A. LISIECKI: MECHANISMS OF HARDNESS INCREASE FOR COMPOSITE SURFACE LAYERS ...
577–583
MECHANISMS OF HARDNESS INCREASE FOR COMPOSITE
SURFACE LAYERS DURING LASER GAS NITRIDING OF THE
Ti6Al4V ALLOY
MEHANIZMI POVE^ANJA TRDOTE POVR[INSKIH SLOJEV
KOMPOZITOV ZLITINE Ti6Al4V MED LASERSKO-PLINSKIM
NITRIRANJEM
Aleksander Lisiecki
Silesian University of Technology, Faculty of Mechanical Engineering, Welding Department, Konarskiego 18A, 44-100 Gliwice, Poland
aleksander.lisiecki@polsl.pl
Prejem rokopisa – received: 2016-06-14; sprejem za objavo – accepted for publication: 2016-12-14
doi:10.17222/mit.2016.106
Titanium matrix composite (TMC) TiNX/Ti surface layers characterized by high microhardnesses in a range from 1000 HV0.2
up to 2330 HV0.2 were produced by the Laser Gas Nitriding (LGN) of Ti6Al4V substrate in a pure nitrogen environment by a
diode laser beam with unique characteristics. Despite the high hardness, crack-free surface layers were produced in a wide range
of processing parameters. It was found that the maximum values of hardness and its depth profile on the cross-section of the sur-
face layer should be considered in relation not only to the population and volume fraction of the dendritic precipitations of tita-
nium nitrides TiN, but also in relation to the atomic ratio N/Ti in the TiNX compounds. Conditions favorable for stoichiometric
-TiN nitrides precipitation exist in the vicinity of the liquid/gaseous nitrogen boundary, because of the highest temperatures in
this region of melt pool and the highest concentration of atomic nitrogen. Therefore, the population of stoichiometric titanium
nitrides is the highest directly under the top surface.
Keywords: laser gas nitriding, titanium alloy, diode laser, metal-matrix composite, hardening
Nitriranje kompozita na osnovi titana (angl. TMC) TiNX/Ti, za katere je zna~ilna visoka mikrotrdota v obmo~ju med 1000
HV0.2 in 2330 HV0.2, je bilo izvedeno s plinskim laserskim nitriranjem (angl. LGN) na substratu zlitine Ti6Al4V s specialnim
diodnim laserskim snopom. Kljub visoki trdoti, so povr{inske plasti proizvedene brez razpok v {irokem obmo~ju procesnih
parametrov. Ugotovljeno je bilo, da je potrebno upo{tevati mejne vrednosti trdote in globino profila na preseku povr{inskega
sloja v povezavi, ne samo s koli~ino in volumskim dele`em dendritov titanovih nitridov (TiN), pa~ pa tudi z atomskim raz-
merjem N/Ti v TiNX spojinah. Najugodnej{i pogoji za izlo~anje stehiometri~nih -TiN nitridov obstajav bli`ini meje
talina/plinasti du{ik. Tam so najvi{je temperature in zato nastaja bazen~ek staljene kovine v prisotnosti najvi{je atomske
koncentracije du{ika. Zato je koli~ina nastalih stehiometri~nih titanovih nitridov najvi{ja tik pod povr{ino.
Klju~ne besede: lasersko-plinsko nitriranje, titanova zlitina, diodni laser, kovinska kompozitna matrica, utrjevanje (kaljenje)
1 INTRODUCTION
Titanium and titanium alloys, in particular, show
great mechanical properties, especially strength-to-
weight ratio and corrosion resistance in different envi-
ronments, compared to other advanced metallic or
composite materials such advanced high-strength steels
(AHSS), stainless steel, nickel- or cobalt-based super-
alloys, aluminium or magnesium alloys.1–6 Titanium
alloys also show good resistance to static, dynamic, and
cyclic alternating loads (fatigue), such as in the case of
vibrations.7–11 One of the most commonly used two-
phase titanium alloys is the + alloy Ti6Al4V, the pro-
perties of which can be controlled through a heat treat-
ment used to adjust the amounts and types of phases.12–16
However, the application of titanium alloys is often
limited due to medium or even poor tribological proper-
ties.1,3,17–22 For that reason different surface treatments or
surface-modification methods are often applied to impro-
ve the surface characteristic of Ti alloys, such hardness,
erosion, friction, cavitation wear resistance, or even to
improve the corrosion resistance.1,23–27 One of such me-
thods is the Laser Gas Nitriding of a titanium substrate in
the solid or liquid state, by Laser Surface Melting (LSM)
in a nitrogen-rich atmosphere or in pure gaseous or even
liquid nitrogen (cryogenic conditions).1,24–27 In the field
of Laser Gas Nitriding of titanium and its alloys, a lot of
research has already been done. Laser nitriding of
titanium substrate leads to the precipitation of hard
titanium nitrides TiN in the metallic matrix, increasing
hardness of the substrate surface. Additionally, the wear
and also corrosion resistance of the substrate can be
significantly increased, as shown by many studies and
many researchers.1,3,11–14,24–27 However, the main problem
widely reported is cracking of hard nitrided layers due to
excessive hardness and stresses, especially in the case of
multi-bead surface layers, when the subsequent layers
are produced by overlapping.1,3,26–29 Thus, the control of
hardness is crucial to ensure high quality and appropriate
service properties of the laser nitrided surface layers.1 S.
Mridha and T. N. Baker25 produced hard a surface layers
with a maximum surface hardness of 1480 HV by laser
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Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)577(2017)
melting a Ti6Al4V substrate in pure nitrogen environ-
ment. They found that the hardness is related to the
dendritic TiN population (concentration) in the layers.
Additionally, they indicate that the microstructure and
thus the surface hardness can be controlled by nitrogen
flow rates and laser energy density. In turn, C. Hu and T.
N. Baker26 produced crack-free overlapped nitrided
layers, and they indicate that the tendency to crack is
related to the volume fraction of titanium nitrides formed
in the melt. They point to the importance of preheating
the substrate before nitriding, which can reduce the
tendency to crack and improve the depth profile.26 M. S.
Selamat et al.27 successfully produced crack-free over-
lapped nitrided surface layers on a Ti6Al4V substrate in
a dilute nitrogen atmosphere comprising 20 % nitrogen
and 80 % argon. However, the surface hardness was
rather in the medium range, from only 500 HV to
800 HV.
In this study, a High Power Direct Diode Laser
(HPDDL) with unique characteristics of the laser beam
was applied for the surface melting of the Ti6Al4V alloy
in a pure gaseous nitrogen environment at different scan-
ning speeds and different laser output powers. Micro-
hardness depth profiles were determined on the
cross-section of the surface layers, while the structure,
chemical and phase compositions were determined and
analyzed.
2 EXPERIMENTAL PART
The tests of Laser Gas Nitriding were carried out on
a substrate of the titanium alloy Ti6Al4V (Grade 5 ac-
cording to ASTM B265), with the nominal chemical
composition of 6.29 Al, 4.12 V, 0.14 C, 0.18 Fe in % of
mass fractions and balance Ti. This titanium alloy is
comprised of a hexagonal close-packed (hcp)  phase
and a body-centered cubic (bcc)  phase, at room tem-
perature, thanks the addition of vanadium that stabilizes
the  phase. The desired properties of this Ti-alloy are
shaped during the heat treatment. The samples prepared
for nitriding tests were 3.0 mm thick and cut from a
hot-rolled sheet into coupons of 40.0 mm × 70.0 mm.
The whole surfaces of specimens were mechanically
ground using 180-grade SiC paper to remove the surface
oxide layers and to ensure uniform conditions for the
absorption of laser radiation. Next the surfaces were
decreased by acetone. The experimental setup was based
on a fully automated and programmable positioning
system coupled with a High Power Direct Diode Laser
(HPDDL) with a rectangular beam spot. In the design of
direct diode laser, the beam is emitted directly from the
laser head, without the need to transmit a laser beam. In
this case the generator of the laser radiation, shaping,
and focusing optics are placed in a compact laser head.
The laser head was mounted on a linear drive, set in a
vertical position. The characteristic features of the
specific HPDDL laser is a rectangular beam spot with
dimensions of 1.8 mm × 6.8 mm, multimode energy
distribution in the longitudinal direction, and a short
wavelength of 808 nm, which is advantageous because of
the high absorption coefficient on the surfaces of metals.
The rectangular beam spot was focused on the top
surface of a sample and set transversely to the scanning
direction. Single stringer beads were produce as a result
of surface melting in the pure gaseous nitrogen. Due to
the width of 6.8 mm, the width of the stringer beads was
in a range from approximately 6.0 mm to 6.5 mm wide.
To provide stable and reproducible conditions of laser
nitriding, and also to prevent the access of oxygen, or
hydrogen from the ambient air, as well as the problem
with degassing of the substrate a gas chamber was
applied with a continuous flow of gaseous nitrogen at a
pressure slightly higher than atmospheric pressure of
about 1.0 bar. The gas chamber was first evacuated and
next filled with gaseous nitrogen at room temperature.
The nitrogen of 99.999 % purity was delivered by an
inlet placed on a side wall, and the free flow of nitrogen
was provided via the chamber and through the outlet
placed on the opposite side wall. The nitrogen flow rate
was kept at 10.0 L/min. The gas chamber was made of
acrylic glass (PMMA), which is completely transparent
for the 808 nm radiation, so the laser beam was delivered
into the chamber through the cover glass (Figure 1).
Table 1: Parameters of laser gas nitriding of the Ti6Al4V alloy by the
HPDDL laser
Surface
layer
Scanning
speed
(mm/min)
Laser out-
put power
(kW)
Energy
input
(J/mm)
Power
density
(W/cm2)
SL1 400 1.8 270 1.5×104
SL2 1000 1.5 90 1.2×104
SL3 1500 1.5 60 1.2×104
Processing parameters were chosen based on previ-
ous investigations in such a way as to provide different
penetration depth, different thermal conditions of
nitriding, and thus different structures and morphologies
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Figure 1: A view of laser melting of Ti6Al4V alloy substrate in the
gas chamber filled with nitrogen by the HPDDL laser beam transmit-
ted via the transparent cover made of acrylic glass (PMMA)
of the test surface layers. In general, the higher the en-
ergy input of the laser nitriding, the higher the penetra-
tion depth. Preliminary tests have shown that when using
the HPDDL laser beam, preheating of the substrate is not
required. The processing parameters and technological
conditions are given in Table 1. After the nitriding trials,
the samples were sectioned and grinded by 180 up to
1500 grit SiC abrasive papers and polished by 0.5 μm di-
amond paste. Kroll’s reagent was applied to reveal the
microstructure on polished cross-sections. The composi-
tion, crystal structure and microstructure of the test sur-
face layers were analysed by optical microscopy (OM),
scanning electron microscopy (SEM), energy-dispersive
spectroscopy (EDS), electron backscatter diffraction
(EBSD), and X-ray diffraction (XRD). Vickers micro-
hardness was measured along the symmetry axis of
cross-sections, and the microhardness depth profiles
were determined. The hardness values in specific regions
of surface layers were correlated with the microstruc-
tures. The results of the investigations are given in Fig-
ures 2 to 8.
3 RESULTS AND DISCUSSION
The surface layers produced by the HPDDL laser sur-
face melting of Ti6Al4V substrate in a pure nitrogen
environment are shiny and have a golden colour. The sur-
face topography, depth of the fusion zone, and morpho-
logy are dependent on the processing parameters, such as
scanning speed and the laser output power. Vickers
microhardness depth profiles were determined with a
load of 200 g on the cross-sections, starting from the
subsurface region, through the fusion zone, heat affected
zone, to the base metal, as shown in Figure 2. As can be
seen, the maximum values of the microhardness were
determined in the subsurface regions of every tested
surface layer. However, the highest value of micro-
hardness 2330 HV0.2 was measured for the surface layer
(SL1) produced at the highest energy input 270 J/mm,
and the lowest scanning speed 400 mm/min (Table 1,
Figure 2). In this case the maximum depth of the fusion
zone was estimated at approximately 1.6 mm. As can be
seen in Figures 3 to 5 the fusion zone is not homo-
geneous, and it can be divided into subregions. These
results are consistent with the previous investigations
described in 1. The first subsurface region A extends to a
depth of 150 μm and consists of densely packed den-
drites perpendicular to the top surface (Figure 3).
However, some of these dendrites reach deeper, up to a
depth of 250 μm or even 300 μm, (Figures 3, 4). It is
also worth nothing that the surface (SL1) is flat, as
shown in Figure 3a. The surface is covered tightly by a
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Figure 3: SEM micrograph of the nitrided surface layer SL1, showing
the: a) composite structure consisting of TiNx dendrites in metallic
matrix and b) XRD pattern of the subsurface region A, and c) Kikuchi
lines of EBSD analysis in the A region indicating TiN phase type of
the composition N(50 % of amount fractions) and Ti(50 % of amount
fractions)
Figure 2: Microhardness profiles determined on cross-sections of
nitrided surface layers compared to the substrate of Ti6Al4V alloy
homogenous layer with a thickness from 8 μm to a maxi-
mum 10 μm. An analysis of the chemical composition
conducted on the depth profile of the cross-section of the
surface layers LS1 indicates the presence of only
nitrogen and titanium to a depth of approximately 8 μm,
(Figures 7, 8). Despite the large scatter of results, the
trend lines show that the nitrogen concentration in this
region is up to 40 % (Figure 7b). The results of the XRD
analysis show the presence of titanium nitrides -TiN in
the subsurface region (Figure 3b). Additionally, the
EBSD analysis indicates that the titanium nitrides have a
stoichiometric atomic ratio N/Ti (50 % of N and 50 % of
Ti), as show in Figure 3c. However, the nitrogen con-
centration decreases rapidly with the depth, as can be
seen in Figure 7b. The structure in the middle region
"B" of the surface layer SL1 shows different morphology
(Figures 3a, 4). In this region the share of dendrites is
considerably smaller, and the orientation of dendrites
becomes random. Additionally, the interdendritic spaces
are filled with needle-like phases, which were identified
by XRD and EBSD analysis as the matrix of martensitic
’-Ti enriched in nitrogen (Figures 3b, 4b). The
microhardness in this region falls down gradually to
approximately 1400 HV0.2. The XRD spectrum taken
from this region indicates presence of ’-Ti(N), -TiN,
and also -Ti2N nitrides. Detailed EBSD analysis con-
firmed the presence of -Ti2N nitrides at the atomic ratio
N/Ti 33.3 % of amount fractions of nitrogen and 66.7 %
of amount fractions of titanium (Figure 4b). Addition-
ally, the EBSD analysis revealed that the titanium
nitrides TiNX in this region do not have the stoichiome-
tric composition. The precipitations of hard titanium
nitrides TiN phase are particularly important because
these phases responsible for the enhancement of hard-
ness and wear characteristics of nitrided layers.27 The
titanium nitride at the stoichiometric ratio of N/Ti (equal
to 1) has a face-centred cubic (fcc) structure with the
lattice constant a = 0.4249 nm. However, the lattice para-
meter depends on the content of nitrogen, but the com-
pound is stable over a wide range of atomic ratio N/Ti
(0.6 < N/Ti < 1.16), as shown by M. S. Selamat et al.27
Similarly the hardness of titanium nitride TiNX depends
on the atomic ratio N/Ti. That is why different values can
be found in the literature. In general, Vickers micro-
hardness values in a range from 2100 HV up to 2400 HV
are often reported for the titanium nitride hard coating.
However, in the case of coatings or thin films deposited
on different substrates the range of microhardness
reported in literature is significantly wider. Depending
on the applied method of coating deposition, micro-
structure, especially grain size, columnar vs. equiaxed
structure, the thickness of the film, the presence of voids,
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Figure 5: SEM micrograph of: a) the bottom region C of the nitrided
surface layer SL1 and b) Kikuchi lines of EBSD analysis in the C re-
gion indicating Ti2N phase type of the composition N (33.3 % of
amount fractions) and Ti (66.7 % of amount fractions)
Figure 4: SEM micrograph of: a) the middle region B of the nitrided
surface layer SL1 and b) Kikuchi lines of EBSD analysis in the B re-
gion indicating TiN0.61 phase type of the composition N(37.9 % of
amount fractions) and b) Ti(62.1 % of amount fractions)
purity, etc., and the microhardness can be significantly
higher than 2400 HV. On the other hand, the results of
hardness nanoindentation of nanocrystalline TiN pre-
sented in the literature show significant difference bet-
ween the single crystal TiN (2900 HV) and the poly-
crystalline TiN (3200 HV).26,27 In the case of the middle
region (B) of the surface layer SL1 the atomic ratio of
TiNX dendrites was determined as 38 % of amount frac-
tions of nitrogen and 62 % of amount fractions of
titanium, so the TiN0.61 phase was identified. Addition-
ally, the quantity (volume fraction) of dendritic precipi-
tations decreases with the depth. Thus, the micro-
hardness changes should be considered in relation to
both the share of titanium nitrides precipitations, as well
as to the atomic ratio N/Ti of the TiNX compounds. Since
the stoichiometric titanium nitrides -TiN have the
highest hardness, the share of these nitrides determines
the maximum hardness of the surface layers. The third
characteristic region C that can be distinguished in the
surface layer SL1 is placed on the bottom, i.e., at the
boundary of fusion zone and the heat-affected zone. In
this specific region, the amount of titanium nitrides is
very small, but the microhardness remains at a high level
of about 510 HV0.2. This is because high cooling rates
resulted in the formation of martensitic structure of
’-Ti(N). However, the titanium metal matrix is enriched
in nitrogen, which is a strong -phase stabilizer and its
solubility in the -Ti phase reaches 23 % of amount
fractions at 1050 °C. Additionally, nitrogen is an inter-
stitial element, which can be considered as strengthening
element. The nitrogen distorts the lattice of ’-Ti matrix
by occupying the available interstitial sites in the struc-
ture. Thus, during the laser gas nitriding in the liquid
state the lattice parameter of ’-Ti phase is affected by
both the rapid cooling rates and the nitrogen content
dissolved in the titanium matrix. On the other hand, in
the region of the heat-affected zone, which reaches the
bottom surface of the 3.0-mm-thick sample, the micro-
hardness is in a range from 450 HV0.2 to 500 HV0.2.
The surface layers SL2 and SL3, produced at signifi-
cantly lower energy inputs of 90 J/mm and 60 J/mm,
respectively, show a different morphology (Figure 6).
The surface layer SL2 is covered by a homogenous layer
of stoichiometric -TiN, but the layer is thinner com-
pared to the surface layer SL1. In this case the thickness
of the homogenous layer is from 2 μm up to 5 μm.
Dendrites perpendicular to the top surface layer extend
to a depth of approximately 100 μm. In contrast to the
surface layer SL1, in this case the dendritic nitrides are
embedded by the metallic matrix of ’-Ti(N) in the
subsurface region (Figure 6a). Therefore, the maximum
value of the microhardness measured in the subsurface
region is just 1340 HV0.2. Next the microhardness falls
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Figure 7: Analysis of the chemical composition on the depth of sur-
face layers LS1 indicates the: a) the line along which the chemical
composition was determined b) change in the composition of individ-
ual elements
Figure 6: SEM micrographs and XRD patterns of the surface layers:
a) SL2, b) SL3
down to approximately 550 HV0.2 and 400 HV0.2 in the
heat-affected zone.
The surface layer SL3 produced at the lowest energy
input is also covered by the characteristic golden layer;
however, the surface layer is very thin and not tight (Fig-
ure 6b). The thickness of the layer is from 1 μm to 3 μm.
The longest dendrites in the subsurface region have ap-
proximately 30 μm. Additionally, the share of titanium
matrix ’-Ti(N) is significantly greater than the dendritic
nitrides. Therefore, in this case the highest microhard-
ness of just 820 HV0.2 was determined in the subsurface
region, and next the microhardness decreased gradually
to the value 500 HV0.2 characteristic for the heat-af-
fected zone.
4 CONCLUSIONS
The morphology of surface layers produced by the
HPDDL laser surface melting of the Ti6Al4V alloy in a
pure nitrogen atmosphere are dependent on the laser pro-
cessing conditions, such as laser power, beam power
density, and scanning speed. However, the highest values
of microhardness were measured directly under the top
surface for each nitrided surface layer. Depth profiles of
microhardness determined on the cross-sections indicate
that the microhardness decreases gradually from the
subsurface region to the base metal of the titanium alloy,
having a microhardness of 330–350 HV.02. The highest
value of the microhardness of approximately
2330 HV0.2 was determined in the subsurface region of
the surface layer with the greatest depth of 1.6 mm,
produced at the highest energy input of 270 J/mm. The
subsurface region consists mainly of closely spaced
dendrites oriented perpendicularly to the surface, and
identified by XRD and EBSD analysis as titanium
nitrides. Additionally, each nitrided surface is covered
tightly by a homogenous layer having a thickness up to
10 μm. The top layer was also identified as the TiN
phase at the stoichiometric or near stoichiometric atomic
ratio of N/Ti. Below the subsurface region the orientation
of dendrites becomes random, and additionally a
needle-like phase occurs. The needle-like structure was
identified as the matrix of martensitic ’-Ti(N) enriched
in nitrogen. The share of the dendritic precipitations and
the needle-like matrix structure changes with depth.
Moreover, the dendrites concentration decreases with the
depth. Thus, it is clear that value of the hardness or the
microhardness is correlated with the share of dendritic
titanium nitrides and the matrix. However, it was found
that the atomic ratio N/Ti of the dendritic nitrides is not
constant for the nitrided layers produced under different
conditions, as well as within a single layer produced at
the highest energy input. The EBSD results showed that
the dendritic nitrides in the subsurface region of the
thicker surface layer are mainly stoichiometric -TiN
with the atomic ratio 50 % of N and 50 % of Ti. Since
the stoichiometric titanium nitrides exhibit the highest
hardness, the share of stoichiometric -TiN determines
the maximum hardness of the surface layers. The share
of stoichiometric -TiN decreases rapidly with the depth,
which is why the highest microhardness of the composite
TiN/Ti layers can be found directly under the top sur-
face, and next the drop of microhardness can be ob-
served.
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A. LISIECKI: MECHANISMS OF HARDNESS INCREASE FOR COMPOSITE SURFACE LAYERS ...
Materiali in tehnologije / Materials and technology 51 (2017) 4, 577–583 583
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS