Chemical bonding in hard boron-nitride multilayers Page: 4 of 16
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indentation depths to less than 10 nm.(89) There are many examples of hardness enhancement in
multilayer and strained layered superlattice coatings. A 75% increase in the microhardness was
measured for single crystal TiN/VN(100) superlattices prepared by reactive magnetron sputtering
for A decreasing from 32 to 7.5 nm.(0) The hardness increased an additional 75% as the A
further decreased to 5 nm, below which the hardness decreased to a rule-of-mixtures (r-o-m)
value. In a second example, the hardness of (TiN)0.7/(NiCr)0.3 multilayers sputter deposited onto
tool steels were assessed using an indentation penetration depth that was less than 10% of the
film thickness. (11) The coating hardness increased to a maximum that was 60% above the r-o-m
value as A decreased to 1.2 nm. Hardness enhancement in multilayers is not always large as
large as for strained layered superlattices.(12) For example, only a 10% increase in hardness is
found for Cu/Cr laminates.(13) Although the B/BN multilayers are not anticipated to form a
superlattice, the use of nanometric A(s) may create a hardness that exceeds the r-o-m value in an
EXPERIMENTS AND RESULTS
The development of B sputter targets has facilitated the investigation of B and BN
deposition without the presence of BN precursor compounds as the hexagonal phase.(14-16)
Films of B and BN deposited at low temperature are prepared to establish the baseline properties
of the multilayer structures which will consist of a hard B layer alternating with a compliant BN
layer. In these experiments, the (hexagonal) BN layer is intended to moderate residual stresses
formed within the coating by the hard layer. The deposition of the B coating proceeds by rf-
sputtering the B target using an unbalanced planar magnetron.05-17) The deposition chamber is
cryogenically pumped to a base pressure of 5.3x10-6 Pa in 12 hrs including a 4 hr, 100 'C bake
out. Sapphire wafers and 20-50nm thick Ni-coatings on Si wafers (Ni/Si) are used as the
substrates which are horizontally positioned 9 cm away from the center of the 6.4cm diameter B
target. The sapphire wafers are heated to 450 'C and the Ni/Si wafers are heated to 215 *C using
a BoralectricTm heater. The sputter gas pressure is nominally selected as 1 Pa (7 mTorr) with a
constant flow rate of 28 cc/min. The deposition rate is monitored with a calibrated 6 MHz Au-
coated quartz crystal. An increase in forward power from 100 to 300 W produces a linear
increase in deposition rate from to 7 to 21 (x10-3) nm s-1. The multilayer synthesis proceeds by
cycling the gas flow composition between Ar and Ar-25%N2 to produce N number of layer pairs.
A 200 W forward power is used to deposit a 0.15-0.16 pm total film thickness t. The low
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Jankowski, A.F. & Hayes, J.P. Chemical bonding in hard boron-nitride multilayers, article, June 1, 1997; California. (digital.library.unt.edu/ark:/67531/metadc708102/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.