by sputtering the Zr-Cu alloyed target in an Ar+N2 mixture using an unbalanced direct-current magnetron system under a total pressure of 0.7 Pa [670-673].

The microhardness of the ZrN/a-Cu composite coatings, which contained approximately 1-2 wt% Cu, exceeds 40 GPa [670]. X-ray diffraction displayed a strong reflection from ZrN nanograins and no reflections from copper. The micro-hardness proved to be very sensitive to the substrate current density and the substrate temperature. The negative substrate bias was found not to have a strong influence on the microhardness, which remains nearly constant for a wide range of negative substrate bias voltages.

A substrate current density (is) higher than 0.8 mA/cm2 is favorable to obtain nc-ZrN/a-Cu superhard nanocomposite coatings. The range of substrate temperatures (Ts) between 200 and 400 °C provides the deposition of the coatings with microhardness higher than 45 GPa. Among these optimal regimes, the highest microhardness (~54 GPa) of the nc-ZrN/a-Cu containing 1-2 at% Cu corresponds to Ts = 300 0C and is = 1 mA/cm2.

These experiments suggest that the incorporation of nitrogen into the deposits leads to the nanocrystalline structure formation of the coatings, where nc-ZrN and a-Cu are components of the nanocomposite. X-ray diffraction indicated only intensive X-ray peaks from ZrN nanograins oriented so that the (111) texture appears. Copper and its compounds have no X-ray peaks, which is evidence of the X-ray amorphous state of the Cu matrix. The average nanograin size evaluated from the integral width of the X-ray peaks was 35 nm.

Replacement of Zr on Cr in combination with Cu has provided similar results [672]. The highest microhard-ness achievable was approximately 35 GPa [672]. The nc-CrN/a-Cu nanocomposite coatings with the highest hardness were sputtered at 300 °C with a substrate bias of -500 V and a substrate ion current density of 1.6 mA/cm2. The optimal composition of the superhard nc-CrN/a-Cu nanocomposite coatings was 1 at% Cu, 48 at% Cr, and 51 at% nitrogen. X-ray diffraction did not display the presence of copper and no crystalline Cu phase was observed.

Superhard nanocomposite coatings of the Al-Cu-N [306] and Al-Si-Cu-N [674] systems have been prepared by the unbalanced magnetron sputtering technique from targets composed of AI+(2-12) at% Cu and AI+(5-9) at% Si+(2-12) at% Cu, respectively. A mixture of Ar+N2 was used under a total pressure of 0.5 Pa. The maximum hardness of approximately 48 GPa is reached at a nitrogen partial pressure of 0.13 Pa and composed of substoichiometric AINx (N/Al = 0.77) with an average nanograin size of 9.5 nm [675]. The AINx nanograins are surrounded by 8.1 at% Cu, which has an X-ray amorphous state.

nc-ZrN/a-Y Nanocomposite coatings of the Zr-Y-N system can achieve a hardness of up to 47 GPa if they possess both an optimal nanostructure and a composition. An unbalanced magnetron sputtering of the alloyed targets of Zr-Y (80/20 at%) and Zr-Y (93/7 at%) has been utilized for the synthesis of this type of coating [676, 677]. Alloying with yttrium has been found to have a positive influence on reducing the grain size and decreasing the starting temperature of oxidation [678, 679]. Coatings of various compositions of Zr-Y-N were deposited in N2 and N2 +Ar atmosphere under a total pressure of 0.5 Pa. The typical coating thickness was approximately 3 ¡m.

The hardness of such coatings suggests dependence on the partial pressure of nitrogen [676]; that is, the partial pressure of nitrogen predominantly influences the nano-structure formation of the coatings. The average nanograin sizes vary between 4 and 20 nm. The superhardness (~41 GPa) is achieved when the average grain size becomes about 20 nm for PN2 = 0.0025 Pa. These deposits were characterized by the (200) X-ray texture of the nc-ZrN phase. The embedding phase containing yttrium is X-ray amorphous. The nc-ZrN/a-Y nanocomposites achieve a maximum microhardness when the incorporation of nitrogen becomes higher.

The ratio of N/(Zr+Y) affects the microhardness of the Zr-Y-N coatings [676]. The highest possible microhardness (~45 GPa) is achieved when the ratio of N/(Zr+Y) reaches about 1.

0 0

Post a comment