The hard coatings investigated in this research were synthesized
using Oerlikon Balzers Rapid Coating System (RCS) in the cathodic arc
ion platting mode [30]. The following coatings were employed: AlCrNbased
[31] coatings (AlCrN monolayer [32] and AlCrN–TiAlN bilayer
[33]) as well as AlTiN PVD coating. AlCrN–TiAlN bilayer coating has an
AlCrN functional layer deposited on the TiAlN supporting layer. TiAlN
layer endures good adhesion and mechanical strength whereas AlCrN
layer exhibits good oxidation resistance, hot hardness and low thermal
conductivity [33]. A pure reactive nitrogen atmosphere was used during
the deposition of all the coatings. The pressure during deposition was
3.5 Pa and the substrate temperature was held at 450 °C. With the
exception of the AlN coating, the DC-substrate bias voltage was held at
−40 V during the deposition. Due to the insulating character of AlN,
for this coating a unipolar pulsed bias voltage of −40 V and 10 kHz
was applied. Targets were operated with ~3.5 kW. The thicknesses
of the coatings were in the range of 1.5 to 3.6 μm [31]. The micromechanical
characteristics of the coatings were measured on WC–Co
substrates using a Micro Materials NanoTest system. Nanoindentation
was performed in a load controlled mode with a Berkovich diamond
indenter calibrated for load, displacement, frame compliance and indenter
shape according to an ISO14577-4 procedure. The area function
for the indenter was determined by indentations to 0.5–500 mN into a
fused silica reference sample. For the nanoindentation of the coatings,
the peak load was 40 mN and 40 indentations were performed for
each coating. This load was chosen to minimize any influence of surface
roughness on the data whilst ensuring that the indentation contact
depth was under 1/10 film thickness so that a coating-only (loadinvariant)
hardness could be measured in combination with coatingdominated
elastic modulus. Nanoindentation was performed at room
temperature. Nano-impact testing was performed with a NanoTest
fitted with a cube corner indenter as an impact probe. The indenter
was accelerated from 12 μm above the coating surface with 20 mN
coil force to produce an impact every 4 s for a total test duration of
300 s. Tests were performed with 20 and 30 mN coil force. The tests
were repeated 10 times at both forces. The coatings' nano-impact
fatigue fracture resistance was assessed by the final measured impact
depth and confirmed by microscopic analysis of impact craters. Four
repeat micro-scratch tests to 5 N were run for each coating as threescan
topography-scratch-topography experiments using a R = 25 μm
probe. In the scratch scan after 250 μm the load was linearly ramped
at 150 mN/s to reach a peak load of 5 N at 420 μm and then kept
constant at this value until the end of the scratch track after 600 μm.
Analysis of the on-load depth and residual depth data after correction
for frame compliance, topography and slope allows true depth data to
be displayed and the location of first coating failure to be determined.
Cutting tests were performed according to the requirements of
the ISO 8688. Machining parameters used are shown in Table 1. A tool
dynamometer (9255B, Kistler) was used to measure the cutting forces.
Comprehensive data on wear performance including flank wear, rake
wear, average cutting forces and chipping intensity was reported. The
coated tool wear and chipping was measured using a Mitutoyo optical
microscope and a magnification of 30×. The worn end mills with a collet
were fitted in a special holder for measurement. The accuracy of measurement
was ±5 microns. End mills were coated by suppliers to provide
the coatings used for testing. The coatings were pre-examined for
surface irregularities using optical and scanning electron microscopy.
Wear behaviour of worn tools was analyzed using SEM/EDS. EDS analysis
was employed as well for studying chemical composition of the
coatings.
A Bruker D8 DISCOVER with DAVINCI.DESIGN diffractometer with a
CoKα tube was used to perform XRD analysis and identify the phases
formed. 2D frames were collected with DIFFRAC. Measurement Centre
Version 3.0 software was integrated to 1D using DIFFRAC.EVA Version
4.0 (all from Bruker-AXS). A pattern search/match was then executed
using the integrated ICDD PDF-2 2011 powder database.