coatings

Article
The Effect of DLC Surface Coatings on Microabrasive Wear of
Ti-22Nb-6Zr Obtained by Powder Metallurgy
Silvio José Gobbi 1 , Jorge Luiz de Almeida Ferreira 1 , José Alexander Araújo 1 , Paul André 1 ,
Vinicius André Rodrigues Henriques 2 , Vladimir Jesus Trava Airoldi 3 and Cosme Roberto Moreira da Silva 1, *

1 Department of Mechanical Engineering, University of Brasilia, Brasilia 70910-900, Brazil;

silviogobbi2@gmail.com (S.J.G.); jorge@unb.br (J.L.d.A.F.); alex07@unb.br (J.A.A.);
paul.ingciv@hotmail.com (P.A.)
2 Department of Aerospace Science and Technology, Aeronautics Institute of Technology,
São José dos Campos 12228-900, Brazil; vinicius@iae.cta.br
3 National Institute of Research, São José dos Campos 12227-010, Brazil; vladimir@las.inpe.br
* Correspondence: cosmeroberto@unb.br

Abstract: Titanium alloys have a high cost of production and exhibit low resistance to abrasive wear.
The objective of this work was to carry out diamond-like carbon (DLC) coating, with dissimilar
thicknesses, on Ti-22Nb-6Zr titanium alloys produced by powder metallurgy, and to evaluate its
microabrasive wear resistance. The samples were compacted, cold pressed, and sintered, producing
substrates for coating. The DLC coatings were carried out by PECVD (plasma-enhanced chemical
vapor deposition). Free sphere microabrasive wear tests were performed using alumina (Al2 O3 )
abrasive suspension. The DLC-coated samples were characterized by scanning electron microscopy
(SEM), Vickers microhardness, coatings adhesion tests, confocal laser microscopy, atomic force
microscopy (AFM), and Raman spectroscopy. The coatings did not show peeling-off or delamination
in adhesion tests. The PECVD deposition was effective, producing sp2 and sp3 mixed carbon
compounds characteristic of diamond-like carbon. The coatings provided good structural quality,
Citation: Gobbi, S.J.; Ferreira, J.L.d.A.; homogeneity in surface roughness, excellent coating-to-substrate adhesion, and good tribological
Araújo, J.A.; André, P.; Henriques,
performance in microabrasive wear tests. The low wear coefficients obtained in this work demonstrate
V.A.R.; Airoldi, V.J.T.; Moreira da
the excellent potential of DLC coatings to improve the tribological behavior of biocompatible titanium
Silva, C.R. The Effect of DLC Surface
alloy parts (Ti-22Nb-6Zr) produced with a low modulus of elasticity (closer to the bone) and with
Coatings on Microabrasive Wear of
near net shape, given by powder metallurgy processing.
Ti-22Nb-6Zr Obtained by Powder
Metallurgy. Coatings 2024, 14, 1396.
https://doi.org/10.3390/
Keywords: biomaterials; dlc coating; titanium bio inert; microabrasive wear
coatings14111396

Academic Editors: Yu Liu, Yali

Gao, Dongdong Zhang and
1. Introduction
Bingbing Wang
Over the past few years, materials science has investigated different types of biomateri-
Received: 27 April 2024 als and their applications to replace or restore the function of compromised or degenerated
Revised: 10 September 2024 tissues or organs [1]. Every year, over 13 million prostheses/medical devices are implanted
Accepted: 23 September 2024 in the US alone [1,2]. Thus, biomaterial helps to improve the quality of life and longevity
Published: 4 November 2024
of human beings. The field of biomaterials has shown rapid growth to keep up with the
demands of an aging population [3]. An acceptable reason for the increase in revision
surgeries is the longer life expectancy [3,4]. Implants are expected to work much longer or
Copyright: © 2024 by the authors.
until the end of life without failure or revision surgery [3]. Thus, developing appropriate
Licensee MDPI, Basel, Switzerland. materials with a high longevity and excellent biocompatibility becomes essential. Titanium
This article is an open access article and its alloys have been widely used in numerous biomedical applications due to a unique
distributed under the terms and combination of desirable properties [5,6]. Such mechanical properties include excellent
conditions of the Creative Commons corrosion resistance, low density, high toughness, and excellent biocompatibility [5–7].
Attribution (CC BY) license (https:// Titanium alloys have proven to be superior in terms of biocompatibility when compared
creativecommons.org/licenses/by/ to stainless steel and cobalt alloys [8,9], thus being the most promising biomaterials for
4.0/). implants [3,10]. The titanium alloy Ti-6Al-4V is the most commonly used for application as

Coatings 2024, 14, 1396. https://doi.org/10.3390/coatings14111396 https://www.mdpi.com/journal/coatings

Coatings 2024, 14, 1396 2 of 19

an implant material [11]. This alloy was originally developed for other applications, such as
the aerospace industry [12]. However, in biomedical applications, both Al and V released
into the bloodstream are related to long-term health problems [9,12]. It has been reported
that Al is an element involved in serious diseases such as Alzheimer’s disease and bone
metabolism (osteomalacia) [4,9]. Some V-free Ti alloys for biomedical applications, such
as Ti-6Al-7Nb and Ti-5Al-2.5Fe, have been developed [9,13]. The modulus of elasticity of
Ti-6Al-4V alloy (~110 GPa) is much lower than that of stainless steel and Co-based alloys (~180
and 210 GPa, respectively) [13]. However, its modulus of elasticity is significantly greater than
that of bone tissue (10–40 GPa), causing the formation of tension shielding that can potentially
cause bone resorption and eventual implant failure [12]. Thus, the development of low modulus
Ti alloys for biomedical applications has evolved in recent years [13,14]. New alloys with high
biocompatibility, low elastic modulus, and tensile strength superior to pure titanium are promis-
ing candidates for implant application [15–18]. Titanium has high reactivity, especially with
oxygen [19]. The cost of machining these alloys is relatively high. The powder metallurgy
process can produce parts with the final shape very close to the desired one, known as
“near-net-shape”, effectively reducing the cost of producing titanium alloys [20,21]. The
powder metallurgy technique also enables the production of parts with controlled porosity
and modulus of elasticity closer to the bone, and, consequently, reduces the tension between
the implant and the bone [14,22,23]. Despite the good mechanical properties, Ti and its
alloys exhibit low tribological performance, such as a high and unstable wear coefficient,
severe adhesive wear, and low abrasive wear resistance, limiting their application [7,24].
Proper surface treatment expands the use of titanium and its alloys, being one of the
most effective methods for improving wear resistance [25,26]. DLC (diamond-like carbon)
coatings show good behavior, with excellent mechanical, tribological, and biocompatible
properties [27]. They are excellent candidates for use as anti-wear coatings due to their
extreme mechanical strength, low friction coefficient, high stability, and excellent biocom-
patibility [28,29]. They have, therefore, a potential medical application to suppress the
generation of particles in implants arising from the implant/bone movement [29]. The
accumulation of wear residues at the implant/bone interface can produce an adverse
cellular response leading to inflammation, the release of harmful enzymes, osteolysis,
infection, implant loosening, and pain [29]. High friction and consequent wear of artificial
hip implants after 10–15 years of implantation are the major issues leading to revision
surgery [30,31]. Therefore, surface modification techniques can be developed to improve
the implant quality considering the lifespan of younger patients. DLC coating has an
amorphous and chemically inert structure composed of two types of carbon hybridization
(sp2 and sp3) that provide high hardness, low friction coefficient, biocompatibility, and,
in addition, it is a solid lubricant [32–34]. As a solid lubricant, the so-called transfer layer
of the graphitic fraction is deposited on the counterpart, preventing wear and providing
negligible wear rates for the DLC coating under tribological conditions [33–35]. The DLC
coating is bio-inert and has good cell adhesion, unlike most biomaterials [36–39]. In this
work, diamond-like carbon (DLC) coating, with dissimilar thickness, was carried out on
Ti-22Nb-6Zr titanium alloys produced by powder metallurgy. Niobium (Nb), titanium
(Ti), and zirconium (Zr) have attracted much attention as implant materials due to their
excellent mechanical properties and biocompatibility. [40]. Although Zr is considered a
neutral element in relation to the alpha and beta phases, some studies demonstrate that
Zr is a stabilizer of the beta phase in the Ti-Nb-Zr ternary system [41]. In the Yudin et al.
study, [42] a powder of Ti-18Zr-15Nb biomedical alloy with spongy morphology and with
more than 95% vol. of β-Ti was obtained by reducing the constituent oxides with calcium
hydride. The influence of the synthesis temperature, the exposure time, and the density of
the charge (TiO2 + ZrO2 + Nb2 O5 + CaH2 ) on the mechanism and kinetics of the calcium
hydride synthesis of the Ti-18Zr-15Nb β-alloy was studied.
Coatings 2024, 14, 1396 3 of 19

2. Materials and Methods

2.1. Production of the Sample of Ti-22Nb-6Zr by Powder Metallurgy
The powders of titanium and zirconium elements were produced from the hydrogena-
tion process. Sponge fines previously washed with organic solvent (acetone) and air-dried
were used. The hydrogenation step for all metals was carried out at 500 ◦ C, in a Thermal
Technology Astro Series 1000 high vacuum oven, Thermal Technology, 2221 Meridian Blvd
Minden, NV 89423, USA, with a maximum temperature of 2500 ◦ C, for approximately
3 h, with a pressure of 100 kPa. After cooling, the friable material was ground under a
133 × 10−3 Pa mechanical vacuum at room temperature. A stainless steel mill was used,
coated with titanium plates and containing titanium balls, aiming to avoid contamination.
All powders were used in their hydrogenated state, aiming to achieve greater activation
of the sintering process through the atomic movement of hydrogen during the process
and reduce costs since the dehydrogenation step is expensive and time-consuming. The
average particle size for the powders was 20 µm. In preparing the substrate of the Ti-
22Nb-6Zr alloy (SBTNZ), the specimens of the alloy under analysis were obtained by the
blended-elemental (BE) technique from the mixture of hydrogenated elemental powders,
followed by a sequence of uniaxial and cold isostatic pressings and vacuum sintering.
These methodologies aimed to achieve the maximum possible densification and optimize
the process parameters. A Mettler Toledo analytical balance model PB3002l, Mettler-Toledo
Indústria e Comércio Ltda, Barueri-SP, Brazil, with a precision of 0.01 g was used to weigh
the powders in the stoichiometry of the alloy. Then, grinding and mixing were carried out
for 15 min in a mechanical shaker. Cylindrical samples measuring 8 mm in diameter by
4 mm in height were prepared for the Ti-22Nb-6Zr alloy using steel dies with a floating
jacket. Compaction was performed using a uniaxial hydraulic press (Marconi, model 0981,
Marconi- Equipamentos para Laboratórios LTDA, Piracicaba-SP, Brazil). The uniaxially
cold compacted specimens were encapsulated under vacuum in flexible latex molds and
introduced into the cylindrical pressure vessel of a cold isostatic press. A pressure of
450 MPa for 30 s was applied. A Paul Weber KIP 100 E isostatic press, Paul-Otto Weber
GmbH, Fuhrbachstraße 4-6/73630 Remshalden, Germany, was used, with a capacity of
100 t, equipped with a cylindrical chamber with a diameter of 50 mm, a useful height
of 160 mm, and a maximum pressure of 500 MPa. A Thermal Technology Inc vacuum
furnace, model 1000-3060-FP 20, Thermal Technology, 2221 Meridian Blvd Minden, NV
89423, USA, with a graphite resistive element and maximum temperature of 2500 ◦ C was
used for sintering. The samples were sintered at 1400 ◦ C under a 10−7 Torr vacuum with a
20 ◦ C/min heating rate. Upon reaching the specified temperature, the samples remained at
this level for two hours. The samples, after sintering, were progressively ground in 240,
400, 600, 800, 1200, and 2400 sandpaper. Polishing was performed in alumina solution,
with a final granulometry of 0.05 µm.

2.2. Deposition of the DLC Coating

The polished substrates were first cleaned by ultrasonic cleaning in distilled water
for 30 min and then in acetone for 20 min before being placed in the deposition chamber.
Next, the DLC coatings were deposited on the titanium alloy substrates using the PECVD
technique—plasma-enhanced chemical vapor deposition. The vacuum chamber for coating
deposition was assembled in Brazil, with an internal volume of 130 L, with a pumping
system composed of a mechanical pump of 90 m3 /h and a 2000 L/s diffuser pump. First,
the substrates were sputter cleaned using argon plasma for 30 min, a flow of 10 sccm, a
pressure of 0.15 Pa, and a self-polarization of −0.6 kV. This treatment with argon plasma
allowed for the elimination of the oxide layer on the metallic surfaces. Then, the DLC
coatings were deposited using acetylene (C2 H2 ) as a precursor gas, with a gas flow of
7.5 sccm, using an applied constant voltage of −0.75 kV and pressure of about 6.6 Pa. For
greater understanding concerning carrying out the tests and analyzing the results found,
the alloy samples under analysis were identified according to the treatment condition of
the DLC coating, as shown in Table 1.
Coatings 2024, 14, 1396 4 of 19

Table 1. Identification of the sample nomenclature and its description.

Sample Identification Description

SBTNZ Sample of uncoated Ti-22Nb-6Zr alloy (substrate).
Sample of Ti-22Nb-6Zr alloy with DLC coating
TNZ1
thickness of 0.487 ± 0.06 µm.
Sample of Ti-22Nb-6Zr alloy with DLC coating
TNZ4
thickness of 4.23 ± 0.08 µm.

2.3. X-ray Diffractometry

In this work, X-ray diffractometry was carried out to identify the crystalline phases
present in the SBTNZ substrate. A Shimadzu diffractometer, model XRD-6000, Shimadzu
corporation, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto, 604-8511, Japan, was used,
with a wavelength of 1.54 Å, generated by a Cu-Kα tube with a 2θ interval between 30◦
and 80◦ , with a step of 0.01◦ and a counting time of 1.5 s per step.

2.4. Vickers Microhardness

The samples of Ti-22Nb-6Zr alloys with and without DLC coating were submitted
to the Vickers microhardness test to evaluate the top microhardness (perpendicular to
the coating deposition surface). For this purpose, Microhardness Emco Test DuraScan
equipment, EMCO-TEST Prüfmaschinen GmbH, Kuchl, Germany, was used.

2.5. Scanning Electronic Microscopy

One of the primary functions of scanning electron microscopy (SEM) was the mea-
surement of the thickness of the DLC coating, consisting of a nanometer-scale for one
deposition condition (TNZ1) and a micrometer-scale for another condition (TNZ4), which
generated two coatings with different thicknesses. In addition, SEM was also used in the
following analyses:
- Microstructural evaluation of the samples, including energy-dispersive mapping;
- Evaluation of the wear mode observed inside the wear caps, considering the micro-
scale analysis.
A JEOL JSM 7100 FA Field Emission Scanning Electron Microscope, Tokyo, Japan,
was used.

2.6. Raman Spectroscopy

Raman spectroscopy was used to characterize the DLC coatings, identifying the types
of bonds. We sought to demonstrate the effective formation of the diamond-like carbon
coating. The equipment used to perform the Raman spectroscopy measurements was a
Jobin-Yvon triple spectrometer, model T64000, HORIBA Advanced Techno, Co., Ltd, Tokyo,
Japan, in the subtractive configuration using an optical microscope (50× objective—spot
around 5 µm). The Raman signal was detected by a CCD (charged couple device) cooled by
liquid nitrogen. The sample was excited using a coherent CW (continuous wave) argon-ion
laser tuned to the 532 nm line with a power of 10 mW on the sample. All spectra were
obtained at room temperature.

2.7. Microabrasive Wear

The microabrasive wear tests of samples with and without deposition of DLC coatings
were carried out with the CSM free sphere equipment, Anton Paar GmbH, Graz, Austria,
using the Calowear model. Its configuration is a free sphere made of 100 Cr6 steel, 20 mm
in diameter, which rotates continuously on the sample’s surface at a constant speed. The
abrasive medium was a suspension composed of alumina (Al2 O3 ) particles in distilled
water at a concentration of 0.40 g of abrasive per cm3 of water. We sought to use abrasive
particles with a hardness higher than the calcium particles present in the debris from bone
Coatings 2024, 14, 1396 5 of 19

degeneration in orthopedic implants. The objective was to test the wear resistance of the
samples under test conditions theoretically more rigorous than those existing in prostheses
implanted in the human body. The mean particle size was 1 µ. The abrasive suspension was
continuously stirred throughout the test by a magnetic stirrer coupled to the microabrasion
device to prevent the decanting of abrasive particles. The mixture was pumped to the
sphere–sample interface using a peristaltic pump connected to the equipment. The abrasive
flow rate was set at approximately one drop every 8 s. The rotation of the drive shaft was
maintained at 280 rpm, generating a velocity between the surface of the sphere and the
sample of approximately 0.195 m s−1 . Wear tests were performed, adjusting to the sliding
distance traveled by the ball. The initial test times for the substrate samples (SBTNZ) and
the samples (TNZ1) were set at 5 min intervals, with the first time being 15 min and the last
time being 40 min. Thus, for the times used, the first sliding distance was 175.66 m and the
last 468.44 m. Therefore, with increments of 58.55 m between intervals. Table 2 presents the
sliding distance and the respective times for each test.

Table 2. Sliding distance and respective times of each test for samples SBTNZ and TNZ1.

Test Time (min) Sliding Distance (m)

15 175.66
20 234.22
25 292.77
30 351.33
35 409.88
40 468.44

For samples with the thicker coating (TNZ4), test times were set at 15 min intervals,
with the first time being 95 min and the last time being 170 min. The increments were
175.66 m between intervals. Table 3 presents the sliding distance and the respective times
of each test.

Table 3. Sliding distance and the respective times of each test for the TNZ4 samples.

Test Time (min) Sliding Distance (m)

95 1112.54
110 1288.20
125 1463.86
140 1639.53
155 1815.19
170 1990.86

2.8. Wear Volume

The wear volume after each interval of the ball sliding over the sample’s surface was
determined using expression (1):

π · b4
V∼
= b≪ϕ (1)
32·ϕ

where:
- b is the diameter of the wear crater;
- ϕ is the diameter of the test sphere.
This equation is used to calculate the spherical crater since its volume is minimal
concerning the volume of the sphere.
Coatings 2024, 14, 1396 6 of 19

2.9. Wear Coefficient (K)

The Archard equation was used to calculate the wear coefficient K of the samples in
the uncoated condition (SBTNZ) and for the coating + substrate systems (TNZ1 and TNZ4)
considering the set (coating + substrate). Archard’s equation for calculating the sample
wear coefficient is shown in expression (2).

π · b4
K= (2)
32·ϕ·S·N

where:
- b is the diameter of the wear crater;
- S is the slip distance;
- ϕ is the diameter of the test sphere;
- N is the normal force to the sample.

2.10. Confocal Laser Microscopy

The reconstruction of the crater generated in the microabrasive tests was carried out
using a confocal laser microscope Olympus model LEXT OLS 4100, Olympus, Tokio, Japan.

2.11. VDI Indentation Test

The study of peeling-off regions (loss of adhesion) of the coatings was carried out
through a load applied with a Rockwell Durometer. In the Rockwell indentation images,
the damage caused to the coatings by the indentation was compared to reference standards
described in the VDI 3198 standard, which has a scale from HF1 to HF6. According to
that standard, an acceptable coating deposition is considered the qualitative verification of
classes HF1 to HF4. However, above class HF4, the coating does not have ideal adhesion,
and consequently, there are peeling-off regions in the deposited layer. Therefore, using the
standard, it is possible to classify the adhesion of the coatings qualitatively.

3. Results and Discussions

3.1. X-ray Diffractometry Results
Figure 1 illustrates the X-ray diffractometry analysis of the ternary alloy (Ti-22Nb-6Zr)
before coating (SBTNZ substrate), showing the α-Ti phase, the β-Ti phase, and niobium.
No intermetallics from the TiNbZr system were detected at the resolution level of the
diffractometer used.
Table 4 presents the results of the quantitative evaluation of the Tiα, Tiβ, and Ni phases
present and the lattice parameters of the alloy developed in this work, using the Rietveld
method. The substrate Ti22Nb6Zr presented 56 wt% Tiβ, 43 wt% Tiα, and 1% Nb.

Table 4. Lattice parameters and phase percentages using Rietveld for the Ti22Nb6Zr substrate.

Lattice Parameters (Å)

Phases % wt Calculated
a B c
Tiβ 56 3.3099 3.3099 3.3099
Tiα 43 2.9789 2.9789 4.7662
Nb 1 3.3369 3.3369 3.3369
Coatings 2024, 14, x FOR PEER REVIEW 7 of 21

Coatings 2024, 14, 1396 7 of 19

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Figure 1. X-ray diffraction patterns of the substrate Ti22Nb6Zr alloy (SBTNZ).

No intermetallics from the TiNbZr system were detected at the resolution level of the
diffractometer used.
Table 4 presents the results of the quantitative evaluation of the Tiα, Tiβ, and Ni
phases present and the lattice parameters of the alloy developed in this work, using the
Rietveld method. The substrate Ti22Nb6Zr presented 56 wt% Tiβ, 43 wt% Tiα, and 1%
Nb.

Table 4. Lattice parameters and phase percentages using Rietveld for the Ti22Nb6Zr substrate.

Lattice Parameters (Å)

Phases % wt Calculated
a B c
Tiβ 56 3.3099 3.3099 3.3099
Tiα 43 2.9789 2.9789 4.7662
Figure 1. 1.
Figure X-ray
Nb diffraction
X-ray patterns
diffraction 1 ofof
patterns thethe
substrate Ti22Nb6Zr
3.3369
substrate alloy
Ti22Nb6Zr (SBTNZ).
3.3369
alloy (SBTNZ). 3.3369

3.2.
No
3.2.Microstructural
intermetallics
Microstructural Analysis
from the TiNbZr system were detected at the resolution level of the
Analysis
Figure
diffractometer 2 shows
used.
Figure 2 shows thethe SEM
SEMmicrograph
micrographofofthethe Ti-22Nb-6Zr
Ti-22Nb-6Zr alloy
alloy produced
produced by powder
by powder
metallurgy,
Table 4 still without
presents the coating.
results The
of thegrain boundaries
quantitative are well
evaluation defined,
of the and
Tiα, the
Tiβ,
metallurgy, still without coating. The grain boundaries are well defined, and the matrix matrix
andisNi is
composed
phases of
present
composed β-phase and
and the and
of β-phase α-phase
lattice in
parameters
α-phase the form of lamellae,
of theofalloy
in the form with
developed
lamellae, low porosity
in porosity
with low this work, levels
using
levels and
andthe
high
Rietvelddensification.
method. The substrate Ti22Nb6Zr presented 56 wt% Tiβ, 43 wt% Tiα, and 1%
high densification.
Nb.

Table 4. Lattice parameters and phase percentages using Rietveld for the Ti22Nb6Zr substrate.

Lattice Parameters (Å)

Phases % wt Calculated
a B c
Tiβ 56 3.3099 3.3099 3.3099
Tiα 43 2.9789 2.9789 4.7662
Nb 1 3.3369 3.3369 3.3369

3.2. Microstructural Analysis

Figure 2 shows the SEM micrograph of the Ti-22Nb-6Zr alloy produced by powder
metallurgy, still without coating. The grain boundaries are well defined, and the matrix is
composed of β-phase and α-phase in the form of lamellae, with low porosity levels and
Figure 2. Scanning electron microscopy overview of the uncoated surface of Ti-22Nb-6Zr alloy,
Figure
high 2. Scanning electron microscopy overview of the uncoated surface of Ti-22Nb-6Zr alloy,
densification.
showing α lamellae and β interlamellar phase.
showing α lamellae and β interlamellar phase.
Figure 3a–c shows the mapping of elements present in the Ti22Nb6Zr alloy obtained
Figure 3a–c shows the mapping of elements present in the Ti22Nb6Zr alloy obtained
by EDS, where it is possible to observe the lower affinity of the α phase for the Nb and Zr
by EDS, where it is possible to observe the lower affinity of the α phase for the Nb and Zr
elements compared to the β phase.
elements compared to the β phase.
Coatings 2024,
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(a)

(b)

(c)
Figure3.
Figure 3. Distribution
Distribution of
ofTi,
Ti,Nb,
Nb,and
andZrZrelements
elementsobtained
obtainedby by
EDS for the
EDS for SBTNZ alloyalloy
the SBTNZ (Ti22Nb6Zr)
(Ti22Nb6Zr)
observing α phase needles (dark areas) distributed in the β matrix (a), EDS spectrum with emphasis
observing α phase needles (dark areas) distributed in the β matrix (a), EDS spectrum with emphasis
on α phase needles (lighter red areas) in the β matrix (dark regions) (b), and EDS imaging with α
on α phase needles (lighter red areas) in the β matrix (dark regions) (b), and EDS imaging with
phase needles (red) with niobium (blue dots) and zirconium (green dots) distributed in the β matrix
phase needles (red) with niobium (blue dots) and zirconium (green dots) distributed in the β
α(c).
matrix (c).
Coatings 2024,
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9 of 20
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3.3.
3.3.DLC
DLC Coatings
Coatings
3.3. DLC
The Coatings
TheDLC
DLCcoating
coating procedure
procedure waswas carried
carried out
out under
under the
the different
different processing
processing conditions
condi-
defined
tionsTheinDLC
the in
defined methodology,
coating givingwas
procedure
the methodology, risecarried
givingtorise
DLC coatings
toout
DLCunder with different
the different
coatings thicknesses.
processing
with different Figure 4
condi-
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presents
Figure 4 the
tions defined image
in the
presents for the for
image coating
themethodology, produced
thegiving
coatingrise on the
to DLC
produced Ti-22Nb-6Zr
coatings
on the alloy
with different
Ti-22Nb-6Zr ininaatreatment
thicknesses.
alloy treat-
Figurecondition
condition
ment 4 presents thegenerated
that generated
that image forathe
a thinner coating
coating
thinner produced
(sample
coating on TNZ1).
the Ti-22Nb-6Zr alloy in a treat-
TNZ1).
(sample
ment condition that generated a thinner coating (sample TNZ1).

Figure 4. Image of the Ti-22Nb-6Zr alloy substrate with the thinner coating (sample TNZ1) with
Figure 4. Image of the Ti-22Nb-6Zr alloy substrate with the thinner coating (sample TNZ1) with
Figure 4. Image
magnification of 1of27,000X.
the Ti-22Nb-6Zr alloy substrate
Mean measured with the thinner
coating thickness: coating
0.487 ± 0.06 µm.(sample TNZ1) with
magnification
magnification of
of 11 27,000 ×.Mean
27,000X. Meanmeasured
measured coating
coating thickness:
thickness: 0.487
0.487 ± 0.06
± 0.06 µm. µm.
Figure 5 depicts the cross-section of a sample of Ti-22Nb-6Zr alloy coated with DLC,
Figure
Figure 5 depicts the cross-section
cross-sectionofofa asample
sample of of Ti-22Nb-6Zr alloy coated with
DLC,DLC,
showing the5substrate
depicts the
and the coating layer. The lighterTi-22Nb-6Zr
area at the alloy
top ofcoated withshows
the image
showing
showing the
the substrate
substrate and the
and the coating
coating layer.
layer. The lighter
Theinlighter area at
area at thethe top of the image shows
the thicker coating surface (TNZ4 sample) seen perspective. Thetop
wavyof the image
surface shows
area un-
the thicker
the thicker
der
coating
the coatcoating
surface
surface
represents
(TNZ4
the (TNZ4
substrate
sample)
sample) seen in perspective. The wavy surface area un-area
seen
base material.
in perspective. The wavy surface
under
der thethe coat
coat represents
represents thethe substrate
substrate basebase material.
material.

Figure 5. Scanning micrography of the cross-section of coated Ti-22Nb-6Zr alloy (sample TNZ4)
Figure 5. Scanning micrography of the cross-section of coated Ti-22Nb-6Zr alloy (sample TNZ4)
showing
Figure 5. the substrate
Scanning and DLC coating.
micrography of the1000X magnification.
cross-section of coated Ti-22Nb-6Zr alloy (sample TNZ4)
showing the substrate and DLC coating. 1000X magnification.
showing the substrate and DLC coating. 1000× magnification.
Coatings 2024, 14, 1396 Figure 5. Scanning micrography of the cross-section of coated Ti-22Nb-6Zr alloy (sample10TNZ4)
of 19
showing the substrate and DLC coating. 1000X magnification.

Thedetailed
The detailedimage
imageofofthe
theTNZ4
TNZ4sample
sampleshows
showsthethedifference
differenceinintexture
texturebetween
betweenthe
the
substrate and
substrate and the coating with
with greater
greatermagnification
magnification(Figure
(Figure6).6).
Being a more
Being ductile
a more ma-
ductile
terial, thethe
material, substrate hashas
substrate a rougher aspect,
a rougher while
aspect, the coating
while section
the coating has ahas
section thinner and more
a thinner and
regular
more appearance.
regular appearance.

Figure6.6.Enlarged
Figure Enlargedimage
imageofofthe
theTNZ4
TNZ4sample
samplecoating
coatingand
andsubstrate
substrateatat7500
7500X magnification.The
× magnification. The
average measured coating thickness was 4.23 ± 0.08 μm.
average measured coating thickness was 4.23 ± 0.08 µm.

Thethickness
The thicknessmeasurements
measurements
of of
thethe coatings
coatings were
were carried
carried outthe
out in inSEM
the SEM software
software and
and Image
Image J (version
J (version 1.5h).1.5h).

3.4. Raman Spectroscopy

Analyses by Raman spectroscopy were carried out on specimens coated with DLC
coatings with a laser wavelength of 532 nm. The D and G bands are obtained by Gaussian
deconvolution, which makes it possible to determine the location of the bands, the full
width of the G band at half maximum (FWHM (G)), and the determination of the ID and IG
intensities. The spectrum could be deconvoluted using two Gaussian components for the
present work. Raman spectra were determined by Gaussian fitting in the 850–1800 cm−1
and are shown in Figures 7 and 8.
The Raman spectrum showed two characteristic bands (D and G) indicative of the
DLC phase formation [43]. In the DLC Raman spectra, the D band is attributed to the
deformation of disordered aromatic rings in the graphitic phase. Its appearance indicates
that it is an amorphous coating [44]. The G band is associated with C = C bonds (crystalline
graphite) located at sp2-hybridized carbon sites. It is common to define D and G bands in
the Raman spectrum of carbon-based materials [44]. The position values of the D and G
bands, the full width at half maximum (FWHM), and the ratio of ID/IG integral intensities
are shown in Table 5.
As described in Table 5 and through the deconvolution of the Raman spectrum, it is
possible to identify the presence of the D band centered at 1371 cm−1 for the TNZ1 condition
and 1380 cm−1 for the TNZ4. They are characteristics of graphite-like materials due to
the symmetrical E2g vibrational mode in graphite materials [6,7]. Thus, the analysis of the
Raman spectrum classifies the layers obtained in this work as diamond-like carbon (DLC)
by both bands typical for DLC-type films. Therefore, the results suggest that the structure
produced was composed of mixed carbon sp2 and sp3, characteristic of DLC materials.
 Coatings2024,
Coatings
Coatings 2024,14,
2024, 14,xxFOR
FORPEER
1396 PEERREVIEW
REVIEW 11 ofof 20
11 20
11 of 19

Figure7.7.7.
Figure
Figure Raman
Raman
Raman spectrum—deconvolution
spectrum—deconvolution
spectrum—deconvolution by Gaussian
byGaussian
by Gaussian lineslines
lines of sample
ofofsample
sample TNZ1.
TNZ1.
TNZ1.

Figure
Figure8.8.8.
Figure Raman
Raman
Raman spectrum—Gaussian
spectrum—Gaussian
spectrum—Gaussian deconvolutionofofthe
deconvolution
deconvolution the TNZ4
ofTNZ4 sample.
the TNZ4 sample.
sample.

Table The
The Ramanspectrum
Raman
5. Position spectrum
of showed
D and Gshowed
bands and two
two characteristic
characteristic
band intensity ratio. bands(D
bands (DandandG) G)indicative
indicativeof ofthe
the
DLCphase
DLC phaseformation
formation[43].
[43].In
Inthe
theDLC
DLCRaman Ramanspectra,
spectra,the theDDband
bandisisattributed
attributedto tothe
thede-
de-
formationof
formation of disordered D aromatic
Band ringsG inBand
thegraphitic
graphitic
Position phase.phase.
FWHM Itsappearance
appearance
(G) indicates
Sampledisordered aromatic −rings in the
−1 ]
Its
−1
indicates
Ratio ID /IG
thatititisisan
that anamorphous
amorphous Position
coating [44].1The
coating[cm
[44]. ]TheGGband
band[cmisis associatedwith
associated with[cmCC==C]Cbonds
bonds(crystalline
(crystalline
graphite)
graphite) locatedatatsp2-hybridized
located
TNZ1 sp2-hybridized
1371.10 carbon carbonsites.
sites. ItItisiscommon
1544.24 commonto to160.51
defineDDand
define andGGbands
bands in
in
1.01
theRaman
the Ramanspectrumspectrumof ofcarbon-based
carbon-basedmaterials
materials[44]. [44].The Theposition
positionvalues
valuesof ofthe
theDDandandGG
TNZ4 1383.59 1550.35 149.46 1.37
bands,the
bands, thefull
fullwidth
widthatathalf
halfmaximum
maximum(FWHM),(FWHM),and andthe theratio
ratioof
ofID/IG
ID/IGintegral
integralintensities
intensities
areshown
are shownin inTable
Table5.5.
3.5. Vickers Microhardness
The microhardness values for the substrates and the DLC coatings are shown in Table 6.
The Ti-22Nb-6Zr alloy substrate showed a microhardness of approximately 324 HV. As
expected, it can be seen that the uncoated alloy (SBTNZ) has microhardness values below
all coated samples analyzed, regardless of the thickness of the coating used.
 by both bands typical for DLC-type films. Therefore, the results suggest that the structure
produced was composed of mixed carbon sp2 and sp3, characteristic of DLC materials.

3.5. Vickers Microhardness

Coatings 2024, 14, 1396 The microhardness values for the substrates and the DLC coatings are shown in Table
12 of 19
6. The Ti-22Nb-6Zr alloy substrate showed a microhardness of approximately 324 HV. As
expected, it can be seen that the uncoated alloy (SBTNZ) has microhardness values below
all coated samples analyzed, regardless of the thickness of the coating used.
Table 6. Vickers microhardness of the substrate (SBTNZ) and the TNZ1 and TNZ4 coatings.
Table 6. Vickers microhardness of the substrate (SBTNZ) and the TNZ1 and TNZ4 coatings.
Vickers Microhardness (HV) (50 gF)
SBTNZ Vickers Microhardness
TNZ1 (HV) (50 gF) TNZ4
SBTNZ
324 ± 0.22
TNZ1
1170 ± 0.35
TNZ4
1750 ± 0.27
324 ± 0.22 1170 ± 0.35 1750 ± 0.27

3.6. Microabrasive Wear of the Substrate (SBTNZ) and Coated Samples (TNZ1 and TNZ4)
3.6. Microabrasive Wear of the Substrate (SBTNZ) and Coated Samples (TNZ1 and TNZ4)
Two-bodyabrasive
Two-body abrasivewear
wearis is caused
caused by by rubbing
rubbing of aofsofter
a softer surface
surface by a by a hard
hard roughrough
surface, while three-body abrasive wear is caused by hard particles entrapped between two
surface, while three-body abrasive wear is caused by hard particles entrapped between
sliding
two surfaces
sliding [45].
surfaces Both
[45]. forfor
Both the analysis
the analysisofofthe
themicroabrasive wearofofthe
microabrasive wear theTi-22Nb-
Ti-22Nb-6Zr
alloy substrate and the samples coated with DLC, the measurements
6Zr alloy substrate and the samples coated with DLC, the measurements of the wear of the wear craters
cra-
were carried out using confocal laser microscopy and scanning electron microscopy.
ters were carried out using confocal laser microscopy and scanning electron microscopy.

3.6.1. Diameter
3.6.1. Diameterofofthe
theCraters
Craters
Foreach
For eachwear
wearcrater
craterproduced,
produced, thethe determination
determination ofexternal
of its its external diameter
diameter was carried
was carried
out through
out throughthetheaverage
average of five
of five diameters.
diameters. Four Four diameters
diameters were measured
were measured in different
in different di-
directions
rections (Figure
(Figure 9a).9a).
TheThe
fifthfifth diameter
diameter waswas obtained
obtained by inserting
by inserting a circle
a circle by thebyconfocal
the confocal
microscopeanalysis
microscope analysisprogram,
program,asas exemplified
exemplified in Figure
in Figure 9b. 9b.

(a) (b)

Figure 9. Measurement of the diameter of a worn crater by averaging (a) four diameters in different
directions; (b) diameter of the inserted circle.

3.6.2. Abrasive Wear Mechanisms

Detailed images of the wear craters were obtained by scanning electron microscopy to
identify the predominant abrasive wear mechanisms in each sample. Micro-rolling abrasive
wear was identified between the grooving abrasive wear risks in the SBTNZ samples and
for the thinnest coating (TNZ1). As for the thicker coating (TNZ4), the predominant wear
mechanism was rolling, as shown in Figures 10–12.
 3.6.2. Abrasive Wear Mechanisms
Detailed images of the wear craters were obtained by scanning electron microscopy
to identify the predominant abrasive wear mechanisms in each sample. Micro-rolling
Coatings 2024, 14, 1396
abrasive wear was identified between the grooving abrasive wear risks in the SBTNZ sam-
13 of 19
ples and for the thinnest coating (TNZ1). As for the thicker coating (TNZ4), the predomi-
nant wear mechanism was rolling, as shown in Figures 10–12.

(a) (b)

Coatings 2024, 14, x FOR PEER REVIEW (c) 15 of 21

Figure
Figure10.
10.Images
Imagesof of
thethe
same wear
same crater
wear at different
crater magnifications
at different ((a) 1200X,
magnifications (b) 3000X,
((a) 1200 ×, (b)and (c)×, and
3000
5500X) for the SBTNZ sample showing wear by grooving abrasion and micro-rolling.
(c) 5500×) for the SBTNZ sample showing wear by grooving abrasion and micro-rolling.

(a) (b)

(a) (b)

(c)
Figure 11. Abrasive
Abrasive wear
wear by
by grooving
grooving abrasion
abrasion and
and micro-rolling
micro-rolling in the same wear crater at
at different
different
magnifications ((a) 1200X, (b) 3000X, and (c) 5500X) for the TNZ1 condition.
magnifications ((a) 1200×, (b) 3000×, and (c) 5500×) for the TNZ1 condition.
 (c)
Coatings 2024, 14, 1396 14 of 19
Figure 11. Abrasive wear by grooving abrasion and micro-rolling in the same wear crater at differ
magnifications ((a) 1200X, (b) 3000X, and (c) 5500X) for the TNZ1 condition.

(a) (b)

Figure 12. RollingFigure 12. Rolling

abrasive wear inabrasive wear
the same in the
wear sameat
crater wear crater at
different different magnifications
magnifications ((a) 1900×((a) 1900X a
(b) 5000X) for the TNZ4
and (b) 5000×) for the TNZ4 condition. condition.

For grooving
For grooving abrasive abrasive
wear, the wear,must
particles the particles must be
be embedded inembedded in the
the specimen orspecimen
in or
the sphere (body). In the samples that showed grooving abrasive wear
the sphere (body). In the samples that showed grooving abrasive wear and micro-rolling and micro-roll
between
between the grooving the grooving
abrasive abrasive
wear risks wear and
(SBTNZ risksTNZ1),
(SBTNZtheand TNZ1), the characteristic
characteristic grooves groo
of micro-grooving prevailed, where it is possible to identify the displacement of mate
of micro-grooving prevailed, where it is possible to identify the displacement of material
adhered to the edges of the groves. However, for samples with a thicker film, TNZ4,
adhered to the edges of the groves. However, for samples with a thicker film, TNZ4, the
mode of wear of the substrate material under the test conditions was by rolling.
mode of wear of the substrate material under the test conditions was by rolling.
3.6.3. Volumes and Wear Coefficients
3.6.3. Volumes and Wear Coefficients
The wear volume as a function of the sliding distance for the Ti-22Nb-6Zr alloy su
The wear volume as a function of the sliding distance for the Ti-22Nb-6Zr alloy
Coatings 2024, 14, x FOR PEER REVIEW strate sample without any coating (SBTNZ), with the thinner coating 20 (TNZ1),
15 ofand and w
substrate sample without any coating (SBTNZ), with the thinner coating (TNZ1), with
increased DLC film thickness (TNZ4) are presented in Figure 13.
increased DLC film thickness (TNZ4) are presented in Figure 13.

Figure
Figure13.
13.Total
Totalwear
wearvolume
volume as as
a function of sliding
a function distance
of sliding for SBTNZ
distance (substrate),
for SBTNZ TNZ1 TNZ1
(substrate), (thinner
(thinner
coating),
coating),and
andTNZ4
TNZ4(thick film).
(thick film).

Through
Throughthe analysis
the analysisof Figure 13, it13,
of Figure was possible
it was to notice
possible a successive
to notice increaseincrease
a successive in
the amount
in the amount of material worn worn
of material for greater sliding sliding
for greater distances. When compared,
distances. the substrate
When compared, the sub-
samples show a more
strate samples showsignificant volume loss
a more significant at all sliding
volume loss at distance measurement
all sliding points,
distance measurement
i.e., the worn
points, volume
i.e., the worn of the substrate
volume is always is
of the substrate greater
always than that of
greater coated
than thatsamples (film
of coated samples
+(film
substrate system),
+ substrate regardless
system), of the coating
regardless thickness,
of the coating indicating
thickness, the effectiveness
indicating of the
the effectiveness of
protective treatment. One way to verify if the steady-state of wear has been
the protective treatment. One way to verify if the steady-state of wear has been reached reached is
through the analysis of the graphs of the wear volume (V) as a function of the sliding
distance (S) (V = f(S)). If the wear volume shows a linear variation with the sliding dis-
tance, it is considered that the steady-state wear regime has been reached. A constant (per-
manent) wear regime is obtained for the condition with constant normal force when the
wear volume is linearly dependent on the sliding distance. In all graphs, the linearity of
the V x S relationship is observed for the results of the data obtained experimentally, in-
Coatings 2024, 14, 1396 15 of 19

is through the analysis of the graphs of the wear volume (V) as a function of the sliding
distance (S) (V = f(S)). If the wear volume shows a linear variation with the sliding dis-
tance, it is considered that the steady-state wear regime has been reached. A constant
(permanent) wear regime is obtained for the condition with constant normal force when
the wear volume is linearly dependent on the sliding distance. In all graphs, the linearity
of the V × S relationship is observed for the results of the data obtained experimentally,
indicating the condition of the steady-state wear regime for the substrate with and without
coatings. Based on Figure 13, which characterizes trends close to a straight line and the
consideration of obtaining the steady-state wear regime, the wear rate was determined, as
shown in Table 7. As previously described, the wear rate of wear wasobtained by deriving
dV
the equation of the curve referring to each of these figures Q = ds .

Table 7. Wear rate for the Ti-22Nb-6Zr alloy substrate and for substrate and systems with DLC
coatings (TNZ1 and TNZ4).

Sample SBTNZ
Equation
Wear Rate (Q)− Converted to [mm3 /m]
V = f(S) − V(m3 ) and S(m)
V = 3.61528 × 10−14 S 3.62 × 10−5 or 0.0000362
Sample TNZ1
V = 6.89591 × 10−15 S 6.90 × 10−6 or 0.0000069
Sample TNZ4
V = 1.06325 × 10−15 S 1.06 × 10−6 or 0.00000106

The wear rate obtained (Q) is the volume of material removed per unit of sliding
distance between the ball and the specimen. Considering the wear rate, the coated samples
showed better wear resistance. The most significant reduction in the wear rate relative to
the substrate was obtained with the highest coating thickness (TNZ4), as shown in Table 8.

Table 8. Reduction in wear rate (%) in relation to the substrate.

Sample Reduction in Wear Rate (%)—in Relation to the Substrate

TNZ1 80.93
TNZ4 97.06

From the graphs constructed to verify that the analyses were carried out under steady-
state wear, it was possible to calculate the substrate’s wear coefficient and substrate + DLC
coating system through Archard’s equation (K = πb4 /(32·Φ·S·N)). Table 9 presents the
results of the wear coefficients of the samples coated with DLC and without coating for the
test conditions used, while Figure 14 shows the graphic representation of these values.
It was observed that the DLC-coated samples showed superior performance when
compared to the uncoated samples. As shown in Table 9 and Figure 14, the sample with
the thicker coating (TNZ4) showed better wear resistance when compared to the respective
substrate, with a wear coefficient 34.68 times lower. This progress was also observed in the
TNZ1 sample, with a wear coefficient (k) 5.53 times lower than its substrate.

Table 9. Values of the wear coefficient of the substrate (SBTNZ) and samples with DLC coating of
different thicknesses (TNZ1, TNZ4).

Sample Wear Coefficient (K)− [m3 /N.m] Standard Deviation

SBTNZ 7.20 × 10−13 1.08 × 10−13
TNZ1 1.30 × 10−13 1.48 × 10−14
TNZ4 2.08 × 10−14 5.54 × 10−15
 Sample Wear Coefficient 𝐊 [m3/N.m] Standard Deviation
SBTNZ 7.20 × 10−13 1.08 × 10−13
TNZ1 1.30 × 10−13 1.48 × 10−14
TNZ4 2.08 × 10−14 5.54 × 10−15
Coatings 2024, 14, 1396 16 of 19

Coatings 2024, 14, x FOR PEER REVIEW 17 of 20

Figure 14. Graphic representation of the wear coefficients calculated by the Archard equation.

Figure3.6.4. Adhesion
14. Graphic of Coatings
representation of the wear coefficients calculated by the Archard equation.
3.6.4. Adhesion of Coatings
In their different coating conditions, the samples were subjected to evaluation of
was In
Itadhesion their different
observed
of DLCthat thecoating
films through
conditions,
DLC-coated samples
Rockwell
the samples were subjected
showed superior
C indentation (HRC).
to evaluation of ad-
performance
Figure 15awhenshows a typical
comparedhesion of DLC films
to the uncoated through
samples. Rockwell C indentation (HRC). Figure 15a shows a typical
indentation generated on theAs shownofinthe
surface Table
TNZ19 and Figure Very
sample. 14, the sample
thin radial with
cracks can be
indentation
the thicker coating generated
(TNZ4) on the surface of the TNZ1 sample. Very thin radial cracks can be
detected. Small chipsshowed better wear
can be observed resistance
only when
at specific compared
points on thetoedge
the respec-
of the indentation
detected.
tive substrate, Small chips
with a wear can be
coefficientobserved only at specific points on the edge of the indentation
for the coating on the titanium34.68
alloy,times lower.inThis
as shown the progress
detailed wasimagealsoinobserved
Figure 15b. No cir-
for thesample,
coatingwith
in the cumferential
TNZ1 on thea titanium
wear alloy,(k)
coefficient as 5.53
shown in lower
times the detailed
than image
its in Figure 15b. No
substrate.
circumferential cracks or or
cracks significant delaminations
significant delaminations around
aroundthe theedge
edgewere
wereobserved.
observed. Interfacial
Inter-
bonds are strong. Even in the region where the substrate
facial bonds are strong. Even in the region where the substrate accumulates accumulates due to to
due deformation
defor-
caused
mationby indentation,
caused minimalminimal
by indentation, chipping is observed
chipping in the coating.
is observed UnderUnder
in the coating. these treatment
these
conditions (TN1), the DLC film showed satisfactory adhesion, with
treatment conditions (TN1), the DLC film showed satisfactory adhesion, with an adhesion an adhesion quality
index
quality index of the coating–substrate pair related to the HF2 condition provided for in VDI
of the coating–substrate pair related to the HF2 condition provided for in the
3198 standard.
the VDI 3198 standard.

Figure 15.
Figure 15. Characterization
Characterizationof the HRCHRC
of the indentation test ontest
indentation the surface
on the of the TNZ1
surface sample
of the TNZ1(a), sample
show- (a),
ing good adhesion (HF2) and thin radial cracks (b).
showing good adhesion (HF2) and thin radial cracks (b).
Typical indentations of Rockwell C hardness from the adhesion test of the DLC coat-
Typical indentations of Rockwell C hardness from the adhesion test of the DLC coating
ing on the TNZ4 sample are shown in Figure 16. Very thin radial cracks can also be ob-
on the TNZ4 sample are shown in Figure 16. Very thin radial cracks can also be observed
served on the edge of these indentations. For both the thinner (TNZ1) and thicker (TNZ4)
on the edge of these indentations. For both the thinner (TNZ1) and thicker (TNZ4) coatings,
coatings, these cracks were favored because titanium alloy substrates are soft and produce
these cracks were favored because titanium alloy substrates are soft and produce great
great deformation under load. However, the resistance to DLC peeling-off proved to be
deformation under load. However, the resistance to DLC peeling-off proved to be quite
quite efficient. Also, for TNZ4, the interfacial bonds are strong, with minimal DLC coating
efficient.
peeling offAlso,
even for TNZ4,
in the the interfacial
area where bonds
the substrate are strong,
accumulates with minimal
(pile-up), i.e., where DLC
therecoating
is
peeling
an amount of material displaced to the edge of the indentation due to surface plastic de-there
off even in the area where the substrate accumulates (pile-up), i.e., where
isformation
an amount of material
during displaced
an adhesion to the edge ofcracks
test. Circumferential the indentation due to around
were not observed surfacetheplastic
deformation during
edge. The thicker an adhesion
coating showed test. Circumferential
the adhesion cracks
quality index of were not observed around
the coating–substrate pair, the
edge.
whichThe
canthicker coatingwith
be associated showed the adhesion
the HF2 quality index
pattern, representing of thecoating
a good coating–substrate
adhesion to pair,
the substrate.
 served on the edge of these indentations. For both the thinner (TNZ1) and thicker (TNZ4)
coatings, these cracks were favored because titanium alloy substrates are soft and produce
great deformation under load. However, the resistance to DLC peeling-off proved to be
quite efficient. Also, for TNZ4, the interfacial bonds are strong, with minimal DLC coating
Coatings 2024, 14, 1396
peeling off even in the area where the substrate accumulates (pile-up), i.e., where there is
17 of 19
an amount of material displaced to the edge of the indentation due to surface plastic de-
formation during an adhesion test. Circumferential cracks were not observed around the
edge. The thicker coating showed the adhesion quality index of the coating–substrate pair,
which
which can
can be
be associated withthe
associated with theHF2
HF2pattern,
pattern, representing
representing a good
a good coating
coating adhesion
adhesion to to
the substrate.
the substrate.

Figure 16. Characterization of the HRC indentation test on the surface of the TNZ4 sample (a),
showing good adhesion (HF2) and small radial microcracks (b).

4. Conclusions
1. X-ray diffraction examinations of the substrate (SBNTZ) showed α-Ti and β-Ti phases.
No intermetallics from the TiNbZr system were detected at the resolution level of the
diffractometer used.
2. Scanning electron microscopy analyses of these substrates showed homogeneous
microstructures with low porosity and high densification. In both cases, lamellae
and well-defined grain boundaries of alpha phase and matrix composed of β phase
prevailed. Spectral mappings via EDS showed the concentration of the beta-stabilizing
element (Nb) in the beta matrix
3. Through the deconvolution of the Raman spectrum, it was possible to identify the
presence of the D band centered at 1371 cm−1 for the TNZ1 condition and 1380 cm−1
for the TNZ4. They are characteristics of graphite-like materials due to the symmetrical
E2g vibrational mode in graphite materials. Thus, the analysis of the Raman spectrum
classifies the layers obtained in this work as diamond-like carbon (DLC) by both bands
typical for DLC-type films. Therefore, the results suggest that the structure produced
was composed of mixed carbon sp2 and sp3, characteristic of DLC materials.
4. Both the thinnest (TNZ1) and the thickest (TNZ4) coatings did not present with DLC
delamination or peeling-off in the Rockwell C indentation adhesion tests provided
for in the VDI 3198 standard. The PECVD process used in the deposition of the DLC
films produced strong coating–substrate interfacial bonds. The quality indices of the
film-substrate pairs reached the HF-2 condition of the standard, as mentioned earlier,
indicative of excellent adherence.
5. The PECVD-deposited DLC coatings using acetylene (C2 H2 ) as a precursor gas, with a
gas flow of 7.5 sccm and constant voltage of −0.75 kV, applied on Ti-22Nb-6Zr titanium
alloy produced by powder metallurgy, showed excellent tribological performance in
microabrasive wear tests compared to the results obtained for the wear resistance of
uncoated substrates.
6. The three-body wear prevailed for the thicker coating (TNZ4), giving rise to rolling
wear with abrasive particles rolling during the test, which is less aggressive. Two
bodies wear was predominant for the thinner coatings (TNZ1).
7. According to the results obtained in this work, the DLC coatings in both thicknesses
provided good structural quality, homogeneity, adhesion, high hardness, and re-
sistance to microabrasive wear. Therefore, the deposited DLC coatings are very
promising to improve the tribological behavior of Ti-22Nb-6Zr titanium biomedical
alloys produced by powder metallurgy
Coatings 2024, 14, 1396 18 of 19

8. Future research aims to evaluate the influence of the microabrasive medium on

wear resistance.

Author Contributions: Conceptualization, S.J.G., V.A.R.H., V.J.T.A. and C.R.M.d.S.; Methodology,

S.J.G., J.L.d.A.F. and J.A.A.; Validation, S.J.G., J.A.A., P.A., V.A.R.H., V.J.T.A. and C.R.M.d.S.; Formal
analysis, P.A. and C.R.M.d.S.; Investigation, S.J.G., J.L.d.A.F., P.A., V.A.R.H., V.J.T.A. and C.R.M.d.S.;
Writing—original draft, S.J.G. and C.R.M.d.S.; Writing—review & editing, C.R.M.d.S. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article.
Conflicts of Interest: The authors declare no conflicts of interest.

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